Modelling of perimeter recombination in GaAs solar cells

To investigate perimeter recombination current in heteroface GaAs solar cells, two models were proposed; the first concerned the analysis of recombination at the surface that intersects the space-charge layer and the second dealt with recombination at the quasi-neutral region. Recombination at the depleted layer surface has a 2kT character and was treated in a similar way to that of the bulk, using the model of Sah, Noyce and Shockley. The electric field at the surface due to Fermi level pinning is different from that of the bulk. We suggested a simple model to obtain an analytical form of the perimeter current at the space-charge region surface that yielded values of the product of the characteristic length by the surface recombination velocity (LsS0) that agreed well with experimental values. The recombination current outside the space-region is of two dimensional nature and has a kT behaviour, the model adopted consisted mainly of solving numerically the bidimensional continuity equation. An effective recombination velocity was introduced to account for bend bending caused by the charged surface states. As the ratio of perimeter to area (P/A) is increased the perimeter current acquired significant proportions, thus the expected 2kT current due to bulk deep levels existing in the depletion layer is two to three orders of magnitude too small to account for.