Directed Energy Conversion of Semiconductor Materials

This invention relates to semiconductive materials, particularly to the conversion of such materials to change their characteristics, and more particularly to the conversion of a limited region of a semiconductive material to permit conduction in solar cells.

It is standard practice to form solar cells by depositing successive patterned layers upon a substrate. Alternatively, the layers are patterned after deposition by removing selected portions to form individual solar cells. The resulting cells are joined externally to form solar arrays. An example is provided by the Borden U.S. Pat. No. 4,278,473. Such arrays are expensive and difficult to manufacture. The patterning processes also reduce the active areas of the cells.

Another type of solar cell is described in the Kuwano et al U.S. Pat. No. 4,281,208 where an unpatterned active layer has electrical interconnections beyond the edge of the layer. This unpatterned structure is easier to manufacture than patterned structures. It is, however, limited in size by the electrode sheet resistance because the interconnection is made at the ends of the cells. As a result, such a panel is impractical for most uses.

Still another type of solar cell is disclosed in Morel et al U.S. Pat. No. 4,517,403 where a continuous semiconductive film carries front and back electrodes. Electrical connection between the electrodes is said to be provided directly through the film itself by diffusion of metal through the film. In particular, Morel uses "stitch bars" on one set of electrodes over which a semiconductive film extends continuously and completely. The stitch bars are tall and rough compared to the thickness of the film. Otherwise the desired conductive passage through the film will not exist. Where the stitch bars are relatively high the application of heat supposedly can cause diffusion of the electrode material, and possibly the material of the stitch bars, into the semiconductive film. The resulting diffused region is allegedly more conductive than the bulk of the film and is said to enhance cell interconnection. The conductive metallic materials allegedly may diffuse into the film as discrete atoms or exist as particulates occupying interstitial sites within the film. It is desirable to apply the heat only at localized areas within preselected areas. A laser beam may be used to melt the contact and the film material to form a eutectic composition of the two materials, or the laser beam may simply heat the materials enough to enhance diffusion.

In practice, heating to produce diffusion does not operate satisfactorily. In particular, heating using a laser beam is incapable of providing the required diffusion. Consequently, in applications of the Morel disclosure it has been necessary to allow the stitch bars of one set of electrodes to extend completely through the semiconductive film into contact with another set of electrodes, since attempted interconnection by diffusion is unworkable.

Accordingly, it is an object of the invention to facilitate the interconnection of electrodes with respect to an intervening semiconductive layer. A related object is to facilitate the production of conductive regions in semiconductive materials.

Still another object of the invention is to overcome the difficulties and limitations associated with the prior art.

Another object is to overcome the difficulties associated with the technique described by Morel where the heating of metallic material is alleged to produce diffusion through an intervening film.

In accomplishing the foregoing and related objects, the invention provides for the directed energy fusion of a selected region of a semiconductor layer to provide a conductive path through the layer.

A device in accordance with the invention includes a continuous layer having opposed surfaces, with one surface having a plurality of discrete electrodes and the opposite surface having a corresponding plurality of discrete electrodes opposite the first set of electrodes. A conductive region extends through the continuous layer from at least one of the electrodes to a corresponding opposite one of the electrodes. The conductive region which extends through the layer from one electrode to another can be transparent. The transparency is caused by channels in the layer extending from said one electrode to the other.

In accordance with one aspect of the invention, the semiconductor layer can be a thin film of controllably doped semiconductor, which can be hydrogenated silicon or germanium. One set of the electrodes is transparent to radiation. Where the electrodes are transparent to light, the device is a photocell. Where the electrodes are transparent to solar energy, the device is a solar cell.

In a method of manufacturing a radiation sensitive device in accordance with the invention, a plurality of spaced-apart electrodes in a first set is formed on a substrate and a continuous layer of semiconductor material is formed over the electrodes and the exposed substrate between the electrodes. A plurality of spaced-apart electrodes in a second set is formed on the semiconductor layer, each electrode of the first set overlying an electrode of a second set to define an intervening radiation sensitive region. An electrical connection is provided through the continuous semiconductor layer between at least one electrode of the first set and a corresponding electrode of the second set. The electrical connection is formed by conversion of the material of said layer, with the power of conversion desirably adjusted to achieve transparency in the layer. The level of transparency is indicated by the existence of pinholes which extend through the layer.

In a method of producing an electrical connection from a front electrode to a back electrode through a semiconductive layer, a semiconductive layer overlies the back electrode and the front overlies the semiconductive layer. A conductive path through the semiconductive layer joins the first electrode to the second electrode. The conductive path is formed by directed energy conversion of the semiconductive region between the first electrode and the second electrode. Laser conversion is desirable, with the laser energy applied to change the structure of the region extending between the front electrode and the back electrode. The change in conductivity of the path between the first electrode and the second electrode can be monitored and used to control the formation of the conductive path. This can be done by controlling the directed energy source. The directed energy can originate at a laser source and the conductivity of the path can be monitored by the transparency.

