Long-term transient and metastable effects in cadmium telluride photovoltaic modules

Thin-film cadmium telluride (CdTe) photovoltaic (PV) technology is poised to begin making significant contributions and impact on terrestrial, electric power generation. However, some outstanding issues such as stability and transient behavior, and their impact on reliability and assessment of performance, remain to be thoroughly addressed, which has prompted some unease among PV industry integrators toward deploying this technology. We explore the issues of long-term stability and transient behavior in the performance of CdTe modules herein, using data acquired from indoor light-soaking studies. We find that measurement of current-voltage parameters and their temperature coefficients are entangled with transient effects. Changes in module power depend on recent operating history, such as electrical bias, and can result in either artificially high or low performance. Both the open-circuit voltage (V-oc) and fill factor (FF) are significantly impacted by metastable behavior that appears to linger for up to tens of hours, and we observe such increased transient effects after modules have undergone several hundred hours of light exposure. We present and analyze data measured under standard reporting conditions and actual operating conditions for six CdTe modules light-exposed and stressed at 65 degrees C nominal temperatures. Copyright (C) 2006 John Wiley & Sons, Ltd.