Can the Staebler-Wronski effect account for the long-term performance of a-Si PV arrays? Bolko von Roedern and Benjamin Kroposki Citation: AIP Conference Proceedings 394, 313 (1997); doi: 10.1063/1.52849 View online: http://dx.doi.org/10.1063/1.52849 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/394?ver=pdfcov Published by the AIP Publishing Articles you may be interested in PVMaT improvements in monolithic a-Si modules on continuous polymer substrates AIP Conf. Proc. 394, 451 (1997); 10.1063/1.52904 PV system field experience and reliability AIP Conf. Proc. 394, 323 (1997); 10.1063/1.52900 Development of a low cost integrated 15 kW A.C. solar tracking sub-array for grid connected PV power system applications AIP Conf. Proc. 394, 827 (1997); 10.1063/1.52878 Effect of Cu doping on the properties of ZnTe:Cu thin films and CdS/CdTe/ZnTe solar cells AIP Conf. Proc. 394, 639 (1997); 10.1063/1.52868 Long‐term stability performance testing of amorphous silicon modules under natural sunlight AIP Conf. Proc. 157, 326 (1987); 10.1063/1.36513
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Can the Staebler-Wronski Effect Account for the Long-Term Performance of a-Si PV Arrays? Bolko von R o e d e m and B e n j a m i n Kroposki National Renewable Energy Laboratory (NREL), Golden, Co 80401-3393 Abstract. We suggest a model for the Staebler-Wronski degradation ofa-Si-based solar cells that can account for long-term performance observations of deployed a-Si photovoltaic modules. The model suggests that the stabilization of the Staebler-Wronski degradation does not occur because an equilibrium between light-induced degradation and thermal or lightinduced recovery is reached. Rather, stabilization occurs because the degradation phenomenon itself is self-limiting, i.e., once enough degradation has been introduced, the degradation process diminishes. This model shows that the module performance and the longterm power output of an a-Si PV array depend not only on the operating conditions, but also, on the temperature history of light exposure. This makes it difficult to accurately predict the exact amount of degradation in the field.
INTRODUCTION Light-induced degradation (Staebler-Wronski effect) of amorphous silicon (a-Si) solar cells and photovoltaic (PV) modules provides a serious challenge for this PV technology to achieve stabilized module efficiencies of 15%, the long-term Department of Energy (DOE) goal. Although light-induced performance changes may occur in many solar cells, only in a-Si is the effect of such magnitude that it has become a research topic of its own. It appears to be an intrinsic effect observable in all hydrogenated amorphous semiconductors, even though the details of the degradation phenomena depend significantly on the details of materials preparation. In most instances, the degradation is observed to stabilize after prolonged light exposure, which allows the fabrication of viable PV devices. However, the power rating of these devices must account for the expected degradation. This makes it important to know how much a given device will degrade. The Staebler-Wronski effect is also observable in the degradation of fundamental material parameters such as the photoconductance (1), the midgap defect density (2), the luminescence (3), or the carrier transport mobilities (4). All these properties degrade with their own time constants (5), but the degradation often follows a stretched exponential relationship, i.e., after 102 to 104 hours of
CP394, NREL/SNL Photovoltaics Program Review, edited by C. Edwin Witt, M. Al-Jassim, and J. M. Gee. AIP Press, New York © 1997
313 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 157.182.150.22 On: Mon, 04 May 2015 16:18:09
exposure to sunlight (100 mW/cm 2 intensity), the degradation diminishes. A very large portion or all of the observable degradation can be reversed upon annealing at temperatures above 150°C. This observation has led to a commonly accepted picture that the degradation stabilizes when an equilibrium with a counteracting annealing process is reached. Thermal anneal rates at typical module operating temperatures (
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 157.182.150.22 On: Mon, 04 May 2015 16:18:09
Can the Staebler-Wronski Effect Account for the Long-Term Performance of a-Si PV Arrays? Bolko von R o e d e m and B e n j a m i n Kroposki National Renewable Energy Laboratory (NREL), Golden, Co 80401-3393 Abstract. We suggest a model for the Staebler-Wronski degradation ofa-Si-based solar cells that can account for long-term performance observations of deployed a-Si photovoltaic modules. The model suggests that the stabilization of the Staebler-Wronski degradation does not occur because an equilibrium between light-induced degradation and thermal or lightinduced recovery is reached. Rather, stabilization occurs because the degradation phenomenon itself is self-limiting, i.e., once enough degradation has been introduced, the degradation process diminishes. This model shows that the module performance and the longterm power output of an a-Si PV array depend not only on the operating conditions, but also, on the temperature history of light exposure. This makes it difficult to accurately predict the exact amount of degradation in the field.
INTRODUCTION Light-induced degradation (Staebler-Wronski effect) of amorphous silicon (a-Si) solar cells and photovoltaic (PV) modules provides a serious challenge for this PV technology to achieve stabilized module efficiencies of 15%, the long-term Department of Energy (DOE) goal. Although light-induced performance changes may occur in many solar cells, only in a-Si is the effect of such magnitude that it has become a research topic of its own. It appears to be an intrinsic effect observable in all hydrogenated amorphous semiconductors, even though the details of the degradation phenomena depend significantly on the details of materials preparation. In most instances, the degradation is observed to stabilize after prolonged light exposure, which allows the fabrication of viable PV devices. However, the power rating of these devices must account for the expected degradation. This makes it important to know how much a given device will degrade. The Staebler-Wronski effect is also observable in the degradation of fundamental material parameters such as the photoconductance (1), the midgap defect density (2), the luminescence (3), or the carrier transport mobilities (4). All these properties degrade with their own time constants (5), but the degradation often follows a stretched exponential relationship, i.e., after 102 to 104 hours of
CP394, NREL/SNL Photovoltaics Program Review, edited by C. Edwin Witt, M. Al-Jassim, and J. M. Gee. AIP Press, New York © 1997
313 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 157.182.150.22 On: Mon, 04 May 2015 16:18:09
exposure to sunlight (100 mW/cm 2 intensity), the degradation diminishes. A very large portion or all of the observable degradation can be reversed upon annealing at temperatures above 150°C. This observation has led to a commonly accepted picture that the degradation stabilizes when an equilibrium with a counteracting annealing process is reached. Thermal anneal rates at typical module operating temperatures (
