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 MAPbI₃ and CsFAMAPbI₃ 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 MAPbI₃ 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°C in nitrogen. To overcome the barrier of the commercialization of metal halide perovskite solar cells, a simple, cost-effective, and generalized strategy that mitigates the intrinsic thermal instability is strongly needed. Here, caffeine is introduced to simultaneously enhance the efficiency and thermal stability of the solar cells based on various kinds of perovskite materials. 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. Ultimately, a champion-stabilized efficiency of 19.8% with 1,300 h thermal stability at 85°C in nitrogen was achieved. “Perovskite is drinking coffee.” The perfect temperature to serve coffee is 85°C. The authors here introduced caffeine, which is the active material of coffee, into the active materials of the perovskite solar cells to enhance their performance and thermal stability under 85°C. A champion-stabilized efficiency of 19.8% with 1,300 h thermal stability at 85°C in nitrogen was achieved. This simple, cost-effective, and generalized strategy increases the commercial prospects of perovskite solar cells.