Growing lightbulbs: how Sheffield and Nanjing are sowing sustainable energy solutions
Individually fragile, the incandescent lightbulb is nonetheless a dauntingly robust technology. Since its popularisation by Edison in the nineteenth century it's spread, carried on the wind around the world: a wildly inefficient technology that still accounts for over $1 billion of revenue a year - and with warm colour representation that people love.
But with China nearing the end of a five year plan to phase out incandescents, the light is going out. 1.35 billion people need a safe, sustainable, way to illuminate their homes, offices, shops and schools.
So what's the alternative?
The Joint Centre for Wide Bandgap Semiconductor Optoelectronics at the University of Sheffield brings together Chinese and UK expertise in service of designing novel semiconductor materials and device technologies. Central to this research is the design of different crystal orientations for LEDs producing sunlight-quality bulbs that are farmed as much as manufactured.
We spoke to Professor Tao Wang, Professor of Advanced Optoelectronics and Head of the Gallium Nitride Centre for Materials & Devices at the University of Sheffield, Doctor Rick Smith - Research Fellow in the Department of Electronic and Electrical Engineering, and researcher at the Gallium Nitride Centre for Materials and Devices and Doctor Yaonan Hou – Research Associate at the Gallium Nitride Centre for Materials and Devices - about the inception, growth and glowing future of sustainable light technology.
This is why the joint centre is very important. If you combine both UK and Chinese students together, encourage them to work together, then we will have discussions, talk to each other, and perhaps work out a different way. It's good for both sides.
Professor Tao Wang
TAO WANG, HEAD OF CENTRE – “The problem is with Edison-style lightbulbs.”
RICK SMITH, RESEARCH FELLOW - “An Edison-style lightbulb is a way of generating heat. Because the metal is glowing white hot it emits light. It emits loads and loads of infrared light – heat, basically - and a tiny fraction of that is visible. So you end up with a couple of percent of visible light coming off.
TAO - “Fluorescent and incandescent lighting tubes are inefficient and their lifetime is not long. We're looking to use semiconductors to create an extremely efficient solid state lighting source to replace them. And not just efficient. Very robust. A single light could last forever...”
RICK - “'Solid state' describes the interaction between electrons and photons. We're interested in getting electrons to give off photons of the right colour. And that happens within a solid – within a crystal. So everything that happens within these crystals is a solid state process.
What we're looking at in solid state lighting is an alternative to getting something hot to make it emit light – we definitely don't want things to get hot. An efficient device will be cool to the touch.”
TAO - “In principle you can achieve a very useful efficiency conversion – put electrical energy in, get optical energy out. It's the ultimate in energy efficiency.”
RICK - “When most people hear 'semiconductor' they think silicon, integrated circuits, large chips performing electronic functions, maybe solar cells. But it's the compound semiconductors – gallium nitride, indium gallium nitride - that we're interested in. These are organic semiconductors.”
TAO - “Our research group is called The Centre for Gallium Nitride Materials and Devices. III-Nitride materials include gallium nitride, aluminum gallium nitride and indium gallium nitride.”
RICK - “We're growing semiconductors in the form of single crystals on a piece of sapphire. We apply a voltage to these crystals to pump electrical energy in, in the form of electrons. The electrons move through this epitaxial structure and they lose a bit of energy. If we can get them to lose a bit of energy in the right layer of the device, the energy that they lose comes out as light. It's not infrared, it's not heat – it's useful light at the colour that we want. And by structuring the composition of these layers we can tune what colour and what wavelength the light comes out.”
TAO - “We're growing this semiconductor material layer by layer, atom by atom. One atomic layer followed by an another atomic layer, stacked up. Around twenty atomic layers, so we will maybe have a structure that is two nanometers thick.”
RICK - “You can think of this kind of material growth like geodes in a cave. Geodes can grow over millennia but we're growing different types of materials in highly controlled lab environments.”
TAO – “It depends on the structure but typically it takes five hours to grow a wafer LED including preparation.”
RICK - “There's a problem though. One of the weaknesses of organics is stability. The promise of these reel to reel, printable, large scale semiconductor devices made out of organic materials hasn't quite been fulfilled yet, in part because of the stability.”
TAO - “Physics tells you what happened. The mechanisms behind things. And it's attractive to find the reason behind things. That's the sort of thing I try to understand.”
RICK - “This is the world of crystallography. Semiconductors are like silicon – cubic crystals – so the atoms are arranged in a cubic lattice. Gallium Nitride is a hexagonal material so the atoms align in hexagons. So you have hexagons stacked on top of hexagons. The atoms are arranged in a three dimensional hexagonal shape.
Most LEDs are grown in a certain direction – the 'c-direction'. These materials grow easily in this direction and the defect density is reasonably low. But there are other problems with growing them in this direction.”
TAO – “At the moment, when you buy a white LED you're generally getting a blue LED with yellow phosphor applied. The phosphor absorbs some of the blue light, converts it to a slightly longer wavelength and this gives the overall impression of a white spectrum. You can get very high efficiency for blue light using the c-direction. But if you go to green or yellow, suddenly the efficiency drops to almost zero. This is what's called the green-yellow gap.
Why is this a problem? Light quality. Natural sunlight consists of seven colours and those seven colours together generate excellent quality of light. We use the colour rendering index to describe the quality of light. Sun is 100%. The colour rendering index of commercial LEDs right now is often less than 70%. Stay in these lights for a long time and you'll feel queasy and eventually it will generate psychological effects.
