ASTM E1125-10(2015)
(Test Method)Standard Test Method for Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells Using a Tabular Spectrum
Standard Test Method for Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells Using a Tabular Spectrum
SIGNIFICANCE AND USE
5.1 The electrical output of a photovoltaic device is dependent on the spectral content of the illumination source, its intensity, and the device temperature. To make standardized, accurate measurements of the performance of photovoltaic devices under a variety of light sources, it is necessary to account for the error in the short-circuit current that occurs if the relative spectral response of the reference cell is not identical to the spectral response of the device to be tested. A similar error occurs if the spectral irradiance distribution of the test light source is not identical to the desired reference spectral irradiance distribution. These errors are accounted for by the spectral mismatch parameter (described in Test Method E973), a quantitative measure of the error in the short-circuit current measurement. It is the intent of this test method to provide a recognized procedure for calibrating, characterizing, and reporting the calibration data for primary photovoltaic reference cells using a tabular reference spectrum.
5.2 The calibration of a reference cell is specific to a particular spectral irradiance distribution. It is the responsibility of the user to specify the applicable irradiance distribution, for example Tables G173. This test method allows calibration with respect to any tabular spectrum.
5.3 A reference cell should be recalibrated at yearly intervals, or every six months if the cell is in continuous use outdoors.
5.4 Recommended physical characteristics of reference cells can be found in Specification E1040.
5.5 Because silicon solar cells made on p-type substrates are susceptible to a loss of Isc upon initial exposure to light, it is required that newly manufactured reference cells be light soaked at an irradiance level greater than 850 W/m2 for 2 h prior to initial characterization in Section 7.
SCOPE
1.1 This test method is intended to be used for calibration and characterization of primary terrestrial photovoltaic reference cells to a desired reference spectral irradiance distribution, such as Tables G173. The recommended physical requirements for these reference cells are described in Specification E1040. Reference cells are principally used in the determination of the electrical performance of photovoltaic devices.
1.2 Primary photovoltaic reference cells are calibrated in natural sunlight using the relative spectral response of the cell, the relative spectral distribution of the sunlight, and a tabulated reference spectral irradiance distribution.
1.3 This test method requires the use of a pyrheliometer that is calibrated according to Test Method E816, which requires the use of a pyrheliometer that is traceable to the World Radiometric Reference (WRR). Therefore, reference cells calibrated according to this test method are traceable to the WRR.
1.4 This test method is a technique that may be used instead of the procedures found in Test Method E1362. This test method offers convenience in its ability to characterize a reference cell under any spectrum for which tabulated data are available. The selection of the specific reference spectrum is left to the user.
1.5 This test method applies only to the calibration of a photovoltaic cell that shows a linear dependence of its short-circuit current on irradiance over its intended range of use, as defined in Test Method E1143.
1.6 This test method applies only to the calibration of a reference cell fabricated with a single photovoltaic junction.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E1125 − 10(Reapproved 2015)
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 to be used for calibration
responsibility of the user of this standard to establish appro-
and characterization of primary terrestrial photovoltaic refer-
priate safety and health practices and determine the applica-
ence cells to a desired reference spectral irradiance
bility of regulatory limitations prior to use.
distribution, such as Tables G173. The recommended physical
requirements for these reference cells are described in Speci-
2. Referenced Documents
fication E1040. Reference cells are principally used in the
2.1 ASTM Standards:
determination of the electrical performance of photovoltaic
E772Terminology of Solar Energy Conversion
devices.
