Device and process for forming highly durable multi-junction solar cells

Abstract

To provide a device and a process for forming highly durable multi-junction solar cells.SOLUTION: A multijunction photovoltaic device includes a first subcell including a base semiconductor layer and a second semiconductor layer, where the base semiconductor layer includes a III-V semiconductor material; and a second subcell on the first subcell including an absorber layer, where the absorber layer includes an organometal halide ionic solid perovskite semiconductor material. The device also includes a passivation layer on at least a portion of a top surface of the first subcell. The device also includes an n-side metal pad contacting the passivation layer on an n-side of the second subcell. The device also includes an interconnect tab contacting the n-side metal pad or contacting the second subcell. The passivation layer may include an oxide layer. The passivation layer may include a material selected from the group including SiO2, Al2O3, TiO2, Ta2O5, HfO2, ZnS, and combinations thereof.SELECTED DRAWING: None

Description

[Technical Field]

[0001] FIELD OF THE INVENTION

[0001] The present teachings relate generally to optoelectronic semiconductor structures such as solar cells and photodetectors (broadly referred to herein as solar cells), and more particularly to heterojunction solar cells and methods for making the same. [Background technology]

[0002]

[0002] A solar cell is a photovoltaic device that can convert incident light energy, and specifically solar energy (sunlight), into electrical energy by the photovoltaic (PV) effect. Due to concerns about pollution, energy security, and limited available resources, there is increasing interest in solar cells. This interest relates to both terrestrial and non-terrestrial (space) applications.

[0003]

[0003] In terrestrial applications, higher solar cell efficiency for converting sunlight to electricity results in a smaller collection area required for a given power output, thus resulting in a lower cost per watt and greater cost-effectiveness for terrestrial photovoltaic systems. The cost per watt of power generation capacity of photovoltaic systems has prevented their widespread use in terrestrial applications. The efficiency of converting sunlight to electricity can be critically important for terrestrial PV systems, as increased efficiency typically reduces the associated electricity generation system components (such as cell area, module or collector area, support structure, and land area) for the system's required power output. For example, in concentrating solar cell systems that concentrate sunlight onto solar cells by a factor of about 2 to about 2000, increased efficiency typically results in a proportional reduction in the area including expensive concentrating optics.

[0004]

[0004] In space applications, the use of nuclear or battery power generally results in a significant increase in the spacecraft payload for a given amount of power required to operate the satellite (artificial satellite, hereafter). Increasing the spacecraft payload in this manner increases launch costs more than linearly. Because solar energy is readily available in space for spacecraft such as satellites, the conversion of solar energy to electrical energy has proven to be a good alternative for increasing payload, and the development of more efficient solar cells can further increase payload capacity.

[0005] To increase power output, solar cells may have one or more photovoltaic junctions (multi-junctions), which are one or more component photovoltaic cells, also called subcells. The subcells / junctions may contain corresponding photovoltaic semiconductor layers with different energy bandgaps and may be stacked so that each subcell can absorb a different portion of the broad energy distribution in sunlight. Incident light energy of different ranges of wavelengths can be converted at corresponding one or more of the junctions.

[0006]

[0006] These component photovoltaic cells or subcells can be connected in series to form a multijunction solar cell, but can also be connected in other electrical configurations, such as in parallel, or in a combination series and parallel junction. The stacked multijunction arrangement is advantageous because each photon absorbed in a subcell corresponds to one unit of charge collected at the subcell operating voltage, which depends approximately linearly on the bandgap of the subcell's semiconductor material. Because output power is the product of voltage and current, an ideally efficient solar cell would have many subcells, each absorbing only photons of energy slightly greater than its bandgap.

[0007]

[0007] In multijunction solar cells, it is often desirable to modify the bandgaps of the semiconductor layers forming the subcells within the multijunction cell, thereby modifying the subcell voltage and wavelength range over which the subcells respond to incident light, e.g., space and terrestrial solar spectra. The specific pad gaps and thicknesses of the layers forming the subcells within the multijunction cell determine the subcell voltage, the current density of each subcell, whether the subcell current densities can be matched to each other as desired in a series-interconnected multijunction cell, and how the combination of subcell bandgaps divides the broad solar spectrum into narrower wavelength ranges to achieve higher solar-to-electrical conversion. A key technical challenge in designing multijunction solar cells is how to achieve an optimal or near-optimal combination of bandgaps for the subcell layers to maximize the efficiency of the multijunction solar cell, and how to achieve the desired wavelength range of subcell response (the wavelength range over which the subcells can collect useful photogenerated current). Often, readily available semiconductors, e.g., semiconductors that are lattice-matched to relatively common and inexpensive substrates, that can be grown with good minority carrier properties such as lifetime and mobility, or that do not introduce unwanted doping or impurities into other parts of the cell, do not have bandgaps that result in the most favorable combination of multijunction subcell bandgaps for conversion of the solar spectrum.

[0008] The most efficient, and therefore dominant, multijunction (MJ) PV cell technology is the GaInP / Ga(In)As / Ge cell structure. Here, the use of parentheses in the Ga(In)As middle subcell material indicates that the incorporation of indium in the middle cell is optional, whereby the middle cell composition can be either GaAs or GaInAs. These monolithic cells can be grown lattice-matched to GaAs or Ge, with the Ge substrate being inactive and only the top two junctions active (two-junction or 2J cells), or all three junctions active (three-junction or 3J cells). Variations of this material system, such as AlGaInP or lattice-mismatched GaInP top cells, may more ideally match the bandgap to the solar spectrum, but practical considerations indicate that lattice-matched GaInP is preferred for large-scale production.

[0009]

[0009] Traditionally, the current generated by each subcell has been controlled by reducing the subcell voltage. Specifically, the alloy composition of the subcell's base semiconductor layer has been designed to obtain the desired amount of current. For example, lower bandgap alloys have been used in the base semiconductor layer to achieve higher current output at the expense of a lower output voltage for the subcell. In monolithic, series-junction, two-junction, and three-junction GaInP / Ga(In)As / Ge solar cells, it is desirable for the GaInP top subcell to have approximately the same photo-generated current density as the Ga(In)As subcell. If the currents are different, the subcell with the lowest photo-generated current limits the current through all the series-interconnected subcells in the MJ cell, and the excess photo-generated current in the other subcells is wasted. Limiting the current in this manner imposes a severe penalty on the MJ cell efficiency. However, one potential approach to improving existing three-junction GaInP / Ga(In)As / Ge multijunction solar cell devices is to add one or more additional junctions.

