Preparation and characterization of solar cells structures

Preparative and analytical work is devoted to preparing new highly efficient solar cells, currently in the form of thin epitaxial multi-layer systems of III-V materials. Multi-junction solar cells are prepared with III-V materials applying the MOCVD technique. They represent the most efficient thin film solar cells. The growth process is monitored via an in-situ optical signal (RDS) allowing for systematic monitoring and for systematic improvements of the growth procedure. The in-situ measured optical signal shows whether desired bulk properties and a specific surface reconstruction have been realized in the MOCVD reactor. A direct relationship is established between a specific optical signal and the corresponding surface reconstruction. This strategy is successful with a unique experimental tool. The latter enables contamination free sample transfer from the MOCVD reactor to ultra-high-vacuum. Thus, signals like LEED, UPS, XPS, AES, STM images, FTIR, and again RDS are measured in ultra-high-vacuum that characterize the specific surface reconstruction. With increasing experience and know-how in this field optical fingerprint signals can be established also for buried interfaces. Easier to handle and more cost effective III-V solar cells can be realized by depositing III-V materials on Si-wafers. Work in this direction has been started, where MBE techniques are employed in addition to MOCVD. Current estimates indicate a competitive cost level for multi-junction III-V cells with conversion efficiencies over 30 percent if they are employed at concentrated sunlight, i.e. in suitable geographical locations.