This is how circuits are born

It is due time that we celebrate our engineers/magicians, who are the actual reason why OSCAR has survived all this time.
Let us keep the magnetometer aside for now. A dedicated explanation of how that works will come on a separate blog post. If we focus on solar cells, you may remember that we talked about measuring their efficiency a while back.
The famous Current-Voltage characteristic you heard about back then, is exactly what we want to obtain for each of our devices while they fly. Those characteristic curves are made by couples of Current and Voltage values, collected -for example- by applying Voltage X (in water analogy, this was the pressure), and reading the current Y that "comes out" of the system (or the flow of water through the pipe).
Sounds easy enough, but it is actually quite complicated to do it, if you have to design the whole circuit...

This is where Steven (forgot about him?) came to save us.
He is designing the measurement instrumentation for us. Roughly, this means that he has programmed a little computer chip to send specific signals to a net of amplifiers, resistors and capacitors. These signals, just like the whole circuit components web, are finely tuned to make sure that the correct Voltage is selected, and that the readout of the Current is accurate and within the limits that the computer chip can withstand without breaking.
Why do we have to be so precise?
Home-built dedicated circuits are normally cheaper than the fancy equipment we would have in a laboratory. That's why we can actually afford to mount them on the balloon and let them crash -maybe very harshly- at the end of the ride. Lab tools are also very heavy and require way too much power to operate... Really not fit for flying with the BEXUS!
They are of course designed to handle a very large variety of "situations" and to be used for many different applications.
That's one downside of our cheaper setup: it does not have such flexibility. We have to build the safety measures ourselves, to make sure we do not let too much current in the chip, for example, or to avoid overheating amplifiers.

But calculating the needed components for the wanted ranges of operation is not all!
You might remember that we want to test a few different kinds of solar cells... As a total, we are going to mount photovoltaic devices from six different materials on our BEXUS gondola!
What does this translate to?
Six times the requirements for operational ranges, six different layouts for the pins with which we need to apply the Voltage and read the Currents, and a whole lot of cables and connections to take care of!
But Steven fixed this, as well: he packed all the cables in some extremely pretty Printed Circuit Boards, which are like thick sheets of copper and insulators, and contain the various connections within them, on separate layers. They also have a shielding copper plate, to avoid signal distortion.
Can you believe that there are the equivalent of 149 cables in these green sheets?

OSCAR's logo is there, you will notice.
This is because our engineers are actually artists at heart ;-)

Now, the work is far from done... the PCBs contain the connections, but they don't have the contacts for the devices on them, for example. And attaching all those tiny pins is quite tedious, besides being potentially dangerous!


PS: have you checked out the short musical version of this story yet? It's on our Youtube channel!

Integration Progress Review highlights

Last week, we had our Integration Progress Review, also called "IPR".
Like for all space projects, the entire process needs to be constantly monitored and reviewed, to make sure things are going smoothly.
Also for REXUS/BEXUS projects, we are constantly supervised by our sponsoring agencies, who make sure that we are on schedule and that nothing is unaccounted for.

So... we mounted our experiment on a mock-up of the gondola (the metallic cubic frame which will be attached to the balloon), and it looks like what you see in the photo above! Say "Hello!" to OSCAR!

During the meeting, the entire team had the chance to meet our ESA mentor, we discussed the progress with the BEXUS 23 payload manager (who is particularly concerned about the safety hazards related to our experiment... not many!).

The experts brought with them a spectrum analyzer, as well, which allowed us to test that our home made Radio Frequency sweep is staying within the range we need to measure the magnetic field. We can thus be certain our measurement (between 2.82 and 2.9 GHz) does not disturb the data transmission to ground (around 2.4-2.5 GHz)... we and the other teams sure need the data to come down!

We can not yet lay back and relax. The newt review is scheduled for mid September, just before flight! By then, everything needs to be ready!
But in the meantime, we can release some stress over a nice beer with ESA mentor!

Testing our way to IPR

IPR (Integration Progress Review) is coming up TOMORROW.

This means we really need to get our experiment ready for flight, and one of the most important things for space-related missions is: TEST!

Make sure you beat Murphy's Law! ... or at least limit its power as much as you can.

