Abstract:
Proposed is a composite photovoltaic device with parabolic collector and different solar cells, wherein the high photoelectric conversion efficiency is achieved along with significant material cost reduction. The device comprises two or three solar cells formed on opposite sides of a transparent substrate, and a parabolic collector attached to the back side of the substrate. First thin film solar cell formed on the front side receives and converts to electricity a short-wavelength portion of the incoming Sun radiation, and transmits the long-wavelength portion. A second solar cell receives and converts to electricity a concentrated long-wavelength portion of the Sun radiation, which is re-directed toward a focal point by the parabolic collector. In one embodiment a third solar cell is included for converting an IR portion of the radiation. Thus, each solar cell utilizes a favorable part of the Sun spectrum, which allows for an enhancement of photoelectric efficiency and significant material cost reduction.

Description:
FIELD OF THE INVENTION 
     The invention relates to a composite photovoltaic (PV) device that comprises a common transparent substrate, two solar cells of different materials and different sizes, and concentrating parabolic collector for providing concentrated radiation for at least one solar cell. More specifically, the invention relates to a spectrum-splitting photovoltaic device, wherein different solar cells absorb and convert to electricity different spectral portions of the Sun radiation, including concentrated portions, thus providing a significantly better utilization of the Sun radiation, higher photoelectric conversion efficiency and significant reduction of photo-active materials. 
     BACKGROUND 
     At the present time one of the main problems of a global solar energy implementation is difficulty in reaching high photoelectric energy conversion efficiency (ECE) at low cost of materials and processes involved in building solar cells and panels. Typically, usage of low cost materials, such as thin films, results in lower ECE, while achieving higher ECE requires expensive materials, e.g., mono-crystalline Silicon (Si), compound materials, such as InGaAs, GaAlAs, and the likes. Some of well known techniques for overcoming high cost of III-V compound materials include using concentrated Sun radiation, which is directed toward smaller solar cells to generate electrical power. Such photovoltaic devices (hereinafter referred to as CPV devices) allow for significantly lower consumption of expensive photovoltaic materials, while providing the same or higher amount of electricity. At the same time CPV devices require additional optical elements (lenses, mirrors) and efficient cooling tools for maintaining optimal PV operation. 
     In order to increase ECE of CPV devices it is common to use PV structure of stacked solar cells (SC), wherein a material band gap decreases from cell to cell in the direction away from the light-receiving surface, thus providing a multijunction solar cell (MJSC). The combination of MJSC and concentrated radiation is described in many references, see e.g. “Current status of III-V concentrating multijunction manufacturing technology and device technology development” by F. Newman, D. Aiken et al.,  Proceedings of  23 rd    European Photovoltaic Solar Energy Conference , Valencia, Spain, (2008). Most commonly used III-V material-based MJSC may also include low band gap layers of a single semiconductor, such as Ge, as described, e.g., in the Patent Application TW200933913, published on Aug. 1, 2011, authors Aiken Daniel J et al. 
     The multijunction approach, described above, is also widely used in a variety of Si and Thin Film (TF)-based PV devices, in which case respective PV devices are commonly called Tandem Solar Cells (TSC). A TSC device typically comprises top SC and bottom SC, each having it&#39;s own band gap, thus providing a sunlight spectrum split between the cells. Some examples of TSC can be found, e.g., in “Tandem photovoltaic device and method of manufacturing the same”, Patent Application CN102237417, published on Nov. 9, 2011, author Seung-Yeop Myong, and in many other references. A TSC may operate at regular (non-concentrated) or CPV conditions. 
     It should be noted that, regardless of operating TSC in a regular or CPV mode, the areas of top SC and bottom SC in a TSC structure are essentially the same (defined by a “stack” dimensions), and a series electrical connection should be provided between top SC and bottom SC. It is, therefore, understood that, in order to form top and bottom cells, all known TSC use respective materials for top cell and bottom cell in the amounts necessary to cover the entire area of a TSC. 
