Plasmonic Enhancement for Colloidal Quantum Dot Photovoltaics

Plasmonic enhancement for colloidal quantum dot photovoltaics

Daniel Paz-Soldan

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto

Toronto, Ontario Canada

Abstract

Plasmonic enhancement for colloidal quantum dot photovoltaics

Daniel Paz-Soldan Master of Applied Science Graduate Department of Engineering University of Toronto 2013

Colloidal quantum dots (CQD) are used in the fabrication of efficient, low-cost solar cells synthesized in and deposited from solution. Breakthroughs in CQD materials have led to a record efficiency of 7.0 %. Looking forward, any path toward increasing efficiency must address the trade-off between short charge extraction lengths and long absorption lengths in the near-infrared spectral region. Here we exploit the localized surface plasmon resonance of metal nanoparticles to enhance absorption in CQD films. Finite-difference time-domain analysis directs our choice of plasmonic nanoparticles with minimal parasitic absorption and broadband response in the infrared. We find that gold nanoshells (NS) enhance absorption by up to 100 % at λ = 820 nm by coupling of the plasmonic near-field to the surrounding CQD film. We engineer this enhancement for PbS CQD solar cells and observe a 13 % improvement in short-circuit current and 11 % enhancement in power conversion efficiency.

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Acknowledgments

This work was made possible by the collective effort of an exemplary group of individuals. First, I thank my supervisor, Professor Edward H. Sargent, for his constant guidance and support. He sets the bar high and encourages us to think big. I am grateful for having had the opportunity to work in a world-class research group in such an exciting field. I am grateful to Dr. Susanna Thon for her leadership and guidance through thick and thin. I owe many thanks to Dr. Anna Lee, mentor and friend, who taught me the virtues of the scientific process and whose contributions were invaluable. I also thank Dr. Michael Adachi for his objective insight and practical know-how with FDTD simulations. I extend thanks to Dr. Mingjian Yuan, Dr. Pouya Maraghechi, Andre´ Labelle, and Haopeng Dong for their helpful advice and important work on the project. I particularly thank David Zhitomirsky, Dr. Illan Kramer, and Dr. Rui Li for guiding my research focus from the very beginning. I am grateful to Dr. Larissa Levina and Elenita Palmiano for CQD synthesis and tireless preparation of materials for the lab on a daily basis. I also thank Damir Kopilovic and Remigiusz Wolowiec for designing and building anything, on-demand, and in a timely fashion. I feel privileged to have collaborated with a diverse and talented group of individuals outside of the Sargent group while at the Uni- versity of Toronto and I am thankful for their valuable contributions: Dr. Aftab Ahmed, Dr. Kun Liu, Dr. Alexander Melnikov, Dr. Peter M. Brodersen, Anjan Reijnders, and Luke Sandilands. I also thank Dr. Neil Coombs and Dr. Ilya Gourevich for training and assistance with electron microscopy. I thank Dr. Sjoerd Hoogland, Dr. Armin Fischer, and Dr. Oleksandr Voznyy for in- sightful discussions. For scientific and non-scientific discussions alike, I thank: Dr. Yuan

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Ren, Dr. Jennifer Flexman, Dr. Shokouh Farvid, Dr. Jin Young Kim, Dr. Ghada Koleilat, Dr. Zhijun Ning, Dr. Silvia Masala, Dr. Philipp Stadler, Alex Ip, Kyle Kemp, Graham Carey, Melissa Furukawa, Lisa Rollny, Xinzheng Lan, Brandon Sutherland, Jixian Xu, Chris Wong, Valerio Adinolﬁ, and Jeannie Ing. I thank my parents, Carlos and Patricia, and my brothers, Carlos Jr. and Mario. I am privileged to have been born into a family that stands behind me in everything I do. Finally I thank Jessica Nguyen, whose constant encouragement and support through everything made this work possible.

Daniel Paz-Soldan University of Toronto iv CONTENTS Plasmonic enhancement for colloidal quantum dot photovoltaics

Contents

Abstract ii

Acknowledgments iii

Acronyms vii

List of Tables viii

List of Figures ix

1 Introduction 1 1.1 Colloidal quantum dots ...... 3 1.1.1 Recent advances in colloidal quantum dot solar cells ...... 4 1.1.2 Limitations ...... 5 1.2 Thesis objectives ...... 7

2 Background 9 2.1 Solar cell fundamentals ...... 9 2.1.1 Figures of merit ...... 10 2.1.2 Quantum efﬁciency ...... 12 2.2 Surface plasmons ...... 12 2.2.1 Localized surface plasmons ...... 14 2.2.2 Effect of ligand ...... 14 2.3 State of the art ...... 15

3 Infrared plasmonic nanoparticles 17 3.1 Optical design considerations ...... 17 3.2 Finite-difference time-domain simulations ...... 19 3.3 Engineering of optical resonances ...... 20 3.3.1 Hemisphere-capped nanorods ...... 20 3.3.2 Arrowhead nanorods ...... 21 3.3.3 Nanoshells ...... 23 3.4 Plasmonic particle spacing in CQD ﬁlms ...... 25 3.5 Conclusions ...... 27

4 Integrating MNPs into CQD solar cells 28 4.1 PbS CQD and NS film deposition ...... 28 4.2 Physical and electrical considerations ...... 29 4.2.1 Surface modification and purification ...... 30 4.2.2 Agglomeration ...... 31

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4.3 Nanoshell film deposition ...... 31 4.3.1 Solvent controls ...... 32 4.3.2 MNP film fabrication by spin casting ...... 32 4.3.3 MNP film fabrication by reservoir drop casting ...... 33 4.4 Conclusions ...... 33

5 Characterization of devices 34 5.1 Design of plasmonic CQD solar cells ...... 34 5.2 Performance of CQD solar cells with NR and ARNR ...... 35 5.3 Optical properties of plasmonic CQD ﬁlms ...... 35 5.4 Device performance ...... 37 5.5 Quantum efﬁciency ...... 38 5.6 Conclusions ...... 39

6 Conclusion 40 6.1 Contributions to the ﬁeld ...... 40 6.2 Future work ...... 41

References 43

A Materials and experimental procedures 48 A.1 PbS CQD synthesis and solvent exchange ...... 48 A.2 Photovoltaic device fabrication ...... 48 A.3 Nanorod synthesis ...... 49 A.4 Arrowhead nanorod synthesis ...... 50 A.5 Nanoshell solution preparation ...... 51

B Measurements and simulations 53 B.1 AM 1.5 photovoltaic performance characterization ...... 53 B.2 EQE measurements ...... 53 B.3 Solution absorption ...... 54 B.4 Film absorption ...... 54 B.5 Double pass ﬁlm absorption ...... 54 B.6 FDTD simulations ...... 54

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Acronyms

AM 1.5 Air Mass 1.5 G solar spectrum MNP Metal Nanoparticle

ARNR Arrowhead Nanorod MPA 1-mercaptopropanoic Acid

CQD Colloidal quantum dot MW Molecular weight

EM Electromagnetic MPP Maximum power point

EQE External Quantum Efﬁciency NR Nanorod

NS Nanoshell FF Fill Factor

OPV Organic Photovoltaics FTO Fluorine doped Tin Oxide PbS Lead sulfide ITO Indium doped Tin Oxide PCE Power Conversion Efficiency IQE Internal Quantum Efficiency PV Photovoltaics J − V Current-Voltage Characteristic PVP Polyvinylpyrrolidone JSC Short-Circuit Current

RS Series Resistance LSPR Localized Surface Plasmon Reso- nance RSH Shunt Resistance

MeOH Methanol VOC Open-Circuit Voltage

Daniel Paz-Soldan University of Toronto vii LIST OF TABLES Plasmonic enhancement for colloidal quantum dot photovoltaics

List of Tables

1 Solar cell performance results with embedded NS with and without pre- sonication ...... 31 2 Solar cell performance results with embedded NS for spin casting and drop casting ...... 32 3 Solar cell performance of devices with embedded NRs ...... 35 4 Solar cell performance of devices with embedded ARNRs ...... 36 5 Device results using nanoshells ...... 37

