Cadmium telluride (CdTe) is one of the most perspective materials for photovoltaic solar energy conversion. Recent achievements show a conversion efficiency of 22.1% [1] for a CdS/CdTe heterojunction. The top limit of the conversion efficiency still has to be reached, once several major problems for these types of cells will be solved. One of the most important tasks, next to interface states (current losses related) and TCO/CdS optimum thicknesses (UV enhancement of the external quantum efficiency), the back contact development issue is still actual and needs further investigations.   CdTe-based solar cells have been prepared by use of ZnO:Al/glass substrates (84% transparent in the visible spectral region). CdS nanometric films have been deposited by Close-Spaced Sublimation (CSS), having thicknesses in the range of 0.1-0.5 µm at a source temperature of 650°C. Cadmium Telluride deposted consecutively at a source temperature of 550°C, resulting in 5 µm thick, p-type polycrystalline films. CdCl2 - ambient based annealing at 385-410°C followed. The device has been completed by applying a Ni or Cu/Ni back contact.   The investigations of the physical processes in CdS/CdTe heterojunctions, having ZnO front contact has been carried out from electrical and photoelectrical characterisations by using a Keithley 4200 SCS setup. J-V-T analysis carried out in the temperature range of 100-370K has proven a tunneling as a dominant mechanism for the charge carrier transport at forward bias. A similar tunneling mechanism at reverse bias is revealed. Both Analysis of the current transport allowed the determination of the tunneling parameters which are influenced by the interface states. The influence of the CdS/CdTe interface states is revealed in addition from the C-V-f analysis. The frequency dependent capacitance measurements allowed the determination of the distribution of traps in the heterojunction. The carrier density profiling revealed a doping level in CdTe films in the range of 1013-1014 cm-3. SCAPS-1D [2] modeling of the physical properties converged to the value of the back contact work function of about 5 eV. Based on the results obtained, an energy band diagram of the investigated CdS/CdTe heterojunction is proposed.