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J. Lynn Davis, Kelley Rountree, and Karmann Mills

RTI International

Phone: 919-316-3325

Email: ldavis@rti.org

Reliability of Lighting Systems:

How Inorganic LEDs can Inform OLEDs

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October 11, 2017

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OUTLINE

▪ Background on reliability and lifetime in lighting systems

▪ Lessons learned from accelerated testing with inorganic LEDs

▪ Standard tests for lumen maintenance (LM-80/TM-21 & LM-84/TM-28)

▪ OLED failure mechanisms

▪ Test standards for OLEDs and inorganic LEDs

▪ Conclusions

▪ Acknowledgements

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Reliability and Lifetime

▪ Claims of long lifetimes in SSL

systems are often based solely

on lumen maintenance

information.

– Ignores system-level impacts

– Ignore usage effects such as

power cycling & power quality

– Customer experience may be

different

▪ Many SSL systems have

expected lifetimes of > 30,000

hours making lab testing

impractical.

▪ Accelerated testing can be used

to reduce lab test duration to

practical times.

Bathtub CurveEvolution of Hypothetic Failure Rate with Time

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Inorganic LEDs: NGLIA and DOE Sponsored Efforts on SSL Reliability

▪ Provides recommendations for reporting and demonstrating luminaire product lifetime for both “Lights Out” and parametric failures.

– A system perspective is important for understanding the true lifetime.

– Avoid products for which reliability claims are based on unreliable proxies for luminaire lifetime, such as the lumen maintenance.

– Use overstress testing to identify design flaws and mfg. defects. Consensus needed on methods.

– Calls for industry collaboration to “understand the issues surrounding true lifetime and reliability.”

https://energy.gov/eere/ssl/led-systems-reliability-consortium

LED Systems Reliability Consortium (LSRC):

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Lessons Learned: Approach to Determining the Reliability of LED Devices

System-level approach consisting of both accelerated stress tests (AST)

and modeling of both entire luminaires and key system components such

as LEDs, drivers, and optical elements

AST Testing

Modeling

Lifetime Perf

Validation in Real World

Background

Literature

Physics of

Failure

Degradation

Mechanisms

Use

Environment

OLED luminaires do not always

fail in a “lights out” fashion as with

other lighting sources

Possible OLED failure modes:• Lights Out Failure – nothing

happens when switch is thrown

• Lumen maintenance – lighting

levels reduced below a lower limit

• Color shift – Change in color of

light

• Energy consumption – change in

electrical properties

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Lessons Learned from Inorganic LEDs: Accelerated Stress Testing

▪ The purpose of accelerated tests is to

simulate aging of systems & components in

a condensed period of time.

▪ Tests should be done in a way that does not

create new failure modes (i.e., “Fried Egg

Syndrome”).

▪ Multiple testing protocols exists in the

electronics industry (JEDEC, IPC, etc.)

– Single stressors (e.g., elevated

temperature, elevated drive currents)

– Two or more stressors (e.g.,

temperature & drive current; heat and

humidity)

– Cycling stressors (e.g., temperature

shock, voltage surge, bending)

Projection at65˚C and 40% RH

7575

8585

6590

Various 6” Downlights

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Lessons Learned from Inorganic LEDs: Robustness or Screening Tests

▪ Screening tests performed on

devices to ensure they meet

some minimal performance

threshold

▪ Provides binary (pass/fail) info

only. Does not provide direct

information on product lifetime

and reliability.

▪ Robustness testing show that

inorganic LEDs systems are

very robust. LED driver is often

the weak link in overall system

reliability.

▪ IES is developing a TM on LED

package robustness testing and

is starting to look at luminaire

robustness.

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Screening test incorporating temperature

(red) & humidity (blue) variations.

Examples

L-Prize: http://www.lightingprize.org

Hammer Test: https://ssl.energy.gov

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Standard Tests for LED Packages, Arrays, and Modules

• LM-80-15: luminous flux measurement at a minimum of two different

temperatures (one of which must be either 55˚C or 85˚C).

• TM-21-11: projects long-term luminous flux maintenance using LM-80

test data. A minimum of 6,000 hours of data is required. Can extrapolate

between temperature but not current

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Standard Tests for LED Lamps and Luminaires

• LM-84-14: luminous flux measurement of luminaires at 25˚C. No explicit

requirement on temperature but two are often used (e.g., 25˚C & 45˚C).

• TM-28-14 can be used to project the long-term luminous flux

maintenance using LM-84 and LM-80 test data. Method 1 requires a

minimum of 6,000 hours of LM-84 data for a full TM-28 projection.

Method 2 requires fewer LM-84 test hours if combined with LM-80 data

(>6,000 hours).

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HPLED MPLED COB

Standard Test Results: Lumen Maintenance of Inorganic LEDs

• More than 200 datasets of inorganic LED LM-80 data were examined.

• Lumen maintenance decay rate (a) can be high particularly in older MP-LEDs.

TM-21-11F(t) = Be-at

For a given Tj and

forward current

Typically performed

at elevated ambient

temperatures.

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-5.0E-06

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a

Temperature (°C )

HPLED MPLED COB

Standard Test Results: Lumen Maintenance of Inorganic LEDs

• More than 200 datasets of inorganic LED LM-80 data were examined.

• Lumen maintenance decay rate (a) can be high particularly in older MP-LEDs.

TM-21-11F(t) = Be-at

For a given Tj and

forward current

Typically performed

at elevated ambient

temperatures.

