Abstract:
Photovoltaic cells with one or more subcells are provided with a wide band gap, pseudomorphic window layer of at least 15 nm in thickness and with an intrinsic material lattice constant that differs by at least 1% from an adjacent emitter layer. This window layer has a higher band gap than a window layer with substantially the same intrinsic material lattice constant as the adjacent emitter layer, which increases the light transmission through the window, thereby increasing the current generation in the solar cell. The quality of being pseudomorphic material preserves a good interface between the window and the emitter, reducing the minority carrier surface recombination velocity. A method is provided for building a wide band gap, pseudomorphic window layer of a photovoltaic cell that has an intrinsic material lattice constant that differs by at least 1% from the adjacent emitter layer.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims benefit under 35 USC 119(e) of U.S. provisional Application No. 61/446,704, filed on Feb. 25, 2011, entitled “PSEUDOMORPHIC WINDOW LAYER FOR MULTIJUNCTION SOLAR CELLS,” the content of which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     NOT APPLICABLE 
     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
     NOT APPLICABLE 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention generally relates to photovoltaic solar cells and, more particularly, to high-efficiency multijunction photovoltaic solar cells based on III-V semiconductor compounds. 
     2. Description of Related Art 
     Multijunction solar cells, based on III-V semiconductor compounds, have demonstrated high efficiencies for the generation of electricity from solar radiation. Such cells have reached 35.8% efficiency under the AMOG spectra (http://www.sharp-world.com/corporate/news/091022.html) and 43.5% (see M. A. Green et al., Progress in Photovoltaics: Research and Applications 19 (2011) 565-572) under concentrated sunlight equivalent to several hundred suns. Such efficiency and power achievements make it possible to apply this technology to the space and terrestrial energy markets. The solar cells with the highest efficiencies to date have employed three subcells having differing energy band gaps and arranged to permit each subcell to absorb a different part of the solar spectrum. Each subcell comprises a functional p-n junction and other layers, such as window and back surface field layers. These subcells are connected through tunnel junctions, with the layers either lattice matched to the underlying substrate or grown over metamorphic buffer layers. 
     Each subcell typically includes a window, emitter, base and back surface field (BSF) and may or may not include other layers. Those of skill in the art will also recognize that it is possible to construct subcells that do not contain all of the foregoing layers. The window and the BSF reflect minority carriers away from their interfaces with the emitter and base layers, respectively, and are well known to be critical to high efficiency carrier collection. The materials and doping levels used for windows are chosen such that the band alignment produces a large energy barrier for the minority carriers with a minimal barrier for majority carriers. This allows majority carriers to diffuse through the window, while minority carriers are reflected. It is critical that the interface between the window and the emitter be very high quality, to minimize the minority carrier surface recombination velocity. The window also typically has a higher band gap than the adjacent emitter in order to minimize its absorption of incident light. 
     For the top subcell, the window can be a major source of current loss. The window absorbs a fraction of the incident light in the solar spectrum that is above its band gap, and generates electron-hole pairs, or photocarriers. These photocarriers are not collected with high efficiency due to the high surface recombination velocity for minority carriers at the top of the window, and the low minority carrier diffusion lengths that are common in window materials. In subcells below the top subcell, the band gap of the window need not be as high as in the top subcell, because the top subcell will already have absorbed the higher energy photons. The window layer of lower subcells may be a source of loss if the upper subcell(s) do not absorb all light above the band gap of this window. 
     The intrinsic material lattice constant is defined as the lattice spacing a material would have as a free-standing crystal. When a semiconductor material has a substantially different intrinsic lattice constant than the substrate or the underlying layers on which is grown, the material will initially adopt the lattice constant of the underlying layers. The semiconductor material is strained, however, and the degree of strain is proportional to the difference in intrinsic material lattice constants between the material and the adjacent material on which it is grown. 
