TW201840013A - Nanostructured subcells with high transparency in multi-junction pv applications - Google Patents

Abstract

A method of making a stacked tandem photovoltaic device including determining nanostructure and index of refraction parameters of subcells of the stacked tandem photovoltaic device to reduce in-plane waveguiding of light incident on the stacked tandem photovoltaic device, providing a first photovoltaic subcell, the first photovoltaic subcell comprising nanostructures based on the step of determining and providing a second photovoltaic subcell based on the step of determining. The first photovoltaic subcell has a first bandgap, the second photovoltaic subcell has a second bandgap and the first bandgap is larger than the second bandgap. Light incident on the stacked tandem photovoltaic device passes through the first photovoltaic subcell before entering the second photovoltaic subcell.

Description

Nanostructure subcell with high transparency in multi-junction PV applications

The present invention is generally directed to photovoltaic cells and, in particular, to series photovoltaic cells.

The III-V semiconductor nanowire is a platform for the next generation of photovoltaic devices. The array of nanowires embedded in the transparent polymer can act as a stand-alone flexible solar cell or stacked on top of a conventional Si bottom cell to form a series structure. To optimize tandem cell performance, high energy photons should be absorbed in the nanowire, while low energy photons should be transmitted and absorbed in the Si cell. The III-V nanowire array has shown promise for photovoltaic devices with 13.8% for InP nanowires and 15.3% for GaAs nanowires. Theoretical modeling has shown that the absorption of light in such arrays can be as nearly as efficient as the corresponding bulk cells.

One embodiment results in a method of fabricating a stacked tandem photovoltaic device that includes determining the nanostructure and refractive index parameters of a subcell of a stacked tandem photovoltaic device to reduce the plane of light incident on the stacked tandem photovoltaic device The inner waveguide provides a first photovoltaic cell, the first photovoltaic cell comprises a nanostructure based on the determining step, and the second photovoltaic cell is provided based on the determining step. The first photovoltaic cell has a first band gap, the second photovoltaic cell has a second band gap, and the first band gap is greater than the second band gap. Light incident on the stacked series photovoltaic device passes through the first photovoltaic cell before entering the second photovoltaic cell. Another embodiment leads to a photovoltaic device comprising a first photovoltaic cell comprising a nanostructure, wherein in the stacked photovoltaic device, the first photovoltaic cell is located above the second photovoltaic cell, the first photovoltaic cell At least one feature of the battery reduces or eliminates reflection losses due to in-plane waveguides of incident light, and wherein the first photovoltaic cell is configured to permit light incident on the stacked tandem photovoltaic device to enter the second photovoltaic The sub-cell passes through the first photovoltaic cell. Another embodiment results in a stacked tandem photovoltaic device comprising a first photovoltaic cell comprising a nanostructure and a second photovoltaic cell. The first photovoltaic cell has a first band gap, the second photovoltaic cell has a second band gap, and the first band gap is greater than the second band gap. At least one feature of the stacked series photovoltaic devices reduces or eliminates reflection losses due to in-plane waveguides of incident light on the stacked series photovoltaic devices. Light incident on the stacked series photovoltaic device passes through the first photovoltaic cell before entering the second photovoltaic cell. Another embodiment elicits a method of operating a stacked tandem photovoltaic device, the method comprising receiving incident light on a first photovoltaic cell comprising a nanostructure such that at least a portion of the incident light passes through the first photovoltaic cell and Entering the second photovoltaic cell, and generating current or voltage from the first photovoltaic cell and the second photovoltaic cell. In one embodiment, the current or voltage in the first photovoltaic cell and the second photovoltaic cell can be generated and output separately (ie, using separate subcell electrodes independently of one another). The first photovoltaic cell has a first band gap, the second photovoltaic cell has a second band gap, and the first band gap is greater than the second band gap, and at least one feature of the stacked series photovoltaic device is reduced or eliminated Reflection loss due to waveguides in the plane of incident light.