[0010]

[0010] The power conversion efficiency of a subcell is a function of both the output voltage and the output current. Therefore, an approach to current matching for solar cell subcells can be based on an overall multi-junction tandem in series, which can produce the maximum current and therefore higher efficiency.

[0011] Recent advances in dye-sensitized solar cells (DSSCs) have created a new material category for solar power conversion. The low-temperature deposition of combined organometallic halides formed in perovskite crystals as polycrystalline layers offers a route to low-cost, highly efficient solar power generation. A recent report indicates that a laboratory efficiency of 20.1% was measured under terrestrial standard conditions (AM1.5G, 25°C), comparable to that of standard silicon wafer-based technology. Significant efficiency gains have led to significant research improvements, and the field is rapidly evolving. However, with each new record, several consistent issues emerge in materials. Many limitations, new materials, and device engineering challenges are being elucidated. For example, overcoming challenges related to radiation resistance, such as the resistance of new materials to UV damage, without sacrificing device performance or increasing cost is of particular interest. Improved multijunction solar (photovoltaic) cells and methods for their fabrication would be a welcome addition to the art.

[0012]

[0012] Current technological developments in tandem solar cells include new materials such as perovskite thin film layers. Perovskites and similar materials are low-cost due to the low deposition temperatures required to form high-quality layers. These temperatures can be below 100°C. The drawback of low formation temperatures is that typical robust assembly methods for connecting cells, such as soldering or welding, can cause the perovskite layers to evaporate or degrade. Therefore, there is a need to incorporate perovskite layers into solar cells while embracing the low-temperature stability of perovskite-based materials. Summary of the Invention

[0013]

[0013] The following presents a simplified summary in order to provide a basic understanding of some aspects of one or more embodiments of the present teachings. This summary is not an extensive overview, and is not intended to identify key or critical elements of the present teachings or to delineate the scope of the disclosure. Rather, the essential purpose of the summary is merely to present one or more concepts in a simplified form as a prelude to the more detailed description that is presented later.

[0014] A multijunction photovoltaic device is disclosed. The multijunction photovoltaic device includes a first subcell that can include a base semiconductor layer and a second semiconductor layer, where the base semiconductor layer can include a III-V semiconductor material, and a second subcell on the first subcell that can include an absorber layer, where the absorber layer can include an organometal halide ionic solid perovskite semiconductor material. The device also includes a passivation layer on at least a portion of a top surface of the first subcell. The device also includes an n-side metal pad in contact with the passivation layer on the n-side of the second subcell. Some embodiments of the multijunction photovoltaic device can include an interconnect tab in contact with the n-side metal pad. The interconnect tab contacts the second subcell. The interconnect tab can include a metal. The n-side metal pad and the interconnect tab can include two different metals. The n-side metal pad and the interconnect tub are on one side of the multijunction photovoltaic device. The passivation layer may include an oxide layer. The passivation layer may include a material selected from the group that may include SiO2, Al2O3, TiO2, Ta2O5, HfO2, ZnS, and combinations thereof. The passivation layer is adjacent to the second subcell. The second subcell does not completely cover the first subcell.

[0015] Another multijunction photovoltaic device is disclosed. The multijunction photovoltaic device may include a first subcell that may include a base semiconductor layer and a second semiconductor layer, where the base semiconductor layer may include a III-V semiconductor material, and a second subcell on a surface of the first subcell that may include an absorber layer, where the absorber layer does not include a perovskite semiconductor material. The device also includes a passivation layer on at least a portion of the top surface of the first subcell. The device also includes an n-side metal pad contacting the passivation layer on the n-side of the second subcell. The device also includes grid fingers contacting the n-side metal pad and the second subcell. Some embodiments of the multijunction photovoltaic device may include an aspect in which the passivation layer is adjacent to the second subcell. In some embodiments, the second subcell does not completely cover the first subcell.

[0016]

[0016] A method for forming a multijunction solar cell is disclosed. The method for forming a multijunction solar cell can include forming a substrate subcell, which can include an n-side, a backside, a base semiconductor layer, and a second semiconductor layer. In this case, the base semiconductor layer can include a III-V semiconductor material. The method also includes forming a passivation layer to protect the n-side of the substrate subcell. The method also includes forming a high-temperature metal deposition on the backside of the substrate subcell. The method also includes forming an interconnect tab in contact with the passivation layer. The method also includes forming a second subcell, which can include an absorber layer. In this case, the absorber layer does not include a perovskite semiconductor material. The method also includes forming a front-side metallization connecting to the pad using low-temperature deposition. Multiple embodiments of the method for forming a multijunction solar cell are shown. In this case, the passivation layer can include an oxide layer. The interconnect tab can contact the second subcell. The substrate subcell can include a base semiconductor layer, which can include Si, GaAs, or Ge. A method for forming a multijunction solar cell can include assembling one or more multijunction solar cells into an array of cells. A method for forming a multijunction solar cell can include forming interconnect tabs by high temperature soldering. A method for forming a multijunction solar cell can include forming interconnect tabs by high temperature welding.

[0017]

[0017] The above features, functions, and advantages can be realized alone in various embodiments or can be combined in other embodiments, further details of which can be understood with reference to the following description and accompanying drawings.

[0018] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present teachings and, together with the description, serve to explain the principles of the disclosure. [Brief explanation of the drawings]

[0019] [Figure 1]

[0019] Figures 1A-1C are cross-sectional views of several aspects of prior art solar cell structures according to the present disclosure. [Figure 2]

[0020] 1A-1D are cross-sectional views of several aspects of multi-junction solar cell structures according to the present disclosure. [Figure 3]

[0021] 1 is a flowchart illustrating a prior art method for manufacturing a solar cell according to the present disclosure. [Figure 4]

[0022] 1 is a flowchart illustrating a method for manufacturing a solar cell according to the present disclosure. [Figure 5]

[0023] FIG. 1 is a diagram of the International Space Station (ISS), which may employ the disclosed multi-junction photovoltaic device(s). DETAILED DESCRIPTION OF THE INVENTION

[0020]

[0024] It should be noted that some details of the drawings have been simplified and illustrated to facilitate understanding of the present disclosure, rather than to maintain strict structural accuracy, detail, and scale.