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List of Figures

1.1 Electricity generation by source in the United States, 2009 ...... 1 1.2 Spectral tuning of quantum dots across the broad solar spectrum ...... 4 1.3 The depleted heterojunction architecture for CQD PV ...... 5 1.4 Measured absorption coefficient spectrum of a PbS CQD film ...... 6 2.1 Basic operation of a solar cell ...... 9 2.2 Equivalent circuit of a solar cell ...... 11 2.3 Dispersion curves (ω) for gold and silver ...... 13 2.4 Simple illustration of a surface plasmon on a metal nanoparticle ...... 14 2.5 The effect of a ligand shell on the surface plasmon of a silver nanoparticle . 15 3.1 Extinction and schematic of hemisphere-capped gold nanorods ...... 20 3.2 FDTD simulation of hemisphere-capped gold nanorods ...... 22 3.3 Extinction and schematic of arrowhead-capped gold nanorods ...... 23 3.4 FDTD simulation of arrowhead-capped gold nanorods ...... 24 3.5 Summary of peak scattering and absorption cross sections for NR and ARNR 25 3.6 Experimental and calculated extinction of gold nanoshells ...... 26 4.1 TEM images of nanoshells before and after purification ...... 30 5.1 Schematic representation and cross sectional TEM image of plasmonic nanoshell CQD solar cell ...... 34 5.2 (a) Single pass absorption spectrum of PbS CQD films with and without embedded NS ...... 36 5.3 External quantum efficiency spectra and J-V characterization of representative samples with and without NS embedded ...... 38 5.4 Internal quantum efficiency spectra of representative samples with and without NS embedded ...... 39 A-1 TEM image of typical NR ...... 50 A-2 TEM image of typical ARNR ...... 51 A-1 Double pass absorption spectrum of PbS CQD films with and without embedded NS ...... 55

Daniel Paz-Soldan University of Toronto ix 1 Introduction Plasmonic enhancement for colloidal quantum dot photovoltaics

1 Introduction

Electricity, however produced, is now considered a basic right in the developed world. We demand it and we fight to keep costs low despite the grave implications for the environment. One of the most significant steps we can take toward reducing climate-changing emissions to sustainable levels is to generate electricity cleanly. In fact, more than 40% of U.S. carbon dioxide emissions come from electricity generation alone [1]. Renewables represent only 9% of total generation in the U.S (Figure 1.1). We rely heavily on conventional sources to fulfill electricity demand even as renewable energy technologies become better, cheaper, and more efficient.

Figure 1.1: Electricity generation by source in the United States in 2009. Only 9% of total generation comes from renewable sources; generation from solar is 0.02% [2].

Solar photovoltaics (PV) has the potential to become a major player in the future energy mix. Of the 120,000 terrawatts illuminating the earth every day [3], it is estimated that

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about 85 terrawatts can realistically be harvested, taking into account land use restrictions and conversion efficiency limits [4]. By comparison, total energy demand is on the order of 15 terrawatts today [2]. While solar today meets only a small percentage of the global electricity demand, this is not due to a lack of available input energy. The fact is that the cost of solar energy generation remains prohibitively high. The gross cost of electricity can be summarized in one number: the cost per watt-peak ($/Wp). This is the actual price of generating electricity at peak power output. In a solar context, the cost per watt peak is the total of the upfront cost of the solar module and the balance of systems (BoS) cost which takes into account the installation, operation and maintenance of the PV system. It is widely agreed that once solar energy can be produced at $1/Wp or about 5-6 cents/kWh it will reach grid parity, making it competitive with conventional energy sources. A combination of high module efficiency and low module cost are the key factors in reaching this threshold [5]. One way to reduce the cost of solar energy is to decrease the thickness of the absorbing layer. The reduced module weight would have the dual benefit of much lower input materials costs as well as decreased installation costs. While crystalline silicon-based PV (c-Si) has laid the foundation for highly efficient solar energy production, significant film thicknesses are required to achieve sufficient light absorption and rigid casings are needed for support. On the other hand, solar cells based on organic materials have the advantage of very high absorption coefficients in the visible range and very thin films (10-50 nm) can be completely absorptive. These materials have achieved reasonably high performance at low cost, but the current best technologies are limited to efficiencies of less than 20% due to their large bandgaps [6]. Colloidal quantum dot (CQD) photovoltaics have been proposed as a promising alter- native, offering the potential for both high-efficiency and low-cost. Within this materials system, complete absorption can be achieved with a few hundred nanometers film thick-

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ness and there is the potential to reach the upper limit efficiencies predicted by Shockley and Queisser in 1962 [7,8]. The potential for flexible, lightweight PV modules offers a significant cost advantage for CQD PV. Additionally, the solution processability of this novel material allows for large-scale, low-temperature, low-cost fabrication techniques with roll- to-roll processing.

1.1 Colloidal quantum dots

In semiconductor colloidal quantum dots the electron and hole wavefunctions are conﬁned to less than the Bohr radius. By adjusting the size of the nanocrystals in colloidal synthesis it is possible to increase the energy of the ﬁrst excitonic transition relative to the bulk material [8]. The implication is that the material bandgap is tunable via careful control of the size and shape of the nanocrystals (Figure 1.2). A variety of quantum dot materials

- In(Sb, As), SnS, FeS2, Cd(Se, Te), Pb(S, Se, Te) - have been demonstrated for myriad applications. This thesis focuses on quantum dots of lead sulfide (PbS). PbS CQDs are synthesized in solution from organo-metallic precursors and capped with long-chain organic ligands to provide colloidal stability. The inter-dot distance in solid-state films made of CQDs determines the transport properties. By selecting a ligand with the desired length one can control the conductivity of the material. Thin films are typically deposited by spin-casting, drop-casting, or dip-coating the solution and applying solid-state ligand exchange. The result is a relatively well ordered film of nanocrystals embedded in a matrix of insulating molecular ligands. The electrical transport properties of such quantum dot films have been studied extensively but the precise mechanisms are in the process of being understood [10–13]. CQDs offer many exciting opportunities for application to photovoltaics. Spectral tunability by the quantum size effect means that CQD films can be precisely engineered to

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Figure 1.2: (a) The standardized AM1.5 spectrum of solar irradiance. (b) Spectral tuning of the band gap of quantum dots is achieved by modulating the diameter (φ) in synthesis. Quantum size effects allow for matching of the absorption to the Sun’s broad spectrum. Reprinted by permission from Macmillan Publishers Ltd: Nature Photonics [9], Copyright 2012. optimally absorb across the broad solar spectrum. This allows for tandem or multi-junction solar cells fabricated from a single material system [14, 15].

1.1.1 Recent advances in colloidal quantum dot solar cells

The CQD PV field has advanced remarkably quickly: since the first reports in 2005 with less than 1% photoconversion efficiency [16], the record has risen to 7.0% [17]. This was achieved by advances in both device architecture and materials chemistry. The architecture

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employed is the depleted heterojunction, a p-n heterojunction formed by n-type titanium dioxide (TiO2) and p-type PbS CQD film. This structure takes advantage of excellent carrier extraction at the front side of the cell - compared to the back, ohmically-contacted side - by extending a depletion region through the CQD film [18]. Another important factor to consider is the formation of trap states at the quantum dot surfaces, which can lead to increased carrier recombination rates and ultimately limits solar cell performance. A hybrid organic-inorganic passivation strategy was shown to dramatically reduce trap density in PbS CQD films, leading to the current record performance [17].

Figure 1.3: (a) Depleted Heterojunction architecture with n-type TiO2 and p-type CQD. (b) The spatial band diagram shows the extraction of photo-generated electrons by acceptor TiO2 and holes by ohmic gold. Reprinted with permission from [18]. Copyright (2010) American Chemical Society.

1.1.2 Limitations

Looking forward, there is significant room for improvement in CQD PV. Figure 1.4 shows the absorption spectrum for a PbS quantum dot film with an excitonic band-edge at 1.3 eV or 950 nm. Light in the near infrared (NIR, 700-1,000 nm) range is not fully absorbed at the optimal film thickness for PbS CQD solar cells of 300 nm. Since nearly half of the solar irradiance is in the NIR, there is much to be gained by increasing the absorption of

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CQD ﬁlms in this spectral region. In fact, Henry calculates that at a 1.3 eV bandgap - close to the optimal bandgap for a single-junction cell - there are enough photons available to generate approximately 36 mA/cm2 of current assuming perfect charge collection [19]; the best devices to date generate about 22 mA/cm2. Only 60% of the available photocurrent is being utilized.

Figure 1.4: Measured absorption coefﬁcient spectrum of a PbS CQD ﬁlm, showing relatively weak absorption in the NIR with absorption lengths on the order of 1 micron.

Examination of the external quantum efficiency (EQE) is revealing: the peak EQE at the exciton is only 30%. While the internal quantum efficiency of short-wavelength photons - which are absorbed very close to the acceptor junction - is high, long wavelength light is much less efficiently absorbed. There is a trade-off between absorption and extraction of photogenerated carriers: the absorption length of infrared photons is at least several

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times greater than the optimal ﬁlm thickness for carrier extraction. This is often called the absorption-extraction compromise and is an important limitation in PbS CQD solar cells. Unless quantum efﬁciency in this range can be improved and the infrared bandgap can be fully utilized, the maximum performance for CQD PV will remain inherently limited.