To optimize the absorption of sunlight in an array of direct bandgap III-V semiconductor nanowire solar cells (i.e., photovoltaic cells), the waveguide modes in individual nanowires can function. As used herein, "solar battery" has the same meaning as "photovoltaic battery." Due to the resonant absorption of the single nanowire modes, the absorption peaks red-shift as the diameter increases. By placing one of these absorption peaks near the band gap of the nanowire material, the absorption in the nanowire can be enhanced in the vicinity of the band gap wavelength region in which the III-V material is present. The absorption coefficient is low. In this way, there is a set of diameters in which the absorption of sunlight in the nanowires can be optimized. At each of these diameters, one of the single nanowire modes is placed near the band gap wavelength. For the selected diameter, the length of the nanowire and the spacing between the nanowire arrays can be optimized. Generally, the optimum spacing increases as the length of the nanowire increases. This behavior is attributed to the following physics: (1) as the spacing increases, the reflection at the top interface of the nanowire array decreases, and (2) as the spacing increases, there is a smaller amount of absorption in the nanowire array. The material, therefore, reduces absorption, and (3) compensates for the decrease in absorption by increasing the length of the nanowire. Therefore, it is preferred to increase the spacing to reduce internal coupling reflection losses due to light entering the array of nanowires. However, the pitch cannot be increased without limitation. This is because at some point the absorption will be much lower than the reflection. The spacing at which the absorption reduction and the reflection decrease are balanced with each other increases as the length of the nanowire increases. Typically, the absorption in the array of nanowires is optimized without highlighting the transmission of photons below the band gap. However, in a tandem solar cell having a nanowire array subcell on top of a lower bandgap subcell, this transmission is quite important. As used herein, the nanowire of a nanowire array comprises a GaAs p-n or p-i-n junction solar cell (or a subcell of a tandem solar cell). The direction of extension of the junction relative to the nanowire may be axial and/or radial. The tandem cell can be formed by epitaxially growing a III-V nanowire directly above the top of the Si cell. The nanowire is a structure having a diameter (or a width in the case of a hexagonal nanowire) of less than 1 micrometer (such as 10-500 nm). The length can be greater than 1 micron. The length to diameter/width ratio can be 10:1 or greater. Alternatively, the III-V nanowires can be separately grown and subsequently embedded in a transparent polymer and/or inorganic dielectric by a nanowire (eg, by embedding a nanowire solar cell containing a pn or pin junction into the dielectric film) Medium) placed on top of the Si battery. The latter method is compatible with Aerotaxy, high throughput, low cost, substrateless nanowire manufacturing techniques, and compatible with conventional VLS-based nanowire growth, which is grown on a strippable polymer film. The combination of the nanowires with the self-growth substrate is subsequently removed. Preferably, for a tandem method of nanowire cells on a planar germanium cell, the nanowire array should efficiently absorb high energy photons and exhibit high transmission of low energy photons to the bottom cell. In the first configuration described above, when the nanowire is located directly above the top of the Si substrate, the Si substrate constitutes a lower bandgap bottom subcell, and the optimal size of the nanowire array will mainly be the nanowire. Depending on the absorption in the subcell. However, in the second configuration embodiment described above, in the "stand-alone" (for example, in which the nanowire is embedded in the free-standing dielectric film), the nanowire array sub-cell cannot be used. Direct contact with the underlying substrate subcell (which may be, for example, a solar subcell containing a pn or pin junction in the crucible). Specifically, in such embodiments disclosed herein, a dielectric material and/or a thin conductive layer between a nanowire array (eg, a nanowire solar subcell) and a substrate solar subcell is included. 1 is a schematic diagram of a stacked tandem photovoltaic cell 100 in accordance with an embodiment. This embodiment includes a first photovoltaic cell 102, a second photovoltaic cell 104, and may include any number of additional photovoltaic cells 106. As illustrated in Figure 1, the first photovoltaic cell 102 is located on top of the stack, i.e., light I (E)_Ph All of the solar spectrum is incident on the top surface of the first photovoltaic cell 102. The second photovoltaic cell 104 is located below the first photovoltaic cell 102 and the additional photovoltaic cell 106 is located below the second photovoltaic cell 104. The first photovoltaic cell 102 has a band gap E₁ . Preferably, the first photovoltaic cell 102 absorbs at least a majority of the energy E_Ph > E₁ Photons (eg, at least 70%, such as at least 90%, such as substantially all photons). In addition, the first photovoltaic cell 102 responds to energy E_Ph Less than the first band gap energy E₁ The light is transparent. The stacked tandem photovoltaic cell 100 is configured such that the second band gap E of the second photovoltaic cell 104₂ Less than the first band gap E₁ . Preferably, the second photovoltaic cell 104 absorbs at least a majority of the energy E_Ph > E₂ Photons (eg, at least 70%, such as at least 90%, such as substantially all photons). In addition, the second photovoltaic cell 104 is responsive to energy E_Ph Less than the second band gap energy E₂ The light is transparent. The same pattern is true for all subsequent additional sub-cells 106. However, the final subcell in the stacked tandem photovoltaic cell 100 need not be transparent. 2A and 2B are schematic views of conventional stacked series photovoltaic cells 200a, 200b. In the conventional stacked series photovoltaic cells 200a, the photovoltaic cells 102, 104, 106 are planar/block photovoltaic devices. Some of the light incident on the first photovoltaic cell 102 is absorbed in the first photovoltaic cell 102, some of the light passes through the second photovoltaic cell 104, and some of the light is from the first photovoltaic cell 102. The interface between the second photovoltaic cells 104 is reflected back. Some of the light incident on the second photovoltaic cell 104 is absorbed in the second photovoltaic cell 104, some of the light passes through the third photovoltaic cell 106, and some of the light is from the second photovoltaic cell 104. The interface between the third photovoltaic cells 106 is reflected back. Light 108 that is neither absorbed nor reflected by the third photovoltaic cell 106 can pass through the third photovoltaic cell 106. A mirror or reflective coating can be applied to the bottom of the third photovoltaic cell 106 as appropriate to reflect light 108 back into the stacked tandem photovoltaic cell 200a. The stacked tandem photovoltaic cell 200b illustrated in Figure 2B is similar to the device illustrated in Figure 2A. However, the stacked tandem photovoltaic cells 200b include a separation layer 103 between the first photovoltaic cell 102 and the second photovoltaic cell 104 and between the second photovoltaic cell 104 and the third photovoltaic cell 106, respectively. And 105. 2C is a schematic diagram of a stacked tandem photovoltaic cell 200c in accordance with an embodiment. In this embodiment, the planar/block photovoltaic devices of the conventional stacked series photovoltaic cells 200a, 200b are replaced by sub-cells 202, 204, 206 comprising nanostructures. In one embodiment, the nanostructure is a nanowire. Unlike conventional planar subcells 102, 104, 106, nanostructure subcells 202, 204, 206 can experience losses by mechanisms that are not present in conventional planar/block photovoltaic cells 102, 104, 106. In particular, the waveguide properties of the nanowires can produce in-plane waveguide modes 210 and/or diffraction stages 212. Although not illustrated, the in-plane waveguide mode 210 and the diffraction stage 212 may be present simultaneously in the same photovoltaic cells 206, 204, 206. The in-plane waveguide mode 210 and the diffraction stage 212 are discussed in more detail below. 