Nanostructured Zinc Oxide Piezoelectric Energy Generators Based on Semiconductor P-N Junctions
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Nanostructured Zinc Oxide Piezoelectric Energy Generators Based on Semiconductor P-N Junctions Joe Briscoe1, Mark Stewart2, Melvin Vopson2, Markys Cain2, Paul M. Weaver2, Steve Dunn1,* 1 Centre for Materials Research, School of Engineering and Materials Science, Queen Mary University of London, E1 4NS, UK. 2 National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK. ABSTRACT ZnO nanorods grown on plastic substrates by chemical methods are combined with both inorganic and organic p-type materials to make flexible p-n junction devices. When bent the devices generate both voltage and current peaks, which is attributed to the piezoelectric effect in the ZnO nanorods. The best device produces a maximum possible power density of 100 nWcm-2. When vibrated at a constant frequency the voltage output by the devices scales linearly with vibration amplitude. Also, when illuminated the output of the devices drops. These effects are consistent with a piezoelectric source of the voltage. INTRODUCTION Piezoelectric materials can generate a voltage when they are compressed, stretched or bent. They therefore have the potential to convert kinetic energy such as movement or vibrations into electrical energy. In this way they could harvest stray energy from the environment to power microdevices or potentially trickle charge batteries for larger devices [1]. Traditionally, ceramic piezoelectric materials such as lead zirconate titanate (PZT) have been investigated for such devices [2]. Kinetic-to-electric energy harvesting using zinc oxide (ZnO) nanostructures has been intensively studied since it was shown a voltage could be generated through bending a ZnO nanorod [3]. This led to the consideration of arrays of ZnO nanorods for use in energy harvesting devices [4-7]. For energy conversion to occur the incorporation of a piezoelectric material into a diode is required, which to date have been largely limited to Schottky diodes between a metal and semiconductor. Here we report a devices made by combining either a polymer or inorganic p-type semiconducting layer with ZnO nanostructures to form a p-n diode. EXPERIMENTAL DETAILS ZnO nanorods were grown on indium-tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates (Aldrich, surface resistivity 60 /square), which were seeded with sputtered ZnO films using a Balzer sputtering system. Substrates were placed in a solution of zinc nitrate (15 mM, Sigma-Aldrich, 98 %) and hexamethylenetetramine (25 mM, Sigma-Aldrich, 99.5 %), which was heated to 90 °C for 4 hours [8,9]. The reaction was repeated 4 times by placing the substrates into fresh solutions. ZnO nanorods were coated with either poly(3,4ethylenedioxythiophene)-poly(styrene-sulfonate) (PEDOT:PSS, Aldrich, 1.3 wt.% in water, conductive grade) by spin-coating, or copper thiocyanate (CuSCN, Aldrich, 99 %) dissolved in propyl sulphide (Aldrich, 97 %), which was spread over the surface and dried at 80 °C [10]. Devices were completed by sputtering gold contact onto the PEDOT:PSS or CuSCN surface.
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