The present invention relates to the manufacture of thin film solar cells and more specifically to the use of an electrically enhanced liquid jet etch process.
References which illustrate the basic structures of thin film solar cells relevant to the present invention include the Hanak U.S. Pat. No. 4,292,092 and the Tyan et al U.S. Pat. No. 4,315,096. Both of these patents are hereby incorporated by reference for all purposes.
As discussed in the two above referenced patents, the series resistance of transparent conductors used on at least the light receiving faces of such thin film solar cells causes a problem where high efficiency or power output is desired. Practical devices have therefore been formed of a large number of serially connected cells. The individual cells are formed on a large substrate by scribing various layers. There are generally three sets layers to be scribed. These include the light receiving face transparent conductor, the actual semiconductor layer, and the back contact which is usually a metal such as aluminum, but which may also be transparent if desired. Each of these layers is deposited as a continuous sheet which is then separated into an appropriate section for each cell by scribing the continuous layer. As discussed in the two above referenced patents, both mechanical scribing and laser scribing have been used. The laser scribing techniques are considered the point of novelty in the Hanak patent.
Mechanical scribing has been found to be difficult or impossible to apply on a production basis. The transparent conductor is usually tin oxide which is quite hard and brittle. It is usually applied directly to a glass substrate. Scribing methods which are capable of cutting through the tin oxide can also cut into the glass substrate. In similar fashion, it is difficult to scribe through semiconductor layers, such as silicon, without also scribing through an underlying tin oxide layer. Other problems are encountered when a back contact made of aluminum is scribed with a mechanical device. The metal contact is relatively soft and therefore easy to cut through, but tends to smear and leave behind a thin layer of metal in the scribe line, thereby electrically shorting adjacent devices, even if the scribe is arranged to cut partially into the underlying semiconductor material.
Laser processing has, on the other hand, been found to be much more easily controlled and therefore reproducible on a production basis. However, laser equipment is relatively expensive to install and operate. In addition, it has proven to be relatively slow and therefore costly in terms of man-power required to produce a given quantity of finished material. This is due to the large amount of scribing which must be performed to produce a commercial scale device. For example, a one foot by one foot module is typically scribed to form twenty-five individual cells. This requires cutting at least twenty-four scribe lines each one foot long for each layer of the device. The substrate must therefore be passed under the laser a total of seventy-two times during its various processing stages. Throughput could be increased by operating a plurality of lasers simultaneously but this is not considered practical in terms of increased capital cost and operating expense.
Thus, it is seen that there is a need for a practical, reproducible and preferrably inexpensive process for scribing the various layers used in thin film solar cells.