ASTM E1125-16(2020)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1125 − 16 (Reapproved 2020) An American National Standard
Standard Test Method for
Calibration of Primary Non-Concentrator Terrestrial
Photovoltaic Reference Cells Using a Tabular Spectrum
This standard is issued under the fixed designation E1125; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.8 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This test method is intended for calibration and charac-
responsibility of the user of this standard to establish appro-
terization of primary terrestrial photovoltaic reference cells to
priate safety, health, and environmental practices and deter-
a desired reference spectral irradiance distribution, such as
mine the applicability of regulatory limitations prior to use.
Tables G173. The recommended physical requirements for
1.9 This international standard was developed in accor-
these reference cells are described in Specification E1040.
dance with internationally recognized principles on standard-
Reference cells are principally used in the determination of the
ization established in the Decision on Principles for the
electrical performance of photovoltaic devices.
Development of International Standards, Guides and Recom-
1.2 Primary photovoltaic reference cells are calibrated in
mendations issued by the World Trade Organization Technical
natural sunlight using the relative quantum efficiency of the
Barriers to Trade (TBT) Committee.
cell, the relative spectral distribution of the sunlight, and a
tabulated reference spectral irradiance distribution. Selection
2. Referenced Documents
of the reference spectral irradiance distribution is left to the
user.
2.1 ASTM Standards:
E490Standard Solar Constant and Zero Air Mass Solar
1.3 Thistestmethodrequirestheuseofapyrheliometerthat
Spectral Irradiance Tables
is calibrated according to Test Method E816, which requires
E772Terminology of Solar Energy Conversion
the use of a pyrheliometer that is traceable to the World
E816Test Method for Calibration of Pyrheliometers by
Radiometric Reference (WRR). Therefore, reference cells
Comparison to Reference Pyrheliometers
calibrated according to this test method are traceable to the
E927Classification for Solar Simulators for Electrical Per-
WRR.
formance Testing of Photovoltaic Devices
1.4 This test method is used to calibrate primary reference
E948Test Method for Electrical Performance of Photovol-
cells; Test Method E1362 may be used to calibrate secondary
taic Cells Using Reference Cells Under Simulated Sun-
and non-primary reference cells (these terms are defined in
light
Terminology E772).
E973Test Method for Determination of the Spectral Mis-
1.5 This test method applies only to the calibration of a
match Parameter Between a Photovoltaic Device and a
photovoltaic cell that shows a linear dependence of its short-
Photovoltaic Reference Cell
circuit current on irradiance over its intended range of use, as
E1021TestMethodforSpectralResponsivityMeasurements
defined in Test Method E1143.
of Photovoltaic Devices
E1040Specification for Physical Characteristics of Noncon-
1.6 This test method applies only to the calibration of a
centrator Terrestrial Photovoltaic Reference Cells
reference cell fabricated with a single photovoltaic junction.
E1143Test Method for Determining the Linearity of a
1.7 The values stated in SI units are to be regarded as
Photovoltaic Device Parameter with Respect To a Test
standard. No other units of measurement are included in this
Parameter
standard.
E1362Test Methods for Calibration of Non-Concentrator
Photovoltaic Non-Primary Reference Cells
This test method is under the jurisdiction of ASTM Committee E44 on Solar,
GeothermalandOtherAlternativeEnergySourcesandisthedirectresponsibilityof
Subcommittee E44.09 on Photovoltaic Electric Power Conversion. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2020.PublishedJuly2020.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1986. Last previous edition approved in 2016 as E1125–16. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
E1125-16R20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1125 − 16 (2020)
E2554Practice for Estimating and Monitoring the Uncer- 3.3.20 O (λ,T)—quantum efficiency, reference cell (%).
D
tainty of Test Results of a Test Method Using Control
3.3.21 r —collimator inner aperture radius (m).
x
Chart Techniques
3.3.22 R—collimator entrance aperture radius (m).
G138Test Method for Calibration of a Spectroradiometer
3.3.23 R —pyrheliometer to integrated spectral irradiance
Using a Standard Source of Irradiance
E
ratio (dimensionless).
G173TablesforReferenceSolarSpectralIrradiances:Direct
Normal and Hemispherical on 37° Tilted Surface
3.3.24 RNG—as a subscript, refers to the minimum-to-
G183Practice for Field Use of Pyranometers, Pyrheliom-
maximum range of an array of values.
eters and UV Radiometers
3.3.25 s—sample standard deviation, reference cell calibra-
2.2 WMO Document:
2 –1
tion value (Am W ).
