Patent Publication Number: US-2010108133-A1

Title: Thin Film Semiconductor Photovoltaic Device

Description:
BACKGROUND 
     The present invention relates to methods and apparatus for providing a photovoltaic device, such as a device in which a thin film, semiconductor photo-sensitive layer is coupled to a transparent substrate. 
     Photovoltaic solar cells are attractive mechanisms for generating electrical energy as they do not produce greenhouse gasses as a byproduct. There are two basic configurations for conventional thin-film photovoltaic solar-cell technology: the superstrate configuration and the substrate configuration. In the superstrate configuration, incident light passes through a transparent material that supports an active semiconductor material disposed thereon. In the substrate configuration, light is incident on the active semiconductor material and then reaches the substrate.  FIG. 1  illustrates a conventional superstrate photovoltaic device  10 , which includes a substrate  12  on which a semiconductor material  14  is disposed. The semiconductor material (which may be crystalline silicon) includes a p-n junction  16 , which has the characteristic of creating unbound charges (electrons and holes) and generating a voltage V across a pair of conductors when light passes through the junction. In this configuration, the substrate is transparent and permits the light to pass to the semiconductor material  14 . 
     The primary issues with conventional solar cell approaches are cost, efficiency, and form factor associated with fabrication of the solar cell. Various single crystal or thin film processes have been developed in an attempt to address these issues. Single crystal solar cells can have high efficiency, but the process is quite expensive. In such cases, especially with expensive III-V solar cells and multi-junction solar cells, solar concentrators are employed. Thin film semiconductor fabrication techniques can be less expensive, but the energy conversion efficiency is normally quite low. As the semiconductor layers are made thin (in order to reduce cost), i.e., less than about 1 um for silicon, the absorption of infrared energy by the cell becomes very low and the efficiency falls quite dramatically. 
     Referring again to  FIG. 1 , in some prior art configurations, a light scattering layer (e.g., formed from a roughened transparent conductive oxide) may be disposed between the substrate  12  and the semiconductor  14 . Another discontinuity (such as a metallization layer) may be disposed on the opposing surface of the semiconductor material  14 . The scattering layer and metallization layer may cause some of the light to become trapped within the semiconductor material  14  because the light would tend to bounce off of the respective layers at respective distributions of angles (light scattering and trapping). While this approach improves solar energy conversion at the p-n junction  16 , complete light trapping is not possible due to scattering light out of the structure. 
     In silicon-based solar cells, which include amorphous, micro- or nano-crystalline, polycrystalline and/or crystalline materials, layer thicknesses are typically less than 5 um and light trapping is critical. For amorphous and microcrystalline silicon solar cells, transparent conductive oxide (TCO) layers are typically used due to the poor conductivity of the doped layers. In the superstrate geometry, the TCO layers are textured (as discussed above) to create a light scattering interface between the TCO and the silicon semiconductor layer. There is a trade-off between the light scattering performance of the surface textures and the electrical transport characteristics of the silicon in the case of microcrystalline silicon. This impacts the light trapping performance of single-junction microcrystalline cells and amorphous/microcrystalline (micromorph) tandem junction cells. The same limitations apply to the substrate geometry. In the case of polycrystalline or crystalline thin film Si solar cells, scattering layers are also employed. Polycrystalline cells may have scattering introduced at the substrate/Si interface in the superstrate geometry while crystalline Si solar cells typically have a planar substrate/Si interface. Again, textured silicon is used at the back reflector to provide scattering. In the substrate configuration, polycrystalline and crystalline silicon solar cells employ scattering at the air/silicon interface and/or the silicon/substrate interface. To eliminate process steps and improve performance, there is a need to provide light trapping without the use of textured surfaces. 
     For the above reasons, the cost of solar energy is about 2-3 times more expensive than conventional grid power. In some solar energy sectors, such as roof top applications in homes, apartment complexes, industrial parks or applications where grid power is not easily available, low weight and form factor may be a significant advantage. Accordingly, there is a need in the art for a new approach to providing photovoltaic solar cells, which enjoy characteristics of low cost, high efficiency, low weight and low form factor. 
     SUMMARY 
     In accordance with one or more embodiments, a photovoltaic device includes: a substantially transparent substrate having first and second major surfaces and a plurality of side surfaces; a thin-film semiconductor layer coupled to the first major surface of the substrate and including first and second major surfaces and at least one photo-sensitive p-n junction therein; and a light directing feature operable to cause incident light to propagate through the substrate and into the semiconductor layer in a waveguide mode such that the light reflects a plurality of times between the first and second major surfaces thereof and impinges upon the p-n junction a plurality of times. 
     The thickness of the semiconductor layer may be less than about 2 um, such as about 1-2 um. The substantially transparent substrate may be formed from at least one of glass, glass ceramic, and polymer. 
