IEC 60904-7:2008

IEC 60904-7
Edition 3.0 2008-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic devices –
Part 7: Computation of the spectral mismatch correction for measurements of
photovoltaic devices
Dispositifs photovoltaïques –
Partie 7: Calcul de la correction de désadaptation des réponses spectrales dans
les mesures de dispositifs photovoltaïques

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IEC 60904-7
Edition 3.0 2008-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic devices –
Part 7: Computation of the spectral mismatch correction for measurements of
photovoltaic devices
Dispositifs photovoltaïques –
Partie 7: Calcul de la correction de désadaptation des réponses spectrales dans
les mesures de dispositifs photovoltaïques

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
K
CODE PRIX
ICS 27.160 ISBN 978-2-88910-324-9
– 2 – 60904-7 © IEC:2008
CONTENTS
FOREWORD.3

1 Scope and object.5

2 Normative references .5

3 Description of method.6

4 Apparatus.7

5 Determination of spectral response .7

6 Determination of test spectrum .7

7 Determination of the spectral mismatch factor .8
8 Report .9
Bibliography.10

60904-7 © IEC:2008 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
PHOTOVOLTAIC DEVICES –
Part 7: Computation of the spectral mismatch correction

for measurements of photovoltaic 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
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6) All users should ensure that they have the latest edition of this publication.
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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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60904-7 has been prepared by IEC technical committee 82: Solar
photovoltaic energy systems.
The text of this standard is based on the following documents:
FDIS Report on voting
82/540/FDIS 82/547/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This third edition cancels and replaces the second edition published in 1998. It constitutes a
technical revision. The main changes with respect to the previous edition are listed below:
• the title has been modified in order to better reflect the purpose of the standard (changed
from “mismatch error” to “mismatch correction”);

– 4 – 60904-7 © IEC:2008
• formulae are now accompanied by explanatory text;

• Clause 3 “Description of method” now describes when it is necessary to use the method
and when it may not be needed. It describes what data must be collected before the

mismatch correction can be calculated;

• Clauses 4, 5 and 6 have added;

• the formula for the mismatch correction has been corrected.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts of 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 publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
60904-7 © IEC:2008 – 5 –
PHOTOVOLTAIC DEVICES –
Part 7: Computation of the spectral mismatch correction

for measurements of photovoltaic devices

1 Scope and object
This part of IEC 60904 describes the procedure for correcting the bias error introduced in the
testing of a photovoltaic device, caused by the mismatch between the test spectrum and the
reference spectrum and by the mismatch between the spectral responses (SR) of the
reference cell and of the test specimen. The procedure applies only to photovoltaic devices
linear in SR as defined in IEC 60904-10. This procedure is valid for single junction devices
but the principle may be extended to cover multijunction devices.
The purpose of this standard is to give guidelines for the correction of measurement bias,
should there be a mismatch between both the test spectrum and the reference spectrum and
between the reference device SR and the test specimen SR.
Since a PV device has a wavelength-dependent response, its performance is significantly
affected by the spectral distribution of the incident radiation, which in natural sunlight varies
with several factors such as location, weather, time of year, time of day, orientation of the
receiving surface, etc., and with a simulator varies with its type and conditions. If the
irradiance is measured with a thermopile-type radiometer (that is not spectrally selective) or
with a reference solar cell, the spectral irradiance distribution of the incoming light must be
known to make the necessary corrections to obtain the performance of the PV device under
the reference solar spectral distribution defined in IEC 60904-3.
If a reference PV device or a thermopile type detector is used to measure the irradiance then,
following the procedure given in this standard, it is possible to calculate the spectral mismatch
correction necessary to obtain the short-circuit current of the test PV device under the
reference solar spectral irradiance distribution included in Table 1 of IEC 60904-3 or any
other reference spectrum. If the reference PV device has the same relative spectral response
as the test PV device then the reference device automatically takes into account deviations of
the real light spectral distribution from the standard spectral distribution, and no further
correction of spectral bias errors is necessary. In this case, location and weather conditions
are not critical when the reference device method is used for outdoor performance
measurements provided both reference cell and test PV device have the same relative
spectral response. Also, for identical relative SR’s, the spectral classification of the simulator
is not critical for indoor measurements.
If the performance of a PV device is measured using a known spectral irradiance distribution,
its short-circuit current at any other spectral irradiance distribution can be computed using the
spectral response of the PV test device.
2 Normative references
The following referenced documents are indispensable for the application 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, Procedures for temperature and irradiance corrections to measured I-V
characteristics of crystalline silicon photovoltaic devices