E816Test Method for Calibration of Pyrheliometers by
1.2 Primary photovoltaic reference cells are calibrated in
Comparison to Reference Pyrheliometers
natural sunlight using the relative spectral response of the cell,
E948Test Method for Electrical Performance of Photovol-
therelativespectraldistributionofthesunlight,andatabulated
taic Cells Using Reference Cells Under Simulated Sun-
reference spectral irradiance distribution.
light
1.3 Thistestmethodrequirestheuseofapyrheliometerthat
E973Test Method for Determination of the Spectral Mis-
is calibrated according to Test Method E816, which requires match Parameter Between a Photovoltaic Device and a
the use of a pyrheliometer that is traceable to the World
Photovoltaic Reference Cell
Radiometric Reference (WRR). Therefore, reference cells E1021TestMethodforSpectralResponsivityMeasurements
calibrated according to this test method are traceable to the
of Photovoltaic Devices
WRR. E1040Specification for Physical Characteristics of Noncon-
centrator Terrestrial Photovoltaic Reference Cells
1.4 Thistestmethodisatechniquethatmaybeusedinstead
E1328Terminology Relating to Photovoltaic Solar Energy
of the procedures found in Test Method E1362. This test
Conversion (Withdrawn 2012)
method offers convenience in its ability to characterize a
E1362Test Method for Calibration of Non-Concentrator
reference cell under any spectrum for which tabulated data are
Photovoltaic Secondary Reference Cells
available. The selection of the specific reference spectrum is
G173TablesforReferenceSolarSpectralIrradiances:Direct
left to the user.
Normal and Hemispherical on 37° Tilted Surface
1.5 This test method applies only to the calibration of a
photovoltaic cell that shows a linear dependence of its short-
3. Terminology
circuit current on irradiance over its intended range of use, as
3.1 Definitions—Definitions of terms used in this test
defined in Test Method E1143.
method may be found in Terminology E772 and Terminology
1.6 This test method applies only to the calibration of a
E1328.
reference cell fabricated with a single photovoltaic junction.
3.2 Symbols:
1.7 The values stated in SI units are to be regarded as
3.2.1 The following symbols and units are used in this test
standard. No other units of measurement are included in this
method:
standard.
λ—Wavelength, nm or µm,
1 2
This test method is under the jurisdiction of ASTM Committee E44 on Solar, For referenced ASTM standards, visit the ASTM website, www.astm.org, or
GeothermalandOtherAlternativeEnergySourcesandisthedirectresponsibilityof contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Subcommittee E44.09 on Photovoltaic Electric Power Conversion. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved March 1, 2015. Published April 2015. Originally the ASTM website.
approved in 1986. Last previous edition approved in 2010 as E1125–10. DOI: The last approved version of this historical standard is referenced on
10.1520/E1125-10R15. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1125 − 10 (2015)
I —Short-circuit current, A, recognized procedure for calibrating, characterizing, and re-
sc
−2
E—Irradiance, Wm , porting the calibration data for primary photovoltaic reference
−2
E—Total irradiance, Wm , cells using a tabular reference spectrum.
t
−2 −1
E(λ)—Spectral irradiance, Wm µm ,
5.2 The calibration of a reference cell is specific to a
−1
R(λ)—Spectral response,AW ,
particular spectral irradiance distribution. It is the responsibil-
−1
R (λ)—Reference cell spectral response, AW ,
r
ity of the user to specify the applicable irradiance distribution,
T—Temperature, °C,
for example Tables G173. This test method allows calibration
−1
α—Temperature coefficient of reference cell I ,°C ,
sc
with respect to any tabular spectrum.
n—Total number of data points,
2 −1
5.3 A reference cell should be recalibrated at yearly
C—Calibration constant,Am W ,
intervals, or every six months if the cell is in continuous use
M—Spectral mismatch parameter,
outdoors.
F—Spectral correction factor, and
S—Standard deviation.
5.4 Recommended physical characteristics of reference
cells can be found in Specification E1040.
