Photoelectric conversion element, optical wireless power supply system

Abstract

To provide a highly efficient photoelectric conversion element for single-wavelength optical wireless power transmission. [Solution] The lower reflector 120 is formed on the substrate 110. The quantum well structure 130 is formed on the lower reflector 120. The upper reflector 140 is formed on the quantum well structure 130. The N-type semiconductor region 150 and the P-type semiconductor region 160 are spaced apart in the in-plane direction of the quantum well structure 130.

Description

Technical Field

[0001] The present disclosure relates to a photoelectric conversion element used for optical wireless power feeding. Background Art

[0002] Wireless power feeding to electronic devices including smartphones and tablet terminals has been put into practical use. Currently, wireless power feeding that has been put into practical use uses electromagnetic waves in the frequency band of several hundred kHz, and electromagnetic induction methods, magnetic field resonance methods, and the like are known. Note that light is essentially a type of electromagnetic wave, but when simply referred to as an electromagnetic wave in this specification, it refers to an electromagnetic wave having a longer wavelength than light.

[0003] Optical wireless power feeding that uses light instead of electromagnetic waves has been proposed. Since light has a characteristic of higher directivity than electromagnetic waves, the use of a beam with a small divergence angle provides an advantage that energy can be propagated with high efficiency over long distances with little attenuation. Prior Art Documents Non-Patent Documents

[0004] Non-Patent Document 1 J. Schubert et. al.“High-Voltage GaAs Photovoltaic Laser Power Converters”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 2, FEBRUARY 2009, pp.170-175 Non-Patent Document 2 Simon Fafard et. al. “Perspective on photovoltaic optical power converters”, J. Appl. Phys. 130, 160901 (2021) Non-Patent Document 3 Henning Helmers et. al. “Overcoming optical-electrical grid design trade-offs for cm2-sized high-power GaAs photonic power converters by plating technology”, Progress in Photovoltaics - 2024 - [Overview of the project] [Problems that the invention aims to solve]

[0005] In optical wireless power transmission systems, where a light beam is shone onto a power receiving element (solar cell) module located in a remote location, power transmission efficiency is crucial. In particular, the photoelectric conversion efficiency of the photoelectric conversion element on the power receiving device side, which is a component of power transmission efficiency, is a limiting factor in the amount of power received, and the energy loss that is not converted into power generates heat. Since low efficiency of the photoelectric conversion element limits the feasibility of the system due to significant heat generation and temperature rise, improving its efficiency is an extremely important issue.

[0006] This disclosure is made in the present circumstances, and one exemplary objective of a certain aspect thereof is to provide a highly efficient photoelectric conversion element for single-wavelength optical wireless power transmission. [Means for solving the problem]

[0007] A photoelectric conversion element in one embodiment of the present disclosure comprises a substrate, a quantum well structure formed on the substrate, and a P-type semiconductor region and an N-type semiconductor region spaced apart in the in-plane direction of the quantum well structure.

[0008] Furthermore, any combination of the above components, or conversion of the expressions in this disclosure between methods, apparatus, etc., are also valid embodiments of the present invention. Moreover, this section does not describe all the essential features of the present invention, and therefore, subcombinations of these described features may also constitute the present invention. [Effects of the Invention]

[0009] According to certain aspects of this disclosure, a highly efficient photoelectric conversion element can be provided. [Brief explanation of the drawing]

[0010] [Figure 1] This is an energy band diagram of a conventional solar cell. [Figure 2] This is an energy band diagram of a conventional solar cell. [Figure 3] Figure 3(a) schematically shows the density of states of a bulk semiconductor, and Figure 3(b) illustrates the pseudo-Fermi level of a bulk semiconductor. [Figure 4] This is a cross-sectional view of the photoelectric conversion element according to the embodiment. [Figure 5] Figure 5(a) schematically shows the density of states of the photoelectric conversion element according to the embodiment, and Figure 5(b) is a diagram illustrating the pseudo-Fermi level of the quantum well structure. [Figure 6] This is a cross-sectional view of a photoelectric conversion element related to the comparative technology. [Figure 7] This is an energy band diagram of a quantum well structure. [Modes for carrying out the invention]

[0011] (Summary of the embodiment) This section outlines some exemplary embodiments of the present disclosure. This outline is intended to provide a basic understanding of the embodiments and to simplify some concepts of one or more embodiments, serving as a prelude to the more detailed descriptions that follow. It is not intended to limit the scope of the invention or disclosure. This outline is not a comprehensive overview of all possible embodiments, nor is it intended to identify essential elements of all embodiments or to delineate the scope of some or all aspects. For convenience, “one embodiment” may be used to refer to one or more embodiments (examples or variations) disclosed herein.

