Tailoring dual-channel anchorable organic sensitizers with indolo[2,3-b]quinoxaline moieties: Correlation between structure and DSSC performance

Abstract

Two new metal-free heteroaromatic organic dyes, FS13 and FS14, with D-D|A-(π-A)2 structural configuration were tailored and synthesized as effective dual-channel anchorable sensitizers in DSSCs. The new entities consist of indolo[2,3-b]quinoxaline (IQ) as the constructing unit containing merged donor–acceptor system (D|A) connected to two cyanoacetic acids as anchoring groups via phenylene as π-linker at 2- and 3- positions of IQ rings. Their experimental properties together with theoretical modeling using density functional theory (DFT) are studied to understand their achievability as feasible sensitizers. The photovoltaic performance and charge transfer properties were assessed with illumination condition as 1.5 AM standard. The DFT study revealed that the isodensity of FS13 and FS14 has been slightly shifted towards di-anchors compared to the mono-anchoring FS08, leading to more distinguished charge separation; however, the closely paralleled anchoring branches bring out high spatial hindrance, turning into the less conjugated system and the lower photocurrent. For the study of co-sensitization effect employing dual-channel dyes with HD-2 ruthenium sensitizer, FS14 displayed the most positive co-sensitization effect and enhanced the overall efficiency by 9.2% from 7.50% to 8.67%. For both novel dyes, the considerably improved photovoltage and fill factor are ascribed to the improved coverage percentage on TiO2 surface and the enhancement of dye anti-aggregation and charge recombination resistance, which in turn corroborates the superior affinity of di-anchors. Furthermore, the dark current resistance estimated from impedance graphs employing co-sensitization methodology coincided with the observed photovoltage, indicating the enormous potential of di-anchoring dyes in ameliorating the photovoltaic performance.