Tin dioxide (SnO) has been demonstrated as an effective electron-transporting layer (ETL) for attaining high-performance perovskite solar cells (PSCs). However, the numerous trap states in low-temperature solution processed SnO will reduce the PSCs performance and result in serious hysteresis. Here, we report a strategy to improve the electronic properties in SnO through a facile treatment of the films with adding a small amount of graphene quantum dots (GQDs). We demonstrate that the photogenerated electrons in GQDs can transfer to the conduction band of SnO. The transferred electrons from the GQDs will effectively fill the electron traps as well as improve the conductivity of SnO, which is beneficial for improving the electron extraction efficiency and reducing the recombination at the ETLs/perovskite interface. The device fabricated with SnO:GQDs could reach an average power conversion efficiency (PCE) of 19.2 ± 1.0% and a highest steady-state PCE of 20.23% with very little hysteresis. Our study provides an effective way to enhance the performance of perovskite solar cells through improving the electronic properties of SnO.

Numerous trap states and low conductivity of compact TiO 2 layers are major obstacles for achieving high power conversion efficiency and high-stability perovskite solar cells. Here we report an effective Na 2 S-doped TiO 2 layer, which can improve the conductivity of TiO 2 layers, the contact of the TiO 2 /perovskite interface, and the crystallinity of perovskite layers. Comprehensive investigations demonstrate that Na cations increase the conductivity of TiO 2 layers while S anions change the wettability of TiO 2 layers, thus improving the crystallinity of perovskite layers and passivate defects at the TiO 2 /PVK interface. The synergetic effects of dopants lead to a champion efficiency as high as 21.25% in unencapsulated perovskite solar cells (PSCs), with much-improved stability. Our work provides new insights on anion dopants in TiO 2 layers, which is usually neglected in previous reports, and also proposes a simple approach to produce low-cost and highperformance electron transport layers for high-performance PSCs.

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