The solar cell output as a function of the thickness of the epitaxial silicon layer. The sharp fall in V_oc of the 1.7 micron cell is due to the fact that modelling of this cell’s J-V and EQE characteristics indicate that it has a more disordered interface layer at the epi-Si/P-c-Si interface than is present in the 2.4 micron cell. Therefore a different symbol is used for this case. The thicker cells have been assumed to have the same parameters as the 2.4 micron cell.