Among the various things to verify are: will our experiment withstand stratospheric temperatures and pressures? Will it withstand vibrations? Ans so on...

OSCAR: "How do we test vibration?"

Experts: "Living in Belgium? Just drive around on any road with your experiment in the back of a truck!"

... And so we did!

We also started testing our magnetometer (Aury). Jarda and Jelle did an awesome job in building her up, and she has already passed our vacuum testing.
See her inside the vacuum bell? She worked for over 5 hours at a pressure of 20 mbar.
Way to go, Aury!

Cross your fingers for us, you all: tomorrow's review is a crucial step towards our balloon flight!

And the OSCAR for the highest power-to-mass ratio goes to...

On the 10th of June, professor Jean Manca (who is endorsing our OSCAR project) gave a talk with this title to the 2nd Dutch Perovskite Workshop.
The event, organized in Delft, aimed to reunite the "perovskite community" within the Netherlands and neighboring countries, to discuss about the progress of research on hybrid perovskite materials (mainly for solar cells applications).

But what are these hybrid perovskite materials, exactly?

The word perovskite itself defines a kind of crystalline structure, which is typical of calcium titanate. The same structure is found on various kinds of materials, with applications ranging from superconductors to batteries to... solar cells. The reason why we refer to hybrid perovskites is because (in perovskites used for solar energy conversion) one of the building blocks of the crystal, the central dark sphere in the figure, is an organic molecule. Hence, we are back to carbon-based!
What is so special about these materials, apart from the fact that they are partly organic and partly inorganic?
They happen to be very fit for photovoltaic applications, as their permittivity ( remember, from some weeks back?) and their charge carrier mobility are quite high. This means that charges promoted to the Conduction Band are immediately separated from the Valence Band, and that they can move very fast to the collecting electrodes and to the outer circuit.

Besides these electrical properties, hybrid organic-inorganic perovskites also have a high absorption coefficient over a broad spectral region, which means that they can absorb light of all visible colors (and also some "invisible colors", like infrared and ultraviolet)... and lots of it!
The high absorption coefficient is a characteristic that hybrid perovskites share with some of their all-organic cousins, and it represents the main reason why solar cells made out of these special semiconductors can be fabricated from very very (let us say "ultra") thin layers.

Going back to outer space applications, we should consider that the cost to bring (anything) far enough from the Earth as to escape the gravity pull is extremely elevated, and scales up with the weight (or, more precisely, the mass) of what we want to send up.
Therefore, it is a smart idea to optimize the mass of the devices we will need on our space bases or space ships, in order to save some money in the transport process.
Here is where ultrathin perovskite solar cells outperform all other rivals.

Perovskite solar cells have higher efficiencies than organic solar cells, but they still do not beat the very expensive inorganic ones in terms of pure power conversion efficiency.
But to look at it from a fairer point of view, we should consider the power that solar cells are able to generate per unit of mass: the power-to-mass ratio!
With this new parameter at hand, we can compare photovoltaic technologies based on their compatibility with "out of Earth's orbit" applications, and we can finally convey the real drive behind our desire to test the reliability of organic-based solar cells in extreme stress conditions.

As a little side note: ultrathin OPV (Organic PhotoVoltaics) are a close second to perovskites... that is why we decided to study them, as well!

Let’s measure solar cells efficiency!

After clearly discussing how organic solar cells convert light into electricity, let us look at how to quantify the efficiency of this process.

Taking one step back, efficiency can be defined as  the ratio between the total generated power (output) and the total light power incident on the device (input), as was already explained in the post “Why carbon... solar cells?”.

But when it comes to actually measuring this, what are our options?

We will refer to the fact that the power generated (or consumed) by any electrical appliance is given by the product of the Voltage drop over the appliance and the Current passing through it.

To understand this in simple terms, let us take a hydraulic circuit analogy: the Voltage corresponds to the pressure of the water inside a pipe, the Current represents the flow of water through that pipe, and the Power is that generated by a mill set in motion by that same water.

If we have no pressure in the tube, the water will not flow out, and there will be no power generated by the mill. If we have no flow of water through the tube (because it is sealed, for example), regardless of the pressure applied, the mill will still produce no power. We can only have an output power for combinations of water pressure and water flow that set the mill in motion!

