Structural studies and photovoltaic investigation of indolo[2,3-b]quinoxaline-based sensitizers/co-sensitizers achieving highly efficient DSSCs

Abstract

Three metal-free organic dyes, coded FS10, FS11, and FS12, were designed and synthesized by employing indolo[2,3-b]quinoxaline (IQ) as the main building block and three different edged donors, viz., triphenylamine, carbazole, and phenothiazine, respectively. In order to comprehend the feasibility of the novel dyes as probable sensitizers/co-sensitizers, they were well-characterized and their photochemical, electrochemical, and theoretical studies were processed. The photovoltaic properties were evaluated under 1.5 AM standard illumination conditions. FS10 acquired the best DSSC performance (η = 5.27%) with supreme V_OC (0.676 V) and FF (70.1%). The contribution from the incorporated IQ was examined theoretically by using DFT and time-dependent DFT, and the results indicated that all the novel dyes displayed clear charge separation at the frontier occupied molecular orbitals. The investigation of the co-sensitization effect was carried out by employing the HD-2 ruthenium sensitizer. The corresponding photovoltaic performance revealed that FS10 gave the most positive co-sensitization effect by improving the efficiency to 8.32% (J_SC = 18.68 mA cm⁻², V_OC = 0.672 V, FF = 66.2%), while HD-2 alone exhibited the efficiency of 7.47% (J_SC = 19.00 mA cm⁻², V_OC = 0.650 V, FF = 60.5%). The increased V_OC and FF resulted from the optimized anchoring mode on the TiO₂ surface with the combination of FS10 and HD-2 and more uniform coverage, leading to the hindrance of dye aggregation and charge recombination. Additionally, despite the reduced J_SC with co-sensitizers, the corresponding IPCE curves displayed more complex increasing as well as decreasing behavior as a function of wavelengths, thus reflecting that sensitizers and co-sensitizers not only compete but also compensate the light harvesting abilities.