Impact of work function cathode on performance and stability of organic solar cells with non-fullerene interlayers

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has been reported over 19% due to the development of novel electron donor polymers and acceptor molecules such as PM6:Y6. In addition, cathode interlayers (CILs) based on non-fullerene structure (e.g., PNDIT-F3NBr and PDINN) have been employed in conventional OSCs to facilitate charge transfer from active layer to electrode. However, metal electrodes for cathode contact have received relatively little attention and the role of CIL/metal interface has been barely investigated in OSCs. While conventional OSCs generally adopt a low work function cathode (e.g., silver and aluminum) for an ideal energy positioning near the of the active material, in this study, gold (Au) with a high work function is utilized as the top electrode rarely explored, resulting in a high open circuit voltage of 0.853 V and PCE of 14% based on a device structure with ITO/PEDOT:PSS/PM6:Y6/PNDIT-F3N-Br/Au. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) reveal a significant upward shift of the apparent work function of Au (ΔΦ Au > 1.0 eV) at the CIL/Au interface, leading to a suitable energy level alignment for charge extraction and efficient device operation. On the other hand, Au diffusion into the PM6:Y6 active blend results in poor long-term stability of OSCs, as evidenced by grazing incidence wide angle X-ray scattering (GIWAXS) and impedance spectroscopy (IS).