Wet-Chemical Synthesis of Thin-Film Solar Cells
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ABSTRACT We have been working on the development of wet-chemical processing methods that can be used to create thin film photovoltaic solar cells. Electrochemically deposition methods have been used to produce copper indium diselenide (CIS) thin films on molybdenum coated polymer substrates. CIS has an extremely high optical absorption coefficient, excellent radiation resistance, and good electrical conductivity and thus has proved to be an ideal absorber material for thin film solar cells. A series of compositionally different p-type CIS films were produced by using different electrochemical deposition potentials. Cadmium sulfide (CdS) window layers were deposited directly on these CIS films using a chemical bath process. CdS is a naturally ntype wide-bandgap semiconductor which has good transparency and is well lattice-matched to CIS. Zinc oxide thin films were grown by electrochemical deposition directly on the CdS films. ZnO is a transparent and conductive thin film that serves as the top contact of the cells. The structural and elemental properties of the individual ZnO, CdS and CIS films were characterized by x-ray diffraction and energy dispersive spectroscopy. The electrical behavior of the CdS on CIS junctions was determined using current versus voltage measurements. We will discuss the performance of these devices based on the physical properties of the component films and the processing methods employed in their fabrication. INTRODUCTION The development of high power-to-weight ratio photovoltaic solar cells for use in space applications is essential to several proposed NASA programs. These cells should be flexible for easy deployment and show good radiation resistance. Thin-film solar cells based on copper indium diselenide grown on polymer substrates are an excellent candidate for such applications [1]. It is also desirable that the methods used to produce these cells are cost-effective, easily scalable, and involve low temperatures, as to be compatible with polymer substrates. Wetchemical methods of producing thin-film photovoltaic materials have the potential to meet these goals. CuInSe 2 (CIS) is a leading candidate for use as a thin-film solar cell absorber layer. CIS has an extremely high optical absorption coefficient, good electrical conductivity, and good radiation resistance [2]. CIS has an optical bandgap of around 1.1 eV, which although not ideal, can be improved by the substitutional doping of Ga for In and/or S for Se [3]. Thin-film solar cells utilizing CulnlnGaxSe 2 (CIGS) absorber layers have reached efficiencies of nearly 18% [4]. The most commonly used junction material for CIS-based solar cells is cadmium sulfide (CdS). The hexagonal for of CdS is well lattice matched to CIS. CdS is naturally n-type and has a good optical transparency and an optical bandgap of approximately 2.4 eV [5]. The CdS films are most commonly deposited using a chemical bath deposition technique [6].
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Mat. Res. Soc. Symp. Proc. Vol. 606 © 2000 Materials Research Society
Zinc oxide (ZnO) is a transparent
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