     CPV devices of all kinds include means for concentrating incoming sunlight, these means are hereinafter referred to as solar collectors. Solar collectors may comprise set of lens, concave spherical mirrors and parabolic mirrors (the latter is hereinafter referred to as concentrating parabolic collector or simply parabolic collector). Relevant to the present invention is the observation that, to the best of our knowledge, all known PV devices and CPV devices utilize either regular (non-concentrated) Sun radiation or concentrated Sun radiation for an entire device, i.e., without splitting sunlight spectrum into the regular and concentrated portions. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high-efficient composite photovoltaic device (hereinafter referred to as “PV device”) comprising a transparent substrate having a front side and a back side, a concentrating parabolic collector attached to the back side of the substrate, and further comprising two or more solar cells made of different materials and having different sizes, formed on the opposite sides of the transparent substrate. The invention allows for a substantial increase of the photoelectric energy conversion efficiency (ECE) and significant reduction of the materials needed to build the PV device. The above advantages result from the following: 1) using first solar cell of the PV device to generate an electrical power by photoelectric conversion of a first (short-wavelength) portion of the Sun radiation, 2) using parabolic collector for concentrating and directing toward second solar cell a second (longer wavelength) portion of the Sun radiation; 3) using second solar cell to generate additional electrical power by photoelectric conversion of the second portion of the Sun radiation. Electrical power produced by the PV device is the total of the powers produced by the first solar cell and by the second solar cell, which results in significant improvement of ECE. Another advantage of the PV device is significant reduction of the materials needed to build high-efficient second solar cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of the PV device of the present invention, comprising a parabolic collector and two solar cells of different materials and sizes formed on the opposite sides of the transparent substrate. 
         FIG. 2  is a partial view of the PV device of the present invention, that shows boundaries and the area of an efficient light absorption in the second solar cell. 
         FIG. 3A  schematically shows conductive electrodes formed on the first solar cell and the second solar cell of the PV device. 
         FIG. 3B  is a top view of the PV device of  FIG. 3A  with the conductive electrodes and connection links formed on the back side of the transparent substrate. 
         FIG. 4  is a top view of a PV module comprising four PV devices connected in series and in parallel. 
         FIG. 5  is a graph that schematically shows the AM1 Solar spectrum along with spectral split and energy distribution between the solar cells of the PV device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, the PV device of the present invention employs the following features: 1) splitting the incoming Sun radiation into two spectral portions; 2) concentrating the longer wavelength portion (or portions), and 3) using two solar cells for photoelectric conversion of the above spectral portions. These features are illustrated by  FIG. 1 , which is a schematic cross-sectional view of main embodiment of the PV device  100 . 
     In  FIG. 1  reference numeral  101  designates a transparent substrate (hereinafter referred to as merely a “substrate”), which is made of material transparent to the Sun radiation (typically of AM1 type), such as, e.g., glass plate, quartz plate or transparent plastic. The substrate  101  has a front side  102  and a back side  103 . The front side  102  may be textured and pre-coated (not shown) for an anti-reflection purpose as commonly used in known PV devices. Furthermore, the PV device comprises a first solar cell formed on the front side of the transparent substrate and a second solar cell formed on the back side of the substrate. A first solar cell  110  is formed on the front side  102 , covering most of the front side. The first solar cell is made of a first material and has a first light-receiving surface  112 , which is exposed to the Sun radiation, as shown by the dashed arrows L1, and a first back surface  113 . The second solar cell  120  is made of a second material and has a second light-receiving surface  122 , an area of the first light-receiving surface  112  of the first solar cell is larger than an area of the second solar cell  122 . 
     The first solar cell  110  has a first thickness, providing photo-active absorption and photoelectric conversion of a first portion of the Sun radiation. The rest of the Sun radiation (hereinafter referred to as a “second portion”) remains unabsorbed in the first solar cell and is transmitted through the substrate, as shown by the dashed arrows marked L2. The first material of the first solar cell is selected from the group of thin film materials that have a band gap in the range of, e.g., 1.5 eV to 2.0 eV and a thickness in the range of, e.g., 200 to 1000 nm. One example of such materials is hydrogenated amorphous silicon (aSi:H). It is understood, that the first portion of the Sun radiation comprises a short-wavelength part of the Sun spectrum, such as the one, shown in  FIG. 5  by reference AA ( FIG. 5  schematically illustrates the AM1 spectrum “S” and spectral splits between different solar cells of the PV device). The second portion of the Sun radiation comprises a longer wavelength part of the Sun spectrum, as shown in  FIG. 5  by reference BB and CC A spectral boundary between the first portion and the second portion (i.e., the longest wavelength that can be absorbed in the first solar cell) is defined by a first band gap of the first material. 
     The PV device further comprises a concentrating parabolic collector  200  (hereinafter referred to as a “parabolic collector”) attached to the back side  103  of the substrate  101 . The parabolic collector  200  has a reflective surface  210  that faces the back side of the substrate. The reflective surface  210  is essentially a parabolic mirror commonly used in optical and optoelectronic devices. A focal point of the reflective surface is marked F in  FIG. 1 . As shown in  FIG. 1 , the reflective surface  210  receives the second portion of the Sun radiation and re-directs the second non-concentrated portion L2 toward the focal point F, thus ensuring concentrating of the second portion L2 in the vicinity of the focal point. It is understood that the second portion L2 becomes concentrated in the vicinity of the focal point F; it is further referred to as “concentrated second portion” and designated by reference L2m of  FIG. 2 . 
