Nanocrystal Sensitized Photovoltaics and Photodetectors ......diameter approximately the size of the...

Nanocrystal Sensitized Photovoltaics and Photodetectors With Performance Enhanced Using Ligand Engineering

David M. Schut, George M. Williams Jr., Stefan Arteaga, Thomas L. Allen, Thomas Novet

Voxtel, Inc., 15985 NW Schendel Avenue, Suite 200, Beaverton OR 97006

ABSTRACT

Nanocrystal quantum dot photovoltaics and photodetectors with performance optimized by engineering the nanocrystals size and the optoelectronic properties of the nanocrystal’s chemical coating are reported. Due to the large surface-to-volume ratio inherent to nanocrystals, the surface effects of ligands used to chemically coat and passivate nanocrystals play a significant role in device performance. However, the optoelectronic properties of ligands are difficult to ascertain, as the band structure of the ligand-capped nanoparticle system is complex and difficult to model. Using density-of-states measurements, we demonstrate that modeling of electropositive and electronegative substituents and use of the Hammett equation, are useful tools in optimizing nanocrystal detector performance. A new particle, the Janus-II nanoparticles, developed using ‘charge-donating’ and ‘charge-withdrawing’ ligands distributed over opposite surfaces of the nanocrystal, is described. The polarizing ligands of the Janus-II nanoparticle form a degeneracy-splitting dipole, which reduces the overlap integral between excitonic states, and thus reduces the probability of carrier recombination, allowing carrier extraction to take place more efficiently. This is shown to allow increased photodetection efficiencies and to allow the capture of multiple exciton events in working photodetectors.

Keywords: nanocrystal, ligand, multiple exciton generation, multiple-exciton capture, Janus-II nanoparticle, photovoltaic, quantum dot

1. INTRODUCTION Commercially available photovoltaics and photodetectors have been dominated by solid-state junction devices optimized for the visible portion of the optical spectrum using crystalline or amorphous silicon, and for the near-infrared (NIR) and short-wavelength infrared (SWIR) using InGaAs, InSb, etc. These materials all profit, in varying degrees, from the experience and material availability resulting from the semiconductor industry, but they remain expensive and difficult to manufacture in large area detector structures. Now there is an increasing awareness of the possible advantages of devices based on solution processed mesoscopic inorganic and organic semiconductors, which offer the prospect of very low-cost fabrication (e.g., via ink jet or screen printing) without expensive and energy-intensive high-temperature and high-vacuum processes, and compatibility with both CMOS wafer and flexible substrates. These detectors are formed from intermediate products, including inorganic semiconductor nanocrystals, conducting metal oxide materials, and organic conductors and semiconductors,1,2,3,4,5,6,7,8 which are made from chemical constituents. Using three-dimensional

Figure 1: Detectivity measurement of a PbS nanocrystal photodetector fabricated using nanocrystals sized (~2.8 nm) with a 1100nm first excitonic peak.

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structured detectors and interpenetrating semiconductor junctions, detection efficiency and energy conversion efficiencies comparable to those of conventional devices have been demonstrated with near background with near background limited (BLiP) performance (Figure 1).

The size-based spectral tuning available from nanocrystal quantum dots make them particularly well suited for optical detection (see Figure 2). Due to the effects of quantum confinement, these absorption properties of the nanocrystals can be spectrally tailored to absorb efficiently over a wide spectral range. This allows for photovoltaics to be better matched to the solar spectrum and for night vision devices to be matched to the night sky radiance, resulting in greater energy conversion and detection efficiencies. Moreover, due to the increased absorptivity coefficient arising from the quantum confinement properties of inorganic nanocrystals, as few as 10 monolayers (<0.1 µm) are needed for full absorption of optical radiation. Because less sensitizing material is needed to absorb the solar radiation, the detector layers can be made thin. This is significant, because a reduction in photoanode thickness, reduces the probability of charge carrier recombination and increases the overall efficiency of the detector material. Furthermore, the reduced absorption layer thickness has the advantage of reduced device weight and raw material costs.

2. BACKGROUND 2.1 Nanocrystal Quantum Dots

Nanocrystal quantum dots are in fact tiny crystals ranging in size from 2 to 10 nm (the size of ≈10–50 atoms) in diameter and consist, in volume, of hundreds of atoms (Figure 3.) Due to the extremely small size of the nanocrystals, “quantum confinement” effects dominate their electrical, optical, magnetic, and structural properties. The opto-electronic and physical properties of a nanocrystal that can be exploited include its wide, size-dependent absorption spectrum and narrow emission peak.

When the nanocrystal’s diameter is smaller than the Bohr radius of an electron-hole pair (aka exciton) formed through a

Figure 2: Absorption curves for various sized NCs showing good first exciton peaks. The diameter and peak wavelength are labeled.

Figure 3: A semiconductor nanocrystal consists of a semiconductor core, several hundred to a few thousand atoms in size, usually covered by a wide bandgap shell layer and an outer ligand coating.





photon interaction with the nanocrystal, the energy states within the nanocrystals are discrete, similar to a 3-D spherical quantum well. The energy levels in a nanocrystal are a function of its diameter — the larger the nanocrystal’s diameter, the smaller the difference between the discrete energy states, until the diameter of the nanocrystal is approximately the size of the Bohr radius, at which point the nanocrystals energy state resemble those of the bulk material.

The quantum confinement effects can be explained by fundamental quantum physics. An electron can be thought of either as a particle or as a wave moving through space. In bulk material, electrons exist at many energy levels — in fact, a continuum of energy levels in valence and conduction bands — because of the numerous atoms in the material. (No electron, however, can exist in the forbidden energy region between the top of the valence band and the bottom of the conduction band.) However, if the particles become smaller than the size of the exciton in the bulk semiconductor (typically ≈10 nm), their electronic structure changes. The electronic properties of such small particles are hence more like those of a giant molecule than an extended solid. The electronic and optical properties of such small particles will depend not only on the material they are composed of, but also on their size.9 The lowest energy optical transition, among other electro-optic properties, will increase significantly due to the quantum confinement of decreasing inorganic cluster size.

