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From these issues it can followed that internal fluorescence followed by light extraction, plays a crucial role in determining theoretical efficiency. At the center of these is the need to engineer the photon dynamics of a PV cell.  **grain boundaries** can alter electronic properties \cite{Yan_2011}  **defect chemistry** Pushing cell conversion efficiencies beyond 20% will require increasing VOC beyond 1V. However the present pathway of processing optimization will likely yield VOC and efficiency converging on 0.9 V and <20%, respectively.\cite{McCandless_2013}  **Back contacts** Degradation  effects are often associated with metal diffusion from the back contact of the cell, which is Cu in most cases.However, cells  with stable back contact can also exhibit instability, as also all the other cell layers are potential sources of impurities causing  instability.Cell degradation due to generation of defects from external influences like particle irradiation, e.g.in space, is another  reason for instability.The development of stable back contacts as well as sources of instability in the cell performance are  discussed with a special focus on the CdS layer and CdTeyCdS interface, which are very sensitive to the accumulation of  impurities and defects.\cite{B_tzner_2004}