Efficient n-type dopants with extremely low doping ratios for high performance inverted perovskite solar cells
N-type doping is an effective way to increase the short circuit current (Jsc) of perovskite solar cells (PSCs), thus enhancing their power conversion efficiency (PCE). However, the largely decreased open circuit voltage (Voc) and fill factor (FF) are always drawbacks in traditional n-type doping. Moreover, efficient n-type dopants are rare to date and it is therefore desirable and urgent to develop new n-type dopants with a better n-type doping ability for high performance PSCs. Here, a series of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole derivatives have been designed and synthesized, which provides a successful case of tailoring the doping ability of n-type dopants. It has been demonstrated that with a stronger n-type dopant, an extremely low doping ratio would be sufficient to boost the Jsc, while enhancing the FF and minimizing the Voc loss. By using the strongest n-type dopant, a PCE of over 18% is achieved in an inverted PSC with a remarkably low doping ratio of 0.05% by weight.