12 June 2012
The conductivity of the molybdenum back contact affects the efficiency of the entire CIGS solar cell. In recent months, researchers from Austrian material supplier Plansee and the Technische Universität (TU) Bergakademie Freiberg, Germany have been investigating defects in molybdenum thin films and their effect on electrical conductivity. At the ICMCTF Conference in California, (US) Harald Köstenbauer, a developer of thin-film materials at Plansee, explained what it is that impairs conductivity in molybdenum thin films. The biggest troublemakers are impurities and incorrect process temperatures during sputtering.
The main influences on the electrical conductivity of molybdenum thin films in CIGS cells are substitutional impurities such as iron, nickel or chromium. At high levels of contamination, the presence of atoms of these materials can reduce the electrical conductivity of the molybdenum thin film by more than 40%. The good news is that the use of pure sputtering targets allows CIGS manufacturers to prevent problems of this nature.
The second largest influence on the electrical conductivity of molybdenum thin films is due to so-called dislocations. These are defects in the crystal lattice. It is these defects that make metals workable. Unfortunately, they also cause distortions to the molybdenum lattice which reduce the electrical conductivity by up to 14%. Here again, the test results have some good news to offer: CIGS manufacturers can simply halve the effect by using a process temperature of 150 °C (about 300°F) instead of working at room temperature.
The third largest factor influencing the electrical conductivity of molybdenum thin films is the presence of small atoms in the molybdenum lattice. These interstitial impurities, which consist of nitrogen, oxygen and argon, reduce the electrical conductivity of molybdenum thin films by up to 12%. It is not possible to avoid the presence of small quantities of these atoms during the coating process. But what about their inclusion in the molybdenum lattice? Once again, the process temperature is the most important factor influencing the presence of these atoms. And, once more, the crucial figure is 150 °C. At this temperature, the small, moving atoms already have enough energy to break free of the molybdenum lattice. At 150 °C, there are almost no more interstitial atoms in the molybdenum thin film.
The test procedure.
Plansee, a manufacturer of metallic and ceramic coating materials, deposited the thin films to be tested using the DC, pulsed DC and high-frequency sputtering methods on soda lime glass, obtained a basic characterization of the layers and measured the electrical resistance. The group headed by Professor David Rafaja of TU Bergakademie Freiberg’s Institute for Materials Science, analyzed the microstructure of the molybdenum thin films. To do this, the researchers used high-resolution procedures such as Transmission Electron Microscopy (TEM) and X-ray diffraction (GAXRD). On the basis of the measured results, Professor Rafaja developed a mathematical model which makes it possible to calculate the effect of the individual influences.
This fall, the research results will be available under the title "Effect of the deposition process and substrate temperature on the microstructure defects and electrical conductivity of molybdenum thin films" in a special issue of the journal Thin Solid Films.
Written by Sandra Henderson, Research Editor, Solar Novus Today