MAPbI3 Deposition by LV-PSE on TiO2 for Photovoltaic Application

Abstract

Hybrid perovskites are one of the most popular materials nowadays due to their very exclusive properties. To mitigate costs, complexity, and environmental impact, in this work, we have prepared methylammonium lead iodide (MAPbI 3 ) films by a two-step Low-Vacuum Proximity-Space-Effusion (LV-PSE). The LV-PSE method exploits the low vacuum and the short diffusion path from the precursor source to have high thermal energy and partial pressure of the sublimated species close to the substrate. To this aim, the substrate is located at a medium distance (∼2 cm) from the melting pots in a low-vacuum chamber at ∼4 × 10 −2  mbar. In the first step, a PbI 2 film is deposited on a substrate; in the second step, the conversion into MAPbI 3 occurs via an adsorption-incorporation-migration mechanism through the evaporation of methylammonium iodide (MAI) reagents. To exploit the potential of the conversion reaction, 190 nm MAPbI 3 layers are deposited on TiO 2 substrates. The layers were characterized in terms of crystal structure by X-ray diffraction (XRD) analyses, which showed the exclusive presence of MAPbI 3 confirming the complete conversion of the PbI 2 film. Scanning Electron Microscopy (SEM) analyses revealed a flat uniform pinhole-free coverage of the substrates and good conformational coverage of the TiO 2 underlayer. Transmission Electron Microscopy (TEM) analyses addressed the formation of the tetragonal phase and the absence of the amorphous phase in the film. Spectroscopic ellipsometry (SE) analyses were used to explore the optical properties and the stability of the MAPbI 3 layer at different temperatures and ambient conditions. As proof of concept, solar cell architectures were prepared using TiO 2 as Electron Transporting Layer (ETL), Spiro-OMeTAD as Hole Transporting Layer (HTL), and Au as a contact to exploit the new up-scalable and clean deposition method. Using just ∼190 nm thick layers, the best efficiency reached with this architecture was 6.30%.