Doped and undoped zinc oxide fims are widely used as transparent window and contact layers in thin film solar cells. Up to now the role of film stress on the layer and solar cell properties was not investigated in detail. In this paper the influence of the energy input from the magnetron sputtering plasma on the structural and electrical layer properties was investigated. The energy input during the deposition could be varied by using a simultaneous RF- and DC-excitation at a fixed sputtering pressure. The energy and the density of the ions bombarding the substrate at floating potential have been measured by a plasma monitor and by a Langmuir probe. Plasma monitor measurements displayed the different plasma potentials and ion energies at a floating substrate. While the DC-discharge is characterized by low plasma potentials and low ion energies, the RF-mode shows up to 2.5 times higher ion energies of EAr++ ≈ 50 eV.
The structural and electrical properties of doped and undoped ZnO-films, deposited with different RF/DC-power ratios can be explained by taking into account the different ion energies and densities during the deposition. Higher ion energies (RF-excitation) improve the structural quality and reduce the film stress, which decreases the specific resistance, both by a higher charge carrier mobility as well as higher carrier concentrations. The lowest resistivities and film stresses were ≈ 6–10-4 Ωcm and 0.5 GPa, respectively. Layers with different film stresses have been used to prepare CuInS2/CdS/ZnO-solar cells. Lower stress in the ZnO-films leads to higher cell efficiencies. For the first time, these results can explain why RF-sputtering is almost exclusively used for the window preparation of thin film solar cells.