IEC TS 60904-1-2:2024

IEC TS 60904-1-2 ®
Edition 2.1 2026-03
TECHNICAL
SPECIFICATION
CONSOLIDATED VERSION
Photovoltaic devices -
Part 1-2: Measurement of current-voltage characteristics of bifacial photovoltaic
(PV) devices
ICS 27.160 ISBN 978-2-8327-1152-1
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 Secretariat Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
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 - IEC Products & Services Portal - products.iec.ch
webstore.iec.ch/advsearchform Discover our powerful search engine and read freely all the
The advanced search enables to find IEC publications by a publications previews, graphical symbols and the glossary.
variety of criteria (reference number, text, technical With a subscription you will always have access to up to date
committee, …). It also gives information on projects, content tailored to your needs.
replaced and withdrawn publications.
Electropedia - www.electropedia.org
The world's leading online dictionary on electrotechnology,
IEC Just Published - webstore.iec.ch/justpublished
Stay up to date on all new IEC publications. Just Published containing more than 22 500 terminological entries in English
details all new publications released. Available online and and French, with equivalent terms in 25 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: sales@iec.ch.
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 6
4 General considerations . 8
5 Apparatus . 8
5.1 General . 8
5.2 Solar simulator with adjustable irradiance levels for single-side illumination . 8
5.3 Solar simulator with adjustable irradiance levels for double-side illumination . 8
5.4 Natural sunlight . 9
5.5 Non-irradiated background . 9
5.6 Temperature sensors . 11
6 Additional I-V characterisations for bifacial devices . 11
6.1 General . 11
6.2 Determination of bifaciality . 12
6.3 Determination of the rear irradiance power gain . 13
6.3.1 General . 13
6.3.2 Outdoor rear irradiance power gain measurement. 14
6.3.3 Indoor rear irradiance power gain measurement with single-side
illumination . 15
6.3.4 Indoor rear irradiance power gain measurement with double-side
illumination . 17
7 I-V characterisation of bifacial PV devices in practice . 17
7.1 General . 17
7.2 I-V measurement of bifacial PV devices . 17
7.3 I-V measurement of bifacial PV devices using a reference bifacial device . 18
8 Report . 20
Bibliography . 21

Figure 1 – Two reference devices (described in IEC 60904-2) to measure irradiance on
front and rear sides of device under test during outdoor measurements . 9
Figure 2 – Scheme of a bifacial PV module and the required non-irradiated background
and aperture . 10
Figure 3 – Suggested points to measure the irradiance at the rear face of a PV module
with 72 cells . 11
Figure 4 – Front and rear-side characterization of bifaciality . 12
Figure 5 – Examples of P as a function of irradiance level on the rear side G (for
max r
outdoor or double-side illumination) or its one-side equivalent irradiance G for a
e
φ
device of bifaciality = 89 % . 16
I
SC
Figure 6 – Transmittances of the device (T ) and its encapsulant (T ) . 18
DUT ENC
Figure 7 – Example of P derived from the measurement of P at STC
max,BNPI max
conditions, P and the BiFi coefficient of the reference used in Formula (10) . 19
max,STC
Table 1 – Maximum power, P , measured at different rear irradiances, G , (double-
max r
-2
sided with G = 1 000 Wm ) or alternatively equivalent front irradiances, G , and the
f E
rear irradiance driven power gain yield, BiFi, derived from the slope of the linear fit on
P (G ) . 16
max r
Table 2 – Example of P derived from the measurement at STC conditions
max,BNPI
(G = 0 and G = 1 000) and the rear irradiance power gain obtained from the bifacial
r f
reference device, BiFi . 19
ref
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Photovoltaic devices -
Part 1-2: Measurement of current-voltage characteristics
of bifacial photovoltaic (PV) devices

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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch or
www.iso.org/patents. IEC shall not be held responsible for identifying any or all such patent rights.
This consolidated version of the official IEC Standard and its amendment has been prepared
for user convenience.
IEC TS 60904-1-2 edition 2.1 contains the second edition (2024-11) [documents 82/2278/DTS
and 82/2309/RVDTS] and its amendment 1 (2026-03) [documents 82/2552/DTS and
82/2579/RVDTS].
In this Redline version, a vertical line in the margin shows where the technical content is
modified by amendment 1. Additions are in green text, deletions are in strikethrough red text.
