A novel theoretical tool for some new low-band-gap copolymers has been developed on the basis of density functional theory (DFT) quantum chemical calculations to model their photophysical properties. These copolymers are constituted of thiophene and benzothiadiazole (PTBT) units essentially as well as their derivatives leading to donor (D)-acceptor (A) structure-types. Firstly, we study the insertion of thiophene as spacer unit into PTBT backbone to reach polydithiophene-alt-benzothiadiazole (PDTBT) copolymer, secondly, bithiophene usual unit was replaced by cyclopentadithiophene entity to obtain poly (cyclopentadithiophene-alt-benzothiadiazole) (PCPDTBT) copolymer and finally, ethynyl and vinylene spacers were added to obtain a novel oligomer models, denoted poly(cyclopentadithiophene-ethynyl-benzothiadiazole) (PCPDTEBT) and poly(cyclopentadithiophene-vinylene-benzothiadiazole) (PCPDTVBT), respectively, and to investigate their effect into the main backbone on various properties by examining structural and electronic properties. PCPDTEBT copolymer presents the optimal properties to be used as an active layer in organic solar cell (OSC). It shows unique combination of properties: a moderate gap (1.55 eV), high extinction coefficients (ε = 3.11 × 105mol-1L cm-1), a low oxidation potential associated with high thermal stability and environmental sustainability. Thus, this copolymer is then blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in bulk-heterojunction solar cell. A model band diagram was established for the proposed solar structure, simulating the energy behavior of such active layer.
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