Passivating buried interface via self-assembled novel sulfonium salt toward stable and efficient perovskite solar cells

The interfacial carrier nonradiative recombination resulting from defects and energy barrier at buried interface hinders further enhancement of power conversion efficiency (PCE) and stability of perovskite solar cells. Herein, we report a simple and effective buried interface passivation strategy based on cation engineering through employing a new type of sulfonium salt ((2-carboxyethyl) dimethyl sulfonium chloride, CDSC) together with reference molecule (3-dimethylamino propionic acid hydrochloride, DPAH) to modify the interface between perovskite and electron transport layers. It is theoretically and experimentally revealed at the atomic scale that CDSC and DPAH chemically interact with both SnO2 and perovskite layer and accordingly well bridge both layers. Both modifiers can not only passivate the defects from the surface of perovskite and SnO2 films, but also reduce interfacial energy barrier via improving energy band alignment. CDSC are certified to be more effective in defect passivation and energy band modulation than DPAH, for the first time revealing that sulfonium cations are superior to commonly adopted ammonium cations. Finally, the DPAH and CDSC-modified devices achieve a PCE of 21.44% and 22.22%, respectively, far outperforming the control device (20.72%). The unsealed devices with CDSC maintain 92.5% of their initial efficiency after thermal aging for 1272 h.