Osram Addresses the LED “Green Gap”.

Ever heard of the “Green Gap” in LEDs?  This is the wavelength band that covers the range from about 520nm – 635nm where there is no commercially available semiconductor material that can be used to make efficient LEDs.  Unfortunately, the optimum green LED primary for display applications, 540nm and the optimum red display primary, 622nm, both fall in this gap.  While it is called the “Green Gap,” the gap actually covers green, yellow and orange wavelengths.
Osram Opto Semiconductors (Regensburg, Germany) addresses the green gap problem in an article in the September issue of Compound Semiconductor titled “Taking aim at the ‘Green Gap’.”  In this article, two authors from Osram, Andreas Löffler and Michael Binder, discuss the origin of the green gap and what Osram is working on to solve the problem.

Figure 1: Efficacy of III-Nitride [e.g. InGaN] (green [blue-gray] data points) and III-phosphide [e.g. AlInGaP] (red data points) LEDs with different wavelengths (data taken from recent publications). The blue lines represent the CIE 1924 [and 1931] luminosity function multiplied by the corresponding value of the wall plug efficiency (WPE). Marked in yellow is the green-yellow range, which is not adequately covered by either the III-nitrides or the III-phosphides. This is the essence of the green gap problem. (Source: Compound Semiconductor, October 2013)

Incidentally, this poor performance in the green for LEDs is the reason for hybrid projectors—you simply cannot build a bright projector  (2000 – 4000 lumens) with a green LED and a reasonable size microdisplay.  (Étendue raises its ugly head!)  A blue laser/green phosphor combination can produce a much brighter spot than a green LED to create the green primary of one of these projectors.  The higher efficiency and lower droop of red and blue LEDs can produce enough light to then balance the green to the desired color temperature.

So how do we make displays with RGB LEDs if we can’t match the optimum wavelength?  Well, the optimum is actually rather broad for green and red.  Acceptable green primary colors made by LEDs can have a peak wavelength anywhere from 520nm to 549 nm and acceptable red primary colors can be made with an LED anywhere 614 to 650nm region.  Peak wavelengths that are actually used are typically 525 nm for green and 623 nm for red.

more info here.