CHARGE TRANSPORT IN ORGANIC ELECTRONIC ......1.2.1 Molecular Rectifiers.....3 1.3 Organic...

CHARGE TRANSPORT IN ORGANIC ELECTRONIC DEVICES: FROM NANO TO MACRO-SCALE

BY

ZACHARY ALAN LAMPORT

A Dissertation Submitted to the Graduate Faculty of

WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES

in Partial Fulfillment of the Requirements

for the Degree of

DOCTOR OF PHILOSOPHY

Physics

May 2018

Winston-Salem, North Carolina

Copyright Zachary A. Lamport 2018

Approved By:

Oana D. Jurchescu, Ph.D., Advisor

Mark E. Welker, Ph.D., Chair

Martin Guthold, Ph.D.

Jed C. Macosko, Ph.D.

Timo Thonhauser, Ph.D.

ii

Acknowledgements

There are so many people that I would like to thank for helping me get to where I am today. First on the list is my advisor and mentor, Dr. Oana Jurchescu. She has had an incredibly positive influence on each facet of my life, starting with allowing me to join her group, to learn, and especially to grow under her direction. Her patience, kindness, and understanding made a world of difference to me as I struggled through difficult situations in my life. I am particularly grateful for her entrusting so many wonderful undergraduate students to me which has simply been an awesome experience for me to learn as a teacher and mentor for my own students. One of the remarkable things about Oana is that she can tell when someone will fit in well with our group, and through this she has put together what is certainly the best, in more ways than one, group at Wake Forest. And of course, I must thank the undergraduate students with whom I have been so very fortunate to work. First among them was Andy Vuong, and I feel that my thanks to him almost requires an apology, as I was still very much learning how to teach my own students, however he deserves my gratitude for all his hard work. Next was Katrina Barth, who worked with me for all four years of her undergraduate degree and I truly cannot say enough about. It has been a great pleasure to watch, and hopefully to help, her growth through the years. Her undying enthusiasm and cheerfulness made each day she was in the lab seem a little brighter, and for that alone I must thank her. Beyond those qualities, her determination and dedication to her work was inspiring to see and I know she is destined for great things both during and after her graduate studies at Duke. The work shown in Chapters 2 and 5 would not have been possible without her, especially impressive considering those are two radically different types of experiments. Following her, Ben Scharmann joined our group and I couldn’t have been happier to have him as a student. His sense of humor and, just generally being a character, made him a welcome addition to an already somewhat eccentric mixing of personalities in our lab group. He has also taken on the responsibility of training a new undergraduate himself which is quite impressive along with his other duties. But of course, he has done tremendous work in the two years that he has worked with me on the molecular rectifier project and the work shown in Chapter 3 shows the fruits of his labors, and his future success at the Georgia Tech campus in Shenzhen, China is assured. The last is Robby Bradford, and unfortunately he and I haven’t had much time together in the lab as Ben was his main instructor, but I can see that he is a quick study and look forward to working with him over this summer. I would be remiss if I did not also thank the students that had come before me. Eric Chapman, in his dual role as lab manager and graduate student, has been a tremendous resource for those of us in the department as well as a genuinely good person to talk to. He has always been willing to help with any question or problem that I had, both in research and in my TA work, and for that I am

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forever grateful. Dr. Jeremy Ward was a great influence on me as he spent so much time teaching me early in my degree, but also he taught me how to teach, which has been invaluable when put into practice with my own students. He had also set a terrific example in how to be an efficient researcher and how to make the most of the time that you have. Dr. Katelyn Goetz was also instrumental in my early career as she taught me everything that I know about the single-crystal growth and fabrication process, for which I am eternally grateful. She was also just a generally fun person to be around in our lab and office and her effect on the group can still be seen today. Dr. Yaochuan Mei provided a sterling example of how creativity in the lab, and the willingness to try everything, can result in fantastic advances in our research. Last but far from least is Dr. Peter Diemer, who after graduating has stayed with our group and really kept the whole place together. Pete has been a great friend through the 6 years that I have known him, and it has been a great experience getting to know both him and his family. His willingness to support me in both research and personally will never be forgotten and I don’t know if I could ever express how much he has helped over the years. And of course, I must thank the students that are still with us, first being Andrew Zeidell. He has become a great friend to me over these past three years and it has been a pleasure getting to know him both inside and out of the lab. He has continued on with the tradition of keeping a sense of humor around the lab and office (whether it is appreciated or not!), but also has an enviable determination with which he pursues his research projects. Then came Colin Tyznik and Hamna Haneef, who, even though they joined the group together, couldn’t be more different. I’ve greatly appreciated conversations with Colin about, well, nearly everything but it has been awesome to find someone who shares my love of fantasy books and will listen to me expound on one series or another. I’m glad to see that he is taking his research in a different direction than the FET which we have come to know and love, because I had done much the same thing and I like to see our group expand its focus. It has been great getting to know Hamna, whose (evil?) sense of humor has made these past two years a ton of fun. I only hope that she is more patient than I was as she is working on her research into single crystal semiconductors. And the last member to join our group is Matthew Waldrip, who has only been with us for about a year but has already been a great addition to the group and I look forward to great things from him in the future. Then I also must thank my brother Brendan and my parents Bert and Janet. They were with me every step of the way, providing inspiration throughout and, often enough in the case of my mom, a much-needed sounding board. There is absolutely no way that I would have made it to where I am today without their constant love and encouragement and the depth of my gratitude cannot be accurately expressed.

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Table of Contents

List of figures .................................................................................................................... vi

List of tables.................................................................................................................... viii

List of abbreviations ........................................................................................................ ix

Abstract ...............................................................................................................................x

Chapter 1 Introduction................................................................................................1

1.1 Electronic Function through Chemistry .......................................................1 1.2 Molecular Electronics ..................................................................................2

1.2.1 Molecular Rectifiers.........................................................................3 1.3 Organic Field-effect Transistors ..................................................................7

1.3.1 OFET Structure and Operation ........................................................8 1.3.2 FET Analysis .................................................................................13 1.3.3 Charge Injection in OFETs ............................................................15 1.3.4 Contact Modification .....................................................................19 1.3.5 Contact Resistance in OFETs ........................................................20

1.4 Thesis Outline ............................................................................................23 References ..............................................................................................................25

Chapter 2 Fluorinated Benzalkylsilane Molecular Rectifiers ...............................33

2.1 Introduction ................................................................................................34 2.2 Experimental ..............................................................................................36

2.2.1 Molecular Structures and Analysis ................................................36 2.2.2 Device Fabrication .........................................................................39 2.2.3 Surface Analysis ............................................................................40 2.2.4 Ab Initio Calculations ....................................................................43 2.2.5 Electrical Characterization .............................................................44

2.3 Results and Discussion ..............................................................................44 2.4 Conclusions ................................................................................................50 References ..............................................................................................................52

Chapter 3 Molecular Diodes from Self-Assembled Monolayers on Silicon ..........56


3.2.1 Device Fabrication .........................................................................60

v

3.2.2 Electrical Characterization .............................................................62 3.2.3 Cyclic Voltammetry .......................................................................62


Chapter 4 Organic Thin Films with Charge Carrier Mobility Exceeding that of Single Crystals ..............................................................................72


4.2.1 Single Crystal Growth....................................................................76 4.2.2 OFET Fabrication and Electrical Characterization ........................77 4.2.3 Theoretical Calculations ................................................................78 4.2.4 Structural Characterization ............................................................78


Chapter 5 Organic Thin-Film Transistors with Charge-Carrier Mobilities of 20 cm2/Vs, Independent of Gate Voltage ...........................................96

5.1 Introduction ................................................................................................97 5.2 Experimental ............................................................................................100

5.2.1 Device Fabrication .......................................................................100 5.2.2 Device Characterization ...............................................................101 5.2.3 Grazing Incidence X-Ray Diffraction ..........................................101

5.3 Results and Discussion ............................................................................102 5.4 Conclusions ..............................................................................................112 References ............................................................................................................114

Curriculum Vitae ...........................................................................................................119

vi

List of Figures

1.1 Device structures on the two relevant length scales ....................................1 1.2 Electrical characteristics for a molecular rectifier .......................................5 1.3 Mechanism of rectification ..........................................................................6 1.4 OFET device structures................................................................................7 1.5 Operation of an OFET from linear to saturation ..........................................9 1.6 Transfer characteristics of an OFET ..........................................................11 1.7 Injection in a p-type OFET ........................................................................16 1.8 Gated-TLM measurement ..........................................................................22

2.1 Chemical structures of fluorinated benzalkylsilanes .................................36 2.2 General synthesis of fluorinated benzalkylsilanes .....................................39 2.3 Contact angle measurements......................................................................41 2.4 AFM measurement on silicon ....................................................................43 2.5 Device structure of molecular rectifiers and electrical measurements ......45 2.6 Representative measurements on SAMs ....................................................47 2.7 Rectification ratio vs. dipole moment ........................................................48

3.1 Chemical and device structures .................................................................61 3.2 Average J-V curves ....................................................................................63 3.3 Histograms of measured rectification ratios ..............................................65 3.4 J-V characteristics of native SiO2 ..............................................................66 3.5 Cyclic voltammetry measurements ............................................................67

4.1 Chemical structure of TMS-BT .................................................................74 4.2 PVT setup and device structure .................................................................76 4.3 Electrical characteristics of single-crystal OFETs of TMS-BT .................80 4.4 Histogram of single-crystal OFET mobilities ............................................81 4.5 Band structure of TMS-BT ........................................................................82 4.6 DFT calculations of TMS-BT ....................................................................83 4.7 GIXD on single crystals .............................................................................84 4.8 Illustration of device structure in reference to crystal structure ................85 4.9 Thin-film OFET and electrical characteristics ...........................................86 4.10 XRD on single crystals and thin films .......................................................87 4.11 Thin-film XRD assuming (001) texture .....................................................88 4.12 Thin-film XRD assuming (02-1) texture ...................................................88

5.1 Chemical structure of diF-TES ADT and electrical characteristics of thin-

film OFETs ..............................................................................................102 5.2 Alternate OFET characteristics ................................................................103

vii

5.3 Mobility vs. gate voltage..........................................................................104 5.4 Mobility and contact resistance vs. deposition rate .................................105 5.5 GIXD measurements and molecular orientation of diF-TES ADT .........106 5.6 Device structure and contact resistance illustration .................................107 5.7 AFM measurements of Au deposited at slow and fast rates ....................108 5.8 Fast Fourier transforms of AFM images ..................................................109 5.9 KPFM measurements of PFBT-treated Au at slow and fast rates ...........110 5.10 Chemical structure of C16IDT-BT and electrical characteristics of thin-film

OFETS .....................................................................................................111

viii

List of Tables

2.1 Properties of SAMs composed of compounds 1-9 ....................................42

ix

List of Abbreviations

Abbreviation Meaning

SAM Self-assembled monolayer

HOMO Highest occupied molecular orbital

LUMO Lowest unoccupied molecular orbital

R Rectification ratio

MO Molecular orbital

EF Fermi level

OPV Organic photovoltaic

OLED Organic light-emitting diode

OFET Organic field-effect transistor

µ Field-effect mobility

DOS Density of states

WF Work function

CT Charge transfer

KPM Kelvin probe microscopy

RC Contact resistance

RCh Channel resistance

TLM Transmission line method

DFT Density functional theory

XRD X-ray diffraction

EGaIn Eutectic gallium-indium

AFM Atomic force microscopy

TMS-BT 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene

PVT Physical vapor transport

GIXD Grazing incidence x-ray diffraction

diF-TES ADT 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene

SKPM Scanning Kelvin probe microscopy

x

Abstract

The electrical properties of devices based on an organic compound result from the structure of the molecules, their solid-state packing, efficiency of charge injection from the electrodes, and the fabrication procedures. The length scales of interest can also vary widely, ranging from a few nanometers in the case of charge transport through single molecules or two-dimensional molecular ensembles, to tens of micrometers in devices focusing on thin films or molecular crystals. The work outlined in this thesis examines the characteristics of electronic devices at both extremes by incorporating organic molecules in molecular rectifiers and organic field-effect transistors (OFETs).

We successfully designed and fabricated molecular rectifiers based on self-assembled monolayers and identified relevant structure-function relationships. We elucidate the dependence of the rectification behavior on molecular length and structure, and found that the degree of rectification is enhanced in shorter molecules and linearly dependent on the strength of the molecular dipole moment. We further developed compounds that, when included into the molecular diodes, rectified current by as much as three orders of magnitude depending on their structure. This performance is on par with that of the best molecular rectifiers obtained on a metallic electrode, but it has the advantage of lower cost and more efficient integration with current silicon technologies, which may yield hybrid systems that can expand the use of silicon towards novel functionalities governed by the molecular species grafted onto its surface.

We then explored charge transport in OFETs using the organic semiconductor 7,14-bis(trimethylsilylethynyl)benzo[k]tetraphene (TMS-BT). We produced thin-film OFETs which exhibited more efficient electronic transport than single crystal devices of the same material, in spite of the inherent presence of grain boundaries. We explained these findings in terms of charge transport anisotropy and electronic trap formation at the interface between the semiconductor and dielectric. We further reduced aggressively the contact resistance in small molecule and polymer OFETs by varying the metal deposition rate, which resulted in over 5 times improved charge carrier mobility compared with the best reported devices with identical composition and structure. The obtained contact resistance normalized over the channel width was 500 Ωcm, and the corresponding devices exhibited charge carrier mobilities of 19.2 cm2/Vs for 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and 10 cm2/Vs for indacenodithiophene-co-benzothiadiazole copolymer (C16IDTBT), with minimal dependence on the gate voltage.

1

Chapter 1 Introduction

1.1 Electronic Function through Chemistry

The synthetic versatility of organic chemistry offers a seemingly limitless variety of

electronic properties and possible device configurations based on covalent bonds and van

der Waals interactions. The reactivity, bonding mechanisms, band gap, and surface energy,

among other properties, can, in principle, be tuned at will by altering the chemical structure.

The work incorporated in this thesis focused on the study of various electrical properties

of such molecules. In each of the compounds discussed here, the feature of interest is the

presence of delocalized, π-conjugated electrons which allows access to the frontier electron

orbitals. Some electronic materials investigated here can covalently bond to a substrate in

order to examine their properties on the molecular scale, i.e. single molecule charge

transport, see for example Figure 1.1a. Other compounds form molecular crystals such that

the π-electrons are delocalized across multiple molecules. This aggregation gives rise to

a) b)

Figure 1.1. (a) Example device structure on the molecular scale, (b) Example device structure

spanning the nanoscale to the microscale.

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interesting properties, which are highly sensitive to the solid-state packing both at the nano-

scale, and the long-range order up to micrometers, see Figure 1.1b. In the first example,

the necessary presence of a σ-bonded alkyl chain allows the exploitation of a molecular

imbalance and the creation of electronically asymmetric devices, as discussed briefly in

Section 1.2 and more in depth in Chapters 2 and 3. The second example is fundamental to

the formation of intrinsic organic semiconductors, the use of which is examined broadly in

Section 1.4, and more specifically in Chapters 4 and 5. In summary, this work will study

molecular electronic devices and organic electronic devices. In the first class, electronic

transport takes place through a single molecular layer, whereas in the second charges move

through an organic semiconductor film consisting of an aggregate of molecules, thus it is

a function of both intramolecular and intermolecular interactions.

1.2 Molecular Electronics

As commercial electronics research has focused on increasing the density of devices on a

microchip, the ultimate goal for the downsizing of electronics is the use of a single

molecule as an active component. Moore’s law, the empirical relation stating that the

density of transistors doubles approximately every two years, is nearing the limits of the

capabilities provided by current, silicon-based electronics. As a fledgling technology,

research into molecular electronics has grown in many directions since the first

experimental proof-of-concept in 1995 [1], but has yet to be adopted in real-world

applications. The most prevalent molecular-scale electronic device in the literature is the

rectifier, with examples of both single-molecule devices [2–7] as well as molecular

ensembles [8–15]. While the behavior of single-molecule devices is by no means

irrelevant, here the focus is on molecular ensembles as this represents a more application-

3

driven technology. The main structure of interest is the self-assembled monolayer (SAM),

which is a single layer of molecules covalently bonded to a substrate. This type of

architecture is relatively straightforward to work with since, regardless of the deposition

method, a single molecular layer will form due to the carefully designed chemical

structures. The compounds are formulated to bond to a surface, but once bonded to the

substrate there are no available bonding sites remaining for additional molecules on top.

Any remaining molecules will only be held to the surface through electrostatic interactions

and can be easily removed through physical means, leaving behind the strongly bound

layer. Once formed, the difficulty therein lies with electrically contacting a single

molecular layer without causing irreparable damage.

