Phosphonic Acid Modification of the Electron Selective Contact: Interfacial Effects in Perovskite Solar Cells

The role electron-transport layers (ETLs) play in perovskite solar cells (PSCs) is still widely debated. Conduction band alignment at the perovskite/ETL interface has been suggested to be an important role for the performance of the solar cells. However, little is known about the effects of work-function shifts on the solar-cell performance, and specifically, the open-circuit voltage (V-OC). Here, the effects of surface modification of SnO2 ETLs using polar phosphonic acids are investigated, including the effects on work function, surface energy, device performance, and device stability in inert atmosphere. The phosphonic acid modifications did not have a large effect on V-OC; however, a sharp decrease in the overall device performance was found, mostly due to reduced fill factors. When exposed to conditions of low oxygen concentration, the phosphonic acid surface modified devices yielded current-voltage (J-V) curves with considerably lower hysteresis than those based on unmodified SnO2. This suggests that this modification method may be valuable for achieving stabilized power conversion efficiency without hysteresis.