Two-step MAPbI3deposition 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 describe an innovative vacuum deposition method to prepare thin CH3NH3PbI3 (MAPbI3) layers for semitransparent perovskite solar cells. The method is based on two-step Low-Vacuum Proximity-Space-Effusion (LV-PSE: working pressure 2-4 × 10-2 mbar; source-substrate distance 1-3 cm) that guarantees high-quality at low production costs. The process 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 energetic MAI concentration. The quality of the final layer arises from this progressive conversion that also exploits the lattice order (texture) of the mother PbI2 layer. Finally, p-i-n solar cells were prepared using ITO/PTAA/MAPbI3/PCBM-BCP/Al architectures with a photo-active layer thickness of 150 nm. 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 to boost the quality of the material. Very importantly, we demonstrated that a 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 thus benefits from the exclusive quality of the MAPbI3 grown with the developed method.