Measurement of ambipolar mobility-lifetime product and its significance for amorphous silicon cells

In order to evaluate correctly the ambipolar diffusion length (L/sub amb/) or the ambipolar drift length (L/sub e/) from a steady-state photocarrier grating (SSPG) diffusion or drift measurement, the condition of charge quasi-neutrality has to be maintained everywhere in the material (ambipolarity condition). This is shown theoretically by calculating the experimentally accessible parameter ( beta ) without assuming that the ambipolarity condition holds. The effect of nonambipolar behavior on the experimental plots, both for diffusion and for drift, is derived. Measured SSPG plots for undoped a-Si:H are given, illustrating both ambipolar and nonambipolar cases.<
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