As the size of a material approaches the nanometer scale, it starts to confine the energy levels at which an electron may exist. To see why, think of an electron as a wave and the nanocrystal as a sphere that confines the electron, with a diameter approximately the size of the Bohr radius. In such a confinement, the electron has to travel within the inside circumference of the sphere so that its waveform does not destructively interfere with itself. This means that the confining circumference must be equal to the electron wavelength or some whole number multiple of the wavelength. At other wavelengths, the wave would interfere with itself and destroy the possibility of existing. Thus, only certain (quantized) wavelengths can exist in such a confinement. The shorter the diameter of the confinement, the shorter the wavelength of the electron. And because energy is inversely proportional to wavelength (directly proportional to frequency), the smaller the nanocrystal, the smaller the allowable wavelength, the greater the frequency, and the bluer the associated photon emission or absorption spectra.

The properties that are tunable and/or enhanced include the onset of absorption (band gap), the peak fluorescence wavelength, and a range of nonlinear effects — including non-linear refractive index, non-linear absorption, and other electro- and magneto-optic effects. Another benefit of nanocrystals is their ability to undergo carrier multiplication, which involves the generation of multiple electron-hole pairs from the absorption of a single photon.

2.2 Nanocrystal sensitized photovoltaics and photodetectors

Because of the benefit of solution processing, spectral tuning, and carrier multiplication, there have been previous attempts to integrate nanocrystal absorbing layers into photodetectors and solar cells. But nanocrystal-sensitized

(a) (b) (c) Figure 4: (A) Effects of ligand length (saturated aliphatic ligand) on tunneling distance. (B) Resonance effects in ligands. Ligands having increased contributions to conjugation exhibit smaller HOMO-LUMO gaps, (right hand side) whereas those without conjugation have larger HOMO-LUMO gaps (left hand side). (C) Inductive effects upon the energy levels of the HOMO and LUMO of the ligand relative to the PbS nanoparticle. By using an electron withdrawing ligand (-NO2), the energy levels have decreased. Using an electron donating ligand, (-CH3), the energy levels have increased.





Hammett Parameter (σ)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

LUM

O E

nerg

y Le

vel (

eV)

-3

-2

-1

0

1

2

3

4

X = S-

X = CO2-

X = NH 2

X - SH

X = CO 2H

Hammett Parameter (σ)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

HO

MO

Ene

rgy

Leve

l (eV

)

-10.5

-10.0

-9.5

-9.0

-8.5

-8.0

-7.5

-7.0

X = NH2

X = SH

X = CO 2H

Figure 5: Correlation graphs of the HOMO (left hand side) and LUMO (right hand side) energy levels as a function of the Hammett Parameter (σ) for benzene (Y-C6H4-X) derivatives.

photodetectors have yet to demonstrate high efficiency. Efficiency is maximized by having a large density of unpopulated states in the photoanode positive of the sensitizer potential, preferably at optimal exoergicity. To date, it is apparent that inadequate or misaligned bands and interfacial engineering, combined with a non-fixed Fermi levels, which vary as a function of light and temperature, have precluded a high rate of photoelectron generation in the nanocrystal sensitizer and efficient electron injection from nanocrystals to the photoanode, where the photosignal can be sensed.

Due to the large surface-to-volume ratio inherent to nanocrystals, the surface effects of ligands used to chemically coat nanocrystals play a significant role in the electron dynamics of the system.10,11 In nanocrystal systems, ligands are most generally used to mediate nanocrystal growth, to impart solubility, and to stabilize the colloid through steric hindrance. For high efficiency, the head groups of the ligands must also passivate surface states.1213 The ligands can also determine a number of important and useful physicochemical properties of the nanocrystals, and can influence the optical and electrical properties of nanocrystal. Since the ligands used to synthesize the nanoparticles are chosen for ease of synthesis and not for their optoelectronic properties, in nanocrystal photodetectors it is beneficial to exchange these ligands for ones that have properties that maximize the electronic performance of the nanoparticle. This is usually accomplished using a synthetic scheme with a chemical (ligand) capping exchange procedure.14 To enhance opto-electronic properties, the native coordinating organic ligands on the surface of the nanocrystals are exchanged or functionalized with a new ligand or ‘chemical cap’.

Despite the critical role ligand caps play in organic–inorganic devices, it was only in 2003 that Louis Brus described the effects of the ligand shell upon the properties of nanocrystals, specifically Si35H36.15 Since then, the effects of stabilizing ligands on the reactivity of colloidal nanocrystals has not been substantively reported on, with two exceptions: Li et al. noted that good capping ligand stabilization appears to diminish catalytic nanocrystal activity in some cases,16 and Narayanan et al. explored the effects of Ostwald ripening and precipitation on catalytic activity.17 Until only a few years ago, researchers were using nanocrystals with largely non-conducting ligands (e.g. oleic acid) used in the nanocrystal synthesis process. More recently, attention has been given to very short ligands, under the rationale that “short” ligands shorten the distance the charge carriers has to travel between the nanocrystal and the photoanode. This general premise certainly has merit, but fails to take into account the specific dynamics and the density of states (DOS) at the photoanode–nanocrystal interface which mediate detector efficiency.

Ligands can be optimized through chain length, extent of conjugation, and degree of inductive effects to achieve desired optoelectronic properties. However, there is currently not a body of work that predicts the optoelectronic properties of ligand-capped nanocrystals. This is largely due to the computational intensity needed to model even the smallest nanocrystals. It becomes untenable to model the entire system of nanocrystals and their corresponding ligands using ab initio methods, and on the other hand, their size scale and asymmetry prevent simple Schrödinger-Poisson solvers from being effective. Here, we show that the photodetection of nanocrystal detectors can be engineered by manipulating the properties of the ligand chemistries. We show that by controlling the type of ligand on the nanocrystals, the carrier multiplication efficiency, multi-exciton lifetime, and charge injection through a device can be optimized, leading to better detectivity and energy conversion efficiency.