3A-3D are schematic illustrations of embodiments of stacked tandem photovoltaic cells 300a-300d. In the stacked tandem photovoltaic cell 300a illustrated in FIG. 3A, the first photovoltaic cell 202 is adjacent to the second photovoltaic cell 204. In this embodiment, the second photovoltaic cell 204 comprises a film made of a material having a high refractive index, and the first photovoltaic cell 202 comprises a film made of a material having a low refractive index. The low refractive index is defined as 1 ≤ n < 2. The high refractive index is defined as n ≥ 2, such as 2 ≤ n ≤ 7, including 2 ≤ n ≤ 4. The stacked tandem photovoltaic cell 300b illustrated in Figure 3B is similar to the embodiment illustrated in Figure 3A. However, in the embodiment illustrated in FIG. 3B, a transparent conductive oxide (TCO) provides a layer 203 between the first photovoltaic cell 202 and the second photovoltaic cell 204. In this embodiment, the thickness of the TCO layer 203 is less than half the wavelength (λ) of the light to be absorbed in the second photovoltaic cell 204. As used herein, at least a majority of photons of a given wavelength or range of wavelengths (eg, at least 70%, such as at least 90%, such as substantially all photons) are understood to be for the real world (ie, non-theoretical devices). Absorbed or transmitted. In the stacked tandem photovoltaic cell 300c illustrated in FIG. 3C, a second TCO layer 205 is optionally provided in addition to the window layer 207. In this embodiment, the window layer 207 is made of a material having a high refractive index. If present, the thickness of the second TCO layer is preferably less than half the wavelength of the light to be absorbed in the second photovoltaic cell 204. Preferably, the window layer 207 is located between the first TCO layer 203 and the second TCO layer 205, between the first photovoltaic cell 202 and the second photovoltaic cell 204. The stacked tandem photovoltaic cell 300d illustrated in Figure 3D is similar to the embodiment illustrated in Figure 3C. However, in this embodiment, the window layer 207 is made of a material having a low refractive index and a thickness greater than λ/2. The window layer 207 has a lower index of refraction than the second photovoltaic cell 204 and a refractive index that is the same as or greater than the first photovoltaic cell 202. One embodiment results in a method of making stacked tandem photovoltaic devices 300a-300d. The method includes determining nanostructures and refractive index parameters of the sub-cells 202, 204 of the stacked series photovoltaic devices 300a-300d to reduce in-plane waveguides of light incident on the stacked tandem photovoltaic devices 300a-300d. The method also includes providing a first photovoltaic cell 202, the first photovoltaic cell 202 comprising a nanostructure based on the steps of determining the nanostructure and refractive index parameters of the subcell 202 (and optionally Based on the step of determining the refractive index parameter of the underlying second subcell 204, such as determining a reduction in the in-plane waveguide, it may be reduced at the expense of a slight decrease in the amount of transmitted solar radiation of the first photovoltaic cell 206. The method also includes providing a second photovoltaic cell 204 based on the step of determining at least the nanostructure and refractive index parameters of the first subcell 202. The first photovoltaic cell 202 has a first band gap, the second photovoltaic cell 204 has a second band gap, and the first band gap is greater than the second band gap. Light incident on the stacked series photovoltaic devices 300a-300d passes through the first photovoltaic cell 202 before entering the second photovoltaic cell 204. In an embodiment, the band gap E of the first photovoltaic cell 202₁ Less than the bandgap of the first photovoltaic cell that is optimized for optical transparency without regard to the in-plane waveguide. In one embodiment, the method further includes forming a first layer 203 having a higher refractive index than the effective refractive index of the photovoltaic cell 202 between the first photovoltaic cell 202 and the second photovoltaic cell 204. In one embodiment, the first layer 203 having a higher refractive index has a thickness less than λ/2, where λ is the wavelength of light to be absorbed in the second photovoltaic cell 204. The effective refractive index of a subcell (such as a nanostructure subcell) is the function of the geometry and material of the subcell. In one embodiment, the first layer 203 having a higher refractive index comprises a transparent conductive oxide. Another embodiment includes forming a window layer 207 between the first photovoltaic cell 202 and the second photovoltaic cell 204, the refractive index of the window layer 207 being higher than the effective refractive index of the first photovoltaic cell 202, and the thickness Less than λ/2. Another embodiment includes forming a second layer 205 comprising a transparent conductive oxide between the first photovoltaic cell 202 and the window layer 207 or the second photovoltaic cell 204, the transparent conductive oxide The thickness of the second layer 205 is less than λ/2. Another embodiment includes forming a window layer 207 between the first photovoltaic cell 202 and the second photovoltaic cell 204, the refractive index of the window layer 207 being lower than the effective refractive index of the second photovoltaic cell 204, and the thickness Greater than λ/2. In other embodiments, the refractive index of the window layer 207 is lower than the effective refractive index of the second photovoltaic cell 204 and the thickness is less than λ/2. Another embodiment includes forming a second layer 205 comprising a transparent conductive oxide between the first photovoltaic cell 202 and the window layer 207 or the second photovoltaic cell 204, the transparent conductive oxide The thickness of the second layer 205 is less than λ/2. The second layer 205 of transparent conductive oxide can be located closer to or closer to the first photovoltaic cell 202 than the window layer 207. In one embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, which is a realistic portion of the refractive index at the wavelength of the transmitted light, and The diameter of the rice noodle is smaller than the diameter of the nanowire of the same composition for optical transparency or absorption optimization regardless of the in-plane waveguide. In another embodiment, the nanostructure comprises a nanowire, a dielectric is provided between the nanowires, the nanowire has a higher refractive index than the dielectric, and the refractive index of the dielectric is optimized for optical transparency or absorption optimization. The same composition of the in-plane waveguide has a low dielectric. In one embodiment, the nanostructure comprises a nanowire, a dielectric is provided between the nanowires, the nanowire has a higher refractive index than the dielectric, and the nanowire comprises a semiconductor material having a ratio of optical transparency The optimization does not take into account the refractive index of the nanowire of the in-plane waveguide and the lower effective refractive index. In another embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, and the nanowire diameter of the fixed nanowire diameter being less than for optical transparency Optimized without regard to the nanowire density of the same composition of the in-plane waveguide. In one embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, and the nanowire diameter of the fixed nanowire density being less than optimized for optical transparency Regardless of the diameter of the nanowire of the same composition of the in-plane waveguide. In another embodiment, the nanostructures are configured in a periodic array, wherein the spacing between the nanostructures is selected such that in transparency (ie, transmitted through the first photovoltaic cell containing the nanostructures) The wavelength range of the relevant wavelength range (eg, 400 to 750 nm) exists for the in-plane k-vector without the in-plane waveguide mode. An embodiment includes tuning a transparency wavelength window with a band gap of a subcell; or placing a subcell exhibiting a reflection problem close enough to adjacent subcells such that the in-plane waveguide mode cannot be "leaked" to adjacent sub-cells Exciting in the battery; or selecting a sufficiently small contrast between the higher refractive index material and the lower refractive index material in each of the subcells exhibiting resonant reflection; or selecting to include a sufficiently low refractive index material to reduce the light direction In-plane directional scattering; or use a periodic system of