Cell Measurements

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Cell measurements at NREL include spectral responsivity and current versus voltage (I-V) of one sun, concentrator, and multijunction devices. Reference cell measurements also include linearity of short-circuit current and total irradiance.

Interested in Cell Measurements?

Request a Measurement

Cell Current versus Voltage

We use I-V measurement systems to assess the main performance parameters for PV cells and modules. 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.

All I-V systems use data acquisition systems and custom software for accurate standardized PV measurements. This includes algorithms developed by our group over the years for calculating I-V characteristics and spectral-mismatch corrections. The custom I-V systems and flash simulators use separate 6½-digit multimeters or high-speed analog-to-digital converters for measuring voltage, current, and intensity; this allows us to correct the current for intensity fluctuations. The systems all have multiple current ranges, allowing a wide variety of commercial and prototype cells and modules to be measured routinely.

For non-concentrator solar cells and multijunction devices, we use:

  • Abet 11048 solar simulator
  • One-sun multisource solar simulator.

For concentrator solar cells, we use:

  • Tunable high-intensity pulsed solar simulator
  • High-intensity pulsed polar simulator
  • Continuous illumination concentrator simulation system.

The following table provides a condensed list of characteristics for cell I-V measurement test beds.

Major Instrumentation for Cell I-V Measurements
System Typical Applications Special Features Light Source Test Bed Voltage Resolution / Limit Current Resolution / Limit

Abet 11048 solar simulator

1-sun I-V measurements for cells and small modules

Wide current and voltage ranges

Filtered 3-kW xenon lamp; 0.1 to ˜10 suns

30 cm diameter; 15° to 50°C

5 µV / ±50 V

±10 pA to ±16 A

One-sun multisource simulator 

Multijunction cells, I-V versus irradiance and temperature

10 separate adjustable wavelength bands; dedicated spectroradiometer

2 xenon and 2 tungsten lamps; fiber optic; up to 1.37 suns

9 cm × 9 cm; 10° to 80°C

0.1 µV / ±40 V

1 nA to ±5 A

Tunable high-intensity pulsed solar simulator 

I-V measurements for concentrator and thermophotovoltaic cells

Spectrally adjustable; ˜1-ms flash; minimal heating; dedicated spectroradiometer

2 xenon flash lamps 30 cm long with mirror; 1 to ˜600 suns

4 cm × 4 cm, <1 cm2 typical); 15° to 80°C

0.1 mV / 20 V

100 µA to 20 A

High-intensity pulsed solar simulator

I-V measurements for concentrator and thermophotovoltaic cells

˜1-ms flash; minimal heating

2 xenon flash lamps 30 cm long with mirror; 1 to 2000 suns

10 cm × 10 cm; 5° to 80°C

0.1 mV / 100 V

500 µA to 50 A

Continuous illumination concentrator simulation system

I-V measurements for concentrator cells

User-controlled bias conditions

1-kW xenon lamp or 3-kW tungsten lamp; 0.1 to 200 suns

˜1-cm diameter for xenon; 5° to 80°C

5 µV / ±10 V

±1 µA to ±10 A

Spectral Responsivity

Spectral responsivity measurement is an important part of the NREL photovoltaic device performance assessment process. Spectral responsivity systems measure how a device responds to selected narrow (spectral) bands of irradiance. Responsivity is measured in units of amps per watt versus wavelength and reported in terms of quantum efficiency—a measure of how efficiently a device converts incoming photons to charge carriers in an external circuit.

We use two spectral responsivity systems:

  • Filter system
  • Grating system.

The following table is a condensed list of major instrumentation characteristics for spectral response measurements.

Major Instrumentation for Spectral Response Measurements

System

Typical Applications

Special Features

Light Source

Wavelength Range

 

Bandwidth

Voltage Bias

Light Bias

Filter spectral responsivity

Spectral responsivity measurements for solar cells and modules; 15°C to 80°C

High flux density; variable beam size; 61 filters on four filter wheels; adjustable chopping frequency

1-kW xenon lamp

280 to 1,900 nm

10 nm full width at half maximum

±40 V

Up to 200 mA

Grating spectral responsivity

Spectral responsivity measurements for small-area and multijunction cells; 15°C to 80°C

3 gratings for visible and infrared; adjustable chopping frequency

75-W xenon lamp

300 to 3,000 nm

>1 nm full width at half maximum

±5 V

Up to 200 mA

Linearity

The linearity of the short-circuit current (Isc) with total irradiance is an important measurement for reference cells because the standards require the reference cell to be linear over its range of operation. NREL measures the linearity of Isc in the range of 0 to 2 suns using two lamps and neutral-density filters. If the sample is linear, then the Isc from one lamp plus the Isc from the other lamp must equal the Isc when both lamps are on. This value is expressed as a percentage deviation from linearity and is typically measured over the range of 0.1 to 1.1 sun. This method is insensitive to the spectrum and spatial nonuniformity of the light from the two lamps at varying light levels.