news digest ♦ Novel Devices


is added. “The holes move inside the tungsten diselenide layer, the electrons, on the other hand, migrate into the molybednium disulphide”, says Mueller. Thus, recombination is suppressed.


This is only possible if the energies of the electrons in both layers are tuned exactly the right way. In the experiment, this can be done using electrostatic fields. Florian Libisch and Joachim Burgdörfer (TU Vienna) provided computer simulations to calculate how the energy of the electrons changes in both materials and which voltage leads to an optimum yield of electrical power.


Tightly packed layers


“One of the greatest challenges was to stack the two materials, creating an atomically flat structure”, says Thomas Mueller. “If there are any molecules between the two layers, so that there is no direct contact, the solar cell will not work.” Eventually, this feat was accomplished by heating both layers in vacuum and stacking it in ambient atmosphere. Water between the two layers was removed by heating the layer structure once again.


Part of the incoming light passes right through the material. The rest is absorbed and converted into electric energy. The material could be used for glass fronts, letting most of the light in, but still creating electricity. As it only consists of a few atomic layers, it is extremely light weight (300 square meters weigh only one gram), and very flexible. Now the team is working on stacking more than two layers - this will reduce transparency, but increase the electrical power.


2D materials


Ultra-thin 2D materials, which consist only of one or a few atomic layers are a hot topic. Research on such materials started with graphene, which is made of a single layer of carbon atoms. Mueller and his team applied their knowledge gained in handling, analysing and improving ultra-thin layers of graphene to other ultra-thin materials to do this work. The team was the first to combine two different ultra-thin semiconductor layers and study their optoelectronic properties.


A hybrid form of perovskite - the same type of material which has recently been found to make highly efficient solar cells - has been used to make low-cost, easily manufactured LEDs, potentially opening up a wide range of applications such as flexible colour displays.


This class of semiconducting perovskites have generated excitement in the solar cell field over the past several years, after Henry Snaith’s group at Oxford University found them to be remarkably efficient at converting light to electricity. In two years, perovskite-based solar cells have reached efficiencies of nearly 20 percent, a level which took conventional silicon-based solar cells 20 years to reach.


Now, researchers from the University of Cambridge, University of Oxford and the Ludwig-Maximilians- Universität in Munich have demonstrated a new application for perovskite materials, using them to make high-brightness LEDs. The results are published in the journal Nature Nanotechnology.


Perovskite is a general term used to describe a group of materials that have a distinctive crystal structure of cuboid and diamond shapes. They have long been of interest for their superconducting and ferroelectric properties. But in the past several years, their efficiency at converting light into electrical energy has opened up a wide range of potential applications.


The perovskites that were used to make the LEDs are known as organometal halide perovskites, and contain a mixture of lead, carbon-based ions and halogen ions known as halides. These materials dissolve well in common solvents, and assemble to form perovskite crystals when dried, making them


150 www.compoundsemiconductor.net Issue VI 2014


Perovskite semiconductor shows promise for low cost


LEDs Material can be easily tuned to emit light in a variety of colours, say researchers


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