WMO-No. 8 Guide to Meteorological Instruments and
3.3.26 T—temperature (°C).
Methods of Observation, Seventh ed., 2008.
3.3.27 T —calibration temperature, reference cell (25°C).
3. Terminology
3.3.28 Z (λ)—pyrheliometer spectral transmittance function
P
3.1 Definitions—Definitions of terms used in this test
(dimensionless).
method may be found in Terminology E772.
3.3.29 λ—wavelength (µm or nm).
3.2 The following symbols and units are used in this test
3.3.30 θ —collimator opening angle (°).
O
method:
3.3.31 θ —collimator slope angle (°).
S
3.3 Symbols:
3.3.32 Θ (λ)—partial derivative of quantum efficiency with
3.3.1 A —collimator aperture identifiers (non-numeric).
D
x
–1
2 –1 respect to temperature (%·°C ).
3.3.2 C—calibration value, reference cell (Am W ).
3.3.3 C—array of calibration values, reference cell
4. Summary of Test Method
2 –1
(Am W ).
4.1 The calibration of a primary photovoltaic reference cell
3.3.4 D—as a subscript, refers to the reference cell to be
consists of measuring the short-circuit current of the cell when
calibrated; as a variable, distance from collimator entrance
illuminated with natural sunlight, along with the direct solar
aperture to reference cell top surface, or to spectroradiometer
irradiance using a pyrheliometer (see Terminology E772). The
entrance optics (m).
ratio of the short-circuit current of the cell to the irradiance is
3.3.5 E—total irradiance, measured with pyrheliometer
called the responsivity, which, when divided by a spectral
–2
(Wm ).
correction factor similar to the spectral mismatch parameter
–2
3.3.6 E—array of measured total irradiance values (Wm ). defined in Test Method E973, is the calibration value for the
−2 −1 –2 –1
reference cell. The spectral correction factor also corrects the
3.3.7 E(λ)—spectral irradiance (Wm µm or Wm nm ).
calibration value to 25°C (see 4.2.2).
–2 –1
3.3.8 E (λ)—measured solar spectral irradiance (Wm µm
S
4.1.1 The relative spectral irradiance of the sunlight is
–2 –1
or WM nm ).
measured using a spectroradiometer as specified in Test
3.3.9 E (λ)—reference spectral irradiance distribution
Method G138 and Test Method E973.
–2 –1 –2 –1
(Wm µm or WM nm ).
4.1.2 A pyrheliometer measures direct solar irrradiance by
3.3.10 F—spectral correction factor (dimensionless).
restricting the field-of-view (FOV) to a narrow conical solid
angle,typically5°,thatincludesthe0.5°conesubtendedbythe
3.3.11 FOV—field-of-view (°).
sun. This calibration method requires that the same irradiance
3.3.12 I—short-circuit current, reference cell (A).
measured by the pyrheliometer also illuminate the primary
3.3.13 I—arrayofmeasuredshort-circuitcurrents,reference
reference cell to be calibrated and the spectroradiometer
cell (A).
simultaneously. Thus, both are required to have collimators
3.3.14 i—as a subscript, refers to the ith current and
(see 6.2).
irradiance data point (dimensionless).
4.1.3 Multiple calibration values determined from I, E, and
E(λ) measurements made on a minimum of three different
3.3.15 j—as a subscript, refers to the jth calibration value
days, are averaged to produce the final calibration result. Each
data point (dimensionless).
data point corresponds to a single E(λ) spectral irradiance.
3.3.16 L—collimator length (m).
4.2 The following is a list of measurements that are used to
3.3.17 n—number of current and irradiance data points
characterize reference cells and are reported with the calibra-
measured during calibration time period (dimensionless).
tion data:
3.3.18 m—number of calibration value data points (dimen-
4.2.1 The relative quantum efficiency of the cell is deter-
sionless).
mined in accordance with Test Methods E1021.
3.3.19 M—spectral mismatch parameter (dimensionless).
4.2.2 Temperature sensitivity of the cell’s short-circuit cur-
rent is determined experimentally by measuring the partial
derivative of quantum efficiency with respect to temperature,
Available fromWorld Meteorological Organization (WMO), 7bis, avenue de la
Paix,CasePostaleNo.2300,CH-1211Geneva2,Switzerland,http://www.wmo.int. as specified in Test Method E973.
