Light-absorbing material, solar cell, and method for producing light-absorbing material

Abstract

To provide a light absorbing material used for solar cells, etc., achieving an improvement (enhancement) in the value of light absorption efficiency, a solar cell including such a light absorbing material, and a production method of such a light absorbing material.SOLUTION: Provided is a light absorbing material including a semiconductor crystal and a dopant rich region regularly arranged in the semiconductor crystal, the dopant rich region including a plurality of dopant ions and having a plurality of resonance absorption wavelengths in a visible region. Also provided is a solar cell including such a light absorbing material as a light absorbing layer. Further provided is a production method in which the diameter of the dopant rich region, the number of contained dopant ions and a distance between dopant ions are determined such that desired resonance absorption energy can be obtained by a molecular orbital method using a variational method.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a light-absorbing material, a solar cell, and a method for manufacturing the light-absorbing material. More specifically, it deals with light absorption by nanometer-sized regions with different electronic states from the surroundings, which are created by ion implantation of dopants into a crystalline semiconductor material. [Background technology]

[0002] Figure 1 shows the energy band diagram of a commonly used Si single crystal solar cell today, as well as a conceptual diagram of the power generation method. Light incident on the solar cell excites electrons in the valence band on the p-type side (left side in Figure 1) of the pn junction, which can be called the light absorption layer, to the conduction band, and the excited electrons pass through the n-type side (right side in Figure 1) and flow to the load, becoming the output current of the solar cell. This output (electron) current then passes through the load and returns to the valence band on the p-type side of the solar cell. The output of the solar cell is the current consumed by the load at this time multiplied by the band gap (the potential difference between the conduction band and valence band). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Isamu Jonoshita, "Creation of quantum dot structures using ion implantation technology," Solar Energy, Japan Solar Energy Society, May 2014, Vol. 40, No. 3, pp. 87-96 (especially p. 94) [Non-patent document 2] Takashi Kita (ed.), "Solar Cell Conversion Efficiency," Corona Publishing, October 2012, p. 40 [Non-patent document 3] Isamu Shironoshita, "Orbital Calculation of Channeling Ions", [online], September 19, 2016, Internet<URL:http: / / www7b.biglobe.ne.jp / ~elbow / > (especially p.10, Fig.32) [Non-patent document 4] Toshiharu Saiki and Yasunori Toda, co-authors, "Nanoscale Optical Properties," Ohmsha, August 2004, pp. 8-10 [Non-Patent Document 5] Masaaki Nakayama, "Optical Properties of Semiconductors," Corona Publishing, August 2013, p. 51 [Non-patent document 6] "Dressed Photon" by Genichi Otsu, Asakura Publishing, March 2013, pp.10-32 [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-025178 [Patent Document 2] Japanese Patent Publication No. 2020-202225 Summary of the Invention [Problem to be solved by the invention]

[0005] As described in Non-Patent Document 2 and the following Reference 1 (particularly the "conventional" sheet on page 5, Appendix 7), when incident light excites electrons in the valence band on the p-type side (left side in Figure 1) to the conduction band, the energy of the incident light that exceeds the band gap is immediately consumed as heat loss and cannot be extracted as solar cell output, resulting in heat loss. Furthermore, light that cannot excite electrons to an energy level exceeding the conduction band cannot be absorbed by electrons in the valence band and is lost through transmission. Due to these losses, assuming the spectrum of incident sunlight on a blackbody surface at 6000°C, the absorption efficiency of light that can be converted into electricity is effectively only 44.3%. [Reference 1] Details of the research related to the present invention are published on the following webpage. Isamu Jonoshita, "Electron energies in nm-scale impurity region and their application for photovoltaics," [online], January 10, 2021, Internet<URL:https: / / regionforsolarcell.wordpress.com>

[0006] The problem to be solved by the present invention is to provide a light-absorbing material having an improved (enhanced) value of the above-mentioned light absorption efficiency, a solar cell including such a light-absorbing material, and a method for manufacturing such a light-absorbing material. [Means for solving the problem]

[0007] One way to solve the above problem is to create a nanometer-scale dopant-rich region (hereinafter sometimes referred to as the DR region) by injecting dopant ions into the light-absorbing material applied to the p-type side (left side in Figure 1) of the pn junction that constitutes the light-absorbing layer of a Si single-crystal solar cell. The following describes a light-absorbing material having a DR region and a method for manufacturing such a light-absorbing material.

