Tracing the Dynamic Chemical Transformations of Spiro-OMeTAD in Operating Perovskite Solar Cells

A significant stability challenge for perovskite solar cells (PSCs) lies in the widely used LiTFSI-doped Spiro-OMeTAD (Spiro) hole transport layer (HTL), in n-i-p configured cells. In this study, we used the sputtering depth profile X-ray photoelectron spectroscopy (SDP-XPS) to systematically examine the cause of instability in n-i-p structured PSCs under shelf-life stability (SLS), open-circuit potential (OCP), and maximum power point tracking (MPPT) conditions. We discover dissociation of the spiro caused by the breakdown of LiTFSI under operational stress, which alters the p-type nature of the spiro and the interface band bending. Specifically, LiTFSI dissociation leads to the formation of byproducts, LiF and Li_XS_YO_Z, across the HTL layer, which initiate the degradation of spiro molecules, especially under MPPT and OCP stress. While the band bending in the SLS device was around 0.6 eV, this dropped to about 0.4 eV in devices stressed by MPPT and OCP, leading to significant decreases in open-circuit voltage and fill factor. This degradation is more severe under OCP and MPPT conditions than under SLS. By providing complex interfacial and chemical insights, this study underscores the necessity for improved dopant stability and a better HTL design to enhance the durability of PSCs.