Multijunction solar cells having a graded-index structure

Abstract

A multijunction solar cells that include one or more graded-index structures disposed directly above the growth substrate beneath a base layer of a solar subcells. In some embodiments, the graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through a solar cell above the graded-index reflector structure is reflected back into the solar subcell by the graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the solar cell above the graded-index reflector structure is transmitted through the graded-index reflector structure to layers disposed beneath the graded-index reflector structure. The second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range.

Claims

1. A multijunction solar cell comprising: an upper solar subcell having an emitter layer and a base layer forming a photoelectric junction, the base layer having a bottom surface; a first graded-index reflector structure disposed beneath the base layer of the upper solar subcell, wherein the first graded-index reflector structure comprises: a first plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a first different semiconductor material, wherein 0<x<1, and 0<y≤0.3 and a mole fraction of aluminum is increased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases; and a second plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As, and a second different semiconductor material, 0<x<1, and 0<y≤0.3 wherein a mole fraction of aluminum is decreased, for each of the plurality of pairs of In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the second plurality of pairs of alternating layers, as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases; and the multijunction solar cell further comprises a lower solar subcell disposed beneath the first graded-index reflector structure, wherein the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction.

2. The multijunction solar cell of claim 1 further comprising a second graded-index reflector structure disposed beneath the base layer of the lower solar subcell; wherein the second graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the lower solar subcell is reflected back into the lower solar subcell by the second graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the lower solar subcell is transmitted through the second graded-index reflector structure to layers disposed beneath the second graded-index reflector structure, wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range.

3. The multijunction solar cell of claim 1, wherein the band gap energy of the first graded-index reflector structure is greater than the band gap energy of the upper solar subcell, and the first graded-index reflector structure is composed of materials that form an optical wavelength reflector structure and the materials are selected and constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the upper solar subcell is reflected back into the upper solar subcell by the first graded-index reflector structure; (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the upper solar subcell is transmitted through the first graded-index reflector structure to layers disposed beneath the first graded-index reflector structure, wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range so that photons of the first spectral wavelength range which have not been absorbed by the upper solar subcell are reflected back into the upper solar subcell for possible absorption therein, thereby increasing the efficiency of the upper solar subcell.

4. The multijunction solar cell of claim 1, wherein the lower solar subcell is composed of germanium and forms a growth substrate of the multijunction solar cell.

5. The multijunction solar cell of claim 4, further comprising a buffer layer disposed between the growth substrate and the first graded-index reflector structure.

6. The multijunction solar cell of claim 1, wherein the increase in mole fraction of aluminum in the In.sub.yAl.sub.xGa.sub.(1-x-y)As layer in the first plurality of pairs of alternating layers ranges from 20% to 90%.

7. The multijunction solar cell of claim 6, wherein the second different semiconductor material is InAl.sub.xGa.sub.1-xAs, where 0<x<0.16.

8. The multijunction solar cell as defined in claim 1, wherein the solar cell is a four junction solar cell comprising four solar subcells including the upper solar subcell and the lower solar subcell, and the average band gap energy of all four subcells is equal to or greater than 1.35 eV.

9. The multijunction solar cell as defined in claim 1, wherein the upper solar subcell is composed of indium gallium aluminum phosphide, and the emitter layer of the lower solar subcell is composed of indium gallium phosphide or aluminum indium gallium arsenide, and the base layer of the lower solar subcell is composed of aluminum indium gallium arsenide, wherein the lower solar subcell has a band gap energy in the range of 1.55 to 1.8 eV.

10. The multijunction solar cell as defined in claim 9, further comprising a third solar subcell disposed below the lower solar subcell, the third solar subcell being composed of indium gallium arsenide and has a third band gap less than that of the lower solar subcell and is lattice matched with the lower solar subcell.

11. The multijunction solar cell as defined in claim 10, wherein the upper solar subcell has a band gap energy of less than 2.15, the lower solar subcell has a band gap energy of less than 1.73 eV; and the third solar subcell has a band gap energy in the range of 1.15 to 1.2 eV.

12. The multijunction solar cell as defined in claim 1, wherein the base layer of the upper subcell is composed of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.142, corresponding to a band gap energy of 2.10 eV, and the emitter layer of the upper solar subcell is composed of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV.