– 6 – 60904-7 © IEC:2008
IEC 60904-1, Photovoltaic devices – Part 1: Measurement of photovoltaic current-voltage

characteristics
IEC 60904-2, Photovoltaic devices – Part 2: Requirements for reference solar devices

IEC 60904-3, Photovoltaic devices – Part 3: Measurement principles for terrestrial

photovoltaic (PV) solar devices with reference spectral irradiance data

IEC 60904-8, Photovoltaic devices – Part 8: Measurement of spectral response of a

photovoltaic (PV) device
IEC 60904-9, Photovoltaic devices – Part 9: Solar simulator performance requirements
IEC 60904-10, Photovoltaic devices – Part 10 Methods of linearity measurement
IEC 61215, Crystalline silicon terrestrial photovoltaic (PV) modules – Design qualification and
type approval
IEC 61646, Thin film terrestrial photovoltaic (PV) modules – Design qualification and type
approval
3 Description of method
For many PV devices, the shape of the I-V characteristic depends on the short-circuit current
and the device temperature, but not on the spectrum used to generate the short-circuit
current. For these devices, the correction of spectrum mismatch or spectral response
mismatch is possible using the following procedure. For other devices, a measurement of the
I-V characteristic shall be done using a light source with the appropriate spectrum.
A correction is not necessary if either the test spectrum is identical to the reference spectrum
(see IEC 60904-3) or if the test specimen’s relative spectral response is identical to the
reference cell relative spectral response. In this case, the reading as obtained from the
reference cell specifies which intensity at the reference spectrum will generate the same
short-circuit current in the test device as the test spectrum.
If there is a mismatch between both spectra and spectral responses then a mismatch
correction should be calculated.
Due to the mismatch in spectra and spectral responses, the reading of the reference cell (see
IEC 60904-2) does not give the intensity of the reference spectrum that generates the short-

circuit current as measured for the test device. One must determine the effective irradiance of
the reference spectrum that generates the same short-circuit current in the test device as
generated by the test spectrum at the measured irradiance G .
meas
G = MM × G (1)
eff at ref spectrum meas
where G is the irradiance as measured by the reference device with its specific spectral
meas
response S (λ) and MM is the spectral mismatch factor as determined in Clause 7.
ref
For a measurement to be referred to the reference spectral irradiance, two correction methods
are possible:
a) If possible, adjust the simulator intensity so that the effective irradiance as determined by
equation (1) equals the reference irradiance G (e.g. 1 000 W/m for STC, as defined in
ref
IEC 61215 and IEC 61646). That is to say that the simulator intensity as measured by the
reference cell using its calibration value given for the reference spectrum has to be set to

60904-7 © IEC:2008 – 7 –
G = G /MM (2)
meas ref
Thus, the inverse mismatch factor 1/MM gives the degree by which the simulator intensity

has to be adjusted, if the device is linear (see IEC 60904-10). Now, the simulator

spectrum +++at this irradiance with its actual simulator spectrum generates the same

short-circuit current as the reference spectrum at the reference intensity. Proceed to

measure the I-V characteristic per IEC 60904-1.

b) Otherwise, measure the I-V characteristic using the given simulator intensity. Determine
the effective irradiance at the reference spectrum using equation (1). Then transfer the I-V

characteristic to the reference irradiance using IEC 60891 with the effective irradiance

determined from equation (1).
Method a) is preferred for simulated sunlight (see IEC 60904-9), as the actual measurement
is performed at the correct short-circuit current, minimising non-linearity errors. Method b) is
usually chosen for outdoor measurements, if the light intensity cannot be easily controlled.
4 Apparatus
4.1 Spectral response measurement set up according to IEC 60904-8.
4.2 Apparatus for measurement of PV current voltage characteristics according to
IEC 60904-1.
4.3 Spectroradiometer capable of measuring the spectral irradiance in the test plane in a
spectral range exceeding that of the spectral responses of the reference and test devices.
NOTE 1 For example spectroradiometer measurements are described in CIE 63 (1984).
NOTE 2 The input head of the spectroradiometer and the test device should have a similar field of view
with a similar dependency of the solid angle.
5 Determination of spectral response
5.1 The relative spectral response of the test specimen shall be measured according to
IEC 60904-8.
5.2 If not available from the calibration documents, the relative spectral response of the
reference device shall be measured according to IEC 60904-8.
6 Determination of test spectrum