Other aspects of the invention will become apparent after considering several illustrative embodiments taken in conjunction with the drawings, in which:

FIG. 1 is a cross-sectional view of a portion of a monolithic solar cell panel in accordance with the prior art;

FIG. 2A is a cross-sectional view of a panel for further processing in accordance with the invention; and

FIG. 2B is a cross-sectional view of the panel of FIG. 2A after modification in accordance with the invention and illustrating schematic view of a portion of the control circuitry for achieving the desired modification in accordance with the invention.

With reference to FIG. 1, there is illustrated a commercial implementation of a monolithic thin film solar panel 10 disclosed in U.S. Pat. No. 4,517,403. The solar panel 10 of FIG. 1 is formed by a transparent substrate 16, a set of transparent front electrodes 18, a continuous thin film 19 of photovoltaic material, and a patterned set of individual back electrodes 17. Each of the back electrodes 17 is separated from adjoining back electrodes and overlies a photovoltaic region of the thin film 19. Ech front electrode 18 is transparent and includes a contact portion or stitch bar 18s.

Each stitch bar 18s extends through the thin film 19 and into contact with one of the back electrodes 17 of a prior photovoltaic region. In theory, according to U.S. Pat. No. 4,517,403 it should be possible to diffuse metallic material through the thin film 19 in order to join the stitch bars 18s to their corresponding back electrodes 17. In practice, metallic diffusion to achieve conduction does not take place and it is therefore necessary to have the stitch bar 18s extend completely through the thin film 19.

The present invention completely avoids the need for stitch bars and the need to have such structure penetrate the thin film of the photovoltaic material into contact with the back electrode. Referring now to structure 20 shown in FIG. 2A, a transparent substrate 26 includes a patterned set of transparent front electrodes 28. It is to be emphasized that the front electrodes 28 do not include any kind of projection, such as the stitch bars 18s of FIG. 1, through the adjoining thin film 29 of hydrogenated photovoltaic material. In addition, the structure 20 of FIG. 2A includes a patterned set of back electrodes 27. The patterning of FIG. 2A may take place in any suitable fashion to separate the adjoining back electrodes 27 and exposes the thin film photovoltaic layer 29.

Once the back electrodes 27 have been formed, a laser 31 is used in the position indicated in FIG. 2B so that the beam 32 passes through an edge of one of the front electrodes 28, illustratively the electrode 28-a through the thin film 29 and into an edge portion of an overlapping back electrode 27, illustrtatively the back electrode 27-a. Since the desired contact of the back electrode 27a with corresponding front electrode 28a through the thin film 29 cannot be achieved by diffusion, i.e., causing the migration of metallic particles from one of the electrodes 27a or 28a to the other electrode, or vice versa, and since the front electrodes 28 do not include any structure corresponding to stitch bars, the invention provides an entirely different technique for realizing a suitable conduction path, illustratively designated 29a in FIG. 2B, through the thin film 29.

The thin film layer 29 is initially opaque to visible light. It has been discovered that if the laser beam 32, e.g. in the near infra-red region, is applied until the path 29a becomes transparent, the material of the path 29a becomes conductive. The laser beam is in general focused on the work using a lens. With a sufficiently high beam intensity a cylindrical lens is required to obtain suitable power densities and realize the aforementioned conducting state. When the conductive state is realized the desired electrical interconnection between front and back electrodes is attained without the need for the stitch bars and any attempt to diffuse metallic particles through the photovoltaic film.

One implementation of the conversion of the thin film material 29 from opacity to transparency is illustrated. A detector is positioned opposite the laser 31. The output of the detector 41 is applied over a control line 42 to the laser 31. After the laser is activated, at low initial power, the transparency of the path 29p is continually monitored. When the transparency reaches a suitable level, a control signal over the line 42 deactivates the laser 31. The conductive path 29p is characterized by the presence of channels which extend from the back electrode to the front electrode. These channels are formed by hydrogen evolution from the hydrogenated semiconductor matter. It is these channels which are responsible for the transparency. It is speculated that by contrast with the supposed diffusion that takes place in the prior art, the present invention permits metallic fingers, e.g., of aluminum to extend into contact with the opposite electrode.

It will be appreciated that the use of a laser source is merely illustrative and that other energy sources such as those making use of molecular beams and other forms of radiation may be used instead. It will also be appreciated that "transparency" is a matter of degree and that, with switch monitoring, films which are not visually transparent can be rendered conductive.

Other aspects of the invention will be apparent to those of ordinary skill in the art. It will be understood that the foregoing description is for illustration only and that suitable modifications may be made without departing from the spirit and scope of the invention.