E1125 − 16 (2020)
4.2.3 Linearity of short-circuit current versus irradiance is G183 provides guidance to the use of pyrheliometers for direct
determined in accordance with Test Method E1143. solar irradiance measurements.
4.2.4 Thefillfactorofthereferencecellisdeterminedusing
6.1.1 Because secondary reference pyrheliometers are cali-
TestMethodE948.Providingthefillfactorwiththecalibration
brated against an absolute cavity radiometer, the total uncer-
data allows the reference cell to be checked in the future for
tainty in the primary reference cell calibration value will be
electrical degradation or damage. reduced if an absolute cavity radiometer is used.
6.1.2 The spectral transmittance function of the pyrheliom-
5. Significance and Use
eter must be considered. For an absolute cavity radiometer
without a window, Z (λ) can be assumed to be one over a very
5.1 The electrical output of a photovoltaic device is depen- P
wide wavelength range. Secondary reference pyrheliometers
dent on the spectral content of the illumination source, its
typically have a window at the entrance aperture, so Z (λ) can
intensity, and the device temperature. To make standardized, P
be assumed to be the spectral transmittance of the window
accurate measurements of the performance of photovoltaic
material.
devices under a variety of light sources when the intensity is
6.1.2.1 Test Method E816 requires absolute cavity radiom-
measured with a calibrated reference cell, it is necessary to
eters to be “nonselective over the range from 0.3 to 10 µm”,
account for the error in the short-circuit current that occurs if
and secondary reference pyrheliometers to be “nonselective
the relative quantum efficiency of the reference cell is not
over the range from 0.3 to 4 µm.”
identicaltothequantumefficiencyofthedevicetobetested.A
similarerroroccursifthespectralirradiancedistributionofthe
6.1.2.2 Commercially available secondary pyrheliometers
testlightsourceisnotidenticaltothedesiredreferencespectral
use a variety of different window materials, and many do not
irradiance distribution. These errors are accounted for by the
meet the 0.3 to 4 µm requirement of Test Method E816. The
spectral mismatch parameter (described inTest Method E973),
transmittance of fused silica (SiO ), for example, has signifi-
which is a quantitative measure of the error in the short-circuit cantvariationsinthe2to4µmregionthatdependonthegrade
current measurement. It is the intent of this test method to
ofthematerial(ultravioletorinfraredgrade).Sapphire(Al O )
2 3
provide a recognized procedure for calibrating, characterizing, transmits beyond 4 µm, but its transmittance is not entirely flat
and reporting the calibration data for primary photovoltaic
over0.4to4µm.Crystallinequartz(SiO )isveryflatover0.25
reference cells using a tabular reference spectrum.
to 2.5 µm, but the transmittance falls to zero by 4 µm. The
pyrheliometer manufacturer should be consulted to obtain the
5.2 The calibration of a reference cell is specific to a
window transmittance data.
particular spectral irradiance distribution. It is the responsibil-
6.1.2.3 The calibration procedure in Test Method E816
ity of the user to specify the applicable irradiance distribution,
places restrictions on allowable atmospheric conditions and
for example Tables G173. This test method allows calibration
doesnotadjustcalibrationresultswithspectralinformation:all
with respect to any tabular spectrum.
pyrheliometers are calibrated with the same procedure regard-
5.2.1 Tables G173 do not provide spectral irradiance data
less of the window material.
for wavelengths longer than 4 µm, yet pyrheliometers (see 6.1)
typicallyhaveresponseinthe4–10µmregion.Tomitigatethis
6.2 Collimators—Tubes with internal baffles, intended for
discrepancy, the Tables G173 spectra must be extended with
pointing toward the sun, that restrict the FOV and are fitted to
the data provided in Annex A2.
the reference cell to be calibrated and the spectroradiometer
(see6.3);anacceptablecollimatordesignisprovidedinAnnex
5.3 A reference cell should be recalibrated at yearly
A1.The collimators must match the FOVof the pyrheliometer
intervals, or every six months if the cell is in continuous use
(see A1.4.1).
outdoors.
6.2.1 Eliminate or minimize any stray light entering the
5.4 Recommended physical characteristics of reference
collimators at the bottoms of the tubes.
cells can be found in Specification E1040.