[0012] A photoelectric conversion element according to one embodiment comprises a substrate, a quantum well structure formed on the substrate, and a P-type semiconductor region and an N-type semiconductor region provided spaced apart in the in-plane direction of the quantum well structure.

[0013] The basic principle of the receiving element in optical wireless power transmission is essentially the same as that of a solar cell, using a semiconductor pn junction. Conventional solar cells using bulk semiconductors are designed to utilize the broad wavelength spectrum of sunlight and use semiconductors with a narrow band gap (energy gap) to absorb as much long-wavelength light (low photon energy) as possible. However, in this design, although short-wavelength light (high photon energy) is absorbed, the large difference between the band gap and photon energy results in significant heat loss due to this difference. In other words, the photoelectric conversion efficiency decreases due to the loss of light energy (photon energy loss).

[0014] In contrast, a photoelectric conversion element according to one embodiment is designed to be optimized for monochromatic light, i.e., a narrow spectrum. Instead of a bulk structure, a quantum well structure is applied to the photoelectric conversion element, and in particular, a compressive strain quantum well, which is advantageous for improving efficiency, is applied. This makes it possible to absorb monochromatic light with high efficiency.

[0015] In semiconductor lasers and optical sensors using quantum well structures, electrode pairs are typically formed on either side of the quantum well structure, and carriers flow in the stacking direction of the quantum well structure. In this case, energy is required for the carriers to overcome the barrier layer. Since semiconductor lasers and optical sensors rely on external power supply, this energy can be supplied from an external source. On the other hand, in the case of photoelectric conversion elements used as solar cells, this energy is lost. Therefore, the above configuration is characterized by using electrodes formed spaced apart in the direction of the substrate plane (in the quantum well plane) to extract power from the quantum well. As a result, carriers accumulated in the well layer move in the in-plane direction rather than in the stacking direction of the quantum well structure, eliminating the need for energy to overcome the barrier layer and improving efficiency.

[0016] In one embodiment, the photoelectric conversion element may further comprise a lower reflecting mirror formed between a substrate and a quantum well structure, and an upper reflecting mirror formed on the quantum well structure. In this configuration, the quantum well structure is sandwiched between two reflecting mirrors, and the photoelectric conversion element operates as a resonant absorption solar cell. This configuration uses resonance characteristics to absorb only light of a specific monochromatic wavelength, and thus is effective only for optical wireless power transmission using monochromatic light.

[0017] The quantum well structure according to one embodiment may be a multiple quantum well structure. This makes it possible to increase the amount of light absorption.

[0018] An optical wireless power transmission system according to one embodiment comprises any one of the photoelectric conversion elements described above, and a light source that irradiates light onto the photoelectric conversion element.

[0019] (Embodiment) Hereinafter, preferred embodiments will be described with reference to the drawings. The same or equivalent constituent elements, members, and processes shown in each drawing are denoted by the same reference numerals, and repeated descriptions will be omitted as appropriate. In addition, the embodiments are illustrative rather than limiting the invention, and not all features and combinations thereof described in the embodiments are necessarily essential to the invention.

[0020] Before describing the photoelectric conversion element according to the embodiment, a conventional solar cell will be described.

[0021] FIG. 1 is an energy band diagram of a conventional solar cell. A conventional solar cell that receives broadband sunlight uses a semiconductor material having a band gap Eg that balances the effects of long-wavelength and short-wavelength light so as to efficiently utilize the entire spectrum of sunlight.