1.2 Thesis objectives

The proposed project is designed to improve photovoltaic conversion efﬁciency of state- of-the-art CQD solar cells using solution-processed colloidal metal nanoparticles (MNPs). The goal of the project is to use plasmonic resonant scattering to enhance NIR absorption in the quantum dot layer, break the absorption-extraction compromise, and build a plasmonic CQD solar cell with enhanced efﬁciency. The objectives of this project can be summarized as follows:

Identify the design considerations for plasmonic enhancement. Optical, electrical, and physical design considerations must be fully explored in order to exploit the plasmonic effect. An integral aspect of this project is to identify the key factors in such a design through simulation and experiment.

Achieve absorption enhancement via plasmon-CQD resonance matching. An advantage of CQDs is the relatively strong absorption at the exciton energy near 1.3 eV for single junctions. This project proposes to overlap the LSPR of the MNPs with the CQD exciton wavelength to signiﬁcantly enhance absorption in the IR - a concept that is tailored for quantum dot photovoltaics.

Realize a fully solution-processed plasmonic CQD solar cell. One of the advantages of CQD PV is the fact that solution processing allows the possibility of large-scale deposition

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on ﬂexible substrates through roll-to-roll techniques. While mechanical techniques such as nanoimprinting, or thermal methods such as annealing, have already been demonstrated for the fabrication of plasmonic nanostructures in PV devices, these methods would detract from the very attractive solution-processed approach. The use of solution-processed MNPs that are easily integrated into the device fabrication process is an approach amenable to improving CQD solar cell performance.

All of the listed objectives are addressed through simulation and experimental analysis. Chapter 2 provides the relevant background and theory. Chapter 3 and 4 describe the design considerations that were explored and optimized. Chapter 5 showcases the ﬁnal design along with complete characterization and analysis. Chapter 6 summarizes the results and provides future directions for work in this ﬁeld.

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2 Background

This chapter presents the necessary background and theory of the concepts covered in the thesis. We begin with a basic discussion of photovoltaic theory and surface plasmons, then move to a literature review of the state of the art in plasmonics for photovoltaics.

2.1 Solar cell fundamentals

Put simply, the function of a photovoltaic cell is to convert electromagnetic energy in the form of light to electrical energy (Figure 2.1a). Light is incident on an absorbing material, with wavelength λ and energy E given by equation 1.

Figure 2.1: (a) Basic operation of a solar cell requires an absorbing layer and efﬁcient extraction of electrons and holes. (b) Solar cell operation is measured by sweeping the voltage across a load and measuring the current. Reprinted by permission from Macmillan Publishers Ltd: Nature [20], Copyright 2012.

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hc E = hν = (1) λ where h is Planck’s constant and c is the speed of light. Absorption of a photon excites an electron to a higher energy state, leaving behind a positive hole. The high energy electron quickly relaxes to the bandedge. It is then collected by a metal contact and travels through an external circuit where it can do work on a load. A transparent conducting oxide is often used at the front of the cell and a reﬂective metal contact at the back to obtain a double pass of light through the absorbing layer.

2.1.1 Figures of merit

Solar cell performance is typically measured on a current density vs voltage (J-V ) plot under simulated solar conditions using the American Society for Testing and Measurement (ASTM) standard G173, Air Mass 1.5 (AM1.5). These conditions correspond to a clear day with sunlight incident upon a sun-facing 37◦-tilted surface with the sun at an angle of 41.81◦ above the horizon. The behaviour of an ideal solar cell diode under illumination is shown in Figure 2.1b and given by Equation 2.

qV J = J exp − 1 − J (2) 0 kT L

where J is the net current density ﬂowing through the diode, J0 is the dark saturation current density, V is the applied voltage across the diode terminals, q is the electron charge, k is Boltzmann’s constant and T is the absolute temperature. JL here is the light generated current density, which shifts the J − V curve into the fourth quadrant. From the diode behaviour it is possible to characterize the most important ﬁgures of merit of a solar cell [21].

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The open-circuit voltage (VOC ) and the short-circuit current density (JSC ) are the in- tercepts of the current and voltage axes, respectively. The maximum power point (MPP ) is the point on the curve at which the J-V product is maximal and is the operating point of the solar cell. The Fill Factor (FF ) is a measure of how close the maximum Jm · Vm product is to the JSC · VOC product and thus hints at the efficiency of charge extraction under forward-biased conditions. FF benefits from a small series resistance (Rs) and a large shunt resistance (Rsh). The power conversion efficiency (η) is the ratio of electrical power out to incident power and is given by Equation 3.

P V J FF η = out = OC SC (3) Pin Pin

We can model solar cell behaviour with an equivalent circuit of discrete components that represent the effect of the different sources and losses (Figure 2.2). The ideal solar cell would be modelled as a current source representing light generated current, JL, in series with a diode. The losses due to resistance are included in series (RS) and in parallel (RSH ).

Figure 2.2: Equivalent circuit of a solar cell where J is the net current ﬂowing through the diode and V is the applied voltage across the terminals [21].

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2.1.2 Quantum efﬁciency

An ideal solar cell absorbs every incident photon and collects every photogenerated carrier. Quantum Efficiency (QE) is a quantitative measure of the ideality of this process at a given photon energy. External Quantum Efficiency (EQE) - or Incident Photon Conversion Efficiency (IPCE) - is the ratio of current density collected to incident photon flux. It is measured at short- circuit conditions under 1 sun illumination and given as a percentage as in Equation 4.

J (λ) J (λ) hc EQE(λ) = SC · 100% = SC · · 100% (4) φIN (λ) PIN (λ) λ

It may be important to separate out the effect of optical losses due to reflection or transmission: the Internal Quantum Efficiency (IQE) - or Absorbed Photon Conversion Ef- ficiency (APCE) - is the ratio of current density collected to absorbed photon flux. The IQE gives a measure of the charge extraction efficiency of a solar cell. IQE can be calculated from EQE and absorption curves as in equation 5.

EQE(λ) IQE(λ) = (5) Abs(λ)

Using EQE and IQE it is possible to spectrally resolve the contributions of each wavelength of the solar spectrum to the overall short-circuit current density. Furthermore, by integrating the EQE curve over the solar spectrum the expected JSC can be accurately calculated.

2.2 Surface plasmons

Surface plasmons are electromagnetic (EM) surface perturbations arising from the resonant oscillation of free electrons in response to an applied ﬁeld. For most bulk metals the resonance condition is met with energies in the ultraviolet range. However, for some noble

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metals (Ag, Au) the surface plasmon resonance (SPR) occurs in the visible range [22]. To first order we can describe the optical properties of noble metals by a free electron gas (plasma) model where an electron sea oscillates against a background of positive ion cores. Lattice potential and electron-electron interactions are ignored in this model. This is an accurate description up to visible frequencies for gold and silver; interband transitions become significant leading to strong absorption at visible frequencies. The response of a given material to an external field at some frequency, ω, is described by its complex dielectric function, (ω) = 1 + i2. The real part 1 relates to the polarizability of the material. The imaginary part 2 relates to the dissipation or absorption of energy in the material [22]. The measured real and imaginary parts of the complex dielectric function are shown in Figure 2.3.

Figure 2.3: Dispersion curves for gold (a) real and (b) imaginary parts; and silver (c) real and (d) imaginary parts [23].

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2.2.1 Localized surface plasmons

Figure 2.4: Surface plasmons are oscillations of free electrons at a metal-dielectric interface in response to an applied electromagnetic ﬁeld. Reprinted with permission from [24]. Copyright (2007) Annual Reviews.

We now distinguish between bulk surface plasmons and localized plasmons on the surfaces of sub-wavelength MNPs. Localized surface plasmons are non-propagating excita- tions of the conduction electrons on the surface of sub-wavelength particles (Figure 2.4). The localized surface plasmon resonance (LSPR) condition originates from the driving force of the applied EM field and the restoring force due to the surface curvature of the particles. The LSPR modes are therefore dependent on the size and shape of the metal nanoparticle, and the optical indices of the particle and the host medium. These plasmons lead to significant field enhancements both inside the particle and in the near-field region around the particle [25].

2.2.2 Effect of ligand

Typical colloidal MNPs synthesized by a bottom-up approach are capped with ligands to passivate their surface and allow dispersibility in the desired solvent. We can view the ligands as a low-index ’shell’ and they have a strong effect on the plasmonic response (Figure 2.5). The ligands introduce a spacer layer which reduces the effect of the optical near ﬁeld and causes a blue shift of the LSPR relative to an uncapped MNP in a high

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index medium. Further, the ﬁeld is effectively conﬁned within the low-index shell which reduces the optical absorption enhancement (gain) in the surrounding medium [26]. Any application of plasmonic nanoparticles requires careful consideration of the ligand shell thickness and the effective medium of the shell.

