A mechanism for enhanced recombination at the p-i junction of a-Si:H solar cells

The existence, in equilibrium, of a large density of dangling bonds at the interface is proposed as the mechanism for enhanced recombination at the p-i junction of a-Si:H solar cells. The existence of this large density results from the fact that the Fermi level, near the p-i interface, lies in the valence band tail state distribution in the gap, and hence tail states above E_F are empty. Holes in these weak bonds create dangling bonds through the relaxation of the lattice, which by the very existence of the weak bond is under stress at that site. Hence, the mere capture of an electron does not heal the bond, since lattice motion is also required. The capture of an electron is followed by hole capture and constitutes a recombination event. These dangling bonds may be identical to the Staebler-Wronski defect, but are established by the equilibrium conditions at the p-i interface, rather than by light. Calculation of the magnitude of the interface recombination velocity (10³ to 10⁴ cm/s) and its effect on open-circuit voltage are discussed and compared to experiment. Ways to minimize the effects on cell properties are also discussed.