4. Summary of Test Method
5.5 Becausesiliconsolarcellsmadeonp-typesubstratesare
4.1 The calibration of a primary photovoltaic reference cell
susceptible to a loss of I upon initial exposure to light, it is
sc
consists of measuring the short-circuit current of the cell when
required that newly manufactured reference cells be light
illuminated with natural sunlight, along with the total solar
soaked at an irradiance level greater than 850 W/m for2h
irradiance using a pyrheliometer. The ratio of the short-circuit
prior to initial characterization in Section 7.
current of the cell to the irradiance, divided by a correction
factor similar to the spectral mismatch parameter defined in
6. Apparatus
Test Method E973, is the calibration constant for the reference
6.1 Pyrheliometer— A secondary reference pyrheliometer
cell. Also, if the temperature of the cell is not 25 6 1°C, the
that is calibrated in accordance with Test Method E816.An
short-circuit current must be corrected to 25°C.
absolute cavity radiometer may also be used. Because second-
4.1.1 The relative spectral irradiance of the sunlight is
ary reference pyrheliometers are calibrated against an absolute
measured using a spectral irradiance measurement instrument
cavityradiometer,thetotaluncertaintyintheprimaryreference
as specified in Test Method E973.
cell calibration constant will be reduced if an absolute cavity
4.2 The following is a list of measurements that are used to
radiometer is used.
characterize reference cells and are reported with the calibra-
6.2 Collimator—A collimator fitted to the reference cell
tion data:
during calibration that has the same field-of-view as the
4.2.1 The spectral response of the cell is determined in
pyrheliometer.An acceptable collimator design is described in
accordance with Test Methods E1021.
Annex A1.
4.2.2 The cell’s short-circuit current temperature coefficient
6.3 Spectral Irradiance Measurement Equipment, as re-
is determined experimentally by measuring the short-circuit
quired by Test Method E973.
currentatvarioustemperaturesandcomputingthetemperature
6.3.1 The spectral range of the spectral irradiance measure-
coefficient (see 7.2.2).
ment shall be wide enough to include the spectral response of
4.2.3 Linearity of short-circuit current versus irradiance is
the cell to be calibrated.
determined in accordance with Test Method E1143.
6.3.2 The spectral range of the spectral irradiance measure-
4.2.4 Thefillfactorofthereferencecellisdeterminedusing
ment shall include 98% of the total irradiance to which the
TestMethodE948.Providingthefillfactorwiththecalibration
pyrheliometer is sensitive.
data allows the reference cell to be checked in the future for
6.3.3 If the spectral irradiance measurement is unable to
electrical degradation or damage.
measure the entire wavelength range required by 6.3.2,itis
5. Significance and Use acceptabletouseareferencespectrum,suchasTablesG173,to
supply the missing wavelengths. The reference spectrum is
5.1 The electrical output of a photovoltaic device is depen-
scaled to match the measured spectral irradiance data over a
dent on the spectral content of the illumination source, its
convenientwavelengthintervalwithinthewavelengthrangeof
intensity, and the device temperature. To make standardized,
the spectral irradiance measurement equipment. It is also
accurate measurements of the performance of photovoltaic
acceptable to calculate the missing spectral irradiance data
devices under a variety of light sources, it is necessary to
using a numerical model.
account for the error in the short-circuit current that occurs if
6.3.4 The spectral irradiance measurement equipment shall
the relative spectral response of the reference cell is not
have the same field-of-view as the pyrheliometer and the
identical to the spectral response of the device to be tested. A
reference cell collimator.
similarerroroccursifthespectralirradiancedistributionofthe
testlightsourceisnotidenticaltothedesiredreferencespectral 6.4 Normal Incidence Tracking Platforms—Tracking plat-
irradiance distribution. These errors are accounted for by the forms used to follow the sun during the calibration and to hold
spectralmismatchparameter(describedinTestMethodE973), the reference cell to be calibrated, the pyrheliometer, the
a quantitative measure of the error in the short-circuit current collimator, and spectral irradiance measurement equipment.
measurement. It is the intent of this test method to provide a The pyrheliometer and the collimator must be parallel within
E1125 − 10 (2015)
60.25°. The platforms shall be able to track the sun within 7.2.3.1 For reference cells that use single-crystal silicon
60.5° during the calibration procedure. solar cells, or for reference cells that have been previously
characterized,theshort-circuitcurrentversusirradiancelinear-
6.5 Temperature Measurement Equipment—An instrument
ity determination is optional.
orinstrumentsusedtomeasurethetemperatureofthereference
7.2.4 Fill Factor—Determinethefillfactorofthecelltobe
cell to be calibrated, that has a resolution of at least 0.1°C, and
calibrated from the I-V curve of the device, as measured in
a total error of less than 61°C of reading.
accordance with Test Methods E948.
