Bifacial all perovskite tandem solar cells for a sustainable energy future

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Solar energy is a clean, renewable and most abundant energy resource, however electric energy from solar cells still only contributes to a very small portion of the global electricity generation. Achieving highly efficient and low-cost photovoltaic modules is critical to make solar energy a truly viable and attractive solution for a sustainable energy future. Tandem solar cells have a much higher thermodynamic efficiency upper limit than single junction devices. Due to low-cost solution processing for both subcells, the all-perovskite tandem solar cells have enormous potential to reduce the levelized cost of energy below those of silicon based single-junction and multijunction cells. Tremendous progress has been made over the past five years to improve the efficiency of all-perovskite tandem solar cells from 10.8% to over 25%, but they have not yet reached the full potential of this technology. This project aims at increasing the equivalent efficiency of all-perovskite tandem cells to over 33% via a bifacial tandem architecture and address the related fundamental material and device challenges. The success of this project can yield a promising renewable energy source with cost competitiveness to fossil energy, thus reducing greenhouse gas emissions and environmental pollution. The project will provide training to graduate and undergraduate students through interdisciplinary research and will bring them an entrepreneur mindset. A strong focus of the project is on increasing participation of minorities and providing early exposure of research to a broad audience through several outreach activities at The University of North Carolina at Chapel Hill. The efficiency of all-perovskite tandem solar cells is primarily limited by the larger than usual open circuit voltage deficit of wide-bandgap perovskite cells and the low external-quantum-efficiency of narrow-bandgap perovskite cells. We propose to develop bifacial all-perovskite tandem cells with a damage-free deposition of transparent electrodes on tin-containing narrow-bandgap perovskite cells which have a much higher efficiency upper limit of over 33%. The bifacial tandem architecture not only enhances the upper limit of realistic power conversion efficiency, but also shifts the optimal bandgap of wide-bandgap cell to a lower value, thus avoids the large open circuit voltage deficit problem for larger bandgap perovskite cells. In addition, we will enhance light harvesting efficiency of the narrow-bandgap perovskite cells. In order to overcome the low external-quantum-efficiency issue in the narrow bandgap perovskite cells, this project will explore the fundamental processes that limit the carrier diffusion length of tin-containing narrow bandgap perovskites and address the related defects with specialized additives. Moreover, this project will provide a comprehensive understanding of the impact of ground reflected albedo effects, weather conditions, and module installation conditions on the efficiency of bifacial all-perovskite tandem cells, in turn, providing valuable feedback to improve solar cell design. This research will address the fundamental understanding of bifacial all-perovskite tandem solar cells and provide a promising renewable energy with low levelized cost of energy.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.