Impact of charge transport layers on the photochemical stability of MAPbI3 in thin films and perovskite solar cells

In this work, we systematically explored the photochemical stability of MAPbI3 interfaces with five different charge transport layers (CTLs): substituted naphthalene (NDI) and perylene (PDI) diimides, fullerene derivatives PC61BM and PC71BM, and widely used hole transport material spiro-OMeTAD. While studying these model systems, we revealed several important interfacial degradation pathways. First, we demonstrated that diimides (both NDI and PDI) strongly accelerate the photodegradation of MAPbI3 since they specifically interact with PbI2 formed as one of the perovskite degradation products. Fullerene derivatives, in turn, absorb another decomposition product, methylammonium iodide (MAI), which also results in accelerated ageing of the perovskite films deposited atop fullerene-based CTLs. However, both fullerene derivatives and spiro-OMeTAD have excellent encapsulation properties and strongly suppress photodecomposition of MAPbI3, when the perovskite films are placed underneath. Unfortunately, the observed impressive perovskite bulk phase stabilization using PC61BM, PC71BM and spiro-OMeTAD coatings does not improve the operation lifetime of photovoltaic cells due to interfacial degradation processes. TOF-SIMS analysis has revealed accumulation of the perovskite decomposition products (presumably CH3I) in the spiro-OMeTAD layer as well as a noticeable degradation at the interface with the electron transport tin dioxide layer. The obtained results emphasize the major challenge in the development of perovskite solar cells with long operation lifetime related to poor stability of the functional perovskite/CTL interfaces.