A separate Final version with all changes accepted is available in this publication.

IEC TS 60904-1-2 has been prepared by IEC technical committee 82: Solar photovoltaic energy
systems. It is a Technical Specification.
This second edition cancels and replaces the first edition published in 2019. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) The scope has been updated and refers to IEC TS 63202-3 for the measurement of
non-encapsulated solar cells.
b) The requirements for the non-uniformity of irradiance have been updated and now refer to
classifications introduced in IEC 60904-9.
c) The requirement for non-irradiated background has been revised.
d) Spectral mismatch corrections are no longer mandatory, unless required by another
standard. Spectral mismatch would have to be considered in the measurement uncertainty.
e) The requirement regarding the calculation of bifaciality has been modified: Equivalent
irradiance shall not be calculated based on the minimum bifaciality value between I and
SC
P , but on the bifaciality of I .
max SC
The text of this Technical Specification is based on the following documents:
Draft Report on voting
82/2278/DTS 82/2309/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/publications.
A list of all parts in the IEC 60904 series, published under the general title Photovoltaic devices,
can be found on the IEC website.
The committee has decided that the contents of this document and its amendment 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, or
– revised.
1 Scope
This part of IEC 60904 describes procedures for the measurement of the current-voltage (I-V)
characteristics of single junction bifacial photovoltaic devices in natural or simulated sunlight.
It is applicable to encapsulated solar cells, sub-assemblies of such cells or entire PV modules.
For measurements of I-V characteristics of non-encapsulated solar cells, IEC TS 63202-3
applies.
The requirements for measurement of I-V characteristics of standard (monofacial) PV devices
are covered by IEC 60904-1, whereas this document describes the additional requirements for
the measurement of I-V characteristics of bifacial PV devices.
This document can be applicable to PV devices designed for use under concentrated irradiation
if they are measured without the optics for concentration, and irradiated using direct normal
irradiance and a mismatch correction with respect to a direct normal reference spectrum is
performed.
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 60891, Photovoltaic devices – Procedures for temperature and irradiance corrections to
measured I-V characteristics
IEC 60904-1, Photovoltaic devices – Part 1: Measurement of photovoltaic current-voltage
characteristics
IEC 60904-2, Photovoltaic devices – Part 2: Requirements for photovoltaic reference devices
IEC 60904-3, Photovoltaic devices – Part 3: Measurement principles for terrestrial photovoltaic
(PV) solar devices with reference spectral irradiance data
IEC 60904-4, Photovoltaic devices – Part 4: Photovoltaic reference devices – Procedures for
establishing calibration traceability
IEC 60904-7, Photovoltaic devices – Part 7: Computation of the spectral mismatch correction
for measurements of photovoltaic devices
IEC 60904-8, Photovoltaic devices – Part 8: Measurement of spectral responsivity of a
photovoltaic (PV) device
IEC 60904-9, Photovoltaic devices – Part 9: Classification of solar simulator characteristics
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols
IEC 62788-1-4, Measurement procedures for materials used in photovoltaic modules – Part 1-4:
Encapsulants – Measurement of optical transmittance and calculation of the solar-weighted
photon transmittance, yellowness index, and UV cut-off wavelength
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 terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
bifacial PV device
PV device, both surfaces of which (front and rear sides) are capable of power generation
3.2
front side
side of the PV device declared by the manufacturer as the front side, which is the side designed
to be oriented toward the sun
Note 1 to entry: If no declaration is provided, the front side is the side with the higher maximum power measured
under standard test conditions (STC).
3.3
rear side
side of the PV device declared by the manufacturer as the rear side, that is the side designed
to point away from the sun
Note 1 to entry: If no declaration is provided, the rear side is the side with the lower maximum power measured
under STC.
