Three teams led by 91探花 researchers have received competitive awards totaling more than $2.3 million from the U.S. Department of Energy Solar Energy Technologies Office for projects that will advance research and development in photovoltaic materials, which are an essential component of solar cells and impact the amount of sunlight that is converted into electricity.

The 91探花teams are led by , a professor of electrical and computer engineering; , a professor of chemical engineering; and , an associate professor of both mechanical engineering and materials science and engineering. All are also researchers with the UW-based , and MacKenzie serves as director of the institute鈥檚 . Dunham and Hillhouse are also members of the 91探花.

Hillhouse and MacKenzie are leading projects to explore the properties and manufacturing potential of thin-film perovskites. These are printable crystalline compounds that are able to harvest photons at power conversion efficiencies almost equal to silicon-based semiconductors used in today鈥檚 solar cells, but at lower costs. But before perovskites can have a global impact on solar energy, researchers need to improve their stability and develop improved, scalable manufacturing methods.

Hillhouse鈥檚 project, awarded $1.5 million, will focus on understanding how the composition, structure, and environmental exposure of pervoskites can affect their stability and performance. This project will apply new photoluminescence imaging and video methods to combinatorial material libraries, which were fabricated at a facility built by Hillhouse with funding from the M.J. Murdock Charitable Trust. His team will use machine learning methods to extract new information from these extremely large datasets, which could reveal the fundamental connections between nanoscopic and microscopic material features and macroscopic solar cell performance and stability. 91探花partners in this work are , professor of statistics, and , director of research at the UW鈥檚 eScience Institute and research associate professor of chemical engineering.

MacKenzie鈥檚 project, awarded nearly $200,000, focuses on perovskite manufacturing using roll-to-roll processing techniques. In the solar energy field, roll-to-roll processing involves additively printing and coating ultra-thin solar-cell components 鈥� including thin-film perovskites 鈥� directly onto rolls of flexible material, much like applying paint to a wall or printing out a document. MacKenzie鈥檚 team will analyze the effectiveness of different techniques for depositing perovskite onto the rolls by rapidly analyzing the films as they are being printed. They will use optical probes and photoluminescence techniques to gather data on how well various roll-to-roll-produced perovskites interact with light. They can use this data to change the ways perovskites are deposited in roll-to-roll processing to manufacture higher-quality, flexible solar cells more efficiently, as well as at the production scales needed to make an economic and environmental impact. His team鈥檚 work will make use of the Washington Clean Energy Testbeds near the 91探花campus, which include world-class roll-to-roll manufacturing facilities supported by the state of Washington and the Washington Research Foundation.

Dunham鈥檚 project, awarded $681,000, will investigate another promising material in photovoltaics research, known by its acronym CIGS 鈥� or copper indium gallium selenide. Like perovskites, CIGS is another strong and efficient absorber of photons from sunlight 鈥� a necessity for any material used in photovoltaic applications. CIGS can also be deposited onto flexible materials for incorporation into thin-film solar cells. Dunham鈥檚 research centers on understanding how variations in CIGS crystalline structure and composition affects how carriers move within the crystal and impact its sunlight-to-energy conversion rate. They plan to use this information to create models for CIGS manufacturing processes and their impact on performance efficiency, which they鈥檒l test and refine in partnership with , a California-based solar energy company.

The awards to 91探花teams are part of from the Solar Energy Technologies Office to develop new technologies and solutions that both reduce solar electricity costs and support growing employment in the solar field. These include projects to boost the performance and reliability of photovoltaic cells, modules and systems 鈥� as well as to reduce materials and processing costs.

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Solar cells are devices that absorb photons from sunlight and convert their energy to move electrons 鈥� enabling the production of clean energy and providing a dependable route to help combat climate change. But most solar cells used widely today are thick, fragile and stiff, which limits their application to flat surfaces and increases the cost to make the solar cell.

鈥淭hin-film solar cells鈥� could be 1/100^th the thickness of a piece of paper and flexible enough to festoon surfaces ranging from an aerodynamically sleek car to clothing. To make thin-film solar cells, scientists are moving beyond the 鈥渃lassic鈥� semiconductor compounds, such as gallium arsenide or silicon, and working instead with other light-harvesting compounds that have the potential to be cheaper and easier to mass produce. The compounds could be widely adopted if they could perform as well as today鈥檚 technology.

