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Photovoltaic Research of the Webergroup

Our research in photovoltaics (PV) encompasses many aspects of materials improvement. Firstly, we investigate bad regions of multicrystalline silicon solar cells, to determine the chemical and electrical nature of transition metal contaminants in crystalline silicon solar cells, as well as their interactions with lattice defects and other elements such as carbon and oxygen. Secondly, we are actively pursuing research to increase our understanding of shunts, which reduce solar cell fill factors. Thirdly, our group's extensive experience in gettering facilitates studies in alluminum gettering for crystalline silicon solar cells. Lastly, the interactions of transition metal precipitates with hydrogen is of real importance for today's research, and our research team is actively pursuing investigations in this area.


Bad Regions

The efficiency of a solar cell device is determined by the summation of its parts, thus poorly performing regions (a.k.a. bad regions) represent a substantial power loss. The goal of our research is to determine the role of transition metals in poorly performing regions of solar cells. Through the use of a variety of techniques, including but not limited to synchrotron-based x-ray fluorescence microprobe (µ-XRF), x-ray beam induced current (XBIC) [1], and x-ray absorption spectroscopy (µ-XAS), the electrical and chemical nature as well as the spatial distribution and concentration of transition metal contaminants in bad regions can be determined with a micron-scale resolution. In addition, we study the correlation between transition metals and lattice defects, such as dislocations and grain boundaries, and their complexes with light elements such as oxygen and carbon. The ultimate goal of this research is to identify process steps which will reduce the damaging effect of transition metal contamination in solar cell devices through understanding the chemical nature, formation mechanisms, and corresponding electrical activities of different types of transition metal precipitates in silicon.

Results from our lab have demonstrated the important role of iron in bad regions, specifically, the interaction with oxygen precipitates. Via x-ray absorption spectroscopy (µ-XAS) studies, it was determined in certain oxygen-rich bad regions that iron interacts with oxygen to form either a silicate or an iron-oxygen complex.[2] Further work will be conducted to determine the roll of heat treatments and processing steps on the formation and dissolution of these iron-oxygen complexes. In addition, the complexes of other transition metals with oxygen and carbon will be investigated.


Shunting Mechanisms

Shunts (local defects of the p-n junction) can noticeably decrease the cell efficiency, mainly by decreasing the short-circuit current ISC, which results in the maximum power point occurring at lower V, consequently reducing the fill factor. Certain process-induced shunting mechanisms are well known, such as scratching through the p-n junction and improper edge passivation of the solar cell. However, the mechanisms of many other types of shunts are still unknown, and the abnormally high ideality factors unexplained.

Recent work from our lab has indicated that transition metals may indeed play a role in certain types of shunting behavior.[3] Current research includes collecting statistically meaningful data on shunts, studying shunts in a variety of silicon types and processes, and modelling.


Gettering

The removal of transition metals from the silicon crystal by means of thermodynamically favored processes is also known as 'gettering.' Our research team's past studies have demonstrated the difficulty to getter transition metals from multicrystalline silicon, material out of which about two fifths of the world's solar cells are manufactured.[4] We perform gettering studies with aluminum (e.g.[5]), a common rear surface contact for solar cells, and compare our experimental results with our powerful gettering simulator.[6] The goal of this research is to determine via what mechanisms and to what extent aluminum gettering is effective for the removal and possible passivation of defects, and to identify the mechanisms of formation of gettering-resistant sites in multicrystallline silicon.

For more information about all of our gettering studies in silicon, please visit our gettering page.


Hydrogen Passivation

The indiffusion of hydrogen has been shown to increase the average minority carrier lifetime of solar cell material. It is believed that hydrogen can passivate transition metal precipitates. However, much about the interaction between hydrogen, heat, and transition metal precipitates is still unknown.

Recent results from our laboratory suggest the interaction between hydrogen and transition metal precipitates is much more complex than previously believed. Future studies will help understand the basic physics of this phenomenon, which in turn will aid industries in optimizing their production processes.


References

In addition to the references below, please have a look at our complete Si publication list.

[1] X-ray beam induced current a synchrotron radiation based technique for the in-situ analysis of recombination properties and chemical nature of metal clusters in silicon. O.F. Vyvenko, T. Buonassisi, A.A. Istratov, H. Hieslmair, A.C. Thompson, R. Schindler, and E.R. Weber. J. Appl. Phys. 91, 3614-7 (2002). online.

[2] Synchrotron-based impurity mapping. S.A.McHugo, A.C.Thompson, C.Flink, E.R.Weber, G.Lamble, B.Gunion, A.MacDowell, R.Celestre, H.A.Padmore, Z.Hussain. J.Cryst.Growth 210, 395-400 (2000).

[3] Analysis of Shunts in Multicrystalline Silicon Solar Cells Using Microprobe X-Ray Fluorescence Technique. T.Buonassisi, O.F.Vyvenko, A.A.Istratov, E.R.Weber, R.Schindler, and G.Hahn in 12th workshop on crystalline silicon solar cell materials and processes, B. L. Sopori, Editor, NREL, Golden, CO, 266-270 (2002).

[4] Gettering of metallic impurities in photovoltaic silicon. S.A.McHugo, H.Hieslmair and E.R.Weber, Appl. Phys. A 64, 127 (1997).

[5] Application of X-ray Fluorescence Technique to Studies of Aluminum Gettering in Silicon. O.F.Vyvenko, T.Buonassisi, A.A.Istratov, and E.R.Weber in 12th workshop on crystalline silicon solar cell materials and processes, B. L. Sopori, Editor, NREL, Golden, CO, 266-270 (2002).

[6] "Gettering simulator: physical basis and algorithm" H.Hieslmair, S.Balasubramanian, A.A.Istratov, and E.R.Weber. Semiconductor Science and Technology 16, 567-574 (2001).


This page is maintained by Tonio Buonassisi. Please email him with updates. Last revised: August 28, 2002

 

 

 

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