CALIBRATION PROCEDURES – STATE OF THE ART

H. OssenbrinkEuropean Commission, DG Joint Research Centre,

Institute for Environment and Sustainability, Renewable Energies Unit, Ispra, Italy

ABSTRACT

Solar Cell Calibration plays an important role fordetermining the efficiency of photovoltaic devices underdefined conditions. Laboratories involved in PVcharacterisation developed a number of calibrationmethods and procedures, which progress towardsdecreasing uncertainty, and worldwide acceptance. Thispaper gives an overview about the methods used, theirspecific features and applicability. The overview is limitedto reference devices, and does not discuss standardperformance measurements on PV devices.

1. INTRODUCTION

The most visible figure-of-merit of photovoltaic solarcells is their efficiency. Since first solar cells flew inspace, measurement of efficiency and power deliveredunder defined conditions was a key to improve solar cells,to understand parameters influencing their performanceand to forecast their economic value. Calibration is todayunderstood as the process, to determine the short-circuitcurrent of photovoltaic devices under reference conditionswith respect to device temperature, irradiance level andspectral distribution. As it would be economically notfeasible, to calibrate all solar cells or modules inproduction with the required precision and uncertainly, useis made of reference cells, which, once thoroughlycalibrated, serve as comparison in a simplifiedmeasurement procedure of short circuit current of thedevice under test. Calibration of reference devices in turncan make use of sophisticated methods, which developedin particular since the use of solar cells in terrestrialapplications. A key element in calibration is the referenceto standard test conditions (STC), which define a solar celljunction temperature of 25°C, an irradiance exposed to of1000 W/m2, and a tabular solar spectral distribution of AirMass 1.5, which corresponds to a sun elevation of 37° atsea level. Whilst temperature and irradiance can beestablished either in the laboratory or under naturalconditions, or at least corrected for, the reference solarspectral distribution is neither to simulate with sufficientmatch, nor can it be found routinely under naturalsunlight. Almost all calibration methods have their own,characteristic approach to take this problem into account.

In contrast to PV solar cells, thermopile detectors,which measure heat generated by the incident light, arespectrally almost unselective, representing practically ablack body, and calibration of such devices can beperformed with less sophisticated methods. A set ofthermopile detectors are also the primary reference for theWorld Radiometric Reference, established by the WorldMeteorological Organization, and serve as a traceability

standard to determine irradiance in the magnitude of solarirradiance.

2. RELEVANCE

In today’s terrestrial PV markets, the sales price of PVmodules is almost entirely related to the maximum powerit can deliver at STC. Competition between manufactureris largely based on the offered unit price per Watt. Themaximum power of the modules leaving the factory isestablished within the quality control systems in place, andrefers either to calibrated reference modules, or tocalibrated reference cells placed together with the deviceunder test in the measurement plane of a solar simulator.Assuming the world-wide sales of PV modules for 2003being 500 MW, a global calibration uncertainty of ±3%for the production module would be the equivalent ofover- or underestimating the delivered modules by±15MW. Assuming average sales price of 4 EUR perWatt, this uncertainty translates to ±60 Mio EUR salesvolume. Moreover, as profit margins in the wholecommercialisation chain are most probably less than 10%,calibration uncertainty can be the decisive factor onprofitability of a PV enterprise.

3. REQUIREMENTS

3.1. Uncertainty

The basic requirement, which all calibration methodsdeveloped so far strive to minimise, is overall calibrationuncertainty.

The major components of uncertainty are:

1. Bias error. This is a shift of calibration valuescaused by factors not corrected for, or by poorexperimental design. Typical bias errors are forinstance drift or ageing of electronic componentsor misalignment to the light source.

2. Precision error: This includes all uncertainties ofsubsystems, such as Voltmeters and Analogue toDigital converters, or temperature controls.

3. Random error: basically this error is can caused bythe random nature of atmosphere or noise in thesignal conditioning.

4. Repeatability: Almost any experiment, whenrepeated shows slight variations, which are beyondthe pure random error. It can be caused by slightchanges in the experimental set-up, by themeasurement sequence or procedure, or even byhuman factors. Repeatability can never be betterthan the random error.

Even though uncertainty can be broken down in theabove factors which can be estimated or measured, acontrol of uncertainty can only established by repeatedintercomparison campaigns (Round-Robins) involving asmany methods and operators as possible [5], [6], [7].

