Two-step MAPbI3 deposition by Low-Vacuum Proximity-Space-Effusion for high-efficiency inverted semitransparent perovskite solar cells

Halide perovskite solar cells can combine high photoconversion efficiency with high transmittance. Herein, we developed an innovative vacuum deposition method to prepare CH3NH3PbI3 (MAPbI3) thin film for semitransparent perovskite solar cells. The method applies a two steps Low-Vacuum Proximity-Space-Effusion (LV-PSE) under low vacuum conditions to produce high-quality thin layers of phase-pure MAPbI3. The parameter optimization was validated by theoretical calculation. We show that, during the process of CH3NH3I (MAI) deposition (second step) on PbI2 (first step) at a given substrate temperature, the conversion of the PbI2 film to MAPbI3 occurs from the top-surface inward via an adsorption-incorporation-migration mechanism guided by the gradient of MAI concentration. The quality of the final layer arises from this progressive conversion that exploits the lattice order of the mother PbI2 layer. Finally, p-i-n solar cells were prepared using ITO/PTAA/MAPbI3/PCBM-BCP/Al architectures with photo-active layer thickness of 150nm. This layer, characterized by an Average Visible Transmittance (AVT) as high as 20%, produced an average efficiency of 14.4% that is a remarkable result considering the transparency vs. efficiency countertrend that indeed demands a proper balance from the quality of the material. Very importantly, we demonstrated that further down scalability of the MAPbI3 layer is feasible as proved by reducing the thickness down to 80 nm. In this specific case, the devices showed an average efficiency of 12.9% withstanding an AVT of 32.8%. This notable efficiency recorded on those extremely thin layers benefits from the exclusive quality of the MAPbI3 grown with the developed method.