Solar cells sensitised by push-pull azo dyes: dependence of photovoltaic performance on electronic structure, geometry and conformation of the sensitizer

Novel Azo dyes possessing varying conjugation lengths and different extents of electron cloud delocalisation were synthesised and characterised and explored as sensitizers for dye-sensitised solar cells (DSSC). The envisaged azo dyes comprising of electron withdrawing and electron-donating moieties which are linked through conjugation bridges of varying lengths facilitated the prevalence of push-pull mechanism in the molecules. Optimisation of the geometry were performed for the synthesised compounds using B3LYP/6-31+G (d,p) level of density functional theory and their computed optical absorption and band gaps were validated with experimental results. The dyes exhibited molar extinction coefficients in the range of 3.2x10(4) to 4.2x10(4) mol(-1) Lcm(-1). The highest occupied molecular orbital (HOMO) was located between -5.53 and -5.03 eV for the various sensitizers synthesised and their lowest unoccupied molecular orbital (LUMO) was located between -2.86 and -3.08 eV. HOMO-LUMO gaps were in the range of 2.02-2.67 eV. Fill factor of the cells varied from 28% to 32% and the power conversion efficiencies ranged from 0.4% to 2.7%. This is the first time reporting of a systematic investigation, correlating the influence of nature and position of substituent, extending of conjugation and geometry of sensitizers on the photo physical properties of the sensitizers and the photovoltaic performance of corresponding DSSC.