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The CELLO technique
uses a modulated light beam with low intensity that is scanned across a solar
cell which is illuminated with flood light and held at constant voltage or a
constant current at a specified point of its global I-V-characteristics. |
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The small signal response to
the modulated light beam under various bias or illumination conditions is
measured and fitted to a model of the solar cell that includes local variations
of all relevant parameters and - and this is new - a lateral coupling between
these parameters. |
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A nearly perfect fit will only
be obtained with a model that is more complicated, but also more realistic,
than standard models of solar cells. Of prime importance, e.g., are local
gradients in the minority carrier life time which are usually neglected. |
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From 4 - 5
measurements for every pixel ("fast scan") on the solar cell and a
suitable fitting routine maps of e.g. the local series- or shunt resistance can
be obtained in addition to the well-known short-circuit current map. |
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Complete
I-V-characteristics of a selected pixel are obtainable by taking a whole set of
small signal responses along the I-V-characteristics of the global solar cell
and by fitting this expanded data set to the model ("slow
scan"). |
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The picture below
shows a map of the series resistance of a solar cell obtained by the fast scan
and a set of three curves (obtained by a local slow scan) per pixel area which
were taken from five adjacent areas on a line between two grid fingers as
indicated by the purple bars. |
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The upper curve shows the fit
to the red datapoints based on the model. |
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The lower curves show the
I-U-characteristics of the global cell (orange line) as a reference and the
calculated local characteristics at the indicated points (blue line). |
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It is obvious that the local
characteristics vary to a considerable degree and that there is room for
improvement of this cell. |
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If you do not get a
clear picture on your screen, you may consider to download the picture and view
it at higher magnification with a suitable software. |
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First
Publications concerning CELLO |
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For more
information contact Dr. Carstensen via e-mail
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