Hello everyone, this is the second blog post on technical aspects of solar cells. This article discusses the specifications manufacturers provide in the datasheets of their solar module. If you ever looked at a data sheet and wondered what these terms meant and want to know more, this blog is for you.
Those interested in photovolatics are sure to have seen a solar module datasheet at some point. This blog is to help you understand the different terms in the specifications. These include – “Maximum Power” (Pmax), “Open Circuit Voltage” (VOC), “Short Circuit Current” (ISC), “Maximum Power Point Voltage” (Vmpp), “Maximum power point current” (Impp).
Of course, there would also be other information such as – component details (dimensions, weight, number of cells, some mechanical characteristics), thermal characteristics (thermal coefficient), system integration parameters etc. These other characteristics will be covered in our forthcoming blogs, but in this blog, we will talk about the different currents and voltages.
The origin of the current in a solar cell was explained in our previous article, “How does a solar cell work”. This blog discussed how doped semiconductors can be joined to make a junction, how this junction has a potential drop (or electric field), how energy (in form of heat or light) creates electron-hole pairs and how the junction’s electric field separates them to generate current.
Take such a P-N junction. In the dark, there is only thermal energy. It creates e-hole pairs and junction’s potential drop drives electrons from P to N-side and holes from N to P-side. This is the thermal generation current, Igen. Now the P-side will have more holes and N-side will have more electrons. This concentration difference drives them back (this is the recombination current, Irec). So the recombination current is in opposite direction to the field and will be hindered by it (this dependence of recombination current on the junction’s potential difference will become important once we attach a load).
In an isolated junction, however, externally no current can flow and so the two currents have no choice but to balance each other: Irec = I gen
If light is shone over this junction, the photons in the light will create more electron-hole pairs – let the resulting current from these be called Iphoto.
Peak Power, Pmax = Vmax*Imax
Efficiency = Peak Power / (Irradiance * Panel Area)
η = Pmax / E*A
For those who want to quickly grasp the essence of the different specifications -
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