CPM measurements on a-Si:H based pin cells-a critical investigation

The subband gap optical absorption spectra of amorphous hydrogenated silicon p-i-n solar cells were investigated by photocurrent spectroscopy in the CPM (constant photocurrent method) mode. A comparative study was made of thin (standard) and thick p-i-n junctions as well as junctions with slightly n- and p-doped active layers. Characteristic dependencies on the applied bias voltages (forward and reverse bias and short-circuit case) were observed. A critical discussion of these results in comparison to standard measurements (I-V curves under illumination, spectral response) is given. It is found that Urbach energies (slope of the exponential tail) are not voltage bias dependent and show, in the cases observed, the expected enhancement in p- nu -n and p- pi -n diodes with respect to p-i-n diodes.