[0022] When the energy of an absorbed photon is defined as Ep (=hν), Ep-Eg corresponds to heat loss E lossThis is the result, where h is the Boltzmann constant and ν is the frequency. For the short-wavelength components with high energy within the broad spectrum of sunlight, this heat loss becomes larger. The theoretical maximum efficiency when using a single semiconductor material is about 30%, while the efficiency of commercially available solar cells remains at around 20%.

[0023] Figure 2 is an energy band diagram of a conventional solar cell. In optical wireless power transmission, light of a specific wavelength (monochromatic) is used for power transmission. By selecting a specific wavelength 1 / ν as low as possible within the range Ep > Eg relative to the semiconductor band gap Eg, the heat loss E loss This can be made smaller. Also, with optical wireless power transfer, the intensity (density) of light can be freely set, so higher efficiency can be achieved compared to solar power generation.

[0024] Even with commercially available solar cells, irradiating them with light of the appropriate single wavelength (laser light or monochromatic LED light) can achieve efficiencies of 30-50%. Furthermore, efficiencies of 69% have been reported through detailed structural design using existing solar cell materials. While these are significantly higher efficiencies than those achieved with sunlight irradiation, they are far from 100%, and further improvements in efficiency are needed.

[0025] This explains why efficiency is limited when using bulk semiconductors in solar cells combined with a single wavelength.

[0026] Figure 3(a) schematically shows the density of states of a bulk semiconductor. The horizontal axis represents the density of states, and the vertical axis represents energy. The density of states can be considered as the absorption coefficient. To reduce heat loss and obtain high efficiency, it is necessary to use a semiconductor with a band gap (Eg ≈ Ep) that can absorb exactly the wavelength of monochromatic light.

[0027] However, if a semiconductor with a band gap (Eg ≈ Ep) that can absorb the wavelength of monochromatic light is used, the light absorption coefficient of the relatively thick semiconductor film (bulk semiconductor) used in typical solar cells is very small. Therefore, a very thick layer is required to efficiently absorb light, making it difficult to form the structure of the device. In order to thin the semiconductor layer to a level that is practically feasible to manufacture, it is necessary to use monochromatic light with a wavelength slightly shorter than the wavelength corresponding to the band gap Eg (generally a wavelength difference Δλ of 10 nm or more) so that the light absorption coefficient becomes sufficiently large. Thus, the wavelength difference Δλ required to thin the semiconductor layer reduces energy efficiency (photon energy loss).

[0028] For example, in a gallium arsenide (GaAs) solar cell with a band gap equivalent wavelength of 880 nm, light at a wavelength of 880 nm can hardly be absorbed, so it is necessary to use light with a wavelength of 800 to 850 nm (Δλ = 30 to 80 nm). Even considering only the difference between this band gap and photon energy (photon energy loss), the upper limit of efficiency is around 92 to 97%, resulting in significant energy loss. That is the first reason. Next, I will explain the second reason.

[0029] Figure 3(b) illustrates the pseudo-Fermi level of a bulk semiconductor. The average energy (pseudo-Fermi energy) of the carriers (electrons and holes) generated in the semiconductor by light absorption, as understood from the thermodynamics of the majority of these carriers, is at an energy position lower than the band gap edge (band edge) Eg. The output voltage of an actual solar cell is not based on the band gap voltage, but on this pseudo-Fermi energy. The voltage drop (voltage loss) corresponding to the difference between the band gap and the pseudo-Fermi energy varies depending on the material and the intensity of the irradiated light (or the density of the generated carriers), but in bulk semiconductors it is generally large, around 0.3 to 0.7 V. This effect further reduces the ideal efficiency upper limit shown earlier, and the calculated efficiency upper limit becomes around 46 to 76%.

[0030] In summary, the factors that reduce the efficiency of solar cells are (1) insufficient light absorption due to photon energy loss and limitations on the thickness of the absorption layer, and (2) voltage loss. Below, we will describe photoelectric conversion elements that can solve these problems and achieve high efficiency.

[0031] Figure 4 is a cross-sectional view of the photoelectric conversion element 100 according to the embodiment. The optical wireless power supply system 200 comprises the photoelectric conversion element 100 and a light source 210. The light source 210 irradiates the photoelectric conversion element 100 with light having a specific wavelength.

