Around the world, researchers are busy working on ways to improve the efficiency of silicon solar cells. Their latest trick? “Pimp” the glass plate that protects the solar cell with a thin-film solar cell made of perovskite. As a result, they expect to achieve an efficiency of 30%.
Silicon solar cells are just too good
For years, two technologies have existed next to each other: silicon solar cells (Si) and thin-film solar cells such as CIGS, CdTe and perovskites (a calcium titanium oxide mineral). Si-cells have the advantage of higher efficiency, while thin-film cells are less expensive. Although not an issue at module level, the lower efficiency of thin-film technology is still problematic at a system level. It was expected that thin-film solar cells would eventually catch up to their silicon counterparts through lower cost per unit of energy generated. However, it now seems that for the next decade at least, this is likely to be limited to a few specific markets. Silicon solar cells are just too good—they are achieving ever higher efficiencies and costs are getting lower because of the large factories where they are produced. This does not mean that thin-film solar cells will not be important in the future. They are, for example, ideally suited for use in building components such as windows, railings, roof tiles, etc. These building integrated PV (BIPV) applications promise to be the best way to achieve climate-neutral buildings.
Figure 1: The efficiency of silicon solar cells can be increased by adding a thin-film solar cell on top. This stack only makes sense when using thin-film solar cells with a high open circuit voltage.
And thin-film solar cells are also going to cause another disruption, namely by giving silicon solar cells an extra boost in conversion efficiency.
From 27% to 30% conversion efficiency
The maximum conversion efficiency of silicon solar cells is estimated to be 26 to 27%. Combining a silicon solar cell with a thin-film solar cell can increase this to 30%. The idea is to stack the two solar cells—the thin-film solar cell sits on top and uses all light except red and infrared light. The latter frequencies are used by the underlying silicon solar cell (Figure 1). If a thin-film solar cell is used with a higher open circuit voltage than Silicon (more than 760 mV (millivolts)) the stacked solar cell will have a higher efficiency than the silicon solar cell alone.
Figure 2: Thin-film solar cells and silicon solar cells can be stacked in two ways. In the “4-terminal stacked” concept, the thin-film solar cell is grown on a glass plate which is then inversely glued to the silicon solar cell. In the “2-terminal tandem” concept, the thin-film solar cell is grown on top of the silicon solar cell.
Researchers from imec have developed two ways to stack the solar cells (Figure 2). The first concept is the “4-terminal stacked” solar cell. This approach relies very strongly on the way silicon solar cells are made today: a glass plate is bonded to the silicon cell with adhesive. Only this time, it is not an ordinary glass sheet but is instead coated with a thin-film solar cell. As both solar cells are electrically isolated from each other, four contacts are therefore required.
The second concept is the “2-terminal tandem” concept. Here, the thin-film solar cell is directly grown on the Si solar cell. They thus form a single structure through which the charge flows.
A 4- terminal stacked solar cell with perovskite and silicon cells
Thin-film solar cells based on perovskites are a logical choice to test the two stacked concepts. Perovskite solar cells not only promise a high efficiency, but their open circuit voltage can go above 1 volt.
The imec researchers’ initial results are very promising. Although they only started working with perovskite solar cells relatively recently, they have already gained a lot of expertise in stacking cells and creating highly efficient silicon solar cells.
Figure 3: The benefit of perovskite solar cells is that you can tune the bandgap by adjusting the amount of bromine. In this way, you can make a solar cell with a bandgap of around 1.8 eV, which, according to calculations, gives the best combination with a silicon solar cell (if you want to achieve an efficiency higher than that of the silicon solar cell alone).
The researchers calculated that the stacked solar cell would perform best if the thin-film solar cell would have a bandgap of about 1.8 eV (electron volt). The advantage of perovskite solar cells is that the bandgap can be tuned by adjusting the amount of bromine in the material. This allows the bandgap to be varied between 1.55 eV and 2.3 eV (Figure 3).
A perovskite cell was thus made with a bandgap of 1.8 eV. The efficiency of this cell is 9%. Further improvements are still needed to increase efficiency to 16%. This would be necessary to create a stack with 27.5% conversion efficiency. The contact materials also need to be further optimized to increase the efficiency of the stacked solar cell. But everything indicates that this is the way to give silicon solar cells the extra boost that our planet needs.
This article was written by David Cheyns, principal scientist of thin-film solar cells at imec.
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