Temperature and light induced degradation effect on a-Si:H photovoltaic PIN devices properties

The purpose of this work is to understand how the recombination of carriers generated by light (at several temperatures) can influence the a-Si:H devices quality. Here we report a comparative analysis between the photovoltaic performances of the PIN diodes and the transport properties of their active i-layers under the same degradation conditions. PIN solar cells were light soaked up to 60 hours. The cell characteristics, the optoelectronic properties and the microstructure parameter (R=I₂₁₀₀/I₂₁₀₀+I₂₁₀₀) as well as the hydrogen content (Ch) and density of states at the Fermi level (g(Ef)) 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 (Flying Spot Technique) measurements. Data show that the rate of defects growth on the i-layer depends on the quality of the material resulting from the equilibrium between light induced and light annealed defects. During light soaking, the evolution of the faction of hydrogen bonded on internal surfaces was analysed trough the change on the I₂₁₀₀ and I₂₁₀₀ infrared intensities of the SiH and SiH₂ stretch modes. The rate of defects growth was correlated with the increase of SiH₂ radicals. It was also observed that thermal annealing do not play a significant role on the material microstructure (the IR absorption spectra did not present any change and no hydrogen was lost during annealing up to 100°C). The results reveal 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.