Figure 2.5: Simulation of absorption enhancement (Gain) spectra in a CQD ﬁlm with 10 nm Ag MNPs capped with ligand shells of different thickneses relative to a bare CQD ﬁlm. The ligand shell is modelled as n = 1.55 which is typical for organic molecules. The effect of the ligand shell is to reduce absorption enhancements and blue shift the LSPR relative to the uncapped MNP. Reprinted with permission from [26]. Copyright (2011) OSA.

2.3 State of the art

The LSPR of gold and silver MNPs have been used as a means to enhance absorption in a variety of thin films. In CQD thin films, Konstantatos et al. have achieved absorption enhancement using metal nanostructures. A 2.4-fold responsivity enhancement near the absorption edge was observed [27]. The electrical effects of gold, silver, and aluminum particles on infrared CQD photodetector sensitivity were investigated. It was shown that the work function of the MNPs is an important factor: gold is ohmic to PbS CQD while silver and aluminum create a Schottky barrier which impedes current flow. For this reason gold was determined to be the best material for plasmonic enhancement [28]. However,

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these nanoparticles were formed by an evaporation and annealing method, which is not amenable to a solution-processing approach. While these strategies which produce nanostructured substrates have shown promise, there is a growing trend toward synthesizing and depositing nanoparticles using wet chemical methods. These methods are particularly useful for dye sensitized and organic solar cells. A recent study by Yang et al. showed near-ﬁeld plasmonic enhancement by Au particles embedded in the recombination layer between the small and large bandgap junctions of a polymer tandem solar cell [29]. Elsewhere, Au nanoparticles were embedded in the titanium dioxide paste of a dye-sensitized solar cell. It was found that a 3 nm silicon dioxide shell needed to be grown on the Au particle to chemically and electrically insulate it from the surrounding medium [30]. Solution processing allows excellent control over particle size and shape and allows the possibility of using anisotropic and non-spherical particles. For example, silver nanoprisms have been proposed as candidate plasmonic enhancers for organic photovoltaics because their SPR frequency is in the NIR range where organic molecules are weak absorbers [31, 32], but enhanced solar cell performace has yet to be achieved. Large silver nanoparticles with small nucleated silver particles on their surface have shown efﬁcient absorption enhancement in amorphous silicon solar cells due to their omni-directional scattering and large scattering-to-absorption ratios [33]. However, typical enhancements with all of these MNPs are in the visible portion of the solar spectrum, and infrared particles have yet to be used for solar cell enhancement. CQD PV is an ideal platform for investigating NIR plasmonic enhancement and for achieving measurable solar cell performance enhancements.

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3 Infrared plasmonic nanoparticles

The aim of the thesis is to use MNPs tuned to the spectral region of low absorption in CQD ﬁlms to enhance ﬁeld intensities and therefore absorption. In this chapter we explore the properties of several plasmonic metal nanoparticles and outline the design principles for application to CQD photovoltaics.

3.1 Optical design considerations

It is desirable to choose metal nanoparticles with a LSPR in the wavelength range where quantum dots absorb less efficiently. Absorption of electromagnetic radiation through a film with thickness, L, at a given wavelength, λ, is characterized by the absorption coefficient, α(λ), through the Beer-Lambert Law (equation 6).

I T = = 10−α(λ)L (6) I0

where I and I0 are the intensity of transmitted and incident light, respectively. The absorption coefficient of a PbS CQD film was measured by spectroscopic ellipsometry and is shown in Figure 1.4. The absorption length, 1/α is the distance at which 1/e or about 63 % of photons have been absorbed. It is clear from this spectrum that several hundred nanometers to a few microns film thickness are required to fully absorb light in the infrared range.

The absorption and scattering cross sections, σabs and σscat of small metal nanoparticles are accurately predicted by a particular solution to Maxwell’s equations in the quasi-static approximation. In this approximation, the size of the particle is much smaller than the

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wavelength of light, so that the phase of the harmonically oscillating EM ﬁeld is approximately constant over the volume of the particles [25]. These are given by [34]:

2π σ = Im[Γ] (7) abs λ

1 2π 4 σ = |Γ|2 (8) scat 6π λ

/ − 1 Γ = 3V m (9) /m + 2 where λ is the wavelength of incident light, V is the volume of the material, Γ is the polarizability and and m are the dielectric constants of the material and the medium, respectively. The SPR occurs at the frequency where the polarizability is maximized.

Re[(ω)] = −2(ω) (10)

The Frohlich¨ Condition, equation 10, predicts the plasmon resonance based on the dipole approximation. The LSPR is thus determined by the complex refractive index of the material, the size and shape of the particle, and the dielectric constant of the surrounding medium. Further, we note that the optical cross section of plasmonic nanoparticles is typically larger than the physical cross section [34]. That is, the interaction volume of photons can be larger than the physical volume of the particle. One of the most important considerations for plasmonic PV is parasitic absorption in the metal nanoparticle. From equations 7 and 8 it is clear that the absorption and scattering cross sections scale with volume V and V 2, respectively. Two competing factors arise here. First, it is important to couple the near ﬁeld to the surrounding medium with maximum efﬁciency. Second, absorption in the nanoparticle is a loss mechanism because this

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absorption will not lead to enhanced photocarrier generation. A simple analysis leads to the hypothesis that scattering must be greater than absorption in order to obtain any beneﬁt from plasmonic enhancement (equation 11).

σ scat > 1 (11) σabs

That is, for each photon incident on the particle the probability that it will be absorbed in the metal nanoparticle must be less than the probability that it will be scattered into the surrounding medium. While there are more complex factors to consider, to first order the criterion in equation 11 must be satisfied to minimize parasitic absorption. Ideally we would like to know a priori which particle size and shape will give maximal scattering and minimal absorption. For large or anisotropic particles, an analytical solution to Maxwell’s equations may not be easily found and electrodynamic (time varying) effects must be taken into account [25]. In these cases the scattering and absorption spectra of the particles in any medium can be accurately predicted by numerical analysis, such as by finite-difference time-domain simulation.

3.2 Finite-difference time-domain simulations

The ﬁnite-difference time-domain (FDTD) method is a numerical analysis technique for solving coupled differential equations on a discretized spatial grid. It is a versatile method for modeling electromagnetic wave interactions in various media for arbitrary geometry. Since solutions are evaluated in the time domain, broadband pulses can be applied. In this thesis, FDTD simulations are used to evaluate the LSPR spectra and the relative scattering- to-absorption cross sections for the candidate metal nanoparticles. Further details are found in Appendix B.6.

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3.3 Engineering of optical resonances

Metal nanoparticles are engineered to have a localized surface plasmon resonance in a useful spectral range. For instance the dipole resonance of spherical gold nanoparticles can be tuned from about 540 nm to 650 nm by increasing the radius from <50nm to about 300 nm. At diameters larger than 150 nm broadband multipole modes arise at lower energies in the near infrared (NIR) spectral range [34]. Very large sphere sizes relative to the quantum dot ﬁlm thickness are needed to obtain good scattering properties. Furthermore it is difﬁcult to achieve NIR absorption enhancement near the CQD exciton with a spherical geometry as typically the LSPR are not trivially extended to the NIR. We must look beyond spherical particles for application to CQD photovoltaics.

3.3.1 Hemisphere-capped nanorods

Figure 3.1: Measured extinction spectrum of as-synthesized hemisphere-capped nanorods. Inset: schematic drawing.

Spectral tuning through the entire NIR range can be achieved by varying the aspect ratio of hemisphere-capped gold nanorods (NRs). The aspect ratio is deﬁned as the ratio of

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the total length from tip-to-tip to the total diameter at the rod’s center. In the case of NRs, as with any elongated particle, two spectrally separated LSPRs arise due to the coherent oscillations of the conduction band electron along each of the particle axes (transverse and longitudinal). Figure 3.2 shows FDTD simulations demonstrating a linear shift of the LSPR with increasing NR length and ﬁxed diameter. The incident radiation was a plane wave polarized along the length of the rod to excite the longitudinal modes. Additionally, the scattering cross section increases with R more quickly than the absorption cross section.

This agrees well with the theory presented in equations 7 and 8, where σscat increases with

2 V while σabs increases with V . The LSPR depends sensitively on the local dielectric environment of the particles. In general, as the medium n increases, the LSPR redshifts. In relatively high index quantum dot ﬁlms (n ∼ 2.5) a redshift of the LSPR of particles embedded in a ﬁlm relative to the LSPR in the native solution can be expected. These trends have been reported in previous studies [35, 36].

