# Patent

## Title
Multi-junction optoelectronic device

## Abstract

An optoelectronic semiconductor device is disclosed. The optoelectronic device comprises a plurality of stacked p-n junctions. The optoelectronic semiconductor device includes a n-doped layer disposed below the p-doped layer to form a p-n layer such that electric energy is created when photons are absorbed by the p-n layer. Recesses are formed on top of the p-doped layer at the top of the plurality of stacked p-n junctions. The junctions create an offset and an interface layer is formed on top of the p-doped layer at the top of the plurality stacked p-n junctions. The optoelectronic semiconductor device also includes a window layer disposed below the plurality stacked p-n junctions. In another aspect, one or more optical filters are inserted into a multi-junction photovoltaic device to enhance its efficiency through photon recycling.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
Under 35 U.S.C. 120, this application is a Continuation-in-Part and claims priority to U.S. application Ser. No. 12/939,077 filed on Nov. 3, 2012 and U.S. application Ser. No. 12/605,108 filed on Oct. 23, 2009.


FIELD OF THE INVENTION
The present invention generally relates to optoelectronic semiconductor devices, and more particularly to multi-junction optoelectronic devices.
BACKGROUND OF THE INVENTION
There is a need for providing optoelectronic devices that have increased efficiency when compared to conventional devices. These devices should, however, be cost effective, easily implemented and adaptable to existing environments. The present invention addresses such a need.
SUMMARY OF THE INVENTION
An optoelectronic semiconductor device is disclosed. The optoelectronic device comprises a plurality of stacked p-n junctions, each of the plurality of stacked p-n junctions comprising a p-doped layer. The optoelectronic semiconductor device further includes a n-doped layer disposed below the p-doped layer to form a p-n layer such that electric energy is created when photons are absorbed by the p-n layer, wherein recesses are formed on top of the p-doped layer at the top of the plurality of stacked p-n junctions; wherein the junctions create an offset and an interface layer is formed on top of the p-doped layer at the top of the plurality stacked p-n junctions. The optoelectronic semiconductor device also includes a window layer disposed below the plurality stacked p-n junctions.
In another aspect, one or more optical filters are inserted into a multi-junction photovoltaic device to enhance its efficiency through photon recycling. The optical filter is specific to the angle and wavelength of the light so that it reflects reemitted photons for recycling while it transmits the incident light for absorption by lower junctions.



BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG. 1 is a diagram which depicts a first embodiment of a multi-junction optoelectronic device.
FIG. 2 is a diagram which depicts a second embodiment of a multi-junction optoelectronic device.
FIG. 3 is a diagram which depicts a third embodiment of a multi-junction optoelectronic device.
FIG. 4 is a diagram which depicts a fourth embodiment of a multi-junction optoelectronic device.
FIG. 5 illustrates the photon recycling operation in a multi-junction optoelectronic device.



