New developments in green LEDs

Abstract

1 Introduction The performance of commercial LEDs has improved tremendously over the past few years. Today commercial LEDs cover the entire spectral range from UV to IR. The brightness of InGaN-LEDs has been increased by more than an order of magnitude over the last 10 years. Internal Quantum Efficiencies (IQE) of 75% with corresponding wall plug efficiencies (WPE) above 50% have been demonstrated for blue LEDs [1]. The OSRAM Opto Semiconductors ThinGaN-technology has pushed the Light Extraction Efficiency (LEE) of LED chips beyond 80%. Thin GaN technology also provides scalability of LED chips: LED brightness and efficiencies can be scaled to larger chip areas without losses. However, the InGaN Internal Quantum Efficiency is neither independent of the energy gap (or emission wavelength) nor of the current density: while IQE of more than 75% can be achieved for blue InGaN LEDs (440 nm, 50 A/cm 2 ) the IQE drops to less than 40% for green InGaN LEDs (540 nm, 50 A/cm²). At high current densities this efficiency loss even worsens especially for long wavelengths ("droop"). Pushing InGaN-LEDs towards red emission, the IQE drops dramatically below 10%. For wavelength above 580 nm the InGaAlP material system provides very efficient yellow, amber and red LEDs. In the wavelength range between 500 nm and 580 nm, InGaAlP LEDs are not efficient anymore due to weak carrier confinement.

Efficiency of True-Green Light Emitting Diodes: Non-Uniformity and Temperature Effects<strong> </strong>

“…It is interesting that two-peak character of the low-temperature efficiency dependence on current 446 has been reported earlier for both blue and green LEDs [7]. Moreover, the efficiency maxima were 447 shifted remarkably to lower currents in green LEDs compared to blue ones, in line with our observa-448 tions.…”

“…Those data may also be interpreted in terms of competition between the current flow through 449 the V-pits and through the {0001}-interfaces of QWs, assuming the efficiency of the main QWs to be somewhat lower than that of the semi-polar QWs on the side walls of the pits. At RT, the low-current 451 efficiency peaks quenched and the efficiency dependence on current gains conventional dome-like 452 shape [7]. Generally, the above mechanism considering V-pits as the origin of AR non-uniformity is ex-pected to be especially pronounced in green LEDs, as higher indium content in the QWs is known to 455 favor the V-pit formation.…”

Evaluation of Light Extraction Efficiency of GaN-Based Nanorod Light-Emitting Diodes by Averaging over Source Positions and Polarizations

“…This "efficiency droop" phenomenon results in limited light output power at high current densities [6][7][8][9]. In addition, the EQE of GaN-based yellow-green LEDs with emission wavelengths from 530 to 600 nm has been reported to be <30%, which has been termed the "green gap" problem [10][11][12].…”

AlGaInP red-emitting light emitting diode under extremely high pulsed pumping

“…The effect of device overheating and efficiency thermal rollover in LEDs can be mitigated by a packaged heat sink and/or by driving the device with short pulses with the duty cycle ≤ 1% 16 . In this paper, we will compare the behaviour of EQE in commercial 620 nm red high-power AlGaInP LED under continuous wave (CW) and microsecond-to-sub-nanosecond current-pulse excitation in the width modulation (PWM) regime up to the current densities of ~4.5 kA/cm 2 .…”