6.5.1 Sensorssuchasthermocouplesorthermistorsusedfor
the temperature measurements must be located in a position
8. Procedure
that minimizes any temperature gradients between the sensor
and the photovoltaic device junction.
8.1 Mountthereferencecelltobecalibrated,thecollimator,
the pyrheliometer, and the spectral irradiance measurement
6.6 Electrical Measurement Equipment—Voltmeters,
equipment on the tracking platforms.
ammeters, or other suitable electrical measurement
instruments, used to measure the I of the cell to be calibrated
8.2 Measuretherelativespectralirradianceofthesun, E(λ),
sc
and the pyrheliometer output, that have a resolution of at least
using the spectral irradiance measurement instrument (see 6.6)
0.02% of the maximum current or voltage encountered, and a
and the procedure of Test Method E973. During the spectral
total error of less than 0.1% of the maximum current or
irradiance measurement, perform the following:
voltage encountered.
8.2.1 Measure the pyrheliometer output, E, and verify that
t
−2 −2
the total irradiance is between 750 Wm and 1100 Wm .
6.7 Spectral Response Measurement Equipment,asrequired
8.2.2 Measure the short-circuit current of the reference cell,
by Test Method E1021.
I .
6.7.1 The wavelength interval between spectral response
sc
8.2.3 Measure the reference cell temperature, T.
data points shall be a maximum of 50 nm.
8.2.4 Repeat 8.2.1 and 8.2.2 at least four times. These
6.8 Temperature Control Block (Optional)—A device to
repetitions must be distributed in time during the spectral
maintain the temperature of the reference cell at 25 6 1°C for
irradiance measurement. To assure temporal stability, the
the duration of the calibration.
short-circuitcurrentofthereferencecellshallnotvarybymore
than 60.2% during the repetitions.
7. Characterization
8.2.5 Average the short-circuit current and total irradiance
7.1 Prior to the characterization measurements, illuminate
valuesfrom8.2.4toobtainthe I and E thatcorrespondstothe
sc t
−2
the reference cell to be calibrated at 1000 Wm for 2 h. This
spectral irradiance measurement.
is necessary to stabilize any light-induced degradation of the
8.3 Perform a minimum of five replications of 8.2.
cell prior to calibration.
8.3.1 The five replications must be performed on at least
7.2 Characterize the reference cell being calibrated by the
three separate days. Therefore, five replications all performed
following methods:
on the same day would not be an acceptable data set for the
7.2.1 Spectral Response—Determine the relative spectral
calibration.
response, R(λ), (optionally the absolute spectral response) of
8.3.2 In order to reduce precision errors through averaging,
the cell to be calibrated in accordance with Test Methods
it is recommended that at least 30 replications of 8.2 be
E1021.
performed.
7.2.2 Temperature Coeffıcient—Determine the temperature
coefficient, α, of the cell to be calibrated as follows:
9. Calculation of Results
7.2.2.1 Using the electrical measurement equipment, mea-
9.1 Each spectral irradiance measurement obtained in 8.2
sure I at four or more temperatures over at least a 50°C
sc
defines one data point.The total number of these data points is
temperature range centered around 35°C. The irradiance shall
−2 −2
denoted as n.
be at least 750 Wm and less than 1100 Wm , as measured
9.1.1 For each data point, calculate the spectral correction
with a second reference cell. Measure the temperature of the
factor, F, using the spectral mismatch parameter calculation,
being calibrated at the same time.
