RUI: Exploring Nonperovskite Phase Growth Mechanisms in Halide Perovskites

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Non-Technical SummaryThe development of lower cost solar cell technologies is crucial for reducing pollution from the energy sector as well as the cost of alternative electricity generation. The technology to best achieve this goal might be so-called perovskite solar cells because of very low material costs and the potential for manufacturing by printing, similar to the printing of newspaper or photographic film. Under laboratory conditions perovskite solar cells show performances on par with current commercially available solar cells. This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, addresses a major unknown for the technology at a fundamental materials chemistry level: long-term durability of perovskite films under real-world conditions. To speed up durability studies - to learn in six months if the solar cells will survive 30 years - it is necessary to be able to understand and predict the various failure modes of the materials system. Prof. Christians’ research focuses on understanding a specific known failure mode of perovskite solar cells, a change in the central material’s crystalline phase or structure leading to a decrease in solar cell performance. Various aspects of the perovskite materials are carefully adjusted to gain a predictive understanding of the conditions under which they degrade and their specific failure mode. The level of understanding gained from this research is expected to serve as a guide to allow researchers to accurately understand, quantify and mitigate the effects of this failure mode during years-long outdoor operation of solar cells. This project also includes mentoring activities undertaken by Prof. Christians at Hope College, a primarily undergraduate institution. It allows undergraduate students to participate in cutting-edge materials research and provides an opportunity for these students to build their skills as research scientists and to present their work at national research conferences. This award will also support work done in tandem with ExploreHope, a campus outreach program, to design and implement a summer STEM research camp, called "Digging into Photovoltaics", for traditionally underrepresented local high school students to give these students experience with solar cells and STEM research.Technical SummaryA material's crystalline phase is one of the most important aspects of its chemical, physical, and electronic properties. Halide perovskites, the absorber material in perovskite solar cells, have corner-sharing metal halide octahedra, but can transform into other crystal phases where these octahedra tilt to become face- or edge-sharing. This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, advances the understanding of the phase transformation from perovskite phases to nonperovskite phases which is important to designing halide perovskite absorbers that remain stable in their perovskite phase during years-long outdoor operation. Nonperovskite phase formation is sensitive to material composition and crystal size, but these parameters are not well understood. Prof. Christians experimentally tracks, induces and studies the phase transformations of halide perovskites materials into their competing nonperovskite phases. The first phase of the project uncovers the energetics, rate, and mechanism of this process. Based on these findings the researchers then explore how compositional and morphological changes to the halide perovskite materials influence these parameters. This work sheds light on the role that external stimuli, such as humidity, play in initiating and catalyzing nonperovskite phase formation. Taken together, this project advances the understanding of halide perovskite phase stability and phase change and will thereby play a role in the research community's search for more stable materials, improved device designs, and better informed accelerated device lifetime protocols.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.