Nanocrystalline Silicon Solar Cells on Flexible Al Substrates

Abstract

Thin film solar cell technologies are gaining more favour over first generation PV technologies such as monocrystalline silicon PV modules. This is due to their special characteristics such as lightweight, flexibility and reduced resource consumption. HyET Solar is a Netherlands based company that makes thin film amorphous silicon modules using a novel processing technique. This construction philosophy involves using a temporary Al substrate to deposit the solar cell layers. In the final step a permanent plastic carrier foil is encapsulated, and the Al foil is removed. HyET Solar has partnered with TU Delft under the FlamingoPV project to produce a-Si:H/nc-Si:H tandem solar cells with efficiencies up to 14%. A starting point for the micromorph cell is a single junction nanocrystalline silicon solar cell on Al foil which has never been attempted at TU Delft. A key concept of cell construction is the use of modulated surface textures (MST) to improve light trapping and growth of high quality nc-Si:H material. MST are optimised on Al foil and used to deposit nc-Si:H layers. The quality of the layers is characterised using various techniques such as Raman spectroscopy, XRD and SEM images. Hydride stretching mode signatures are a non-intrusive way of characterising device grade nc-Si:H layers without undertaking the arduous fabrication process at HyET. Investigations show the substrate texture features have an influence on the material properties. Results on Al foil show both different and similar trends compared to glass substrates. The silane concentration is an important parameter which influences the nature of deposited material. By tuning this parameter, different signatures of the hydride stretching modes are observed. Initial results suggest that just any seed layer is non-conducive to growth of the nc-Si:H material. Seed layers are deposited using silane concentration grading technique. These graded seeds show excellent microstructure homogeneity and reproducibility. The seed layers also enhance the (220) preferential growth orientation of nanocrystalline grains. The graded seed layers are incorporated into p-i-n cells which will serve as good p-i interface, this resulted in high quality material with good thickness homogeneity and high crystalline fractions over 75%. The nc-Si:H solar cells show high shunt resistance values that are orders of magnitude higher than HyET’s modules. This indicates high quality material with limited shunts and cracks which were developed using graded seed layers and MST. When completing the cells at HyET, several challenges were faced due to lack of experience with nc-Si:H solar cells. These include but are not limited to, laser scribing settings and final etching of the Al foil. These issues need to be ironed out at HyET before processing new samples which is out of the control of PVMD. To overcome this bottleneck, an alternate finishing process is researched at TU Delft, despite first results of this alternative approach being unsuccessful, we are confident it can be perfected with more dedicated research.