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

Abstract

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-c]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 Δ_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.e., −4.85 eV), while their ESOP values are generally above the TiO₂ conduction band (i.e., −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 λ_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 E_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).