Theoretical insights on the rigidified dithiophene effects on the performance of near-infrared cis-squaraine-based dye-sensitized solar cells with panchromatic absorption

As compared with the promising near-infrared (NIR) absorbing trans-squaraines (SQ), cis-squaraines (HSQ) possess special absorption spectra showing better performance and are being attracted as promising NIR chromophores in dye-sensitized solar cells (DSSCs). For further extending the absorption toward the NIR region, a rigidified dithiophene spacer 4H-Silolo[3,2-b:4,5-b']dithiophen/e,4,4,-dimethyl (DTS) was introduced into the skeleton of HSQ (HSQ-DTS). A comprehensive analysis on the light harvesting ability, intramolecular charge separation, interfacial electron injection and dye regeneration were conducted by means of density functional theory (DFT) and time-dependent DFT calculations on the dye@(TiO2)(38) system. Our results demonstrated that as compared with HSQ, HSQ-DTS could exhibit panchromatic absorption with more favorable charge separation ability, more injected electrons with inhibited charge recombination process due to the larger distance between the cation hole and the semiconductor surface. The superior performance of dye HSQ-DTS satisfied our original design intent. Therefore, based on these inspiring results, another six different rigidified dithiophene spacers (16) were also investigated to screen out more efficient cis-squaraine sensitizers. We find 3 with 4H-Phospholo [3,2-b:4,5-b']dithiophene,4-ethynyl-,4-oxide possesses the best performance with the maximum short-circuit current (J(sc)(max))achieved 31.60 mAcm(-2), which is expected to support future experimental and theoretical studies.