The Structure and Passivation Effects of Double-Positioning Twin Boundaries in CdTe
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The Structure and Passivation Effects of Double-Positioning Twin Boundaries in CdTe Yanfa Yan, M.M. Al-Jassim, and K.M. Jones National Renewable Energy Laboratory, Golden, CO 80401, USA ABSTRACT Using the combination of high-resolution transmission electron microscopy, first-principles density-functional total-energy calculations, and image simulations, we studied the atomic structure and passivation effects of double-positioning (DP) twin boundaries in CdTe. The DP twin boundaries are found to contain more Te dangling bonds than Cd dangling bonds, resulting in energy states in the bandgap that are detrimental to the electronic properties of CdTe. We found that I, Br, Cl, S, and O atoms present passivation effects on the DP twin boundaries to differing degrees, whereas H does not passivate the boundaries. Of all these impurities, I and Cl atoms present the best passivation effects on the DP twin boundaries. The superior passivation effects are realized by either terminating the Cd atoms with dangling bonds, or substituting the Te atoms with dangling bonds in the DP twin boundaries in CdTe by Cl and I atoms. INTRODUCTION Polycrystalline cadmium telluride (CdTe) has been recognized as a promising photovoltaic material for thin-film solar cells because of its near-optimum bandgap of ~1.48 eV and its high direct-absorption coefficient. However, it has been challenging to grow high-quality materials. Various growth methods, such as molecular-beam epitaxy (MBE) [1], metalorganic chemical vapor deposition (MOCVD) [2], hot-wall epitaxy [3], and close-spaced sublimation (CSS) [4], have been used to grow CdTe films. But none of these growth methods can easily eliminate the formation of extended defects, especially stacking faults and twin boundaries [5]. However, in a finished CdS/CdTe solar cell, many of the extended defects are found to be inactive. It is believed that these extended defects are passivated during device fabrication. So far, the mechanism of defects passivation in CdTe is still unknown. The possible impurity sources for the passivation in CdTe solar cells include I, Cl, Br, S, O, and H. The Cl source comes from the post-deposition heat treatment in CdCl2, a critical process to achieving high efficiency. To obtain good cell performance, CdS/CdTe back surfaces often need to be treated in bromine (Br) methanol prior to the application of back contacts. In this case, Br can also diffuse into CdTe to become an impurity source. In a finished CdS/CdTe solar cell, interdiffusion often occurs at the interface. S is found to diffuse into CdTe, providing the S source. S is known to passivate CdTe surfaces. In addition, CSS-grown CdTe thin films are often reported to contain high concentration of O. All these impurities—Cl, Br, S, O, H, and Na—can be potential sources for passivating the extended defects. So far, there is no systematic investigation of the passivation effects of these impurities. In this paper, we report on our studies of the atomic structure and passivation effects of double-positioning (DP) tw
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