DETAILED DESCRIPTION
The present invention generally relates to optoelectronic semiconductor devices, and more particularly to multi-junction optoelectronic devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.
FIG. 1 depicts a multi-junction optoelectronic device 100 in accordance with an embodiment. 100 is a single-sided photovoltaic device and therefore contains both contacts, such as the p-metal contact 302 and the n-metal contact 312, disposed on the same side of optoelectronic device 100, as described by other embodiments herein. An anti-reflective coating (ARC) layer 202 may be disposed over a window layer 402. The ARC layer 202 contains a material that allows light to pass through while preventing light reflection from the surface of the ARC layer 202.
A p-n junction structure 401 of higher bandgap has been added above a structure 431 between the ARC layer 202 and the front window layer 106. Structure 401 is comprised of a window layer 402 (for example AlInP, AlGaInP, or AlGaAs), an n-type material 404 (for example InGaP or AlGaAs), a p-type material 406 (for example, InGaP or AlGaAs), and back-surface field or back side window layer 408 (for example, AllnP, AlGaInP, or AlGaAs). This structure is electrically and optically connected to structure 431 through a tunnel junction structure 431. Structure 431 is comprised of a highly p-type doped layer 422 (for example, InGaP or AlGaAs), and a highly n-type doped layer 424 (for example, InGaP or AlGaAs). One of ordinary skill in the art readily recognizes a variety of materials listed could differ from the examples listed herein. Furthermore, the p-n junction formed in structure 401 could be a homojunction or a heterojunction; that is, both the n-type doped layer 404 and p-type doped layer 406 could be the same material, or could be different materials, and that would be within the spirit and scope of the present invention. Also the doping could be inverted, with p-type material at the top of the device, facing the sun, and n-type material at the bottom. One or more additional p-n structures could be added to structure 401 in a similar fashion, either above or below structure 401, and possibly coupled to the rest of the device through a tunnel junction layer or layers.
For some embodiments, the n-type front window 106, the p-type emitter layer 110, and/or the p-type contact layer 112 may be roughened or textured. Each of the n-type front window 106, the p-type emitter layer 110, and/or the p-type contact layer 112 may be roughened for example by an etching process, such as a wet etching process or a dry etching process. This roughening may also be achieved by other processes, such as by using lattice mismatched materials during epi growth to induce strain. Texturing may be achieved for example by applying small particles, such as polystyrene spheres, to the surface of the n-type front window 106 before applying the ARC layer 202. By roughening or texturing the n-type front window 106, the p-type emitter layer 110, and/or the p-type contact layer 112, different angles are provided at the interface between the ARC layer 202 and the n-type front window 106, which may have different indices of refraction. In this manner, more of the incident photons may be transmitted into the n-type front window 106 rather than be reflected from the interface between the ARC layer 202 and the n-type front window 106, because some angles of incidence for photons are too high according to Snell's Law. Thus, roughening or texturing the n-type front window 106, the p-type emitter layer 110, and/or the p-type contact layer 112 may provide increased trapping of light.
As shown in FIG. 1, both the p-metal contact 302 and the n-metal contact 312 are on the back side of the optoelectronic device 100 while the ARC layer 202 is on the sun side or front side of the optoelectronic device 100 that receives light 132. The p-metal contact 302 contains a p-metal contact layer 304 disposed on a p-metal contact layer 306, while the n-metal contact 312 contains an n-metal contact layer 308 disposed on an n-metal alloy contact 310, in some embodiments described herein.
In some embodiments, the optoelectronic device 100 may be formed from a gallium arsenide based cell. In one example, a resist mask may be formed on the exposed surface of the p-type contact layer 112 and pattern recesses and holes may be formed during a photolithography process. The pattern recesses and holes extend through the p-type contact layer 112, the p-type emitter layer 110, the n-type back window 117, and the graded layer 115, and partially into the n-type absorber layer 108. Thereafter, the resist mask is removed to reveal the n-type absorber layer 108 and the p-type contact layer 112 as the exposed surfaces on the back side of the optoelectronic device 100, as viewed from the two-dimensional perspective towards the back side of the optoelectronic device 100. The sidewalls of the recesses and holes reveal exposed surfaces of the p-type contact layer 112, the p-type emitter layer 110, the n-type back window 117, and the graded layer 115, and partially into the n-type absorber layer 108.
In one embodiment, the p-metal contact layer 306 is formed on a portion of the exposed the p-type contact layer 112, and the n-metal alloy contact 310 is formed on a portion of the exposed the n-type absorber layer 108. Thereafter, the insulation layer 216 may be deposited over the surface of the optoelectronic device 100, such as to cover all exposed surfaces including the p-metal contact layer 306 and the n-metal alloy contact 310. Subsequently, the exposed surfaces of the p-metal contact layer 306 and the n-metal alloy contact 310 are revealed by etching pattern holes into the insulation layer 216 by a lithography process. In some embodiments, the p-metal contact layer 306 and the n-metal alloy contact 310 are formed prior to separating the gallium arsenide based cell from a growth wafer during an epitaxial lift off (ELO) process while the insulation layer 216 is formed subsequent to the ELO process. The p-metal contact layer 304 may be formed on the p-metal contact layer 306 and a portion of the insulation layer 216, while the n-metal contact layer 308 may be formed on the n-metal alloy contact 310 and other portions of the insulation layer 216 to form the optoelectronic device 100, as depicted in FIG. 1. In some examples, the p-metal contact layer 304 and the n-metal contact layer 308 may be formed containing the same compositional layers of material as each other and in other examples, the p-metal contact layer 304 and the n-metal contact layer 308 are simultaneously formed on the optoelectronic device 100 during the same metallization steps.
