Perovskite-Based Solar Cells

TLDR: Time-resolved transient absorption and photoluminescence are used to show that the effective diffusion lengths are indeed relatively large in CH3NH3PbI3, about 100 nm for both electrons and holes—a high value for a semiconductor formed from solution at low temperature.

Abstract: Photovoltaic (PV) cells that convert sunlight directly into electricity are becoming increasingly important in the world's renewable energy mix. The cumulative world PV installations reached around 100 GWp (gigawatts) ( 1 ) by the end of 2012. Some 85% use crystalline Si, with the rest being polycrystalline thin film cells, mostly cadmium telluride/cadmium sulfide ones. Thin-film cells tend to be cheaper to make with a shorter energy payback time. However, they do have the disadvantage, one that may become crucial when considering the terawatt range, that most of them contain rare elements like tellurium (as rare as gold), indium, and gallium. A newcomer to the PV field ( 2 ) has rapidly reached conversion efficiencies of more than 15% (see the figure). Based on organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH3NH3PbI3), these perovskites tend to have high charge-carrier mobilities ( 3 , 4 ). High mobility is important because, together with high charge carrier lifetimes, it means that the light-generated electrons and holes can move large enough distances to be extracted as current, instead of losing their energy as heat within the cell. On pages 344 and 341 of this issue, Xing et al. ( 5 ) and Stranks et al. ( 6 ) use time-resolved transient absorption and photoluminescence to show that the effective diffusion lengths are indeed relatively large in CH3NH3PbI3, about 100 nm for both electrons and holes—a high value for a semiconductor formed from solution at low temperature.