Photoelectric conversion element

Abstract

To provide a photoelectric conversion element capable of achieving high voltage while avoiding reduction in photoelectric conversion efficiency.SOLUTION: The photoelectric conversion element includes a plurality of light absorption layers stacked on each other along a first axis and each including a compound semiconductor, and a tunnel junction layer provided between adjacent light absorption layers among the plurality of light absorption layers, the plurality of light absorption layers have band gaps different from each other, and the band gaps monotonically change along the first axis.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a photoelectric conversion element. [Background technology]

[0002] Research is being conducted into the use of photoelectric conversion elements in optical wireless power supply using laser light, for example, laser light with a wavelength in the 1064 nm band. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Hidefumi Akiyama, "Diagnosis and design of multi-junction solar cells and evaluation of absolute luminescence value and luminescence quantum efficiency," Applied Physics, Vol. 84, No. 4 (2015) [Non-patent document 2] Tomoyuki Miyamoto, "Optical wireless power transfer realized by optical beams" (2021) Summary of the Invention [Problem to be solved by the invention]

[0004] In optical wireless power transmission, it is desirable to obtain a higher voltage. However, simply increasing the number of light absorption layers included in the photoelectric conversion element to achieve a higher voltage results in a decrease in photoelectric conversion efficiency.

[0005] An object of the present disclosure is to provide a photoelectric conversion element that can be used at a high voltage while avoiding a decrease in photoelectric conversion efficiency. [Means for solving the problem]

[0006] The photoelectric conversion element of the present disclosure includes a plurality of light absorbing layers stacked along a first axis, each containing a compound semiconductor, and a tunnel junction layer provided between adjacent ones of the plurality of light absorbing layers, wherein the plurality of light absorbing layers have different band gaps, and the band gaps vary monotonically along the first axis. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to increase the voltage while avoiding a decrease in photoelectric conversion efficiency. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view showing a photoelectric conversion element according to a first embodiment. [Figure 2] FIG. 4 is a cross-sectional view showing a photoelectric conversion element according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] First, the reason why the photoelectric conversion efficiency decreases when an attempt is made to increase the voltage in a photoelectric conversion element will be explained.

[0010] One possible structure for achieving high voltage is one in which multiple light-absorbing layers are stacked with a tunnel junction layer sandwiched between them. The light-absorbing layers include p-type and n-type layers. Here, the multiple light-absorbing layers are assumed to have the same composition. In a photoelectric conversion element with such a structure, the smaller the variation in current obtained from each light-absorbing layer, the more current loss can be reduced. On the other hand, between multiple light-absorbing layers, the intensity of light reaching them decreases the further away from the surface onto which light is irradiated. Therefore, to reduce the variation in current obtained from each light-absorbing layer, the light-absorbing layers farther from the surface onto which light is irradiated must be made thicker.

[0011] When a compound semiconductor is used as the material for the light-absorbing layer, the light-absorbing layer is formed by epitaxial growth on a substrate, but there is a limit to the thickness that can be lattice-matched to the substrate. Therefore, if the sum of the thicknesses of the light-absorbing layer and the tunnel junction layer is set to a thickness that is equal to or less than the thickness that can be lattice-matched to the substrate and the light-absorbing layer is made thicker as it moves away from the surface onto which light is irradiated, the light-absorbing layer closest to the surface onto which light is irradiated inevitably becomes significantly thinner, resulting in a decrease in photoelectric conversion efficiency. The decrease in photoelectric conversion efficiency is due to the fact that the diffusion of the depletion layer becomes more pronounced as the light-absorbing layer becomes thinner.

[0012] The band gap of the optical absorption layer of a photoelectric conversion element for optical wireless power supply using laser light in the 1064 nm wavelength band is set to 1.165 eV or less. InGaAsP is an example of a material for the optical absorption layer with a band gap of 1.165 eV or less. When a semiconductor layer including multiple InGaAsP layers is epitaxially grown on an InP substrate as a growth substrate, the thickness of the semiconductor layer that can be lattice-matched with the substrate is a maximum of several micrometers.

[0013] For example, if the number of InGaAsP layers used as the light absorption layers is eight and the total thickness of the eight light absorption layers is 2500 nm, and the light absorption layers further from the light-irradiated surface are made thicker to reduce current variations, the thickness of the light absorption layer closest to the light-irradiated surface will be 100 nm or less. On the other hand, if the InGaAsP layer is less than 200 nm thick, the photoelectric conversion efficiency will be significantly reduced due to the effects of depletion layer diffusion. Therefore, if multiple light absorption layers have the same composition, it will be impossible to obtain a high voltage while avoiding a decrease in photoelectric conversion efficiency.

