Abstract:
Highly efficient photon recycling photosensitive optoelectronic device (POD) structures are disclosed which may include optical concentrating non-imaging collectors. Such device structures may be utilized with both organic and inorganic photoconverting heterostructures to enhance photoconversion efficiency. These photo recycling POD structures are particularly well suited for use with organic photoactive materials.

Description:
FIELD OF INVENTION 
     The present invention generally relates to thin-film photosensitive optoelectronic devices. More specifically, it is directed to photosensitive optoelectronic devices, e.g., solar cells, with structural designs to enhance photoconversion efficiency by optimizing optical geometry for use with optical concentrators. 
     BACKGROUND OF THE INVENTION 
     Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. 
     The falloff in intensity of an incident flux of electromagnetic radiation through a homogenous absorbing medium is generally given by: 
       I=I   0   e   −ax   (1) 
     where I 0  is the intensity at an initial position, α is the absorption constant and x is the penetration depth. Thus, the intensity decreases exponentially as the flux progresses through the medium. Accordingly, more light is absorbed with a greater thickness of absorbent media or if the absorption constant can be increased. Generally, the absorption constant for a given photoconductive medium is not adjustable. For certain photoconductive materials, e.g., 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), or copper phthalocyanine (CuPc), very thick layers are undesirable due to high bulk resistivities. However, by suitably re-reflecting, or recycling, light several times through a given thin film of photoconductive material the optical path through a given photoconductive material can be substantially increased without incurring substantial additional bulk resistance. However, a solution is needed which efficiently permits electromagnetic flux to be collected and delivered to the cavity containing the photoconductive material while also confining the delivered flux to the cavity so that it can absorbed. 
     Less expensive and more efficient devices for photogeneration of power have been sought to make solar power competitive with presently cheaper fossil fuels. Therefore organic photoconductors, such as CuPc and PTCBI, have been sought as materials for organic photosensitive optoelectronic devices (OPODs) due to potential cost savings. The high bulk resistivities noted above make it desirable to utilize relatively thin films of these materials. However, the use of very thin organic photosensitive layers presents other obstacles to production of an efficient device. As explained above, very thin photosensitive layers absorb a small fraction of incident radiation thus keeping down external quantum efficiency. Another problem is that very thin films are more subject to defects such as shorts from incursion of the electrode material. Co-pending U.S. patent application Ser. No. 09/449,801 entitled “ Organic Photosensitive Optoelectronic Device With an Exciton Blocking Layer”  (hereinafter “&#39;801 Application”) incorporated herein by reference describes photosensitive heterostructures incorporating one or more exciton blocking layers which address some of the problems with very thin film OPODs. However, other solutions are needed to address the problem of low photoabsorption by very thin films, whether the films are organic or inorganic photoconductors. 
     It has been known to use optical concentrators, as known as Winston collectors, in the fields of solar energy and radiation detection. Such concentrators have been used primarily in thermal solar collection devices wherein a high thermal gradient is desired. To a lesser extent, they have been used with photovoltaic solar conversion devices. However, it is thought that such applications have been directed to devices wherein photoabsorption was expected to occur upon initial incidence of light upon the active photoconductive medium. If very thin photoconductor layers are used, it is likely that much of the concentrated radiation will not be absorbed. It may be reflected back into the device environment, absorbed by the substrate or merely pass through if the substrate is transparent Thus, the use of concentrators alone does not address the problem of low photoabsorption by thin photoconductive layers. 
     Optical concentrators for radiation detection have also been used for the detection of {haeck over (C)}erenkov or other radiation with photomultiplier (“PM”) tubes. PM tubes operate on an entirely different principle, i.e., the photoelectric effect, from solid state detectors such as the OPODs of the present invention. In a PM tube, low photoabsorption in the photoabsorbing medium, i.e., a metallic electrode, is not a concern, but PM tubes require high operating voltages unlike the OPODs disclosed herein. 
     The cross-sectional profile of an exemplary non-imaging concentrator is depicted in FIG.  1 . This cross-section applies to both a conical concentrator, such as a truncated paraboloid, and a trough-shaped concentrator. With respect to the conical shape, the device collects radiation entering the circular entrance opening of diameter d 1  within ±θ max  (the half angle of acceptance) and directs the radiation to the smaller exit opening of diameter d 2  with negligible losses and can approach the so-called thermodynamic limit. This limit is the maximum permissible concentration for a given angular field of view. A trough-shaped concentrator having the cross-section of FIG. 1 aligned with its y axis in the east-west direction has an acceptance field of view well suited to solar motion and achieves moderate concentration with no diurnal tracking. Vertical reflecting walls at the trough ends can effectively recover shading and end losses. Conical concentrators provide higher concentration ratios than trough-shaped concentrators but require diurnal solar tracking due to the smaller acceptance angle. (After  High Collection Nonimaging Optics  by W. T. Welford and R. Winston, (hereinafter “Welford and Winston”) pp 172-175, Academic Press, 1989, incorporated herein by reference). 
