Inverse charge funnelling in a two-dimensional semiconductor paves the way to highly efficient solar cells
The amount of energy received from the Sun by the surface of Earth in one hour, if converted into electricity, would power the whole world for one day. However, such energy conversion cannot be realised at present due to physical limitations in solar cells. Solar panels work by converting the energy of the light into electrical charges inside a material (typically a semiconductor such as Silicon), which are then extracted by metal electrodes into an external circuit, such as a battery or a mobile phone. Current technology for Silicon solar cells allows only 24% of the energy of the impinging light to be converted into electricity. This is due to fundamental limitations which impose a theoretical maximum efficiency of 34%, known as Shockley–Queisser limit.
At the University of Exeter we pioneered a novel technique which could overcome the aforementioned limits. The physical phenomenon, known as “charge-funnelling”, allows the efficient extraction of photo-generated charges into an external circuit using a non-uniform band-gap profile in a strain-engineered semiconductor. A simple analogy helps explain the concept. If we try to fill a bottle with a narrow neck (our battery) with a liquid (our energy) by simply pouring it from a bucket (the Sun) we will not get much in the bottle. Conversely, if we place a funnel on top of the bottle we obtain a much more efficient transfer of liquid. This is exactly what we do in our work, using a material called Hafnium Disulphide (chemical formula HfS2). The funnel is created by modifying the composition of the material using an intense laser light. Such modification induces strain in the material which, in turn, modifies the energy of the bands where the electrons move, creating the equivalent of a charge funnel.
Charge funnelling can enable so-called “hot-carrier” solar cells with a power conversion efficiency above 60%. This would allow to power entire cities with a fraction of the space (and cost) nowadays required with silicon-based solar panels. Furthermore, the use of atomically-thin materials (materials which are between one and three atoms thick) will enable flexible and wearable solar cells, which could be embedded into fabric products such as clothing, parasols, umbrellas, etc.
Future research will focus on developing a scalable technique to engineer charge funnelling in large-scale devices. This comes with the challenge of materials production and fabrication compatibility with existent technology.
More about our discovery can be found this short video:
You can read the full paper in Nature Communications at the following link (open access): https://www.nature.com/articles/s41467-018-04099-7