Significant light absorption enhancement by a single heterocyclic unit change in the π-bridge moiety from thieno[3,2-b]benzothiophene to thieno[3,2-b]indole for high performance dye-sensitized and tandem solar cells

The molecular design of organic sensitizers is one of the fundamental factors for high-efficiency dye-sensitized solar cells (DSSCs). In this study, we first utilize the alkylated thieno[3,2-b]indole (TI) moiety as the π-bridge unit to enhance the π-bridge capability of the thieno[3,2-b]benzothiophene (TBT) used in organic sensitizers. To improve the spectral response of the SGT-130 reference dye, we strategically designed and synthesized two novel TI-based organic sensitizers, SGT-136 and SGT-137, through a simple change of the π-bridge unit. By replacing the TBT with the alkylated TI moiety, SGT-136 and SGT-137 could have a red-shifted absorption band and upshifted highest occupied molecular orbital (HOMO) energy level. As a result, the SGT-137-based DSSC exhibits a higher PCE (12.45%) than that based on SGT-130 (9.83%) owing to the improvement of current density and retardation of charge recombination by the hexyl substituted TI unit. These results indicate that the TI moiety is a good candidate for remarkable π-electronic mediators in D–π–A organic sensitizers with the characteristic of facile synthesis compared to other complicated π-bridges. Furthermore, the parallel-connected tandem device with SGT-137 and SGT-021 porphyrin-based DSSCs shows a significantly improved current density (22.06 mA cm−2) and PCE (14.64%), which is the highest value reported for organic-based tandem solar cells to date.