Screen-printed Cu 3 BiS 3 -polyacrylic Acid Composite Coatings
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Screen-printed Cu3 BiS3 -polyacrylic acid composite coatings H. Hu, O. Gomez-Daza, and P. K. Nair Departamento de Materiales Solares, Centro de Investigaci´on en Energ´ıa, Universidad Nacional Aut´onoma de M´exico, Temixco 62580, Morelos, M´exico (Received 24 October 1997; accepted 6 April 1998)
A technique for preparing electrically conductive coatings of Cu3 BiS3 powder in polyacrylic acid matrix is presented. Bi2 S3 powder obtained by chemical precipitation was introduced into a freshly prepared CuS chemical deposition bath. After the initial nucleation period, CuS started to deposit on the Bi2 S3 surface. The as-obtained CuS–Bi2 S3 powder was mixed with polyacrylic acid aqueous solution, and the resulting mixture was used as a paste to form a screen-printed composite coating. Up to 200 ±C the film behaves like a simple CuS film; the sheet resistance is around 100 V and the crystallized phase in the composite is CuS (Covellite). When the temperature is equal or higher than 250 ±C, atomic diffusion at the CuS–Bi2 S3 interface is promoted, leading to the formation of the ternary compound Cu3 BiS3 (Wittichenite) in the composite film. The formation of the compound depends on the temperature, relative abundance of the Bi2 S3 and CuS components in the CuS–Bi2 S3 pigment, as well as on the annealing atmosphere.
The compound Cu3 BiS3 , which in the mineral form is called Wittichenite,1,2 can be formed by interfacial atomic diffusion in chemically deposited Bi2 S3 and CuS thin films by annealing for 1 h at 200–300 ±C.3 Thin films of Cu3 BiS3 show good thermal stability (.300 ±C), are p-type semiconductors, and possess a high optical absorbance in the visible region. These characteristics may place them among the p-absorber coatings suitable for application in heterojunction solar cell structures in a similar role to that of the well-investigated CuInSe2 absorber layer.4 The deposition of Bi2 S3 or CuS thin films by a chemical bath technique5,6 is accompanied by precipitation of the compound in the bath. In the case of CuS, electrically conductive CuS precipitate has been used to form a thick coating by a screen-printing technique7 or to form a composite coating with a polyacrylic acid aqueous solution by casting and also by a screenprinting method.8,9 The Bi2 S3 precipitate, on the other hand, is an electrical insulator. By introducing it into a CuS chemical bath, a Bi2 S3 –CuS precipitate is formed in the bath, and it was shown in the earlier work3 that the same ternary compound Cu3 BiS3 can also be obtained in powder samples by annealing the precipitate at temperatures between 250 ±C and 300 ±C. However, the electrical properties of the powder as a function of experimental parameters remain to be studied. In this work screen-printed Bi2 S3 -CuS coatings are prepared using an aqueous polyacrylic acid (PAA) solution as a binder. The reason we choose this polymer is that unlike other screen-printed films, no flux material is needed fuse the inorganic pigment (the precipitate
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