氧族元素对D-A和D-π-A共聚物光吸收谱红移的第一性原理研究

D-A型共聚物作为有机聚合物太阳能电池的电子给体材料近年来引起广泛关注. 本文以苯并二噻吩(BDT)为电子给体单元, 苯并噻二唑(BT)为电子受体单元来模拟D-A共聚体; 并用噻吩环作为π桥, 构造出D-π-A(PBDT-DTBX, X = O, S, Se, Te)结构. 采用第一性原理的密度泛函理论, 系统地计算相应的电子结构和光吸收谱. 比较不同氧族元素和噻吩π-键桥对聚合物光吸收谱的影响. 研究结果表明: D-A共聚体中当X位元素以O, S, Se, T_e 替换时, 其体系的最高占有分子轨道(HOMO)能级变化不大, 最低未占有分子轨道(LUMO)能级逐渐靠近费米能级, 带隙逐渐减小. 在可见光区有两个较强的吸收峰, 随着X位元素原子序数增大, 位于4.0 eV左右的光吸收峰位基本不变, 另一光吸收峰强度明显增大并发生红移. 与D-A结构相比, D-π-A结构的带隙均有所减小, 其中X为Te时带隙最小; 光吸收峰强度随着氧族元素原子序数的增大也明显增大并发生红移. 通过比较光吸收系数和相应态密度, 结果表明, 4.0 eV 左右的光吸收峰主要是BDT单元的贡献, 氧族元素的改变主要影响519.4-703.9 nm范围的光吸收.

First-principle study of the optical absorption spectra of chalcogen on D-A and D-π-A copolymers

D-A type copolymer as an organic polymer solar cell electronic material in recent years has attracted wide attention. In order to improve the efficiency of energy conversion, many active layer materials, especially the donor materials, have been designed and synthesized. By inducing the different donor and acceptor units, the absorption spectrum can better match with the solar spectrum and the carrier mobility can increase. In this paper, by using the density functional theory method, we investigate the electronic structures and optical absorption spectra of D-A and D-π-A copolymers. Benzodithiophene (BDT) as the electron donor unit, and dibenzothiophene (BT) as the electron acceptor unit are used to simulate D-A (PBDT-BX, X = O, S, Se, Te) copolymer systems; and D-π-A (PBDT-DTBX, X = O, S, Se, Te) structures are constructed with thiophene ring as a bridge between D and A. Firstly, our calculation results indicate that when X is replaced separately by elements O, S, Se and T_e in D-A copolymers, the LUMO levels move close to the Fermi level, while the changes of the HOMO energy levels are relatively small, resulting in the band gap decreasing gradually. Then, the analysis of the density of states (DOS) shows that the contribution of LUMO comes from the BT unit and HOMO from the BDT unit. Also the difference in charge density shows that the thiophene ring enhances the charge transfer between BT and BDT. Besides, the studies of the optical absorption spectrum reveal that there appear two strong absorption peaks with the increase of atomic number of X, of which one is at about 4.0 eV and has no obvious change, and the other increases intensively and is red-shifted. Moreover, compared with the D-A structure, the D-π-A structure has the band gap that will decrease obviously and has a lowest value when X is Te. The optical absorption peak also increases significantly as the atomic number of oxygen group elements increases and peak position is red-shifted. The range of optical absorption peak is mainly from 703.9 to 519.4 nm. According to the absorption spectrum and DOS the optical absorption peak at about 4.0 eV is mainly contributed by the BDT unit. Overall, our findings provide a good understanding of mechanism about the red-shift of optical absorption spectra of copolymers and can serve as guidance for organic polymer design in photovoltaic cell experimentally.