Measurements & Characterization • National Center for Photovoltaics

Device Performance

We measure the performance of PV cells and modules with respect to 

standard reporting conditions—defined as a reference temperature 

(25°C), total irradiance (1000 Wm-2), and spectral irradiance 

distribution (IEC standard 60904-3). Typically, these are “global” 

reference conditions, but we can measure with respect to any 

reference set.

To determine device performance, we conduct two general categories 

of measurements: spectral responsivity (SR) and current versus 

voltage (I-V). We usually perform these measurements using standard 

procedures, but we develop new procedures when required by new 

technologies.

We also serve as an independent facility for verifying device 

performance for the entire PV community. We help the PV community 

solve its special measurement problems, giving advice on solar 

simulation, instrumentation for I-V measurements, reference cells, 

measurement procedures, and anomalous results. And we collaborate 

with researchers to analyze devices and materials. 

Our facility for calibrating primary reference cells is one of four 

facilities certified in accordance with the world photovoltaic scale 

and that cooperates internationally to provide the PV community 

with a path of traceability to standards. We are accredited by the 

American Association for Laboratory Accreditation (A2LA) to ISO17025 standards for primary reference cell, secondary reference 

cell, and secondary module calibrations for samples that meet the 

requirements. We help develop consensus U.S. and international PV 

standards through participation in ASTM and IEC standards groups.

TYPICAL MEASUREMENT FLOWS

For devices that come to us for measurement, we typically follow 

a procedure that ensures quality measurement and follow up. After 

logging in the device based on information from a cover letter or 

request form, we measure its area, which is crucial for determining 

its efficiency. We then obtain its spectral responsivity. For cells, we 

measure the spectral responsivity with one of two systems. For 

modules, however, spectral responsivity is generally provided to us by 

the manufacturer or we measure it on a representative cell. 

Next, we use the information on spectral responsivity to calculate the 

spectral mismatch between the test device and a primary reference 

cell for the simulator that will be used for the subsequent I-V 

measurement. This is done because the intensity of the light source 

in a simulator system is set with a reference cell; and the correction 

factor for the spectral mismatch enables measurement of the sample’s 

performance with respect to a reference spectrum.

We then measure the I-V characteristics of the device under simulated 

conditions. For modules, we also measure I-V performance under 

natural sunlight, which enables us to determine module response 

under “real” conditions. After measurement, the results are translated 

to standard conditions. For concentrator cells, we measure the I-V 

characteristics as a function of light level. We can also adjust the 

spectrum to a limited extent for multijunction concentrators to make 

the current matching as close as possible to reference conditions.

After all measurements have been made, we carefully review the 

results for anomalies or procedural errors. Finally, we prepare a report 

for the client—which can be as simple as a presentation of data tables 

or as involved as a document that contains description, analysis, data, 

and recommendations.

SPECTRAL RESPONSIVITY (SR) SYSTEMS

Spectral responsivity systems measure how a device responds 

to selected narrow (spectral) bands of irradiance. Responsivity 

is measured in units of AW-1 versus wavelength and reported in 

terms of quantum efficiency (QE)—a measure of how efficiently a 

device converts incoming photons to charge carriers in an external 

circuit. These systems measure the spectral response at different 

temperatures (10° to 80°C) voltages (±15 V), light levels (0 to few 

suns), and different chopping frequencies (<0.2 to 400 Hz).

We use two SR systems—a grating system and a filter system. The 

grating system is used for applications where a narrow bandwidth 

and wavelength interval is required. The grating system has a typical 

minimum beam size of 1 mm x 3 mm, making it ideal for absolute 

spectral responsivity measurements. The filter system is used for solar 

cells and modules and has much more power in the beam, with a 

10-nm typical bandwidth, but has a 20- to 50-nm wavelength interval 

from 290 to 2000 nm.

certificate number 2236.01 

Accredited for primary reference 

cell, secondary reference cell, and 

secondary module calibration

Spectral responsivity measurement system using 10-nm-bandwidth 

interference filters

Mike Linenberger/PIX14476 
Although important differences exist between the two systems, the 

basic procedures are similar: A wide-spectrum light source is chopped 

and filtered or diffracted into a discrete succession of narrow spectral 

bands, each of which is directed onto the test device. The device 

current produced from the monochromatic light is converted to an ac 

voltage signal. A lock-in amplifier locks into the chopper frequency of 

the light signal and measures the corresponding ac voltage produced 

by the light. Using a time-periodic light signal and a lock-in amplifier 

allows us to distinguish signals produced by the relevant spectral band 

from those that may be produced by other light sources. 