13. The multijunction solar cell as defined in claim 1, further comprising a third solar subcell disposed below the lower solar subcell, and further comprising a grading layer disposed between the lower solar subcell and the third solar subcell, wherein the grading interlayer is compositionally step-graded with between one and four steps and is composed of In.sub.xGa.sub.1-xAs or (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with 0<x<1, 0<y<1, and x and y selected such that the band gap energy is in the range of 1.15 to 1.41 eV throughout its thickness.

14. The multijunction solar cell as defined in claim 13, wherein the grading interlayer has a graded band gap energy in the range of 1.2 to 1.35 eV.

15. The multijunction solar cell as defined in claim 1, wherein either (i) the emitter layer; or (ii) the base layer and emitter layer, of the upper solar subcell have different lattice constants from the lattice constant of the lower solar subcell.

16. A multijunction solar cell as defined in claim 4, wherein a lattice constant of each pair of alternating layers increases over a lattice constant of the growth substrate.

17. The multijunction solar cell of claim 1, wherein the multijunction solar cell is an upright multijunction solar cell, an upright metamorphic multijunction solar cell, or an inverted metamorphic multijunction solar cell.

18. A multijunction solar cell comprising: an upper solar subcell having an emitter layer and a base layer forming a photoelectric junction; a first graded-index reflector structure disposed beneath the base layer of the upper solar subcell, wherein the first graded-index reflector structure comprises: a first plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a first different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum is increased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases, a second plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a second different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum repeats for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the second plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases; and a third plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a third different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum is decreased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the third plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases; a lower solar subcell disposed beneath the first graded-index reflector structure, wherein the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction.

19. A method of fabricating a multijunction solar cell comprising: forming an upper solar subcell having an emitter layer and a base layer that form a photoelectric junction, the base layer having a bottom surface; forming a lower solar subcell having an emitter layer and a base layer that form a photoelectric junction; and forming a reflector structure using successive layers with a changing or graded index of refraction disposed above the emitter layer of the lower solar subcell and beneath the base layer of the upper solar subcell, wherein forming the reflector structure includes: forming a first plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a first different semiconductor material, wherein 0<x<1, and 0<y≤0.3 and a mole fraction of aluminum is increased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases; and forming a second plurality of pairs of alternating layers of In.sub.yAl.sub.xGa.sub.(1-x-y)As, and a second different semiconductor material, 0<x<1, and 0<y≤0.3 wherein a mole fraction of aluminum is decreased, for each of the plurality of pairs of In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the second plurality of pairs of alternating layers, as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 is a graphical illustration of peak reflection vs. number of periods for exemplary DBRs with alternating layers having (i) a larger index of refraction difference (solid line), and (ii) a smaller index of refraction difference (dashed line).

(3) FIG. 2 is a graphical illustration of approximate bandwidth of the reflector for exemplary DBRs with alternating layers having (i) a larger index of refraction difference (solid line), and (ii) a smaller index of refraction difference (dashed line).

(4) FIG. 3 illustrates the structure of a simulated DBR on the left, and the index of refraction change as a function of depth from the rear of the device. The oscillating layers near the front of the device represent the DBR pairs.

(5) FIG. 4 is a plot illustrating simulated reflectance for DBRs with 12 pairs of alternating layers (dashed line) and 8 pairs of alternating layers (solid line) of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As, centered at 815 nm.

(6) FIG. 5 is a plot illustrating the simulated reflectance for a DBR with 12 pairs of alternating layers of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As both without (dashed line) and with (solid line) an overlying 2500 nm thick layer of GaAs that absorbs the majority of incoming light up to 870 nm.

(7) FIG. 6 is an index of refraction profile for a graded-index reflector structure with 20 pairs of alternating layers of Al.sub.xGa.sub.(1-x)As/Al.sub.15Ga.sub.85As, wherein 0<x<1, and x is gradually increased and decreased on each side of the center 6 pairs of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As.

(8) FIG. 7 is a plot of the simulated reflectance for a DBR with 12 pairs of alternating layers of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As (solid line) compared with a graded-index reflector structure with 20 pairs of alternating layers of Al.sub.xGa.sub.(1-x)As/Al.sub.0.15Ga.sub.0.85As, wherein 0<x<1, and x is gradually increased and decreased by 10% for each pair on both sides of the center 6 pairs of alternating layers of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As (dashed line).

(9) FIG. 8 is a plot of the simulated reflectance for the DBR (dashed line) and the graded-index reflector structures (solid line) of FIG. 7 after an absorbing 2500 nm thick GaAs cell is deposited above the reflective layers. Note the large reduction in side lobe reflection losses.

