Unveiling the origin of performance reduction in perovskite solar cells with TiO₂ electron transport layer: Conduction band minimum mismatches and chemical interactions at buried interface

Direct evidence of chemical interaction and origin of electron accumulation at a “buried” methylammonium lead triiodide (CH₃NH₃PbI₃, hereafter “MAPI”)/TiO₂ interface is presented in this study for the first time. Despite the high power conversion efficiency of perovskite solar cells (PSCs) using a TiO₂ electron transport layer, the MAPI/TiO₂ interface is believed as an electron accumulation position during device operation. To elucidate the cause of the electron accumulation, the energy level alignment at the MAPI/TiO₂ interface should be understood. However, a buried MAPI/TiO₂ interface forms after a thick MAPI layer deposition; thus, the electronic structure of the MAPI/TiO₂ interface cannot be measured using surface-sensitive photoelectron spectroscopy in a conventional stack-up manner. In this study, we investigated the electronic structure of a buried MAPI/TiO₂ interface by removing the MAPI and organic layers using solvent immersion. As a result, we reveal that a conduction band minimum (CBM) mismatch occurs owing to the Ti–O–Pb bonding on the TiO₂ surface. The Ti–O–Pb bonds form by the Pb ions penetrating during the spin coating of the MAPI solution. When a [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) layer was inserted, the CBM mismatch was removed owing to the high work function of PCBM.