It is long overdue that we spend some time to explain how
our solar cells work, and why is it so interesting to mount them on the BEXUS
balloon for a trip to the stratosphere.
We all saw movies like The Martian, where
entire space bases are run for years on solar power. This sounds like a smart
idea, right? Stars are going to radiate light no matter whether we use it or
not, so we can just choose to collect it for our energetic needs.
What does a solar panel (or, to give it a more technical
name, a photovoltaic panel) need to be like, in order for it to be considered
for space applications?
Mainly, it needs to be very efficient in converting light to
electricity, which means that the ratio between the total generated power and
the total light power incident on the device has to be as high as possible. To
give you an idea of how high this ratio normally is, for traditional solar
panels, like the ones you see on roof-tops, the efficiency can be as good as
25%, but rarely exceeds this value. For space, they combine the use of
expensive materials, with the stacking of more solar panels one on top of the
other, and they can reach final efficiencies above 45%.
A second very important requirement for technologies that
are to fly to space is the ability to withstand “stress” without degrading. If
the extremely low (or, sometimes, extremely high) temperatures that we find in
space risk to reduce the efficiency of the solar cells or to irreversibly break
them, then they cannot be used in outer space, as their reliability would not
be meeting the strict standards set for aerospace missions. The same is valid
for other kinds of potentially degrading conditions, like the high level of
radiation and the very high vacuum.
Our aim, with flying solar cells to the stratosphere, is to
have a first feel of their stability, when subject to such harsh and un-earthly
conditions.
You may ask: “if, in The Martian, Matt Damon was already
using those panels on Mars... can’t we assume they are OK for use in the
stratosphere?”
The answer is certainly “Yes, we can!”, but what we want to
do is not testing Matt Damon’s solar panels... we want to test solar panels
made out of new plastic materials, which can be processed from solution, i.e.
printed, cheaply fabricated. The potentially low production cost is not the
only appealing feature of plastic solar panels. They also could be made in
different colors, semi-transparent, very thin and even flexible!
You know from daily situations that plastics are unfortunately not very durable, if compared with metals or rocks. That is why plastic photovoltaics’ reliability is one of their weak points.
When we refer to plastics, we can talk about “organic
materials”, as the substances we use are largely composed of a carbon atoms. In
our special case, what we need to have is an organic compound (which
can be a polymer, made of many repeating units, or a single molecule) that
behaves like a traditional semiconductor. Traditional semiconductors, like
silicon, are at the base of all electronics devices. As you may guess:
substituting a traditional semiconductor with a plastic one can serve purposes
that go beyond the sole production of photovoltaic modules...
... But that is another story :)
Stay tuned for a closer look to the working principle of
organic-based solar cells!
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