Abstract:
An MTPV thermophotovoltaic chip comprising a photovoltaic cell substrate, micron/sub-micron gap-spaced from a juxtaposed heat or infrared radiation-emitting substrate, with a radiation-transparent intermediate window substrate preferably compliantly adhered to the photovoltaic cell substrate and bounding the gap space therewith.

Description:
This application is a continuation of U.S. patent application Ser. No. 12/152,195 filed on May 12, 2008. U.S. patent application Ser. No. 12/152,195 is incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to sub-micrometer gap thermophotovoltaic devices (MTPV) for generating electrical power, wherein a heat or infrared source or emitter is spaced from a photovoltaic cell surface by a gap preferably of less than one micrometer (hereinafter sometimes referred to as micron/sub-micron gaps), obviating the far-field limitation of Planck&#39;s Law and allowing the system to function as though its black body emissivity were greater than unity and thereby achieving photocurrents many times those obtained in conventional far-field thermo-photovoltaic cells (TPV), as described in U.S. Pat. Nos. 6,084,173 and 6,232,546, and the paper entitled “Micron-gap ThermoPhoto Voltaic (MTPV)”, DiMatteo et al., Proceedings of the Fifth TPV Conference, 2002, all incorporated herein by reference; the invention being more particularly concerned with the novel interposition of an infrared-transmitting window in the gap adjacent the photovoltaic cell, and improvements resulting therefrom in cell structure and in the fabrication or manufacture of such structures. 
     BACKGROUND OF INVENTION 
     To avoid thermal shorting, the MTPV system is preferably operated in a vacuum enclosure or housing H which enables an evacuated gap G; and gap spacers—made, for example, of silicon dioxide—are employed to set the gap between the emitter and the photovoltaic cell receiver in a manner which minimizes heat transfer through the spacers. Phonons or non-radiated energy carriers are a source of inefficiency though they transfer energy from the source; but they do not have the individual energy to excite electrons across the bandgap. 
     As described in the above referenced paper, a previous method of forming the spacers between the heat emitter and the photovoltaic cell substrate was to grow a thick oxide on the emitter chip and pattern the oxide through such methods as photolithography and plasma etching into cylindrical spacers, with the spacers to be about six microns in diameter; but a disadvantage of this technique is that the spacers permit too large a heat loss from the emitter, reducing the efficiency of conversion of heat to electricity and increasing the cooling requirements on the photovoltaic cell. 
     Another disadvantage arises in the use of micrometer gap thermophotovoltaic devices of large area, requiring, for example, brazing individual chips to create a “tiled” surface as, for example, in U.S. Pat. No. 6,232,546. A single large emitter chip and photovoltaic cell cannot be used because the emitter is operated at about 1000° C. and the photovoltaic cell must be kept at room temperature to function effectively as a collector of photons and a generator of electrons. The difference in thermal expansion between the heater and the photovoltaic cell as the heater chip is heated from room temperature to of the order of 1000° C., can break the spacers or distort the geometry during the temperature excursion if there is such a rigid attachment. 
     An approach to solve this problem is to use an array of laterally spaced hollow tubes of thermally resistant material disposed in wells formed in the heat emitter substrate, each carrying a flange on top and serving as a spacer extending into the gap—as indicated at S in the drawings—a structure that lends itself to fabrication by established microfabrication methods such as lithography and plasma etching, particularly with a silicon emitter substrate and silicon dioxide spacers. 
     A more facile and less complicated and less costly construction is now, however, provided by the present invention, as later described in detail. 
     OBJECTS OF THE INVENTION 
     It is accordingly an object of the present invention to provide a new and improved micron/submicron gap thermophotovoltaic device and method of manufacturer and assembly, with a lower fabrication cost, and for precise and uniform setting of the gap dimension, and that shall not be subject to the above-described and other limitations of the prior art. 
     Another object of the present invention is to provide a new and improved thermophotovoltaic device of the character described using a sub-micron gap, preferably evacuated, with an integrated intermediate radiation transparent window preferably compliantly adhered to the photovoltaic device and bounding the gap formed therewith. 
     Other and further objects are described hereinafter and are pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     In summary, the invention embraces a sub-micrometer gap thermo-photovoltaic chip structure comprising a photovoltaic cell substrate, micron/sub-micron gap-spaced from a juxtaposed heat or infrared radiation-emitting substrate, with a radiation-transparent intermediate window substrate preferably compliantly adhered to the photovoltaic cell substrate and bounding the gap space therewith. 
     Preferred and best mode embodiments are hereinafter presented in detail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in connection with the accompanying drawings, in which: 
         FIG. 1  is an enlarged schematic view of a prior art sub-micron gap thermo-photovoltaic device; 
         FIG. 2  is a similar view of the invention in preferred form; 
         FIG. 3  is a view of the invention incorporating additional functionality imparted by up-conversion layers; and 
         FIGS. 4( a ) and 4( b )  present a view of the invention additionally incorporating wiring and bypass diodes. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, a sub-micrometer gap thermophotovoltaic device is shown on an enlarged and exploded fragmentary schematic view, including a photovoltaic cell layer or substrate  2 , a juxtaposed heat or infrared source emitter chip  1 , at least one and preferably an array of spacers S located between the emitter chip  1  and PV cell  2 , and a gap of sub-micrometer separation labeled G maintained by the spacers. The spacers may be disposed upon either the emitter chip  1  (“hot side” spacers) or PV cell  2  (“cold side” spacers). 
     While conventional TPV systems involve conversion of infrared light emitted by a blackbody into electricity via the use of photovoltaic (PV) cells, MTPV systems utilize a preferably sub-micron evacuated gap between the (“hot side”) emitter and the PV cell (“cold side”) to achieve enhanced radiative transfer as compared to conventional far-field TPV systems, as earlier mentioned. 