     Furthermore, a second solar cell  120  is formed on the back side  103  of the substrate between the focal point F and the reflective surface  210 . The second solar cell  120  is made of a second material and has a second light-receiving surface  122  that faces the reflective surface  210 , and a second back surface  123  that faces the back side  103  of the substrate  101 . The second solar cell  120  has a second thickness, providing photo-active absorption and photoelectric conversion of the concentrated second portion L2m of the Sun radiation. The second back surface  123  and the back side  103  may carry pre-deposited conductive electrodes and connection links, described below. 
     The second solar cell  120  is positioned in the vicinity of the focal point F in such a way, that the projection of the focal point F onto the second back surface  123  in a direction perpendicular to the back side  103  is within the second back surface  123 . This is illustrated by the reference axis O′-O′ of  FIG. 1 . For example, the focal point can be positioned inside the substrate  101 , and between the second back surface  123  and the front side  102 , as shown in  FIG. 1 . It is understood that, in order to receive the most of the concentrated second portion L2m, generated by the reflective surface  210 , the second solar cell should provide the second back surface  123  (and, preferably, the central area of it) to be aligned with the focal point F along the axis O-O′. It is further understood that the second light-receiving surface  122  can be pre-textured and pre-coated (not shown) for anti-reflection purposes as commonly used in known PV devices. 
     As shown in  FIG. 1 , the second light-receiving surface  122  receives a concentrated second portion L2 of the Sun radiation, therefore, the first light-receiving surface of the first solar cell may be substantially larger than the second light-receiving surface of the second solar cell. More specifically, an area ratio of the first light-receiving surface to the second light-receiving surface may be in the range of 10 to 200, depending on the proximity of the second light-receiving surface to the focal point F. This important feature of the PV device ensures a significant reduction of the amount of the second material needed to form the second solar cell, i.e., a substantial cost reduction of the device. 
     According to the present invention, a band gap of the first material of the first solar cell  110  is wider than a band gap of the second material of the second solar cell  120 . The second material of the second solar cell is selected from the group of mono crystalline or polycrystalline materials that have a band gap in the range of, e.g., 0.7 eV to 1.5 eV, and having a thickness in the range of, e.g., 50 μm to 250 μm. One example of the second material is monocrystalline Silicon (c-Si). As a result, the second solar cell provides an efficient light absorption and photo-electric conversion of the second portion of the Sun radiation, which comprises a longer wavelength part of the Sun spectrum, such as the one shown in  FIG. 5  by reference BB. 
     The laws of geometric optics impose certain limitations on the amount of the second portion L2 of the sun radiation that can be used (in concentrated form) in the second solar cell. These limitations are shown in  FIG. 2 , which is a partial view of the PV device of the present invention, that shows boundaries of an efficient light absorption in the second solar cell. For simplicity the PV device is symmetrical with the position of the origin O′ aligned to the center of the reflective surface  210  and to the center of the second solar cell  120 . Reference Xf (shown for the right side only) designates the boundaries of the reflective surface, which is approximately equal to the width of the first solar cell). Reference Am designates the maximum angle of reflection for the light rays of the second portion L2, that will be received by the second light-receiving surface  122 . Consequently, the light ray marked L2m and the reference Xm designate boundaries of the concentrated part L2m of the portion L2 that will be received and converted to electricity by the second solar cell. It is understood, that, although references Am and Xm are shown only for the left side of the cross-sectional view, similar boundaries exist on the right side. It is further understood, that light rays of the second portion L2 of the Sun radiation, which hit the reflective surface  210  at the angles smaller than Am, will not be received by the second solar cell. 
     Boundary position Xm depends on the geometrical parameters of the PV device such as parabolic factor A (which is defined by a parabolic equation of the reflective surface Y=AX 2 ), width of the second solar cell  120  (half of which is shown in  FIG. 2  by reference d), and the distance between the focal point F and the back surface  123  of the second solar cell  120  designated by reference t. Calculations based on the laws of the geometric optics result in the following equation:
 
 Xm= 1/2 A [SQRT( Z   2 +1)− Z]   (1)
 
tan( Am )= Z   (2)
 
where A is parabolic factor of the reflective surface, Z=t/d. Apparently, for Z&lt;&lt;1, which corresponds to the close proximity of the back surface of the second solar cell to the focal point (as compared to the second solar cell size) the equation (1) results in Xm being approximately equal to Xf. It is understood that in that case the most of the light portion L2 is utilized by the second solar cell, and the energy losses are negligible.
