IEC TR 60747-5-12:2021

Semiconductor devices - Part 5-12: Optoelectronic devices - Light emitting diodes - Test method of LED efficiencies

IEC TR 60747-5-12:2021(E) discusses the terminology and the measuring methods of optoelectronic efficiencies of single light emitting diode (LED) chip or package without phosphor. White LEDs for lighting applications are out of the scope of this part.
This technical report provides guidance on
- terminology of optoelectronic efficiencies of single LED chip or package without phosphor, such as the power efficiency (PE), the external quantum efficiency (EQE), the voltage efficiency (VE), the light extraction efficiency (LEE), the internal quantum efficiency (IQE), the injection efficiency (IE), and the radiative efficiency (RE);
- test methods of optoelectronic efficiencies of the PE, the EQE, the VE, the LEE, and the IQE;
- review of various IQE measurement methods reported so far in view of accuracy and practical applicability;
- the measuring method of the LED IQE based on the temperature-dependent electroluminescence (TDEL);
- the measuring method of the LED IQE based on the room-temperature reference-point method (RTRM);
- the measuring method of the radiative and nonradiative currents of an LED;
- the relationship between the IQE and the VE, which leads to introduction of a new LED efficiency, the active efficiency (AE) as AE = VE × IQE.

General Information

• Status: Published
• Publication Date: 12-Oct-2021
• ICS: 31.080.99 - Other semiconductor devices
• Technical Committee: SC 47E - Discrete semiconductor devices
• Drafting Committee: WG 9 - TC 47/SC 47E/WG 9
• Current Stage: PPUB - Publication issued
• Start Date: 11-Nov-2021
• Completion Date: 13-Oct-2021

Standards Content (Sample)

IEC TR 60747-5-12 ®
Edition 1.0 2021-10
TECHNICAL
REPORT
colour
inside
Semiconductor devices –
Part 5-12: Optoelectronic devices – Light emitting diodes – Test method of LED
efficiencies
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

IEC Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always
committee, …). It also gives information on projects, replaced have access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and
and French, with equivalent terms in 18 additional languages.
once a month by email.
Also known as the International Electrotechnical Vocabulary

(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc

If you wish to give us your feedback on this publication or
need further assistance, please contact the Customer Service
Centre: [e-mail].
IEC TR 60747-5-12 ®
Edition 1.0 2021-10
TECHNICAL
REPORT
colour
inside
Semiconductor devices –
Part 5-12: Optoelectronic devices – Light emitting diodes – Test method of LED

efficiencies
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 31.080.99 ISBN 978-2-8322-1035-9

– 2 – IEC TR 60747-5-12:2021 © IEC 2021
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative reference . 8
3 Terms and definitions . 8
3.1 General terms and definitions . 9
3.2 Terms and definitions relating to the optoelectronic efficiencies . 9
3.3 Terms and definitions relating to measuring the efficiencies . 11
3.4 Terms and definitions relating to measuring current components . 12
3.5 Abbreviated terms . 12
4 LED efficiencies. 13
4.1 General . 13
4.2 Theoretical background of optoelectronic efficiencies . 15
4.3 Separate measurement of various efficiencies . 20
4.4 Requirements for accurate and reliable IQE measurement . 20
4.5 Classification of IQE measurement methods . 21
5 Conventional IQE measurement methods: features and limitations . 22
5.1 Calculation of the LEE . 22
5.2 Temperature-dependent photoluminescence (TDPL) . 22
5.3 Intensity-dependent photoluminescence (IDPL) or simply photoluminescence
(PL) . 23
5.4 Temperature-dependent time-resolved photoluminescence (TD-TRPL) . 26
5.5 Time-resolved photoluminescence (TRPL) . 28
5.6 Time-resolved electroluminescence (TREL) . 34
5.7 Constant ABC model . 39
5.8 Constant AB model . 45
6 Standard IQE measurement method I: TDEL . 46
6.1 Temperature-dependent electroluminescence (TDEL) method . 46
6.2 Temperature-dependent radiant power . 46
6.3 Evaluation of the IQE . 47
6.4 Validity of the TDEL: examples of blue LEDs . 49
6.5 Sequence of IQE determination by the TDEL . 50
6.6 Summary of the TDEL . 51
7 Standard IQE measurement method II: RTRM . 51
7.1 Room-temperature reference-point method (RTRM) . 51
7.2 Recombination coefficients, A, B, and C in semiconductors . 52
7.3 Strategy of the IQE measurement just at an operating temperature . 53
7.4 Theoretical background of the RTRM . 54
7.5 Example of the RTRM . 56
7.6 Comparison of IQEs by the TDEL and the RTRM . 59
7.7 Summary of the RTRM . 60
8 The RTRM versus the TDEL and the constant ABC model: comparisons . 60
9 LED performance issues related to the IQE measurement . 67
9.1 Various LED efficiency measurement . 67
9.2 Radiative and nonradiative currents . 70
9.3 The active efficiency (AE): IQE versus forward voltage . 74

