Interfacial Fields in Organic Field-effect Transistors and Sensors

8/11/2019 Interfacial Fields in Organic Field-effect Transistors and Sensors

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INTERFACIAL FIELDS IN ORGANIC FIELD-EFFECTTRANSISTORS AND SENSORS

By

Thomas J. Dawidczyk

A dissertation submitted to Johns Hopk ins University in conformity with the

requirements for the degree o f Doctor o f Philosophy

Baltimore, Maryland

M arch, 2013

2013 Thomas J. Dawidczyk


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UMI Number: 3572759

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ABSTRACT

Organic electronics are currently being commercialized and present a v iable alternative to

conventional electronics. These organic materials offer the ability to chemically manipulate the

molecule, allowing for more facile mass processing techniques, which in turn reduces the cost.

One application where organic semiconductors (OSCs) are being investigated is sensors. This

work evaluates an assortment of n- and p-channel semiconductors as organic field-effect

transistor (OFET) sensors. The sensor responses to dinitrotoluene (DNT) vapor and solid along

with trinitrotoluene (TNT) solid were studied. Different semiconductor materials give different

magnitude and direction of electrical current response upon exposure to DNT. Additional OFET

parameters mobility and threshold voltage further refine the response to the DNT with

each OFET sensor requiring a certain gate voltage for an optimized response to the vapor. The

pattern o f responses has sufficient diversity to distinguish DNT from other vapors.

To effectively use these OFET sensors in a circuit, the threshold voltage needs to be

tuned for each transistor to increase the efficiency of the circuit and maximize the sensor

response. The threshold voltage can be altered by embedding charges into the dielectric layer of

the OFET. To study the quantity and energy of charges needed to alter the threshold voltage,

charge carriers were injected into polystyrene (PS) and investigated with scanning Kelvin probe

microscopy (SKPM) and thermally stimulated discharge current (TSDC). Lateral heterojunctions

of pentacene/PS were scanned using SKPM, effectively observing polarization along a side view

of a lateral nonvolatile organic field-effect transistor dielectric interface. TSDC was used to

observe charge migration out of PS films and to estimate the trap energy level inside the PS,

using the initial rise method.

The process was further refined to create lateral heterojunctions that were actual working

OFETs, consisting of a PS or poly (3-trifluoro)styrene (F-PS) gate dielectric and a pentacene

OSC. The charge storage inside the dielectric was visualized with SKPM, correlated to a


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ACKNOW LEDGEM ENT

I would like to thank my PhD advisor, Howard Katz, who was always been available to

help with guidance and support; 1 am very fortunate to have had him as a mentor. He is my role

model for a scientist, mentor and teacher.

I am thankful of the work done by the staff members in Materials Science, specifically

Marge Weaver, Jeanine Majewski, Dot Reagle and Mark Koontz. They are the primary resources

for all graduate students in the department. Additionally, I want to thank the members o f my PhD

committee: En Ma, Jonah Erlebacher, Nina Markovic, Daniel Reich and Bob Cammarata.

The Katz research group members, past and present, have been ideal research

companions. Their varied expertise helped further my knowledge through insightful discussions

about research and experiments. Additionally, Recep Ozgun and Gary Johns were great

collaborators for my thesis research. JHU Applied Physics Lab researchers Andrew Mason and

Joan Hoffman were always available if I had questions. I am grateful to have been able to work

with Subodh Mhaisalkar, Nripan Matthews, and Verawati Tjoa at Nanyang Technical University.

Lastly Bill Wilson was a great resource for spectroscopy.

I would not have been able to complete my PhD if it were not for my entire family. My

parents have been extremely encouraging and supporting during this process and my brother ,

Dan, has been a great sounding board. There are no words to convey the importance my wife,

Charli, has had in my success at JHU. Her faith in me and my intellect has helped me

tremendously during my time at JHU.


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CONTENTS

ABSTRACT............................................................................................................................................ ii

ACKNOWLEDGEMENT................................................................................................................... iv

LIST OF TABLES.............................................................................................................................. viii

LIST OF FIGU RES ............................................................................................................................. ix

CHAPTER I: Introduction to Organic Semiconductors (OSCs) and Devices .......................... 1

1. Introduction to Organic Semiconductors.................................. 1

1.1 Organic Semiconductor Electronic Structure......................................................................... 1

1.2 Applications of Organic Semiconductors............................................................................... 2

1.3 Operation of Field-effect Transistors...................................................................................... 3

1.4 Charge Density in a MIS Capacitor......................................................................................... 5

1.5 Charge Transport in the Transistor Channel .......................................................................... 6

1.6 Mobility in OFETs................................................................................................................... 7

1.7 Extracting Mobility and Threshold Voltage........................................................................... 8

1.8 Threshold Voltage in OFETs .................................................................................................. 8

1.9 Semiconductor-Dielectric Interface and Traps...................................................................... 9

1.10 Thin Film Deposition and Morphology.............................................................................. 10

2. Mechanisms of Threshold Voltage Shifting............................................................................... 12

2.1 Electrostatic Threshold Voltage Shifting............................................................................. 12

2.2 Non Electrical Threshold Voltage M odulation .................................................................... 16

2.3 Operational Stability............................................................................................................... 17

CHAPTER II: Chemical Sensing with Organic Field-Effect Tran sistors ................................. 18

1. Sensor Testing................................................................................................................................ 18

2. Previous Work on OSC Vapor Sensing ....................................................................................... 18

3. Enhanced Sensor Response with Receptors.............................................................................. 21

4. Sensing Mechanism...................................................................................................................... 22

5. DNT Vapor Sensing...................................................................................................................... 24

5.1 Experimental Setup.............................................................................................................. 25


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5.2 DNT Sensor Results............................................................................................................... 25

5.3 Proposed Sensing Mechanism s............................................................................................. 52

6 . TNT Sensing........................................ ........................................................................................ 53

7. Sensing Multiple Vapors.............................................................................................................. 56

8 . Maximum Sensitivity of an OFET as a Function of Gate Voltage ........................................... 59

9. Future Work................................................................................................................................... 59

CHAPT ER III: Storing Charge in Dielectrics............................................................................... 61

1. Gate Bias Stress............................................................................................................................ 61

2. SKPM Operation........................................................................................................................... 61

3. Previous SKPM Experiments on OFETs.................................................................................... 62

4. Visualizing the Stored Charge ..................................................................................................... 65

4.1 Experimental setup.................................................................................. 65

4.2 SKPM Scans of the Interfaces............................................................................................... 67

4.3 Analyzing the SKPM Scans.................................................................................................. 74

5. Quantities and Energies of Injected Cha rge ............................................................................... 76

7. Conclusions and Future Work..................................................................................................... 78

CHAPTER IV: C orrelating the Surface Potential to T hresholdVoltage Shifts....................... 80

1. Lateral Transistors........................................................................................................................

80

2. Fabricating Lateral OFETs........................................................................................................... 80

3. Testing Lateral OFETs Containing PS ........................................................................................ 84

4. Changing the Dielectric in Lateral OFETs................................................................................. 96

4.1 Lateral OFETs with a F-PS Dielectric.................................................................................. 96

4.2 Lateral OFETs with a PMMA D ielec tric........................................................................... 103

5. Lateral OFETs with an Air Gap Dielectric ............................................................................... 106

6 . Spectroscopy................................................................................................................................ 107

7. Conclusions and Future W ork.................................................................................................... 109

CHAP TER V: Refining Gate Dielectric Prop erties to W ithstan dGate Bias Stress.............. I l l

1. Gate Bias Stress ........................................................................................................................ I l l

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1.1 Extrinsic Factors Contributing to Gate Bias Stress............................................................I l l

1.2 Intrinsic Factors Contributing to Gate Bias Stress ............................................................ 112

2. Mechanisms of Gate Bias Stress................................................................................................. 113

3. The Influence of the Dielectric on Gate Bias Stress ................................................................. 115

3.1 The Effect of Dielectric Addi tives...................................................................................... 119

3.2 F-PS Dielectric Gate Bias Response................................................................................... 129

4. Conclusions and Future W ork.................................................................................................... 130

BIBLIOGRAPHY: ............................................................................................................................. 132

VITA..................................................................................................................................................... 141

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LIST OF TABLES

1 The response of four different electrode pairs on a PQT12 sensor to DNT and TNT transfer

from a PDMS stamp...................................................................................................................... 56


