Correlation of Light-induced Enhancement of Open-Circuit Voltage and Structural Change of Heterogeneous Silicon Solar Ce
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Correlation of Light-induced Enhancement of Open-Circuit Voltage and Structural Change of Heterogeneous Silicon Solar Cells Jeffrey Yang, Kenneth Lord, Baojie Yan, Arindam Banerjee, and Subhendu Guha United Solar Systems Corp., 1100 West Maple Road Troy, MI 48084-5352 U.S.A. Daxing Han and Keda Wang Department of Physics and Astronomy, University of North Carolina Chapel Hill, NC 27599-3211 U.S.A. ABSTRACT We observe a significant light-induced increase in the open-circuit voltage, Voc, of thin-film silicon solar cells whose intrinsic (i) layer consists of an amorphous and microcrystalline mixed phase. The increase depends on the i-layer thickness, the i-layer deposition temperature, the initial Voc values, and the light-soaking intensity. An increase of as large as 150 mV is observed. The original Voc is restored after subsequent thermal annealing. In-situ photoluminescence (PL) spectroscopy is used to investigate this metastable phenomenon. We find that the PL intensity and peak-energy position associated with the amorphous component of the heterogeneous material increase upon light soaking, suggesting a structural change. We propose that a reduction of microcrystalline volume fraction or size is responsible for the Voc enhancement. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) alloy materials and solar cells near the onset of microcrystallinity have received a great deal of attention due to their superior film properties and solar cell performance [1]. The effect of hydrogen dilution during film growth was first reported by Guha et al. in 1981 [2]. The technique has now been widely used to reach the amorphous-tomicrocrystalline transition [3-10]. We have previously shown that the onset of the transition depends on deposition conditions such as hydrogen-dilution ratio and film thickness [7]. The transition region is easily identified by monitoring the open-circuit voltage (Voc) of the solar cell. As one approaches the threshold of transition from the amorphous region, Voc increases slightly, reaches a maximum, then drops down sharply into the transition region. The sudden drop in Voc is also accompanied by a large dispersion in the Voc values. As the dilution ratio or film thickness is further increased, Voc eventually converges to a typical microcrystalline silicon value of ~0.5 V. In this study, we report results on a series of light-soaking experiments on solar cells prepared in the amorphous-to-microcrystalline transition region. The solar cells exhibit initial Voc's ranging from greater than 1 V to less than 0.5 V, encompassing the amorphous, the mixedphase, and the microcrystalline silicon intrinsic (i) layers. We find that after one-sun light soaking, cells in the amorphous and microcrystalline regions show a small reduction in Voc, while those in the mixed phase display significant Voc enhancement [11]. An increase of nearly 140 mV after one-sun illumination at 50 °C is observed for the most heterogeneous cells. We also find that the change in Voc (∆Voc) depends on the i-layer thickness, the i-layer
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