sufficiently small time period to prevent excitation of the in-plane waveguide. In one embodiment, the first photovoltaic cell absorbs, for example, a majority (such as greater than 80%) of light having a greater energy than the first bandgap and less than 50% (such as less than 20%) of energy less than the first bandgap. . An example stacked tandem photovoltaic device that can achieve such results is illustrated in Figures 3A-5B and described in more detail above and in the "Solar Cell Design Considerations" section below. An embodiment includes adapting the dispersion of the in-plane waveguide mode such that it cannot be excited by the method allowed by the k-vector in the relevant wavelength range of transparency, or allowing the in-plane waveguide mode to be selected by the k-vector, but by The actual excitation strength of the decreasing modality of the scattering geometry of the array is adjusted. Another embodiment includes selecting a diameter of the nanowire to prevent resonant reflection when the nanostructure comprises a nanowire. One embodiment leads to stacked tandem photovoltaic devices 300a-300d. The device includes a first photovoltaic cell 202 that includes a nanostructure. The device includes a second photovoltaic cell 204. The first photovoltaic cell 202 has a first band gap E₁ The second photovoltaic cell 204 has a second band gap E₂ And the first band gap E₁ Greater than the second band gap E₂ . The first photovoltaic cell 202 and the second photovoltaic cell 204 are configured based on determining the nanostructure and refractive index parameters of the first subcell 202 and the second subcell 204 of the stacked tandem photovoltaic devices 300a-300d, Thereby the in-plane waveguides of the light incident on the stacked series photovoltaic devices 300a-300d are reduced. Light incident on the stacked tandem photovoltaic devices 300a-300d passes through the first photovoltaic cell 202 before entering the second photovoltaic cell 204, as will be discussed above and below in the "Solar Cell Design Considerations" section. Described in more detail. In an embodiment, the band gap E of the first photovoltaic cell 202₁ Less than the bandgap of the first photovoltaic cell that is optimized for optical transparency without regard to the in-plane waveguide. An embodiment further includes a first layer 203 having a higher refractive index between the first photovoltaic cell 202 and the second photovoltaic cell 204. In one embodiment, the first layer 203 having a higher refractive index has a thickness less than λ/2, where λ is the wavelength of light to be absorbed in the second photovoltaic cell 204. In one embodiment, the first layer 203 having a higher refractive index comprises a transparent conductive oxide. An embodiment further includes a window layer 207 between the first photovoltaic cell 202 and the second photovoltaic cell 204, the window layer 207 having a higher refractive index than the first photovoltaic cell 202 and having a thickness less than λ /2. Another embodiment includes a second layer 205 comprising a transparent conductive oxide between the first photovoltaic cell 202 and the second photovoltaic cell 204, the second layer 205 of the transparent conductive oxide having a thickness less than λ/2 . Another embodiment includes a window layer 207 between the first photovoltaic cell 202 and the second photovoltaic cell 204, the window layer 207 having a lower index of refraction than the first photovoltaic cell 202 and having a thickness greater than λ/ 2. Another embodiment includes a second layer 205 comprising a transparent conductive oxide between the first photovoltaic cell 202 and the second photovoltaic cell 204, the second layer of transparent conductive oxide having a thickness less than λ/2. In one embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, and the diameter ratio of the nanowires being optimized for optical transparency without regard to in-plane The diameter of the nanowire of the same composition of the waveguide is small. In one embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, and the refractive index of the dielectric being optimized for optical transparency without regard to the in-plane waveguide The dielectric is low. In one embodiment, the nanostructure comprises a nanowire, a dielectric is provided between the nanowires, the nanowire has a higher refractive index than the dielectric, and the nanowire comprises a semiconductor material having a ratio of optical transparency Optimization does not take into account the lower refractive index of the nanowire of the in-plane waveguide. In one embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, and the nanowire density of the fixed nanowire diameter being less than optimized for optical transparency Regardless of the nanowire density of the same composition of the in-plane waveguide. In one embodiment, the nanostructure comprises a nanowire providing a dielectric between the nanowires, the nanowire having a higher refractive index than the dielectric, and the nanowire diameter of the fixed nanowire density being less than optimized for optical transparency Regardless of the diameter of the nanowire of the same composition of the in-plane waveguide. In an embodiment, the nanostructures are configured in a periodic array in which the spacing between the nanostructures is selected such that there is no in-plane waveguide mode for the in-plane k-vectors in the relevant wavelength range of transparency. As described above, in one embodiment, the first photovoltaic cell absorbs, for example, a majority (such as greater than 80%) of energy greater than the first bandgap and less than 50% (such as less than 20%) of energy less than the first Band gap light. The optimization of the absorption in the nanowire array subcells is not sufficient to optimize the combined absorption of the upper bandgap photons in the nanowire array and the transmission of the underlying bandgap photons by the nanowire array. First, it is considered that the nanowire array is completely embedded in the dielectric film. As calculated by volume averaging, the refractive index of the nanowire array is more efficient than that of the dielectric on the top and bottom surfaces. In this case, the light waves can be excited in the plane of the array of nanowires. That is, the excitation of the in-plane waveguide mode can be observed. When the waves are coupled out of the film, they can result in enhanced reflection compared to the case where the in-plane waveguide modes are not excited. The resonant excitation of the in-plane waveguide modes can result in significant reflection losses of the underlying bandgap photons. However, since these upper bandgap photons are heavily absorbed in the nanowire, the in-plane mode typically does not result in a large amount of reflection loss of the upper bandgap photons. If the size of the nanowire reported for the nanowire placed directly above the Si substrate is the only guidance for device design, the resonant reflection loss of the lower bandgap photons results in a less efficient device. The inventors have discovered that the resonant excitation of the in-plane waveguide mode depends on the diameter of the nanowire, the array spacing, and the material surrounding the nanowire and the material between the nanowires. In principle, the physics underlying resonant reflection from the in-plane waveguide mode can be complex. When the diameter and time period of the nanowire are changed, the dispersion of the in-plane waveguide mode and the excitation intensity of the nanowire change. In order to prevent the resonant excitation of the in-plane waveguide mode, there are two ways: (1) adapting the dispersion of the in-plane waveguide mode so that it cannot be excited by the method allowed by the k-vector in the relevant wavelength range of transparency, or (2) Allowing the in-plane waveguide mode to be selected by the k-vector, but decreasing the actual excitation strength of the mode by adapting the scattering geometry of the array. Solar cell design considerations The following applies to the above methods and devices: 1) Basic background of k-vector matching: the excitation mode of the waveguide mode matches one of the allowed k-vectors in the array to the plane of the in-plane waveguide mode Depends on the k vector. In spacingP In the periodic array, for the normal incident light we consider, the minimum allowed in-plane k-vector is 2π/P . The in-plane k-vector of the in-plane waveguide mode generally decreases as the wavelength increases. Therefore, for a given pitch and diameter, there is a maximum wavelength due to the in-plane k vector 2π/P k vector larger than any in-plane waveguide mode 2π/P The waveguide mode cannot be excited above the maximum wavelength. 