     As the thickness of such a semiconductor layer is increased, the accumulated strain increases until a critical thickness is reached, after which it becomes energetically favorable to relax and relieve strain through dislocation, i.e., departure of the atoms from their normal crystalline structures. The critical thickness depends upon many factors, including the materials involved, the substrate and/or underlying layers, growth technique and growth conditions. For a difference in intrinsic material lattice constants of about 1%, U.S. Pat. No. 4,935,384 to Wanlass et al. teaches that the critical thickness is around 15 nm. Below that critical thickness, Wanlass reports that the semiconductor layer is considered pseudomorphic, or fully strained, and the semiconductor layer holds the lattice constant of its substrate or underlying layer in the plane perpendicular to the growth direction. Typically, such a layer will have a different lattice constant in the direction of growth, with all lattice constants different from the material&#39;s intrinsic material lattice constant. The semiconductor layer is considered fully relaxed when sufficient dislocations have formed that the layer has been essentially restored to its intrinsic material lattice constants. In general, layers may be fully strained, fully relaxed, or partially strained and partially relaxed when grown on top of a substrate or layers with a substantially different lattice constant. This discussion has assumed that the materials have a cubic crystal structure in which the intrinsic material lattice constant is the same in all three crystal directions. An analogous discussion is appropriate for materials which are not cubic. 
     The prior art is primarily photovoltaic cells with window layers that have nominally the same intrinsic material lattice constants as the cell layers beneath them. For a given alloy system, choosing the lattice constant fixes the material composition and therefore its relevant properties such as its band-gap energy. For example, fully disordered Al x In 1-x P has substantially the same intrinsic material lattice constant as a GaAs substrate where x=0.52. This composition has an indirect band gap of 2.29 eV and a direct band gap of 2.37 eV at 300K. Strained, pseudomorphic window layers are mentioned by Wanlass et al. and King et al. (U.S. Pat. No. 7,119,271), but, as mentioned above, the teaching is that the critical thickness is 15 nm for a 1% difference in lattice constants. A thickness of 15 nm or less is too thin for practical use in many multijunction solar cells; hence, King et al. focus on window layers that are fully relaxed rather than pseudomorphic. 
     King et al. use a fully relaxed, high-band-gap window layer that incorporates dislocations to achieve relaxation in photovoltaic cells. While relaxation via dislocations have been claimed to improve interface quality and minimize defect transport, the greater body of work in the literature shows that dislocations are non-radiative recombination centers that degrade the quality of the material and reduce its current collection efficiency. In addition, defects at the interface of the emitter and window can increase the surface recombination velocity of the minority carriers and further degrade the solar cell efficiency. Thus, fully relaxed window layers are not ideal for high efficiency solar cells. 
     To improve the efficiency of high efficiency solar cells, it is desirable to maximize the band gap of the window layer of the top subcell, which typically reduces the light absorption in the window and increases the current of the solar cell, while avoiding dislocations that would be produced by relaxation. 
     SUMMARY 
     According to the invention, a photovoltaic solar cell with one or more subcells is provided, wherein at least one subcell has a wide-band-gap, pseudomorphic window layer of at least 15 nm in thickness and with an intrinsic material lattice constant that differs by at least 1% from an adjacent emitter layer. This window layer has a higher band gap than a window layer with substantially the same intrinsic material lattice constant as the adjacent emitter layer, which increases the light transmission through the window, thereby increasing the current generation in the solar cell. The quality of being a pseudomorphic material preserves a good interface between the window and the emitter, reducing the minority carrier surface recombination velocity, resulting in higher efficiency. 
     In a method according to the invention, a wide band gap, pseudomorphic (Al)In(Ga)P window layer of a photovoltaic cell is grown that has a lattice constant that differs by at least 1% from the adjacent emitter layer. The method utilizes growth temperatures of 300-550 degrees Celsius with growth rates of at least 0.1 microns per hour to deposit a layer of 15-60 nm of thickness that is fully strained. The elemental and molecular source material used to grow this layer has at least 99.9999% purity. Molecular beam epitaxy is a preferred technique for depositing the window layer, with a background pressure less than 10 −5  Torr. Relaxation, or the formation of dislocations to accommodate the change in lattice constant, is inhibited by the use of this growth method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic cross-section of an example of a single junction solar cell according to the invention. 
         FIG. 2A  shows a schematic cross-section of a photovoltaic cell with three subcells, where each subcell comprises a window, emitter, base and back surface field connected by tunnel junctions. 
         FIG. 2B  shows a schematic cross-section of one subcell of a multijunction solar cell, according to the invention. 
