Polarization effect of dye-sensitizers on the current density and photovoltaic efficiency of co-sensitized DSSCs using metal-free and metal-based organic dyes
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Co-sensitization of metal-free (Eosin and Coumarin) and metal-based (N3 and N719) organic dye combinations were employed with TiO2 nanoparticles based photoanodes for the fabrication of dye-sensitized solar cells. The charge injection and oxidized dye regeneration process in mono and co-sensitized system were explored from the constructed energy level diagram using cyclic voltammetry analysis. Electrochemical impedance measurements were performed to determine interfacial charge transfer kinetics such as chemical capacitance, recombination resistance and electron lifetime. Current density–Voltage (J–V) characteristics resulted an improved efficiencies in co-sensitized DSSCs due to increased short circuit current density (Jsc). The studies on polarization effect was also evaluated using conductivity plot of impedance spectroscopy. Here, we report for the first time, the internal polarization of dye-sensitizers contributes a small current in the total obtained current density (Jsc) of a device besides the conduction current which enhances the PCE. Especially, high polarization effect of coumarin dye by its resonance polar structure increasing the conductivity in intermediate frequency region of conductivity plot and thus the current density gets enhanced. Improved efficiency of ~ 5.4% was realized in pure metal-based dye co-sensitization by its optimized electron concentration in CB TiO2 to balance charge injection and recombination.
This work was supported by University Grants Commission (UGC-India) Research funding [Grant No.: 41-915/2012 (SR); Dt-26 July 2012]. The authors thanks UGC-India for the financial support and Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, India for the photovoltaic (J–V) measurements.
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