3.2.Costs

There are not to many calibration laboratories in theworld, which routinely are engaged in solar cellcalibration. It is in all their interest to contain costs forequipment purchase and set-up as low as possible.

3.3. Duration

The time required to execute a complete calibrationincluding data analysis and verification is not only themajor factor for customer satisfaction, but the associatedlabour costs may well limit calibration throughput to veryfew cells per year, worsening the economic balance of acalibration laboratory quite considerably.

3.4. Availability

Ideally, a calibration task shall be executedimmediately after receipt of the test devices. Outdoormethods, however, rely on actual weather conditions,which often are variable. Also, outdoor measurementsshould not lead to another bias error caused by climaticconditions, which are difficult to correct for, such ashumidity or excessive atmospheric aerosol content.

3.5. Universal Applicability

Solar reference cells may incorporate differentphotovoltaic technologies, which might interfere withspecific features of the calibration method. Typicalexamples are slow response to fast light-pulses or choppedlight, or impossibility to control the junction temperaturein case of thin-film reference devices.

4. METHODS AND STANDARDS

In the following, the variety of calibration methodswill be briefly described, with reference to publishedstandards.

4.1.Thermopile Detectors

Thermopile detectors serve to determine solarirradiance regardless of spectral distribution. They have incommon that solar irradiance is converted into heat in ablack absorber. The temperature rise in relation to areference temperature is proportional to the incomingirradiation. Temperatures are measured by a number ofthermocouples connected in series (“Pile”) in order toincrease the thermovoltage measured to practical levels ofsome millivolts.

Thermopile detectors are different by their field ofview: Pyranometers have a hemispheric field of view,allowing measurement of solar irradiance form the wholesky, whilst pyrheliometers apply collimation to restrict thefield of view to typically 5°. The apparent angle of the

sun’s disc is in comparison 0.33°. Pyrheliometersconsequently need to be mounted a tracking platformensuring alignment with the sun’s position. Aclassification of these instruments is published as ISO19060.

4.1.1. PyrheliometersISO 19059 describes the calibration method for

pyrheliometers, to be performed under natural sunlight.Both the test and reference pyrheliometer a mounted on asun-tracking platform, and the values read. The standardpre-scribes minimum irradiance levels as well as a dataanalysis procedure. This measurement method and itsstandard has a very high for the whole chain of calibration,as the reference pyrheliometer can also be one of theprimary or secondary references in use world-wide withinthe World Radiometric Reference. Such primaryreferences are “Practical Absolute Cavity Radiometers(PACRAD), which absorb incident light on a idealblackbody, realised as an precision aperture in front of acavity, which houses a usually cone-shaped, highlyabsorptive surface. A second, identical cavity within thesame case is shielded from sunlight and electrically heatedto track the temperature of the first cavity, which isexposed to sunlight. The electrical energy required forheating of the reference cavity divided by the aperture areaof the first cavity establishes the irradiance value.Uncertainty of the method depends to a large extent on thetime-constant of the instruments involved and thecorresponding reading intervals where a compromise hasto be found between precision and atmosphericfluctuations. Figure 1 one shows measured uncertainties ofthe Joint Research Centre’s Cavity Radiometers,established in a number of intercomparisons with theWorld Radiometric Reference.

Figure 1. Uncertainty and calibration factors relative tothe World Radiometric Reference in a 10-yr period of thetwo JRC Reference Cavity Radiometers. Uncertainties arless ±0.2%

4.1.2. PyranometersCalibration of Pyranometers with their hemispheric

field of view is referenced to pyrheliometers and requiresa set-up, which accounts for the diffuse irradianceabsorbed. All test methods apply for this purpose ashading device aligned such that no direct irradiance fromthe sun can fall onto the pyranometers thermopile surface.The shading device is constructed such that its geometry

World Radiometric Reference (WRR) Factors

0.9980

0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

1.0020

1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004Year

WR

R F

acto

r

PMO6 81109 PMO6 911204

81109 Initial Calibration

Davos

911204 Initial Calibration

Davos

81109Re-Calibration

DavosIPC-IX (2000)Davos

NPC1999

NPC2001

NPC2002

(diameter and distance to the pyranometer) matches thereceiving geometry of the pyrheliometer. After reading thediffuse irradiation value of the pyranometer under shadedcondition, the shade is removed and a reading of the totalirradiance is taken. The difference between total anddiffuse irradiance corresponds to the direct irradiance asdetermined by the pyrheliometer and established thecalibration value of the pyranometer. Calibrationuncertainty is much influenced by the design of theshading device, and the linearity of the pyranometer undertest. ISO 19846 describes the calibration methods, takinginto account to set-ups: one where the pyranometer undertest is mounted co-planar with the pyrheliometer, and asecond where the pyranometer is mounted fixedhorizontally, and the shadowing device follows the sun.The first method is mechanically simpler, but may needcorrections for ground albedo effects.

