Unraveling the Molecular Size Effect on Surface Engineering of Perovskite Solar Cells

Surface engineering in perovskite solar cells, especially for the upper surface of perovskite, is widely studied. However, most of these studies have primarily focused on the interaction between additive functional groups and perovskite point defects, neglecting the influence of other parts of additive molecules. Herein, additives with -NH3+ functional group are introduced at the perovskite surface to suppress surface defects. The chain lengths of these additives vary to conduct a detailed investigation into the impact of molecular size. The results indicate that the propane-1,3-diamine dihydroiodide (PDAI2), which possesses the most suitable size, exhibited obvious optimization effects. Whereas the molecules, methylenediamine dihydroiodide (MDAI2) and pentane-1,5-diamine dihydroiodide (PentDAI2) with unsuitable size, lead to a deterioration in device performance. The PDAI2-treated devices achieved a certified power conversion efficiency (PCE) of 25.81% and the unencapsulated devices retained over 80% of their initial PCE after 600 h AM1.5 illumination. The additive with the best-matched molecular size for selected perovskite exhibits the most effective passivation effect, while the additives with the mismatched size have a negative effect on the device performance simply because of the size difference. image