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

Abstract

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 (ΔGinject), charge transfer (CT) character, and vertical dipole moment (μnormal). Further investigation of the adsorbed WCH-SQ11-1 (i.e. the WCH-SQ11-1/(TiO2)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.