Controllable growth of perovskite films by room-temperature air exposure for efficient planar heterojunction photovoltaic cells

A two-step solution processing approach has been established to grow void-free perovskite films for low-cost high-performance planar heterojunction photovoltaic devices. A high-temperature thermal annealing treatment was applied to drive the diffusion of CH₃NH₃I precursor molecules into a compact PbI₂ layer to form perovskite films. However, thermal annealing for extended periods led to degraded device performance owing to the defects generated by decomposition of perovskite into PbI₂. A controllable layer-by-layer spin-coating method was used to grow "bilayer" CH₃NH₃I/PbI₂ films, and then drive the interdiffusion between PbI₂ and CH₃NH₃I layers by a simple air exposure at room temperature for making well-oriented, highly crystalline perovskite films without thermal annealing. This high degree of crystallinity resulted in a carrier diffusion length of ca. 800 nm and a high device efficiency of 15.6 %, which is comparable to values reported for thermally annealed perovskite films. A breath of fresh air: Simple room-temperature air exposure can drive the interdiffusion between perovskite precursor layers and crystallize the perovskite thin films. The obtained perovskite films show high crystallinity and well-aligned orientation. The devices with and without a TiO₂ electron transporting layer yielded high efficiencies of 15.6 % and 13.8 %, respectively.