How Do Surface Polar Molecules Contribute to High Open‐Circuit Voltage in Perovskite Solar Cells?

To date, the improvement of open‐circuit voltage (VOC) offers a breakthrough for the performance of perovskite solar cells (PSCs) toward their theoretical limit. Surface modification through organic ammonium halide salts (e.g., phenethylammonium ions PEA+ and phenmethylammonium ions PMA+) is one of the most straightforward strategies to suppress defect density, thereby leading to improved VOC. However, the mechanism underlying the high voltage remains unclear. Here, polar molecular PMA+ is applied at the interface between perovskite and hole transporting layer and a remarkably high VOC of 1.175 V is obtained which corresponds to an increase of over 100 mV in comparison to the control device. It is revealed that the equivalent passivation effect of surface dipole effectively improves the splitting of the hole quasi‐Fermi level. Ultimately the combined effect of defect suppression and surface dipole equivalent passivation effect leads to an overall increase in significantly enhanced VOC. The resulted PSCs device reaches an efficiency of up to 24.10%. Contributions are identified here by the surface polar molecules to the high VOC in PSCs. A fundamental mechanism is suggested by use of polar molecules which enables further high voltage, leading ways to highly efficient perovskite‐based solar cells. A passivation strategy using polar molecular phenmethylammonium iodide (PMAI) is introduced to enhance performance of FA‐MA hybrid (FA: HC(NH2)2; MA: CH3NH3) perovskite solar cells. The dipole‐induced equivalent passivation effect is revealed successfully by PMAI and an applicable role of polar molecular passivators is declared. Overall, combining the molecular‐bonding defect passivation effect, the PMAI leads to a tremendous increase over 100 mV in VOC.