JP3173016U - Grid design of III-V compound semiconductor solar cells - Google Patents

Abstract

An improved III-V compound semiconductor multi-junction solar cell having a grid configuration that enables power generation with a maximum DC power per square centimeter of the cell area of greater than 35 milliwatts.
A photovoltaic solar cell that generates energy from the sun, comprising a germanium substrate that includes a first photoactive junction and forms a lower subcell 10, and an intermediate between a gallium arsenide disposed on the germanium substrate. A subcell 20, an indium gallium phosphide upper subcell 30 disposed over the intermediate subcell 20, and a surface grid including a plurality of spaced gridlines 45, each gridline 45 being thicker than 7 microns, The grid lines have a trapezoidal cross section with a cross-sectional area of 45 to 55 square microns.
[Selection] Figure 2

Description

  The present invention relates generally to the design of solar cells in space or terrestrial concentrating photovoltaic systems for converting sunlight into electrical energy, and more particularly to arrangements including grid configurations on solar cells.

  Commercially available silicon solar cells for terrestrial photovoltaic applications have efficiencies in the range of 8-15%. Compound semiconductor solar cells based on III-V compounds have an efficiency of 28% under normal operating conditions. Furthermore, it is well known that by collecting solar energy on a III-V compound semiconductor photovoltaic cell, the efficiency under concentrating exceeds 37% and the efficiency of the photovoltaic cell is increased.

  Currently, terrestrial photovoltaic power generation systems use silicon solar cells in view of their low cost and widespread use. Although III-V compound semiconductor solar cells are widely used in satellite applications, their power-weight efficiency is more important than watt unit cost considerations when selecting such devices. Such III-V compound semiconductor solar cells have not yet been designed to meet the optimal coverage of the solar spectrum present on the Earth's surface (known as air mass 1.5 or AM1.5D).

  In both silicon and III-V compound semiconductor solar cell designs, the first electrical contact is typically placed on the front side of the light absorber or solar cell, and the second contact is placed on the back side of the solar cell. Is done. A photoactive semiconductor is disposed in the light absorber of the substrate and comprises one or more pn junctions that generate an electron current when light is absorbed in the solar cell. Conductive grid lines extend over the top surface of the solar cell to capture this stream of electrons and are connected to front contacts or bonding pads.

  An important aspect that identifies solar cell design is the physical structure (composition, band gap, and layer thickness) of the semiconductor material layers that make up the solar cell. Often, solar cells are fabricated with vertically multi-junction structures in order to convert as much sunlight spectrum as possible utilizing materials with different band gaps. One embodiment of a multijunction structure useful for design according to the present invention is a triple junction solar cell structure comprising a germanium lower subcell, a gallium arsenide (GaAs) intermediate subcell, and an indium gallium phosphide (InGaP) upper subcell.

US Pat. No. 6,680,432

  It is an object of the present invention to provide an improved III-V compound semiconductor for terrestrial power generation applications having a grid configuration that allows for maximum DC power per square centimeter of cell area in the sun of solar radiation AM1.5D to exceed 35 milliwatts. It is to provide a junction type solar cell.

  The object of the present invention is an improved III-V compound semiconductor multi-junction type for space power applications having a grid configuration that allows a maximum DC power per square centimeter of solar cell of solar radiation AM0 to exceed 35 milliwatts. It is to provide a solar cell.

  Another object of the present invention is to provide a grid structure on the front surface of a III-V semiconductor solar cell in order to provide a large current for concentrating photovoltaic applications.

  Some embodiments may achieve fewer than all of the above objectives.

  In summary, the present invention provides a concentrating photovoltaic solar cell that generates energy from the sun, having a condensing lens that creates a light concentration greater than 500 ×, and a solar cell in the path of the converging beam. Provide configuration. The solar cell includes a germanium substrate having a first photoactive junction and forming a lower subcell; a gallium arsenide intermediate subcell disposed on the substrate; and an AM1.5 disposed on the intermediate subcell. An indium gallium phosphide upper subcell having a band gap for maximizing absorption in the spectral region; and a surface grid disposed on the upper subcell and having a plurality of spaced grid lines; The grid lines have a thickness greater than 7 microns and each grid line has a trapezoidal cross section with a cross-sectional area of 45 to 55 square microns.

