Short description of the doctoral thesis:

"Development of ultra-thin multi junction solar cells with highest efficiencies"

The highest efficiencies for the conversion of sunlight to electricity (up to 41.1%) today are achieved with multi junction solar cells made of III-V-semiconductors. For these solar cells, three pn-junctions of the lattice-matched materials Ga0.5In0.5P, Ga0.99In0.01As and Ge are grown epitaxially on top of each other. These triple junction solar cells are used in concentrator systems which have been increasingly adopted over the last years. They have the potential to further reduce the costs of photovoltaic electricity in the future.

The solar cells are grown in an epitaxial process on 150-250 µm thick germanium substrates. Germanium is one of the rarest elements on earth and its costs make up a large share in the total costs for multi junction solar cell. Therefore, the research for new and cheaper alternatives for the germanium substrate is crucial to develop and extend this technology in the future.

During this Ph.D thesis a new solar cell technology will be developed. It is based on the inverted growth of a multi junction solar cell and the following separation of the grown layers. The concept of inverted metamorphic solar cells contains the growth of an "upside-down" multi junction solar cell structure on a GaAs substrate with a subsequent transfer on another substrate (e.g. a cover glass). Depending on the technology used for the transfer, the substrate can be reused for new epitaxies which would lead to significant cost savings. Etching techniques will be used at first to remove the substrate but will be replaced later with deattachement techniques. The resulting solar cell structure is only 5-10 µm thick. Other than the reduction of costs this technique improves the application in concentrator systems since the heat accumulating in the solar cell can dissipate faster through the base plate of the module which increases lifespan and efficiency.

Another advantage of inverted metamorphic structures is the high theoretical efficiency combined with the high material quality due to low defect densities in the crystal. In the presently produced lattice-mismatched triple junction solar cells from ISE, with Ga0.35In0.65P/Ga0.99In0.01As/Ge the GaInP top cell as well as the GaInAs middle cells are grown with a different lattice constant compared to Ge. By this means efficiencies of up to 37.6% were reached. For this, a special buffer layer is inserted on top of the Ge substrate, in which the lattice constant is continuously changed. It is very difficult to relax the crystal completely in this buffer layer when trying to avoid the spread of dislocations into the electrically active layers at the pn-junction at the same time. Some dislocations are produced at all times which reduce the quality of the successive solar cell. In contrast, using the inverted metamorphic structure, the first two subcells of GaInP and GaAs are grown lattice matched on GaAs and therefore are virtually free of defects. The lattice constant is not changed until the third Ga1-xInxAs subcell. First tests at the National Renewable Laboratory have demonstrated, that this structure is superior and that bottom cells of GaInAs with a band gap energy down to 1.0 eV can be produced. This approach combines the already well understood lattice matched semiconductor structures on GaAs substrate with a theoretically even better GaInAs subcell offering a 1.0 eV band gap. Moreover, the costs of the multi junction solar cell can be reduced even further if the GaAs substrate is reused. This promising methodology will be evaluated during this Ph.D thesis.