[0021]

[0025] Reference will now be made in detail to exemplary embodiments of the present teachings, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same, similar or like parts.

[0022]

[0026] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the embodiments are approximations, the numerical values ​​set forth in the specific embodiments are reported as precisely as possible. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in each testing measurement. Moreover, all ranges disclosed herein should be understood to include any and all subranges subsumed therein. For example, a range "less than 10" can include any and all subranges between (and including) a minimum value of zero and a maximum value of 10, i.e., any and all subranges having a minimum value greater than or equal to zero and a maximum value less than 10, e.g., 1 to 5. In certain cases, the numerical values ​​recited for parameters can be negative. In this case, an exemplary range recited as "less than 10" can take negative values, such as -1, -2, -3, -10, -20, -30, etc.

[0023]

[0027] Furthermore, one or more of the actions described herein may be performed in one or more separate actions and / or phases. Furthermore, to the extent that the terms "including," "includes," "having," "has," "with," or variations thereof are used in either the detailed description or the claims, these terms are intended to be as inclusive as the term "comprising." Furthermore, in the description and claims herein, the term "on" when used in connection with two materials, such as one "on" the other, means that there is at least some contact between the materials, whereas "over" means that the materials are in close proximity to each other, possibly with one or more additional intervening materials, and that contact is possible but not required. Neither "on" nor "over," as used herein, implies any directionality. The phrase "at least one of" is used to mean that one or more of the listed items may be selected. As used herein, the phrase "one or more" (e.g., of "A, B, and C") means any of A, B, or C alone, any combination of two of these, e.g., A and B, B and C, and A and C, or any combination of three of A, B, and C.

[0024]

[0028] As used herein, the terms "doped" or "doping" (and variations thereof) in reference to semiconductor materials or layers comprising semiconductor materials refer to the introduction or presence of impurity "dopant" elements intentionally added to, for example, an extrinsic semiconductor, to cause a change in the intrinsic electrical conductivity of the semiconductor material.

[0025]

[0029] As understood by those skilled in the semiconductor arts, the "type" of dopant refers to the relative amount of valence electrons of the impurity atom compared to the atoms of the semiconductor in which it is present or into which it is introduced. That is, a p-type dopant has fewer valence electrons than the semiconductor, and an n-type dopant has more valence electrons than the semiconductor. In common usage, "p-type doped" indicates that the semiconductor has been doped with a p-type dopant to increase the number of positive charge carriers relative to the number of negative charge carriers in the semiconductor, and "n-type doped" indicates that the semiconductor has been doped with an n-type dopant to increase the number of negative charge carriers relative to the number of positive charge carriers in the semiconductor.

[0026]

[0030] As understood by those skilled in the semiconductor arts, a "level" of dopant refers to the concentration of dopant impurities within a semiconductor. For example, as used herein, "lightly doped" refers to a doping level of about 10 14 from about 10 15 dopant atoms / cm 3 "Moderately doped" indicates a doping level of approximately 10 16 from about 10 17 dopant atoms / cm 3 "Heavily doped" indicates a doping level of approximately 10 18 from about 10 19 dopant atoms / cm 3 Thus, as used herein, the designation "p+" or "n+" indicates that the corresponding semiconductor is moderately doped p-type or n-type, and the designation "p++" or "n++" indicates that the corresponding semiconductor is heavily doped p-type or n-type, respectively.

[0027]

[0031] The following examples are described by way of example only and with reference to the following drawings. Those skilled in the art will appreciate that the following description is exemplary in nature and that various changes to the parameters described herein are possible without departing from the scope of the examples. The specification and examples are intended to be considered exemplary only. The various examples are not necessarily mutually exclusive, as an embodiment may be combined with one or more other embodiments to form a new embodiment. Structures depicted in the drawings may include additional features not shown for simplicity, while features that are shown may be removed or modified.

[0028]

[0032] Perovskite materials are relatively new and are known for their applications in the fabrication and use of solar cells or photovoltaic devices. Perovskite materials are low-cost, do not require expensive processing equipment, do not require high-temperature processing, etc. Devices incorporating perovskites can provide more efficient solar cells at lower costs. Typically, tandem solar cells or multijunction solar cells are preferred. Tandem solar cells, as described, can include a top junction and a bottom junction, most often on a silicon substrate. Other embodiments include perovskites in conjunction with III-V materials. Advantageously, perovskites can be deposited at temperatures around 100°C, compared to 600-700°C for III-V materials.

[0029]

[0033] Disadvantages of perovskites include their low-temperature processing, which can lead to their degradation. Most of the thermal stress occurs during the assembly process, which involves connecting multiple cells together. Indeed, such connections can be a weak link in the manufacturing process. For example, silicon-based panels are soldered and wired together, whereas space solar cells can be brazed or welded at higher temperatures using short pulses of very high current. When employed in perovskite-containing cells or devices, these techniques can vaporize the perovskite at temperatures between 300 and 400 °C in a matter of seconds. A drawback of low formation temperatures is that typical robust assembly methods for connecting cells together, such as soldering or welding, can vaporize or degrade the perovskite layer. Thus, typical state-of-the-art perovskite tandem solar cells lack a robust method for interconnecting cells that meets the requirements for durability in space environments, including temperature cycling. Overall, other potential devices using upper subcells or sets of subcells with lower robustness or lower process temperatures can be considered.

