Thin film solar cells: Materials science at interfaces

Interfaces are important for the efficiencies of thin film solar cells. In particular for polycrystalline chalcogenide semiconductors as Cu(In,Ga)(S,Se)(2) and CdTe the existing physical concepts, which describe the electronic properties of semiconductor interfaces, are not sufficient. The increased complexity is mostly due to the non-abruptness of the interfaces and the strong tendency for the formation of defects. For the CdTe thin film solar cell a very relevant interface for their operation and efficiency is the CdTe/CdS semiconductor hetero junction. The properties of the semiconductor interfaces have been characterised systematically with photoelectron spectroscopy (XPS/UPS) in integrated ultra high vacuum (UHV) systems for sample preparation and analysis. Withal the key topic is the experimental determination of the band alignment at the semiconductor interfaces. For high efficiency CdTe solar cell production CdCl2 activation is of major importance. The effects of the CdCl2 treatment step on CdTe solar cells had been not completely understood so far. To investigate its influence the activation process has been transferred into the integrated UHV system. We will report about chemical and electronic modifications of the CdTe/CdS hetero interface due to in-situ CdCl2 activation performing sputter depth profiles in combination with X-ray photoelectron spectroscopy (XPS).