LATTICE MATCHABLE ALLOY FOR SOLAR CELLS

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BACKGROUND OF THE INVENTION

[0004] The present invention relates to multij unction solar cells, and in particular to high efficiency solar cells comprised of III-V semiconductor alloys.

[0005] Multijunction solar cells made primarily of III-V semiconductor alloys are known to produce solar cell efficiencies exceeding efficiencies of other types of photovoltaic materials. Such alloys are combinations of elements drawn from columns III and V of the standard Periodic Table, identified hereinafter by their standard chemical symbols, names and abbreviation. (Those of skill in the art can identify their class of semiconductor properties by class without specific reference to their column.) The high efficiencies of these solar cells make them attractive for terrestrial concentrating photovoltaic systems and systems designed to operate in outer space. Multijunction solar cells with efficiencies above 40% under concentrations equivalent to several hundred suns have been reported. The known highest efficiency devices have three subcells with each subcell consisting of a functional p-n junction and other layers, such as front and back surface field layers. These subcells are connected through tunnel junctions, and the dominant layers are either lattice matched to the underlying substrate or are grown over metamorphic layers. Lattice-matched devices and designs are desirable because they have proven reliability and because they use less semiconductor material than metamorphic solar cells, which require relatively thick buffer layers to accommodate differences in the lattice constants of the various materials. As set forth more fully in U.S. Patent Application No. 12/217818, entitled "GalnNAsSb Solar Cells Grown by Molecular Beam Epitaxy," which application is incorporated herein by reference, a layer made of GalnNAsSb material to create a third junction having a band gap of approximately 1.0 eV offers a promising approach to improving the efficiency of

multijunction cells. Improvements are nevertheless to be considered on the cell described in that application.

[0006] The known highest efficiency, lattice-matched solar cells typically include a monolithic stack of three functional p-n junctions, or subcells, grown epitaxially on a germanium (Ge) substrate. The top subcell has been made of (Al)GalnP, the middle one of (In)GaAs, and the bottom junction included the Ge substrate. (The foregoing nomenclature for a III-V alloy, wherein a constituent element is shown parenthetically, denotes a condition of variability in which that particular element can be zero.) This structure is not optimal for efficiency, in that the bottom junction can generate roughly twice the short circuit current of the upper two junctions, as reported by J.F. Geisz et al, "Inverted GalnP / (In)GaAs / InGaAs triple-junction solar cells with low-stress metamorphic bottom junctions," Proceedings of the 33 ^rd IEEE PVSC Photovoltaics Specialists Conference, 2008. This extra current capability is wasted, since the net current must be uniform through the entire stack, a design feature known as current matching. [0007] In the disclosure of above noted U.S. Patent Application No. 12/217,818, it was shown that a material that is substantially lattice matched to Ge or GaAs with a band gap near 1.0 eV might be used to create a triple junction solar cell with efficiencies higher than the structure described above by replacing the bottom Ge junction with a junction made of a different material that produces a higher voltage. [0008] In addition, it has been suggested that the use of this 1 eV material might be considered as a fourth junction to take advantage of the entire portion of the spectrum lying between 0.7 eV (the band gap for germanium) and 1.1 eV (the upper end of the range of bandgaps for the ~1 eV layer). See for example, S. R. Kurtz, D. Myers, and J. M. Olson, "Projected Performance of Three and Four- Junction Devices Using GaAs and GalnP," 26th IEEE Photovoltaics Specialists Conference, 1997, pp. 875-878. Ga _1-x In _x NyAsi _-y has been identified as such a 1 eV material, but currents high enough to match the other subcells have not been achieved, see, e.g. , A. J. Ptak et al, Journal of Applied Physics 98 (2005) 094501. This has been attributed to low minority carrier diffusion lengths that prevent effective photocarrier collection. Solar subcell design composed of gallium, indium, nitrogen, arsenic and various concentrations of antimony (GalnNAsSb) has been investigated with the reported outcome that antimony is helpful in decreasing surface roughness and allowing growth at higher substrate temperatures where annealing is not necessary, but the investigators reported that antimony, even in small concentrations is critical to be avoided as detrimental to adequate device performance. See Ptak et al, "Effects of temperature, nitrogen ion, and antimony on wide depletion width GalnNAs," Journal of Vacuum Science Technology B 25(3) May/June 2007 pp. 955-959. Devices reported in that paper have short circuit currents far too low for integration into multijunction solar cells. Nevertheless, it is known that Gaj. _x In _x N _y Asi _-y-z Sb ₂ with 0.05 < x < 0.07, 0.01 <y < 0.02 and 0.02 < z < 0.06 can be used to produce a lattice-matched material with a band gap of approximately 1 eV that can provide sufficient current for integration into a multijunction solar cell. However, the voltages generated by subcells containing this material have not exceeded 0.30 V under 1 sun of illumination. See D.B. Jackrel et al, Journal of Applied Physics 101 (1 14916) 2007. Thus, a triple-junction solar cell with this material as the bottom subcell has been expected to be only a small improvement upon an analogous triple junction solar cell with a bottom subcell of Ge, which produces an open circuit voltage of approximately 0.25 V. See H. Cotal et al, Energy and Environmental Science 2 (174) 2009. What is needed is a material that is lattice-matched to Ge and GaAs with a band gap near 1 eV that produces an open circuit voltage greater than 0.30 V and sufficient current to match (Al)InGaP and (In)GaAs subcells. Such a material would also be advantageous as a subcell in high efficiency solar cells with 4 or more junctions.

