Role of photodegradation on the μτ product and microstructure of the a-Si: H pin devices

PIN solar cells were light soaked up to 60 hours. The cell characteristics, the optoelectronic properties and the microstructure parameter (R = I₂₁₀₀/I₂₁₀₀+I₂₀₀₀) as well as the hydrogen content (C_H) and density of states (g(E_f)) of the active i-layer were monitored throughout the entire light induced degradation process and compared with the correspondents μτ product (for both carriers) inferred through steady photoconductivity and FST measurements. Data show a strong correlation between the decrease of μτ product for electron and the increase of the fraction of hydrogen bonded on internal surfaces (R increases from 0.1 to 0.4) suggesting structural changes during the light induced defects' formation. For holes, the μτ product remains approximately constant and only dependent on the initial hydrogen content. As g(E_f) increases, μτ presents an asymmetrical decrease showing that electrons are more sensitive to defects' growth than holes. We also observe that the rate of degradation is faster for samples having the lowest defect densities, R and C_H, showing that the amount of degradation is not a simple function of the photon exposure (Gt product) but also depends on the material microstructure.