6.2.2 The receiving aperture of the reference cell collimator
5.5 High-quality silicon primary reference cells are ex-
shallbesizedsuchthattheentireopticalsurfaceoftheprimary
pected to be stable devices by nature, and as such can be
reference cell to be calibrated is completely illuminated,
considered control samples. Thus, the calibration value data
including the window (see Specification E1040). Thus, for a
points(see9.3)canbemonitoredwithcontrolcharttechniques
reference cell with a 50 mm square window, the collimator
according to Practice E2554, and the test result uncertainty
would require a receiving aperture radius equal to:
estimated. The control charts can also be extended with data
2 2
=50 150 ⁄2 5 35.4 mm
points from previous calibrations to detect changes to the
reference cell or the calibration procedures.
6.3 Spectroradiometer, as required by Test Methods G138
and E973 for direct normal solar spectral irradiance measure-
6. Apparatus
ments.
6.1 Pyrheliometer— A secondary reference pyrheliometer 6.3.1 The wavelength range of the spectral irradiance mea-
that is calibrated in accordance with Test Method E816,oran surementshallbewideenoughtospanthewavelengthrangeof
absolute cavity radiometer. See also World Radiometric Ref- the quantum efficiency of the cell to be calibrated (see 6.7.3)
erence in Terminology E772 and the World Meteorological and the spectral sensitivity function of the pyrheliometer (see
Organization (WMO) guide WMO-No.8, Chapter 7. Practice 6.1.2).
E1125 − 16 (2020)
6.3.2 If the spectral irradiance measurement is unable to 6.7.4 The full-width-at-half maximum bandwidth fo the
measure the entire wavelength range required by 6.3.1 and monochromatic light source shall be 10 nm or less.
6.3.2, it is acceptable to use a reference spectrum, such as
6.8 Temperature Control Block (Optional)—A device to
TablesG173,tosupplythemissingwavelengths.Thereference
maintain the temperature of the reference cell at 25 6 1°C for
spectrum is scaled to match the measured spectral irradiance
the duration of the calibration.
data over a convenient wavelength interval within the wave-
length range of the spectral irradiance measurement equip-
7. Characterization
ment. It is also acceptable to calculate the missing spectral
7.1 Becausesomesiliconsolarcellsaresusceptibletoaloss
irradiance data using a numerical spectral irradiance model.
of short-circuit current upon initial exposure to light, newly
6.3.2.1 Note that the reference spectrum is also required to
manufactured reference cells shall be light soaked prior to
include the wavelengths specified by 6.3.1: see 5.2.1.
initial characterization, as follows:
6.4 Normal Incidence Tracking Platforms—A platform or
7.1.1 Measure the short-circuit current and the cell area of
platforms that hold the reference cell to be calibrated, the
the reference cell to be calibrated according to Test Method
pryheliometer,andthespectroradiometerduringthecalibration
E948, with respect to standard reporting conditions corre-
procedure. Using two orthogonal axes, such as azimuth and
sponding to the reference spectral irradiance distribution (see
elevation (that is, altazimuthal mount), the platforms must
5.2 and Table 1 of Test Method E948).
follow the apparent motion of the sun such that the angle
7.1.2 Connect the reference cell to the electrical measure-
between the sun vector and the normal vector is less than 0.1°
ment equipment (see 6.6) and prepare to record short-circuit
(that is, the tracking error). The collimators (including that of
current versus time.
the pyrheliometer) define the normal vector and shall be
7.1.3 Illuminate the reference cell with either natural sun-
parallel to each other within 60.25°.
light or a solar simulator (see Specification E927); the spectral
6.4.1 The tracking error tolerance is dependent on the FOV
irradiance is not critical, nor is the cell temperature.
and slope angle of the pyrheliometer and the collimators (see
7.1.4 Record the short-circuit current of the reference cell
A1.4.1); WMO-No. 8 states that 0.1° is acceptable for the
when the current is greater than 85 % of the current measured
recommended FOV of 5° and slope angle of 1°.
in 7.1.1.
6.5 Temperature Measurement Equipment—The instrument
7.1.5 Integrate the short-circuit currents recorded in 7.1.4
orinstrumentsusedtomeasurethetemperatureofthereference
with time to calculate the total charge generated.
cell to be calibrated must have a resolution of at least 0.1°C,
–2
7.1.6 Discontinue the illumination when 22 MCm have
andatotaluncertaintyoflessthan 61°Cofreadingwhensuch
been generated. For an Si solar cell with a short-circuit current
uncertainty is combined with the uncertainty of the sensors
–2 –2
density of 300 Am at 1000 Wm , this amount of charge
themselves.
requires approximately 20 h of illumination.
