Above 23% Efficiency by Binary Surface Passivation of Perovskite Solar Cells Using Guanidinium and Octylammonium Spacer Cations

One of the important factors in the performance of perovskite solar cells (PSCs) is effective defect passivation. Dimensional engineering technique is a promising method to efficiently passivate non‐radiative recombination pathways in the bulk and surface of PSCs. Herein, a passivation approach for the perovskite/hole transport layer interface is presented, using a mixture of guanidinium and n‐octylammonium cations introduced via GuaBr and n‐OABr. The dual‐cation passivation layer can provide an open‐circuit voltage of 1.21 V with a power conversion efficiency of 23.13%, which is superior to their single cation counterparts. The mixed‐cation passivation layer forms a 1D/2D perovskite film on top of 3D perovskite, leading to a more hydrophobic and smoother surface than the uncoated film. A smooth surface can diminish non‐radiative recombination and enhance charge extraction at the interface making a better contact with the transport layer, resulting in improved short‐circuit current. In addition, space charge‐limited current measurements show a three times reduction in the trap‐filled limit voltage in the mixed‐cation passivated sample compared with unpassivated cells, indicating fewer trapped states. The shelf‐life stability test in ambient atmosphere with 60% relative humidity as well as light‐soaking stability reveal the highest stability for the dual‐cation surface passivation.