In a published online this spring in the journal , scientists at the 91探花 report that a prototype semiconductor thin-film has performed even better than today鈥檚 best solar cell materials at emitting light.

鈥淚t may sound odd since solar cells absorb light and turn it into electricity, but the best solar cell materials are also great at emitting light,鈥� said co-author and 91探花chemical engineering professor , who is also a faculty member with both the UW鈥檚 and .聽 鈥淚n fact, typically the more efficiently they emit light, the more voltage they generate.鈥�

The 91探花team achieved a record performance in this material, known as a lead-halide perovskite, by chemically treating it through a process known as 鈥渟urface passivation,鈥� which treats imperfections and reduces the likelihood that the absorbed photons will end up wasted rather than converted to useful energy.

鈥淥ne large problem with perovskite solar cells is that too much absorbed sunlight was ending up as wasted heat, not useful electricity,鈥� said co-author , a 91探花professor of chemistry and chief scientist at the CEI. 鈥淲e are hopeful that surface passivation strategies like this will help improve the performance and stability of perovskite solar cells.鈥�

Ginger鈥檚 and Hillhouse鈥檚 teams worked together to demonstrate that surface passivation of perovskites sharply boosted performance to levels that would make this material among the best for thin-film solar cells. They experimented with a variety of chemicals for surface passivation before finding one, an organic compound known by its acronym TOPO, that boosted perovskite performance to levels approaching the best gallium arsenide semiconductors.

鈥淥ur team at the 91探花was one of the first to identify performance-limiting defects at the surfaces of perovskite materials, and now we are excited to have discovered an effective way to chemically engineer these surfaces with TOPO molecules,鈥� said co-lead author , a postdoctoral researcher at the Massachusetts Institute of Technology who conducted this research as a 91探花chemistry doctoral student. 鈥淎t first, we were really surprised to find that the passivated materials seemed to be just as good as gallium arsenide, which holds the solar cell efficiency record. So to double-check our results, we devised a few different approaches to confirm the improvements in perovskite material quality.鈥�

DeQuilettes and co-lead author , who conducted this research as a doctoral student in chemical engineering, showed that TOPO-treating a perovskite semiconductor significantly impacted both its internal and external photoluminescence quantum efficiencies 鈥� metrics used to determine how good a semiconducting material is at utilizing an absorbed photon鈥檚 energy rather than losing it as heat. TOPO-treating the perovskite increased the internal photoluminescence quantum efficiencies by tenfold 鈥� from 9.4 percent to nearly 92 percent.

鈥淥ur measurements observing the efficiency with which passivated hybrid perovskites absorb and emit light show that there are no inherent material flaws preventing further solar cell improvements,鈥� said Braly. 鈥淔urther, by fitting the emission spectra to a theoretical model, we showed that these materials could generate voltages 97 percent of the theoretical maximum, equal to the world record gallium arsenide solar cell and much higher than record silicon cells that only reach 84 percent.鈥�

These improvements in material quality are theoretically predicted to enable the light-to-electricity power conversion efficiency to reach 27.9 percent under regular sunlight levels, which would push the perovskite-based photovoltaic record past the best silicon devices.

The next step for perovskites, the researchers said, is to demonstrate a similar chemical passivation that is compatible with easily manufactured electrodes 鈥� as well as to experiment with other types of surface passivation.

鈥淧erovskites have already demonstrated unprecedented success in photovoltaic devices, but there is so much room for further improvement,鈥� said deQuilettes. 鈥淗ere we think we have provided a path forward for the community to better harness the sun鈥檚 energy.鈥�

Other co-authors are , a postdoctoral researcher at the University of California, Berkeley; , who recently completed his 91探花undergraduate degree in materials science and engineering; and , who just completed his doctoral degree with the 91探花Department of Chemistry and the CEI. The research was funded by the U.S. Department of Energy, the National Science Foundation, the 91探花, the 91探花Clean Energy Institute, the 91探花Molecular Engineering & Sciences Institute and the University of California, Berkeley.

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For more information, contact Ginger at 206-685-2231 or dginger@uw.edu and Hillhouse at 206-685-5257 or h2@uw.edu.

Grant numbers: DE-SC0013957, DGE-1256082, DE-EE0006710, ECC-1542101.