[0008] First, an embodiment of the present invention will be described. A light-absorbing material according to an embodiment of the present invention includes a semiconductor crystal and a dopant-rich region regularly arranged in the semiconductor crystal, the dopant-rich region containing a plurality of dopant ions and having a plurality of resonant absorption wavelengths in the visible range.

[0009] Here, the light absorbing material may be capable of absorbing light in a continuous energy range in the visible range.

[0010] The probability density function of light absorption of the light absorbing material preferably has a maximum value at an energy level within ±10% of the energy level at which the solar light intensity is at its maximum.

[0011] The semiconductor crystal is a silicon crystal, the dopant is phosphorus, the dopant-rich region has a diameter of 5 nm or more and 7 nm or less, contains 4 to 10 dopant ions and an average of 7 dopant ions, and the distance between the dopant ions is 2 nm or more.

[0012] The solar cell according to the embodiment of the present invention may include the light absorbing material according to the embodiment of the present invention as a light absorbing layer.

[0013] In the method for producing a light-absorbing material according to an embodiment of the present invention, when producing the light-absorbing material according to an embodiment of the present invention, the diameter of the dopant-rich region, the number of dopant ions contained therein, and the distance between the dopant ions are determined by a molecular orbital method using a variational method so that a desired resonance absorption energy is obtained.

[0014] In this manufacturing method, it is preferable to form the dopant-rich region by forming a crystal film on the surface of the semiconductor crystal, the crystal film having a lattice constant different from that of the semiconductor crystal, and injecting the dopant ions into the semiconductor crystal through the crystal film.

[0015] Furthermore, the diameter of the dopant-rich region, the number of dopant ions contained therein, and the distance between the dopant ions may be controlled by adjusting the ion implantation amount, ion energy, and heat treatment conditions after the ion implantation.

[0016] Here, as detailed in the manufacturing method, it is preferable to apply the same method as that described in detail in Patent Document 2, as summarized below. Here, the semiconductor crystal is not limited to a Si single crystal, and the dopant is not limited to phosphorus ions. A method for producing a light-absorbing material may include a first step of forming a structure in which lattice matching points, where channels of the semiconductor single crystal, where nuclei do not exist on a straight line in the semiconductor single crystal, overlap with channels of the crystal film, where nuclei do not exist on a straight line in the crystal film, are regularly arranged; and a second step of injecting ions that will become dopants in the semiconductor single crystal into the semiconductor single crystal via the crystal film, into the semiconductor single crystal, so that ions that have been injected into the surface of the crystal film travel through the channels of the crystal film, pass through the lattice matching points, and travel through the channels of the semiconductor single crystal to the interior of the semiconductor single crystal where they stop, thereby producing a structure in which DR regions are regularly distributed at a period corresponding to the period of the lattice matching points.

[0017] Here, the ions are preferably those that become n-type dopants in the semiconductor single crystal. In this case, the ions are preferably phosphorus ions. The semiconductor single crystal is preferably made of a p-type semiconductor. The semiconductor single crystal is preferably a Si single crystal, and the crystal film is preferably a Ge crystal film.

[0018] In the second step, the depth at which the DR region is formed in the semiconductor single crystal and the size of the DR region can be controlled by adjusting the kinetic energy of the ions to be implanted and the temperature and time of annealing after ion implantation, and the doping density in the DR region can be controlled by adjusting the amount of ions implanted.

[0019] The light-absorbing material thus obtained comprises a semiconductor single crystal made of one of a p-type semiconductor, an i-type semiconductor, and an n-type semiconductor, and a DR region made of any other of these semiconductors, the DR region being a nanometer-scale region in which the semiconductor single crystal is doped with a dopant and regularly arranged in the semiconductor single crystal.

[0020] Furthermore, the light-absorbing material obtained as described above can be suitably used to form a solar cell. In this case, the solar cell may include, as a light-absorbing layer, a light-absorbing material in which the semiconductor single crystal is p-type and the DR region is n-type.

