In this project funded by the Divisions of Chemistry, Materials Research, and Mathematical Sciences, Professors Matthew D. Law, John C. Hemminger, and John Lowengrub of the University of California-Irvine will study the use of iron pyrite in thin-film photovoltaics (PV). Pyrite or iron persulfide (FeS2) is an under-researched, extremely promising semiconductor for use as the light-absorbing layer. There are two key hurdles to utilizing this material for solar energy conversion: the difficulty in synthesizing high-quality, phase-pure pyrite and the low photovoltage of pyrite devices. This research will combine novel phase field crystal (PFC) and density functional theory (DFT) models with targeted synthetic and surface characterization efforts to grow device-quality pyrite thin films by sintering layers of solution-deposited pyrite nanocrystals and by chemical vapor deposition (CVD), and to fix the pyrite photovoltage by passivating mid-gap states via judicious annealing and coordination chemistry approaches.
This collaboration between experts in continuum and atomistic crystal modeling (Prof. Lowengrub, Mathematics), pyrite growth (Prof. Law, Materials Science), and surface characterization (Prof. Hemminger, Chemistry) provides a new approach to developing semiconductor materials that are earth-abundant and environmentally friendly. Such properties are important for the large-scale implementation of solar energy as an alternative energy source. Undergraduate, graduate and postdoctoral researchers working in this team gain interdisciplinary perspectives not often encountered in standard academic curricula. To complement the research effort, the team is developing an intensive, month-long summer course on crystal growth for ~20 high school students annually as part of the California State Summer School for Mathematics and Science (COSMOS) at UCI. This course provides students with the opportunity to learn mathematical theory, use state-of-the-art simulation tools, and learn about crystallography and solar energy in the laboratory. The research team is also involved in the design and implementation of an interactive educational exhibit on solar energy and energy use, including real-time monitoring of an operating photovoltaic array, at the Discovery Science Center of Orange County. This site is expected to be visited by 70,000 K-12 students and 445,000 members of the community each year.
This SOLAR project, which was carried out by a collaborative team of 25 researchers at the University of California, Irvine, with support from scientists at the National Renewable Energy Laboratory and several other institutions, focused on fundamental studies of iron pyrite (cubic iron disulfide) to improve the properties of this earth-abundant, nontoxic semiconductor for solar cell applications. The goal of the project was to combine several types of mathematical modeling with synthesis and thorough surface characterization to (i) grow device-quality pyrite thin films by sintering layers of solution-deposited pyrite nanocrystals and molecular inks and by chemical vapor deposition (CVD), and (ii) understand the reason for the historically low photovoltage of pyrite devices, then fix the pyrite photovoltage by passivating midgap and surface states using various annealing and coordination chemistry approaches. The project resulted in 24 papers in peer-reviewed scientific journals, scores of talks at scientific conferences, 3 patent filings, and training of approximately 20 graduate students and postdoctoral fellows for STEM careers in academia and industry. The PI worked with Discovery Science Center (DSC) of Santa Ana, CA. to design a suite of exhibits on solar energy to be built at DSC for K-12 education. A partial list of key outcomes of the project include the following: We showed the existence of a strong hole inversion layer at the surface of high-quality n-type single crystals of pyrite grown by a flux method. The presence of the hole inversion layer was corroborated by photoemission spectroscopy and measurements as a function of crystal thickness. The inversion layer can explain both the low photovoltage of pyrite solar cells and the near-universal heavy p-type conductivity of pyrite thin films that have together perplexed researchers for the past thirty years. We found that the thickness and conductivity of the inversion layer can be modified by mechanical and chemical treatments of the pyrite surface, suggesting that it is possible to eliminate this hole-rich layer by passivating surface states and subsurface defects. Furthermore, an analysis of high-temperature electrical conductivity and optical transmission data suggest that the electronic band gap is >0.80 eV at room temperature, confirming that photovoltages in excess of 500 mV should be attainable from pyrite under solar illumination. Several numerical methods were developed for phase-field crystal, modified phase field crystal, and dynamical density functional models of crystals. We also developed a thermodynamically and mechanically consistent method for modeling freezing (solid-liquid phase transitions) that accounts for melt flow on crystallization at the nanoscale, and numerical methods needed to solve the model equations, in order to simulate the synthetic conditions used in pyrite molecular inks. We developed accurate and efficient methods to simulate sintering of multiple nanocrystallites using a phase-field approach. Our results indicate that the difference between the simple grain growth and sintering can be seen in terms of densification, with the latter being more densely packed. The sintering model can be been used to predict ideal operating conditions for achieving film uniformity. We used systematic density functional theory studies and model analyses to show that the band gap of iron pyrite (FeS2) can be increased from 1.0 to 1.2−1.3 eV by replacing 10% of the sulfur atoms with oxygen atoms (i.e., 10% O impurities). To improve the fundamental understanding of the pyrite electronic structure and optical properties, we used spectroscopic ellipsometry to determine the pseudodielectric function spectrum of natural pyrite single crystals. We calculated the stabilities and electronic properties of a large number of neutral and charged defects in pyrite, including natives, nonmetals, and transition metal elements. We determined the influence of the surface stoichiometry and morphology on the band gap of the pyrite FeS2(001) surface, showing that sulfur-deficient surfaces retain most of the bulk band gap but sulfur-rich surfaces have gaps less than 0.3 eV. We synthesized the first phase-pure, stable colloidal pyrite nanocrystals and showed that polycrystalline pyrite films can be made from solution. We made the first phase-pure, dense, large-grain pyrite films from a hydrazine-free molecular ink and characterized their basic optical and electrical properties. CVD was used to grow device-quality pyrite films on molybdenum-coated glass for the first time. These films were extensively studied. We found that marcasite is a tenacious impurity in pyrite, but proper sulfur annealing can always eliminate marcasite and pyrrhotite phase impurities in pyrite thin films. Optical measurements and modeling suggest that the band gap of marcasite as at least as large as that of pyrite, contrary to previous belief. We also found that alkali metal leaching from glass substrates is crucial for the nucleation and growth of pyrite over marcasite. In particular, sodium or potassium are required to make high-quality films. Through density functional calculations, we investigate the segregation of an isloated sulfur vacancy from interior sites outward to the (100) surface of a pyrite crystal.
