Band-gap tunable thiadiazolo[3,4-g]quinoxaline derivatives as non-fullerene acceptors in organic photovoltaic cells processed from low toxic ethanol/anisole mixtures
The synthesis, characterisation and application as electron-acceptors in organic photovoltaic cells of new [1,2,5]thiadiazolo[3,4-g]quinoxaline-based small molecules with oligo(ethylene oxide) solubilising groups and tunable optical bandgaps are reported. A new p-type polymer consisting of a polythiophene derivative substituted with analogous solubilising groups was also synthesised to perform as a donor, in combination with the small molecules, for bulk-heterojunction solar cells using ethanol/anisole mixtures as low toxic solvents to process the active blends. Comparison of the structural, electrochemical and photophysical properties of the small molecules elucidated on the structural dependence of their optical bandgaps, ranging from 1.31 to 2.25 eV. The active layers composed of the polythiophene donor and the small molecules showed well-intermixed morphologies, and the best performing blends showed a fibrillar-type morphology, indicative of more ordered molecular packings. Blends processed from the 50 : 50 (v/v) mixture of ethanol and a low toxic solvent, anisole, yielded low power conversion efficiencies of 0.39% in non-optimized organic solar cells. Although the efficiencies are low, these are the first reported results of OSCs with active layers composed of non-fullerene acceptors processed from ethanol-based solvent formulations. This study provides guidelines for the design of non-fullerene acceptors for the fabrication of OSCs processed from environmentally friendly solvents.