[0021] Here, a brief description will be given of the light absorption that can be achieved by a light-absorbing material having a DR region. Figure 2 shows an example of the distribution of phosphorus ions in a Si crystal film for the light-absorbing material with the DR region described above. By performing appropriate annealing after ion implantation, these phosphorus ions are activated by replacing silicon atoms in the silicon crystal. The energy of the electron orbitals in the DR region can be calculated using the Ritz variational method. The calculation results show that, under certain conditions, the distribution of the potential barriers in these DR regions exhibits a range similar to the energy distribution of visible light photons. This potential barrier allows the DR region to confine electrons within the region. Electrons trapped in the DR region exhibit a specific, high optical absorption for light with the same energy as the potential barrier. In other words, each DR region has its own unique potential barrier, and it specifically absorbs light with the same energy as its potential energy. [Effects of the Invention]

[0022] The light-absorbing material according to the embodiment of the present invention is a light-absorbing material having an improved (enhanced) light absorption efficiency value compared to conventional materials. The optical absorption coefficient is an index that indicates how easily a substance absorbs light. Assuming a probability density function of 0.5, the optical absorption coefficient of the optical absorption layer containing the DR region is shown in Figure 3. When the optical energy is approximately 3.3 eV or less, absorption by the DR region is dominant over absorption by the band gap. Furthermore, as shown in Figure 4, the optical absorption by the Si layer containing the DR region according to the present invention differs from conventional absorption by the band gap. Light is absorbed only when the optical energy is equal to the potential barrier of the DR region. When light is absorbed under the above conditions, neither heat loss nor transmission loss occurs, resulting in essentially loss-free absorption. [Brief explanation of the drawings]

[0023] [Figure 1] This is an energy band diagram of a silicon single crystal solar cell that is currently in common use, and a conceptual diagram of the power generation method. [Figure 2] FIG. 3 is a diagram showing an example of the distribution of phosphorus atoms (or phosphorus ions) in a Si crystal film produced by a method according to an embodiment of the present invention. [Figure 3] FIG. 10 is a diagram showing changes in the optical absorption coefficient for absorption by the DR region and absorption by the Si crystal band gap. [Figure 4] FIG. 1 is a diagram illustrating the concept of light absorption by a Si layer including a DR region. [Figure 5] This figure shows the change in the potential barrier with respect to the number of phosphorus ions under each condition, where (a) shows the case where the domain size is changed, and (b) shows the case where the distance between phosphorus ions is changed. [Figure 6] FIG. 1 is a diagram showing the relationship between the energy state of electron orbitals in the DR region and the potential barrier. [Figure 7] FIG. 10 is a diagram showing the relationship between the potential barrier and the probability density function in the DR region. [Figure 8] FIG. 10 is a graph showing a relationship curve between light energy and light absorption coefficient α. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, a light-absorbing material, a solar cell, and a method for producing a light-absorbing material according to embodiments of the present invention will be described in detail.

[0025] The inventors have been researching optimal materials and manufacturing methods for these materials with the aim of improving the light absorption efficiency of solar cells. Patent Document 1 discloses a method for creating very small (nanometer-sized) regions in which ions are abundantly implanted into a crystalline material. By implanting oxygen ions into crystalline silicon from four directions using a ZnTe crystal film as a filter, nanometer-sized semiconductor quantum dots (SiO2 insulating regions) can be created regularly, densely, and uniformly.

[0026] Furthermore, in Patent Document 2, the implanted ions in Patent Document 1 are changed from oxygen ions to phosphorus ions (P + ) and changing the filter film to a germanium crystal film, it is possible to reduce the amount of ion implantation. The size of the ion implantation region is about 1.5 times larger than that in Patent Document 1, and the distance between the ion implantation regions is also longer by the same ratio.

[0027] In the present invention, based on Patent Documents 1 and 2, the behavior of electrons in ion-implanted regions in Si crystals was studied in detail with the aim of proposing a method for more efficient light absorption, and a method for improving the unavoidable losses in conventional photovoltaic power generation was devised.

