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

Abstract

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 (J_SC = 13.08 mA cm⁻²) generation. However, the dye with a fluorene linker exhibited high open circuit voltage (V_OC = 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.