Unsymmetrical squaraine dye containing dithieno[3,2-b:2′,3′-d] pyrrole as a π-spacer: A potential photosensitizer for dye-sensitized solar cells

Designed with electron-rich and electron-deficient groups as π-linkers, a series of novel unsymmetrical squaraine dyes (SQ) for dye-sensitized solar cells (DSSCs) have been simulated using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Dye/TiO₂ complexes were also calculated to explore various properties on the interface. The results show that, compared with the prototype dye of WCH-SQ11, the modified WCH-SQ11-1, obtained by replacing the 3,4-ethylenedioxythiophene (EDOT) unit in WCH-SQ11 with dithieno[3,2-b:2′,3′-d]pyrrole (DTP), keeps a good balance in various key properties including light harvesting efficiency (LHE), driving force for electron injection (ΔG_inject), charge transfer (CT) character, and vertical dipole moment (μ_normal). Further investigation of the adsorbed WCH-SQ11-1 (i.e. the WCH-SQ11-1/(TiO ₂)₃₈ complex) reveals a red-shifted absorption spectrum compared to the free dye. The analysis of the molecular orbitals responsible for the maximum absorption indicate a direct electron injection mechanism for the WCH-SQ11-1/(TiO₂)₃₈ system.