Solution-Processed Cu₉S₅ as a Hole Transport Layer for Efficient and Stable Perovskite Solar Cells

Organic–inorganic perovskite solar cells have seen tremendous developments in recent years. As a hole transport material, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is widely used in n–i–p perovskite solar cells. However, it may lead to the perovskite film degradation due to the dopant lithium bis((trifluoromethyl)sulfonyl)amide (Li-TFSI), which has strong hydrophilicity. Cu9S5 is considered as a superior p-type transport material, which also has a favorable energy level matching with the highest occupied molecular orbital of Spiro-OMeTAD. Herein, a solution-processed organic–inorganic-integrated hole transport layer was reported, which is composed of the undoped Spiro-OMeTAD and Cu9S5 layer. Since there is no Li-TFSI doping, it is extremely conductive to the long-term stability of the solar cells. In the meantime, we proposed a method to adjust the lowest unoccupied molecular orbital (LUMO) of SnO2 via nitrogen implantation (N:SnO2). The LUMO of SnO2 can be tuned from −4.33 to −3.91 eV, which matches well with the LUMO of CH3NH3PbI3 (−3.90 eV), and thus helps to reduce hysteresis. The modified hole and electron transport layers were applied in n–i–p perovskite solar cells, which achieve a maximum power conversion efficiency (PCE) of 17.10 and 96% retention of PCE after 1200 h in air atmosphere without any encapsulation.