Buffer-layer Effect on Mixed-Phase Cells Studied by Micro-Raman and Photoluminescence Spectroscopy
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Buffer-layer Effect on Mixed-Phase Cells Studied by Micro-Raman and Photoluminescence Spectroscopy Andrea Hilchey, Chris Lawyer, Keda Wang, and Daxing Han, Dept of Physics & Astronomy, Univ of North Carolina at Chapel Hill, Chapel Hill, NC 27599 Baojie Yan, Guozhen Yue, Jeffrey Yang, and Subhendu Guha, United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI 48084 ABSTRACT We use micro-Raman and photoluminescence (PL) spectroscopy to study the effects of an aSi:H buffer layer at the i/p interface of the mixed-phase silicon solar cells. We find that the signature of the crystalline 520 cm-1 mode still appears on the Raman spectrum for the cells with a 100 Å thick a-Si:H buffer layer; but it completely disappears for cells with a 500 Å thick a-Si:H buffer layer. At 80 K, the PL spectral lineshape reflects the features of the electronic states in the band tails. The characteristics of the PL spectra of the mixed-phase cells are a narrower main band than the standard a-Si:H band and an extra low energy band from the grain boundary region. As the thickness of the a-Si:H buffer layer increases, the PL main band becomes broader, and the low energy band is depressed. We find that, after light soaking, the PL main band is slightly broadened for the cells with no a-Si:H buffer layer, almost no change for the cells with a 100 Å thick buffer layer, and a remarkable decrease in total PL intensity for the cells with a 500 Å thick buffer layer. In addition, the PL intensity of the defect band increases after light soaking for the cells with a 500 Å thick buffer layer, where light-induced defect generation in the a-Si:H buffer layer masks the changes in the mixed-phase intrinsic layer. The Raman and PL results are consistent with previous observations of the effect of an a-Si:H buffer layer on the performance and metastability against light soaking for mixed-phase solar cells. INTRODUCTION A light induced open-circuit voltage (Voc) increase has been observed in mixed-phase hydrogenated silicon solar cells, where the intrinsic layer contains a small amount of nanocrystalline grains in the amorphous matrix [1-2]. An increase of photoluminescence (PL) intensity and a blue shift of the peak energy at 80 K have been observed previously in an in-situ light soaking experiment. The Voc increase was initially explained by light-induced structural changes, i.e. an expansion of the amorphous phase and a reduction of the crystalline phase [1-2]. However, the structural change model [3] explains the light-induced enhancement of volume on the order of 0.10.01%, while the observed Voc increase is as large or greater than 15%. Both x-ray diffraction (XRD) and Raman did not find any observable light-induced structural changes in the mixed-phase materials [4]. Meanwhile, micro-Raman found non-uniform distribution of the nanocrystalline grains on a micrometer-scale in the mixed-phase cell [5]. Experimental results also showed [4] that the Voc enhancement can be suppressed by applying a reverse bias during light soaking, which suggeste
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