Patent Application
20110146788
The invention relates generally to the area of semiconductor photovoltaic (PV) cells. More specifically, the invention relates to the area of PV cells wherein the semiconductor absorber material employed is cadmium telluride (CdTe).
The solar spectrum “sunlight” contains a distribution of intensity as a function of frequency. It can be shown that the conversion efficiency for utilizing sunlight to obtain electricity via semiconductors is optimized for semiconducting band-gaps that range from about 1.4-1.5 electron volt (eV). The semiconducting band-gap of CdTe, is a good match for this requirement. Quite generally, in the interest of brevity of the discussions herein, PV cells including CdTe as the photo-active material may be referred to as “CdTe PV cells.”
Commercial feasibility of large-scale CdTe PV installations has been demonstrated, and the cost of electricity obtained from such large-scale CdTe PV installations is approaching grid parity. Commercial feasibility of smaller scale, that is, area confined, installations remains a challenge within the art due to the relatively poor overall efficiency of such smaller scale installations. Despite significant academic and industrial research and development effort, the best efficiencies for CdTe PV cells have been stagnant for close to a decade, at about 16.5%, even as the entitlement-efficiency of CdTe PV cells for the solar energy spectrum is about 23%. These conversion efficiency numbers may be compared to the overall efficiency of typical currently available commercial large-scale CdTe PV installations including such CdTe PV cells, which conversion efficiency is lower at about 10-11%.
Improvement in the overall efficiency of CdTe PV installations will enhance their competitiveness compared to traditional methods of generating electricity such as from natural gas, or coal. It is evident that improvement in overall efficiency will enable CdTe PV technology to successfully penetrate markets where small-scale area confined installations are required, such as markets for domestic PV installations.
A CdTe PV cell design that is more reliable and enables conversion efficiencies in excess of the efficiencies of present generation CdTe PV cells, would therefore be highly desirable.
Embodiments of the invention are directed to a photovoltaic cell.
A photovoltaic (PV) cell comprising, a plurality of ultrafine structures electrically coupled to, and embedded within, a polycrystalline photo-active absorber layer comprising p-type cadmium telluride (CdTe).
A photovoltaic solar cell, comprising, an optical-window-electrode (OWE) layer, a plurality of ultrafine structures comprising an n-type semiconductor electrically coupled to, and embedded within and substantially along a thickness direction of, a polycrystalline photo-active absorber layer comprising p-type cadmium telluride (CdTe), and a electrode layer comprising a p-type semiconductor.
A photovoltaic (PV) cell comprising, a plurality of ultrafine structures electrically coupled to, and embedded within, a polycrystalline photo-active absorber layer comprising a p-type compound semiconductor.
These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.
In the following description, whenever a particular aspect or feature of an embodiment of the invention is said to comprise or consist of at least one element of a group and combinations thereof, it is understood that the aspect or feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
As discussed in detail below, embodiments of the invention are directed to improved photovoltaic (PV) cell designs. Particular embodiments of the invention proposed here provide for a PV cell including a photo-active absorber layer including n-type CdTe, and having an efficiency that is enhanced over the efficiencies of presently available CdTe PV cells (about 16.5%). Embodiments of the CdTe PV cell disclosed herein may display an efficiency that approaches, or even exceeds, about 20%. The photo-active absorber layer disclosed herein is the part of the PV cell where the conversion of electromagnetic energy of incident light, for instance, sunlight, to electrical energy occurs.
However, as discussed in more detail below, quite generally, it is envisaged that the designs and concepts proposed herein will be useful for the development of PV cells including a photo-active absorber layer that includes compound semiconductors other than CdTe. Non-limiting examples of compound semiconductors include gallium arsenide, indium gallium arsenide, indium phosphide, copper indium gallium selenide, copper indium sulphide, Cu(In,Ga,Al,Ag)(S,Se)2, or copper zinc tin sulphide.
