Importance of the transport isotropy in μc-Si:H thin films for solar cells deposited at low substrate temperatures

The influence of the substrate temperature during μc-Si:H deposition on the material structure and optoelectronic properties was explored in the range from 150 to 350 °C. The low temperature material is especially interesting with regard to the suppressed oxygen-related donor formation and high efficiency of the resulting solar cells. Surprisingly, in this material, conductivity is the same parallel and perpendicular to the substrate (as measured by dc and ac techniques). The same is true for the ambipolar diffusion length which was measured by steady-state photocarrier grating (SSPG) (L∥) and by surface photovoltage (SPV) (L⊥) methods. Finally, the relevance of the SPV method extended to the measurement in complete thin film solar cell structures is demonstrated.