Degradation of ultrathin CdTe films with SWCNT or Graphene back contact

The degradation of ultrathin film solar cells based on CdS/CdTe materials and back contacted with nanolayers are analysed using SCAPS. The ultrathin films suffer from uncompleted photo-absorption and fast degradation. The instability in performance was mainly attributed to the back contact materials which cause roll-over and cause mobile ions inter-diffusion. Thus, in this work, three different nanolayers such as single walled carbon nanotubes and Graphene are considered as the metal-free back contacts with wide controllable work function for the CdTe films. The simulations show that the roll-over in characteristics of the device disappears when the work function of the nanolayer increases by a proper doping. The current density-voltage curves showed promising results when the CdTe thickness was thinned down to 0.7 µm. Surface coverage of the grain boundaries at the interface of CdTe/nanolayer can reduce the contact series resistance and improve the carrier collection. However, the inter-sheet resistance of the nanolayers should be re-optimized. Finally, the time dependent approach was applied to simulate the defect generation under stress condition where the Cu-doped nanolayers showed faster degradation while the nanolayer back contacted devices showed higher stability.