3.4
bifaciality
property expressing the ratio between the main characteristics of the rear side and the front
side of a bifacial PV device quantified by specific bifaciality coefficients
Note 1 to entry: Unless otherwise specified, the bifaciality refers to standard test conditions (STC). The bifaciality
of the performance parameters is expressed as:
φ
– Short-circuit current bifaciality:
Ι
SC
– Open-circuit voltage bifaciality: φ
V
OC
– Maximum power bifaciality: φ
P .
max,BiFi
3.5
equivalent irradiance
G
E
irradiance required to illuminate the front of the device under test, so that it produces the same
-2
power output as if it were illuminated from the device front with irradiance 1 000 Wm and from
the rear with irradiance G
r
3.6
rear face irradiance
G
r
irradiance arriving at the rear face of the DUT
3.7
bifacial nameplate irradiance
BNPI
irradiance at which nameplate characteristics are reported for bifacial modules, specifically
-2 -2
1 000 Wm on the module front and 135 Wm on the module rear
3.8
maximum power at BNPI
P
max,BNPI
maximum power output of the DUT under BNPI
Note 1 to entry: The quantity can be measured or calculated.
3.9
short-circuit current at BNPI
I
SC,BNPI
short-circuit current of the DUT under BNPI
Note 1 to entry: The quantity can be measured or calculated.
3.10
open-circuit voltage at BNPI
V
OC,BNPI
open-circuit voltage of the DUT under BNPI
Note 1 to entry: The quantity can be measured or calculated.
3.11
rear irradiance power gain
BiFi
quantity which indicates the power gain, in addition to that obtained at STC conditions, per unit
of rear irradiance
Note 1 to entry: Rear irradiance power gain is the slope derived from the linear fit of the P versus rear irradiance,
max
G .
r
-2 2
Note 2 to entry: BiFi is expressed in W/(Wm ) or m .
3.12
relative rear irradiance power gain
BiFi
rel
rear irradiance power gain, BiFi normalized by front-side irradiance and maximum power output
at STC
Note 1 to entry: BiFi is unitless.
rel
3.13
BiFi
ref
rear irradiance power gain of the bifacial device used as a reference
4 General considerations
The final performance of bifacial PV devices in a power plant depends not only on the spatial
distribution of the irradiance incident onto the front surface, but additionally on that incident
onto the rear surface of the device, which is strongly affected by site-specific conditions, such
as albedo, reflective surface size, the racking system, the device's elevation and its tilt angle.
Owing to these dependences and in order to obtain comparable measurement results, I-V
characterisation is extended to quantify the bifaciality of the device and the rear irradiance
power gain. Bifaciality is an intrinsic property of the device, unlike the site-specific conditions
such as albedo. The measurement conditions for bifacial devices should strive to generate extra
photocurrent proportional to their bifaciality. In general, this can be achieved with a test
spectrum close to the reference spectrum such as provided by natural sunlight or with a solar
simulator whose irradiance level is adjustable, a high albedo and minimal near object shading.
However, in practice, measurement conditions differ from the ideal and will deviate from the
reference conditions. This document sets limits on the permissible deviations for obtaining valid
measurements. In any case, the deviations of the measurement conditions from the reference
conditions shall be accounted for in the analysis of measurement uncertainty.
5 Apparatus
5.1 General
In addition to the apparatus requirements described in IEC 60904-1, one of the equipment sets
described in 5.2, 5.3 and 5.4 meeting the requirements for a non-irradiated background as
described in 5.5 is necessary for the characterisation of bifacial devices.
5.2 Solar simulator with adjustable irradiance levels for single-side illumination
A solar simulator, as defined in IEC 60904-9, with adjustable irradiance level shall be used for
the I-V characterisation of bifacial devices. Simulators shall be able to provide irradiance levels
-2 -2
above 1 000 Wm (typically up to 1 200 Wm ). The solar simulator's non-uniformity of
irradiance shall be Class B or better in accordance with IEC 60904-9 and shall maintain its
classification at irradiance levels used for the characterisation of bifacial devices. The non-
uniformity of irradiance, the spectral distribution and the temporal instability of irradiance shall
be measured at the irradiance levels used for the characterisation of bifacial devices.
5.3 Solar simulator with adjustable irradiance levels for double-side illumination
A solar simulator, as defined in IEC 60904-9, with the additional capability to simultaneously
illuminate the bifacial device on both sides shall be used. The non-uniformity, the spectral
distribution and the temporal instability of irradiance shall be measured on both sides while the
irradiance on the opposite side of the device under test is eliminated by appropriate measures
-2
as described in 5.5. In cases where a contribution larger than 5 Wm on the opposite side is
present, this contribution shall be corrected and incorporated into the evaluation of
-2
measurement uncertainty. In cases where a contribution lower than 5 Wm from the opposite
side is present, it is recommended that the contribution also be corrected (see 5.5) if its
magnitude is known. For individual measurements the non-uniformity of irradiance shall be
Class B or better in accordance with IEC 60904-9 and shall maintain its classification on both
sides, at the irradiance levels used for the characterisation of bifacial devices.
