Performance enhancement of high temperature SnO₂-based planar perovskite solar cells: Electrical characterization and understanding of the mechanism

Mg doping into high temperature processed (HTP) SnO₂ as an electron selective layer (ESL) significantly improves perovskite solar cell (PSC) performance including power conversion efficiency (PCE), open circuit voltage, short circuit current (J_SC) and fill factor. An optimum Mg content (7.5%) affords a uniform, smooth and dense Mg-doped SnO₂ film with high electron mobility and its corresponding PSC displays less hysteresis and achieves a highest steady-state PCE of 14.55%, nearly 92.8% enhancement compared to that with undoped SnO₂. Electrical measurements show that suitable Mg doping dramatically reduces free electron density and substantially increases the electron mobility of pristine SnO₂. The mechanism of efficiency enhancement for PSCs is proposed as follows: the low free electron density causes suppression of carrier recombination and high electron mobility facilitates fast extraction of electrons from perovskite to ESLs, contributing to an improved J_SC. Impedance analysis strongly supports the proposed mechanism and reveals that the higher is the electron mobility, the higher is the electron collection efficiency, and the higher are the J_SC and PCE. The HTP SnO₂ with a suitable Mg content can be an excellent ESL for PSCs and might well be a suitable candidate of ESLs for CdTe, CuInGaSe and other photovoltaic devices involved in HTP treatment.