Molybdenum, commonly used as the back-contact material in CIGS solar cells, has been the subject of an in-depth study by sputtering target manufacturer Plansee, working in collaboration with TU Bergakademie Freiberg. Plansee’s results, presented at the ICMCTF Conference, identified the process errors and defect types present in molybdenum thin films that can have a detrimental effect on electrical conductivity.

Impurities and incorrect process temperatures during sputtering were said to be the main influences on electrical conductivity of the material. Impurities such as iron, nickel and chromium can, at high enough levels, reduce the electrical conductivity of the molybdenum thin film by more than 40%. Counteracting this effect is possible by ensuring the high purity of sputtering targets in the CIGS manufacturing process.

Dislocations, or defects in the molybdenum crystal lattice, are said to be another major influence on electrical conductivity of molybdenum films. These dislocations, although necessary to enable the workability of the metals, can reduce the electrical conductivity by up to 14%. Plansee and TU Bergakademie Freiberg’s findings in this regard show that this effect can be halved by employing a process temperature of 150°C instead of room temperature.

Use of this higher temperature can also help reduce the effect of unavoidable interstitial impurities that collect on the lattice – usually consisting of nitrogen, oxygen and argon – which can reduce the electrical conductivity of the films by up to 12%. At 150°C, the tiny atoms are sufficiently energized to break free of the molybdenum lattice.

The testing of the molybdenum material was conducted by depositing the thin films on soda lime glass, thereby allowing the extraction of a basic characterization of the layers, measurement of the films’ electrical resistance and analysis of the films’ microstructure using Transmission Electron Microscopy (TEM) and X-ray diffraction (GAXRD). The group was headed by Professor David Rafaja of TU Bergakademie Freiberg’s Institute for Materials Science and Harald Köstenbauer, a developer of thin-film materials at Plansee.