[0014] Based on this knowledge, the inventors of the present application have come up with the following embodiment.

[0015] (First embodiment) First Embodiment Fig. 1 is a cross-sectional view showing a photoelectric conversion element according to the first embodiment.

[0016] The photoelectric conversion element 1 according to the first embodiment includes a plurality of light absorbing layers each containing a compound semiconductor, stacked along a first axis, and a tunnel junction layer provided between adjacent light absorbing layers. More specifically, the photoelectric conversion element 1 includes eight light absorbing layers 11, 12, 13, 14, 15, 16, 17, and 18, and seven tunnel junction layers 31, 32, 33, 34, 35, 36, and 37. The light absorbing layers 11 to 18 are stacked along the first axis.

[0017] The tunnel junction layer 31 is provided between the light absorption layer 11 and the light absorption layer 12. The tunnel junction layer 32 is provided between the light absorption layer 12 and the light absorption layer 13. The tunnel junction layer 33 is provided between the light absorption layer 13 and the light absorption layer 14. The tunnel junction layer 34 is provided between the light absorption layer 14 and the light absorption layer 15. The tunnel junction layer 35 is provided between the light absorption layer 15 and the light absorption layer 16. The tunnel junction layer 36 is provided between the light absorption layer 16 and the light absorption layer 17. The tunnel junction layer 37 is provided between the light absorption layer 17 and the light absorption layer 18. The tunnel junction layers 31 to 37 are, for example, made of C-GaAs having a thickness of 50 nm. 0.50 Sb 0.50 layer and 50nm thick Si-In 0.53 Ga 0.47 It is a laminated body in which an As layer is bonded.

[0018] The photoelectric conversion element 1 further includes a positive electrode (+ electrode) 41 and a negative electrode (- electrode) .

[0019] The light absorbing layers 11 to 18 each include a p-type layer and an n-type layer. The p-type layer and the n-type layer form a pn junction. The p-type layer of the light absorbing layer 11 contacts the positive electrode 41, and the n-type layer contacts the tunnel junction layer 31. The p-type layer of the light absorbing layer 12 contacts the tunnel junction layer 31, and the n-type layer contacts the tunnel junction layer 32. The p-type layer of the light absorbing layer 13 contacts the tunnel junction layer 32, and the n-type layer contacts the tunnel junction layer 33. The p-type layer of the light absorbing layer 14 contacts the tunnel junction layer 33, and the n-type layer contacts the tunnel junction layer 34. The p-type layer of the light absorbing layer 15 contacts the tunnel junction layer 34, and the n-type layer contacts the tunnel junction layer 35. The p-type layer of the light absorbing layer 16 contacts the tunnel junction layer 35, and the n-type layer contacts the tunnel junction layer 36. The p-type layer of the light absorbing layer 17 is in contact with the tunnel junction layer 36, and the n-type layer is in contact with the tunnel junction layer 37. The p-type layer of the light absorbing layer 18 is in contact with the tunnel junction layer 37, and the n-type layer is in contact with the negative electrode 42. The light absorbing layer 11 has a surface 11A that is irradiated with light L. The surface 11A faces the positive electrode 41.

[0020] The light absorption layers 11 to 18 are, for example, InGaAsP layers. For example, the wavelength of the light L is 1064 nm, and the band gaps of the light absorption layers 11 to 18 are all 1.165 eV or less. The light absorption layers 11 to 18 have different band gaps, which vary monotonically along the first axis. That is, the band gaps become monotonically smaller as the distance from the surface 11A onto which the light L is irradiated increases. The light absorption layers 11 to 18 also have different absorption coefficients, which vary monotonically along the first axis. That is, the absorption coefficients become monotonically larger as the distance from the surface 11A onto which the light L is irradiated increases. The light absorption layers 11 to 18 have the same thickness. For example, the light absorption layers 11 to 18 all have the same thickness of 312.5 nm. Table 1 below shows the thicknesses, band gaps, and absorption coefficients of the light absorption layers 11 to 18. The band gap of InGaAsP is at least 0.753 eV, and cannot be made smaller than 0.753 eV.