[0030]

[0034] Therefore, interconnect methods and means are a major challenge for cells or devices containing perovskites. For example, silver tabs or silver inks are not robust, and space applications require these cells and devices to withstand thermal environments and be extremely robust. The higher temperature range must be able to withstand at least 150°C, and the lower temperature range must be able to withstand at least -100 to -200°C. Therefore, the present disclosure provides a method by which the location where the welded interconnect bond is made can be separated from the use and assembly of the perovskite-containing components. In some embodiments, the deposition of an insulating contact on the silicon, such as an oxide, silicon dioxide, aluminum oxide, or aluminum nitride insulator, helps prevent the top contact of the device from touching the bottom contact of the device and shorting it out. This further provides a weldable or solderable connection. Further embodiments include the use of two different metals for the n-side metal and the metal grid fingers (interconnect tabs), without the need to use the same metal. The metal may be deposited and sintered at 200-400°C, followed by perovskite deposition. High-temperature processing is completed first, followed by connection formation with the low-temperature section. The n-side metal contact may include low-resistivity metals such as silver, copper, or alloys, or complex structures such as multilayer structures or alloys. It should be noted that aluminum is typically not suitable for this contact material. The interconnect tab, also known as a grid finger, may be aluminum or a different metal from the n-side metal contact and should also be low-resistivity. These two metal pads do not need to be the same material or solderable. For example, different materials may be included, and different deposition techniques, such as screen printing, evaporation, or deposition, may be used. Typical metals include, but are not limited to, noble metals such as Cu, Ag, and Au for conductivity. However, the metal stack may be composed of multiple alloy layers, which may include metals that act as dopants, such as Fe, Ge, Zn, or metals that act as diffusion or adhesion layers, such as Ti, Ni, W, Pt, Pd, as is commonly known to those skilled in the art.

[0031]

[0035] The methods and devices disclosed herein provide devices that sacrifice some active area because the portion of the cell that includes the insulating tab or portion is thus impervious to solar energy because the perovskite layer is only a partial layer. This slight efficiency sacrifice is made in exchange for increased throughput and interconnectivity between cells. In embodiments of the present disclosure, the number of cells is not limited, and arrays ranging from two to 1000 cells may be used. In still other embodiments, the interconnects may also be used as bus wires, for example, to extract and transmit power to a battery. While perovskites are described as the low-temperature component in the cells and devices of the present disclosure, other low-temperature materials may also be used, including copper indium gallium diselenide, cadmium zinc telluride, and the like. Additional semiconductor systems are contemplated, including chalcogenide thin-film systems such as CdS, CdSe, ZnTe, and ZnSe, as well as polymer-based solar cell materials.

[0032]

[0036] This disclosure provides devices and processes for forming durable multi-junction solar cells. Current state-of-the-art developments in tandem solar cells include novel materials such as perovskite thin film layers. In tandem solar cells, perovskites (PVK) can be layered on top of a silicon substrate to form perovskite / silicon 2J (two-junction) devices. Such devices offer a low-cost route to highly efficient photovoltaics over conventional cells. Perovskites and similar materials are low-cost due to the low deposition temperatures required to form high-quality layers. These can be as low as 100°C or lower.

[0033]

[0037] This disclosure utilizes a robust subcell below the perovskite layer. Typically, this subcell is fabricated from single-crystal silicon to form the bottom subcell of a Pvk / Si 2J device. A typical assembly process involves forming contacts using metal deposited on the top and bottom of the cell. In some embodiments, these contact pads are formed on the silicon subcell. The key to forming all contacts on the bottom subcell is the addition of an insulating layer below the top contact. This layer allows for connection of the top layer of the tandem upper subcell without creating a short path across the top junction. A final step may include adding one or more contact fingers on top of the perovskite top subcell, which then connects the pads to the top layer of the pn junction diode. This may reduce the area of ​​the top junction relative to the bottom junction.

[0034]

[0038] Because serial interconnection is also a challenge, the described methods can include both contacts on one side of the device. In some embodiments, top-to-top (sun-facing) contacts can be used, or alternatively, both contacts can be on the bottom (away from the sun). Furthermore, suitable serial interconnects can include using flex circuits with embedded traces to avoid direct welding or soldering to adjacent cells during serial interconnection. Similar techniques can be employed in the fabrication of additional 2J or higher multijunction devices, where the top subcell is susceptible to high-temperature damage from metal deposition or welding or soldering assembly steps. Some embodiments can include a bonded (spared or lifted) III-V cell (AlGaAs or AlGaInP / AlGaAs / GaAs) on a silicon subcell for 2J, 3J, or 4J devices, or a II-VI polycrystalline device on a passivated Si or Ge subcell. These include 1.7 eV / 1.1 eV 2J (1.4-1.9 eV (~1.7 eV preferred) CuGaInSe or CdZnTe thin film devices) on silicon, which may require a conductive layer between the upper and lower subcells (a tunneling device is provided on the Si side before depositing the II-VI alloy subcell). Alternatively, on germanium, there are GaAs / Ge 2J with low-cost 1.9 eV II-VI alloy devices (1.9-2.0 eV CdZnTe polycrystalline devices).

[0035]

[0039] Generally, the solar cells described herein include multijunction solar cells that utilize at least one III-V semiconductor and, in some embodiments, at least one perovskite material layer. These layers may be located within a single subcell and / or within different subcells of the same multijunction photovoltaic device to improve current density. In one embodiment, the III-V semiconductor layer(s) are configured to act as ultraviolet (UV) absorbers / attenuators to minimize or prevent any damage to the perovskite material layer. More specifically, the high-efficiency multijunction photovoltaic cells described herein may be used, for example, in satellites, manned or unmanned spacecraft, and space probes. In one approach, in a GaInP / GaAs / Ge three-junction photovoltaic device, the weakest junction affected by radiation (e.g., the GaAs subcell) may be replaced with a perovskite subcell (e.g., GaInP / perovskite / Ge) to improve overall radiation tolerance. In certain embodiments, the perovskite material layer is not present, and instead of a perovskite material, the multijunction solar cell may include other materials that can be processed at low temperatures, e.g., below 100°C, such as materials including, but not limited to, copper indium gallium diselenide (CuInGaSe), cadmium zinc telluride (CdZnTe), CdS, CuS, SuSe, CdSe, HgCdTe, etc.

[0036]

[0040] In the radiative performance of conventional solar cells, such as III-V and IV solar cells, two primary physical properties contribute to the preservation of solar cell performance: first, the amount of damage the material sustains in terms of defects created in the material per ionizing particle, and second, the location of the defects within the energy gap and, therefore, the availability of the defects to act as non-radiative recombination sites. Thus, the solar cells described herein may utilize induced lead halide materials, as these materials have shown superior performance over conventional solar cell materials.

