Thiadiazole based π-conjugated small molecule as donor material for highly stable and efficient bulk heterojunction organic solar cells

This paper demonstrates the designing of π-conjugated donor small organic molecule 4,7-bis(5-((E)-3,5-dimethoxystyryl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BTD-2OMe) and its synthesis through the multistep synthetic routes from commercially available low cost precursor materials. The π-conjugated thiophene units interconnected with vinylene in BTD-2OMe favor the red shift absorption spectrum. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis of BTD-2OMe present excellent thermal stability up to ∼403 °C with only 3% weight loss and good crystallization of organic chromophore, respectively. The absorption band in solid state shows the red shift due to J-aggregation and a moderate optical bandgap of ∼1.81 eV with HOMO-LUMO energy levels of −5.35 and −3.54 eV has been obtained. The bulk heterojunction organic solar devices (BHJ-OSCs) have been fabricated and are systematically investigated with different BTD-2OMe:PC₇₁BM (1:1, 1:2, and 1:3, w/w) active layers. The optimized BTD-2OMe:PC₇₁BM (1:2, w/w) device with improved active layer morphology and balanced energy level alignment achieves a high short circuit current density (Jsc) of ∼13.13 mAcm⁻² and open-circuit voltage (Voc) of ∼0.924 V resulting in power conversion efficiency (PCE) of ∼6.72%. Importantly, the optimized BTD-2OMePC₇₁BM (1:2, w/w) device poses significantly high device stability of up to 15 days under ambient temperature by retaining ∼95% of PCE from its initial value.