Each system is controlled by a computer. Once the operator sets the 

parameters, the computer does the rest—runs the procedure through 

the selected wavelength range, acquires the data, calculates QE, saves 

the data, and updates the directory in a standardized manner as a tabdelimited text file.

CURRENT VERSUS VOLTAGE (I-V) SYSTEMS

I-V measurement systems determine the output performance of 

devices, including: open-circuit voltage (Voc), short-circuit current (Isc), 

fill factor (FF), maximum power output of the device (Pmax), voltage 

at maximum power (Vmax), current at maximum power (Imax), and 

conversion efficiency of the device (η). Some I-V systems may also be 

used to perform dark I-V measurements to determine diode properties 

and series and shunt resistances.

For cells, we use three I-V systems to measure performance: two 

systems for solar concentrator cells—a continuous-illumination 

concentrator and a high-intensity pulsed solar simulator—and a 

Spectrolab X-25 solar simulator for non-concentrator solar cells 

and multijunction devices. The Spectrolab X-25 test stage also 

accommodates small modules.

For modules, we use four I-V systems. Two systems are used for 

measurements under simulated conditions: a Spire 240A pulsed 

solar simulator and a large-area continuous solar simulator. And two 

are used for measurements under outdoor conditions: the standard 

outdoor measurement system and the Daystar DS-10/125 portable I-V 

curve tracer.

All I-V measurements are made using 4-terminal Kelvin connections.

This enables the use of separate channels for voltage and current 

measurements, which minimizes measurement errors by eliminating 

voltage-drop losses that could result from resistances due to cables, 

connections, and wiring.

All I-V systems use data acquisition systems and custom software 

for accurate standardized PV measurements. This includes 

algorithms developed by the group over the years for calculating 

I-V characteristics and, for most systems, for making the spectralmismatch corrections. The custom I-V systems and flash simulators 

use separate meters for measuring voltage, current, and intensity; this 

allows us to correct the current for intensity fluctuations. The custom 

systems all have multiple current ranges, allowing dark I-V on cells 

and modules to be routinely measured.

The combined systems capabilities allow us to accurately measure 

the characteristics of any conceivable photovoltaic cell or module 

technology. The overlapping capabilities of many of the systems allow 

for cross checking and understanding the various error sources of the 

test beds.

Continuous Illumination Concentrator

This system has two light sources, giving it the dual capability of 

measuring concentrator solar cells and thermophotovoltaic (TPV) cells. 

For concentrator cells, it uses a 1-kW short-arc xenon lamp. The light 

from the xenon source is reflected off a mirror onto a concentrator lens 

mounted on a translation stage. The system can be adjusted to achieve 

concentration ratios of 0.1 to 200 suns over an area that ranges from 4 

cm2

 to less than 0.1 cm2.

For TPV measurements, the system uses a 3-kW tungsten light source. 

The high infrared (IR) range of this light source makes it suitable for 

TPV cells, which have low bandgaps and respond best to the IR portion 

of the spectrum. 

System

Continuous 

illumination 

concentrator

High-intensity 

pulsed solar 

simulator

Spectrolab X-25 

solar simulator

Typical Applications

I-V measurements for 

concentrator and TPV cells

I-V measurements for 

concentrator and TPV cells

1-sun I-V measurements for 

cells and small modules

Special Features

Spectrally adjustable; 

user-controlled bias 

conditions

Spectrally adjustable; 

minimal heating

Spectrally adjustable; wide 

current and voltage ranges

Light Source

1-kW Xe or 3-kW 

tungsten; 0.1 to 

200 suns

2 Xe flash lamps 30 cm 

long with mirror; 1 to 

2000 suns

Spectrolab X-25 filtered 

3-kW Xe; 0.2 to 

10 suns

Voltage

Resolution/Limit

5 μV to ±10 V

0.1 mV to 100 V

5 μV to ±50 V

Current

Resolution/Limit

±1 μA to ±10 A

500 μA to 50 A

±10 pA to ±16 A

Test Bed

~1-cm diameter for 

Xe, 5 cm x 10 cm for 

IR lamp; 5° to 80°C

10 cm x 10 cm; 

5° to 80°C

30 cm x 30 cm; 

5° to 50°C

MAJOR INSTRUMENTATION FOR CELL I-V MEASUREMENTS

System

Grating spectral 

responsivity

Filter spectral 

responsivity

Typical Applications

SR measurements for 

small-area TPV cells

SR measurements for 

solar cells and modules

Special Features

3 gratings for visible and IR; 

adjustable chopping frequency

High flux density; variable beam 

size; 61 filters on four filter wheels; 

adjustable chopping frequency

Light Source

250-W tungsten

1-kW Xe

Bandwidth

>1 nm FWHM

10 nm FWHM

Voltage

Bias

±5 V

±40 V

Light

Bias

Up to 200 mA

Up to 200 mA

Wavelength

Range

400 to 3,000 nm

280 to 1,900 nm

MAJOR INSTRUMENTATION FOR SPECTRAL RESPONSE MEASUREMENTS
High-Intensity Pulsed Solar Simulator (HIPSS)