8.1, of Test Method E973. To perform this calculation, replace
7.2.2.2 Divide each value of I by the normalized instanta-
sc
the reference cell spectral response R (λ) with unity (this
r
neous irradiance level at the time of each measurement.
represents the spectral response of the py
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E1125 − 10 E1125 − 10 (Reapproved 2015)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method is intended to be used for calibration and characterization of primary terrestrial photovoltaic reference cells
to a desired reference spectral irradiance distribution, such as Tables G173. The recommended physical requirements for these
reference cells are described in Specification E1040. Reference cells are principally used in the determination of the electrical
performance of photovoltaic devices.
1.2 Primary photovoltaic reference cells are calibrated in natural sunlight using the relative spectral response of the cell, the
relative spectral distribution of the sunlight, and a tabulated reference spectral irradiance distribution.
1.3 This test method requires the use of a pyrheliometer that is calibrated according to Test Method E816, which requires the
use of a pyrheliometer that is traceable to the World Radiometric Reference (WRR). Therefore, reference cells calibrated according
to this test method are traceable to the WRR.
1.4 This test method is a technique that may be used instead of the procedures found in Test Method E1362. This test method
offers convenience in its ability to characterize a reference cell under any spectrum for which tabulated data are available. The
selection of the specific reference spectrum is left to the user.
1.5 This test method applies only to the calibration of a photovoltaic cell that shows a linear dependence of its short-circuit
current on irradiance over its intended range of use, as defined in Test Method E1143.
1.6 This test method applies only to the calibration of a reference cell fabricated with a single photovoltaic junction.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E772 Terminology of Solar Energy Conversion
E816 Test Method for Calibration of Pyrheliometers by Comparison to Reference Pyrheliometers
E948 Test Method for Electrical Performance of Photovoltaic Cells Using Reference Cells Under Simulated Sunlight
E973 Test Method for Determination of the Spectral Mismatch Parameter Between a Photovoltaic Device and a Photovoltaic
Reference Cell
E1021 Test Method for Spectral Responsivity Measurements of Photovoltaic Devices
E1040 Specification for Physical Characteristics of Nonconcentrator Terrestrial Photovoltaic Reference Cells
E1328 Terminology Relating to Photovoltaic Solar Energy Conversion (Withdrawn 2012)
E1362 Test Method for Calibration of Non-Concentrator Photovoltaic Secondary Reference Cells
G173 Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface
This test method is under the jurisdiction of ASTM Committee E44 on Solar, Geothermal and Other Alternative Energy Sources and is the direct responsibility of
Subcommittee E44.09 on Photovoltaic Electric Power Conversion.
Current edition approved June 1, 2010March 1, 2015. Published July 2010April 2015. Originally approved in 1986. Last previous edition approved in 20052010 as
E1125 – 05.E1125 – 10. DOI: 10.1520/E1125-10.10.1520/E1125-10R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1125 − 10 (2015)
3. Terminology
3.1 Definitions—Definitions of terms used in this test method may be found in Terminology E772 and Terminology E1328.
3.2 Symbols:
3.2.1 The following symbols and units are used in this test method:
λ—Wavelength, nm or μm,
I —Short-circuit current, A,
sc
−2
E—Irradiance, Wm ,
−2
E —Total irradiance, Wm ,
t
−2 −1
E(λ)—Spectral irradiance, Wm μm ,
−1
R(λ)—Spectral response, AW ,
−1
R (λ)—Reference cell spectral response, AW ,
r
T—Temperature, °C,
−1
α—Temperature coefficient of reference cell I , °C ,
sc
n—Total number of data points,
2 −1
C—Calibration constant, Am W ,
M—Spectral mismatch parameter,
F—Spectral correction factor, and
S—Standard deviation.
4. Summary of Test Method
4.1 The calibration of a primary photovoltaic reference cell consists of measuring the short-circuit current of the cell when
illuminated with natural sunlight, along with the total solar irradiance using a pyrheliometer. The ratio of the short-circuit current
of the cell to the irradiance, divided by a correction factor similar to the spectral mismatch parameter defined in Test Method E973,
is the calibration constant for the reference cell. Also, if the temperature of the cell is not 25 6 1°C, the short-circuit current must
be corrected to 25°C.