     Other aspects, features, advantages, etc. will become apparent to one skilled in the art when the description of the invention herein is taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purposes of illustrating the various aspects of the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1  is a side view of a photovoltaic device in accordance with the prior art; 
         FIG. 2  is a perspective view of a photovoltaic device in accordance with one or more aspects of the present invention; 
         FIG. 3  is a side view of a photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 4  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 5  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 6  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 7  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 8  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 9  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 10  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 11A  is a side view of a further photovoltaic device in accordance with one or more further aspects of the present invention; 
         FIG. 11B  is a side view of an alternative light directing element for use with the photovoltaic device of  FIG. 11A ; and 
         FIG. 12A ,  12 B are graphs illustrating simulation results of certain parameters of merit associated with a basic operational concept of the photovoltaic devices of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, wherein like numerals indicate like elements, there is shown in  FIG. 2  a perspective view of a photovoltaic device  100  in accordance with one or more embodiments of the present invention. The photovoltaic device  100  includes a substantially transparent substrate  102  having first and second major surfaces  108 ,  110  and a plurality of side surfaces, generally forming a right parallelepiped. A semiconductor layer  104  is coupled to the first major surface  108  of the substrate  102  and includes at least one photo-sensitive p-n junction  106 . 
     The structure  100  is considered herein to exhibit composite waveguide characteristics because, as will be discussed in detail below, light propagates within the structure in a waveguide mode. Indeed, light may propagate within the semiconductor layer  104  between first and second major surfaces thereof in one or more waveguide modes (as opposed to light scattering). Additionally or alternatively, light may propagate within the composite structure of the substrate  102  and the semiconductor layer  104  in one or more waveguide modes. The light propagation in one or more waveguide modes is not the same as light scattering or trapping (of the prior art). In light scattering or trapping the light reflects off of a discontinuity at respective distributions of angles (which results in the escape of significant light energy from the cell). In contrast, light propagation in one or more waveguide modes exhibits the characteristic of substantially total internal reflection and the escape of very little or no light energy. 
     It is understood that the structural and electrical details of the photo-sensitive p-n junction  106  are relatively complex, but are very well known and understood in the art. Thus, for the sake of brevity and clarity, such details (including formation techniques, location of electrical conductors, etc.) will be omitted from this description. It is noted, however, that in solar-cell technologies, p-n junctions are formed in semiconductor materials to convert solar radiation into electrical current. These p-n junctions separate the electron-hole pairs created by the absorption of radiation to generate useful electrical current for an external load. Depending on the semiconductor material and process used, various type of solar-cell designs have been developed in the art. Some are simple p-n junctions, while others are more complex and are optimized for higher efficiency. Such more complicated junctions include p-i-n junctions. In some cases, p+ and n+ layers are added to the p-n and/or p-i-n junctions for improved charge collection and electrode/solar cell fabrication. In this application, when a p-n junction is referred to, it may include any of the various junctions indicated above, others known from existing literature, and/or those developed hereafter. 
     As illustrated by the dashed arrows in  FIG. 2 , incident solar energy (light) may enter the substrate  102 , specifically through a side surface or one of the major surfaces  108 ,  110 , as depicted by light ray A. Depending on the angle of the light ray A, a light ray A′ will reflect off of the discontinuity between the substrate  102  and the semiconductor  104  at an angle, and a light ray B will enter the semiconductor layer  104  at a particular angle. The light ray B will reflect off of a far major surface of the semiconductor layer  104  as ray B′. Without any scattering structures, the ray B′ will have the characteristic of a total internal reflection of ray B. (The light ray A′ will reflect off of the major surface  110  back toward the semiconductor layer  104  and the propagation pattern will continue.) Depending on the angle of the light ray B′, the light will either escape the semiconductor layer  104  as light ray C, or propagate in a direction parallel to the major surfaces of the semiconductor layer  104  in a waveguide mode, as illustrated by rays B, B′, B″, etc. As will be discussed later herein, the photovoltaic device  100  may include a light directing feature that is operable to cause the incident light ray (or rays) A to propagate through the substrate  102  and into the semiconductor layer  104  at or above a critical angle such that the waveguide action results in light rays B, B′, B″, etc. The waveguide action arises from the angle of the light rays (as discussed above) and respective dielectric constant discontinuities proximate to the first and second major surfaces of the semiconductor layer  104 . Thus, as the incident light reflects a plurality of times between the first and second major surfaces of the semiconductor layer  104 , the solar energy thereof impinges upon the p-n junction  106  a plurality of times. 
     For any light rays that were not initially coupled into the semiconductor layer  104 , such as ray A′, or any light rays leaving the semiconductor layer  104  and entering the substrate  102 , such as ray C, such rays may reflect off of the major surface  110  of the substrate and back to the semiconductor layer  104 , such as ray D. It is desirable to design the structure such that rays that were not initially coupled into the semiconductor layer  104 , such as ray A′, or other light rays, such as ray C, reflect off of the interface of surface  110  with total internal reflection. Thus, depending on the angles of reflection, such light may re-enter the semiconductor layer  104  and propagate therein in the waveguide mode discussed above. 
     As noted above, light may alternatively or additionally propagate within the composite structure of the substrate  102  and the semiconductor layer  104  in one or more further waveguide modes. This characteristic is achieved when the light rays, such as ray D, propagates through the substrate  102  and into the semiconductor layer  104  at or above a critical angle such that rays E, E′, D′, D″ result in further rays E″, E′″, etc. 