3.3.2 Arrowhead nanorods

The shape of the nanorod cap can have a profound effect on the plasmon properties of the particle. Arrowhead nanorods (ARNR) and have been fabricated in-house (Figure 3.3 shows a solution-phase extinction spectrum). It was hypothesized that the irregular pyrami- dal shape at the nanorod caps would lead to dephasing of the propagating plasmons along the longitudinal axis and a concomitant decrease of the absorption cross section, when compared to hemisphere-capped nanorods. An analysis was performed by FDTD simulation and the results are presented in Figure 3.4. The incident radiation was a plane wave polarized along the length of the rod to excite the longitudinal modes. It was found that the ARNR longitudinal resonances occur at considerably longer wavelengths compared to NR of equal length. Furthermore, the

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Figure 3.2: FDTD simulation results for hemisphere-capped gold nanorods. (a) Schematic of simulated NR with r = 20 nm. (b) Electric ﬁeld intensity (E2) proﬁle for L = 80 nm NR at λ = 785 nm. (c-d) Scattering and absorption cross sections increase and the maxima redshift as a function of increasing rod length in a medium of water n = 1.33.

LSPR peak full-width at half-maximum (FWHM) was signiﬁcantly larger than for NR. This property is promising for broadband absorption enhancement for solar cell applications. The peak scattering and absorption cross sections of the particles simulated in this study are summarized in Figure 3.5. It is noted that the maximum scattering cross section of the ARNR is less than that of the NR. Furthermore, while the scattering cross section increases with size as expected, in the case of ARNR the scattering is almost always smaller than the absorption whereas NRs seem to perform better in terms of scattering for (L>50 nm). However, in all cases the scattering and absorption cross sections are, at best, comparable

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Figure 3.3: Measured extinction spectrum of as-synthesized arrowhead nanorods. In- set: schematic drawing. to each other. In fact it has been observed experimentally and verified numerically that the scattering-to-absorption ratio of NRs is low due to the presence of a strong electric field inside the particle, as seen in the electric field intensity profiles. Therefore, while NR and ARNR have strong field interactions and wide spectral tunability, only a small enhancement of the absorption of quantum dot films can be expected when these types of MNPs are employed.

3.3.3 Nanoshells

Broadband visible to NIR localized surface plasmons can be sustained on the surface of a metal nanoshell (NS), a small dielectric core with a thin metallic shell [37]. The origin of the speciﬁc plasmon spectra of these complex nanostructures can be understood by a simple hybridization model. The model considers the plasmon response of a gold nanoshell as the hybridization of a gold sphere plasmon and a dielectric void plasmon in a gold substrate. The result is two distinct modes that come from the symmetric (’bonding’) and

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Figure 3.4: FDTD simulation results for arrowhead gold nanorods. (a) Schematic of simulated ARNR with r = 20 nm, H = 28 nm and W = 56 nm. (b) Electric ﬁeld intensity (E2) proﬁle for L = 80 nm ARNR at λ = 988 nm. (c-d) Scattering and absorption cross sections increase and the maxima redshift as a function of increasing rod length in a medium of water n = 1.33.

anti-symmetric(’anti-bonding’) states due to the interaction of the induced charge density on the inside and outside surface of the gold shell [38]. The two hybridized modes can be expressed as spherical harmonics of order l; for each order l>0 the resonant frequencies of these modes can be expressed as [25]:

" s # ω2 1 a2l+1 ω2 = p 1 ± 1 + 4l(l + 1) (12) l,± 2 2l + 1 b

where a, b are the inner and outer radii of the shell, respectively. For convenience, particles of this geometry will be denoted by (a, b) with a, b in nanometres.

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Figure 3.5: Peak scattering and absorption cross sections for various ARNR (a) and NR (b) geometries.

Figure 3.6 shows the experimental extinction spectrum as well as the calculated scattering and absorption cross section spectra of a solution of gold nanoshells (60, 7.5) dissolved in methanol (n = 1.33). The dipole and quadrupole modes are distinguishable. For NS of this size, the scattering-to-absorption ratio is far superior to that of spheres of similar size, and to that of NRs or ARNR.

3.4 Plasmonic particle spacing in CQD ﬁlms

Thus far, we have examined the properties of individual nanoparticles. However, ensembles of coupled nanoparticles constitute a different system and the optical response differs from that of an isolated nanoparticle. For instance, in a recent report it was found end- to-end assembly of gold nanorods leads to a red shift of the longitudinal LSPR, while side-by-side assembly leads to blue shift of the longitudinal LSPR and red shift of the transverse LSPR [12]. It was shown by FDTD simulation and experimentally via surface- enhanced Raman spectroscopy (SERS) that side-by-side assembly of gold nanorods leads to destructive interference of the radial component of the surface plasmon modes in the

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Figure 3.6: Experimental and calculated extinction, and calculated scattering and absorption spectra of (60, 7.5) nanoshells in a medium of index 1.33. Inset: schematic drawing. cluster, resulting in field cancellation in the gap between the nanorods [39]. Similarly, we expect the plasmon modes of nanoshells to become coupled if the MNPs in our films become too closely spaced and the absorption and scattering properties of the ensemble will change [40]. For application to photovoltaics it might be possible to properly engineer ensembles of coupled particles for enhanced electric field intensity; however, the controlled design of such a system was outside the scope of this thesis. Instead we aim to design a system where the ensemble-average behaviour of the plasmonic nanoparticles is comparable to that of the isolated nanoparticles. The optimal coverage would ensure that the entire plane of CQD

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ﬁlm could interact optically with the nanoparticles while keeping minimal nanoparticle coverage to reduce recombination effects that could negatively impact the electrical properties of the device. We can estimate the minimum inter-particle spacing to ensure that all light interacts with the plasmonic particles from the calculated extinction cross section in Figure 3.6. The geometric (areal) cross section of (60, 7.5) nanoshells is πr2 = 2 × 10−14 m2 and the peak extinction cross section is 1.4 × 10−13 m2. The ratio of the optical cross section is therefore 7 times larger than the geometric cross section at the LSPR, and the ideal areal density of nanoshells is about 14%. This is in reasonably good agreement with previous estimations that 30% surface coverage is needed to scatter almost all incident ir- radiation [41]. In this spacing regime, the ensemble-average behaviour is estimated to be comparable to that of the isolated nanoparticles due to minimal coupling.

3.5 Conclusions

The studies in this chapter help us to understand the important optical properties for applications of plasmonic metal nanoparticles to solar cells. We are able to engineer the plasmon response of these particles by selecting appropriate sizes and shapes. We are also able to use simulation to evaluate the scattering-to-absorption ratio to determine which material would be the least lossy. We now focus on gold nanoshells in future chapters and look to exploit their scattering properties for CQD ﬁlms.

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4 Integrating MNPs into CQD solar cells

In this chapter we explore how best to integrate plasmonic nanoparticles into a CQD solar cell. The overall goal is to increase the absorption cross section of CQD ﬁlms in order to boost device performance. The main challenge to overcome is maintaining the integrity of the thin ﬁlm after plasmonic nanoparticle deposition.

4.1 PbS CQD and NS ﬁlm deposition

We begin with a description of the deposition process of a uniform, dense film of quantum dots. PbS CQD solar cells are fabricated from the solution phase and a film is built up via a layer-by-layer process by spin casting. A single layer comprises CQD spin cast on a substrate with thickness determined by the rotational speed of the spin coater and the concentration of the CQD solution. Next a solution of mercaptopropionic acid (MPA) is used to flood the CQD layer. This allows complete exchange of the long oleic acid with MPA on the surfaces of the dots. The ligand exchange step is essential to creating a conductive, dense film. Finally, a washing step with methanol (MeOH) is used to remove excess ligand and prepare the surface for the subsequent layer. 12 PbS CQD layers were used in this study. A plasmonic PbS CQD solar cell should deviate minimally from this established and facile layer-by-layer process. In order to embed the NS within the CQD film, we chose to deposit the NS layer in between PbS CQD layers, keeping all other processing conditions the same. The main obstacle in this case was ensuring complete coverage of the NS layer with CQD to ensure that the NS were completely embedded. This was important for several

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reasons. The first is that the evaporated top metallic contact is most effective when it directly contacts the PbS CQD film. Second, we wanted to ensure we had a contiguous CQD film to facilitate charge carrier transport around the nanoshells. Lastly we aimed to maximize the coupling of the plasmonic near-field to the CQDs, requiring that the NS be embedded directly in the film. We found that four layers of the above layer-by-layer process were needed to achieve complete coverage. One other factor to consider was the location of the NS along the CQD film thickness. By empirical observation and theoretical considerations we found that it was best to place the NS near the back of the device. We hypothesized that the additional surfaces introduced by embedding NS could lead to enhanced recombination effects which would contribute to reduced open-circuit voltage. The carriers near the TiO2/PbS interface are particularly sensitive to these recombination effects. Additionally, we reasoned that introducing NS near the front of the cell could lead to losses due to Fano interference effects as has been found in previous reports [27, 42]. These consideration led us to deposit the NS after the eighth CQD layer, followed by four CQD layer to embed the NS within the film. Details of the NS deposition method will be described in the present chapter.

