Most solar cells are made from crystalline silicon. Crystalline silicon is expensive because it has to be very pure to make efficient solar cells. Unfortunately, nobody knows exactly how pure and that is why manufacturers play it safe. In other words: they generally use too pure and therefore too expensive silicon. New data on the relation between solar cell performance and impurity content are very important for cost saving. Until now, data from the 1970s have been used.

In the meantime, photovoltaic (PV) solar energy has made tremendous technological progress. New results are presented by Dr. Gianluca Coletti, senior researcher at ECN, which may enable significant cost savings for the PV industry sector. They show how the silicon purity affects the performance of crystalline silicon solar cells.

Understanding the relation between impurity and performance
The availability of silicon of an (assumed) sufficient chemical purity has been an important limiting factor for rapid growth and price reduction during part of the past decade, and may become one again in the future. There is a strong need for low-cost and low-investment cost technologies for the production of silicon for PV applications.

However, such low-cost production technologies will most likely compromise the purity of the resultant silicon. The aim of the studies presented in this recent work is to understand the complex relation between the impurity content of the silicon starting material (the "feedstock") and the resulting solar cell device performance. The study takes into account the current as well as future photovoltaics technology developments.

Applicable to industry
Thanks to this work it is possible to determine the solar cell performance as function of the impurity content in the feedstock (see graph). It also points out which metal impurities have the highest impact on the solar cell efficiency. Therefore the results of this study can directly be deployed by the PV industry.

The effect of the silicon quality on the cell performance is of major importance in optimizing the silicon purification. It also determines the material requirements for advanced future solar cell devices, taking into consideration the wafer substrate thickness for cost reduction. Furthermore, standardization of material specifications is a key factor to foster industry growth and innovation. This study supports the formulation of standard specifications for silicon material to be used by industry.

Optimization of costs
For the first time the effects of a whole range of (metal) impurities in the silicon starting material ('feedstock') on the efficiency of state-of-the-art and advanced solar cells are described in detail and reported in a comprehensive way. This way it becomes possible to purify the silicon in more targeted ways and to optimize the costs. Since 'good is good enough', unnecessary steps in the purification processes can thus be avoided. The study shows the difference material properties needed for state-of-the-art and advanced devices