Effect of auxiliary donors and position of benzothiadiazole on the optical and photovoltaic properties of dithieno[3,2-b:2′,3′-d] pyrrole-based sensitizers

Organic dyes used as sensitizers in dye-sensitized solar cells offer endless opportunities for modulation of cell efficiency through structural diversifications. Despite the countless number of dyes developed, structure-property analysis by varying chromophores and their topology in dyes is useful to formulate design principles. Herein, we report six new organic dyes featuring dithienopyrrole (DTP), benzothiadiazole (BTD) and cyanoacrylic acid units. The effect of auxiliary donors such as triphenylamine or diphenylaminofluorene and the position of benzothiadiazole on the optical and photovoltaic properties were evaluated by appropriate design of dyes. Incorporation of additional donor enhanced the spectral response of the dyes by elongating the conjugation length and supplementing the donor-acceptor interaction. Moving the BTD unit from the position between DTP and acceptor (D-D-A-pi-A) to a position between the auxiliary donor and DTP (D-A-D-A) altered the absorption as well as electrochemical properties. Theoretical calculations revealed that the later molecular configuration ensures sufficient overlap of HOMO and LUMO and facilitate the charge transfer propensity from arylamine donor to acceptor which is beneficial for light harvesting. Among the dyes, a D-A-D-A dye possessing triphenylamine exhibited highest power conversion efficiency (eta = 7.57%). The structure-property relationship is supported by electron transfer kinetics elucidated by electrochemical impedance spectroscopic studies. The best performing dye displayed high charge recombination and sufficiently low charge transfer resistance in the device.