Room‐Temperature Cubic Phase Crystallization and High Stability of Vacuum‐Deposited Methylammonium Lead Triiodide Thin Films for High‐Efficiency Solar Cells

Abstract

Methylammonium lead triiodide (MAPI) has emerged as a high‐performance photovoltaic material. Common understanding is that at room temperature, it adopts a tetragonal phase and it only converts to the perfect cubic phase around 50–60 °C. Most MAPI films are prepared using a solution‐based coating process, yet they can also be obtained by vapor‐phase deposition methods. Vapor‐phase‐processed MAPI films have significantly different characteristics than their solvent‐processed analogous, such as relatively small crystal‐grain sizes and short excited‐state lifetimes. However, solar cells based on vapor‐phase‐processed MAPI films exhibit high power‐conversion efficiencies. Surprisingly, after detailed characterization it is found that the vapor‐phase‐processed MAPI films adopt a cubic crystal structure at room temperature that is stable for weeks, even in ambient atmosphere. Furthermore, it is demonstrated that by tuning the deposition rates of both precursors during codeposition it is possible to vary the perovskite phase from cubic to tetragonal at room temperature. These findings challenge the common belief that MAPI is only stable in the tetragonal phase at room temperature.

Fully Vacuum‐Processed Perovskite Solar Cells on Pyramidal Microtextures

“…In a recent report it was shown that different ratios lead to a changing morphology and crystalline phase for the MAPbI 3 films. [34,35] In addition, the different ratios can affect the growth of the perovskite and consequently the charge carrier transport, as it was published by Buonassisi et al [36] X-ray diffraction (XRD) measurements were performed on the films to examine the formation of perovskite phases and crystallinity ( Figure S2, Supporting Information). In all evaporated films, the characteristic peaks of the perovskite were observed.…”

Preparation of methylammonium lead iodide (CH3NH3PbI3) thin film perovskite solar cells by chemical vapor deposition using methylamine gas (CH3NH2) and hydrogen iodide gas

“…These are high-volume phases, indicated to be predominantly tetragonal when found in high quantities, as well as a low-volume phase, predominantly cubic. 21 Their quantities show a strong dependence on the treatment step, where the low-volume phase becomes predominant after MA treatment.…”

Historical Analysis of High‐Efficiency, Large‐Area Solar Cells: Toward Upscaling of Perovskite Solar Cells

“…Although a higher efficiency than that of the solution process has not yet been demonstrated for non-solution-based processes, attempts to apply such processes on perovskite solar cells were made. [69,74,80,83,[225][226][227] As many researchers already recognize, it is difficult to directly apply the deposition technology used in other thin-film solar cells to perovskite solar cells. First, perovskite materials are known to have a fast crystallization rate at relatively low temperatures.…”

“…Although a higher efficiency than that of the solution process has not yet been demonstrated for non‐solution‐based processes, attempts to apply such processes on perovskite solar cells were made. [ 69,74,80,83,225–227 ]…”