     The above reflection action of the light impinging upon the p-n junction  106  numerous times has the advantageous effect of significantly increasing the efficiency of the photovoltaic device  100  over conventional techniques. Indeed, the photovoltaic device  100  may be considered to be in vertical waveguide configuration, which increases the absorption path length for high efficiency and small form factors. This is so because the light penetrates into the semiconductor layer  104  and is partially absorbed on each reflection and consequently generates more electron-hole pairs. With multiple reflections, the radiation can be very efficiently absorbed. In essence, this approach decouples a previously accepted limiting relationship between the semiconductor layer thickness and the solar light absorption. Thus, very efficient light absorption, of even the infrared spectrum, is obtained in thin-film construction. Consequently lower cost thin-film fabrication of solar cells may be achieved without compromising cell efficiency. 
     A principle of this approach is that the light focused and guided in the substrate  102  bounces on and reflects through the p-n junction  106  several times depending on the incident angle and thickness of the substrate  102 , and the properties of the semiconductor layer  104 , use of light guiding materials/structures, etc. On each impingement, solar radiation crosses the active solar cell and becomes absorbed, generating electron-hole pairs. Within a few millimeters of propagation (in a direction parallel to the major surfaces  108 ,  110 ) within the semiconductor layer  104 , the light can bounce several times and the effective path length of the solar light in the active medium is increased. The path length can be approximated by the following formula: 
       (path)/(number of reflections)˜2*t/sin(theta), 
     where theta is the internal angle of the radiation in the active semiconductor layer  104 , and t is the thickness of the active semiconductor layer  104 . Even for a substrate height of a few millimeters, the effective path length through the active semiconductor layer  104  can be significant multiples of the active semiconductor layer  104  thickness and can lead to complete or near complete absorption of the solar radiation including the long wavelengths. 
     The above reflection action of the light impinging upon the p-n junction  106  numerous times has the advantageous effect of significantly increasing the efficiency of the photovoltaic device  100  over conventional techniques. This is so even when the semiconductor layer  104  is of thin-film construction, such as being less than about 1 um thick. As generally accepted in the art, thin-film and thick-film solar cells are defined by the process and physical thickness of the active semiconductor layers used in the solar cells. In the context of the waveguide solar cells disclosed in this application, the cells are differentiated based on the absorption of the solar radiation of interest in a single pass. Solar radiation at ground level consists of a range of wavelengths from UV to near infrared. 
     Depending on the semiconductor material  104  used and its band gap, there is a wavelength range covered by the solar cell. The absorption coefficient varies from a very large value to a small value as a function of solar wavelength, particularly near the band edge. For example, for single crystal silicon, the wavelength range of interest is from around 350 nm to about 1100 nm. The absorption coefficient for single crystal silicon at 400 nm is about 8.89E+04 cm-1. In contrast, the absorption coefficient for single crystal silicon at 900 nm is only 2.15E+02 cm-1. If 900 nm radiation falls on a 1 um (0.0001 cm) thick single crystal silicon solar cell, only about 2% of the radiation is absorbed in a single pass through such a cell, whereas almost 99% of the light is absorbed at 400 nm. In this case, a 1 um thickness cell would not be able to absorb a majority of the radiation at 900 nm in a single pass and can be considered as too thin for single pass geometries. Such a solar cell is considered a “thin-film”solar cell in the context of the waveguide solar cells being disclosed here. Silicon thicknesses of about 100-200 um are needed to absorb a majority of the radiation up to 1100 nm in a single pass and cells with such thickness are considered “thick-film” solar cells in this context. 
     In one or more embodiments herein, the semiconductor material of the layer  104  may be in the form of amorphous, micro- or nano-crystalline, polycrystalline, or substantially single-crystal material. The term “substantially” is used in describing the layer  104  to take account of the fact that semiconductor materials normally contain at least some internal or surface defects either inherently or purposely added, such as lattice defects or a few grain boundaries. The term substantially also reflects the fact that certain dopants may distort or otherwise affect the crystal structure of the semiconductor material. For the purposes of discussion, it may be assumed that the semiconductor layer  104  is formed from silicon. The above features (and those described later herein) may be applied using other inorganic semiconductor materials such as the type III-V GaAs, copper indium gallium diselenide, InP, etc. Still other semiconductor materials may be employed, such as the IV-IV (i.e. SiGe, SiC), the elemental (i.e. Ge), or the II-VI (i.e. ZnO, ZnTe, etc.). Thin Film organic semiconductors can also be employed with proper consideration. 
     The substantially transparent substrate  102  may be formed from glass, glass-ceramic, polymers, etc. For example, the substrate  102  may be formed from an oxide glass or an oxide glass-ceramic, such as glass substrates containing alkaline-earth ions. The glass may be silica-based, such as, substrates made of CORNING INCORPORATED GLASS COMPOSITION NO. 1737 or CORNING INCORPORATED GLASS COMPOSITION NO. EAGLE 2000®. 
     When the semiconductor layer  104  is, for example, silicon and the substrate  102  is formed of a glass or glass ceramic material, then the semiconductor layer  104  may be bonded to the substrate  102  using any of the existing techniques. Among the suitable techniques is bonding using an anodic bonding process. A suitable anodic bonding process is described in U.S. Pat. No. 7,176,528, the entire disclosure of which is hereby incorporated by reference. 