     SUMMARY AND OBJECTS OF INVENTION 
     The present invention discloses photosensitive optoelectronic device structures which trap admitted light and recycle it through the contained photosensitive materials to maximize photoabsorption. The device structures are particularly suited for use in combination with optical concentrators. 
     It is an object of this invention to provide a high efficiency photoconversion structure for trapping and converting incident light to electrical energy. 
     It is a further object to provide a high efficiency photoconversion structure incorporating an optical concentrator to increase the collection of light. 
     It is a further object to provide a high efficiency photoconversion structure in which the incident light is admitted generally perpendicular to the planes of the photosensitive material layers. 
     It is a further object to provide a high efficiency photoconversion structure in which the incident light is admitted generally parallel to the planes of the photosensitive material layers. 
     It is a further object to provide a high efficiency photoconversion structure utilizing generally conical parabolic optical concentrators. 
     It is a further to provide a high efficiency photoconversion structure utilizing generally trough-shaped parabolic optical concentrators. 
     It is a further object to provide a high efficiency photoconversion structure having an array of optical concentrators and waveguide structures with interior and exterior surfaces of the concentrators serving to concentrate then recycle captured radiation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will be more readily apparent from the following detailed description of exemplary embodiments taken in conjunction with the attached drawings. 
     FIG. 1 is a cross-sectional profile of a prior art radiation concentrator for use in conjunction with the present invention. 
     FIGS. 2A-2E depict embodiments of device structures in accord with the present invention in which light is accepted in a direction generally perpendicular to the planes of the photosensitive layers. 
     FIG. 2A is side view of a perpendicular type embodiment with a concentrator attached. 
     FIG. 2B is top down view of FIG. 2A along line A—A having a circular aperture for use with a conical concentrator. 
     FIG. 2C is a top down view of FIG. 2A along line A—A having a rectangular aperture for use with a trough-shaped concentrator. 
     FIG. 2D is a perspective representation of a collection of perpendicular type PODs with conical concentrators. 
     FIG. 2E is a perspective representation of a collection of perpendicular type PODs with trough-shaped concentrators. 
     FIG. 3 is a cross-sectional view of a portion of an array of perpendicular type PODs with concentrators wherein the concentrators reflect light on their interior and exterior surfaces. 
     FIGS. 4A-4E depict embodiments of device structures in accord with the present invention in which light is accepted in a direction generally parallel to the planes of the photosensitive layers. 
     FIG. 4A is side view of a parallel type embodiment with a concentrator attached. 
     FIG. 4B is end-on view of FIG. 4A along line B—B having a circular aperture for use with a conical concentrator. 
     FIG. 4C is a end-on view of FIG. 4A along line B—B having a rectangular aperture for use with a trough-shaped concentrator. 
     FIG. 4D is a perspective representation of a parallel type POD with a conical concentrator. 
     FIG. 4E is a perspective representation of a parallel type POD with a trough-shaped concentrator. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 2A, a cross-sectional view which can correspond to two different device structures is depicted. Both structures permit light to be introduced into a reflective cavity, or waveguide, containing photosensitive layers such that the light is initially incident in a direction generally perpendicular to the planes of the photosensitive layers so this type of structure is generally referred to herein as a “perpendicular type structure”. A perpendicular type structure can have two types of preferably parabolic cross-section concentrators as described above—“conical” and “trough-shaped”—FIGS. 2B-2E provide different views on conical versus trough-shaped structures whose common cross-section is shown in FIG.  2 A. The same numerals are used for corresponding structure in each of FIGS. 2A-2E. 
