Thin films of copper indium disulphide and zinc-based buffer layers for application in photovoltaic devices

Johnston, David (2005) Thin films of copper indium disulphide and zinc-based buffer layers for application in photovoltaic devices. Doctoral thesis, Northumbria University.

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Abstract

Thin films of copper indium disulphide and zinc sulphide were fabricated, for use as absorber and window layers respectively for photovoltaic devices. The copper indium disulphide was produced in two stages. In the first stage, a copper indium alloy was produced on molybdenum-coated glass substrates by sputtering from metallic targets. A number of slides were placed together on each platten, giving a total area for deposition of approx. 100 cm2. A rotating turntable allowed a large number of very thin layers of copper and indium to be deposited alternately,
resulting in very good mixing to form the alloy. A range of powers were applied to the copper and indium targets, to produce films of different compositions, including stoichiometric, copper-rich and indium-rich material. Rutherford backscattering analysis showed a high degree of compositional uniformity, both over the area of the deposited film, and with depth through the film.
Three different methods were used to convert the copper indium alloy to copper indium disulphide. The first method involved evaporation of sulphur inside a graphite box, in which the substrate (with a copper indium precursor film) was enclosed. The sulphur diffused to fill the enclosed space, thus eliminating the limitations of line-of-sight evaporation. A strip heater was used in the initial experiments. Subsequently, a tube furnace was used to provide a more uniform and stable temperature. This allowed substrates up to 25 mm x 25 mm to be used. Temperatures up to 450C were used. X-ray diffraction showed essentially
complete conversion to copper indium disulphide for films heated to 400C for 40 minutes. This relatively low temperature allowed soda-lime glass, rather than borosilicate glass, to be used for the substrates. Both uniformity and adhesion to the substrate were good.
The second method also involved heating sulphur in the tube furnace to produce vapour. The size of the substrates used was as above - 25 mm x 25 mm. In this case, the sulphur vapour was transported over the substrates in a flowing argon stream. This provided an alternative mechanism for overcoming line-of-sight limitations. A similar range of temperatures was used, again allowing soda-lime glass to be used. X-ray diffraction again showed essentially complete conversion to copper indium disulphide for films heated to 400C for 40 minutes. Adhesion to the substrate was good. Uniformity was good, though less so than for material
produced using the graphite box.
A third method involved electrolytic conversion using a sulphur-containing solution. Both water and ethanediol were used as solvents. X-ray diffraction of films produced using water as a solvent showed a high degree of conversion to
copper indium disulphide. However, uniformity and adhesion were poor. Films produced with ethanediol as a solvent had improved uniformity and adhesion, but showed only partial conversion to copper indium disulphide. This was improved,
to some extent, by a subsequent anneal in vacuum at 400C for 30 minutes.
Zinc sulphide was produced by chemical bath deposition. Tri-sodium citrate was used as a complementary complexing agent, as a non-hazardous substitute for hydrazine hydrate, which had been used in previous work. The films produced
showed high optical transmittance (~ 90 %) over the wavelength range longer than 330 nm. They had good uniformity and adhesion to the substrate. Inclusion
of aluminium compounds in the deposition solution, in combination with a postdeposition anneal in vacuum at temperatures up to 400C, resulted in a significant
reduction in the resistivity of the films (~ 1 ohm for a 1cm2 device).
Partially-completed devices, consisting of copper indium disulphide and zinc sulphide layers, were sent to Sri Venkateswara University, Tirupati, India, where zinc oxide layers were added, to form complete photovoltaic devices. Voltage-current characteristics showed open circuit voltages of 300 to 400 mV and short circuit current densities of approx. 10 mAcm-2. The corresponding fill factor was 0.4.

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