[0037]

[0041] For example, the amount of damage a material receives from radiation particles (e.g., atomic displacement from the lattice) is proportional to the mass of the atoms. In GaAs, both Ga and As are typically displaced from the lattice by high-energy electron and proton beams from elastic collisions, known as displacement damage. Without being limited to a particular theory, it is believed that because the central atoms and ions that make up perovskite materials are more massive than the constituent atoms, e.g., Ga and As, in the subcells of conventional III-V photovoltaic devices, the displacement damage caused by such particle radiation is reduced compared to perovskite materials and GaAs materials. Therefore, perovskite materials are believed to have fewer centers than comparable GaAs solar cells at the same radiation dose.

[0038]

[0042] Furthermore, in photovoltaic devices, the band gap energy of the recombination region controls the absorption of photons. However, it is known that the energy states introduced through defects in the crystal lattice of perovskite materials are outside this band gap. Specifically, defects at the grain boundaries of organolead iodide perovskite do not generate effective non-radiative recombination centers. Therefore, without being limited to a particular theory, it is believed that in photovoltaic devices for space radiation environment applications, i.e., in perovskite subcells of solar cells used in outer space, even if defects are generated by particle radiation, they are ineffective as recombination sites. Therefore, it is further believed that perovskite solar cells are largely unaffected by space radiation.

[0039]

[0043] Multijunction Photovoltaic Devices

[0044] FIG. 1 is a cross-sectional view of several embodiments of prior art solar cell structures according to the present disclosure. Referring to FIG. 1, several embodiments of a photovoltaic device are shown as solar cell structure 100. Solar cell structure 100 includes a tandem solar cell 100′. Solar cell structure 100 may include a first (front or top) contact layer, illustrated as an n-side metal contact 108, and / or a second (back or bottom) contact layer 106. Metal contact 108 may be disposed over a front side of solar cell structure 100, and a second p-side contact layer 106 may be disposed over a back side of solar cell 100′. When the front side of solar cell structure 100 is exposed to electromagnetic radiation, such as solar radiation, solar cell 100′ may generate a voltage between its respective front and back sides. In several embodiments, n-side metal contact 108 may alternatively be embodied as a p-side contact, and second contact layer 106 may be an n-side contact in an inverted orientation. In other embodiments, not shown here, one or more layers of anti-reflective coating layers may be included in the solar cell structure 100 .

[0040]

[0045] The solar cell 100′ may include one or more subcells, i.e., the solar cell 100′ may include a multijunction solar cell. The subcells may be referred to by the order in which light enters each subcell when it enters the front surface of the solar cell structure 100. For example, in FIG. 1 , the front subcell may also be referred to as the top subcell 104, and the back subcell (adjacent to the back surface) may also be referred to as the bottom subcell 102. In the case of an alternative exemplary solar cell structure, the subcell between the top subcell 104 and the bottom subcell 102 may be referred to as the middle subcell. In general, the cell structure may have n subcells that may be electrically connected in series, where n may be equal to 1 for a single-junction cell, or n may be any integer greater than or equal to 2 for a multijunction solar cell. Multiple subcell structures are known to those skilled in the art but are not illustrated herein for clarity.

[0041]

[0046] One or more tunnel junctions may connect the subcells. For example, in one embodiment of a multijunction solar cell, one or more tunnel junctions may be disposed between the top subcell 104 and the middle subcell to electrically connect the subcells in series. In one embodiment, one or more tunnel junctions may be disposed between the middle subcell and the bottom subcell 102 to electrically connect the subcells in series. Generally, each of the n subcells in a multijunction solar cell, such as the solar cell structure 100 of FIG. 1, may be connected in series to an adjacent subcell(s) by one or more tunnel junctions to form a monolithic two-terminal series-interconnected multijunction cell. In a two-terminal configuration, it may be desirable to design the thickness and bandgap of the subcells so that each subcell has approximately the same current at the maximum power point of its current-voltage curve, so that one subcell does not severely limit the current of the other subcells. Alternatively, the subcells may be contacted by additional terminals, such as metal contacts to the laterally conductive semiconductor between the subcells. This allows the formation of three-terminal, four-terminal, and generally m-terminal multijunction cells (where m is an integer greater than or equal to 2 and less than or equal to 2n, and n is the number of active subcells in the solar cell structure). The subcells can be interconnected in a circuit using these additional terminals, thereby effectively utilizing most of the photogenerated current density available in each subcell. Such effective utilization can lead to high efficiency multijunction cells, even when the photogenerated current densities vary significantly among the various subcells.

[0042]

[0047] One or more subcells may have the same or different substructures based in part on their location within the solar cell structure 100, for example, depending on their configuration as the top subcell 104, at least one middle subcell (when present), or the bottom subcell 102.

[0043]

[0048] For example, in one embodiment of the solar cell structure 100, including a multijunction solar cell, the solar cell 100' includes a first subcell and a second subcell. The first subcell includes a base semiconductor layer and another semiconductor layer. The base semiconductor layer includes a p-type semiconductor, such as a p-type III-V semiconductor material, and the other semiconductor layer may include an emitter semiconductor layer. The emitter semiconductor layer may include an n-type semiconductor material. The second subcell includes an absorber layer. In this case, the absorber layer includes an organometal halide ionic solid perovskite semiconductor material. Generally, the organometal halide ionic solid perovskite semiconductor material of the absorber layer can be represented by the formula ABX3, where A includes an initiator ion, B includes a group IV ion, and X includes a halide ion. The organic ion may include methylammonium (MA), formamidine (FA), at least one alkali metal, or a combination thereof. In this case, the alkali metals may include cesium (Cs), rubidium (Rb), or both. The group IV ions may include Pb + , Sn + , or a combination thereof. The halide ion may include Cl - , Br - , I - , or a combination thereof. In one example, the organometal halide ionic solid perovskite of the absorber layer of the second subcell may include methylammonium lead iodide (CHNHPbI), methylammonium lead bromide (CHNHPbBr), methylammonium lead chloride (CHNHPbCl), methylammonium tin iodide (CHNHSnI), methylammonium tin bromide (CHNHSnBr), formamidinium lead iodide (NHCH=NHPbI), or a mixture thereof. Generally, the n-type material should have a lower valence band level (Ev) than the perovskite-containing absorber layer to limit the injection of minority carrier holes into the emitter. Further applicable low-temperature deposition materials may include polymeric materials suitable for PV applications.

