Theoretical design and analysis of D–π–A–A′ organic dyes for enhanced efficiency in DSSCs by modifying donor (D) and acceptor (A) moieties

This study presents a comprehensive investigation of ten different dyes using DFT and TD-DFT calculations. The dyes feature a D-π-A-A' architecture with a thiophene π-bridge, cyanoacrylic acid serving as both an electron-acceptor and anchoring group, and a selection of electron-donor and electron-acceptor moieties. The electron-donor moieties explored include coumarin (COU), triphenylamine (TPA), indoline (IN), carbazole (CAR), diphenylamine (DPA), tetrahydroquinoline (THQ), triazatruxene (TAT), azulene (AZ), and julolidine (JUD). Besides, the electron-acceptor moieties consist of quinoline (QN), [1,2,5]thiadiazole[3,4-<i>c</i>]pyridine (PY), phthalimide (PTM), benzothiadiazole (BTZ), naphthalenediimide (NDI), benzothiazole (BZ), and pyridoquinazolinone (PYQ). The dyes are labeled with the following designations: COU-QN, TPA-PY, IN-PTM, CAR-BTZ, DPA-NDI, THQ-BZ, TAT-BZ, TPA-PYQ, AZ-QN, and JUD-BTZ. Among these dyes, TPA-PYQ shows the lowest <i>Δ</i> _H-L value of 1.674 eV, which decreases further to 1.281 eV upon binding with the Ti₅O₁₀ cluster. Band-alignment plots indicate that all dyes have GSOP values below the redox potential of the I^-/I₃ ^- electrolyte (<i>i.e.</i>, -4.85 eV), while their ESOP values are generally above the TiO₂ conduction band (<i>i.e.</i>, -4.05 eV), with the exception of AZ-QN. The negative adsorption energies suggest effective chemisorption of the dye-clusters on the TiO₂ surface, facilitating electron transfer from the dye's LUMO to the conduction band of TiO₂. Additionally, absorption studies reveal that the <i>λ</i> _max of the dyes shifts towards the red region when complexed with Ti₅O₁₀. Dye-clusters such as JUD-BTZ-Ti₅O₁₀, TPA-PY-Ti₅O₁₀, and TPA-PYQ-Ti₅O₁₀, with lower <i>E</i> _b values, exhibit enhanced exciton dissociation and charge transfer, leading to improved performance. These findings suggest that the designed dyes may act as promising candidates for the development of dye-sensitized solar cells (DSSCs).