10 Conclusion: test method of optoelectronic efficiencies of LEDs . 80
Bibliography . 81

Figure 1 – Sequence of the efficiency measurements . 20
Figure 2 – Theoretical model for analysing the TRPL experiment . 30
Figure 3 – Schematic TRPL response and its interpretation in terms of various lifetimes . 32
Figure 4 – Temporal responses of the TRPL for three samples . 33
Figure 5 – Fitted results of the measured TRPL response . 34
Figure 6 – Schematic diagram of the pulse current injection. 35
Figure 7 – Square of 1τ as a function of current density for a bias voltage . 39
EL
Figure 8 – Estimated IQE (left axis) and measured EQE (right axis) versus current
density . 39
Figure 9 – Experimental EQE curve of a blue LED . 42
Figure 10 – Normalized EQE curves (solid lines) and experimental data (rectangular
symbols) for different IQE peak values as a parameter for a blue LED emitting at
460 nm . 42
Figure 11 – SRH nonradiative carrier lifetime as a function of the C
τA= 1
( )
SRH
coefficient calculated from Equation (82) . 43
Figure 12 – Experimental EQE curve of a blue LED . 43
Figure 13 – Temperature characteristics of an LED . 47
Figure 14 – IQEs as a function of current at various operating temperatures from room
to cryogenic measured by the TDEL method . 49
Figure 15 – Two different cases of normalized EQE curves as a function of current at
various temperatures . 50
Figure 16 – Sequence of the IQE measurement by the TDEL method . 51
Figure 17 – Comparison between the conventional ABC model and the improved AB
model . 54
Figure 18 – Calculation procedure from a relative EQE curve to an IQE curve with the
RTRM . 54
Figure 19 – IQE calculation procedure as a function of current based on the RTRM. 57
Figure 20 – Example of the IQE calculation based on the RTRM . 59
Figure 21 – Comparison of the IQEs evaluated by (a) the TDEL and (b) the RTRM . 60
Figure 22 – Radiant power versus current of a blue LED sample measured at various
temperatures . 61
Figure 23 – Normalized intensities on linear and log scales measured at various
temperatures . 62
Figure 24 – I-V characteristics at various temperatures . 63
Figure 25 – Calculated a as a function of current for various temperatures. I at
2 ref
300 K is the current giving the minimum value of a in region II. . 64
Figure 26 – IQEs obtained by the RTRM (symbols) and the TDEL (solid lines) at
various temperatures . 64
Figure 27 – Comparison of the IE obtained from a at 300 K (left axis) and the
theoretical IE for constant I (right axis) . 65
leak
Figure 28 – Normalized EQE and the fitting by the constant ABC model . 66
Figure 29 – Ratio of the SRH, radiative, Auger recombination currents to the total
current . 66

– 4 – IEC TR 60747-5-12:2021 © IEC 2021
Figure 30 – Radiant power and forward voltage as a function of forward current . 68
Figure 31 – Calculation of the mean photon energy from the emission spectra . 69
Figure 32 – LED efficiencies as a function of forward current . 70
Figure 33 – Sequence of the radiative and nonradiative current measurements . 72
Figure 34 – IQE and forward voltage as a function of forward current . 72
Figure 35 – Radiative current and forward voltage as a function of forward current . 73
Figure 36 – Nonradiative current and forward voltage as a function of forward current . 73
Figure 37 – Total forward current, radiative current, and nonradiative current plotted as
a function of forward voltage . 74
Figure 38 – Distribution of the IQE and V for 31 blue MQW LEDs . 76
F
Figure 39 – Optoelectronic characteristics of three samples under consideration . 77
Figure 40 – Separated radiative and nonradiative current densities of samples 1 and 2 . 78
Figure 41 – Separated radiative and nonradiative current densities of samples 1 and 3 . 79

Table 1 – LED items and their measuring methods listed in IEC 60747-5-6:2016 . 14
Table 2 – Summary of efficiency items defined in IEC 60747-5-8:2019 . 19
Table 3 – Various LED IQE measurement methods . 22
Table 4 – Parameters in IQE and current density versus voltage curves . 77
Table 5 – Comparison of recombination mechanisms between samples . 79

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
Part 5-12: Optoelectronic devices – Light emitting diodes –
Test method of LED efficiencies

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 60747-5-12 has been prepared by subcommittee 47E: Discrete semiconductor devices,
of IEC technical committee 47: Semiconductor devices. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
47E/741/DTR 47E/748/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.

