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).
