Analysis of Charge Transport and Recombination Studied by Electrochemical Impedance Spectroscopy for Dye-sensitized Sola
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1270-GG04-05
Analysis of Charge Transport and Recombination Studied by Electrochemical Impedance Spectroscopy for Dye-Sensitized Solar Cells with Atomic Layer Deposited Metal Oxide Treatment on TiO2 Surface * Braden Bills , Mariyappan Shanmugam, Mahdi Farrokh Baroughi and David Galipeau Department of Electrical Engineering and Computer Science South Dakota State University, Brookings, SD 57007, USA ABSTRACT The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the porous titanium oxide (TiO2) nanoparticles back into the electrolyte solution, which occurs almost exclusively through the interface. This and the fact that DSSCs have a very large interfacial area makes their performance greatly dependant on the density and activity of TiO2 surface states. Thus, effectively engineering the TiO2/dye/electrolyte interface to reduce carrier losses is critically important for improving the photovoltaic performance of the solar cell. Atomic layer deposition (ALD), which uses high purity gas precursors that can rapidly diffuse through the porous network, was used to grow a conformal and controllable aluminum oxide (Al2O3) and hafnium oxide (HfO2) ultra thin layer on the TiO2 surface. The effects of this interfacial treatment on the DSSC performance was studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy (EIS) measurements. INTRODUCTION DSSCs have been under development for almost two decades and are showing great potential to compete with conventional p-n junction solar cells in terms of efficiency, cost and simple, fast, low-temperature and non-vacuum fabrication procedures. However, major challenges still persist including narrow band absorption dyes, poor stability, lack of robustness in large scale production and the corrosive nature of the liquid electrolyte. Another significant problem limiting the performance of DSSCs is photogenerated carrier loss that occurs between the photoelectrode and the electrolyte [1]. Carrier loss is fundamentally different in DSSCs compared to conventional solar cells. Recombination between electrons and holes in conventional solar cells occurs mainly in the bulk of usually only one material (absorber), where recombination in DSSCs is due to electron injection from semiconducting nanoparticles to liquid electrolyte occurring almost exclusively through the interface. This and the fact that DSSCs have a much larger (~1000 times) interfacial area compared to conventional solar cells makes their performance greatly dependant on the density and activity of surface states. Thus, effectively engineering the interfaces to reduce carrier losses is critically important for improving opencircuit voltage (VOC), short-circuit current (JSC), fill factor (FF) and efficiency (η). Recent works have shown DSSC performance enhancements when a wide range of wet chemical processed materials were deposited onto the mesoporous TiO2 network [2]. Another technique to treat surface defects is to use oxygen, nitrog
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