Characterization of metal-free D-(π-A)2 organic dye and its application as cosensitizer along with N719 dye for efficient dye-sensitized solar cells

The optical, electrochemical and density functional theory molecular simulation of a metal-free D-(pi-A)(2), i.e., 3,30( 5,5'-(9-hexyl-9H-carbazole-3,6-diyl)bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid) denoted as D has been investigated. A stepwise cosensitization of D with N719 dye is adopted to enhance the power conversion efficiency of dye-sensitized solar cells. The metal-free dye possesses strong absorption in the 370-450 nm wavelength range and effectively overcomes the competitive light absorption by I-3(-)/I-. The N719/Dcosensitized dye-sensitized solar cell shows a power conversion efficiency of about 7.24 %, which is higher than the dye-sensitized solar cells based on either N719 (5.78 %) or D (3.95 %) sensitizers. The improved power conversion efficiency of the cosensitized dye-sensitized solar cell is attributed to the combined enhancement of both short-circuit photocurrent and open-circuit voltage. The short-circuit photocurrent improvement is attributed to the increase in the both light-harvesting efficiency of the cosensitized photoanode and charge collection efficiency of the dye-sensitized solar cell. However, the open-circuit voltage is improved due to better adsorption and surface coverage of TiO2 on cosensitization and an associated reduction in the back electron recombination with increased electron lifetime. These effects are analyzed using electrochemical impedance spectroscopy and dark current-voltage measurements of the dye-sensitized solar cells.