The current theoretical limit is slightly above 33%, but with new technologies that push quantum efficiency, a conversion of more than 60% can be reached.
Solar panels have been giving us in recent months one of lime and another of sand. On the one hand, China’s overwhelming dominance in its production has caused a notable drop in prices. It is increasingly affordable for the user to buy blown panels for self-consumption and place them on the roof or even on the balcony. In fact, the price has fallen so much that there are those who choose to buy panels and place them as garden fences rather than traditional wooden ones.
The negative side is that the theoretical limit of conversion in conventional crystalline silicon panels is being reached. The good news is that they are researching new solar cells that would reach an extreme quantum efficiency level of 190%. There are several things to say about this.
The Shockley-Queisser limit. In 1961, physicists William Shockley and Hans Queisser established a theoretical limit to the amount of light energy that could be converted into electricity. Over the years, the limit rose to 33.7%, where it was established as a kind of standard and barrier on the energy that could be converted into electricity. This means that of the 1,000 W/m² incident, what can be converted to electricity will be 337 W/m².
Obviously, there has been a lot of rain since then and this limit only applies to single-ply systems, but with new panels and materials, we have seen the barrier be pushed considerably. An example is perovskite panels, with which conversion efficiencies of 33.9% have been achieved. And a couple of years ago 47.6% efficiency was achieved by combining a layer of gallium indium phosphide and gallium/aluminum arsenide, along with another layer of gallium indium arsenide phosphide and gallium indium arsenide .
Maximize efficiency. They are technologies that can leave behind the limitations of single-junction solar cells, and that is precisely where we are right now, in that search for efficiency. This can be achieved by making more use of current solar panels and giving second chances to old solutions or improving the efficiency of panels with new technologies.
In China they are already experimenting with solutions that would allow an efficiency of just over 50%, but that barrier could be further blurred with the solar cells they are researching at Lehigh University.
Quantum efficiency vs conversion. Here we must distinguish between efficiency in converting light energy into electricity and extreme quantum efficiency. This refers to the ability of photovoltaic cells to convert photons into usable electrons. Currently, that extreme quantum efficiency (or EQE) is 100%, meaning that each photon results in one usable electron.
Research from Lehigh University takes this much further. Trying to find the technology for next generation panels, they have created a quantum material that, integrated as an active layer in a solar cell device, gave an 80% absorption and a high rate of photoexcited carriers. The EQE was between 110% and 190% over a wide range of solar waves, including the near-infrared and visible light spectrum. Taking advantage of all possible slots is another objective to improve conversion efficiency.
Promising. Along with this finding, the team observed that, by adjusting the thickness of this active layer, higher optical activity and greater EQE are achieved at wavelengths from 600 to 1,200 nm. The theoretical result is a conversion efficiency to electricity of up to 63%. And that is the truly impressive result, since it is what would allow solar panels to be more efficient in the electricity production process.
The team recognizes that there is work ahead, but that in the industry there are already several well-established ways to improve the EQE and efficiency of panels. It is something that is being achieved thanks to the use of textured surfaces that minimize losses due to reflection, multilayer designs to capture a broader spectrum of sunlight or advanced materials such as new semiconductors or the aforementioned perovskites. They are also focusing on internal quantum efficiency, which is governed by optical losses due to light reflection on the front of the solar cells.
What is clear is that once the price of current solar panels (commercial ones do not usually come close to the theoretical limit of 33.7%) is at rock bottom, the industry is focusing on making them much more efficient. The big advantage? Take advantage of light energy and produce more electricity without exponentially increasing the number of panels installed in photovoltaic plants.
Image | Raysonho
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