Chemical\nInteraction at the MoO₃/CH₃NH₃PbI_3–<i>x</i>Cl<i>_x</i> Interface

Abstract

The\nlimited long-term stability of metal halide perovskite-based\nsolar cells is a bottleneck in their drive toward widespread commercial\nadaptation. The organic hole-transport materials (HTMs) have been\nimplicated in the degradation, and metal oxide layers are proposed\nas alternatives. One of the most prominent metal oxide HTM in organic\nphotovoltaics is MoO₃. However, the use of MoO₃ as HTM in metal halide perovskite-based devices causes a severe\nsolar cell deterioration. Thus, the formation of the MoO₃/CH₃NH₃PbI_3–<i>x</i>Cl<i>_x</i> (MAPbI_3–<i>x</i>Cl<i>_x</i>) heterojunction is systematically\nstudied by synchrotron-based hard X-ray photoelectron spectroscopy,\nscanning electron microscopy, energy-dispersive X-ray spectroscopy,\nand Raman spectroscopy. Upon MoO₃ deposition, significant\nchemical interaction is induced at the MoO₃/MAPbI_3–<i>x</i>Cl<i>_x</i> interface: substoichiometric\nmolybdenum oxide is present, and the perovskite decomposes in the\nproximity of the interface, leading to accumulation of PbI₂ on the MoO₃ cover layer. Furthermore, we find evidence\nfor the formation of new compounds such as PbMoO₄, PbN₂O₂, and PbO as a result of the MAPbI_3–<i>x</i>Cl<i>_x</i> decomposition and\nsuggest chemical reaction pathways to describe the underlying mechanism.\nThese findings suggest that the (direct) MoO₃/MAPbI_3–<i>x</i>Cl<i>_x</i> interface\nmay be inherently unstable. It provides an explanation for the low\npower conversion efficiencies of metal halide perovskite solar cells\nthat use MoO₃ as a hole-transport material and in which\nthere is a direct contact between MoO₃ and perovskite.