 
     For any given parameters of the PV device a ratio S of the second light portion L2 to the concentrated part L2m can be calculated as follows:
 
 S =( Xm ) 2 /( Xf ) 2 =SQRT( Z   2 +1)− Z   (3)
 
     Example 1 
     d=0.5 cm, t=0.1 cm. S=0.65 (65% of the second light portion L2 is utilized by the second solar cell). 
     Example 2 
     d=0.5 cm, t=0.02 cm, S=0.9 (90% of the portion L2 utilized). 
     According to the present invention, the following parameters of the PV device can be used: width of the first solar cell—10 cm, Xf=9 cm, half width of the second solar cell d=1 cm, focal point position F (relative to the origin O′)=4.5 cm, t=0.02 cm. It should be noted that in this case an area ratio of the first light-receiving surface of the first solar cell to the second light-receiving surface of the second solar cell is approximately 100, which results in proportionate reduction of the amount of the second material needed to form second solar cell. Similar analysis applies to the third material of the third solar cell, which will be described below. 
       FIG. 3A  schematically shows conductive electrodes to the first solar cell  110  formed on the light-receiving surface  112  and on the first back surface  113  of the first solar cell. It also shows conductive electrodes to the second solar cell  120  formed on the second back surface  123  of the second solar cell. In  FIG. 3A  references numeral  111 ,  115   a  and  115   b  designate conductive electrodes to the thin film first solar cell formed on the first light-receiving surface  112 . A transparent conductive electrode  111  (made, e.g. of ITO) may be formed on the surface  112  prior to forming metal electrodes  115   a  and  115   b . The transparent electrode  111  is electrically connected to the conductive electrode of one polarity, e.g., to the electrode  115   a . In order to provide the electrode  115   b  of the opposite polarity (which must be isolated of the electrode  115   a ), a small area of the first solar cell may be etched off and isolated prior to forming the conductive electrodes  115   b , which is shown in  FIG. 3   a  as the hatched region  115   c . Also, the electrode  115   b  can be partially formed on the first back surface of the first solar cell. 
     References numeral  125   a  and  125   b  of  FIG. 3A  designate conductive electrodes to the second solar cell  120  formed on the second back surface  123 . It is understood that in the main embodiment of the PV device the second solar cell  120  is formed, e.g., as a Si-based backside solar cell with the electrodes of both polarities formed on the second back surface  123 . Such design allows for the second light-receiving surface to be fully exposed to the concentrated light portion L2m (described above), which results in higher ECE of the PV device. 
       FIG. 3B  shows the top view of the front side of the PV device of  FIG. 3A  with the conductive electrodes of  FIG. 3A  and connection links formed on the front side of the substrate and on the back side of the substrate. The connection links are in contact with conductive electrodes to the first solar cell and to the second solar cell and provide separate output connections for the first solar cell and for the second solar cell. In the drawing the first solar cell is shown as the hatched square  110 , and an inner border of the parabolic collector (i.e., an edge contour of the reflective surface) is shown as a dashed circle  201 . According to one aspect of the invention the conductive electrodes to the first solar cell can be formed in the corners of the first solar cell, as shown by references numeral  115   a  and  115   aa  for one polarity and by references numeral  115   b  and  115   bb  for the opposite polarity of the electrodes. In order to provide output connections for the second solar cell, the conductive electrodes to the second solar cell  125   a  and  125   b  are complemented by connection links  135   a  and  135   b  formed on the back side of the substrate (ref.  103  of  FIG. 1 ) and connected to the electrodes  125   a  and  125   b  respectively. 
     It is understood that shapes, dimensions, and locations of the conductive electrodes and the connection links are shown in  FIG. 3A  and  FIG. 3B  only schematically, and any reasonable modifications are allowed provided that these modifications do not depart from the scope of the present invention. It is further understood that, in order to increase the first light-receiving surface exposed to the Sun radiation L1 (for the first solar cell) and the second light-receiving surface exposed to the concentrated second portion of the Sun radiation L2m (for the second solar cell), linear dimensions of the conductive electrodes and links should be made as small as possible (at least for their opaque parts). Materials of the conductive electrodes and connection links can be selected from the group of metals, such as, e.g., Al, AlSi, Ni, Ag, Ti, Cu and their alloys, as well known in art. 
