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Thursday, 23 Nov 2017

Tandem solar cells using perovskite could reach 30% efficiency

Efficient solar cells are the key to low-cost renewable electricity and ancillary costs are reduced when the solar cells efficient improve which leads to smaller size photovoltaics

23 Nov 2015 | Editor

Researchers at EMPA have come up with a procedure that makes it possible to produce thin film tandem solar cells in which a thin perovskite layer is used. The processing of perovskite takes place at just 50 degrees Celsius and according to the researchers such a process is potentially applicable for low cost roll-to-roll production in future.

The semi-transparent perovskite solar cell absorbs UV, blue and yellow visible light, allowing red light and infrared radiation to pass through. Based on this principle, a double-layer "tandem solar cell" can be built with an efficiency that is much higher than single-layer solar cells.

Stacking two solar cells one on top of the other, where top cell is semi-transparent, which efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity. Up to now, the sophisticated technology needed for the procedure was mainly confined to the realm of Space or Concentrated Photovoltaics (CPV). These "tandem cells" grown on very expensive single crystal wafers are considered not attractive for mass production and low cost solar electricity.

The research team working under Stephan Buecheler and Ayodhya N. Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa-Swiss Federal Laboratories for Material Science and Technology has now succeeded in making tandem solar cells that are based on polycrystalline thin films, and the methods are suitable for large area low cost processing, Flexible plastic or metal foils could also be used as substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells with low cost processes.

EMPA - Perovskite/CIGS tandem solar cell

Figure: EMPA - Perovskite/CIGS tandem solar cell

The researchers created the top solar cell perovskite film via a low-temperature procedure at just 50 degrees Celsius. This the researchers believe holds the promise for energy-saving and cost-saving manufacturing processes.

The tandem solar cell yielded an efficiency rate of 20.5% when converting light to electricity.

The EMPA researchers are confident that it has lots more potential to offer for better conversion of solar spectrum into electricity.

The key to this dual success was the development of a 14.2% efficient semi-transparent solar cell, with 72% average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals.

The perovskite is grown on a thin interlayer made of the substance abbreviated as PCBM (phenyl-C61-butyric acid methyl ester) is used. The perovskite film is prepared by a combination of vapour deposition and spin coating onto this layer, followed by an annealing at a "lukewarm" temperature.

This perovskite crystal absorbs blue and yellow spectrum of visible light and converts these into electricity. By contrast, red light and infrared radiation simply pass through the crystal. As a result, the researchers can attach a further solar cell underneath the semi-transparent perovskite cell in order to convert the remaining light into electricity.

For the lower layer of the tandem solar cell, the EMPA researchers use a CIGS cell (copper indium gallium diselenide). Based on the CIGS cells, small-scale production is already under way for flexible solar cells.

While very good single-layer polycrystalline solar cell may practically convert a maximum of 25% of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30%.

Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory., said, "A lot of research work is needed before that will be possible, what we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells."
Stephan Bücheler, who coordinates the lab research in Tiwari’s team, "When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity."

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