         FIG. 3  shows the measured external quantum efficiency (EQE), and internal quantum efficiency (IQE) of two GaInP solar cells with no anti-reflection coating (ARC). 
         FIG. 4  shows the measured reflectivity of two GaInP solar cell structures with no anti-reflection coating (ARC). 
       FIG.  5 A 1  shows the short-circuit current (J sc ) calculated using the external and internal quantum efficiency data for the two GaInP solar cells, and the difference in J sc  between the two solar cells. 
         FIG. 5A  shows the measured internal quantum efficiency (IQE) of the GaInP subcells of two multijunction solar cells. 
       FIG.  5 A 2  shows the short-circuit current (J sc ) calculated using the external and internal quantum efficiency data for the two GaInP subcells of multijunction solar cells, and the difference between in J sc  between the two subcells. 
         FIG. 5B  shows the measured external quantum efficiency (EQE) of the GaInP subcells of two multijunction solar cells. 
         FIG. 6  shows the measured reflectivity of the two multijunction solar cells. 
         FIG. 7  shows a triple axis x-ray diffraction scan of a triple junction solar cell with a fully strained AlInP window in the top subcell, according to the invention. 
         FIG. 8  shows a reciprocal space map along the asymmetric direction (224) of a triple junction solar cell on a GaAs substrate with one subcell having a fully-strained Al 0.73 In 0.27 P window according to the invention. 
         FIG. 9  shows a reciprocal space map along the symmetric direction (004) of a triple junction solar cell on a GaAs substrate with one subcell having a fully-strained Al 0.73 In 0.27 P window according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the invention, a photovoltaic cell having one or more subcells is provided, wherein at least one of the subcells has a pseudomorphic window layer that has an intrinsic material lattice constant that differs by at least 1% from the adjacent emitter layer of the subcell. The window layer is on the order of between 15-60 nm in thickness. The window layer has a higher band gap than materials or compositions that have substantially the same intrinsic material lattice constant as the adjacent emitter layer. Having an intrinsic material lattice constant that differs by at least 1% from the adjacent emitter layer, rather than a smaller amount, maximizes the increase in band gap in the window layer. Higher band gaps are desirable because they produce a larger increase in solar cell efficiency, by reducing the fraction of the solar spectrum that can be absorbed by the window layer. 
     As an example,  FIG. 1  shows a schematic cross-section of a photovoltaic cell with one subcell, also known as a single junction solar cell, according to the invention. Layers are not drawn to scale. The cell includes a thin window  12  (e.g., Al x In 1-x P), an emitter  13  (e.g., (Al)GaInP), a base  14  (e.g., GaInP) and back surface field  15  (e.g., AlGaInP), in addition to buffer layer(s)  16 , a substrate  17  and cap  11  and contact layers  10 . The back contact is not shown. The emitter, base and back surface field have substantially the same intrinsic material lattice constant as the substrate. The window layer has an intrinsic material lattice constant that differs from that of the adjacent emitter, as well as the substrate, by at least 1%. This means that subtracting the intrinsic material lattice constant of the window from that of the substrate and dividing this number by the intrinsic material lattice constant of the substrate yields an absolute value of at least 0.01 (or 1%). The window layer is fully strained with no relaxation via dislocations. In one embodiment, the window layer is Al x In 1-x P (x&gt;0.65) of between 15 and 60 nm in thickness, and the substrate is GaAs or Ge. Al x In 1-x P with x&gt;0.65 has a wider band gap than the composition of x=0.52 that has substantially the same intrinsic material lattice constant as the substrate, increasing the transmission of light to the emitter and base and increasing the photovoltaic cell efficiency. The current collection efficiencies in the emitter and base are higher than in the window layer, for reasons discussed above. Increasing the transmission of light to the emitter and base, and decreasing absorption in the window, will increase the solar cell&#39;s overall current collection and efficiency. 