1.2.1 Molecular Rectifiers

Conduction through a nanometer-scale organic insulator occurs through a tunneling

mechanism which can be approximated by a simplified Simmons model [16–18]:

𝐽𝐽 = 𝐽𝐽0𝑒𝑒−𝛽𝛽𝛽𝛽 (1.1)

where J is the current density, J0 is the extrapolated zero-distance tunneling current, β is

the tunneling decay constant, and d is the distance between electrodes. This mechanism,

however, assumes that tunneling is the only effect by which charges can transfer from one

electrode to the other. In the landmark publication by Aviram and Ratner [19], it was

suggested that molecular-scale junctions can facilitate charge transport of a more complex

variety. In their example, electron donor and electron acceptor units were connected by a

σ-bonded tunneling bridge to keep separate the highest occupied molecular orbital

(HOMO) and lowest unoccupied molecular orbital (LUMO). The highly anisotropic

4

electronic levels were hypothesized to allow a different amount of current at opposing

biases, an effect known as rectification with the figure of merit being the rectification ratio

R:

𝑅𝑅 = 𝐽𝐽𝑓𝑓𝑓𝑓𝛽𝛽𝐽𝐽𝑟𝑟𝑟𝑟𝑟𝑟

(1.2)

with Jfwd and Jrev the forward and reverse bias current density, respectively. An example of

rectifier J-V characteristics is provided on a linear scale in Figure 1.2a and a logarithmic

scale in Figure 1.2b.

The first experimental evidence of such an effect was introduced by the Metzger

group [9, 20], however it soon became clear that this molecular design needed

improvement to approach the degree of rectification available in macroscale, silicon-based

diodes. A significant step forward came in the form of a rectifier utilizing a single

molecular orbital in the metal-insulator-metal structure [21]. If there exists a frontier

electronic level, either the HOMO or LUMO, near the Fermi levels of the two associated

electrodes, this level can participate in charge transport across the junction. In the example

from Kornilovitch, Bratkovsky, and Williams [21], they achieve rectification through a

single molecular orbital (in this case the LUMO) through molecular asymmetry, where the

MO is located to one side of the molecule as can be seen in Figure 1.3a. At reverse bias,

Figure 1.3b, most of the applied potential is dropped over the insulating chain resulting in

a reduced total conductivity. At forward bias, Figure 1.3c, the applied potential allows

conduction through the MO before tunneling through the insulating chain. It is also

possible that at one bias polarity the MO falls between the contact Fermi levels, resulting

in the MO participating in transport, and at the opposite bias it falls outside of the contact

5

Fermi levels, resulting in entirely tunneling current. Lastly, rectification can occur due to

differences at the contacts, including asymmetric electrode work functions, resembling a

Schottky diode [11], as well as differences in the molecule/electrode interaction [22, 23],

rectification in real devices is usually a combination of these effects. The literature contains

many examples of monolayer rectifiers, but only a few have reached 3 orders of magnitude:

a donor-acceptor pair self-assembled on Au reached R = 3000 [24], a molecular

heterojunction showed a maximum of R = 1000 [25], and 6.3 × 105 using a two-level

ferrocene compound in conjunction with a Schottky diode [26]. Chapters 2 and 3 will

present a series of examples which can be described by this transport model.

Figure 1.2. Rectifier J-V characteristics on (a) Linear scale and (b) Logarithmic scale.

a) b)

6

Figure 1.3. Energy level diagrams showing the mechanism of rectification in a two-

component molecule and slightly offset contact Fermi levels with a dotted line representing

tunneling and a solid line representing hopping at (a) Zero bias, (b) Reverse bias, and (c)

Forward bias.

a)

b)

c)

7

1.3 Organic Field-Effect Transistors

The field of organic electronics has exploded in popularity since its humble beginnings in

1960 with the identification of the organic semiconductor [27, 28], with many

commercially available products already on the market. The structure of organic

semiconductors departs quite heavily from the covalently-bonded inorganic

semiconductors, where the former are either molecular crystals formed of small molecules

or conjugated polymers and held together by weak van der Waals bonds. The

semiconducting properties of these materials arise from the conjugated π-orbitals present,

which facilitate electron delocalization and charge transport between molecules. It is

important to note that organic electronics was never meant to replace the existing silicon-

based technologies but to supplement by enabling applications inaccessible to rigid

electronics. The weak bonding of these organic materials allows for low-temperature

solution processing, opening the realm of soft plastics [29–36], textiles [37], and paper

[38–40] for use as substrates. These qualities have led to the introduction of various organic

photovoltaics (OPVs) [41–43], organic light-emitting diodes (OLEDs) [44, 45], various

pressure and gas sensors [46, 47], as well as the organic field-effect transistor (OFET) [48,

49]. Since the earliest OFETs were presented in the 1980s [50–53], device performance

has radically improved due to a better understanding of the chemical and solid-state

structures of the semiconductor, the effects of the dielectric layer, and the injection

processes at the contacts. In this section, the OFET is introduced and the components,

physics, materials, methods, and non-ideal characteristics are detailed.

8

1.3.1 OFET Structure and Operation

The OFET is comprised of 5 different electrically active layers assembled on a substrate:

the organic semiconductor, the gate dielectric, and 3 electrodes, namely the gate, source,

and drain electrodes. These layers are generally arranged in one of 4 device architectures

(Figure 1.4 a-d): bottom gate, bottom contact (BGBC), bottom gate, top contact (BGTC),

top gate, bottom contact (TGBC), and top gate, top contact (TGTC). These 4 structures are

divided into two categories, coplanar (BGBC and TGTC) and staggered (BGTC and

TGBC). Here, coplanar refers to the source, drain, and conducting channel being located

a) b)

c) d)

Figure 1.4. OFET device structures: (a) Bottom gate, bottom contact, (b) Bottom gate,

top contact, (c) Top gate, bottom contact, (d) Top gate, top contact.

9

on the same plane, and in the staggered structures the conducting channel is offset from the

plane of the source and drain contacts.

The operation of an OFET relies on the application of two potentials, the gate-

source voltage (VGS) and the drain-source voltage (VDS), with the source electrode held at

ground. For the following discussion, the semiconductor is assumed to be p-type, where

the majority charge carriers are holes, though for n-type, where the charge carriers are

electrons, the polarities are simply reversed. With no VGS being applied, there is no charge

accumulation at the semiconductor-dielectric interface, i.e. the device is turned “off.” At

the simplest level, an applied VGS polarizes the dielectric causing the accumulation of

charge carriers at the semiconductor-dielectric interface: the transistor turns “on.” An

applied VDS forces these accumulated charge carriers from the source to the drain electrode

where the drain current (ID) is measured. The charge density in the transistor channel and,

thus, the current, are modulated by the magnitude of the field applied, VGS, hence the “field-

effect” terminology. In a real device, a small, negative VGS is sometimes required first to

fill charge traps at the semiconductor-dielectric interface before free charge carriers can

accumulate in the conduction channel. This trap-filling potential is known as the threshold

voltage (VTh) and can result from sources like crystal defects, impurities, and interfacial

roughness. In addition, small amounts of dopants in the semiconductor and surface dipoles

can result in a positive threshold voltage wherein the device is already “on” at VGS = 0V

and requires a positive VGS to reach the “off” state.

Figure 1.5a shows a typical evolution of the drain current with increasingly negative

drain-source voltage where each curve is measured at a fixed, negative, gate-source

voltage, this type of measurement is known as the “output characteristics” or “transport.”

10

When VDS < |VGS – VTh| the device is in the linear regime, Figure 1.5b; the gate-source

voltage dictates the charge density in the conduction channel, thus the drain current will

increase linearly with the drain-source voltage and the device acts as a gate voltage-

controlled variable resistor. However, as VDS increases and the magnitude approaches that

of VGS, the shape of the conduction channel changes due to the two interacting potentials.

At the critical point where VDS = |VGS – VTh|, the area near the drain electrode is depleted of

free charge carriers and the channel becomes pinched off, Figure 1.5c. As the drain-source

voltage further increases, the competing effects of the increasing potential forcing charges

from source to drain and the growing depletion zone near the drain cause a saturation of

ID, Figure 5d, aptly named the saturation regime.

b)

d)

c)

a)

Figure 1.5. (a) Ideal OFET output characteristics, (b-d) BGBC structures with the

conduction channel in purple, (b) In the linear regime, (c) At pinch-off, (d) In the

saturation regime.

11

Figures 1.6a and 1.6b show in black the drain current on a logarithmic scale as a

function of gate-source voltage with drain-source voltage held constant in the linear and

saturation regimes, respectively. In blue, Figure 1.6a shows the drain current on a linear

scale and Figure 1.6b shows the square-root of drain current on a linear scale. This type of

measurement is known as the “transfer” characteristics. The field-effect mobility, µ, a

measure of how quickly charge carriers move in response to an external electric field, is

calculated from the slope of the red line in Figure 1.6a, 𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝜕𝜕𝐺𝐺𝐺𝐺

, for the linear regime, and

from the slope of the red line in Figure 1.6b, 𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝜕𝜕𝐺𝐺𝐺𝐺

, for the saturation regime, as detailed in

Section 1.3.2. The mobility µ has units of cm2/Vs, as a reference the mobility of amorphous

silicon is approximately 1 cm2/Vs and that of single-crystal silicon is 100-1000 cm2/Vs,

depending on the conditions. Also shown in Figure 1.6b is the threshold voltage, VTh, which

is usually calculated by finding the intercept of 𝐼𝐼𝐷𝐷 = 0 and the red line, although there are

Figure 1.6. Evolution of ID at fixed, negative, VDS as a function of increasingly negative VGS

in (a) The linear regime and (b) The saturation regime.

a) b)

12

many different methods of calculating VTh [54]. The threshold voltage is a result of trap

states being filled by the applied VGS and the density of trap states increases with decreasing

temperature due to the reduction in thermal energy required to eject charges from a trap.

Therefore, VTh is dependent on temperature, allowing an estimation of the density of

interfacial trap states, Nit, through measurements at varying temperature and application of

Equation 1.3:

𝑁𝑁𝑖𝑖𝑖𝑖 =𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝑘𝑘𝐵𝐵𝑞𝑞

𝜕𝜕𝑉𝑉𝑇𝑇ℎ𝜕𝜕𝜕𝜕

(1.3)

where kB is Boltzmann’s constant, T is the temperature, q is the elementary charge, and

Cdiel is the gate dielectric capacitance per unit area [55]. In addition to the threshold voltage,

another indication of charge traps can be seen in the quantity known as the sub-threshold

swing or inverse sub-threshold slope (S), the inverse slope of the orange line in Figure 1.6b.

S is a measure of how fast ID increases with VGS at fixed VDS and has units of V/decade, or

the amount that VGS must increase to cause a 10-fold increase in ID. S depends on the

interfacial trap states, and the gate dielectric as defined in Equation 1.4:

𝑆𝑆 =𝑘𝑘𝐵𝐵𝜕𝜕𝑇𝑇𝑇𝑇(10)

𝑞𝑞𝑁𝑁𝑖𝑖𝑖𝑖𝑞𝑞2

𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑+ 1 . (1.4)

A smaller value of S indicates that the device will have a sharp turn-on and the theoretical

lower limit of S at room temperature (T = 300K) is 60 mV/dec as the first term in Equation

1.4 approaches zero.

Another signature of the existence of traps is device hysteresis, where there is a

significant difference in current characteristics in the forward voltage sweep and the

reverse voltage sweep. This difference can occur when charges become trapped and then

13

released on the forward and reverse sweeps, respectively, and also in the presence of highly

polar dielectrics. A series of extremely useful methods for characterizing the trap states in

an OFET have emerged whereby the trap density as a function of energy in the band gap

(trap density of states, or trap DOS) is calculated from the standard transistor measurements

[56, 57].

1.3.2 FET Analysis

To analyze the electrical properties of an OFET, an assumption called the gradual channel

approximation is made. Here the electric field between the source and gate electrode is

much larger than the electric field between the source and drain electrode. This is generally

accomplished by ensuring that the gate dielectric thickness, d, and the channel length, L,

satisfy the relation 𝐿𝐿𝛽𝛽≥ 10, which becomes more relevant in devices with very short

channel lengths [58]. With this relation in place, the gate-source voltage dictates the charge

accumulation in the semiconductor, these charges reside at the semiconductor-dielectric

interface, and the potential distribution can be approximated as one-dimensional across the

channel. The total charge density that is accumulated at the semiconductor-dielectric

interface is given by, with x the distance across the channel:

𝑄𝑄 = −𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝑉𝑉𝑖𝑖𝑡𝑡𝑖𝑖𝑡𝑡𝑑𝑑(𝑥𝑥), 𝑉𝑉𝑖𝑖𝑡𝑡𝑖𝑖𝑡𝑡𝑑𝑑(𝑥𝑥) = 𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) (1.5)

where Vtotal(x) is the total potential in the channel resulting from an applied VDS and VGS.

However as mentioned earlier, a small potential, VTh, is required to fill traps before mobile

charge carriers are accumulated and thus the more relevant expression for mobile charge

carriers becomes:

𝑄𝑄𝑚𝑚𝑡𝑡𝑚𝑚𝑖𝑖𝑑𝑑𝑟𝑟 = −𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑(𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) − 𝑉𝑉𝑇𝑇ℎ). (1.6)

14

Then, as these charges are mobile along the x-direction between the source and drain

electrodes, the current ID through the drain is defined as:

𝐼𝐼𝐷𝐷 = 𝜇𝜇𝑄𝑄𝜇𝜇𝐸𝐸𝑥𝑥, 𝐸𝐸𝑥𝑥 = −𝑑𝑑𝑉𝑉𝑑𝑑𝑥𝑥

(1.7)

where Ex is the electric field in the direction of current flow, W is the width of a contact or

the channel width, and µ is the charge-carrier mobility. Substituting Equation 1.6 into

Equation 1.7 gives:

𝐼𝐼𝐷𝐷𝑑𝑑𝑥𝑥 = 𝜇𝜇𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑(𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) − 𝑉𝑉𝑇𝑇ℎ)𝑑𝑑𝑉𝑉 (1.8)

and, assuming that the charge-carrier mobility µ does not vary with applied potential,

integrating Equation 1.8 along the channel from x = 0 to the channel length L, and from V

= 0 to VDS as such:

𝐼𝐼𝐷𝐷𝑑𝑑𝑥𝑥𝐿𝐿

0= 𝜇𝜇𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑 (𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉(𝑥𝑥) − 𝑉𝑉𝑇𝑇ℎ)𝑑𝑑𝑉𝑉

𝜕𝜕𝐷𝐷𝐺𝐺

0

results in:

𝐼𝐼𝐷𝐷,𝑑𝑑𝑖𝑖𝑙𝑙 =𝜇𝜇𝐿𝐿𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑 (𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ)𝑉𝑉𝐷𝐷𝐺𝐺 −

12𝑉𝑉𝐷𝐷𝐺𝐺2 . (1.9)

Equation 1.9 is valid in the linear regime, however the drain current in the linear and

saturation regimes will follow different relations due to the pinch-off at saturation which

results in an effective maximum drain-source voltage of 𝑉𝑉𝐷𝐷𝐺𝐺 = 𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ. Substituting this

into Equation 1.9 yields for the saturation drain current:

𝐼𝐼𝐷𝐷,𝑠𝑠𝑡𝑡𝑖𝑖 =𝜇𝜇2𝐿𝐿

𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑(𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ)2. (1.10)

15

Then, taking the derivative with respect to VGS and rearranging Equations 1.9 and 1.10

gives:

𝜇𝜇𝑑𝑑𝑖𝑖𝑙𝑙 =𝐿𝐿

𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝜇𝜇𝑉𝑉𝐷𝐷𝐺𝐺𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝑉𝑉𝐺𝐺𝐺𝐺

(1.11)

𝜇𝜇𝑠𝑠𝑡𝑡𝑖𝑖 =2𝐿𝐿

𝜇𝜇𝐶𝐶𝛽𝛽𝑖𝑖𝑟𝑟𝑑𝑑𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝑉𝑉𝐺𝐺𝐺𝐺

2

(1.12)

which are the two relations used to find the charge-carrier mobility in the linear and

saturation regimes, respectively. In a perfect device the values for the µlin and µsat are

identical, however in devices which have a significant barrier to injection, µlin is

substantially lower than µsat. Any resistance at the contacts will result in a reduced effective

VDS, and in the linear regime, the potential between source and drain electrodes is already

quite small compared to the saturation regime. Consequently, the linear regime is affected

to a larger degree by contact resistance, the causes of which are discussed in Section 1.3.5.

