Cell Measurements

Interested in Cell Measurements?

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.

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 (V_oc), short-circuit current (I_sc), fill factor (FF), maximum power output of the device (P_max), voltage at maximum power (V_max), current at maximum power (I_max), 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
System Abet 11048 solar simulator	Typical Applications 1-sun I-V measurements for cells and small modules	Special Features Wide current and voltage ranges	Light Source Filtered 3-kW xenon lamp; 0.1 to ˜10 suns	Test Bed 30 cm diameter; 15° to 50°C	Voltage Resolution / Limit 5 µV / ±50 V	Current Resolution / Limit ±10 pA to ±16 A
One-sun multisource simulator	Typical Applications Multijunction cells, I-V versus irradiance and temperature	Special Features 10 separate adjustable wavelength bands; dedicated spectroradiometer	Light Source 2 xenon and 2 tungsten lamps; fiber optic; up to 1.37 suns	Test Bed 9 cm × 9 cm; 10° to 80°C	Voltage Resolution / Limit 0.1 µV / ±40 V	Current Resolution / Limit 1 nA to ±5 A
System Tunable high-intensity pulsed solar simulator	Typical Applications I-V measurements for concentrator and thermophotovoltaic cells	Special Features Spectrally adjustable; ˜1-ms flash; minimal heating; dedicated spectroradiometer	Light Source 2 xenon flash lamps 30 cm long with mirror; 1 to ˜600 suns	Test Bed 4 cm × 4 cm, <1 cm²typical); 15° to 80°C	Voltage Resolution / Limit 0.1 mV / 20 V	Current Resolution / Limit 100 µA to 20 A
System High-intensity pulsed solar simulator	Typical Applications I-V measurements for concentrator and thermophotovoltaic cells	Special Features ˜1-ms flash; minimal heating	Light Source 2 xenon flash lamps 30 cm long with mirror; 1 to 2000 suns	Test Bed 10 cm × 10 cm; 5° to 80°C	Voltage Resolution / Limit 0.1 mV / 100 V	Current Resolution / Limit 500 µA to 50 A
System Continuous illumination concentrator simulation system	Typical Applications I-V measurements for concentrator cells	Special Features User-controlled bias conditions	Light Source 1-kW xenon lamp or 3-kW tungsten lamp; 0.1 to 200 suns	Test Bed ˜1-cm diameter for xenon; 5° to 80°C	Voltage Resolution / Limit 5 µV / ±10 V	Current Resolution / Limit ±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
System Filter spectral responsivity	Typical Applications Spectral responsivity measurements for solar cells and modules; 15°C to 80°C	Special Features High flux density; variable beam size; 61 filters on four filter wheels; adjustable chopping frequency	Light Source 1-kW xenon lamp	Wavelength Range 280 to 1,900 nm	Bandwidth 10 nm full width at half maximum	Voltage Bias ±40 V	Light Bias Up to 200 mA
System Grating spectral responsivity	Typical Applications Spectral responsivity measurements for small-area and multijunction cells; 15°C to 80°C	Special Features 3 gratings for visible and infrared; adjustable chopping frequency	Light Source 75-W xenon lamp	Wavelength Range 300 to 3,000 nm	Bandwidth >1 nm full width at half maximum	Voltage Bias ±5 V	Light Bias Up to 200 mA

Linearity

The linearity of the short-circuit current (I_sc) 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 I_sc in the range of 0 to 2 suns using two lamps and neutral-density filters. If the sample is linear, then the I_sc from one lamp plus the I_sc from the other lamp must equal the I_sc 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.