Synthesis and computational study of coumarin thiophene-based D-π-A azo bridge colorants for DSSC and NLOphoric application

Abstract This article is comprised of designing and synthesis of three coumarin-based donors (D)-π-azo-acceptor (A) colorants with −COOH and -SO3H anchoring groups for application in dye-sensitized solar cell (DSSC). Prototype DSSC devices were prepared with these colorants as sensitizers and were characterized through IV and EIS measurements to explore the practical performance. The well-performing colorant NAA 236 exhibit a short-circuit current density, Jsc of ∼10.28 mA cm−2, with an open-circuit voltage, Voc of ∼0.62 V. The electrochemical impedance spectroscopy (EIS) and electron lifetime (τe) analysis showed effective performance with Rrec (141.61 Ω) and τe10.5 ms for the NAA 236 colorant respectively. The power conversion efficiency (PCE) of 4.5 % (AM 1.5 G, 100 mW cm−2) is achieved for NAA 236 that has the −COOH (an anchoring group) at the para position of the azo group with fill factor FF of ∼ 68.7. Theoretical Density Functional Theory (DFT) and Time-Dependent TD-DFT calculations were performed using the method B3LYP/6-311+g(d,p). The HOMO-LUMO energy, vertical excitation energy, Global Reactivity Descriptors (GRD) i.e. electrophilicity index (ω), hyper-hardness(Γ) and Light-harvesting efficiency (LHE) property were assessed through optimized geometry, and it showed a linear trend with Power Conversion Efficiency (PCE). All the sensitizers demonstrate excellent non-linear optical (NLO) response and display a direct relation with photovoltaic performance.