“God made the bulk...
“... the surface was invented by the devil!” (Wolfgang Pauli, German theoretician, Nobel prize in physics 1945)
This is exactly the situation we are currently facing with thin film solar cells based on the chalcopyrite material Cu(In,Ga)Se2. But before going into this, a short summary on the chalcopyrite story is necessary.
About thirty years ago, Si and GaAs based solar cells were already established for space applications such as powering satellites. Suddenly, the oil crisis in the mid-eighties came up and everybody talked about how to reduce the dependency on oil and other conventional energy sources.
During this time, people - and not only idealists and visionaries - started to seriously think about renewable energies in order to reduce mankind's “footprint” on earth. Society became more and more aware that ”more of the same” could have fatal consequences in the long run. In this atmosphere the research community also discovered this field and began to work on it. Since then, dealing with renewable energies gained more and more attraction so that nowadays it is widely accepted.
Coming back to the chalcopyrites (which are often abbreviated by CIGSe); this material is very interesting for the photovoltaic research community since it promises high efficiencies at a low material consumption.
They are mostly used in thin film photovoltaic devices, which consist of several layers of functional materials with a total thickness of 5-10 µm. Compared to the average thickness of a human hair with about 100 µm this is incredibly thin. Most important for such a device is the CIGSe layer, with a thickness of just 2 µm, where the sunlight is effectively absorbed. The principle is well established, but efficiency is still an issue. Losses in the solar cell can be directly addressed to bulk material properties and to interface or surface properties of the layered structure. And here, we are coming back to the beginning of this article citing Pauli’s words: “God made the bulk; the surface was invented by the devil!”
After 20 years of research it is still a mystery - the properties of the chalcopyrite surface are not fully understood yet. A possible way to overcome this challenge is the direct analysis of the freshly prepared chalcopyrite material by surface sensitive methods such as photo-emission spectroscopy (PES). But in our case, the vacuum based deposition system is located in Berlin-Wannsee, whereas the ultrahigh vacuum (UHV) based analysis system is located about 30 km away in Berlin-Adlershof. This is why we came up with the idea of a vacuum based transfer unit consisting of a dedicated chamber which is kept under vacuum using an independent, battery assisted pumping system.
However, things are not as easy going as expected. The sample holder systems of both, the deposition and the analysis system, are not compatible to each other. Therefore, special handling in vacuum is needed where the sample is taken out of the specimen holder of the deposition system by a special grip and then put onto the holder of the analysis system. Despite mechanical and technical problems, we were able to successfully test our transfer system with a simple dummy glass sample in respect to feasibility.
Now, there remains only the final proof using the real chalcopyrite absorber and transferring it from Wannsee to Adlershof for subsequent investigations - this is planned in the near future!