Photoinduced Topotaxial Exchange Reactions in Cadmium Sulphide Thin Films
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PHOTOINDUCED TOPOTAXIAL EXCHANGE REACTIONS IN CADMIUM SULPHIDE THIN FILMS
T.J. CUMBERBATCH, P.E. BARDEN AND J.KNIGHTLEY Engineering Department, Cambridge University, Trumpington Street, Cambridge, England CB2 lPZ
ABSTRACT A CO 2 laser, operating at 10.6pm, has been used to promote the growth of topotaxial layers of the chalcocite phase of cuprous sulphide in cadmium sulphide thin films immersed in an organic solution of a cuprous salt at ambient temperature. Cuprous sulphide growth is initiated by the laser beam which passes through the liquid and raises the surface temperature of the CdS allowing ion exchange to take place without changing the surface topography. An investigation into different combinations of organic solvents and cuprous/cupric salts has revealed that cuprous iodide in acetonitrile or propionitrile yields the fastest growth rates. The results are compared with those obtained from junctions fabricated using lasers operating in the visible frequency range.
INTRODUCTION One of the most attractive features of the cuprous sulphide-cadmium sulphide solar cell is the ease with which the p-type photon absorber layer can be grown using a topotaxial ion exchange reaction. Cuorous sulphide has a very high optical absorption coefficient leading to a thickness requirement of only Q.2pm for efficient solar cells: furthermore, the constituent elements are abundant and non-toxic. Conventionally, the heterojunction is fabricated by immersing a polycrystalline cadmium sulohide layer into an acidic aqueous solution of cuprous chloride at 90°C for about five seconds; an ion exchange reaction leads to the formation of a topotaxial cuprous sulphide layer. However, this technique has the disadvantage of accelerated growth down the cadmium sulphide grain boundaries and is very sensitive to the immersion period. An alternative approach is to coat the surface of the cadmium sulphide with a thin film of evaporated cuprous chloride which is subsequently heated to form the heterojunction. This method has the disadvantage of beino a vacuum based process and has been shown to produce discontinuities in the p-type layer at grain boundaries. Problems with both growth techniques and the stability of the heterojunction mean that cuprous sulphide is no longer considered to be a serious contender for thin film solar cells. However, the two principal alternative materials, cadmium telluride and copper indium diselenide require more sophisticated deposition procedures, much thicker layers (several microns) and contain highly toxic ingredients. In principle, the disadvantages of the "wet dip" technique can be overcome by using organic solutions of cuprous salts, in which the cuprous ion is stabilised by the solvent and localised rapid heating (see Figure 1). If the temperature profile within the CdS is sufficiently steep, then this should lead to a dramatic reduction in the grain boundary penetration. The principal reaction may be written simply: Cu 2 S + Cd2+
CdS + 2Cu + with an unwanted side reaction: CdS + Cu2+
+
Mat. Res. Soc
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