NOTE A reflective cloth can be positioned directly under the device under test to minimise artefacts arising from
non-uniformity of irradiance at the rear face.
Figure 1 – Two reference devices (described in IEC 60904-2) to measure irradiance on
front and rear sides of device under test during outdoor measurements
5.4 Natural sunlight
In addition to the general measurement requirements described in IEC 60904-1, at least two
additional PV reference devices shall be used. The first additional PV reference device shall
comply with IEC 60904-2 to measure the irradiance level on the rear side. The second additional
PV reference device shall be used to measure the non-uniformity of irradiance of the rear side.
Their spectral responsivity should be as close as possible to that of the devices under test, or
spectral mismatch corrections shall be applied according to IEC 60904-7. If spectral mismatch
corrections are not made, a specific component shall be considered in the evaluation of
measurement uncertainty. In addition, the front side non-uniformity of irradiance is practically
negligible and can be assumed to be 0 %. The rear side non-uniformity of irradiance shall be
10 % or better in accordance with IEC 60904-9. To minimize the non-uniformity of irradiance at
the rear side, a reflective cloth should be optionally positioned directly under the device under
test (Figure 1).
5.5 Non-irradiated background
To measure the individual I-V characteristics of both front and rear surfaces of bifacial devices,
the contribution from the light incident on the opposite side of the device under test shall be
-2
5 Wm or less during the measurement by creating a non-irradiating background. The
-2
background is considered to be non-irradiating if the irradiance does not exceed 5 Wm , at any
point, on the non-exposed side of the device under test. The contribution of remaining
background illumination shall be compensated during I-V corrections.
Figure 2 – Scheme of a bifacial PV module and the required
non-irradiated background and aperture
In order to fulfil this requirement, in the case of PV modules, it is recommended to limit the size
of the test area to that of the device under test using baffles as illustrated in Figure 2. Low
reflective materials may be placed against the non-exposed side to reduce the non-irradiated
background irradiance.
To measure the irradiance on the non-exposed side, measurements shall be performed at the
non-illuminated side. The minimum point density is six points per square meter. The points
selected for measurement of non-uniformity of irradiance shall be placed in the middle of a cell
at the module's rear face. Furthermore, for c-Si PV modules the distance between neighbouring
points horizontally and vertically shall not exceed the area covered by four full-size cells. It is
noted that the area covered by two half-cut cells counts as the area of a full-size cell. The area
covered by three third-cut cells counts as a full-size cell and so on. For thin film technologies,
the maximum distance between the two nearest points shall not exceed 840 mm. For the
measurement of non-illuminated background, the PV reference device shall be positioned
between the non-reflective surface and the rear side of the module. Figure 3 shows an example
of the location of measurement points on a 72-cell PV module. For the measurement of non-
illuminated background, the detector shall be positioned against the rear surface of the module.
The size of the detector shall be at least half the size of the solar cells used in the module under
test.
The criterion for non-irradiated background shall be verified once per optically equivalent
module design or when the optical configuration of the measurement system undergoes
modifications. Module designs can be considered as optically equivalent if all of the following
are the same:
– transparent area fraction,
– total module area, number of cells and cell spacing,
– encapsulation package: this includes the glass (type, thickness, texturing, and spectral
transmission), anti-reflective coatings, encapsulant, and backsheet (type, colour, and
spectral back-reflection).
The measurement shall be repeated only for devices with higher fraction of transparent areas.
Figure 3 – Suggested points to measure the irradiance at
the rear face of a PV module with 72 cells
5.6 Temperature sensors
Care shall be taken to minimize the shadowing if placing sensors to measure the temperature
of bifacial devices under natural sunlight or using double-side illumination. When contact
sensors are used, sensors should be placed preferably along the solar cell busbars and fixed
with transparent thermal tape. Wires shall be placed in such a way that shading on the PV
module is minimized (i.e. shortest route to edge). Alternatively, contactless infrared
thermometer (IRT) or equivalent cell temperature calculation can be used as described in
IEC TS 62446-3 and IEC 60904-5, respectively.
