Amorphous silicon based betavoltaic devices
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Amorphous silicon based betavoltaic devices N. Wyrsch1, Y. Riesen1, A. Franco1, S. Dunand1, H. Kind2, S. Schneider2, C. Ballif1 1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), mb-microtec, Freiburgstrasse 634, 3172 Niederwangen, Switzerland. ABSTRACT Hydrogenated amorphous silicon betavoltaic devices are studied both by simulation and experimentally. Devices exhibiting a power density of 0.1 μW/cm2 upon Tritium exposure were fabricated. However, a significant degradation of the performance is taking place, especially during the first hours of the exposure. The degradation behavior differs from sample to sample as well as from published results in the literature. Comparisons with degradation from beta particles suggest an effect of tritium rather than a creation of defects by beta particles. INTRODUCTION Batteries based on radioactive sources, using betavoltaic effect, may offer advantages compared to classical chemical batteries for applications requiring very long lifetime. The betavoltaic principle is based on the direct generation of an electrical voltage and current upon irradiation of a diode by beta particles. Such a principle has been used for power generation in few satellites, but never commercialized. However, betavoltaics has recently seen a renew interest for the powering of micro-devices [1,2]. For safety reasons, it is attractive to use tritium (T) as a low energy beta particle emitter (with 5.7 keV average energy) but it then limits considerably the power density of such a device. Such a low power is compensated by the relatively long T half-lifetime 12.3 years, which should allow for batteries with high energy density. Furthermore, T is industrially available, relatively cheap and can be embedded, if needed, in a solid matrix. Recently, a new battery design has been proposed comprising a 3D porous silicon diode to increase the surface interaction between the active volume (containing T) and the semiconductor device to improve performances [3]. We here explore the possibility to use hydrogenated amorphous silicon (a-Si:H) thin film device together with tritium to fabricate betavoltaic battery. In contrast to the solution incorporating a 3D porous silicon diode design, a-Si:H silicon diodes potentially offer several advantages. a-Si:H cells can be deposited on thin and (eventually) flexible substrates in order to create a 3D module design by stacking or folding such devices. They also exhibit higher performances at low excitation levels compared to c-Si diodes. A few experiments were already performed by other groups using either tritiated amorphous silicon [4] or diodes exposed to tritium [5]. In the latter case, a rapid degradation of the cell was observed, attributed to a replacement of hydrogen in a-Si:H T, leading to an increase of dangling bonds upon decay of tritium atoms into helium [6]. In the present work we will present the potential, the theoretical and practical performance of a-Si-H betavoltaic devices and analyzed the degradation of various a-Si:H
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