Info
Info
News Article

Illuminating The Way To Brighter LED Solid-state Lighting

Density functional theory indicates that the rigidity of the crystalline host structure is a key factor in the efficiency of phosphors used to create white LEDs. Currently, the best phosphors possess a highly rigid structure


By determining simple guidelines, researchers at UC Santa Barbara's Solid State Lighting & Energy Centre (SSLEC) have made it possible to optimise phosphors – a key component in white LED lighting – allowing for brighter, more efficient lights.


"These guidelines should permit the discovery of new and improved phosphors in a rational rather than trial-and-error manner," says Ram Seshadri, a professor in the university's Department of Materials and Department of Chemistry and Biochemistry.


The results of this research, performed jointly with materials professor Steven DenBaars and postdoctoral associate researcher Jakoah Brgoch, appear in The Journal of Physical Chemistry.





The researchers behind the breakthrough, from left to right : Steve DenBaars, Jakoah Brgoch and Ram Seshadri


LED lighting has been a major topic of research due to the many benefits it offers over traditional incandescent or fluorescent lighting. LEDs use less energy, emit less heat, last longer and are less hazardous to the environment than traditional lighting. Already utilised in devices such as street lighting and televisions, LED technology is becoming more popular as it becomes more versatile and brighter.


Most, if not all of the recent advances in solid-state lighting (SSL) have come from devices based on GaN LEDs.


In solid-state white lighting technology, phosphors are applied to the LED chip in such a way that the photons from the blue GaN LED pass through the phosphor, which converts and mixes the blue light into the green-yellow-orange range of light. When combined evenly with the blue, the green-yellow-orange light yields white light.


Seshadri explains that for a good phosphor "we mean something that is efficient, something that takes the blue photons that go in and push out yellow or orange photons and the same number of them rather than wasting some of the blue photons. The challenge is two-fold - not just having a material that is efficient at room temperature - but also a material that retains its efficiency at elevated temperatures. And our studies suggest a solution to both."


Art to science


Until recently, the preparation of phosphor materials was more an art than a science, based on finding crystal structures that act as hosts to activator ions, which convert the higher-energy blue light to lower-energy yellow/orange light.





This illustration demonstrates how bright blue LED light, shone through its complementary yellow phosphor, yields white light


"So far, there has been no complete understanding of what make some phosphors efficient and others not," Seshadri notes. "In the wrong hosts, some of the photons are wasted as heat, and an important question is: How do we select the right hosts?"


As LEDs become brighter, for example they are used in vehicle front lights, they also tend to get warmer, and, inevitably, this impacts phosphor properties adversely.


"Very few phosphor materials retain their efficiency at elevated temperatures," Brgoch says. "There is little understanding of how to choose the host structure for a given activator ion such that the phosphor is efficient, and such that the phosphor efficiency is retained at elevated temperatures."


However, using calculations based on density functional theory, which was developed by UCSB professor and 1998 Nobel Laureate Walter Kohn, the researchers have determined that the rigidity of the crystalline host structure is a key factor in the efficiency of phosphors.


Sheshadri says, "We have found through a combination of computational studies as well as experimental studies that the best phosphors have rather rigid structures."


The images below show examples of a rigid phosphor which is highly connected  and a phosohor where the units  are not connected and the structure is what Sheshadri describes as "somewhat floppy."





Rigid connected phosphor      (Credit UCSB)





"Floppy" unconnected phosphor  (Credit UCSB)


This new breakthrough will also allow the determination of structural rigidity which can be computed using density functional theory, allowing materials to be screened before they are prepared and tested.


This breakthrough puts efforts for high-efficiency, high-brightness, SSL on a fast track. Lower-efficiency incandescent and fluorescent bulbs – which use relatively more energy to produce light – could become antiquated fixtures of the past.


"We can now start looking for cheaper materials from which we can construct the same rigid hosts and that should decrease the cost of phosphors and by looking at increasingly rigid hosts we can also start finding materials for niche applications where very high brightness is key, things like the front lighting of cars and even perhaps stadium lighting," concludes Sheshadri.


"Our target is to get to 90 percent efficiency, or 300 lumens per watt," says DenBaars, who also is a professor of electrical and computer engineering and co-director of the SSLEC. Current incandescent light bulbs, by comparison, are at roughly 5 percent efficiency, and fluorescent lamps are a little more efficient at about 20 percent.


"We have already demonstrated up to 60 percent efficiency in lab demos," DenBaars concludes.


Further details of this work has been published in the paper, " Article Proxies from Ab Initio Calculations for Screening Efficient Ce3+ Phosphor Hosts," by Jakoah Brgoch et al in the Journal of Physical Chemistry, 2013, 117 (35), pp 17955–17959. DOI: 10.1021/jp405858e


 



AngelTech Live III: Join us on 12 April 2021!

AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!

Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.

2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.

We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.

We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.

Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.

Having attracted 1500 delegates over the last 2 online summits, the 3rd event promises to be even bigger and better – with 3 interactive sessions over 1 day and will once again prove to be a key event across the semiconductor and photonic integrated circuits calendar.

So make sure you sign up today and discover the latest cutting edge developments across the compound semiconductor and integrated photonics value chain.

REGISTER FOR FREE

VIEW SESSIONS

Info
×
Search the news archive

To close this popup you can press escape or click the close icon.
×
Logo
×
Register - Step 1

You may choose to subscribe to the Compound Semiconductor Magazine, the Compound Semiconductor Newsletter, or both. You may also request additional information if required, before submitting your application.


Please subscribe me to:

 

You chose the industry type of "Other"

Please enter the industry that you work in:
Please enter the industry that you work in:
 
X
Info
X
Info
{taasPodcastNotification}
Live Event