6 Additional I-V characterisations for bifacial devices
6.1 General
The procedure for the measurement of the I-V characteristics of a bifacial PV device is based
on the same basic principles as in IEC 60904-1 but requires some additional considerations
and also provides supplementary characteristics specific to bifacial devices. For instance, the
bifaciality, φ, relative rear irradiance power gain, BiFi and the maximum power output of the
rel
device under test at BNPI shall be characterised.
The measurement conditions for I-V characteristics of bifacial devices require more attention
than for monofacial devices as the measurement results for bifacial devices are prone to
measurements artefacts arising from the deviation of measurement conditions from the
reference conditions. For instance, the parasitic reflections towards the rear side of the device
under test can significantly increase the measurement uncertainty.
The measured I-V characteristics shall be corrected for spectral mismatch, background
irradiance and temperature, wherever possible. As the spectral responsivity of the device may
vary between front and rear, it is advisable to measure separately the spectral responsivity from
the front and rear side of the device according to IEC 60904-8 and apply separate spectral
mismatch corrections for the front and rear. Corrections of measured I-V characteristics are
allowed within a range of ±10 % of the target irradiance. The uncertainty of the corrections and
furthermore the uncertainty arising from corrections which are not possible or have been omitted
shall be considered in the evaluation of measurement uncertainty.
6.2 Determination of bifaciality
In order to determine the bifaciality of the test specimen, the main I-V characteristics of the
front and the rear sides shall be measured at STC, as schematised in Figure 4 (with
–2
G = 1 000 Wm ). A non-irradiated background, as described in 5.5, shall be used in order to
avoid the illumination of the non-exposed side. Ideally a spectrally matched device should be
used to determine the irradiance for measurements of front and rear.

Figure 4 – Front and rear-side characterization of bifaciality
When the rear side of PV modules is measured, care shall be taken to reduce (as practicably
as possible) any shading caused from temperature sensors, cables, nameplate, junction box
and frame. Manufacturers are responsible for positioning of junction box, frame and nameplate.
The measurement setup shall be verified by visual inspection to ensure that no additional
shading is introduced, for example by temperature sensors (see also 5.5), cables or other
external objects.
When the rear I-V curve is measured, the module frame can cause a deviation between the
measurement plane and the reference plane. If the reference plane and the measurement plane
cannot be brought into alignment, the offset in irradiance intensity shall be determined and
corrected. It is noted that the reference plane of irradiance is determined based on the position
of solar cells contained in the module, and not its frame.
φ
Short-circuit current bifaciality, , is the ratio between the short-circuit current generated
I
SC
exclusively by the rear and front side of the bifacial device respectively. Both currents are
–2
measured at STC (1 000 Wm , 25 °C, with the IEC 60904-3 reference solar spectral irradiance
distribution AM1.5):
I
SCr
φ =
(1)
I
SC
I
SCf
where
φ
is the short-circuit current bifaciality. It is usually expressed as a percentage;
I
SC
I is the short-circuit current when the device is illuminated only on the rear side, at STC;
SCr
I is the short-circuit current when the device is illuminated only on the front side, at STC.
SCf
The bifaciality, as defined in this document, of the device cannot be determined by double-sided
illumination, due to the influence of series resistance that introduces performance losses when
-2
the device is exposed to irradiance higher than 1 000 Wm and performance gains when the
-2
irradiance is lower than 1 000 Wm . When double-sided illumination is practiced, it is
suggested to report instead the relative rear irradiance power gain (see 6.3).
For certain technologies and applications, it is advisable to measure the bifaciality at different
levels of irradiance. For example, it is typical for c-Si technologies that the bifaciality reduces
with reducing irradiance. When the bifaciality is measured at a different irradiance, then the
irradiance shall be stated in subscripts omitting SI units. For example, the bifaciality of a device
-2
of front and rear irradiance shall be expressed as φ .
