Amphiphilic Carbon Dots Suppress Iodide Ion Migration in Perovskite Solar Cells

Despite considerable advancements in the power conversion efficiency (PCE) of lead halide perovskite solar cells (PSCs), their operational durability remains a pivotal challenge for their widespread commercialization. One of the primary sources of instability of PSCs is the halide anion migration in the perovskite layer, causing a lack of stability of the perovskite crystal structure, particularly when subjected to increased temperature and moisture, which results in reduction in the PCE. Undesirable ion migration can be eliminated through advancements in PSC architecture, such as proper design of electron and hole transport layers and incorporation of nanoparticle additives such as carbon dots (CDs) into the perovskite layer. This study has shown that negatively charged amphiphilic CDs with many aliphatic, carbonyl, and hydroxyl groups at the surface are effective in suppressing the iodide migration process and thus improve the performance and stability of PSCs based on FACsPbI3 (FA stays for the formamidinium cation). Introduction of these CD additives affects the crystallization process of FACsPbI3 perovskite films, causing an increase of the perovskite grain size by 81% and at the same time a diminished appearance of the undesired PbI2 phase as compared to the reference sample without CDs. Furthermore, incorporation of CDs into perovskite films enables us to adjust their energy level structure, facilitating charge carrier extraction in PSCs. As a result, PSCs based on the FACsPbI3 perovskite films with amphiphilic poly(ethylene glycol)-covered CDs demonstrate an increase in the maximum short-circuit current and suppressed hysteresis between forward and reverse scans. The latter effect is attributed to the passivation of defects, which results in the reduction of the ion migration pathways and the amount of I- anions at the interface between the electron transport layer and the perovskite active layer. These improvements result in a maximum PCE of such PSCs of 15%, which is 29% higher as compared to the maximum value of PCE for the reference device without any CDs.