[0021] [Table 1]

[0022] In the photoelectric conversion element 1, the band gap varies monotonically along the first axis, and the further away from the surface 11A the light-absorbing layer is, the smaller the band gap and the larger the light absorption coefficient. This reduces the variation in the current obtained in the light-absorbing layers 11-18, thereby reducing current loss. Furthermore, by using light-absorbing layers 11-18 with a thickness that is unlikely to reduce photoelectric conversion efficiency, such as a thickness of 200 nm or more, the total thickness of the light-absorbing layers 11-18 and the tunnel junction layers 31-37 can be set to a thickness that allows lattice matching with the growth substrate. Therefore, the photoelectric conversion element 1 can obtain a high voltage by multi-layering while avoiding a reduction in photoelectric conversion efficiency.

[0023] Furthermore, the light absorbing layers 11 to 18 have the same thickness, which makes the design easy.

[0024] (Second embodiment) A second embodiment will be described. The second embodiment differs from the first embodiment mainly in the configuration of the light absorption layer. Fig. 2 is a cross-sectional view showing a photoelectric conversion element according to the second embodiment.

[0025] The photoelectric conversion element 2 according to the second embodiment has light absorbing layers 21, 22, 23, 24, 25, 26, 27 and 28 instead of the light absorbing layers 11, 12, 13, 14, 15, 16, 17 and 18 in the photoelectric conversion element 1. The light absorbing layers 21 to 28 are stacked one on top of another along the first axis.

[0026] Like the light absorbing layers 11 to 18, the light absorbing layers 21 to 28 each include a p-type layer and an n-type layer. The p-type layer and n-type layer form a pn junction. The p-type layer of the light absorbing layer 21 contacts the positive electrode 41, and the n-type layer contacts the tunnel junction layer 31. The p-type layer of the light absorbing layer 22 contacts the tunnel junction layer 31, and the n-type layer contacts the tunnel junction layer 32. The p-type layer of the light absorbing layer 23 contacts the tunnel junction layer 32, and the n-type layer contacts the tunnel junction layer 33. The p-type layer of the light absorbing layer 24 contacts the tunnel junction layer 33, and the n-type layer contacts the tunnel junction layer 34. The p-type layer of the light absorbing layer 25 contacts the tunnel junction layer 34, and the n-type layer contacts the tunnel junction layer 35. The p-type layer of the light absorbing layer 26 contacts the tunnel junction layer 35, and the n-type layer contacts the tunnel junction layer 36. The p-type layer of the light absorbing layer 27 is in contact with the tunnel junction layer 36, and the n-type layer is in contact with the tunnel junction layer 37. The p-type layer of the light absorbing layer 28 is in contact with the tunnel junction layer 37, and the n-type layer is in contact with the negative electrode 42. The light absorbing layer 21 has a surface 21A that is irradiated with light L. The surface 21A faces the positive electrode 41.

[0027] The light absorption layers 21 to 28 are, for example, InGaAsP layers. For example, the wavelength of the light L is 1064 nm, and the band gaps of the light absorption layers 21 to 28 are all 1.165 eV or less. The light absorption layers 21 to 28 have different band gaps, which vary monotonically along the first axis. That is, the band gaps become monotonically smaller as the distance from the surface 21A irradiated with the light L increases. The light absorption layers 21 to 28 also have different absorption coefficients, which vary monotonically along the first axis. That is, the absorption coefficients become monotonically larger as the distance from the surface 21A irradiated with the light L increases. The light absorption layers 21 to 28 have different thicknesses, with the light absorption layer having a smaller band gap being thicker. That is, the thickness increases in the order of the light absorption layers 21 to 28. The thicknesses of the light absorption layers 21 to 28 increase, for example, exponentially. Table 2 below shows the thicknesses, band gaps, and absorption coefficients of the light absorption layers 21 to 28.

[0028] [Table 2]

[0029] In the photoelectric conversion element 2, the band gap varies monotonically along the first axis, and the further away from the surface 21A the light absorbing layer is, the smaller the band gap and the larger the light absorption coefficient. This reduces the variation in the current obtained in the light absorbing layers 21-28, thereby reducing current loss. Furthermore, by using the light absorbing layers 21-28 with a thickness that is unlikely to reduce photoelectric conversion efficiency, for example, a thickness of 200 nm or more, the total thickness of the light absorbing layers 21-28 and the tunnel junction layers 31-37 can be set to a thickness that allows lattice matching with the growth substrate. Therefore, the photoelectric conversion element 2 can obtain a high voltage by multi-layering while avoiding a reduction in photoelectric conversion efficiency.

