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!