Sputtered ZnTe:N and ZnO:Al for Solar Cell Electrodes and Recombination Junctions
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Sputtered ZnTe:N and ZnO:Al for Solar Cell Electrodes and Recombination Junctions Jennifer Drayton1, V. Parikh1, G. Rich1, A. Gupta1, T. Osborn2, R.G. Bohn1, A.D. Compaan1, B.E. McCandless3, P.D. Paulson3 1 Department of Physics and Astronomy, The University of Toledo, Toledo, OH 2 Department of Chemistry, The University of Toledo, Toledo, OH 3 Institute of Energy Conversion, The University of Delaware, Newark, DE ABSTRACT We have used sputtered ZnTe:N and ZnO:Al as transparent electrodes for CdTe based solar cells. ZnTe:N is reactively sputtered with 3% N2 in the sputter gas. The ZnTe:N films typically have transmission near 85% above 750 nm and resistivity as low as 10 ohm-cm with fine grains of approximately 30 nm diameter. ZnO:Al is sputtered from a ZnO:Al2O3 (2%) target. The ZnO:Al films have resistivity as low as 4 x 10-4 ohm-cm with 80-95% transmission over the visible spectrum. To test the stability of the films, ZnTe:N films were annealed in argon, dry air and nitrogen at a range of temperatures from 200°C to 500°C. Annealing decreased the resistivity for temperatures up to 350°C and increased for temperatures greater than 350°C. ZnO:Al films were annealed in dry air at temperatures from 300°C to 550°C. Though the resistivity increased at higher annealing temperatures, there was no change in the transmission. Important to the function of a tandem solar cell is a transparent contact, like ZnTe:N/ZnO:Al, to replace the traditional metal contact to the CdS/CdTe solar cell. We investigate the ZnTe:N/ ZnO:Al bilayer as a possible recombination junction. The rf sputtered ZnTe:N and ZnO:Al films were characterized by AFM, STM, XRD, transmission, 4-point probe, and Hall measurements. INTRODUCTION Films used in tandem solar cells have to be able to retain their desirable properties when exposed to conditions necessary for subsequent layers needed to complete the structure. These include exposure to gases and temperatures used in fabrication that may adversely affect the film stability. In order to ascertain how ZnTe:N and ZnO:Al films would be affected by these two parameters, films were annealed over a range of temperatures in different ambients. Nitrogen-doped, p-type ZnTe is a good candidate to serve as a copper-free back contact to CdS/CdTe solar cells used in tandem solar-cell structures [1]. Aluminum-doped, n-type ZnO is a low-resistivity, transparent conducting oxide (TCO) [2] which could be used as an intermediate layer along with ZnTe:N in a 2-terminal tandem solar cell. In this configuration, these two films would form a tunnel/recombination junction as shown in Figure 1. Recombination/tunnel junction structures with the ZnTe:N/ZnO:Al interface were fabricated on glass substrates with Ni and Al contacts in order to isolate the junction separately from the rest of the solar cell. Current-voltage properties of the recombination junction structure should show whether the ZnTe:N/ZnO:Al interface has the current characteristics of a tunnel/recombination junction.
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