2.3 Modeling ligands inductive, resonance, and tunneling properties on photodetector performance

The band structure of the ligand-capped nanoparticle system is extremely complex: in order to calculate it to any degree of accuracy, one would have to know the Hamaker constant of the ligand, the work function of the ligand-nanocrystal





Table 1: Linear fit parameters and correlation of model results of various ligand types to Hammett parameter, σ. Representative ligands from each class were modeled and the properties fit to a linear regression to characterize performance (see Figure 5 for example.)

Ligand Class Calculated Parameter Linear Fit Equation Correlation Coefficient (r2)

4-substituted anilines (Y=NH2)

HOMO Y = -0.5317X – 8.0444 0.3985LUMO Y = -0.8631X – 0.3010 0.7157Band Gap (Eg) Y = -0.3314X + 7.7434 0.1886Total Energy Y = -11333X – 50882 0.2685Net Charge (C1) Y = 0.0183X – 0.0532 0.0545Net Charge (C4) Y = -0.0533X – 0.0968 0.5503

4-substituted benzenethiols (Y=SH)

HOMO Y = -0.5815X – 8.4303 0.7313LUMO Y = -0.8955X – 0.7172 0.7881Band Gap (Eg) Y = 0.314X + 7.7131 0.2763Total Energy Y = -11332X – 51073 0.2685Net Charge (C1) Y = 0.0148X – 0.1816 0.8351Net Charge (C4) Y = -0.2769X – 0.0499 0.3514

4-substituted benzenethiolates (Y=S-)

HOMO Y = -0.4918X – 3.4824 0.8787LUMO Y = -1.086X + 2.3801 0.7512Band Gap (Eg) Y = -0.5942X + 5.8625 0.5551Total Energy Y = -11340X – 50754 0.2688Net Charge (C1) Y = 0.0099X + 0.0174 0.8704Net Charge (C4) Y = -0.2879X – 0.1147 0.3813

4-substituted benzoic acids (Y=CO2H)

HOMO Y = -1.0036X – 9.1260 0.7333LUMO Y = 0.6679X – 1.1355 0.8786Band Gap (Eg) Y = 0.3356X + 7.9905 0.2661Total Energy Y = -11331X – 63069 0.2685Net Charge (C1) Y = 0.0178X – 0.1637 0.8634Net Charge (C4) Y = -0.0046X – 0.0665 0.6441

4-substituted benzoates (Y=CO2-)

HOMO Y = -0.3185X – 4.6709 0.8957LUMO Y = -0.5399X + 1.6398 0.0397Band Gap (Eg) Y = -0.2214X + 6.3108 0.007Total Energy Y = -11337X – 62742 0.2687Net Charge (C1) Y = -0.0228x – 0.1122 0.0044Net Charge (C4) Y = 0.1152X – 0.1142 0.6184

bond, the curvature and morphology of the nanoparticle surface, the spacing of ligands within the ligand shell, etc. As a substitute, we use molecular models and measurements of ligands assigned electropositive and electronegative coefficients using the Hammett equation. We use the Hammett equation as a parameter to calculate the effects of different substituents on different classes of ligands, and to accurately predict their contributions to the optoelectronic properties of the nanocrystals that they are attached to.

The Hammett equation describes linear free-energy relationships that compare reaction rates and equilibrium constants for reactions of structurally similar chemical derivatives, using just two parameters: a substituent constant and a reaction constant. The Hammett equation is defined as the log of the ratio of the rate constant for the acid dissociation of a substituted benzoic acid (K) to the rate constant of the acid dissociation of the nonsubstituted benzoic acid (K0); this is equal to the product of the Hammett parameter, σ, and the reaction parameter, ρ:18

σρ=)log(0K

K (1)

Although it was originally used for the determination of the acid dissociation constants of benzoic acids, the Hammett equation has been used to determine the electronic contributions of substituents to a reaction or property. There are several different constants available to use in modeling ligands, such as the Hammett constants ρm, ρp, and ρ* (which is also known as the Taft constant). The ρm and ρp Hammett constants are used to denote the effect of substituents in the meta position of an aromatic system (ρm) and the para position of an aromatic system (ρp). The ρ* Taft constant is used for alkyl substitution rate adjustments. Because of the effects of substituents on the rate of reaction, including resonance





and inductive contributions, ρmp and ρpm, respectively, they can be used to determine the extent of each effect upon the overall reactivity.

By utilizing molecular modeling, in conjugation with the Hammett parameters/constants – the ability to predict what the ligands effect on the optoelectronic properties of the nanoparticles can be enhanced. In our implementation, the extent of electron-withdrawing or -donating activity by the ligand end-groups is determined by the electronegativity or polarity of the end-groups on the conjugated ligand, as well as by the work function of the bond tethering the ligand to the nanocrystal core. The strength of this interaction affects the displacement of charge across the conjugated bonds between the ligands and the nanocrystal core, resulting in an overlapping of wavefunctions and the creation of either highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) conduction states, depending on whether the capping ligand’s end-group makes it a charge donor or acceptor, respectively. This electronic configuration is somewhat analogous to that produced in doped semiconductors, in which the presence of charged impurities enables electronic conduction throughout the doped region of the semiconductor material through artificial states.

We categorize ligands into two major categories: conjugated aromatic ligands, which are dominated by resonance and inductive effects, and saturated aliphatic ligands, which have properties generally dominated by tunneling effects.

2.3.1 Inductance and Resonance Effects

In modeling the conjugated aromatic systems, it is interesting to see the effect of the substituents on the ligand and their effects on the electronic properties of the ligands.

By using the Hammett equation and calculating ligand inductance and resonance parameters such as: the HOMO energy level, the LUMO energy level, the electronic population of the atomic orbitals, and the net charge of a given atom, etc., it is possible to accurately predict the electronic effects of various ligands connected to specific nanocrystals. By establishing this relationship to ligand-coated nanocrystals, we are able to use the Hammett equation as a tool to correctly distinguish the separate effects of the ligand’s functional groups on parameters. This allows us to predict trends even from relatively sparse data. Following this, we apply multiple theoretical calculations, including AM1, MINDO3, PM3, and ab initio modeling of the structures.

