Phase transition induced recrystallization and low surface potential barrier leading to 10.91%-efficient CsPbBr3 perovskite solar cells

High efficiency and long-term stability are vital for further development of perovskite solar cells (PSCs). PSCs based on cesium lead halide perovskites exhibit better stability but lower power conversion efficiencies (PCEs), compared with organic-inorganic hybrid perovskites. Lower PCE is likely associated with trap defects, over-growth of partial crystals and irreversible phase transition in the films. Here we introduce a strategy to fabricate high-efficiency CsPbBr3-based PSCs by controlling the ratio of CsBr and PbBr2 to form the perovskite derivative phases (CsPb2Br5/Cs4PbBr6) via a vapor growth method. Following post-annealing, the perovskite derivative phases as nucleation sites transform to the pure CsPbBr3 phase accompanied by crystal rearrangements and retard rapid recrystallization of perovskite grains. This growth procedure induced by phase transition not only makes the grain size of perovskite films more uniform, but also lowers the surface potential barrier that existsbetween the crystals and grain boundaries. Owing to the improved film quality, a PCE of 10.91% was achieved for n-i-p structured PSCs with silver electrodes, and a PCE of 9.86% for hole-transport-layer-free devices with carbon electrodes. Moreover, the carbon electrode-based devices exhibited excellent long-term stability and retained 80% of the initial efficiency in ambient air for more than 2000 h without any encapsulation.