  In another aspect, the present disclosure provides a photovoltaic solar cell that generates energy from the sun. The solar cell includes a germanium substrate having a first photoactive junction and forming a lower subcell; a gallium arsenide intermediate subcell disposed on the substrate; and indium gallium disposed on the intermediate subcell A phosphide upper subcell; and a surface grid having a plurality of spaced grid lines, each grid line having a thickness greater than 7 microns, each grid line having a cross-sectional area of 45 to 55 square microns And has a trapezoidal cross section.

  In another aspect, the present disclosure provides a photovoltaic solar cell configured to generate energy from the sun, the energy solar cell having a first photoactive junction and forming a lower subcell A substrate; a gallium arsenide intermediate subcell disposed on the substrate; an indium gallium phosphide upper subcell disposed on the intermediate subcell; and a plurality of spaced grids disposed on the upper subcell A surface grid having lines, the grid lines having a thickness greater than 7 microns.

  In some embodiments, the surface gridlines have a trapezoidal cross-sectional shape with an upper width of about 4.5 microns and a lower width of about 7 microns.

  In some embodiments, the surface gridlines have a pitch between centerlines of about 100 microns.

  In some embodiments, the surface grid lines are comprised of a plurality of parallel grid lines covering the top surface.

  In some embodiments, the surface grid lines have a total surface area covering 5% or more of the surface area of the upper subcell and less than 10% of the surface area.

  In some embodiments, the surface grid lines have a total surface area of a grid pattern that covers about 6% of the surface area.

In some embodiments, the solar cell has an open circuit voltage (V _OC ) greater than 3.0 volts, a short circuit response greater than 0.13 amps / watt, a fill factor (FF) greater than 0.70. A maximum DC power of over 35 milliwatts per square centimeter of cell area, and a conversion efficiency of over 35% / sun with solar radiation AM1.5D.

In some embodiments, the solar cell has an open circuit voltage (V _OC ) greater than 3.0 volts, a short circuit response greater than 0.13 amps / watt, a fill factor (FF) greater than 0.70. And a maximum DC power of over 35 milliwatts per square centimeter of cell area and a conversion efficiency of over 35% / sun with solar radiation AM0.

  In some embodiments, the upper, middle and lower subcell band gaps are 1.9 eV, 1.4 eV and 0.7 eV, respectively.

  In some embodiments, the upper subcell has a sheet resistance of less than 300 ohms / square.

  In some embodiments, the sheet resistance of the upper subcell is about 200 ohms / square.

  In some embodiments, the tunnel diode layer disposed between solar cell subcells has a thickness suitable for a current density in the tunnel diode of 15-30 amps / square centimeter.

  Some embodiments of the present invention will incorporate or implement some of the aspects and features outlined in the foregoing summary.

It is an expanded sectional view of the solar cell for ground comprised by the prior art. FIG. 3 is an enlarged cross-sectional view of a terrestrial solar cell constructed in accordance with the teachings of the present disclosure. FIG. 6 is a graph showing solar cell conversion efficiency as a function of grid line thickness for a solar cell surface area of 1 square centimeter, AM 1.5D spectrum, 500 sun illumination. FIG. FIG. 5 is a graph showing the conversion efficiency of a solar cell as a function of grid line thickness at a surface area of 60 square centimeters, AM0 spectrum, 1 sun illumination.

  The details of the invention will now be described using its exemplary aspects and embodiments. In the drawings and the following description, like reference characters are used to identify similar or functionally similar elements, and the main features of the exemplary embodiments are shown in simplified schematic form. Furthermore, the drawings do not represent all features of an actual embodiment, nor represent the relative dimensions of the illustrated elements, and are not drawn to scale.

  The general semiconductor structure design of triple junction III-V compound semiconductor solar cells is described in detail in US Pat. No. 6,680,432, incorporated herein by reference.

