Effect of Donor Modification on Photo-Physical and Photo-Voltaic Properties of N-Alkyl /Aryl Amines Based Chromophores
Six push-pull sensitizers (MD1 to MD6) having rhodanine-3-acetic acid acting as an electron withdrawing group and N-alkyl /aryl amine as a donor moiety have been planned and prepared to rationalize the influence of donor alteration on absorption/emission properties and photon to current conversion efficiency (η). The structures of all the sensitizers were ascertained by 1H-NMR, 13C-NMR and CHN analysis. The photophysical parameters of these sensitizers were recorded in nine solvents having different polarity to study the solvatochromic behavior of the dyes. Nanocrystalline mesoporous TiO2 based DSSCs were constructed by employing MD1 to MD6. The overall photon to current conversion efficiency was obtained in the range of 2.75% to 5.06% where MD2 chromophore shows the highest efficiency of 5.06%. All these efficiencies were correlated with structural modification of donor moiety to establish a structure-property relationship. The improved light to the power conversion efficiency of MD2 is because of high short circuit current Jsc (12.70 mA), high resistance in electron recombination and longer lifetime of injected electrons. The rigidity of donor moiety (MD1) results in selective stabilization of HOMO energy level which increases band gap and shows a decrease in efficiency along with a blue shift in photophysical properties. N-alkylation at donor moiety (MD4-MD6) causes an increase in molar absorptivity but affects the conjugation and shows a negative impact on efficiency. Lack of planarity in the pi-conjugation (MD3) has a negative impact on photophysical and photovoltaic properties. Extensive DFT study reveals the experimental absorption of all the dyes have very good correlation with theoretical vertical excitation obtained by time-dependent density functional theory (TD-DFT).