Functional tuning of organic dyes containing 2,7-carbazole and other electron-rich segments in the conjugation pathway

Organic dyes containing a triarylamine donor, a cyanoacrylic acid acceptor and a conjugation pathway composed of 2,7-carbazole, thiophene and fluorene have been synthesized and characterized as sensitizers for TiO2-based dye-sensitized solar cells. The effect of the nature of the conjugation bridge on the optical, electrochemical and photovoltaic properties has been investigated. Elongation of conjugation by the insertion of 2,7-carbazole or 2,7-fluorene π-linkers led to an increase in the molar extinction coefficients while the use of terthiophene red-shifted the absorption. Also the lengthening of the π-bridge helped to raise the lowest unoccupied molecular orbital of the dyes relative to the conduction band of TiO2. This increased the thermodynamic driving force for the electron injection from the excited dyes into the conduction band of TiO2. The trends in the optical properties of the dyes were also substantiated by TDDFT computations. DSSCs based on a dye containing terthiophene in the conjugation bridge exhibited the highest power conversion efficiency (4.9%) in the series attributable to its higher photocurrent (JSC = 13.08 mA cm−2) generation. However, the dye with a fluorene linker exhibited high open circuit voltage (VOC = 632 mV) and prolonged electron lifetime for the device. The role of the π-bridge in the charge transfer and electron recombination kinetics in the DSSC was elucidated by electrochemical impedance spectroscopic studies.