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

In order to evaluate correctly the ambipolar diffusion length (L_amb) or the ambipolar drift length (L_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 (β) 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.