Organolead halide perovskites are a new class of photovoltaic materials for use in thin film solar cells, but suffer from rapid degradation under light and humidity. Recently-developed quasi-2D Ruddlesden-Popper phase perovskites show improved electrochemical stability, but the mechanism behind their stability is not well understood. Here, we have used time-of-flight secondary ion mass spectrometry (ToF-SIMS), photoluminescence spectroscopy (PL), electrochemistry, and other techniques to study the chemistry and performance of both 2D and 3D phases of methylammonium lead triiodide (MAPI) solar cells in relation to humidity exposure. ToF-SIMS depth profiles of 3D MAPI planar solar devices (n-i-p, C60/perovskite/PEDOT:PSS) show the formation of a significant hydrolysis-based degradation layer at the C60 interface of the perovskite film after humidity exposure, while 2D MAPI device depth profiles do not. Isotopic D2O vapor studies confirm the degradation layer is caused by ambient humidity, while electrochemistry confirms it is directly related to loss of electrochemical cell performance. The growth of this hydrolysis layer is found to be inhibited in 2D MAPI devices by the formation of a thin protective layer composed of more thermodynamically-stable 2D phases, clearly observed in the PL spectra, which acts to protect the less stable 2D bulk. To confirm that this newly-formed 2D material improves device stability, we fabricated solar devices consisting of 3D MAPI films coated with a thin layer of the 2D perovskite, and show that these cells are more stable than un-treated 3D MAPI devices. As a result, we conclude that using 2D perovskite as a protective interface may be a simple way to limit humidity-related chemical damage and improve the stability of perovskite materials.

Probing the degradation chemistry and enhanced stability of 2D organolead halide perovskites

2019 Spring National Meeting

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