Improved electrochemical performance of dye-sensitized solar cell via surface modifications of the working electrode by electrodeposition

Abstract

Modifications of the working electrode with TiO2 blocking or coating layers are carried by electrodeposi- tion in TiCl3 precursor solution. The results suggest that the electrodeposited TiO2 blocking layer provides excellent agglutination between the FTO substrate and the active TiO2 layer. In addition, the electrodeposited TiO2 coating layer enhances the interconnections between the TiO2 nanoparticles and the FTO substrate, and therefore it increases the electron transport efficiency. The morphology and crystalline structure of the electrodeposited TiO2 layers are char- acterized by SEM, TEM, and XRD. The electrochemical impedance spectroscopy measurements show that the improved DSSC performance with the electrodeposited coating layer is mainly due to the increase in the lifetime of the conduction band electron in the TiO2 film. The photoelectron conversion efficiency of DSSC is increased from 3.47% to 5.38% by employing the TiO2 electrodeposited working electrode. � /I3 � ) redox electrolyte, and a platinum-coated transparent conducting oxide (TCO) glass as a counter electrode. The electrons excited from the dye are transferred step by step through the conducting band of TiO2 nanoc- rystallites and FTO electrode. The dye is regenerated by electron donation from the redox system of the electrolyte. The iodide is re- covered by the reduction of triiodide on the platinum layer of the counter electrode. In practice, the FTO surface might not be fully covered with the mesoporous TiO2 film due to the porous nature of the TiO2 film. The exposed surface area of the FTO to the electro- lyte may lead to charge recombination between the TiO2 and the FTO interfaces. Electron transfer to the FTO is possibly intercepted by the reduction of the electrolyte interacting with the electrode. Another weakness of mesoporous TiO2 films is the inadequate con- nection between TiO2 nanoparticles, leading to an increase in elec- tron transfer resistance (6-8). To solve this recombination and elec- tron transfer resistance problem, previous studies have demonstrated that it is effective to introduce a compact and thin TiO2 blocking layer onto the transparent conducting glass substrate (9-11) using a range of preparation methods such as sputtering (11,12), sol-gel (13, 14), and spray-coating (15,16) techniques. A TiCl4 post-treatment of the TiO2 photoanode is a common modification method for ob- taining a reliable thin TiO2 coating layer to improve the cell perfor- mance such as current density, fill factor, and conversion efficiency (17-19). In this study, an electrodeposition strategy was used to modify the mesoporous TiO2 photoanode in two schemes: i) a blocking layer prepared first by the electrodeposition on the FTO substrate and then TiO2 photoanode fabricated, and ii) a TiO2 photoanode fabri- cated first and then the electrodeposition performed for covering the interfaces on the FTO substrate and mesopores between TiO2 nanoparticles. Experimental parameters for TiO2 electrodeposition were applied voltage and electrodeposition time. The blocking layer can reduce the electron recombination phenomena from FTO to electrolyte by separating effectively at a specific layer thickness (20). In addition, a thin TiO2 coating layer formed by the electrodeposi- tion can not only reduce electron recombination from FTO to elec- trolyte, but also increase electron transport between TiO2 nanoparticles because the TiCl3 precursor solution can penetrate deep into the ex- posed FTO surface and the mesoporous TiO2 nanoparticles. The photovoltaic properties of the DSSCs with the TiO2 modified photo anode by electrodeposition were characterized and compared with those of the unmodified photo anode case.