Reaction Temperature and Partial Pressure Induced Etching of Methylammonium Lead Iodide Perovskite by Trimethylaluminum

Abstract

Al 2 O 3 atomic layer deposition (ALD), which uses trimethylaluminum (TMA) as the metal precursor, shows promise in improving the environmental stability of hybrid halide perovskites. However, it is not yet entirely clear how TMA, a strong Lewis acid, reacts with fresh perovskites and how the reaction affects the nucleation of ALD Al 2 O 3 . Here, the effects of reaction temperature and partial pressure of TMA on the mechanisms of TMA/CH 3 NH 3 PbI 3 reactions are investigated. Our real time mass gain data and in situ mass spectrometry data show that the TMA/CH 3 NH 3 PbI 3 reaction can either remove mass or accumulate mass onto CH 3 NH 3 PbI 3 substrates, depending strongly on the reaction temperature and partial pressure of TMA. The TMA/CH 3 NH 3 PbI 3 reaction probably generates TMA−CH 3 NHx adduct compounds, which protects CH 3 NH 3 PbI 3 from TMA by forming a shell at 25 °C in the vacuum process. However, these adduct compounds decompose at higher temperatures (e.g., 75 °C). This product layer is much thicker than a monolayer, suggesting the interface formed between Al 2 O 3 coating and CH 3 NH 3 PbI 3 is blurring and messy. These results have not yet, but should be, carefully considered to correctly interpret the effect of ALD Al 2 O 3 treatment on optoelectronic properties of CH 3 NH 3 PbI 3 .

The chemistry and energetics of the interface between metal halide perovskite and atomic layer deposited metal oxides

“…[26][27][28] Furthermore, by means of in situ quartz crystal microbalance (QCM) and IR spectroscopy, it was shown that the TMA molecules etch the CH 3 NH 3 PbI 3−x Cl x perovskite surface at 75°C. 27 A similar behavior was reported by Yu et al 29 Synchrotron XPS analysis was employed by Kot et al, 30 which indicated that the interaction of ALD precursors occurs only at the surface of the CH 3 NH 3 PbI 3 and FA 0.85 MA 0.15 Pb(Br 0.15 I 0.85 ) 3 perovskite (where FA corresponds to the formamidinium cation) during the growth of Al 2 O 3 at room temperature. 23,24 In parallel, they showed that there is covalent bonding at the CH 3 NH 3 PbI 3 /Al 2 O 3 interface.…”

SnOx Atomic Layer Deposition on Bare Perovskite—An Investigation of Initial Growth Dynamics, Interface Chemistry, and Solar Cell Performance

“…The quick and persistent mass gain from the TDMASn pulse exposure, however, suggests that the added mass during the pulse was a spontaneous and continuous reaction between the precursor and the perovskite surface (Figure S3 in the Supporting Information). A similar result was also shown in a previous study, where a modified surface was found in conjunction with a mass gain during the low partial pressure TMAl exposures of perovskite . The initial exposure of TDMASn during the ALD processes in this study was found to leave a surface that gave a smaller mass gain for the following SnO x cycles as compared to the film-on-film growth regime.…”

“…A similar result was also shown in a previous study, where a modified surface was found in conjunction with a mass gain during the low partial pressure TMAl exposures of perovskite. 13 The initial exposure of TDMASn during the ALD processes in this study was found to leave a surface that gave a smaller mass gain for the following SnO x cycles as compared to the film-on-film growth regime. Thus, after the initial exposure of TDMASn, there seems to be less-than-expected −DMA ligands at the surface, which would reduce the number of sites that H 2 O can react with and in turn would limit the amount of −OH groups that TDMASn can react with in the next pulse, effectively reducing the mass gain per cycle.…”

“…It does however rely on chemical reactions. So far, most studies that perform ALD directly on perovskites have shown that the employed chemistry is too reactive and damages the perovskite surface or even bulk. − The amount of available ALD chemistries and growth conditions is however large, − and there are thus a lot of possibilities to alter the growth process in order to form a more intact perovskite/inorganic interface. Studies have also shown that even if the surface does become modified, thin layers of Al 2 O 3 can still be grown by ALD directly on the perovskite and improve both solar cell performance and long-term stability. , A parallel study also showed that TiO 2 could successfully be deposited on (Cs,FA)­Pb­(Br,I) 3 …”

Enhanced Nucleation of Atomic Layer Deposited Contacts Improves Operational Stability of Perovskite Solar Cells in Air

“…Several studies have reported that excessive exposure to metal-organic ALD precursors can damage the perovskite material by etching away organic cations from the perovskite surface. [55][56][57][58] We used the same analysis to investigate the nucleation of ALD SnO 2 on C 60 that has been coated with a thin layer of PEIE. The thickness of the PEIE layer is approximately 2 nm as measured by spectroscopic ellipsometry and can be controlled by varying the concentration of the PEIE solution ( Figure S3).…”