IEC TS 63202-2:2021

Photovoltaic cells - Part 2: Electroluminescence imaging of crystalline silicon solar cells

IEC TS 63202-2:2021 specifies methods to detect and examine defects on bare crystalline silicon (c-Si) solar cells by means of electroluminescence (EL) imaging with the cell being placed in forward bias. It firstly provides guidelines for methods to capture electroluminescence images of non-encapsulated c-Si solar cells. In addition, it provides a list of defects which can be detected by EL imaging and provides information on the different possible methods to detect and differentiate such defects.

General Information

• Status: Published
• Publication Date: 15-Dec-2021
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 82 - Solar photovoltaic energy systems
• Drafting Committee: WG 8 - TC 82/WG 8
• Current Stage: PPUB - Publication issued
• Start Date: 16-Dec-2021
• Completion Date: 17-Jan-2022

Overview

IEC TS 63202-2:2021 - Photovoltaic cells, Part 2 - provides standardized methods for electroluminescence (EL) imaging of crystalline silicon (c‑Si) solar cells. This technical specification describes how to capture EL images of non‑encapsulated (bare) c‑Si cells placed in forward bias, how to interpret EL features, and how to use EL together with other techniques when imaging alone is inconclusive. While focused on bare c‑Si cells, the guidance is generally applicable to wafer solar cell technologies.

Key topics and technical requirements

• Scope & purpose: Detection and examination of defects on bare c‑Si solar cells using EL imaging under forward bias; guidance on capture, interpretation, and reporting.
• Imaging system components:
  • Camera detector (CCD/CMOS; cooling recommended for better SNR).
  • Lens compatible with near‑IR emission (no coatings that block band‑gap wavelengths).
  • Dark room / imaging studio to eliminate ambient light.
  • Power supply for forward bias injection and a computer with image processing software.
• Detector sensitivity & SNR: For crystalline silicon the detector should be sensitive in the 900–1 100 nm wavelength range. Images shall achieve an SNR of 45 or better (determined per IEC TS 60904‑13:2018); SNR must be reported.
• Imaging procedures: Camera settings, positioning, exposure and imaging steps, plus image correction (vignetting, barrel distortion) and evaluation of dynamic range, sharpness, and modulation transfer function (MTF).
• Image evaluation & interpretation:
  • Principles of electroluminescence and radiative recombination.
  • Diagnostic indicators: series resistance, minority‑carrier lifetime and diffusion length, shunt resistance.
  • Assignment of root causes and qualitative image interpretation; catalog of EL‑detectable defects (cracks, broken fingers, shunts, concentric patterns, etc.).
• Reporting: Required documentation of imaging conditions, SNR, and findings.

Applications and users

IEC TS 63202-2:2021 is intended for:

• PV cell manufacturers and process engineers for production quality control and failure analysis.
• Test laboratories and research institutions performing cell characterization.
• Module assembly technicians assessing effects of EL features on downstream processes.
• Field and QA inspectors integrating EL as part of acceptance and diagnostic workflows.

Practical uses include defect detection during R&D, inline inspection, root‑cause analysis of performance loss, and decisions about cell pass/fail for module assembly.

Related standards

• IEC TS 60904-13:2018 - Electroluminescence of photovoltaic modules (methods for modules and SNR measurement referenced).
• IEC TS 61836:2016 - Terms, definitions and symbols for solar PV.
• IEC TS 62446-3 - PV systems - Outdoor infrared thermography (complementary thermal inspection).

Keywords: IEC TS 63202-2:2021, electroluminescence imaging, EL imaging, crystalline silicon solar cells, photovoltaic cells, SNR, PV defect detection, EL camera, forward bias.

Standards Content (Sample)

IEC TS 63202-2 ®
Edition 1.0 2021-12
TECHNICAL
SPECIFICATION
colour
inside
Photovoltaic cells –
Part 2: Electroluminescence imaging of crystalline silicon solar cells
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IEC TS 63202-2 ®
Edition 1.0 2021-12
TECHNICAL
SPECIFICATION
colour
inside
Photovoltaic cells –
Part 2: Electroluminescence imaging of crystalline silicon solar cells

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-1061-1

– 2 – IEC TS 63202-2:2021 © IEC 2021
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Imaging . 7
4.1 Apparatus . 7
4.1.1 General . 7
4.1.2 Electroluminescence imaging camera . 7
4.1.3 Dark room imaging studio or environment . 8
4.1.4 Power supply . 9
4.1.5 Image processing and displaying software . 9
4.2 Procedure . 10
4.2.1 Camera settings and positioning . 10
4.2.2 Camera setting . 10
4.2.3 Imaging . 11
4.2.4 Image correction . 12
5 Evaluation of EL images . 12
5.1 Principles of electroluminescence . 12
5.2 Image interpretation . 12
5.2.1 Series resistance . 12
5.2.2 Minority carrier lifetime and diffusion length . 13
5.2.3 Shunt resistance . 13
5.2.4 Assignment of root cause . 13
5.2.5 Qualitative image interpretation . 13
6 Reporting. 18
Bibliography . 19

