Effects of silole content and doping on the electronic structures and excitation energies of silole/thiophene cooligomers

摘要

The effects of siloles and doping with positive and negative charges on the electronic structures and band gaps of the silole/thiophene copolymers are studied employing the density functional theory and the time-dependent density functional theory with B3LYP functional. The optimized geometries, the calculated excitation energies, and the extrapolated band gaps agree with the available experimental results. The decreasing trend of the calculated band gaps with the increasing silole content in the silole/thiophene copolymers is in good agreement with experimental observations. Doping significantly shortens the carbon-carbon distances along the backbones and lowers the excitation energies of thiophene oligomers and silole/thiophene cooligomers relative to those of the neutral ones. Moreover, the low oxidized states have the lower vertical excitation energies than the high oxidized ones. There also exists the even/odd effect in excitation energies of the charged oligomers. The doped copolymers with higher silole content are suggested to be the potential conducting polymers with narrow band gaps. The injected charges by doping lead to the formations of polarons or bipolarons, which are characterized by the distortions in geometries, fluctuations in the bond order waves and spin densities. The even/odd effect in excitation energies is also qualitatively explained by the different widths of subgaps split by the formations of polaron and biopolaron. [References: 34]