The HIPSS is a commercial system with a temperature-controlled 

vacuum plate that has an electrically isolated voltage contact and 

that can accommodate 10 cm x 10 cm cells. The system is used to 

measure I-V characteristics of both concentrator solar cells and TPV 

cells. Its light source is two low-pressure xenon arc lamps that are 

adjusted between 1200 and 3200 V. They deliver 1-ms pulses of light 

with an intensity of up to 2 x 106

 Wm-2 and a spatial nonuniformity of 

±3% over the area of 17 cm x 3 cm. The beam is adjustable to provide 

concentrations of 1 to 2000 suns.

Spectrolab X-25 Solar Simulator

This test bed is used to measure the 1-sun I-V characteristics of all 

cells we evaluate. The Spectrolab X-25 measures I-V characteristics of 

PV cells as large as 30 cm 

x 30 cm. It uses a 3-kW 

xenon arc lamp filtered to 

provide a standard AM1.5 

Global reference spectrum 

(IEC 60904-3 Class A). The 

irradiance is adjustable 

from 0.2 to 10 suns (for 

smaller areas) and has 

a spatial uniformity of 

±3% at 1-sun (i.e., at a 

beam diameter of 30 cm 

x 30 cm). The spectral 

irradiance of the unit is 

adjustable with filters to 

achieve current matching for 2- and 3-junction cells. The test device 

and monitor cell temperatures are controlled separately.

The measurement uncertainty for device efficiency ranges from 

±2% to ±5%, depending on sample size, geometry, and number of 

junctions. This range takes into account the ±1% uncertainty of the 

reference cell, and the uncertainties in the spectra, intensity, spectral 

mismatch, and electronics. 

Spire 240A Pulsed Solar Simulator

This simulator uses a long-arc pulsed xenon lamp, flashed at a 

maximum rate of 15 Hz, that is filtered to simulate the global reference 

spectrum (IEC60904-3, class A), and whose irradiance has a spatial 

nonuniformity of ±3% over an area of 61 cm x 122 cm. Temperature 

measurements are made with a spring-loaded thermocouple in contact 

with the back of the module. 

The original software has 

been modified to store data 

and update the directory 

in our standardized tabdelimited text format.

Temperature coefficients 

can be measured using an 

adjustable heating blanket. 

The I-V characteristics as 

a function of lamp intensity 

in the range of 0.15 to 1.3 

suns can be measured using 

screen filters.

Large-Area Continuous 

Solar Simulator (LACSS)

The large-area continuous solar simulator (LACSS) is a customdesigned system that is used for measuring module performance. It 

uses a Spectrolab model X200 continuous 20-kW xenon short-arc 

lamp to provide a filtered light source to simulate the global spectrum 

(IEC 60904-3, class A). It is capable of measuring modules as large 

as 152 cm x 122 cm with spatial nonuniformity of ±3%. Sample 

temperature is measured with a spring-loaded, type T thermocouple at 

the back of the module.

Standard Outdoor 

Measurement System (SOMS)

The standard outdoor measurement system is our principal system for 

measuring module performance under prevailing outdoor conditions. 

We attempt to make these measurements under a clear sky, with 

the irradiance between 950 and 1050 Wm-2, and as close to 25°C 

as possible. For each 

measurement, we use a 

pyranometer and a silicon 

reference cell in a module 

package to determine 

the total irradiance, and 

a spectroradiometer to 

determine spectral content 

of the sunlight between 350 

and 1100 nm. Once the 

parameters have been set 

by the operator, the system 

System

Spire 240A 

pulsed solar 

simulator

Large-area 

continuous solar 

simulator

Standard outdoor 

measurement 

system

Daystar DS-10/125 

portable I-V curve 

tracer

Typical Applications

I-V measurements (1-sun 

modules) under simulated 

conditions

I-V measurements (1-sun 

modules) under simulated 

conditions

Flat-plate and concentrator 

I-V measurements under 

outdoor conditions

I-V measurements under 

outdoor conditions

Special Features

Commercial system; 25°C 

(20° to 60°C possible)

User-controlled bias conditions

2-axis positioning; meteorological 

parameters; spectral irradiance 

measured; user-controlled bias 

Portable; may be powered either 

with a 120 Vac line or with a 

12 Vdc battery

Light Source

Spire 240A Xe flash lamp; 