1.3.3 Charge Injection in OFETs

Choosing the proper contacts for an OFET is of the utmost importance to achieving optimal

device performance. The contact interface between the source electrode and the

semiconductor is the site of carrier injection, and so it plays a significant role in device

performance. This is especially true when downscaling devices, as contact effects dominate

at small transistor channel size [59]. The goal is to obtain an ohmic contact, or one that

results in a negligible voltage drop and should be able to supply as much current as needed

and act as a charge carrier reservoir. There are three dominant models of charge injection:

thermionic emission, field emission, and hopping through gap states. Thermionic injection

16

occurs when charge carriers have sufficient energy to overcome the injection barrier, and

is applicable as long as the applied voltage is not large or the contact is heavily doped [60].

If these conditions apply, then field emission is the more appropriate model. The energy

barrier at the interface becomes thin enough that charges are able to quantum-mechanically

tunnel through to the semiconductor conduction band, even without necessarily having

energy greater than the injection barrier. Lastly, defects can induce gap states between WF

and the HOMO/LUMO level; the charge carrier can hop through these states into the

semiconductor.

Contact injection is largely determined by the alignment between the work function

of the contact, WF, and the HOMO/LUMO levels of the semiconductor, Figure 1.7. The

injection barrier ΦB is the difference between WF and the HOMO for p-type transistors (or

WF and the LUMO for n-type transistors) and in general should be minimized. Indeed,

Figure 1.7. An illustration of the energetics at the semiconductor/dielectric interface.

17

since most organic semiconductor materials are intrinsically ambipolar, the alignment of

WF with the HOMO/LUMO levels can determine if a device is p-type, n-type, or ambipolar.

In p-type devices WF of the electrode aligns with the HOMO of the organic semiconductor,

in n-type with the LUMO, and ambipolar transport requires that WF lies in between the

HOMO and LUMO levels.

Alignment of the contact work function and the semiconductor HOMO/LUMO

levels is not trivial. Although the work function of a metal is equal to its Fermi energy EF

in vacuum, work function is a surface property and is subject to environmental effects. The

electron cloud of a bare metal extends slightly into vacuum, creating an electric field

oriented away from the metal. Any charge entering or leaving the metal will have to

overcome the potential of this electric field, an effect that the work function accounts for.

When a new material is brought near to or in contact with the metal, the electron cloud of

the new material pushes back the electron cloud of the metal. This changes the magnitude

of the electric field at the surface and therefore the work function. This is known as the

push-back or pillow effect [61]. Additionally, if the two materials have different Fermi

energies, then they will establish a common Fermi energy (i.e. thermal equilibrium) by

exchanging carriers. A depletion or accumulation zone (depending on the relative Fermi

energies) forms in the semiconductor creating an additional electric field. This is

commonly known as band bending [62–65]. This new electric field also alters the work

function, shifting the injection barrier. Since the Fermi energy of an intrinsic

semiconductor is positioned between the HOMO and LUMO energies, attempting to match

a metal with a work function close to one of these bands will cause significant charge

transfer to occur to establish equilibrium. This in turn shifts the HOMO and LUMO levels

18

and an energy barrier remains. This effect is known as Fermi level pinning [66, 67]. With

these and other effects, it is impossible to predict the energy level alignment at the

contact/semiconductor interface given only the contact work function and semiconductor

HOMO/LUMO levels, making the contact selection difficult.

Metallic contacts are often used due to their chemical stability, reliable processing,

and well-known properties. These include gold, silver, copper, platinum, calcium, and

aluminum, among others, and are generally deposited through thermal or electron-beam

evaporation in high vacuum. In the pursuit of all-organic or printable devices, several

groups have experimented with using organic metals as the electrodes or as a surface

treatment to other metallic electrodes [68]. One example is poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a solution-processable

macromolecular salt [69]. Depending on the processing conditions, PEDOT:PSS can

exhibit high conductivities (up to 3065 S/cm) while remaining sufficiently transparent for

solar cell applications [70]. Another class of compounds that falls into the organic metal

family are charge-transfer (CT) complexes, which are formed through the combination of

electron donor and acceptor compounds and were first discovered in 1973 [71–73]. CT

complexes have garnered considerable attention due, in part, to the relative ease of work

function modification through chemical substitution, as exemplified by the transition from

p-type, to n-type, and finally to ambipolar behavior on the same semiconductor through

manipulation of the donor and acceptor constituents [74]. One issue that arose with the use

of CT complexes, however, is their low solubility in common organic solvents. One

approach to overcome this challenge was introduced by Hiraoka, et al. through the use of

“double-shot inkjet printing,” where the more soluble donor and acceptor components were

19

individually deposited and the CT complex was allowed to form on the substrate [75]. π-

conjugated carbon in the form of carbon nanotubes or graphene is also gaining considerable

interest as an electrode material in OFETs due to the reduced injection barrier stemming

from the interfacial morphology as well as possibly aiding in the growth of an overlaid

organic semiconductor [76–82].

1.3.4 Contact Modification

Chemical modifications of metallic contacts can create a more favorable interface for

charge injection into the organic semiconductor. Metal oxides such as molybdenum oxide

and titanium oxide were used to modify the work function by a large degree in either

direction [67, 83, 84]. Charge transfer salts [68, 71] and SAMs [85–90] can also shift the

work function of the source and drain electrodes, and can also impact the semiconductor

morphology [90, 91]. The most convenient method of work function tuning for metallic

contacts is through SAMs, usually of polar, thiol-based small molecules [66]. The SAM

adheres spontaneously to the metal surface as the thiol group forms a covalent bond with

metals. The presence of the SAM modifies the local electric field at the surface of the

electrode by creating a dipole at the interface. The electric field of this dipole, combined

with the polar dipole of the SAM itself, change the work function of the contact. The sign

and magnitude of the SAM dipole determines whether the work function is shifted up or

down. In general, alkane terminals decrease the work function while halogenated terminals

increase the work function. The work function can be further fine-tuned using a blend of

SAMs [92]. The surface potential, and therefore the work function, resulting from a SAM

assembled on an electrode can be measured through non-contact methods such as

ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe microscopy (KPM).

20

1.3.5 Contact Resistance in OFETs

It is important to consider the non-idealities that may be present in real OFETs to aid in

device design as well as to ensure that any extracted quantities are physically meaningful.

There exist two main contributions to the overall device resistance: charge injection from

the contacts into the organic semiconductor layer, and charge transport in the

semiconducting channel [93]. With the advent of new semiconducting materials with high

intrinsic mobilities and correspondingly low channel resistances, RCh, the impact of contact

resistance, RC, became much more significant. In addition, the focus on device downscaling

has exacerbated this issue as the channel resistance decreases yet the contact resistance

remains constant with smaller channel length. For an OFET to function as intended, the

resistance due to the contacts must be substantially lower than the resistance due to the

channel. If this is not the case, the voltage drop at the contacts results in a smaller effective

potential across the channel. While the injection barrier is the main source of contact

resistance RC, additional factors can have a significant effect. The bulk resistance RC,bulk,

or the resistance due to the depleted semiconductor between the contacts and the

conducting channel (as opposed to the contact interface resistance RC,int, or the resistance

to charge injection from contacts into the semiconductor) can be reduced through device

geometry. In a staggered device geometry, Figures 1.4b and c, injection occurs along the

surface parallel to the channel; as the gate voltage is increased, more of this surface

participates in injection due to increased channel conduction, effectively reducing RC. In

contrast, coplanar devices, Figures 1.4a and d, inject directly into the channel along the

surface perpendicular to the channel. This eliminates RC,bulk, but the injecting surface is

limited to the thickness of the conduction channel, typically only a few nanometers. High

21

mobility also aids in carrier injection, reducing RC,int, while simultaneously decreasing the

bulk transport resistance RC,bulk [60]. However, contact resistance still remains dominant in

high mobility devices. Generally, long channel-length (50-100 µm) devices with mobilities

around 1 cm2/Vs will not display significant contact effects as long as RC is less than a few

kΩcm, however as mobilities increase past 5 cm2/Vs, RC must be less than 1 kΩcm. At

small channel length, the restriction on these resistance values is even greater, especially

at high frequencies [94].

A large and non-ohmic contact resistance can result in quite non-ideal electrical

measurements, easily seen as an “S” shape in the ID1/2 vs. VGS curves where the slope will

abruptly increase and then fall back to a steady state, leading to an overestimation of the

field-effect mobility if the value for 𝜕𝜕𝐼𝐼𝐷𝐷𝜕𝜕𝜕𝜕𝐺𝐺𝐺𝐺

is extracted at this peak. This phenomenon has

captured the attention of many and is attributed to a large injection barrier at the contacts

[95, 96]. The injection barrier then disappears with increasing VGS as the greater potential

aids in the extraction of charges. This effect can also be seen when the contact resistance

begins higher than the channel resistance, before dropping with increasing VGS. Chapter 5

focuses on the contact resistance of OFETs and a new method for minimizing the resistance

at injection.

Several techniques have been developed to measure contact resistance both directly

and indirectly. An example of an indirect method to determine the resistance due to the

injecting contacts is the gated transmission line method (gated TLM), which relies on the

assumption that the organic semiconductor is uniform and isotropic, and thus the channel

resistance scales linearly with the channel length. This technique uses linear regime transfer

22

measurements on many devices of differing channel length to evaluate the total resistance

of the device Rdevice. This value is given by the series combination of the contact resistance

(independent of the channel length, L), and the channel resistance (directly proportional to

the channel length) and follows Equation 1.13:

𝑅𝑅𝛽𝛽𝑟𝑟𝑟𝑟𝑖𝑖𝑑𝑑𝑟𝑟 = 𝑅𝑅𝐶𝐶 + 𝑅𝑅𝐶𝐶ℎ(𝐿𝐿). (1.13)

Plotting the width-normalized resistance vs. channel length results in a straight line with

the y-intercept equal to RCW, Figure 1.8. To minimize the errors due to the fact that each

device turns on at a different voltage and determine the correct value for the device

resistance, the drain current must be measured at a particular value of VGS called the

overdrive voltage (𝑉𝑉𝐺𝐺𝐺𝐺 − 𝑉𝑉𝑇𝑇ℎ = 𝑉𝑉𝑡𝑡𝑟𝑟𝑟𝑟𝑟𝑟𝛽𝛽𝑟𝑟𝑖𝑖𝑟𝑟𝑟𝑟). The use of the overdrive voltage ensures

consistency between devices which may have differing VTh, so that the magnitude of the

drain current is taken at the same distance away from the threshold voltage. The drain-

Figure 1.8. Example Gated TLM graph showing the determination of the contact resistance.

23

source voltage is then divided by ID to obtain the total device resistance which, multiplied

by the channel width as a normalization factor, is plotted with many devices as shown in

Figure 1.8.

A more direct method of measuring contact resistance uses scanning Kelvin probe

microscopy (SKPM) which in this case uses a metal-coated atomic force microscope

(AFM) tip to measure the surface potential across the channel of a device in operation [97,

98]. By measuring the drain current and the voltage drop at the contacts, the contact

resistance can be extracted. It is important to note that this technique can only be used in

certain device configurations where the contacts and the channel are accessible with the

SKPM tip.

1.4 Thesis Outline

Chapter 2 presents 9 newly developed compounds and details their incorporation into a

molecular-scale metal-insulator-metal structure designed to rectify current. The

compounds are each allowed to form SAMs on silicon wafers and contacted using a

eutectic gallium-indium (EGaIn) probe tip. The measured electrical properties of each

compound, along with the dipole moment calculated using density functional theory

(DFT), revealed a structure-function relationship. Depending on the molecular length, the

degree of rectification was positively correlated with the strength of the associated dipole.

Chapter 3 expands upon the work outlined in Chapter 2 and introduces an additional 8

newly-synthesized compounds using the same device structure. The new experimental

design resulted in the highest rectification ratio obtained in SAMs formed on silicon, up to

a maximum of 2635.

24

In Chapter 4, the device under study is the OFET, where the organic components contain

larger conjugated structures than the rectifiers above. The organic semiconductor here is

processed from the vapor phase in the form of a single crystal where electrical

measurements reveal significant anisotropy and a maximum mobility of 0.3 cm2/Vs. The

combination of DFT and X-ray diffraction (XRD) measurements revealed the main

direction for hole transport in the plane of the measurement, although not necessarily

parallel to the conduction channel. Through solution processing, thin-film OFETs were

produced, resulting in an enhancement of the high-mobility crystallographic direction

parallel to the conduction channel and an associated increase in mobility.

Chapter 5 details the production of thin-film OFETs, also referred to as organic thin-film

transistors (OTFTs), with aggressively reduced contact resistances, and associated

increased mobilities, through modifications to the contact deposition rate. Using two model

semiconductors, the small molecule 2,8-difluoro-5,11-bis(triethylsilylethynyl)

anthradithiophene (diF-TES ADT) and the copolymer indacenodithiophene-co-

benzothiadiazole (C16IDT-BT), contact resistances of 500 Ωcm and 200 Ωcm were

obtained, respectively. The reduced injection barrier in these devices resulted in record-

high charge carrier mobility of 19.2 cm2/Vs for diF-TES ADT, and 10 cm2/Vs in C16IDT-

BT.

Together, these results represent significant developments in the fields of both molecular

electronics and organic electronics, achieving record performance in silicon-based

molecular rectifiers as well as small-molecule OFETs.

25

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33

Chapter 2 Fluorinated Benzalkylsilane Molecular

Rectifiers

The introduction of electrical components on the molecular scale introduces a host of new

challenges, however the simplicity of the self-assembled monolayer (SAM) alleviates

many of these issues through the ease of fabrication and the lack of a necessary alignment.

We examine the electrical properties of nine new alkylated silane SAMs of the form

(EtO)3Si(CH2)nN=CHPhX where n = 3 or 11 and X = 4-CF3, 3,5-CF3, 3-F-4-CF3, 4-F, or

2,3,4,5,6-F, and explore their rectification behavior in relation to their molecular structure.

The electrical properties of the films were examined in a metal/insulator/metal

configuration, with a highly-doped silicon bottom contact and a eutectic gallium-indium

liquid metal (EGaIn) top contact. The junctions exhibit high yields (> 90%), a remarkable

resistance to bias stress, and current rectification ratios (R) between 20 and 200 depending

on the structure, degree of order, and internal dipole of each molecule. We found that the

rectification ratio correlates positively with the strength of the molecular dipole moment

and it is reduced with increasing molecular length.

This work was adapted from Zachary A. Lamport, Angela D. Broadnax, David Harrison, Katrina J. Barth, Lee Mendenhall, Clayton T. Hamilton, Martin Guthold, Timo Thonhauser, Mark E. Welker, and Oana D. Jurchescu, “Fluorinated Benzalkylsilane Molecular Rectifiers,” Sci. Rep., Vol. 6, p. 38092, 2016.

34

2.1 Introduction

Molecular electronics has been an area of great interest as device technology closes

on the limits of the often-mentioned Moore’s law, with consumer-available

technology already in the tens of nanometers range. The idea of molecular

electronics was proposed as a means to surpass the challenges present in

downscaling existing technologies[1–3]; however, commercially viable examples

have yet to be introduced [4–8]. Since 1974, when Aviram and Ratner proposed the

first rectification mechanism for a molecular structure containing a donor-acceptor

pair separated by a σ-bonded tunnelling bridge, the field has witnessed spectacular

growth [9]. A rectifier is a device which allows a large amount of current to pass

through at one bias while restricting current flow at the opposite bias. While the

above-mentioned theoretical work extrapolated the inorganic solid state p-n junction

miniaturized to the size of a single molecule and relied on the manipulation of the

molecule’s energy levels, several different mechanisms for enhanced rectifications

have subsequently been proposed [10–13]. Further experimental work has also

demonstrated that the donor-acceptor pair may not always be necessary, and that

systems with a single π-bonded component and a σ-bonded insulating chain can

provide a more consistent, as well as a greater degree of, rectification [14–16].

Significant research effort was dedicated toward the examination of both single-

molecule and molecular assemblies as rectifiers, with clear advantages exhibited by

both [17–20]. Single molecule devices allow for a direct comparison with the

theoretical calculations due to the lack of complexity in such systems. Nevertheless,

Nijhuis et al. produced convincing arguments for the observed rectification in

35

ferrocene-terminated alkanethiolate molecules assembled on surfaces in monolayer

patterns [21–23]. Other self-assembled monolayers and Langmuir-Blodgett films

provide ease of fabrication due to the molecule’s penchant for assembling on a well-

chosen surface. This allows for the formation of many devices on a single substrate

simultaneously and without the difficult alignment task present in single-molecule

structures. The research to improve the rectifying behaviour of molecular diodes,

however, has proceeded mostly by trial and error and a relation between the

molecular structure and resulting strength of rectification has not been clearly

demonstrated.