Figure 1 – Typical setup of an electroluminescence imaging system . 7
Figure 2 – Image of multi-crystalline silicon solar cell with SNR value of 45 . 8
Figure 3 – EL images of PV solar cell with broken fingers . 14
Figure 4 – EL images of PV cell with different grades of concentric circles in mono-
crystalline silicon . 15
Figure 5 – EL images of PV cell with local shunting . 16
Figure 6 – EL images of PV mono-crystalline silicon solar cells . 16
Figure 7 – EL images of PV multi-crystalline silicon solar cells. 17
Figure 8 – EL images of PV cell . 17

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC CELLS –
Part 2: Electroluminescence imaging
of crystalline silicon solar cells

FOREWORD
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IEC TS 63202-2 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
82/1912/DTS 82/1951/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.

– 4 – IEC TS 63202-2:2021 © 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 63202 series, published under the general title Photovoltaic cells,
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.
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PHOTOVOLTAIC CELLS –
Part 2: Electroluminescence imaging
of crystalline silicon solar cells

1 Scope
This part of IEC 63202 specifies methods to detect and examine defects on bare crystalline
silicon (c-Si) solar cells by means of electroluminescence (EL) imaging with the cell being
placed in forward bias. It firstly provides guidelines for methods to capture electroluminescence
images of non-encapsulated c-Si solar cells. In addition, it provides a list of defects which can
be detected by EL imaging and provides information on the different possible methods to detect
and differentiate such defects. When EL imaging alone cannot provide conclusive information
for the presence of a type of defect, suggestions are also made to utilize a combination of other
methods.
Finally, this document provides some information on potential effects when using cells with
specific EL features in module assembly. Although this document mainly addresses bare c-Si
solar cells, it is generally applicable to all wafer solar cell technologies.
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 61836:2016, Solar photovoltaic energy systems – Terms, definitions and symbols
IEC TS 62446-3, Photovoltaic (PV) systems – Requirements for testing, documentation and
maintenance – Part 3: Photovoltaic modules and plants – Outdoor infrared thermography
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 61836, together
with the following, apply.
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
electroluminescence
EL
light emission by radiative recombination of excited charge carriers in a semiconductor device
resulting from electrical voltage applied to the semiconductor in forward bias
3.2
open circuit
for a given terminal pair, electric circuit without a continuous path between the two terminals of
the pair
– 6 – IEC TS 63202-2:2021 © IEC 2021
Note 1 to entry: A cell exhibits an “open circuit” if defective or damaged so that no current can flow through it when
attached to an external circuit at the cell electrical connection points.
Note 2 to entry: A PV cell itself is in open circuit condition if one or all of the cell electrical connection points are
not connected to electrical terminations or current is not flowing as defined in IEC TS 61836:2016,3.4.56.
3.3
forward bias
forcing current flow with a power supply where the leads are connected to those of the same
polarity (+ and -) on the solar cell
3.4
barrel distortion
geometric distortion of a rectangular raster causing its boundaries to appear convex
3.5
vignetting
reduction of an image's brightness at the periphery compared to the image centre caused by
the lense of the imaging system
3.6
dynamic range
DR
ratio between the maximum output signal level and the noise floor
Note 1 to entry: Noise floor is the root mean square noise level in a black image.
3.7
sharpness
S
minimum real dimension from the darkest black to the point which could provide a contrast of
50 % over the brightest white in terms of EL brightness
Unit: mm
Note 1 to entry: The sharpness is used as the index of the resolvable object size.
3.8
gray-scale value
numeral used to define the different levels between the brightest white and the darkest black
Note 1 to entry: EL images will correspond to a weighted average value (mean gray value).
3.9
modulation transfer function
MTF
ratio of contrast of input images over the output image’s contrast
Note 1 to entry: MTF is a function of spatial frequency.
Note 2 to entry: MTF is applied to evaluate the resolution of the images in terms of frequency over the test area.
Note 3 to entry: MTF is also referred to as spatial frequency contrast sensitivity function.
3.10
field of view
FOV
area of a scene that is imaged on the camera sensor
Note 1 to entry: The geometry of a field of view is usually rectangular and corresponds to the camera sensor shape.

3.11
angle of incidence
angle between the normal to the reference surface of the FOV (the cell) and the camera optical
axis
3.12
angle of view
maximum angle between any two incident rays from the detector camera to any arbitrary points
on the FOV
Unit: dimensionless, usually expressed in degrees
Note 1 to entry: FOV describes the angular extent of a given scene that is imaged by a camera.
4 Imaging
4.1 Apparatus
4.1.1 General
A general description of an EL imaging system and its required apparatus is given in this clause.
Figure 1 shows a typical setup for EL imaging. It consists of a camera detector with appropriate
lens or filters that senses the EL emission from the device under test, a power supply connected
to the DUT which injects current into it for EL emission, a dark chamber that reduces stray light
from ambient and a computer that controls all the components. Under normal test conditions,
the device under test shall be forward biased in the dark such as to achieve a current flow,
similar to the AM1.5G short-circuit current. The resulting radiative recombination causes a light
emission of the cell which is captured by the camera detector.