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LIST OF FIGURES

1.1 Schematic view of a bottom contact, bottom gate OFET. S refers to the source electrode while

D refers to the drain electrode......................................................................................................... 3

1.2 Output (left) and transfer (right) curves for an n-channel transistor. Note the y-axis in the

right graph is for the square root of the drain current .................................................................. 4

1.3. Energy band diagram of a p-channel MIS capacitor (a) at thermal equilibrium and (b) in

accumulation mode with a negative gate voltage applied............................................................ 6

1.4 A point-to-grid corona triode schematic. A corona is generated near the point and ions flow

through the grid and impart their charge in the dielectric layer.Error! Bookmark not

defined.4

2.1 Schematic of an analyte delivery system for a OFET vapor sensor. The arrows symbolize the

analyte...................................................................................... Error! Bookmark not deflned.9

2.2 The effective voltage seen by the molecular semiconductor as the distance from the dielectric

interface is increased. (13)............................................................................................................ 24

2.3 Chemical structures of DNT (left) and TNT (right)...................................................................... 25

2.4 The chemical structure of bis-CF3 NTCDI (a) and response of a 6 nm thick OSC film FET

upon exposure to DNT vapors at various concentrations.......................................................... 27

2.5 The chemical structure of C2PhF5 NTCDI (a) and response of a 15 run thick OSC film FET

upon exposure to DNT vapor....................................................................................................... 30

2.6 The chemical structure of 6 PTTP6 (top) and response of a 6 nm thick OSC film FET upon

exposure to DNT vapor (bottom)............................................ 3Error! Bookmark not defined.

2.7 The chemical structure of 2PTTP2 (a) and response of a 4.5 nm thick OSC film FET upon

exposure to DNT vapor................................................................................................................. 33

2.8 The chemical structure of CuPc (a). The mobility (b) and threshold voltage (c) response o f a

20 nm thick OSC film FET upon exposure to 4 LPM DNT vapor........................................... 35


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2.9 The chemical structure of a-6T (top) with the corresponding response (bottom) upon exposure

to 4 LPM DNT vapor.................................................................................................................... 36

2.10 The chemical structure of PTPTP (top) with the corresponding response (bottom) upon

exposure to 4 LPM DNT vapor.................................................................................................... 37

2.11 The chemical structure of PQT12 (a) and response of an OFET upon exposure to DNT

vapor. The response o f mobility (b); response o f threshold voltage (c)................................... 39

2.12 The chemical structure of PIFTBT-10 (a) and response of an OFET upon exposure to DNT

vapor............................................................................................................................................... 41

2.13 The response of mobility (a) and threshold voltage (b) to DNT. The gray bars indicate air

flowrate over DNT at 4 LPM ....................................................................................................... 43

2.14 The response of NTCDI, pentacene and two bilayer devices to DNT. The normalized

mobility (a) and normalized response o f the threshold voltage (b).......................................... 45

2.15 The chemical structure of H060PT (top) and the OFET response to DNT vapor for a 3nm

thick 6PTTP6 film with 3 nm o f H 06 0P T and 6PTTP6co-evaporated on top o f it 46

2.16 The chemical structure of porphyrin (top) and the two different device structures that have

porphyrin as a receptor.................................................................................................................. 48

2.17 The normalized mobility (a) and threshold voltage (b) response to 4 LPM DNT vapor for

different blend ratios..................................................................................................................... 49

2.18 The normalized mobility (a) and threshold voltage (b) response to DNT o f bilayer films with

different spin coating speeds for the PS/porphyrin layer........................................................... 50

2.19 The mobility (a) and threshold voltage (b) response to DNT of bilayer films with different

weight concentrations for the porphyrin layer........................................................................... 51

2.20 Schematic of the procedure for testing TNT and DNT transfer to an OFET sensor. The

sensor would have the PDMS stamp placed on it to get a baseline source drain cu rre nt 54


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2.21 The output curves before and after exposure to DNT (a) and TNT (b). The TNT consistently

lowered the source drain current more than DNT ...................................................................... 55

2.22 The mobility (a) and threshold voltage (b) response of a 25 nm bis CF3 NTCDI film to

DMMP and DNT vapor................................................................................................................ 58

2.23 The normalized current response of a 15nm C2PhF5 NTCDI film to DMMP and DNT

vapor. Note that the delivery gas for the DMMP vapor was nitrogen while the delivery.... 59

3.1 The SKPM scans of bottom (a & c) and top (b & d) contact pentacene FETs. The device

geometries are shown for top and bottom contact devices ........................................................ 63

3.2 The transfer curves at various time intervals of an OFET after a continuous gate bias of -60 V

(a). The source drain current was -5 V during gate biasing...................................................... 64

3.3 The procedure used to fabricate the lateral in terfaces between PS and pentacene.....................6 6

3.4 The step-by-step procedure for measuring the surface potential of the lateral interfaces.

During Step 1 SKPM scans are taken at each voltage step........................................................ 67

3.5 SKPM scans of a PS-pentacene interface before (a) and after (b) +200 V charging for 10

minutes. The interface between the PS and pentacene is where the large drop in .................68

3.6 The SKPM line scans of a PS-pentacene interface before (a) and after (b) -200 V charging for

10 minutes. Again, the interface between the PS and pentacene ............................................. 70

3.7 The SKPM line scans for samples charged at +100 V (a) and -150 V (b) for 5 minutes. In

both cases the surface potential reached the limit o f the SKPM so the flat lines .................... 71

3.8 The SKPM line scans for gold samples before charging (a) and after being charged at +100 V

(b) and -100 V (c) for 10 minutes................................................................................................ 73

3.9 A schematic of the surface potential for the PS and pentacene. The PS is always on the left of

the schematic. The surface potential of the uncharged samples with small ........................... 74

3.10 A schematic showing the charges entering the PS during positive charging, then being

trapped in the PS after charging................................................................................................... 75

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3.11 The change in surface potential difference between PS and pentacene at each incremental

applied bias, as shown in Steps 1 and 3 from Figure 3.4........................................................... 76

3.12 The TSDC for samples corona charged with a V^a o f 150 V. Note the different

temperatures where the peaks occur, the glass transition temperature for PS is ~100 C. ... 77

4.1 The output characteristics of lateral transistors shifted by application of either polarity of

charging voltage to the gate.......................................................................................................... 81

4.2 Schematic of the fabrication process for a lateral OFETs. First, 50 nm gold e lectrodes with a

4-5 nm Cr adhesion layer are deposited on the Si substrate via photolithography (a) ........... 83

4.3 Optical image of the lateral OFET showing the pentacene covering the left side of the image

and the PS covering the right side ................................................................................................ 84

4.4 Height scans of the two lateral OFETs in Figure 4.5. The source (S), drain (D) and gate (G)

electrodes can be easily seen and are labeled ............................................................................. 85

4.5 SKPM surface potential scans of lateral OFETs. Similar to the optical and height images the

source and drain electrodes are always at the left and the gate is at the far right ..................... 86

4.6 SKPM scans showing the surface potential o f the same lateral OFET immediately after

transistor testing and 1 hour later................................................................................................. 88

4.7 Transfer curves for the samples from Figure 4.4 before and after charging. The lines after

charging to -100 V (a) and +100 V (b) are in red....................................................................... 89

4.8 Output curves for the lateral OFETs before (a,b) and after (c,d) 100 V charging.A sample

was tested before (a) and after (c) charging at -100 V for 10 minutes............................ 90

4.9 SKPM surface potential scans of lateral OFETs. Similar to the previous SKPM scans the

source and drain electrodes are always at the left and the gate is at the far right.................... 91

4.10 Transfer curves for the samples from Figure 4.8 before and after charging. The lines after

charging to -50 V (a) and +50 V (b) are in red ........................................................................... 92


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Chapter I: Introduction to Organic Semiconductors (OSCs) and Devices

1. Introduction to Organic Semicond uctors

M olecular materials for electronics applications are on the cusp of bein g com me rcially

viable as alternatives to conventional electronics. The interest in these m olecular m aterials has

been increasing ex po nent ially ove r the prev ious tw o decades, draw ing in terest from bo th

academia and corporate institutions. One m ajor draw o f these organic materials is the ability to

chemically manipulate the molecule, which will allow for more facile mass processing

techniques. The ease of processing the molecular materials can offset the reduced performance

compared to conventional high quality inorganic semiconductors. Som e benefits of organic

semiconductors are low cost; low temperature and large area processing; and large scale

manufacturing on a flexible substrate. One application where organic semicond uctors are

currently emerging is electronic displays, while applications such as flexible solar cells, printed

memory, and white lighting are close to comm ercialization.

