Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2002-05-14
2004-02-10
Le, Dung (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S186000, C257S440000, C257S083000, C257S084000, C438S022000, C438S048000
Reexamination Certificate
active
06690041
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the design and manufacture of and includes monolithically integrated diodes for use in various applications including in planar, thin-film, photovoltaic devices such as solar cells.
2. Description of the Prior Art
Photovoltaic (PV) cells generate an electric current when exposed to light (a photocurrent). PV cells thus have a wide variety of potential uses, including functioning as a power supply in certain terrestrial and space applications, acting as a photosensor to detect either a binary on-off presence of light as is required for certain security systems, and acting as a photosensor to detect varying intensities and/or wavelengths of light as is required for a variety of photographic and videographic applications. PV cells may produce photocurrent in response to a wide range or specific narrow band of the electromagnetic spectrum, including that band defined by the visible spectrum. In some applications, a PV cell that is transparent or translucent to a first frequency of light but produces a photocurrent in response to a second frequency is placed on top of a PV cell that produces a photocurrent in response to a first frequency. Although a PV cell produces current when exposed to light, when shadowed or shaded, it behaves as a diode. When a PV cell is described herein as being in a shadowed or shaded state, it means that it is not receiving sufficient light of the relevant wavelength or wavelengths to produce photocurrent.
A common practice in configuring PV cells, particularly when used in a power supply, is to place multiple cells in series. When a portion of the series or string of cells is shadowed, the shadowed cell (or cells) acts as a diode in reverse bias to the remainder of the series or string. As a result, the cell (or cells) acting as diodes in reverse bias tends to gain heat, which consequently damages that cell and potentially damages nearby cells. This heating is evidence of what is termed “destructive breakdown,” which is detrimental to a cell. Moreover, the energy consumed in heating and breaking down the cell is wasted, thus producing a less efficient device.
One method that has been applied to overcoming the problem of shadowed cells is to incorporate diodes into the design of the series or string. These diodes traditionally have been discrete components that have been attached to the PV cell by soldering or similar techniques. The addition of these discrete diodes provides an alternate electrical path in the event of a shadowed or shaded cell.
Unfortunately, the additional diodes add to a module's weight, thickness, complexity, and cost of manufacture, while decreasing its reliability. Moreover, many of the connection techniques used to attach discrete diodes impose additional constraints (e.g., rigidity) on the cell, further limiting its usefulness. Examples of designs that utilize discrete diodes include U.S. Pat. Nos. 6,255,793, 6,103,970, and 4,577,051. Other techniques which have been used include diodes which are integrated into the design of the cell. These may rely on “C” or “S” shaped interconnects to connect adjacent cells. An example of such a design is U.S. Pat. No. 5,616,185. These designs reduce the active area of the cell, thereby reducing efficiency.
Other designs involving integrated diodes make use of both sides of the PV cell. Some examples of this type of design include U.S. Pat. Nos. 5,580,395 and 4,323,719. Because these designs include additional layers on the bottom or reverse of the cell, they make the cell inherently thicker, and consequently heavier. Additionally, these designs increase the complexity of the design, and the cost of both materials and manufacture. Other techniques for creating integrated diodes do so by means of special doping techniques. Examples of this kind of design include U.S. Pat. Nos. 5,990,415, 5,389,158, 5,389,158, 5,248,346, and 4,933,022.
Another technique for creating integrated diodes includes adding additional partial layers to the surface of the cell and connecting these “integrated diodes” to the cell using integrated circuit techniques. An example of this type of design is U.S. Pat. No. 4,759,803.
An ideal design would provide an individual bypass diode for each PV cell, however, cost, size, and weight constraints may limit the designer to include only one bypass diode for every two or more series PV cells. In these configurations, the total number of cells which are in parallel to each bypass diode may be bypassed if only one of the PV cells of the group is shadowed, yielding less than optimal efficiency. Moreover, the larger bypassed voltage may require more robust diodes to avoid exposing the bypass diode to a voltage greater than or equal to its breakdown voltage.
SUMMARY OF THE INVENTION
The present invention relates to the design and manufacture of and includes monolithically integrated diodes for use in various applications including planar, thin-film PV devices such as solar cells. The design of the present invention is exemplified by an embodiment based upon a string of PV cells into which diodes have been monolithically integrated. The design produces a series or string of PV cells that have reduced weight, thickness, cost, and complexity while achieving increased reliability compared to the prior art. Other advantages of the present invention include its ability to be used in flexible thin film devices, its ability to extend the life span of PV cell series or strings, and its ability to increase manufacturing output. The integrated diode of the present invention is also capable of acting as a bypass diode, or as a blocking diode, to prevent the reverse flow of current from, for example, the electrical bus or parallel series or string to which the series or string may be connected.
One embodiment of the present invention, for example, overcomes the problem of discrete components through a monolithic integration technique which permits diodes to be fully integrated without the need for discrete components.
One embodiment of the present invention is created by providing a photovoltaic cell. This cell may comprise a substrate, and deposited on this substrate there may be a conducting layer, upon which a p-type absorber layer may be deposited. The cell may further comprise an n-type window layer deposited on this p-type absorber layer. The substrate may, for example, comprise an electrically insulating top surface, such as Upilex®, polyimide, polyphenylene benzobis oxazole (PBO), polyamide, polyether ether ketone, or metallic foils coated with one of these electrically insulating materials. The conducting layer may, for example, comprise molybdenum, transparent conductive oxides, brass, titanium, nickel, or nickel-vanadium. The p-type absorber layer may, for example, comprise a device made of copper-indium-gallium-selenide, copper-indium-selenide, copper-aluminum-selenide, copper-indium-aluminum-selenide, or other compounds like those mentioned that substitute (in whole or part) silver for copper or sulfur for selenide or both. The n-type window layer may, for example, comprise cadmium sulfide, cadmium zinc sulfide, zinc sulfide, zinc selenide, cadmium selenide, zinc-indium selenide, or indium selenide. Next, one may remove a portion of the cell including the n-type window layer, the p-type absorber layer, and the conducting layer, thereby producing a trench or groove defined by the edges of the remaining portions of the cell. A preferred way to remove these layers is by means of laser scribing. Other techniques to remove these layers include, for example, chemical etching, electronic etching (such as, for example, e-beam writing or electron scribing), and mechanical scribing. Chemical etching may require masking to prevent unwanted removal of other portions of the layers. These techniques are sometimes referred to as scribing, and the resulting trench or groove as a scribe.
Depending on the method of removal used, it may be desirable to clean the groove or trench to remove debris or other by-p
Armstrong Joseph H.
Wiedeman Scott
Woods Lawrence M.
Global Solar Energy, Inc.
Le Dung
Preston Gates Ellis & Rouvelas Meeds LLP
LandOfFree
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