(10) FIG. 9 is a schematic representation of an exemplary multijunction solar cell having a graded-index reflector structure according to the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(11) Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale.

(12) A variety of different features of multijunction solar cells (as well as inverted metamorphic multijunction solar cells) are disclosed in the related applications noted above. Some, many or all of such features may be included in the structures and processes associated with the “upright” solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a multijunction solar cell grown on a single growth substrate. More specifically, in some embodiments, the present disclosure relates to four junction solar cells with direct band gaps in the range of 2.0 to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eV or less, for the middle subcells, and (iii) 0.6 to 0.8 eV indirect bandgaps for the bottom subcell, respectively.

(13) The present disclosure provides an unconventional four junction or multijunction design that leads to a surprising significant performance improvement over that of traditional three or four junction solar cells, even those that incorporate a DBR layer under the second subcell. This performance gain is especially realized at high temperature and after high exposure to space radiation by the proposal of incorporating a superior semiconductor reflective structure under one or more of the subcells, thus specifically addressing the problem of ensuring continues adequate efficiency and power output at the “end-of-life”.

(14) The use of a Distributed Bragg Reflector (or DBR) is known for use in solar cells. A DBR is a specially formulated sequence of thin film semiconductor layers that offers a high degree of reflectance over a specific wavelength range. The central wavelength in free space (air), λ.sub.central, of the reflectivity band can be adjusted by employing two or more materials that have a difference in their optical index of refraction. The reflector may be realized by placing repeating alternating pairs of each material. The thickness of each layer is ¼ of the central wavelength inside the material, or t=λ.sub.central/{4n(λ.sub.central)}, where t is the layer thickness and n(λ.sub.central), is the index of refraction of the layer at the central wavelength.

(15) DBR reflectivity is calculated from Maxwell's equations via the transfer matrix method. It can also be approximated by the following equation:

(16) $R={\left[\frac{{n_o\left(n_2\right)}^{2N}-{n_s\left(n_1\right)}^{2N}}{{n_o\left(n_2\right)}^{2N}+{n_s\left(n_1\right)}^{2N}}\right]}^2,$
where n.sub.0,1,2,s are the refractive indices of the originating material, the two alternating materials, and the substrate material; and N is the number of repeated pairs. The frequency bandwidth, Δf.sub.o, of the reflection band can be approximated by:

(17) $\frac{\Delta f_0}{f_0}=\frac{4}{\pi }\arcsin \left(\frac{n_2-n_1}{n_2+n_1}\right),$
where f.sub.o is the central frequency of the band. Adding and subtracting Δf.sub.o from f.sub.o and converting the two frequencies into wavelength via c=fλ, one arrives at approximate bandwidth of reflector in free space.

(18) FIG. 1 is a graphical illustration of peak reflection vs. number of periods for exemplary DBRs with alternating layers having (i) a larger index of refraction difference (solid line), and (ii) a smaller index of refraction difference (dashed line). FIG. 2 is a graphical illustration of approximate bandwidth of the reflector for exemplary DBRs with alternating layers having (i) a larger index of refraction difference (solid line), and (ii) a smaller index of refraction difference (dashed line).

(19) Increasing the number of pairs in a DBR increases the mirror reflectivity as illustrated in FIG. 1, while increasing the refractive index contrast between the materials in the Bragg pairs increases both the reflectivity (at a given number of periods) and the bandwidth as illustrated in FIG. 1 and FIG. 2. Stated otherwise, a higher index of refraction change may require fewer alternating pairs to reach a high reflectivity and may produce a broader bandwidth, which may be precisely what is desired for economical incorporation into a multijunction solar cell. For fiber optics, the desired bandwidth is extremely narrow so a very small index of refraction change may be preferable, and thus, hundreds of alternating pairs may be used to reach a high degree of reflectivity.

(20) When the transfer matrix method is used to calculate the DBR reflectance as a function of wavelength, a more accurate result may be obtained when dispersive material properties are used (i.e., index of refraction and absorption coefficient as a function of wavelength). Simulated results from the computer program VERTICAL (developed at Sandia National Laboratories for VCSELS, 1996) are illustrated in FIG. 3. FIG. 3 illustrates the structure of a simulated DBR on the left, and the index of refraction change as a function of depth from the rear of the device. The oscillating layers near the front of the device represent the DBR pairs.