1.1 Organic Semiconductor Electronic Structure

A conjugated bonding scheme, alternating double and single carbon-carbon bonds, has

been shown to be a very effective de sign in molecu la r el ec tronics .( l) The ke y is de sign in g an

aromatic system that packs face-to-face, and limits the more favorable edge-to-face interactions.

The main method used to preferentially create face-to-face packing is to build aromatic systems

that have a large x-surface to circumference ratio that limits perfect herringbone packing (all

contact between molecules is edge-to-face). Generally side chains are added to increase

solubility, but they can also force face-to-face interactions by steric hindrance that does not allow

for efficient edge-to-face interactions. The face-to-face mole cular contact of OSCs is preferred

because it increases the overlapping of 7i-orbitals. The charge transport in organic materials may

have band character in high quality single crystals. In this case, the highest occupied m olecu lar

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orbital (HOMO) can be approximated as the valence band from inorganic semiconductors, and

the lowest unoccupied molecular orbital (LUMO) in organic semiconductors (OSCs) corresponds

to the conduction band in inorganic semiconductors. However, thermally activated hopping is

prop osed for p olycrystalline OSCs , which are us ed in mos t applications o f o rg an ic elec tron ics.

The closer adjacent molecules become, the more likely there will be interactions between

their respective molecular orbitals. This close proximity helps with charge tran sfer from

molecule to molecule. Generally the charge mobility increases with a more o rdered an d closer

packed organic semicon ductor system. Eve n closely pack ed organic se m ic onductor s are sti ll a

weakly interacting system, which has been shown to display a thermally activated mobility with

Arrhenius behavior.(2) Two models that use this Arrhenius behavior to describe charge transport

are a multiple trapping and release model(3) and a model, which assumes hopping between

localized states.(4)

Just as inorganic semiconductors can transport either holes or electrons, organic

semiconductors can be primarily p- (hole transport) or n-channel (electron transport). Their

be havior is governed by the HOMO and LUMO overlap with the elec trodes and the

environmental stability of the charge carriers. In general, an n-channel materia l will hav e a high

electron affinity, while p-channel materials have lowe r ionization potentia ls.(l) Unlik e inorganic

semiconductors which have been deliberately doped to become n-type or p-type, organic

semiconductors have an intrinsic ability to transport holes or electrons. Ho we ver, organic

materials that were once thought to only transport holes have exhibited n-channel behavior. (5)

This occurred in vacuum and only when special care was taken to reduce the nu m ber o f traps at

the dielectric/semiconductor interface.

1.2 Applications of Organ ic Semiconductors

Organic semiconductors are being investigated in three major fields: photovoltaics;

emissive devices, such as light emitting diodes; and circuits. For circuits, the device architecture

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is generally in the form of a transistor or a diode. In this work, I will be fo cusing solely on

transistors. Organic field-effect transistors (OFETs) are the primary device s that are used in

active circuits, performing logic and switching operations. Figure 1.1 shows a cross sectional

view of an OFET. Most OFETs are operated in accumulation mode because a majority o f

organic semiconductors have an inherent preference f or p- or n-channel conduction in air.

^Con ducting channel

s | 1Da a Figure 1.1 Schematic view of a bottom contact, bottom gate OFET. S refers to the source electrode while D

refers to the drain electrode.

1.3 Operation of Field-effect Transistors

A transistor can be considered a metal-insulator-semiconductor (MIS) combined with a

two terminal resistor. In the resistor, the two term inals are called the sourc e and drain w ith

the semiconductor acting as the resistive material. To modulate the resistance of the

semiconductor layer, a voltage is applied to the gate electrode, which polarizes the dielectric and

causes charge carriers to accumulate at the dielectric/semico nductor interface (this is noted as the

conducting channel in Figure 1.1). As the charge carrier density increases so does the

conductivity of the resistor. Since the resistance can be transferred from the gate terminal, the

device was called a transfer resistor or transistor .

The gate electrode can be patterned on an insulating substrate or the entire substrate may

be conductive, lik e a single crystal o f highly doped sil icon . Both organic an d inorga nic in su la ting

layers are used for the dielectric. SiC>2 and AI2O 3 are common inorganic dielectrics while

polymeric materia ls like po lystyrene (PS) , po ly(m ethy l meth acry late) (PMMA), or Cytop (a


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KLUMO

H O M O HOMO

Metal f Semiconductor

Insulator

At Equilibrium (a) A tV g V g)

occurs. This regime is the result o f a charge depletion region near the drain electrode. Id during

the saturation regime is given in equation ( 1 .2 ):

depicted in Figure 1.1. Charges ma y be inserted and remo ved from the cond uction channel b y the

1,- j -nC, ( n - V r H - y ( 1 . 1 )

. where L is the length of the channel between the sou rce and drain electrodes, W is the w idth o f

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W / \2^ - ^ A * q v f - v r ) (1.2)

The two terms V T and p w ill be discussed in detail later o n, but the mo bility o f a material

can be thought of as the speed o f charges per applied field. W hile the V t can be conside red the

gate voltage where the field-effect mobility determines the cha rge transport in the OF ET channel.

Generally organic semiconductors have field-effect mobilities in the range of 10 ' 3 - 10 cm2/V s in

transistors, while crystalline silicon can have a mo bility o f 1000 cm2/Vs an d am orph ous silicon

mobility is around 1 cm 2/Vs.

1.6 Mobility in OFETs

The mobility of a semiconductor is the velocity of the charges per unit of electric field

betw een the source and drain co ntacts. Th ere are nu merou s methods to ex trac t th e mob ility in

semiconductors, such as time of flight, space charge limited current, and, as described in this

work, field-effect transistors. How ever each test method produces a different m obility value that

cannot be used interchangeably in the other methods. Furthermore in the case of field-effect

transistors, the mobility value should be considered device mobility not a material mobility.

Num erous variables in transistor prep aration can al te r the mobili ty in the OFET (e .g. the

dielectric choice, grain boundaries in the semiconductor, traps in the semiconductor, charge

injection into the semiconductor). Additionally, just the choice of a top contact or bottom contact

geometry can change the device mobility. Top contact devices have the m etal for the contacts

diffuse into the semiconductor creating a more complicated electronic interface than a simple

metal-semiconductor junction. While the bottom contact interface may be cleaner, the packing o f

the semiconductor on top of the source or drain electrodes m ay be poo rer than o n top o f the

dielectric.(6 ) This results in lower device mobility for bottom con tact devices com pared to the top

contact devices. In the work done by Kym issis et al., the higher surface energy o f the SiC>2 was

shown to result in more intermolecular interactions compared to the lower surface energy of the

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metal contacts. However, self-assembled mo nolayers on top of the metal contacts can improv e

the surface morphology of the semiconductor in bottom contact geometries.(7)

Different metal contacts can be used to alter the charge injection into the

semiconductor.(8 ) For an n-channel semiconductor the metal work function should be close to the

LUMO energy level in the semiconductor, while for p-channel semiconductors the metal work

function should be closer to the HO M O energy level. W hile simply altering the metal work

functions sounds like a simple idea, ultraviolet photoemission spectroscopy showed that there are

chemical interactions between the metal contact and the semiconductor.(9) These chemical

interactions have an influence on the charge injection from the me tal into the se micond uctor.

