Soluble hexamethyl-substituted subphthalocyanine as a dopant-free hole transport material for planar perovskite solar cells.

Boron subphthalocyanine (SubPc) has special physical and chemical properties, originating from its non-centrosymmetric, near-planar taper structure and large conjugated system; it can act as an alternative to the small molecule hole-transporting material 2,2',7,7'-tetrakis-(,-di--methoxyphenylamine)-9,9'-spirobifluorene in perovskite solar cells (PSCs). To achieve a higher solubility in common organic solvents and a more suitable highest occupied molecular orbital energy level that aligns with the valence band of the perovskite material, a SubPc molecule with a hexamethyl substitution at its peripheral position (Me-SubPc) was successfully designed and synthesized in a one-step method. Completely solution processed PSCs were fabricated with only a small hysteresis, a power conversion efficiency of 6.96% and of 0.986 V.