Phase Stability of Quasi-2D Hybrid Perovskite Films by Chemical Vapor Deposition

A promising approach to tackle the environmental instability in three-dimensional (3D) hybrid perovskite (HP) solar cells is to reduce its dimensionality to two-dimensions (2D). However, the anisotropic charge-transport properties of 2D HP have a negative impact on the solar cell power conversion efficiency. A resolve to this is to use a quasi-2D HP absorber film, which retains the effective charge-transport properties of the 3D material. Chemical vapor deposition (CVD) is an established solvent-free technique for the conformal deposition of air-stable perovskite layers and devices. We report on the low-pressure CVD of a quasi-2D HP thin film by a consecutive 2D to quasi-2D conversion, using butylammonium (BA) as the long-chain organic cation to produce BA₂PbI₄, followed by its exposure to the methylammonium (MA) small cation, to produce BA₂MA_n-1Pb_nI_3n+1. Absorption and emission spectroscopy confirm the presence of the constituent MAPbI₃ and BA₂PbI₄ peaks and the formation of a quasi-2D HP. Synchrotron-based grazing incidence x-ray diffraction (XRD) measurements from 50 – 300 K provides insight into the phase stability of the material. Rietveld refinement of the XRD spectra confirm the presence of the n = 1 and n = 2 Ruddlesden-Popper (RP) phases and the unanticipated n = 1 Dion-Jacobson (DJ) phase throughout the temperature range. Furthermore, no phase changes or lattice volume expansion is observed for the RP and DJ phases, which speaks to the phase stability of the material throughout the temperature range.