New acceptor–donor–acceptor (A–D–A) type copolymers for efficient organic photovoltaic devices

Abstract

Three new conjugated systems alternating acceptor–donor–acceptor (A–D–A) type copolymers have been investigated by means of Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) at the 6-31g (d) level of theory. 4,4′-Dimethoxy-chalcone, also called the 1,3-bis(4-methoxyphenyl)prop-2-en-1-one (BMP), has been used as a common acceptor moiety. It forced intra-molecular S⋯O interactions through alternating oligo-thiophene derivatives: 4-AlkylThiophenes (4-ATP), 4-AlkylBithiophenes (4-ABTP) and 4-Thienylene Vinylene (4-TEV) as donor moieties. The band gap, HOMO and LUMO electron distributions as well as optical properties were analyzed for each molecule. The fully optimized resulting copolymers showed low band gaps (2.2–2.8eV) and deep HOMO energy levels ranging from −4.66 to −4.86eV. A broad absorption [300–900nm] covering the solar spectrum and absorption maxima ranges from 486 to 604nm. In addition, organic photovoltaic cells (OPCs) based on alternating copolymers in bulk heterojunction (BHJ) composites with the 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]–C61 (PCBM), as an acceptor, have been optimized. Thus, the band gap decreased to 1.62eV, the power conversion efficiencies (PCEs) were about 3–5% and the open circuit voltage Voc of the resulting molecules decreased from 1.50 to 1.27eV.