Interdiffusion Control in Sequentially Evaporated Organic-Inorganic Fully Vacuum Deposited Perovskite Solar Cells

Vacuum deposition of metal halide perovskite is a scalable and adaptable method. In this study, we adopt sequential evaporation to form the perovskite layer and reveal how the relative humidity content, during the annealing step, impacts its crystallinity and the photoluminescence quantum yield (PLQY). By controlling the humidity, we achieved a significant enhancement of 50 times in PLQY from 0.12% to 6%. This improvement equates to a 70 milielectronvolt (meV) increase in the implied open-circuit voltage (Voc) for a photovoltaic device. We discover that the enhancement in PLQY corresponds to an increase in crystallinity and photophysical properties by combining structural methods like in-situ grazing incidence wide angle X-ray scattering (GIWAXS) and three-dimensional time-of-flight secondary ion mass spectroscopy (3D ToF-SIMS) with spectroscopic methods like time-resolved photoluminescence (TRPL). Our results show that annealing in a controlled humid environment improves the organic and inorganic halides' interdiffusion throughout the bulk, which in turn significantly reduces non-radiative recombination by an order of magnitude. Specifically, the non-radiative recombination rate in the bulk is reduced from 1.7x10⁷s^-1to 3.3x10⁶ s^-1, and at the interface between the perovskite and hole transport layer from 1.5x10⁷ s^-1 to 4.3x10⁵s^-1. We also observe a significant enhancement in vertical charge transport mobility from 0.9 to 2.6 cm²(Vs)^-1. We further demonstrate that the enhanced intermixing results in fully vacuum-deposited p-i-n perovskite solar cells (PSCs) with a maximum power point tracked efficiency of 21% under simulated air mass (AM) 1.5G 100 mWcm-2 irradiance. Additionally, PSCs exhibit superior stability under full spectrum simulated solar illumination at 85°C and in an open-circuit condition for over 200 hours.