Super-thin optoelectronic diodes could finally make flexible, wearable tech a reality
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Research into novel materials is usually focused on improving a particular technology, but a trio of just published studies take aim at improving the functionality multiple technologies by making better diodes. This could have huge impacts in all areas of optoelectronics, which encompasses photovoltaic cells, light emitting diodes (LEDs), and photodetectors. Researchers are exploring the possibility of using extremely thin “2D” sheets of semiconducting transition metals to make these devices incredibly thin and flexible.
We have already vastly slimmed down optoelectronic technologies — there are demos of flexible photovoltaics and OLED screens all the time. However, the materials being investigated in these studies are considerably thinner and potentially more useful. A diode just one molecule thick could fundamentally change the way we interact with technology. Displays, sensors, and solar cells could be cheaper and available in more places — even built into your clothing.
The research undertaken at MIT, TU Vienna, and University of Washington focused on a material known as tungsten diselenide. This substance has a crystalline structure made of a single atomic layer of tungsten enclosed by two layers of selenium. This makes tungsten diselenide an extremely durable material even when it’s shaved down to a single layer, which is exactly what the researchers did. Luckily, the same mechanical cleaving process used for graphene works for tungsten diselenide, so there was no problem producing the base of these semiconductor diodes.
A diode is simply a semiconductor device that allows current to only flow in one direction. There are two different classes of semiconducting materials used in diodes — p-type or n-type (essentially positive or negative). You can only have one or the other because the properties of a diode are based on doping the base material with atoms like germanium or boron. These super-thin diodes created from tungsten diselenide are unique because they aren’t stuck being one or the other — these materials can flip between the two classes as needed. Bringing this thin film layer close to a metal electrode allows the charge state to be controlled electrically. At the same time, it can be flexed or folded over to fit on any surface.
The applications for such a material are vast — it can be used to produce all three types of optoelectronic devices. The MIT study went so far as to produce proof-of-concept versions of all three — an LED, a photodetector, and a simple photovoltaic cell [Research paper: doi: 10.1038/nnano.2014.25 - "Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide"]. The LED produces a tiny amount of light, and the solar cell only had an efficiency of about 0.2%, but it all used a single material tuned without chemical doping.
If scientists can perfect tungsten diselenide diodes we could see a range of technologies suddenly become workable. Flexible transparent displays based on this research could be built into vehicle windshields or smart devices like glasses or contact lenses — Google Glass is a little bulky anyway, right? We could also finally have flexible solar cell technology that’s cheap enough to apply to any suitable surface. If safety can be assured, these technologies could even be worn around on the skin. Who wouldn’t want a lick-and-stick arm display with built-in solar panel?