[0044]

[0049] FIG. 2 illustrates a cross-sectional view of several embodiments of multijunction solar cell structures according to the present disclosure. FIG. 2 shows a cross-section of an exemplary perovskite / silicon 2J tandem solar cell 200. The multijunction photovoltaic device includes a substrate layer 202 incorporating a silicon n-on-p bottom junction 202 and overlying a perovskite n-on-p top junction layer 204. Opposite the substrate layer 202 is a p-side contact 206, which in this example is on the backside. The perovskite n-on-p top junction layer 204 does not completely cover the bottom junction of the substrate layer 202 because a portion of it is covered with a passivation layer 212 to prevent shorting between the perovskite n-side contact and the n-side of the silicon bottom junction. This passivation layer 212 or insulating layer has an n-side metal contact pad 208 attached to its exposed surface. In other examples, the orientation can be reversed to place it on the p-side. Attached to the metal contact 208 is an interconnect tab 210. The interconnect tabs 210 are shown in this example as metal grid fingers or transparent conductors in contact with the contacts 208 and the perovskite n-on-p top junction layer 204. Additionally, a portion of the substrate layer 202 is etched to add p-side contacts 214 to facilitate connections between multiple solar cells 200.

[0045]

[0050] In some embodiments of the multijunction photovoltaic device of the present disclosure, the first subcell includes a base semiconductor layer and a second semiconductor layer. In this case, the base semiconductor layer includes a III-V semiconductor material. The second subcell includes an absorber layer. In this case, the absorber layer includes an organometal halide ionic solid perovskite semiconductor material. In some embodiments, the absorber layer does not include an organometal halide ionic solid perovskite semiconductor material. Also, an insulating passivation layer is present on at least a portion of the top surface of the first subcell. An n-side metal pad contacts the passivation layer on the n-side of the second subcell. As disclosed herein, when the interconnect tab includes a metal, illustrative examples of the metal include low-resistivity metals such as silver, copper, various alloys, or complex structures such as multilayer structures or alloys.

[0046]

[0051] The insulating or passivation layer, in some embodiments, may include an oxide or sulfide layer. Illustrative examples may include materials such as insulating oxides and sulfides, such as, but not limited to, SiO2, Al2O3, TiO2, Ta2O5, HfO2, ZnS, and combinations thereof. In some embodiments, the n-side metal pad and the interconnect tab are on one side of the multijunction photovoltaic device. For example, the n-side metal pad and the interconnect tab are both on the n-side of the second subcell, or alternatively, on the p-side of the multijunction photovoltaic device. The passivation layer may be deposited or layered adjacent to the second subcell or where the second subcell does not completely cover the first subcell.

[0047]

[0052] In some general embodiments of the devices described herein, the solar cell structure comprises a two-junction solar cell structure, such as that of Figure 1 or Figure 2. In a first embodiment of the two-junction solar cell structure, a first subcell comprising a III-V semiconductor material in the base semiconductor layer may be configured as an upper subcell, such as an upper subcell. Accordingly, a second subcell comprising an organometal halide ionic solid perovskite semiconductor material in the absorber layer may be configured as a lower subcell, such as a lower subcell.

[0048]

[0053] Perovskite materials may be susceptible to UV light. A UV absorber layer may be disposed between the organometal halide ionic solid perovskite semiconductor material and a source of electromagnetic energy, including UV light. This arrangement allows some to all of the UV light reaching the device to be absorbed within the subcell containing the UV-absorbing active layer, minimizing or preventing the UV light from penetrating and / or degrading the perovskite layer. Thus, in a first embodiment expression, a III-V semiconductor may be selected to absorb UV light and serve as a UV protectant for the perovskite in the bottom subcell. Thus, the bandgap of the base semiconductor may be larger than the bandgap of the organometal halide ionic solid perovskite. In a second embodiment of the two-junction solar cell structure, a second subcell containing an organometal halide ionic solid perovskite semiconductor material within an absorber layer may be configured as the top subcell, such as top subcell 104. In a representation of the second embodiment of the two-junction solar cell, the bandgap of the base semiconductor layer can be smaller than the bandgap of the organometal halide ionic solid perovskite.

[0049]

[0054] In some embodiments, the solar cell structure includes a three-junction solar cell structure. The three-junction solar cell structure can be similar to the solar cell structure of FIG. 1 or FIG. 2, but the solar cell structure includes an additional third subcell as a middle subcell. In a first embodiment of the three-junction solar cell, the second subcell includes an organometal halide ionic solid-state perovskite semiconductor material in the absorber layer and can be configured as a top subcell, such as a top subcell. The third subcell can be configured as a bottom subcell, such as a bottom subcell. Thus, the first subcell includes a III-V semiconductor material in the base semiconductor layer and can be configured as a middle subcell, such as a middle subcell. In the representation of the first embodiment of the three-junction solar cell, the bandgap of the base semiconductor can be greater than the bandgap of the organometal halide ionic solid-state perovskite. In some embodiments of the multi-junction solar cell or photovoltaic device, all of the subcells are in electrical contact with each other.

[0050]

[0055] Although not shown in FIG. 1 or FIG. 2 , additional layers may be included without departing from the scope of the present disclosure. For example, a traditional feature of PV cells has been the use of a window layer over an emitter layer disposed on the base of the PV cell. The primary function of the window layer is to reduce minority carrier recombination (i.e., passivate the front surface of the emitter). Furthermore, the optical properties of the window material must be such that it transmits as much light as possible to the underlying cell layers, where photogenerated charge carriers can be more efficiently collected. Alternatively, if significant light is absorbed within the window, the lifetime of minority carriers within the window must be long enough for the carriers to be efficiently collected at the p-n junction between the emitter and base of the PV cell. Similarly, back surface field (BSF) structures below the PV cell base have also been used to reduce minority carrier recombination at the back surface of the base. With respect to the window, the BSF structure must have optical properties that allow most of the light that can be used by the subcells below the BSF to be transmitted by the BSF. and / or the minority carrier characteristics in the BSF must be such that electrons and holes generated by light absorption in the BSF are efficiently collected at the pn junction of the PV cell.

