2-Thiopheneformamidinium-Based 2D Ruddlesden-Popper Perovskite Solar Cells with Efficiency of 16.72% and Negligible Hysteresis

Formamidinium (FA)-based 3D perovskite solar cells (PSCs) have been widely studied and they show reduced bandgap, enhanced stability, and improved efficiency compared to MAPbI(3)-based devices. Nevertheless, the FA-based spacers have rarely been studied for 2D Ruddlesden-Popper (RP) perovskites, which have drawn wide attention due to their enormous potential for fabricating efficient and stable photovoltaic devices. Here, for the first time, FA-based derivative, 2-thiopheneformamidinium (ThFA), is successfully synthesized and employed as an organic spacer for 2D RP PSCs. A precursor organic salts-assisted crystal growth technique is further developed to prepare high quality 2D (ThFA)(2)(MA)(n)-1PbnI3n+1 (nominal n = 3) perovskite films, which shows preferential vertical growth orientations, high charge carrier mobilities, and reduced trap density. As a result, the 2D RP PSCs with an inverted planar p-i-n structure exhibit a dramatically improved power conversion efficiency (PCE) from 7.23% to 16.72% with negligible hysteresis, which is among the highest PCE in 2D RP PSCs with low nominal n-value of 3. Importantly, the optimized 2D PSCs exhibit a dramatically improved stability with less than 1% degradation after storage in N-2 for 3000 h without encapsulation. These findings provide an effective strategy for developing FA-based organic spacers toward highly efficient and stable 2D PSCs.