Eliminating Halogen Vacancies Enables Efficient MACL-Assisted Formamidine Perovskite Solar Cells

Methylammonium chloride (MACl) additive is almost irreplaceable in high-performance formamidine perovskite photovoltaics. Nevertheless, Some of the problems that can arise from adding MACl are rarely mentioned. Herein, it is proposed for the first time that the addition of MACl would cause the non-stoichiometric ratio in the perovskite film, resulting in the halogen vacancy. It is demonstrated that the non-synchronous volatilization of methylamine cations and chloride ions leads to the formation of halogen vacancy defects. To solve this problem, the NH4HCOO is introduced into the perovskite precursor solution to passivate the halogen vacancy. The HCOO- ions have a strong force with lead ions and can fill the halogen vacancy defects. Consequently, the champion devices' power conversion efficiency (PCE) can be improved from 21.23% to 23.72% with negligible hysteresis. And the unencapsulated device can still retain >90% of the initial PCE even operating in N-2 atmosphere for over 1200 h. This work illustrates another halogen defect source in the MACl-assisted formamidine perovskite photovoltaics and provides a new route to obtain high-performance perovskite solar cells.

Automated summary

Methylammonium chloride (MACl) additive is vital in high-performance formamidine perovskite photovoltaics. However, the addition of MACl can cause non-stoichiometric ratio and halogen vacancy defects in the perovskite film. This study introduces NH4HCOO to passivate the halogen vacancy and demonstrates improved power conversion efficiency in the devices, even after prolonged operation.