Solution-processed perovskite films usually contain a large quantity of grain boundaries, which decrease the film crystalline quality and introduce severe defect recombination, hindering performance of the devices based on them. Therefore, preparation of high-quality films to achieve high power conversion efficiencies remains a great challenge for perovskite solar cells. Due to high abilities of self-assembly and morphology-tuning for liquid crystal molecules, a single-molecule liquid crystal 4-cyano-4′-pentyl biphenyl (5CB) was employed as additive in CH(NH₂)₂PbI₃ (FAPbI₃) precursor solution to increase the perovskite grain size and decrease the grain boundaries. In addition, the cyano group in 5CB passivates the uncoordinated Pb²⁺ in the perovskite films, which reduces the trap density concentration and inhibits the nonradiative recombination. The resulting perovskite solar cells with 0.2 mg/mL 5CB in the precursor achieve an efficient power conversion efficiency of 21.27% with an open circuit voltage of 1.086 V, a current density of 24.17 mA/cm², and a fill factor of 80.96%. In conclusion, introducing single-molecule liquid crystal as additive is a facile and efficient strategy for improving the performance of FAPbI₃ solar cells.