Correlation of Material Properties and Open-Circuit Voltage of Amorphous Silicon Based Solar Cells

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Correlation of Material Properties and Open-Circuit Voltage of Amorphous Silicon Based Solar Cells Baojie Yan, Jeffrey Yang, Guozhen Yue, and Subhendu Guha United Solar Systems Corp., Troy, MI 48084, U.S.A. ABSTRACT Correlation of hydrogenated amorphous silicon (a-Si:H) alloy material properties and solar cell characteristics have been studied experimentally and by computer simulation. Simulation results show that all three solar cell parameters, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), decrease with increased defect density. For a given intrinsic layer thickness, a larger band gap (Eg) results in a higher Voc but a lower Jsc. However, FF does not depend on band gap. This allows us to distinguish the effect of change in band gap from that in defect density on the variation in Voc. For solar cells with good interface characteristics, a linear relation FF = βVoc + γ is obtained by light soaking experiments and simulation with different defect densities. The slope β is in the range from 2 to 3 V-1 depending on cell properties and light soaking condition, and the intersect γ depends mainly on the band gap. Comparing cells made with high H2 dilution to no H2 dilution, we find that a 58 mV enhancement in Voc with H2 dilution is due to both widening of band gap and reduced defect density. Simulation results also show that a narrower valence band tail leads to a higher Voc. We did not include this effect in the analysis due to lack of available data for correlation between H2 dilution and band tail narrowing. INTRODUCTION Hydrogen (H2) dilution is now widely used to improve the performance of hydrogenated amorphous silicon (a-Si:H) based solar cells since the first report in 1981 by Guha et al. [1]. The best solar cells are made with a dilution ratio just before the onset of microcrystalline silicon formation [2]. The main improvements are on open-circuit voltage (Voc) and fill factor (FF). It has been found that a-Si:H films made with high hydrogen dilution include chain-like structures and improved medium range order [3]. However, how these changes in material property affect the device performance is not well understood. For example, it is not clear as to how much of the enhancement of Voc is due to a wider band gap, narrower band tails, or lower defect density. In addition, the limiting factor of Voc has become an interesting topic lately [4,5]. Imperfect interfaces between the doped (n or p) layer and the intrinsic (i) layer would definitely affect Voc, and are reflected on the insensitivity of Voc upon light soaking. However, for a device with reasonably good interface, the bulk properties of the i layer become the dominant factor. In this case, all the three characteristic parameters, short circuit density (Jsc), Voc, and FF, of a solar cell degrade after light soaking. Schiff and his co-workers reported that the density of defect states does not affect the Voc very much [4], which is inconsistent with most of our experimental results [6]. In this study, we report

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