Effects of the core unit on perylene-diimide-based molecular acceptors in fullerene-free organic solar cells

Perylene diimide (PDI)-based molecular acceptors, denoted PDI-TVT-PDI, PDI-FTVT-PDI, and PDI-CNTVT-PDI, which have different core units, were synthesized and evaluated as n-type materials in fullerene-free organic solar cells (OSCs). The effects of the addition of fluorine and cyano groups to (E)-1,2-di-(thiophen-2-yl)ethene (TVT) in the synthesized acceptors were investigated based on the optical and electrochemical properties and computational calculations, as well as observation of the morphologies of the blend films. Among the synthesized acceptors, PDI-CNTVT-PDI showed a high absorption coefficient, balanced hole/electron mobility ratio, predominant face-on structure, and smooth morphology in the blends with poly[4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b; 4,5-b'] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th). The fabricated OSCs using PTB7-Th/PDI-CNTVT-PDI exhibited a higher power conversion efficiency (PCE, 4.32%) than those fabricated with PTB7-Th/PDI-TVT-PDI and PTB7-Th/PDI-FTVTPDI. The purpose of our study is to provide a facile strategy to understand the impact of the core units to enable the design of PDI-based molecular acceptors.