LED initiative reduces green gap phenomenon
The 'Hi-Q-LED' project, which was coordinated by Osram Opto Semiconductors and funded by Germany's Federal Ministry of Education and Research (BMBF), has claimed to have reached a breakthrough in terms of green LED performance. According to them, they were able to mimimise the 'green gap' phenomenon, which is the drop in efficacy in the green spectral range.
The result of the 'Hi-Q-LED' project is a green-emitting LED based on indium gallium nitride (InGaN) semiconductors that achieved record efficacy of 147lm/W at a wavelength of 530nm and a spectral width of 35nm. In addition, another green LED developed by combining a blue chip with a phosphor converter has achieved a record-breaking efficacy exceeding 200lm/W.
As part of the 'LED Lead Market Initiative' funded by the BMBF, the working group for 'Efficient LED Solutions with High Colour Rendering Indices' in the 'Hi-Q-LED' project headed by Osram Opto Semiconductors has developed two green LED prototypes.
LEDs show a significant efficacy drop in the green spectral range an effect known as the green gap phenomenon.
Conventional LEDs show a significant efficacy drop at wavelengths above 500nm a phenomenon known as the 'green gap.' Research activities in the framework of the project have enabled the development of a narrowband green LED with a record efficacy of 147lm/W for a chip size of 1mm2 and a driving current of 350mA (current density: 45A/cm2). The LED has a central wavelength of 530nm and a forward voltage of 2.93V at this current density. A reduction of the carrier density in the light-emitting layers and a significantly improved material quality were the key factors behind this breakthrough. Thanks to a significantly reduced dependency of the efficacy on the operating current compared to conventional green LEDs, the LED prototype shows improved performance at higher current densities and achieves as much as 338lm at 125A/cm2.
InGaN-based LEDs, in which the light output is generated by an InGaN semiconductor exclusively, offer a much more narrowband emission with a spectral width of about 35nm compared to green LEDs that are based on phosphor conversion. This breakthrough is an enabling technology for highly efficient projection systems requiring a high colour rendering index, stressed Andreas Lffler, project manager at Osram. After all, a high colour rendering index or an increased colour gamut means a more vivid, higher-quality image.
The second approach of the project, which was to create a new, even more efficient green LED, comes into play in cases where the spectral bandwidth of the LED is not critical. Record-breaking figures demonstrated were 209lm/W (210lm) with a chip size of 1mm2, a central wavelength of 540nm, a forward voltage of 2.88V and a driving current of 350mA (current density: 45A/cm). For a current density of 125A/cm2, it proved possible to increase the light output to above 500lm. Despite this high current density, the efficacy of these devices amounts to 160lm/W. The efficacy peaks at 1.5A/cm with a maximum of 274lm/W. According to Osram research engineer Thomas Lehnhardt, the performance figures have been achieved thanks to the optimised interaction of chip and converter technologies: Continuous improvement of the blue LED chips, an optimised excitation wavelength and an increased degree of conversion of the phosphor converter are the winning combination underlying the record-breaking LED, explained Lehnhardt.
Comparison of green InGaN-based and conversion LED Luminous flux and efficacy as a function of operating current for an InGaN-based green LED and a conversion LED. Source: Osram
For the moment, the unprecedented figures achieved by the two LED prototypes still can only be ranked as development data. Further time will be needed to develop products based on the findings of the research project with optimised price and performance that are well suited for mass production.
- Paul Buckley
EE Times Europe