Nontoxic (CH3NH3)3Bi2I9 perovskite solar cells free of hole conductors with an alternative architectural design and a solution-processable approach

Abstract

Methylammonium iodide bismuthate ((CH3NH3)3Bi2I9) (MBI) perovskite was introduced as a new lead-free and air-stable absorber for hole conductor-free perovskite solar cells. The two-step soaking-assisted sequential solution (2-S) method was adopted to fabricate MBI films for the first time. We compared the formation processes and final morphologies of the MBI films fabricated using the 1-S and 2-S methods on planar and mesoporous TiO2 layers, respectively. We also investigated the effects of the morphologies of MBI films and device architectural design on device performance. Results showed that the MBI films fabricated using the 2-S method achieved a superior coverage both on the compact TiO2 and mesoporous TiO2 layers. The mesoporous structure devices presented higher power conversion efficiencies than the planar structure devices. In addition, all devices exhibited excellent thermal and long-term stabilities. The presented architectural design and solution-processable approach could inspire further research and practical applications on lead-free organic-inorganic hybrid perovskite solar cells.