[0030] Furthermore, in the photoelectric conversion element 2, the smaller the band gap of the light absorption layer, the thicker it is. Therefore, in the photoelectric conversion element 2, the band gap of the light absorption layer 28, which has the smallest band gap, is made relatively larger than 0.753 eV, and the light absorption layer 28 is made relatively thick, thereby making it possible to reduce current loss more than in the photoelectric conversion element 1. In this way, according to the second embodiment, it is possible to further reduce current loss.

[0031] In the present disclosure, the composition of the compound semiconductor used in the light absorption layer is not particularly limited. Examples of compound semiconductors used in the light absorption layer include quaternary compound semiconductors such as InGaAsP and InAlGaAs. In addition, the wavelength of the irradiated light is not limited, and the composition of the compound semiconductor used in the light absorption layer can be appropriately selected depending on the wavelength of the irradiated light.

[0032] The band gap of the light absorbing layer is not limited as long as it is equal to or less than the energy corresponding to the wavelength of the irradiated light, but it is preferable that the difference between the band gap of the light absorbing layer closest to the surface irradiated with light and the energy corresponding to the wavelength of the irradiated light is small. The thickness of the light absorbing layer is preferably equal to or greater than the thickness at which the decrease in photoelectric conversion efficiency is suppressed, for example, 200 nm or more. Furthermore, the upper limit of the thickness of the light absorbing layer is not particularly limited as long as lattice matching is maintained, and for example, the thickness of the light absorbing layer is several μm or less.

[0033] Various aspects of the present disclosure are summarized below as appendices.

[0034] (Appendix 1) a plurality of light absorbing layers stacked on top of each other along a first axis, each of the light absorbing layers including a compound semiconductor; a tunnel junction layer provided between adjacent light absorption layers among the plurality of light absorption layers; and the plurality of light absorbing layers have different band gaps; the bandgap varies monotonically along the first axis; Photoelectric conversion element.

[0035] (Appendix 2) The compound semiconductor is a quaternary compound semiconductor. Attachment 1: The photoelectric conversion element according to claim 1.

[0036] (Appendix 3) The compound semiconductor is InGaAsP or InAlGaAs. 3. The photoelectric conversion element according to claim 2.

[0037] (Appendix 4) The band gap is 1.165 eV or less in all cases. 4. The photoelectric conversion element according to any one of claims 1 to 3.

[0038] (Appendix 5) The thicknesses of the light absorbing layers are equal to each other. 5. The photoelectric conversion element according to any one of claims 1 to 4.

[0039] (Appendix 6) The smaller the band gap of the light absorbing layer, the larger the thickness. 5. The photoelectric conversion element according to any one of claims 1 to 4.

[0040] (Appendix 7) The thickness of each of the plurality of light absorbing layers is 200 nm or more. 7. The photoelectric conversion element according to any one of claims 1 to 6.

[0041] Although the embodiments have been described in detail above, the present disclosure is not limited to the specific embodiments, and various modifications and changes are possible within the scope of the claims. [Explanation of symbols]

[0042] 1, 2 Photoelectric conversion element 11, 12, 13, 14, 15, 16, 17, 18, 21, 22, 23, 24, 25, 26, 27, 28 Light absorbing layer 11A, 21A side 31, 32, 33, 34, 35, 36, 37 Tunnel junction layer 41 Positive electrode (+ electrode) 42 negative electrode (-electrode)

Claims

1. a plurality of light absorbing layers stacked on top of each other along a first axis, each of the light absorbing layers including a compound semiconductor; a tunnel junction layer provided between adjacent light absorption layers among the plurality of light absorption layers; and the plurality of light absorbing layers have different band gaps; the bandgap varies monotonically along the first axis; Photoelectric conversion element.

2. The compound semiconductor is a quaternary compound semiconductor. The photoelectric conversion element according to claim 1 .

3. the compound semiconductor is InGaAsP or InAlGaAs; The photoelectric conversion element according to claim 2 .

4. The band gap is 1.165 eV or less in all cases. The photoelectric conversion element according to claim 1 .

5. The thicknesses of the light absorbing layers are equal to each other. The photoelectric conversion element according to claim 1 .

6. The smaller the band gap of the light absorbing layer, the larger the thickness. The photoelectric conversion element according to claim 1 .

7. The thickness of each of the plurality of light absorption layers is 200 nm or more. The photoelectric conversion element according to claim 1 .

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