For solar cells, it works exactly the same way: we will need to look at the power generated for “good combinations” of Voltage and Current. To find these “combinations”, we need to know the relationship between Current and Voltage in our solar cell. That is: we need to know what Current we will get out of it, if we apply a certain Voltage (back to water analogy: what is the flow we can have out of the pipe, for a certain applied pressure?). This kind of relationship is called a Current-Voltage characteristic.

Such a characteristic can be plotted in curves like the one above.

How do we read such a curve?

The top quadrant corresponds to Power Dissipation, because we can see that by multiplying the Current value for each point on the red curve with the corresponding Voltage value, we obtain a negative “generated power”, which corresponds to a positive “consumed power”. Be careful, sign conventions may vary... just keep the concept in mind!

In this region, the solar cell does not produce any power because no light is shining on it.

Once we turn the light on, the Current-Voltage characteristic changes into the light blue curve, which now represents Power Generation (because, taking the product between Current and Voltage in the bottom quadrant, we obtain a positive value). Of course, the more intense the incident light, the more power the solar cell can generate, as we see with the darker blue curves.

But don’t be fooled: the solar cell produces more power... but there is also a greater input power from the light!

So, in general, the efficiency of a photovoltaic device can be determined for whatever amount of light we use to promote power generation.

This is, again, a simplification. Defects in the materials (quite often present in organic layers)  can lead to changes in the shape of the characteristic curve for various light intensities. In such cases, the ratio between output power and input power will not necessarily remain constant for varying intensities of light.

How do Organic PhotoVoltaics work?

Last week, we explained our interest for organic (carbon based) solar cells, which can potentially be produced very cheaply, via printing or other roll-to-roll compatible deposition techniques.

As you already read, the organic materials we are dealing with are good substitutes of traditional (inorganic) semiconductors. But what are semiconductors?

Let us imagine that all materials are energetically divided in “bands”, on which charges are disposed. Normally, all charges sit in the Valence Band, where they cannot displace themselves within the material. The charges with enough energy to reach the Conduction Band can, on the other hand, travel around the material and participate in the electric current conduction.

Looking at the schematics, we can imagine that, the larger the Energy Band Gap, the more difficult it will be for a charge to move from the Valence Band to the Conduction Band. In the case of conductors, no energy is require for charges to move, while an insulator has such a large Band Gap that the charges can never really end up in the Conduction band. Semiconductors sit in between, which means that small amounts of energy can get a charge to be mobile.

The mechanism at the basis of solar cells’ functioning is that of using the energy of incoming light to promote charges from the Valence Band to the Conduction band of the semiconductor, to eventually drive them out of the device and collect them.

In organic solar cells, things are slightly more complicated, as we need to combine two organic semiconductors in order to be able to generate current.
Why so?

When we have a charge inside a material, the electric field created by it has a certain interaction with the surroundings, which depends on a material property called the permittivity. In organic semiconductors, the low permittivity is responsible for the fact that, once a charge moves to the Conduction Band, it cannot fully detach from the Valence Band. To make sure that we can get the charge to fully participate to conduction, we can mix our (organic) semiconductor 1 with (organic) semiconductor 2, which has a Conduction Band at a slightly lower energy. Like this, our charge can “relax” down to the lower energy of the Conduction Band of semiconductor 2, and effectively leave the hole it left behind in semiconductor 1.

The mix of the two semiconductors represents the photo-active layer of the organic solar cell, which actively converts the energy of light into electric charges. This is not the only layer that makes up a photovoltaic device: we also need electrodes to collect the produced charges and to bring them to the outside circuit. Typically, some “buffers” are introduced between the photoactive layer and the electrodes. These are there to protect the organic semiconductor from eventual electrode-induced damage, and to efficiently attract the charges (again, by letting them relax down a staircase of energy levels).

The final outcome is a structure of 5 layers, deposited on a substrate, and covered by an encapsulation. But do not be fooled, the total thickness is still going to be much less than a millimeter! This is a property of organic based solar cells, which fall inside the “Thin-film PhotoVoltaics” family.

Be back next week, to know how we measure solar cells’ efficiency!

Why Carbon... Solar Cells?

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!