Dr. Nikolaus Weinberger
Thin Layer Technology
Our research group is involved in the development of new photovoltaic technologies, with a focus on thin-film photovoltaics. Thin-film solar cells are thinner than a human hair and about 100 times thinner than conventional (silicon-based) solar cells. The advantages are: low material consumption, low energy consumption in production, low weight and flexibility. Production on film (flexible) opens up a wider range of applications and facilitates the integration of photovoltaics, e.g. in vehicles, buildings and airplanes. In addition, so-called roll-to-roll processes can be used for production. Our research group carries out comprehensive material analyses in order to describe fundamental relationships between production and performance. In addition to developing new materials and manufacturing processes, we also test them on our photovoltaic outdoor test stand.
Left: Flexible thin-film solar cell manufactured as part of the NoFrontiers project; Right: Electron microscope image of the layer structure of the solar cell (cross-section).
Our research group specialises in the investigation of different layers using scanning electron microscopy (SEM). This technology enables images to be taken at enormous magnification (up to 1,000,000 times), allowing us to image the layers, some of which are only nanometres thick. With the help of SEM-based methods such as cathodoluminescence (CL), energy dispersive X-ray spectroscopy (EDX) and electron beam induced current (EBIC) measurements, we can investigate the band transitions, elemental composition and current collection in the solar cell with a lateral resolution of less than 10 nanometres. Together with classical analysis methods, this allows us to describe fundamental relationships. The investigation of so-called microcells using high-vacuum mini robots enables us to investigate the relationship between morphological defects and device performance.
Micro solar cells for investigating the influence of inhomogeneities using mini-robots integrated in an electron microscope.
Cross-sections of the layer structure of a CIGS thin-film solar cell: polished cross-section including substrate (right); broken cross-section of the layer structure (top left) Plasma polished cross-section of the layer structure (bottom left).
Images of copper selenide whiskers on CIGS absorbers, taken with a scanning electron microscope.
Vacuum chamber for magnetron sputtering of doped sub-stoichiometric metal oxide layers (left), plasma during TiO2-x coating (right).
In general, we deal with magnetron sputtering of doped substoichiometric metal-oxide layers (Cu:NiOx, Nb:TiO2-x, Nb:Nb2O5-x, ...), but also with purely metallic and tribological layers. We are also developing new absorber layers for thin-film solar cells. There is currently an intensive dynamic in photovoltaic research in the field of tandem cells. This involves combining two solar cells with customised band gaps. We are developing a hole-selective layer between the solar cells based on Cu:NiO.
We are engaged in the process development of roll-to-roll processes, focussing on homogeneity and overcoming challenges for industrial upscaling. Thin-film technology makes it possible to deposit solar cells on foils (e.g. steel or polyimide) . in this area, we have been cooperating with the company Sunplugged for over 10 years. Particularly in the production of polycrystalline, multi-component absorber layers using the roll-to-roll process, there are complex correlations between machine parameters and the resulting layer properties or the performance of the solar cells.
Schematic representation of the industrial roll-to-roll Hybrid CIGS process at Sunplugged.