– 6 – IEC TR 60747-5-12:2021 © IEC 2021
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 60747 series, published under the general title Semiconductor
devices, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.

INTRODUCTION
The latest international standards for light emitting diode (LED) devices are
IEC 60747-5-6:2016, IEC 60747-5-8:2019, IEC 60747-5-9:2019, IEC 60747-5-10:2019, and
IEC 60747-5-11:2019, where terminology and measuring methods of basic electrical and optical
characteristics of LEDs are given.
This technical report gives guidance on the terminology and the measuring methods of various
efficiencies of single light emitting diode (LED) chip or package without phosphor. White LEDs
for lighting applications are out of the scope of this part of IEC 60747-5-12.
The efficiencies whose measuring methods are described in this technical report are the power
efficiency (PE), the external quantum efficiency (EQE), the voltage efficiency (VE), the internal
quantum efficiency (IQE), and the light extraction efficiency (LEE). To measure these
efficiencies separately, one needs the measurement data of the internal quantum efficiency
(IQE).
The IQE is a key performance parameter that represents the quality of epitaxial wafers and
contains essential information on operational mechanisms. Requirements for accurate and
reliable IQE measurements are suggested. The various IQE measurement methods reported so
far are reviewed in detail from a theoretical and practical point of view. Subsequently, the
technical limitations for these IQE measurement methods to meet the requirements for accurate
and reliable IQE measurements are discussed.
In particular, two different measuring methods of the IQE that can meet the requirements are
described in detail both experimentally and theoretically. They are known as the temperature-
dependent electroluminescence (TDEL) and the room-temperature reference-point method
(RTRM).
A measuring procedure of PE, EQE, VE, IQE, and LEE are demonstrated. But the injection
efficiency (IE) and the radiative efficiency (RE) are described for definitions only.
Separate knowledge of various efficiencies of the LED chip or package is able to improve
optoelectronic performances of LED chip itself and to design LED application systems such as
LED lamps more efficiently and reliably.

– 8 – IEC TR 60747-5-12:2021 © IEC 2021
SEMICONDUCTOR DEVICES –
Part 5-12: Optoelectronic devices – Light emitting diodes –
Test method of LED efficiencies

1 Scope
This technical report discusses the terminology and the measuring methods of optoelectronic
efficiencies of single light emitting diode (LED) chip or package without phosphor. White LEDs
for lighting applications are out of the scope of this part.
This technical report provides guidance on
– terminology of optoelectronic efficiencies of single LED chip or package without phosphor,
such as the power efficiency (PE), the external quantum efficiency (EQE), the voltage
efficiency (VE), the light extraction efficiency (LEE), the internal quantum efficiency (IQE),
the injection efficiency (IE), and the radiative efficiency (RE) [1] ;
– test methods of optoelectronic efficiencies of the PE, the EQE, the VE, the LEE, and the
IQE [1];
– review of various IQE measurement methods reported so far in view of accuracy and
practical applicability;
– the measuring method of the LED IQE based on the temperature-dependent
electroluminescence (TDEL) [2];
– the measuring method of the LED IQE based on the room-temperature reference-point
method (RTRM) [3];
– the measuring method of the radiative and nonradiative currents of an LED [4];
– the relationship between the IQE and the VE, which leads to introduction of a new LED
efficiency, the active efficiency (AE) as AE = VE × IQE.
2 Normative reference
The following document is referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60747-5-6, Semiconductor devices – Part 5-6: Optoelectronic devices – Light emitting
diodes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
___________
Numbers in square brackets refer to the Bibliography.