Table 1 gives a breakdown on the linearity and correlation fit of calculated properties of the following ligands: 4-substituted anilines, 4-substituted benzenethiols, 4-substituted benzenethiolates, 4-substituted benzoic acids and 4-substituted benzoates. Figure 5 shows two graphs of the HOMO and LUMO values of the substituted benzenes (Y-C6H4-X) illustrating the inductive/resonance effects of a number of aromatic ligands. The graph on the left depicts the correlation between the HOMO energy level as a function of the Hammett Parameter (σ). The graph on the right is the LUMO energy level as a function of the Hammett Parameter. As the graphs depict, there is definitely a trend between

Figure 6: Orbital contributions to the HOMO for: biphenylcarboxylic acid (substituent in 8-position, X = -H), 8 – methyl biphenyl carboxylic acid (electron donating, X = -CH3), and 8-nitrobiphenylcarboxylic acid (electron withdrawing, X = -NO2).

Figure 7: Orbital contributions to the LUMO for ligands of Figure 8.





Figure 9: Orbital contributions to the LUMO for: octanethiol (C8-SH), dodecanethiol (C12-SH) and octadecanethiol (C18-SH).

the energy levels of the HOMO and LUMO and the Hammett Parameter. However, when trying to apply a linear fit to these graphs, the correlation does not fit well in all cases. While this does not preclude the usefulness of this approach to predict the properties of the ligands upon the optoelectronic properties of the nanocrystals, it makes it difficult to assign a quantitatively values to each ligand, which predicts the exact effect on the nanocrystal’s optoelectronic properties. There are many ligand/nanoparticle interactions which contribute to the device performance, any of which may dominant the calculations. Until these factors are deconvoluted from within the data set, a quantitative assessment may well nigh be impossible to obtain. Nevertheless, useful information can be garnered using the Hammett Parameter calculations, which enable us to predict the outcome of a ligand attachment to the optoelectronic properties of the nanoparticle.

In examining substituent effects on the HOMO contribution of biphenylcarboxylic acid (Figure 6), when X = -H and –CH3, the ligand should be a good hole carrier since the orbitals are all next to each other (the different colors represent different phases, not a separation between the orbitals) and are continuous. However, when X = -NO2, there is a discontinuity of the orbitals, meaning that it will not be a good hole transport ligand.

Likewise, on examination of Figure 7, which represents the LUMO contributions of biphenylcarboxylic acid, when X = -H and –NO2, good electron transport is exhibited since all the orbitals are continuous. However, when X = -CH3, there is a degree of discontinuity in the orbitals, which signifies that this ligand will not conduct electrons as efficiently as the other two ligands.

2.3.2 Tunneling Effects

Using saturated aliphatic ligands the dielectric constant around the nanoparticle can be manipulated by increasing or decreasing the ligand length dependent carrier tunneling distance between nanoparticles. For example, a C18 saturated acid (octadecanoic acid) has a large carrier extraction tunneling distance, whereas a C4 saturated acid (butanoic acid) has a shorter tunneling distance, which should allow for much better carrier extraction. These ligands are shown in the models of Figure 8 and Figure 9, which show a thiol chelating atom. In addition to the thiol chelating atoms, carboxylic acid (-CO2H), and the amine (-NH2) were also modeled. In all of these ligands, regardless of the length of the alkyl chain or the chelating head of the ligand, there is always a 4-carbon contribution to the orbitals making up the HOMO/LUMO system. This means that any ligand of C4 or smaller will exhibit good carrier extraction properties from the nanocrystal into the host matrix material. This has been shown experimentally.19,20,21,22

2.4 Type II Janus Nanocrystals

The use of conjugation can be used to manipulate the effects on the optoelectronic properties of nanoparticles. By increasing the degree of conjugation, the HOMO-LUMO gap can be decreased, allowing easier injection of carriers from the nanocrystal into the LUMO or the HOMO of the ligand. Likewise, by changing the substituents on the ligand, making them either electron withdrawing or electron donating, it is possible to increase the relative energy levels of the HOMO and LUMO, either increasing them (electron donating) or decreasing them (electron withdrawing) relative to the nanoparticle.

Using ligand parameters derived using the Hammett Parameter, we have developed a new ligand-engineered nanoparticle, the “type II Janus” (Janus-II) nanocrystal. In the Janus-II nanocrystal ‘charge-donating’ and ‘charge-

Figure 8: Orbital contributions to the HOMO for ligands of Figure 9.





withdrawing’ ligands are distributed over opposing surfaces of the nanocrystal; one ligand type is attached to one hemisphere of the nanocrystal and the other ligand to the other hemisphere of the nanocrystal. The ionic compensation of the two ligand types forms a degeneracy-splitting dipole, which reduces the overlap integral between excitonic states. The breaking of the degeneracy reduces the time-evolving overlap integral of the states, and thus the recombination probability, which in turn allows more efficient carrier extraction.

The two different ligands used in the Janus-II nanoparticles are a linking ligand, L1 and a conjugating ligand, L2. The L1 ligand has multiple roles. First, it has to physically bind the nanoparticle to the photoanode. In order to bind efficiently, the ligand must have at least two functional groups: one functional group binding selectively to the nanoparticle while the other binds to the electron transport layer. Without this selectivity in binding, the nanoparticles will aggregate and provide poor coverage of the photoanode (e.g. a TiO2 electron transport layer), resulting in delocalization of the exciton across multiple nanoparticles. Second, the ligand must also be a good electron carrier. For this, the ligand must either be fairly short to allow for tunneling, or have an extended conjugated chain through which electrons can easily pass.