4.2 Physical and electrical considerations

A ligand shell of polyvinylpyrrolidone (PVP, MW ' 40,000) was used to maintain colloidal stability of NS in polar solvents; methanol was used for our studies. An excess of PVP was added to allow temporal colloidal stability and avoid aggregation of NS in solution. However, deposition of such a solution was found to lead to considerably reduced device performance due to several factors. First, excess ligand acts as an insulating layer preventing extraction of photogenerated carriers. Local variations in ligand density could lead to non-uniform drying patterns. Furthermore, a dense ligand shell will conﬁne the E

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ﬁeld and limit coupling to the CQDs. On the other hand, removal of ligands leads to undesired agglomerates in solution and in ﬁlm. Also, a thin passivating dielectric layer is useful to avoid recombination of carriers on the gold nanoparticle surface. A tradeoff exists for the optimal surface chemistry of the nanoparticles.

4.2.1 Surface modiﬁcation and puriﬁcation

Surface modification of the nanoshells and purification of excess ligand were achieved by centrifugation of the nanoshell solutions. A fixed volume of nanoshell solution is centrifuged at a set speed and time, causing the capped particles to ’crash’ out of solution. The supernatant containing excess PVP ligand and other impurities is removed. In order to minimize particle agglomeration we would like to avoid removing capping ligands from the particle surfaces. To achieve this, relatively slow centrifugation speeds were used. Sev- eral centrifugation steps were then needed to ensure complete purification. Transmission electron microscopy (TEM) was used to visualize the effect of each centrifugation step (Figure 4.1). A relatively low voltage (70 kV) was used to detect the ligands which appear as a shadow around the nanoshell particles.

Figure 4.1: Representative low-voltage TEM images of nanoshells deposited on a TEM grid: (a) as-synthesized NS, (b) ﬁrst centrifugation step and (c) second centrifugation step. Scale bar for a-c, 2 microns.

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4.2.2 Agglomeration

As mentioned before, bare nanoshells without ligands are prone to agglomeration in solution. We often observed dimers, trimers, and agglomerates of nanoshells, even in the as-synthesized solution. While it is difficult to eliminate these in a nanoscale colloidal system, it is desirable to minimize this type of particle coupling. We expected that the optical and electrical characteristics of multiparticle systems would be altered from that of individual particles. Sonication of the nanoshell solutions in an ultrasonic bath was performed immediately before NS film deposition. It was hypothesized that this applied energy would break the agglomerated particles apart from each other, leading to more uniform film characteristics. Table 1 shows the results of a controlled study where device peformance of plasmonic CQD devices was compared when sonication was applied. The effect is small but the trend shows improved device properties after the ultrasound treatment to the NS solution. Sonication has been used for deagglomerating and dispersing nanoparticles evenly in solution for other applications [43].

Table 1: Solar cell performance results with embedded NS with and without pre- sonication.

Device VOC (V) JSC (mA/cm2) FF (%) PCE (%) Std. Dev. in PCE Nanoshells 0.57 21.6 48.9 5.4 0.3 Nanoshells w. son. 0.58 21.6 51.9 6.0 0.5

4.3 Nanoshell ﬁlm deposition

Here we describe the methods explored for depositing nanoshells in a uniform, dense, uncoupled array on a PbS quantum dot ﬁlm without etching or degrading the underlying surface.

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4.3.1 Solvent controls

It was important to establish that the solvents used for NS deposition would not have a deleterious effect on device performance. Metal nanoparticles are often synthesized in aqueous solution, but it is known that water vapor affects the doping levels of PbS CQD devices as it may act as an electron acceptor. This may have an undesired effect on solar cell performance [44]. Instead we sought to establish a nanoparticle loading method using MeOH as the compatible solvent. This solvent was chosen because the layer-by-layer process employs MeOH in the washing step already, and the low boiling point (65◦C) allows for fast evaporation. A set of solar cells were fabricated to test the effect of the added solvent layer. The results of these solvent controls are shown in Table 2. Spin casting has a negligible effect on device performance, and drop casting has a small negative effect on device performance, although the results are within one standard deviation of each other.

Table 2: Results for solvent controls where methanol was used to determine the effect on device performance.

VOC (V) JSC(mA/cm2) FF (%) PCE (%) Std. Dev. in PCE Control 0.59 20.9 48.6 6.2 0.3 Slow spin 0.60 21.2 48.4 6.3 0.5 Drop cast 0.60 21.8 47.1 6.0 0.4

4.3.2 MNP ﬁlm fabrication by spin casting

From the solvent controls it was evident that spin casting of MeOH did not deteriorate the quality of the CQD ﬁlm. However, it was determined that spin casting would be very wasteful as most of the solution does not stick to the surface. We tested slow spin casting, but found that this led to non-uniform deposition. For these reasons we established that spin casting was not an optimal deposition method.

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4.3.3 MNP ﬁlm fabrication by reservoir drop casting

Drop casting of the plasmonic nanoparticle solution results in minimal waste and uniform films. Our method of NS deposition leads to consistently repeatable film quality over a large area (∼3 cm2). We employ a circular reservoir and deposit a controlled-volume solution with fixed NS concentration. To minimize exposure of the CQD surface to the MeOH solution, solvent evaporation was performed in low vacuum (∼10−3 Torr). By controlling the rate of evaporation we were able to obtain consistent and uniform films.

4.4 Conclusions

In summary, the optimal conditions for nanoshell deposition on a colloidal quantum dot surface were found. Careful control of the NS surface chemistry was achieved to produce well-dispersed, puriﬁed, colloidally stable solutions with minimal aggregation. A reservoir drop-casting method was found to obtain consistent uniform, dense, largely uncoupled arrays of nanoshells on a quantum dot surface. With these methods in place, we are now able to fabricate PbS CQD solar cells with integrated plasmonic nanoparticles.

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5 Characterization of devices

In this chapter we perform full optical and electrical characterization of plasmonic CQD solar cells and compare to a relevant set of controls. From these results we elucidate the mechanism of improved performance in the enhanced devices.

5.1 Design of plasmonic CQD solar cells

Figure 5.1: Cross sectional TEM image after focused ion beam (FIB) milling and schematic representation of a plasmonic CQD solar cell, showing spectral matching of the NS plasmon with the PbS quantum dot absorption spectrum. Scale bar, 100 nm.

We sought to signiﬁcantly enhance the absorption cross section of our PbS quantum dot ﬁlms and thus improve the performance of our best solar cells. To achieve this goal, we employed the depleted heterojunction stack introduced in chapter 2 as the platform for our plasmonically enhanced devices. We integrated plasmonic nanoshells directly into the

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active CQD layer, allowing for efficient coupling of the broadband plasmonic near and far field into the PbS film. The design of our solar cell is shown in Figure 5.1. The nanoshells are embedded into the CQD film. The total PbS film thickness is 400 nm and the NS are located 200 nm above the TiO2 electrode. Using the design considerations from chapters 3 and 4 we optimized these devices to achieve performance exceeding that of a bare control.

5.2 Performance of CQD solar cells with NR and ARNR

From considerations of the scattering-to-absorption ratios in nanorods, one would expect no benefit to solar cell performance using embedded gold NR and ARNR. We verified this experimentally by applying our best processing to make plasmonic nanorod devices as in Tables 3 and 4. From the figures of merit of 3 devices of each control and plasmonic samples, we observed no significant enhancement in any figure of merit for these device sets.

Table 3: Average performance of hemisphere-capped nanorod devices.

Device VOC (V) JSC (mA/cm2) FF (%) PCE (%) Control sample 1 0.60 20.4 50.3 6.0 Control sample 2 0.57 20.2 51.0 5.9 Control sample 3 0.58 21.5 44.4 5.6 NR sample 1 0.59 21.1 43.8 5.3 NR sample 2 0.57 21.5 44.1 5.5 NR sample 3 0.59 20.3 48.4 5.8

5.3 Optical properties of plasmonic CQD ﬁlms

The absorption spectra in a single pass through our thin ﬁlms were measured using integrating sphere spectrophotometry. The spectra of two representative samples with and without nanoshells are shown in Figure 5.2. We observe a broadband absorption enhance-

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Table 4: Average performance of arrowhead capped nanorod devices.

