Spectral responsivity (SR) measurement is an important part of the National Renewable Energy Laboratory (NREL) photovoltaic (PV) 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 (QE) — a measure of how efficiently a device converts incoming photons to charge carriers in an external circuit. SR 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 × 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.
The following table is a condensed list of major instrumentation characteristics for spectral response measurements.
|System||Typical Applications||Special Features||Light Source||Wavelength Range||Bandwidth||Voltage Bias||Light Bias|
|Grating spectral responsivity||SR measurements for small-area thermophotovoltaic cells||3 gratings for visible and IR; adjustable chopping frequency||250-W tungsten||400 to 3,000 nm||>1 nm FWHM||±5V||Up to 200 mA|
|Filter spectral responsivity||SR measurements for solar cells and modules||High flux density; variable beam size; 61 filters on four filter wheels; adjustable chopping frequency||1-kW Xe||280 to 1,900 nm||10 nm FWHM||±40V||Up to 200 mA|
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 alternating current (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 tab-delimited text file.
Spectral responsivity measurement is an important part of the NREL device performance measurement process.
For additional information contact Keith Emery, 303-384-6632.