Molecular design and theoretical investigation on the thieno[3,2-b]thienobis(silolothiophene)-based low band gap donor polymers for efficient polymer solar cell

Donor-acceptor (D-A) copolymers have been proved to be excellent candidates for efficient polymer solar cells. In this paper, a series of D-A polymers with the same donor unit of Si4T and different acceptor units are theoretically designed. Two novel strategies (extending the length of π-conjugation and using the electron-deficient groups) have been considered for the conjugated polymer design. The energy levels and band gaps are theoretically investigated using the confirmed density functional theory/time-dependent density functional theory method. The results show that, compared with two original polymers, the newly designed D-A polymers have better predicted performances with smaller band gaps and lower highest occupied molecular orbital energy levels. When combined with fullerene derivatives (PCBM) for organic solar cells, these polymers can produce power conversion efficiencies as high as ∼10%, estimated by Scharber diagrams.