  As shown in the example of FIG. 1, the lower subcell 10 includes substrates 11 and 12 formed of p-type germanium (Ge), and this lower portion also functions as a base layer of the subcell 10. A metal contact layer (or pad) 50 is formed at the bottom of the base layer 11 and provides electrical contact to the multijunction solar cell. Further, the lower subcell 10 includes, for example, an n-type Ge emitter region 12 and an n-type nucleation layer 13. The nucleation layer 13 is disposed on the substrates 11 and 12, and the emitter layer 12 is formed in the Ge substrate by diffusion of the dopant from the top layer to the Ge substrate, thereby forming the top 12 of the p-type germanium substrate. The n-type region 12 is changed. A heavily doped n-type gallium arsenide layer 14 is deposited on the nucleation layer 13 to provide a source of arsenic dopant to the emitter region 12.

The growth substrate and base layer 11 is preferably a p-type Ge growth substrate and base layer, but other semiconductor materials can also be used as the growth substrate and base layer, or only as the growth substrate. Examples of such substrates include, but are not limited to, GaAs, InP, GaSb, InAs, InSb, GaP, Si, SiGe, SiC, Al ₂ O ₃ , Mo, stainless steel, soda lime glass, And SiO ₂ .

  Highly doped p-type aluminum gallium arsenide (AlGaAs) and (GaAs) tunnel junction layers 14 and 15 are disposed on the nucleation layer 13 to form a tunnel diode to form a lower subcell and an intermediate subcell 20. Provide a low resistance path in between.

The intermediate subcell 20 comprises a heavily doped p-type aluminum gallium arsenide (AlGaAs) back surface field (BSF) layer 16, a p-type InGaAs base layer 17, and a heavily doped n-type indium. A gallium phosphide (InGaP ₂ ) emitter layer 18 and a highly doped n-type indium aluminum phosphide (AlInP ₂ ) window layer 19 are provided.

  The window layer generally has the same doping type as the emitter, but has a higher doping concentration than the emitter. Furthermore, it is desirable for the window layer to have a larger band gap than the emitter in order to suppress light generation and injection of minority carriers in the window layer and reduce recombination. Otherwise, recombination will occur in the window layer. AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlGaInSb, AlGaInSb, AlGaInSb, AlGaInSb A variety of different semiconductor materials, including AlGaInNAs, ZnSSe, CdSSe, and other materials within the scope of the present invention can be used as the window layer, emitter, base and / or BSF layer of photovoltaic cells. Should be noted.

  The InGaAs base layer 17 of the intermediate subcell 20 includes, for example, about 1.5% indium. Other compositions can be used as well. The base layer 17 is formed on the BSF layer 16 after the BSF layer is deposited on the tunnel junction layers 14 and 15 of the lower subcell 10.

  The BSF layer 16 is provided to reduce recombination loss in the intermediate subcell 20. The BSF layer 16 drives minority carriers from the heavily doped region near the back surface to minimize the effects of recombination loss. In this way, the BSF layer 16 reduces recombination losses on the back side of the solar cell and reduces recombination at the base layer / BSF layer interface. After the emitter layer 18 is deposited, a window layer 19 is deposited on the emitter layer 18 of the intermediate subcell 20. The window layer 19 in the intermediate subcell 20 also helps reduce recombination loss and improves cell surface passivation of the underlying junction.

Prior to depositing the upper subcell 30 layer, a heavily doped n-type InAlP ₂ tunnel junction layer 21 and p-type InGaP ₂ tunnel junction layer 22 are each deposited on the intermediate subcell 20 to form a tunnel diode. To do.

  In a high concentration terrestrial solar cell embodiment, the tunnel diode layer disposed between the subcells has a thickness adapted to support a current density of 15-30 amps / square centimeter through the tunnel diode.

In one embodiment, the upper subcell 30 comprises a highly doped p-type indium gallium aluminum phosphide (InGaAlP) BSF layer 23, a p-type InGaP ₂ base layer 24, a heavily doped n-type InGaP ₂ emitter layer. 25, and a heavily doped n-type InAlP ₂ window layer 26. After the BSF layer 23 is formed on the tunnel junction layers 21 and 22 of the intermediate subcell 20, the base layer 24 of the upper subcell 30 is deposited on the BSF layer 23. After the emitter layer 25 is formed on the base layer 24, a window layer 26 is deposited on the emitter layer 25 of the upper subcell. A cap layer 27 is deposited on the window layer 26 of the upper subcell 30 and patterned into separate contact regions.

