We use it every day. It helps us to see and helps us do things and helps us move around in the world. We’re completely dependent on it for so many things yet we take it completely for granted. In fact, it’s hard to imagine what life was like before it existed.
It’s the simple light bulb. An invention so great that a sudden brilliant idea is actually called a “light bulb moment.”
The often revered, and sometimes reviled, Thomas Edison played an important part in developing the first light bulb but even that wasn’t a light bulb moment. His patent for an improved electric light came after 75 years of hard work by several scientists and engineers, all scrambling to find the best way to run electrical current through a filament and get it to glow. His invention was so successful that it hardly changed in the past one hundred years.
Today there’s a better way to do it.
Light bulbs based on light-emitting diode (LED) technology are already ten times more energy efficient and last twenty times as long compared to old-fashioned Edison-style incandescent bulbs. Researchers in the Materials Department at UC Santa Barbara are working hard to get even more bang for the buck from these high-tech light sources.
A small team lead by Professors James Speck and Claude Weisbuch from the Center for Energy Efficient Materials (CEEM), along with collaborators at École Polytechnique in Paris, have developed a new technique to tackle one of the trickiest technological mysteries for LEDs: efficiency droop. Their recent discovery could have very exciting implications in terms of how we think about and use this new way of making light.
But just like Edison’s tricky filament, the devil is in the details.
An LED works by passing electric current, a stream of electrons, through layers of semiconductor material called a diode. In a perfectly efficient LED, every electron passing through the diode releases its energy in the form of light.
How an electron creates light at all involves some magic of quantum physics. Albert Einstein won the Nobel Prize in 1921 for explaining the so-called photoelectric effect. Magic aside, it suffices to say that if you want to get as much light out of an LED as possible, you must account for the electrons.
In a real LED, not every electron does it’s supposed to. You apply more and more current and the LED doesn’t emit as much light as it should. The device actually gets less efficient the harder you turn it on. The efficiency, for lack of a better word, droops. This is a problem for LED bulb designers who want to squeeze the most light out of each chip. It’s especially a problem if they want to replace Edison-style bulbs which, despite being really inefficient, happen to be really bright.
Where are the electrons in an LED going if they’re not making light? This is exactly the question UCSB researchers have been trying for years to answer.
Shuji Nakamura, currently a Professor in UCSB’s Materials and Electrical & Computer Engineering Departments and co-director of the Solid State Lighting & Energy Center with Professor Steven DenBaars, has played an important role in seeing these tiny light emitters go mainstream. While still a researcher in Japan in the late 1990’s, Nakamura was the first to demonstrate a modern blue LED using electrically injected diode made of a semiconductor alloy called Gallium Nitride (GaN). Three years later he was also one of the first to observe efficiency droop. For two decades, the cause of LED efficiency droop has been hotly debated.
One explanation, theorized in 2011 by UCSB Professor Chris van de Walle and his research team, puts the blame on misbehaving electrons. Instead of releasing their energy as light, some diode electrons transfer their energy to another electron instead. Think of billiard balls colliding in a game of pool, as one ball transfers motion to another. These pesky energetic electrons are called hot electrons or Auger electrons. The more Auger electrons, the less light you get. They end up eventually losing their energy through heat.
There have been several experiments trying to prove the existence of Auger electrons in an LED, but actually measuring them has been nearly impossible. Yet Speck, Weisbuch and their collaborators have managed to directly measure Auger electrons for the very first time.
Justin Iveland, a Materials graduate student who worked on this project for the past two years, joked that the most important piece of equipment was the transatlantic airliner that let him travel to Paris to collaborate with researchers in the Laboratoire de Physique de la Matière Condensée at École Polytechnique. The French-based research team has over 30 years experience in taking the kind of careful electrical measurements that this experiment required.
It wasn’t easy. It required very carefully prepared samples under a really high vacuum, to start. The hardest part of all? “Getting everything right,” said Weisbuch. Yet their success “illustrates the benefit of teamwork through both an international collaboration and a DOE Energy Frontier Research Center,” he explained in a UCSB press release.
With persistence and hard work, the team was able to tune the sample and equipment just right to detect Auger electrons. The higher they pumped the LED, the more the efficiency drooped, and the more Auger electrons they measured. Professor Speck, the Seoul Optodevice Chair in Solid State Lighting at UCSB, explains, “it’s the first direct measurement of Auger electrons in any semiconductor. Based on our data and analysis it offers direct proof that Auger is the dominant mechanism for LED droop.” They call this kind of discovery unambiguous, which is possibly a nicer way to say: “I told you so.”
Although LED makers have engineered some ways to work around the droop problem, not knowing why it happens in the first place has been a big hurdle. This is why this latest discovery is so important. By finally identifying the cause of the efficiency droop problem as Auger electrons, engineers and scientists can focus on actually solving it.
It’s amazing to think that the first demonstration of the modern LED happened only 20 years ago. Today you could walk into the Apple Store and shell out $60 for a tricked-out LED bulb from Philips that changes colors with an iPad app. Or you could walk into Home Depot and pick up a decent LED bulb for $10. They even come in a pleasing array of colors like “warm white,” unlike the compact fluorescent bulbs everyone loves to loathe.
In just a few more years the options will be brighter, cheaper and more adaptable, thanks to contributions like those made in research labs around the world, even right here in Santa Barbara.
Most technological breakthroughs don’t depend on light bulb moments. Thomas Edison himself said: “Genius is one percent inspiration, ninety nine percent perspiration,” a testament to the dedication and hard work that scientific discoveries require.
The next breakthrough in LEDs? Trust me, they’re working on it.