     A tiled approach may also be employed, where multiple semiconductor layers  104  are disposed on one or more of the major surfaces of the substrate  102  in spaced apart relation. In such a configuration the respective electrodes are coupled in parallel and/or series to achieve the desired voltage and current magnitudes. 
     With reference to  FIG. 3 , which is a side view of an alternative photovoltaic device  100 A, variations in the structural characteristics of the device may be made to further improve the conversion of light energy into electrical power. The photovoltaic device  100 A is of similar construction as the photovoltaic device  100  of  FIG. 2 , however, the device  100 A includes at least two thin-film semiconductor layers  104 A,  104 B, at least one such layer  104  coupled to each of the first and second major surfaces  108 ,  110  of the substrate  102 , and each layer  104  including at least one photo-sensitive p-n junction  106 A,  106 B. Under this configuration, the waveguide characteristic of the composite waveguide structure is operable to cause the waveguide action within each of the semiconductor layers  104 A,  104 B and the reflection of the light to impinge upon the respective p-n junctions  106 A,  106 B of each of the thin-film semiconductor layers  104 A,  104 B a plurality of times. 
     The illustrated light propagation in each of the semiconductor layers  104 A,  104 B has been simplified for purposes of discussion. It is noted, however, that the waveguiding action within the semiconductor layers  104 A,  104 B alone or within one or more composite structures may be achieved (as was discussed with respect to  FIG. 2 ). In the illustrated embodiment of  FIG. 3 , one example of a composite structure is the combination of the substrate  102  and the semiconductor layer  104 A (where waveguiding would occur through the composite structure as discussed with respect to FIG.  2 —rays C, D, E, E′, D′, D″ E″, E′″, etc.). Alternatively or additionally another composite structure may be the combination of the substrate  102  and the semiconductor layer  104 B, where again the waveguiding would occur through the composite structure. A further example of a composite structure would include the substrate  102  and both semiconductor layer  104 A,  104 B. In such a case, the waveguide mode propagation may include: (i) a ray directed from the substrate  102  into the semiconductor layer  104 A, (ii) a reflected ray directed from the semiconductor layer  104 A into the substrate  102  and further into the semiconductor layer  104 B, (iii) a reflected ray directed from the semiconductor layer  104 B into the substrate  102  and further into the semiconductor layer  104 A, and (iv) repeating. 
     Reference is now made to  FIG. 4 , which is a side view of a further photovoltaic device  100 B in accordance with one or more further aspects of the present invention. The illustrated light propagation in the structure  100 B has been simplified to avoid repetition, however, the waveguiding action within the semiconductor layer  104  (and/or composite structure) may be achieved as was discussed with respect to  FIG. 2 . In the illustrated embodiment, additional optical mechanisms may be employed to enhance the absorption of solar energy and electrical power generation. For example, one or more lenses, prisms, reflectors, scattering surfaces, etc. that redirect the solar radiation for improved waveguide action and light trapping may be employed. Additionally, the concentrator optics may be transmissive, reflective or diffractive, and may be of imaging or non-imaging construction. 
     More specifically, the photovoltaic device  100 B may include a light collecting device that is operable to direct solar light toward one of the plurality of side surfaces of the substrate  102  such that the solar light is coupled into the substrate  102  in the waveguide mode. The light collecting device may be a solar concentrator  120  having a focal axis F directed toward the one of the plurality of side surfaces of the substrate  102 . Notably, the focal axis F is transverse to a normal axis N of the photovoltaic device  100 B (and may be close to perpendicular thereto). 
     Alternatively, or additionally, the light collecting device may include a convex edge  122  characteristic of one or more of the side surfaces of the substrate  102 . The curvate characteristic of the edge  122  will tend to improve the collection of light into the waveguide mode, either alone or in combination with the concentrator  120 . 
     The composite waveguide includes the transparent substrate  102 , and the semiconductor layer  104 . In addition, the composite waveguide can include various other intermediate layers that serve various other functions. For example, the composite waveguide can contain one or more transparent conductor layers or other dielectric layers in between the substrate  102  and the semiconductor layer  104 . These layers can serve the function of charge collecting electrodes and/or antireflection coatings or bonding agents. 
     The intermediate layers or other layers may advantageously include the option of putting selective scattering/diffractive features in optimal locations as opposed to the entire illuminated p-n junction surface as in prior art. The scattering/diffractive features are employed to further induce waveguiding within the semiconductor layer  104 . They can also operate to facilitate additional light trapping. One of the constraints on the intermediate layers of the composite waveguide is that they should not introduce unnecessary losses and they should facilitate as much of the absorption as possible in the p-n junction  106  for maximum efficiency. 