2) The maximum in-plane k-vector of any in-plane waveguide mode usually follows the diameter of the high-refractive-index material nanowireD This increases because it increases the volume averaged refractive index of the array in which the waveguide modes propagate. Furthermore, for a given pitch, the longest wavelength at which the in-plane waveguide mode can be excited is red-shifted into the transparency region of the nanowire as D increases, and resonant reflections can occur. Or in other words, the resonant excitation decreases as the diameter D decreases. This behavior can be seen in Figure 15C, whereD When the reduction is less than 200 nm, the resonant excitation decreases. 3) The excitation of the in-plane waveguide mode depends on the diffraction of light from the direction of normal incident light to the vertical direction. The diffraction is achieved by the refractive index contrast between the nanowires and the medium between the nanowires. Furthermore, it is desirable that this scattering increases as the amount of high refractive index included in the array (starting from a small diameter) increases, because this increases the volume of the scattering region. The trace of this behavior can be seen in Figure 15C, where for a fixed pitch, the resonant excitation followsD Decrease and decrease. 4) For a given array of nanowires, find the two effects described above in points (2) and (3). As the time period increases, the excitation of the waveguide mode allows for a smaller diameter by the k vector selection rule. However, as the diameter decreases, the actual excitation strength of the in-plane waveguide mode decreases. Thus, in the D < 200 nm region in the right panel of Figures 15A-15C, there are actually two different mechanisms for preventing resonant reflection. For smaller values of P, the k-selection rule blocks the excitation (2π/P is greater than the largest k-vector of any in-plane mode). For larger values of P, the resonant excitation is instead decremented by reducing the ratio of diameter to time period, which is expected to reduce the excitation strength of the in-plane mode. In fact, starting with a very small P, the diameter of the resonant excitation appears withP Increase and decrease, in accordance with point (2). However, forP > 600 nm, withP As the diameter increases, the diameter of the resonant excitation increases, which is in accordance with point (3) above. 5) As discussed above, the excitation of the array mode relies on the diffraction of light from the direction of normal incident light to the vertical direction. The diffraction is achieved by the refractive index contrast between the nanowires and the medium between the nanowires. Therefore, along with the refractive index of the medium between the nanowiresn Initial valuen = 1.5 increase to better match the semiconductor nanowiren ≈ 3.5, the excitation of the in-plane waveguide mode is reduced (Figures 11A-11F). 6) The in-plane waveguide mode can leak into the high refractive index region adjacent to the nanowire array (Figs. 8A-8E). Therefore, resonant reflection has not been found in prior art modeling of absorption and transmission of nanowire arrays on Si substrates. Thus, in one embodiment, the nanowire solar cell membrane is designed to attenuate in-plane excitation. Furthermore, the resonant excitation of the waveguide mode can be prevented by increasing the refractive index of the material around (eg, between, above and below) the array of nanowires. 7) It can be shown that if the nanowire array rises above the high refractive index substrate, it is greater than aboutλ /3 (λ The distance of the wavelength of the light in the vacuum, the waveguide mode will not effectively leak into the high refractive index substrate. In other words, a true near field coupling to the high refractive index region is required to prevent strong excitation of the waveguide mode. Therefore, the high refractive index layer should be close to the nanowire, for example, less thanλ /3. 8) Due to the band gap defining the wavelength region of the bandgap photon below, there is a connection between the band gap of the nanowire and the diameter of the nanowire and the array spacing to avoid the resonance of the in-plane waveguide mode. excitation. Generally, as the band gap energy of the nanowire increases, the diameter and spacing need to be scaled down in the same manner as the bandgap wavelength is scaled down to avoid resonance reflection. This is because the shortest wavelength of the nanowire is not absorbed. small. 9) If a transparent conductive oxide is used at the top and bottom of the array of nanowires, the resonant excitation of the in-plane modes can result in large absorption losses in such layers, for example due to leakage. Other design considerations include: 1) For a given pitch, the diameter of the nanowire that is too small cannot be chosen because the absorption in the nanowire is too weak. 2) It is not possible to select a too large diameter because the subsequent resonant reflection reduces the transparency of the underlying bandgap photons. 3) Based on (1) and (2), the optimal absorption has a limitation in diameter, which is more stringent than when only optimizing the absorption in the nanowire. 4) For the nanowire array subcell on the Si substrate, the optimum diameter is the smallest diameter that still provides the optimum value for absorption in the nanowire. 5) To achieve a larger diameter by reducing the resonant reflection loss, we can: i) place the nanowire directly above the top of the high refractive index substrate or where solar energy needs to be and/or optimize the absorption of the transmission wavelength of light. Above the battery (for example, a dice battery). In this case, the waveguide mode "leaks" into the substrate and/or the underlying subcell to prevent its resonant excitation; or ii) uses an embedded material having a higher refractive index, preferably matching the material to the nanowire. To excite the in-plane waveguide mode, the device should have a refractive index difference in the plane of the nanowire array. By decrementing this difference, the excitation of the in-plane mode can be decremented. 6) In addition, if the in-plane waveguide mode is strongly excited, the use of indium tin oxide (ITO) on the top and bottom of the nanowire can lead to strong absorption of ITO. For absorption in the nanowire, according to the prior art, the minimum diameter of the absorption in the optimized nanowire array is typically also the overall optimum value of the absorption of the series cells of the nanowire array subcells directly above the dice battery. Therefore, for tandem applications, we will choose this diameter to optimize the size of the nanowires in the film. However, the problem of possible resonant reflections from in-plane waveguide modes will be particularly manifested in larger diameters. The results and simulations disclosed herein are produced on periodic systems (hexagonal and square arrays). However, the random array also exhibits an excitation of the in-plane mode (this is because the k-vector match is relaxed). Thus, the embodiments disclosed above can include subcells having randomly oriented nanowires. The optical response of stacked tandem photovoltaic cells 300a-300d can be modeled using Maxwell's equations. The optical response of the underlying material by its wavelength (λ) dependent refractive index (or equivalently by its dielectric function) may be included. For incident light, a normal incidence plane wave directed toward the array of nanowires is selected. In this calculation, the reflection coefficient R(λ) and the transmittance T(λ) of the system can be determined. The overall absorption ratio of the system is given by A(λ) = 1 - R(λ) - T(λ). As used herein, the reflectance, transmittance, and absorption ratio are defined as the fraction of incident light of a given wavelength that is reflected, transmitted, or absorbed. A power flow F(z, λ) in the varying z-plane (z along the axis of the vertically erected nanowire) can also be obtained. For example, the ITO top layer takes up space by z₁ < z < z₂ Defined, nanowire space by z₂ < z < z₃ Defined, and the ITO underlying space is defined by z₃ < z < z₄ Defined. Subsequently, the absorption ratio in the ITO layer and NWs is determined by A_{ITO , top} (λ) = [F₁ (z,λ) - F₂ (z,λ)] / I_Inc (λ), A_ITO ,_Bot (λ) = [F₃ (z,λ) - F₄ (z,λ)] / I_Inc And A_NWs (λ) = [F₂ (z,λ) - F₃ (z,λ)] / I_Inc (λ) defined, where I_Inc (λ) is the incident power flow [its unit is W/(m² Nm)]. A stacked battery that selects a current matching constraint between the nanowire subcell and the Si subcell. Preferably, the current generation performance of the two batteries is maximized. However, the absorption in the nanowire array cell will cause a decrease in the short circuit current in the Si cell. In principle, Shockley-Queisser detailed equilibrium efficiency calculations can be performed for a combined series battery. However, this kind of detailed balance calculation is not practical when changing a large number of parameters. The analysis can instead be limited to methods that limit the variation in absorption in Si cells. To calculate the incident intensity, use AM1.5D direct and ring day 1 solar spectrum I_{AM1 . 