     Accordingly, light incident from the top of these embodiments enters into one or more concentrator structures  2 B 01  (conical) or  2 C 01  (trough-shaped). The light admitted to each concentrator is then reflected into an aperture  2 B 02  or  2 C 02  in top reflective layer  203 . As shown in FIGS. 2B and 2C, aperture  2 B 02  is a generally circular shaped opening for use with a conical concentrator, and aperture  2 C 02  is a generally rectangular shaped opening for use with a trough-shaped concentrator. Only the bottom surface of layer  203  need be reflective so the top surface may be non-reflective and/or be optionally coated with, for example, a protective layer to enhance weather resistance. Passivated oxides or polymer coatings, for example, may be suitable protective coatings. After passing through the aperture, the admitted radiation is trapped in a waveguide structure formed between top layer  203  and bottom reflective layer  204 . The space between the two layers is occupied by several layers comprising one of the inorganic photosensitive optoelectronic devices known in the art made, for example, from silicon, or one of the organic photosensitive optoelectronic devices (“OPODS”) such as those disclosed in co-pending U.S. patent application Ser. Nos. 09/136,342, 09/136,166, 09/136,377, 09/136,165, 09/136,164 to Forrest et al. (the “Forrest Applications”), which are herein incorporated by reference in their entirety or in the &#39;801 Application. 
     More specifically, in an exemplary embodiment of an OPOD with an optical concentrator geometry, and with reference particularly to FIG. 2A, below top layer  203  is a transparent, insulating layer  205  of, for example, glass or plastic, through which the light admitted by aperture  2 B 02  or  2 C 02  initially traverses. On its initial pass, the light then traverses a transparent electrode  206  of, for example, degenerately doped ITO. On its initial pass, the light then traverses one or more active layers  207  which may include one or more rectifying junctions, or exciton blocking layers for efficient conversion of optical energy to electrical energy. An exciton blocking layer can comprise 2,9-dimethyl4,7-diphenyl-1,10-phenanthroline (BCP), as disclosed in the &#39;801 Application. Any light which is not absorbed on this pass is reflected by layer  204  back through active layers  207 , transparent electrode  206 , and transparent insulating layer  205  to be reflected off of top layer  203  to repeat the cycle again until the light is completely absorbed. Layer  204  is typically a metallic film such as silver or aluminum which also can serve as the lower electrode. Alternatively, the lower electrode could be in whole or part a transparent conductive material such as degenerately doped ITO in conjunction with a reflective metallic film which in turn could optionally be deposited upon a substrate such as glass, metal or plastic. FIG. 2A depicts two typical incident light rays. Those of ordinary skill in the art will appreciate that there are numerous other possible trajectories for incident radiation and that the ray depicted is merely for illustration. 
     The process of trapping the admitted light until it is absorbed enhances the efficiency of the photoconversion and may be referred to as “optical recycling” or “photon recycling”. A structure designed to trap light within may generally be called a waveguide structure, or also an optical cavity or reflective cavity. The optical recycling possible within such optical cavities or waveguide structures is particularly advantageous in devices utilizing relatively high resistance organic photosensitive materials since much thinner photoactive layers may be used without sacrificing conversion efficiency. 
     In FIGS. 2D and 2E, a certain number of PODs with concentrators are shown in one integrated structure. Those of ordinary skill in the art would appreciate that the number of PODs in such integrated structures may be increased as desired. In FIG. 2E it should be appreciated that the trough-shaped concentrator are shown having open ends. Optionally, the ends of a trough are closed with a structure having a reflecting surface facing the interior of the trough to help capture additional light into the apertures. Vertical or sloped planar surfaces may be used. Also, each end of the trough may be closed with a shape generally resembling half of a parabolic cone. Such structures permit the trough interior surface to be smoothly curved in its full extent. 
     It should be appreciated in the array of perpendicular-type PODs depicted in FIGS. 2D and 2E with reference to FIG. 2A, that after the admitted light has entered an aperture  2 B 02  or  2 C 02 , the light will not be reflected back across the plane defined by the top surface of the top reflective layer  203 . Therefore, the space between the exterior of the concentrator and top layer  203  may be empty or filled with a non-transparent material. For mechanical stability, it is preferable that at least part of this volume should be filled with material to provide support for the concentrator. Also, it should be appreciated that the FIG. 2A,  2 D,  2 E structures as described above utilize three separate reflective surfaces for, respectively, the interior of the concentrator, the upper reflective surface of the waveguide structure and the lower reflective surface of the waveguide structure. In FIG. 3, an alternative array structure is depicted in cross-section which can utilize a single reflective film to provide both the concentrator reflections and the “upper” waveguide reflections. The concentrator/reflector  301  is a reflective layer, typically metal such as silver or aluminum, deposited on a layer  302  of molded or cast transparent insulating material such as plastic or glass. Layer  302  is made with the shape of the concentrator array formed into it. The transparent upper electrode, one or more photosensitive layers, labeled collectively as  303 , and lower reflective layer (optionally also the lower electrode)  304  complete the waveguide structure. This arrangement allows the manufacture of a POD concentrator array to begin with a preformed bare concentrator array structure. Thereafter, a double-duty reflective coating can be deposited on the concentrator side of the array structure and the photoactive and conductive layers for extraction of photogenerated current can all be deposited on the lower surface using masking and photolithographic techniques. Since physical support is provided by layer  302 , reflective layer  301  can be made much thinner than would be possible if layer  301  were needed to be a partly or completely self-supporting concentrator/reflector. 