3.1 General terms and definitions
3.1.1
radiant power
Φ
e
change in radiant energy with time
Note 1 to entry: The unit used is: W. Radiant power is also known as "radiant flux".
[SOURCE: IEC 60050-845:2020, 845-21-038, modified – Note 1 has been expanded.]
3.1.2
spectral distribution
Φ
density of a radiant power , with respect to wavelength, λ , at the wavelength λ
e
dΦλ
( )
e
Φ =
e,λ
dλ
[SOURCE: IEC 60050-845:2020, 845-21-029, modified – In the definition, "a radiant or luminous
or photon quantity X ()λ " has been replaced by "a radiant power Φ ". In the formula, X has
e
been replaced by Φ . Notes have been deleted.]
e
3.1.3
mean photon energy
hν
mean energy that each photon carries
Φ
e
hν=
λ
Φ dλ
e,λ
∫
hc
where
h is the Planck constant;
c is the speed of light in vacuum
[SOURCE: IEC 60747-5-8:2019, 3.1.3]
3.2 Terms and definitions relating to the optoelectronic efficiencies
3.2.1
power efficiency
η
PE
Φ
ratio of the radiant power (coupled to free space), , to the electrical power consumed by the
e
LED, V I , where V is the forward voltage and I is the forward current of the LED
F F F F
Φ
e
η =
PE
VI
F F
Note 1 to entry: Power efficiency is also known as the "wall-plug efficiency". Power efficiency is identical to the
"radiant efficiency" when the power dissipated by any auxiliary equipment is excluded from the electrical power.

– 10 – IEC TR 60747-5-12:2021 © IEC 2021
[SOURCE: IEC 60747-5-8:2019, 3.2.1]
3.2.2
voltage efficiency
η
VE
ratio of the mean photon energy emitted from the LED to the electron energy given by the
forward voltage of the LED, V
F
hν
η =
VE
qV
F
where
q is the elementary charge.
Note 1 to entry: Voltage efficiency can be greater than 1 at very low forward currents.
[SOURCE: IEC 60747-5-8:2019, 3.2.2]
3.2.3
external quantum efficiency
η
EQE
ratio of the number of photons emitted into the free space per unit time to the number of
electrons injected into the LED per unit time
Φ hν
e
η =
EQE
Iq
F
[SOURCE: IEC 60747-5-8:2019, 3.2.3]
3.2.4
internal quantum efficiency
η
IQE
ratio of the number of photons emitted from the active region per unit time to the number of
electrons injected into the LED per unit time
Φ hν
e,active
η =
IQE
Iq
F
where
Φ is the radiant power emitted from the active region.
e,active
[SOURCE: IEC 60747-5-8:2019, 3.2.4]
3.2.5
light extraction efficiency
η
LEE
ratio of the number of photons emitted into the free space to the number of photons emitted
from the active region
Φ
e
η =
LEE
Φ
e,active
[SOURCE: IEC 60747-5-8:2019, 3.2.5]
3.2.6
injection efficiency
η
IE
ratio of the number of electrons injected into the active region per unit time to the number of
electrons injected into the LED per unit time
I
F,active
η =
IE
I
F
where
I is the portion of the forward current injected into the active region.
F,active
[SOURCE: IEC 60747-5-8:2019, 3.2.6]
3.2.7
radiative efficiency
η
RE
ratio of the number of photons emitted from the active region per unit time to the number of
electrons injected into the active region per unit time
Φ hν
e,active
η =
RE
Iq
F,active
[SOURCE: IEC 60747-5-8:2019, 3.2.7, modified – The specific use in angle brackets as well as
the note have been removed.]
3.3 Terms and definitions relating to measuring the efficiencies
3.3.1
peak EQE point
set of operating conditions of the forward current and radiant power at which the EQE is the
maximum for a given temperature.
Note 1 to entry: The forward current and radiant power at the peak EQE point are denoted as I and Φ ,
peak peak
respectively.
[SOURCE: IEC 60747-5-9:2019, 3.1.6]
3.3.2
cryogenic temperature
temperature range below 200 K
[SOURCE: IEC 60747-5-9:2019, 3.1.7]