In an alternative embodiment, the p-metal contact 302 and the n-metal contact 312 may be fabricated, in whole or in part, and subsequently, the insulation layer 216 may be formed over and on the sidewalls of the recesses between and around the p-metal contact 302 and the n-metal contact 312. In another alternative embodiment, the insulation layer 216, in whole or in part, may be formed on the optoelectronic device 100 prior to forming the p-metal contact 302 and the n-metal contact 312.
Despite all the contacts, such as the p-metal contact 302 and the n-metal contact 312, being on the back side of the optoelectronic device 100 to reduce solar shadows, dark current and its stability with time and temperature may still be concerns when designing an efficient photovoltaic device, such as the optoelectronic device 100. Therefore, for some embodiments, an insulation layer 216 may be deposited or otherwise formed on the back side of the optoelectronic device 100. The insulation layer 216 contains an electrically insulating material or grout which helps to reduce the dark current within the optoelectronic device 100.
The insulation layer 216 may contain an electrically insulating material or grout, such as silicon oxides, silicon dioxide, silicon oxynitride, silicon nitride, polysiloxane or silicone, sol-gel materials, titanium oxide, tantalum oxide, zinc sulfide, derivatives thereof, or combinations thereof. The insulation layer 216 may be formed by a passivation method, such as by a sputtering process, an evaporation process, a spin-coating process, or a CVD process.
In another embodiment, the insulation layer 216 eliminates or substantially reduces electrical shorts from occurring between the p-metal contact 302 and the n-metal contact 312. The insulation layer 216 contains an electrically insulating grout and/or other electrically insulating material that has an electrical resistance of at least 0.5 MΩ (million ohms) or greater, such as within a range from about 1 MΩ to about 5 MΩ, or greater. Exemplary grouts or other electrically insulating materials may contain a polymeric material, such as ethylene vinyl acetate (EVA), polyimide, polyurethane, epoxy, derivatives thereof, or combinations thereof. In one example, the electrically insulating grout contains a photosensitive polyimide coating. In another example, the electrically insulating grout contains a thermal set of polymeric material.
In many embodiments, the n-metal alloy contact 310 may be formed by a low temperature process, which includes low temperature deposition processes followed by a low temperature, thermal anneal process. Suitable contact materials deposited within the n-metal alloy contact 310 by low temperature deposition processes may include palladium, germanium, palladium germanium alloy, titanium, gold, nickel, silver, copper, platinum, alloys thereof, or combinations thereof, among others.
In another embodiment, the n-metal alloy contact 310 may contain multiple layers of conductive materials including a palladium germanium alloy. The n-metal alloy contact 310 is disposed between the n-type absorber layer 108 and the n-metal contact layer 308 for providing a strong ohmic contact therebetween. The palladium germanium alloy within the n-metal alloy contact 310 allows a high conductivity of the electric potential from the gallium arsenide materials within the n-type absorber layer 108, across n-metal alloy contact 310, and to the n-metal contact layer 308. The n-metal alloy contact 310 can also contain a metallic capping layer which can be provided, for example, on the palladium germanium alloy layer. In some embodiments, the capping layer can include an adhesion layer and a high conductivity layer. For example, the adhesion layer can allow the conductivity layer to adhere to the alloy layer. In some examples, the adhesion layer may contain titanium, tin, zinc, alloys thereof, or combinations thereof and the high conductivity layer may contain gold, silver, nickel, copper, aluminum, alloys thereof, or combinations thereof, or a stack of multiple different metal layers and/or alloy layers. In one example, the n-metal alloy contact 310 contains a high conductivity layer containing gold disposed on an adhesion layer containing titanium, which is disposed on a palladium germanium alloy.
Similar fabrication methods and embodiments as described above are also fabricated for the p-metal contact layer 304 and/or the n-metal contact layer 308 on the optoelectronic device 100. Some of the example embodiments include n-metal alloy contact 304, p-metal contact 302, n-metal contact 312, n-metal alloy contact 310, and other layers suitable for use with contact layers of the cell 300 are described in copending patent application Ser. No. 12/939,050, entitled, “Metallic Contacts for Photovoltaic Devices and Low-Temperature Fabrication Processes Thereof,” filed on Nov. 3, 2010, and which is incorporated herein by reference. Other types, structures, and materials of metal contact layers can also be used with the optoelectronic device 100.
The contact layers can contain Group III-V materials, such as gallium arsenide (GaAs), depending on the desired composition of the final photovoltaic unit. According to embodiments described herein, the contact layers may be heavily n-doped. In some embodiments, the doping concentration may be within a range greater than about 5×1018 cm−3, for example, from greater than about 5×1018 cm−3 to about 1×1019 cm−3. The high doping of the contact layers of the cell 300 allows an ohmic contact to be formed with a later-deposited metal layer without any annealing step performed to form such an ohmic contact, as described below.
In some embodiments, the contact layers may be gallium arsenide (GaAs) doped with silicon (Si). For example, some embodiments using a high-growth rate in forming the layers of the structure can use a silicon dopant (as an n-dopant) to bring the doping concentration to 5×1018 cm−3 or greater. For example, a precursor disilane can be introduced in a fast growth rate process to deposit the silicon. In other embodiments selenium (Se) or tellurium (Te) can be used as a dopant in the formation of the layers of structure.