[0028] Figure 2 shows a conceptual diagram of the proposed DR region. The implanted ions are phosphorus ions (P + ), then P + Once the charge is lost, the phosphorus atoms become phosphorus atoms. After the appropriate heat treatment (annealing), the phosphorus atoms replace the Si atoms in the crystal structure and are incorporated into the crystal structure as dopants, becoming phosphorus ions. The free electrons in the Si crystal region around the phosphorus ions then trace orbits with specific energies. These specific energies can be calculated as an approximation using the molecular orbital method using the Ritz variational method.

[0029] Figure 2 shows an example in which phosphorus ions are randomly arranged in a Si crystal under the following conditions, which were obtained as an appropriate set of parameters through trials. Condition 1) The number of phosphorus ions in the DR region is 4 to 10 (average value is 7). Condition 2) The size within the DR region (diameter when approximated to a circle) is approximately 5 nm, 6 nm, and 7 nm. Condition 3) The distance between individual phosphorus ions within the DR region is 2 nm or more. These conditions can be achieved by properly controlling the ion implantation amount and ion energy, and by properly controlling the temperature and time of the post-ion implantation heat treatment (annealing).

[0030] <Calculation method> As mentioned above, to approximate the energies of the electronic levels in the DR region, we use the "Ritz variation method" according to the following procedure.

[0031] (Step 1) Calculation of the effective Bohr radius and effective Rydberg energy In a silicon crystal, phosphorus atoms become phosphorus ions, which generate an electric field around them similar to that of a hydrogen nucleus in a vacuum. However, in the case of phosphorus ions in a silicon crystal, the vacuum is the silicon crystal structure, and in this case the effective Bohr radius a B and the effective Rydberg energy E R is calculated by the following formula:

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[0032] (Step 2) Calculation of overlap integral, Coulomb integral, and resonance integral In the Ritz variational method, it is necessary to solve the following determinant (3).

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[0033] If i≠j, then H ij is called the "resonance integral" and is calculated by equation (6).

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[0034] (Step 3) Eigenvalue E φ Ask for. determinant |H ij -E φ S ij The value of | is calculated for each energy value. This is the "E φ As a function of ", the determinant |H ij -E φ S ij In this case, x is "E φ ", y is |H ij -E φ S ij | value. Furthermore, when considering a linear combination, this curve will have several intersections with the x-axis, and the number of intersections indicates the number of electronic energy states in the DR region, which is equal to the number of phosphorus ions in the region. The energy of the electronic orbitals in the region can then be calculated as x. As detailed in Reference 1, we calculated the minimum value of the electronic energy (maximum as an absolute value) of each DR region in Figure 2 and the potential barrier in the DR region.

[0035] <Calculation results> The above calculation results revealed the following: Result 1) The more phosphorus ions there are in each DR region, the smaller the potential barrier becomes (the absolute value becomes larger, assuming the energy of a free electron is 0). Result 2) The smaller the size of each DR region, the smaller the potential barrier becomes (the absolute value becomes larger, assuming the energy of a free electron is 0). Result 3) The potential barrier values ​​of the individual DR regions range from -6 eV to -0.2 eV. Result 4) Within each DR region, there are several electron energy states around 0 eV.

[0036] Figure 5(a) shows the relationship between the size of the DR region and the potential barrier when the region size is 4 nm, 5 nm, 6 nm, and 7 nm. Figure 5(b) shows this relationship when the distance between phosphorus ions is 2 nm or more, 2.5 nm or more, and 3 nm or more. Results 1) to 3) are clear from Figure 5. These graphs show the change in the potential barrier under each condition.

[0037] Result 4) will be explained in detail using Figure 6. Calculations using Ritz's variational method show that there are several electron energy states near 0 eV, and a resonance energy can be defined for each of these. This means that when photons matching the resonance energy come near the DR region, these photons are absorbed by electrons in the DR region, and electrons in the DR region are raised to a state of around 0 eV, resulting in multiple states. The existence of multiple such states increases the probability of the existence of a "photon matching the resonance energy." This is because electrons whose difference with the energy of a free electron (0 eV) is less than room temperature energy (25.8 meV, 300 K) can easily escape from the DR region.

