Structural modulation of D-D-π-A type metal-free organic sensitizers for high-performance dye-sensitized solar cells: The influence of alkyl and alkoxy chains.

Charge recombination, which causes photocurrent loss, is a critical factor limiting the performance of dye-sensitized solar cells (DSSCs). Introducing alkyl or alkoxy chains into molecular structures has been established as an effective strategy to mitigate this process. Therefore, this study systematically investigated the influence of alkoxy groups in triphenylamine (TPA) and alkyl/alkoxy phenyl groups in phenothiazine (PTZ) on DSSC performance using density functional theory (DFT). The photophysical and electrochemical properties of four molecules, TP-C6, TP-PhOC6, C6O-TP-C6, and C6O-TP-PhOC6, were computationally analyzed. Results show that the alkoxy phenyl groups in PTZ and the absence of alkoxy groups in TPA within TP-PhOC6 enhance molecular planarity, induce redshifted absorption spectra, and improve charge transfer performance. Adsorption simulations further reveal that dye@TiO2 system exhibits a reduced bandgap and redshifted absorption, thereby enhancing spectral response and facilitating electron injection. Moreover, the application of an external electric field is shown to significantly improve electron transfer, optical properties, and interfacial electron injection at the dye/TiO2 interface. Photoelectric performance predictions confirm that TP-PhOC6 achieves the highest power conversion efficiency (PCE) of 6.509%, attributed to its high short-circuit current density (Jsc = 11.96 mA cm-2) and open-circuit voltage (Voc = 0.650 V). In the molecular design, TPA was substituted with carbazole in the donor unit, and the benzene ring in the π bridge was replaced by thiophene. The thiophene substitution was found to broaden the light absorption range and enhance light-harvesting efficiency, resulting in a higher Jsc. Consequently, the designed molecule exhibits superior PCE compared to the experimental molecules. Theoretical simulations further validate the feasibility of developing high-performance DSSCs.