Size effects of a graphene quantum dot modified-blocking TiO₂ layer for efficient planar perovskite solar cells

Research on the addition of suitable materials into perovskite solar cells (PSCs) for improved performance is as important as the fabrication of efficient perovskite films themselves. An attempt to enhance the performance of planar-type perovskite solar cells was performed by introducing graphene quantum dots (GQDs) onto a blocking TiO₂ layer via O₂ plasma treatment. Furthermore, the bandgap of the GQDs was tuned through their size control and the effects of the GQD size on cell performance were explored. The GQDs can induce fast electron extraction and the formation of the improved perovskite quality. The devices with appropriately sized-GQDs showed an average of 10% enhancement compared with those of cells without GQDs and achieved 19.11% as the best power conversion efficiency (PCE). Furthermore, GQDs contributed to the reduction in the extent of current-voltage hysteresis, which was attributed to the planar structure.