     There are, however, several challenges in the manufacturing of a MTPV systems capable of generating commercially significant (&gt;1 kW) power levels. First, one must achieve a very small and preferably uniform gap G between the emitter  1  and the PV cell  2 . Second, this operation must be repeated many times to achieve an integrated MTPV system capable of achieving high power levels; the exact number of times depending upon the power level specified, the size of each PV cell  2  and emitter chip I, and the power density and efficiency of the system. Finally, in some embodiments, the formation of the submicron gap G between the emitter  1  and the PV cell  2  may occur after these components are fully processed, therefore requiring accommodating during assembly of the resulting bow and surface irregularities. 
     The present invention, illustrated in  FIG. 2 , admirably solves these problems by utilizing an intermediate “window” material  3  between the PV cell  2  and the emitter  1 , where the sub-micron gap G is formed between the emitter  1  and the window  3 . The PV cell  2  or array or plurality of cells is shown integrated onto the back or outside surface of the window  3 , as by a preferably somewhat compliant adhesion layer. 
     The window material  3  should be transparent to the radiation emitted by the emitter chip  1 . In general, this requires a material with a band gap larger than the infrared (greater than approximately 1.0 electron-Volt) and a low density of free carriers or defects. The window material layer  3  should also have a high refractive index, preferably equal to or larger than the refractive index of the emitter and PV cell. Window materials for the invention include single crystalline semi-insulating GaAs, single crystalline semi-insulating InP, float-zone Si, or lightly doped Si. 
     The adhesive layer  4 , moreover, must be able to bond the PV cell  2  to the window material  3  without voids, cracking or delaminating. In theory, adhesive layers with a coefficient of thermal expansion matched to the PV cell and window are ideal; but, in reality, this is difficult to achieve given the inherent trade-off between melting point and coefficient of thermal expansion—adhesives must melt at low temperature, so they will generally exhibit high coefficients of thermal expansion, since melting point and thermal expansion are both fundamentally a function of atomic bond strength. Alternately, the bond can be engineered to minimize the total thermal mismatch energy between the adhesive. PV cell, and window, by well-known methods such as low temperature or anodic bonding. 
     The adhesive layer  4  should also preferably exhibit a high refractive index (&gt;1.4 and preferably &gt;2) and high transmission in the infrared. Suitable adhesive layers  4 , for the purpose of present invention include epoxies, filled elastomers, solder glasses such as those containing lead oxide, and chalcogenide glasses. Chalcogenide glasses are amorphous solid materials, composed of such elements as germanium, selenium, tellurium, arsenic, indium, sulfur, and antimony. They are preferable because of their high refractive indices, high infrared transmission, and low softening points. The adhesive material of layer  4  preferably also exhibits some compliancy, as before stated. 
     In some embodiments of the invention, additionally, the adhesive material may serve a dual function, both as the previously described adhesive to adhere or hold the PV cell to the window material, and also as a material to provide for up-conversion of incoming photons. Up-conversion involves the use of low energy photons to promote electrons in a material up one or more energy levels. When the electron relaxes to a lower energy state, it emits a photon of a higher energy. These up-conversion layers may be used to tailor the incoming infrared light spectrum to the PV cell. Typically, they are formed via introduction of rare earth compounds or ions, based on such elements as yttrium or erbium, into the adhesive material layer  4 . Alternately, up-conversion can be achieved through incorporation of semiconductor quantum dots into the adhesive material layer  4 . In this embodiment of  FIG. 2 , the up-conversion functionality is incorporated into the adhesive material layer  4 , as schematically represented at D in  FIG. 3 . 
     Among the advantages of this modified MPTV structure of the invention are that the window material of layer  3  can be a single-crystal wafer that has very good surface roughness and overall flatness, enabling formation of a uniform gap G over a large area. Secondly, though many different PV cells may then be integrated onto the window, they will all be subject to the same uniform gap G formed between the emitter layer  1  and the window layer  3 . Third, the adhesive layer  4  can provide an insulating base that enables integration of wiring and bypass diodes into the MPTV package. Such an arrangement is schematically shown in  FIGS. 4( a ) and 4( b ) . In these figures, the line A-A′ is used to demonstrate the correspondence between cross-sectional view of  FIG. 4( a )  and the bottom view of  FIG. 4( b ) . The wires, schematically indicated as W, and bypass diodes, indicated as BD, utilize the insulating adhesive layer  4  as a substrate. Finally, PV cells layer  2  with non-uniform surfaces can be accommodated by the adhesive material, as its compliancy assures that the entire PV cell can be adhered or “stuck” to the window even if its surface is irregular. 
     The use of such a window construction. moreover, also readily allows the formation of gap spacers on the either the “hot” side or the “cold” side. In the prior art as shown in  FIG. 1 , the use of “cold” side spacers requires that the spacers are formed on the PV cell. In this embodiment, if the PV cell is a smaller area than the emitter, then the achievement of a uniform gap requires that identically sized spacers are formed on individual PV cells and the cells are placed proximate to the emitter with a substantially uniform gap between the emitter and each PV cell. In practice, this is difficult to achieve given process non-uniformities and variability in mechanical loading of each PV cell within the housing. In the current invention, as shown in  FIG. 4 , if “cold” side spacers are utilized, they are formed on a single window layer instead of on multiple individual PV cells. This completely opens up the type of material that can be used for the emitter layer  1 . For example, a selective emitting substance such as tungsten silicide can be deposited on a refractory substrate such as zirconia. 
     The technique of the invention is applicable to both front and rear illuminated PV devices, although the requirements on the window material may be very different for the two cases. 
     Further modifications will also occur to those skilled in this art, and such are considered to fall within the spirit and scope of the invention as defined in the appended claims.