JP2015228413A - Solar battery of high conversion efficiency and preparation method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel middle-band type solar battery arranged so that electrons (hot carriers) having a high temperature within a middle band are used.SOLUTION: A solar battery comprises: a substrate; a p-type semiconductor layer; a light absorption layer; an n-type semiconductor layer; and an electrode. The light absorption layer includes a quantum well, a quantum wire or a quantum dot, and it is formed from a compound semiconductor, a metal nitride, a sulfur compound, a selenide, a metallic arsenic compound or a metal phosphorus compound, or an alloy semiconductor composed of a combination thereof, which includes at least one element selected from Cu (copper), In (indium), Ga (gallium) and Al (aluminum). A quantum well, a quantum wire, a quantum dot or a doped impurity forms a middle level between an upper end energy level of a valence band, and a lower end energy level of a conduction band. Further, of carriers of the middle level, hot carriers having an energy higher than the lower end energy level of the conduction band are taken out from the electrode as a current.

Description

  The present invention relates to a solar cell that suppresses transmission loss and heat loss to improve the conversion efficiency of solar energy.

In recent years, solar cells have been attracting attention as a clean energy source that does not emit CO ₂ , and the spread thereof is progressing. At present, the most popular solar cell is a single junction solar cell using silicon. A single-junction solar cell is composed of a pn junction in which a p-type semiconductor and an n-type semiconductor are joined. The energy difference between the upper end of the valence band (VB) and the lower end of the conduction band (CB) is referred to as band gap energy (E _g ). When light is incident having a higher energy E _g in a semiconductor, the light is absorbed. When light is absorbed, electrons present in VB are excited to CB, and holes are generated in VB. The generated electrons and holes move to the n-type semiconductor side and the p-type semiconductor side by the electric field in the depletion layer generated by the donor ion of the n-type semiconductor and the acceptor ion of the p-type semiconductor, respectively. Thereafter, electrons can be taken out as a current by flowing out to an external circuit through the electrodes.

A single-junction solar cell (hereinafter simply referred to as a solar cell) has some inevitable losses, so that the incident light energy is not converted into electrical energy by 100%. The main loss, as shown in Figure 1, when the light having a smaller energy than E _g is incident photons of light becomes a transmission loss not absorbed, also, the incident light having a larger energy than E _g In this case, electrons and holes are excited with higher energy than the CB lower end and VB upper end, respectively, but before the excited carriers reach the electrode, excess energy is lost due to phonon scattering or the like. End up.

  As described above, the energy conversion efficiency of a solar cell has an upper limit of conversion efficiency that is difficult to exceed, mainly due to the existence of inevitable losses such as heat loss and transmission loss. Usually, the upper limit of the energy conversion efficiency of a solar cell is about 30%. If the above unavoidable loss can be suppressed, a great improvement in conversion efficiency can be expected.

  Here, when obtaining the conversion efficiency of the solar cell, a standard sunlight spectrum is required. The sunlight spectrum represents how far the sunlight has passed through the atmosphere, and an air mass (AM) is used. The AM of sunlight incident at an angle θ with respect to the normal of the earth surface is given by 1 / cos θ. AM1.5 is the solar spectrum in a mild climate and is the standard spectrum for determining solar cell efficiency.

FIG. 2 shows the output of the solar cell and the ratio of each loss to the incident light energy in the Shockley-Queisser model of the solar cell under AM1.5 non-condensing. The theoretical conversion efficiency is obtained by theoretical calculation at the time of detailed equilibrium in consideration of carrier recombination. As shown in the figure, the maximum theoretical conversion efficiency of the solar cell under AM1.5 non-condensing, that is, the Shockley-Quisser theoretical limit value (SQ theoretical limit) is when E _g is 1.34 eV. About 33%.

  A solar cell having a structure exceeding the SQ theoretical limit is called a third generation solar cell, and research is currently underway. Known examples of third generation solar cells include hot carrier solar cells (see, for example, Non-Patent Document 1) and intermediate band solar cells (see, for example, Patent Documents 1 and 2).

A hot carrier type solar cell is a solar cell composed of a carrier generation layer and an energy selective electrode (SEC). The SEC, by forming adjacent the electron transport layer and the hole transport layer with a narrow E _g the carrier generation layer, which is a carrier having a specific energy realizing be taken out as a current through the moving layer It is. In this solar cell, the carrier excited when absorbing light having an energy higher than E _g is taken out via SEC before energy relaxation, thereby suppressing heat loss and increasing efficiency. A conceptual diagram of the hot carrier solar cell is shown in FIG. The theoretical conversion efficiency of the hot carrier solar cell under AM1.5 non-condensing is assumed to be about 66%, and it is attracting attention as a solar cell that realizes high efficiency next generation, similar to the intermediate band solar cell described later. ing. However, the present situation is that the device has not yet been made so that carriers can be taken out as current before energy relaxation.

