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Marrying two sub-cells, often of different materials, to improve cell efficiency in moving sunlight to energy is called a tandem cell. Relegated in the past to applications that use optics and large amounts of direct sunlight, such as space applications, tandem cells are no longer that. They are becoming more basic in construction with some companies combining a more traditional photovoltaic material such as silicone with perovskite.
For example, Oxford Photovoltaics of Oxford, UK is already testing perovskite with both silicon and copper indium gallium selenide (CIGS) solar cells in which the perovskite top absorbs visible light but transmits infrared and near infrared light to the CIGS bottom. Tests showed a 20% relative improvement in electricity generated.
Several groups have demonstrated working tandem devices made of a silicon cell and a perovskite cell, but the efficiencies have been limited because the range of the solar spectrum that the perovskite absorbed did not align with the range of the silicon. Attempts to tweak the range of light the perovskite absorbed led to instabilities within the material’s structure that compromised performance.
Oxford came up with a method that relies on substituting certain ions in the material with others made of cesium to achieve desired photovoltaic properties while maintaining structural stability.
Oxford is aiming to release a tandem cell to silicon panel manufacturers in 2017.
MIT and Stanford University
This is not the only tandem type that can be commercialized. Any use of two to even three materials of different band gaps using one control circuit can be engineered to absorb and convert more energy. In earlier versions of tandems, different cell parts contained their own wiring and control circuits to allow the parts to be tuned independently. By contrast, the new combined versions should be much simpler to make and install.
For example, graduate students at the Massachusetts Institute of Technology (MIT) and Stanford University, led by MIT Associate Professor of Mechanical Engineering Tonio Buonassisi, PhD, added to a layer of silicon to a semi-transparent layer of perovskite that can absorb higher-energy particles of light. Both layers are connected together as a single device that requires only one control circuit. This new design could be both simpler to make and install, the researchers stated.
Image Credits: Felice Frankel/MIT
Specifically to make them, a layer of methylammonium-lead(II)-iodide perovskite is stacked on top of crystalline silicon. The entire cell was one square centimeter in size and consisted of a 200-micron-thick silicon layer topped with a one-micron-thick perovskite layer. The device also incorporates layers of other materials on top of and between the perovskite and silicon to assist with the flow of electric charge
Perovskite absorbs higher-energy visible photons, while the silicon absorbs lower-energy infrared photons. According to the researchers, dividing the spectrum of sunlight between specialized absorbing layers is more efficient than letting a single layer attempt to convert the entire spectrum by itself.
Improving the Tandems Efficiency
One tradeoff is still that the current capacity is limited by the lesser of the two parts. To address this, there is work underway to optimize the output of the proof-of-concept cell.
In the initial version, the efficiency is 13.7%, but the researchers state that the technology could ultimately achieve a power efficiency of more than 35%. Another hurdle they are working on is creating a durable enough material for commercial manufacture, given that perovskite corrodes in moisture and air and needs to be modified for production use.
Mirroring this work was the 2015 Ph.D. thesis of Alice Furlan, Ph.D. in the Molecular Materials and Nanosystems group, under Professor René Janssen at the University of Technology Eindhoven. She tested how to best combine two or three different layers of semi-conductive plastic, looking at the electrical connections between the different layers to measure where losses were the greatest. Ultimately combining plastic cells within thin layers of amorphous silicon yielded an efficiency of 13.2%.
Overall, plastic is a strong absorber of infrared light, while silicon converts well the light from the visible and ultraviolet spectrum.
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