Today, commercial solar cells are essentially made of Silicon, which is either amorphous, polycrystalline or monocristalline for the most efficient ones. Instead of organic photovoltaic solar cells to which interest began in 1990, Silicon photovoltaic phenomena is known since 1839 and has been used in solar panels in 1954 essentially for aerospace technologies.

However, organic cells have made major step forward over nearly 20 years of research and begin since a few years to be industrialized. This comes from the fact that they present several advantages and not the least even if there are also some inconveniences like their poor yield in current conversion. Currently, the yield is between 2 and 5 or 7% (instead of 10 to 30% for silicon) but, the low cost of this kind of solar cells makes them attractive enough for little and/or specific application. An other point that increases the interest for this technology, is the fact that they are flexible and eventually transparent (grätzel cells). Also, the flexible characteristic of organic solar cells have been joined by some Silicon based solar cells for a few years, but the yields of these ones are in the same range than those of organic ones.

 

After this short prewiew of this technology, let's see how it works .

Here is an animation which shows what is happening when light comes inside the cell :

Let's have some explanations about the different steps you can observe.

1. Photons in sunlight hit the solar cell and are absorbed by semiconducting materials (π-conjugated polymer).

2. Delocalized electrons (which make the double bond) are knocked and passed into an excited state. At this time, there is a creation of an electron/hole pair (the hole is the name given to a positive charge) called exciton.

3. The excited state of the electron allows it to flow through the material and due to the special composition of the cell ( potential difference between the two electrodes), electrons can only move in a single direction which is opposite of the hole direction.

4. Electron goes out of the solar panel to produce usable electricity before going back to the cell and recombine with the hole.

 

Let's have an overview of some phenomena, which induce a relatively low conversion yield :

• The first one appears at the creation of exciton. As you can see in the animation, when the separation between positive and negative charge is not large enough, we have a recombination of this pair inside the polymer layer and therefore no electricity is produced. So, to perform this separation, we use two different materials, which can be compared to the classical P/N junction in inorganic solar cell and other electronic devices. One of these materials called "electron donor" in which excitons are made and where the holes are well conducted. The second one called "electron acceptor" should be a good electron conductor.
• The second major problem inducing the low conversion yield, comes from the conception of the cell. It specially concerns the connection between the polymer layer and the electrode, because if we have a weak affinity with these two materials, electron/hole movement will be stopped. This affinity problem occurrs essentially with the cathode, because it's generally made of inorganic compound such as ITO (Indium Tin Oxide). So to prevent this compatibility’s problem, it’s common to use a third material, which has a proper affinity with both polymer and electrode. This material is usually PEDOT-PSS mixed polymers, it has the advantage of being a good conductor and to be transparent for light. Here is its structure:

Therefore, everything being considered, here is a typical organic solar cell structure:

• Protective substrate: must be transparent in the interesting range of sunlight spectrum. Could be glass, PET, PC or PMMA and more..

• Electrode : they shouldn't be the same due to the potential difference needed. The cathode one must be transparent as well as the protective layer and also a well conductor, usually an inorganic alloy such as ITO or other oxide. The anode doesn't need to be transparent and is usually high conducting metal such as Aluminum, Gold and others.

• Electron donor layer : it's the active material and it should absorb the light in a maximum range as much as possible, it must be a π-conjugated structure and it is usually polymer (MEH-PPV or P3HT) or Dyes (small organic molecule).
• Electron acceptor layer : should be a good electron conductor, generally a carbon backbone based such as fullerene and carbon nanotube. The most common one is the PCBM, a fullerene derivative with higher solubility and compatibility. Other kind of electron conductor can be used such as perylene derivative.

Here is an example of an organic solar cell

 

For more information about the process or other indications about Organic Solar Cells, just have a look at our link page.