SUMMARY OF THE INVENTION

[0009] According to the invention, an alloy composition is provided that has a bandgap of at least 0.9 eV, namely, Gai _-x In _x N _y As _1-y-z Sb _z with a low antimony (Sb) content and with enhanced indium (In) content and enhanced nitrogen (N) content as compared with known alloys of GalnNAsSb, achieving substantial lattice matching to GaAs and Ge substrates and providing both high short circuit currents and high open circuit voltages in GalnNAsSb subcells suitable for use in multijunction solar cells. The composition ranges for Gai.

xIn _x N _y As _{1 -y-z} Sb _z are 0.07 < x < 0.18, 0.025 <y < 0.04 and 0.001 < z < 0.03. These composition ranges employ greater fractions of In and N in GalnNAsSb than previously taught and allow the creation of subcells with bandgaps that are design-tunable in the range of 0.9 - 1.1 eV, which is the range of interest for GalnNAsSb subcells. This composition range alloy will hereinafter be denoted "low-antimony, enhanced indium-and-nitrogen GalnNAsSb" alloy. Subcells of such an alloy can be grown by molecular beam epitaxy (MBE) and should be able to be grown by metallorganic chemical vapor deposition

(MOCVD), using techniques known to one skilled in the art.

[0010] The invention described herein reflects a further refinement of work described in U.S. Patent Application Serial No. 12/217,818, including the discovery and identification of specific ranges of elements, i.e., a specific alloy mix of the various elements in GalnNAsSb that improve significantly the performance of the disclosed solar cells.

[0011] The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1A is a schematic cross-section of a three junction solar cell incorporating the invention.

[0013] Figure IB is a schematic cross-section of a four junction solar cell incorporating the invention. [0014] Figure 2A is a schematic cross-section of a GalnNAsSb subcell according to the invention.

[0015] Figure 2B is a detailed schematic cross-section illustrating an example GalnNAsSb subcell.

[0016] Figure 3 is a graph showing the efficiency versus band gap energy of subcells formed from different alloy materials, for comparison.

[0017] Figure 4 is a plot showing the short circuit current (J _sc ) and open circuit voltage (VQ _C ) of subcells formed from different alloy materials, for comparison.

[0018] Figure 5 is a graph showing the photocurrent as a function of voltage for a triple junction solar cell incorporating a subcell according to the invention, under 1-sun AM1.5D illumination.

[0019] Figure 6 is a graph showing the photocurrent as a function of voltage for a triple junction solar cell incorporating a subcell according to the invention, under AM1.5D illumination equivalent to 523 suns. [0020] Figure 7 is a graph of the short circuit current (J _sc ) and open circuit voltage (V _oc ) of low Sb, enhanced In and N GalnNAsSb subcells distinguished by the strain imparted to the film by the substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Figure 1 A is a schematic cross-section showing an example of a triple junction solar cell 10 according to the invention consisting essentially of a low Sb, enhanced In and N GalnNAsSb subcell 12 adjacent the Ge, GaAs or otherwise compatible substrate 14 with a top subcell 16 of (Al)InGaP and a middle subcell 18 using (In)GaAs. Tunnel junction 20 is between subcells 16 and 18, while tunnel junction 22 is between subcells 18 and 12. Each of the subcells 12, 16, 18 comprises several associated layers, including front and back surface fields, an emitter and a base. The named subcell material (e.g., (In)GaAs) forms the base layer, and may or may not form the other layers.

[0022] Low Sb, enhanced In and N GalnNAsSb subcells may also be incorporated into multij unction solar cells with four or more junctions without departing from the spirit and scope of the invention. Figure IB shows one such four-junction solar cell 100 with a specific low Sb, enhanced In and N GalnNAsSb subcell 12 as the third junction, and with a top subcell 16 of (Al)InGaP, a second subcell 18 of (In)GaAs and a bottom subcell 140 of Ge, which is also incorporated into a germanium (Ge) substrate. Each of the subcells 16, 18, 12, 140 is separated by respective tunnel junctions 20, 22, 24, and each of the subcells 16, 18, 12, 140 may comprise several associated layers, including optional front and back surface fields, an emitter and a base. The named subcell material (e.g., (In)GaAs) forms the base layer, and may or may not form the other layers.