1.7 Extracting Mobility and Threshold Voltage

To derive the field-effect mobility and V T for a specific OFET , a transfe r curve is

recorded. A Vd necessary to attain saturation at every V g measu red will applied to the dev ice and

the gate voltage will be swept, recording the la. By utilizing equation (1.2) we see there are only

two variables: p and VT. If we take the square root o f the equation and make the p lot see n in

Figu re 1.2 the Vt can be seen as the x-intercep t and the slope is pro por tiona l to p l/2. This

assumes that the field-effect mobility is independent of gate voltage(lO) and the contact

resistance is n egligible.(l 1)

1.8 Threshold V oltage in OFETs

There is a possibility that trap states exist in the semiconductor and/or at the dielectric-

semiconductor interface in OFETs.(12) To accumulate charges in the channel and begin to have

charge transport between the source and drain these trap states mu st first be filled. The p resence

of these traps will slow the transition from intrinsic to field-effect mobility. These traps states

may have different densities and may be at various energetic levels in the semiconductor band

gap, resulting in gate dependence on the field-effect mob ility. The traps are distributed


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throughout the semiconductor film, but may be more concentrated at grain boundaries or at the

dielectric-semiconductor interface. As the gate voltage is increased the traps are f illed by the

injected charges, this decreases the number of traps available to slow the motion of charges,

which allows for higher field-effect mobility at higher gate voltage s.(3,4, 13, 14)

For inorganic semiconductor-based electronics, a trap can be considered a localization of

energy levels present in the band gap of the semiconductor (either in the bulk or at the surface),

which belong to an impurity, a defect in the semicon ductor or both. These traps can be shallow

or deep depending on where the energy level of the trap aligns with the semiconductor bandgap.

Shallow traps have en ergy level differe nces with the ba nd gap ed ge of about 1 -2 tim es k BT, wh ile

deep traps may be 3-5 times kBT. These charge traps play a major role in the transp ort o f charges

in disordered semiconductors such as amorphou s silicon, oxide based semico nductors a nd OSCs.

1.9 Semicond uctor-Dielectric Interface and Traps

The interface between the semiconductor and dielectric is crucial to the performance of

the semiconductor in OFETs because most accumulated charge is localized within the first 3-5

monolayers of semiconductor.(13) The presence of traps, dopants, impurities and morphology

play a major role in de term ining the mob ility an d threshold vo ltag e of the OFET. As men tion ed

previously, ca refu l control o f the dielec tr ic-sem icon du ctor in terfac e can ev en co nt ro l th e po la rity

of the semiconductor.(5) By utilizing a trap free, hydrophobic benz ocyclo buten e (BC B)

dielectric, instead of the traditional Si0 2 , n-channel behavior was observed in common p-channel

semiconductors such as regioregular poly(3-hexylthiophene) (P3HT). Silanol (Si-OH ) groups on

the hydrophilic Si0 2 dielectric act as electron traps. Even pentacene, a very com m on p-channel

OSC, was shown to exhibit n-channel behavior by using a poly(vinyl alcohol) (PVA) dielectric

that created interface states which helped in electron transport in pentacene.(15)

Both the VT and mobility in OFETs may be influenced by a variety o f factors such as

surface dipoles, surface energy, roughness of the interface and the energetic disorder at the

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interface.(16-18) There has been substantial work on surface dipoles and the influence o f polar

groups at the dielectric-semiconductor interface on OFE T properties.( 19-29) T he imp act is

mainly on threshold voltage but there may be additional traps, which influence the mobility as

well. The surface energy can alter the orientation of the OSC molecules and correspon ding film

morphology.(16) Lower surface energy dielectrics aid in increasing the interconnection between

grains, yielding improved mobilities.(30-34) Charge accumulation in bottom gate transistors can

be drastically redu ced by ro ug her dielectric surfaces. (16, 35 -39) The ro ug her surfac e leads to a

more disordered OSC, either by a reduction in grain size, increased voids, or greater molecular

disorder from the defects in the intermolecular packing.(38, 40, 41) The increa sed surface

roughness o f the dielectric has been associated with in an increase in the num ber o f traps states at

the interface.(42) Also, higher permittivity dielectric layers will alter charge transport by

broadening the Gaussian De nsity o f States (D OS) in di sord ered OSC s.(43) H ig he r mob ility,

lower V j and sm aller hysteresis were seen in transistors on low k dielectrics. This is attributed to

the randomly oriented dipoles inside the dielectric.

1.10 Thin Film Deposition and Morph ology

With so much depending on the semiconductor-dielectric interface, the device fabrication

plays a critical role in the eventual pe rforman ce o f the OFET. There are tw o m ain way s to

deposit thin films onto the substrates: thermal evaporation and solution processing. In this w ork

small molecules will generally be thermally evaporated while larger more soluble molecules and

po lymer wi ll be solut ion processed . The main technique us ed for so lu tion pro ce ss in g is spin

coating, but eventual large-scale production will require other fabrication techniques such as

gravure, offset, or flexographic printing.

Thermal evaporation is a type o f physical vap or deposition where substrates a re placed in

a deposition chamber under a vacuum pressure on the order of 10'6 torr. The material to be

deposited is placed in a thermally crucible. The crucible is then placed into a wire filament

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between two electrodes. Cu rrent is pa ssed th ro ug h th e fi lamen t resulting in re sistiv e hea ting and

the material inside the crucible is sublimed. Due to the very low pressure, m elting o f the material

is uncomm on for symmetrical and rigid organic materials. The sublimated organ ic mo lecules

then condense on the substrate placed above the crucible source. Most deposition rates for the

organic and metal thermal evaporation are appro xim ately 0.5 A/s.

The solution processing technique requires OSCs that are soluble in an appropriate

solvent, which is not always feasible. Drop casting is where a solution o f OSC is d ropped onto a

substrate and the solvent is allowed to evaporate away. The resulting film thickness is de pendent

on the total amount o f solute in the solution. For spin coating a m ore viscous solution is required

for thicker films, which is why spin coating generally requires polymers. W hile the mo rphology

of films made via drop casting and spin coating can be refined with thermal and s olvent a nnealing

they still never achieve the crystallinity o f thermally evapo rated films (partly due to the m olecular

structure differences, small molecule vs. polymer).

Small molecule OSCs form polycrystalline films with each grain being a single crystal.

While polymers may have some crystallinity, they will never be fully crystalline and will contain

free volume and amorphous regions. The difference in crystal structure is one reason that small

molecules still show greater mobility values than their poly m er counterparts. The interface

be tw een each grain is called a grain bo un da ry . The re is gr ea te r energetic disord er at th e grain

boundaries, which lim its the mobility co mpa re d to the more ordered interio r o f the grain, so grain

boundaries influence the OFET mob ility trem endo us ly. (44)

For improved charge transport, highly crystalline films are preferred because they

increase the jr-orbital overlapping of adjacent molecules. Most small molecule OSCs will orient

themselves so that the n-orbitals overlap, which means the long axis of the m olecule will be close

to perpendicular to the substrate. The film structure is controlled by both intermo lecular V an der

Waals forces and interfacial energy between the substrate and OSC m olecule. Tw o approach es to


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improve the crystallinity of the OSC are the utilization of self-assembled mono layers (S AM s) on

the substrate and thermal and/or solvent annealing. Alkyl silanes are very com mo n SA Ms for

OSC films on silicon dioxide dielectric layers.(45) The silicon portion o f the SAM will

chemically bond to the Si0 2 while the alkyl side chain will extend outw ards from the S i0 2

interface. This SAM layer will alter the OSC surface interactions, impro ving the crystalline

packing in the OS C film. Th ermal an ne aling o f substrates during sm al l molec ule th ermal

evaporations can also lead to larger grain sizes, but each OSC system is different and requires an

optimized substrate temperature.(46, 47)

2. Mechanism s of Threshold Voltage Shifting

For some applications, such as inverters and sensors, there is an ideal, non-zero threshold

voltage that the OFET should have for optimum performance. The reasons for the non-zero Vt

will be discussed later, but for these applications, the threshold voltage is optim ized e ither during

fabrication or once the OFET has been manufactured. There are numerous mechanisms where a

controlled shift in the threshold voltage can be achieved, but all the m echanisms can be classified

in one o f two broader categories: electrostatic and nonelectrical threshold voltage shifting.

2.1 Electrostatic Threshold V oltage Sh ifting

2.1.1 S urface Dipoles a t the Dielectric Interface

As discussed earlier, the SAM on the dielectric layer can improve the OSC film

morphology, but a dipolar SAM can be placed at the dielectric interface with the OSC or the gate

electrode and cause a depletion or accumulation o f charges inside the OSC. T he stored cha rges in

the dielectric layer will cause the accumulation/depletion of charges in OSC depending on the

po larity o f the charge. When functional en d grou ps were used on the SA M at th e dielec tr ic-O SC

*interface, the threshold voltage of a p-channel OFE T could be shifted more n egative fo r a NH 2

terminated SAM and more positive for a fluorinated SAM.(21) The same shifting behavior was

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shown for an n-channel OSC, which means the p-channel device was easier to turn on with the

fluorinated SAM (accumulation of charges in the OSC), while the n-channel device was harder to

turn on (depletion of charges in the OSC). The amine term inated SAM had the opposite eff ect

and made the n-channel device easier to turn on and the p-channel device hard er to turn on. Since

the SAM layers are permanently on the devices the shift in thresho ld voltage is permanen t.