[0038] Next, we consider the distribution range of the potential barrier for each DR region. Even if all of the above conditions 1) to 3) are the same, the potential barrier value has a certain distribution range. In addition to this distribution range, the distribution range of the potential barrier can also be expanded by changing conditions 1) to 3). DR regions with different sizes and charge densities are allowed to coexist, and the coexistence of these DR regions expands the distribution range of the potential barrier. If this distribution range is made wider than the energy distribution range of visible light, it becomes possible to absorb the energy of all visible light. Therefore, the distribution range of the potential barrier is extremely important in the present invention.

[0039] To illustrate this, the relationship between the potential barrier and the probability density function is shown in Figure 7. (For the data for this graph, see Appendix 4 in Reference 1.) The solid curves in Figure 7 show the potential barrier (horizontal axis) and probability density function (vertical axis) under the conditions of "an average number of phosphorus ions in a region of 7, a region size of approximately 5 nm, and a center-to-center distance between phosphorus ions of 2.5 nm or more." The dashed curve is the curve under the conditions of "an average number of ions of 7, a size of approximately 6 nm, and a center-to-center distance between phosphorus ions of 2.5 nm or more," and the dotted curve is the curve under the conditions of "7, approximately 7 nm, and 2 nm or more."

[0040] Figure 7 also shows the AM1.5 energy spectrum as part of the solar energy spectrum at the surface of Japan. In each of the three probability density function curves, it can be seen that the distribution range of the potential barrier in the DR region covers the AM1.5 energy range as a rough distribution.

[0041] (For details of the formulas (7) to (11), see Non-Patent Documents 4 and 5.) Furthermore, we will consider the optical absorption coefficient of light-absorbing materials. The optical absorption coefficient is a measure of how easily a material absorbs photons, and the value of this optical absorption coefficient is extremely important. Even if the resonance energy of the potential barrier in the DR region spans the entire range of visible light, if the optical absorption coefficient due to the band gap is dominant over the optical absorption coefficient due to the DR region, the optical absorption due to the band gap will be large, resulting in loss.

[0042] The value of the light absorption coefficient α of this light absorption system can be calculated using equation (7).

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[0043] Under this condition, if the DR region is considered as an electric dipole, the resonant frequency can be calculated using the following equation (11).

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[0044] Table 1 shows an example of calculation using equations (7) to (11). Figure 8 (Fig. 5 in Reference 1) shows the relationship curve between the light energy and the light absorption coefficient α. From these relationships, we can see what kind of light absorption will occur. From the curve in Figure 8, the value of α is determined by the amount of light energy that reaches the resonance condition (ω i =ω), and is nearly 0 in other cases. This means that only light of a specific energy is absorbed in a specific region that resonates with that light with a particularly large optical absorption coefficient α. In other words, light absorption in the DR region is only possible without loss in the resonance region. Since the potential barrier value in the DR region is approximately -6 eV to -0.2 eV, light in this region can be absorbed highly efficiently without loss.

[0045] [Table 1]

[0046] Next, the resonance condition (ω i =ω) is examined. The full width at half maximum (FWHM) is usually used to express the resonance range. From equations (7) and (8), it can be seen that the important value that determines the value of α is ε2, and the full width at half maximum of ε2 is expressed by γ. And the relaxation time is 1 ns (= 1 × 10 -9 [s]), the half-width is converted to energy units using equation (12).

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[0047] On the other hand, as shown in Figure 7, if the probability density function when the potential barrier is near the resonance energy is approximately 50%, the probability that the potential barrier in a certain region is within the half-width of the resonance region of a certain light is only 0.000033%. Because of this extremely low probability, the value of N0 in equation (10) becomes small. As a result, the value of ε2 also becomes small, and the value of α also becomes small. With this small value of α, as shown in Table 1, the thickness of the light absorption layer required to absorb 90% of the number of photons is on the order of 100 μm. This is not thin compared to absorption by a normal band gap, and it cannot be said that light absorption by the DR region is advantageous.

[0048] However, there are the following factors that can improve (thin) this necessary thickness, in other words, increase the substantial value of α.