– 12 – IEC TR 60747-5-12:2021 © IEC 2021
3.3.3
critical cryogenic temperature
T
c
cryogenic temperature at which the peak EQE shows the maximum value
[SOURCE: IEC 60747-5-9:2019, 3.1.9]
3.3.4
normalized variables of X and Y
converted quantities of current and radiant power as follows:
X= Φ I /Φ I
( )
( )
e F e peak
YI= / I
F peak
[SOURCE: IEC 60747-5-10:2019, 3.1.7]
3.3.5
coefficients of a and a
1 2
coefficients of the quadratic equation of Y in X, i.e., Y = a X + a X
1 2
Note 1 to entry: a and a change slowly enough according to the forward current as compared to X and Y, but should be
1 2
treated as a function of the forward current in the data analysis.
[SOURCE: IEC 60747-5-10:2019, 3.1.8]
3.3.6
reference point
operating point at which a is minimum
Note 1 to entry: a , X, and Y at the reference point are represented by a , X , and Y , respectively. The current
2 2,ref ref ref
at the reference point is denoted as I .
F,ref
[SOURCE: IEC 60747-5-10:2019, 3.1.9]
3.4 Terms and definitions relating to measuring current components
3.4.1
radiative current
I
rad
current that is consumed by the radiative recombination process in the LED
[SOURCE: IEC 60747-5-11:2019, 3.1.2]
3.4.2
nonradiative current
I
nonrad
current that is consumed by the nonradiative recombination processes in the LED
[SOURCE: IEC 60747-5-11:2019, 3.1.3, modified – The notes have been removed.]
3.5 Abbreviated terms
AE active efficiency
CW continuous wave
EL electroluminescence
EQE external quantum efficiency
IDPL intensity-dependent photoluminescence
IE injection efficiency
IQE internal quantum efficiency
LED light emitting diode
LEE light extraction efficiency
MQW multiple quantum well
PE power efficiency
PL photoluminescence
QW quantum well
RE radiative efficiency
RTRM room-temperature reference-point method
SRH Shockley-Read-Hall
TDEL temperature-dependent electroluminescence
TDPL temperature-dependent photoluminescence
TD-TREL temperature-dependent time-resolved electroluminescence
TD-TRPL temperature-dependent time-resolved photoluminescence
TREL time-resolved electroluminescence
TRPL time-resolved photoluminescence
VE voltage efficiency
4 LED efficiencies
4.1 General
LEDs are now found in numerous applications owing to advantages such as low power
consumption, small size, long lifetime, and fast switching. LEDs are available in various spectral
ranges including ultraviolet, visible, and infrared wavelengths, based on different material
systems [5]-[7]. Although the LEDs have simple pn junctions with a long history of researches
since the early 1960s, there still remain multiple issues in relation with the device configurations
and materials. In order to analyse any possible device issues, accurate characterization of the
device is essential.
Many parameters have been utilized for LED devices to quantify the device performance:
parameters obtained from simple current-voltage (I-V) and light-current (L-I) measurements
constitute a basis. However, they don’t typically give enough details about a device under test
[8]-[15]. Since many device parameters are interrelated, more extensive characterization is
required to form a complete picture of any possible cause behind a problem in the device and
to remedy it [16],[17]. If there is any measure implemented to remedy and enhance the device
performance, it is often difficult to judge whether the intended effects have been achieved by
simple checking of the output parameters such as I-V, L-I, and the emission spectrum.
IEC 60747-5-6:2016 lists terminology and measuring methods of basic electrical and optical
characteristics of LEDs as categorized in Table 1.

– 14 – IEC TR 60747-5-12:2021 © IEC 2021
Table 1 – LED items and their measuring methods listed in IEC 60747-5-6:2016
Measurement
LED Characteristics Items
Method
Forward voltage (V )
6.2
F
Reverse voltage (V )
6.3
R
Differential resistance (r )
Electrical 6.4
f
Reverse current (I )
6.5
R
Capacitance (C )
6.6
t
Junction temperature (T )
6.7
j
Temperature
Thermal resistance (R )
6.7
th(j-X)
Response time 6.8
Frequency
Frequency response and cut-off frequency (f )
6.9
c
Luminous flux (Φ )
6.10
V
Radiant power (Φ )
6.11
e
Luminous intensity (I )
Optical
6.12
V
Radiant intensity (I )
6.13
e
Luminance (L )
6.14
V
Emission spectrum, peak emission wavelength (λ ), spectral
p
Spectral 6.15
half bandwidth (Δλ)
Chromatic Chromaticity 6.16
Directivity 6.17
Directional
Illuminance (E )
6.18
V
Quality Evaluation Quality evaluation test and inspection 8