2.1.2 Static Charge in Dielectrics

The main method for threshold voltage shifting in this work is to store charge in the bulk

of the dielectric. The charge may be stored in the dielectric layer for prolong p eriods o f time,

meaning this is a fairly permanent change in the OFE T threshold voltage. One m ethod used to

store electrostatic charge in the dielectric is corona charging.(48-51) Figure 1.4 shows a

schematic of a corona triode charging setup. A dielectric on a conductive substrate is placed in a

chamber. The substrate is grounded and a potential is applied to a grid above the substrate. A

corona voltage is applied (generally around 10 kV). The blue lines represe nt the electrostatic

field lines in air. This corona voltage generates a corona at the point due to the breakd ow n o f air.

In the case of a negative polarity point electrode, only negative ions can reach the surface o f the

dielectric. Once the negative ions reach the surface o f the dielectric, they donate th eir electrons

and become neutral. This leaves a layer o f trapped electrons underneath the dielectric surface.

(52) The grid, consisting of a metal wire mesh, is utilized to control the ch argin g o f the surfac e

and improve uniformity of charges embedded into the dielectric. Ions will pass through the grid

and deposit onto the surface of the dielectric until the surface potential matche s the grid potential.

Once the surface potential of the dielectric m atches the grid po tential there is no electric field and

the ions will just flow to the grid. In addition to the corona voltage and grid v oltage, ch arging

temperature and time can be varied to alter the surface potential.

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Corona i

voltage T

Grid

Grid I

voltage ^x

Dielectric

Conductive Substrate

Figure 1.4 A point-to-grid corona triode schematic. A corona is generated near the point and ions flow through

the grid and impart their charge in the dielectric layer. The blue lines show the electrostatic field lines in air. A

metallic wire mesh grid is used for uniform charge distribution.

2.1.3 Ferroelectric Polarization

Polymer and ceramic ferroelectric transistors exhibit threshold voltage shifts from a

po larization response analo gous to a ferrom ag ne tic magne tic response. (53-63)T he ferroelectrics

show a hysteresis due to remnant polarization from an intrinsic dipole alignm ent. To achieve

electrical polarization ferroelectrics must be insulating and cannot be metallic. The po larization

will occur above the coercive field (Ec) and below the dielectric breakdown. The im print time,

time under the electric field required to alter the polarization, will vary from system to system,

with amorphous materials never actually polarizing due to the completely random dipole

orientation. The threshold voltage shift is the remnant polarization causing eithe r depletion or

accumulation o f charges in the OSC. Ferroelectrics are mainly used for m em ory applications,

allowing the threshold voltage to be shifted so the OFET is either ON or OFF at a certain

gate voltage.

2.1.4 Charging o f Emb edded Floating Gates

Floating gate devices are a mod ified version o f the conv entional m etal-oxide-

semiconductor field-effect transistor (MO SFET ), where a second gate electrode is placed betw een

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2.2 Non Electrical Threshold Voltage Modulation

2.2.1 M echanical Force

Hydrostatic pressure can reversibly shift the threshold voltage and mobility in polymer

OSC devices.(75) The threshold voltage was seen to become more negative with increasing

pressure, while the mobility o f the tran sistor s in crea sed four fo ld over th e same pressure rang e.

The increase in mobility was attributed to the increase in 7t-orbital overlap, due to the shorter

inter-a tom ic. distances while under comp ression, while the shift in thresh old v oltage was

suggested to be from an increase in trap density.

2.2.2 Photopolarization and Photoin duce d Charge Transfer

Photoinduced charge transfer from a single crystal OSC to the dielectric layer has been

shown, with the illumination time playing a majo r role in the threshold voltage shift.(79) Shorte r

illumination times were able to alter the threshold voltage, w hile only gate bias stress wa s see n in

the longer exposures. At the shorter time scales the rate o f thresho ld voltage shifts were correlated

to the wavelength and intensity of the light. The idea that this is a photoconductive effec t is not

valid, since upon illumination the current decreases instead of increasing. Podzo rov et al.

attributed the threshold voltage shift to hot charge carriers going from the OSC into the dielectric

layer. The hot charge carriers are generated by the absorption of photons with energies g reater

than the band gap of the semiconductor. The emb edded charges in the dielectric layer the n shift

the threshold voltage.

When a SAM o f a dipolar nonlinear optical chrom ophore was used on a Si0 2 dielectric

layer the initial un-illuminated dipole of 8 Debye could-be increased with photo-illumination to

approximately 20 Debye in its excited, charge separated state.(80) The increase in dipole will

accumulate or deplete the charges in the semicond uctor. How ever the directio n o f the gate sw eep

influences the threshold voltage, indicating there may be a photoinduced charge transfer to the

SAM.

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2.3 Operational Stability

For organic electronics to be a viable commercial product, the devices need to be reliable

and stable. The intrinsic stability of the semicon ductor and the role o f the gate dielec tric are both

critical factors in the stability of the device. Organic sem iconductors are sensitive to

environmental factors such as oxygen, water, various polar molecules, oxidizing agents, and

reducing agents. Additionally, some gate dielectrics may trap charge carriers over time, causing a

shift in the threshold voltage o f the transistor; this phenomena is called the bias stress effect. The

bias stress effect gradually will tu rn a tran sistor from ON to O FF m ak in g th is a ch al lenging

problem for a pplication s like display dr iver s, where the tran sistor is used to op erate a pixe l.

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Chapter II: Chemical Sensing with Organic Field-Effect Transistors

1. Sensor Testing

When OSCs are made into field-effect transistors they are commonly operated in

accumulation mode, meaning induced majority charge carriers are mainly responsible for

conduction. Like most organic materials, OSCs are insulators under ordinary ambient conditions,

bu t have the prop erty that th eir cond uc tivity can increase marke dly on the ap plica tion o f voltage,

electromagnetic radiation, or heat. The change in conductivity possible for an OS C is the product

o f the charge density induced by the energetic input times the mobilities o f the charges under

electric fields. Vapors may alter the effective voltage seen by the semiconductor or the mobility

of the charge carrier inside the semiconductor. To create a vapor sensor from an OF ET the

change in output signal will need to be monitored under both normal conditions and exposure to

the vapor.

There are two methods to measure the OFET characteristics: output or transfer curves.

With an output curve the drain current is m easured vs. the drain voltage at stepp ed g ate voltages.

This generates multiple curves, since there is one for each gate voltage. W hen a transfer curve is

taken the OFET is generally operated in the saturation regime. In the saturation regim e, the drain

current is independent o f the drain voltage, which is held fixed during the measur emen t. The gate

voltage is swept and the drain current is measured generating only one curve compared to the

output curve measurements that generate multiple curves. In this work, transfer curves are used

to monitor the sensor, since the mobility and threshold voltage can be quickly m easured. This

minimizes the gate bias stress, which will also alter the OFET output.

2. Previous Work on OSC V apor Sensing

Due to the reactivity of both charge carriers and domain bou ndarie s w ith extrinsic

chemical species, OSCs can be highly responsive to environmental chemical agents (analytes).

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When OSCs are positioned between a pair of electrodes and their native conductivities are

modulated by chemical exposure, the device is termed a chemiresistor. W hen a third, gate

electrode is used to preset, or scan, a range of conductivities that are further perturbed by an

analyte, the device becomes and OFET sensor. A schematic o f an OFET sensor, integrated with a

gaseous analyte delivery system, is shown in Figure 2.1.

Analyte

Flow

meter

Air

OSC

Valve

0 $ 4

Dielectric

Gate

Figure 2.1 Schematic of an analyte delivery system for a OFET vapor sensor. The arrow s sym bolize the analy te.