  The cap layer 27 serves as an electrical contact from the upper subcell 30 to the metal grid layer 40. The sheet resistance of the upper subcell is less than 300 ohms / square, and in some embodiments, about 200 ohms / square centimeter. The doped cap layer 27 is a semiconductor layer such as a GaAs or InGaAs layer. Further, an antireflection coating 28 may be provided on the surface of the window layer 26 between the contact regions of the cap layer 27.

  Grid lines 40 in prior art solar cells generally extend between two bus bars on opposite sides of the cell. In the prior art, grid lines generally have a thickness or height of 5 microns or less, a width of about 5 microns, and a pitch of about 100 microns (ie, the distance between the centers of adjacent grid lines). The total surface area of the grid pattern is 5.0% to 10.0% of the surface area of the upper subcell.

  As shown in the example of FIG. 2, the solar cell of the present embodiment has substantially the same semiconductor layers 11 to 27, metal contact layer 50, and antireflection coating layer 28 as the solar cell of FIG. The detailed description is not repeated.

  In some embodiments of the present disclosure, grid lines 45 extend between two bus bars on opposite sides of the cell. In some embodiments, each grid line 45 has a trapezoidal cross section with a cross-sectional area of 45-55 square microns, and the size of each conductor is a relatively high current conduction produced by a highly concentrated solar cell. Is adapted to.

  The grid lines have a thickness or height of 7 microns or more, a width of about 5 microns, and a pitch of about 100 microns (distance between the centers of adjacent grid lines). In some embodiments, the grid lines have a trapezoidal cross section with an upper side of about 4.5 microns and a base of about 7 microns.

  The total surface area of the grid pattern is 5.0% to 10.0% of the surface area of the upper subcell. The grid pattern and grid line dimensions are selected to carry a relatively high current generated by the solar cell. In some embodiments, the total surface area of the grid pattern is 6.0% of the surface area of the upper subcell.

  In some embodiments, for example, for terrestrial power applications, a condensing lens 60 or other optical component is placed on the solar cell and is incident on the surface of the solar cell up to a magnification of 500 × or higher. Used to focus.

In some embodiments, the resulting solar cell has a band gap of 1.9 eV in the upper subcell, 1.4 eV in the middle subcell, and 0.7 eV in the lower subcell. In some embodiments, when illuminated by sunlight concentrated at a magnification greater than 500 ×, the solar cell is 3.0 watts so as to produce a maximum DC power of greater than 35 milliwatts per square centimeter of cell area. Open circuit voltage (V _OC ) above volt, short circuit responsiveness above 0.13 amps / watt, fill factor (FF) above 0.70, 25 degrees Celsius, air mass 1.5 (AM1.5D) or equivalent ground spectrum with a conversion efficiency of 35% or more.

  FIG. 3 is a graph showing solar cell conversion efficiency as a function of grid line thickness at a solar cell surface area of 1 square centimeter, an AM 1.5D spectrum, and 500 sun illumination. Such a solar cell (identified as a model CTJ) is a concentrating power generation system that uses a lens or other optical component to focus a solar beam incident on the solar cell at a magnification of 500 times or more. Suitable for ground use. The use of thick grid lines (eg, 7 microns thick or more) substantially improves the conversion efficiency of solar cells. From a production or reliability point of view using current production techniques, it may be difficult to achieve a grid thickness at the extreme edge of the graph (ie, 10 microns or more) due to limitations in lithography and processing conditions. This does not detract from the teachings of the disclosure.

  FIG. 4 is a graph showing the conversion efficiency of a solar cell as a function of grid line thickness at a surface area of 60 square centimeters, an AM0 spectrum, 1 sun illumination. Such a solar cell (identified as model ZTJ) is suitable for space applications in a power generation system operating with one sun (ie, no use of incident solar beam expansion). The use of thick grid lines (eg, 7 microns thick or more) substantially improves the conversion efficiency of the solar cell. From a production or reliability point of view using current production techniques, it may be difficult to achieve a grid thickness at the extreme edge of the graph (ie, 10 microns or more) due to limitations in lithography and processing conditions. This does not detract from the teachings of the disclosure.

