Acceptor–Donor–Acceptor Small Molecules Based on Indacenodithiophene for Efficient Organic Solar Cells

Abstract

Four A-D-A type small molecules using 4,4,9,9-tetrakis(4-hexylphenyl)- indaceno[1,2-b:5,6-b′]dithiophene as central building block, bithiophene or terthiophene as π-bridges, alkyl cyanoacetate or rhodanine as end acceptor groups were synthesized and investigated as electron donors in solution-processed organic solar cells (OSCs). These molecules showed excellent thermal stability with decomposition temperatures over 360 °C, relatively low HOMO levels of −5.18 to −5.22 eV, and strong optical absorption from 350 to 670 nm with high molar extinction coefficient of 1.1 × 10⁵ to 1.6 × 10⁵ M^–1 cm^–1 in chloroform solution. OSCs based on blends of these molecules and PC₇₁BM achieved average power conversion efficiencies of 2.32 to 5.09% (the best 5.32%) after thermal annealing. The effects of thiophene bridge length and end acceptor groups on absorption, energy level, charge transport, morphology, and photovoltaic properties of the molecules were investigated.