Fishing – trapping – and vermin destroying
Patent
1994-03-16
1995-12-05
Fourson, George
Fishing, trapping, and vermin destroying
148DIG64, 136256, 136260, H01L 218254, H01L 3104
Patent
active
054729106
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the production of ohmic contacts to p-type IIB/VIB semiconductor materials.
The production of thin film IIB/VIB semiconductors, e.g. CdTe, by electrodeposition and their use in photovoltaic cells is disclosed in GB 1 532 616.
The use of heat treatment to convert thin films of near intrinsic or n-type semiconductor based on elements of Group IIB of the Periodic Table and Te to p-type semiconductor is disclosed in U.S. Pat. No. 4,388,483.
In order to make photovoltaic cells with good electrical properties it is necessary to make a good ohmic contact to the layer of IIB/VIB semiconductor. U.S. Pat. No.4,456,530 discloses a method of obtaining good ohmic contacts on p-type semiconductor by etching the semiconductor film surface with an acidic solution, and then etching with a strong basic solution, and finally depositing a metal layer. The acidic etch may be an acid solution of an oxidising agent such as the well-known dichromate etch. The strong basic etch may be hydrazine monohydrate.
We have found that the stability on prolonged light exposure of cells provided with ohmic contacts using the method of U.S. Pat. No. 4,456,530, i.e. their resistance to ageing, is not always as high as is desirable and that the electrical characteristics may be impaired as result of a reduced shunt resistance and increased series resistance.
We have now found an alternative method of providing ohmic contacts to layers of IIB/VIB semiconductors.
According to the present invention a method of making an article having an ohmic contact to p-type IIB/VIB semiconductor is characterised by the successive steps of: substantially free of oxide groups, of the layer of p-type semiconductor,
The preferred method of applying the invention is to deposit a conductive layer on the surface which has been treated in accordance with the invention. An alternative, less preferred, method is to form a conductive layer, and then to deposit a very thin layer of IIB/VIB semiconductor on the conductive layer. This layer of IIB/VIB semiconductor is then subjected to the treatment of the present invention. Finally a thicker layer of IIB/VIB semiconductor layer is deposited on the treated thin layer of IIB/VIB semiconductor which provides an ohmic contact to the underlying conductive layer.
The semiconductor is a IIB/VIB semiconductor, i.e. a semiconductor containing at least one element from Group IIB and at least one element from Group VIB. In this specification references to IIB and VIB are references to the Periodic Table of the Elements as appearing in "Advanced Inorganic Chemistry" by Cotton & Wilkinson, 4th Edition, in which Group IIB includes Cd, and Group VIB includes Se and Te. The preferred semiconductors are compounds of Cd and Te, which may also contain Hg, as disclosed in U.S. Pat. No. 4,548,681. In addition to compounds of Cd,Te, and Hg it is also possible to use CdTe doped with small quantities of Cu, Ag, and Au as disclosed in EP 244 963.
The p-type semiconductor layer may be a thin film semiconductor, but may also be a single crystal material. It is particularly preferred to apply the process of the present invention to a p-type polycrystalline thin film semiconductor obtained by the heat treatment of a layer of IIB/VIB semiconductor obtained by electrodeposition.
The IIB/VIB semiconductor layer may form part of a multi-layer article intended to be formed into a photovoltaic cell. Such an article may be obtained by depositing CdS on a transparent conducting substrate (for example using electrodeposition or electroless or vacuum deposition), and then electrodepositing the IIB/VIB semiconductor on the CdS layer.
In order to obtain the p-type semiconductor required by the process of the present invention the electrodeposited material is heat-treated using known methods to form p-type material. If the heat treatment is carried out in the presence of a gas containing oxygen such as air then a surface layer containing oxides of Cd and Te is produced. This oxide-containing layer is undesirable and if it is formed
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Johnson Daniel R.
Oktik Sener
Ozsan Mehmet E.
Patterson Michael H.
Bilodeau Thomas G.
BP Solar Limited
Fourson George
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