Humidity controlled crystallization of thin CH₃NH₃PbI₃ films for high performance perovskite solar cell

Control of crystallization of a solution-processed perovskite layer is of prime importance for high performance solar cells. In spite of the negative effect of water on perovskite solar energy conversion in several previous works, we observed that humidity plays a critical role to develop a thin uniform, dense perovskite film with preferred crystals, in particular, in a device with architecture of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC₇₁BM/LiF/Al fabricated by two-step sequential spin-coating process. Humidity controlled spin-coating of CH₃NH₃I on the pre-formed PbI₂ layer was the most influential process and systematic structural investigation as a function of humidity revealed that grains of CH₃NH₃PbI₃ perovskite crystals increase in size with their preferred orientation while film surface becomes roughened as the humidity increases. The performance of a device was closely related to the humidity dependent film morphology and in 40% relative humidity, the device exhibited the maximum power conversion efficiency of approximately 12% more than 10 times greater than that of a device fabricated at 20% humidity. The results suggest that our process with controlled humidity can be another efficient route for high performance and reliable perovskite solar cells.