Synthesis, modeling and photovoltaic properties of a benzothiadiazole based molecule for dye-sensitized solar cells

A benzothiadiazole based solution-processable organic dye D1 (N⁴-Phenyl-N⁴,N⁷,N⁷-tri{4-[(2-ethoxycarbonyl)-2-cyanovinyl]phenyl}benzo[c][1,2,5]thiadiazole-4,7-diamine) was synthesized. Dye-sensitized solar cells (DSSCs) were fabricated based on D1 and a commercially available benzothiadiazole based RK1 dye. The photovoltaic parameters of these cells were studied under simulated AM 1.5 illumination (100 mW cm⁻²). The DSSC sensitized by RK1 dye provided a power conversion efficiency of 5.7 % with high values of short-circuit photocurrent density, open-circuit photovoltage, and fill factor. Under the same conditions, the cell sensitized with D1 dye showed a very poor performance due to the lower values of short-circuit photocurrent density and open-circuit photovoltage. In order to provide insight into poor performance of the DSSC sensitized by D1, molecular structure of D1 was analyzed by density functional theory. Its frontier orbitals were calculated to investigate effectiveness of charge transport and reasons of the poor performance. This computational study revealed that the photovoltaic performance of the benzothiadiazole based DSSCs can be enhanced significantly by inserting a phenyl ring between the benzothiadiazole unit and the anchoring group.