Model for Staebler-Wronski Degradation Deduced from Long-Term, Controlled Light-Soaking Experiments
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Model for Staebler-Wronski Degradation Deduced from Long-Term, Controlled Light-Soaking Experiments Bolko von Roedern and Joseph A. del Cueto National Renewable Energy Laboratory (NREL) 1617 Cole Blvd., Golden, CO 80401-3393, U.S.A. ABSTRACT Long-term light-soaking experiments of amorphous silicon photovoltaic modules have now established that stabilization of the degradation occurs at levels that depend significantly on the operating conditions, as well as on the operating history of the modules. We suggest that stabilization occurs because of the introduction of degradation mechanisms with different time constants and annealing activation energies, depending on the exposure conditions. Stabilization will occur once a sufficient accumulation of different degradation mechanisms occurs. We find that operating module temperature during light-soaking is the most important parameter for determining stabilized performance. Next in importance is the exposure history of the device. The precise value of the light intensity seems least important in determining the stabilized efficiency, as long as its level is a significant fraction of 1-sun. INTRODUCTION Since about 1991, amorphous silicon (a-Si) research program guided by the National Renewable Energy Laboratory (NREL) has specified that all a-Si solar cell or photovoltaic (PV) module performances should be reported after stabilization. At that time, it was recommended to light-soak for 1000 hours with a device temperature of 50oC to establish stabilized performance. NREL carried out four light-soak tests using controlled exposure conditions to assess the details of degradation upon light-soaking [1,2]. Since about 1994, it became clear that outdoor exposure in Colorado typically resulted in higher degradation − typically twice as much − than exposure at 50oC continuous light-exposure [2,3]. In previous publications, we surmised that this behavior was a fundamental a-Si property; because the same degradation trends were observed in modules from different manufacturers and with different initial efficiencies [4]. It was argued that this behavior could not be explained by a single degradation mechanism in which thermal or light-induced annealing would balance the light-induced degradation. Instead, at least two mechanisms have to be involved: a fast one that can be annealed at typical module operating temperatures, and a slow one that does not recover measurably when annealing temperatures are limited to values below 70oC [4]. In a previous publication, we demonstrated that temperature and the temperature history during light-exposure clearly affected stabilized module efficiency [5]. In this work, we have extended the exposure temperatures further to lower values, resulting in further module degradation. Also, we have chosen to include a controlled light-soak cycle at ½-sun (about 500 instead of 1000 W/m2) to investigate our previous speculation that both exposure temperature and light-intensity affect the rates at which the fast and the slow mechanism can be introduced dur
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