Perovskite solar cells based on chlorophyll hole transporters: Dependence of aggregation and photovoltaic performance on aliphatic chains at C17-propionate residue

Four chlorophyll (Chl) derivatives with C3(1)-hydroxy and esterifying hydrocarbon groups including methyl, hexyl, dodecyl, and phytyl at the C17-propionate residue, were synthesized and examined as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). These hydrocarbon groups are not directly conjugated with the chlorin pi-system, and have a negligible effect on the electronic absorption spectra and energy levels of mono-meric Chl derivatives. However, the different chain lengths of these hydrocarbon groups affect their solubility in nonpolar solvents, which in turn has an influence on the intermolecular organization and crystallinity of their J-aggregates. As a result, these Chl derivative solid films show slightly different electronic absorption, energy levels, and surface morphology with the exception of the methyl ester, but they all show similar hole-extraction abilities as HTMs. Among the four Chl derivatives examined here, the average power conversion efficiencies are ordered as follows: hexyl ester (13.06 +/- 0.72%).2; dodecyl ester (12.52 +/- 1.60%) > methyl ester (10.33 +/- 1.23%) > phytyl ester (9.90 +/- 0.93%), which are consistent with the order of the short-circuit current density (J(sc)) and fill factor (FF). The highest photovoltaic performance (J(sc) FF) in PSCs based on the hexyl and dodecyl esterifying ZnChls can be explained by the most efficient internal/interfacial charge transfer.