Effects of bis-carbazole based D–π-A sensitizers on solar energy capture in DSSCs

Abstract

A new series of molecular engineered dyes having a carbazole (CA) based bulky donor was synthesized and compared to a previously studied set of dyes having a diphenyl amine (DP) donor for their use in dye sensitized solar cells. In this work, their photophysical, electrochemical, and kinetic properties were measured and molecular modeling methods were used to predict their electronic and physical properties. Regarding the latter, TDDFT studies adopting different exchange correlation functionals were performed to predict solar capture behavior of these sensitizers. The calculated electronic energies showed that the introduction of the bulky CA donor gave a greater negative shift on E_HOMO rather than E_LUMO, imparting the CA series of dyes a red shifted absorption. On the other hand, tuning of the spacer unit by using different heterocylic rings was sensitive to the shift of E_LUMO shift. Better light absorption and driving force of devices based on the CA series were counteracted with negative effects related to the electron injection yield and electron recombination lifetime. The origin of the lower efficiency of the CA dye based devices was investigated in terms of dye load, charge recombination lifetime, in tandem with modeling studies. Results from assessing the effects of different heterocyclic rings, as a π conjugation unit on photovoltaic performance showed that the thiophene unit gave the highest current. On the other hand, CA- and DP-based dyes having an N-methyl pyrrole unit afforded the highest V_oc. The best DSSC performance in the CA series was achieved by the thiophene derivative, where η = 3.05% with J_sc = 6.29 mA cm⁻², V_oc = 673 mV and FF = 0.72 under 100 mW m⁻² irradiation.