YAONAN HOU, RESEARCH ASSOCIATE - “There was a time in my childhood when I always played - when China was not as developed as it is now. And I’m from a village so lots of boys and girls played together. But one day I wandered further and laid down by myself in a field of wheat. I was laid there by myself looking at the sky. At that time the air was not polluted. It was totally blue, like in the UK. And suddenly finding the sun in the sky was quite beautiful. These lights above us [indicates florescent ceiling lights] use phosphors. People developed them to be energy saving but they’re not great for our eyes.”
TAO - “The colour and quality of the light we use is very important for us as human beings. Of course efficiency is extremely important but we also have to think about the quality of the light. Natural sunlight is the best. Other universities are still working on the c-plane. But right now we are moving onto the semipolar gallium nitride, which uses the m-plane. I call this the next generation of gallium nitride.”
RICK – “The green-yellow problems are c-plane's problems. They don't exist at all in m-plane.”
TAO - “We can get yellow colour using m-plane by tuning it with indium. This isn't something you can easily do with c-plane gallium nitride.
So m-plane is better in terms of health and also the colour rendering. You can use m-plane to get high efficiency yellow colour. But m-plane is difficult to create.”
YAONAN - “Tao has strong perseverance. He can stick on his research and concentrate on his research. Many top universities’ research groups finally gave up researching semipolar growth because it’s really really difficult to synthesise this semipolar gallium materials with high efficiency. He stuck with it for five years and finally got that in the last year.”
TAO - “We started semipolar gallium nitride growth in the year 2008 – seven years ago. At that time most people wanted to build blue LEDs. For the blue LED, c-plane nitride should be good enough but while people are focussed on how to improve the efficiency, they never think about the quality of the light.
I believe with technological improvement, efficiency will naturally go up and up and up. But it's not useful to have a solution that is efficient but harmful. This is a scientist's job. We have to look at something that industry would possibly think about later. Not right now. Industry just want to sell high efficiency high emitting diode products to make money. But as a scientists we have to think further. I think this is our duty. To think about the quality of the light...”
RICK - “This is a very international research area. We're benefitting from blue sky research done in Japan 25 years ago. Their breakthroughs opened up this area that's done all over the world.
We live in a global community now. Research is moving so quickly we communicate through letters rather than journals. The complementary expertise and skills of Sheffield and Nanjing means we can approach a problem from more directions than if we possibly could if we were isolated.”
YOUNAN HOU - “In this centre we have different subgroups. We have the 'growers' – the folks on material growth. We also have a subgroup for device fabrication. We fabricate different kinds of devices - like solar cells for electrodes, laser diodes, LEDs. We also have a characterisation group that focuses on the photonic properties of the materials and devices...”
RICK - “Having that full cycle is one of our benefits compared to other research groups in the UK. But that can be massively expanded by what our partners in China can do with their process. But for me it's more than just the lab. It's about a working relationship.”
TAO - “This is why the joint centre is very important. If you combine both UK and Chinese students together, encourage them to work together, then we will have discussions, talk to each other, and perhaps work out a different way. It's good for both sides.”
Science and technology should bring people better lives and not be harmful. I see it as scientists' duty to lower or minimise these sort of things.
Dr Yaonan Hou
YOUNAN HOU – “I graduated from the Institute of Physics at the Chinese Academy of Sciences three years ago, researching high performance optoelectronic devices like LEDs and laser components and energy harvesting devices. Things were already developing quickly when I graduated. I wanted to research these solid state lighting. III-nitrides seemed the most likely candidate for this.”
TAO - “What I judge students on is whether they can make a contribution to society.
We don't just want to create a new technology here in Sheffield. We're not just working with industry on knowledge transfer. We also want to do the fundamental science - to understand the science behind this research. To further our understanding.
My ideas will work with industry to do the knowledge transfer to secure the funding in order to support the fundamental science. We want to do science and technology both together, not just one.”
RICK – “You have to link back to the fundamental science. Understanding the effect a certain technology has on affecting the quality of a material. You have to view that through the fundamental science.”
TAO – “Right now students want to do MBAs (Master of Business Administration) and make money. Completely different from our generation – we wanted to pay loyalty to the country and contribute to the country – we have to do science and technology to make the country become more...
As I become older I'm trying to think about my duty as a human being and as a scientist. How to contribute to society. Not just to think about yourself.”
YAONAN - “Science and technology should bring people better lives and not be harmful. I see it as scientists' duty to lower or minimise these sort of things.”
RICK – “The thing about nitride semiconductors is it’s a newer research area. We had an explosion – silicone generated the technology that enabled the information age. Then other compound semiconductors enabled infrared so optical communication techniques, vast connectivity through optical fibre networks.
Nitrides are a much younger material class – we're talking early 90's for the breakthroughs that enabled this research area. So you answer a question and open up significantly more.
We're still at a stage of discovery. Obviously a project is focussed on end goals, so we've got ideas about improving the efficiency of lighting, about generating ultraviolet emitting LEDs that have higher efficiency than current ones, we're looking at solar energy harvesting to generate hydrogen from water, hybrid organic/inorganic devices to get higher efficiency devices. But I don't view research as having an end goal. The journey is the joy of it.”
YAONAN - “When I was small I was always staring at the light in my room. An incandescent light – red, orange. And when my father turned off the light I would immediately just start crying. And I would just keep crying until he turned on the light again.
Currently we’re fabricating gallium materials into devices with emission lighting from green and yellow to amber. You can have blue, green, amber together to create white light. And the light is quite beautiful. This is phosphor free, all III-nitrides materials-based LEDs. Next step is to integrate them together to form a single cheap light emitting diode. This is sustainable. Efficient. And even more beautiful than the light I remember as a child.”
Find out more
Joint Research Centre for Wide Bandgap Semiconductor Technologies – a collaboration between the University of Sheffield in the UK and Nanjing University in China.