6.5.1 Sensorssuchasthermocouplesorthermistorsusedfor
7.2 Characterize the reference cell to be calibrated by the
the temperature measurements must be located in a position
that minimizes any temperature gradients between the sensor following methods:
and the photovoltaic device junction. 7.2.1 Quantum Effıciency—Determine the relative quantum
efficiency (optionally the absolute quantum efficiency) of the
6.6 Electrical Measurement Equipment—Voltmeters,
reference cell to be calibrated at 25°C in accordance with Test
ammeters, or other suitable electrical measurement
Methods E1021 and the requirements of 6.7.
instruments, used to measure the short-circuit current, I,ofthe
7.2.1.1 Repetition of 7.2.1 is optional if the quantum effi-
celltobecalibratedandthepyrheliometeroutput, E,musthave
ciencyhasbeenpreviouslymeasuredinaccordancewith7.2.1.
a resolution of at least 0.02% of the maximum current or
7.2.2 Partial Derivative of Quantum Effıciency with Respect
voltage encountered, and a total uncertainty of less than 0.1%
to Temperature—Determine the working temperature range of
of the maximum current or voltage encountered.
the reference cell to be calibrated and measure its Θ (λ)
6.6.1 The electrical measurement equipment should be able D
according to Annex A1 of Test Methods E973.
to record a minimum of 50 to 100 data points during the
calibration time period (see 8.1).
NOTE 1—Test Method E973 requires all quantum efficiency measure-
ments needed for Q (λ,T ) and Θ (λ) be measured with the same
6.7 Quantum Effıciency Measurement Equipment, as re- D 0 D
multiplicative calibration or scaling factors.
quired by Test Method E1021 for spectral responsivity mea-
surements and the following additional requirements: 7.2.2.1 Repetition of 7.2.2 is optional if Θ (λ) has been
D
previously measured in accordance with 7.2.2.
6.7.1 The wavelength interval between successive quantum
efficiency data points shall be 10 nm or less. 7.2.3 Linearity—Determine the short-circuit current versus
irradiance linearity of the cell being calibrated in accordance
6.7.2 For reference cells made with direct bandgap semi-
conductors such as GaAs, it is recommended that the wave- with Test Method E1143 for the irradiance range 750 to 1100
−2
Wm .
length interval be no greater than 5 nm.
6.7.3 The low- and high-wavelength endpoints of the quan- 7.2.3.1 For reference cells that use single-crystal silicon
tum efficiency measurement shall span all wavelengths for solar cells, or for reference cells that have been previously
whichthemeasuredquantumefficiencyaregreaterthan1%of characterized, the short-circuit current versus irradiance linear-
the maximum quantum efficiency. ity determination is optional.
E1125 − 16 (2020)
7.2.4 Fill Factor— Determine the fill factor of the cell to be 9.2.1 Computethemeanshort-circuitcurrent,where nisthe
calibrated from the I-V curve of the device, as measured in number of current values measured in each repetition of 8.3.1:
accordance with Test Methods E948.
n
I 5 I 5 I (3)
^ &
j j i
(
n
i51
8. Procedure
9.2.2 Compute the mean irradiance, where n is the number
8.1 Selectthetimeperiodforasinglecalibrationdatapoint.
of current values measured in each repetition of 8.3.2:
Two factors must be considered: (1) the response time of the
E 5 ^E & (4)
pyrheliometer, and (2) the time required for the spectroradi- j j
ometer to measure a single spectral irradiance.
9.2.3 Compute the short-circuit current range in percent:
8.1.1 Pyrheliometers have response times (defined as the
maxI 2 minI
j j
time required for the instrument to indicate 95 % of a step
I 5 200 (5)
RNGj
maxI 1minI
j j
change of input irradiance) on the order of 1 to 30 s. It is
recommended that the calibration time period span the manu- 9.2.4 Compute the irradiance range in percent:
facturer’s specified response time by a factor of at least five.
maxE 2 min E
j j
8.1.1.1 Absolute cavity radiometers are self-calibrating in- E 5 200 (6)
RNGj
max E 1min E
j j
struments that rely on periodically blocking all light with
–2
9.2.5 Discard any data points for which E is <750Wm or
shutters; the blocked periods must be considered when select-
j
–2
>1100 Wm .
ing the calibration time period.
9.2.6 Discard any data points for which I is >1 %.
8.1.2 Spectroradiometers that use mechanically rotated dif-
RNGj
9.2.7 Discard any data points for which E
...