Molecular Engineering and Theoretical Investigation of Novel Metal-Free Organic Chromophores for Dye-Sensitized Solar Cells

In this work we report design and synthesis of three new metal free D-D-A-π-A type dyes (E_1-3) with different acceptor/anchoring groups, as effective sensitizers for nanocrystalline titanium dioxide based dye sensitized solar cells. All the three dyes carry electron donating methoxy group as an auxiliary and indole as a principal donor, cyanovinylene as an auxiliary acceptor and thiophene as a π-spacer. Whereas, cyanoacetic acid, rhodanine-3-acetic acid and 4-aminobenzoic acid perform as acceptor/anchoring moieties, respectively in the dyes E_1-3. Though the dye containing 4-aminobenzoic acid unit (E₃) exhibits comparatively lower λ_max, it shows the highest power conversion efficiency arising from the higher electron life time and good light-harvesting capability. The DFT studies reveal a better charge separation between the HOMO and LUMO levels of E₃, further substantiating the experimental results. Among the three dyes, E₃ shows the best photovoltaic performance with short-circuit current density (J_sc) of 9.35 mA cm^-2, open-circuit voltage (V_oc) of 620 mV and fill factor (FF) of 0.71, corresponding to an overall conversion efficiency of 4.12% under standard global AM 1.5G.