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Massachusetts Institute of Technology

Department of Electrical Engineering and Computer Science

Proposal for Thesis Research in Partial Fulfillment

of the Requirements for the Degree of

Doctor of Philosophy

Title: Performance Limits of Quantum Dot and Emerging Thin-Film Solar Cells

Submitted by: Joel Jean 102 6th St. Cambridge, MA 02141

Signature of Author: __________________________

Date of Submission: August 1, 2016

Expected Date of Completion: June 2017

Problem Statement:

Emerging thin-film solar photovoltaics (PV) can be deployed at multi-terawatt scale at low cost to help mitigate anthropogenic climate change. This work will investigate charge extraction and performance limits in colloidal quantum dot (QD) and other emerging thin-film PV technologies. We propose to develop strategies to engineer charge extraction in QDPV, including an inverse method to map spatial variations in extraction efficiency in an operating cell and a nanowire-based device architecture to improve extraction. We further propose to use photothermal deflection spectroscopy (PDS) to characterize sub-bandgap states and band tailing in PbS QD solids, and to evaluate the ultimate detailed-balance efficiency limit for QD solar cells.

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Table of Contents

Summary of Proposed Thesis 3 Motivation and Background 4

Emerging thin-film photovoltaics 4 Colloidal quantum dot solar cells 5

Preliminary Work and Related Contributions 6 Long-term potential of solar PV and R&D directions for emerging thin-film PV 6 Charge extraction in PbS QD solar cells 7 Open-circuit voltage and efficiency limits for QD solar cells 7

Proposed Work 8 Proposed Project 1: Development of an inverse method to map charge extraction efficiency in thin-film solar cells 8 Proposed Project 2: Characterization of band tailing and ultimate efficiency limits of QDPV using photothermal deflection spectroscopy (PDS) 9

Timeline and Milestones 11

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Summary of Proposed Thesis

The proposed thesis consists of two main parts:

Part 1: Evaluating the long-term potential of emerging thin-film photovoltaic technologies Part 2: Elucidating charge extraction and performance limits in colloidal quantum dot solar cells

In Part 1, we assess the current status and future prospects of solar photovoltaics, focusing on emerging thin-film technologies. We first examine historical PV module and system cost trends—dominated by crystalline silicon technologies—and calculate the levelized cost of solar PV-generated electricity today. Location-specific PV generation costs are used in an online tool (CoalMap) that evaluates the current and future cost-competitiveness of solar against wind and legacy thermal coal generation in the United States.

Turning back to PV cell technologies, we review the landscape of solar PV technologies and introduce a framework for classifying PV technologies based on absorber material complexity. We analyze the scalability of each technology based on materials availability and current global production rates. Finally, we discuss high-value performance metrics for emerging thin-film PV, including weight-specific power and flexibility. As a proof of concept, we demonstrate an in situ PV fabrication process leading to record-thin, lightweight, and flexible organic solar cells.

In Part 2, we investigate two outstanding technological challenges—inefficient charge extraction and uncertain ultimate performance limits—for thin-film solar cells based on colloidal quantum dots (QDs), which use abundant materials, can be processed simply, and are compatible with lightweight, flexible substrates.

Inefficient charge transport is a major barrier to high efficiency in QD solar cells. Like most emerging thin-film PV technologies, QD solar cells trade off light absorption and charge extraction. Inefficient extraction limits absorber layer thicknesses, leading to incomplete absorption. To decouple absorption from extraction in QD solar cells, we introduce an ordered bulk heterojunction device architecture based on ZnO nanowires and PbS QDs. We further propose to develop a method for inferring the spatial profile of charge extraction efficiency in any thin-film solar cell, using basic film and device measurements. This technique could help inform strategies to further enhance charge extraction in QDPV.

Even if charge transport and extraction could be improved dramatically, however, it is unclear whether QD solar cells can ever achieve competitive efficiencies approaching the Shockley-Queisser limit. Today’s QDPV devices exhibit low open-circuit voltages (VOCs) compared to their nominal bandgaps. Low voltages signify an inability to separate quasi-Fermi levels adequately, due to excess recombination or the presence of sub-bandgap states. Here we propose to use photothermal deflection spectroscopy—a highly sensitive pump-probe technique for measuring UV-vis-NIR absorption—to characterize sub-bandgap absorption and band tailing in PbS QD films under different processing conditions. This study could uphold or diminish the future potential of QD solar cells and help identify directions for further investigation.