       FIG. 2A  shows a schematic cross-section of an exemplary photovoltaic cell consisting of three subcells (which may be identified as the top cell, middle cell and bottom cell, where the direction references proximity to the light source above), where each subcell consists of a window  23 ,  28 ,  33 , emitter  24 ,  29 ,  34 , base  25 ,  30 ,  35 , and back surface field (BSF)  26 ,  31 ,  36  layer. These subcells are connected by tunnel junctions  27 ,  32 . The substrate  38 , buffer layer(s)  37 , cap  21 , top contact  20  and anti-reflection coating  22  (AR) are also shown. The back contact is not shown.  FIG. 2B  shows a schematic cross-section of a subcell of a photovoltaic cell corresponding to elements  23 ,  24 ,  25  and  26 , according to the invention. The pseudomorphic window layer has a thickness of at least 15 nm and an intrinsic material lattice constant that differs by at least 1% from that of the adjacent emitter layer. In a preferred embodiment of the invention, the window layer is comprised of Al x In 1-x P with x&gt;0.65, where x=0.52 indicates the composition that has substantially the same intrinsic material lattice constant as the Ga 0.51 In 0.49 P emitter and base, and the GaAs or Ge substrate. Compared to x=0.52, the transmission of light to the emitter and base is increased with a window of Al x In 1-x P with x&gt;0.65, which increases the photovoltaic cell efficiency. 
     In some embodiments, the subcell incorporating the invention will be the top subcell of a photovoltaic cell. In this case, the window may be directly adjacent to the anti-reflection coating, as illustrated by window  23  in  FIG. 2A . In this case the window layer may also be considered part of the anti-reflection coating. For the example of an AlInP window, the Al x In 1-x P refractive index helps to reduce the difference between the low refractive indices of the anti-reflection coating layers and the rest of the semiconductor layers in the solar cell structure, which have high refractive indices. 
     The invention provides a method for producing a fully-strained Al x In 1-x-y Ga y P window layer in a photovoltaic cell, where the Al x In 1-x-y Ga y P material has a lattice constant that differs from the lattice constant of the adjacent emitter layer by at least 1%. For example, with a Ga 0.5 In 0.49 P emitter, an Al x In 1-x P window with x&gt;0.65 has an intrinsic material lattice constant that differs from that of the emitter by at least 1%. According to the processing method, growth temperatures are between 300 and 550 degrees Celsius with a growth rate of at least 0.1 microns per hour. The source material for the window consists of elemental aluminum, elemental indium, elemental gallium and molecular phosphorus that are each of at least 99.9999% purity. The background pressure of the reactor is less than 10 −5  Torr. With these nonequilibrium growth conditions, dislocation formation is kinetically limited, so fully strained layers with thicknesses of 15-60 nm may be obtained. In a particular embodiment of the invention, molecular beam epitaxy is used to form the fully strained window layers. 
       FIG. 3  shows the measured external quantum efficiency (EQE), and internal quantum efficiency (IQE) of two photovoltaic cells with the structure shown in  FIG. 1 . The cells have no anti-reflection coatings. The EQE is measured using calibrated detectors and it is verified with a standard reference solar cell. The Al x In 1-x P windows for these structures had thicknesses of approximately 35 nm and had x=0.52 and x=0.70, where Al 0.70 In 0.30 P is a fully strained layer according to the invention. The difference in intrinsic material lattice constant between the windows and the GaAs substrate is ˜0% (x=0.52) and 1.3% (x=0.70). The IQE is obtained from the EQE using the measured reflectivity shown in  FIG. 4  for the two samples, respectively. The EQE response for the sample with x=0.70 is higher than for the sample with x=0.52 for the wavelength range 350 nm to ˜450 nm, because the transmission of light through the window is higher. This increase in the EQE translates to an increase in the cell current of 0.4 mA/cm 2  under the AM1.5D spectrum. However, the reflectivity for the sample with x=0.70 in the window is lower than the sample with x=0.70 (see  FIG. 4 ). Thus, the improvement in current is lower when it is calculated using the IQE data (0.2 mA/cm 2 ) under the AM1.5D spectrum (FIG.  5 A 1 ). 
