Large Area Cu(In,Ga)Se 2 Films and Devices on Flexible Substrates Made by Sputtering
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F14.34.1
Large Area Cu(In,Ga)Se2 Films and Devices on Flexible Substrates Made by Sputtering Dennis R. Hollars, Randy Dorn, P. D. Paulson, Jochen Titus and Robert Zubeck Miasolé San Jose, CA 95131, U.S.A.
ABSTRACT A reactive sputtering process was developed for the production of Cu(In,Ga)Se2 films on a moving stainless steel substrate, in simulation of the operation of a roll coater. Cu, In and Ga fluxes were provided through magnetron sputtering and were reacted in a flux of Se on the heated substrate. CdS films were deposited either by chemical bath deposition (CBD) or by sputtering. Devices of the type steel/Cr/Mo/CIGS/CdS/ZnO/Ag were completed by sputtering ZnO layers and by screen printing grid lines. We made devices with efficiency values above 9%. A uniformity study was performed on a CIGS film and on small area devices made from it. The target length was 12”. Targets of this size are expected to produce a uniformly thick deposit over a range of 6-8”. The film thickness was 2.54 µm over a range of 6” with a standard deviation σ of 0.04 µm. The film composition was uniform over a range of 16”. The values of Cu/III and Ga/III were 0.84 and 0.31, with σ values of 0.02 and 0.01, respectively. The efficiency of allsputtered devices was uniform over a range of 12”, well beyond the 6” wide range of constant CIGS film thickness. Their efficiency was 6.6% on average with σ=0.6%. INTRODUCTION Cu(In,Ga)Se2, also known as CIGS, thin films have great potential for the fabrication of high efficiency photovoltaic devices. Several research groups routinely fabricate CIGS solar cells with efficiency values greater than 16% using various deposition techniques. However, the laboratory successes are not translated into the manufacturing scenario, mainly because of the complex process steps adopted for the fabrication of high efficiency devices, which may not be suitable for large-scale manufacturing. Successful commercialization of this technology demands a manufacturing process that is easily scalable, easy to control and that has a large process window. Co-evaporation of the elements and selenization of sputtered metal precursors are the dominant contemporary CIGS preparation methods. The preparation of compositionally uniform CIGS films over large areas by co-evaporation of the elements requires intelligent process control. Alternatively, CIGS films can be prepared by sputtering [1-4]. This approach has been somewhat neglected, probably due to the more rapid increases in device efficiency achieved by co-evaporation of the elements and by selenization of metallic precursor layers. The higher stability of the sputtering process can be employed to deposit uniform CIGS films and hence makes its application to the production of CIGS-based solar cells very promising. As of today the dominant substrate material in CIGS manufacturing is glass, which may pose severe handling and transportation problems for large scale manufacturing. Flexible substrates are better suited for large scale manufacturing. Several groups have reported wor
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