(21) The central wavelength of the DBR is designed to be 800 nm in the example. The substrate is GaAs (mole fraction of aluminum=0) and the DBR has 12 periods of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As. In some embodiments, the Al.sub.0.9Ga.sub.0.1As is deposited on the substrate first with an index of refraction n=3.04 and thickness t=67.1 nm. The Al.sub.0.15Ga.sub.0.85As follows, which may have an index of refraction n=3.51 and thickness t=57.8 nm. The pair may be repeated 12 times and be coated with a standard dual layer antireflection coating consisting of TiO.sub.2 t=60 nm and SiO.sub.2 t=100 nm.

(22) The resulting reflectivity of the structure above is shown in FIG. 4 for both 12 pairs and 8 pairs of alternating layers. FIG. 4 is a plot illustrating simulated reflectance for DBRs with 12 pairs of alternating layers (dashed line) and 8 pairs of alternating layers (solid line) of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As, centered at 815 nm. One can see that by increasing the number of pairs, the peak reflectance increases. In addition, the full width half maximum of the reflection band is approximately 100 nm, broadening slightly for the 8-pair device.

(23) If a thick semiconductor layer is deposited above the DBR, most of the incident light may be absorbed up to the bandgap energy of the layer. In some embodiments, any light that is not absorbed by the material will reflect off the DBR, effectively doubling the optical thickness of the semiconductor layer over the wavelength region where the DBR reflectance is near unity. FIG. 5 is a plot illustrating the simulated reflectance for a DBR with 12 pairs of alternating layers of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As both without (dashed line) and with (solid line) an overlying 2500 nm thick layer of GaAs that absorbs the majority of incoming light up to 870 nm. The plot shows that only the remnants of the DBR side lobes are visible above 870 nm.

(24) The numerous reflectance peaks above 870 nm result in a lower transmission of light into the material below the DBR stack. In the case of a triple junction solar cell, where the DBR is deposited beneath the 2.sup.nd (In)GaAs subcell, the 3.sup.rd Ge subcell may not be greatly affected by the resulting light loss since it has an approximately 40% excess in photocurrent. Nevertheless, reducing the side lobe reflection peaks may increase the photocurrent of the Ge subcell and improve the triple junction FF and device performance slightly.

(25) In designs that require a high degree of current matching in all subcells, the resulting side lobe reflection losses may be so problematic that any performance gain made at EOL by the employment of a DBR may be lost by the reduced current in the subcell below. Thus, it is clear that there is a need for an internal reflector that offers the performance of a traditional DBR but does not result in excessive side lobe losses.

(26) Disclosed herein are multijunction solar cells that include a graded-index reflector structure. Compared to a DBR, it has been found that graded-index reflector structures can reduce or eliminate side lobe reflectivity. Thus, graded-index reflector structures are sometimes referred to as apodized reflectors.

(27) Apodized or graded-index reflector structures are sometimes used in the fiber optics industry. In the reflector design of the present disclosure, instead of having repeating pairs of high and low index materials, the index of refraction difference is gradually increased as the number of periods increase. Then, one or more periods of the maximum index difference pairs are layered, followed by a gradual decrease in the index of refraction difference.

(28) FIG. 6 is in one embodiment an index of refraction profile for a graded-index reflector structure with 20 pairs of alternating layers of Al.sub.xGa.sub.(1-x)As/Al.sub.15Ga.sub.85As, wherein 0<x<1, and x is gradually increased and decreased on each side of the center 6 pairs of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As.

(29) In FIG. 7, to demonstrate the improvement over a DBR structure, a 20-pair graded-index reflector structure is compared to a 12-pair traditional DBR. FIG. 7 is a plot of the simulated reflectance for a DBR with 12 pairs of alternating layers of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As (solid line) compared with a graded-index reflector structure with 20 pairs of alternating layers of Al.sub.xGa.sub.(1-x)As/Al.sub.0.15Ga.sub.0.85As, in one embodiment, wherein 0<x<1, and x is gradually increased and decreased by 10% for each pair on both sides of the center 6 pairs of alternating layers of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As (dashed line). The mole fraction of aluminum of the lower index Al.sub.0.2Ga.sub.0.8As is increased by 10% for each pair from Al.sub.0.2Ga.sub.0.8As/Al.sub.0.15Ga.sub.0.85As to Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As. Then, 5 pairs of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.15Ga.sub.0.85As are repeated to increase the maximum reflectivity value. Finally, the mole fraction of aluminum is decreased by 10% for each pair as before to result in the graded-index profile. In some embodiments, the result is a reflectivity profile that has the same peak as the 12-period DBR, a slightly narrower bandwidth, and most importantly, no side lobes in the infrared above 870 nm.

