Temperature-Dependent Open-Circuit Voltage Measurements and Light-Soaking in Hydrogenated Amorphous Silcon Solar Cells
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A21.8.1
Temperature-Dependent Open-Circuit Voltage Measurements and Light-Soaking in Hydrogenated Amorphous Silcon Solar Cells Jianjun Liang,1 E. A. Schiff,1 S. Guha,2 B. Yan,2 and J. Yang2 1 Department of Physics, Syracuse University, Syracuse, NY 13244-1130 USA 2 United Solar Ovonic Corp., Troy, MI 48084 USA ABSTRACT We present temperature-dependent measurements of the open-circuit voltage VOC(T) in hydrogenated amorphous silicon nip solar cells prepared at United Solar. At room-temperature and above, VOC measured using near-solar illumination intensity differs by as much as 0.04 V for the as-deposited and light-soaked states; the values of VOC for the two states converge below 250 K. Models for VOC based entirely on recombination through deep levels (dangling bonds) do not account for the convergence effect. The convergence is present in a model that assumes the recombination traffic in the as-deposited state involves only bandtails, but which splits the recombination traffic fairly evenly between bandtails and defects for the light-soaked state at room-temperature. Recombination mechanisms are important in understanding light-soaking, and the present results are inconsistent with at least one well-known model for defect generation. INTRODUCTION The open-circuit voltage VOC is often the simplest solar cell parameter to understand. Experimentally, VOC is relatively independent of the thickness of a-Si:H nip solar cells. With ideal p and n layers, VOC may be identified with bulk photocarrier recombination processes in the intrinsic material. A simple understanding of recombination processes in a-Si:H solar cells would be valuable for two reasons. First, it would help establish which materials parameters actually determine the efficiency of working cells. Second, the metastable degradation of a-Si:H cells under illumination (the Staebler-Wronski effect) is undoubtedly mediated by photocarrier recombination – so correctly identifying the recombination processes occurring under solar illumination would be crucial to correctly identifying the microscopic mechanism underlying metastability. In the present work, we have studied the temperature-dependence of VOC under strong illumination in cells deposited at United Solar Ovonic Corp.. The results exhibit an interesting “convergence” effect: the differences in VOC for the as-deposited and the light-soaked states at higher measurement temperatures essentially disappear below 250 K. We show that a reasonably simple “bandtail+defect” recombination model accounts for these VOC measurements. In particular, the as-deposited state seems well-described by valence bandtail recombination (and neglecting defect recombination). The light-soaked state apparently involves a nearly equal combination of bandtail and defect recombination. We do not believe that this picture is a satisfactory model for photocarrier recombination at very low excitation densities; more than two decades of research at low excitation densities has revealed daunting complexities that are certainly not accommod
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