Synergistic Modulation of Composite Donors and π-Spacers in Porphyrin Sensitizers for Enhanced Charge Transfer and Photovoltaic Efficiency: A DFT/TD-DFT Study

Abstract

Molecular engineering of porphyrin sensitizers is a key approach to enhancing the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). In this study, we systematically investigate the synergistic effects of donor engineering (triphenylamine, indoline, and their methoxy modifications) and acetylene π-spacer on the intramolecular charge transfer (ICT) and photovoltaic performance of D-π-A-type porphyrin dyes using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We designed five novel porphyrin dyes (L, T, T-L, TO-L, and T-LD) and performed detailed calculations to analyze their frontier molecular orbitals, absorption spectra, electron-hole separation dynamics, and interfacial coupling features on the (TiO2)9 cluster surface. The results demonstrate that the composite donor strategy effectively optimizes energy level alignment and facilitates directional charge transfer. Among these, the methoxymodified composite donor dye (TO-L) exhibits the best overall performance, with a strong electron-donating effect leading to the highest electron injection driving force (Gin = -2.24 eV) and optimal interfacial energy level matching, achieving a high open-circuit voltage of 1.12 V and a maximum theoretical conversion efficiency of 9.84%. Comparative analysis reveals that the introduction of the rigid acetylene π-bridge (T-LD) significantly extends the conjugated system, resulting in the largest absorption redshift (443.58 nm) and enhanced light-harvesting efficiency. This study provides a microscopic understanding of the relationship between molecular modifications and photovoltaic performance, offering valuable theoretical design guidance for future improvements in porphyrin-based sensitizer efficiency.