Using the Hammett equation and molecular modeling as a guide, we identified three bi-functional candidate electron withdrawing saturated aliphatic ligands that fit all these requirements when configured with PbS Janus-II nanocrystals: 3-mercaptopropionic acid (MPA), 2,3-dithiopropane sulfonate (DT), and cysteine (Cys). All three have good tunneling properties and are well suited for interaction and reaction with PbS and Si or TiO2 photoanode layers. Figure 12 provides the chemical structures for these three ligands; all three binding ligands have a three-carbon chain backbone, and all have a thiol group that binds well to the PbS nanocrystal. Both MPA and Cys have a carboxylic acid group that selectively binds to a TiO2 photoanode and DT has a sulfonic acid group that can bind to TiO2. We also identified an aromatic ligand, 1,2,4,5-benzenetetracarboxylic acid, which has good resonance properties combined with electron withdrawing properties.

The conjugating ligand (L2) used to complete the Janus particle, is chosen to create a local field that stabilizes the exciton in the nanoparticle (Figure 13). The Hammett equation shows that the following ligands from the benzoic acid series, ranked in terms of resonance and inductive effects governing electron-donating properties, are suitable for this task: #1) N,N-(dimethylamino)phenylboronic acid; #2) p-toluic acid; #3) benzoic acid; #4) p-cyanobenzoic acid; #5) p-nitrobenzoic acid. These ligands are members of the 4-substituted benzoic acids (Y=CO2H) ligand class shown in Table 1, and were chosen because of their inductive and resonance properties. We also identified a series of bi-dentate phthalic acid series ligands, which strongly bind to PbS. These included, 4-sulfophthalic acid, a moderately electron

(a) (b) (c) (d) (e) (f) Figure 10: Chemical structures of various L2 ligands. (a) p-nitrothiophenol (electron withdrawing), (b) p-chlorothiophenol (electron withdrawing), (c) thiophenol (electron donating), (d) p-methylthiophenol (electron donating), (e) p-methoxythiophenol (electron donating), and (f) p-aminothiophenol (electron donating).

Figure 11: UPS spectra taken of the ligand functionalized nanocrystals. S = 4-(N,N-dimethylamino)phenylboronic acid, T = p-cyanobenzoic acid, U = p-toluic acid, V = benzoic acid, W = p-nitrobenzoic acid, and X = 3-mercaptopropionic acid. Expanded view of the spectra showing the leading edge of the binding energy for each of the nanocrystal involved in the study.





donating ligand and 4-aminophthalic acid a very strongly electron donating ligand (see Table 2)

2.4.1 Verification of Janus-II Nanocrystal Functionality

We demonstrate the performance of Janus-II nanoparticles in working photovoltaic device structures. Using a silicon substrate and a 1,2,4,5-benzenetetracarboxylic acid L1 ligand, a series of L2 ligands, including oleic acid (the as synthesized native ligand on the PbS nanocrystal), a strong dielectric aliphatic ligand with electron donating properties, 4-sulfophthalic acid (moderately electron donating), and 4-aminophthalic acid (strong electron donating ligand) were tested. In order to characterize the energy levels of the multi-component nanocrystal detectors in the presence of the phthalic acid series ligands, x-ray photoelectron spectra (XPS) and ultraviolet photoelectron spectra (UPS), obtained using a Thermo Fisher Scientific Escalab 250, were used to characterize the DOS of Janus-II nanoparticle detector systems.

The work function, ϕ, of a conductive sample can be calculated from a UPS spectrum as follows:

ϕ = hν – (EF – EC) (2)

This measurement characterizes the work function of the conjugated bond with the core of the nanoparticle and the charge displacement effected by the electronegativity or polarity of the ligand end-group. This allows us to compare a number of different end-group configurations, predicting and then ultimately allowing the engineering of the relative effects of electron acceptor/donor levels, capacity for secondary bonding, and close packing ability in nanocrystal photodetectors.

Shown in the UPS data of Figure 12 are valence band profiles of three different Janus-II nanocrystals configured in Silicon–L1–PbS–L2 detector structures, where L1 was 1,2,4,5-benzenetetracarboxylic acid and L2 was chosen to be one of the following ligands: Oleic acid (L2a), 4-sulfophthalic acid (L2b), and 4-aminophthalic acid (L2c). The L2 samples were chosen to exhibit a quantum-confined stark effect (QCSE), which varied in magnitude in proportion to the ligand-induced dipole across the nanocrystal. References including bare silicon (R1), 1,2,4,5-benzenetetracarboxylic acid

Binding Energy (eV)

0 2 4 6 8 10 12 14 16

Nor

mal

ized

Inte

nsity

0.0

0.2

0.4

0.6

0.8

1.0oleic acid

4-sulfophthalic acid

4-aminophthalic acid

Figure 12: UPS measurements showing valence-band DOS for Si–L1– NANOCRYSTAL–L2, with a clear shift as a function of ligand type. The electron donation trend is as follows: 4-aminophthalic acid > 4-sulfophthalic acid > oleic acid.

Figure 13: Bi-functionalized Janus NC showing electron-withdrawing and electron-donating ligands, which align the HOMO and LUMO levels to the TiO2 to promote electron injection.





coated Silicon (R2), and Oleic acid coated Silicon (R3) were obtained to isolate baseline signals from the results.

If net work-function modulation were the dominant effect, the sulfonated (L2b) and amino-terminated (L2c) nanocrystals would both shift in their valence band energies, but in opposite directions. If, however, a dipole effect was present, the shift would occur in the same direction with respect to the more neutral oleic acid reference, since UPS should not distinguish between a dipole oriented towards or away from the substrate. The electron donating amino group would be expected to produce a larger shift than the sulfonate, regardless of the direction of the shift.

The results of the UPS measurements are shown in Figure 14. Clearly, the sulfonated and amino-terminated compounds result in shifts in the same direction, both moving toward a lower-binding-energy state. We may, therefore, conclude that an electric dipole has been applied across the nanocrystal film. The magnitude of the shift is significant. Even without full optimization of the ligand packing density, a shift of almost 1 eV was obtained. Since this is more than twice the bandgap of the bulk PbS semiconductor, the large shift indicates that a high degree of spatial segregation of the electron-rich and hole-rich portions of the excitonic wavefunction was achieved.