6.1 Mount the input head of the spectroradiometer in the position where the test device will
subsequently be mounted, or as close as possible to that location. It shall be mounted
coplanar to the test specimen within ± 2°.
6.2 Record the spectrum of the light source. For simulator measurements, steps of 2 nm or
less with 2-5 nm bandwidths are recommended. For outdoor spectra, steps and bandwidth of
up to 10 nm are allowable. Verify that the total irradiance does not vary by more than ± 2 %
during this measurement. If necessary, apply a linear intensity correction to all measurement
points with respect to the actual total irradiance. Alternatively, several scans can be taken,
they shall agree within ± 2 %. Then determine the average relative spectrum.
6.3 If the acquisition time for a full spectrum is larger than the acquisition time for the I-V
characteristic, or if the light source is not spectrally stable over time (e.g. flash simulators or
natural sunlight), special care must be given to determine the correct test spectrum.

– 8 – 60904-7 © IEC:2008
NOTE 1 A pulsed simulator may not be spectrally stable during the I-V measurement period. Also, at the rising

and falling edge of the pulse, the spectrum may be different from the spectrum during the designated measurement
time. Therefore, it may not be correct to measure the spectrum with an integration time including the rise and tail of

the pulse.
NOTE 2 Outdoor spectra may not be stable due to changes in the atmospheric conditions.

7 Determination of the spectral mismatch factor

Determine the spectral mismatch factor from

E (λ)S ( λ) dλ E (λ) S (λ) dλ
ref ref meas sample
∫ ∫
(3)
MM =
E (λ )S (λ ) dλ E (λ) S (λ) dλ
meas ref ref sample
∫∫
where
E ( λ ) is the irradiance per unit bandwidth at a particular wavelength λ , of the reference
ref
spectral irradiance distribution, for example as given in IEC 60904-3;
E ( λ ) is the irradiance per unit bandwidth at a particular wavelength λ , of the spectral
meas
irradiance distribution of the incoming light at the time of measurement;
S (λ) is the spectral response of the reference PV device;
ref
S (λ) is the spectral response of the test PV device.
sample
All integrals must be performed in the full spectral range where the reference device and the
sample are spectrally sensitive.
NOTE 1 The spectral irradiance distributions and the spectral responses can be given on an absolute or relative.
scale.
NOTE 2 Due to the irregular shape of the solar and simulator spectra, spectral responses should be interpolated
to the wavelength points of the spectral irradiance measurements, not vice versa.
NOTE 3 Equation 3 is valid for single junction devices, but may be used for multi-junction devices. For multi-
junction devices, the calculation must be performed for each junction in the device, using its spectral response
including the spectral filtering caused by the junctions above the junction under consideration. The test report
should specify the mismatch factors and the relative current generation of the individual junctions.
NOTE 4 The integral boundaries should be the boundary wavelengths of the SR.
In the case, that absolute spectra and absolute spectral responses are used for the analysis,
Equation 3 can be interpreted as

I I
sc,ref,E sc,sample,E
ref meas
MM = (4)
I I
sc,ref,E sc,sample,E
meas ref
where
I is the short-circuit current of the test sample under the reference spectrum;

sc, sample, E
ref
I  is the short-circuit current of the reference device under the reference
sc, ref, E
ref
spectrum;
I is the short-circuit current of the test sample under the measured spectrum;

sc, sample, Emeas
I is the short-circuit current of the reference device under the measured

sc, ref, E
meas
spectrum
because I = E()λ S()λ dλ
sc
∫
60904-7 © IEC:2008 – 9 –
8 Report
The following information should be given in the test report according to IEC 60904-1.

a) If the spectral mismatch is used for the irradiance correction of a measurement based on

IEC 60904-1 or another relevant standard, the calculated spectral mismatch factor, the

identification o
...