Coating Evaporated MAPI Thin Films with Organic Molecules: Improved Stability at High Temperature and Implementation in High-Efficiency Solar Cells

Methylammonium lead iodide (MAPI) has proven to be an exceptional light-absorber for single-junction thin-film solar cells.Nonetheless, degradation induced by environmental agents (air, moisture, heat) limits the stability of this hybrid perovskite.Here, we demonstrate that coating evaporated MAPI thin films with different hydrophobic molecules leads to a significant improvement in their stability.We especially investigated the degradation of MAPI and the subsequent formation of PbI 2 at 150 °C by in situ XRD analysis and showed that this transformation is remarkably slowed down in films coated with trioctyl phosphine oxide and tridodecyl methylammonium iodide.This enhances the processability of such films, which is an important aspect for the fabrication of thin-film devices.Eventually, we demonstrate that such protected films can be implemented in single-junction n-i-p solar cells without any loss in the device efficiency.2][3][4] The outstanding performances of these devices can be linked to several photophysical properties of perovskites, such as their high absorption coefficient, narrow bandgap, and high charge diffusion lengths, 5,6 together with the ease of fabrication and low cost of precursors. 7Nonetheless, the widespread use of MAPI is currently severely hindered by its well-known instability.Indeed, oxygen, water (moisture), and heat among other factors are known to lead to a fast degradation of MAPI thin films. 8,9In order to mitigate this instability, one possible approach is to alter the material composition, as already demonstrated in many reports.5][16][17][18] These substitutions can lead to rather complex formulations comprising up to seven different ions whose relative amounts 47 need to be well controlled. 195][16][17][18] Hence, it is critical to explore new ways of 51 enhancing the stability of MAPI films without necessarily 52 altering their overall composition.Because degradation often 53 initiates at the surface of the films through its exposure to 54 external species (e.g., moisture), 20 one possible strategy is to 55 chemically modify the perovskite surface with different organic 56 molecules.Yang et al. 21showed that coating the surface of 57 solution-processed MAPI thin films with different hydrophobic 58 molecules leads to an increased stability in a high-moisture 59 environment at room temperature.The same approach has 60 recently been developed by Wang et al., 22 while deQuilettes et 61 al. 23 found that surface modification with Lewis base molecules 62 leads to efficient passivation of the nonradiative surface trap 63 states.Huang et al. also noticed an increase of initial efficiency