Photovoltaics on Wire
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PHOTOVOLTAICS ON WIRE M. Rojahn, M. Rakhlin and M. B. Schubert University of Stuttgart, Institute of Physical Electronics, Pfaffenwaldring 47, 70569 Stuttgart, Germany Email: [email protected] ABSTRACT This contribution presents an approch to use wire-like substrates for thin-film devices. Based on plasma deposition processes for metals, hydrogenated amorphous silicon (a-Si:H) and transparent conductive oxides, solar cells are fabricated onto metal wires coated by a dielectric. Photolithography using a N2-laser and etching steps serve to pattern the thin film layers. Scanning electron microscope pictures and current-voltage characteristics of the device are demonstrated. Simple geometric simulations show the spectrally resolved intensity of the light that is scattered into the a-Si:H layers as a function of the diameter ratio of the substrate material and the deposited thin films. Finally, we discuss light concentrating and light trapping designs of sensors on transparent fibre substrates. INTRODUCTION Thin film sensor- or solar cell devices based on amorphous silicon (a-Si:H) have been developed for 30 years. Using plasma enhanced chemical vapor deposition (PECVD), sputter- and evaporation techniques, such devices are often cheaper and exhibit a much higher photosensitivity than their crystalline counterparts. As a result of the non-crystalline, but rather network-like arrangement of atoms, amorphous silicon has a 20-30 times higher light absorption in the visible spectral range. Combined with recent experimental and theoretical studies which focus on the low-temperature deposition of a-Si:H [1], we deposit thin a-Si:H devices on a variety of flexible substrates such as plastic foils, etc. In reference [2] a solar cell arrangement was presented in which p- and n-type crystalline silicon layers are formed at 500-1000°C around a conductive cylindrically shaped substrate in order to form a pn-junction c-Si solar cell. Reference [3] proposed an a-Si:H solar cell design on metallic wires. The photo-voltaic active layers are evaporated onto the substrate and a number of organic substances are suggested as sensitizers to widen the usable spectrum of light to be converted into electricity. Cu(In, Ga)Se2 solar cells on glass tubes are disclosed in reference [4]. Non planar lithographic (NPL) techniques using the electron beam of an scanning electron microscope (SEM) were addressed in reference [5] and reference [6] presented an optical photo-resist exposure system for NPL based on a mercury lamp as the ultraviolet (UV) radiation source. Standard thin film technologies can be applied to wire–like substrates to form solar cells or sensors. Such devices can be manufactured with a simple radial symmetry around the substrate, or patterned to give the device preference to some angle of incident radiation or laterally patterned on the wire. Single wires can be arranged to nets which are transparent and wind permeable with advantages for large surface/ or area building applications, provided that they are no more e
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