Two-Step MAPbI₃ Integration by Low-Vacuum Proximity-Space-Effusion in Inverted Semitransparent Perovskite Solar Cells for Tandem Applications

Hybrid Organic-Inorganic Perovskites are worldwide one of the most investigated materials today, due to their unique properties impacting various fields as Photovoltaics, (PV), Light Emission Devices(LEDs) and Photodetectors. In PV, thanks to the wide absorption range and excellent charge carrier lifetime and diffusion of the material, Perovskite Solar Cells (PSCs) were able to reach the exceptional goals of 26.1%. All their exceptional properties, are strictly related to the possibility of preparing high quality films in terms of crystallinity, thickness and low defect density, are strictly related. In this perspective, although vacuum deposition methods are more suitable in terms of industrial throughput, they were less investigated, despite some seminal work demonstrating how hybrid organic inorganic perovskite films prepared by co-evaporation possess higher stability and performances with respect films prepared by spin coating techniques. 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 about 10^-2mbar conditions to produce high-quality thin layers of phase-pure MAPbI₃. The parameter optimization was supported by process simulation. 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.<br/>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 MAPbI₃grown with the developed method.<br/>Lastly, integration of LV-PSE MAPbI₃ layers into monolithic Si-Perovskite tandem solar cells have been simulated with the Seftos software by Fluxim, using as input the optical constants of the material measured by Spectroscopic Ellipsometry. Current matching is achieved for a MAPbI₃ thickness of 300 nm, demonstrating that the integration is feasible even with this medium-bandgap perovskite, provided semi-transparency preserves performances and low lattice defects as in the LV-PSE material.