       FIG. 5  is a side view of a further photovoltaic device  100 C in accordance with one or more further aspects of the present invention. To amplify the aforementioned increase in the absorption path length and/or to reduce the height of the active semiconductor layer  104  (the vertical dimension in  FIG. 5 ) for lower cost, the light that reaches the side surface at the bottom of the substrate  102  may be reflected or scattered using a light reflective element  124 . The expanded view of the light reflective element  124  in  FIG. 5  reveals that at least one light reflective element  124  is dispose proximate to at least one of the plurality of side surfaces (e.g., the bottom side surface) of the substrate  102 . The light reflective element  124  is operable to cause the light that has reflected a plurality of times between the first and second major surfaces  108 ,  110  in the waveguide mode to reverse or redirect its propagation direction and reflect a further plurality of times between the first and second major surfaces  108 ,  110  in a waveguide mode and to impinge upon the p-n junction a further plurality of times. Although the light reflective element may take on many forms, one example is a prismatic structure (scattering) structure. Other forms may include lenses, prisms, reflectors, scattering surfaces, diffractive surfaces, etc. The length of the panel may be on the order of 10s of centimeters and absorb all or nearly all the available solar radiation. 
     It is preferable to redirect the light along the long dimension of the substrate  102  and also to be within the numerical aperture of the composite waveguide. Prismatic and diffractive features may be better able to do that compared to random scattering structures. The objective of these structures is to redirect the light to increase the effective absorption in the p-n junctions and not to re-scatter out of the composite waveguide solar cell. 
     Generally, the thickness of the semiconductor layer  104  is in the range of about 1-10 um, whereas the thickness of the substrate  102  is on the order of 100s of microns. The refractive indices of the substrate  102  and semiconductor layer  104  are such that the distance between the bounces of the waveguide rays of the structure are determined by the formula discussed above. The height of the substrate  102  should be between a few millimeters to a couple of centimeters in order to achieve a substantial number of bounces and resultant high light absorption in the p-n junctions  106 . The absorption may be substantially improved when the light is waveguided within a semiconductor layer  104  of the 1-2 um. In such case, each bounce takes only a few microns and the height of the substrate  102  need only be a few tens to a few hundreds of microns to achieve a significant number of bounces and high absorption. 
       FIG. 5 . illustrates that placing selective diffractive or selective scattering features  125  close to the entry facet (in this case near the top edge of the composite waveguide structure  100 C) facilitates the waveguiding within the semiconductor layer  104 . After the first entry into semiconductor layer  104 , when the light ray is redirected by the diffractive/selective scattering surface  125  into a shallower angle (which is greater than the critical angle dictated by the refractive indices of the substrate  102  and the semiconductor layer  104 ), the ray will not re-enter the substrate  102 . Instead, the light ray will totally internally reflect within the semiconductor layer  104  and waveguiding is achieved. For the waveguiding to commence and not re-scatter, the scattering feature  125  should be in bands that are only a few microns wide with a few tens of microns wide gaps near the entry point. With a silicon layer  104  bonded to the substrate  102 , the outside surface of the silicon layer  104  is accessible for such patterning before further processing. The diffractive or scattering features may be alternatively or additionally placed on additional transparent dielectric or passivation layers. 
     With reference to  FIG. 2 , in order to reduce the total height required for the substrate  102 , scattering may be added, for example, at the air/substrate interface at major surface  110 . This scattering may be present along the entire length of surface  110  or only some fraction of the length beginning at an edge where light is incident and extending toward an opposite edge. This may eliminate the need to provide a light redirecting surface at  124  ( FIG. 5 ) and may be simpler to implement than processing the very thin edge of  124 . 
       FIG. 6  is a side view of a further photovoltaic device  100 D in accordance with one or more further aspects of the present invention. In this embodiment, at least a pair of photovoltaic devices  100 - 1 ,  100 - 2 , each of substantially the same construction as the photovoltaic device  100  of  FIG. 2 , are employed. Again, the illustrated light propagation in the structure  100 D has been simplified to avoid repetition, however, the waveguiding action within the semiconductor layer  104 - 1 , the semiconductor layer  104 - 2  (and/or composite structures) may be achieved as was discussed with respect to  FIG. 2 . At least first and second semiconductor layers  104 - 1 ,  104 - 2  are disposed facing one another in a spaced apart configuration forming a gap G therebetween. The gap is formed via respective rods  130 A,  130 B. The rods  130 A,  130 B disposed in the gap G may be operable to space the first and second semiconductor layers  104 A,  104 B apart and/or focus at least some solar light into the gap G such that the light propagates in a waveguide mode along the gap impinging upon the respective p-n junctions a plurality of times. The gap volume may be filled with high index material, or may be filled with a gas or fluid, such as air. Thus, in addition to obtaining multiple reflections within each semiconductor layer  104  via light entering through edges of the respective substrates  102 , the incident light will enter the gap, thereby further entering the semiconductor layers  104 , waveguiding therein, and impinging upon the respective p-n junctions a plurality of times. This results in increased absorption even with thin semiconductor layers  104  of about 0.5-1.0 um. The gap may be between about 0.1-0.7 mm. 
     Turning again to the various aspects of the light propagation within the device  100 D, a composite structure may be defined to include the two semiconductor layers  104 - 1 ,  104 - 2  and the gap G. In such an example, light propagation in one or more waveguide modes may be defined by: a ray B directed from the gap G into the semiconductor layer  104 - 1  (a reflective ray may also bounce back into the gap initiating further propagation modes), (ii) a reflected ray directed from the semiconductor layer  104 - 1  back into the gap G and further into the semiconductor layer  104 - 2 , (iii) a reflected ray directed from the semiconductor layer  104 - 2  back into the gap G and further into the semiconductor layer  104 - 1 , etc. 