10: Lower subcells 11, 17, 24: Base layers 12, 18, 25: Emitter layers 14, 15, 21, 22: Tunnel junction layers 16, 23: Back surface field (BSF) layers 19, 26: Windows Layer 20: Intermediate subcell 27: Cap layer 28: Antireflection coating 30: Upper subcell 40, 45: Grid line 50: Metal contact layer (or pad)
60: Condensing lens

Claims (20)

A concentrating photovoltaic solar cell that generates energy from the sun,
A condenser lens that creates a light concentration greater than 500x;
A solar cell in the path of the light concentrated light beam,
The solar cell is
A germanium substrate having a first photoactive junction and forming a lower subcell;
An intermediate subcell of gallium arsenide disposed on the germanium substrate;
An upper subcell of indium gallium phosphide disposed over the intermediate subcell and having a band gap for maximizing absorption in the AM1.5 spectral region;
A surface grid disposed on the upper subcell and having a plurality of spaced grid lines;
The grid lines have a thickness greater than 7 microns, each grid line has a trapezoidal cross-section with a cross-sectional area of 45-55 square microns, and the relatively high current produced by the solar cell. A solar cell characterized by being adapted for conduction.   The solar cell of claim 1, wherein the trapezoidal shape has an upper width of about 4.5 microns and a lower width of about 7 microns.   The solar cell of claim 1, wherein the grid lines have a center-to-center pitch of about 100 microns.   The solar cell according to claim 1, wherein the grid pattern includes a plurality of parallel grid lines covering the upper surface.   2. The solar cell according to claim 1, wherein the total surface area of the grid pattern covers 5% or more of the surface area of the upper subcell and less than 10% of the surface area of the upper subcell.   The solar cell of claim 1, wherein a total surface area of the grid pattern covers about 6% of a surface area of the upper subcell. The solar cell has an open circuit voltage (V _OC ) of 3.0 volts or more, a short circuit response of 0.13 amps / watt or more, a fill factor (FF) of 0.70 or more, and a square centimeter of the cell area. The solar cell according to claim 1, characterized in that it produces a maximum DC power of more than 35 milliwatts per unit and has a conversion efficiency of more than 35% / sun with solar radiation AM1.5D.   The solar cell of claim 1, wherein the upper subcell has a band gap of 1.9 eV, the intermediate subcell has a bandgap of 1.4 eV, and the lower subcell has a band gap of 0.7 eV.   The solar cell of claim 1, wherein the upper subcell has a sheet resistance of less than 300 ohm / square.   The solar cell of claim 9, wherein the sheet resistance of the upper subcell is about 200 ohms / square. Further comprising a tunnel diode layer disposed between the subcells of the solar cell;
The solar cell of claim 1, wherein the tunnel diode layer has a thickness adapted to support a current density in a tunnel diode of 15 to 30 amperes per square centimeter. A photovoltaic solar cell that generates energy from the sun,
A germanium substrate having a first photoactive junction and forming a lower subcell;
An intermediate subcell of gallium arsenide disposed on the germanium substrate;
An upper subcell of indium gallium phosphide disposed on the intermediate subcell;
A surface grid disposed on the upper subcell and having a plurality of spaced grid lines;
The grid line has a thickness greater than 7 microns, and each grid line has a trapezoidal cross section having a cross-sectional area of 45 to 55 square microns.   The solar cell of claim 12, wherein the trapezoidal shape has an upper width of about 4.5 microns and a lower width of about 7 microns.   The solar cell of claim 12, wherein the grid lines have a center-to-center pitch of about 100 microns.   The solar cell according to claim 12, wherein the grid pattern is composed of a plurality of parallel grid lines covering the upper surface.   The solar cell according to claim 12, wherein the total surface area of the grid pattern covers 5% or more of the surface area of the upper subcell and less than 10% of the surface area of the upper subcell.   The solar cell of claim 12, wherein the total surface area of the grid pattern covers about 6% of the surface area of the upper subcell.   The solar cell of claim 12, wherein the upper subcell has a band gap of 1.9 eV, the intermediate subcell has a bandgap of 1.4 eV, and the lower subcell has a bandgap of 0.7 eV.   The solar cell of claim 12, wherein the upper subcell has a sheet resistance of less than 300 ohms / square.   The solar cell of claim 19, wherein the sheet resistance of the upper subcell is about 200 ohm / square.

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