We have developed nine new fluorinated benzalkylsilane molecules of varying

internal molecular dipole due to the imposed length and termination and studied their

electrical properties in relation to their chemical structure. We incorporated the self-

assembled monolayers (SAMs) composed of these molecules into molecular diodes and

obtained high yields and reproducible rectification ratios as high as 200, with good bias

stress stability. We calculated the molecular dipole using density functional theory (DFT)

and we found that the rectifying behavior is stronger in the case of short molecules and is

enhanced by the internal dipole. Our results provide evidence that the molecular structure

impacts the electrical properties of molecular diodes through the internal dipole

characteristic to each molecule and may promote a rational design of compounds that can

function as high-performance molecular rectifiers.

36

2.2 Experimental

2.2.1 Molecular Structures and Analysis

The examined molecules were fluorine-substituted benzaldehyde imine-terminated

trialkoxysilanes, designed such that each contains a triethoxysilane group that

ensures the attachment on the Si/SiO2 surface, an alkyl chain with either 3 or 11 CH2

groups, a phenyl group to facilitate the formation of an “intermolecular top-link” to

enhance ordering, and a fluorine-containing head-group providing different amounts

Figure 2.1. Chemical structures of molecules 1-9

37

and orientations of fluorine atoms [24]. The chemical structures are included in

Figure 2.1, as follows: (E)-1-(4-(trifluoromethyl)phenyl)-N-(11-(triethoxysilyl)

undecyl)methanimine (molecule 1), (E)-1-(3,5-bis(trifluoromethyl) phenyl)-N-(11-

(triethoxysilyl)undecyl)methanimine (molecule 2), (E)-1-(4-fluorophenyl)-N-(11-

(triethoxysilyl)undecyl) methanimine (molecule 3), (E)- 1-(3-fluoro-4-

trifluoromethyl)phenyl)-N-(11-(triethoxysilyl)undecyl)methanimine (molecule 4),

(E)-1-(4-(trifluoromethyl)phenyl)-N-(3-(triethoxysilyl)propyl)methanimine

(molecule 5), (E)-1-(3,5-bis(trifluoromethyl)phenyl)-N-(3-(triethoxysilyl)propyl)

methanimine (molecule 6), (E)-1-(4-fluorophenyl)-N-(3-(triethoxysilyl)propyl)

methanimine (molecule 7), (E)- 1-(3-fluoro-4-trifluoromethyl)phenyl)-N-(3-

(triethoxysilyl)propyl)methanimine (molecule 8) and (E)-1-(perfluorophenyl)-N-(3-

(triethoxysilyl)propyl) methanimine (molecule 9). (E)-1-(perfluorophenyl)-N-(3-

(triethoxysilyl)undecyl) methanimine (the long chain analogue of molecule 9) was

also investigated; its properties, however, varied significantly from sample to

sample, as well as for the same film, and therefore we decided not to include it in

our analysis. The molecules were synthesized from amino trialkoxysilanes by using

the condensation of commercially available amino alkyl triethoxysilanes with

substituted benzaldehydes to prepare the precursors, following established

procedures, Figure 2.2 [25, 26]. We will refer to molecules 1, 2, 3, and 4 as the “long

molecules” (their lengths are between 2 and 2.2 nm) and molecules 5 through 9 as

the “short molecules” (their lengths are between 0.9 and 1.1 nm). The length was

evaluated using Spartan software, measured from the Si atom to the most distal atom

of the head-group for the lowest energy conformer of each molecule [27]. The

38

molecules were synthesized from amino trialkoxysilanes by using standard

procedures, see Figure 2.2. With the exception of one imine the yields for all of these

reactions were in the 68-87% range and the only purification of the products required

after the reaction was filtration and removal of residual solvent under high vacuum.

The new compounds were characterized by 1H and 13C nuclear magnetic resonance

(NMR) and elemental analysis or high-resolution mass spectrometry. The proton

nuclear magnetic resonance (1H NMR) spectra were obtained using a Bruker Avance

300 MHz spectrometer operating at 300.1 MHz or a Bruker Avance 500 MHz

spectrometer operating at 500.1 MHz. 13C NMR spectra were obtained using a

Bruker Avance 300 MHz spectrometer operating at 75.5 MHz. 1H and 13C NMR

spectra were referenced to the residual proton or carbon signals of the respective

deuterated solvents. All elemental analyses were performed by Atlantic Microlabs

Inc., Norcross, GA. High-resolution mass spectrometry was performed at the Mass

Spectrometry Facility at Northwestern University, Evanston, IL.

All reactions were carried out under an atmosphere of nitrogen.

Aminoundecyltriethoxysilane was purchased from Gelest and used as received.

Deuterated solvents were purchased from Cambridge Isotope Laboratories and dried

over molecular sieves. Aminopropyltriethoxysilane and the benzaldehydes were

purchased from Aldrich Chemical Company and used as received. Anhydrous

sodium sulfate and HPLC grd hexane were purchased from Fisher/Acros and used

as received.

39

2.2.2 Device Fabrication

The SAMs were deposited on highly-doped silicon wafers with native SiO2 formed

at their surface by either solution or vapor-based techniques. The substrates were

cleaned in hot acetone, then hot isopropanol (IPA), followed by a 10-minute

exposure to UV-ozone and subsequent rinse in DI water before being dried in a

stream of nitrogen. The UV-ozone step serves to both remove organic contaminants

and to increase the density of hydroxyl groups on the surface. The monolayers were

formed in a nitrogen (<0.1 ppm H2O, <0.1 ppm O2) glovebox. We found that for the

long SAMs (molecules 1, 2, 3, and 4) self-assembly from a 4 mMol solution in

chloroform at 30º C provided the films of highest quality. The short SAMs

(molecules 5 through 9) were amenable to deposition by vapor treatment, where the

silicon wafer was placed into a sealed jar along with 11 µL of the pure SAM

compound at 30º C. Both methods typically took 18 to 24 hours. To remove the

SiRO

OROR

(CH2)n

N

+

O H

R3

R4

R5

R2

R1H

R3

R4

R5

R2

R1SiRO

OROR

(CH2)n

NH2Na2SO4

dry solvent

1 R = Et, n = 11, R3 = CF3, R1, 2, 4, 5 = H, 73%

2 R = Et, n = 11, R2 = R4 = CF3, R1, 3, 5 = H, 75%

3 R = Et, n = 11, R3 = F, R1, 2, 4, 5 = H, 46%

4 R = Et, n = 11, R3 = CF3, R2 = F, R1, 4, 5 = H, 93%

5 R = Et, n = 3, R3 = CF3, R1,2,4,5 = H, 82%

6 R = Et, n = 3, R2 = R4 = CF3, R1,3,5 = H, 85%

7 R = Et, n = 3, R3 = F, R1,2,4,5 = H, 87%

8 R = Et, n = 3, R3 = CF3, R2 = F, R1, 4, 5 = H, 87%

9 R = Et, n = 3, R1-R5 = F, 68%

40

excess adsorbed molecules on the monolayer surface, the samples were thoroughly

rinsed in chloroform followed by IPA and then immediately dried in a stream of

nitrogen.

The conical-tip EGaIn contact, which was used as a top soft contact for our

molecular rectifiers, was created using a Micromanipulator probe holder, and a 0.5

mm diameter Micromanipulator probe tip. EGaIn was placed onto a sacrifical copper

strip into which the probe tip was slowly lowered and then raised with the aid of the

contact angle goniometer.

2.2.3 Surface Analysis

In order to evaluate the quality of the SAM and the best method for assembly, water

contact angle measurements were conducted using a Ramé-Hart Model 200 Contact

Angle Goniometer; the results are displayed in Table 2.1 and in Figure 2.3. A high

value of the contact angle generally coincides with a higher degree of order and/or

a more dense film [28]. Indeed, the contact angles measured for all the SAMs were

around 70°. These values agree with the measurements performed on other

fluorinated SAMs deposited on SiO2 [29]. For reference, we have also measured the

contact angle on untreated substrates, and that is displayed in the same figure. It can

be observed that the contact angle for bare native SiO2 is < 10º.

We employed atomic force microscopy (AFM) measurements to characterize

the surface roughness prior to SAM deposition. We measured a sample of 1 cm x 1

cm and imaged a surface area of approximately 2 x 2 µm in several spots using a

Nanoscope IIIA AFM (Veeco Instruments), see Figure 2.4. This surface is of similar

41

quality to that obtained using template stripped or flip-chip laminated metallic

electrodes [22, 30]. The roughness value is over an order of magnitude smaller than

the height of the molecules studied here, including the shortest one, and therefore it

is largely inconsequential with regard to the assembly of a film, allowing for the

formation of a highly ordered film when the processing parameters are carefully

tuned. A graphical illustration of this can be seen in Figure 2.4c, where we

schematically show the assembly of a long and a short molecule studied here and

the respective difference in the length scales of the substrate and molecules.

Figure 2.3. Contact angle measurements conducted on monolayers of molecules 1-9 assembled

on native SiO2. The contact angle of clean native SiO2 is shown for reference in panel 10.

42

The work function measurements on highly doped silicon were performed

using a Trek model 325 electrostatic voltmeter configured for Kelvin probe

measurements and calibrated using highly-ordered pyrolytic graphite (HOPG) as a

standard. The calculations followed from Equation 2.1:

𝜙𝜙 = −𝑒𝑒−(𝑉𝑉𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 − 𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻) + 𝜙𝜙𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 (2.1)

where e is the elementary charge, 𝑉𝑉𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 is the measured signal from the substrate

(Si with a native layer of SiO2), 𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 is the measured signal from HOPG, and

𝜙𝜙𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 is the work function of HOPG (𝜙𝜙𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 = 4.48 𝑒𝑒𝑉𝑉) [31]. Four samples were

evaluated, in at least 8 spots, and the results were consistent: we obtained a work

function of 𝜙𝜙𝑆𝑆𝑆𝑆 = 4.65 ± 0.05 𝑒𝑒𝑉𝑉 for the highly doped Si substrates with a thin

layer of native oxide on their surface.

Table 2.1: Contact angles, average rectification ratios, and dipole moments of molecules 1-9

Molecule Contact Angle (°) R μ (D)

1 74 23 ± 8 4.50

2 72 82 ± 24 4.64

3 70 36 ± 13 2.71

4 79 80 ± 25 5.29

5 66 183 ± 41 4.42

6 74 159 ± 38 4.59

7 77 34 ± 19 2.48

8 78 192 ± 42 5.40

9 70 104 ± 10 3.21

43

2.2.4 Ab Initio Calculations

We performed ab initio calculations at the density functional theory level, using

VASP [32, 33]. We used the standard projector augmented wave (PAW)

pseudopotentials provided by VASP along with the Perdew-Burke-Ernzerhof (PBE)

exchange-correlation functional [34, 35]. All calculations were done with a kinetic-

energy cutoff of 400 eV and an energy convergence criterion of 10-4 eV.

Calculations were performed in orthorhombic unit cells with a vacuum of at least 2

nm between periodic images. All structures were relaxed until the maximum force

on any atom was less than 10 meV/angstrom. Dipole moments were calculated using

Figure 2.4. (a) Isometric view of a single AFM scan, (b) general line profile from AFM scan,

and (c) schematic representation, to scale, of the SAM on a device substrate showing the

significant difference in length for both the long (left) and short (right) molecules compared to

the surface roughness.

a) c)

b)

44

VASP's built-in functionality to calculate and correct for the dipole moment in all

directions (i.e. the IDIPOL and LDIPOL tags).

2.2.5 Electrical Characterization

The electrical measurements were taken on metal/SAM/metal structures using a

eutectic gallium-indium (EGaIn) conical-tip top contact in which the silicon

substrate was held at ground and the bias was applied to the EGaIn contact in

ambient. In this configuration, the forward bias condition corresponds to a positive

potential applied at the fluorinated end of the molecules. The eutectic liquid metal

EGaIn was utilized as a soft top-contact because its use is non-damaging,

reproducible, and well-studied in this context [22, 23, 36–43]. A schematic of the

device structure used in this study is included in Figure 2.5a. This figure, however,

denotes an ideal structure of the device, while we are aware that our SAM layers

may exhibit a lower degree of order, different orientation with respect to the

substrate, and possible step-edges due to substrate imperfections, impurities, etc.

2.3 Results and Discussion

In Figure 2.5b we show the dependence of the current density (J) on the applied

voltage (V) for one film of molecule 3, in 25 different spots. The relative standard

deviation for these measurements is < 1.4. Measurements on different spots on the

films are consistent, suggesting that our SAMs are uniform. In addition, for a

particular SAM to be considered viable as a rectifier, it must be able to withstand

many repetitive measurements with minimum changes in the electrical

45

Figure 2.5. (a) Schematic of device structure showing a SAM sandwiched between the

native SiO2 bottom contact and EGaIn top contact. (b) Current density versus the applied

voltage on 25 different spots in a 1 cm x 1 cm area on a film consisting of molecule 3. (c)

Bias stress measurements on molecule 6 over 50 consecutive measurements in a single

spot.

a)

b)

c)

46

characteristics. This could involve a variability and instability in the measured

output characteristics or, in the worst-case scenario, a “breaking” of the monolayer,

indicating that there is now an irreversible conductive path between the two

electrodes. The good operational stability, as clearly seen in Figure 2.5c for molecule

5, where we show the results of 50 consecutive measurements, indicates that these

monolayers are robust to bias stress (the relative standard deviation is less than 0.21).

The difference between Figure 2.5b and Figure 2.5c at low voltages originates from

the fact that charging effects may occur when repeated measurements are taken on

the same spot on the film, causing noise and asymmetries in the measured current.

We note, however, that applying higher voltages may result in irreversible damage

of the SAM film. The breakdown field in these SAMs range from approximately 20

MV/cm for molecule 9 to 50 MV/cm for molecule 5.

Figure 2.6 shows typical current-voltage (J-V) characteristics for each of the

9 molecules, taken during the forward and reverse bias conditions. We measured at

least 10 devices for each type of SAM, in at least 25 spots each, and obtained similar

results. As expected, the magnitude of the measured current depends on the

molecular structure, with the long molecules generally exhibiting lower

conductivities and less pronounced asymmetries between the forward and reverse

bias conditions.

We define the rectification ratio, R, as the ratio between the J value during

the forward bias measurement, (V = +2V) and the J value during the reverse bias

measurement, (V = -2V):

𝑅𝑅= 𝐽𝐽(𝑉𝑉= +2𝑉𝑉)𝐽𝐽(𝑉𝑉=−2𝑉𝑉)

(2.2)

47

In Table 2.1 we list the R values obtained for the nine molecules. In order to

relate the electrical properties with the molecular structure, we determined the

electrical dipole of each molecule using DFT calculations; results are listed in Table

2.1. In Figure 2.7 we plot the dependence of R on the molecular dipole for all the

molecules investigated in this study. Results obtained for short molecules are

included in red, while those for long ones are shown in grey. First, note the high

value of R obtained for molecule 5 and 8, R5 = 183 ± 41 and R8 = 192 ± 42. These

values are on par with some of the highest reported in the literature, with a

thiophene-1,1-dioxide oligomer exhibiting an R of ~200, a ferrocene-alkanethiolate

with an R =150, and ferrocene placed on a monolayer of β-cyclodextrin

demonstrating an R of 170 [15, 38, 44]. We note, however, that larger values can

Figure 2.6. Representative current density versus applied voltage curves films of molecules 1-9.

48

also be obtained [45, 46]. Nevertheless, our molecules have the additional advantage

of being very low-cost since the precursors are common organic molecules, being

easy to synthesize, and requiring minimal purification steps. This figure also denotes

that a stronger rectification is achieved in the case of short molecules, in agreement

with previous reports [47, 48]. Another key point that can be observed from Figure

2.7 is that for molecules of similar length, the rectification ratio correlates positively

with the internal dipole moment of each molecule. This trend can originate from the

fact that the molecular dipole creates a local electric field proportional to the

magnitude of the dipole moment. This field contributes to the total net field

experienced by the molecule under external bias, modifying the shape of the

Figure 2.7. Rectification ratio vs. the dipole moment of each molecule. In red we show the

results corresponding to the short molecules and in grey for the long molecules.