Of various device parameters, efficiencies contain the most important information on the device
performance and any possible problem in it. IEC 60747-5-8 defines various efficiencies relevant
to the LED devices. The overall efficiency of the LED device is characterized by the power
η
efficiency (PE), . The PE is rather simple to measure and serves as a useful parameter
PE
representing how efficient the device is in converting the electrical power to the desired radiant
power. However, in many cases, one needs to know more details than the PE to infer limiting
factors in device performance.
The PE can be decomposed into its constituent factors, which are the voltage efficiency (VE),
η η
, and the external quantum efficiency (EQE), . The EQE is then decomposed into the
VE EQE
η
light extraction efficiency (LEE), , and the internal quantum efficiency (IQE), η . The IQE
LEE IQE
is in turn separated into the injection efficiency (IE), η , and the radiative efficiency (RE), η .
IE RE
The PE, VE, and EQE are measurable by using experimental data of current, voltage, radiant
power, and spectra. On the other hand, a standard method of measuring the IQE has not been
known since the advent of LEDs in 1960s, before the publication of IEC 60747-5-9:2019 and
60747-5-10:2019.
The EQE is a measurable quantity once the mean photon energy is obtained. It can be limited
by either the IQE or the LEE. Thus, a separate measurement of the IQE and the LEE is
extremely useful not only to improve the device performance but also to elucidate the operating
mechanisms of an LED device. The optimization of the epitaxial structure and the growth
condition is a typical method for achieving a high IQE. A high LEE is achieved by reducing the