[0049] 1) There are multiple electron energy states around 0 eV Some of the electronic energy states in the DR region are near 0 eV in electronic energy. This means that there are some resonant states, which have slightly different resonant energies from the other states. Taking this factor into account, the value of N0 in Eq. (10) is smaller than the value of N calculated by Eq. (13). eff can be replaced by

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[0050] 2) Influence of adjacent DR regions (See Non-Patent Document 6 for the following equations (14) to (19).) Next, we consider the influence of these neighboring DR regions. If the distance between the DR regions is short enough, energy transfer occurs via so-called "near-field light" (dressed photons). The exchange energy can be calculated using the following equation (14). As shown in Fig. 6 of Reference 1 and in Appendix 5, this value can be as large as 1 meV.

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[0051] As can be seen from the following equation (17) (see Non-Patent Document 6), which is the basis of equation (14), most of the energy transfer is concentrated when the resonance energy of the DR region is near the energy of the light.

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[0052] When the energy of the incident light is slightly greater than the resonance energy, the negative value indicates the emission of energy from this DR region. In other words, when the energy of the incident light is slightly greater near the resonance energy of the DR region, the energy for exciting electrons in the DR region decreases by emitting energy to the outside. However, the value of this emitted energy becomes 0 when the energy for electron excitation becomes equal to the resonance energy. In other words, the energy for electron excitation is automatically adjusted to the resonance energy. (See graph 2 in Appendix 5 of Reference 1)

[0053] As mentioned above, the resonance region is very narrow, and when the relaxation time is 1 ns, the resonance region is only 6.5 × 10 -7 However, if energy transfer is a factor in adjusting the resonance conditions, the resonance region is about 1 meV (1.0 × 10 -3 This directly affects the thickness required for light absorption in solar cells, and can be reduced by a factor of 1000. The calculations of the effects of items 1) and 2) above and the results are shown in the right column of Table 1 and in Appendix 6 of Reference 1.

[0054] [Conclusion] A light-absorbing material in which a DR region containing multiple dopants such as phosphorus ions is formed in a semiconductor crystal such as a silicon crystal has multiple resonant absorption wavelengths in the visible range, and can also absorb light in a continuous energy range in the visible range.

[0055] Furthermore, we investigated the type of configuration suitable for a photovoltaic power generation method using the DR region using the method described herein. From this investigation, we reached the following conclusions. As shown in Figure 7, when comparing the AM1.5 light spectrum with the solid curve (the curve for the conditions of "an average number of phosphorus ions of 7, a DR region size of approximately 5 nm, and a center-to-center distance between phosphorus ions of 2.5 nm or more"), it is easy to see that their peak positions on the x-axis are almost identical. Light with a frequency that provides the maximum energy corresponding to this peak can be absorbed most efficiently. This also indicates that if a p-type Si crystal layer including a DR region under these conditions is used as a light absorption layer, light energy can be efficiently absorbed. The specific parameters of the DR region (e.g., the size of the DR region, the number of phosphorus ions contained, the distance between the phosphorus ions, etc.) can be set so that the maximum value of the probability density function of light absorption is located in an energy range of ±10% of the energy at which the solar light intensity is at its maximum. If even higher efficiency is desired, a multilayer structure can be considered in which a light absorber having a DR region of a light absorption layer designed under different conditions is stacked, such as those shown by the dashed or dotted lines in Figure 7. Whether to adopt a single-layer or multilayer structure and the structural conditions of each layer (average number of phosphorus ions, size of the DR region, center-to-center distance between phosphorus ions) can be determined depending on the circumstances, such as the desired light absorption efficiency.

[0056] To actually manufacture a light-absorbing material having a designed DR region, a method similar to that disclosed in Patent Document 2 can be used. Specifically, a crystalline film with a lattice constant different from that of a semiconductor crystal, such as Si, is formed on the surface of the semiconductor crystal, and dopant ions, such as phosphorus, are implanted into the semiconductor crystal through the crystalline film to form the DR region. In this process, parameters such as the size of the dopant-rich region, the number of dopant ions contained, and the distance between the dopant ions can be controlled by adjusting the ion implantation dose, ion energy, and heat treatment conditions after ion implantation.

