donderdag 23 juni 2016

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.

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