0.15 to 1.3 suns; filtered 

to AM1.5 global spectrum

Spectrolab X200 filtered 

20-kW Xe lamp; 0.1 to 

20 suns

Sunlight

Sunlight

Voltage

Resolution/Limit

0.1 mV to 100 V

5 μV to ±300 V

5 μV to ±300 V

12 mV to 600 V

Current

Resolution/Limit

0.5 mA to 20 A

±1 μA to ±60 A

±1 μA to ±60 A

5 mA to 125 A

Test Bed

61 cm x 122 cm

152 cm x 122 cm

200 cm x 300 cm

unlimited

MAJOR INSTRUMENTATION FOR MODULE I-V MEASUREMENTS

ISO 17025 accredited test bed for calibrating 

secondary reference cells

Mike Linenberger/PIX14458

Large-area continuous solar simulator (LACSS)

Jim Yost/PIX14091

Standard outdoor measurement system (SOMS)

Measurements and Characterization Team
measures the I-V characteristics, the temperature (using a thermocouple 

at the back of the module), and calculates other performance 

characteristics.

Prototype Concentrator Evaluation Test Bed

We use this test bed to evaluate I-V characteristics throughout the 

day of prototype concentrator cells and modules. The site includes 

instruments to measure sample and air temperature, wind speed, and 

direct-beam irradiance. The system also records data from the RMIS 

meteorological station (see http://rmis.nrel.gov) with the individual 

module I-V curves. The data are analyzed for performance as a 

function of time, temperature, and light level. This test bed data set is 

also used to evaluate translation equations and energy rating methods.

Daystar DS-10/125 Portable I-V Curve Tracer

Like the SOMS, this system is used to make module I-V measurements 

under prevailing outdoor conditions. But typically, we only use it when 

the sample is too cumbersome or impractical to disassemble and 

remount on the SOMS test bed. Total irradiance is measured using a 

pyranometer mounted in the same plane as the sample. 

SHORT-CIRCUIT CURRENT LINEARITY WITH IRRADIANCE

To rate photovoltaic devices, a reference detector is required whose 

response is linear with total irradiance. The detector determines the 

linearity of the short-circuit current (Isc) versus the total irradiance (Etot) 

by illuminating a reference cell with two lamps. The irradiance range 

is about 0.01-sun to several suns. A device is linear if the current 

measured with both lamps illuminating the cell is the same as the 

sum of the currents with each lamp illuminating the cell. The twolamp method is insensitive to the light spectra or spatial nonuniformity 

changing with irradiance.

PRIMARY REFERENCE CELL CALIBRATION

We calibrate primary reference cells for in-house use, for use by 

other national laboratories, and to provide our clients and partners 

with a path for traceability to standards. Our laboratory is one 

of only four facilities in the world certified to calibrate reference 

cells in accordance with the world photovoltaic scale, and these 

measurements are accredited to ISO 17025 standards.

Most of the cells we select for calibration are obtained from 

organizations that have established reputations for making cells for 

reference and whose cells have a history of high quality and stability. 

When we make a reference cell for calibration, we carefully choose 

materials and structures. We then make the cell in accordance with 

stringent procedures that ensure quality and stability. Once we make 

or obtain a reference cell, we subject it to carefully devised calibration 

procedures that minimize errors due to measurement errors due to 

spectral correction.

To calibrate the cells, we concurrently measure short-circuit current, 

total irradiance, and spectral irradiance outdoors with the same field 

of view (5.0°). Total irradiance is measured with an Eppley HF primary 

absolute cavity radiometer. Spectral irradiance is measured with a 

LICOR LI-1800 spectroradiometer. From these measurements, we 

calculate an average uncorrected calibration value, which relates 

the cell’s short-circuit current to total irradiance. The atmospheric 

parameters and cell temperature are also measured. Once a valid 

calibration value is obtained, the short-circuit current is corrected for 

temperature and spectrum to the standard conditions.

James Kiehl/NREL

Prototype concentrator evaluation test bed contains prototype cell assemblies on 

right and variety of lens and mirror-based concentrator modules on left

1617 Cole Boulevard, Golden, Colorado 80401-3305

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Measurements and Characterization is within the National Center for Photovoltaics at NREL

Web site: www.nrel.gov/pv/measurements

Phone: (303) 384-6675 • Fax: (303) 384-6604

Contract No. DE-AC36-08GO28308

NREL/BR-520-40123 • October 2009

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 10% post consumer waste.

NOTICE—This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes 

any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, 

process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed 

herein do not necessarily state or reflect those of the United States government or any agency thereof.

Test bed for calibrating primary reference cells

Jim Yost/PIX14100