[0032] The photoelectric conversion element 100 comprises a substrate 110, a lower reflector 120, a quantum well structure (MQWS: Multi Quantum Well Structure) 130, an upper reflector 140, an N-type semiconductor region 150, an N-side electrode 152, a P-type semiconductor region 160, and a P-side electrode 162.

[0033] The substrate 110 can be made of gallium arsenide (GaAs) when the wavelength is 500 nm to 1000 nm, indium phosphide (InP) when the wavelength is 1000 nm to 1700 nm, or gallium nitrogen (GaN) or sapphire (Al2O3) when the wavelength is 300 nm to 500 nm. However, since the substrate can be made of a material that can be attached and held after the upper structure is formed, dielectrics such as glass, metals, and organic materials are also possible.

[0034] The lower reflector 120 is formed on the substrate 110. The lower reflector 120 may be a dielectric DBR (Distributed Bragg Reflector), a semiconductor DBR, or a metallic reflective film.

[0035] The quantum well structure 130 is formed on the lower reflector 120. The quantum well structure 130 includes a well layer 132 and a barrier layer 134. Examples of material combinations for the well layer 132 and the barrier layer 134 include a combination of gallium indium arsenide (GaInAs) and gallium arsenide (GaAs), or a combination of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). Generally, a single layer is often insufficient for absorbing light. Therefore, it is preferable to employ a multi-layer quantum well structure with multiple layers for the quantum well structure 130, thereby increasing the amount of light absorbed.

[0036] The upper reflector 140 is formed on the quantum well structure 130. The upper reflector 140 may be a dielectric DBR (Distributed Bragg Reflector) or a semiconductor DBR.

[0037] The N-type semiconductor region 150 and the P-type semiconductor region 160 are provided spaced apart in the in-plane direction, sandwiching the quantum well structure 130 of the photoelectric conversion element 100.

[0038] The above describes the configuration of the photoelectric conversion element 100.

[0039] Figure 5(a) schematically shows the density of states of the photoelectric conversion element 100 according to the embodiment. The horizontal axis represents the density of states, and the vertical axis represents energy.

[0040] As shown in Figure 3(a), the density of states function that determines the energy distribution of carriers in bulk semiconductors is quadratic, but the quantum well structure has a stepped density of states, as shown in Figure 5(a). Therefore, as shown in Figure 5(a), the optical absorption coefficient is large even at the band edge. This makes it possible to make the photon energy almost match the effective band gap of the quantum well, thereby reducing photon energy loss and heat loss.

[0041] Figure 5(b) illustrates the pseudo-Fermi level of a quantum well structure. When the density of states function is stepwise, the pseudo-Fermi energy approaches the band edge for the same carrier density. As a result, the voltage loss becomes smaller than that of bulk semiconductors (0.3-0.7V). Furthermore, as the volume carrier density increases, this voltage loss decreases, so for the same number of carriers per area generated by the same light intensity per area, a thinner absorption layer will have a higher volume carrier density. The fact that quantum wells have a higher volume carrier density than bulk structures due to their thin structure also contributes to the reduction of voltage loss.

[0042] The magnitude of the stepped density of states is also influenced by the effective mass of the semiconductor material. In particular, a smaller effective mass results in a smaller light absorption coefficient, but the pseudo-Fermi energy increases for the same volume carrier density, thus reducing voltage loss. Therefore, voltage loss can be reduced by using a material with a small effective mass or by reducing the effective mass of holes by introducing crystal strain (in the case of compression-strained quantum wells). Applying strained quantum wells is more effective than changing the material, as it is easier.

[0043] By combining these effects, a structure is created that improves efficiency compared to conventional solar cells using bulk semiconductors.

[0044] The multiplication of quantum well structures, particularly compressive strain quantum wells, is burdensome to manufacture. Therefore, by sandwiching a quantum well structure 130 between one or a few layers of quantum wells and providing a lower reflector 120 and an upper reflector 140, it functions as a resonant absorption solar cell. The wavelength of light emitted by the light source 210 is determined based on the resonance wavelength. This configuration is particularly advantageous in optical wireless power transmission using monochromatic light because it utilizes resonance characteristics to absorb only light of a specific monochromatic wavelength.