     Those skilled in the art will appreciate from the foregoing that other composite structures may be defined within the device  100 D, such as at least one of: (i) the first substrate  102 - 1  and the first semiconductor layer  104 - 1 ; (ii) the second substrate  102 - 2  and the second semiconductor layer  104 - 2 ; (iii) the gap G and the first semiconductor layer  104 - 1 ; (iv) the gap G and the second semiconductor layer  104 - 2 ; (v) the gap G and the first and second semiconductor layers  104 - 1 ,  104 - 2 ; and (vi) combinations of the above. 
       FIG. 7  is a side view of a further photovoltaic device  100 E in accordance with one or more further aspects of the present invention. The combination substrate  102  and semiconductor layer  104  may be any suitable ones of the aforementioned configurations or those later described herein. The photovoltaic device  100 E further includes a light collecting device  132  operable to direct solar light toward one of the plurality of side surfaces of the substrate  102  such that the solar light is coupled into the substrate  102  and then into the semiconductor layer  104  in the waveguide mode. By way of example, the light collecting device  132  includes a substantially cylindrical rod having a longitudinal slot  134  extending along a wall thereof. The substrate  102  and semiconductor layer  104  are located within the slot  134  such that one of the plurality of side surfaces of the substrate  102  abuts a bottom  136  of the slot  134 . The slot  134  is shown in exaggerated form (showing a gap with the substrate  102  and layer  104 ) although in a practical device a snug fit is preferred. 
     A depth of the slot  134  is such that the substrate  102  and layer  104  are positioned at an appropriate distance from the top of the rod  132  for optimum light collection. In this regard, the rod  132  includes optical properties that cause the solar light to couple into the substrate  102  in the waveguide mode. For example, the rod  132  may be of the tracking or non-tracking type concentrator variety, where the material of the rod  132  may be a high index (hence high NA) material, such as glass, transparent polymer, and/or plexiglass. The rod  132  may be shaped for better packing array and for reducing aberrations. Alternatively or additionally, portions of the surface of the rod  132  may be modified for further optical properties that cause the solar light to couple into one of the major surfaces of the substrate  102 . For example, element  138  may be roughening, grooving, coating, (reflective and/or scattering), etc. for redirecting and trapping the light. This may cause light that would otherwise leave the rod  132  to redirect toward the substrate  102  and layer  104 . Alternatively or additionally, the element  138  may include one or more reflectors located proximate to an outside surface of the rod  132 , which direct light exiting the wall of the rod  132  back toward the substrate  102  and semiconductor layer  104 . A number of the rod concentrators  132  may be stacked side by side for scaling up over an area. 
     One or more further reflectors  139 A,  139 B ( FIG. 8 ), which are secondary tapered concentrators, may also be employed alone or in combination with the rod  132  and/or the concentrator  120  to form a further embodiment  100 F. The secondary tapered concentrators  139 A,  139 B may be employed to collect light not focused onto the substrate  102  and trapped. This is especially desirable with non-tracking concentrators  132 . The reflectors  139 A,  139 B may include 1-D refractive or reflective tapers and/or light funnels with light trapping structures. They can be corrugated 1-D linear or parabolic light funnels for light weight with diffractive or refractive focusing elements. The inside surfaces tapers may be coated with highly reflective coatings or dielectric mirrors. Such designs would be suitable for low concentration factors. 
     The reflectors  139 A,  139 B each include a first edge  137 A,  137 B, located proximate to the substrate  102  and semiconductor layer  104 . The reflectors  139 A,  139 B angle away from the respective first edges  137 A,  137 B toward respective opposite edges  135 A,  135 B. This arrangement operates such that the reflectors  139 A,  139 B cause the light to reflect back toward the substrate  102  and semiconductor layer  104  and to couple into one of the major surfaces  108 ,  110  of the substrate  102 . 
     The long axis of the cylindrical lens may be oriented the East-West direction so that the long length of the rod  132  allows the capture of the solar radiation as the sun moves over the horizon during the day. For low concentration designs, the high NA of the rod  132  may capture the radiation without significant efficiency reduction even if the illumination is not on axis during the seasonal changes of sun&#39;s position on the horizon. 
       FIG. 9  is a side view of a further photovoltaic device  100 G in accordance with one or more further aspects of the present invention. In this embodiment, a light collecting device  140  includes refractive, focusing and tapered concentrators combined in a monolithic fashion. In particular, the light collecting device  140  includes a wedge-shaped rod having a longitudinal slot  134  extending along a narrow edge thereof. The substrate  102  and semiconductor layer  104  is located within the slot  134  such that one of the plurality of side surfaces of the substrate  102  abuts a bottom  136  of the slot  134 . The wedge-shaped rod  140  may be made using transparent polymer or glass materials. Plexiglass or polymer materials may provide lower cost, easier shaping, and lighter weight; however, they may not be as durable and may have absorption of the shorter wavelengths of solar radiation. Glass may be potentially more durable, and have lower UV or blue absorption but it can be more difficult to shape, and can be costly for high index material. 
