Role of Mesoporous TiO 2 Surface States and Metal Oxide Treatment on Charge Transport of Dye Sensitized Solar Cells
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Role of Mesoporous TiO2 Surface States and Metal Oxide Treatment on Charge Transport of Dye Sensitized Solar Cells Mariyappan Shanmugam,1 Braden Bills and Mahdi Farrokh Baroughi Department of Electrical Engineering and Computer Science South Dakota State University, Brookings, SD-57007, USA ABSTRACT Photovoltaic performance of dye sensitized solar cell (DSSC) was enhanced by 19 and 69 % compared to untreated DSSC by treating the nanoporous titanium dioxide (TiO2) by ultra thin Aluminum oxide (Al2O3) and Hafnium oxide (HfO2) grown by atomic layer deposition method. Activation energy of dark current, obtained from the temperature dependent current-voltage (I-VT), of the untreated DSSC was 1.03 eV on the other hand the DSSCs with Al2O3 and HfO2 surface treatment showed 1.27 and 1.31 eV respectively. A significant change in the activation energy of dark current, over 0.24 eV for Al2O3 treatment and 0.28 eV in case of HfO2 treatment, suggest that density and activity of surface states on nanoporous TiO2 was suppressed by ALD grown metal oxides to result improved photovoltaic performance. Further the enhanced DSSC performance was confirmed by external quantum efficiency measurement in the wavelength range of 350-750 nm. INTRODUCTION DSSCs have shown great potential to compete with conventional p-n junction solar cells in terms of conversion efficiency (η), cost, simple, fast and low-temperature fabrication procedures [13]. However, major challenges with DSSCs include narrow band absorption dyes, poor stability, lack of robustness in large scale production, and corrosive nature of the liquid electrolyte which affect the platinum coated electrode over time [4, 5]. Another significant problem associated with DSSCs is photogenerated carrier recombination loss that occurs at the interface between solid inorganic semiconductor, usually n type mesoporous TiO2, organic dye, and liquid electrolyte [8, 9]. The JSC, open circuit voltage (VOC) and fill factor (FF) of DSSCs are greatly dependent on the density and activity of defects at the interface. Modeling and experimental evidences were presented to prove the charge transfer and recombination losses at the TiO2/dye/electrolyte interface were the most dominant factor in DSSC performance [10]. A. Kay et al. has reported that the density of trap states and their activity can effectively be engineered by treating the mesoporous TiO2 using wet chemical processed ZnO, TiO2, ZrO2, MgO, Al2O3, and Y2O3 [11]. E. Palomares et al. has reported significant enhancement in DSSC performance using wet chemical processed Al2O3 treated mesoporous TiO2 shown by increased JSC from 8.1 to 10.9 mA/cm2, increased VOC from 705 to 750 mV and improved efficiency from 3.8 to 5% [12]. Wet chemical processed TiCl4 surface treatment on mesoporous TiO2 also yielded a significant change in JSC from 9.5 to 11.2 mA/cm2, VOC from 680 to 690 mV and improved cell efficiency from 4.3 % to 5.1 %, and was observed to change the morphology and surface area of the mesoporous TiO2 resulting in enhanced dye l
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