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

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 TiO 2 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. Novel organic sensitizers were designed and synthesized by employing indolo[2,3- b ]quinoxaline (IQ) as the main building block. IPCE graphs indicated that both competition and compensation of photon harvesting co-exist during the co-sensitization.