Fraunhofer Institute for Solar Energy Systems ISE
We develop various photovoltaic cell designs optimized for laser power applications. On our two MOVPE reactors, we grow our semiconductor structures epitaxially so that the semiconductor material is tailored to fit the laser wavelength. In our clean room laboratories, we use the most modern process technology available for fabricating photovoltaic cells. Our services include customizing the cell design to the given operating and boundary conditions required by the specific application. The customer can specify the desired dimensions, geometry and contacting technology.
Independent of the incident laser power, we optimize the front contact grid to minimize shading and reduce losses in current collection.
In the area of cell development we also focus on increasing the device output voltage to e.g. 3.3 V, 5 V or 12 V, as typically required for electrical applications. The output voltage of a laser power converter based on GaAs is about 1 V. The use of suitable power electronics can be one way to achieve higher voltages for a reliable power supply. An elegant alternative to power electronics is the application of advanced cell architectures, in which several subcells are monolithically connected in series to increase the output voltage.
One possibility to implement this is to vertically stack several subcells on top of each other, as for high-efficiency solar cells. In such multi-junction cells, the individual photovoltaic cells are epitaxially deposited on top of each other, forming a vertical stack. The series connection is established by integrating a tunnel diode between the different cells.
A further concept is lateral segmentation. In this technology, the photovoltaic cell structure is grown on a semi-insulating substrate. During wafer processing, the semiconductor material is structured by etching trenches that separate the photovoltaic cell area in several segments. The segments, which are now electrically isolated from each other, are subsequently connected in series outside the photoactive area. In principle, such monolithic interconnected modules (MIM) can be realized with any number of subcells connected in series or parallel, based on the application’s requirements. We have realized multi-segment cells with 2, 4, 6 and 12 series-connected segments. The voltage output increases by this factor accordingly.