Modification of benzoindenothiophene-based organic dye with fused thiophenes for efficient dye-sensitized solar cells

Recently low-energy-gap benzoindenothiophene (BIT)-based organic dyes have been experimentally sensitized to dye-sensitized solar cells (DSSCs) with impressive 10.9% power conversion efficiency. This paper presents a computational study of the modification of BIT-based dyes with fused thiophene moieties to build novel low band gap sensitizers. Density functional theory (DFT), tight-binding DFT, and time dependent DFT (TDDFT) approaches are used to demonstrate the electronic and optical properties of the BIT dyes and dye/(TiO₂)₄₆ complexes. Our calculations show that the structural modification by using fused thiophenes can effectively lower the band gap of the BIT dyes by 0.07-0.12 eV and affect the optical properties of BIT dyes. Enlarging the thiophene unit in BIT with thienothiophene and dithienothiophene improves the oscillator strength by 14%-25%, while the lowest-energy absorption peak basically remains at 559 nm. The incorporation of cyclopentadithiophene unit leads to a significant 47 nm red-shift of absorption peak and a 25% enhanced oscillator strength, compared to the original BIT dye. Those fused thiophenes modified BIT dyes also demonstrate ideal molecular orbital distribution patterns and ultra-fast injection time at the dye/(TiO₂)₄₆ interface. Our calculations provide useful guidance for the molecular design of novel naphthalene-based dyes for DSSC optimizations.