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Brewing an explanation for LED droop
Materials Today Volume 16, Number 5 May 2013
NEWS
News Brewing an explanation for LED droop Light-emitting diodes (LEDs) work less efficiently at high drive currents and now an international team has confirmed what is to blame: Auger recombination. James Speck and Claude Weisbuch of the Center for Energy Efficient Materials at University of California, Santa Barbara, and colleagues at CNRS-E´cole Polytechnique in France, explain that the well-known phenomenon of LED ‘‘droop’’ in which the devices light emission increases sub-linearly as the current is raised above a threshold. The droop effect has held back the adoption of LEDs as a viable lighting alternative to incandescent and fluorescent bulbs because the cost per lumen is too high as many LEDs operating at low current are being used in a single bulb to keep the efficiency high. Scientists had plenty of theories as to the origins of LED droop, with a leading one being that a complex non-radiative process known as Auger recombination caused droop in nitride semiconductor LEDs, first suggested by Lumiled’s Shen and colleagues, and shown theoretically plausible by UCSB’s Van de Walle and colleagues. In this process collisions between injected electrons and holes lead to the generation of hot electrons instead of photons, which
Credit: E´cole Polytechnique Ph. Lavialle.
further leads to a loss of the excess electron kinetic energy by chip heating. The research team has now demonstrated this phenomenon using an LED with a specially prepared surface – a caesiated InGaN/ GaN LED – that allowed them to directly measure the energy spectrum of electrons inside the LED once they are emitted outside the LED into vacuum. The results showed unambiguously the signature of energetic electrons associated with the Auger process. ‘‘The drop in electroluminescence efficiency
shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process,’’ the team reports [Iveland et al., Phys. Rev. Lett. 110 (17) (2013) 177406]. The team suggests that the new insight into LED droop will help technologists design a new class of LED that will have significantly higher light emission efficiencies. Such devices have great potential for domestic and commercial lighting that is bright but uses a lot less electricity than incandescent bulbs and even than low-wattage compact fluorescent tubes. Estimates suggest that the US alone could reduce its power production by 50 GW if LEDs were to replace conventional lighting across the nation. ‘‘This was a very complex experiment – one that illustrates the benefits of teamwork through both an international collaboration and a DOE Energy Frontier Research Center,’’ Weisbuch says. He is based at UCSB but also a CNRS faculty member at Polytechnique and enlisted the support of Lucio Martinelli and Jacques Peretti there. UCSB graduate student Justin Iveland was a key member of the team also working both at UCSB and CNRS. David Bradley
Size is everything when it comes to batteries Batteries just a few millimeters in size could be packed into a box the size of a mobile phone and jump start a dead car battery or recharge a phone in seconds, according to US researchers [King et al., Nat. Commun. (2013) doi:10.1038/ncomms2747]. William 158
King and colleagues at the University of Illinois Urbana-Champaign explain that their microbatteries out-perform even the best supercapacitors and could revolutionize compact, mobile electronics and communications.
The team reports a new class of lithiumion battery that exploits the battery’s microscopic internal architecture to simultaneously optimize ion and electron transport for high-power delivery using threedimensional interdigitated electrodes. The
1369-7021/06/$ - see front matter http://dx.doi.org/10.1016/j.mattod.2013.05.009
NEWS
News Brewing an explanation for LED droop Light-emitting diodes (LEDs) work less efficiently at high drive currents and now an international team has confirmed what is to blame: Auger recombination. James Speck and Claude Weisbuch of the Center for Energy Efficient Materials at University of California, Santa Barbara, and colleagues at CNRS-E´cole Polytechnique in France, explain that the well-known phenomenon of LED ‘‘droop’’ in which the devices light emission increases sub-linearly as the current is raised above a threshold. The droop effect has held back the adoption of LEDs as a viable lighting alternative to incandescent and fluorescent bulbs because the cost per lumen is too high as many LEDs operating at low current are being used in a single bulb to keep the efficiency high. Scientists had plenty of theories as to the origins of LED droop, with a leading one being that a complex non-radiative process known as Auger recombination caused droop in nitride semiconductor LEDs, first suggested by Lumiled’s Shen and colleagues, and shown theoretically plausible by UCSB’s Van de Walle and colleagues. In this process collisions between injected electrons and holes lead to the generation of hot electrons instead of photons, which
Credit: E´cole Polytechnique Ph. Lavialle.
further leads to a loss of the excess electron kinetic energy by chip heating. The research team has now demonstrated this phenomenon using an LED with a specially prepared surface – a caesiated InGaN/ GaN LED – that allowed them to directly measure the energy spectrum of electrons inside the LED once they are emitted outside the LED into vacuum. The results showed unambiguously the signature of energetic electrons associated with the Auger process. ‘‘The drop in electroluminescence efficiency
shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process,’’ the team reports [Iveland et al., Phys. Rev. Lett. 110 (17) (2013) 177406]. The team suggests that the new insight into LED droop will help technologists design a new class of LED that will have significantly higher light emission efficiencies. Such devices have great potential for domestic and commercial lighting that is bright but uses a lot less electricity than incandescent bulbs and even than low-wattage compact fluorescent tubes. Estimates suggest that the US alone could reduce its power production by 50 GW if LEDs were to replace conventional lighting across the nation. ‘‘This was a very complex experiment – one that illustrates the benefits of teamwork through both an international collaboration and a DOE Energy Frontier Research Center,’’ Weisbuch says. He is based at UCSB but also a CNRS faculty member at Polytechnique and enlisted the support of Lucio Martinelli and Jacques Peretti there. UCSB graduate student Justin Iveland was a key member of the team also working both at UCSB and CNRS. David Bradley
Size is everything when it comes to batteries Batteries just a few millimeters in size could be packed into a box the size of a mobile phone and jump start a dead car battery or recharge a phone in seconds, according to US researchers [King et al., Nat. Commun. (2013) doi:10.1038/ncomms2747]. William 158
King and colleagues at the University of Illinois Urbana-Champaign explain that their microbatteries out-perform even the best supercapacitors and could revolutionize compact, mobile electronics and communications.
The team reports a new class of lithiumion battery that exploits the battery’s microscopic internal architecture to simultaneously optimize ion and electron transport for high-power delivery using threedimensional interdigitated electrodes. The
1369-7021/06/$ - see front matter http://dx.doi.org/10.1016/j.mattod.2013.05.009