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Fundamental Limits to

Efficiency of LEDs

Steve DenBaars

Solid State Lighting and Energy Center Materi l M t ials and ECE D d Departments t t University of California, Santa Barbara

Fundamental Limits to Efficiency

Wall-Plug Efficiency is composed of

Injection Efficiency Internal Quantum Efficiency Extraction Efficiency Electrical Efficiency

wallplug inj int elec extraction

UCSB researchers believe that SSL Sources are primarily Limited by : 1) Internal Efficiency Droop Auger Recombination and Non-Radiative

Recombination at Defects are Main Causes

Recombination Rates on New Crystal Planes need to be explored

Non Polar Semipolar C Plane

Non-Polar, Semipolar, C-Plane

2) Thermal Droop-> Carrier Overflow, Non-Radiative at Defects

Bulk GaN for Defect Free Structure

3) Light Extraction-> Approximately 80%, could be improved to >95%

Ph t i C Photonic Crystals, Hi h R fl ti it Mi t l Higher Reflectivity Mirrors, L Low L Loss contacts t t

4) Electrical Efficiency

Ohmic Losses, Driver Losses, Driveless (f>100Hz)

Current GaN LED EQE Status

(

(Low Current External Quantum Efficiencies)

)

80

70

60

50

40

30

20

10 10

0 1998 2000 2002 2004 2006 2008 2010

Roughened, VLED PSS2 PS Flip Chip S Std

EQE(%)

Year

Key Research Area in LEDs

Efficiency Droop

Internal Efficiency Decreases with

Current Density

Auger Recombination Shen, APL 91 p.141101 (2007)

K. T. Delaney, P. Rinke, and C. G. Van y, ,

UPDATE!!

0.8 0.6 Efficiency 0.4 0.2

Quantum Efficiency

de Walle, Appl. Phys. Lett. 94, 191109 (2009

Polarization/Leakage Kim, APL 91 p.183507 p 183507 (2007) Carrier Overflow to Defects

Peak Efficiency

0 10 20 30 40 50 Current Density (A/cm²) 60 70

0

Recomb. Rate=An + Bn2 +Cn3 A and C are non-radiative A=Defects C A C= Auger C ffi i t Coefficient

Auger Non-Radiative Recombination

Fundamental understanding of Auger in LEDs F d t l d t di fA i LED (UCSB Theorists C. Van de Walle) 2electron, one hole

Auger Rate Cn3 fits Lumileds Experiments

28 Cover Article Compound Semi, August 2009

Identify Interband Process is dominant in Nitrides

Defect Free Bulk GaN

Fundamental lower defect density 100/cm2 vs 100,000,000/cm2 Current Sapphire Freedom to choose optimum growth plane Robust Vertical Devices on High Thermal Conductivity substrate

LED on Bulk GaN substrate

LED structure on foreign substrate

1E+8defects/cm2

1E+5/cm2

LED Electrical Efficiency, Target > 98%

Besides contact losses critical flicker freq CFF < 75Hz i i l fli k f H Drivers losses are biggest problem AC-LED drive 100Hz should be enough

Simply driven by AC without additional componets Over 35,000 hrs of lifetime not reduced by electrical t components

Temporal Contrast Sensitivity Function (TSF) for various adapting fields, Adler's Physiology of the eye, Clinical Application. St. Louis: The C. V. Mosby Company, 1987

Semi-Polar and Non-Polar GaN LEDs

Polari ation fields in polar QWs create problems i green and bl e can Polarization in create problems in green, n blue,

semipolar solve the green gap?

60% Exte ernal Quantum Efficien ncy 50% 40% 30% 20% 10%

UV

SSLEC 2008 By UCSB

InxGa1xN

Nonpolar

C-plane C l

Semipolar UCSB

450 550 Peak Wavelength (nm) 650

0% 350

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Fundamental Limits to Efficiency of LEDs

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