Surface photovoltage measurements in μc-Si:H: Manifestation of the bottom space charge region.

We discuss results of surface photovoltage (U) measurements for d = 10 µm thick layers of undoped hydrogenated microcrystalline silicon (µc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient a and a large penetration depth α[sup -1], a photovoltage peak appears on a curve U= U(α). This peak is located at α...d[sup -1] and its occurrence depends critically on the substrate material. The peak is present in a µc-Si:H film grown on p + crystalline silicon (c-Si), on the other hand it is missing in the µc-Si:H samples grown on n[sup +] c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface µc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when n is below a certain threshold value n[sub th]. In the simplified case when d/L ... 1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found n[sub th] ≈ 0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of n. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells. [ABSTRACT FROM AUTHOR]