     The wedge-shaped rod  140  includes optical properties that cause the solar light to be directed toward the one of the plurality of side surfaces of the substrate  102  and to direct light that would otherwise not couple into the substrate  102  back toward the substrate  102  and semiconductor layer  104 . The wedge-shaped rod  140  may includes a convex domed surface  142  opposite the slot  134  defining a focal axis directed toward the side surface of the substrate  102 . The wedge-shaped rod  140  includes at least one side surface, and preferably a pair of surfaces  144 ,  146  that extend from the narrow edge or end of the rod  140  outward and angled away from the substrate  102  and semiconductor layer  104  toward respective edges  142 A,  142 B of the convex domed surface  142 . The respective side surfaces  144 ,  146  are operable to direct light that would otherwise not couple into the substrate  102  back toward the substrate  102  and the semiconductor layer  104 , such as to couple into one of the major surfaces  108 ,  110  of the substrate  102 . The side surfaces  144 ,  146  may be diffractive to focus the light in desired directions. 
       FIG. 10  is a side view of a further photovoltaic device  100 H in accordance with one or more further aspects of the present invention. The combination substrate  102  and semiconductor layer  104  may be any suitable ones of the aforementioned configurations or those later described herein, the illustrated structure being the basic photovoltaic device  100  of  FIG. 2 . The photovoltaic device  100 H further includes a light collecting device  150  operable to direct solar light toward one of the plurality of side surfaces of the substrate  102  such that the solar light is coupled into the substrate  102  and into the semiconductor layer  104  in the waveguide mode. 
     By way of example, the light collecting device  150  includes an integrating-type hollow cylinder having a cylindrical wall  152  defining an interior volume  154 . The substrate  102  and the thin-film semiconductor layer  104  are located at least partially within the interior volume  154 . The cylindrical wall  152  includes a slot  156  extending longitudinally, defining an aperture for the solar light to enter the interior volume  154 . The cylindrical wall  152  includes a reflective interior surface that may direct the light toward one of the plurality of side surfaces of the substrate  102  such that the solar light is coupled into the substrate  102  in the waveguide mode. Alternatively or additionally, the reflective interior surface of the wall  152  may direct the light back toward the substrate  102  and semiconductor layer  104  and to couple into one of the major surfaces  108 ,  110  of the substrate  102 . 
     The slot  156  is operable to allow for the focal spot movement during the course of the day, which would allow non-tracking solar panels without significant reduction in efficiency. Additionally, the photovoltaic device  100 H may further include a solar concentrator  120  as previously discussed herein to direct light into the slot  156 . 
       FIG. 11A  is a side view of a further photovoltaic device  100 I in accordance with one or more further aspects of the present invention. This embodiment is a variation of the waveguiding and trapping geometry, where the photovoltaic device is in a horizontal orientation rather than a vertical orientation. The photovoltaic device  100 I includes a substrate  102  having first and second major surfaces  108 ,  110  and a plurality of side surfaces. One or more semiconductor layers  104 A,  104 B,  104 C, are coupled to the first major surface  108  of the substrate  102  and include at least one photo-sensitive p-n junction  106 . 
     It is noted that this embodiment (as well as the other embodiments of the invention discussed above) may support one or more p-n junctions  106 . These junctions can be of homogeneous or heterogeneous type. The semiconductor layer  104  may be selected to cover a broad range of wavelengths for efficient usage of all of the solar spectrum. For example, single crystal silicon may be used with amorphous silicon, Si—Ge, Ge, GaAs, etc. The single crystal silicon may also be combined with polymer semiconductors. This approach provides an advantage in any solar cell where the semiconductor layer  104  has insufficient absorption in a single pass. 
     As illustrated in  FIG. 11A , the semiconductor layer  104  may include a stacked, multi-junction configuration  104 D,  104 E or the semiconductor layer  104  may be spatially separated  104 A,  104 B,  104 C. 
     A light collecting device of the structure  101 I includes one or more solar concentrators  120 A,  120 B, each having a focal axis F operable to direct solar light toward the first major surface  108  of the substrate  102 . The light entry areas on the surface  108  of the substrate  102  may include AR coatings to collect light at different angles of incidence and different spectrums. The light collecting device also includes one or more corresponding reflective elements  121 A,  121 B operable to direct the light entering the substrate  102  through the first major surface  108  transversely with respect to the focal axis of the solar concentrators  120 A,  120 B such that the solar light is coupled into the semiconductor layers  104  in a waveguide mode. 
     As with some of the other embodiments discussed above, the illustrated light propagation in the structure  100 I has been simplified to avoid repetition, however, the waveguiding action within any of the semiconductor layers  104  and/or any of numerous composite structures may be achieved as was discussed above. 
     The horizontal waveguide configuration  101 I may be implemented in several embodiments. In the embodiment of FIG.  11 A, the light redirecting element  121  is built into the substrate  102 . This may be a shaped reflective/diffractive cavity. 