5} (λ), scaled to 1000 W/m² Incident intensity. To convert the spectrum X(λ) to the corresponding short-circuit current (density), use the following equation:. (1) In this paper, q is the basic charge, h is the Planck constant, and c is the speed of light in vacuum. To calculate the short-circuit current in the nanowire array_NWs Estimated upper limit, using equation (1) X(λ) = A_NWs (λ), λ_low = 280 nm, below this AM1.5 direct and ring-day spectroscopy exhibits negligible intensity, and λ_high = λ_NWs . In this paper, λ_NWs = hc/E_{Bg , NWs} To have the band gap energy E of the nanowire_{Bg , NWs} The bandgap wavelength of the nanowire. To calculate the short-circuit current of the Si battery_Si The estimated upper limit assumes that all transmitted light can be coupled into the underlying Si cell. In this case, use equation (1) X(λ) = A_Si (λ) = T(λ), λ_low = 280 nm and λ_high = λ_{Bg , Si} = hc/E_{Bg , Si} ≈ 1107 nm, where E_{Bg , Si} = 1.12 eV. Consider E_{Bg , Si} < E_{Bg , NWs} Case. That is, consider λ_{Bg , Si} > λ_{Bg , NWs} Case. In this case, λ_{Bg , NWs} < λ < λ_{Bg , Si} The light is intended for absorption in Si cells. For the specific purpose of this study, we considered the loss of light. Therefore, use equation (1) X(λ) = R(λ), λ_low = λ_{Bg , NWs} And λ_high = λ_{Bg , Si} Give the reflection loss of the photon specified for the Si battery._{Si , R} -loss. In addition, if the ITO layer is present in the modeled system, by using equation (1) X(λ) = A_{ITO , top} (λ) + A_{ITO , Bot} (λ), λ_low = λ_{Bg , NWs} And λ_high = λ_{Bg , Si} , we consider j_{ITO , Si} For the absorption of ITO of the specified photons in the Si battery. For E_{Bg , NWs} = 1.43 eV (λ_{Bg , NWs} 867 867 nm) GaAs nanowire, under the assumption of perfect absorption, ie for λ < λ_{Bg , NWs} A_NWs (λ),j_NWs Upper limit = 31.1 mA/cm² . Similarly, assume that for λ < λ_{Bg , NWs} T(λ) = 0 and for λ_{Bg , Si} < λ < λ_{Bg , NWs} GaAs nanowire with T(λ) = 1, j_Si = 12.7 mA/cm² . It should be noted that in selecting E with the above perfect absorption and transmission_{Bg , NWs} = 1.7 eV nanowire, j_NWs = 21.7 mA/cm² And j_Si = 22.1 mA/cm² . The response of the GaAs nanowire array on the top of the Si substrate or completely within the n = 1.5 refractive index film was compared (see Figures 5A, 5B of the schematic). The nanowires were placed in a hexagonal array and the results averaged for x and y polarized light. Figures 6A-6C show the calculation of the optical response of a GaAs nanowire array on top of a Si substrate. The length of the nanowire is 3000 nm, but the result is for varying nanowire lengths (have resulted in lower/higher j_NWs The main difference) is similar. Optimize the short-circuit current in the nanowire array for diameters of D ≈ 170 nm or D ≈ 400 nm. For large diameters (D > 200 nm) considering the GaAs nanowires completely in the n = 1.5 film (Figs. 7A-7F), find j_{Si , R} - Significant value of loss. That is, there may be a large amount of reflection that reduces the transmittance T. Upon increasing the length of the nanowires from 500 nm and 6000 nm (Figs. 7D-7F), it was found that the resonant reflection exhibited more and more fringes, as expected from a larger number of in-plane waveguide modes with thicker waveguide regions. This reflection can be attributed to the in-plane waveguide resonance in the x-y plane of the nanowire array. Resonant reflection was found above when the nanowire was completely embedded in the n = 1.5 film (Fig. 7C). However, this resonance reflection was not observed when the nanowire was on the Si substrate of n ≈ 3.5 (the right panel of Fig. 6C). The behavior of the in-plane waveguide mode is known to leak into the high refractive index substrate. This "leakage" prevents modal resonant excitation, which explains the lack of resonant reflection when the nanowire is on the Si substrate. In Figures 8A-8E, the results show the varying refractive index of the substrate. As expected, as the refractive index increases, the resonant reflection decreases. It also shows that the area of the resonant reflection is reduced to a smaller spacing of the array. Since the in-plane waveguide mode appears to leak into the substrate, if the refractive index of the substrate is sufficiently high, the inventors have also studied how the "leakage" can occur by a lower refractive index space. For a spacer thickness s > λ/3, the in-plane waveguide mode does not appear to be effectively coupled into the substrate and instead can cause resonant reflection (Figures 9A-9E). In addition, the inventors investigated whether the resonance reflection does not recover in the case of including a high refractive index superstate (Figs. 10A-10D). In a planar system, it is not possible to excite an in-plane waveguide mode. In order to couple light to the vertical direction, there must be a change in refractive index in the in-plane direction. Therefore, when attempting to match the refractive index of the material between the nanowires to the refractive index of the nanowire, the excitation of the in-plane waveguide mode and the resonance reflection are decremented due to the resonant excitation. When the refractive index is increased from n = 1.5 to n = 3.5 (which is approximately the refractive index of GaAs at λ ≈ 1000 nm), the resonant reflection is decremented (Figs. 11A-11F). When the ITO layer is present in the system, the in-plane waveguide mode can result in resonant absorption within the ITO (Fig. 11E). If the ITO-nano-ITO stack is placed on a Si substrate (assuming a 100 nm thick ITO layer, for this desired in-plane waveguide mode leakage into the substrate, to prevent resonant excitation, as seen in Figures 12A-12F) Above, the ITO absorption loss can be reduced. In Figures 11A-11F, compare GaAs (E_{Bg , NWs} = 1.43 eV) and GaAsP (where the composition is selected to give E_{Bg , NWs} = 1.7 eV) Optical response of the nanowire array. As discussed above, a higher band gap with a GaAsP nanowire results in a lower j_NWs And also lead to higher j_Si (For the nanowire geometry strongly absorbed by the nanowire). Resonant reflections have also been found to have higher values than GaAs (due to GaAsP in this region, j_Si The GaAsP nanowires can be higher, which also leads to potentially higher losses. It has also been found that for GaAsP, the diameter below which the resonant reflection is not seen is reduced. In order to prove that the resonant reflection by resonant excitation is not excluded from the hexagonal array by the in-plane waveguide mode, a square array is considered (Figs. 12A-12C). In this paper, resonant reflections are similarly found for hexagonal arrays. In addition, the range between studies has been extended to a higher value. In order to increase the range of the distance, D < 200 nm has also been found as a "good" region, in which the resonance reflection is not obvious (except for a small area of about 550 nm in the spacing, where a uniform smaller D is required to "prevent" Resonant reflection). By the above criteria, a stand-alone nanowire photovoltaic film (for example, a film) can be prepared and provided for a stacked series photovoltaic device. The membrane can be placed over a bulk photovoltaic cell (such as a bulk germanium cell) to form a tandem photovoltaic device. In another embodiment, a pre-existing photovoltaic panel can be upgraded by placing a first photovoltaic cell (eg, a film containing a nanostructure) over a photovoltaic panel. Although the foregoing relates to certain preferred embodiments, it should be understood that the invention is not limited thereto. It will be apparent to those skilled in the art that various modifications may be made to the disclosed embodiments and such modifications are intended to be within the scope of the invention. All publications, patent applications, and patents cited herein are hereby incorporated by reference in their entirety.

