This review discusses recent progress and the rational design criteria used in the structural design of organic dyes and porphyrin photosensitizers for DSSCs. The effects of molecular structural engineering on the photophysical and electrochemical properties, photovoltaic parameters, and efficiency of DSSCs are presented.

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.