Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
1999-08-06
2001-05-15
Chapman, Mark (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S293000
Reexamination Certificate
active
06232545
ABSTRACT:
In prior thermophotovoltaic (TPV) circuit designs, GaSb TPV cells are soldered down onto metal pads on a copper substrate where a thin insulating layer isolates the metal pads from the copper substrate. Flexible interconnects are then bonded from pads on the top of the cell over to pads on the circuit. Finally, mirrors are attached over the lead bond areas. Since the leads have to carry large currents in TPV circuits, the lead area is fairly large. Consequently, the mirrors typically cover 40% of the circuit area.
There are two disadvantages to this configuration. First, although the lead attach step can be automated, it is still quite time consuming. Eliminating the lead bonding step is desirable. Second, the mirror area leads to a loss in system efficiency. This system efficiency loss comes about as follows. If radiation heat transfer were the only heat transfer mechanism, then the mirror area would not have a negative impact because the radiation hitting the mirrors is simply returned to the infrared (IR) emitter in a TPV system. However, heat transfer also occurs by conduction and convection through the air space between the IR emitter and the cell circuit. Recent measurements indicate that this heat transfer process is as much as 30% of the total heat transfer rate. The mirror area is additional area that does not contribute to electric power production but does receive heat through the air heat transfer mechanisms. It is therefore desirable to reduce the mirror area and to increase the fractional active cell area.
An alternative circuit concept for solar photovoltaic application is to shingle the cells, with the top of one cell attached to the underside of the next cell. Unfortunately, this idea has been tried without success. The problem is that during thermal cycling, the substrate material expands at a different rate than the cell so that the rigid bond joint eventually is pulled apart forming an open circuit. A need exists for flexible leads that avoid this failure mechanism.
SUMMARY OF THE INVENTION
The solar photovoltaic shingle concept did not work because the silicon solar cells are quite large and silicon has a very low thermal expansion coefficient (4.2×10
−6
/° C.) compared to substrate materials. Furthermore, the temperature excursions for space solar panels are very large. However, GaSb TPV cells are smaller and have a higher thermal expansion coefficient of 7×10
−6
/° C. Also, there have been recent developments in the field of GaAs microwave device packaging. The thermal expansion coefficient for GaAs is 6.5×10
−6
/° C.
Packaging materials now available for GaAs devices are AlSiC composites and metal laminates. AlSiC composites consist of SiC particles in an aluminum matrix. This material may be cast in various shapes and has a thermal expansion coefficient of 7.5×10
−6
/° C. An example of a metal laminate is Cu/Invar/Cu.
One preferred embodiment of a TPV shingle circuit utilizes a terraced AlSiC substrate. An insulating film is deposited over the terraces and then copper pads are deposited on the terrace top faces. GaSb TPV cells are then bonded to the copper pads and connected in series in a shingle pattern.
There are alternate means of creating shingled circuits. For example, a Cu/Invar/Cu laminate sheet may be used. The terraces may then be formed with a thermally conductive epoxy, and then the cell shingle pattern may be fabricated. In a second alternate, an iron
ickel alloy with 46% nickel (alloy 46) may be used as the shingle circuit substrate. This material has a CTE of 7.3×10
6
/C which is similar to that of AlSiC. The terraces may be machined, cast or forged into the substrate surface. An insulator layer is then deposited, followed by the deposition of the metal pads and then the placement of the shingled cells.
The AlSiC material may also be used as a substrate for a GaAs/GaSb mechanically stacked concentrator solar cell circuit. In this case, four contacts are required to the front and back of the top cell and to the front and back of the bottom cell. In prior art designs, the back of the bottom cell is soldered down to a circuit and three flexible leads run from pads on the circuit to the front of the bottom cell and to the back and front of the top cell. The two cell front leads can be done normally as for a single chip. However, the lead to the back of the top cell requires special handling. Using an AlSiC circuit with a ridge, the backs of both cells may be soldered in one operation and the fronts of both cells may be bonded in a second operation, just as if the stack were a single chip.
The AlSiC microstructure is composed of a continuous Al-metal phase with discrete SiC particulate phase. The AlSiC composite microstructure is fully dense with no void space creating a hermetic material. The leak rates are better than 10
−9
atm cc/s measured on 0.010-inch material cross sections. This hermeticity level allows fabricated AlSiC packages to provide environmental protection of functional components.
The unique AlSiC material properties result from a combination of the constituent material properties. AlSiC properties are tailored by varying the ratio of these constituents. The integrated circuit (IC) coefficient of thermal expansion (CTE) compatible AlSiC compositions have an Sic particulate content between 50-68 vol %.
AlSiC composite materials have thermal conductivity values that are similar to aluminum metal, and CTE values that are similar to alumina. These attributes make AlSiC packages ideal for direct active device attachment to maximize thermal dissipation and improve product reliability.
AlSiC strength and stiffness compare favorably to traditional packaging materials. The ultimate bending strength of AlSiC is two to three times greater than Al-metal.
The active cell area/total area ratio, or packing factor, is quite high for this configuration since no space is taken up by interconnects and traces.
Bi—Sn solder was selected since it has a thermal coefficient of expansion (TCE) of 15 rather than 27 for Sn62, and so is closer to the GaSb TCE of about 7.
The current is usually directed through the back metal and directly into a thick copper circuit trace. There is only about 1 micron thickness of backside metal on the cells for this reason. The fill factor for this configuration may be improved with thicker backside metal.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.
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Avery James E.
Fraas Lewis M.
Keyes Jason B.
Samaras John E.
Chapman Mark
Creighton Wray James
JX Crystals Inc.
Narasimhan Meera P.
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