ISO 20256:2026

International
Standard
ISO 20256
First edition
Space systems — Solar cells —
2026-04
Calibration procedures
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references. . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 8
5 General requirements and procedures for AM0 primary reference solar cells . 8
5.1 General .8
5.2 Classification .8
5.2.1 General .8
5.2.2 Single-junction AM0 primary reference solar cell .8
5.2.3 Multi-junction AM0 primary reference solar cell .11
5.2.4 High altitude calibration procedures . 12
5.2.5 Synthetic calibration procedures . 12
5.3 Selection . 12
5.4 Temperature measurement . 12
5.5 Electrical connections. 13
5.6 Calibration . 13
5.7 Data sheet . 13
5.8 Marking .14
5.9 Packaging .14
5.10 Care of AM0 primary reference solar cells .14
6 General requirements and procedures for AM0 secondary reference solar cells . 14
6.1 General .14
6.2 Solar simulator . 15
6.3 Calibration procedure .16
7 AM0 solar spectral irradiance reference .16
7.1 General .16
7.2 AM0 solar spectral irradiance .16
Annex A (informative) Calibration methods for AM0 primary reference solar cells .18
Annex B (normative) AM0 solar spectral irradiance .31
Annex C (normative) Measurement method of the spectral irradiance .90
Annex D (informative) Solar cell holders .97
Bibliography .101

iii
Foreword
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SC 14, Space systems and operations.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
This document is consistent with the principles associated with photovoltaic solar cells established by
IEC/TC 82, Solar photovoltaic energy systems. It provides specific requirements and procedures that apply
to the generation of primary and secondary reference solar cells used for solar cell calibration for space
applications. Wherever possible the document refers to existing standards such as the IEC 60891 and
the IEC 60904 series which cover all aspects of solar cell calibration and measurements for terrestrial
applications.
The primary calibration methods given in Annex A serve as examples of acceptable calibration techniques.

v
International Standard ISO 20256:2026(en)
Space systems — Solar cells — Calibration procedures
1 Scope
This document specifies the requirements and procedures for the calibration of primary and secondary
reference solar cells under the air mass zero (AM0) spectrum. It is applicable to both single-junction and
multi-junction solar cells.
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, Measurement of photovoltaic current-voltage characteristics
IEC 60904-2, Requirements for photovoltaic reference devices
IEC 60904-4, Reference solar devices — Procedures for establishing calibration traceability
IEC 60904-7, Computation of the spectral mismatch correction for measurements of photovoltaic devices
IEC 60904-8, Measurement of spectral responsivity of a photovoltaic (PV) device
IEC 60904-8-1, Measurement of spectral responsivity of multi-junction photovoltaic (PV) devices
IEC 60904-9, Solar simulator performance requirements
IEC 60904-10, Methods of linearity measurement
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60904-7, IEC 60904-10 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
absolute cavity radiometer
highly precise instrument used to measure solar irradiance
Note 1 to entry: The absolute cavity radiometer is a combined electrical substitution and cavity radiometer where
the electrical substitution inequivalence, efficiency of the cavity, the area of the entrance aperture, radiative and
conductive losses, and other energy exchanges are accounted for such that the electrically substituted heating can be
absolutely equated to the radiant heating of the detector.

Note 2 to entry: Most currently existing absolute cavity radiometers are designed for the measurement of direct solar
irradiance. The World Radiation Reference (WRR) scale for solar irradiance observations used in many meteorological
and climatological applications is defined by a group of well-characterized absolute cavity radiometers maintained by
the World Radiation Center (WRC) in Davos, Switzerland.
3.2
air mass
AM
length of path through the Earth's atmosphere traversed by the direct solar beam, expressed as a multiple of
the path traversed to a point at sea level with the sun directly overhead
Note 1 to entry: The value of air mass is 1 at sea level with a cloudless sky when the sun is directly overhead and the air
pressure P = 1,013 × 10 Pa.
At any point on the earth surface, the value of the air mass m is given by:
AM
 P 
m  1/sin
AM  
P
 0 
where
P is the local air pressure in pascal;
P is 1,013 x 10 , in pascal;
θ is the solar elevation angle, in degrees.
3.3
air mass zero
AM0
absence of atmospheric attenuation of the solar irradiance at one astronomical unit (3.6) from the Sun
3.4
AM0 standard solar cell
calibrated solar cell (3.26) used to measure irradiance or to set simulator irradiance levels in terms of an air
mass zero (AM0) (3.3) reference solar spectral irradiance (3.28) distribution
3.5
angle of incidence
angle between the direct irradiant beam and the normal to the active surface
3.6
astronomical unit
AU
unit of length approximately equal to the mean distance between the Sun and the Earth with a currently
accepted value of (149 597 870 691 ± 3) m
[SOURCE: ISO 21348:2007, 2.1, modified — The abbreviated term "ua" and Note 1 to entry have been
removed.]
3.7
centroidal wavelength
λ
g
wavelength that represents the band