The contact layers may be formed at a thickness of about 10 nm or greater, such as about 50 nm. In some embodiments, the contact layer 20 can be formed prior to an ELO process that separates the structure from the growth wafer. In some alternate embodiments, the contact layers can be formed at a later stage subsequent to such an ELO process.
In the embodiment of FIG. 1, the current generated by the top cell must be equal to the current generated by the bottom cell. This is because of the electrical connection therebetween. Hence, although the optoelectronic device 100 operates effectively it is desirable sometimes to remove this requirement.
FIG. 2 is a diagram which depicts a second embodiment of a multi-junction optoelectronic device 200. In this embodiment, which has similar elements to those in FIG. 1, there are two additional contacts 313 and 315. Contact 313 is electrically coupled to contact 312′ and contact 315 is coupled to contact 302′. In this embodiment, the current flow is between contacts 312′ and contact 313 for the first cell and between contacts 315 and 302′ for the second cell. In this embodiment, the current flows can be independent from each other by placing an insulator 450 therebetween. Therefore the cells can operate with independent currents dependent upon the application of the device.
FIG. 3 is a diagram which depicts a third embodiment of a multi-junction optoelectronic device. The device 600 includes a semiconductor structure 1101. In an embodiment, the semiconductor structure 1101 comprises an n-layer 1112 and p-layer 1114 coupled together. For example, the n-layer is an n-emitter GaAs layer 1112 and the p-layer is a p-BSF (Back Surface Field) AlGaAs layer 1114. However, one of ordinary skill in the art readily recognizes a variety of materials including but not limited to GaAs, AlGaAs, InGaP, InGaAs, and alloys thereof, etc., could be utilized for either of these layers and that would be within the spirit and scope of the present invention. Furthermore, the junction formed between the two layers does not have to be a heterojunction, that is, both the n-layer 1112 and p-layer 1114 could be the same material (both layers being GaAs or both layers AlGaAs, for example) and that would be within the spirit and scope of the present invention. Also the doping could be inverted, with p-type material at the top of the device, facing the sun, and n-type material at the bottom. Furthermore, the optoelectronic device could be comprised of multiple p-n layers grown in series, for example, to form a multi-junction solar cell.
In another embodiment, a solar cell includes an absorber layer made of gallium arsenide (GaAs) and having only one type of doping, and an emitter layer made of a different material than the absorber layer and having a higher bandgap than the absorber layer. An intermediate layer is provided between the absorber layer and the emitter layer, the intermediate layer having the same type of doping as the absorber layer, wherein the intermediate layer includes a material gradation from GaAs at a side closer to the absorber layer, to the different material of the emitter layer at a side closer to the emitter layer. A heterojunction is formed between the emitter layer and the absorber layer, and a p-n junction is formed between the emitter layer and the absorber layer and at least partially within the different material at a location offset from the heterojunction. The p-n junction causes a voltage to be generated in the cell in response to the device being exposed to light at a front side of the solar cell.
In other embodiments, as shown in FIG. 4, an intermediate layer 114 may be formed between the n-type absorber layer 108′′ and the p-type emitter layer 110′′. The intermediate layer 114 can provide a material transition between the n-type absorber layer 108 and the p-type emitter layer 110′′.
FIG. 4 shows a portion of one embodiment 150 of cell 140 including absorber layer 108′′, an intermediate layer 114, and emitter layer 110′′. In some embodiments, the intermediate layer 114 contains the same or substantially the same material as the emitter layer 110′′, e.g., such as aluminum gallium arsenide in embodiments in which the emitter layer 110′′ contains aluminum gallium arsenide. In addition, the intermediate layer 114 has the same type of doping as the absorber layer 108′′. For example, the intermediate layer may have the formula of molar ratios of AlxGa1-xAs, for example a molar ratio of Al0.3Ga0.7As, and be n-doped within a range from about 1×1016 atoms/cm3 to about 1×1019 atoms/cm3, for example 1×1017 atoms/cm3. The dopant concentrations can be the same or substantially the same as the n-type absorber layer 108′′. In some embodiments the intermediate layer 114 can have a thickness of about two depletion lengths, where a depletion length is the width of the depletion region formed around the p-n junction. For example, in some embodiments the intermediate layer 114 can have a thickness in the range of about 0 to 200 nm.
This embodiment of the cell 140 provides a structure that allows the p-n junction that generates voltage for the cell to be offset from the heterojunction provided by materials having different bandgaps. For example, the p-n junction 152 is at the interface between the n-type and p-type materials of the emitter layer 110′′ and the intermediate layer 114. Thus, in one described embodiment, the p-n junction is provided at least partially within the higher-bandgap material of which the emitter layer 110′′ and intermediate layer 114 are composed (e.g., AlGaAs), and the heterojunction 122 is located at the interface between the intermediate layer 114 and the absorber layer 108′′ (e.g., the interface between GaAs and AlGaAs). This offset provides some advantages over a coincident p-n junction and heterojunction. For example, the offset p-n junction provided between the AlGaAs layers can reduce barrier effects of an interface between the AlGaAs and GaAs layers. In some embodiments, a majority of the absorber layer 108 is outside of a depletion region formed by the p-n junction.
In some embodiments, the heterojunction 154 is located within two depletion lengths of the p-n junction 152. For example, a depletion region may be about 1000 Å (100 nm) wide in some embodiments. The depletion region typically still has a depletion effect past this region, within about two depletion region widths (depletion lengths) of the p-n junction. A heterojunction located further than this distance from the p-n junction may not allow the depletion effect to span the heterojunction interface and a barrier may thus exist.
