Defense and dissertation
The culmination of 5 years at Stanford
Polycrystalline Tandem Photovoltaics
An in-depth look at metal-halide perovksite solar cells for tandem applications. The opportunity for perovskites in tandems is discussed, followed by discussion of the advantages and disadvantages of different tandem designs, stabilizing the perovskite device, and finally cost-modeling of the perovskite and perovskite/silicon tandems to identify the commercial opportunity for these devices
Polycrystalline Tandem Photovoltaics
A 45 minute high-level presentation of perovskite solar cells and their applications in tandem structures. Mechanically-stacked tandems (4-terminal), monolithic tandems (2-terminal), stabilization of the perovskite, and cost-modeling is discussed. This presentation was given in partial fulfillment of the requirements of a PhD at Stanford University.
Nature Energy, 2017
As the record single-junction efficiencies of perovskite solar cells now rival those of copper indium gallium selenide, cadmium telluride and multicrystalline silicon, they are becoming increasingly attractive for use in tandem solar cells due to their wide, tunable bandgap and solution processability. Previously, perovskite/silicon tandems were limited by significant parasitic absorption and poor environmental stability. Here, we improve the efficiency of monolithic, two-terminal, 1-cm2 perovskite/silicon tandems to 23.6% by combining an infrared-tuned silicon heterojunction bottom cell with the recently developed caesium formamidinium lead halide perovskite. This more-stable perovskite tolerates deposition of a tin oxide buffer layer via atomic layer deposition that prevents shunts, has negligible parasitic absorption, and allows for the sputter deposition of a transparent top electrode. Furthermore, the window layer doubles as a diffusion barrier, increasing the thermal and environmental stability to enable perovskite devices that withstand a 1,000-hour damp heat test at 85 ∘C and 85% relative humidity.
Advanced Materials, 2016
A sputtered oxide layer enabled by a solution-processed oxide nanoparticle buffer layer to protect underlying layers is used to make semi-transparent perovskite solar cells. Single-junction semi-transparent cells are 12.3% efficient, and mechanically stacked tandems on silicon solar cells are 18.0% efficient. The semi-transparent perovskite solar cell has a T 80 lifetime of 124 h when operated at the maximum power point at 100 °C without additional sealing in ambient atmosphere under visible illumination.
MRS Bulletin, 2015
A method to cost-effectively upgrade the performance of an established small-bandgap solar technology is to deposit a large-bandgap polycrystalline semiconductor on top to make a tandem solar cell. Metal-halide perovskites have recently been demonstrated as large-bandgap semiconductors that perform well even as a defective and polycrystalline material. We review the initial experimental and modeling work performed on these tandems. We also discuss in-depth the challenges of perovskite-based tandems and the innovations needed from the solar research community to propel perovskite-based tandems into the high-efficiency (>25%) regime and reach commercial competitiveness.
Applied Physics Letters, 2015
With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a Voc as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.
Energy & Environmental Science, 2015
A promising approach for upgrading the performance of an established low-bandgap solar technology without adding much cost is to deposit a high bandgap polycrystalline semiconductor on top to make a tandem solar cell. We use a transparent silver nanowire electrode on perovskite solar cells to achieve a semi-transparent device. We place the semi-transparent cell in a mechanically-stacked tandem configuration onto copper indium gallium diselenide (CIGS) and low-quality multicrystalline silicon (Si) to achieve solid-state polycrystalline tandem solar cells with a net improvement in efficiency over the bottom cell alone. This work paves the way for integrating perovskites into a low-cost and high-efficiency (>25%) tandem cell.
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Physical Chemistry Chemical Physics, 2014
A method for achieving complete pore-filling in solid-state dye-sensitized solar cells termed melt-infiltration is presented: after the customary solution-processed deposition of spiro-OMeTAD, the device is heated above the glass transition temperature of spiro-OMeTAD to soften the material and allow capillary action to pull additional spiro-OMeTAD from the overlayer reservoir into the pores. The pore-filling fraction increases from 60–65% to 90–100% as a result of melt-infiltration. The organic D–π–A dye used in this study is found to withstand the thermal treatment without performance loss, unlike ruthenium-based dyes. Through our experiments, we find that the 4-tert-butylpyridine (tBP) additive, commonly used in dye-sensitized solar cells, evaporates from the device during heat treatment at temperatures as low as 85 °C. This significantly impacts device performance, potentially excluding its use in commercial applications, and demonstrates the need for a more thermally stable tBP alternative. Melt-infiltration is expected to be a viable method for achieving complete pore-filling in systems where volatile additives are not required for operation.
Chemistry of Materials, 2013
A major limitation of solid-state dye-sensitized solar cells is a short electron diffusion length, which is due to fast recombination between electrons in the TiO2 electron-transporting layer and holes in the 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) hole-transporting layer. In this report, the sensitizing dye that separates the TiO2 from the Spiro-OMeTAD was engineered to slow recombination and increase device performance. Through the synthesis and characterization of three new organic D-π-A sensitizing dyes (WN1, WN3, and WN3.1), the quantity and placement of alkyl chains on the sensitizing dye were found to play a significant role in the suppression of recombination. In solid-state devices using Spiro-OMeTAD as the hole-transport material, these dyes achieved the following efficiencies: 4.9% for WN1, 5.9% for WN3, and 6.3% for WN3.1, compared to 6.6% achieved with Y123 as a reference dye. Of the dyes investigated in this study, WN3.1 is shown to be the most effective at suppressing recombination in solid-state dye-sensitized solar cells, using transient photovoltage and photocurrent measurements.
Shock Waves, 2012
The effect of incident shock wave strength on the decay of interface introduced perturbations in the refracted shock wave was studied by performing 20 different simulations with varying incident shock wave Mach numbers (M ~ 1.1− 3.5). The analysis showed that the amplitude decay can be represented as a power law model shown in Eq.7, where A is the average amplitude of perturbations (cm), B is the base constant (cm−(E−1), S is the distance travelled by the refracted shockwave (cm), and E is the power constant. The proposed model fits the data well for low incident Mach numbers, while at higher mach numbers the presence of large and irregular late time oscillations of the perturbation amplitude makes it hard for the power law to fit as effectively. When the coefficients from the power law decay model are plotted versus Mach number, a distinct transition region can be seen. This region is likely to result from the transition of the post-shock heavy gas velocity from subsonic to supersonic range in the lab frame. This region separates the data into a high and low Mach number region. Correlations for the power law coefficients to the incident shock Mach number are reported for the high and low Mach number regions. It is shown that perturbations in the refracted shock wave persist even at late times for high incident Mach numbers.
Body language video
Presented as part of the class GSBGEN 315 at Stanford
Making Body Language Your Superpower - an instructional video on using body language effectively. Presented by Stanford graduate students Matt Levy, Colin Bailie, Jeong Joon Ha, and Jennifer Rosenfeld. Created as an exemplary final project in Lecturer JD Schramm's Strategic Communication course in March 2014. Body language - both the speaker's and the audience's - is a powerful form of communication that is difficult to master, especially if the speaker is nervous. This video will teach you how to use your body language effectively, even if you are nervous. This video will also show you how to read the audience's body language and what you should do when they look bored or disconnected from the presentation. Use these tools to enhance your nonverbal communication abilities and better connect with your audiences.