There are a number of methods describing transfer ofcalibration between reference and field pyranometers,such as in ISO 19846 an ASTM 824.

4.2.Silicon Solar Cells

Different from thermopiles, solar cells are spectrallysensitive. Photons with wavelengths greater than 1050 nmare not absorbed at all in silicon cells, and for wavelengthbelow this band-gap limit the quantum efficiency varies.Consequently, even if solar irradiance is constant asmeasured by a thermopile detector, which integrates overall wavelengths, the short circuit current of a solar cellmay vary with the relative spectral distribution. Thespectral distribution varies with atmospheric conditions,and even if the standard sun elevation of 37° definingAirmass 1.5 can be found at least twice a day, thevariability of the diffuse and direct component of sunlightmay bias the measurement results. However, simulatedsunlight is even more difficult to match with the standardAir Mass 1.5 spectral distribution, and requires in any casethorough correction of spectral mismatch.

4.2.1. Simulated SunlightThis method requires a spectroradiometer to determine

the spectral distribution of the light source during thecalibration. The short-circuit current of the reference solarcell is measured under the simulator’s light together withthe spectral irradiance. Together with the spectralresponsivity data of the reference cell measured along themethod described under 4.2.3, a spectral mismatchcalculation can be performed to correct for the differencesbetween the simulator and the tabulated standard spectralspectral distribution. The absolute irradiance duringcalibration can be derived either from the absoluteirradiance calibration for each wavelength of thespectroradiometer, or from an intermediate transferthermopile, limited by filters to the wavelength range ofthe spectroradiometer.

The advantage of the method is that it can be appliedfor any solar cell technology, in particular when a steady-state simulator is used. Major uncertainty source is thetemperature control of the solar cell, and the calibrationuncertainty of the spectroradiometer. For a full descriptionof the method, refer to ISO/DIS 15387.

4.2.2. Natural Sunlight4.2.2.1.Direct Sunlight Method. This method follows inprinciple the calibration of pyrheliometers, described in4.1.1. above, with the test pyrheliometer replaced by thereference solar cell. The solar cell is mounted with anaperture close matching the geometry of the referencepyrheliometer. In addition, spectral irradiance of the directsunlight is measured, in order to perform a spectralmismatch correction, which requires the spectralresponsivity of the solar cell. If meteorological conditionsare good and repeatable, the mismatch factor can be small(<2%) and most of all, a constant for differentmeasurement campaigns. In this case, thespectroradiometer can be omitted, or replaced by themeasurement of selective wavelengths, which allow amodel to compute the actual spectral irradiance. One ofthe principal disadvantages of this method is on one handthe limited field of view of the reference cell which mightnot correspond to the later use, the other being the fact thatthe spectral irradiance of the direct sun light is differentfrom the tabulated standard IEC 60904-3. Furthermore,practical considerations limit this method to referencesolar cells smaller than about 25 cm2, as the collimatorsystem required for larger cells becomes rather unhandy.In taking readings one needs to consider the settling timeconstants for the reference pyrheliometer are at least threeorders of magnitude slower than of a solar cell.

4.2.2.2 Global Sunlight Method. This method is anadoption of the above described pyranometer calibrationmethod 4.1.2. The silicon reference cell is mounted on asun-tracking device together with a pyranometer andpyrheliometer. The pyranometer is shaded such to measureonly the diffuse component of sunlight, the direct one isgiven by the pyrheliometer readings. The method avoidsthe principal disadvantage of the direct sunlight method,and might benefit from the closer match of the actualspectral distribution with the standard reference spectrum.If atmospherical conditions are carefully selected, one canachieve high repeatability even without the measurementof the spectral irradiance.

Fig.2. Set-Up for calibration comparison of the global anddirect calibration method. Mounted are: Collimator forreference cell, uncollimated reference cell, pyranometer ascontrol, shadowed pyranometer for diffuse irradiance, twopyrheliometers and two cavity radiometers (from left).