[0045] Further advantages of the photoelectric conversion element 100 become clear when compared with comparative technologies.

[0046] Figure 6 is a cross-sectional view of a photoelectric conversion element 100R relating to a comparative technology. The photoelectric conversion element 100R is equipped with an upper electrode 104 and a lower electrode 102 in place of the N-type semiconductor region 150 and P-type semiconductor region 160 of the photoelectric conversion element 100 according to the embodiment, and is configured to extract power from the upper electrode 104 and the lower electrode 102.

[0047] In other words, in the comparative technique, a pn junction is formed in the thickness direction of the quantum well (plane-of-paper, vertical direction), and carriers flow in the thickness direction of the quantum well.

[0048] Figure 7 shows the energy band diagram of the quantum well structure 130. In the quantum well structure 130, the carriers generated by absorbing light, i.e., electrons and holes, are energetically bound to the barrier layer 134 that constitutes the quantum well. In order to extract these bound carriers as electrical power, additional energy is required to cross the quantum well barrier. This energy depends on the quantum well structure, but is approximately 0.1 to 0.2 V in voltage terms.

[0049] In optical sensors and semiconductor lasers, similar to comparative technologies, a structure in which the quantum well structure 130 is sandwiched between upper and lower electrodes in the stacking direction is common. This is because it assumes external power supply and allows additional energy to be applied from the outside.

[0050] However, since solar cells are not designed to rely on external power supply, the energy needed to overcome barriers must be supplied by the electromotive force, resulting in a voltage loss of approximately 0.1 to 0.2V, which reduces power conversion efficiency.

[0051] In contrast, in this embodiment, instead of forming a pn junction in the stacking direction (up and down direction) that sandwiches the conventional quantum well structure, a pn junction is formed in the in-plane direction of the semiconductor substrate. As a result, carriers flow in the in-plane direction and do not need to overcome the barrier of the quantum well, so voltage loss does not occur in principle, and it becomes possible to extract power with high efficiency.

[0052] (modified version) In this embodiment, a multi-layer quantum well structure was used as the quantum well structure 130, but a single-layer quantum well structure may also be used. A single-layer quantum well structure can be used if the absorption coefficient of the single-layer quantum well is relatively high, or if sufficient energy can be extracted by multiple reflections between the upper reflector 140 and the lower reflector 120, even if the absorption coefficient is not very high.

[0053] In the embodiments described, a resonant absorption solar cell was constructed in which a multiple quantum well structure 130 is sandwiched between a lower reflector 120 and an upper reflector 140. However, this disclosure is not limited thereto. That is, the lower reflector 120 and the upper reflector 140 may be omitted, or only the lower reflector 120 may be provided.

[0054] Although this disclosure has been described using specific terminology based on embodiments, these embodiments merely illustrate the principles and applications of this disclosure, and many modifications and changes in arrangement are permitted in the embodiments, without departing from the spirit of the invention as defined in the claims. [Explanation of Symbols]

[0055] 100 Photoelectric conversion elements 110 circuit boards 120 Lower reflector 130 Quantum well structure 132 Well layer 134 Barrier layer 140 Upper reflector 150 N-type semiconductor region 152 N side electrode 160 P-type semiconductor region 162 P side electrode

Claims

1. circuit board and A quantum well structure formed on the aforementioned substrate, The quantum well structure comprises a P-type semiconductor region and an N-type semiconductor region spaced apart in the in-plane direction, A photoelectric conversion element characterized by comprising the following features.

2. A lower reflecting mirror formed between the substrate and the quantum well structure, An upper reflecting mirror formed on the quantum well structure, The photoelectric conversion element according to claim 1, further comprising the features described above.

3. The photoelectric conversion element according to claim 1 or 2, characterized in that the quantum well structure is a multiple quantum well structure.

4. A photoelectric conversion element according to claim 1 or 2, A light source that irradiates the aforementioned photoelectric conversion element with light, A wireless optical power supply system characterized by comprising the following features.

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