Industrial Ph.D.: Critical wafer parameters for the efficiency and degradation of solar cells

In his Ph.D. project, Elkem’s Espen Hvidsten Dahl found that even small concentrations of sodium in silicon may decrease a solar cell’s efficiency.

Espen Hvidsten Dahl has written a Ph.D. thesis at Aarhus University within a project run by Elkem and partially financed by the Norwegian Research Council (NFR). The Ph.D. is titled Identification of critical ingot parameters for the efficiency and degradation of solar cells. Espen, who works as an R&D Engineer in Elkem Technology, defended and passed his Ph.D. thesis in June 2013. 

What were the results?

The most important finding in the project, I will say, is that we have documented that even relatively small concentrations of sodium (< 1 ppm) in silicon may lead to a degradation of the solar cell efficiency. This has not been the main opinion in the field. Among other results the project established that iron, due to well-developed and optimized refining processes, did not act as a dominant impurity in Elkem Solar Silicon® (ESS™). In addition we studied optimal material quality improvement by thermal treatment, techniques commonly referred to as gettering, and described inherent material properties reducing this potential. In the end we did an overall assessment between ESS™ and commercial polysilicon (market base product) . Although there are differences in the composition of impurity elements and structural defects, we did not find any major qualitative differences. This is even better visualized by the good performance indicators of installed ESS™ based solar panels worldwide. We believe there could be several, so far not described in detail, beneficiary mechanisms in the metallurgical refining process that makes Elkem Solar Silicon®  so competitive. 

Why does it matter?

The main goal of such projects as the one I have worked on, is to improve the knowledge base on structural defects in solar cell grade multicrystalline silicon in general, and ESS™ in particular. You will never reach a final conclusion, but in order to reduce the cost of production and push the efficiency of solar cells, these projects are vital. If we listen to the IPCC (the Intergovernmental panel on climate change) and its latest report, we need a transition from a fossil fuel-based society to a society based on a mix of renewable energies, including solar, and we need it fast. Every advantage we can give solar energy in order to become more competitive will help, both to secure this essential transition and for the prosperity of Elkems Solar energy related products. 

What methods did you use?

I investigated wafers that were based on Elkem’s ESS™ product. Wafers can be said to be the base product for solar cell processing, with the dimensions and structural specification as the final solar cells. At the wafer level you would like to know if the electrical, chemical and structural properties are within the requirements of the customer (solar cell producer). In my work I have combined a range of different characterization techniques in order to study how structural defects and impurities can interplay to reduce the potential efficiency of the final solar cell. Among these methods I will especially mention Deep Level Transient Spectroscopy (DLTS) that describe how defects influence the efficiency of the solar cell, even at concentrations below 1 ppb. The electrical quality of the wafers can be determined by the Microwave-Photoconductive Decay (µ-PCD) technique, where the lifetime of the electrical carriers in the material are determined. This lifetime is related to the solar cell efficiency. In order to describe the bulk chemical composition, I have used a Secondary Ion-Mass Spectrometry (SIMSthat can tell me, down to roughly 1 ppb, what kind of elements I may find in the material. In addition, I have used different types of optical and electron microscopy like TEM and SEM besides other typical characterization techniques related to this field.  

What are the next steps?

Although there has not been scheduled any continuation of my work, there are similar projects ongoing within Elkem. Research is carried out both internally at Elkem facilities in Kristiansand, and in cooperation with different universities and technical institutions in and outside of Norway. It is essential both for Elkem and the industry in general to carry out such research to stay or become competitive in a market that currently is at a standstill. It is my hope that both Elkem and other actors in the industry may find interest in the results from the project. 

Left: Image of a wafer based on ESS™ that reveals individual grains in the structure

Right: TEM image revealing a metal containing cluster detected in one of the silicon wafers.

Elkem and the Industrial Ph.D. scheme

  • The Industrial Ph.D. scheme for funding for industry-oriented doctoral research fellowships was established to facilitate the recruitment of researchers to Norwegian industry.
  • The Research Council launched the Industrial Ph.D. scheme in 2008, with the first years as a trial period. It is modelled on the Industrial Ph.D. Programme in Denmark, which was established in 1972.
  • At present, Elkem has one employee working on an Industrial Ph.D. project. The company has plans for two new projects, scheduled to start in 2014.


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