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Motivation and Background

Emerging thin-film photovoltaics Solar photovoltaics are the fastest-growing energy technology today and a leading candidate for carbon-free electricity generation at the terawatt scale. Global PV deployment is dominated by crystalline silicon wafer-based technologies, which benefit from high power conversion efficiencies, abundant materials, and proven manufacturability. Rapid declines in PV electricity costs approaching wholesale grid parity have been observed in many locations. However, maintaining cost-competitiveness in a future of high solar penetration will require deep reductions in PV module, auxiliary hardware, deployment, and integration costs. Ubiquitous deployment of solar PV will likely require lightweight and flexible module designs that are inaccessible with today's commercial technologies.

Emerging thin-film solar cell technologies provide new functionality today and could reshape the solar landscape tomorrow. From metal-halide perovskites to molecular organic materials and colloidal quantum dots, emerging nanomaterials are structurally complex but relatively simple to process (Figure 1). They open the door to new formats for deploying solar power. Low-temperature processing allows lightweight, thin substrates to be used, leading to high power-to-weight ratios (>100 W/kg) and flexible cells that are easy to transport, install, and stow. Flexible, monolithically integrated modules could be cheaply manufacturable and extremely durable, with no wafers to break or solder joints to fail. In the long term, emerging thin-film PV technologies may overcome many of the limitations of today's deployed technologies at low cost, assuming improvements in efficiency and stability are realized.

Figure 1: Material complexity framework for classifying PV technologies. Emerging thin-film solar cells can be manufactured and deployed at low cost, with form factors that are unachievable with deployed wafer and thin-film technologies.

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Colloidal quantum dot solar cells Colloidal quantum dot photovoltaic (QDPV) technologies offer several unique advantages over existing technologies. QD absorber bandgaps can be tuned through quantum size effects, which could enable multijunction devices within a single material system. QD solar cells use abundant and high-volume-produced materials (lead and sulfur). They are compatible with lightweight and flexible substrates using simple, low-cost, and low-temperature processing (Figure 2).

Several outstanding scientific and technological challenges, however, could limit the long-term potential of QDPV. The most efficient and stable QD solar cells today use toxic lead-based absorbers, which raises environmental health and safety concerns. Development of fast deposition methods for QD films has been limited, and slow layer-by-layer spin-coating remains the leading processing technique. Perhaps most importantly, the most efficient QD solar cells today (11.3% as of July 2016) exhibit low open-circuit voltages and are far less efficient than other thin-film technologies (e.g., CdTe, CIGS, and perovskites). It is unclear whether QDPV can ever approach the theoretical detailed-balance limit, largely due to energetic disorder.

Overcoming energetic disorder is a fundamental challenge for the development of efficient QD solar cells. Disorder arises from random QD packing, surface dipoles, size polydispersity, off-stoichiometry, variations in surface ligand coverage and passivation, ligand states, and ligand- or impurity-induced states. These phenomena and their consequences are often difficult to characterize and to control in QD films and devices. Device-level consequences of energetic disorder include inefficient charge transport and extraction, reduced VOC due to high sub-bandgap state densities, and potentially low ultimate efficiency limits due to broadening of the band tail—characterized by the Urbach energy—and high saturation current density.

Figure 2: Sunglasses with integrated semi-transparent quantum dot PV array. These QDPV arrays based on PbS QDs produce an output of roughly 4 mW and are shown powering LCD clocks under fluorescent room lighting. Larger-area devices will require further development of scalable deposition methods for active layers and electrodes.

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Preliminary Work and Related Contributions

Several background projects related to and in support of the proposed thesis work have been completed and are listed below.

Long-term potential of solar PV and R&D directions for emerging thin-film PV • Analysis of historical cost trends and potential for future deployment and cost decline

under nationally determined climate change mitigation targets o J. Trancik, P. R. Brown, J. Jean, G. Kavlak, and M. Klemun, “Technology Improvement and

Emissions Reductions as Mutually Reinforcing Efforts: Observations from the Global Development of Solar and Wind Energy,” MIT technical report (2015).

• Analysis of solar PV technology, economics, R&D, and U.S. solar policy o R. Schmalensee, V. Bulović, R. Armstrong, C. Batlle, P. Brown, J. Deutch, H. Jacoby, R. Jaffe, J.

Jean, R. Miller, F. O’Sullivan, J. Parsons, J. I. Pérez-Arriaga, N. Seifkar, R. Stoner, and C. Vergara, “MIT Future of Solar Energy Study,” MIT Energy Initiative (2015).

• Analysis of location-dependent levelized cost of electricity (LCOE) from solar PV, wind, and legacy coal generation in the U.S.

o J. Jean, D. C. Borrelli, and T. Wu, “Mapping the Economics of U.S. Coal Power and the Rise of Renewables,” MITEI Working Paper Series, WP-2016-01 (2016).

• Perspective article introducing material complexity framework for PV technology classification and analysis of materials scalability for emerging thin-film PV

o J. Jean, P. R. Brown, R. L. Jaffe, T. Buonassisi, and V. Bulović, “Pathways for Solar Photovoltaics,” Energy & Environmental Science 8, 1200–1219 (2015).