In a follow up experiment, several substituted benzoic acid ligands were used to create Janus particles. As in the previous experiment, ligands with electron donating groups, shift the electron binding energy down compared to electron withdrawing groups ro. As is shown in Figure 15, over a 3.5 eV difference was measured between the different ligands used in the experiments. As predicted by the Hammett Parameter analysis, the magnitude of the shift in the DOS followed the trend in the electron donating ability of the ligand [4-(N,N-dimethylamino)phenylboronic acid > p-toluic acid > benzoic acid > p-cyanobenzoic acid > p-nitrobenzoic.]. The data indicates that efficient carrier extraction can be accomplished with ligands possessing electron-donating properties somewhere between p-cyanobenzoic acid and benzoic acid. Substituents that fall within this range include: acetoxy (-O2CCH3), acetyl (-C(O)CH3), bromo (-Br), carbomethoxy (-CO2CH3), chloro (-Cl), fluoro (-F), phenyl (-C6H5), and trifluoromethyl (-CF3).

2.4.2 Janus-II Photodetector Characterization

The PbS nanoparticles synthesized using oleic acid as the capping and size mediating ligand. The molecular modeling Table 2: All samples prepared on bare silicon substrates. Samples 1-3 are references for samples 4-6, which are expected to have dipole moments of differing magnitudes.

Sample First Ligand (L1) Nanocrystal Second Ligand (L2) R1 None None n/a R2 1,2,4,5-benzenetetracarboxylic acid None n/a R3 Oleic acid None n/a L2(a) 1,2,4,5-benzenetetracarboxylic acid PbS Oleic acid (weak electron donating; strong

dielectric) L2(b) 1,2,4,5-benzenetetracarboxylic acid PbS 4-sulfophthalic acid (resonant, moderate

electronegative) L2(c) 1,2,4,5-benzenetetracarboxylic acid PbS 4-aminophthalic acid (resonant, very

electronegative)

Figure 14: (Left) UPS spectra taken of the ligand functionalized nanocrystals. S = 4-(N,N-dimethylamino)phenylboronic acid, T = p-cyanobenzoic acid, U = p-toluic acid, V = benzoic acid, W = p-nitrobenzoic acid, and X = 3-mercaptopropionic acid. (Right) Expanded view of the spectra showing the leading edge of the binding energy for each of the nanocrystal involved in the study.





-1.00 eV

0.00 eV

1.00 eV

2.00 eV

3.00 eV

5.0 eV

Pote

ntia

l vsN

HE (e

V)

EVB

EF

TiO2

VB

CB

ECB

PbS NanocrystalEg= 1.39 eVEg= 1.27 eVEg= 0.96 eVEg= 0.85 eV

HOMO

LUMO

L1

L2

Cathode Figure 15: Band diagram of PbS Janus-II photodetector.

and Hammett analysis indicate that 3-mercaptopropionic acid (MPA) allows better electronic transport than the oleic acid used to synthesize the nanocrystals. This is not surprising given the difference in chain lengths (C3 for the MPA vs. C18 for the oleic acid). For that reason, MPA was used as the linking (L1) ligand. The MPA ligand exchange was accomplished by adding oleic acid capped PbS nanoparticles to a basic solution (pH > 10) of MPA in ammoniacal methanol. The particles were centrifuged out of solution and washed with acetone to remove excess ligand. High initial pH was necessary to remove the proton from the thiol group to insure the nanoparticle bound to the thiol rather than having the MPA’s carboxylic acid group bind to the nanoparticle.

One of the major issues in nanocrystal photodetectors is the ability to achieve high coverages of quantum dots onto the TiO2 mesostructures. In fact, coverages of <50% are frequently reported for these materials.23,24,25 By placing the MPA onto the nanoparticle before reacting the PbS(MPA) nanoparticle with a fresh and clean surface of TiO2, efficient coverage of the TiO2 surface with the PbS(MPA) was accomplished. After soaking the TiO2 surface with fresh MPA capped nanoparticles, the bound PbS particles were converted to Janus-II nanocrystals by treating the electrode with a solution of the conjugating ligand (L2.)

By substituting the hemisphere opposite the MPA with first an electron-donating ligand (4-toluic acid) and then an electron-withdrawing ligand (4-nitrobenzoic acid), we demonstrated that we could achieve 7.7x improvement in Isc and a 5x improvement in photovoltaic efficiency. Next, we demonstrated a series of substituted benzenethiols, with either electron donating or electron withdrawing functional groups, including: p-nitrothiophenol, p-chlorothiophenol, thiophenol, p-methylthiophenol, p-methoxythiophenol, and p-aminothiophenol (chemical structures for each ligand given in Figure 16). The electrochemical response for the detectors fabricated using Janus-II nanoparticles is shown in Figure 16. From this series, it is possible to see that for most of the ligands, the electrochemical response fits the expected curve (where H < methyl ≈ methoxy < amino in terms of electron-donating capability). However, the poor response of the p-aminothiophenol shows that the picture is more complicated than simply choosing an electron-donating group, resonance and inductive effects, as well as other factors, need to be included in the model.

2.5 Harvesting Multiplied Charge Carriers

2.5.1 Multiple Exciton Generation

Multiple exciton generation (MEG) has been observed in bulk semiconductors,26,27,28,29 where it occurs via impact ionization. In this process, a high-energy electron-hole pair created by photon absorption decays toward the band edges by transferring its excess energy through the creation of additional electron–hole pairs. However, the efficiency of MEG in bulk semiconductors is too low to be beneficial for photovoltaics, because the cross section for MEG is low on account of the stringent momentum conservation rule it needs to fulfill,30 and because competing processes such as phonon-assisted decay are very efficient in bulk semiconductors.31 Hopes were recently expressed that impact ionization might be more efficient in semiconductor nanocrystals,30 because momentum conservation rules would be relaxed by the lack of translational symmetry. Furthermore, it was suggested that the competing process of phonon-assisted carrier relaxation might be inhibited in nanocrystals, due to the sparse density of electronic levels. Optical evidence of MEG has been reported in a variety of semiconductor nanocrystals, including PbSe,,32,33,34,35 PbS, 32 PbTe,36 CdSe,32,37 InAs,3839 and Si.40 However, the quantum confined properties of the nanocrystal increase the recombination rate of the electron and





Figure 16: Effects of L2 ligands on photovoltaic performance. More electron-donating ligands result in better I-V.

hole carriers as the number of excitons created in the nanocrystal increases. This makes carrier extraction difficult in working photodetectors.

