SPECTRAL SENSITIVITY ANALYSES OF TANDEM MODULES USING STANDARD FLASHER AND DYNAMIC LED BACKLIGHT

Abstract

As the solar spectra are changing during the day, seasons and weather conditions, the accurate annual energy rating has to be based not only on the nominal power at Standard Test Conditions STC but also on the sensitivity of power versus spectrum. The aim of the work is to measure the spectral sensitivity of tandem modules within the current/voltage scan. A standard industrial flasher was used to measure amorphous / microcrystalline (a- Si/uc-Si) tandem modules. This system was equipped with a special developed dynamic LED backlight at ZHAW. Test samples were analyzed using different shapes of dynamic backlight superposing the standard flasher STC spectra. The change of the LED backlight in the millisecond regime according to triangular, rectangular and sinusoidal shapes were applied. The analyzes were manly performed on a-Si/uc-Si tandem modules (1.1m x 1.3m). A triangle backlight characteristic was applied, increasing the IR LED light from 0% to 7% of ISC of the crystalline Silicon reference cell. Immediately after that period of 1.25ms, the IR LEDs are turned off and the blue LED decay from again 7% of ISC to 0% remaining on top of the STC spectra. This analysis shows significantly different spectral sensitivity at this specific time of changing from IR to blue. On the one hand the current of the module at maximum power point (MPP) changes proportionally with the IR intensity increase. On the other hand the tandem module's short circuit current shows no significant changes at the same IR intensity variation. The same method of analyzes are applied to a-Si/uc-Si tandem modules operating during 4 months and 34 months respectively, under real outdoor conditions. It is evident that this outdoor degraded modules show only marginal changes of the module current at MPP for the same increase of this dynamic IR LED backlight. Thus, a method together with the setup is presented and confirmed to measure the state of degradation of this kind of tandem module during the flasher period of a few milliseconds. The use of the presented dynamic LED backlight has the potential, in the industrial module production process, to improve the output of a standard STC flash during several milliseconds by providing information about the current matching of top and bottom tandem cells on a module scale.