Rd 

 
g

Rd

where
λ is the wavelength in nm;
R(λ) is the response of spectroradiometer to monochromatic light with a wavelength of λ and of
constant radiant flux
Note 1 to entry: This characteristic applies to input optics having a band sensitivity to wavelength.
3.8
junction temperature
T
j
temperature of p-n junction in a solar cell (3.26) that affects the performance, conversion efficiency and
lifetime
Note 1 to entry: In the absence of light, cell and junction temperature are equivalent. In case of illumination, the
junction temperature cannot be directly measured.
Note 2 to entry: T can be derived from applying the methods described in IEC 60904-5.
j
3.9
component solar cell
specially manufactured solar cell (3.26), with only one active junction and the same spectral responsivity
(3.30) as one sub cell (3.32) of a multi–junction solar cell
3.10
current-voltage characteristics
relationship between the output current of a solar cell (3.26) and the output voltage, at a particular
temperature and irradiance
I = f(V)
where I is the output current and V is the output voltage
3.11
device temperature
T
d
temperature of a solar cell (3.26) read off from a temperature sensor fixed to the solar cell
Note 1 to entry: This temperature is typically different from the cell or junction temperature (3.8) as soon as the solar
cell is being illuminated.
Note 2 to entry: Reference devices often are equipped with temperature sensors fixed to them. In general, there is
an offset between the device temperature and the cell temperature which can be determined using the methods
described in IEC 60904-5. When the device temperature of the reference device during calibration and application is
set to the same value no corrections are needed. Nominally identical cells (in terms of spectral responsivity (3.30) then
have slightly different spectral responsivities when they are not adjusted to the same junction temperature T (3.8).
j
This adds to the mismatch factor.

3.12
irradiance
E
e
density of incident radiant flux with respect to area at a point on a real or imaginary surface, expressed by
E = dΦ /dA
e e
where Φ is radiant flux and A is area on which the radiant flux is incident
e
Note 1 to entry: It is expressed in W/m .
[SOURCE: ISO 16378:2022, 3.6]
3.13
irradiation
exposure of a substance to energetic particles that penetrate the material and have the potential to transfer
energy to the material
[SOURCE: ISO 23038:2018, 3.6]
3.14
linearity
property that two physical quantities or properties vary linearly
EXAMPLE With respect to a solar cell (3.26), relation of short-circuit current with irradiance (3.12) shows
linearity.
3.15
multi-junction solar cell
photovoltaic cell that consists of multi p-n junctions made of different semiconductor materials to absorb
different portions of the solar spectrum, thereby improving the conversion efficiency of the solar cell (3.26)
compared to a single-junction solar cell (3.25)
Note 1 to entry: Each layer in a multi-junction solar cell is tuned to capture a specific range of wavelengths, allowing
for the conversion of more sunlight into electrical energy.
3.16
open circuit voltage
V
oc
voltage, of a photovoltaic (PV) reference device, generated by a solar cell (3.26) without electrical loading at
a particular temperature and irradiance (3.12)
3.17
primary reference solar cell
instrument which a laboratory uses to calibrate secondary references, compared at periodic intervals to a
secondary standard (3.23)
Note 1 to entry: Often primary reference solar cells can be realised at much lower costs than secondary standards.
Note 2 to entry: Typically, a solar cell (3.26) is used as a reference solar device for the measurement of natural or
simulated solar irradiance (3.12). In the context of this document the term “primary reference solar cell” is used
(instead of primary reference).
3.18
primary standard
standard that is designated or widely acknowledged as having the highest metrological qualities and whose
value is accepted without reference to other standards of the same quantity
Note 1 to entry: They are usually maintained by national metrology institutes (NMIs) or similar organisations
entrusted with maintenance of standards for physical quantities. Often referred to also just as the “primary”, the
physical implementation is selected such that long-term stability, accuracy and repeatability of measurement of the
quantity it represents are guaranteed to the maximum extent possible by current technology.

