ISFH CalTeC - Solar cells
Dr. Karsten Bothe
Tel.: +49(0)5151-999 425
Area measurement system based on a calibrated high resolution document scanner.
Setup for the measurement of the current voltage characteristic
Differential spectral response (DSR) measurement setup
Measurement procedure and technical details
The ISFH CalTeC (Calibration and Test Center) is accredited according to ISO 17025 for the calibration of solar cells by the national accreditation body of the Federal Republic of Germany DAkkS (Deutsche Akkreditierungsstelle) under the registration number D-K-18657-01-00. The subjects of the accreditation are the measurements of the spectral responsivity (SR) and the determination of the characteristic parameters (Isc, Voc, FF and η) of the current voltage (IV) curve of solar cells measured under standard test conditions (STC) as defined in the IEC60904 standards. All parameters are reported with the accompanied uncertainty following from an approved measurement uncertainty analysis. In order to ensure traceability to SI-units all reference devices are calibrated at the national metrology institute of Germany, PTB (Physikalisch-Technische Bundesanstalt).
The calibration service of the ISFH CalTeC focuses on the measurement of wafer-based silicon solar cells from laboratory up to industrial formats. The procedure for a calibrated solar cell measurement consists of three tasks:
Task 1: Area measurement
The area of the solar cell under test is required for the calculation of the efficiency h. Thus, the first task is the measurement of the solar cell area. For this task a calibrated flatbed scanner is used.
Task 2: Measurement of the spectral responsivity
For SR measurements the ISFH CalTeC Solar Cell Laboratory is equipped with a grating monochromator based measurement setup allowing differential spectral response measurements at a large number of different wavelengths with a spectral bandwidth of about 10 nm. The monochromatic light field has an area of (160×160) mm2. The setup comprises an array of halogen lamps allowing bias light intensities of up to 1100 W/m2 (1.1 suns). To address non-linearities in the solar cells under test we always measure the differential spectral response over the whole wavelength range at five bias light intensities E between 0 W/m² and 1000 W/m2 and calculate the relative spectral response for each wavelength by integration over E.
The spectral responsivity (SR) is required for the calculation of the spectral mismatch factor fMM. The spectral mismatch correction compensates differences in the current generation between the target spectrum (usually AM1.5G) and the spectrum of the sun simulator used for the measurement of the current voltage curve of the solar cell under test.
Task 3: IV Measurement
IV measurements are carried out using the light from an AAA solar simulator (WACOM WXS-156 S-L2) which comprises a two lamp system (halogen and xenon). The light field has an area of (160×160) mm2. On a motorized x-axis a spectroradiometer and the measurement unit for the solar cell under test as well as for the WPVS reference solar cell are mounted. This allows a precise control of the intensity and spectrum of the solar simulator.
The measurement of the current voltage curve is divided in five steps:
- The WPVS reference solar cell is positioned below the solar simulator light field and its intensity is adjusted in order to reproduce the short circuit current Isc.WPVS.STC of the reference cell determined during calibration at PTB. Afterwards the spectrometer is positioned below the light field and class A conformity is tested.
- We compensate for spectral mismatch (fMM) and light field inhomogeneity (fhom). The WPVS reference solar cell is positioned below the solar simulator and the light intensity is set to Isc.WPVS.corr = Isc.WPVS.STC / fMM / fhom.
- The solar cell under test is mounted and electrically contacted with kelvin probes to avoid any shading. The solar cell under test is positioned below the solar simulator and the short circuit current Isc.probes is measured. Afterwards contact bars are mounted and the short circuit current is increased such the current Isc.probes measured with probes is reproduced.
- The solar cell under test is positioned below the solar simulator light field and the 25°C equivalent open circuit voltage is determined by applying the Voc–t-method. For this, the temperature of the solar cell under test is adjusted in the dark to 25°C measured at the solar cell rear using a tactile PT-1000 temperature sensor. Afterwards the solar simulator high speed shutter is opened and the open circuit voltage is measured as function of time. The maximum of the resulting curve Voc.max is the best approximated value for Voc at 25°C under illumination.
- The shutter of the IV tester remains open. The temperature of the measurement chuck is adjusted until the continuously measured Voc equals Voc.max. The current voltage curve is measured using a four-quadrant current voltage source.
A calibration certificate is issued for the SR measurement and for the IV measurement. All parameters are reported with the accompanied uncertainty.
Please note that calibrated measurements will only by performed if standard testing conditions as defined in the IEC 60904 standards can be ensured through the whole measurement procedure. The temporal stability of the solar cells lies in the responsibility of the customer.
|Number of cells||Price in € per Cell (SR and IV)|