OFETs that have not been sealed, or packaged, will generally have their output current

change upon exposure to polar vapors, including relative humidity from water vapor.(57) The

electrical contacts o f pentacene OFETs were shown to be susceptible to humidity. The shorter

channel length OFETs resulted in higher sensitivity, while the ON/OFF ratio (conductance with a

gate voltage versus without the gate voltage) could vary by an order of mag nitude with v arying

humidity levels. Another pentacene-based OFET showed a current decrease o f approxim ately

80% when the relative humidity was varied from 0% to 30%.(52) Wh en the relative humidity

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was 75%, the OFET no longer functioned. Electron con duction materials are suscep tible to

oxygen as well as humidity, because of the ability of both to quench, or trap, the electrons. For

example, in perylenetetracarboxylic diimide (PT CDI), ox ygen was shown to decrease the electron

mobility and density.(83)

Hole transporting copper phthalocyanine (CuPc) OFETs showed a greater response for

more polar vapors; with the response further increasing when the OSC layer thickness was

reduced.(54) An array of CuPc and other phthalocyanines were exposed to various analytes.(55)

By changing the copper core to cobalt, nickel, zinc, and H 2 the array was seen to have a

sensitivity that was correlated to the Lewis base b inding enthalpy of the analyte, for the m etal

phthalocyanines; and to the an alytes h yd ro gen bo nd ing enthalpy , for the ph th alocy an in e with no

metal core.

Using different OSCs and 16 analytes of various polarities, a response fingerprint was

developed for an array of OFETs. This fingerprint gives a distinct response from the arra y for

each of the various analytes.(86) These devices were then show n to operate in circuits and the

sensitivity was calculated for 1 ppm .(57) Ano ther application of the fingerprin t approa ch has

been used to de tect vo lat ile organic compo un ds for fo od quality analysis.(55) Pen tace ne, po ly (3 -

hexylthiophene) (P3HT) and poly(3-octylthiophene) devices were exposed to acetic acid,

octanoic acid, ethanol, propanol and other vapors giving unique responses for each vapor. The

carbonyl group was shown to have a greater response. Wh en the side chain of the analyte w as

lengthened, the analyes had less of an effect on the output current o f the OFET . This depe ndence

on side chain length is due to the lower diffusivity in the OSC film. Other studies have show n that

the sensing array method could give lower detection limits, compared to single

me asurem ents. (59)

Contact effects were investigated in an a,co-dihexylsexithiophene (D H a6 T) film exposed

to butanol vapor.(90) The higher sensitivity ON state showed that the sensing enhan cem ent was

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from the channel transport in the OFET and the contact resistance and leakage current had lesser

roles, while the less sensitive lower gate voltages showed the contact resistance played a larger

role. Additionally, the OSC will interact more w ith longer alkyl side chains; the lon ger side chain

means there is higher mass absorbed onto the OSC .(97)

A decrease in film thickness increases sensor response to the analyte, from the faster

diffusion to the conduction channel.(92) There is a minim um thickness requ ired for a ch annel to

form in the OFET but once that thickness is met additional OSC thickness only redu ces the sensor

response. Further work showed that even wh en the dielectric thickness wa s reduce d to ju st the

native oxide (~2nm S i0 2) the thin OSC layer was very sensitive.(46)

3. Enhanced Sensor Response with Receptors

When polar molecules are adsorbed onto the OFET they introduce traps into the OSC

film; these traps alter the source drain current. W hile these molecules may ad here to th e grain

boundaries or other defect sites in the film , there is no specific binding to the OSC film . I f the

binding str ength were increased , the nu mbe r o f analytes on the fi lm would in crease , re su lt in g in a

stronger signal. One o f the attractive features o f OSC s is the ability to covalen tly bo nd a receptor

group to an existing semiconducting material. This allows the receptor to ge t closer to the charge

conduction channel, which will increase the sensitivity of the OFET. By placing an ether group

on a polythiophene side chain the response to ethanol was increased com pared to a poly thiophen e

which just had an alkyl side chain.(97)

To better sense dimethyl methylphosphonate (DMMP), a simulant for the nerve agent

sarin, a hydroxylated bithiophene oligomer was used as a receptor.(93) The OS C used was 5,5-

bis(4-he xy lpheny l)-2 ,2-bithiophene (6 PTTP6 ) in a 1:1 ratio with the receptor that seem ed to give

a single phase film, when examined with scanning electron microscopy and x-ray diffraction

(XRD). The current decrease occurred faster upon exposure to DM MP and w as greater in

magnitude for the device with the incorporated receptor. The decrease in curren t can be

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attributed to stronger bonding o f the DM MP to the receptor and a change in the local electric field

from binding to the receptor. The sensitivity could be enhanced by incorporating a receptor with

the 6 PTTP6 and decreasing the film thickness, reducing the detection limit from 150 ppm to 5

ppm.(92)

Electron transporting materials have also been used with receptors to detect DMMP. (94)

An OSC film o f NTCD I had receptors of a hydroxphenylated NTCDI dep osited on top o f the

film. This receptor formed islands on the OSC due to the difference in surfac e energy w ith the

NT CD I. Eve n though the receptor was no t fu lly inco rporated in to the fi lm it st ill sh ow ed a more

selective response to DM MP.

To further show how receptor groups can influence the response of a sensor, two

different side chain receptors were incorporated onto an OSC core and placed on top of an

existing OSC film.(95) These receptors made the sensor respond differently to two enantiom ers

of a chiral alcohol citronellol. This shows the selectivity that is achievable by ca refully selecting

the receptor molecule. In another study, 2 mon olayers (lOnm) films of 5,50-bis-(7-do decyl-9 H-

fluoren-2-yl)-2,20-bithiophene (DDFTTF) were exposed to various volatile organic compounds.

(96) The receptors used were calix[8 ]arene (C[8 ]A) and c-methyl calyx[4]-resorcinarene

(CM[4]RA), which were previously shown to work as size selective cavities in other host-guest

sensors. C[8 ]A showed a ethyl acetate detection limit five times lower than then pure DDFTTF

film.

4. Sensing Mechanism

Since the charge carriers at the boundaries between grains dominate the charge transport

in longer channel devices, the sensing mechanism is dominated by the formation of dipole-

induced traps at the grain boundaries when analyte is adsorbed. This analyte-in duced trap

decreases the source-drain current for most polar analytes, because there is always a region

around the dipole where charge carriers are at lower energy, and there is an activation barrier for

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them to move away from the lower energy region. There are fewer grain boundarie s in short

channel devices, so the analyte molecules that diffuse to the electrode-OSC interface will alter the

source-drain current. The electrode-OSC interface is the region where the majority o f the

resistance occurs for short channel devices. W hen interfacial resistances do not determ ine the

current-voltage relationships, then dipole adsorption or diffusion into the bulk OSC results in

current-lowering traps. Analytes with a high redox activity can act as quen chers or dop ants for

the main semiconducting domains, lowering or raising current levels, respectively. An electron

withdrawing analyte can act as a dopant for a p-type OSC and a trap for an n-typ e OSC. The

doping mechanism is particularly useful for sensors because of the rareness and distinction o f a

tum-on signaling for analytes. Two recent examples o f this are the use of n-channel OS Cs to

obtain current increases in response to ammonia vapor.(97, 98)

As discussed earlier, the extent to which an analyte can alter the charge transport varies

with the analytes location in the OSC film. The charge carriers furthest awa y from the gate can

be screened by the lower OSC layer from up to 90% o f the appl ied gate vo ltag e, which means

that the charge carriers are operating at a voltage much lower than the applied gate voltage. (11)

Figure 2.2 shows the drop in the effective voltage seen by the OSC as the distance from the

dielectric interface increases. Electrostatic m odeling has suggested that a cluster o f polar analytes

can have a potential of a few tenths of a volt, which means that carriers further from the gate can

be trapped by analytes .(93) Analytes may be more prevalent at grain boundaries of the OS C than

the bulk, depending on the ability of the analyte to intercalate into the bulk O SC. The m ain

benefits o f a transistor sensor over a ch em iresis to r are the ab ility to set the in it ia l charge ca rr ie r

density with the gate electrode, the ability for multiple parameters to be measured, instead of a

pure current measurement as in the case o f a chem iresistor, an d the ab il ity to inco rp orate the

transistor into cascaded logical circuits. A transistor allows for current, thresho ld voltage ( V t ) ,

and mobility (p) to be measu red.(/7)

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semiconductorinsulatorV

Vo

Vo=Vs

Vi

V2V = 0

0 d n d x

Figure 2.2 The effective voltage seen by the molecular semiconductor as the distance from the dielectric

interface is increased. (13)

5. DNT Vapor Sensing

Currently there is substantial effort in sensing for compounds used in explosives.

