D-A-π-A based organic dyes for efficient DSSCs: A theoretical study on the role of π-spacer

A series of novel D-A-pi-A organic dyes with different pi-spacers for dye-sensitized solar cells (DSSCs) are designed based on hetero-tri-arylamine donor-based dye DP2. The key parameters of all the dyes affecting short-circuit current (J(SC)) and open-circuit voltage (V-OC) are theoretically investigated in detail using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to reveal structure-property relationships. The theoretical results indicate that extending pi-spacer is beneficial to improve light-absorbing capacities with larger responsive range of spectra and molar extinction coefficients (epsilon). The designed dyes DP2-1-DP2-5 exhibit a good balance in various important properties including light harvesting efficiency (LHE), electronic injection driving force (Delta G(inject)), regeneration driving force (Delta G(reg)), reorganization energy (lambda(total)) and vertical dipole moment (mu(normal)) and number of photoinjected electrons (n(c)) compared to reference dye DP2. Among all the designed dyes, DP2-3-DP2-5 containing pi-spacers with branched chain exhibit obvious red shifted absorption band, smaller regeneration driving force (Delta G(reg)) and larger excited state lifetime (tau(0)). This study is expected to provide new clues for the experimental synthesis of highly efficient metal-free organic dyes for DSSCs.