Note 2 to entry: The World Radiometric Reference (WRR) as realized by the World Standard Group (WSG) of cavity
radiometers is the accepted primary standard for the measurement of solar irradiance (3.12).
In the context of this document the Sun is defined as an artefact by convention and thus can be considered as a primary
standard itself. The Sun is considered stable, i.e. it shall be assumed that there are no variations of its irradiance in
time. Nevertheless, whenever calculations or corrections are made that make use of the AM0 spectrum tabulated in
Annex B, the related uncertainties of the AM0 reference spectrum and the total solar irradiance (TSI) (3.34) value shall
be taken into account in the uncertainty budget.
3.19
pyranometer
radiometer normally used to measure global sunlight irradiance (3.12) on a horizontal plane
Note 1 to entry: A pyranometer can also be used at an angle to measure the total sunlight irradiance on an inclined
plane, which in this case includes an element caused by radiation reflected from the foreground.
3.20
pyrheliometer
radiometer, complete with a collimator, used to measure direct sunlight irradiance (3.12)
Note 1 to entry: This instrument is sometimes called normal incidence pyrheliometer (NIP).
3.21
relative spectral responsivity
s(λ)
rel
spectral responsivity (3.30) normalized to a specified reference value, s
m
ss / s
m
rel
Note 1 to entry: The specified reference value, s , can be an average value, a maximum value or an arbitrarily chosen
m
value of s(λ).
3.22
secondary reference solar cell
measurement device in use for daily routine measurements or to calibrate working references, calibrated at
periodic intervals against a primary reference
Note 1 to entry: The most common secondary references for the measurement of natural or simulated solar irradiance
(3.12) are photovoltaic (PV) cells and PV modules. Secondary references are normally used by calibration and testing
laboratories, but sometimes also in industrial production.
[SOURCE: IEC 60904-4:2019, 3.4]
3.23
secondary standard
device, which by periodical comparison with a primary standard (3.18), serves to maintain conformity to SI
units at other places than that of the primary standard
Note 1 to entry: It does not necessarily use the same technical principles as the primary standard, but strives to
achieve similar long-term stability, accuracy and repeatability.
Note 2 to entry: Typical secondary standards for solar irradiance (3.12) are cavity radiometers which participate
periodically (normally every 5 years) in the International Pyrheliometer Comparison (IPC) with the WSG, thereby
giving traceability (3.35) to WRR. Direct traceability to SI radiometric scale can also be available for these instruments.
[SOURCE: IEC 60904-4:2019, 3.2]
3.24
short circuit current
I
sc
output current of a solar cell (3.26) in the short-circuit condition at a particular temperature and irradiance
(3.12)
3.25
single-junction solar cell
photovoltaic cell that consists of a single p-n junction to convert sunlight into electrical energy
Note 1 to entry: The conversion efficiency of a single-junction solar cell is limited by the bandgap of the semiconductor
material used, which determines the range of the solar spectrum it can absorb and convert into electricity.
3.26
solar cell
basic photovoltaic semiconductor device that converts sunlight directly into electrical energy, and that is
the fundamental building block of solar panels
3.27
solar elevation angle
θ
angle between the direct solar beam and the horizontal plane
Note 1 to entry: This angle is measured in radians.
3.28
spectral irradiance
E(λ)
irradiance (3.12) per unit wavelength
-2 -1
Note 1 to entry: The spectral irradiance is expressed in W·m ·(nm) .
3.29
spectral match of solar cells
matching of relative spectral responsivity (3.21) of two solar cells (3.26)
3.30
spectral responsivity
s(λ)
short-circuit current (density) generated by unit irradiance (3.12) at a particular wavelength as a function of
wavelength
Note 1 to entry: For the spectral power responsivity which refers to the short-circuit current density generated by unit
-1
irradiance, the unit is A·W . For the spectral irradiance (3.28) responsivity which refers to the short-circuit current
-1 2
generated by unit irradiance, the unit is A·W ·m .
3.31
STC
standard test conditions
test conditions at cell temperature of 25 °C under AM0 solar spectral irradiance (3.28)
3.32
sub cell
individual solar cell (3.26) junction in a multi-junction solar cell (3.15)
3.33
temperature coefficient of short-circuit current
α
change of the short-circuit current of a solar cell (3.26) as a function of the change of cell temperature
Note 1 to entry: α is expressed in amperes per Kelvin (A/K).

