Hexylthiophene-Functionalized Carbazole Dyes for Efficient Molecular Photovoltaics: Tuning of Solar-Cell Performance by Structural Modification

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Novel organic dyes (MK dyes), which have a carbazole derivative as an electron donor and a cyanoacrylic acid moiety (═C(—C≡N)COOH) as an electron acceptor and an anchoring group, connected with n-hexyl-substituted oligothiophenes as a π-conjugated system, were designed and synthesized for application in dye-sensitized solar cells (DSSCs), which are one of the promising molecular photovoltaics. The photovoltaic performance of the DSSCs based on MK dyes markedly depends on the molecular structure of the dyes in terms of the number and position of n-hexyl chains and the number of thiophene moieties. Retardation of charge recombination caused by the existence of n-hexyl chains linked to the thiophene groups resulted in an increase in electron lifetime. As a consequence, an improvement of open-circuit photovoltage (Voc) and hence the solar-to-electric power conversion efficiency (η) of DSSCs was achieved upon addition of n-hexyl chains to the thiophene groups. In addition, the adsorption condition (amount of dye molecules and/or dye aggregate thickness) on the nanoporous TiO2 electrode, depending on the number of hexyl chains, strongly affected the performance of DSSCs. A larger amount and/or thicker aggregate of dye molecules brought about longer electron lifetime, which resulted in higher Voc, and slower diffusion of I3− ions in the nanoporous TiO2 electrode, which led to lower short-circuit photocurrent (Jsc) and fill factor (FF). In the result of thorough investigation on the series of MK dyes, a DSSC based on MK-2 consisting of n-hexyl-substituted quarter-thiophene produced 8.3% of η (Jsc = 15.22 mA cm−2, Voc = 0.73 V, and FF = 0.75) under 100 mW cm−2 simulated AM1.5G solar irradiation.