[0023] By way of further illustration, Fig. 2A is a schematic cross-section in greater detail of a GalnNAsSb subcell 12, according to the invention. The low Sb, enhanced In and N GalnNAsSb subcell 12 is therefore characterized by its use of low Sb, enhanced In and N GalnNAsSb as the base layer 220 in the subcell 12. Other components of the GalnNAsSb subcell 12, including an emitter 26, an optional front surface field 28 and back surface field 30, are preferably III-V alloys, including by way of example GalnNAs(Sb), (In)(Al)GaAs, (Al)InGaP or Ge. The low Sb, enhanced In and N GalnNAsSb base 220 may either be p-type or n-type, with an emitter 26 of the opposite type.

[0024] To determine the effect of Sb on enhanced In and N GalnNAsSb subcell performance, various subcells of the type (12) of the structure shown in Fig. 2B were investigated. Figure 2B is a representative example of the more general structure 12 in Fig. 2A. Base layers 220 with no Sb, low Sb (0.001 < z < 0.03) and high Sb (0.03 < z < 0.06) were grown by molecular beam epitaxy and were substantially lattice-matched to a GaAs substrate (not shown). These alloy compositions were verified by secondary ion mass spectroscopy. The subcells 12 were subjected to a thermal anneal, processed with generally known solar cell processing, and then measured under the AM1.5D spectrum (1 sun) below a filter that blocked all light above the GaAs band gap. This filter was appropriate because a GalnNAsSb subcell 12 is typically beneath an (In)GaAs subcell in a multijunction stack (e.g., Figs. 1 A and I B), and thus light of higher energies will not reach the subcell 12. [0025] Figure 3 shows the efficiencies produced by the subcells 12 grown with different fractions of Sb as a function of their band gaps. The indium and nitrogen concentrations were each in the 0.07 to 0.18 and 0.025 to 0.04 ranges, respectively. It can be seen that the low Sb, enhanced In and N GalnNAsSb subcells (represented by triangles) have consistently higher subcell efficiencies than the other two candidates (represented by diamonds and squares). This is due to the combination of high voltage and high current capabilities in the low Sb, enhanced In and N GalnNAsSb devices. (See Figure 4). As can be seen in Figure 4, both the low and high concentration Sb devices have sufficient short-circuit current to match high efficiency (Al)InGaP subcells and (In)GaAs subcells (> 13 mA/cm ² under the filtered AMI .5D spectrum), and thus they may be used in typical three-junction or four-jur.ction solar cells 10, 100 without reducing the total current through the entire cell. This current-matching is essential for high efficiency. The devices without Sb have relatively high subcell efficiencies due to their high open circuit voltages, but their short circuit currents are too low for high efficiency multijunction solar cells, as is shown in Figure 4.

[0026] Figure 4 also confirms that Sb has a deleterious effect on voltage, as previously reported for other alloy compositions. However, in contrast to what has been previously reported for other alloy compositions, the addition of antimony does NOT decrease the short circuit current. The low Sb-type subcells have roughly 100 mV higher open-circuit voltages than the high Sb-type subcells. To illustrate the effect of this improvement, a triple-junction solar cell 10 with an open circuit voltage of 3.1 V is found to have 3.3% higher relative efficiency compared to an otherwise identical cell with an open circuit voltage of 3.0 V. Thus, the inclusion of Sb in GalnNAs(Sb) solar cells is necessary to produce sufficient current for a high efficiency solar cell, but only by using low Sb (0.1 -3%) can both high voltages and high currents be achieved. [0027] Compressive strain improves the open circuit voltage of low Sb, enhanced In and N GalnNAsSb subcells 10, 100. More specifically, low Sb, enhanced In and N GalnNAsSb layers 220 that have a lattice constant larger than that of a GaAs or Ge substrate when fully relaxed (<0.5% larger), and are thus under compressive strain when grown

pseudomorphically on those substrates. They also give better device performance than layers with a smaller, fully relaxed lattice constant (under tensile strain).

[0028] Figure 7 shows the short circuit current and open circuit voltage of low Sb, enhanced In and N GalnNAsSb subcells grown on GaAs substrates under compressive strain (triangles) and tensile strain (diamonds). It can be seen that the subcells under compressive strain have consistently higher open circuit voltages than those under tensile strain.

[0029] Low Sb, enhanced In and N, compressively-strained GalnNAsSb subcells have been successfully integrated into high efficiency multijunction solar cells. Figure 5 shows a current-voltage curve of a triple-junction solar cell of the structure in Figure 1 A under AM1.5D illumination equivalent to 1 sun. The efficiency of this device is 30.5%. Figure 6 shows the current- voltage curve of the triple junction solar cell operated under a

concentration equivalent to 523 suns, with an efficiency of 39.2%.

[0030] The invention has been explained with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in the art. It is therefore not intended for the invention to be limited, except as indicated by the appended claims.