3.34
total solar irradiance
TSI
mean level of solar radiation that falls on a unit area normal to the line from the Sun per unit time outside
the Earth’s atmosphere at one astronomical unit (3.6)
-2 -2 [4],[5]
Note 1 to entry: The value and one-sigma uncertainty are presently taken to be 1 361,1 W·m ± 0,5 W·m .
3.35
traceability
requirement for any photovoltaic (PV) reference solar device to tie its calibration value to SI units in an
unbroken and documented chain of calibration transfers including stated uncertainties
Note 1 to entry: The WRR has been compared several times to the SI radiometric scale. While in previous comparisons
the two scales were found to be indistinguishable within the uncertainty of the comparison, the latest comparison of
scales established that there is a systematic shift between the scales, with WRR reading 0,34 % higher irradiance (3.12)
than the SI scale. The uncertainty of this shift was given as 0,18 % (k = 2). Therefore, traceability to WRR automatically
provides traceability to SI units. However, the shift between the scales may be corrected for those measurements
traceable to WRR. The uncertainty of the scale comparison shall be included into the uncertainty budget. Essentially
there are two possibilities for those measurements traceable to SI units via the WRR. Firstly, no correction is applied
for the scale difference and a larger uncertainty of 0,3 % (rectangular distribution) shall be used. Secondly, an explicit
correction of the scale difference amounting to 0,34 %. In this case the uncertainty contribution is 0,18 % (k = 2). The
value of 0,34 % for the scale difference is the latest available at time of publication of this standard. The scientific
literature should be checked for possible updates of this difference and its uncertainty. In particular, it is possible that
in the future the WRR is adapted to take account of this difference and bring it into line with SI units. In this case no
further correction shall be applied.
[SOURCE: IEC 60904-4:2019, 3.6, modified — Note 1 to entry has been added.]
3.36
wavelength width of slit
effect of the transmission wavelength band on the spectroradiometer
w
i
b 
i
d
i
f
i
d
where
b is the wavelength width of the entrance slit, in nm;
i
w is the mechanical width of the entrance slit, in mm;
i
dθ /dλ is the degree of dispersion of input side, in rad/nm;
i
f is the focal length of the collimator, in mm;
i
w
b 
d
f
d
where
b is the wavelength width of the exit slit, in nm;
w is the mechanical width of the exit slit , in mm;
dθ /dλ is the degree of dispersion of exit side, in rad/nm;
f is the focal length of input optics, in mm
4 Symbols and abbreviated terms
CNES Centre national d'études spatiales (French Space Agency)
EQE External quantum efficiency
ESA European Space Agency
GMT Greenwich mean time
NASA-GRC National Aeronautics and Space Administration – Glenn Research Center
PTB Physikalisch-Technische Bundesanstalt
PV photovoltaic
SI unit Système international d’unités (International System of units)
TM telemetry
WRR World Radiometric Reference
WSG World Standard Group
5 General requirements and procedures for AM0 primary reference solar cells
5.1 General
This clause gives requirements for the classification, selection, packaging, marking, calibration and care of
air mass zero (AM0) primary reference solar cells. Thereby, this clause closely adheres to the philosophy
described in IEC 60904-2 and IEC 60904-4 which shall apply with the amendments provided in 5.2 to 5.10.
5.2 Classification
5.2.1 General
Reference solar cells are specially calibrated devices which are typically used to set sun simulators to AM0
equivalent irradiance conditions in order to measure the performance of other solar cell devices under AM0
conditions. In this standard two different types of AM0 primary reference devices are distinguished, single-
junction primary reference solar cells and multi-junction primary reference solar cells.
Traceable (synthetic) calibration procedures are necessary to transfer calibration from a standard or
reference measuring (solar) irradiance (such as absolute cavity radiometer, pyrheliometer and pyranometer
or a reference photodiode traceable to SI) to a PV reference solar device. Such procedures shall be in
accordance with IEC 60904-4. In addition, a direct measurement under quasi extraterrestrial illumination
conditions is defined as a possible (high altitude) calibration procedure.
5.2.2 Single-junction AM0 primary reference solar cell
A reference device with only one junction being electrically active.
This includes also component cells which are representative of a sub cell in a multi-junction solar cell. Note
that component cells (unless being the top component cell) can show an unwanted sensitivity in a wavelength
[6],[7]
range in which the photons are absorbed by an upper electrically inactive layer (Figure 1 ). This is due to
the fact that the upper layers are still optically active, i.e. they absorb photons which recombine radiatively,
thereby emitting a photon which then can be re-absorbed in the layer below and that is possibly the active
junction. This phenomenon is called photon recycling or optical coupling. Photon recycling in reference solar
cells shall be removed since the photon recycling leads to a component cell which is not identical to the

