Effects of different functional groups on the optical and charge transport properties of copolymers for polymer solar cells

Abstract

The ground state properties of polymers BSeTT, QTT, BDT-DTBTBPz (Pz), BDT-DTBTBQx (Qx) and their derivatives D1, D2, D3, including frontier molecular orbital, energy gaps, ionization potentials (IPs), electron affinities (EAs) and electron or hole reorganization energies, have been investigated with DFT-B3LYP/6-31G(d) method. Based on optimized structures, the absorption and fluorescence spectra of the seven oligomers are simulated with TD-DFT-Cam-B3LYP/6-31G(d) method. The external electric field is taken into account in view of the practical application. Moreover, developed visualized charge difference density is applied to present the charge density redistribution for the fullerene/polymer complexes. More deeply, the electronic coupling matrix, reorganization and Gibbs free energy are estimated to calculate the rates of the charge transfer and recombination for the fullerene/polymer complexes. Results show that the lengths of conjugated chains obviously affect the energy levels of HOMO and LUMO, and the ionization potentials and electron affinities. By evaluating the rates of the charge transfer and recombination for the fullerene/polymer complexes, reorganization energy and spectrum nature, it is found that the molecule BDT-DTBTBPz and designed molecule (D2) have the best optical and electronic properties among the investigated system, indicating that introducing appropriate functional group can effectively improve the photoelectric characteristics.