Silicon based solar technology has made tremendous improvements in the last decade. In order to boost the efficiency of silicon solar cells to come closer to the theoretical efficiency limit, carrier recombination specifically at the metal contacts needs to be further reduced. To tackle this challenge, contact passivating structures based on polycrystalline silicon (poly-Si) layers have been investigated in recent years. The majority of the poly-Si based passivating contacts have been achieved by chemical vapor deposition techniques like LPCVD or PECVD (Low Pressure or Plasma Enhanced Chemical Vapor Deposition). These techniques involve the use of hazardous materials (e.g. silane, phosphine etc.) and high temperature processing. Further, these poly-Si layers have been successfully implemented on the rear side of the solar cells, although their implementation on the front side of the cells is more challenging. This is due to the significant light absorption by these layers, reducing the available light for carrier generation in the active part of the solar cell. In order to use these layers on the front side, the patterning of poly-Si layers is required, such that these layers remain only under the metal contacts. This requires ex-situ processing of masking and etching of unwanted poly-Si regions.
Very recently polysilicon layers deposited by physical vapor deposition methods with similar passivation quality (to CVD methods) haven been proposed. These methods include magnetron sputtering or electron-beam evaporation. Using these techniques, poly-Si layers could be deposited at room temperature without the use of any harmful precursor. Doped (n or p-type) poly-Si layers could be deposited by means of a suitably doped silicon target without the need of phosphine/diborane gases or ex-situ doping. In addition, sputtering allows single-side poly-Si deposition unlike LPCVD techniques. Magnetron sputtering is especially attractive for various reasons including a) the possibility to deposit layers over a large area with very good uniformity, b) conformal deposition over rough surfaces, c) maintaining the composition of the target material in the deposited layer d) the possibility to use low cost polycrystalline target material, and e) high throughput for thin layers. Importantly, the use of room temperature and directional deposition for sputtering enables the use of an in situ mask for patterned deposition. This could enable the easy implementation of polysilicon layer at the front side of the solar cell without ex-situ pattering steps. Such patterned deposition of poly-Si by PVD methods has not been demonstrated yet.
In order to study the viability of sputtering of poly silicon for the application of passivating contacts, the following issues need to investigated in this thesis:
- Crystalline quality of sputtered layers in terms of amorphous content, crystallite size and defects.
- Doping characterization of polysilicon layers (in-situ and ex situ) by means of sheet resistance, SIMS or ECV
- Surface and contact passivation by doped poly-Si layers.
- Patterning of the layers by masking and assessment of the contamination coming from the deposition process of patterned layers.
Type of project: Internship, Thesis
Duration: 6-12 months
Required degree: Master of Engineering Technology, Master of Science, Master of Engineering Science
Required background: Energy, Nanoscience & Nanotechnology, Physics
Supervising scientist(s): For further information or for application, please contact: Sukhvinder Singh (Sukhvinder.Singh@imec.be)
Allowance only for students from a non-Belgian university