In the discussions herein, the term “ultrafine structures” will be understood to include structures such as nanowires, nanotubes, quantum wires, quantum dots, nanowalls, and combinations thereof of such structures, as also any other structure capable of displaying physical properties similar to one or more physical properties of the aforementioned structures. Quite generally, the term “ultrafine structures” will be understood to include any structure wherein a smallest physical dimension of, or feature of, the structure has a spatial extent “aus” of less than about 1 micrometer. It will be appreciated that, for instance, a “mesh,” for instance, of nanowires, wherein the mesh extends over a spatial extent in excess of aus, but wherein at least a portion of the nanowires have at least one physical dimension that is substantially less than aus, is a specific non-limiting example of an ultrafine structure according to the present definition. Further, it will be understood that, for instance, ultrafine structures, wherein a spatial extent, of at least some of the features, such as pores or voids, characterizing the ultrafine structures, is less than about 1 micrometer, fall within the purview of the present invention. In particular embodiments of the invention disclosed herein, the ultrafine structures may be disposed on a substrate, for instance, a thin-film, of the same material from which the ultrafine structures are formed. In more particular embodiments of the invention disclosed herein, the substrate may be formed from a dielectric material, which substrate may also serve as a template for the growth of the ultrafine structures.
Quite generally, in the interest of brevity of the discussions herein, PV cells including CdTe as the photo-active material may be referred to as “CdTe PV cells.” The type of doping (p-type or n-type) of the CdTe will be specified, although is non-limiting.
Those of skill in the art would be aware that the effective carrier density of CdTe is a function of the defects and impurities present therein. Also, the photo-active absorber layers employed in CdTe PV cells are substantially polycrystalline, that is, they are composed of a multitude of grains with grain boundaries mediate the grains. Those of skill in the arts of photovoltaics and thin films would appreciate that the grain boundaries are regions of defects. The presence of these defects leads to the creation of additional electronic states within the semiconducting band-gap of CdTe. These additional states, which may be acceptor states, or donor states, can affect the physical properties, for instance, the optical and electrical properties, of the photo-active absorber layer.
As will be discussed in context of embodiments of the instant invention, a control of the active function, that is, the function related to charge carrier transport, of the grain boundaries, can result in enhanced conversion efficiency levels beyond those possible for present generation CdTe PV cell designs.
It will be appreciated, that the feasibility of any proposed scheme for such control of the active function of the grain boundaries, will be augmented, if the materials and processes enabling the scheme, are compatible with extant CdTe PV cell design and fabrication paradigms.
p-type CdTe is currently the most commonly used material in PV cells where the photo-active material is CdTe. Present generation CdTe PV cells utilize p-type CdTe to form an absorber layer. The absorber layer is the part of the PV cell where the conversion of electromagnetic energy of incident light (for instance, sunlight), to electrical energy (that is, to electrical current), occurs. Present generation CdTe PV cells, however, suffer from multiple issues that have hampered the development of high performance CdTe PV cells with efficiencies approaching the entitlement-efficiency of the CdTe PV cells for the solar energy spectrum.
Joule dissipation is among the factors limiting the efficiency of PV cells that include CdTe absorber layers, such as the CdTe PV cell 100. Furthermore, the optical window layer, commonly used in present generation CdTe PV cells is also a seat of loss of efficiency since it absorbs a portion of incident light energy flux before it reaches the CdTe absorber layer, which absorbed portion of light energy flux is therefore unavailable, for generation of photo-generated charge carriers, that is, for the generation of electrical current. For instance, one of the commonly used materials to form optical window layers in present generation CdTe PV cells that include p-type CdTe absorber layers, is n-type CdS. It is known that typical n-type CdS optical window layers (having typical thickness of about 50-200 nanometers) result in a loss of about 2-5 milli Amperes per square centimeter (mA/cm2) of electrical current due to absorption of incident light flux. If schemes can be found, to mitigate the above discussed dissipation modes within the CdTe absorber layer, as well as to address the loss of incident light energy flux in the optical window layer, then it is likely that CdTe PV cells that are designed according to such schemes would possess efficiencies enhanced over the efficiencies of present generation CdTe PV cells.
As discussed in detail below in relation to
Quite generally, the substrate 208 coupled to the plurality of ultrafine structures 202 serves as the negative electrode for an electrical load (not shown) connected to the PV cell 200, while the electrode layer 234 coupled to the photo-active absorber layer 204 serves as a positive electrode for the electrical load (not shown) connected to the PV cell 200.
The plurality of ultrafine structures 202 include at least one n-type material, wherein the n-type material includes semiconductors, or transparent conducting oxides. In one embodiment of the invention, a carrier density within the n-type material lies within a range from about 1016 per cubic centimeter (/cm3) to about 1018/cm3. In one embodiment of the invention, a carrier density within the n-type material can be up to about 1021/cm3. In one embodiment of the invention, the plurality ultrafine structures comprise an n-type material coated with an n-type semiconductor. Non-limiting examples of the n-type semiconductor include n-type cadmium sulphide. In one embodiment of the invention, the n-type semiconductor has a carrier density of that lies within a range from about 1015/cm3 to about 1019/cm3. In one embodiment of the invention, the n-type semiconductor has a carrier density that lies within a range from about 1015/cm3 to about 2×1020.
