Photovoltaic (PV) modules and cells - Measurement of diode ideality factor by quantitative analysis of electroluminescence images

IEC TS 63109:2022 specifies a method to measure the diode ideality factor of photovoltaic cells and modules by quantitative analysis of electroluminescence (EL) images. This document provides a definition of the term diode ideality factor n, as the inverse of increment ratio of natural logarithm of current as a function of applied voltage, which is related to the fill factor FF, and is useful as an effective indicator to represent the output efficiency of photovoltaic cells and modules with the other key parameters open circuit voltage Voc and short circuit current Isc.

General Information

Status
Published
Publication Date
24-Mar-2022
Current Stage
PPUB - Publication issued
Completion Date
25-Mar-2022
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IEC TS 63109:2022 - Photovoltaic (PV) modules and cells - Measurement of diode ideality factor by quantitative analysis of electroluminescence images
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IEC TS 63109
Edition 1.0 2022-03
TECHNICAL
SPECIFICATION
Photovoltaic (PV) modules and cells – Measurement of diode ideality factor by
quantitative analysis of electroluminescence images
IEC TS 63109:2022-03(en)
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---------------------- Page: 2 ----------------------
IEC TS 63109
Edition 1.0 2022-03
TECHNICAL
SPECIFICATION
Photovoltaic (PV) modules and cells – Measurement of diode ideality factor by
quantitative analysis of electroluminescence images
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-1090-3

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 63109:2022 © IEC 2022
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms and definitions ...................................................................................................... 7

4 Procedures for quantitative analysis of EL intensity ......................................................... 8

4.1 General ................................................................................................................... 8

4.2 Samples.................................................................................................................. 9

4.3 Apparatus ............................................................................................................... 9

4.4 EL image capturing and camera calibration ............................................................. 9

4.5 Procedures of analysing data to derive n values (refer to Annex A) ......................... 9

5 Measurement report ........................................................................................................ 9

Annex A (normative) EL intensity dependence on the injection current ............................... 11

A.1 General ................................................................................................................. 11

A.2 Derivation of diode ideality factor .......................................................................... 11

Annex B (informative) Examples of measurements of diode ideality factor n ....................... 13

B.1 General ................................................................................................................. 13

B.2 Examples of n value of cells .................................................................................. 13

B.2.1 Example 1 – Module without defect................................................................ 13

B.2.2 Module with defect ......................................................................................... 15

Annex C (informative) Diode ideality factor n as an indicator of the output

performance of PV modules – Measurement using proposed single diode model – ............... 19

C.1 General ................................................................................................................. 19

C.2 Practical single diode model ................................................................................. 20

C.3 Concise derivation method of n using photo response parameters ........................ 26

Bibliography .......................................................................................................................... 28

Figure 1 – Scheme for labeling position of cells in a module viewed from the light-

facing side according to coordinates (i,j) ............................................................................... 10

Figure A.1 – Electroluminescence intensity dependence on injection current ........................ 12

Figure B.1 – EL image (module without defect) ..................................................................... 13

Figure B.2 – EL intensity dependence on injection current (module without defect) ............... 14

Figure B.3 – EL image (aged module) ................................................................................... 15

Figure B.4 – EL intensity dependence on injection current (aged module) ............................. 15

Figure B.5 – Diode ideality factor n of 3,F ............................................................................. 16

Figure B.6 – EL image (defective module) ............................................................................. 17

Figure B.7 – EL intensity dependence on injection current (defective module) ...................... 17

Figure B.8 – Diode ideality factor n of 4,E ............................................................................. 18

Figure C.1 – Equivalent circuit model in dark considering series resistance R and

shunt resistance R ............................................................................................................. 20

Figure C.2 – Equivalent circuit model in dark for the practical single diode model ................. 20

Figure C.3 – Schematic I-V characteristic in dark using linear coordinates ............................ 21

Figure C.4 – Schematic I-V characteristic in dark using semi-logarithmic scales ................... 21

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IEC 63109:2022 © IEC 2022 – 3 –
Figure C.5 – Equivalent circuit model under photo irradiation considering series

resistance R ........................................................................................................................ 23

Figure C.6 – Equivalent circuit model under photo irradiation for practical single diode

model ................................................................................................................................... 23

Figure C.7 – Photo response showing I – V characteristic flowing through the load ....... 24

ph ph

Figure C.8 – Diode current as a function of the diode voltage ............................................... 25

Figure C.9 – Semi-logarithmic plot of diode current versus diode voltage .............................. 25

Figure C.10 – Schematic consideration of photo-response change with increasing the

diode ideality factor n ............................................................................................................ 26

Table B.1 – Performance of module without defect (module A) (at STC) ............................... 14

Table B.2 – Performance of aged module (module B) (at STC) ............................................. 16

Table B.3 – Performance of PID module (at STC) ................................................................. 18

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– 4 – IEC 63109:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC (PV) MODULES AND CELLS –
MEASUREMENT OF DIODE IDEALITY FACTOR BY QUANTITATIVE
ANALYSIS OF ELECTROLUMINESCENCE IMAGES
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,

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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 TS 63109 has been prepared by IEC technical committee 82: Solar photovoltaic energy

systems. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
82/1955/DTS 82/1992/RVDTS

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 Specification is English.

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.
---------------------- Page: 6 ----------------------
IEC 63109:2022 © IEC 2022 – 5 –

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.
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– 6 – IEC 63109:2022 © IEC 2022
INTRODUCTION

EL (Electroluminescence) diagnosis technique has been widely used for the evaluation of

photovoltaic cells and modules photographically. EL images can identify various kinds of

deficiencies, such as cracks and pin-holes in substrates, breakdown and detachment of

electrodes, etc. In addition to these qualitative inspections, the quantitative analysis of EL

intensity can reveal the electronic performance of photovoltaic cells [1] to [7] . The EL intensity

is proportional to the total number of minority carriers in photovoltaic cell bodies. The injection

of minority carriers is governed by the I-V characteristics of pn junctions following the diode

rectification formula, which yields that the EL intensity dependence upon the injection current

will derive the diode ideality factor [8].

