Experimental study on a p-i-n structure for thermoelectric applications

Franziska Maculewicz, University of Duisburg-Essen & CENIDE, Duisburg, Germany
Khaled Arzi, University of Duisburg-Essen & CENIDE, Duisburg, Germany
Timo Wagner, University of Duisurg-Essen & CENIDE, Duisburg, Germany
Roland Schmechel, University of Duisburg-Essen, Duisburg, Germany

Common thermoelectric generators are built-up from unipolar n- and p-type legs. But it was suggested by theoretical studies to use p-i-n structures for the thermoelectric energy conversion already ten years ago. One essential argument was the ability of this structure to harvest the thermal energy of bipolar charge carriers. Here we present an experimental study on such a structure. Silicon p-i-n diodes are fabricated by laser sintering of highly doped boron and phosphorus silicon nanoparticles forming p- and n-layer on the sides of an intrinsic silicon wafer. The I-V characteristics were measured up to hot site temperature of about 300°C. Despite such a p-i-n structure with internal temperature gradient is a quite complex 2-dimenional problem, the essential characteristics could be explained by a relative simple equivalent circuit of lumped elements. It is shown, that the generated electric power is caused by two different physical mechanism. One is based on the regular Seebeck effect that occurs mainly in the p- and n-type regions. But, even if the p- and n-type legs are electrically short-circuited, there is a remaining output power that raises monotonically with the hot site temperature. This indicates the presents of second independent energy conversion process. This process is assigned to the separation of thermally excited bipolar charge carriers within the i-region. Such a mechanism was already predicted, but has never been demonstrated experimentally. Only if both mechanism are taken into account, the measured device characteristic can be explained. The extracted electric power from the i-region by bipolar charge separation is compared to the Seebeck-effect of the bare i-region. This comparison underlines again the different nature of both mechanism.

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