Molecular design of benzo[c][1,2,5]thiadiazole or thieno[3,4-d]pyridazine-based auxiliary acceptors through different anchoring groups in D-π-A-A framework: A DFT/TD-DFT study

Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, a novel series of D-π-A-A porphyrin sensitizers adsorbed on the TiO2 cluster has been investigated. The D-π-A-A configurations contained a donor of iminodibenzyl, π-linker of Zn-porphyrin, and two kinds of auxiliary acceptors (labeled as BT and TP), as well as five types of anchoring groups. The ground-state geometries, electronics, optics, and charge transfer properties of all free dyes including injection driving force (ΔGinj), regeneration energy (ΔGreg), and light-harvesting efficiency (LHE) were calculated. Additionally, reorganization energy (λtotal), electron affinity (EA), and ionization potential (IP) were also reported to confirm the transport properties of the designed dye. Furthermore, the interfacial system of dye@(TiO2)48 was further discussed to reveal the complexation energy of the system by considering the adsorption energy (Eads) between dyes and (TiO2)48. The results showed that the insertion of different auxiliary acceptors in the D-π-A-A motif and variations in the anchoring groups resulted in red-shift absorption along with the increase in photovoltaic properties. These results suggested that BT with the rhodanine-3-acetic acid group (BT4) and BT with 2-(1,1-dicyanomethylene)rhodanine group (BT5) and TP with the same rhodanine-3-acetic acid and 2-(1,1-dicyanomethylene)rhodanine groups (TP4 and TP5) had shown better properties among other candidates for DSSCs. BT4-BT5 and TP4-TP5 had pronounce effect on optical and charge transfer properties by showing small HOMO-LUMO gaps, red-shifted spectra (λmax), greater LHE and ICT characters, and maximum-negative Eads. Thus, our theoretical investigation using these auxiliary moieties can be helpful for the precise structural modifications and design for developing efficient dyes in dye-sensitized solar cells (DSSCs).