100‧‧‧Stacked series photovoltaic cells

102‧‧‧First photovoltaic cell

103‧‧‧Separation layer

104‧‧‧Second photovoltaic cell

105‧‧‧Separation layer

106‧‧‧The third photovoltaic cell battery

108‧‧‧Light

200a‧‧‧Stacked series photovoltaic cells

200b‧‧‧Stacked series photovoltaic cells

200c‧‧‧Stacked series photovoltaic cells

202‧‧‧Nano structure subcell

203‧‧‧First conductive oxide layer

204‧‧‧Nano structure subcell

205‧‧‧Second conductive oxide layer

206‧‧‧Nano structure battery

207‧‧‧ window layer

210‧‧‧In-plane waveguide mode

212‧‧‧Diffraction level

300a‧‧‧Stacked tandem photovoltaic cells

300b‧‧‧Stacked series photovoltaic cells

300c‧‧‧Stacked series photovoltaic cells

300d‧‧‧Stacked tandem photovoltaic cell

1 is a schematic diagram of a stacked tandem photovoltaic cell according to an embodiment. 2A and 2B are schematic views of a conventional stacked series photovoltaic cell. 2C is a schematic diagram of a stacked tandem photovoltaic cell according to an embodiment. 3A-3D are schematic views of an embodiment of a stacked series photovoltaic cell. 4 is a schematic view showing the following nanowire subcells: (1) incident light, (2) reflected light, (3) transmitted light, and (4) a waveguide in a plane of a nanowire array. 5A is a schematic illustration of an array of GaAs nanowires on top of a Si substrate, in accordance with an embodiment. Figure 5B is a schematic illustration of a GaAs nanowire array within a film having a refractive index n = 1.5. 6A to 6C are current density curves as a function of the diameter and pitch of the nanowires of the GaAs nanowire array directly above the Si substrate, the Si substrate having a film of n = 1.5 between the nanowires and on the top. In one embodiment, the GaAs nanowire of the nanowire array comprises a GaAs pn or pin junction solar cell. Figure 6A illustrates the current density of the nanowire j _NWs . Figure 6B illustrates the current density of the germanium substrate: j _Si . Figure 6C illustrates the current density of the reflection loss j _{Si , R - loss} . 7A through 7F are current density curves as a function of the diameter and spacing of the nanowires of the array of GaAs nanowires having a film of n = 1.5 between the nanowires and on the top. Figure 7A illustrates the current density of the nanowire j _NWs . Figure 7B illustrates the current density of the germanium substrate: j _Si . Figure 7C illustrates the current density of the reflection loss j _{Si , R - loss} . Figure 7D illustrates the reflection loss j _{Si , R - loss} current density of a nanowire of length 500 nm. Figure 7E illustrates the reflection loss j _{Si , R - loss} current density of a nanowire of length 3000 nm. Figure 7F illustrates the reflection loss j _{Si , R - loss} current density of a nanowire of length 6000 nm. 8A to 8E are current density curves as a function of the diameter and pitch of the nanowires of the GaAs nanowire array on the substrate having a varying refractive index. The nanowire has n = 1.5 material between it and on the top. Figure 9A is a schematic diagram illustrating the reflection loss in a nanowire array on a tantalum substrate. Figure 9B is a schematic diagram illustrating the reflection loss in a nanowire array having a spacer layer on a tantalum substrate. Figure 9C is a current density curve as a function of the diameter and spacing of the nanowires of the GaAs nanowire array on the germanium substrate. Figure 9D shows the current density as a function of the gap size between the nanowires of the array and the substrate. The nanowires of the array have a diameter of 300 nm, a pitch of 500 nm, and a length of 3000 nm. Figure 9E shows the current density as a function of the gap size between the nanowires of the array and the substrate. The nanowires of the array have a diameter of 180 nm, a pitch of 350 nm, and a length of 3000 nm. Figure 10A is a schematic illustration of a nanowire subcell having an encapsulation layer having a refractive index of n = 1.5 on a substrate having a refractive index of n = 3.0. Fig. 10B is a graph showing the current density curve as a function of the diameter of the nanowire and the pitch of the reflection loss of the device of Fig. 10A. Figure 10C is a schematic illustration of a nanowire subcell having an encapsulation layer having a refractive index of n = 3.0 on a substrate having a refractive index of n = 3.0. Figure 10D is a graph showing the current density curve as a function of the diameter of the nanowire and the pitch of the reflection loss of the device of Figure 10C. Figure 11A is a schematic diagram of a nanowire subcell having a membrane layer having a refractive index of n = 1.5-3.5 above and below the nanowire and a window layer having a refractive index of 1.5. 11B to 11F are current density curves showing the change in refractive index of the diaphragm layer as a function of the diameter and pitch of the nanowire of the subcell illustrated in Fig. 11A. 12A is a schematic diagram of a nanowire subcell having a window layer having a refractive index of 1.5 above and below the nanowire and an indium tin oxide (ITO) layer and a refractive index of 1.5 above and below the ITO layer. 12B to 12F are current density curves showing the diameter and pitch of the nanowires of the subcells illustrated in Fig. 12A, illustrating the reflection and absorption loss in the presence of the ITO layer. 13A is a schematic view of a nanowire subcell having a window layer having a refractive index of 1.5 above or below a thin film of a nanowire or an indium tin oxide (ITO) layer and an ITO layer. 13B and FIG. 13C are current density curves showing the diameter and pitch of the nanowires of the subcells illustrated in FIG. 13A in the presence of ITO layers and germanium substrates. 14A to 14C are current density curves as a function of the diameter and pitch of the nanowires of the GaAs nanowire array in the film having a refractive index of n = 1.5 on the substrate. Figure 14A illustrates the current density of the nanowire. Figure 14B illustrates the current density of the germanium substrate in the bottom cell of the crucible. Figure 14C illustrates the current density due to reflection losses. 14D to 14F are current density curves as a function of the diameter and pitch of the nanowires of the 1.7 eV bandgap GaAsP nanowire array in the film having a refractive index of n = 1.5 on the germanium substrate. Figure 14D illustrates the current density of the nanowire. Figure 14E illustrates the current density of the germanium substrate in the bottom cell of the crucible. Figure 14 illustrates the current density due to reflection loss. 15A to 15C are current density curves as a function of the diameter and pitch of the nanowires of the square GaAs nanowire array in the film having a refractive index of n = 1.5 on the substrate.

Claims (40)