4.1.1 The relative spectral irradiance of the sunlight is measured using a spectral irradiance measurement instrument as specified
in Test Method E973.
4.2 The following is a list of measurements that are used to characterize reference cells and are reported with the calibration
data:
4.2.1 The spectral response of the cell is determined in accordance with Test Methods E1021.
4.2.2 The cell’s short-circuit current temperature coefficient is determined experimentally by measuring the short-circuit current
at various temperatures and computing the temperature coefficient (see 7.2.2).
4.2.3 Linearity of short-circuit current versus irradiance is determined in accordance with Test Method E1143.
4.2.4 The fill factor of the reference cell is determined using Test Method E948. Providing the fill factor with the calibration
data allows the reference cell to be checked in the future for electrical degradation or damage.
5. Significance and Use
5.1 The electrical output of a photovoltaic device is dependent on the spectral content of the illumination source, its intensity,
and the device temperature. To make standardized, accurate measurements of the performance of photovoltaic devices under a
variety of light sources, it is necessary to account for the error in the short-circuit current that occurs if the relative spectral response
of the reference cell is not identical to the spectral response of the device to be tested. A similar error occurs if the spectral
irradiance distribution of the test light source is not identical to the desired reference spectral irradiance distribution. These errors
are accounted for by the spectral mismatch parameter (described in Test Method E973), a quantitative measure of the error in the
short-circuit current measurement. It is the intent of this test method to provide a recognized procedure for calibrating,
characterizing, and reporting the calibration data for primary photovoltaic reference cells using a tabular reference spectrum.
5.2 The calibration of a reference cell is specific to a particular spectral irradiance distribution. It is the responsibility of the user
to specify the applicable irradiance distribution, for example Tables G173. This test method allows calibration with respect to any
tabular spectrum.
5.3 A reference cell should be recalibrated at yearly intervals, or every six months if the cell is in continuous use outdoors.
5.4 Recommended physical characteristics of reference cells can be found in Specification E1040.
5.5 Because silicon solar cells made on p-type substrates are susceptible to a loss of I upon initial exposure to light, it is
sc
required that newly manufactured reference cells be light soaked at an irradiance level greater than 850 W/m for 2 h prior to initial
characterization in Section 7.
E1125 − 10 (2015)
6. Apparatus
6.1 Pyrheliometer— A secondary reference pyrheliometer that is calibrated in accordance with Test Method E816. An absolute
cavity radiometer may also be used. Because secondary reference pyrheliometers are calibrated against an absolute cavity
radiometer, the total uncertainty in the primary reference cell calibration constant will be reduced if an absolute cavity radiometer
is used.
6.2 Collimator—A collimator fitted to the reference cell during calibration that has the same field-of-view as the pyrheliometer.
An acceptable collimator design is described in Annex A1.
6.3 Spectral Irradiance Measurement Equipment, as required by Test Method E973.
6.3.1 The spectral range of the spectral irradiance measurement shall be wide enough to include the spectral response of the cell
to be calibrated.
6.3.2 The spectral range of the spectral irradiance measurement shall include 98 % of the total irradiance to which the
pyrheliometer is sensitive.
6.3.3 If the spectral irradiance measurement is unable to measure the entire wavelength range required by 6.3.2, it is acceptable
to use a reference spectrum, such as Tables G173, to supply the missing wavelengths. The reference spectrum is scaled to match
the measured spectral irradiance data over a convenient wavelength interval within the wavelength range of the spectral irradiance
measurement equipment. It is also acceptable to calculate the missing spectral irradiance data using a numerical model.
6.3.4 The spectral irradiance measurement equipment shall have the same field-of-view as the pyrheliometer and the reference
cell collimator.
