Density Functional Theoretical and Time‐dependent Density Functional Theoretical Study on Thiophene–Benzothiadiazole‐based Polymers

Abstract

Bulk hetero-junction polymer solar cells, which consist of low bandgap organic polymers as an electron donor and fullerene derivatives as an electronic acceptor, have attracted much attention because of their many advantages. They include low weight, flexibility, portability, cost-effective lowtemperature processing, and tunable optoelectronic properties, to name a few.1 In order to improve the performance of the photovoltaic cell, much effort has been dedicated in terms of the design of new materials, device architectures, and processing techniques.2 In the design of new molecular structure of the repeat unit, electronic properties such as the highest occupied molecular orbital (HOMO) level and bandgap are important because they are related to the efficiency of the photovoltaic cell. A quantitative prediction of these electronic properties prior to the actual synthesis would be of great help by minimizing the effort of trial-and-error synthesis. 3,4Recently, we reported the molecular design and photovoltaic property relationships of low bandgap polymer based on a thiophene (T)-benzothiadiazole (BT) alternating structure. 5The electronic effects of substituents on the T or BT units in the polymer on the photovoltaic cell performance were investigated experimentally 5 and computationally. 6 These studies showed that the introduction of an alkyl side chain on the T or BT ring breaks the coplanarity of a polymeric backbone as a result of the steric repulsion, although it can increase the solubility of the polymer in organic solvent. On the other hand, the introduction of an alkoxy substituent on the electrondeficient BT moiety improves solubility while maintaining the deep HOMO level and backbone coplanarity via SÁ Á ÁO noncovalent coulomb interaction (between sulfur in thiophene and alkoxy oxygen) compared to the unsubstituted T-BT structure, which also leads to good interchain π-π packing between two polymer chains. Based on these results, we have been working on the further modification of the T and BT rings while keeping the SÁ Á ÁO noncovalent interaction. 7,8 In the present study, density functional theory (DFT, Jaguar quantum chemistry software, 9 B3LYP/6-31G * * level and B3LYP/6-311G * * level) and time-dependent DFT (TD-DFT) calculations at the same levels were performed to calculate the HOMO levels and the bandgaps of the repeat units comprising T-BT alternating copolymers in order to investigate the optical and electronic properties of the polymers. Bandgaps were estimated either from the difference of the eigenvalues of the Kohn-Sham orbitals or directly from the TD-DFT calculations.In Scheme 1, the chemical structures of repeat units having various T-BT linkages that were considered in the present study are depicted. Methyl or methoxy substituents were chosen to simplify the calculations. One and two repeat units were considered to investigate the dependence of the quantities on the length of the polymerization. Figure 1 shows the calculated torsion angle profiles and optimized structures. The torsion ...