Theory of amorphous silicon solar cell (a): numerical analysis

A theoretical analysis of recombination kinetics and space charge distribution in amorphous silicon is carried out with a view to bring out the underlying physics. A uniform excitation with a flat quasi-Fermi level and a constant np product has been used as a probe to estimate the relative importance of various parameters. Recombination rates have been calculated for various ratios of capture rates for Coulomb attractive and neutral traps. In practice a large ratio of capture rates exists and for this case two peaks of recombination maxima are found to lie in the space charge regions corresponding to transitions at the energy level E1 (for D+–D0 transition) at the p–i edge and for E2 energy level (corresponding to D0–D− transition) at the i–n interface. A two independent level model therefore holds to a good approximation. The dangling bond density is found to determine both the space charge distribution and the recombination rate. Based on space charge density distribution i-layer can be divided in the five parts. The two recombination rate peaks are found to exist at the p–i and i–n space charge transitions respectively. This enables us to develop a simple model for the i-layer of the p–i–n diode.