Investigation of a Pin-Structure Germanium Photovoltaic Cell

The fabrication and initial testing of an alloy-junction planar pin thermophotovoltaic cell are discussed. The cell develops an open-circuit voltage of 125 mV for an incident intensity of 0.1 watts per sq cm from a thermal source at 1375C. The results of a pulse method of measuring the incremental excess-carrier life-time and the ambipolar diffusion coefficient at high-injection levels in germanium are presented. The lifetime falls from about 25 microsec at low-injection levels to about 5 microsec at a carrier concentration of 2 x 10 to the 17th power/cu cm. The concentration dependence of the ambipolar diffusion coefficient is shown to agree with prior independent measurements of the high-level carrier mobilities, if the Einstein relation is employed. Two theoretical problems relating to the two-dimensional, high-injection-level flow of excess carriers are discussed. One treats the base region of a bipolar transistor, the other is based on the planar TPV structure. The numerical results of the bipolar transistor problem show that minority-carrier flow is almost exactly longitudinal, and that the usual lumped models for two-dimensional flow are grossly in error. A method of correcting these models is suggested. Approximate closed-form solutions are developed for the planar structure. (Author)