(30) If a thick semiconductor layer is deposited above the reflector structure, most of the incident light may be absorbed up to the bandgap energy of the layer. In some embodiments, any light that is not absorbed by the material will reflect off the reflector structure, effectively doubling the optical thickness of the semiconductor layer over the wavelength region where the graded-index reflectance is near unity. FIG. 8 is a plot of the simulated reflectance for the DBR (dashed line) and the graded-index reflector structures (solid line) of FIG. 7 after an absorbing 2500 nm thick GaAs solar subcell is deposited above the reflective layers. Note the large reduction in side lobe reflection losses. Note the large reduction in side lobe reflection losses. The plot shows that only the remnants of the DBR side lobes are visible above 870 nm.

(31) The lack of side lobes opens up the opportunity to use internal reflectors under multiple subcells, or even every subcell, in a multijunction solar cell.

(32) Turning to the multijunction solar cell device of the present disclosure, FIG. 9 is a cross-sectional view of a first embodiment of a multijunction solar cell 100 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322, such as presented in to the disclosure of U.S. patent application Ser. No. 15/873,135 filed Jan. 17, 2018, or Ser. No. 16/504,828 filed Jul. 8, 2019. For simplicity, only the top two subcells A and B are illustrated in the Figure, but one, two or more additional subcells may be included in the solar cell 100 in other embodiments within the scope of the present disclosure.

(33) As shown in the illustrated example of FIG. 9, the bottom subcell D includes a growth substrate 300 formed of p-type germanium (“Ge”) which also serves as a base layer. A back metal contact pad 350 formed on the bottom of base layer 300 provides the bottom p type polarity electrical contact to the multijunction solar cell 100. The bottom subcell D, further includes, for example, a highly doped n-type Ge emitter layer 301, and an n-type indium gallium arsenide (“InGaAs”) nucleation layer 302. The nucleation layer is deposited over the base layer, and the emitter layer 301 is formed in the substrate 300 by diffusion of dopants into the Ge substrate 300, thereby forming the n-type Ge layer 301. Heavily doped p-type aluminum indium gallium arsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”) tunneling junction layers 304, 303 may be deposited over the nucleation layer to provide a low resistance pathway between the bottom and middle subcells.

(34) In one embodiment, a first alpha layer 504, preferably composed of InGaP or other suitable material, is deposited over the tunnel diode 303/304, to a thickness of between 0.25 and 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the bottom subcell 301/300, or in the direction of growth into the adjacent middle subcell C and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

(35) A metamorphic layer (or graded interlayer) 500 is deposited over the alpha layer 504 (if present) using a surfactant. Layer 500 is preferably a compositionally step-graded series of InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from the bottom subcell 300/301 to the middle subcell C while minimizing threading dislocations from occurring. The band gap of layer 500 is constant throughout its thickness, in the range of 1.22 to 1.34 eV or other appropriate band gap.

(36) In the surfactant assisted growth of the metamorphic layer 500, a suitable chemical element is introduced into the reactor during the growth of layer 500 to improve the surface characteristics of the layer. In one embodiment, such element may be a dopant or donor atom such as selenium (Se) or tellurium (Te). Small amounts of Se or Te are therefore incorporated in the metamorphic layer 500, and remain in the finished solar cell. Although Se or Te are the preferred n-type dopant atoms, other non-isoelectronic surfactants may be used as well.

(37) Surfactant assisted growth results in a much smoother or planarized surface. Since the surface topography affects the bulk properties of the semiconductor material as it grows and the layer becomes thicker, the use of the surfactants minimizes threading dislocations in the active regions, and therefore improves overall solar cell efficiency.

(38) As an alternative to the use of non-isoelectronic one may use an isoelectronic surfactant. The term “isoelectronic” refers to surfactants such as antimony (Sb) or bismuth (Bi), since such elements have the same number of valence electrons as the P atom of InGaP, or the As atom in InGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactants will not typically be incorporated into the metamorphic layer 406.