• Demonstration of vapor-deposited parylene substrates and in situ fabrication process for lightweight and flexible organic solar cells (Figure 3)

o J. Jean, A. Wang, and V. Bulović, “In Situ Vapor-Deposited Parylene Substrates for Ultra-thin, Lightweight Organic Solar Cells,” Organic Electronics 31, 120–126 (2016).

Figure 3: In situ vapor-deposited parylene substrates for lightweight, flexible thin-film solar cells. (left) Organic solar cells on 1-µm-thick parylene membranes can be supported by a soap bubble, highlighting their low weight and conformability. (right) Large-area films are easily released from carrier wafers and may be compatible with high-throughput processing.

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Charge extraction in PbS QD solar cells • Demonstration of ordered bulk heterojunction device architecture using ZnO nanowires

to enhance photocurrent in PbS QDPV (Figure 4) o J. Jean, S. Chang, P. R. Brown, J. J. Cheng, P. H. Rekemeyer, M. G. Bawendi, S. Gradečak, and

V. Bulović, “ZnO Nanowire Arrays for Enhanced Photocurrent in PbS Quantum Dot Solar Cells,” Advanced Materials 25, 2790–2796 (2013).

o H. Park*, S. Chang*, J. Jean, J. J. Cheng, P. T. Araujo, M.-S. Wang, M. G. Bawendi, M. Dresselhaus, V. Bulović, J. Kong, and S. Gradečak, “Graphene Cathode-based ZnO Nanowire Hybrid Solar Cells,” Nano Letters 13, 233–239 (2012).

• Development and validation of transfer-matrix optical model in work on global optimization of multilayer antireflection coatings for c-Si solar cells

o P. Azunre, J. Jean, C. Rotschild, V. Bulović, S. G. Johnson, and M. A. Baldo, “Guaranteed Global Optimization of Thin-Film Optical Systems,” in preparation.

Open-circuit voltage and efficiency limits for QD solar cells • Investigation of radiative sub-bandgap states in relation to VOC deficit in QDPV

o C.-H. M. Chuang, A. Maurano, R. E. Brandt, G. W. Hwang, J. Jean, T. Buonassisi, V. Bulović, and M. G. Bawendi, “Open-Circuit Voltage Deficit, Radiative Sub-Bandgap States, and Prospects in Quantum Dot Solar Cells,” Nano Letters 15, 3286–3294 (2015).

Figure 4: Ordered bulk heterojunction QDPV for enhanced charge extraction. Vertical arrays of ZnO nanowires can decouple light absorption from carrier collection in PbS quantum dot solar cells and increase power conversion efficiencies by 35%.

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Proposed Work

We propose two projects—described in detail below—to elucidate limits to charge extraction and ultimate performance for PbS QD solar cells.

Proposed Project 1: Development of an inverse method to map charge extraction efficiency in thin-film solar cells Project Goal Develop a semi-empirical method for inferring the 1-D charge extraction probability profile in QD and other thin-film solar cells

• Investigate effect of process parameters, device structure, and operating voltage on charge extraction in QDPV

• Estimate carrier transport lengths (i.e., depletion width, diffusion length) in QDPV

Purpose This technique would allow characterization of position-dependent charge extraction in any thin-film solar cell using only standard device and film measurements (EQE, film thicknesses, optical constants).

Specific Background To our knowledge, no experimental method currently exists for characterizing the spatial variation in charge extraction efficiency in an operating thin-film solar cell.

• Different measurements and device structures are generally required to estimate key parameters related to charge extraction (e.g., depletion width and diffusion length).

• Related experimental techniques include electron beam-induced current (EBIC), which measures the spatially-resolved charge collection probability of e-beam-generated carriers, and Mott-Schottky analysis, which estimates the depletion width, built-in voltage, and doping density but requires a Schottky device architecture.

Optical interference effects play a major role in the spatial distribution of light absorption and carrier generation in emerging thin-film PV.

• Many emerging thin-film PV suffer from inefficient charge extraction. The highest-performing devices often trade off light absorption to improve carrier collection, using sub-optimal film thicknesses that lead to strong cavity interference effects at wavelengths with lower absorption coefficients (e.g., near the QD bandgap).

Optical modeling can provide insight into optical behavior and charge collection in thin-film PV devices.

• Optical modeling using the transfer-matrix method (TMM) has been applied widely to polymer and small-molecule organic solar cells but used less often for QD solar cells.

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By combining 1-D optical modeling (charge generation vs. position and wavelength) with external quantum efficiency (EQE) measurements (charge extraction vs. wavelength), we can deduce the charge extraction efficiency (i.e., the probability that a photogenerated carrier will contribute to photocurrent) as a function of position inside an operating solar cell.