It is clarified that, even though in
Particular non-limiting embodiments of the invention include a plurality of ultrafine structures that include a plurality of metallic ultrafine structures conformally coated with an n-type semiconductor. For instance,
An electric field 218 is generated across a p-n junction 220 between the photo-active absorber layer 204 comprising p-type CdTe and the plurality of ultrafine structures 202 comprising a n-type semiconductor, according to principles that would be known to one of skill in the art of photovoltaics. In the embodiment presently under discussion, the electric field 218 would be directed substantially “inwards” from bulk portions 226 of the p-type CdTe layer 204 towards the individual ultrafine structures of the plurality of ultrafine structures 202.
On the basis of the descriptions provided, it should be evident to one of skill in the art that the p-n junction 220, is “distributed,” along the interface regions 228 between the plurality of ultrafine structures 202, and the bulk portions 226 of the photo-active absorber layer 204. Evidently, the interface regions 228 are “spread-out” across the bulk of the photo-active absorber layer 204. PV cell designs according to embodiments of the present invention possess several enhancements over present generation PV cell designs, as are now discussed.
As will be discussed in more detail in relation to
It is also envisaged that, since the p-n junction interfaces have effectively been “transferred” to within the bulk of the photo-active absorber layer 204 (as opposed to the p-n junction 112 which lies along the edge 120 of the photo-active absorber layer 102 of CdTe PV cell 100), embodiments of the present invention, may be exempt from the requirement of a n-type CdS optical window layer (of type 104) in order to perform a photovoltaic function. In such embodiments of the invention therefore, the substrate 208 may itself be formed from a material (such as a transparent conducting oxide) having appropriate electrical and optical characteristics to serve as an OWE layer. This lifting of the requirement of the CdS optical window layer would likely contribute to an increase in the efficiency of the CdTe PV cell as per the discussions herein. It is clarified, however, that CdTe PV cell designs comprising an OWE layer comprising n-type CdS fall within the purview of the present invention.
The open circuit voltage “VOC” and the short circuit current density “JSC” are among the standard figures of merit on the basis of which performance of PV cells may be evaluated. Embodiments of the invention include PV cells (for instance, of type 200) that are capable of generating an open circuit voltage “VOC” of up to about 0.85 Volt. Alternate embodiments of the invention include PV cells (for instance, of type 200) that are capable of generating a short circuit current density “JSC” of up to about 0.025 Ampere/cm2. Due to such range of achievable values of VOC and JSC, it is envisaged that PV cells according to embodiments of the present invention, will be capable of achieving a JSC of up to about 0.028 A/cm2, leading to efficiencies (for conversion of light energy to electrical energy) of greater than about 16% and up to about 20%.
In one embodiment of the invention, the plurality of ultrafine structures 202 include cadmium telluride (CdTe), cadmium sulphide (CdS), zinc telluride (ZnTe), zinc selenide (ZnSe), cadmium selenide (CdSe), zinc sulphide (ZnS), indium selenide (In2Se3), indium sulphide (In2S3), or zinc oxyhydrate. In one embodiment of the invention, at least a portion of the plurality of ultrafine structures 202 comprise a structure comprising at least one nanowire, or at least one nanotube, or at least one quantum wire, or at least one quantum dot, or at least one nanowall. In particular, individual ultrafine structures of the plurality of ultrafine structures 202 may be formed as a “pillar,” or “string” of quantum dots. In one embodiment of the invention, at least one physical dimension of at least a portion of the plurality of ultrafine structures is less than about 1 micrometer. In more particular embodiments of the invention, at least one physical dimension of at least a portion of the plurality of ultrafine structures is less than about 500 nanometers. In more particular embodiments of the invention, at least one physical dimension of at least a portion of the plurality of ultrafine structures is less than about 200 nanometers. In more particular embodiments of the invention, at least one physical dimension of at least a portion of the plurality of ultrafine structures is less than about 100 nanometers.
As discussed, in one embodiment of the invention, the plurality of ultrafine structures include an n-type semiconductor material. In one embodiment of the invention, the plurality of ultrafine structures comprise a material having a semiconducting band-gap substantially greater than the semiconducting band-gap of the p-type CdTe of the photo-active absorber layer 204. In one embodiment of the invention, the plurality of ultrafine structures comprise a material having a semiconducting band-gap substantially equal to the semiconducting band-gap of the p-type CdTe from which is comprised the photo-active absorber layer 204.