(39) In one embodiment of the present disclosure, the layer 500 is composed of a plurality of layers of InGaAs, with monotonically changing lattice constant, each layer having the same band gap, approximately in the range of 1.22 to 1.34 eV. In some embodiments, the constant band gap is in the range of 1.28 to 1.29 eV.

(40) The advantage of utilizing a constant bandgap material such as InGaAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors.

(41) Although the described embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer 500 for reasons of manufacturability and radiation transparency, other embodiments of the present disclosure may utilize different material systems to achieve a change in lattice constant from the middle subcell to the bottom subcell 300/301. Other embodiments of the present disclosure may utilize continuously graded, as opposed to step graded, materials. More generally, the graded interlayer may be composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter less than or equal to that of the third solar subcell C and greater than or equal to that of the bottom solar cell and less than or equal to that of the third solar cell, and having a bandgap energy greater than that of the third solar cell.

(42) In some embodiments, a second alpha layer 507, preferably composed of n+ type InGaP with a different composition than the first alpha layer 504, is deposited over metamorphic buffer layer 406, to a thickness from 0.25 to 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the bottom subcell, or in the direction of growth into the middle subcell, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.). A break is shown above alpha layer 507 since other layers may be provided therein as suggested in the related applications noted above, and in the text that follows.

(43) In some embodiments, Distributed Bragg reflector (DBR) layers (now shown) are then grown adjacent to and between the second alpha layer 507 and the adjacent middle solar subcell C as illustrated in U.S. patent application Ser. No. 15/681,144 hereby incorporated by reference. The DBR layers are arranged so that light can enter and pass through the middle solar subcell and at least a portion of which can be reflected back into the middle solar subcell by the DBR layers.

(44) For some embodiments, distributed Bragg reflector (DBR) layers can be composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction. For certain embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.

(45) For some embodiments, distributed Bragg reflector (DBR) layers includes a first DBR layer composed of a plurality of n type or p type Al.sub.xGa.sub.1-xAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of n type or p type Al.sub.yGa.sub.1-yAs layers, where 0<x<1, 0<y<1, and y is greater than x.

(46) In some embodiments, the middle subcell C (not shown) is disposed above alpha layer 507 includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer, a p-type InGaAs base layer, a highly doped n-type indium gallium phosphide (“InGaP2”) emitter layer and a highly doped n-type indium aluminum phosphide (“AlInP2”) window layer. The InGaAs base layer of the subcell C can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer is formed over the BSF layer after the BSF layer is deposited over the DBR layers.

(47) A window layer is deposited on the emitter layer of the subcell C. The window layer in the subcell C also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the subcell B, heavily doped n-type InGaP and p-type AlGaAs (or other suitable compositions) tunneling junction layers may be deposited over the subcell C.

(48) Before depositing the layers of the top cell A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers may be deposited over the subcell B, which are not illustrated to simplify the drawing.

(49) In an embodiment, the top subcell A includes a highly doped p-type indium aluminum phosphide (“InAlP”) BSF layer, a p-type InGaAlP base layer 110, a highly doped n-type InGaAlP emitter layer 105 and a highly doped n-type InAlP2 window layer. The base layer 110 of the top subcell A is deposited over the BSF layer after the BSF layer is formed over the tunneling junction layers of the subcell B. The window layer 160 is deposited over the emitter layer 105 of the top subcell A after the emitter layer 105 is formed over the base layer 110.

(50) A cap or contact layer 161 may be deposited and patterned into separate contact regions over the window layer 160 of the top subcell A. The cap or contact layer 161 serves as an electrical contact from the top subcell A to the metal grid layer 162. The doped cap or contact layer 161 can be a semiconductor layer such as, for example, a GaAs or InGaAs layer.

(51) After the cap or contact layer 161 is deposited, the grid lines 162 are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 161.

(52) It should be understood that the multijunction solar cell 100 can include additional layers above, below, or in between the illustrated layers, and that such additional layers have not been illustrated in FIG. 9 for simplicity and clarity.

(53) As illustrated in FIG. 9, light enters the multijunction solar cell through the top. The multijunction solar cell includes a cover glass upper solar subcell A composed of emitter layer 105 and base layer 110. In some embodiments, the depicted upper solar subcell A can represent and include a wide variety of suitable solar subcells such as silicon subcells or III-V compound semiconductor solar subcells.

(54) A first graded-index reflector structure 150 as described in the present disclosure is disposed beneath base layer 110 of upper solar subcell A.

