The suppression of perovskite surface defect recombination is very critical in obtaining high-efficiency perovskite solar cells (PSCs). Ammonium salts with long carbon chains or forming two-dimensional (2D) perovskites are usually used to passivate defects. However, they might limit carrier extraction or transport due to the electrical insulation of long-chain organic ligands. Herein, we propose a small molecule of thiophene-2-acetamide (TAA) to passivate surface defects of a methylammonium lead iodide (MAPbI 3 , where MA is methylammonium) perovskite film. The results suggest that the S atom of thiophene suppresses Pb 0 defects and the −C�O group passivates the uncoordinated Pb 2+ by forming chemical bonds with Pb 2+ . The −NH 2 group further enhances the interaction between TAA and perovskite surface defects by forming hydrogen bonds with I − . Moreover, carrier extraction is promoted significantly from the perovskite film to the hole-transport layer. The incorporation of the TAA additive is found to enhance the crystallization of films via a strong interaction between the −C�O group and Pb 2+ of the perovskite precursors. The synergistic effect of surface passivation and enhanced crystallization has been successfully demonstrated in the champion MAPbI 3 PSCs, resulting in a significantly enhanced conversion efficiency from 18.35% to 20.62%. Notably, the device stability under 1-sun illumination and high humidity (∼85%) in air has significantly been improved.

SnO2 film is one of the most widely used electron transport layers (ETL) in perovskite solar cells (PSCs). However, the inherent surface defect states in SnO2 film and mismatch of the energy level alignment with perovskite limit the photovoltaic performance of PSCs. It is of great interesting to modify SnO2 ETL with additive, aiming to decrease the surface defect states and obtain well aligned energy level with perovskite. In this paper, anhydrous copper chloride (CuCl2) was employed to modify the SnO2 ETL. It is found that the adding of a small amount of CuCl2 into the SnO2 ETL can improve the proportion of Sn4+ in SnO2, passivate oxygen vacancies at the surface of SnO2 nanocrystals, improve the hydrophobicity and conductivity of ETL, and obtain a good energy level alignment with perovskite. As a result, both the photoelectric conversion efficiency (PCE) and stability of the PSCs based on SnO2 ETLs modified with CuCl2 (SnO2-CuCl2) is improved in comparison with that of the PSCs on pristine SnO2 ETLs. The optimal PSC based on SnO2-CuCl2 ETL exhibits a much higher PCE of 20.31% as compared to the control device (18.15%). The unencapsulated PSCs with CuCl2 modification maintain 89.3% of their initial PCE after exposing for 16 days under ambient conditions with a relative humidity of 35%. Cu(NO3)2 was also employed to modify the SnO2 ETL and achieved a similar effect as that of CuCl2, indicating that the cation Cu2+ plays the main role in SnO2 ETL modification.

Interface modification has been proved to be an effective method to improve the performance of perovskite solar cells (PSCs). In this paper, tetramethylammonium hexafluorophosphate (TMAPF6) is employed to modify the interface of SnO2/perovskite. Fluorine (F) in PF6− will fill the oxygen vacancy by interacting with Sn in SnO2. Meanwhile, TMA+ and PF6− in TMAPF6 will effectively fill the MA+ and I− vacancy in the interface. TMAPF6 modification enhances the hydrophobicity of the SnO2 surface, promotes the growth of high-quality perovskite film with large grain size, and then significantly suppresses the non-radiative recombination of PSCs. Furthermore, the TMAPF6 modification introduces a better energy level alignment between SnO2 and perovskite layer, enabling a more efficient electron extraction. As a result, the TMAPF6-modified MAPbI3 PSC achieves a significantly increasing power conversion efficiency (PCE) from 18.62 % to 20.92 % and an improved stability with only 15 % PCE drop after 600 hours of storage in air. This work develops an efficient interface modification molecule to increase the efficiency of PSCs, which would be a promising strategy for the large-scale commercialization of the photovoltaic devices.