'Graphene sandwich' unlocks solar cell properties of 2D crystal

Part of a raft of new research into possible uses for the atom-thick sheets of carbon atoms first discovered at Manchester, the new discovery shows how graphene can help harness the properties of light-senstive two-dimensional crystals, which could give rise to devices such as light-sensitive walls that could power whole buildings.

Since Andre Geim and Kostya Novosolev’s discovery that graphene can be made by peeling off single layers of graphite mechanically, several other similar single-layer materials have been found, the team says in a paper published in the journal Science. Moreover, stacking these crystals together with graphene has proved to be a useful way of unlocking the properties of these materials; for example, it’s allowed the team to create materials that could be used in flexible electronics.

The latest research uses a class of materials called transition metal dichalcogenides (TMDCs). These are actually three-layer materials, consisting of a single-atom thick lattice of transition metal atoms between two single-atom layers of sulphur, selenium or tellurium. The bonds within each layer are very strong, but those between the layers are quite weak — a similar structure to graphite. Examples of these materials are tungsten disulphide (WS₂), molybdenum disulphide (MoS₂) and niobium diselenide (NbSe₂).

TMDCs are used industrially as lubricants and to protect surfaces, but they have unusual electronic properties which stem from the way atoms bond to their neighbours within and between the sheets — in particular, they are very good at absorbing light — a 300nm thick film can absorb 96 per cent of the light that shines on it. WS₂ is particularly interesting, the team says, because it is a very stable material and the electronic structure of its atoms allow it to absorb visible light.

This means that it should be a very promising candidate for solar cells, but previous attempts to use it failed because it proved very difficult to make the electrons freed by solar photons to flow out of the material.

Graphene seems to be the key to this problem, acting as a transparent electrode on either side of a layer of TMDC in a three-dimensional crystalline sandwich. The graphene has to be ‘doped’ in the same way as semiconductors in an electronic component — replacing some carbon atoms with electron-rich elements in the layer on one side, and electron-poor elements on the other — and the whole structure sandwiched between layers of another material often used with graphene, hexagonal boron nitride, which stabilises it and enhances its properties.

Using WS2 as the TMDC layer, the team, led by lead author Liam Britnell at Manchester, created a PV material which could be made flexible, by mounting it on a PET film, or rigid, by mounting it on silica. These produce ‘suprisingly large’ photocurrents when a laser is shone on them, the team says: currents of up to 3µA were generated by laser power of around 75µW, and the stack showed an extrinsic quantum efficiency — the ratio of free electrons generated to the number of photons hitting the surface — of around 30 per cent.

‘It was impressive  how quickly we passed from the idea of such photosensitive heterostructures to the working device,’ Britnell commented. ‘It worked practically from the beginning, and even the unoptimised structures showed very respectable characteristics.’

Kostya Novosolev believes this could be the beginning of a new phase of graphene research. ‘We are excited about the new physics and new opportunities which are brought to us by heterostructures based on 2D atomic crystals,’ he said. ‘The library of available 2D crystals is already quite rich. As we create more and more complex heterostructures, so the functionalities of the devices will become richer, entering the realm of multifunctional devices.’