       FIG. 5A  shows IQE and  FIG. 5B  shows the measured EQE for GaInP subcells of multijunction photovoltaic cells with anti-reflection coatings (ARC). The structure of the subcells is shown in  FIG. 2B , and the GaInP and (Al)InGaP layers have substantially the same intrinsic material lattice constants as the GaAs substrate. The Al x In 1-x P windows for these structures had thicknesses of approximately 35 nm and had x=0.52 and x=0.70, where Al 0.70 In 0.30 P is a fully strained layer according to the invention. The difference in intrinsic material lattice constants between the windows and the adjacent emitter layers is ˜0% (x=0.52) and 1.3% (x=0.70). The IQE is obtained from the EQE with the measured reflectivity shown in  FIG. 6  for the two samples, respectively. Again, there is an increase in the EQE and IQE for the short wavelength range, and a corresponding increase in the subcell current. The increase in subcell current is 0.5 mA/cm 2  under the AM1.5D spectrum, whether the current is determined from IQE or EQE (see FIG.  5 A 2 ). For a multijunction solar cell with three subcells, the bottom subcell often has higher current production than the upper and middle subcells, which may be current-matched. Thus, an increase in top subcell current may be split between the top and middle subcells by increasing the top subcell band gap or decreasing the top subcell thickness. Then the 0.5 mA/cm 2  gain in current according to this embodiment of the invention would increase the total multijunction cell current by 0.25 mA/cm 2 . 
     Reciprocal space maps and triple-axis rocking curves are well known high resolution, x-ray diffraction techniques for studying strain and relaxation in semiconductor epilayers. When a layer is pseudomorphically grown on a substrate that has a different intrinsic material lattice constant, the adoption of the layer&#39;s lattice to that of the substrate causes a tetragonal distortion in the film&#39;s unit cells. A Bragg reflection of the epitaxial film—substrate system will split into two reflection peaks, one due to the layer and one to the substrate. This is clearly seen in  FIG. 7 , which shows a triple-axis x-ray diffraction scan of a multijunction solar cell with the top subcell having a fully strained AlInP window layer according to the invention. The Ga 0.51 In 0.49 P emitter that is adjacent to the AlInP window layer has substantially the same intrinsic material lattice constant as the GaAs substrate. The scan, taken in the (004) direction, shows the substrate peak at 0 arc sec and the Al 0.73 In 0.27 P layer peak near 4300 arc sec. The Al 0.73 In 0.27 P composition and thickness (42 nm) are determined for the fully strained layer from fitting the triple-axis x-ray diffraction scan using known lattice constants for GaAs and the AlInP alloy system. The difference in intrinsic material lattice constant between the AlInP film and the GaAs substrate is 1.5%. 
     In order to derive the degree of relaxation as well as the AlInP composition, more structural information is needed than is obtained from the measurement of a single triple-axis rocking curve. This is where a reciprocal space map is useful. Reciprocal space mapping is performed such that the Bragg reflection under investigation is fully mapped in a confined area in Q space. Reciprocal space maps may be obtained by joining together successive one-dimensional scans in Q space. A fully strained layer with a different intrinsic material lattice constant will have reciprocal lattice points along the vertical line that passes through the substrate. A fully relaxed layer will have reciprocal lattice points along a line connecting the substrate and the origin of the reciprocal space. For a fully relaxed epilayer on a substrate, the Qx for the epilayer will be different than that of the substrate. 
       FIGS. 8 and 9  show reciprocal space maps done along two orientations of the multijunction solar cell with the top subcell having a Al 0.73 In 0.27 P window with a thickness of 42 nm according to the invention, with an intrinsic material lattice constant that differs from the substrate and adjacent emitter by 1.5%. The maps show that the A 1   0.73 In 0.27 P window is pseudomorphic (i.e., fully strained).  FIG. 8  shows the reciprocal space map along the (224) direction, which is the asymmetric scan.  FIG. 9  is a representation along the (004) direction, which is the symmetric scan. For the space map along the (224) direction, the Q x  for the substrate and the AlInP layer is the same, and the reciprocal lattice point of the AlInP layer lies along the vertical line normal to the surface. This means that the layer is fully strained and has the same in-plane lattice parameter as the substrate. In the case of symmetric scan, there is nonzero component of the scattering vector along the substrate normal. This means that the symmetric θ-2θ scan is along the y-axis in the reciprocal space while the rocking curve theta scan is along the x-axis in the reciprocal space. A θ-2θ originating from a set of planes which are not parallel to the substrate normal will be in the x-y plane. Thus, there is no indication of relaxation or tilt in the layer viewed with respect to the substrate. 
     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.