49

tunnelling barrier, and, consequently, of the current magnitude. Although a detailed

interpretation of this dependence is beyond the scope of this experiment, the results

suggest that for molecules of similar length, the rectification increases with the

molecular dipole, regardless of structural details of each compound. The trend is

monotonic for the case of short molecules, and we note that there is a spread in our

data for the long molecules. On one hand, this may originate from variations in

molecular orientation and the degree of order of the SAMs on the surfaces, which

results from the competition between intermolecular interactions of the SAM

molecules and molecule-substrate interactions. We attempted to characterize the

ordering of the SAMs through polarized modulated-infrared reflection absorption

spectroscopy (PM-IRRAS) and X-ray reflectivity measurements, both of which

were unsuccessful due to the small size of the molecular layers. On the other hand,

local molecular torsions and changes in the conformation were shown to

significantly impact the value of the rectification ratio [49, 50]. Several other factors

may contribute in the observed asymmetries. First, the two contacts are not perfectly

symmetric: a strong, Si-O covalent bond is established between the SAM and the

bottom contact, while the top contact interacts with the SAM via a weak, van der

Waals bond established between the end-group of the SAM and the Ga2O3 layer

formed at the surface of the EGaIn electrode [22]. The strength of this bond, and

therefore the rate of charge tunnelling, may vary with changing the chemistry of the

SAM fluorinated functional group. In addition, these contacts have slightly different

work-functions (𝜙𝜙Si= 4.65 ± 0.05 eV, as determined from Kelvin probe

measurements described above, while 𝜙𝜙EGaIn = 4.1-4.2 eV [51]), which impact the

50

injection of charge carriers. Second, additional dipole moments may be formed

through band-bending when physical contact is established between the SAM and

electrodes. Nevertheless, the dependence is clearly distinguished, and this suggests

that tailoring the internal molecular dipole via molecular design may be a powerful

tool in controlling the rectification ratios in molecular diodes. This result may seem

to contradict that of Yoon and collaborators, who showed that the rectification

behaviour of molecular diodes is independent of their molecular dipole [12]. In that

study, however, the molecules had various lengths, and therefore the dependence

may have been masked. Further studies, focusing on the examination of properties

of different classes of compounds—including compounds with dipole moments in

opposite directions—as well as studies on SAMs of different anchoring and terminal

groups would clarify the generality and limitations of this method.

2.4 Conclusions

We have designed and measured nine new rectifying self-assembled monolayers of

different lengths and polar terminations. We found that these fluorinated

benzalkylsilane molecules exhibit high yield when incorporated in molecular

diodes, coupled with robust rectification ratios ranging from 20-200, depending on

their structure. In addition, they show good uniformity over large surface areas and

excellent bias stress stability. We found an increase in the rectification strength with

enhancing the internal molecular dipole of each molecule. Our results suggest that

the electrical properties of molecular diodes can be controlled by tailoring the

molecular structures of the constituent molecules. This is a significant step toward

51

controlling the performance of molecular rectifiers through manipulating specific

structure-property relationships for use in macroscopic electronics as diodes, half-

wave rectifiers, AC/DC converters, or other charge-restricting elements.

52

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56

Chapter 3 Molecular Diodes from Self-Assembled

Monolayers on Silicon

Silicon-based electronics have been the subject of intense study driven by the

commercialization of the digital age. Unfortunately, the technology has nearly reached the

stage where the current modes of thinking have been outpaced by the demand for smaller

devices than doped silicon and similar materials can account for. To continue the trend of

shrinking length scales, the field of molecular electronics has bloomed. Research efforts

are aimed at understanding structure-property relationships in order to develop molecules

with predictable and stable electronic properties. Here, we demostrate efficient current

rectification in a new class of compounds that form self-assembled monolayers on silicon.

Integration of molecular electronics with silicon may yield hybrid systems that can expand

the use of silicon towards novel functionalities governed by the molecular species grafted

onto its surface. The new molecules that we introduce here consist of one alkyl chain and

a benzene termination with varius substituents and result from a simple, yet robust and

high-yield synthetic procedure. We found that when incorporated in molecular diodes,

these coumpounds can rectify current by as much as three orders of magnitude depending

on their structure. This performance is on par with that of the best molecular rectifiers

obtained on a metallic electrode, but it has the advantage of lower cost and more efficient

integration with current technologies.

This work was adapted from Zachary A. Lamport, Angela D. Broadnax, Ben Scharmann, Andrew DelaCourt, Hui Li, Scott M. Geyer, Mark E. Welker, and Oana D. Jurchescu, To be submitted, 2018.

57

3.1 Introduction

Electronic devices are ubiquitous to our lives and the quest for ever-smaller and better

devices, where commercially available silicon-based technologies have reached their

physical limits, has spurred the rise of molecular electronics. The field of molecular

electronics began in 1974 with the pivotal paper by Aviram and Ratner where it was

suggested that a single molecule consisting of one electron donor and one acceptor unit

connected by a σ-bonded tunneling bridge could rectify current, thus forming a molecular-

scale diode [1]. Since then, different strategies were proposed to obtain these molecular-

scale devices in multilayers, self-assembled monolayers (SAMs), and a variety of single-

molecule techniques. In 1993, Martin, et al. showed that the rectification behavior in a

donor-acceptor system is due to the organic layer itself, rather than work function mismatch

between contacts resulting in a Schottky diode [2]. Although the type of structure used in

this work differed slightly from the one proposed by Aviram and Ratner, their result was

extremely important in showing that the electrical properties of several molecular layers

can result in macroscopically significant measurements. Using the same zwitterionic

structure, Metzger, et al. proved that rectification is possible not only through multilayered

films but also through an organic monolayer [3]. In 2002, Chabinyc, et al. demonstrated

that the donor-acceptor structure is not a prerequisite for current rectification, heralding the

introduction of a new class of molecular rectifiers [4]. There have also been many examples

of single-molecule devices, including mechanically-controlled break junctions [5],

scanning tunneling microscopy (STM) [6, 7], and conducting probe atomic force

microscopy (CP-AFM) [8, 9]. The field has seen an explosion of interest as the fabrication

and analysis techniques have become more sophisticated, including the realization of a

58

memory circuit based on molecular junctions [10]. One of the drawbacks of working with

single molecular layers is the difficulty in establishing electrical contact to an already-

deposited layer, as an evaporated metal has the tendency to form filaments through to the

surface underneath, destroying the original, intended functionality [11]. A solution to this

issue was presented by Akkerman, et al., who introduced a multilayered top electrode

composed of the conducting polymer poly(3,4-ethylenedioxythiophene):

polystyrenesulfonate (PEDOT:PSS) and evaporated gold, where the soft contact of spin-

coated PEDOT:PSS precludes the formation of conducting filaments through the SAM

[12]. Chiechi, et al. pioneered the use of eutectic gallium-indium (EGaIn), a soft contact

that allowed to reproducibly obtain high-yield molecular devices [13]. The molecular

rectifiers based on SAMs take advantage of the spontaneous assembly of the molecular

layer on the substrate via chemisorption from solution or gas phase, a process that is both

simple and efficient.

Most current molecular rectifiers use thiol-terminated SAMs, which can only bond

to metallic substrates. This is due to the fact that the bond energy associated with the newly

formed Au-S covalent bond is very small compared to that of the Si-C or Si-O-C

characteristic in the case of monolayers on silicon, thus allowing the thiolate to migrate

between different Au sites. This molecular diffusion results in enhanced order, a property

which is not easily obtained in molecular layers that bind to silicon [14]. Nevertheless, the

procedures commonly used for metal deposition (i.e. thermal or electron-beam

evaporation) typically yield films with surface roughness that can match or even exceed

the lengths of the SAMs, particularly in the case of the short-chained compounds. In that

case, the measured electrical characteristics can be significantly altered by the

59

overwhelmingly greater conductivity of a metal-metal junction. To circumvent this issue,

the template-stripping method was developed wherein metal is deposited onto a silicon

wafer and, through the use of a release layer, the metal layer is inverted and transferred to

a different substrate [15]. While this process has proven effective for fabrication of high

performance Au-molecule-metal junctions, its complexity adds significantly to the costs of

production of these devices. Transition to silicon substrates could eliminate this issue since

a typical roughness of a silicon substrate without any post-processing is lower than 0.1 nm

[16], thus allowing its use without additional steps beyond cleaning. Another benefit to the

use of silicon as a substrate is the breadth of knowledge available from decades of work in

the commercial semiconductor industry and easier integration with current technologies

for hybrid devices [17]. The group of Wheeler provided a series of examples where devices

have been reliably assembled on the nanometer scale through chemically etching a

multilayered silicon-silicon oxide structure and bridging the gap with coated Au

nanoparticles [18, 19]. While, indeed, this work exploited the thiolates diffusion discussed

earlier, it also showed that the sub-10 nm scale can be achieved in silicon-based devices.

Following these studies, Ashwell, et al. used a similar procedure with amino-terminated

compounds to form molecular-scale devices which eschewed the metal nanoparticle used

in earlier studies, showing that compounds which covalently bonded to silicon are viable

options [20, 21]. Lenfant, et al. then gave an example of a sequentially-assembled

monolayer on silicon with an e-beam evaporated top contact, where a single molecular

orbital participated in charge transport leading to current rectification, but unfortunately

the rectification was very modest (maximum rectification ratio of 37 and yield of 50-70%)

[22]. Au-thiol-silicon molecular devices have been fabricated by using the flip-chip

60

lamination technique, which represented a significant step forward towards the

development of silicon-based molecular junctions [17]. Although there are convincing

reasons to work with molecular electronics on silicon, the vast majority of work has

focused on thiolates for the reasons outlined above, which can only be assembled on

metals. Here, we report on a series of newly developed molecules which can efficiently

assemble on silicon substrates to form high-performance molecular rectifiers. The

benzalkylsilane molecules consist of a 3-carbon alkyl chain and methanimine group

connected to a benzene ring with various substituents, and result from a very simple, and

high-yield synthetic procedure. We obtained a maximum rectification ratio of R = 2635 in

(E)-1-(4-cyanophenyl)-N-(3-(triethoxysilyl)propyl)methanimine, a value that rivals some

of the best molecular diodes on metallic surfaces. The ease of synthesis and straightforward

assembly on the surface of silicon makes these compounds compatible with current

electronic devices, and may expand the use of silicon technologies towards novel

functionalities.

3.2 Experimental


The eight different compounds, labelled 1-8, consisted of the same base structure: a

triethoxysilane, a 3-carbon alkyl chain, and a methanimine group attached to a benzene

ring. The substituents on the benzene ring were different for each system, as follows: 1: 4-

cyano, 2: 2,4-methoxy, 3: 2,6-difluoro, 4: 3-fluoro 4-cyano, 5: 3,5-difluoro, 6: 4-

dimethylamino, 7: 4-methoxy, 8: 4-nitro, see Figure 3.1a. SAMs comprised of these

61

molecules were constructed on 1 cm × 1 cm wafers of highly-doped silicon (≤0.005 Ωcm).

The silicon wafers were cleaned first by immersion in hot acetone for 10 minutes followed

by a rinse of fresh acetone and isopropyl alcohol (IPA), then a 10-minute immersion in hot

IPA followed by a rinse of fresh IPA and dried in a stream of nitrogen. They were then

subjected to a 10-minute UV/Ozone treatment, rinsed thoroughly with deionized water,

and dried in a stream of nitrogen. The samples were then introduced to a nitrogen glovebox

(<0.1 ppm H2O, <0.1 ppm O2) and submerged in a 4 mMol solution of one of compounds

1-8 in chloroform for 18-24 hours. Following the SAM deposition, the samples were

removed from the glovebox, rinsed thoroughly with fresh chloroform and IPA, and dried

Figure 3.1. (a) The general chemical structure of compounds 1-8. For compounds 1: R2, R3,

R5, R6 = H, R4 = CN; 2: R2, R3, R5 = H, R4, R6 = OCH3; 3: R3, R4, R5, = H, R2, R6 = F; 4: R2,

R3, R6 = H, R4 = CN, R5 = F; 5: R2, R4, R6 = H, R3, R5 = F; 6: R2, R3, R5, R6 = H, R4 =

N(CH3)2; 7: R2, R3, R5, R6 = H, R4 = OCH3; 8: R2, R3, R5, R6 = H, R4 = NO2, (b) Device

structure of the molecular rectifiers showing the liquid metal EGaIn probe tip, the SAM

under test, and the highly-doped silicon substrate.

a) b)

62

in a stream of nitrogen. The samples were immediately measured using an EGaIn probe tip

as the top electrode. The probe was constructed by depositing a small amount of EGaIn

onto a sacrificial copper strip into which a 0.5 mm diameter tungsten probe was lowered

and then gently raised using a Micromanipulator.

3.2.2 Electrical Characterization

Figure 3.1b shows the molecular rectifier structure where the bottom electrode was highly-

doped silicon upon which SAMs of compounds 1-8 were assembled, and the top electrode

was a cone of the liquid metal EGaIn. A bias was applied to the EGaIn electrode with the

silicon electrode held at ground, and the current was measured at forward and reverse bias,

of up to +2 V and -2 V. The figure of merit is the rectification ratio R, which is the ratio of

current density at forward bias compared to the current density at negative bias:

𝑅𝑅 =𝐽𝐽𝑉𝑉𝑓𝑓𝑓𝑓𝑓𝑓 = + 2 𝑉𝑉𝐽𝐽(𝑉𝑉𝑟𝑟𝑟𝑟𝑟𝑟 = − 2 𝑉𝑉)

(3.1)

where the current density is obtained by dividing the measured current by the area of the

EGaIn probe tip.

3.2.3 Cyclic Voltammetry

In order to determine the position of the highest occupied molecular orbital (HOMO),

cyclic voltammetry measurements were carried out using 0.1 M tetrabutylammonium

tetrafluoroborate in acetonitrile, a Pt counter-electrode, and an Ag/AgCl reference. The

system was calibrated using the Fc/Fc+ redox potential (0.6 V vs. Ag). The scans were

taken at a scan rate of 20 mV/s using the silicon wafer treated with the SAM under

examination as the working electrode.

63


In Figures 3.2a and 3.2b we show the average current density (J) as a function of applied

voltage (V) through SAMs of compound 1, on both a linear and a logarithmic scale. It can

be observed that charge transport is more efficient in the forward bias regime, similar to

the case of a solid-state diode. The average current density curve shown in Figure 3.2 was

obtained by taking the average current density from 100 J-V curves obtained on different

samples (multiple spots on one sample, as well as several SAM films).

The small standard deviation in current density points towards a great uniformity

in the electrical properties of the films, which is of paramount importance in any new

technology. Similar J-V curves were obtained on the other compounds, but with different

rectification strengths. It is also important to note that each compound rectified current in

the same direction, meaning that the current density at positive bias was always higher than

that at negative bias. At least 70 measurements resulting from a minimum of 5 samples

Figure 3.2. (a) The average J-V curve for compound 1 on a linear scale, showing the

diode-like rectification, (b) The average J-V curve for compound 1 on a logarithmic

scale where the scale bars show the standard deviation.

a) b)

64

were characterized for each type of SAM and the results are summarized in Figure 3.3,

where for each molecule we list the average and maximum rectification ratio obtained. The

highest rectification strength was obtained on compound 1, Rmax = 2635, with an average

of 1683 ± 458. To the best of our knowledge, this value represents the best rectification

ratio obtained from a SAM on silicon. In addition to the high performance showed in

devices, these compounds are synthetically simple to make, utilizing condensation

chemistry. For the rest of the molecules examined, the results are as follows: compound 2

Rmax = 1248, Ravg = 727 ± 323, compound 3 Rmax = 2118, Ravg = 711 ± 440, compound 4

Rmax = 1418, Ravg = 619 ± 230, compound 5 Rmax = 653, Ravg = 264 ± 162, compound 6 Rmax

= 933, Ravg = 235 ± 162, compound 7 Rmax = 590, Ravg = 221 ± 106, compound 8 Rmax =

367, Ravg = 212 ± 77. The variability in the properties of these compounds is by no means

a linear relationship, as not only does the rectification depends on the relative molecular

orbital’s energetic position and the physical asymmetry of each molecule, but also the local

electric field, the molecular disorder, and the molecular density. For example, although

compounds 2 and 3 show fairly large, and similar, standard deviations it is probable that

they arise from different sources. Compound 2 is asymmetric along the axis of the

molecule, which can introduce an intramolecular tilt thereby increasing disorder. However,

compound 3 is symmetric, but has a longer span in the plane of the substrate due to the

width of the molecule, which can decrease the molecular density on the surface.

To elucidate the contribution of the difference in work functions of the silicon and

EGaIn electrodes to current rectification we also evaluated a blank sample, in which the

EGaIn was in direct contact with the silicon wafer; the resulting J-V curve is shown in

Figure 3.4. The work functions of the EGaIn and silicon electrodes are 4.1-4.2 eV and 4.65

65

eV, respectively, raising the possibility of a Schottky mechanism when comparing forward

to reverse bias [16, 23]. However, as the electrical characteristics in Figure 3.4 show, there

is negligible current rectification in this sample, and therefore the observed rectification is

solely the result of the presence of the molecule.

Below we discuss the mechanism through which these compounds rectify current.

The asymmetries between the positive and negative biases result from the difference

between current resulting from strictly tunneling and current arising from tunneling plus

hopping transport, similar to the case of molecular rectifiers on Au, see Figure 1.3 in

Chapter 1 [24, 25]. At reverse bias, charges can only tunnel from one electrode to the other,

with the SAM acting as a tunneling barrier. On the contrary, in the forward bias regime,

the HOMO of the π-conjugated tailgroup is accessible between the two electrodes resulting

in charges moving through a hopping mechanism, as well as tunneling through the

remaining σ-bonded alkyl chain. Due to the considerable inefficiency of tunneling

Figure 3.3. Histograms showing the measured rectification ratios of compounds 1-8.

