donderdag 16 juni 2016

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!

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