Dinitrotoluene (DNT), can be used as a simulant for the more hazardous trinitrotoluene (TNT), as

shown in Figure 2.3. DNT is also a byproduct o f the manufacturing o f TNT. DN T is a yellow

solid with very low vapor pressure at room temperature. The saturation vapor pressure of DNT at

room tem perature is 173 parts per billion (ppb), while he vapor pressure o f TNT is orders o f

magnitude lower. (99, 10G)

Both n- and p-channel semiconductors are used to fabricate OFETs that are responsive to

DNT. Different responses are obtained from devices with various semiconductor materials and

receptor layers. The pattern of responses includes sufficient diversity to set it apart from other

vapors that might give similar responses to som e OFE Ts in the set, but n ot all o f them. T his

allows for the possibility o f distinguishing DNT from other vapors in a man ner that c ould be

further developed using mechanistic principles elucidated in this work.

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n o 2 o 2N n o 2

n o 2 n o 2

DNT TNT

Figure 2 J Chemical structures of DNT (left) and TNT (right).

5.1 Exp erimen tal Setup

A testing setup similar to the DMMP sensor experiments previously performed in the

group using a vapor delivery system and testing chamber was fabricated to analyze the DNT

vapor response for individual OFET sensors. (93) Figure 2.1 show s a schem atic d iagram o f the

test chamber and DNT vapor delivery system. The vapor delivery system can be switched

be tween air and DNT vapor. Whe n it is sw itch ed to air-on ly ; pure , fil tered ai r at co ntro lled flow

rate is delivered into the testing chamber. To deliver the DN T analyte to the OFE T sensor, the

airflow is switched so the airflow is now directed above the DN T solid in a bu bbler con tainer and

carries a certain amount o f DNT vapor into the testing chamber. The DN T v apor g enerated by

this system should be much lower than the saturation vapor of DNT, less than or equ al to 100

ppb.

5.2 DNT Sensor Results

5.2.1 n-Channel Sma ll Molecule Semiconductors

N ,N -Bis (3 ,5 -b is (t ri fluoro m ethyl) p hen ylm eth yl)n aphth a le ne-l ,4 ,5 ,8 -te tr acarboxyli c

diimide (Bis-CF3 NTCDI) semiconductor. Bis-CF3-NTCDI is an air stable n-channel

semiconductor (101)with fairly high electron mobility. The increased air stability is beneficial in

sensor applications where the OFET is not packaged and the higher mobility allows for lower

voltage OFET operation Figure 2.4a shows the chemical structure o f the bis-CF3 NT CD I

molecule. The electron conducting bis-CF3 NTCDI is expected to have its source-drain current

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partially quenched whe n it is expo sed to elec tron withd rawing analytes. Add itio nal ly , the charge

carrier mobility would be expected to decrease. Ano ther benefit for sensing applications is that

the minimum thickness of OSC required to achieve OFET o peration with this com poun d is very

thin, 6 nm. As previously reported for DMM P, the thickness of the OSC layer of OFETs

effectively determines the time taken for analytes to diffuse through the semiconductor layer to

the conduction channel, and hence affects response time o f sensors.(92) Organ ic sem iconductors

with thinner threshold thickness are thu s'more desirable for fast sensing.

Heavily doped silicon substrates with 300 nm o f thermally grow n S i0 2 had an

octyltriethocylsilane (OTS) SAM on the Si0 2 surface. The bis-CF3 NT CD I was thermally

evaporated onto the OTS surface. Top contact gold source-drain electrodes were dep osited by

thermal evaporation. The electrodes were evaporated through a shadow mas k with a spacing of

L= 270 pm and W = 6.5 mm. Figure 2.4b shows the norm alized source drain curr ent at a Vd and

Vg of 100V o f an OFET with 6 nm bis-CF3 NTCDI film. Devices with 6 nm bis-CF3 NTCDI

exhibited the best response signals upon exposure to D NT vapors, using the sam e d elivery system

as described above, at various concentrations. The gray squares indicate w hen 4 liters per minu te

(LPM) o f airflow was directed over the DNT . Yellow , pink, and blue denote flowrates of 2 LPM ,

6 LPM, and 8LPM respectively.

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1.00

0.95

0.90

0.85

0.80

0 10 20 30 4 0 50 6 0 70 80 90 100 110 120

Time (mins)

0.038

0.037

p 0.036

0.035

0.034

0.033

10 20 30 40 50 60 70 80 90 100 110 120

Time (mins)

-50.5

-51.0-

-52.5-

-53.010040 60

Time (mins)

120

(b)

(C)

( d )

Figure 2.4 The chemical structure of bis-CF3 NTCDI (a) and response of a 6 nm thick OSC film FET upon

exposure to DNT vapors at various concentrations. The normalized response of source-drain saturation current

(Id at/I


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concentration. The source-drain current of the OFE T decreased rapidly wh en D NT vap or was

delivered into the testing chamber, and the current leveled o ff at a min imu m valu e after

approximately 180 seconds. At this point, the total amount o f analyte accum ulated in the

semiconductor layer began to saturate, and the rate of analyte desorption is countered by the

adsorption rate. Once the chamber was vented with air, the source-drain current in the device

recovered quickly. The change o f field effect mo bility follow ed the respon se of source-drain

current when the OFET sensor was exposed to DNT vapors and air periodically. The magnitude

o f mobility decrease is also proportional to DNT v apor concentrations. Th e thres hold v oltage

response, while small, for the device can also be observed as shown in Figure 2.4d. Gentle air

flow over the device was sufficient to recover some o f the initial current. I f the reco very needed

to be faster it could be accelerated by using a higher a ir flow rate or slightly heating the d evice.

N ,N -B ls (p en ta flu oro phenyle th yI) naph th a lene-1 ,4 ,5 ,8 - te t ra c a rb o x y li c - d iimid e

(C2PhF5 NTCDI) semiconductor. OFETs with this n-channel semiconductor (102) are

relatively unstable compared to bis-CF3 NTCDI when the thickness of semiconductor layer is

ultrathin. The instability may be due to discontinuities in the thin film, and since the cu rrent m ust

traverse relatively small area grain boundaries with a very high resistance, the OSC will heat up

causing possible decomposition. In an exemplary OFE T sample, 15 nm C2PhF5 N TC D I (Figure

2.4a) was deposited on Si0 2 /Si substrates. Again, top gold electrodes were deposited through

shadow masks with the same w:l ratio. This device exhibited stable performance in air, as

opposed to a 10 nm thickness, which did not, indicating 15 nm is the approximate threshold

semiconductor thickness for this material. D NT vapor response w as then assessed w ith this 15 nm

device.

This device displayed a large response upon exposure to DNT vapors, as shown in Figure

2.5. The current response did show some gate voltage dependence. At lower gate voltage, the

relative curren t Id,Sa/Id,o changed m ore, but the base line also drifts m uch mo re. The res po nse o f

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the mobility also tracked that of the current. Upon e xposure to DNT vap or, m obility decreased

immediately, and started to increase once the testing chamber was vented with air. Interestingly,

the threshold voltage VT did not show a response to DNT vapor. In Figure 2.5d, the curve is a

straight line. VT kept increasing during the entire measurem ent, no m atter whe ther DN T v apor or

air was delivered into the testing chamber. This is notably d ifferent from wh at was observed from

bis-CF3 NT CD I, which displayed obviou s VT respon se . The DNT seems to slo w the m otion o f

the electrons but not trap them for periods longer than the time scale of the measurem ents. This

is an example of OFETs desirably responding in different ways upon exposure to the sam e

analyte vapor, to help generate a pattern o f responses associated with a particular analyte o f

interest.

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(S

A/

4 8 12 16 20 24 28 32 36 40

Time (mins)

0.054

0.052

0.050

\0.048

0.046Mobility

0.044 (c)

0 4 8 12 16 20 24 28 32 36 40

Time (mins)

>

> '

( d )

0 4 8 12 16 20 24 28 32 36 40

Time (mins)

Figure 2.5 The chemical structure of C2PhF 5 NTC DI (a) and response o f a 15 nm thic k OSC film F ET upon

exposure to DNT vapor. The normalized response of source-dra in saturation current (b); response of m obility

(c); response of threshold voltage (d). The gray bars a re at an air flowrate o f 4 LPM.


