The goal of the course is to give students awareness of the largest alternative form of energy and how organic / polymer solar cells can harvest this energy. The course provides an insight into the theory behind organic solar cells and describes the three main research areas within the field i.e. materials, stability and processing. NOTE: This course is a specialized course on organic solar cells. If you are looking for a more general solar cell course, we strongly recommend Introduction to solar cells (https://www.coursera.org/learn/solar-cells).
Electrical Defects
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Do curso por Technical University of Denmark (DTU)
Organic Solar Cells - Theory and Practice
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Na lição
Stability and Degradation
By the end of this module you should: -Be able to identify different kinds of degradation behaviours -Know about decay curves, including concepts such as burn-in and T80 -Distinguish between different ISOS standards.
Conheça os instrutores
Eva BundgaardPhD, Senior Scientist
DTU Energy
Frederik C. KrebsPhD, Professor
DTU Energy
Mikkel JørgensenPhD, Senior Scientist
DTU Energy
Morten Vesterager MadsenResearcher
DTU Energy
In this lesson you will learn about electrical defects in organic solar cells.
Many of the other degradation mechanisms that you have learned about in the
previous lessons, will of course, eventually manifest
themselves as a decrease in electrical output.
But this is a secondary effect.
A more direct electrical defect may be in the form of particles or delamination
features in the layers of the device
that create electrical shorts and open circuits.
In order to investigate these defects, we will show you two different
and complimentary 2D mapping techniques that can be used to identify them.
One of the main reasons that organic solar
cells are prone to electrical defects is that they
are thin film devices with a layer thicknesses
on the order of tens to hundreds of nanometers.
The distance between the two outer electrodes
are usually much less than a micron apart.
It means that even small particles may
form a conducting path between them, and in
that region of the film there will be
a reduced resistance or even an electrical short.
In the latter case, all the current will pass
through the short circuit, and the cell will not work.
The cohesive strength is small, and the difference in
surface energy is large of some of the organic layers.
This increases the chance for delamination which of
course opens the electrical circuit and inhibits charge transport.
Electrical characterization of solar cells by measuring
the current versus voltage response, gives information about
the device as a whole, but it cannot show where in the cell the defects occur.
To do this, we have two technologies.
Light Beam Induced Current, and lock-in thermography,
that can map out the solar cell.
Light Beam Induced Current, or LBIC for short, use a laser to
scan over the cell, while the electrical output is measured in each point.
These values are then translated into a range of colors, and
a map of the solar cell is constructed by a computer.
Lock-in thermography, on the other hand, produces a thermal map of the solar cell.
In this example, you can see how
delamination defects can be detected with LBIC.
Polymer solar cell module, with 16 individual serially connected
cells printed on a plastic substrate was intentionally crumbled.
Delamination starts, at the sharply folded
edges, where the mechanical stresses are largest.
Despite this abuse the solar cell still worked, but as you can see from
the LBIC image, dark blue, nonfunctional areas
have appeared where delamination disrupted the electrical connection.
In another example, a single solar cell is shown.
In this case, smaller delaminated areas are clearly revealed by the
LBIC image that cannot be seen in the photo of the cell.
Electrically conducting particles that span the whole depth of the solar
cell can create a low resistive area or even a short circuit.
It means that the electrical current that is created by the solar
cell can now run backwards and partially or fully stop it from working.
In roll-to-roll produced polymer solar cells, for instance, the first
or front electrode is printed as a fine silver grid.
Unfortunately, the ink sometimes contains larger spike-like
silver particles that can protrude through the device.
To minimize this problem, the back electrode was
also printed as a grid structure, so that the
only places where the top and bottom electrode could
meet was at the intersections of these two grids.
The concentration of larger silver particles is still
high enough, so that occasionally, shorts will develop.
In the LBIC images, a very small resistance in a full
short circuit means that the output from the whole cell disappears.
If the resistance is merely decreased somewhat, it affects a smaller area.
This can be seen in the highlighted area of the LBIC image as darker areas in
the vicinity of the grid electrodes, particularly where they meet.
I have invited an expert, Dr. Dieter Karg, from DCG Systems
to talk about lock-in thermography to detect electrical defects in OPV.
>> Thank you, Mikkel.
The title of my presentation is Localizing electrical active
defects in solar cells and modules by Lock-In Thermography.
The short word is LIT.
The figure shows a comparison of different types
of visible light, and IR radiation at room temperature.
In the visible range between 400 and 700 nano meters and in the
near infrared range up to two micro meters, the dominating radiation
is reflected radiation coming from sun, maps and things like that.
Instead of this, in the thermal infrared starting at approximately two
micrometers, the radiation splits up and reflected and emitted radiation.
In the next figure, the emitted thermal radiation of solid set
surfaces is shown, at room temperature, as a function of the wavelengths.
As mentioned before, this type of
radiation starts at approximately 2 micrometers.
Infrared cameras suited for the detection of thermal radiation are commercially
available in the 3 to 5 micrometers and the 8 to 12 micrometers wavelength range.
For lock-in thermography, we use an infrared camera,
in the 3 to 5 micrometers wavelength range.
The principle of lock-in thermography, is that the devices like solar cells,
or modules, or LEDs, are periodically excited with a fixed frequency.
In the case of organic PV, and organic LED devices,
electrical or optical excitation sources are used.
In the mean time, the infrared camera continuously makes images.
These images are correlated with the
weighting factors cosine, and sine, and added.
Leading to the lock-in result images, 0 degree and 90 degree.
Both lock-in result images have information about
depth and intensity of an electrical defect.
By doing simple math, you can separate depth and intensity from each other.
The image corresponding with the depth is called lock-in phase.
The image corresponding with the intensity is called lock-in amplitude.
The next figure shows a schematic drawing of a typical lock-in thermographic system.
Including excitation source, device, infrared camera,
and computer - how it is used normally.
Under electrical or optical excitation, electrical
defects in solar cells and modules
generates thermal radiation between 100 micro kelvin
and 1 kelvin local temperature increase.
The sensitivity of state of the art infrared
cameras is between 15 mK and 20 mK.
And, therefore, not high enough for small electrical defects.
So lock-in thermography principle increases the
sensitivity of these cameras to 1 mK.
At 1 second measurement time, and to,
for example, to 100 microkelvin, at 100 seconds.
In this figure, the lock-in amplitude of an OPV module is shown as example.
Three cells of the module are heavily shunted.
Here is shown a microscopic LIT amplitude image of a serious shunt
associated with one of the fingers in the front silver grid of the OPV module.
I want to shortly summarize my presentation
for LIT, a fast and sensitive infrared
camera into 3 to 5 micrometers or 8 to 12 micrometers wavelength is necessary.
LIT is a very sensitive method to
localize electrical defects in solar cells and modules.
Also microscopic LIT measurements with a spatial
resolution of one to five micrometers are possible.
Thank you for your interest.
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