respective sub cell in a multi-junction cell (Figure 2), and - even worse - results in an unacceptable non-
[6]
linearity of the reference solar cell (Figure 3 ).
Key
EQE external quantum efficiency, expressed in per cent
λ wavelength, expressed in nanometres
r ratio of signals after/before electron irradiation
1 measurement before electron irradiation
2 measurement after electron irradiation
3 wavelength region showing photon recycling
NOTE The photon recycling from the upper layers is suppressed due to the degradation of the material quality
after irradiating the sample with electrons. The dashed line corresponds to the right axis and gives the ratio between
the measurement performed after and before irradiation.
Figure 1 — EQE of a Ge component cell before and after (electron) irradiation

Key
EQE external quantum efficiency, expressed in per cent
λ wavelength, expressed in nanometres
1 measurement of a Ge sub cell
2 measurement of a Ge component cell
Figure 2 — Comparison of the EQE of the Ge sub cell (as part of triple-junction solar cell) and the
corresponding component cell
Key
EQE external quantum efficiency, expressed in per cent
λ wavelength, expressed in nanometres
r ratio of signals without/with bias light, expressed in per cent
1 measurement of a Ge component cell in darkness
2 measurement of a Ge component cell with bias light
3 wavelength region of photon recycling with non-linear dependence on bias light conditions
NOTE While the measurement is perfectly reproduced in the region where the Ge sub cell in a 3J cell is typically
sensitive (from 900 nm on) the EQEs differ by over 10 % in the other regions. This is evidence to the fact that the
photon recycling and thus the spectral responsivity in this region depend on the level of intensity.
Figure 3 — EQE of a Ge component cell measured with bias light and in the dark
As shown in Figure 1 photon recycling can be removed by irradiating the solar cells with electrons. This is
quite simple for Ge cells since the Ge cell itself is almost not affected by the particle irradiation. However, for
other layers the particle irradiation can not only remove the photon recycling but also would change the EQE
of the active cell. In order to avoid that, it is suggested to try to irradiate these cells with low energy protons
which stop in the layers above the active junction.
5.2.3 Multi-junction AM0 primary reference solar cell
A reference device with more than one junction being electrically active. Multi-junction primary reference
solar cells are typically used as final verification when adjusting sun simulators to AM0 conditions (using
single-junction component cells). The reference value in case of multi-junction reference solar cells is
typically not limited to the short circuit current but the full I-V curve is required.
NOTE Multi-junction AM0 primary reference solar cells can only be calibrated using high altitude methods. This
is due to the fact that solar simulators on ground can never be matched closely enough to the AM0 spectrum and a
mismatch correction on the measured calibration value is considered to introduce a too high uncertainty for multi-
junction solar cells.
5.2.4 High altitude calibration procedures
Calibration procedures based on quasi extraterrestrial AM0 conditions. Hereby, “quasi extraterrestrial AM0
1)
conditions” shall be understood as conditions found above the tropopause (around 15 km above sea level).
NOTE High altitude AM0 conditions are characterized by actually using the sun as a calibration reference.
However, according to the international definition of primary reference standards and primary reference
measurement procedures the sun cannot be considered a primary reference standard unless it is by convention
agreed that it is defined as an artefact for the AM0 irradiance level. High altitude AM0 conditions are obviously the
most ideal conditions to calibrate primary reference solar cells and are considered the only conditions under which
multi-junction solar cells can adequately be calibrated. However, it shall be highlighted that also methods that are
based on high altitude calibrations typically apply certain corrections which strongly depend on the altitude at which
the solar cells are calibrated.
Examples for high altitude calibration methods, using high altitude balloon or aircraft, can be found in
Annex A.
5.2.5 Synthetic calibration procedures
Calibration procedures that are based on conditions found on ground and characterised by applying as a
reference a radiometer, a standard detector or a standard light source traceable to SI units. Synthetic AM0