(55) Lower solar subcell B is disposed beneath the first graded-index reflector structure 150. Lower solar subcell B can be composed of the same materials or different materials than upper solar subcell A. In some embodiments, lower solar subcell B can include a wide variety of suitable solar subcells such as silicon subcells or III-V compound semiconductor solar subcells.

(56) In some embodiments, the multijunction solar cell can optionally include a second graded-index reflector structure 250 as described in the present disclosure disposed beneath base layer 210 of solar subcell B.

(57) For example, in some implementations, a multijunction solar cell includes an upper solar subcell, a first graded-index reflector structure, and a lower solar subcell. The upper solar subcell has an emitter layer and a base layer forming a photoelectric junction, and the base layer has a bottom surface. The first graded-index reflector structure is disposed beneath the base layer of the upper solar subcell and includes a first plurality of pairs of alternating layers and a second plurality of pairs of alternating layers. The first plurality of pairs of alternating layers are of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a first different semiconductor material, wherein 0<x<1, and 0<y≤0.3, and mole fraction of aluminum is increased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases. The second plurality of pairs of alternating layers are of In.sub.yAl.sub.xGa.sub.(1-x-y)As, and a second different semiconductor material, 0<x<1, and 0<y≤0.3, and a mole fraction of aluminum is decreased, for each of the plurality of pairs of In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the second plurality of pairs of alternating layers, as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases. The lower solar subcell is disposed beneath the first graded-index reflector structure, and the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction.

(58) The multijunction solar cell can further include a second graded-index reflector structure disposed beneath the base layer of the lower solar subcell. The second graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the lower solar subcell is reflected back into the lower solar subcell by the second graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the lower solar subcell is transmitted through the second graded-index reflector structure to layers disposed beneath the second graded-index reflector structure, wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range.

(59) In some implementations, the band gap energy of the first graded-index reflector structure is greater than the band gap energy of the upper solar subcell, and the first graded-index reflector structure is composed of materials that form an optical wavelength reflector structure and the materials are selected and constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the upper solar subcell is reflected back into the upper solar subcell by the first graded-index reflector structure; (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the upper solar subcell is transmitted through the first graded-index reflector structure to layers disposed beneath the first graded-index reflector structure. The second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range so that photons of the first spectral wavelength range which have not been absorbed by the upper solar subcell are reflected back into the upper solar subcell for possible absorption therein, thereby increasing the efficiency of the upper solar subcell.

(60) In some implementations, the lower solar subcell is composed of germanium and forms a growth substrate of the multijunction solar cell.

(61) In some implementations, the multijunction solar cell includes a buffer layer disposed between the growth substrate and the first graded-index reflector structure.

(62) In some implementations, the increase in mole fraction of aluminum in the In.sub.yAl.sub.xGa.sub.(1-x-y)As layer in the first plurality of pairs of alternating layers ranges from 20% to 90%. In some implementations, the second different semiconductor material is InAl.sub.xGa.sub.1-xAs, where 0<x<0.16.

(63) In some implementations, the solar cell is a four junction solar cell comprising four solar subcells including the upper solar subcell and the lower solar subcell, and the average band gap energy of all four subcells is equal to or greater than 1.35 eV.

(64) In some implementations, the upper solar subcell is composed of indium gallium aluminum phosphide, and the emitter layer of the lower solar subcell is composed of indium gallium phosphide or aluminum indium gallium arsenide, and the base layer of the lower solar subcell is composed of aluminum indium gallium arsenide, wherein the lower solar subcell has a band gap energy in the range of 1.55 to 1.8 eV.

(65) In some implementations, the multijunction solar cell includes a third solar subcell disposed below the lower solar subcell. The third solar subcell can be composed of indium gallium arsenide, can have a third band gap less than that of the lower solar subcell and can be lattice matched with the lower solar subcell.

(66) In some implementations, the upper solar subcell has a band gap energy of less than 2.15, the lower solar subcell has a band gap energy of less than 1.73 eV, and the third solar subcell has a band gap energy in the range of 1.15 to 1.2 eV.

(67) In some implementations, the base layer of the upper subcell is composed of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.142, corresponding to a band gap energy of 2.10 eV, and the emitter layer of the upper solar subcell is composed of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV.