     It should be appreciated that as just described, layer  301  has reflecting surfaces on both its interior and exterior parabolic surfaces. Optionally, layer  301  could be two separate coatings on the interior and exterior of a generally conical or trough-shaped base material such as molded or cast plastic or glass. This implementation more easily permits the concentrator interior and exterior surface shapes to be slightly different thus permitting independent optimization of the concentrator reflections and the waveguide structure reflections. 
     In FIGS. 4A-4E, different versions of structures which permit light to be introduced into a reflective cavity, or waveguide, containing photosensitive layers such that the light is initially incident in a direction generally parallel to the planes of the photosensitive layers so that this type of structure is generally referred to herein as a “parallel type structure”. As with perpendicular type structures, parallel type structures can have both generally “conical” and “trough-shaped” type concentrators. FIGS. 4B-4E provide different views on conical versus trough-shaped structures whose common cross-section is shown in FIG.  4 A. The same numerals are used for corresponding structure in each of FIGS. 4A-4E. 
     Accordingly, light incident from the top of these embodiments enters into one or more concentrator structures  4 B 01  (conical) or  4 C 01  (trough-shaped). The light admitted to each concentrator is reflected into an aperture  4 B 02  or  4 C 02  at the base of each concentrator. As shown in FIGS. 4B and 4C, aperture  4 B 02  is a generally circular shaped opening for use with a generally conical concentrator, and aperture  4 C 02  is a generally rectangular shaped opening for use with a trough-shaped concentrator. The remaining structure is now described with respect to a typical incident light ray but those of ordinary skill in the art will appreciate that there are numerous other possible trajectories for incident radiation and that the ray depicted is merely for illustration. The typical ray enters a transparent, insulating layer  403  of, for example, glass or plastic. The typical ray then reflects off reflective layer  404 , which is typically a metallic film of, for example, silver or aluminum. The reflected ray then retraverses part of transparent layer  403  and then traverses transparent conductive layer  405  which serves as one electrode of the device and is typically a conductive oxide such as degenerately doped ITO. The typical ray then traverses the photoactive layers  406  which are photosensitive rectifying structures such as those described in the Forrest Applications or the &#39;801 Application or inorganic photosensitive optoelectronic structures made from, for example, silicon. Any optical intensity in the typical ray that has not been absorbed reflects off upper reflective layer  407  which may be a metallic reflective film of, for example, silver or aluminum and typically serves as an electrode layer. Optionally, the electrode function may be served in part or whole by a second transparent electrode with the reflective function provided by a separate layer. 
     In attaching the concentrator structure to the POD, care should be taken to avoid shorting of the electronically active layers. This can be accomplished by providing a thin insulating protective coating around the edges of the photoconductive layers. It should be further appreciated that a reflective coating may be optionally located around the edges of the device to reflect light back into the device. More specifically, in FIG. 4A, a reflective layer (not shown) electrically insulated from the electronically active layers would optionally be placed at the right end of FIG. 4A so that (as illustrated) the typical ray could reflect back toward the concentrator. It should be appreciated that the proportions of the device depicted in the figures are merely illustrative. The device may be made generally thinner vertically and longer horizontally with the result that most light is absorbed before it ever reaches the edges of the device opposed to the concentrator. Only light which is truly normal to the plane of the aperture and thus truly parallel to the planes of the photoactive layers would have a substantial probability of reaching the edge opposite the concentrator. This should represent a small fraction of the incident light. 
     In FIG. 4B, aperture  4 B 02  is illustrated as covering a section of only transparent insulating layer  403 . Provided the instructions above relating to preventing electrical shorts between electrically active layers is heeded, the concentrator  4 B 01  and aperture  4 B 02  may be disposed to allow direct illumination of transparent electrode  405  or photoactive layers  406 . In FIG. 4C, it should be appreciated that transparent insulating layer  403  is not depicted since generally rectangular aperture  4 C 02  is shown as completely overlapping layer  403  in the view shown. As with aperture  4 B 02 , aperture  4 C 02  may be varied in size along with concentrator  4 C 01  to provide direct illumination of more or less of the interior of the POD. 
