Category: Blog

GaN-based LEDs find widespread applications, but they exhibit maximum efficiency only at very low current. The electrical-to-optical power conversion efficiency drops dramatically with higher input current. This so-called efficiency droop has been investigated for many years, and it still represents a key challenge to solid-state lighting, according to the DOE SSL R&D Plan published in May 2015. 

Trouble is, we often don’t know for sure what causes this efficiency droop. Different microscopic mechanisms have been proposed, most prominently thermionic electron leakage from the light-emitting active layers and Auger recombination inside these layers, respectively. Droop analysis is mainly based on modeling and very few direct measurements of either mechanism are published thus far.

The first direct evidence for Auger recombination in InGaN quantum wells was provided only two years ago by measuring high-energy (hot) electron emission from the LED surface (http://arxiv.org/abs/1304.5469). The authors believe that these hot electrons are generated by Auger recombination inside the active region and subsequently travel all the way to the LED surface. Another experiment was conducted independently by German researchers based on the assumption of a much shorter hot electron travel distance (http://epub.uni-regensburg.de/28841/2/ApplPhysLett_103_071108.pdf).  In that case, hot Auger electrons release their energy quickly and are captured by a neighboring active layer. However, the Auger signal is relatively weak in both cases and there is no direct evidence that the Auger process is strong enough to single-handedly cause the measured efficiency droop.

Electron leakage was first observed in 2008 on ultraviolet LEDs by measuring additional light emission from p-doped layers, which indicates electrons traveling beyond the active region. A few similar reports followed, but none was able to demonstrate a leakage magnitude that fully explains the measured efficiency droop. Interestingly, a group from Korea observed electron leakage when the LED is cooled down to cryogenic  temperatures (http://dx.doi.org/10.1063/1.3703313). The blue LEDs used in this study did not exhibit any leakage at room temperature. This result was quite unexpected since we usually believe that higher temperatures make it easier for electrons to escape from the active region.

Advanced computer simulation was recently able to explain this phenomenon (http://www.nusod.org/piprek/piprek15apl2.pdf). Due to the high ionization energy of Mg acceptors used for p-doping, rising temperatures free more holes and improve the hole conductivity significantly. This was confirmed experimentally by a group from Finland (http://dx.doi.org/10.1109/TED.2015.2391117). As a result, the hole injection into the active layers is enhanced, fewer electrons need to leak out to find holes, and the efficiency rises with higher temperature.

On the other hand, if Auger recombination is causing the droop, the simulations show a declining efficiency with higher temperature.  Thus, the competing efficiency droop mechanisms have the opposite effect on the efficiency when the LED temperature rises. The temperature sensitivity of the LED efficiency therefore offers a simple way to distinguish between both droop mechanisms. Published measurements commonly show a declining efficiency with higher temperature. Thus, Auger recombination can be considered the primary mechanism behind the efficiency droop in these devices.

In fact, I am not aware of any efficiency measurement that shows the opposite trend, i.e., a growing efficiency with rising temperature. Please send me an e-mail if you know of such a case (piprek@nusod.org).

In the post-recession world, it’s still a difficult and long road to success for LED lighting start-ups.  Despite an attractive and growing market, getting new technology to market is difficult and time consuming, leaving little hope for a quick and successful exit.  Yet this is the perfect time for disruptive innovations to take root, as the product design is still evolving.

Start-ups along all parts of the lighting value chain, from basic materials to advanced user interfaces, proves that technology innovation is shaping the digital lighting industry into something that is going to be far different than today.

Start-ups are exploiting the intersections of technologies and are proposing new business models that are forcing the lighting industry to change.  The advent of “digital lighting” has opened up vast potential for start-ups, but there is still trouble getting to real, sustained revenue. Most digital lighting start-ups require development of both hardware and software to realize their concept and that requires more time and money than most venture funding sources are willing to provide. Not to mention that customers expect products to be complete, integrated and “hardened” for commercial use. Lighting already exists for most purposes so potential customers can always stick with the status quo. Your hurdle is to get the chance to demonstrate your new and valuable innovation.  But what’s the path to success?

Major lighting manufacturers have had to rethink the way they’ve always done development and quickly take up and implement solid state lighting technology innovations. LED lighting has created the need to work with, and within, the innovation ecosystem.  Most lighting companies, like OSRAM, have created a specialized group to provide a startup-friendly interface to the company. This new function helps to increase the chances for appropriate consideration of your innovation by the organization.

Working with established lighting companies could be critical as they know about key issues for bringing a lighting product to market such as regulatory, market channels, integration with existing controls (or other infrastructure), performance and design for manufacture.  Very often, start-ups with brilliant teams and solid ideas fail to realize their ultimate success by approaching customers while still missing pieces of the formula.  There is truly a win-win solution through strategic partnerships such as joint development agreements (JDA) that can provide business, technical and financial support for the start-up and benefit the corporation by assimilating innovative technology and genes into the organization.

It can be challenging to work with a major lighting company.  For example, lighting companies have long used IP as a critical business tool and that remains true today.  You should expect negotiation around ownership rights on any IP developed cooperatively.  But they understand that IP is often the only asset that underpins a start-up.  Working with smaller companies, the majors look to create value through speed to market, exclusivity or other creative ways to create an advantage in the market.

Exciting times.  Start-ups with new and disruptive technologies are a sure sign of fundamental change in the lighting industry ecosystem.  But there are many hurdles to bring a successful technology or product to market, and there is a lot to learn and leverage from established companies like their deep understanding of lighting, global market channels, large-scale manufacturing and involvement in the regulatory process.  Working with strategics might not be as hands-off as you’d like but that could be a good thing in the long run.  The rigor and resources could be just what you need to ensure market success.