66

transport and its dependence on the length of the tunneling barrier, the abbreviated

tunneling plus hopping mechanism results in a higher measured current. Note, however,

that the rectification properties could also arise from molecular asymmetry, where the

position of the relevant molecular orbital is located closer to one end of the molecule [24].

Even if the molecular orbital is accessible at both bias polarities, the larger voltage drop

across the σ-bonded tunneling barrier results in different electrostatic conditions on the

conjugated portion of the molecule at equal and opposite biases [24]. In reality, the

rectification through SAMs is a combination of these two mechanisms, though it is also

affected by disorder in the monolayer, the type of bond between the molecules themselves

and the contacts, and any potential work function mismatch between the contacts.

Figure 3.4. J-V characteristics on a Si:SiO2:EGaIn junction, showing that the choice of

electrodes results in negligible rectification. The voltage sweep was taken from -1 V to 1

V due to the current reaching the compliance of the instrument at higher voltages.

67

To understand the differences in rectification strengths observed in Figure 3.3, we

conducted cyclic voltammetry measurements on the two compounds which gave the

highest rectification ratios, 1 and 2, the results are shown in Figure 3.5. The onset of

oxidation gives a value for the HOMO level of compound 1 of E1,HOMO = -6.3 eV (see

Figure 3.5a), and E2,HOMO = -6.1 eV for compound 2 (see Figure 3.5b). This difference of

ΔE = 0.2 eV plays a critical role in the current rectification as the electrostatic asymmetry

of these compounds is the defining factor here, and a difference in HOMO level values

results in a difference in the potential barrier to charge transport. Of course, as mentioned

above, the physical separation of the metallic contacts and the molecular orbital will play

a role as the system is highly asymmetric. The strong electron withdrawing cyano group in

compound 1 effectively pulls the HOMO level closer to the end of the molecule near the

EGaIn contact, resulting in the orbital being exposed to the greatest potential at positive

bias. This significant spatial asymmetry could result in an easier charge injection at positive

bias which causes the extreme asymmetry in current density at opposing biases.

Figure 3.5. (a) Cyclic voltammetry measurement on compound 1, (b) Cyclic

voltammetry measurement on compound 2.

a) b)

68

3.4 Conclusions

We have demonstrated efficient molecular rectifiers assembled on silicon substrates. The

new compounds rectify current to varying degrees, with the highest recorded rectification

ratio showing an average of nearly 1700 and a maximum of 2635. To date, this is the

highest rectification ratio obtained on a molecular junction fabricated on silicon. We have

identified the likely causes behind the large differences in electrical properties between the

compounds, laying the groundwork for future studies to enhance their efficacy. Not only

do they contribute superior performance, but they are relatively simple to produce and

manufacture. This work represents substantial progress towards the use of molecular-scale

devices in commercially-viable electronics.

69

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[25] C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, “Theory of electrical rectification in a molecular monolayer,” Phys. Rev. B - Condens. Matter Mater. Phys., Vol. 64, p. 1, 2001.

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Chapter 4 Organic Thin Films with Charge Carrier

Mobility Exceeding that of Single Crystals

The performance of organic field-effect transistors (OFETs) depends heavily upon the

intrinsic properties and microstructure of the semiconducting layer, the processes taking

place at the semiconductor/dielectric interface, and the quality of contacts. In this article,

we report on 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene single crystal and thin-

film OFETs and compare their properties. We find that the single crystals exhibit a

pronounced anisotropy in electrical characteristics, with a maximum field-effect mobility

of 0.3 cm2/Vs. Through density functional theory (DFT) calculations we identified the

main direction for hole transport, which was confirmed by X-ray diffraction (XRD)

measurements as parallel to the plane of the single crystal facet where the transport was

probed. By processing the material as a thin-film semiconductor, the content of high-

mobility direction probed within the transistor channel was enhanced. The control of film

morphology, coupled with a different design of the device structure allowed us to obtain

an order of magnitude higher charge carrier mobilities and a very small spread in device

performance.

This work was published as Zachary A. Lamport, Ruipeng Li, Chao Wang, William Mitchell, David Sparrowe, Detlef-M. Smilgies, Cynthia Day, Veaceslav Coropceanu, and Oana D. Jurchescu, “Organic thin films with charge-carrier mobility exceeding that of single crystals,” J. Mater. Chem. C, Vol. 5, p. 10313, 2017.

73

4.1 Introduction

Organic semiconductors have drawn considerable interest due to the inherent processing

flexibility afforded by their van der Waals intermolecular forces and the intrinsic synthetic

versatility of organic chemistry which allows for a near-limitless array of materials

depending, in principle, only on the desired function. The weakly bonded organic crystals

are amenable to manufacturing near room temperature from solution, allowing for a diverse

assortment of viable substrates. At the same time, these weak interactions result in a

reduced performance due, in part, to increased defect probability and stronger interactions

between the lattice and charge carriers. A strenuous experimental and theoretical effort into

material development produced significant improvements in organic device properties over

the past decades. For example, the charge-carrier mobility in organic field-effect transistors

(OFETs) has increased several orders of magnitude [1, 2]. The mobility of such a device,

however, is not simply a function of the intrinsic material properties but the film

microstructure, the quality of the contacts, and the processes at the

semiconductor/dielectric interface play a consequential role. In the example of a rigorously

studied organic semiconductor, diF-TES ADT (2,8-difluoro-5,11-bis(triethylsilylethynyl)

anthradithiophene), a change of dielectric from SiO2 to the polymer Cytop allowed an order

of magnitude increase in mobility due to a reduction in the trap density in the

semiconductor film [3]. In the same material, surface modifications with self-assembled

monolayers (SAMs) were used to tune both the film microstructure and device contact

resistance, which resulted in OFETs with mobilities up to several orders of magnitude

greater [4–9]. Fabricating devices using single crystals corresponded with an additional

order of magnitude increase in mobility [10]. OFETs made using semiconductors in a

74

single crystal form avoid many of the issues which arise in thin films because single

crystals possess a highly-ordered microstructure and grain boundaries are nonexistent.

These properties allow single-crystal OFETs to access the fundamental limits of charge

transport in a material, provided that the contacts are optimized [11], setting an upper limit

on the possible electrical characteristics for a new material when processed in a different

fashion [12–19]. While single-crystal organic semiconductors will often set benchmark

electrical properties for a compound, they are highly anisotropic, i.e. the charge-carrier

mobility is highly dependent on the crystallographic direction along which the

measurements are conducted [15, 20–22].

Here, we examine a compound that shows significant anisotropy in the ab-plane

such that the mobility varies by at least one order of magnitude depending on the direction

of measurement. We then tune the microstructure of the thin film to enhance the content of

the high-mobility molecular orientation with respect to the transistor channel, and by doing

Figure 4.1. Chemical structure of 7,14-bis (trimethylsilylethynyl) benzo[k]tetraphene.

75

so we consistently obtain mobilities greater than 1 cm2/Vs. The material of focus is the

nonlinear acene 7,14-bis(trimethylsilylethynyl) benzo[k]tetraphene (TMS-BT) (chemical

structure shown in Figure 4.1), which was synthesized following the procedures reported

elsewhere [23]. Nonlinear acenes have received considerable interest recently due to their

enhanced environmental stability as compared to the linear acenes, while maintaining the

favorable electronic properties which originally drew attention to the class of polycyclic

aromatic hydrocarbons [24–28]. For example, Zhang et al. produced a nonlinear acene that

exhibited a field-effect mobility as high as 6.1 cm2/Vs and identified the origin of the

additional stability present in this class of compounds [27]. They suggested that because a

nonlinear acene contains a greater number of aromatic Clar sextets than a linear acene, the

stability is increased in accordance with Clar’s sextet rule. In our system, single crystal

OFETs were fabricated on a SiO2 dielectric and mobilities between 0.0004 and 0.3 cm2/Vs

were obtained. To determine the source of this disparity in device performance, we

conducted density functional theory (DFT) calculations. This analysis confirmed the

presence of charge transport anisotropy and informed us on the direction of high mobility.

Using this information, we were then able to carefully tune processing parameters to

enhance the content of the high-mobility molecular orientation in thin films, as confirmed

by X-ray diffraction (XRD) data. By controlling the microstructure of the thin films and

implementing a device structure incorporating Cytop as the top-gate dielectric, we

consistently obtained mobilities greater than 1 cm2/Vs.

76

4.2 Experimental

4.2.1 Single Crystal Growth

The physical vapor transport (PVT) method was used to grow single crystals, as seen in

Figure 4.2a [29]. The quartz tube was cleaned using detergent followed by DI water, high-

purity acetone, then high purity isopropanol (IPA), and baked overnight at approximately

300°C. The starting material was then placed in the tube at the hottest part of the furnace

and left under flowing high-purity argon at room temperature overnight. Before bringing

the furnace to the sublimation temperature, the temperature was raised above 100°C for

two hours to remove any remaining water. TMS-BT was heated to approximately 195°C

under 50 mL/min high-purity argon flow and within a day of reaching the sublimation

temperature, thin, yellow-green platelets were obtained in the cold part of the tube, the

Figure 4.2. (a) A depiction of the physical vapor transport setup used to grow TMS-BT, (b)

Schematic representation of the single-crystal OFET device structure, (c) An image of TMS-BT

laminated between the source and drain contacts.

a)

b) c)

77

crystallization region. Once the crystals formed, the furnace temperature was reduced by

10°C every 20 minutes to room temperature to reduce thermal shock in the crystals upon

cooling the growth tube.

4.2.2 OFET Fabrication and Electrical Characterization

For single-crystal OFETs, a bottom-gate, bottom-contact structure was used where the

bottom gate consisted of highly-doped silicon with 200 nm of thermally grown SiO2 as the

gate dielectric. Source and drain contacts were patterned by photolithography or shadow

mask and deposited by e-beam evaporation (5 nm Ti/45 nm Au). No significant difference

was noted in the performance of the devices as a function of contact patterning method.

After metal evaporation, the substrates were cleaned in hot acetone, hot IPA, followed by

a 10-minute exposure to UV-ozone and rinsed with DI water. The contacts were then

treated with 2,3,4,5,6-pentafluorobenzenethiol (PFBT) using a 30mM solution in ethanol

for 30 minutes followed by a 5-minute sonication in ethanol. The single crystals were then

laminated by hand between the source and drain contacts to complete the OFET structure,

as seen in Figures 4.2b and 4.2c. The thin-film devices were prepared with a top-gate,

bottom-contact structure on a glass substrate. Source and drain contacts were processed

similar to the case of single-crystal devices. TMS-BT was spin-coated onto the samples

from a 10 mg/mL solution in mesitylene. The Cytop top-gate dielectric was then spin-

coated and baked at 100°C for 2 minutes on a hot plate to obtain an 800-850 nm thick layer,

onto which a silver top gate was evaporated. Electrical characterization was performed

using an Agilent 4155C Semiconductor Parameter Analyzer in ambient and dark.

78

4.2.3 Theoretical Calculations

The electronic structure of the TMS-BT crystal was obtained on the basis of experimental

crystal structure using the B3LYP functional and the 6-31G basis set. The Brillouin zone

was sampled using a uniform 10 × 4 × 4 k-point grid. The effective masses (mij) of holes

were calculated by means of Equation 1:

1𝑚𝑚𝑖𝑖𝑖𝑖

= 1ℏ2

𝜕𝜕2𝐸𝐸𝜕𝜕𝜕𝜕𝑖𝑖𝜕𝜕𝜕𝜕𝑖𝑖

(4.1)

Here E is the band energy, ℏ is Planck’s constant, k is the hole wave vector, and i and j are

the Cartesian coordinates in reciprocal space. The inverse effective mass tensor was

calculated by means of Sperling’s centered difference method with ∂k = 0.06/bohr. The

effective transfer integrals for holes were evaluated at the B3LYP/6-31G(d,p) level and

based on the fragment orbital approach combined with a basis set orthogonalization

procedure [30]. The periodic boundary conditions calculations were performed by means

of the Crystal 14 package [31] while the transfer integrals were obtained using the Gaussian

09 package [32].

4.2.4 Structural Characterization

X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXD) were performed

at beamline G2 at the Cornell High Energy Synchrotron Source (CHESS). Both thin-film

and single-crystal samples were characterized on a Psi-Circle surface diffractometer [33]

using an X-ray energy of 11.2 keV (λ = 1.071 Å) from a Be single-crystal beam-splitting

monochromator. XRD was carried out in the θ-2θ mode from 2° to 20° with a step of 0.02°

and recorded using a 640-element linear diode-array detector. The diffraction peaks were

79

analyzed by fitting Gaussian distributions using Origin Pro software. Two molecular

orientations were identified in thin films and their relative content was calculated from the

ratio of integrated peak area and structure factor, according to ref. [8].

GIXD was performed on a surface diffractometer in powder mode, with the sample

being rotated 360° around the surface normal for each angle step of the data collection [34].

A set of Soller slits was mounted on the detector arm to provide an in-plane resolution of

0.16°. Scattered X-rays were collected with a linear diode array. The incident angle was

fixed at 0.15° and the in-plane scattering angle ν covered a range from 2 to 30° with a step

size of 0.1°. The diode array covered 10° of out-of-plane scattering angle δ such that a two-

dimensional scattering map I(ν,δ) of the sample was obtained in a single scan. The structure

was determined by fitting peak positions and parallel crystallographic planes [34, 35].


Included in Figures 4.3a and 4.3b are typical electrical measurements recorded on the

single-crystal OFETs. The evolution of drain current (ID) vs. drain-source voltage (VDS) at

varying gate-source voltages (VGS) shows linear behavior typical of Ohmic contacts before

the expected transition to the saturation regime in Figure 4.3a. These measurements

indicate weak or negligible contact effects in these devices. The gate-sweep measurement

of the drain current in the saturation regime at VDS = -40V is seen in Figure 4.3b, with the

square root of drain current in black corresponding to the left axis and the drain current on

80

a log scale in blue on the right axis, shows a fairly sharp turn-on with a subthreshold swing

of S = 1.3 V/dec a current on/off ratio greater than 105, a field-effect mobility of µ = 0.3

cm2/Vs, though the threshold voltage is quite high at VTh = -15V. This high threshold

voltage is a common characteristic of our single-crystal devices and this is most likely due

to the high density of trapping states at the surface of the SiO2 dielectric. We measured

over 30 single-crystal OFETs and a summary of the field-effect mobilities is included in

Figure 4.4. It can be observed that there is a spread of approximately three orders of

Figure 4.3. (a) The evolution of the drain current vs. drain-source voltage at varying gate

voltage, (b) A gate-source voltage sweep in the saturation regime at constant drain-source

voltage.

a)

b)

81

magnitude in measured mobilities, with a minimum of µmin = 0.0004 cm2/Vs, a maximum

of µmax = 0.3 cm2/Vs and an average of µavg = 0.03 cm2/Vs. This large variance in mobility

can be attributed to different factors including crystal and contact quality, as well as

anisotropy in the electrical properties along the different crystal directions as has been

reported in other systems [15, 20–22].

We conducted DFT calculations in order to elucidate the electronic properties of

TMS-BT and relate the results along the different crystallographic directions to the

measured spread in the electrical properties. The electronic structure calculations, Figures

4.5 and 4.6, suggest that the largest electronic couplings (46 meV) for holes occurs along

the a crystallographic axis. As a consequence, the smallest effective mass (𝑚𝑚1 = 1.01 𝑚𝑚0,

where 𝑚𝑚0 is the electron mass in vacuum) and the main direction for hole transport are also

identified with this direction. The calculations also reveal that the next smallest effective

mass component, which is found along [210] direction, is 2.38 𝑚𝑚0 suggesting that charge

Figure 4.4. Single-crystal OFET mobility statistics obtained for over 30 devices.

82

transport for holes has a quasi-two-dimensional character, as illustrated in Figure 4.6. XRD

measurements performed on single crystals of TMS-BT found a series of (00l) peaks (l =

1, 2, 3, 4) showing a (001) orientation of the crystals laminated on a silicon wafer substrate.

This assignment is supported by the GIXD intensity map which only shows reflections due

to a (001) texture (Figure 4.7). The observed texture shows that the ab-plane of the crystal

is parallel to the surface of the crystal, and therefore to the substrate, hence electrical

measurements on such laminated single crystals largely probe charge transport in the ab-

plane. From the known bulk structure [23], we conclude that the TMS-BT molecules are

essentially oriented along the crystal surface normal, as shown in Figure 4.8, where we

depict one possible orientation of the molecules with respect to the device structure.

