Bulk heterojunction gifts bismuth-based lead-free perovskite solar cells with record efficiency

Bismuth-based lead-free perovskite solar cells are promising alternatives to the lead-based organic-inorganic hybrid cells which suffer from the environmental toxicity of lead and poor ambient stability, but the devices based on single-component ternary bismuth halides exhibit inferior power conversion efficiency. Herein, for the first time we construct bulk heterojunction (BHJ) bismuth-based perovskite solar cells with the photoactive layer consisting of in-situ phase-separated Cs3Bi2I9 and Ag3Bi2I9 components, achieving a record efficiency of approximate 3.6% and an unprecedented open-circuit voltage reaching 0.89 V. Formation of BHJ structure leads to increased crystal grain size of Cs3Bi2I9 and optimized grain orientation of Ag3Bi2I9, and a type-II energy band alignment is achieved, benefiting exciton separation and charge carrier transport. Cs3Bi2I9-Ag3Bi2I9 BHJ devices exhibit superb thermal stability, retaining similar to 90% of the initial efficiency after 450 h heating under 85 degrees C in glove box. Moreover, the universality of BHJ concept in boosting device performance of perovskite solar cells based on other reported AgxBiyIx+3y light-absorbers is verified. Our proof-of-concept breakthrough paves the way toward high-efficiency lead-free perovskite solar cells.