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5.2.2 p-Ch annel Small Molecule Semicondu ctors

5,5-bis(4-hexylphenyI)-2,2-bithiophene (6 P T T P 6 ) semiconductor. 6 PTTP6 has been

used as a p-channel semiconductor by other group m embers. {103, 104)The chem ical structure of

6PTTP6 can be seen in Figure 2.6. Devices made from two molecular mo nolay ers (6 nm) of

6PTTP6 were deposited by thermal evaporation, follow ed by the deposition of go ld top electrodes

through shadow masks. Substrate temperature was nom inally kept at room tem perature. The

botto m pa rt o f Figure 2.6 shows the respon se o f th is device up on exposure to DNT va por at a

flowrate of 4 LPM. The response to DNT can be seen, how ever the signal to noise ratio is poorer

than other OFET sensors. Current decreased right after the exposure to DNT vapor, and partially

recovered after the chamber was vented with air. When the 6 PTTP6 OFET sensor is compared to

the bis-CF3 NT CDI sensor, it is not considered as sensitive to DN T.

1.05Vg=-4QV

Vg=-50V

Vg=-60VVg=-7QV

Vg=-80V1.00

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1.02-

1.01-

1.00-o 0.99-TJ

0.98-

*o~ 0.97-

0.96

0.95-

0.94

0.93-

*82saos,

-V =-40V - V -60V

- Vg= -80V,

DNT

(b)

CO

I&15

0.02230

0.02225

0.02220

0.02215

0.02210

0.02205-

0.02200

0.02195

Time (mins)

n

fT

-------

c

r

-----

pcPo

ce-

0

DNT

(c)

0 2 4 6 8 10 12 14 16

-25.2

-25.6-

> ~ -26.0-

DNT

-26.4-

0 2 4 6 8 10 12 14 16

(d )

Time (mins)

Figure 2.7 The chemical structure of 2PTTP2 (a) and response of a 4.5 nm thick OSC film FET upon exposure

to DNT vapor. The normalized response of source-drain saturation current (Id,sat/Id^at,o) (b); response of

mobility (c); response of threshold voltage (d). The gray bars are at an air flowrate o f 4 LPM.

Co pper phthalocyanine (CuPc) sem iconductor. CuPc is another p-channel small

molecule semiconductor, but unlike the other semiconductors discussed so far it has a co ppe r core

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inside the molecular structure as seen in Figure 2.8a. Some groups had shown that Cu Pc was

responsive to DNT vapor but due to the better packing o f the CuPc the responses w ere muc h

lower than other- OSCs. (105-108) 20 nm o f CuPc was deposited at an elevated substrate

temperature of 75 C since lower temperature depositions resulted poor quality films without

field effect behavior. The films were deposited on hexamethyldisilazane (HM DS) treated

substrates. 50 nm gold electrodes were therm ally evaporated and defined by the same w:l ratio

shadow mask. The mobility response and normalized current response to the DNT vapor at 4

LPM was very small and did not recover as well compared to the bis CF3 NTCDI OFET sensors.

The threshold voltage plot in Figure 2.8c shows gate bias stress occurring but no response to the

DNT vapor.

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1.32x10(/)

4

Eo

CM

-QO

Cu

(a)

7 *

(b)

5 10 15 20Time (min)

> -10

_ (c)

5 10 15 20Time (min)

Figure 2.8 The chemical structure of CuPc (a). The mobility (b) and threshold voltage (c) respon se of a 20 nm

thick OSC film FET upon exposure to 4 LPM DNT vapor. The gate and drain voltage are at -50V for the scans

above.

Alpha-sexithiophene (-6 T) and PT PT P semiconductors. Some small molecule

organic semiconductors either show negligible or no response to DNT vapor like the previously

discussed CuPc. Two additional examples are a-6 T and PTPTP shown in Figure 2.9 and 2.10

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respectively. The top of the figures show the molecu lar structure of the OSC . Alp ha-

sexithiophene is one of the earliest p-channel se micon ductors.(709) An OF ET w ith 15 nm a- 6T

semiconductor layer only exhibited a slight decrease in source-drain current upon exposure to

DNT vapor. PTPTP is another commonly used p-channel semiconductor. This material displayed

no response to DNT vapor, as shown in the bottom o f Figure 2.10.

Figure 2.9 The chemical structure of a-6T (top) with the corresponding response (bottom) upon exposure to 4

LPM DNT vapor.

a l p h a - s e x i t h i o p h e n e

0 2 4 6 8 10 12T i m e ( m i n s )

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P T P T P

1.4

1.2

2 1.0

_! 0.8^ 0.6

0.4

0.20.0

0 2 4 6 8 10 12

T i m e ( m i n s )

Figure 2.10 The chemical structure of PTPTP ( top) with the corresponding response (bottom) upon exposure to

4 LPM DNT vapor.

5.2.3 p-Ch annel Polymer Semiconductors

Poly(3,3 -didodecylquaterthiophene) (PQT12) semiconductor. OFETs based on

the relatively stable p-channel polym er semicon ductor poly(3 ,3 -didodec ylquaterthioph ene)

(PQT12) (110-113) were fabricated by spin coating. Fig. 2.11a shows the chemical structure of

the PQT12 molecular repeat unit. The polymer was dissolved in ehlorobenzene at a concentration

of 4 mg/mL. T he PQT12 solution was heated to 80 C to achieve a completely dissolved solution ,

filtered through a 0.45 pm po ly(tetrafluoroethylene) (PTFE) filter, and sonicated w hile cooling to

room temperature. The polym er was deposited onto the Si/S i0 2 substrates by spin coating at

2000 RPM for one minute. Gold electrodes were then thermally evaporated using the same w:l

ratio shadow mask as the previous OFE Ts (1 = 270 pm and w = 6.5 mm).

| DNT Vapor

-

Vg=-40V- o - Vg=-60V

Vg50V~Vg70V Vg=-80V

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c 12h25

12n 25 (a)

CO

0.046-

0.045-

0.044-Eo. 0.043-

$0.042-

j O

0.041-

0.040-

8LPM 6LPM 4LPW 4LFM 2LPM

20 30 40

Time (mins)

6LPM 6LPM 4LPM 4LPM 2LPM

(C)

20 30 40

Time (mins)

Figure 2.11 The chemical structure of PQT12 (a) and response of an OFET upon exposure to DNT vapor. The

response of mobility (b); response of threshold vo ltage (c). The blue, pink, gray and yellow b ars indica te air

flowrates of 8, 6, 4 , and 2 LPM respectively. The gate and drain voltages each at -100 V for the scans.

(PIFTBT-10) semiconductor. Another p-channel polymer synthesized for solar cell

applications was utilized as an OFET senso r.(/74 ) The chem ical structure of this polym er can be

seen in Figure 2.12a. The PIFTBT-10 O FETs were fabricated by spin coating. The po lym er was

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dissolved in chlorobenzene at a concentration o f 7 mg/m L an d gently heated to a chieve a

completely dissolved solution. The solution was then filtered through a 0.45 p m PT FE filter and

allowed to cool. The polymer was deposited onto the Si/SiC>2 substrates by spin coating at 500

RPM for 20 seconds and 2000 RPM for 40 seconds. The device electrodes were then deposited in

the same manner as the PQ T12.

The normalized source-drain current at different gate voltages, field-effect mobility and

threshold voltage response to the DNT vapor can be seen in Figure 2.12. The device showed

reproducible response to exposure to DNT vapor, with the change in mobility slightly larger for

PIFTB T than PQT12. How ever the PIFTBT -10 is less air stable, with grea ter bias stress

observed. W hile the response is not as fast as the PQT 12 OFET it still is fairly quic k with a t90=

1.5 minutes. The device was more responsive at higher gate voltages, which is somewhat more

atypical. The response at higher gate voltages can be due the larger gate bias stress observed in

the PIFTBT-10.

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of threshold voltage (d). The gray bars are at an air flowrate o f 4 LPM. Th e gate and drain voltages are each at

-100 V (c & d)

5.2.4 Com binedp-Channel and n-C hann el OFETs

PQT 12 w ith car bo n black. Pure PQT 12 devices were fabricated the way previously

describe

Interfacial Fields in Organic Field-effect Transistors and Sensors

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Transcript of Interfacial Fields in Organic Field-effect Transistors and Sensors