Additive MXene and dominant recombination channel in perovskite solar cells

Two-dimensional (2D) MXenes have emerged as attractive materials for application in perovskite solar cells (PSCs). Their unique properties arising from terminating functional groups to tune the work function (WF) of the layers in a PSC lead to a notable improvement in the photovoltaic efficiency. The ideality factor (nid) is used to infer the dominant recombination type in the PSCs. We study the effect of the added MXene in the bulk absorber and the electron transport layer (ETL), and at the interfaces on the ideality factor (nid), and then the obtained nid is used to infer the dominant recombination type in the PSCs. By using the concept of intensity-dependent quasi -Fermi level splitting (QFLS), conduction and valence band offsets, and drift-diffusion simulations, we show that the bulk recombination in the MXene-free cells dominates. Considering the obtained nid values, we explain that adding MXene just into the absorber layer leads to a suppressed bulk recombination compared with the case of MXene-free device. Meanwhile, devices with added MXene into the ETL and at the ETL/absorber interface show a notable reduced interface recombination. The study of the interfacial energy offsets and their correlation with the interface and bulk recombination currents reveals that in the cells with MXene, higher nid values are desirable and correlate with better cell performance.