[0051]

[0056] Furthermore, in perovskite cells, the use of buffer layers and / or hole transport material (HTM) layers allows electrons or holes to pass from the perovskite absorber layer and block carriers on the other side. This arrangement enforces one-way (diode) behavior. The n-side buffer layer and HTM layer in a perovskite subcell can each have a corresponding bandgap value equal to or greater than the bandgap value of the perovskite layer. When formed adjacent to the perovskite layer, the HTM layer can have a valence band energy (Ev) substantially equal to the valence band energy (Ev) of the perovskite layer. When formed adjacent to the perovskite layer, the n-side buffer layer interface can have an Ec substantially equal to the conduction band energy (Ec) of the perovskite layer.

[0052]

[0057] The bottom subcell can be disposed on a growth substrate. The growth substrate can be electrically active or can be electrically active, thereby forming one of the n subcells in a multi-junction photovoltaic device. Additional layers, such as a support layer, a reflective layer, an anti-reflective coating (ARC), and / or a cover glass layer, can also be included in the solar cell structure 100.

[0053]

[0058] Several variations of photovoltaic structures can be used as subcells within a solar cell structure, such as structure 100 of Figure 1 or solar cell 200 structure of Figure 2. Several other components, substructures, or additional layers, such as emitter-base subcells, base semiconductors, emitter semiconductor layers, window layers, BSF layers, emitter-absorber subcells, absorber layers, HTM layers, absorber-base subcells, buffer layers, emitter active substrate subcells, base semiconductor substrates, nucleation layers, etc., are described in U.S. Patent No. 10,861,992, which is incorporated herein by reference in its entirety.

[0054]

[0059] FIG. 3 is a flowchart illustrating a prior art method for fabricating solar cells according to the present disclosure. Prior art methods for forming a multijunction solar cell 300 begin with forming a substrate subcell 302. The substrate subcell typically comprises silicon, germanium arsenide, or germanium, Mo (molybdenum), InP, SiGe alloys, silicon-on-glass (SoG), germanium-on-insulator (GeoI), or a combination thereof. Next, steps include forming one or more upper subcells 304 via epitaxy or deposition and processing the subcells into solar cells 306 by adding a metal grid to the top of the multijunction cell and metal to the backside. Materials and configurations for several embodiments of other solar cell devices, such as those described herein, can be fabricated by this method. After processing the subcells into solar cells 306, the cells can be diced into shapes 308 and tested 310, along with adding front-side interconnect tabs 312 by soldering or welding at high temperatures. Exemplary temperature ranges for this step include approximately 100° C. to 500° C., or above 150° C. to below 300° C. To complete a multi-junction photovoltaic device or array thereof, vitrification 314, addition of backside interconnects 316, and assembly of one or more cells into strings of cells 318 may be performed.

[0055]

[0060] FIG. 4 is a flowchart illustrating a method for fabricating a solar cell according to the present disclosure. The present disclosure provides a method 400 for fabricating a solar cell. The method 400 includes forming a substrate subcell 402 and depositing an insulator 404, including masking, steps that may include backside metallization. This formation of the insulator can protect the n-side of the substrate cell and allow for higher-temperature metal deposition on the insulator. Further steps include adding a front-side interconnect tab 406 by soldering or welding, as described above, and forming one or more upper subcells 408 via epitaxy or deposition. The formation of the upper subcell occurs after the interconnects are attached. Fabricating a photovoltaic device in this manner separates high-temperature operations, such as welding or soldering, from low-temperature-sensitive steps, including the formation of one or more upper subcells, which use materials that may degrade or otherwise be damaged when exposed to the high-temperature operations referenced herein. Next is processing the subcells into cells 410, which may include front-side metallization connecting to pads using low-temperature deposition, and dicing the cells to shape 412. Dicing may alternatively be performed before forming the top subcell (as shown in step 408). Next, testing 414 the cells may be performed. Further steps of assembling 420 one or more cells into strings of cells may then be performed, such as vitrifying 416, adding backside interconnects 418, and assembling one or more multijunction solar cells into an array of cells, which may result in a completed multijunction photovoltaic device or array thereof.

[0056]

[0061] In exemplary embodiments, a method for forming a multijunction solar cell includes forming a substrate subcell including an n-side, a backside, a base semiconductor layer, and a second semiconductor layer, where the base semiconductor layer includes a III-V semiconductor material. The method further includes forming a passivation layer to protect the n-side of the substrate subcell, forming a high-temperature metal deposition on the backside of the substrate subcell, forming a front-side interconnect tab in contact with the passivation layer, and forming a second subcell including an absorber, where the absorber includes a perovskite semiconductor material. The method further includes forming a front-side metallization connecting to the pads using low-temperature deposition. The high-temperature metal deposition is performed at a first temperature ranging from about 100°C to about 400°C, about 300°C to about 500°C, or about 300°C to about 800°C. The low-temperature second-temperature metallization can be performed at a temperature that does not degrade or decompose any perovskite or other high-temperature-sensitive materials. The lower second temperature deposition may be performed at a temperature ranging from about 100 to 500°C, or from above 150°C to below 300°C. The lower second temperature is performed at a temperature or temperature range such that the perovskite-based material, or other materials susceptible to degradation at high temperatures, are not degraded or destroyed. Some embodiments include those in which the passivation layer comprises an oxide layer. The interconnect tab, when formed by the method of the present disclosure, contacts the second subcell in the multijunction solar cell. In some embodiments, the substrate subcell comprises a base semiconductor layer comprising Si, GaAs, or Ge. In certain embodiments, the sequence of processes in the method may be alternating or may include additional applicable steps.