4.2.3. Spectral Responsivity Calibrations4.2.3.1.Absolute Responsivity. Spectral responsivitycalibrations are based on the fact, that the calibration valueof a solar cell can be computed for any reference solarspectral distribution, if the absolute responsivity for eachwavelength interval is known. All methods for spectralresponsivity are based on short-circuit current measured ofsolar cells exposed to near monochromatic light of knownwavelength. This can be achieved by applying band-passfilters of a monochromator between the light source andthe solar cell. However, as irradiance of filtered light islow, it needs to be insured that the solar cell is linear in itsshort circuit, as the number of photons converted per unitof time and surface area is up to three orders of magnitudesmaller than at exposure to natural sunlight. Theapplication of additional white bias light up to 1000 W/m2

can resolve this problem, but requires then in additiontemperature control of the cell to be calibrated. Theabsolute, monochromatic irradiance can be determinedeither when both the light source level and the absorptivityof the monochromator or filters are known, or byswitching the cell under test with a reference detectorcalibrated for selected wavelengths. This method isdescribed as “Differential Spectral Responsivity Method”in ISO/DIS 15387.

4.2.3.2 Relative Responsivity. If the uncertainty of theabsolute responsivity from the previous described methods4.2.3.1. is insufficient, the measurements can be verifiedby one or more of the previous natural sunlight or solarsimulator methods, as in the spectral mismatch calculationonly relative responsivity is required.

5. TRACEBILITY ISSUES

According to international agreements, all calibrationvalue of commercial significance must refer to SI-Units.In particular, calibration laboratories accredited accordingto ISO 17025 need to maintain an unbroken chain oftraceability. IEC 904-1 refers to the World RadiometricReference as the primary standard, as the SI radiometricscale is demonstrated only for irradiance levels far belownatural sunlight. Comparison [4] have shown meanwhilethe equivalence of both radiometric references. Aninternational standard is in preparation, IEC 904-4 PWI,which shall define procedures for establishing thetraceability of the calibration of photovoltaic referencedevices. It will define requirements for calibrationprocedures used in maintenance of the traceability chain,such as uncertainty, and list some of methods in use.

6. UNCERTAINTY LIMITS

All calibration procedures need to establish andregularly confirm their uncertainty limits either throughverification measurements or Round-Robinintercomparisons.. References [1], [2] and [3] areexamples of such uncertainty estimates. Fromintercomparisons held in recent years, uncertaintiesbetween ±1.0% and ±2.0% are realistic and worldwideachievable. A current Round-Robin focuses on theestablishment of a “World Photovoltaic Scale (WPVS)”,which would consist in a set of standardised reference

cells compared repeatingly, which shall serve thecalibration laboratories as a primary laboratory reference[8].

7. CONCLUSIONS

Calibration methods in use today have evolved tobetter uncertainties, which are consistently near ±1.5%.The different methods, when carefully applied, haveshown in various intercomparisons that the calibrationfactors achieved are in good agreements. Theestablishment of the World Photovoltaic Scale, togetherwith a new international standard on traceability ofcalibrations to SI units will underpin fair and transparentcommercialisation of photovoltaic technology on all theworld markets.

8. ACKNOWLEDGENTS

The author wishes to thank W. Zaaiman of JRC, Italyfor much of the material referenced in this publications,and K. Emery of NREL, USA for many fruitfuldiscussions.

9. REFERENCES

9.1. Selected Standards Publications

9.1.1.Solar Spectra and Light SimulationIEC 60904-3 Measurement principles for terrestrial

photovoltaic (PV) devices with referencespectral irradiance data.

IEC 60904-9 Solar Simulator requirements.ISO 9845-1 Reference solar spectral irradiance at

ground at different receiving conditions –Part1: Direct normal and hemisphericalsolar irradiance for Air Mass 1.5., 1992.

ASTM 891 Tables for Terrestrial Direct Normal SolarSpectral Irradiance for Air Mass 1.5

ASTM 892 Tables for Terrestrial Solar SpectralIrradiance for Air Mass 1.5 for a 37° TiltedSurface.

ASTM 927 Specification for Solar Simulation forTerrestrial Photovoltaic Testing.