In this embodiment, on a top side of the semiconductor structure 1101 are a plurality of contact members 1103a-1103n. Each of the top-side contact members 1103a-1103n comprise an optional antireflective coating (ARC) 1102, a n-metal contact 1104 underneath the optional ARC 1102, and a gallium arsenic (GaAs) contact 1106 underneath the n-metal contact. On a back side of the semiconductor structure 1101 is a plurality of non-continuous contacts 1115a-1115n. Each of the non-continuous contacts 1115 includes an optional contact layer 1116 coupled to the back side of the semiconductor structure 1101 and a p-metal contact 1118 underneath contact layer 1116. An optional ARC layer 1120 may also be present on the back side of the device. These may also be a textured layer 1121, between the ARC 1120 and the p-layer 1114.
Similar to FIG. 1, the contact layers can contain Group III-V materials, such as gallium arsenide (GaAs), depending on the desired composition of the final photovoltaic unit. According to embodiments described herein, the contact layers may be heavily n-doped. In some embodiments, the doping concentration may be within a range greater than about 5×1018 cm−3, for example, from greater than about 5×1018 cm−3 to about 1×1019 cm−3. The high doping of the contact layers of the cell 300 allows an ohmic contact to be formed with a later-deposited metal layer without any annealing step performed to form such an ohmic contact.
An additional p-n junction structure 601 of higher bandgap is included in this device. Structure 601 is comprised of a window layer 602 (for example, AllnP, AlGaInP, or AlGaAs), an n-type material 604 (for example, InGaP or AlGaAs), a p-type material 606 (for example, InGaP or AlGaAs), and back-surface field or back side window layer 608 (for example, AllnP, AlGaInP, or AlGaAs). This structure is electrically and optically connected to structure 1101 through a tunnel junction structure 1131. Structure 1131 is comprised of a highly p-type doped layer 1102 (for example, InGaP or AlGaAs), and a highly n-type doped layer 1104 (for example, InGaP or AlGaAs).
One of ordinary skill in the art readily recognizes that a variety of materials listed could differ from the examples listed herein. Furthermore, the p-n junction formed in structure 601 could be a homojunction or a heterojunction, that is, both the n-layer 604 and p-layer 606 could be the same material, or could be different materials, and that would be within the spirit and scope of the present invention. Also the doping could be inverted, with p-type material at the top of the device, facing the sun, and n-type material at the bottom. One or more additional p-n structures could be added to structure 1101 in a similar fashion, either above or below structure 1101, and could be possibly coupled to the rest of the device through a tunnel junction layer or layers.
Copending application Ser. No. 13/223,187, entitled “Photon Recycling in an Optoelectronic Device”, which is incorporated herein in its entirety by reference, describes in detail the advantages of the properties that make for effective photon recycling in a single junction optoelectronic device. Accordingly, in an embodiment, the process of photon recycling within the multi-junction optoelectronic device is utilized to enhance its operation.
Photon recycling is the process by which a photon is absorbed, or generated within the semiconductor layers of an optoelectronic device, such as a photovoltaic device, so that the device can generate an electron-hole pair which then radiatively recombines to create another photon. This photon can then create another electron-hole pair, and so on. Under open-circuit conditions, this process can repeat itself many times—this is photon recycling. For an optoelectronic device this can create a much higher probability that photo-generated carriers are collected, increasing the effective lifetime of the device. Similarly, for a device such as an LED, this can greatly increase the probability that generated photons will escape the semiconductor.
Photon recycling requires a device with very low carrier losses to non-radiative recombination processes in the semiconductor, and very low photon losses due to processes that do not allow the photons to escape out through the front of the device. As such, it is associated with highly-efficient devices in general, particularly devices that have very low dark-current. For an optoelectronic device under open-circuit conditions, the carrier density within the device can be greatly increased due to the recycling as described above, which in turn will lead to a greatly increased Voc. Indeed, in terms of an electrical output, a high Voc is the primary signature of photon recycling. Photon recycling can also boost other performance metrics of the device, such as the maximum-power operating voltage Vmax, the associated current density Jmax, the short-circuit current density Jsc, as well as the overall device efficiency.
To describe the operation of a multi-junction optoelectronic device in more detail refer now to the following discussion in conjunction with the accompanying FIG. 5.
FIG. 5 illustrates the photon recycling operation in a multi-junction optoelectronic device 1200. The device 1200 is a simplified version of device 1100 of FIG. 3. The device 1200 includes a top cell 1204 and a bottom cell 1208. A tunnel junction 1206 is located between the top cell 1204 and the bottom cell 1208. A first metal layer 1202 is coupled to the top cell 1204 and a textured reflector layer 1210 is underneath the bottom cell 1208. A dielectric layer 1212 is located under the textured reflector layer 1210. A second metal layer 1214 is located under the dielectric layer 1212. Finally, a backing material 1216 is under the second metal layer 1214.
In the multi-junction optoelectronic device 1200, incident sunlight is provided as shown by arrow 1. As is well understood, sunlight has multiple wavelengths, many of which will be absorbed by the top cell 1204. However, the wavelengths that are not absorbed by the top cell 1204 will enter the bottom cell 1208 as indicated by arrow 2. In an embodiment, the top cell 1204 has a higher energy than the bottom cell 1208. In principle, if the material and quality is fine enough, a large density of these photons can be generated at the wavelength specified by the bandgap of the material, in both the top and bottom cells 1204 and 1208.