methods are restricted to the calibration of single-junction solar devices (which includes component solar
cells).
Examples for synthetic calibration methods can be found in Annex A.
5.3 Selection
-2
Solar cells shall be irradiated with the TSI (1 361,1 W/m and AM0 spectrum in accordance with Annex B)
for 48 h. The cells shall be kept at 25 °C ± 5 °C during the test. Electrical performance measurements shall be
performed on the cells before and after this test to demonstrate their stability.
AM0 primary reference solar cells shall be stable devices, i.e. their photovoltaic characteristics shall not
change from the initial calibration to re-evaluation by more than 1 %.
The output signal of the AM0 primary reference solar cell shall vary linearly with irradiance (see NOTE
concerning component cells in 5.2.2), as defined in IEC 60904-10, over the range of interest.
NOTE 1 If the final purpose of the AM0 primary reference solar cells is to be used for multi-junction cell
measurements, a set of primary reference solar cells contains as a minimum the respective component cells. The
adjustment of sun simulators used for measuring multi-junction solar cells are typically performed with component
cells. Sometimes a final check of the simulator settings is performed with a multi-junction primary reference cell.
Thus, a complete set of primary reference solar cells contains the full multi-junction plus the respective component
cells.
NOTE 2 The end-of-life (EOL) performance of solar cells in space applications plays an important role. Therefore, an
enhanced set of primary reference solar cells would also contain additional subsets of cells which have been irradiated
with particles to different fluence levels.
It is recommended to provide spare cells for each AM0 primary reference solar cell.
5.4 Temperature measurement
Means shall be provided for measuring the AM0 primary reference solar cell device or junction temperature
to an accuracy of ±1 °C.
1) Any method that flies below 30km altitude can suffer from higher uncertainties which are related to non-linearities
[19]
in the Langley extrapolation when performing the residual ozone corrections.

5.5 Electrical connections
Any measurement resistor incorporated into the AM0 primary reference solar cell shall be a high
temperature stability resistor with a low value in order to allow the cell to operate close to its short-circuit
current (the requirements defined in IEC-60904-2 on built-in shunt resistors shall apply). On the other
hand, the electrical connections to the AM0 primary reference solar cell without resistance shall consist of
a four-wire contact system (Kelvin probe). In accordance with the requirements of IEC 60904-1, the 4-wire
connection of the primary reference solar cell shall start at the cell bus bars.
5.6 Calibration
Each AM0 single-junction primary reference solar cell shall be calibrated in terms of its short-circuit current
at STC.
Each AM0 multi-junction primary reference solar cell shall be calibrated by determining the full I-V curve of
the cell at STC.
Some possible methods of calibrating AM0 primary reference solar cells are described in Annex A.
Whenever current-voltage characteristics are recorded in the frame of the respective calibration method the
requirements defined in IEC 60904-1 shall be followed, using the AM0 spectrum as the reference spectrum
in accordance with Annex B.
The relative spectral responsivity and the temperature coefficient of each AM0 primary reference solar cell
shall be measured in accordance with IEC 60904-8, IEC 60904-8-1 and IEC 60891, using the AM0 spectrum
as the reference spectrum in accordance with Annex B).
NOTE IEC 60904-8-1 covers methods to measure spectral responsivity of multi-junction solar cells. Other
documents that can be read in conjunction are References [8], [9], [10] and [11].
5.7 Data sheet
Each time an AM0 primary reference solar cell is calibrated, the following information shall be recorded on
a data sheet:
— identification number
— type (high altitude AM0 primary reference solar cell or synthetic AM0 primary reference solar cell)
— cell manufacturer
— material type
— type of package
— type and dimension of cell(s)
— circuit diagram, in particular of any connectors
— calibration organization
— site and date of calibration
— method of calibration (refer to standard)
— radiometer or standard lamp characteristics (where applicable)
— simulator characteristics (where applicable)
— type of temperature sensor (where applicable)
— relative spectral responsivity

