Materials and solar cell development
The research group develops and investigates materials for the application in thin film solar cells. The focus is on alloys based on amorphous and microcrystalline silicon, i.e., on a-Si:H and µc-Si:H, as well as, for instance, a/µc-SiGe:H, a/µc-SiO:H and a/µc-SiC:H, which all belong to the so-called ''disordered semiconductors''.
One of the recent research topics is the development and fabrication of microcrystalline silicon-oxide alloys (µc-SiOx:H ) as a multi-functional material with a wide range of applications in solar cells. The material is a phase mixture of microcrystalline silicon and amorphous silicon oxide, and combines in this form high optical transparency with – in the case of doped layers - large electrical conductivity (s. Dissertation Lambertz).
The materials are fabricated by methods relying on controlled decomposition and synthesis of gaseous components, respectively, which are subsequently deposited at comparably low temperatures of 200°C as thin films on glass, metal, or plastic substrates. The deposition methods such as Plasma-Enhanced Chemical Vapour Deposition (PECVD) or Hot-Wire Chemical Vapour Deposition (HWCVD) are optimized under consideration of the desired material properties and investigated with regard to potential technological application and industrial up-scaling (s. Process-technology).
A characteristics of the materials fabricated in this way is the structural disorder, which distinguishes them from crystalline semiconductors. The disorder leads to the formation of structural defects affecting in an essential way the optical and electronic properties and therefore also the applicability in solar cells. We study these defects by methods like, e.g., electron spin resonance (ESR) or photocurrent spectroscopy and investigate the connection between the deposition conditions on the one hand and the optoelectronic properties on the other hand.
Finally, the materials developed are used in solar cells. Today, these comprise, besides the traditionally studied thin film silicon solar cells, also so-called heterostructure solar cells, which consist of a combination of crystalline silicon with thin films and different hybrid systems with novel materials such as perovskites and organic semiconductors. A further application of greatest actuality is the use of multi-junction solar cells in monolithic systems for the photoelectrocatalytic water splitting.
The solar cells are deposited on both rigid substrates (glass, metal) and flexible substrates (metal or plastic foils). Besides the fabrication of single junction solar cells, a major focus is on the development of multi-junction solar cells that are optimally matched to the solar spectrum and thus provide higher conversion efficiencies. The complex layer sequences, with layer thicknesses down to a few nanometers only, require the optimization and control of every single fabrication process step. The prerequisite for such developments are the available deposition systems, which have been developed on highest technical and scientific level at IEK-5 together with the industrial partners.