EELS measurements of boron concentration profiles in p-a-Si and nip a-Si solar cells

The p-type Si layer in a-Si and μc-Si solar cells on foil needs to fulfil several important requirements. The layer is necessary to create the electric field that separates the photo-generated charge carriers; the doping also increases the conductivity to conduct the photocurrent to the front contact; on the other hand, the p-layer should transmit the incident light efficiently to the intrinsic absorber layer. We show that it is possible to study TEM samples prepared, for analysis of possible layer defects, by focussed ion beam milling to detect boron and carbon concentrations as low as 10²⁰ cm^-3, using core-loss EELS combined with numerical analysis. We control the band gap and activation energy of p-a-SiC by varying the B₂H₆ and CH₄ flow during deposition in the process chamber. We have found a linear relation between the activation energy of the dark conductivity E_act and the optical band gap E₀₄. Modelling shows that the optimum efficiency in nip solar cells is obtained when the p-a-SiC band gap is slightly larger than the band gap of the absorber layer. We have assessed the potential of core-loss EELS for detecting B and C concentrations as low as 10²⁰ cm^-3 in a spatially resolved manner, and of low-loss EELS as a probe of the local variations in plasmon energy.