50,000 hr

to L70

10,000 hr

to L70

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Elevated Ambient Test Results: Chromaticity Shifts MP-LEDs

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CSM-1 CSM-2 CSM-3 CSM-4 CSM-5 Other

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Color Shift Mode (CSM)

Color Shift Modes for CALiPER LED PAR38 Lamps

COB HP-LED PLCC Hybrid

CSM Terminal Shift Direction

CSM-1 Blue

CSM-2 Green

CSM-3 Yellow

CSM-4 Yellow then Blue

CSM-5 Red

Source: CALiPER 20.5 (2016)

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Yellow Emitter

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Yellow Shift

Blue

Green

Yellow

Yellow

To Blue

Red

CSM = Chromaticity shift mode

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OLED Failures: Literature Findings for OLED Display Panels

▪ Lumen depreciation of OLEDs has been shown

to depend on both current and temperature.

𝐴𝐹 =𝑇𝑡𝑒𝑠𝑡𝑇𝑜𝑝

𝑒

𝐸𝑎𝑘𝑏

1𝑇𝑜𝑝

−1

𝑇𝑡𝑒𝑠𝑡

𝑒 𝐼𝑜𝑝 𝐶 +𝐷𝑇𝑜𝑝

𝑒𝐼𝑡𝑒𝑠𝑡 𝐶+

𝐷𝑇𝑡𝑒𝑠𝑡

AF = Acceleration Factor

Ttest = accelerated test temperature

Itest = accelerated illuminance

Top = normal operating temperature

Iop = normal illuminance

▪ The impact of temperature is slightly larger

than that of current.

▪ OLED luminaire and panel manufacturers

optimize drive current, but that may be

problematic in a controlled luminaire lab test.

Elevated ambient testing is more likely.

Source: Kim, Oh, Youn, Kwon, IEEE Trans.

Industrial Elec. 64 (2017) 2325.

Distributions have the same

shape parameter, b.

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OLED Accelerated Testing for Lumen Maintenance

▪ An accelerated test has been proposed for

luminous flux maintenance of OLED panels

consisting of a combination of mildly

elevated temperature and drive current.

▪ Lumen maintenance model as a linear

combination of two exponential decay

curves.

▪ Activation energies for the second (i.e.,

longer acting) component were constant

with temperature & current indicating a

consistent decay mechanism.

▪ Similar acceleration factors for temperature

and current, but combined effect is

greatest.Source: Yashioka, SID Digest 2014, p. 642.

Green CBP:Ir(ppy)3 emitter

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Possible Causes of Parametric Failures in OLED Devices

▪ Electrical-related causes

– Inadequate headroom in driver output

voltage

– Component failure or degradation that

adversely affects output V and I

– Dark/inactive pixels

▪ Materials-relative causes

– Accumulation of degradation products

that quench luminescence (a few % can

have a big impact)

▪ Deep trap states

▪ Non-radiative recombination centers

– Degradation of stack components and

interfaces resulting in impedance

increases

– Oxidation and layer delamination

reactions attributable to moisture ingress

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Examples of OLED Luminaire Failures

Inactive pixels possibly

caused by moisture ingress

Source: CALiPER Report 24 (Sept. 2016)

Shorted Panel

Breakage (my fault)

Color Variation

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Lumen Maintenance in OLED Panels and Luminaires

▪ GOAL: Accelerated testing on OLED panels and luminaires to study and

model reliability issues and provide information to the industry and potential

users.

y = 1.003e-3.279x10-5(time)

R² = 0.9280

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Average Luminous Flux Maintenance for Acuity Chalina Luminaire

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Chromaticity shifts in OLED luminaires

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• Chromaticity shift proceeded in the blue direction. CSM-1 shift

• Spectral changes point to reduction in emissions from red emitters,

relative to the blue emitter, as the cause for chromaticity shift.

• Agrees with findings from Sugimoto et al., SID 2016 Digest P-162.

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Current OLED Standards

▪ IEC has developed standards for basic

operation of OLEDs

– IEC 62868:2015 – safety requirements

– IEC 62922:2016 - performance

requirements for OLED tiles and panels in

lighting

– IEC TS 62972:2016 – terms and definitions

▪ The Illuminating Engineering Society (IES)

Testing Procedures Committee (TPC)

currently has no standards development

activity for photometric properties of OLEDs

in IES.

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Conclusions

▪ OLED devices are just beginning to reach a level of maturity where product

reliability needs to be considered on a system level and not just the OLED

panel.

▪ Lessons learned during the development of inorganic LED lighting systems

can be leveraged. Often electronics such as drivers and controls are the

weak links in these LED lighting systems.

– Lumen maintenance is not a proxy for lifetime in inorganic LED

systems and may not be an adequate proxy in OLED systems

either.

▪ Reliability testing standards establish guidelines for independent testing of

products that is required by Energy Star, most governments and

municipalities, etc. Several different test may be used.

▪ Once appropriate standards are established for OLEDs, third party testing

can use a variety of environmental stressors. Room temperature and

elevated ambient (35°C to 50°C range) will likely be part of the test matrix.

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Acknowledgements

▪ This works was funded in part by the Department of Energy (Award No. DE-

EE0005124) and KeyLogic (Subcontract No. DE-FE0025912)

▪ Disclaimer: This report was prepared as an account of work sponsored by an

agency of the United States Government. Neither the United States

Government nor any agency thereof, nor any of their employees, makes any

warranty, express or implied, or assumes any legal liability or responsibility

for the accuracy, completeness, or usefulness of any information, apparatus,

product, or process disclosed, or represents that its use would not infringe

privately owned rights. Reference herein to any specific commercial product,

process, or service by trade name, trademark, manufacturer, or otherwise

does not necessarily constitute or imply its endorsement, recommendation, or

favoring by the United States Government or any agency thereof. The views

and opinions of authors expressed herein do not necessarily state or reflect

those of the United States Government or any agency thereof.