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-π-A organic dyes with different π-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 π-spacer is beneficial to improve light-absorbing capacities with larger responsive range of spectra and molar extinction coefficients (ε). The designed dyes DP2-1–DP2-5 exhibit a good balance in various important properties including light harvesting efficiency (LHE), electronic injection driving force (ΔG ^inject ), regeneration driving force (ΔG ^reg ), reorganization energy (λ _total ) and vertical dipole moment (μ _normal ) and number of photoinjected electrons (n _c ) compared to reference dye DP2. Among all the designed dyes, DP2-3–DP2-5 containing π-spacers with branched chain exhibit obvious red shifted absorption band, smaller regeneration driving force (ΔG ^reg ) and larger excited state lifetime (τ ₀ ). This study is expected to provide new clues for the experimental synthesis of highly efficient metal-free organic dyes for DSSCs.