Enhanced VOC of two-dimensional Ruddlesden–Popper perovskite solar cells using binary synergetic organic spacer cations

Abstract

Two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs), with the general formula (RNH₃)₂A_n−1B_nX_3n+1, could realize promising device stability as compared with their three-dimensional counterparts. However, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) with 2D RPPs is relatively low, especially the open circuit voltage (V_OC) despite the large band gap of 2D RPPs. Herein, to reduce the V_OC losses and enhance the PCE, we propose the use of synergetic organic spacer cations with n-butylammonium (BA⁺, CH₃(CH₂)₃NH₃⁺) as the major spacer cation and octylammonium (OA⁺, CH₃(CH₃)₇NH₃⁺) as the additive spacer cation. Scanning electron microscopy reveals that 2D RPP films with mixed organic spacer cations (with a 2 : 0.03 molar ratio of BA : OA in precursor solution) are more uniform and denser. Furthermore, it is found that the 2D RPPs with OA cations exhibit enhanced charge transport by suppressing low-n phases, which is beneficial for high V_OC. With the assistance of the OA spacer cation, the V_OC is notably increased from 0.94 V to approximately 1.1 V. PSCs with BA–OA 2D RPP achieve the highest PCE of 11.90%, which is higher than that based on pure BA 2D RPP (10.81%). The unencapsulated devices with BA–OA 2D RPPs retain 63% and 93% of their original PCE after being kept in air with a humidity of 30% ± 5% at a room temperature of 20 °C ± 5 °C for 410 h and in a N₂ glove box over 1224 h, respectively. This work provides a simple idea for achieving high quality 2D RPP films, and highlights the importance of organic spacer cations in obtaining highly performed PSCs.