The Role of Diammonium Cation on the Structural and Optoelectronic Properties in 3D Cesium–Formamidinium Mixed‐Cation Perovskite Solar Cells

Incorporating diammonium cations, which electrostatically connect the adjacent inorganic slabs ([PbI 6 ] 4− ), into 3D perovskite is recently proposed to develop high‐performance perovskite solar cells (PSCs). However, due to limited studies, the effects of these organic cations on the perovskite structural and optoelectronic properties are yet to be understood. Herein, a diammonium cation, propane‐1,3‐diammonium (PDA), is first proposed to modulate the cesium–formamidinium (Cs–FA)‐mixed cation perovskite. By increasing the PDA content, the efficiency of the Cs 0.15 FA 0.85 − x PDA x PbI 3 PSC first increases and then drastically decreases. The highest power conversion efficiency (PCE) of 18.10% obtained by Cs 0.15 FA 0.83 PDA 0.02 PbI 3 is superior to that of the Cs 0.15 FA 0.85 PbI 3 (16.82%). Through systematic investigations, it is revealed that the PDA content–dependent efficiency is attributed to a competition between the enhanced defect passivation and emerged excitonic effect with an increased PDA content. Moreover, the encapsulated Cs 0.15 FA 0.83 PDA 0.02 PbI 3 device exhibits almost 1.5 times increased stability than the Cs 0.15 FA 0.85 PbI 3 counterpart, with 83% of its initial efficiency retained after 500 h exposure, under continuous light soaking at 60 °C in ambient air. This study provides a practical strategy to enhance the device stability without sacrificing the efficiency and deepens our understanding on effects of diammonium cation incorporated in 3D perovskite.