Cell Current Versus Voltage (I-V)
The National Renewable Energy Laboratory Device Performance group uses three I-V systems to measure performance of individual solar cells. We use two systems for solar concentrator cells — a continuous-illumination concentrator and a high-intensity pulsed solar simulator. For non-concentrator solar cells and multijunction devices we use a Spectrolab X-25 solar simulator. The Spectrolab X-25 test stage also accommodates small modules.
The following table provides a condensed list of characteristics for cell I-V measurement instrumentation.
Major Instrumentation for Cell I-V Measurements
|System||Typical Applications||Special Features||Light Source||Test Bed||Voltage Resolution/Limit||Current Resolution/Limit|
|Continuous illumination concentrator||I-V measure-
ments for concentrator and thermophotovoltaic (TPV) cells
|Spectrally adjustable; user-controlled bias conditions||1-kW Xe or 3-kW tungsten; 0.1 to 200 suns||~1-cm diameter for Xe, 5 cm × 10 cm for infrared (IR) lamp; 5° to 80°C||5 µV to ±10 V||±1µA to ±10 A|
|High-intensity pulsed solar simulator||I-V measurements for concentrator and TPV cells||Spectrally adjustable; minimal heating||2 Xe flash lamps 30 cm long with mirror; 1 to 2000 suns||10 cm × 10 cm; 5° to 80°C||0.1 mV to 100 V||500 µA to 50 A|
|Spectrolab X-25 solar simulator||1-sun I-V measurements for cells and small modules||Spectrally adjustable; wide current and voltage ranges||Spectrolab X-25 filtered 3-kW Xe; 0.2 to 10 suns||30 cm × 30 cm; 5° to 50°C||5 µV to ±50 V||±10pA to ±16 A|
Continuous Illumination Concentrator Simulation System
This system has two light sources, giving it the dual capability of measuring concentrator solar cells and 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 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.
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. It can accommodate 10 cm × 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 × 106 Wm-2 and a spatial nonuniformity of ±3% over the area of 17 cm × 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 photovoltaic (PV) cells as large as 30 cm × 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 × 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.
For additional information contact Keith Emery, 303-384-6632.