Arrows 4-12 represent photons that are present within the device 1200 during operation of the device 1200. The photon recycling operation will be described in more detail herein below. The photons represented by arrows 4 and 9 are a typical radiative recombination, so the incident sunlight generates an electron hole 3, and an electrical current is generated therefrom defined by the bandgap of the top cell 1204 or bottom cell 1208. As is seen, photons as represented by arrows 5 and 10 result from electron hole recombination, proceed out of the top cell 1204 and bottom cell 1208, respectively. Photons can also reflect off the top and of the bottom surfaces of the top and bottom cells 1204 and 1208 as the photons represented by arrows 7,8,11 and 12, respectively. The photon represented by arrow 12 is reflected by the textured reflector layer 1210 such that the angle of incidence of the reflector 1210 causes the photon to be reflected in a different angle.
The photon represented by arrow 7 shows light that hits the top cell 1204 first but, unlike the photon represented by arrow 5, the photons represented by arrow 7 is trapped by the total internal reflection caused by the refractive index contrast between the top cell 1204 and the air. The photon represented by arrow 8 shows a similar phenomenon at the back side of the cell 1204. The tunnel junction 1206 could, in principle, have maximum reflectivity for light at the bandgap wavelength of the top cell 1204. If some additional factor is not added, the photons represented by arrows 8 and 11 are going to be fairly weak, because there is not much reflectivity off of the tunnel junction 1206.
Therefore, the tunnel junction 1206 must be thin and with a high bandgap, as transparent as possible, so that as little light is lost to the tunnel junction 1206 as possible, in which case the photon represented by arrow 6 can go through to the bottom cell 1208. Therefore the photon represented by arrow 6 would be absorbed because it has a higher energy than the bandgap energy of the bottom cell 1208. In other words, recombination that results in a photon represented by arrow 6 is not wasted, even though it is not generating current in the top cell 1204, because it sends current to the bottom cell 1208.
In a system in accordance with the present invention, one or more optical filters can be inserted into the tunnel junction 1206 of the multi-junction photovoltaic device 1200 or added on top of tunnel junction 1206. The one or more optical filters are specific to the angle and wavelength of the light, such that each of them are engineered to reflect photons with energy greater than the bandgap energy of the p-n junction material below it, while at the same time transmitting photons of lower energy.
The one or more optical filters may be implemented as a part of the deposition process of the active material, such as during the epitaxial growth process. This may consist of multiple thin layers of compatible materials with different refractive indexes, all with band gaps above the upper junction to avoid absorption of relevant light. The contrast in the refractive index may be combined with specific layer thickness so that the structure is anti-reflective at the normal angle through destructive interference of the reflected light over a relative broad wavelength range in order to allow the transmission of the incident light. The reflections are made constructive when the light is sufficiently away from the normal angle beyond the total-internal-reflection escape cone angle of the optoelectronic device at the wavelength of the band gap of the upper junction, so that photon recycling is obtained. In addition, total internal reflection from the filter layers can also be used to enhance the reflection of off-normal light.
Ideally the recycled photons of the top cell 1204 will stay inside the top cell 1204, because they provide higher energy if the photons are in the top cell 1204. At the same time, the lower energy photons still need to get through. This is because the top cell 1204 is more or less transparent to the recycled photons of the bottom cell 1208. Depending on what angle the light is coming out, it is possible to lose the recycled photons. Also, ideally the recycled photons of the bottom cell 1208 will stay inside of the bottom cell 1208.
One problem with the light trapping is that if the cell 1208 is too thin, the incident light from the large arrows 1 and 2 will largely just bounce straight back off the reflector 1210 and straight back out the front of the cell 1208. The textured reflection layer or light-scattering layer 1210 of this embodiment can be utilized so that the light can be bounced sideways into the cell 1208 as opposed to bouncing straight back out of the front of cell 1208.
One of ordinary skill in the art readily recognizes a variety of materials listed could differ from the examples listed herein. Furthermore, the p-n junction formed in structure could be a homojunction or a heterojunction, that is, both the n-layer and p-layer could be the same material, or could be different materials, and that would be within the spirit and scope of the present invention. Also the doping could be inverted, with p-type material at the top of the device, facing the sun, and n-type material at the bottom. One or more additional p-n structures could be added to the structure in a similar fashion, either above or below structure, and possibly coupled to the rest of the device through a tunnel junction layer or layers.
While the foregoing is directed to embodiments of the inventions, other and further embodiments of the inventions may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. Accordingly, the figures are intended to be illustrative rather than definitive or limiting. In particular many design elements could change, including but not limited to:
the optoelectronic device could be p-on-n rather than n-on-p,
the structure could include more than two junctions,
the optoelectronic device could be a homojunction,
the tunnel junctions could be made of GaAs or InGaP or other material,
other layers within the cell could be exchanged with different materials, e.g. AlGaAs or AlGaInP instead of AllnP, etc., and
the reflector could be purely metal as well as a dielectric plus metal.
Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.