Figure 4.5. Electronic band structures and densities of states of TMS-BT. The points of high

symmetry in the first Brillouin zone are in crystallographic coordinates: Γ = (0, 0, 0); X = (0.5,

0, 0); Y= (0, 0.5, 0); Z = (0, 0, 0.5); V = (0.5, 0.5, 0); U = (0.5, 0, 0.5); T = (0, 0.5, 0.5).

83

In this case, the high-mobility direction [100], determined by DFT (marked in red), is

perpendicular to the direction of charge transport (yellow). This crystal orientation with

respect to the source/drain contacts results in subdued electrical properties in the single-

crystal OFETs. Since we have not oriented the crystals with respect to the electrodes, the

large anisotropy in the electrical properties of this material in the ab-plane, which was

probed in OFET measurements, provides an explanation for the large spread in mobilities

seen in Figure 4.4. The highest mobilities likely were obtained when measuring along the

[100] direction and the lowest mobilities correspondingly measured nearly perpendicular

to this high-mobility direction. We do not exclude, however, the contributions from the

inherent differences in the crystal and contact quality.

t12= 46 meV

t34= 37 meV

t13= t24 = 10 meV

m1= 1.01 m0

m2= 2.38 m0

Figure 4.6. Illustration of dominant charge transport pathways for holes in TMS-BT crystals.

The smallest effective mass is observed along the stacking direction (a axis, indicated by a red

arrow), and the next smallest effective mass component is along the [210] direction (indicated

by a blue arrow). The numbers in the crystal structure label the molecules used for the electronic

coupling calculations.

84

In order to enhance the content of the high mobility [100] direction in devices, we

turn to processing. Controlling the solid-state packing by processing and/or post-

processing takes advantage of the weak intermolecular bonds characteristic to organic

crystals, which often allow to fine-tune the crystal structure by manipulating

solvent/substrate, solute/substrate, solvent/vapor, and solute/solvent interactions. Such

efforts have been a staple of the field of organic electronics and morphology modification

has been accomplished by many different techniques [7, 36–43]. By tuning the processing

conditions of TMS-BT layer in organic thin-film transistors (OTFTs), as well as the device

structure (nature of the constituent layers) and architecture (order in which the layers

Figure 4.7. Indexation of the grazing-incidence diffraction pattern of a single crystal. Laminated

single crystals show a predominant (001) texture.

85

are deposited), we were able to control the molecular orientation such that the content of

high-mobility orientation was enhanced along the direction where the transport was

measured. The structure of the OTFT can be found in Figure 4.9a, where the bottom-gate

dielectric SiO2 was replaced with a Cytop top-gate dielectric. Typical performance is

shown in Figure 4.9b. This particular device exhibits a more than three-fold increase in

mobility compared to the best single-crystal device, µ = 1 cm2/Vs. Measurements taken on

a large data set yielded an average mobility of µ = 0.95 ± 0.04 cm2/Vs, with a very small

spread. To identify the source of the increased mobility and consistency between multiple

films, we evaluated the structure of the TMS-BT films using XRD measurements. We

found that the films pack in the same polymorph as the single crystals (Figure 4.10a), but

Figure 4.8. An illustration of the crystal structure in reference to the fabricated single-crystal

OFETs, showing the fast-growth direction of the single crystals in yellow and the high mobility

direction in red.

86

consist of a mix of two orientations, (021) (Figure 4.10b), and (001) (Figure 4.10c) in a

ratio of 1:1.4. While, as expected, the unique molecular orientation in single crystals yields

to the presence of (00l) peaks only, thin films showed two peaks, (001) and (021). The

latter peak position is about 0.5° away from the (002) peak (Figure 4.10a inset), clearly

indicating the presence of the (021) orientation in thin films. GID maps, Figures 4.11 and

4.12, revealed a larger amount of mosaicity in the films as indicated by the arc-shaped

reflections, hence overlap of reflections was a problem. From our fits, we can state that

both the (001) and (021) orientations provide reflections compatible with the XRD data. In

a)

Figure 4.9. (a) Device structure of thin-film OFETs using Cytop as the gate dielectric, (b) Gate-

source voltage sweep for a typical thin-film device.

b)

87

addition, there may be a small distortion of the thin-film lattice compared to the bulk single-

crystal lattice which is commonly observed in triclinic materials.

In both of these orientations, the main direction for hole transport as determined by

DFT calculations lies in the plane of the thin film parallel to the substrate, as marked with

a red arrow in Figures 4.10b and 4.10c, and is therefore accessible in OTFT measurements.

Since a large number of grains are present in the films, and their alignment is randomly

Figure 4.10. (a) X-ray diffraction measurements of single crystals and thin films, including

insets showing the (001), (002), and (02𝟏𝟏) peaks, (b) a side-view of TMS-BT in (02𝟏𝟏)

orientation with the functional side-groups removed for clarity, (c) a top-view of TMS-BT in

(001) orientation. The red arrows correspond to the high-mobility direction as calculated by

DFT.

a)

b) c)

88

Figure 4.12. Indexation of a thin film diffraction pattern assuming the bulk structure and a

(02-1) texture.

Figure 4.11. Indexation of a thin film diffraction pattern assuming the bulk structure and a (001)

texture.

89

distributed, the probability of charge carriers accessing the high-mobility direction is

greatly increased and, consequently, the performance is consistently better in the thin-film

devices. On the contrary, the high-mobility direction can be accessed in single-crystal

devices only if that direction is first identified and the crystal is aligned such that that

particular crystalline axis is parallel with reference to the source and drain electrodes.

While the OTFTs obtained by spin coating yielded more consistent mobilities, the

additional improvements in performance are a result of device design. The staggered

contact geometry lowers the contact resistance, while the Cytop polymer top-gate dielectric

creates a very low density of traps. Collectively these modifications account for the

increase in charge-carrier mobility from 0.3 cm2/Vs, which was the best obtained in single

crystals to over 1 cm2/Vs in thin films [3, 44]. We had attempted to utilize this device

structure in our single-crystal OFETs in order to directly compare the different

semiconductor deposition methods using the same interface, but experienced significant

adhesion issues when laminating single crystals on the highly hydrophobic Cytop layer;

we do note, however that this has been accomplished in different systems [45].

4.4 Conclusions

Single-crystal field-effect transistors of TMS-BT on SiO2 dielectric exhibit a maximum

field-effect mobility of 0.3 cm2/Vs and a spread in performance of three orders of

magnitude. Through DFT calculations the high-mobility direction was identified to be

along the a crystallographic axis. XRD measurements conclude that the single crystals

form in a pure (001) orientation such that the high-mobility direction is in the plane of the

crystal surface. Through variations in thin-film processing, OTFTs of TMS-BT that contain

90

a mix of (001) and (021) orientations were produced. Both generated molecular

orientations contain the high-mobility direction in the plane of the device. By controlling

the film microstructure and replacing the bottom-gate SiO2 dielectric with a top-gate Cytop

dielectric we fabricated OFETs that consistently exhibit mobilities of 1 cm2/Vs. This work

gives an example of thin-film transistors that outperform single-crystal devices of the same

material though optimized film processing and device architecture. The results underline

the fact that single crystals provide the best performance of a material only if the high-

mobility direction is accessible, the device structure and dielectric/semiconductor-interface

are optimized, and the contacts are of high quality.

91

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[44] P. V. Necliudov, M. S. Shur, D. J. Gundlach, and T. N. Jackson, “Modeling of organic thin film transistors of different designs,” J. Appl. Phys., Vol. 88, p. 6594,

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2000.

[45] W. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, and B. Batlogg, “Organic small molecule field-effect transistors with CytopTM gate dielectric: Eliminating gate bias stress effects,” Appl. Phys. Lett., Vol. 90, p. 92104, 2007.

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Chapter 5 Organic Thin-Film Transistors with Charge-

Carrier Mobilities of 20 cm2/Vs, Independent

of Gate Voltage

The field of organic electronics has focused on the development of high mobility

semiconductors to improve device performance and increase the viability of these devices

in the commercial marketplace. The charge-injecting contacts, however, have received

much less attention as the route to high-performing devices. In this study, we focus on the

deposition parameters of Au source and drain electrodes to greatly reduce the contact

resistance and vastly improve device performance in organic field-effect transistors

(OFETs). We use a well-studied small-molecule semiconductor in 2,8-difluoro-5,11-

bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), and, by reducing the rate of

electrode deposition, reduce the contact resistance to 500 Ωcm and increase the field-effect

mobility to 19.2 cm2/Vs. To confirm that this method is suitable for use in OFETs beyond

small-molecule semiconductors, we produce devices using the polymer semiconductor

indacenodithiophene-co-benzothiadiazole (C16IDT-BT) and achieve a contact resistance of

200 Ωcm and field-effect mobilities up to 10 cm2/Vs.

This work was adapted from Zachary A. Lamport, Katrina J. Barth, Hyunsu Lee, Martin Guthold, Lee J. Richter, Iain McCulloch, John E. Anthony, and Oana D. Jurchescu, To be submitted, 2018.

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5.1 Introduction

The promise to impact contemporary applications has sparked great interest in the study of

organic electronic and opto-electronic devices. The rich chemistry of organic materials,

low manufacturing cost and compatibility with flexible and stretchable substrates, provide

an opportunity to incorporate electronics in non-traditional areas such as clothing [1], paper

[2], flexible and rollable displays [3–5], or bio-integrated applications [6, 7]. Research

efforts have focused on manipulating the chemical structure and degree of order of the

semiconductor layer, understanding and controlling the processes taking place at device

interfaces, light manipulation, integration with biological systems and more. These efforts

have been largely successful, and as the field has seen radical improvements, an increasing

number of products are approaching the marketplace. For example, recently developed

solution-processable small-molecule and polymer semiconductors incorporated in organic

field-effect transistors (OFETs) have reached charge-carrier mobilities μ previously

reserved for inorganic materials [8–13]. But with this remarkable progress also come great

challenges. A direct consequence of enhancing the intrinsic mobility of the organic

semiconductor layer is that the contributions of the contact effects to the overall device

performance now can become significant. Understanding and reducing these contributions

is of the utmost importance since they limit the OFET performance, particularly in the

linear regime, where they would operate for applications such as the active matrix displays.

This issue becomes more severe as the channel dimensions are minimized, since the

channel resistance scales positively with the channel length, while the contact resistance is

independent of this variable. In addition, the development of new materials also hinges on

a correct evaluation of mobility. The equations adopted from silicon MOSFETs for the

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characterization of OFET operation assume negligible contact resistance, and thus they fail

when the devices are severely limited by contacts. In this case it is thus impossible to access

the intrinsic properties of materials and to provide meaningful feedback for material design

[14–16].

The impact of contacts on OFET performance was recognized as early as 1996, by

Lin and co-workers [17], and several other groups have focused on this topic [18–26].

Recently, Klauk identified contact resistance as the single largest hurdle to overcome in

the pursuit of high-frequency OFETs [27]. The contact resistance results from the fact that

a small fraction of the voltage applied between the source and drain electrodes is necessary

to inject the charges from the electrode surface to the organic semiconductor layer. The

magnitude of this potential drop at the contact depends on several factors. The energetic

mismatch between the electrode work function and the electron affinity (n-type)/ionization

potential (p-type) of the organic semiconductor influences the injection process and

reduction of contact resistance by chemically tailoring this surface with self-assembled

monolayers (SAMs) has proven effective [28, 29]. Often, however, these modifications

not only alter the work function, but also the surface energy of the electrodes, therefore

impacting the morphology of the films deposited on these surfaces [30–32]. Charge

injection layers and contact dopants have also been proposed to enhance injection by

increasing the charge carrier concentration at the electrodes [21, 25, 33, 34]. The geometry

of the device can also play a role, with coplanar OFETs typically exhibiting higher contact

resistance than staggered structures [19]. For top-contact transistors, methods such as

nanotransfer printing or flip-chip lamination were introduced in order to avoid the

degradation of the semiconductor layer under the contact, which would lead to the

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formation of parasitic resistances [35, 36]. In addition to these methods to reduce contact

resistance, the use of organic materials such as graphene, reduced graphene oxide, carbon

nanotubes or charge transfer salts has grown in popularity due not only to the favorable

work function but also a reduction in the injection barrier stemming from interfacial dipoles

[33, 37–44]. Recently, Uemura, et al. found that contact-annealing can greatly reduce the

contact resistance in bottom-gate, top-contact devices, and it can also eliminate the non-

ideal current-voltage curves arising from gated Schottky contacts [15]. Here, we reduced

aggressively the contact resistance in small molecule and polymer OFETs by varying the

metal deposition rate in conjunction with using a pentafluorobenzene thiol (PFBT)

treatment, which resulted in over 5 times improved charge carrier mobility compared with

the best previously reported devices with identical composition and structure. The obtained

contact resistance normalized over the channel width was as low as 200 Ωcm, and the

OFETs exhibited charge carrier mobilities of 19.2 cm2/Vs for 2,8-difluoro-5,11-

bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and 10 cm2/Vs for

indacenodithiophene-co-benzothiadiazole copolymer (C16IDTBT), with minimal

dependence on the gate voltage. To understand this drastic improvement in device

performance, we performed grazing incidence X-ray diffraction (GIXD) measurements on

the organic semiconductor films to evaluate if the contact deposition rate results in

variations in the film morphology and/or microstructure, and found no major differences

in the structure of the semiconductor layer. This result suggests that the improvements in

device performance originate only from the differences in the electrode properties. Indeed,

the metal grain size correlates negatively with the deposition rate, as confirmed by atomic

force microscopy (AFM) measurements, thus creating different environments for the SAM

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attachment and also impacting its final structure. Evaluation of the PFBT/Au using

scanning Kelvin probe microscopy (SKPM) indicated that there exist local variations in

the work function of the electrodes fabricated using a low deposition rate, pointing to the

existence of regions with more efficient charge injection due to enhanced PFBT order, a

feature which is absent in the samples obtained via fast deposition.

5.2 Experimental


Bottom-contact, top-gate devices were fabricated on an insulating surface consisting of a

200 nm thermal silicon oxide. The wafers were cleaned by immersion in hot acetone for

10 minutes, then rinsed with fresh acetone and isopropyl alcohol (IPA), followed by

immersion in hot IPA for 10 minutes and an additional rinse using fresh IPA and dried in

a stream of nitrogen. Then the substrates were exposed to a UV-Ozone treatment for 10

minutes, rinsed thoroughly using deionized water and dried in a stream of nitrogen. The

source and drain contacts were patterned by shadow mask and consisted of a 5 nm titanium

adhesion layer deposited by e-beam evaporation at a rate of 1 Å/s followed by 40 nm of

thermally evaporated gold at varying deposition rates. These contacts were then treated for

30 minutes using a 30mM solution of room-temperature PFBT in ethanol followed by a 3-

minute sonication in fresh ethanol and a thorough ethanol rinse and dried in a stream of

nitrogen. The substrates were then brought into a nitrogen glovebox (<0.1 ppm O2, <0.1

ppm H2O) where the organic semiconductor layer was deposited immediately. A 16.5

mg/mL solution of diF-TES ADT in chlorobenzene was spin-coated at 1000 RPM for 80 s

and placed under vacuum for 90 minutes to remove additional solvent. C16IDT-BT was

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spin-coated at 2000 RPM for 60 s from a 10 mg/mL solution in chlorobenzene before

annealing at 100°C for 10 minutes. Samples were then brought back into the glovebox to

apply the Cytop top-gate dielectric, which was spin-coated at 2000 RPM for 60 s and then

annealed at 55°C overnight. A 40 nm gold top gate was then applied using electron beam

evaporation at a rate of 1 Å/s.

5.2.2 Device Characterization

The transistor characterization measurements were carried out in the dark and under

ambient conditions using an Agilent 4155C Semiconductor Parameter Analyzer. AFM and

KPFM measurements were taken using an Asylum MFP-3D Bio AFM (Asylum Research,

USA) in ambient atmosphere. For AFM, a silicon cantilever (Nanosensors PPP-NCLR,

force constant: 21-98 N/m, resonance frequency: 146-236 kHz) was used in tapping mode

with a feedback setpoint of 500 mV, and 1 µm × 1 µm images were taken at a rate of 0.5

Hz. KPFM measurements used a silicon cantilever with a Ti/Ir coating (Oxford Instruments

ASYELEC.01-R2, force constant: 1.4-5.8 N/m, resonance frequency: 58-97 kHz) at a nap

height of 5 nm, and 20 µm × 20 µm images were taken at a rate of 1 Hz.