total internal reflection at the LED surface. The LEE and the IQE can be separately obtained
from the EQE if one of them is known. In general approaches, the LEE is theoretically calculated
and the IQE is experimentally measured. Theoretical calculation of the LEE is limited to specific
cases in practice. This is because the LEE is very sensitive to the microscopic parameters such
as the complex refractive index of each material, the layer structure, or the randomly textured
surface [18]-[20]. Thus, direct measurement of the IQE as a function of current is more practical
and has been pursued actively.
This technical report focuses on direct measurement methods of the IQE as a function of current
in single LED chip or package without phosphor. Various characterization techniques for
measuring the LED IQE are critically reviewed and compared. After the limitations of the existing
IQE measurement techniques are reviewed, the room-temperature reference-point method
(RTRM) is presented as a most accurate and practical IQE measurement method. The RTRM
is then applied to various LED devices to show how the IQE measurement techniques can be
utilized to analyse their optoelectronic performances quantitatively.
4.2 Theoretical background of optoelectronic efficiencies
Usually, an LED is electrically driven by a power supply producing the voltage V and the
F
forward current I . The total electrical power supplied is VI . The LED operation is more clearly
F F F
-
qV I q q
understood when VI is expressed as ( )( ) , where is the elementary charge of 1.6x10
FF
F F
C: Iq is the total number of electrons injected into an LED per second, and qV is an
F F
average electrical potential energy of each electron given by the forward voltage V . In the ideal
F
case, each electron energized by a power supply emits one photon without any energy loss so
that both quantum particles should have the same energy qV . In a real case, however, there
F
are many sorts of electrical and optical energy loss mechanisms during the electrical-to-optical
energy-conversion process.
For LEDs, various efficiencies can be defined as a measure of different conversion processes
[5]-[7]. The overall efficiency of an LED device can be characterized by the PE. The PE, η ,
PE
is defined as the ratio of the radiant power (coupled to the free space), Φ , to the input electrical
e
power VI , i.e.,
F F
radiant power Φ
e
η ≡= (1)
PE
electrical power VI
F F
The PE represents how efficiently an LED device can convert the electrical energy to the optical
energy and thus is the most important efficiency parameter. The electrical energy that is not
converted to the optical energy is wasted as heat. There are various factors that can affect the
PE and these factors are characterized by various other efficiency parameters. From now on,
the efficiency parameters that constitute the PE are described in detail.
The VE, η , represents the ratio of the average photon energy emitted from the LED to the
VE
average electron energy supplied by the power source:
mean photon energy hν
η ≡=,
(2)
VE
mean electron energy qV
F
where
hν
– 16 – IEC TR 60747-5-12:2021 © IEC 2021
and recombination process. Recently, it was also pointed out that space charges of electrons
and holes accumulated near the active region also contribute additional forward voltage for a
desired forward current [21]-[23]. In order to improve the voltage efficiency, both low forward
I
F
turn-on voltage and small series resistance are required. The VE is experimentally measurable
so that the value can be improved by a sort of feedbacks to fabrication processes and LED chip
designs.
The mean photon energy is defined as follows:
hν
Φ
e
hν≡
,
∞ λ (3)
Φ λ dλ
( )
e,λ
∫
hc
c
Here, h is the Planck constant, is the speed of light in vacuum, and λ is the free-space
∞
λ
Φ λ dλ
( )
wavelength. Note that the integral represents the number of photons emitted
e,λ
∫
hc
from the LED per second. The average photon energy should be approximately equal to the
bandgap energy of the active region of the LED.
Another is the EQE, η , defined as the ratio of the number of photons emitted into the free
EQE
space per unit time to the number of electrons injected into the LED per unit time, i.e.,
Φ hν
e
η = ,
EQE
Iq
F
(4)
where
Φ hν is the number of photons emitted into free space per second;
e
Iq is the number of electrons injected into LED per second.
F
Note from Formulae (2) and (4) that the multiplication of the VE and the EQE gives the PE, i.e.,
η ηη⋅
.
(5)
PE VE EQE
η
The IQE, , is defined as the ratio of the number of photons emitted from the active region
IQE
per unit time to the number of electrons injected into the LED per unit time:
Φ hν
e,active
,
η =
(6)
IQE
Iq
F
where
Φ
is the radiant power emitted from the active region;
e,active
is the number of photons emitted from active region per second;
Φ hν
e,active
Iq is the number of electrons injected into LED per second.
F
The IQE is greatly dependent on crystal growth and the epitaxial layer structure. The IQE is one
of the key performance indicators of LEDs.
=
The ratio of the number of photons emitted into the free space to the number of photons emitted
from the active region is defined as the light extraction efficiency (LEE), η :
LEE
Φ
e
.
η =
(7)
LEE
Φ
e,active
where
Φ is the number of photons emitted into free space per second;
e
Φ
is the radiant power emitted from the active region.
e,active
The LEE is a measure of the photon loss during the propagation from the active region into free
space. In an ideal LED, all photons emitted by the active region should escape from an LED
die. However, in a real LED, not all the radiant power emitted from the active region is emitted
into the free space by such factors as total internal reflection. The trapped light inside the LED
chip is eventually absorbed by the device, generating heat. Increasing the LEE is one of the
main endeavours many chip manufactures are dedicated to so that they could improve the PE.
Using the EQE and the IQE, the LEE can be expressed as
η
EQE
η =
LEE
η
IQE
Or
η ηη⋅
(8)
EQE IQE LEE
η
The IE, , is defined as the ratio of the electrons injected into the active region per unit time
IE
to the number of electrons injected into the LED per unit time, expressed as
I
F,active
η =
(9)
IE
I
F
where
I is the number of electrons injected into active region per second;
F,active
I is the number of electrons injected into LED per second.
F
Here, I is the portion of the forward current injected into the active region. The IE is a
F,active
measure for how many electrons recombine in the active QW region compared with the total
injected electrons into an LED. It depends on the current level as well as the LED structure
itself. The IE is determined by nonradiative recombination rates occurring outside the active
QW regions. Possible leakage currents are semiconductor surface current, defect-related
tunnelling current, and electron overflow from the QW active region to p-clad region [24]-[26].
The surface leakage current is initially observed at around zero bias voltage and it shows
relatively symmetric current-voltage curve for forward and reverse biases. Special surface
treatment and passivation techniques have been utilized to suppress the surface leakage
current. The electron blocking layer in the epitaxial growth has been introduced to reduce the
electron overflow from the active region to the p-type clad layer at high-level current injection
[27]. Current spreading is important to improve the current injection efficiency in terms of
reducing current density over an entire LED surface. The nonuniform current injection increases
=
– 18 – IEC TR 60747-5-12:2021 © IEC 2021
both the Joule heating and the nonradiative recombination processes and eventually lowers the
light power efficiency [28],[29].
I η
With respect to , one can define the RE, :
F,active RE
Φ hν
e,active
η = ,
(10)
RE
Iq
F,active
where
Φ hν is the number of photons emitted from active region per second;
e,active
Iq is the number of electrons injected into active region per second.
F,active
which is different from the IQE by the factor IE:
η ηη⋅
(11)
IQE IE RE
For a high RE, it is necessary to increase the radiative recombination rate and decrease the
nonradiative recombination rate.
η
Lastly, the active efficiency (AE), , can be defined as the ratio of the radiant power emitted
AE
from the active region to the electrical power supplied to the LED:
Φ
e,active
η = ,
(12)
AE
VI
F F
wh
...