The proposed analysis method is not intended to give the criteria for the diagnosis of cells and

modules, but the measured values of n are informative for stakeholders to share a common

view about degradation phenomena among themselves. This standard measurement technique

may be useful for the following stakeholders:

a) Manufacturers ‒ checking validity of samples for both development and quality control (refer

to Annex C).

b) Power producers ‒ checking suspicious modules for potential failures (refer to Annex B).

c) Reuse ‒ evaluation of value of second-hand modules (refer to Annex B).
____________
Numbers in square brackets refer to the Bibliography.
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IEC 63109:2022 © IEC 2022 – 7 –
PHOTOVOLTAIC (PV) MODULES AND CELLS –
MEASUREMENT OF DIODE IDEALITY FACTOR BY QUANTITATIVE
ANALYSIS OF ELECTROLUMINESCENCE IMAGES
1 Scope

This document specifies a method to measure the diode ideality factor of photovoltaic cells and

modules by quantitative analysis of electroluminescence (EL) images.

This document provides a definition of the term diode ideality factor n, as the inverse of

increment ratio of natural logarithm of current as a function of applied voltage, which is related

to the fill factor FF, and is useful as an effective indicator to represent the output efficiency of

photovoltaic cells and modules with the other key parameters open circuit voltage V and short

circuit current I .

This document is only applicable to crystalline silicon photovoltaic cells and modules.

2 Normative references

The following documents are 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 TS 60904-13:2018, Photovoltaic devices – Part 13: Electroluminescence of photovoltaic

modules
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC TS 61836 and the

following 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
3.1
electroluminescence

near infra-red light (NIR) and shortwave infra-red (SWIR) light emitted by crystalline silicon

photovoltaic cells under current injection in forward bias

Note 1 to entry: The dependence of EL intensity upon injection current is explained in Annex A.

[SOURCE: Reference [6] and IEC TS 60904-13:2018, 3.1]
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– 8 – IEC 63109:2022 © IEC 2022
3.2
dark I-V

diagram representing the dependence of the current passing through the diode (i.e. the

photovoltaic cell) in the dark versus the applied voltage
3.3
diode ideality factor

inverse of increment ratio of natural logarithm of current as a function of applied voltage; value

is normalized by thermal voltage
Note 1 to entry: Thermal voltage: V =
where
k is the Boltzmann constant;
T is the temperature;
e is the electron charge.
4 Procedures for quantitative analysis of EL intensity
4.1 General

The diode ideality factor n is an important metric to represent the electronic quality of pn

junctions based on the material physics. In general, it is defined by the diode current formula

(1):
𝑉𝑉
� �
(1)
𝑛𝑛×𝛽𝛽
𝐼𝐼 =𝐼𝐼 ×𝑒𝑒
where
I is the dark saturation current;
β is the thermal voltage.

The value of n reflects the current transport mechanisms through the diodes and is considered

to be parametric variable. It should be noted that n has been revealed to be related to the fill

factor FF [9] to [11], and will be an effective indicator to represent the output efficiency of

photovoltaic cells and modules with other key parameters of the open circuit voltage V and

the short circuit current I .

Usually n is derived from the slope of semi-logarithmic plot of the dark diode current as a

function of the applied voltage. Electrical lead wires are needed to measure current voltage (I‑V)

characteristics, and so the measurement of independent cells composing modules is very

difficult.

This newly proposed method utilizing quantitative analysis of EL images has the following novel

features:

• Non-contact and remote sensing measurement for both indoor and outdoor applications: It

can be used for modules after different accelerated stress tests and/or aged ones installed

in the fields.

• Non-destructive method for modules containing multiple cells: Independent measurement of

each cell is simultaneously possible by successive EL image capturing at various injection

current values.
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IEC 63109:2022 © IEC 2022 – 9 –

• The EL intensity dependence on the injection current is analysed to derive n based on a

conventional solar cell diode model and dark I-V curve analysis. The use of EL intensity,

rather than voltage, simplifies the analysis because the lumped series resistance parameter

does not need to be known in order to perform the analysis.
4.2 Samples
Preparation of correlated sample cells and modules is recommended.
4.3 Apparatus
Apparatus of taking EL images shall meet the requirements in IEC TS 60904-13.
4.4 EL image capturing and camera calibration

Taking a sequence of EL images is described in IEC TS 60904-13. EL intensity is measured at

various injection current values in the range of 1 % ~ 100 % of I (short circuit current). In

order to keep the injection current at the designated value during measurements the current

shall be set at the appointed value under the constant current (CC) mode control. The

fluctuation of sample temperature during measurements yields slight changes in current-voltage

characteristics of samples. Cameras with a linear intensity response shall be used. If non-linear,

this may be corrected to achieve a linear intensity response function.
4.5 Procedures of analysing data to derive n values (refer to Annex A)

The EL intensity of the test specimens should be taken without changing the capturing

conditions, i.e., the configuration of the position of test specimens and the camera and the

camera parameter settings (shutter speed, diaphragm, and focal length, brightness and contrast

in the software of image capturing).

The EL images should be corrected as described in IEC TS 60904-13. Next, select some cells

suitable for the desired analysis from the El images. Then, for those cells, calculate the average

intensity of whole cell area including the electrode part, and use it as EL intensity. The EL

intensity L is
...

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