Photovoltaic panel and cell with fine fingers and method of manufacture of the same

This patent application claims priority from the following provisional United States patent applications:

Application No. 61/012,795, filed Dec. 11, 2007 entitled, “PHOTOVOLTAIC CELL WITH FINE FINGERS AND METHOD OF MANUFACTURE OF THE SAME,” assigned attorney docket number 3253/135, and naming Brown Williams, Christopher E. Dubé, and Andrew Gabor as joint inventors,

Application No. 61/046,045, filed Apr. 18, 2008 entitled, “PHOTOVOLTAIC CELL WITH TABS FOR REFLECTING LIGHT TOWARD SUBSTRATE,” assigned attorney docket number 3253/162, and naming Brown Williams as the sole inventor,

Application No. 61/079,178, filed Jul. 9, 2008, entitled, “EFFICIENT PHOTOVOLTAIC CELL,” assigned attorney docket number 3253/164, and naming Christopher E. Dubé, Stephen Fox, Andrew Gabor, and Brown Williams as joint inventors,

The disclosures of these three provisional United States patent applications are incorporated herein, in their entireties, by reference.

This patent application is related to the following United States patent application:

U.S. patent application Ser. No. ______, filed on the same date as this patent application, assigned attorney docket number 3253/182, naming Brown Williams as inventor, and entitled, “SHAPED TAB CONDUCTORS FOR A PHOTOVOLTAIC CELL,” the disclosure of which is incorporated herein, in its entirety, by reference.

The invention generally relates to photovoltaic cells and panels and, more particularly, the invention relates to photovoltaic cell efficiency using fine fingers.

Photovoltaic cells convert light into electrical energy. To that end, a photovoltaic cell has a doped substrate that, when exposed to light, generates charge carriers, such as electrons. Conductors (referred to in the art as a “tabs”) coupled with the substrate conduct these electrons to another device, thus producing an electrical current. As an example, one common cell technology collects the charge carriers by forming a plurality of conductive fingers on the substrate. The fingers conduct the collected charge carriers to one or more of the tabs for transmission to another device.

Problems arise when any of the fingers break—i.e., when a discontinuity forms along any of the fingers—or when the fingers otherwise have some kind of discontinuity. This is especially problematic when screen printing the fingers onto the substrate. If that happens, electrons may have to travel farther to reach another, less proximate tab, thus losing energy. These phenomena undesirably reduce cell efficiency.

Those in the art have responded to this problem by forming relatively wide, robust fingers. For example, to the inventors\' knowledge, current state of the art, commercially acceptable cell finger printing processes do not form fingers narrower than about 100-120 microns (i.e., having average widths of less than about 100-120 microns). Forming fingers with such large widths often desirably reduces the incidence of breakage.

Wide fingers typically also are more conductive than narrower fingers for a given height. Accordingly, wide fingers should deliver improved current flow from the cell—to deliver commercially acceptable performance. Undesirably, however, wide fingers present another problem; namely, they cover a greater area of the top surface of the cell than that covered by narrower fingers. Thus, as known by those in the art, such greater coverage necessarily reduces the amount of light striking its photosensitive top surface, consequently offsetting efficiency gains by increased finger conductivity.

In accordance with one embodiment of the invention, a photovoltaic cell has a photosensitive substrate and a plurality of fingers in ohmic contact with the substrate. At least one of the fingers has an average width of less than about 95 microns. The cell also has a plurality of busbars in ohmic contact with the fingers. At least two of the busbars that are spaced less than about 38 millimeters apart intersect the at least one finger (i.e., the finger(s) having an average width of less than about 95 microns).

As a lower limit, the at least two busbars may be spaced no less than about six millimeters apart. Moreover, in certain applications, no busbar on the substrate may be spaced more than about 38 millimeters from another adjacent busbar. For example, the plurality of busbars may be generally equally spaced apart a given distance of no more than about 15 millimeters.

Any of a number of different substrates may suffice. For example, the substrate may be a sheet wafer formed from doped polysilicon (e.g., a string ribbon wafer having a thickness of no greater than about 300 microns). Each busbar and each finger may be formed from a screen printed conductive material. Moreover, the plurality of fingers preferably are formed from only a single layer of conductive material.

Each of the plurality of intersected fingers may have an average height of no greater than about twenty microns, and an average width of no greater than about sixty microns. Alternatively, each of the plurality of fingers may have an average width of no greater than about 65 microns. In a similar manner, the at least one finger may have an average width of no less than about 35 microns.