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Thin Film Silicon Solar Cells on Glass
Group members:
Dr. F. Sculati-Meillaud, S. Hänni, Dr. E. Moulin, Dr. B. Niesen, Dr. J.-W. Schüttauf
Running projects: OFEN, CTI, EU - Cheetah, FP7-Fast-Track
Keywords: thin film silicon, amorphous silicon, microcrystalline silicon, micromorph, solar cells
Background
The "Thin Film Silicon Solar Cells on glass" group focuses on the development of high efficiency hydrogenated amorphous (a-Si:H) and microcrystalline (µc-Si:H) silicon single-junctions and multi-junctions solar cells in the superstrate configuration (p-i-n). The first µc-Si:H solar cells were pioneered at the PV-Lab in 1994. Microcrystalline silicon is of particular interest when combined with amorphous silicon in a solar cell tandem configuration, commonly called "micromorph", as the different optical band gaps of these materials allow for the active conversion of a larger part of the solar spectrum. In such combination, a top a-Si:H cell absorbs light with wavelengths up to 800 nm while the µc-Si:H bottom cell can absorb light in the near infrared region, allowing for larger attainable conversion efficiencies. This "micromorph" serially connected tandem cell was first introduced at the PV-Lab in the mid 1990s, and since then, an increasing number of research institutes and companies have adopted this concept.
Figure 1. SEM image (left) and schematic (right) of the cross section of a micromorph solar cell in the superstrate configuration. From bottom to top: glass - LPCVD textured ZnO - thin a-Si:H top cell (250 nm) - thin ZnO based intermediate reflector (200 nm) - 1.5 μm thick μc-Si:H bottom cell and back contact in LPCVD ZnO.
Microcrystalline silicon is a complex material that exhibits a wide range of possible microstructures depending on both the deposition conditions and substrate material. Many efforts have been laid in the past years in understanding the growth mechanisms of this material and the relationship between microstructure and solar cell efficiency. On the other hand, the amorphous nature and the metastable properties of hydrogenated amorphous silicon material also lead to different controversial issues remaining discussed, so that research remains active in this domain too. Finally, in order to achieve high efficiency with thin films of silicon, a high light confinement is fundamental and advanced cell designs integrating textured front-electrodes obtained via LPCVD deposition of ZnO or via the nano-imprinting fabrication of transparent nanostructures covered by highly transparent conductive oxides are developed, together with advanced solar cell designs allowing for high electrical properties on such rough substrates.
The main research areas of the group are as follows:
Increasing thin film silicon solar cells efficiency by:
Characterizing and improving the material quality (by modifying e.g. deposition processes) of microcrystalline silicon deposited in small-area laboratory PECVD systems and in semi-large area industrial PECVD systems.
Improving the stability of a-Si:H solar cells to achieve larger stabilized efficiencies: protocrystalline materials and low deposition rate material to reduce the material degradation, or reduction of the impact onto the cell performance of the Staebler-Wronski effect.
Implementing and assessing the light trapping potential in single to multi-junctions solar cells of new morphologies. Developments of novel concepts and materials to realize textured front electrode and substrate.
Developing advanced concepts for optical confinement in the multi-junctions solar cells
Improving the cell design to retain high electrical properties for cells deposited on highly textured substrates
Testing different multi-junctions combinations and in that respect developing high-gap and low-gap materials
Improving the solar cell design to ensure reduced cost of ownership for thin film silicon solar modules. One of the focus is for example on the development of high quality high deposition rate µc-Si:H material to decrease deposition time and hence production costs (strong collaboration with industries).
Developing a better understanding of the relationship between plasma conditions, material quality and solar cell efficiency (in collaboration with the plasma group).
Developing and implementing material and solar cell and modules characterization techniques