Role of Additive in Perovskite Solar Cells

The role of additives, including formamidinium thiocyanate (FASCN) and crown ether of 18-crown-6 (18C6), in the perovskite solar cell (PSCs) is investigated. In the first work, we observe a significant enlargement in the perovskite domain size after the incorporation of thiocyanate ions (SCN-). Moreover, a trace amount of SCN- across the perovskite film measured by the time-of-flight secondary ion mass (ToF-SIMS) spectrometer identifies the SCN- existing inside the perovskite bulk, especially near the film bottom. Additive of FASCN effectively suppresses the bulk recombination in perovskite film, leading to an improvement on open-circuit voltage and fill factor for the PSCs. Our studies confirm the existence of SCN- in the final film and the passivation effect of SCN- for enhancing the device performance. In the second work, we add a crystallization controlling agent of 18C6 in the doctor-bladed perovskite film to improve its film quality. Liquid-phase scanning electron microscopy (L-SEM) is conducted to study the effects of 18C6 on the perovskite precursor and the doctor-bladed perovskite film. The L-SEM image of the pristine perovskite precursor reveals self-assembly of micelles, while the addition of 18C6 effectively suppresses the size of the perovskite-based micelles in the perovskite precursor. This effect results in uniform-sized micelles in the precursor and a reduced crystal growth rate to achieve a high-quality perovskite film with preferred crystallinity, an enhanced perovskite domain size, and a smooth surface. Addition of 18C6 significantly improves the power conversion efficiency of doctor-bladed PSCs.