TiO 2 /dye/electrolyte Interface Engineering by Atomic Layer Deposited Ultra Thin SiO 2 for Improved Dye Sensitized Sola
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TiO2/dye/electrolyte Interface Engineering by Atomic Layer Deposited Ultra Thin SiO2 for Improved Dye Sensitized Solar Cell Performance Mariyappan Shanmugam1, Braden Bills and Mahdi Farrokh Baroughi Department of Electrical Engineering and Computer Science South Dakota State University, Brookings, SD-57007, USA ABSTRACT The short circuit density (JSC) and open circuit voltage (VOC) of dye sensitized solar cells (DSSCs) were improved from 9.8 to 17.8 mA/cm2 and 728 to 743 mV by depositing an ultra thin SiO2 layer on mesoporous TiO2 using Atomic Layer Deposition (ALD) method. X ray photoelectron spectroscopy confirmed the growth of SiO2 on mesoporous TiO2 surface. It was also observed that the enhancement in DSSC performance highly depends on the thickness of the ALD grown SiO2 layers on mesoporous TiO2. Compared to the reference DSSC which used untreated TiO2, incorporation of 5 ALD cycles (about 5 atomic layers) of SiO2 on mesoporous TiO2 resulted in 80 % enhancement (E) in the photoconversion efficiency from 4 to 7.2%. It is believed that the deposition of the ultra thin SiO2 film on mesoporous TiO2 modifies the density and activity of the surface states and an optimized layer thickness (5 cycles) leads to significant improvement in the DSSC performance. The enhanced photovoltaic performance was confirmed by dark and illuminated I-V and external quantum efficiency (EQE) measurements. Key words: Dye sensitized solar cells, atomic layer deposition, surface defects. INTRODUCTION Nanocrystalline TiO2 based DSSCs were introduced in 1991 with photoconversion efficiency of 11% [1]. DSSCs have shown great potential to compete with inorganic p-n junction solar cells in terms of efficiency (η), cost, low-temperature, and non-vacuum fabrication procedures [2, 3]. Major interfaces such as TCO/TiO2, TCO/dye/TiO2 and TiO2/electrolyte in DSSCs with large amount of defects play a significant role in the performance of DSSCs. Further, the photovoltaic performance of DSSCs is mainly governed by the TiO2/dye/electrolyte interface because at this interface major photovoltaic mechanisms such as photocarrier injection from the lowest unoccupied molecular orbital (LUMO) of the organic dye to conduction band (EC) of the TiO2, and electron recombination from the EC of the TiO2 to either the highest occupied molecular orbital (HOMO) level of the oxidized dye molecules or the redox electrolyte through surface defect states occur. Presence of such electronically active surface states at the TiO2/dye/electrolyte interface plays a crucial role on the performance of DSSCs [4, 5, 6]. Electron recombination at the TiO2/dye/electrolyte interface is an important performance limiting factor affecting JSC, VOC, FF and efficiency (η) of DSSCs significantly. Wet chemical processed metal oxide layers on mesoporous TiO2 can control carrier recombination from the EC of the TiO2 to oxidized HOMO level of the dye and liquid electrolyte [7, 8]. 1
Corresponding author: Tel.: +1(605)651-1804, Fax: +1(605)688-4401 E Mail: Mariyappan.Shanmugam@sdsta
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