The back contact on CdTe based solar cells is thought to be responsible for the non-idealities in the cell characteristics. Numerical modelling of the thin film cells seems to support this assumption: a simple model can explain all gross qualitative non-idealities such as rollover and fill-factor loss in I-V curves, the shape and frequency dependence of C-V curves. A more straightforward way to get convinced by the importance of the back contact is the use of monochromatic light which penetrates to this contact. For CdTe, this comes down to light of about 840 nm. The influence of the light in this wavelength region on the I-V characteristics is studied in this paper. It was found that the I-V curves were only influenced by photons within this wavelength region. This can be explained by the generation of a new charge carrier distribution in the neighbourhood of the contact.