Photovoltaics based on CuInSe2 and related materials have the highest performance of any thin film devices. However, many questions remain to be answered concerning their operation.  In particular the effect of grain boundaries on performance is of great interest.  The devices typically consist of polycrystalline p-type Cu(In,Ga)(S,Se)2 (CIGS) semiconductor alloys deposited on Mo-coated soda-lime glass substrates.  The CIGS is coated with intrinsic CdS by a chemical bath deposition method.  Finally, a top n-type transparent conducting oxide contact is applied.  The devices are generally thought to be limited by recombination of carriers in the space-charge region, although the details of defects involved in this recombination and the properties of the CIGS/CdS heterojunction are still very unclear.  In addition to polycrystalline CIGS devices, a limited number of devices have been produced using single-crystal epitaxial layers of CIGS on GaAs.  The performance of these devices is generally found to be inferior to that of the polycrystals, in spite of controlled variation in surface orientation and surface polarity based on selection of the GaAs substrate orientation.  The lower performance of the single crystal devices is probably not due to less optimization of the process conditions and is likely primarily the result of the grain boundaries themselves.  Two major questions must be resolved to understand this remarkable result, first how the grain boundaries can fail to have a detrimental effect on the device by mediating carrier recombination, as is typical in other semiconductors, and second what benefits they provide that are not available in single crystals.

GaAs bicrystal substrate                          X-SEM of CuInSe2 bicrystal

This project consists of growth of epitaxial bicrystals of CIGS on bicrystal GaAs substrates and the characterization of the structure of the grains, the grain boundaries, and the surface morphology near the boundary.  The benefit of growing epitaxial bicrystals is that it becomes possible to probe the behavior of an isolated grain boundary in detail.  In conventional thin film CIGS polycrystals the grain size is typically less than ~2 micrometers, while exciton diffusion lengths and depletion widths are thought to be of the order of 0.5 micrometers.  Therefore, it is nearly impossible to probe the electronic properties of bulk grains in these materials.  The bicrystals described here provide samples suitable for such characterization.  The optical and electronic properties of these bicrystals are also being studied. Temperature-dependent Hall effect measurements, photluminescence, and cathodoluminescence studies are also being conducted on these samples.

                Grain boundary TEM                            AFM Image of CuInSe2 grain boundary