Determination of minority-carrier lifetime and recombination currents in solar cells, from high frequency response of a bipolar JFET structure

A novel method is presented for measuring the minority-carrier recombination lifetime in the base layer of a solar cell as well as the recombination current in the emitter of the solar cell. The method is based on a three-terminal device structure, to be fabricated onto the solar cell under test, that incorporates the n⁺pp⁺ (or p⁺nn⁺) structure that forms the BSF (back surface field) solar cell. The basic structure is similar to a vertical junction field effect transistor (JFET), in which the gate and drain layers form the BSF cell, and the source layer is the third terminal added for the purpose of measurement. On the basis of a small-signal analysis of the test structure, it is shown that measurement of the cut-off frequency of the small-signal current gain as a function of the drain current allows an easy determination of the lifetime in the base layer, as well as the recombination velocities of the source and gate layers.