measured at 200 Wm
I
SC− 200
The bifaciality of open-circuit voltage and maximum power shall be measured at STC and are
calculated as described below:
V
OCr
φ =
(2)
V
OC
V
OCf
P
maxr
φ =
(3)
P
max
P
maxf
where
φ is the open-circuit voltage bifaciality. It is usually expressed as a percentage;
V
OC
φ is the maximum power bifaciality. It is usually expressed as a percentage;
P
max
V is the open-circuit voltage when the device is illuminated only on the rear side, at STC;
OCr
V is the open-circuit voltage when the device is illuminated only on the front side, at STC;
OCf
P is the maximum power when the device is illuminated only on the rear side, at STC;
maxr
P is the maximum power when the device is illuminated only on the front side, at STC.
maxf
When the bifaciality of open-circuit voltage or maximum power are measured at a different
irradiance, the irradiance shall be stated as subscripts omitting SI units. For example, the
-2
bifaciality of maximum power for a device measured at 200 Wm of front and rear irradiance
shall be expressed as φ .
P
max− 200
It is recommended to measure the bifaciality on multiple samples and to provide its dispersion.
6.3 Determination of the rear irradiance power gain
6.3.1 General
The gain in power generation due to additional rear irradiance on the bifacial device under test
shall be determined as a function of the rear side irradiance level. To this end, outdoor or indoor
measurement procedures shall be applied as described below.
–2 -2
The bifacial device under test shall be measured at STC, i.e., 1 000 Wm (G = 0 Wm ), AM1.5
r
and 25 °C junction temperature. The front side irradiance shall be within ±10 % of the target
-2
irradiance and corrected to this target value (1 000 Wm ) according to IEC 60891. Note that
this irradiance range is more restrictive than that of IEC 60891, which allows for a larger
correction range (±30 %).
Additionally, the P of the device under test shall be measured:
max
–2
a) In the case of double-sided illumination with G = 1 000 Wm on the front side plus at least
f
two different rear side irradiance levels ;
G
r
i
b) In the case of single sided illumination: with at least two different equivalent irradiance levels
on the front side according to Formula (6) and Formula (7);
G
E
i
−2 −−22
with, in both cases (i = 1, 2, …; for instance, 0 ≤ r r
1 2
G –2
r
and – > 100 Wm ).
2 G
r
The rear irradiance power gain, BiFi, is the slope derived from the linear fit of the P versus
max
G data series (see the example in Figure 5 and Table 1). This linear least squares fit shall be
r
forced to cross the P axis at P and its non-linearity shall be considered in the
max max,STC
uncertainty estimation. The relative rear irradiance power gain BiFi is calculated by
rel
-2
normalizing BiFi by front irradiance, G = 1 000 Wm and front P , at STC:
max
ΔP
BiFi=
(4)
ΔG
r
−2
1 000 W m
BiFi = BiFi (5)
rel
P
max,STC
φ
NOTE It is planned for next edition of IEC 61215-1 to require either or BiFi to be reported.
I
SC
rel
Besides BiFi , the bifacial maximum power, P , shall be reported and corresponds to
rel max,BNPI
-2
the bifacial nameplate irradiance (BNPI) with front irradiance, G = 1 000 Wm and rear
f
-2
irradiance, G = 135 Wm . P shall be obtained according to Formula (6).
r max,BNPI
P P +⋅BiFi G
(6)
max,BNPI max,STC r
Alternative methods, which can predict the bifacial gain and produce a full I-V curve by means
of translating known I-V characteristics to target irradiance and temperature, are given in
IEC TS 63202-3:2023, Clause A.1 and Clause A.2.
6.3.2 Outdoor rear irradiance power gain measurement
In order to perform outdoor measurement of the rear irradiance power gain, the non-uniformity
of irradiance on the rear side shall be below 10 %.
=
In order to improve the uniformity of irradiance on the rear side, it is recommended to elevate
the device under test to higher positions, e.g. to a distance of 0,5 m to 1,0 m between the
bottom edge of the device and the ground. A diffusely reflective material may be used to
increase the reflection uniformity of the surface behind the device.
6.3.3 Indoor rear irradiance power gain measurement with single-side illumination
In order to perform indoor measurement of the rear irradiance power gain, a solar simulator
with adjustable irradiance levels for single-side illumination, as described in 5.2 can be used.
To this end, a non-irradiated background is required as described in 5.5.
φ
The equivalent irradiance levels are determined as functions of the bifaciality coefficient
I
SC
according to Formula (7):
−2
G 1 000 Wm+φG⋅
(7)
ErI
SC
Example: A device with bifaciality of φ = 80 %, shall be irradiated, on the front side at
I
SC
−2 −2
G = 1160 Wm to provide the equivalence of G = 200 Wm .
