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The preparation of thin film graded band gap solar cells

By R Radojcic


Thin film solar cells with a band gap graded in\ud the thickness direction were prepared by vacuum\ud evaporation of various alloys of Cadmium Sulphide (CdS)\ud and Cadmium Telluride (CdTe) onto glass substrates.\ud Theoretical computer analyses were carried out and\ud show that a solar cell with a graded band gap surface\ud layer ought to be a more efficient photovoltaic converter\ud than a conventional homojunction device. A vacuum\ud evaporator capable of multiple simultaneous co-evaporations\ud was built and used initially to deposit only the CdS\ud and CdTe films and subsequently mixed and graded\ud films of Cadmium Sulphide Telluride (CdS\ud x\ud Te 1-x).\ud The composition of the mixed and graded films was\ud controlled by a set of shutters built above the sources,\ud rather than by the control of the temperature of the\ud sources. The electronic properties of the films were\ud adjusted by co-evaporation of dopant materials such\ud as Cadmium (Cd), Indium (In) and Copper (Cu), and\ud measured by Hall Effect measurements. The physical\ud properties of the films such as the band gap, crystal\ud phase and grain size were investigated by optical\ud transmission measurements, X-ray analysis and scanning\ud electron microscopy respectively. Finally, p-n junctions\ud with n-type graded band gap surface layers on top\ud of either pure p-type CdTe films or mixed p-type\ud CdS0.5Te0.5 films were prepared. However, the\ud photoresponse of these structures was low, probably\ud due to very short minority carrier life-times, lack\ud of low resistance contacts and cross diffusion of\ud dopant materials. Nevertheless, the characteristics\ud of individual materials and the spectral response\ud of the devices indicated that if these problems could\ud be solved, a successful solar cell could be made

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