Roof tiles are a thing of the past: Today, you can see large black and blue rectangles on more and more Swiss roofs that convert sunlight into electricity. The black-blue colour comes from silicon crystals, as the majority of solar cells available today are based on this semiconductor material. However, silicon is not the only way to produce solar cells – and possibly not the best.
Silicon-based photovoltaic cells are now so advanced that they are reaching the limits of their efficiency. Although a few more percentage points could be achieved, the theoretical upper limit for the efficiency of a single silicon cell is 33 per cent. In practice, it is somewhat lower, as small energy losses inevitably occur during the construction and operation of the cells.
The reason for this limited efficiency is due to the material properties of silicon. The so-called band gap of the material means that only photons with a certain energy can be converted into electricity. If the energy of the photon is too high, it cannot be fully “utilised” by the solar cell.
Two layers are better than one
Solar cells made of other materials offer a way to circumvent this limitation, says Empa researcher Fan Fu. The group leader in the Laboratory for Thin Films and Photovoltaics is researching highly efficient solar cells made of perovskite. A perovskite single cell alone does not achieve a higher efficiency, because perovskite as a semiconductor also has a limited band gap. The real strength of this innovative material lies in the fact that this band gap – unlike silicon – can be controlled by varying the composition of the perovskite material.
If two perovskites with different band gaps are processed into thin-film solar cells and “stacked” on top of each other, the result is a so-called tandem solar cell. One perovskite layer “catches” the photons with high energy, the other those with low energy. This theoretically allows efficiencies of up to 45 per cent to be achieved – significantly higher than the 33 per cent of single cells. Alternatively, a perovskite layer can also be combined with a silicon layer to create a highly efficient tandem cell.
However, Fu and his team are currently mainly researching pure perovskite tandem cells, including as part of the EU research project “SuPerTandem”, in which a total of 15 leading European research institutions and companies are involved. The aim of the project is to develop flexible perovskite tandem modules with an efficiency of over 30 per cent, which can also be produced using scalable and cost-effective processes. This is another strength of perovskite solar cells: “Silicon solar cells usually require high-purity silicon monocrystals that are produced at high temperatures,” explains Fu. “Perovskite thin films, on the other hand, can be printed, vaporised or deposited from solution, with a correspondingly low CO2 footprint. Small defects that occur in the process have little impact on their optoelectronic properties.”
The potential benefits of projects such as “SuPerTandem” are enormous, because the higher the efficiency, the cheaper the solar system will be at the end of the day. “The cell itself accounts for less than 20 per cent of the cost of a PV system,” says Fu. “The remaining 80 per cent is accounted for by the cabling, the inverters, the control system and, of course, the labour required for installation.” If the efficiency of the individual cells is increased, a smaller – and therefore cheaper – PV system is sufficient for the same electricity production. Thin-film cells made of perovskite can also be produced on lightweight flexible films instead of on heavy, rigid glass plates like silicon cells. This means they can also be used in more locations, for example on car roofs or on buildings with a low load-bearing capacity.
From the lab to the roof
This great potential of perovskite solar cells must now be utilised. In addition to “SuPerTandem”, Fan Fu’s team is also working on two Swiss projects. In a project funded by the Swiss National Science Foundation (SNSF), the Empa team is working to better understand the fundamental properties and challenges of perovskite solar cells, which contribute to their efficiency and stability. And in a project with the Swiss Federal Office of Energy (SFOE), they are immediately putting their existing knowledge into practice by scaling up the tandem cells developed at Empa.
What else do we need to do to ensure that the black and blue squares on the roof are soon joined by reddish perovskite films? “First of all, we have to scale up the perovskite cells from the current prototypes of a few centimetres in size to industrial sizes,” says Fu. It is also important to effectively protect the still somewhat sensitive cells from the effects of the weather. The Empa researcher is optimistic that both will be achieved in the next five to ten years. “We are making good progress and there is a lot of interest from industry,” says the scientist. “Research has only been working on perovskite solar cells for just under 15 years. After all, research into silicon cells has been going on for almost 70 years.”
