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Effects of Alkylammonium Choice on Stability and Performance of Quasi-2D Organolead Halide Perovskites

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journal contribution
posted on 12.05.2020, 19:45 authored by Bryan R. Wygant, Alexandre Z. Ye, Andrei Dolocan, C. Buddie Mullins
The long-term instability of typical organolead halide perovskites has led to increased interest in Ruddlesden–Popper phase (RPP) perovskites. These materials have been shown to possess high stability under a variety of conditions, including humidity, making them interesting candidates for stable perovskite devices. Here, we report the increased moisture stability of a methylammonium lead triiodide-based (MAPI) RPP perovskite containing n-hexylammonium (hexyl-MAPI) as compared to an otherwise-identical material synthesized using n-butylammonium (butyl-MAPI), attributed to decreased halide mobility within the material. Despite only small differences in chemical composition, hexyl-MAPI photovoltaic devices show a significantly lower performance loss compared to butyl-MAPI devices when exposed to 78% RH, using both Au and reactive Ag electrodes. We find evidence that both perovskites develop a passivation layer composed of low-n perovskite phases at the film surface following exposure to humidity, but only butyl-MAPI films exhibit clear spectroscopic evidence of distinct low-n phases. Analysis of full devices using time-of-flight secondary ion mass spectrometry provides additional evidence of the passivation layer and shows hexyl-MAPI leaches less iodide during moisture exposure. Halide mobility measurements further confirm this observation and show that the activation energy of halide mobility in n = 2 hexyl-MAPI (74± 6 kJ/mol) is larger than that in butyl-MAPI (60± 4 kJ/mol). Together, these results show that increasing the barrier to halide mobility in perovskite materials reduces the rate of iodide leaching and indicates that RPP perovskite phases could be used to increase the stability of perovskite photovoltaic devices, regardless of the metal contact.