     FIGS. 4D and 4E are perspective illustrations of exemplary embodiments of the present invention in parallel type structures. In FIG. 4D, a generally conical concentrator is illustrated delivering incident light to a parallel type structure having an upper reflective layer  408 , one or more photosensitive layers and rectifying structures including one or more transparent electrodes labeled collectively as  409 , and a transparent insulating layer and bottom reflective layer labeled collectively as  410 . FIG. 4E depicts a generally trough-shaped concentrator  4 C 01  delivering light to a similar POD structure in which the layers are labeled accordingly. It should be appreciated that the trough-shaped concentrator is shown having open ends which may optionally be closed off with a reflecting surface as described above with regard to FIG.  2 E. 
     The concentrator may be formed of only metal or of molded or cast glass or plastic which is then coated with a thin metallic film. With the parallel type structure, the waveguide photoabsorbing structures are more readily manufactured separately from the concentrator structures with the pieces being attached subsequently with suitable adhesive bonding materials. An advantage of the perpendicular type structure, as described above, is that its manufacture can begin with preformed concentrator structures which are used as the substrate for further build up of the device. 
     It should be appreciated that the terms “conical” and “trough-shaped” are generally descriptive but are meant to embrace a number of possible structures. “Conical” is not meant to be limiting to an shape having a vertical axis of symmetry and whose vertical cross-section would have only straight lines. Rather, as described above with reference to Welford and Winston, “conical” is meant to embrace, among other things, a structure having a vertical axis of symmetry and a generally parabolic vertical cross section as depicted in FIG.  1 . Further, the present invention is not limited to concentrators, either “conical” or “trough-shaped”, having only smoothly curved surfaces. Rather, the general conical or trough shape may be approximated by some number of planar facets which serve to direct incident light to the exit aperture. 
     Further, the generally circular and generally rectangular apertures are preferred for use with optical concentrators but are exemplary. Other shapes for apertures are possible particularly in the perpendicular type structure. Concentrators having generally parabolic sloped sides may be fitted to a number of aperture shapes. However, the 3D parabolic and trough-shaped concentrator with their respective circular and rectangular apertures are preferred. 
     It should also be appreciated that the transparent insulating layer, e.g.,  205  or  403 , is present to prevent optical microcavity interference effects. Therefore, the layer should be longer than the optical coherence length of the incident light in all dimensions. Also, the transparent insulating layer can be placed on either side of the photoactive layers. For example, in FIG. 2A, the aperture could be in layer  204  and layer  203  could be just a reflecting layer. Accordingly, any concentrator  2 B 02  or  2 C 02  would be placed over the aperture wherever it may lie. This would have the effect of permitting the admitted light to reach the photoactive layers initially before reaching the transparent insulating layer. For many possible photoactive materials, however, the embodiments specifically disclosed herein, e.g., FIG. 2A, are preferred since they allow the transparent insulating layer to protect the underlying photoactive layers from the environment. Exposure to atmospheric moisture and oxygen may be detrimental to certain materials. Nonetheless, those of ordinary skill in the art would understand this alternate version of the device with the benefit of this disclosure. 
     While the particular examples disclosed herein refer preferably to organic photosensitive heterostructures the waveguide and waveguide with concentrator device geometries described herein suitable as well for other photosensitive heterostructures such as those using inorganic materials and both crystalline and noncrystalline photosensitive materials. The term “photosensitive heterostructure” refers herein to any device structure of one or more photosensitive materials which serves to convert optical energy into electrical energy whether such conversion is done with a net production or net consumption of electrical energy. Preferably organic heterostructures such as those described in the Forrest Applications are used. 
     Also, where a reflective electrode layer is called for herein, such electrode could also be a composite electrode comprised of a metallic layer with an transparent conductive oxide layer, for example, an ITO layer with a Mg:Ag layer. These are described further in the Forrest Applications. 
     It should be appreciated that the terms “opening” and “aperture” are generally synonymous and may be used herein somewhat interchangeably to refer to the entrance or exit of a concentrator as well as a transparent hole or window which allows radiation to reach the interior of a POD. Where it is necessary to draw a distinction between the two types of openings or apertures, for example, in the claims, antecedent context will provide the suitable distinction. 
     Thus, there has been described and illustrated herein waveguide structures for PODs and use particularly in conjunction with optical concentrators. Those skilled in the art, however, will recognize that many modifications and variations besides those specifically mentioned may be made in the apparatus and techniques described herein without departing substantially from the concept of the present invention. Accordingly, it should be clearly understood that the form of the present invention as described herein is exemplary only and is not intended as a limitation on the scope of the present invention.