[0057] The current limit of conversion efficiency for solar cells using the band gap of Si crystal is said to be 29%. Photon energy is converted into electron energy by the band gap of the pn junction of the Si crystal, but if the photon energy is larger than the band gap, that amount of energy is lost as heat energy. Also, if the photon energy is smaller than the band gap, the photon passes through without being absorbed. On the other hand, as can be seen by comparing Figures 1 and 4, the mechanism by which photon energy is absorbed by the DR region is completely different. Most photons are absorbed by electrons in the DR region, which has a potential barrier that matches their energy. Therefore, since the energy of the light and the energy given to the electrons are the same, theoretically neither the heat loss nor the transmission loss shown in Figure 1 occurs.

[0058] Finally, the basic difference between a solar cell having a light absorbing layer made of a light absorbing material including a DR region according to an embodiment of the present invention and a so-called quantum dot solar cell will be described. Both quantum dot solar cells and solar cells using light-absorbing materials with DR regions absorb the energy of light by exciting the electrons inside them (in the process of extracting them from the quantum dots or DR regions), but the methods for extracting the electrons and transporting them outside the solar cell differ.

[0059] In quantum dot solar cells, electrons are extracted from the light-absorbing layer containing quantum dots by the tunneling effect, which allows the electron wave function to be transmitted between quantum dots. To ensure that the wave functions are consistent between quantum dots, the quantum dots must be uniform in size. Furthermore, because the quantum dots absorb energy according to their wave functions, heat loss and transmission loss occur, similar to absorption due to band gaps. However, because the energy of the wave function can be changed by the size of the quantum dots, stacking light-absorbing layers of consistent size for each layer makes it possible to create solar cells with low heat loss and transmission loss, similar to compound solar cells with multiple band gaps.

[0060] On the other hand, electron extraction by the DR region occurs when the potential barrier and the absorbed energy match. By appropriately configuring the DR region conditions (average number of phosphorus ions, region size, and center-to-center distance between phosphorus ions), multiple energy levels can be established near 0 eV. Furthermore, energy transfer between adjacent regions broadens the absorbable light energy range. As a result, visible light with a wide energy range can be resonantly absorbed. In theory, no heat or transmission loss occurs during this resonant absorption process. Furthermore, the transport of extracted electrons occurs via the diffusion of extracted electrons and the electric field created by the p-n junction, just like in conventional crystalline silicon solar cells. This process is essentially unaffected by the size of the DR region, the number of dopants (phosphorus ions) within the region, or the distance between dopants.

[0061] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present invention.

Claims

1. A semiconductor crystal that is a silicon crystal; and dopant-rich regions regularly arranged in the semiconductor crystal, The dopant-rich region is a plurality of dopant ions, the dopant ions being phosphorus ions; The diameter is 5 nm or more and 7 nm or less, the number of the dopant ions contained is 4 or more and 10 or less, the distance between the dopant ions is 2 nm or more; The light-absorbing material has a plurality of resonant absorption wavelengths in the visible range and is capable of absorbing light in a continuous energy range in the visible range with only a single layer.

2. 2. The light-absorbing material according to claim 1, wherein a probability density function of light absorption of the light-absorbing material has a maximum value at an energy of ±10% of the energy at which the intensity of sunlight is maximum.

3. A light-absorbing material as described in claim 1 or claim 2, wherein the dopant-rich region contains an average of seven dopant ions.

4. A solar cell comprising the light-absorbing material according to claim 1 in a single layer as a light-absorbing layer.

5. A method for producing the light-absorbing material according to any one of claims 1 to 3, wherein the diameter of the dopant-rich region, the number of dopant ions contained therein, and the distance between the dopant ions are determined by a molecular orbital method using a variational method so as to obtain a desired resonance absorption energy.

6. forming a crystal film having a lattice constant different from that of the semiconductor crystal on the surface of the semiconductor crystal; The method for producing a light-absorbing material according to claim 5 , wherein the dopant-rich region is formed by implanting the dopant ions into the semiconductor crystal through the crystal film.

7. 7. The method for producing a light absorbing material according to claim 6, wherein the diameter of the dopant-rich region, the number of dopant ions contained therein, and the distance between the dopant ions are controlled by adjusting the ion implantation amount and ion energy when implanting the dopant ions, and the conditions of heat treatment after ion implantation.

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