Device VOC (V) JSC (mA/cm2) FF (%) PCE (%) Control sample 1 0.56 20.4 49.3 5.8 Control sample 2 0.58 20.9 51.7 6.3 Control sample 3 0.58 20.5 52.9 6.4 ARNR sample 1 0.59 21.1 50.7 6.5 ARNR sample 2 0.55 20.4 47.4 5.3 ARNR sample 3 0.58 20.4 49.8 6.0 ment of as much as 100 % centred at the plasmonic LSPR near 820 nm. By subtracting the absorption curves we can approximately account for the absorption enhancement due primarily to the plasmonic inclusions (Figure 5.2b). The resonance is red-shifted relative to that measured in solution due to the higher index of the surrounding medium. If we consider the high scattering-to-absorption ratio of nanoshells, we may expect the measured absorption enhancement to mostly originate from absorption in the quantum dot ﬁlm and not from parasitic absorption in the plasmonic nanoparticles. Using the integrating sphere method, however, it is not trivial to decouple the relative contributions from these effects.

Figure 5.2: Single pass absorption spectrum of PbS CQD ﬁlms with and without NS embedded as measured by integrating sphere UV-Vis-NIR absorption. (b) The absorption enhancement due to the NS is shown by the subtraction of the two absorption curves.

It is important to note here that single-pass absorption does not fully represent the sys-

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tem employed for the plasmonic CQD solar cells. In a working device, the top contact is used as a fully reflective mirror and therefore we obtain a double pass through our devices. However, the absorption enhancement due to the plasmonic effect in this case is convolved with Fabry-Perot resonances and the specific contribution from plasmonic effects is difficult to interpret. The double-pass absorption spectra of two representative samples are shown in Appendix B, Figure A-1.

5.4 Device performance

The performance of nanoshell devices was evaluated by the current-density curves measured under simulated solar illumination. The results from our best control and nanoshell devices are shown in Figure 5.3a, demonstrating a consistent and signiﬁcant trend of enhancement relative to the control. For the best control and nanoshell devices there was

an enhancement of 3.6 % in VOC , 13.2 % in JSC , -3.9 % in FF (degradation) and 11.4 % in PCE. These results show that the enhancement in performance is primarily due to

enhanced JSC , while there is no statistically signiﬁcant enhancement or degradation of the

other figures of merit, VOC and FF . This trend indicates that we were able to maintain the fidelity of our thin films after NS integration and simultaneously enhance the density of photogenerated carriers by enhancing the CQD film absorption.

Table 5: Device results using nanoshells.

Device VOC (V) JSC (mA/cm2) FF (%) PCE (%) Control sample 1 0.56 20.5 53.3 6.1 Control sample 2 0.56 21.6 51.6 6.2 Control sample 3 0.54 21.2 53.5 6.2 Nanoshell sample 1 0.56 23.8 52.4 6.9 Nanoshell sample 2 0.56 24.0 50.9 6.8 Nanoshell sample 3 0.58 24.5 49.6 6.9

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Figure 5.3: (a) Device performance of champion plasmonic device and control characterized by current-voltage characteristic under AM 1.5 simulated solar illumination showing enhanced performance with embedded NS. (b) External quantum efﬁciency spectra of representative samples with and without embedded NS.

5.5 Quantum efﬁciency

We employ external and internal quantum efficiency measurements to analyze the enhancement in photocurrent in plasmonic CQD solar cells. The results of EQE measurements of two representative samples are shown in Figure 5.3b. We observe a strong correlation between the enhanced absorption in Figure 5.2 and the enhanced quantum efficiency in Figure 5.3b. In fact the peak EQE enhancement ∼35 % occurs at a wavelength near 880 nm, very close to the wavelength of peak absorption enhancement. This is a strong indication of resonant enhancement due to the effect of the plasmonic nanoshells. Further, we show IQE spectra of representative samples (Figure 5.4) and observe that the IQE is relatively lower in the NS case. IQE takes into account the effect of optical losses on the external quantum efficiency, allowing one to compare purely the charge extraction properties of the device. From this measurement we see that charge collection was slightly deteriorated after integrating NS, probably due to a small amount of parasitic absorption in the NS. Despite this, we observed enhanced current and performance. We can therefore conclude that there was an overall optical enhancement in the CQD film.

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Figure 5.4: Internal quantum efﬁciency spectra of representative samples with and without NS embedded.

5.6 Conclusions

A successful demonstration of the plasmonic effect for CQD photovoltaics was presented. Complete device characterization shows that the enhancement observed was indeed due to a plasmonic effect. We have conﬁrmed our hypothesis that solar cell performance can be improved by increasing the absorption of CQD ﬁlms using solution-processed metal nanoparticles.

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6 Conclusion

The field of colloidal quantum dot photovoltaics has advanced rapidly, recently reaching photoconversion efficiencies of 7 % [17]. The inherent cost advantages presented by the solution processability of this unique solar material will one day allow CQD photovoltaics to compete directly with legacy technologies. However, further improvements in device efficiency are needed to reach this goal. The approach to performance enhancement adopted in this thesis was to address the absorption-extraction compromise; that is, at the optimal device thickness for charge collection, absorption of near infrared light is largely incom- plete. We employ plasmonic enhancements from solution-processed metal nanoparticles embedded in the CQD film, coupling the near- and far-field scattered light directly to the absorbing layer. Simulation and theory directed the choice of gold nanoshells with broadband, infrared LSPR and maximal scattering-to-absorption ratio. Careful manipulation of the surface chemistry and proper loading of the nanoshells on the quantum dot surface was key to maintaining efficient charge collection. Finally, these design principles were applied to PbS CQD solar cells, leading to enhanced absorption and ultimately improved device efficiency.

6.1 Contributions to the ﬁeld

This thesis encompasses several major contributions to the ﬁelds of plasmonics and photovoltaics:

1. Design criteria for plasmonic enhancement using solution-cast metal nanoparticles were presented. FDTD simulations and theory were employed to characterize a va-

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riety of particles with tunable plasmonic response in the NIR spectral range. We found that nanorods, while they are easily tunable, are far too lossy and did not lead to enhanced photovoltaic performance. Further, our results were used to qualify gold nanoshells as an optimal material for application to solar cells with appropriately- tuned LSPR and a superior scattering-to-absorption ratio.

2. A consistent and repeatable method for preparing and depositing metal nanoparticles which maintained efﬁcient charge collection through the CQD layer was outlined. This manipulation at the nanoscale was an essential step to exploiting the plasmonic effect.

3. To the author’s best knowledge, this thesis presents the world’s ﬁrst plasmonic colloidal quantum dot solar cell. While the principles outlined here are applied only to CQD photovoltaics, we note that this work is relevant to solution-processed plasmonics for all photovoltaic technologies as well as other opto-electronic device applications.

6.2 Future work

Suggestions for future research directions follow.

1. This thesis introduced the topic of tuning plasmonic resonances to quantum dot ﬁlm absorption curves. It would seem beneﬁcial to employ a wider dispersity of plasmonic nanoparticle shapes or sizes. In theory this would lead to, in effect, a more broadband plasmonic enhancement and a further improvement in performance. This strategy would require careful consideration of the scattering-to-absorption ratios for the different particles.

2. In the ﬁeld of plasmonic enhancement for solar cell applications, future research

Daniel Paz-Soldan University of Toronto Page 41 of 55 6 Conclusion Plasmonic enhancement for colloidal quantum dot photovoltaics

should focus on controlled, periodic self-assembly of metal nanoparticles. This would open the possibility of exploiting the surface plasmon polaritons which arise from the long range order in such a system. We expect this could lead to more sig- niﬁcant absorption and performance enhancements.

3. In the field of CQD photovoltaics, transport in PbS films should be improved through further improvements in quantum dot surface passivation. It is noted that the absorption- extraction compromise could also be addressed from the other direction - if charge collection could be efficient at 1 micron film thickness, a planar architecture would be the most simple and efficient.

Daniel Paz-Soldan University of Toronto Page 42 of 55 REFERENCES Plasmonic enhancement for colloidal quantum dot photovoltaics

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Daniel Paz-Soldan University of Toronto Page 47 of 55 A Materials and experimental procedures Plasmonic enhancement for colloidal quantum dot photovoltaics

A Materials and experimental procedures

A.1 PbS CQD synthesis and solvent exchange

PbS quantum dots were synthesized according to a previously published method [45]. A solution-phase metal halide (Cd) treatment was applied. For metal halide treatment, 1.0 ml metal halide precursor was introduced into the reaction flask after sulphur source injec- tion during the slow cooling process. A 6:1 Pb:Cd molar ratio was maintained during the synthesis. When the reaction temperature reached 3035◦C, the PbS CQD were isolated by the addition of 60 ml of acetone then centrifugation. The nanocrystals were then purified by dispersion in toluene and reprecipitation with acetone/methanol (1:1 volume ratio), then re-dissolved in anhydrous toluene. The PbS CQDs in toluene were transferred to inert atmosphere N2-glovebox and precipitated by centrifugation with methanol (1:1 volume ratio). The supernatant was removed and the PbS were dried under low vacuum. Then the dried quantum dots were redispersed in toluene at about 100 mg/mL. This washing process was repeated twice more with the final redispersion in octane at 50 mg/mL. The final exchanged solutions were stored in sealed vials in inert atmosphere before use.

