A new thermoelectric concept using large area PN junctions
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A new thermoelectric concept using large area PN junctions R. Chavez, A. Becker, V. Kessler, M. Engenhorst, N. Petermann, H. Wiggers, G. Schierning, R. Schmechel Faculty of Engineering and Center for NanoIntegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 47057 Duisburg, Germany ABSTRACT A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines thermally induced charge generation and separation in a space charge region with the conventional Seebeck effect by applying a temperature gradient parallel to the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. The investigated PN junction devices are produced by stacking n-type and p-type nanopowder prior to a densification process. The nanoparticulate nature of the densified PN junction lowers thermal conductivity and increases the intraband traps density which we propose is beneficial for transport across the PN junction thus enhancing the thermoelectric properties. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator. INTRODUCTION In recent years non-fossil fuels and efficiency of energy conversion have gained great interest as the world prepares to fulfill the projected energy demand. In regards to the later, thermoelectricity could play an important role by allowing a practical form of heat waste recovery as nearly 60% of the energy consumed in the U.S.A. is wasted in the form of heat1. Although efforts are being done to incorporate thermoelectric generators (TEGs) in automobiles, among other difficulties, the relatively low efficiencies, usually between 5-15% 1, have refrained the application of thermoelectric generators to deep space missions 2,3. However, the reduction of the thermal conductivity in nanostructured materials has brought improvements to the thermoelectric material’s figure of merit 4–6 and thus the efficiency of the energy conversion process defined by the operation temperatures and the figure of merit which is usually written as: σα 2T zT = (1)
κ
where T is the temperature, ı is the electrical conductivity, Į the Seebeck coefficient and ț the thermal conductivity. Conventional thermoelectric generators consist of p and n-type semiconductor materials thermally connected in parallel and electrically connected in series, where a heat source is applied on one side and part of the thermal flux is converted into electrical energy by means of the Seebeck effect (Figure 1a).
3
Span et. al. have proposed a
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