Role of Additives on the Performance of CsPbI3 Solar Cells

The role of additives in the performance of CsPbI3 perovskite solar cells (PSCs) was investigated. Different kinds of cations and anions were used as additives in a N,N-dimethylformamide (DMF) solution containing CsI and PbI2 (1:1 molar ratio). These include HI, HBr, HCI, NH4I, NH4Br, and NH4CI. Additive cations (H+ and NH4+) as well as halide ions (I-, Br-, and Cl-) are important for the properties of PSCs. Especially, the addition of iodine ion showed good characteristics compared to Br- and Cl-. Among the CsPbI3 layers prepared with different kinds of additives and annealed at different temperatures, the X-ray diffraction peaks of CsPbI3 were clearly found at 14 degrees and 28 degrees for the sample annealed at 150 degrees C with 50 mu L of HI, suggesting the formation of a cubic structure at the low temperature of 150 degrees C. The field emission scanning electron microscopy images indicate that the surface of the perovskite layer with hydrogen halide additive (H+-based additive) is more uniform than that with ammonium additive. The roughness profiles determined by atomic force microscopy indicate that the CsPbI3 film with HI additive shows the least roughness among the samples with H+-based additives. Therefore, the best power conversion efficiency (PCE) of 4.72% is obtained for CsPbI3 PSCs annealed at 150 degrees C with HI (50 mu L). The H+-based additives seem to react with PbI2 in DMF solution, increasing the solubility of PbI2 and thus lowering the processing temperature. Furthermore, the PCE of CH3NH3PbI3-xClx PSCs decreased from 7.45 to 0.23%, whereas that of CsPbI3 PSCs with 50 mu L of HI only decreased from 3.55 to 2.78% after exposing the samples to air for 3 h. These results indicate that H+-based additives, especially HI, have more impact on the CsPbI3 PSCs in terms of lowering the processing temperature and improving the performance.