In 2008 Martin Bivour
graduated with an excellent Master in regenerative energy systems from the university of applied science Berlin. From this Bachelor on he aimed at working in the photovoltaic sector. He wrote his Bachelor thesis at the Helmholtz-Zentrum Berlin für Materialien und Energie and the Q-Cells SE. His Master thesis was completed at the Helmholtz-Zentrum Berlin für Materialien und Energie where he evaluated a novel solar cell concept. By several internships during his academic studies, for example at the Institute of Energy Conversion in Newark, Delaware, USA, he gained valuable experience at leading research centres. Beyond his commitment to his studies he also attained useful skills through the active participance at seminars and conferences.
For the conversion of radiation energy into electrical energy the separation of photo-generated charge carriers in the absorber, electrons and holes, via selective contacts is required. By connecting the electron- and hole contacts of the solar cell via an external circuit, electrons and holes can recombine, thereby doing electrical work. A part of these photo-generated carriers will already recombine inside the solar cell: in the bulk of the absorber, on the surfaces and within the selective contacts. This limits the solar cell efficiency. In the course of this work this loss mechanism will be investigated and reduced for silicon solar cells with multicrystalline absorbers. By combining efficient defect engineering for the multicrystalline absorbers with the formation of the selective contacts by heterocontacts the problem of recombination may be reduced.
Using heterocontacts permits the decoupling of the process of defect engineering from the process of selective contact formation, both of which are essential for high solar cell efficiency. This is possible due to the fact that the process temperature for forming the heterocontacts is much lower than the process temperature for the defect engineering. Therefore the thermal induced degradation of the electronically superior absorber resulting from the process of the formation of the selective contacts can be avoided. This means there is no need to find a trade-off between both processes and the optimum process for defect engineering leading to minimum recombination losses in the absorber can be used.
In this work the selective contacts are defined by a layer system consisting of doped and undoped amorphous, nano- or microcrystalline silicon deposited by PECVD (Plasma Enhanced Chemical Vapour Deposition) onto the absorber surfaces. Compared to the conventionally used homocontacts, these heterocontacts allow, in addtion to the selective carrier transport, the exellent passivation of the absorber surfaces and therefore have the potential to lower both the recombination losses at the interface and within the selective contacts. For silicon solar cells with monocrystalline absorbers this technique leads to the highest open circuit voltages, up to 740mV. The successful transfer of this technique from monocrystalline to multicrystalline absorber is the main challenge of this work.