Different theoretical models have been proposed in the literature to explain the high efficiency of carrier multiplication in semiconductor nanocrystals.32,41 Ellingson et al.42 and Shabaev et al.43 propose a coherent multiexciton model in which absorbed photons instantaneously generate a coherent superposition (oscillations) between the single- and biexciton states that are almost degenerate. The model ignores the effects of the single-/bi-exciton density of states (DOS) by considering only one single exciton and one bi-exciton states coupled through Coulomb interactions. The enhancement of MEG, according to this model, requires a strong Coulomb coupling between single- and bi-exciton states. Schaller et al.32 proposes a second-order perturbation theory, where multi-excitons are directly formed upon light absorption via transitions to virtual single-exciton states. This approach, referred as the Direct Photogeneration Model, predicts two pathways for direct bi-exciton production during the primary photon absorption event. The first pathway, introduced by Schaller, et al. 32, describes resonant bi-exciton generation via virtual single-exciton states. The second pathway, considered by Rupasov and Klimov44, accounts for the non-vanishing Coulomb matrix elements between the exciton vacuum (filled valence band) and biexciton states. This coupling leads to the stabilization of bi-exciton populations through resonant intraband optical transitions. The actual enhancement of QE comes from the increased bi-exciton DOS compared with the single-exciton DOS.

Atomistic modeling by Allan and Delerue41,45 and Franceschetti, An, and Zunger46 presented actual electronic structure calculations of the rates of several exciton-decay processes in nanocrystals. These calculations showed that the impact-ionization mechanisms, which had been previously dismissed as the source of carrier multiplication in nanocrystals47 leads to very fast (sub-picosecond) direct carrier multiplication rates. Luo et al48 used these models to develop a figures of merit for carrier multiplication, which is proportional to the ratio between the biexciton density of states and the single-exciton density of states, restricted to single-exciton and bi-exciton states that are coupled by Coulomb interactions. For a given material, it was also shown that the figure of merit trend is to decrease with decreasing size, suggesting that MEG may actually be suppressed by quantum confinement. In light of this, they conclude that increases in nanocrystal MEG efficiency relative to corresponding bulk materials should be attributed to the suppression of competing relaxation mechanisms, rather than the intrinsic rate of the MEG process.

2.5.2 Multiple Exciton Capture

Multiple exciton capture was demonstrated using PbS nanoparticles formed using a MPA linking (L1) ligand on a planar TiO2 structure. To simplify measurements, single crystal TiO2, surfaces were used. Atomic force microscopy (AFM) was used to measure coverage of the quantum dots after sensitization. Incident current efficiency (IPCE) spectra of a size-series (4.5 ± 0.3 nm, 3.1 ± 0.3 nm, 2.5 ± 0.3 nm) of PbS nanocrystals on the same 001 anatase electrode showed sensitization to the near-IR. The photocurrent response for each smaller quantum dot size showed distinct excitonic behavior at nearly the same photon energies observed in the solution absorbance spectrum suggesting that the band gap energy or size of the quantum dots is not altered upon adsorption to the TiO2 electrode. The larger PbS nanocrystals (9.9 ± 0.8 nm) did not show transferred photocurrent, presumably because the first excited state is now below the TiO2 conduction band.





The APCE values were calculated as a function of excitation energy and the ratio of the excitation energy to the particle band gap (Ehν/Eg) for three sizes of PbS quantum dots.49 The APCE values remained fairly constant up to about 2.8 eV, indicating no increase in the quantum yield despite crossing the threshold of illumination with photon energies twice the band gap for the largest PbS quantum dots (0.96 eV × 2 = 1.92 eV). Illumination with energies above 2.8 eV, corresponding to 2.9 and 3.2 times their band gap, the APCE values increased and exceeded unity, clearly demonstrating multiple exciton collection.

3. CONCLUSION The effects of the resonance, inductance, and tunneling properties of ligands on the optoelectronic properties of nanocrystal nanocrystal photodetectors and photovoltaic devices is evidenced by the XPS and UPS DOS measurements on nanocrystal detector structures and in working photovoltaic devices. Using ligand electron-donating and electron-withdrawing properties predicted by parameters derived using the Hammett Equation parameters, we were able to predict the relative magnitude of the density-of-states shift for a variety of ligand classes. Using these properties, a new nanoparticle, the Janus-II nanocrystal was demonstrated. The polarized ligands of the Janus-II nanoparticle form a degeneracy-splitting dipole, which reduces the overlap integral between excitonic states, breaks degeneracy, reduces the time-evolving overlap integral of the states, and thus reduces the probability of carrier recombination, allowing carrier extraction to take place more efficiently. This is shown to allow increased photodetection efficiencies and to allow the capture of multiple exciton events in working photodetectors.

Using the predictive nature of the Hammett equation, increased charge efficiency was demonstrated in working photovoltaic structures, and the capture of carriers generated in multiple exciton generation events was shown..

