In recent years, perovskite solar cells have achieved a power conversion efficiency of >26%. This improvement is due to the exceptional material properties of the perovskite, such as tunable light absorption, high carrier mobility, long diffusion length, and low exciton binding energy. Moreover, researchers focus on diversifying the material composition using mixed-cation and halide formulation, developing different device structures like n-i-p and p-i-n. At the same, perovskite materials suffer from short-term stability under moisture, migration of cations and anions as well as metal from the electrodes. This leads to irreversible degradation of the perovskite.

The single junction n-i-p structure perovskite solar cells with SpiroOMeTAD as a hole transporting layer is a typically used transport layer, although the interface between SpiroOMeTAD and perovskite is hard to stabilize: Reports show that the hygroscopic LiTFSI salts in SpiroOMeTAD tend to aggregrate on the film due to the strong moisture absorbing effect from the air. Additionally, the Li ion was shown to diffuse into the perovskite layer and electron transporting layer to alter their election properties.

To overcome the ion migration, atomic layer deposition of AlO_x have been investigated proving to increase stability and power conversion efficiency. However, there is still a lack of an in-depth explanation of the role of AlO_x at SpiroOMeTAD perovskite interface. In our work, when the ALD of AlO_x is employed between the perovskite and SpiroOMeTAD, the device performance increased to 19.5% relative to the pristine interface of 18%. The overall improvement of the AlO_x interface modified PSCs resulted from many factos such as the removal of surface defect states, and formation of AlF from reaction between AlO_x and LiTFSI in SpiroOMeTAD, which were confirmed by the PL and HAADF STEM repectively. At the same time, the AlO_x interface modified PSCs have better wettability for better deposition of SpiroOMeTAD, contact angle reduced 9° for target devices in comparison with the control devices, confirmed by contact angle experiments with various ALD-AlO_x thickness. Our investigation on AlO_x modified PSCs with techniques including SEM, HAXPS, and ToF-SIMs reveals that ALD layer positively interacts chemically with both perovskite and SpiroOMeTAD, resulting in better transfer of photogenerated charge carriers along with limiting the migration of ions and protecting the perovskite against moisture.

Keywords: Atomic layer deposition, perovskite solar cell, ALD-AlO_x at interface of SpiroOMeTAD and perovskite.