9.1.2. Calibrations (Thermopiles)ISO 9059 Calibration of field pyrheliometers by

comparison to a reference pyrheliometer”,1990

ISO 9060 Specification and classification ofinstruments for measuring hemisphericalsolar and direct radiation, 1990.

ISO 9846 Solar Energy - Calibration of apyranometer using a pyrheliometer.

ISO 9847 Solar Energy - Calibration of fieldpyranometers by comparison to a referencepyranometer.

ASTM 816 Method for Calibration of SecondaryReference Pyrheliometers andPyrheliometers for Field Use

ASTM 824 Method for Transfer of Calibration fromReference to Field Pyranometers

ASTM 913 Method for Calibration of ReferencePyranometers with Axis Vertical by theShading Method

ASTM 941 Test Method for Calibration of ReferencePyranometers with Axis Tilted by theShading Method

ASTM 1144 Test Method for Calibration of Non-Concentrator Terrestrial PhotovoltaicPrimary Reference Cells under DirectIrradiance

9.1.3. Calibrations (Solar Cells))IEC 60904-2 Requirements for reference solar cells.IEC 60904-4 Procedures for establishing the traceabilityof the calibration of photovoltaic reference devices. (PWI:Proposed Work Item)IEC 60904-6 Requirements for reference solar modules.IEC 60904-7 Computation of spectral mismatch error

introduced in the testing of a photovoltaicdevice.

IEC 60904-8 Guidance for the measurement of spectralresponse of a photovoltaic (PV) device.

ISO/DIS15387 Space systems – Single-junction spacesolar cells – Measurement and calibrationprocedures

ASTM 1144 Test Method for Calibration of Non-Concentrator Terrestrial PhotovoltaicPrimary Reference Cells under DirectIrradiance

ASTM 1039 Method for Calibration andCharacterisation of Non-ConcentratorTerrestrial Photovoltaic Reference Cellsunder Global Irradiation

ASTM 1040 Specification for Physical Characteristicsof Non-Concentrator TerrestrialPhotovoltaic Reference Cells

ASTM 1125 Test Method for Calibration of PrimaryNon-Concentrator Terrestrial PhotovoltaicReference Cells using a Tabular Spectrum

ASTM 1362 Test Method for Calibration of Non-Concentrator Photovoltaic SecondaryReference Cells

9.1.4. Electrical Performance Measurements IEC 60891 Procedure for temperature and irradiance

corrections to measured I-V characteristicsof crystalline silicon photovoltaic

IEC 60904-1 Measurement of photovoltaic current-voltage characteristics.

IEC 60904-5 Determination of the equivalent celltemperature (ECT) of photovoltaic (PV)devices by the open-circuit voltage ASTM1021 Methods for Measuring theSpectral Response of Photovoltaic Cells

ASTM 973 Test Method for Determination of theSpectral Mismatch Parameter between aPhotovoltaic Device and a PhotovoltaicReference Cell

ASTM 1143 Test Method for Determination theLinearity of a Photovoltaic Device withrespect to the Test Parameter

9.2. Literature

[1] K. Whitfield and C.R. Osterwald, Procedure for thedetermination of Photovoltaic Module OutdoorElectrical Performance, Prog. Photovolt: Res.Appl.2001; 9:87-102.

[2] J.S. Henzing and W.H. Knap, Uncertainty inpyranometer and pyrheliometer measurements atKNMI in De Bilt, TR-235, ISBN: 90-369-2195-3,2001.

[3] W. Zaaiman and A. Realini, Photovoltaic DeviceCalibration Intercomparison TestReport,TNI.98.18, 1998. European Commission,Joint Research Centre

[4] J. Romero, N.P.Fox and C. Froehlich: Improvedcomparison of the World Radiometric Referenceand the SI radiometric scale. Metrologia, 1995/96,32, 523-524

[5] The results of the 1984/1985 Round Robin calibrationof reference solar cells for the Summit WorkingGroup on Technology, Growth and Employment,EUR, European Commission, Joint ResearchCentre 0613, 1986.

[6] The results of the PEP’87 Round Robin Calibration ofReference Solar Cells and Modules, PTB Bericht,PTB-Opt-31, ISBN 3-89429-067-6, 1990.

[7] The results of the PEP’93 Intercomparison ofReference Cell Calibrations and NewerTechnology Performance Measurements,NREL/TP-520-23477, March 1998.

[8] The results of the First World Photovoltaic ScaleRecalibration, NREL/TP-520-27942, March 2000.