In one embodiment of the invention, a carrier density within the photo-active absorber layer 204 lies within a range from about 1014/cm3 to about 1016/cm3. In one embodiment of the invention, a carrier density within the plurality of ultrafine structures 202 lies within a range from about 1015/cm3 to about 1019/cm3.
In one embodiment of the invention, the PV cell 200 further includes a contact electrode layer 250 comprising a p-type material in electrical contact with the photo-active absorber photo-active absorber layer 204 comprising p-type CdTe. In one embodiment of the invention, the contact electrode layer 250 comprises a transparent conducting oxide. In particular embodiments of the invention, the contact electrode layer 250 is configured to function as an optical-window-electrode (OWE) layer. In one embodiment of the invention, the contact electrode layer 250 has higher carrier density than the photo-active absorber layer 204. In one embodiment of the invention, the contact electrode layer 250 has a carrier density greater than about 5×1017/cm3. In one embodiment of the invention, the contact electrode layer 250 comprises a material comprising BaCuXF, wherein ‘X’ comprises sulphur, selenium, or tellurium. In one embodiment of the invention, the contact electrode layer 250 comprises a material comprising LaCuOX, wherein ‘X’ comprises sulphur, selenium, or tellurium. In one embodiment of the invention the contact electrode layer 250 comprises a material comprising XCuO(S1-y,Sey), wherein ‘X’ comprises praseodymium, neodymium, or a lanthanide, and ‘y’ can assume values between zero and one. In one embodiment of the invention the contact electrode layer 250 comprises Sr2Cu2ZnO2S2 or Sr2CuGaO3S.
Based on the discussions herein, those of skill in the art may now appreciate that, quite generally, embodiments of the invention include, a PV cell (for instance, of type 200) including, a plurality of ultrafine structures (for instance, of type 202) electrically coupled to, and embedded within, a polycrystalline photo-active absorber layer (for instance, of type 204) comprising p-type CdTe.
Bifacial PV cell embodiments, wherein light energy flux incident upon the substrate 304 (208), as well as upon the p-type contact electrode layer 334 (250), is used for photo-current generation, fall within the purview of the present invention. Within such bifacial embodiments, the substrate 304 (208) as well as the contact electrode layer 334 (250) serve as OWE layers. In such bifacial embodiments, a light energy flux 308, and light energy flux 340, may be allowed to be incident, on the substrate 304 (208), and on the contact electrode layer 334 (250), to result in generation of electron-hole pairs. The specific attributes required of the substrate 304 (208), and of the contact electrode layer 334 (250), to realize such “bifacial” embodiments of the PV cell are discussed herein. However, it is pointed out that, PV cells wherein photo-current generation occurs substantially only due to light energy flux incident upon one or the other, of the substrate 304 (208), or the contact electrode layer 334 (250), fall within the purview of the present invention.
As discussed, a substantial portion of the photo-current generation occurs within the photo-active absorber layer 204, and within which photo-active absorber layer 204 are embedded the plurality of ultrafine structures 202. It is noted that, within embodiments of the invention disclosed herein, the plurality of ultrafine structures 202 serve to collect the photo-generated holes generated substantially within the photo-active layer 204.
As discussed at least in context of the embodiments of the invention depicted in
It view of the discussions herein, it may be appreciated that, embodiments of the invention include a photovoltaic solar cell (for instance, of type 200), including an OWE layer (for instance, a substrate of type 208), a plurality of ultrafine structures (for instance, ultrafine structures of type 202) electrically coupled to, and embedded within and substantially along a thickness direction (for instance, of type 224) of, a polycrystalline photo-active absorber layer (for instance, of type 204) comprising p-type CdTe, and a electrode layer (for instance, of type 234) comprising p-type semiconductor.
In view of the discussions herein, it may now be appreciated that, quite generally, embodiments of the invention include a PV cell (similar in structure to the PV cell 200) including a plurality of ultrafine structures (for instance, of type 202) embedded within a photo-active absorber layer (similar in structure to the photo-active absorber layer 204) comprising a p-type compound semiconductor. Non-limiting examples of compound semiconductors include gallium arsenide, indium gallium arsenide, copper indium gallium selenide, Cu(In,Ga,Al,Ag)(S,Se)2, copper zinc tin sulphide, and combinations thereof.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