• Spatially resolved charge extraction profiles could inform efforts to improve charge extraction in QDPV.

• This method is related to the dynamic inner collection efficiency (DICE) analytical method previously developed for amorphous silicon solar cells. DICE assumes Beer-Lambert (exponential) absorption, which is not appropriate for the incompletely absorbing films used in QDPV and other thin-film solar cells.

Research Plan • Develop TMM optical model • Validate model against other optical models and experimental measurements on simple,

well-behaved structures (bare substrates and simple coatings) • Develop efficient QDPV baseline devices • Characterize optical constants and thicknesses for all layers in QDPV device stack using

spectroscopic ellipsometry • Measure total device absorption in integrating sphere to correct for parasitic absorption • Implement inverse problem solver to extract spatial charge extraction probability profile • Vary EQE measurement parameters (e.g., white-light intensity, bias voltage, device

architecture, illumination direction) and incorporate more data as needed • Estimate depletion width and diffusion lengths in QDPV active layer • Apply model to QDPV with different processing conditions • Apply model to perovskites and other thin-film PV technologies

Proposed Project 2: Characterization of band tailing and ultimate efficiency limits of QDPV using photothermal deflection spectroscopy (PDS) Project Goal Use PDS to measure sub-bandgap absorption and characterize band tailing in PbS QD films

• Investigate effect of QD size, polydispersity, ligand choice, annealing temperature, and environmental factors (e.g., humidity) on Urbach energy and sub-bandgap absorption

Purpose This study could confirm the potential of QD-based solar technologies and identify directions for further investigation or relegate QDPV to niche, low-efficiency applications.

Specific Background The sharpness of the band edge (i.e., the Urbach energy) is strongly correlated with the maximum VOC and efficiency of solar cell absorbers.

• Many techniques exist for characterizing band-edge sharpness and sub-bandgap states.

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• Optical techniques include PDS, UV-vis spectrophotometry, photoluminescence (PL) spectroscopy with variable excitation wavelength, and temperature-dependent transient PL spectroscopy.

• Optoelectronic techniques include intensity-modulated photocurrent spectroscopy (IMPS), Fourier-transform photocurrent spectroscopy (FTPS), and internal quantum efficiency (IQE).

PDS is a proven technique that has been used to measure Urbach energies and sub-bandgap absorption in many disordered PV materials, including a-Si, perovskites, organics, and QDs.

• PDS directly measures the non-radiative recombination and hot-carrier thermalization following light absorption.

• Advantages of PDS include high sensitivity (not sensitive to scattering) and high dynamic range (≥104).

• Disadvantages of PDS include restrictions on sample geometry and long measurement times.

To our knowledge, no PDS measurements have been performed on QD films exhibiting the best solar PV device performance (e.g., PbS QDs with TBAI ligands). Urbach energies of leading QDPV materials have not yet been characterized.

Research Plan • Explore available techniques for measuring QD absorption • Take preliminary PDS measurements on QDs using existing system (with collaborators) • Build and automate PDS optical setup • Calibrate and test system using materials with well-characterized absorption (Si, GaAs) • Characterize system stability and noise floor • Take PDS measurements on PbS QDs

o Investigate effect of material and process parameters (e.g., polydispersity, dot size, ligand choice, air exposure, annealing) on band tailing

o Correlate Urbach energy with QDPV device performance o Evaluate VOC and efficiency limits of PbS QDPV based on detailed balance

• Investigate band tailing in other thin-film PV materials (e.g., QD-perovskite solids)

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Timeline and Milestones

*Institute deadlines in italics

July 2016 – February 2017 Complete charge extraction and PDS experiments

• Milestones for charge extraction modeling: o Transfer matrix optical model developed and validated o Efficient (≥7%) baseline QDPV devices re-established o EQE measurement setup with white-light bias complete o Inverse problem solver implemented and validated on semitransparent QDPV o Model applied to QDPV with varying thickness and other device parameters

• Milestones for PDS measurements:

o Optical set-up for PDS complete o PDS system calibrated using reference sample o System stability and noise floor characterized o PDS measurement validated using known materials o PDS spectra measured for PbS QDs with different processing conditions o PDS measurements compared with other available techniques (UV-vis, EQE, PL) o VOC and efficiency limits of QDPV calculated from measured Urbach energies o PDS absorption spectra measured for other PV materials

January – April 2017 Write thesis

February 10, 2017 Degree application deadline

March 2017 Prepare presentation for thesis defense

April 7, 2017 Thesis title submission deadline

April 2017 Thesis defense

May 5, 2017 PhD thesis submission deadline

June 9, 2017 MIT Commencement