[0057]

[0062] FIG. 5 is a diagram of the International Space Station (ISS), which may employ one or more of the disclosed multi-junction photovoltaic devices. In yet a further embodiment, the article may include a solar array. The article may be a sea, land, air, or space vehicle, or may be a satellite system. The solar array may be operatively coupled to the article. In one such example, the article is a satellite. In one example, the article is the International Space Station (ISS) 500, as shown in FIG. 5, with a solar array 502 operatively coupled to the ISS. The solar array 502 may include at least one photovoltaic device including a III-V material layer and, in certain examples, at least one perovskite material layer. In this case, the III-V material layer may be a layer in a first subcell, and the at least one perovskite material layer may be a layer in a second subcell. Alternatively, the III-V material layer and the at least one perovskite material layer may be layers within a single subcell of a multi-junction photovoltaic device comprising multiple subcells. For example, the solar cell array 502 may include at least one photovoltaic device as described above with respect to each of the preceding figures.

[0058]

[0063] While the teachings herein have been illustrated in connection with one or more embodiments, changes and / or modifications can be made to the examples without departing from the spirit and scope of the appended claims. For example, while a process may be described as a series of acts or events, it will be understood that the teachings herein are not limited by the ordering of such acts or events. Some acts may occur in a different order than described herein and / or concurrently with acts or events other than those described herein. Also, not all process steps may be required to implement a methodology in accordance with one or more aspects or embodiments of the present disclosure. It will be recognized that structural components and / or process steps may be added, or existing structural components and / or process steps may be removed or modified. Furthermore, one or more of the acts described herein may be performed in one or more separate acts and / or phases. Furthermore, to the extent that the terms "including," "includes," "having," "has," "with," or variations thereof are used in either the detailed description or the claims, these terms are intended to be as inclusive as the term "comprising." The phrase "at least one of" is used to mean that one or more of the listed items may be selected. Furthermore, in the description and claims herein, the term "on" when used in connection with two materials, such as one "on" the other, means that there is at least some contact between the materials, whereas "over" means that the materials are in close proximity to each other, possibly with one or more additional intervening materials, and that contact is possible but not required. Neither "on" nor "over," as used herein, implies any directionality. The term "conformal" describes a coating material in which the angle of the underlying material is protected by the conformal material. The term "about" indicates that the recited value may be varied somewhat without causing non-compliance of the process or structure with respect to the illustrated embodiment.The terms "couple," "coupled," "connect," "connection," "connected," "connecting," and "connecting" refer to a "direct connection" or a "connection via one or more intermediate elements or members." Finally, "exemplary" indicates that the description is used as an example, rather than implying ideality. Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure herein. It is intended that the specification and examples be considered exemplary only, with a true scope and spirit of the present teachings being indicated by the following claims.

Claims

1. 1. A multi-junction photovoltaic device comprising: a first subcell including a base semiconductor layer and a second semiconductor layer, the base semiconductor layer including a III-V semiconductor material; a second subcell on the first subcell comprising an absorber layer, the absorber layer comprising an organometal halide ionic solid perovskite semiconductor material; a passivation layer (212) on at least a portion of the top surface of the first subcell; and a n-side metal pad contacting the passivation layer (212) on the n-side of the second subcell.

2. The multijunction photovoltaic device of claim 1 further comprising an interconnect tab in contact with said n-side metal pad.

3. The multijunction photovoltaic device of claim 2 wherein the interconnect tab contacts the second subcell.

4. The multijunction photovoltaic device of claim 2 , wherein the interconnect tabs comprise a metal.

5. The multijunction photovoltaic device of claim 4 , wherein the n-side metal pad and the interconnect tub comprise two different metals.

6. The multi-junction photovoltaic device of claim 1 , wherein the passivation layer (212) comprises an oxide layer.

7. The passivation layer (212) is made of SiO 2 , Al 2 O 3 , TiO 2 , Ta 2 O 5 , HfO 2 7. The multi-junction photovoltaic device of claim 6, comprising a material selected from the group consisting of: ZnS, ZnS, and combinations thereof.

8. The multijunction photovoltaic device of claim 2 , wherein the n-side metal pad and the interconnect tab are on one side of the multijunction photovoltaic device.

9. The multijunction photovoltaic device of claim 1 , wherein the passivation layer (212) is adjacent to the second subcell.

10. The multijunction photovoltaic device of claim 1 , wherein the second subcell does not completely cover the first subcell.

11. 1. A multi-junction photovoltaic device comprising: a first subcell including a base semiconductor layer and a second semiconductor layer, the base semiconductor layer including a III-V semiconductor material; a second subcell on the first subcell comprising an absorber layer, the absorber layer not comprising a perovskite semiconductor material; a passivation layer (212) on at least a portion of the top surface of the first subcell; an n-side metal pad contacting the passivation layer (212) on the n-side of the second subcell; and a multijunction photovoltaic device comprising a grid finger contacting the n-side metal pad and the second subcell.

12. The multijunction photovoltaic device of claim 11 , wherein the passivation layer (212) is adjacent to the second subcell.

13. 12. The multijunction photovoltaic device of claim 11, wherein the second subcell does not completely cover the first subcell.

14. 1. A method of forming a multijunction solar cell, comprising: forming a substrate subcell including an n-side, a backside, a base semiconductor layer, and a second semiconductor layer, the base semiconductor layer comprising a III-V semiconductor material; forming a passivation layer (212) to protect the n-side of the substrate subcell; forming a metal deposit on a backside of the substrate subcell at a first temperature; forming an interconnect tab in contact with said passivation layer (212); forming a second subcell comprising an absorber layer, the absorber layer not comprising perovskite semiconductor material; and A method of forming a multijunction solar cell comprising forming a front-side metallization connecting to the pad at a second temperature using metal deposition.

15. The method for forming a multijunction solar cell of claim 14, wherein the passivation layer (212) comprises an oxide layer.

16. 15. The method for forming a multijunction solar cell of claim 14 wherein said interconnect tab contacts said second subcell.

17. 15. The method for forming a multijunction solar cell of claim 14, wherein the substrate subcell comprises a base semiconductor layer comprising Si, GaAs, or Ge.

18. 15. The method for forming a multijunction solar cell of claim 14, further comprising assembling one or more multijunction solar cells into an array of cells.

19. 15. The method for forming a multijunction solar cell of claim 14, further comprising forming said interconnect tabs by high temperature soldering.

20. 15. The method for forming a multijunction solar cell of claim 14, further comprising forming said interconnect tabs by welding at high temperature.

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