donderdag 30 juni 2016

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!

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