Homogeneously Miscible Fullerene inducing Vertical Gradient in Perovskite Thin‐Film toward Highly Efficient Solar Cells

Abstract Fullerene‐based n‐type charge‐collecting materials have emerged as a solution for high‐performance perovskite solar cells. However, their application to perovskite solar cells is limited in the device architecture and only a small amount of fullerene additives have been introduced to the device system, because of the immiscibility of the fullerene species with polar solvents. To overcome this, triethylene glycol monomethyl ether chain‐attached fullerene derivatives are synthesized and applied to normal‐type perovskite solar cells. The newly synthesized fullerenes exhibit excellent solubility in polar solvents. A novel approach to introducing miscible fullerenes into perovskite devices and inducing a favorable vertical gradient is proposed. Forming an overcoat on an electron‐transporting layer and waiting for a few minutes, the fullerene derivatives progressively permeate into the fullerene‐doped perovskite active film. By fabricating perovskite solar cells combining direct mixing, overcoating, and waiting techniques, a remarkably high device efficiency of 23.34% is achieved. The high performance is attributed to the fullerene additives with a vertical gradient passivating the perovskite defect sites effectively and the overcoat enhancing the charge transfer. The device performance is certified by a national laboratory, which is the highest efficiency among the fullerene additives‐used perovskite solar cells.