E r
The same approach may be applied to assess the low-light behaviour of bifacial PV devices,
−2 −2
e.g. for a measurement at 200 Wm and 40 Wm on the front and rear side of a device
−2 −2
respectively with 80 % bifaciality, shall be measured at G = 200 Wm and G = 40 Wm or
f r
−2
G = 232 Wm .
E
=
,BNPI
Figure 5 – Examples of P as a function of irradiance level on the rear side G
max r
(for outdoor or double-side illumination) or its one-side equivalent irradiance G
e
φ
for a device of bifaciality = 89 %
I
SC
Table 1 – Maximum power, P , measured at different rear irradiances, G ,
max r
-2
(double-sided with G = 1 000 Wm ) or alternatively equivalent front irradiances,
f
G , and the rear irradiance driven power gain yield, BiFi,
E
derived from the slope of the linear fit on P (G )
max r
φ
G G P P BiFi
BiFi
I
SC r E max max,BNPI rel
-2 -2 -2
W W (unitless)
Wm Wm W/(Wm )
0 1 000 296
60 1 048 312
100 1 080 325
89 % 125 1 100 331 0,264 5 0,8935
135 1 108 - 331,7
200 1 160 349
250 1 200 360
The value of P can be derived from BiFi using Formula (4) as follows:
max,BNPI
22−
P P + BiFi ⋅=G 296 W+ 0,264 5 m⋅135 Wm 331,7 W (8)
max,BNPI max,STC r
= =
The same value for P may be also derived from BiFi based on Formula (5):
maxBNPI rel
P
296 W
max,STC -2
P P + BiFi ⋅ ⋅=G 296 W+ 0,893 5⋅ ⋅135 Wm = 331,7 W
(9)
max,BNPI max,STC rel r
-2 -2
1 000 Wm 1 000 Wm
6.3.4 Indoor rear irradiance power gain measurement with double-side illumination
Double-side illumination, as described in 5.3, can alternatively be applied to determine the rear
irradiance power gain. In this case, measurements at two different levels at least as described
in 6.3.1 are used to assess the power gain yield, BiFi.
In order to avoid unwanted reflections between the two light sources, non-reflective masking
around the module is recommended. Possible irradiance non-uniformity added by such
unwanted reflections shall be assessed in the analysis of measurement uncertainty. The
contribution of irradiance on the opposite side determined according to 5.3 can be used to do
so.
7 I-V characterisation of bifacial PV devices in practice
7.1 General
Two cases are to be considered for the I-V characteristics measurement of bifacial devices. In
the first case, the bifaciality coefficients of the test specimen are not known. This is usually the
case for newly developed or modified devices and PV test and calibration laboratories perform
the measurements. The second case corresponds usually to PV production environments,
where reference devices of the same technology as the devices to be tested are available.
The determination of the bifaciality coefficients and the measurement of the rear irradiance
power gain of the reference devices are to be performed in PV laboratories whereas these
characteristics can be used to assess the PV production output. The main differences are
described in 7.2.
7.2 I-V measurement of bifacial PV devices
In order to assess bifacial devices, in addition to the measurements described in IEC 60904-1
and the requirements of IEC 60904-2 and IEC 60904-4, the bifaciality coefficients and the rear
irradiance power gain shall be determined according to the procedures described in this
document.
In order to determine the bifaciality coefficients and the rear irradiance power gain the following
parameters are required:
– I , V and P as functions of at least two irradiance levels larger than zero on the rear
sc oc max
side G or its one-side equivalent irradiance G .The two rear irradiance levels shall be
r E
–2
chosen so that BNPI (corresponding to front irradiance, G = 1 000 Wm and rear
f
–2 -2
irradiance, G = 135 Wm ) is within the range, for instance 0 ≤ < 100 Wm ,
G
r r
-2 -2
100 Wm ≤ G < 200 Wm and sufficiently distinct from each other,
r
-2 –2 –2
– > 100 Wm . For example, = 50 Wm and = 150 Wm is adequate.
G G G G
r r r r
2 1 1 2
– The I , V and P at BNPI shall be either measured at BNPI or
...