Figure 1 – Typical setup of an electroluminescence imaging system
4.1.2 Electroluminescence imaging camera
4.1.2.1 Camera detector
Detectors are typically light-sensing pixels consisting of charge coupled devices (CCD) or
complementary metal oxide semiconductor (CMOS) devices arranged in a focal-plane array. To
achieve better signal-to-noise ratio (SNR) by means of reducing device dark current originating
from thermally generated charges, they may be cooled, usually with thermoelectric cooling.
Semiconductor light absorber materials in the detector shall be sensitive to the EL emission of

– 8 – IEC TS 63202-2:2021 © IEC 2021
the device under test. For crystalline silicon the detector shall be sensitive in the wavelength
range between 900 nm and 1 100 nm and able to achieve SNR of 45 or better, determined
using the method detailed in IEC TS 60904-13:2018,4.3. An example image with SNR of 45
below 45 are not suitable to be qualitatively interpreted.
is given in Figure 2. Images with SNR
Determined SNR for images obtained shall be reported per Clause 6.
SNR = 45
Figure 2 – Image of multi-crystalline silicon solar cell with SNR value of 45
4.1.2.2 Lens
Lenses shall be free of absorption filters or coatings that remove the infrared near the band-
gap of the semiconductor material to be examined. For crystalline silicon this means:
wavelengths between 900 nm and 1 100 nm shall not be attenuated by absorption filters or
coatings. Optical glass is generally suitable when measuring Si based solar cells. Lenses vary
from telephoto to wide-angle in focal length. Choices will depend on the specific application and
geometric considerations when capturing the image. Wide-angle lenses that have short focal
lengths used in conjunction with the higher resolution cameras capture a larger FOV. The
camera may be placed much closer to the solar cell under test, which is useful when space is
a consideration. Some wide-angle lens optics, however, may cause undesirable barrel distortion
in the images that will require correction by post-processing.
Lenses with longer focal lengths generally have less barrel distortion and can therefore more
accurately image a solar cell, in which case the resulting images may not require correction by
post processing. Lenses may feature components that correct for the difference between visible
and infrared wavelengths, which can facilitate focusing.
NOTE Lenses typically have adjustable aperture with their size generally referred to by a f-number. Ignoring
differences in light transmission efficiency, a lens set to a greater f-number has less light gathering area and projects
less electroluminescence signal to the image sensor. Depth of field increases with increasing f-number. Image
sharpness is related to f-number through two different optical effects; aberration, due to imperfect lens design, and
diffraction, which is due to the wave nature of light. Many wide-angle lenses will show significant vignetting at the
edges when using a smaller f-number.
4.1.2.3 Filters
Filters on the camera lens may be used to help cut light of extraneous wavelengths from being
detected. Long-pass filters above 850 nm may be used when imaging near band-edge EL from
silicon.
4.1.3 Dark room imaging studio or environment
While not mandated, a darkened environment is favoured for high quality images. Precautions
should be taken to eliminate stray light entering the imaging studio, such as with use of hard
walls, curtains, baffles, and sealing of any gaps with material that are of light absorbing nature
(black). If a filter is used on the camera, then LED lighting may be used that emits light only in

the spectrum that is cut by the filter. If stray light is present, an image subtraction procedure,
as discussed in 4.1.5.2, will be necessary. Stray light should be avoided in the dark room.
Laboratory measurements, for consistency, are recommended to be performed at solar cell
temperature between 23 °C and 27 °C. Temperature shall be measured using instrumentation
with an accuracy of ± 1 °C and repeatability of ± 0,5 °C. Only images taken at temperature
ranges of the same span are to be used for quantitative comparison with the purpose of
identifying solar cell degradation or cell-to-cell differences (see also 4.2.2.5). As current
injection increases the cell temperature, it is recommended that the cell temperature is
stabilized by passing current until the temperature reaches equilibrium before capturing EL
images. Furthermore, temperature may also affect the sensitivity and signal-to-noise ratio of
the detector. Therefore, once the temperature of the solar cell under test has been stabilized,
images captured in sequence can be used to identify if the camera detector temperature has
been also stabilized. When the camera exposure and thus the current injection time is short
enough to avoid cell heating, it may not be necessary to thermally stabilize the sample by
current injection. An estimate of temperature increase of the cell can be calculated from the
total power applied and the silicon heat capacity of the cells’ volume.
4.1.4 Power supply
An electric DC power supply capable of applying current equal to the short-circuit current (I )
SC
under standard test condition (STC) of the solar cell under test is necessary.
It is recommended to measure voltage and current using instrumentation with an accuracy of
± 2 % of the open-circuit voltage and short-circuit current or better. Cabling from the cell leads
shall be of sufficient gauge to maintain less than 2 % voltage drop over the leads, or
alternatively, a four wire configuration can be used to separately supply current and measure
voltage at the terminals of the cell under test.
4.1.5 Image processing and displaying software
4.1.5.1 Assignment of image colours
Lowest EL signal should be represented by black and the highest EL signal in the image should
be represented by white; however, the image data of the active cell area shall not exist in the
upper extreme to avo
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