Solar Cells Work Better in Tandem

Solar Cells Work Better in Tandem

- in Cell Processing, Cells, Manufacturing

Stacking two solar cells on top of each other means that solar energy can be ‘harvested’ in two stages, allowing sunlight to be converted into electricity more efficiently. Researchers at Empa, the Swiss Federal Laboratories for Materials Science and Technology, have now come up with a thin-film process for producing such tandem solar cells, in which one of the films comprises a thin layer of perovskite. Because the perovskite layer is fabricated at temperatures of just 50°C, this process could be used for low cost roll-to-roll production of solar cells in the future.

What is true for double-bladed razors is also true for solar cells: two layers are better than one. The top layer of a tandem solar cell is semi-transparent and efficiently converts large energy photons into electricity, while the bottom layer converts the remaining or reflected low energy photons. This allows a larger portion of the light energy to be converted into electricity.

Up to now, tandem cells have been produced by growing the layers on very expensive single crystal wafers, but this is not considered an attractive process for mass production. The research team working under Stephan Buecheler and Ayodhya Tiwari from the Laboratory for Thin Films and Photovoltaics at Empa has now succeeded in making tandem solar cells based on polycrystalline thin films, using a method that is suitable for large area, low cost processing, Flexible plastic or metal foils could also be used as the substrate in future. This marks a major milestone on the path to mass production of high-efficiency solar cells using low cost processes.

As the researchers report in Nature Communications, the secret to the new process is creating the top layer from a perovskite film produced with a low-temperature procedure at just 50°C, offering an energy- and cost-efficient production stage for future manufacturing. The resultant tandem solar cell boasted an efficiency rate of 20.5% when converting light to electricity, but the Empa researchers emphasize that this process has lots more potential to provide even better conversion of sunlight into electricity.

This high conversion rate was achieved through the development of a 14.2% semi-transparent solar cell, with 72% average transparency, made from methylammonium lead iodide deposited in the form of tiny perovskite crystals. The perovskite crystals are grown on a thin interlayer made of PCBM (phenyl-C61-butyric acid methyl ester); each PCBM molecule contains 61 carbon atoms interconnected in the shape of a soccer ball.

The perovskite film is prepared by a combination of vapor deposition and spin coating onto the PCBM layer, followed by annealing at a ‘lukewarm’ temperature. The perovskite crystals absorb blue and yellow wavelengths of visible light and convert these into electricity. By contrast, red light and infrared radiation simply pass through the crystals.

To capture this red light and infrared radiation and convert it into electricity, the researchers attach a further layer underneath the semi-transparent perovskite layer. This layer is made from copper indium gallium diselenide (CIGD), which the team has been researching for years.

A standard solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation energy is lower, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted into heat and lost. By combining substances with differing bandgaps, a tandem solar cell like Empa’s perovskite CIGS cell can efficiently convert a larger share of the incident solar energy to electricity.

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%. That’s according to Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that 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, asserts that the race for efficiency in solar cell research is certainly not just an academic pursuit. “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.”

This story is adapted from material from Empa, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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