Caffeine Improves the Performance and Thermal Stability of Perovskite Solar Cells

To increase the commercial prospects of metal halide perovskite solar cells, there is a need for simple, cost-effective, and generalized approaches that mitigate their intrinsic thermal instability. Here we show that 1,3,7-trimethylxanthine, a commodity chemical with two conjugated carboxyl groups better known by its common name caffeine, improves the performance and thermal stability of perovskite solar cells based on both MAPbI(3) and CsFAMAPbI(3) active layers. The strong interaction between caffeine and Pb2+ ions serves as a molecular lock'' that increases the activation energy during film crystallization, delivering a perovskite film with preferred orientation, improved electronic properties, reduced ion migration, and greatly enhanced thermal stability. Planar n-i-p solar cells based on caffeine-incorporated pure MAPbI(3) perovskites, which are notoriously unstable, exhibit a champion-stabilized efficiency of 19.8% and retain over 85% of their efficiency under continuous annealing at 85 degrees C in nitrogen.