The ambipolar diffusion length measured by the surface photovoltage technique

The surface photovoltage (SPV) technique is often used to determine the diffusion length in hydrogenated amorphous silicon (a-Si:H). The use of this technique for a-Si:H is disputed because it has been shown by computer simulations that the drift component is not negligible compared to the diffusion current in SPV measurements on a-Si:H. For crystalline semiconductors the SPV technique gives the minority-carrier diffusion length while the drift current is negligible. We found from the solution of the transport equations that the SPV technique can be used in the case of a-Si:H, but gives the ambipolar diffusion length which consists of the mobility and lifetime of both holes and electrons. However, the term diffusion length is misleading since the hole drift current and the hole diffusion current are of the same order of magnitude for a-Si:H. This result is in accordance with the results of the computer simulations mentioned earlier. The drift current assists the diffusion current and increases the measured diffusion length by a factor (2)1/2 compared to the situation with diffusion current alone.