## Claims

1. An optoelectronic semiconductor device comprising:
a first side and a second side, wherein the device is configured to receive light at the second side of the device;
a plurality of stacked p-n layers between the first side and the second side of the device, each of the plurality of stacked p-n layers comprising a p-doped layer and an n-doped layer, with a p-n junction between the p-doped layer and the n-doped layer such that at least one of the plurality of stacked p-n layers generates electrical energy when photons are absorbed by that p-n layer in response to the device being exposed to a light source on the second side of the device;
wherein each p-n layer has a first side and a second side, and the first side of each p-n layer is closer than its second side to the first side of the device;
wherein the p-n junction within each of the at least one electrical energy generating p-n layers is located closer to the first side of that p-n layer than it is to the second side of that p-n layer, and the p-n junction in the at least one electrical energy generating p-n layer that is located closest to the first side of the device is a heterojunction;
a window layer disposed closer to the second side of the device than each of the plurality of stacked p-n layers; and
a reflector disposed closer to the first side of the device than the plurality of stacked p-n layers, which provides for photons to be redirected to the plurality of stacked p-n layers to be absorbed and converted into electrical energy.

2. The optoelectronic device of claim 1, wherein the reflector comprises a dielectric reflector.

3. The optoelectronic device of claim 1, wherein the reflector comprises a textured reflector.

4. The optoelectronic device of claim 1 wherein the reflector has reflectivity such that light trapping, leading to enhanced photon recycling, is enabled and the performance including the open circuit voltage of the device is enhanced.

