Enhanced VOC of two-dimensional Ruddlesden–Popper perovskite solar cells using binary synergetic organic spacer cations†
Two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs), with the general formula (RNH3)2An−1BnX3n+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 (VOC) despite the large band gap of 2D RPPs. Herein, to reduce the VOC 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 VOC. With the assistance of the OA spacer cation, the VOC 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.