A method of preparing a stacked tandem photovoltaic device, comprising: determining a nanostructure and a refractive index parameter of a subcell of the stacked tandem photovoltaic device to reduce an in-plane waveguide of light incident on the stacked tandem photovoltaic device Providing a first photovoltaic cell, the first photovoltaic cell comprising a nanostructure based on the determining step; and providing a second photovoltaic cell based on the determining step, wherein the first photovoltaic cell has a a band gap, the second photovoltaic cell has a second band gap, and the first band gap is greater than the second band gap, wherein light incident on the stacked series photovoltaic device enters the second photovoltaic device The sub-cell passes through the first photovoltaic cell. The method of claim 1, wherein the band gap of the first photovoltaic cell is less than the bandgap of the first photovoltaic cell for optical transparency optimization regardless of the in-plane waveguide. The method of claim 1, the method further comprising forming a first layer having a higher effective refractive index than the first photovoltaic cell between the first photovoltaic cell and the second photovoltaic cell. The method of claim 3, wherein the first layer having a higher effective refractive index has a thickness less than λ/2, wherein λ is a wavelength of light to be absorbed in the second photovoltaic cell. The method of claim 4, wherein the first layer having a higher refractive index comprises a transparent conductive oxide. The method of claim 5, the method further comprising forming a window layer between the first photovoltaic cell and the second photovoltaic cell, the window layer having a higher effective refractive index than the first photovoltaic cell And the thickness is less than λ/2. The method of claim 6, the method further comprising forming a second layer comprising transparent conductive between the at least one of the first photovoltaic cells and the window layer or the second photovoltaic cell The oxide, the second layer of the transparent conductive oxide has a thickness less than λ/2. The method of claim 5, the method further comprising forming a window layer between the first photovoltaic cell and the second photovoltaic cell, the window layer having a lower refractive index than the second photovoltaic cell And the thickness is greater than λ/2. The method of claim 8, the method further comprising forming a second layer comprising transparent conductive oxidation between the at least one of the first photovoltaic cells and the window layer or the second photovoltaic cell The second layer of the transparent conductive oxide has a thickness less than λ/2. The method of claim 1, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and the nanowires The diameter is smaller than the diameter of the nanowire optimized for optical transparency regardless of the in-plane waveguide. The method of claim 1, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and the refractive index of the dielectric is low Optimized for optical transparency without regard to the dielectric of the in-plane waveguide. The method of claim 1, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and the nanowires comprise A semiconductor material whose refractive index is lower than that for an optical transparency optimization regardless of the refractive index of the in-plane waveguide. The method of claim 1, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and the diameter of the fixed nanowires is The nanowire density is less than the nanowire density optimized for optical transparency without regard to the same composition of the in-plane waveguide. The method of claim 1, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and the density of the fixed nanowires The diameter of the nanowire is smaller than the diameter of the nanowire optimized for optical transparency without regard to the same composition of the in-plane waveguide. The method of claim 1, wherein the nanostructures are configured in a periodic array, wherein a spacing between the nanostructures is selected such that there is no in-plane waveguide for the in-plane k-vector in the relevant wavelength range for transparency The modality exists. The method of claim 1, the method further comprising: tuning a transparency wavelength window by using a band gap of the sub-battery; or placing a sub-cell exhibiting a reflection problem close enough to the adjacent sub-cells such that the in-plane waveguide mode cannot be due to Exciting by leaking into the adjacent subcell; or selecting a sufficiently small contrast between the higher refractive index material and the lower refractive index material in each of the subcells exhibiting resonant reflection; or selecting to include sufficiently less high refraction Rate the material to reduce the scattering of light in the in-plane direction; or use a periodic system of sufficiently small time period to prevent excitation of the in-plane waveguide. The method of claim 1, the method further comprising: adapting the dispersion of the in-plane waveguide mode such that it cannot be excited by a method allowed by the k-vector in the relevant wavelength range for transparency; or by using a k-vector The excitation is selected to excite the in-plane waveguide modes, but the actual excitation strength of the modes is decremented by adapting the scattering geometry of the array. The method of claim 1, the method further comprising selecting a diameter of the nanowire to prevent resonant reflection, wherein the nanostructures comprise nanowires. A photovoltaic device comprising a first photovoltaic cell comprising a nanostructure, wherein in the stacked photovoltaic device, the first photovoltaic cell is located above the second photovoltaic cell, the first photovoltaic cell At least one feature that reduces or eliminates reflection losses due to in-plane waveguides of incident light, and wherein the first photovoltaic cell is configured to permit light incident on the stacked tandem photovoltaic device to enter the second The photovoltaic cell is passed through the first photovoltaic cell. The photovoltaic device of claim 19, wherein the first photovoltaic cell comprises a freestanding film comprising the nanostructures embedded in a dielectric matrix. The photovoltaic device of claim 20, wherein the dielectric substrate comprises a polymer matrix, and the nanostructures comprise a III-V semiconductor nanowire comprising a p-n or p-i-n junction. The photovoltaic device of claim 21, wherein the III-V semiconductor nanowire GaAs or GaAsP nanowires, and wherein the first photovoltaic cell is configured to be located in the cell comprising a bulk germanium battery Two photovoltaic cells above the battery. A stacked tandem photovoltaic device comprising: the first photovoltaic cell of claim 19; and the second photovoltaic cell, wherein the first photovoltaic cell has a first band gap, the second The photovoltaic cell has a second band gap, and the first band gap is greater than the second band gap, wherein at least one feature of the stacked series photovoltaic device reduces or eliminates incidence due to the stacked series photovoltaic device The reflection loss caused by the waveguide in the plane of light, and the light incident on the stacked series photovoltaic device, passes through the first photovoltaic cell before entering the second photovoltaic cell. The stacked tandem photovoltaic device of claim 23, wherein the band gap of the first photovoltaic cell is less than a bandgap of the first photovoltaic cell for optical transparency optimization regardless of the in-plane waveguide. The stacked tandem photovoltaic device of claim 23, the device further comprising a first layer having a higher refractive index between the first photovoltaic cell and the second photovoltaic cell. The stacked tandem photovoltaic device of claim 25, wherein the first layer having a higher refractive index has a thickness less than λ/2, wherein λ is the wavelength of light to be absorbed in the second photovoltaic cell. The stacked tandem photovoltaic device of claim 26, wherein the first layer having a higher refractive index comprises a transparent conductive oxide. The stacked tandem photovoltaic device of claim 27, the device further comprising a window layer between the first photovoltaic cell and the second photovoltaic cell, the window layer having a ratio of the first photovoltaic cell Higher refractive index and thickness less than λ/2. The stacked tandem photovoltaic device of claim 28, further comprising a second layer comprising a transparent conductive oxide between the first photovoltaic cell and the second photovoltaic cell, The thickness of the second layer of transparent conductive oxide is less than λ/2. The stacked tandem photovoltaic device of claim 27, the device further comprising a window layer between the first photovoltaic cell and the second photovoltaic cell, the window layer having a ratio of the first photovoltaic cell Lower refractive index and thickness greater than λ/2. The stacked tandem photovoltaic device of claim 30, further comprising a second layer comprising a transparent conductive oxide between the first photovoltaic cell and the second photovoltaic cell, The thickness of the second layer of transparent conductive oxide is less than λ/2. The stacked tandem photovoltaic device of claim 23, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and The diameter of the nanowires is smaller than the diameter of the nanowires for the optical transparency optimization without regard to the same composition of the in-plane waveguide. The stacked tandem photovoltaic device of claim 23, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and The dielectric has a lower refractive index than a dielectric optimized for optical transparency without regard to in-plane waveguides. The stacked tandem photovoltaic device of claim 23, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and The nanowires contain a semiconductor material whose refractive index is lower than that for the optical transparency optimization regardless of the refractive index of the in-plane waveguide. The stacked tandem photovoltaic device of claim 23, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and fixed The nanowire density of the nanowire diameter is less than the nanowire density optimized for optical transparency without regard to the same composition of the in-plane waveguide. The stacked tandem photovoltaic device of claim 23, wherein: the nanostructures comprise nanowires, a dielectric is provided between the nanowires, the refractive indices of the nanowires are higher than the dielectric, and fixed The nanowire diameter of the nanowire density is smaller than the diameter of the nanowire for the same composition optimized for optical transparency without regard to the in-plane waveguide. The stacked tandem photovoltaic device of claim 23, wherein the nanostructures are configured in a periodic array, wherein the spacing between the nanostructures is selected such that in-plane k in the relevant wavelength range for transparency The vector has no in-plane waveguide mode. The stacked tandem photovoltaic device of claim 23, wherein the first photovoltaic cell absorbs more than 80% of light having energy greater than the first band gap and less than 20% has energy less than the first band gap Light. A method of upgrading a photovoltaic panel comprising placing a first photovoltaic cell as claimed in claim 19 above the photovoltaic panel. A method of operating a stacked tandem photovoltaic device, comprising: receiving incident light on a first photovoltaic cell comprising a nanostructure such that at least a portion of the incident light passes through the first photovoltaic cell and enters a photovoltaic cell; and generating a current or voltage from the first photovoltaic cell and the second photovoltaic cell; wherein: the first photovoltaic cell has a first band gap, and the second photovoltaic cell Having a second band gap, and the first band gap is greater than the second band gap, and at least one feature of the stacked series photovoltaic device reduces or eliminates reflection losses due to the in-plane waveguide of the incident light.