Characterization of an Ultra Thin Dense Hafnium Oxide Compact Layer with Electrochemical Impedance Spectroscopy for Dye-
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Characterization of an Ultra Thin Dense Hafnium Oxide Compact Layer with Electrochemical Impedance Spectroscopy for Dye-Sensitized Solar Cell Application 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 photoelectrode back into the electrolyte solution. An atomic layer deposited (ALD) hafnium oxide (HfO2) ultra thin, a few nanometers, compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the performance of DSSCs were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy (EIS) measurements. Further, the theory of electron recombination at the TCO/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO2 compact layer. 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 conversion efficiency, cost and simple, fast, and low-temperature fabrication procedures. However, a significant problem limiting the performance of DSSCs is the back-reaction of photogenerated electrons from the photoelectrode back into the electrolyte solution, where the photoelectrode is comprised of the porous TiO2 nanoparticle network and the TCO contact. It was shown that contact between the electrolyte and TCO can create an electrical short leading to poor photovoltaic performance [1]; sol-gel processed TiO2 compact layers were then introduced to prevent such electrical shorts [2]. However, electrolyte could still diffuse through the nanoparticles of the sol-gel processed compact layer, resulting in the need to use a thicker compact layer to further separate the TCO and electrolyte in order for the layer to still be effective. But a thick compact layer can interrupt the injection of electrons from the porous TiO2 nanoparticles to TCO by acting as a resistive layer due to the formation of a Schottky barrier [3]. Recently, the ALD method, which has the ability for fast, low temperature processing, fine control of thickness and high purity of material precursors, was used for depositing a TiO2 compact layer, but this compact layers was still relatively thick [4]. Therefore, photovoltaic performance can be enhanced with a compact layer that can effectively retard the recombination of electrons with the electrolyte without interfering with the injection of electrons from porous TiO2 to TCO. This paper presents the fabrication and characterization of DSSCs with a thin ALD HfO2 compact layer. EXPERIMENTAL DETAILS DSSCs with ALD HfO2 and sol-gel processed TiO2 compact layers were fabricated, where the latter will be used as the reference. Deposition of 30 cycles of HfO2 onto commercially available indium tin oxide (ITO) coated g
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