Molecular design and theoretical investigation of new metal-free heteroaromatic dyes with D-π-A architecture as photosensitizers for DSSC application

- Designed and synthesized four new metal-free carbazole based chromophores with D-π-A architecture for DSSC application.
- The dyes P1-2 possess higher molar extinction coefficient, though they show lower absorption maxima, when compared to P3-4.
- The negative free energy for electron injection as well as photovoltaic performance of dyes P1-4 follows the same order.
- As per DFT simulations, P1 carrying cyanoacetic acid as anchoring unit possesses better charger separation in its FMO levels.

Herein, we report design, synthesis and photovoltaic performance of four new metal-free heteroaromatic dyes (P1-4) with D-π-A architecture carrying electron donating carbazole core connected to four different electron withdrawing/anchoring groups, viz. cyanoacetic acid, rhodanine-3-acetic acid, barbituric acid and thiobarbituric acid and phenylene ring as a π-spacer. The newly designed P1-4 were synthesized from carbazole derivative using Suzuki cross coupling approach followed by Knoevenagel condensation reaction. Their structures were confirmed by FTIR, NMR, Mass spectral and elemental analyses. The dyes were subjected to optical and electrochemical studies in order to investigate their absorption/emission behavior as well as HOMO/LUMO energies. The UV-vis spectral studies reveal that the P1-4 showed λmax at 412, 439, 458 and 489 nm, respectively. Their optical band-gap is in the range of 2.17 to 2.61 eV and fluorescence quantum yield is in the order of 44-70%. From energy level diagram, it is clear that all the dyes possess good thermodynamic feasibility for electron injection into CB edge of TiO2 as well as their regeneration from electrolyte system. The photovoltaic performance studies indicate that among the tested dyes, P1 anchored with cyanoacetic acid displayed the highest IPCE (32%), resulting in improved PCE (1.94%), JSC (4.68 mA cm−2), VOC (0.588 V) and FF (70.3%) values, when compared to other dyes. Finally, DFT studies were performed using Turbomole 7.1 V software to investigate their electron cloud delocalization in HOMO/LUMO levels and theoretical absorption spectral data. The results reveal that the dye P1 showed effective charge separation in its FMO levels, which has reflected in its ICT behavior and hence P1 displayed the improved photovoltaic performance.

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