Effects of Grain Boundaries on Performance of Hydrogenated Nanocrystalline Silicon Solar Cells
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Effects of Grain Boundaries on Performance of Hydrogenated Nanocrystalline Silicon Solar Cells Tining Su1, David Bobela2, Xixiang Xu1, Scott Ehlert1, Dave Beglau1, Guozhen Yue1, Baojie Yan1, Arindam Banerjee1, Jeff Yang1, and Subhendu Guha1 1 United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084, U.S.A. 2 National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401, U.S.A. ABSTRACT We investigate the effect of hydrogenation of grain boundaries on the performance of solar cells for hydrogenated nanocrystalline silicon (nc-Si:H) thin films. Using hydrogen effusion, we found that the amplitude of the lower temperature peak in the H-effusion spectra is strongly correlated to the open-circuit voltage in solar cells. This is attributed to the hydrogenation of grain boundaries in the nc-Si:H films. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H) thin films are getting a great deal of attention for use in the bottom cell in multi-junction solar cells. The nc-Si:H based cell has excellent long wavelength response, and is less vulnerable to light-induced degradation than hydrogenated amorphous silicon (a-Si:H) based cells [1-4]. One disadvantage of the nc-Si:H solar cell is its relatively low open-circuit voltage (Voc). A typical value of Voc for nc-Si:H solar cells is about 0.5 V, and is often much lower. This compares poorly to about 1 V for a-Si:H cells. The lower than optimal Voc for the nc-Si:H solar cells can be attributed to many reasons, such as crystalline volume fraction and the defect density in i-layer, as well as ambient degradation. Ambient degradation refers to degradation of Voc and short-circuit current density (Jsc) observed for certain cells when exposed to the ambient conditions for an extended period of time. Electron-spin-resonance (ESR) in nc-Si:H films shows that the neutral defects mostly reside at the grain boundaries [5]. These defects can act as recombination centers, resulting in lower Voc. In addition, it has been suggested that oxygen related defects at the grain boundaries causes increased dark current, and hence also reduced Voc [6]. It has been suggested that improved hydrogenation of grain boundaries can improve Voc [7]. Also, depositing the i-layer at lower temperatures has been shown to improve Voc [6]. This was attributed to increased hydrogen concentration in the film resulting in better hydrogenation of the grain boundaries. Although it is well known that lower deposition temperature tends to reduce the crystalline volume fraction, and increase hydrogen incorporation in the films, it does not necessarily increase the degree of hydrogenation of the grain boundaries. To our best knowledge, no direct evidence has been reported that demonstrates a positive correlation between increased hydrogen concentration and increased hydrogenation of the grain boundaries. To distinguish hydrogen at the grain boundaries from that in the amorphous phase, sophisticated techniques are required for typical local probes, such as 1H NMR; and interpretation of the r
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