     It may be more advantageous, and the substrate strength may be better maintained, if an alternative, shaped cavity is employed. In this alternative, a shaped “redrawn cane” of suitable material is attached to the bottom of the substrate  102  at suitable locations. For example, as shown in  FIG. 11B , a prismatic cane  123  may be drawn from a large glass blank, shaped into a prism and heated to redraw to a final dimension of about 1-2 mms. Low cost techniques similar to fiber redrawing technologies may be used to produce the redirecting structures. The outside facets of the prism  123  may be coated with metal or dielectric reflective coatings in a batch process. The size and facet angles of the redirecting prism  123  may be designed based on the parameters of the substrate  102 , the concentrator lens  120  parameters, and the seasonal variation of the sun&#39;s movement over the horizon, etc. Depending on which side of the redirecting prism  123  facet the solar radiation is focused, the light would be redirected to the left or right side toward the semiconductor layers  104 . This is an advantage in collecting the radiation and maintaining the efficiency even with seasonal variations of the sun&#39;s movement over the horizon. 
     The advantages of the horizontal composite waveguide solar cells structure  101 I include: 
     Scalability to large panels. The substrates  102  may be quite large and the semiconductor layer  104  may be either bonded or deposited in long length with a few mms or cms width with 1-2 mm gaps ( FIG. 11A ).
 
Reduced height compared to the vertical waveguide configuration. This form factor advantage may be important in some of roof-top applications where previously concentrator designs were not considered practical.
 
Potentially lower assembly and processing cost as the horizontal approach does not involve cutting or wire sawing large panels into 5-10 mm high strips as would be involved in the vertical design of  FIG. 2 .
 
     It may be easier to incorporate some of the AR coating and electrical interconnections steps in this geometry. A distinguishing feature of the composite waveguide approach compared to conventional light trapping structures is the separation of the light entry locations from the light waveguiding regions of the solar cell. In the prior art substrate (or superstrate) configurations (e.g.,  FIG. 1 ), the light falls across the entire active surface of the solar cell. If employed, the light trapping/scattering features need to be disposed across the entire active surface. If any sections do not have such features, the light incident on those areas will not scatter and will have only a single pass through the p-n junction. Also, any light that is trapped in one area may re-scatter out of the solar cell by the scattering features in adjacent areas. 
     In contrast, the light entry and waveguiding sections are separated in one or more of the embodiments herein. For example, in  FIG. 2 , the entry point for the light is the edge facet of the composite waveguide (top edge of the substrate  102 ) and the active p-n junction surface is separated from this facet and is orthogonal to it. Similarly, in  FIG. 11A , the light entry surface and active p-n junction surfaces are separated. The light entry is facilitated by the redirecting optics  121  into the composite waveguide. This can be combined with low concentration optics, such as  120 , shown in  FIG. 4 . This approach provides a number of advantages over the prior art. A significant advantage is the fact that the light trapping is not dependent on the scattering features. This eliminates the re-scattering problem present in the prior art designs. The approach also provides flexibility in the p-n junction placement. For example, p-n junctions can be fabricated on both sides of the transparent substrates as shown in  FIG. 3 . The composite waveguide approach also provides the flexibility in placing the scattering, diffractive surfaces in only selective locations to further improve the waveguiding within the semiconductor layer without the concern about re-scattering. Since the semiconductor layer is only 1-2 um generally, waveguiding within that layer rather than the entire substrate composite waveguide would lead to light absorption within 100-200 um rather than several mms otherwise required. (As was explained with reference to  FIG. 5 .) Additionally, since the entry facet does not include the very high index semiconductor layer, it is easier to design the AR coatings for the entry facet of the transparent substrate, which is generally a lower index glass or polymer. This allows a better optimization over a broader wavelength and angular range. 
       FIG. 12A ,  12 B are graphs illustrating simulation results of certain parameters of merit associated with a basic operational concept of the photovoltaic devices of the present invention. The results show the maximum achievable current density (MACD) versus incident angle at the top surface of the cell without concentrator optics in the configuration of  FIGS. 2 and 3 . These results indicate that the photovoltaic structures described herein are operable to absorb a significant amount of solar light, including the long wavelengths, even for semiconductor layer thicknesses of less than about 1 um and vertical heights as small as about 2 mm. For the wavelength range considered in the model of 300-1200 nm, the maximum MACD value is 45.9 mA/cm 2 . Values above 30 mA/cm 2  are considered good and above 35 mA/cm 2  are considered very good. The horizontal axes on the graphs of  FIGS. 12A and 12B  indicate the incident angle on the edge surface of the substrate  102  (e.g., see  FIG. 2 ). The range of angles between 0 and 45 degrees represent the angle associated with a potential concentrator  120 , such as shown in  FIG. 4 .  FIG. 12A  illustrates the performance (line  202 ) of a planar reflector and the performance (line  204 ) of a Lambertian reflector located at  124  of  FIG. 5 . This simulation is for the case of a 0.7 mm wide and 10 mm tall substrate  102 . The semiconductor material  104  is a 1 um thick silicon layer.  FIG. 12B  shows the difference between single side structure  100  (line  206 ) and double side structure  100 A (line  208 ). This simulation is for the case of a 0.7 mm wide and 2 mm tall substrate  102  and a Lambertian reflector at  124  of  FIG. 5 . The device performance is a strong function of the width of the substrate  102 . For narrow substrates  102 , less height is required for very good performance compared with wider substrates. Devices with a substrate thickness of 0.2 mm and glass height of 5.0 mm have MACD values in excess of 40 mA/cm 2 . The model does not take into consideration several potential loss mechanisms, including highly doped silicon loss, contact shadowing loss, and metal contact absorption loss. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.