A.2 Photovoltaic device fabrication

PbS CQD PV devices were fabricated on FTO-coated glass substrates (25.0 ± 0.1 mm × 25.0 ± 0.1 mm, Pilkington, TEC 15). Before spin-coating, the FTO substrates were sonicated in a mixture of Triton in de-ionized (DI )water (2% by volume) and then in isopropanol and DI water sequentially. A ZnO solution (Alfa AesarNanoshield ZN-2000,

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50% in H2O colloidal dispersion) was diluted to 20% in DI H2O with resistivity greated than 13 MΩ·cm. The solution was sonicated for 2 hours before use and vortexed. FTO substrates were coated with 300 µL by spin casting (speed = 2,000 r.p.m. , time = 20 sec, accel = 1.0 sec, deaccel = 1.0 sec), and pre-annealed at 150 ◦C for 30 minutes.

A titanium tetrachloride (TiCl4) treatment was then applied. TiCl4 was diluted to a 60 mM solution in DI water. The TiO2 films were immersed in this solution and placed in a 70 C oven for 30 min in air ambient. The substrates were then removed, rinsed with de-ionized water, and fired at 520◦C in a furnace for 1 h in air ambient. A layer-by-layer spin coating process was used for all PV CQD film deposition in this thesis. Under an ambient atmosphere, 2 drops of PbS CQD through a 0.22 µm filter were dropped on the ZnO/TiO2 substrate, and spin cast at 2,500 r.p.m. A solid-state ligand exchange with MPA was done by flooding the surface for 3 sec, then spin coated dry at 2,500 r.p.m. Finally two washes with MeOH were used to remove unbound ligands. Each device consisted of 12 such layers. Top electrodes were deposited using an Angstrom Engineering A˚ mod deposition system in an Innovative Technology glovebox. The contacts consisted of 10 nm thermally evaporated molybdenum trioxide deposited at a rate of 0.2 A/s,˚ followed by electron-beam deposition of 50 nm of Au deposited at 0.4 A/s,˚ and finally 120 nm of thermally evaporated silver deposited at 1.0 A/s.˚

A.3 Nanorod synthesis

The seed-mediated growth method developed by Nikoobakht and El-Sayed was adapted for gold nanorod (NR) synthesis in-house [46]. For the seed solution, small gold nanoparticles were obtained by the reduciton of HAuCl4 (0.12 mL, 15mM) dissolved in a cetyltrimethy- lammonium bromide (CTAB) (2.5mL, 0.20M), with cold sodium borohydride (NaBH4) in

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1.0 mL DI H2O, in an ice-cold water bath. For the growth solution, CTAB (5.0 mL, 0.20M)

was mixed with an aqueous solution of HAuCl4 (0.50 mL, 15 mM), AgNO3 (280 µL, 4.0 mM) and 4.0 mL of DI water. Ascorbic acid (0.12 mL, 0.788 M) was added dropwise to the solution. The seed solution was aged for 5 min and was added to the growth solution, thus initiating the NR growth. The color of the solution mixture changed from clear to deep purple after incubation for 10 h at 27 ◦C. The mixture was centrifuged at 8,500 r.p.m. for 30 min (Eppendorf centrifuge 5430), and the supernatant was removed and precipitated NRs were re-dispersed in DI water. This step was then repeated once. A 1.0 mL solution of polyvinylpyrrolidone (PVP) (100 mg/mL, MW 5000) was added to the NR solution. The solution was centrifuged at 10,000 r.p.m. for 20 min and re- dispersed in methanol. This centrifugation and re-dispersion was repeated 3 times.

Figure A-1: TEM image of typical NR. Scale bar, 150 nm.

A.4 Arrowhead nanorod synthesis

Gold nanorods were synthesized as described above. The arrowhead NRs (ARNRs) were synthesized following the procedure developed by Xiang et al [47]. A 1.5 mL volume of

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NRs solution was mixed with CTAB (1.0 mL, 0.20M), HAuCl4 (32 µL, 15 mM), AgNO3 (120 µL, 4.0 mM), and ascorbic acid (60µL, 0.788M). The mixture was diluted to a volume

◦ of 6.0 mL with DI H2O and ste in a water bath at 30 C for 12 hours. The solution was centrifuged (20 min at 6,500 r.p.m.) and the supernatant removed. This was repeated three to ﬁve times. A typical TEM image of the ARNR is shown in Figure A-2. Exchange to PVP-capped ARNR dispersed in methanol was carried out in the same way as for NR (above).

Figure A-2: TEM image of typical ARNR. Scale bar, 100 nm.

A.5 Nanoshell solution preparation

Gold nanoshells were purchased from nanoComposix, Inc., and delivered as a dried redis- persible powder capped with PVP (MW 40,000) [48]. The quoted silica core diamater as

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measured by TEM was 119.2 +/− 4.7 nm and shell thickness was measured as 14.3 nm. The hydrodynamic diameter was quoted as 168.1 nm (Malvern Zetasizer Nano ZS). The dried powder was re-dispersed in MeOH at a concentration of 30 mg/mL. This solution was centrifuged at 1,000 r.p.m. for 15 min and re-dispersed twice. The NS solution was used immediately after centrifugation to minimize the time for possible agglomeration and precipitation. Solutions were used within 2-3 h for PV device fabrication. Sonication for 40 sec in an ultrasonic bath was done immediately before NS deposition.

Daniel Paz-Soldan University of Toronto Page 52 of 55 B Measurements and simulations Plasmonic enhancement for colloidal quantum dot photovoltaics

B Measurements and simulations

B.1 AM 1.5 photovoltaic performance characterization

All PV and EQE device measurements were done under inert N2-ﬂow. Current-voltage measurements were done using a Keithley 2400 source meter. A solar simulator (Sci- encetech, Class A, intensity = 100 mW/cm2). The source intensity was measured with a Melles-Griot broadband power meter through a circular 0.049 cm2 aperture. The spectral mismatch factor between the measured and actual solar spectrum was calculated to be approximately 10%, thus a correction factor of 0.90 was applied to all current J measurements. The uncertainty in the AM 1.5 characterization was estimated to be ± 7%.

B.2 EQE measurements

External quantum efficiency measurements were obtained by applying chopped (220 kHz) monochromatic illumination (400 W xenon lamp through a monochromator with order- sorting filters) collimated and co-focused with a 1-sun intensity illumination source on the device of interest. The power was measured with calibratd Newport 818-UV and Newport 818-IR power meters. The response from the chopped signal was mesaured using a Stan- ford Research Systems lock-in amplifier at short-circuit conditions. The uncertainty in the EQE measurements was estimated to be ± 8%.

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B.3 Solution absorption

A Varian CARY 500 UV-Vis-NIR spectrophotometer was used for all absorption measurements. For solution absorption a diluted solution of interest was loaded into a quartz cuvette. The baseline 100% transmission curve was the cuvette with pure solvent and 0% transmission curve was taken with the source blocked by a metal plate.

B.4 Film absorption

For film absorption measurements, an integrating sphere is used. Substrates used were ITO which had minimal absorption (<5% over all wavelength of interest). The baseline 100% transmission curve was taken as the substrate of interest, at a small angle (approx. 25◦) to the input source to collect the specular reflection. Samples were prepared on clean ITO by spin-coating a layer-by-layer film.

B.5 Double pass ﬁlm absorption

Double pass film absorption was measured by depositing a MoO3/Au/Ag reflector on the CQD film, as in device fabrication. The baseline 100% curve in this case was taken as 2× the single pass absorption of a bare ITO substrate. The double pass absorption measurement is shown in Figure A-1.

B.6 FDTD simulations

Finite-difference time-domain (FDTD) simulations were carried out using software pack- age Lumerical FDTD solutions version 7 (http://www.lumerical.com). Scattering and absorption cross sections were determined following the Mie scattering method in [49]. A total-ﬁeld scattered-ﬁeld (TFSF) plane wave source surrounds the particle of interest. A

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Figure A-1: Double pass absorption spectrum of PbS CQD films with and without NS embedded as measured by integrating sphere UV-Vis-NIR absorption. broadband (λ = 400 - 1200 nm) source, polarized along the cylinder axis, was injected. The region is surrounded by Perfectly Matched Layers (PML) which absorb all most incident radiation over a wide range of angles. Two analysis groups, one inside the source boundary (measuring total field) and one outside the source boundary (measuring scattered field) calculate the optical cross sections.

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