Effects of Mg Content on Zn1-xMgxO:Al Transparent Conducting Films
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Effects of Mg Content on Zn1-xMgxO:Al Transparent Conducting Films Xiaonan Li, Hannah Ray, Craig L. Perkins, and Rommel Noufi 5200, NREL, 1617 Cole Blvd., Golden, CO, 80401 ABSTRACT Conductive zinc oxide (ZnO) films are used extensively as transparent electrodes in thin-film photovoltaic solar cells. Compared with the widely used indium tin oxide (ITO) and tin oxide (SnO2), ZnO has a smaller optical bandgap. ZnO is commonly used as a front contact for copper indium gallium diselenide (CIGS) solar cells, but it forms a small, unfavorable conduction-band offset with the CdS layer. The optical bandgap of ZnO could easily be engineering by alloying with MgO or CdO. In this work, we try to optimize the ZnO for CIGS solar cells. The optical and electrical properties of Zn1-xMgxO:Al films fabricated by co-sputtering were studied. Two targets: ZnO:Al and MgO, were used. The ratio of ZnO/MgO was varied continuously on the 6”x6” glass substrate, and the effects of composition on the properties of the Zn1-xMgxO:Al films were investigated. The carrier concentration and mobility of the Zn1-xMgxO:Al films decreased quickly with increasing Mg content. However, the optical properties of the Zn1-xMgxO:Al films do not vary linearly with Mg content, as reported by most papers. The observed optical bandgap of Zn1-xMgxO:Al films is actually first narrowed, then increased with the Mg content. The shift in optical bandgap from narrow to wide occurs at around a composition of x = 0.07. After the point of x = 0.07, the bandgap width start increase but film sheet resistance already too low. Our result therefore suggests that the alloyed Zn1-xMgxO:Al does not benefit the CIGS solar cell. INTRODUCTION The energy conversion efficiencies for thin-film photovoltaics such as multijunction amorphous silicon (a-Si), copper indium gallium diselenide (CIGS), and cadmium telluride (CdTe) solar cells have reached the point such that any improvement of the top window layer would significantly enhance the efficiency values. The goal of this study of the ZnO transparent conducting oxide (TCO) window layer is to optimize its properties for CIGS solar cells. ZnO is used as a front-contact layer for CIGS solar cells because of its high optical transmittance and low fabrication temperature. However, a conduction-band offset of -0.3 eV between ZnO and CdS reduces the cell open-circuit voltage (Voc) [1]. On the other hand, if the ZnO bandgap can be widened, some current could still be gained in the short-wavelength range. Furthermore, if both the short-circuit current density (Jsc ) and Voc of the device are improved, then the fill factor (FF) will also be improved. Thus, widening the bandgap of ZnO by lifting the conduction band would improve the CIGS cell efficiency. The band structure of ZnO can be engineering by alloying with MgO to form new compounds with appropriate optical and electronic properties. Studies have shown that bandgap engineering is possible for zinc magnesium oxide alloys, Zn1-xMgxO (0 < x < 1). Choopun et al. and Matsubara et
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