Journal Article organic solar cells morphology organic photovoltaics side-chain engineering ternary thermal annealing Plant Proteins RNA, Plant Recombinant Proteins Research Support, Non-U.S. Gov't Review alkyl-substituted fluorothiophene side chains all-small-molecule organic solar cells antisolvents conjugated D-A copolymer donors difluoroquinoxaline A-unit digestive ripening driving force energy alignment energy level alignments fatigue behavior fullerene-free functional units interpenetrating networks ion migration methyl benzoate multiple acceptors multiple donors non-fullerene