5. The optoelectronic device of claim 1 where the polarity of all layers is reversed.

6. An optoelectronic semiconductor device comprising:
a first side and a second side, wherein the device is configured to receive light at the second side of the device;
a plurality of stacked p-n layers, between the first side and the second side of the device, each of the plurality of stacked p-n layers comprising a p-doped layer and an n-doped layer, with a p-n junction between the p-doped layer and the n-doped layer such that at least one of the plurality of stacked p-n layers generates electrical energy when photons are absorbed by that p-n layer in response to the device being exposed to a light source on the second side of the device;
wherein each p-n layer has a first side and a second side, and the first side of each p-n layer is closer than its second side to the first side of the device;
wherein the p-n junction within each of the at least one electrical energy generating p-n layers is located closer to the first side of that p-n layer than it is to the second side of that p-n layer;
a window layer disposed closer to the second side of the device than each of the plurality of stacked p-n layers; and
a heterojunction is formed within one or more of the p-n layers.

7. The optoelectronic device of claim 6, wherein the one or more heterojunction is at a location offset from its corresponding p-n junction.

8. The optoelectronic device of claim 7, wherein the location offset of the one or more heterojunction from its corresponding p-n junction is provided by an intermediate layer located between the p-doped layer and the n-doped layer, the intermediate layer being of the same material as either the n-doped layer or the p-doped layer, or being comprised of a different material, or being a layer of graded composition.

9. The optoelectronic device of claim 6, wherein the one or more heterojunction is located within two depletion lengths of its corresponding p-n junction.

10. The optoelectronic device of claim 6, where the polarity of all layers is reversed.

11. An optoelectronic semiconductor device comprising:
a first side and a second side, wherein the device is configured to receive light at the second side of the device;
a plurality of stacked p-n layers, between the first side and the second side of the device, each of the plurality of stacked p-n layers comprising a p-doped layer and an n-doped layer, with a p-n junction between the p-doped layer and the n-doped layer such that at least one of the plurality of stacked p-n layers generates electrical energy when photons are absorbed by that p-n layer in response to the device being exposed to a light source on the second side of the device;
wherein each p-n layer has a first side and a second side, and the first side of each p-n layer is closer than its second side to the first side of the device;
wherein the p-n junction within each of the at least one electrical energy generating p-n layer is located closer to the first side of that p-n layer than it is to the second side of that p-n layer, and the p-n junction in the at least one electrical energy generating p-n layer that is located closest to the first side of the device is a heterojunction;
a heavily doped front contact layer disposed on the plurality of stacked p-n junctions;
at least one metal layer disposed on the front contact layer; and
wherein an ohmic contact is formed between the at least one metal layer and the front contact layer without annealing being necessary to achieve the ohmic contact.

12. The optoelectronic device of claim 11, wherein the front contact layer includes gallium arsenide.

13. The optoelectronic device of claim 11, wherein the front contact layer has a doping concentration of at least 5×1018 cm3.

14. The optoelectronic device of claim 11, where the polarity of all layers is reversed.

15. An optoelectronic semiconductor device comprising:
a first side and a second side, wherein the device is configured to receive light at the second side of the device;
a plurality of stacked p-n layers, between the first side and the second side of the device, each of the plurality of stacked p-n layers comprising a p-doped layer and an n-doped layer, with a p-n junction between the p-doped layer and the n-doped layer such that at least one of the plurality of stacked p-n layers generates electrical energy when photons are absorbed by that p-n layer in response to the device being exposed to a light source on the second side of the device;
wherein each p-n layer has a first side and a second side, and the first side of each p-n layer is closer than its second side to the first side of the device;
wherein the p-n junction within each of the at least one electrical energy generating p-n layer is located closer to the first side of that p-n layer than it is to the second side of that p-n layer, and the p-n junction in the at least one electrical energy generating p-n layer that is located closest to the first side of the device is a heterojunction;
a window layer disposed closer to the second side of the device than each of the plurality of stacked p-n layers; and
one or more filter layers that act as optical filters between two or more of the plurality of stacked p-n layers, such that when the second side of the device is exposed to the light source, each of the one or more filter layers is engineered to:
reflect photons with energy greater than or equal to the bandgap energy of the p-n layers between that filter layer and the second side of the device, and
concurrently transmit photons with energy lower than the bandgap energy of the p-n layers between that filter layer and the second side of the device.