LATTICE MATCHABLE ALLOY FOR SOLAR CELLS

Abstract

An alloy composition for a subcell of a solar cell is provided that has a bandgap of at least 0.9 eV, namely, Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z with a low antimony (Sb) content and with enhanced indium (In) content and enhanced nitrogen (N) content, achieving substantial lattice matching to GaAs and Ge substrates and providing both high short circuit currents and high open circuit voltages in GaInNAsSb subcells for multijunction solar cells. The composition ranges for Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z are 0.07x0.18, 0.025y0.04 and 0.001z0.03.

Claims

1. An electron generating junction comprising a semiconductor alloy composition, wherein the semiconductor alloy composition is Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, wherein, the content values for x, y, and z are within composition ranges as follows: 0.07x0.18, 0.025y0.04 and 0.001z0.03; the content levels are selected such that the semiconductor alloy composition exhibits a bandgap from 0.9 eV to 1.1 eV; and a short circuit current density Jsc greater than 13 mA/cm.sup.2 and an open circuit voltage Voc greater than 0.3 V when illuminated with a filtered 1 sun AM1.5D spectrum in which all light having an energy greater than the bandgap of GaAs is blocked.

2. The electron generating junction of claim 1, wherein the semiconductor alloy composition is characterized by a thickness from 1 m to 2 m.

3. The electron generating junction of claim 1, wherein the semiconductor alloy composition is characterized by a thickness greater than 1 m.

4. The electron generating junction of claim 1, wherein the semiconductor alloy composition is substantially lattice matched to GaAs.

5. The electron generating junction of claim 1, wherein the semiconductor alloy composition is substantially lattice matched to Ge.

6. The electron generating junction of claim 1, wherein the semiconductor alloy composition is n-doped.

7. The electron generating junction of claim 1, wherein the semiconductor alloy composition is p-doped.

8. The electron generating junction of claim 1, wherein the semiconductor alloy composition is in the form of a layer of semiconductor material.

9. The electron generating junction of claim 1, wherein the content values are selected such that the semiconductor alloy composition is lattice matched to GaAs or Ge.

10. A diode comprising the electron generating junction of claim 1.

11. A photodiode comprising the electron generating junction of claim 1.

12. A photodetector comprising the electron generating junction of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1A is a schematic cross-section of a three junction solar cell incorporating the invention.

[0011] FIG. 1B is a schematic cross-section of a four junction solar cell incorporating the invention.

[0012] FIG. 2A is a schematic cross-section of a GaInNAsSb subcell according to the invention.

[0013] FIG. 2B is a detailed schematic cross-section illustrating an example GaInNAsSb subcell.

[0014] FIG. 3 is a graph showing the efficiency versus band gap energy of subcells formed from different alloy materials, for comparison.

[0015] FIG. 4 is a plot showing the short circuit current (J.sub.sc) and open circuit voltage (V.sub.oc) of subcells formed from different alloy materials, for comparison.

[0016] FIG. 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.

[0017] FIG. 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.

[0018] FIG. 7 is a graph of the short circuit current (J.sub.sc) and open circuit voltage (V.sub.oc) of low Sb, enhanced In and N GaInNAsSb subcells distinguished by the strain imparted to the film by the substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0019] FIG. 1A 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 GaInNAsSb 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.

[0020] Low Sb, enhanced In and N GaInNAsSb subcells may also be incorporated into multijunction solar cells with four or more junctions without departing from the spirit and scope of the invention. FIG. 1B shows one such four-junction solar cell 100 with a specific low Sb, enhanced In and N GaInNAsSb 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.

[0021] By way of further illustration, FIG. 2A is a schematic cross-section in greater detail of a GaInNAsSb subcell 12, according to the invention. The low Sb, enhanced In and N GaInNAsSb subcell 12 is therefore characterized by its use of low Sb, enhanced In and N GaInNAsSb as the base layer 220 in the subcell 12. Other components of the GaInNAsSb 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 GaInNAs(Sb), (In)(Al)GaAs, (Al)InGaP or Ge. The low Sb, enhanced In and N GaInNAsSb base 220 may either be p-type or n-type, with an emitter 26 of the opposite type.

[0022] To determine the effect of Sb on enhanced In and N GaInNAsSb subcell performance, various subcells of the type (12) of the structure shown in FIG. 2B were investigated. FIG. 2B is a representative example of the more general structure 12 in FIG. 2A. Base layers 220 with no Sb, low Sb (0.001z0.03) and high Sb (0.03z0.06) 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 GaInNAsSb subcell 12 is typically beneath an (In)GaAs subcell in a multijunction stack (e.g., FIGS. 1A and 1B), and thus light of higher energies will not reach the subcell 12.

[0023] FIG. 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 GaInNAsSb 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 GaInNAsSb devices. (See FIG. 4). As can be seen in FIG. 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.sup.2under the filtered AM1.5D spectrum), and thus they may be used in typical three junction or four-junction 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 FIG. 4.

[0024] FIG. 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 GaInNAs(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.

[0025] Compressive strain improves the open circuit voltage of low Sb, enhanced In and N GaInNAsSb subcells 10, 100. More specifically, low Sb, enhanced In and N GaInNAsSb 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).

[0026] FIG. 7 shows the short circuit current and open circuit voltage of low Sb, enhanced In and N GaInNAsSb 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.

[0027] Low Sb, enhanced In and N, compressively-strained GaInNAsSb subcells have been successfully integrated into high efficiency multijunction solar cells. FIG. 5 shows a current-voltage curve of a triple junction solar cell of the structure in FIG. 1A under AM1.5D illumination equivalent to 1 sun. The efficiency of this device is 30.5%. FIG. 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%.

[0028] 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.