REFERENCES

[1] O’Regan, B. and Gra¨tzel, M., [Nature], 335, 737 (1991). [2] Gra¨tzel, M., [Nature], 414, 338 (2001). [3] Hagfeldt, A. and Gratzel, M., [Acc. Chem. Res.], 33, 269 (2000). [4] Bach, U., Lupo, D., Comte, P., Moser, J. E., Weissortel, F., Salbeck, J., Spreitzert, H., and Gra¨tzel, M., [Nature], 395, 544 (1999). [5] Brabec, C. J. and Sariciftci, N. S., [Mater. Today], 3 (2000). [6] Halls, J. J. M., Pickler, K., Friend, R. H., Morati, S. C., and Holmes, A. B., [Nature], 376, 498 (1995). [7] Tennakone, K., Kumara, G. R., Kottegoda, I. R., and Perera, V. S. P., [Chem. Commun.], 15 (1999). [8] Sayama, K., Suguhara, H., and Arakawa, H., [Chem. Mater.], 10, 3825 (1998). [9] Murphy, C. and Coffer, J., [Appl. Spectr.] (2002). [10] Zhang, J. Z., [Acc. Chem. Res.], 30, 10, 423 (1997). [11] Guyot-Sionnest, P., Wehrenberg. B., and Yu, D. J., [Chem. Phys.], 123, 074709 (2005). [12] Du, H., et. al. [Nano Lett.] (2002). [13] Donega, C., et. al. [J. Phys. Chem.] (2003). [14] Alivisatos, A., [Nature] (2005). [15] Zhou, Friesner, Brus: (a) [Nanoletters] (2003). (b) [J. Am. Chem. Soc.] (2003). [16] Li, Y., and El-Sayed, M., [J. Phys. Chem.] (2001). [17] Narayanan, R., and El-Sayed, M., [J. Am. Chem. Soc.] (2003). [18] Carey, F., and Sundberg, R., [Advanced Organic Chemistry], Vol. A., Kluwer (2000). [19] Suganuma, Y., and Dhirani, A.-A., [J. Phys. Chem.], 109, 15391-15396 (2005). [20] Porter, V.J., Mentzel, T., Charpentier, S., Kastner, M.A., and Bawendi, M.G., [Phys. Rev B], 73, 155303 (2006). [21] Blackburn, J. L., Selmarten, D. C., Ellingson, R. J., Jones, M., Micic, O., and Nozik, A. J. [J. Phys. Chem. B], 109, 2625-2631 (2005). [22] Tang, J., Hinds, S., Kelley, S. O., and Sargent, E. H. [Chem. Mater.], 20(22), 6906-6910 (2008). [23] Guijarro, N., Lana-Villarreal, T., Mora-Sero, I., Bisquert, J., and Gomez, R. [J. Phys. Chem. C], 113, 4208 (2009). [24] Mora-Sero, I., Gimenez, S., Moehl, T., Fabregat-Santiago, F., Lana-Villarreal, T., Gomez, R., and Bisquert, J. [Nanotechnology], 19, 424007 (2008). [25] Gimenez, S., Mora-Sero, I., Macor, L., Guijarro, N., Lana-Villarreal, T., Gomez, R., Diguna, L.J., Shen, Q., Toyoda, T., and Bisquert, J., [Nanotechnology], 20, 295204 (2009).





[26] Kolodinski, S., Werner, J. H., Wittchen, T., Queisser, H. J. Appl. Phys. Lett. 1993, 63, 2405. [27] Geist, J., Gardner, J. L., and Wilkinson, F., [J. Phys. Rev. B], 42, 1262 (1990). [28] Vavilov, V. S., [J. Phys. Chem. Solids], 8, 223 (1958). [29] Christensen, O., [J. Appl. Phys.], 47, 689 (1972). [30] Nozik, A., [J. Physica E], 14, 115 (2002). [31] Shah, J., [Solid-State Electron], 21, 43 (1978). [32] Schaller, R. D., Agranovich, V. M., and Klimov, V. I., [Nat. Phys.], 1,189 (2005). [33] Schaller, R. D., and Klimov, V. I., [Phys. Rev. Lett.], 92, 186601 (2004). [34] Schaller, R. D., Petruska, M. A., and Klimov, V. I., [Appl. Phys. Lett.], 87, 253102 (2005). [35] Schaller, R. D., Sykora, M., Pietryga, J. M., and Klimov, V. I., [Nano Lett.], 6, 424, (2006). [36] Murphy, J. E., Beard, M. C., Norman, A. G., Ahrenkiel, S. P., Johnson, J. C., Yu, P., Micic, O. I., Ellingson, R. J., and Noizk, A. J. [J. Am. Chem. Soc.], 128, 3241 (2006). [37] Schaller, R. D., Petruska, M. A., and Klimov, V. I., [Appl. Phys. Lett.], 87, 253102 (2005). [38] Pijpers, J. J. H., Hendry, E., Midler, M. T. W., Fanciulli, R., Savolainen, J., Herek, J. L., Vanmaekelbergh, D., Ruhman, S., Mocatta, D., Oron, D., Aharoni, A., Banin, U., and Bonn, M. J., [Phys. Chem. C], 111, 4146 (2007). [39] Schaller, R. D., Pietryga, J. M., and Klimov, V. I., [Nano Lett.], 7, 3469 (2007). [40] Beard. M. C., Knutsen, K. P., Yu, P., Luther, J. M., Song, Q., Metzger, W. K., Ellingson, R. J., and Nozik, A. J., [Nano Lett.], 7, 2506 (2007). [41] Allan, G., and Delerue, C., [Phys. Rev. B], 73, 205423 (2006). [42] Ellingson, R. J., Beard, M. C., Johnson, J. C., Yu, P., Micic, O. I., Nozik, A. J., Shabaev, A., and Efros, A. L., [Nano Lett.], 5(5), 865-871 (2005). [43] Shabaev, A., Efros, A. L., and Nozik, A. J., [Nano Lett.], 6, 2856 (2006). [44] Ruposov and V. I., Klimov, V. I., [Phys. Rev. B], 76, 125321 (2007). [45] Allan, A., and Delerue, C., [Phys. Rev. B], 77, 125340 (2008). [46] Franceschetti, A., An, J., and Zunger, A., [Solar Cells], , 6(10), 2191-2195 (2006). [47] Beard, M. C., Knutsen, K. P., Yu, P., Luther, J. M., Song, Q., Metzger, W. K., Ellingson, R. J., and Nozik, A. J., [Nano Lett.], 7, 2506 (2007) [48] Luo, J. W., Franceschetti, A., and Zunger, A., “Nanocrystals: Theoretical Screening of Candidate Materials Based on Band-Structure Effects” [Nano Lett.] 8, 3174–3181 (2008). [49] Justin B. Sambur, et al., “Multiple Exciton Collection in a Sensitized Photovoltaic System,” Science 330, 63 (2010).





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