Strain-less Perovskite Film Engineered by Interfacial Molecule for Stable Perovskite Solar Cells

Although perovskite solar cells (PSCs) have demonstrated power conversion efficiency (PCE) as high as 26%, instability caused by the heterointerfaces has been an issue. Since the thermal expansion coefficient of perovskite is higher than that of SnO₂, in-plane tensile strain formed at the perovskite/SnO₂ interface is one of the causes of the instability of PSCs. We report here an effective methodology to regulate the strain via surface modification of the SnO₂ layer with a bifunctional molecule of phosphorylethanolamine (PEA) bearing phosphate and amine groups linked by an alkyl chain. The grazing incidence X-ray diffraction data showed that an in-plane tensile strain observed upon deposition of the perovskite film on the bare SnO₂ layer was substantially released by modifying the SnO₂ surface with PEA. The strain-less interface resulted in an increase in PCE from 22.87% to 24.35%. Moreover, the unencapsulated device stability was better for the PEA-modified SnO₂ than for the unmodified one: 93% of initial PCE after 1700 h versus 66% of initial PCE. Light-soaking stability of the device with the PEA-modified SnO₂ was superior to the one with the unmodified SnO₂.