— temperature coefficient of short-circuit current
— calibration value (or full I-V curve) at AM0 reference conditions
— measurement conditions
— measurement uncertainty
— shunt resistor nominal resistance and temperature coefficient (where applicable)
For reference cells without fixed electrical connection to the cell, the following information shall be recorded
on the data sheet:
— illustration of type, shape and location of electrical contacts during calibration
5.8 Marking
Each AM0 primary reference solar cell shall carry a clear, indelible identification number for cross-reference
to its data sheet.
5.9 Packaging
For packaging of AM0 primary reference solar cells IEC 60904-2:2023, Clause 10 applies. Examples of
suitable cell packages are shown given in Annex D.
NOTE Ideally, the packaging of AM0 primary reference cells reflects the intended application, i.e. if the primary
reference cells and derivatives (secondary reference cells and working references) thereof are going to be used
to characterise bare solar cells, the primary cells are bare cells whereas solar cell assemblies (solar cells with
coverglasses) are characterised using reference cells with respective coverglasses.
5.10 Care of AM0 primary reference solar cells
All AM0 primary reference solar cells shall be kept at temperatures below 50 °C during operation and
storage. The cells should be kept in darkness during extended storage periods.
The window of a packaged reference device shall be kept clean and scratch-free.
Uncovered AM0 primary reference solar cells shall be preserved from damage, contamination and
degradation. A possible way is storing them under dry nitrogen conditions.
AM0 primary reference solar cells shall be verified on an annual basis (by cross-checks against other
primary reference solar cells). AM0 primary reference solar cells in frequent use shall be cross-checked at
shorter intervals. If there is a change in the calibration values (either I or - in case of multi-junction solar
SC
cells - any other photovoltaic parameter derived from the full I-V curve) beyond ±1 % the cells should be
recalibrated.
A reference device exhibiting any defect which can impair its function shall not be used.
If the calibration value of a reference device has changed by more than 5 % of the initial calibration it shall
not be used as a reference device.
NOTE This paragraph follows closely the recommendations given IEC 60904-2:2023, Clause 11.
6 General requirements and procedures for AM0 secondary reference solar cells
6.1 General
AM0 secondary reference solar cells shall be calibrated in simulated sunlight against an AM0 primary
reference solar cell.
In order to minimize measurement uncertainties AM0 primary reference solar cells and AM0 secondary
reference solar cells shall be manufactured in an identical way as the AM0 primary reference solar cells and
hence have the same properties:
— AM0 primary reference solar cells and AM0 secondary reference solar cells shall be spectrally matched
or at least very similar.
— AM0 primary reference solar cells and AM0 secondary reference solar cells shall have identical active
area.
— AM0 primary reference solar cells and AM0 secondary reference solar cells shall have identical angle
of incidence effects, if the simulated sunlight illumination contains a significant proportion of diffuse
irradiance (E /E >0,1).
diff total
6.2 Solar simulator
The solar simulator shall be classified in accordance with IEC 60904-9 in terms of spectral distribution
match, irradiance non-uniformity on the test plane and temporal instability. With respect to the spectral
distribution match, however, the following adaption is made: The spectral distribution match of the solar
simulator is defined by the deviation from AM0 reference spectral irradiance as laid down in this standard.
For the following wavelength intervals of interest, the percentage of total irradiance (up to the wavelength
of interest) is specified in Table 1. To verify the spectral distribution of the solar simulator the spectral
irradiance shall be
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