6.4 Normal Incidence Tracking Platforms—Tracking platforms used to follow the sun during the calibration and to hold the
reference cell to be calibrated, the pyrheliometer, the collimator, and spectral irradiance measurement equipment. The
pyrheliometer and the collimator must be parallel within 60.25°. The platforms shall be able to track the sun within 60.5° during
the calibration procedure.
6.5 Temperature Measurement Equipment—An instrument or instruments used to measure the temperature of the reference cell
to be calibrated, that has a resolution of at least 0.1°C, and a total error of less than 61°C of reading.
6.5.1 Sensors such as thermocouples or thermistors used for the temperature measurements must be located in a position that
minimizes any temperature gradients between the sensor and the photovoltaic device junction.
6.6 Electrical Measurement Equipment—Voltmeters, ammeters, or other suitable electrical measurement instruments, used to
measure the I of the cell to be calibrated and the pyrheliometer output, that have a resolution of at least 0.02 % of the maximum
sc
current or voltage encountered, and a total error of less than 0.1 % of the maximum current or voltage encountered.
6.7 Spectral Response Measurement Equipment, as required by Test Method E1021.
6.7.1 The wavelength interval between spectral response data points shall be a maximum of 50 nm.
6.8 Temperature Control Block (Optional)—A device to maintain the temperature of the reference cell at 25 6 1°C for the
duration of the calibration.
7. Characterization
−2
7.1 Prior to the characterization measurements, illuminate the reference cell to be calibrated at 1000 Wm for 2 h. This is
necessary to stabilize any light-induced degradation of the cell prior to calibration.
7.2 Characterize the reference cell being calibrated by the following methods:
7.2.1 Spectral Response—Determine the relative spectral response, R(λ), (optionally the absolute spectral response) of the cell
to be calibrated in accordance with Test Methods E1021.
7.2.2 Temperature Coeffıcient—Determine the temperature coefficient, α, of the cell to be calibrated as follows:
7.2.2.1 Using the electrical measurement equipment, measure I at four or more temperatures over at least a 50°C temperature
sc
−2 −2
range centered around 35°C. The irradiance shall be at least 750 Wm and less than 1100 Wm , as measured with a second
reference cell. Measure the temperature of the being calibrated at the same time.
7.2.2.2 Divide each value of I by the normalized instantaneous irradiance level at the time of each measurement.
sc
NOTE 1—The normalized instantaneous irradiance can be determined by dividing the second reference cell’s I by its calibration constant.
sc
7.2.2.3 Determine the temperature coefficient by performing a least-squares fit of the I versus T data to a straight line. The
sc
slope of the line divided by the value of the current from the least-squares fit at 25°C is the temperature coefficient, α.
7.2.3 Linearity—Determine the short-circuit current versus irradiance linearity of the cell being calibrated in accordance with
−2
Test Method E1143 for the irradiance range 750 to 1100 Wm .
7.2.3.1 For reference cells that use single-crystal silicon solar cells, or for reference cells that have been previously
characterized, the short-circuit current versus irradiance linearity determination is optional.
7.2.4 Fill Factor— Determine the fill factor of the cell to be calibrated from the I-V curve of the device, as measured in
accordance with Test Methods E948.
E1125 − 10 (2015)
8. Procedure
8.1 Mount the reference cell to be calibrated, the collimator, the pyrheliometer, and the spectral irradiance measurement
equipment on the tracking platforms.
8.2 Measure the relative spectral irradiance of the sun, E(λ), using the spectral irradiance measurement instrument (see 6.6) and
the procedure of Test Method E973. During the spectral irradiance measurement, perform the following:
−2 −2
8.2.1 Measure the pyrheliometer output, E , and verify that the total irradiance is between 750 Wm and 1100 Wm .
t
8.2.2 Measure the short-circuit current of the reference cell, I .
sc
8.2.3 Measure the reference cell temperature, T.
8.2.4 Repeat 8.2.1 and 8.2.2 at least four times. These repetitions mus
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