Technological innovations in increasing solar cell efficiency – Part 2
In this concluding part we shall briefly look at the innovations that improved the solar cell efficiency over the years.
Starting from the 1990’s we see how each innovation improved efficiency incrementally. As we saw earlier, using the data from NREL’s Cell Efficiencies chart, we found that the efficiency had improved by 0.7% point every year. In this hard fought battle to absorb more and more photons in lesser and lesser area, many innovations have played a key role.
We shall look at a few of them today:
1.????? n-type solar cell (1970’s):
N-type solar cells are made from silicon that has been doped with phosphorus, which makes it negatively charged – implying a higher density of electrons. This higher concentration increases the open-circuit voltage. This helps it to generate more electricity from the same amount of sunlight. P-type cells have a boron doping. Boron is more susceptible to Light Induced Degradation (LID) – formation of defects at the junction. Because of phosphorus, N-type cells are less susceptible to light-induced degradation (LID). This improves performance and efficiency.
HIT stands for Heterojunction with Intrinsic Thin Layer. It is a type of solar cell that combines the advantages of both n-type and p-type cells. The heterojunction is a layer of amorphous silicon that is sandwiched between the n-type and p-type layers. The amorphous silicon layer is not doped, implying no electrons and holes. This makes it a passivated layer – meaning it hinders the recombination of electrons and holes. This layer helps to reduce recombination of electrons and holes, which improves the efficiency of the cell. HIT cells have achieved efficiencies of over 24% and they are also relatively inexpensive to manufacture making it a promising technology. It has, however, a complex and specialised manufacturing process.
[1] Image from SANYO HIT-N235SE10 Flier.
PERC or Passivated Emitter Rear Contact is a technology that improves the efficiency of solar cells by passivating, or covering, the rear surface of the cell. The rear surface is made of silicon – a semiconductor. This can lead to recombination of holes and electrons. PERC cells use a process called screen printing to deposit a layer of aluminum on the rear surface of the cell. This layer of aluminum forms a barrier that prevents electrons from recombining. This prevents electrons from recombining before they can be collected, which increases the efficiency of the cell. PERC cells have achieved efficiencies of over 22%.
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Bifacial solar cells collect sunlight from both the front and back of the cell. This is because the back surface of the cell is also reflective, and can reflect some of the sunlight that hits it back into the cell. Bifacial cells can achieve efficiencies that are 10-20% higher than monofacial cells (which only collect sunlight from the front). It also has reduced shading loses. However, Bifacials are expensive to make, because of the silver / aluminium reflective layer underneath the cell. Further, bifacials perform better in direct sunlight than in diffused light. This is because the reflective material on the back surface is less effective in diffused light.
[1] https://www.solarchoice.net.au/blog/news/bifacial-solar-cells-the-two-sides-of-the-story-050515/
TOPcon stands for Tunnel Oxide Passivated Contact. It is a technology that improves the efficiency of solar cells by creating a tunnel oxide layer at the rear surface of the cell. A very thin layer, about 2 nanometres thick layer of SiO2 is deposited on using atomic layer deposition. This ALD can be achieved under lower temperatures. This thin layer of SiO2 allows electrons to tunnel through it, which means that they can pass through the layer without colliding with the atoms in the layer. The tunnel oxide layer helps to reduce recombination by creating a barrier that prevents electrons from recombining with holes – improving cell efficiency. Further, TOPcon cells can be manufactured using same equipment as traditional solar cells. TOPCon cells have achieved efficiencies of over 25%.
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