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https://phys.org/news/2014-01-perovskite-solar-cells-cheaper-materials.html#:~:text=Although%20the%20perovskite%20material%20itself,that%20of%20gold%20and%20platinum. Facile and low-cost synthesis of a novel dopant-free hole transporting material that rivals… A Spiro fluorene-based dopant-free