(68) In some implementations, the multijunction solar cell includes a third solar subcell disposed below the lower solar subcell, and a grading layer disposed between the lower solar subcell and the third solar subcell. The grading interlayer is compositionally step-graded with between one and four steps and is composed of In.sub.xGa.sub.1-xAs or (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with 0<x<1, 0<y<1, and x and y selected such that the band gap energy is in the range of 1.15 to 1.41 eV throughout its thickness. In some instances, the grading interlayer has a graded band gap energy in the range of 1.2 to 1.35 eV.

(69) In some implementations, either (i) the emitter layer; or (ii) the base layer and emitter layer, of the upper solar subcell have different lattice constants from the lattice constant of the lower solar subcell. In some implementations, a lattice constant of each pair of alternating layers increases over a lattice constant of the growth substrate.

(70) In some implementations, a multijunction solar cell includes an upper solar subcell, a first graded-index reflector structure, and a lower solar subcell. The upper solar subcell has an emitter layer and a base layer forming a photoelectric junction. The first graded-index reflector structure is disposed beneath the base layer of the upper solar subcell. The first graded-index reflector structure includes a first plurality of pairs of alternating layers, a second plurality of pairs of alternating layers, and a third plurality of pairs of alternating layers. The first plurality of pairs of alternating layers are of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a first different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum is increased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases. The second plurality of pairs of alternating layers are of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a second different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum repeats for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the second plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases. The third plurality of pairs of alternating layers of are In.sub.yAl.sub.xGa.sub.(1-x-y)As and a third different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum is decreased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the third plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases. The lower solar subcell is disposed beneath the first graded-index reflector structure, and the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction.

(71) A method of fabricating a multijunction solar cell can include, in accordance with some implementations, forming an upper solar subcell having an emitter layer and a base layer that form a photoelectric junction, wherein the base layer has a bottom surface. The method also can include forming a lower solar subcell having an emitter layer and a base layer that form a photoelectric junction. The method further can include forming a reflector structure using successive layers with a changing or graded index of refraction disposed above the emitter layer of the lower solar subcell and beneath the base layer of the upper solar subcell. Forming the reflector structure can include forming a first plurality of pairs of alternating layers and forming a second plurality of pairs of alternating layers. The first plurality of pairs of alternating layers are of In.sub.yAl.sub.xGa.sub.(1-x-y)As and a first different semiconductor material, wherein 0<x<1, and 0<y≤0.3 and a mole fraction of aluminum is increased for each of the In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases. The second plurality of pairs of alternating layers are of In.sub.yAl.sub.xGa.sub.(1-x-y)As, and a second different semiconductor material, 0<x<1, and 0<y≤0.3, wherein a mole fraction of aluminum is decreased, for each of the plurality of pairs of In.sub.yAl.sub.xGa.sub.(1-x-y)As layers in the second plurality of pairs of alternating layers, as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases.

(72) As illustrated in FIG. 9, an antireflective (ARC) dielectric coating layer 163 is disposed over the entire surface of the upper surface of subcell A including over the grid lines 162. As illustrated in FIG. 9, a cover glass 165 is attached to the upper surface of the multijunction solar cell by adhesive 164.

(73) In some embodiments, the multijunction solar cell is an upright multijunction solar cell. In some embodiments, the multijunction solar cell is an upright metamorphic multijunction solar cell. In some embodiments, multijunction solar cell is an inverted metamorphic solar cell.

(74) There can be practical reasons for not including graded-index reflector structures below high bandgap subcells. In some embodiments, a properly designed reflector may be comprised of materials that have slightly higher bandgap energy than the material above. For example, for an upper (In)GaAs subcell with bandgap energy 1.41 eV, a Al.sub.0.15Ga.sub.0.85As layer with a bandgap energy of 1.61 eV as the lowest bandgap material in the reflector may be used. Otherwise, the reflector may absorb the incident light that is desired to be reflected. In some embodiments, for high bandgap materials like InGaP at 1.9 eV, materials are 2.0 eV and higher. In the III-V material system lattice matched to GaAs, bandgap energies only go up to approximately 2.3 eV. In some embodiments, the graded-index reflector structure could be made from AlInGaP and InAlP. However, the change in index of refraction could be only 0.2 between the two materials, making both the bandwidth narrow and the required number of pairs high.

(75) All references and publications cited herein are expressly incorporated herein by reference in their entirety into this disclosure, except to the extent they may directly contradict this disclosure. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof. The disclosed embodiments are presented for purposes of illustration and not limitation.