5.2.3 Grazing Incidence X-Ray Diffraction

GIXD was carried out at the Cornell High Energy Synchrotron Source (CHESS) on a

surface diffractometer in powder mode. The samples were rotated 360° for each step of the

measurement, using a set of Soller slits on the detector arm before a linear diode array,

resulting in an in-plane resolution of 0.16°. The in-plane scattering angle ν was varied from

2 to 30° with a step size of 0.1°, keeping the incident angle fixed at 0.15°. The linear diode

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array had a 10° range in the out-of-plane scattering angle δ to form a two-dimensional

scattering map I(ν, δ) on each scan.


The chemical structure of diF-TES ADT is shown in Figure 5.1a and the electrical

characteristics of a device made using a Au deposition rate of 0.5 Å/s in Figures 5.1b and

c. In Figure 5.1b we show the evolution of the drain current (ID) as a function of the gate-

source voltage (VGS) in the saturation regime, with the drain-source voltage (VDS) held

constant at -40 V. The blue line corresponds to ID on a log scale (right axis) and the black

open circles correspond to the square root of ID (left axis). The red line serves as a visual

aid to show that the square root of ID follows a linear relation with VGS, as expected from

the gradual channel approximation, and indicates the section of the curve where the

mobility was calculated. Figure 5.1c shows the evolution of ID with VDS, where each line

Figure 5.1. (a) Chemical structure of diF-TES ADT, (b) Drain current as a function of gate-

source voltage in a diF-TES ADT device with L = 100 µm and W = 200 µm, in the saturation

regime, (c) Drain current as a function of drain-source voltage for the same device.

a) b) c)

103

is measured at a different VGS, and demonstrates linearity at low VDS and a clear transition

from the linear to saturation regime. Both these features are emblematic for low contact

resistances. This device exhibits a charge-carrier mobility of μsat = 19.2 cm2/Vs, a current

on/off ratio of Ion/Ioff = 5.9 ∙ 103, and a threshold voltage of VTh = 3.3 V. Larger Ion/Ioff ratios

are possible, as shown for example in Figure 5.2, where Ion/Ioff = 5 ∙ 107.

In order to make sure that the mobility is not overestimated, we also evaluated its

dependence on VGS. Mobility overestimation can occur in the case of gated Schottky

contacts, where there is a large injection barrier at the contacts which is overcome by an

increasing VGS. This relation causes a peak in the apparent mobility when the injection

barrier is overcome, before decreasing to a more realistic value [14–16]. As can be

observed in Figure 5.3, the mobility in our device first increases with increasing VGS,

followed by a plateau at higher VGS. Such a dependence has been observed in other high

mobility systems such as C10DNTT thin films or rubrene single crystals and was attributed

Figure 5.2. Drain current as a function of gate-source voltage in a diF-TES ADT device,

showing an on/off ratio greater than 107.

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to the presence of electronic traps in the organic semiconductor layers [15, 45]. The

mobility evaluated in the linear regime for the device presented in Figure 5.1 was µlin =

16.0 cm2/Vs. The contact resistance has a greater effect on the effective device mobility in

the linear regime and the close correspondence recorded between the linear and saturation

mobilities hint to a low contact resistance, in agreement with the linear curves obtained in

the low-VDS range of the output characteristics in Figure 5.1c. A quantitative analysis of

the contact resistance and its effect on device properties will be provided later.

The evolution of the average mobility with the contact deposition rate is depicted

in Figure 5.4a. It can be observed here that the maximum average mobility, µsat,avg = 14.6

± 3.3 cm2/Vs, was obtained when a rate of 0.5 Å/s was used, decreasing to an average

mobility of µsat,avg = 3.24 ± 0.49 cm2/Vs at a rate of 3.0 Å/s. The lower values coincide

with those reported using the same methods, materials, device architecture, where devices

Figure 5.3. Mobility vs. gate-source voltage for the device in shown in Figure 5.1.

105

fabricated with a contact deposition rate of 2 Å/s resulted in an average mobility of µsat,avg

= 1.5 cm2/Vs and a maximum mobility of µsat,max = 3.14 cm2/Vs [46].

To understand the reason behind the improvements in mobility, we first performed

GIXD measurements on the diF-TES ADT films deposited on PFBT/Au. The results for

the rate of 0.5 Å/s are included in Figure 5.5a, while those for 2 Å/s are shown in Figure

5.5b. It can be observed that in both cases the series of (001) peaks can be distinguished,

confirming that the molecules are “edge-on” oriented, as illustrated in Figure 5.5c. These

findings are in agreement with earlier reports [30, 47, 48]. The dominant (001) orientation

is a result of the PFBT-treated Au acting as a templating mechanism, with the fluorine

atoms of the PFBT molecules interacting with the fluorine atoms of diF-TES ADT

molecules, and this orientation is the most favorable direction for charge transport.

Figure 5.4. (a) Average field-effect mobility vs. contact deposition rate, (b) Width-

normalized contact resistance as a function of contact deposition rate.

a)

b)

106

Since no major differences were observed in the morphology of the film as a

function of contact deposition rate, we further focused on the quantitative analysis of the

changes in the contact resistance. The total device resistance Rdevice is given by the channel

resistance, RCh (a quantity which is proportional to the channel length), and the contact

resistance RC, as shown in equation 5.1. RC in a staggered structure, such as the ones studied

here and depicted in Figure 5.6a, has two main contributions, the interface resistance, Rint,

and the bulk resistance, Rbulk, which can be seen in Figure 5.6b. In general, Rint is a result

of the properties of the electrode surface, including the energy level mismatch between

electrode and semiconductor, and the presence of interfacial dipoles, whereas Rbulk governs

the transport of the injected charges through the organic semiconductor, from the

electrode/semiconductor interface to the accumulation layer. Thus, Rbulk strictly depends

on the conductivity of the semiconductor in the direction perpendicular to the channel and

Figure 5.5. GIXD measurements for diF-TES ADT on patterned Au deposited at (a) 0.5

Å/s and (b) 2 Å/s. (c) The (001) molecular orientation of diF-TES ADT.

a) b) c)

107

the thickness of the semiconducting layer. The relations between the Rdevice, RCh, Rint, and

Rbulk in the linear regime are as follows:

𝑅𝑅𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 = 𝑅𝑅𝐶𝐶ℎ(𝐿𝐿) + 𝑅𝑅𝐶𝐶 (5.1)

𝑅𝑅𝐶𝐶 = 𝑅𝑅𝑑𝑑𝑖𝑖𝑖𝑖 + 𝑅𝑅𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 (5.2)

We evaluated the contact resistance for the devices corresponding to each contact

deposition rate using gated transmission line method (gated TLM), based on equation 5.1,

and the results are displayed in Figure 5.4b. The gated TLM was conducted by first finding

VTh using the second-derivative method [49], and the value of ID in the linear regime was

then taken at an overdrive voltage of VGS -VTh = -40 V. It is clear that the increase in the

average field-effect mobility as a function of contact deposition rate is mirrored by the

inverse trend in contact resistance. The devices obtained using a fast deposition rate of 3

Å/s exhibit large contact resistance, RC = 3.1 kΩcm, which yields a mobility of µsat,avg =

3.24 ± 0.49 cm2/Vs. By slowing down the deposition process to a rate of 0.5 Å/s, we

Figure 5.6. (a) Schematic of the bottom-contact, top-gate device structure used in our devices,

(b) An illustration of the different sources of resistance in our devices.

a) b)

108

reduced the contact resistance by 6 times, to 500 Ωcm. Consequently, the mobility in these

devices is very high. This outcome suggests, along with the nearly identical GIXD results,

that the drastically improved device performance is a result of the enhanced charge

injection provided by the lower deposition rates, which yields lower contact resistance. The

resistance due to the distance between injection and the conduction channel, Rbulk, should

be unchanged between our devices fabricated using various contact deposition rates

because the semiconductor has been verified to have the same crystal structure. This

indicates that the main improvement in our devices lies in the resistance of the interface

between the contact and the semiconductor, or Rint.

Through AFM measurements, shown in Figures 5.7a and b, we found that the slow

deposition rate (0.5 Å/s in this case) yields a larger metal grain size than the faster rates

(here 2.5 Å/s), as confirmed by the 2D fast Fourier transforms, Figures 5.8a and b. When

the Au film formation is slow (similar to a simultaneous deposition and annealing), enough

Figure 5.7. (a) AFM image of Au deposited at 0.5 Å/s, (b) AFM image of Au deposited

at 2.5 Å/s.

a) b)

109

time and energy is allowed for each Au particle to reorganize to a more favorable energy

state before additional particles reach the substrates, effectively “trapping” those

underneath. This process resembles that of Uemura et al., but in that case the annealing

step was performed post-deposition, while here the deposition and annealing are

simultaneous [15]. Nevertheless, the result is the same: lowering of the contact resistance.

Interestingly, the RMS roughness of the two films are very similar (0.64 ± 0.04 nm for 0.5

Å/s, 0.69 ± 0.02 nm for 2.5 Å/s), which suggests that there is no change in the height

variation, but the in-plane variation is the determining factor in the mobility improvements.

To determine if this in-plane variation had any impact on the work function of PFBT-

treated Au films, and therefore on the injection barrier, we first conducted macroscale

Kelvin probe measurements and found no difference in the work function of the treated

Au, both 0.5 Å/s and 2.5 Å/s gave φAu, PFBT = 5.3 eV. Details on the determination of work

function based on Kelvin probe measurements were provided elsewhere [50]. To examine

the local features of the PFBT/Au surface potential, we performed scanning Kelvin probe

a) b)

Figure 5.8. 2D fast Fourier transforms to show the relative peak frequency in AFM images,

and therefore the distance between metal particles, on (a) 0.5 Å/s Au and (b) 2.5 Å/s Au.

110

microscopy (SKPM) measurements on the same samples, the results are displayed in

Figures 5.9a and b. The surface potential of the PFBT-treated Au deposited at 0.5 Å/s

exhibited local peaks, a feature that does not appear in the sample obtained via a fast

deposition rate. This observation indicated that while the average surface potential is very

similar for all samples, variations exist on small length scales, which are probably masked

when macroscopic measurements are carried out. The AFM and SKPM measurements

together point towards the larger grain size allowing the PFBT self-assembled monolayer

to achieve a longer-range order, and as a consequence the surface potential map displays

local maxima. These local maxima correspond to a larger work function, allowing more

efficient injection deeper into the highest occupied molecular orbital (HOMO) than the

surrounding area, and thus providing lower local resistance to injection.

Figure 5.9. KPFM images of PFBT-treated Au deposited at (a) 0.5 Å/s showing small regions of

higher surface potential and (b) 2.5 Å/s showing a more homogeneous surface potential

distribution.

a) b)

111

To discern whether this effect also applies to other systems, we fabricated an

additional set of several samples using the well-studied copolymer indacenodithiophene-

co-benzothiadiazole (C16IDT-BT), the structure of which is displayed Figure 5.10a. The

contact fabrication method that yielded the best performance in the small molecule OFETs

was adopted here, i.e. deposition rate of 0.5 Å/s. C16IDT-BT has been incorporated as the

semiconductor in many studies reaching a maximum charge carrier mobility of µ = 3.6

cm2/Vs [51]. Figures 5.10b and c show the evolution of the drain current at constant drain-

source voltage while varying the gate-source voltage, and the drain current as a function of

drain-source voltage with VGS held constant, respectively. This device exhibited a charge-

carrier mobility of 10 cm2/Vs, which is ~3x greater than the best mobility reported for this

material in this geometry and with Cytop as dielectric. Other device parameters include

Ion/Ioff =3 ∗ 104, S = 2.9 Vdec-1 and VTh = 7.4 V. These device properties are coupled with

a low contact resistance of 200 Ωcm, similar to the case of the small molecule device.

Figure 5.10. (a) Chemical structure of C16IDT-BT copolymer, (b) Drain current as a function of

gate-source voltage for an C16IDT-BT device, (c) Drain current as a function of drain-source

voltage.

a) b) c)

112

These results show that the reduced contact deposition rate has a strong, positive effect on

the device performance in polymer semiconductors as well.

5.4 Conclusions

In summary, we have fabricated a series of OFETs where we varied the bottom-contact

deposition rate between 0.5 Å/s and 3 Å/s and obtained massively improved charge-carrier

mobility when using a rate of 0.5 Å/s reaching a value of 19.2 cm2/Vs, along with a

precipitous drop in contact resistance. We conducted GIXD measurements and found no

difference in the microstructure of the films deposited over the substrates fabricated using

the various contact deposition rates. AFM measurements confirmed a larger grain size in

Au deposited at 0.5 Å/s, and KPFM measurements on the same surfaces exhibited local

maxima in the surface potential. These local maxima can provide channels of lower

resistance to injection into the semiconductor, drastically improving device performance.

To conclude, we have greatly improved the performance of our OFETs through

modification of the contact deposition rate in both small-molecule and polymer

semiconductors. The contact resistance of these devices is significantly decreased with

decreasing deposition rate, from 3.1 kΩcm characteristic to a fast rate, to 500 Ωcm or less

obtained at a rate of 0.5 Å/s. The obtained contact resistance is one of the lowest obtained

in OFETs, and here it results solely from a minor modification in the source and drain

deposition procedure. Our results underline the importance of minutious device design in

achieving high performance organic devices. The reduced contact resistance following this

procedure in conjunction with the use of other semiconductor deposition methods which

113

produce even higher quality semiconductor films could result in even greater enhanced

device performance.

114

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Curriculum Vitae Education

2004 - 2008 Mt. Lebanon High School, Pittsburgh, PA

2008 - 2010 Wake Forest University

2010 - 2012 B.S. Physics, Pennsylvania State University

Minor in Mathematics

2012 - 2018 Ph.D. Physics, Wake Forest University, Winston-Salem, NC

Advisor: Dr. Oana D. Jurchescu

Patents

M. W. Welker, O. D. Jurchescu, and Z. A. Lamport, Invention Disclosure 08/30/2017, “Substituted Benzalkylsilane Molecular Rectifiers.”

Publications 1. Z. A. Lamport, H. F. Haneef, M. Waldrip, S. Anand, and O. D. Jurchescu,

Submitted, J. Appl. Phys. 2018

2. Z. A. Lamport, K. J. Barth, H. Lee, M. Guthold, L. J. Richter, I. McCulloch, J. E. Anthony, D. M. DeLongchamp, and O. D. Jurchescu, To be submitted, July 2018.

3. Z. A. Lamport, B. C. Scharmann, A. D. Broadnax, M. E. Welker, and O. D. Jurchescu, To be submitted, July 2018.

4. A. D. Broadnax, Z. A. Lamport, B. C. Scharmann, O. D. Jurchescu, and M. E. Welker, J. Organomet. Chem. Vol. 856, p. 23, 2018.

5. Z. A. Lamport, R. Li, C. Wang, W. Mitchell, D. Sparrowe, D.-M. Smilgies, C. Day, V. Coropceanu, and O. D. Jurchescu, J. Mater. Chem. C Vol. 39, p. 10313, 2017.

6. Z. A. Lamport, A. D. Broadnax, D. Harrison, K. J. Barth, L. Mendenhall, C. T. Hamilton, M. Guthold, T. Thonhauser, M. E. Welker, and O. D. Jurchescu, Sci. Rep Vol. 6, p. 38092, 2016.

7. J. W. Ward, Z. A. Lamport, and O. D. Jurchescu, ChemPhysChem Vol. 16, p. 1118, 2015.

8. P. J. Diemer, Z. A. Lamport, Y. Mei, J. W. Ward, K. P. Goetz, W. Li, M. M. Payne, M. Guthold, J. E. Anthony, and O. D. Jurchescu. Appl. Phys. Lett. Vol. 107, p. 103303, 2015.

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Presentations 1. Organic thin-films with charge-carrier mobilities of 20 cm2/Vs, independent of gate

voltage Materials Research Society (MRS) Spring Meeting Oral Presentation in Phoenix, AZ (2018)

2. Organic Thin Films with Charge Carrier Mobility Exceeding that of Single Crystals American Physical Society (APS) March Meeting Oral Presentation in New Orleans, LA (2017)

3. Fluorinated Benzalkylsilane Molecular Rectifiers International Conference of Electroluminescence and Optoelectronic Devices (ICEL) Oral Presentation in Raleigh, NC (2016)

4. The effect of internal molecular dipole moment on the properties of molecular rectifiers Solar Energy Research Center (SERC) Conference Poster Presentation in Charlotte, NC (2016)

5. Fluorinated benzalkylsilane molecular rectifiers Materials Research Society (MRS) Spring Meeting Poster Presentation in Phoenix, AZ (2016)

6. Evaluation of the Density of Trap States at the Semiconductor-Dielectric Interface in Organic Field-Effect Transistors Electronic Materials Conference (EMC) Oral Presentation in Santa Barbara, CA (2014)

CHARGE TRANSPORT IN ORGANIC ELECTRONIC ......1.2.1 Molecular Rectifiers.....3 1.3 Organic...

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