Effect of Nitrogen Cation as "Electron Trap" at π-Linker on Properties for p-Type Photosensitizers: DFT Study

On the basis of thieno(3,2-b)thiophene and dithieno[3,2-b:2',3'-d]thiophene (T2 and T3 moieties) as π-linker, the A, D and S series dyes were designed to investigate the effect of the introducing N⁺ as an "electron trap" into T2 and T3 on the properties of the dyes. The optimized structures, electronic and optical properties were investigated by the density functional theory (DFT) and time-dependent DFT (TD-DFT). The results show that the properties of the dyes are sensitive to the N⁺ position in π-linkers. D series dyes with electron-withdrawing units located near the donor have better properties than the corresponding A series with the electron-withdrawing units located near the acceptor. For A and D series, the N⁺ modified dye named T2N+1-d displays the largest red shift of the UV-vis absorption, the maximum integral values of the adsorption-wavelength curves over the visible light, the highest light harvesting efficiency (LHE, 0.996), and the strongest adsorption energy (-44.33 kcal/mol). T2N+1-d also has a large driving force of hole injection (ΔG_inj, -0.74 eV), which results in a more efficient hole injection. Bearing a lengthier π-linker than T2N+1-d, the properties of T2N+1-s are further improved. T2N+1-d moiety or its increased conjugated derivatives may be a promising π-linker.