Electrical Response of Wet Chemically Grown ZnO Nanorods for Photovoltaic Application
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Electrical Response of Wet Chemically Grown ZnO Nanorods for Photovoltaic Application Julian Tornow and Klaus Schwarzburg Dynamics of Interfacial Reactions, Hahn-Meitner-Institut Berlin GmbH, Berlin, Germany
ABSTRACT Wet chemically prepared arrays of ZnO nanorods in electrolyte solution were investigated with respect to their capacitive behavior. The ZnO nanorod electrode could be well described by a frequency independent capacitive and a conductive element in parallel. The capacitive element is determined as space charge capacitance in the nanorods. However the dependency of the nanorod electrodes capacitance on the applied voltage bias is more complicated than for flat semiconductor electrodes. INTRODUCTION ZnO nanorod solar cells were reported recently as an alternative oxide substrate for a dye sensitized solar cell (DSSC) [1,2] and for solar cells with extremely thin solid state absorbers [3]. Replacing the network of TiO2 nano colloids in standard DSSC [4] is expected to bring improvements in charge carrier transport due to the much higher electron diffusion coefficient of ZnO [1]. Also electrolytes or hole conductors with large viscosities can penetrate easier into the channels between the nanorods than into the caves in a nano colloid network. The electrical behavior of nanorod solar cells might be different from the one in a nano colloid solar cell, since effects as a space charge layer are proposed to occur [1] Photocurrent transient measurements on ZnO nanorod dye solar cells have shown a slow electrical response in the ms time range which could be identified as an effect of a large RC-time constant [5]. Within this paper experimental work is presented that gives more details on the capacitive response of the ZnO nanorod substrates. EXPERIMENTAL Zinc oxide nanorod electrodes were prepared by covering a fluorine doped tin oxide glass substrate (Hartford Glass) with a compact layer of sputtered ZnO and subsequently placing this substrate in an aqueous solution containing Zn(NO3)2 (0.01M) and NaOH (0.36M) at 80°C. Using different growth times of 5 to 120 minutes resulted in rod lengths of less than 0.5µm up to more than 2µm. The samples were post annealed at 450°C for one hour. The geometry of the nanorods was investigated by using scanning electron microscopy (LEO Gemini) and measuring length, diameter and spatial density of the nanorods from the SEM images. Typical rod diameters are about 10 nm for the thinnest to 200 nm for the thickest nanorods.
Impedance measurements were performed by contacting the nanorod electrode with an electrolyte typically used in DSSC (0.05 M I2, 0.5 M LiI, 0.5 M 4-tertbutylpiridine in acetonitrile). Measurements were performed with an impedance analyzer (Agilent 4294A) in a frequency range of 40 Hz to 5 MHz with a 20 mV source amplitude. Capacitance values were obtained by fitting the measured impedance data with a non-linear-least-square fit routine. The impedance of the platinum counter electrode was checked by measuring a cell where the nanorod electrode was
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