Theoretical studies of photovoltaic properties of five new silol dithiophene based D2-A-D1-A-D2 donors

Organic solar cell of silol dithiophene based D₂-A-D₁-A-D₂/PC₇₁BM (D: donor part; A: acceptor part; 1 and 2 denote different units) possesses promising power conversion efficiency. Researchers have studied D₂-A-D₁-A-D₂ molecules carefully, including the effects of the different number of terminal thiophenes, the different central moiety (D₁), and the length of the alkyl chain. However, there are few investigations, especially theoretical studies, on the influences of different A (acceptor) units on the properties of D₂-A-D₁-A-D₂ molecule. In the present work, we have designed and modeled five new D₂-A-D₁-A-D₂ (D₂ = bithiophene and D₁ = silol dithiophene) donors by changing A units (A = diketopyrrolopyrrole, naphtho1,2-c:5,6-c′]bis1,2,5]thiadiazole, 5-fluoro-2,1,3-benzoselenadiazole, benzobisthiadiazole, and thiazolo5,4-d]thiazole). We have applied density functional theory (DFT) and time-dependent DFT to predict their ground-state electronic structures and the UV–vis spectra, and the open circuit voltages (Vocs) of organic solar cells of D₂-A-D₁-A-D₂/PC₇₁BM. Based on the calculated results, we find that bithiophene thiazolo5,4-d]thiazole siloldithiophene (BTTS) (D₂ = bithiophene, A = thiazolo5,4-d]thiazole, D₁ = silol dithiophene) possesses the highest lowest unoccupied molecular orbital (−2.60 eV) and the lowest highest occupied molecular orbital (−5.33 eV) energies, and the strongest absorption in the visible region. Besides, the solar cell of BTTS/PC₇₁BM has the highest Voc of 1.02 V. These results indicate that it may be a promising donor. In contrast, bithiophene benzobisthiadiazole siloldithiophene (BBBS) (A = benzobisthiadiazole) has low absorption strength in the visible region, which indicates that it may not be a suitable donor material.