Evaluation of Cu Back Contact Related Deep Defects in CdTe Solar Cells

CdTe solar cells with back contacts formed by either (i) 15nm Cu evaporation followed by application of carbon conductive paste embedded with micron sized Cu particles in ZnTe powder and (ii) only with the above mentioned conducting paste, were evaluated. A Cu-related deep level defect with an activation energy of Ea ∼ 0.57 eV was observed for Cu evaporated back contact cells and an intrinsic defect with an activation energy Ea ∼ 0.89 eV was found for cells prepared only by ZnTe:Cu embedded carbon paste. Frequency dispersion in C-V measurements confirms the presence of Cu-related deep level traps for cells with Cu evaporated back contact whereas no such defects were observed in carbon paste contact. The behavior was believed to be due to diffusion of excess Cu from the contact. It was further observed that majority carrier deep level traps (Cu-related or intrinsic) contribute differently to the degradation of electronic properties of the CdTe solar cells.