In situ interface engineering via gradient rubidium doped perovskite for efficient electron-transport-layer-free photovoltaics

Electron-transport-layer (ETL) free perovskite solar cells (PSCs) have demonstrated great potential for industrialization due to their simple and economical device configurations. However, the performance of ETL-free PSCs still lags behind that of ETL-containing devices, primarily due to sluggish charge extraction and severe interfacial charge recombination at the transparent conducting oxide (TCO)/perovskite interface. Herein, we have developed an in situ interface engineering strategy by gradient rubidium (Rb) doped perovskite for efficient ETL-free photovoltaics. Rb+ is introduced into a 1.63 eV bandgap perovskite, in which the Rb+ ions predominantly accumulate at the bottom of perovskite films. The smaller Rb+ cation at the bottom will reduce the lattice constant and enhance the orbital coupling between Pb and I/Br atoms, leading to matched interface energy-level with a barrier-free contact and reduced the interfacial charge recombination. Consequently, the Rb based ETL-free PSCs achieves a maximum efficiency of 21.2 % with significantly increased open circuit voltage and fill factor compared to the control device. This work provides a simplified and efficient interface engineering method without additional interface layer, which have great potential in practical applications.