Characterization of Nanoporous TiO 2 Surface Defects by Temperature Dependent Electron Transport Studies on Dye Sensitiz
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Characterization of Nanoporous TiO2 Surface Defects by Temperature Dependent Electron Transport Studies on Dye Sensitized Solar Cells Mariyappan Shanmugam1 and Mahdi Farrokh Baroughi Department of Electrical Engineering and Computer Science South Dakota State University, Brookings, SD-57007, USA ABSTRACT Temperature dependent, dark current-voltage characteristics (I-V-T) of dye sensitized solar cells (DSSCs) were used to study the TiO2 surface defects on photovoltaic performance. Three kinds of DSSCs were used for this study. (1) DSSC with no surface treatment, (2) DSSCs with Al2O3 and (3) HfO2 treatments. Activation energy of charge transport, obtained from the I-V-T of the three DSSCs, suggest that Al2O2 and HfO2 surface treatment on the nanoporous TiO2 effectively suppressed the electron capture by the surface states from the conduction band of TiO2 and the consecutive electron transfer to the electrolyte in which the TiO2 is permeated. The reference DSSC showed activation energy of 1.03eV while the DSSCs with Al2O3 and HfO2 surface treatment showed 1.27 and 1.31eV respectively. The higher activation energy, in case of HfO2 surface treatment, suggest that electronically active TiO2 surface states present near to the redox potential of the electrolyte was reduced and hence resulted in improved photovoltaic performance than Al2O3 treated DSSC and the reference DSSC. Key words: Surface treatment, activation energy, charge transport, recombination INTRODUCTION Colloidal, nanocrystalline TiO2 based DSSC was introduced by M. Gratzel’s group in 1991 with photoconversion efficiency of 11.3% [1]. It is a hybrid solar cell with organic dye molecules adsorbed on the large surface area of TiO2 permeated in a liquid hole conducting media usually I-/I3redox electrolyte system. Significant problems associated with DSSCs are poor stability due to liquid, corrosive nature of electrolyte, dyes with narrow absorption band and defective TiO2 surface. All these issues associated with DSSC fabrication are considered to be a major obstacle for mass production and lower cost. Major challenges involve with Charge transport, recombination and accumulation at various interfaces in DSSCs were studied by different characterization methods such as photocurrent transient spectroscopy and electrochemical impedance spectroscopy analysis [2-4]. Electro chemical impedance spectroscopic studies have shown that charge transfer resistance, transport resistance and chemical capacitance can be changed by modifying the surface of TiO2 [57]. Various metal oxide surface treatment on TiO2 improved the photovoltaic performance by suppressing the electron recombination from the conduction band of TiO2 to electrolyte [8]. Density and activity of surface states associated with TiO2 play a vital role on electron transport in the bulk of TiO2. Thickness of the metal oxides used to treat the porous TiO2 network is an important factor that determines the tunneling current of photoinjected electrons from the dye to TiO2. Recent 1
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