     For assembling PV devices of the present invention into PV module additional connection links formed on the front side of the substrate and on the back side of the substrate, providing separate output connections for the first solar cell and for the second solar cell as schematically shown in  FIG. 4 , which is a top view of a PV module segment that comprises four PV devices connected in series and in parallel. The additional connection links  145   a ,  145   b ,  155   a , 155   b ,  145   ab , and  155   ab  are in contact with conductive electrodes to the first solar cell and to the second solar cell of  FIG. 3   a  and  FIG. 3   b  and provide separate output connections for the first solar cell and for the second solar cell. 
     The PV module of  FIG. 4  comprises four individual PV devices C1, C2, C3, and C4, in which first solar cells and second solar cells of individual devices separately connected in parallel and in series by the connection links. More specifically, connection links  145   a  (for simplicity shown only on the left side of the devices C1 and C2) provide parallel connection between first solar cells of the devices C1 and C2; connection links  145   b  (shown only on the right side of the devices C3 and C4) provide parallel connection between first solar cells of the devices C3 and C4. Furthermore, the conductive link  145   ab  provides series connection of the first solar cell of the device C2 to the first solar cell of the device C3. References numeral  110 A and  110 B designate output terminals for a group of the first solar cells of the PV module. 
     Similarly, second solar cells of the devices C1 and C2 are connected in parallel by the connection links  155   a , and second solar cells of the devices C3 and C4 are connected in parallel by the connection links  155   b ; conductive link  155   ab  provides series connection between the second solar cells of the devices C2 and C3. References  120 A and  120 B designate output terminals for a group of the second solar cells of the PV module. It is understood that for achieving a highest possible ECE of PV modules, which are assembled of PV devices of the present invention, a group of first solar cells and a group of second solar cells must be connected separately to respective output terminals. It is further understood that illustrated PV module is only an example, and a variety of connection patterns can be designed to incorporate PV devices of the present invention into PV modules. 
     The PV device of the present invention operates as follows ( FIG. 1  to  FIG. 4 ). Incoming regular (non-concentrated) Sun radiation L1 (e.g., of AM1 spectrum) is received by the first light-receiving surface  112  of the first solar cell  110 . Short-wavelength first portion of the L1 (shown as AA in  FIG. 5 ) is absorbed and converted to electricity in the first solar cell  110 , providing an output electrical power P1 through the conductive electrodes  115   a ,  115   b , 115   aa ,  115   bb  ( FIG. 3   a  and  FIG. 3   b ). The second longer wavelength portion L2 of the Sun radiation remains unabsorbed in the first cell and transmitted through the substrate  101  toward the parabolic collector  200 . It is understood that the second portion L2 in this embodiment is a total of the spectral segments BB and CC of  FIG. 5 . 
     The parabolic reflective surface  210  re-directs light rays of the second portion L2 toward the focal point F. Consequently, a concentrated second portion L2m ( FIG. 2 ) of the second portion L2 is received by the second light-receiving surface  122  of the second solar cell  120 . A proper choice of the PV device&#39;s parameters ensures that the portion L2m is close to the entire second portion L2, as described above in reference to  FIG. 2 . 
     It is also understood that due to the high concentration ratio (provided by the parabolic collector) an area of the second solar cell is substantially smaller than an area of the first solar cell. It is further understood that due to the longer wavelength spectrum of the portion L2, ECE of the second solar cell (made, e.g., of c-Si) can be significantly higher than ECE of the same cell at “normal” conditions, i.e., regular Sun spectrum. The second solar cell provides an output electrical power P2 through the conductive electrodes  125   a , 125   b  and connection links  135   a , 135   b  ( FIG. 3   a  and  FIG. 3   b ). Since the conductive electrodes and the connection links are formed separately for the first solar cell and for the second solar cell, an amount of electrical power P produced by the PV device is total of P1 and P2, i.e., P=P1+P2, which allows for achieving enhanced ECE. For the PV device with previously described parameters of the first and second solar cells a total ECE is expected to be in the range of 22-25%. 
     Although the invention is shown and described with reference to specific examples, it is understood that these examples should not be construed as limiting the areas of application of the invention and that any changes and modifications are possible provided that these changes and modifications do not depart from the scope of the attached patent claims. For example, in the embodiment with two solar cells the focal point can be positioned both inside and outside the transparent substrate. The front side of the substrate can be pre-textured and pre-coated for anti-reflection purposes, as well as the first and second light-receiving surfaces. Locations and patterns of the conductive electrodes and links can be defined in many different ways providing minimal resistive losses in metal lines, and minimum of shading.