  Non-Patent Document 1 discloses the dependence of quantum well parameters (composition ratio, width, number) on the carrier temperature increase due to light collection in a carrier generation layer having a quantum well. Non-Patent Document 1 relates to a hot carrier type solar cell, and an energy selection electrode that can pass only carriers in a very narrow energy range such as a commonly used resonant tunneling structure (see FIG. 1 of Non-Patent Document 1). “ESC” is described, but is synonymous with “SEC”) (see FIG. 4).

Next, an intermediate band solar cell (IBSC) will be described. A conceptual diagram of the intermediate band solar cell is shown in FIG.
As shown in FIG. 5, the intermediate band solar cell has a structure in which an allowable band of electrons called an intermediate band (IB) is inserted between VB and CB of the solar cell. In order to form the intermediate band, a method of doping a transition metal with a high concentration in a semiconductor host semiconductor and a method of forming a quantum nanostructure (quantum well, quantum dot, etc.) have been studied. By inserting the intermediate band, in addition to the electron excitation from the valence band to the conduction band of the conventional solar cell, new electron excitation from the valence band to the intermediate band and from the intermediate band to the conduction band becomes possible. . By enabling new electronic excitation, it becomes possible to absorb light having energy smaller than the energy _EVC between the valence band and the conduction band and to perform photoelectric conversion. For this reason, the intermediate-band solar cell is expected to have a high energy conversion efficiency exceeding the SQ limit by suppressing transmission loss as compared with a conventional solar cell.

Intermediate band solar cell, under AM1.5 non _{condensing, E VC} is 2.1 eV, intermediate band (IB) - Energy _{E IC} between the conduction band (CB) is the theoretical conversion efficiency when 0.75eV The maximum value is about 48%. Intermediate band solar cells are attracting attention because they greatly exceed approximately 33% of the theoretical conversion efficiency of solar cells. However, the maximum value of the conversion efficiency at the experimental value is about 18% under non-condensing, and cannot exceed about 29%, which is the maximum conversion efficiency at the experimental value of the Si solar cell. One of the reasons why the conversion efficiency of an actual intermediate band solar cell is low may be that carrier excitation efficiency by light between the intermediate band (IB) and the conduction band (CB) is poor. In an intermediate band solar cell using an actual InAs / GaAs quantum dot, an increase in current is observed by causing excitation of carriers between the intermediate band (IB) and the conduction band (CB) using a strong laser. However, the current increase is insufficient. In order to increase the efficiency of the intermediate band solar cell, the carrier between the intermediate band (IB) and the conduction band (CB) is efficiently excited by light, or the current is increased by a method other than light. New solar cell structures are required.

  As described above, an intermediate band solar cell in which an intermediate band (IB) is provided between a valence band (VB) and a conduction band (CB) of a solar cell has been studied as a method for suppressing transmission loss. However, although the transition from the intermediate band (IB) to the conduction band (CB) is realized by light absorption, the light absorption coefficient is actually very small, and the effect of suppressing transmission loss by the intermediate band is realized. Required a mechanism for pumping electrons from the intermediate band (IB) to the conduction band (CB) more efficiently.

JP 2012-195381 A JP 2012-089756 A

L. C. Hirst et al., Hot Carriers in Quantum Wells for Photovoltaic Efficiency Enhancement, IEEE JOURNAL OF PHOTOVOLTAICS, VOL.4, NO.1, pp.244-252, JAN. 2014.

As described above, the current intermediate band type solar cell has a problem that the efficiency of electron excitation between the intermediate band and the conduction band by light is not good, and it is more efficient to realize the effect of suppressing the transmission loss by the intermediate band. A mechanism to pump electrons from the intermediate band to the conduction band was required.
In view of such circumstances, an object of the present invention is to provide a new intermediate band solar cell using electrons having a high temperature in the intermediate band, that is, hot carriers.

  As a result of earnestly examining the improvement in efficiency of the intermediate-band solar cell in the third generation solar cell, the present inventors have used energy other than light in the past to increase the temperature of the electrons, so that other than light can be obtained. It was found that the current can be increased by the method.

That is, the solar cell of the present invention has an intermediate level between the energy level at the upper end of the valence band and the energy level at the lower end of the conduction band, and among the carriers of the intermediate level, the energy level at the lower end of the conduction band. Hot carriers having higher energy are taken out as current.
As carriers present in the intermediate level become hot carriers having a high carrier temperature, the occupation probability of carriers at high energy increases. In other words, since electrons in the intermediate level become hot carriers having a high electron temperature, electrons in the intermediate level can exist at a higher energy than the energy level at the lower end of the conduction band. Electrons that exist stochastically at an energy higher than the energy level can be taken out as a current.
In the case of an intermediate band solar cell, the electrons in the intermediate band (IB) become hot carriers, so that the electron distribution function spreads to the high energy side, and the electrons in the intermediate band (IB) have a conduction band (CB). It becomes possible to exist with higher energy than the energy at the lower end. Among the electrons excited between the valence band (VB) and the intermediate band (IB), electrons that are present stochastically at an energy higher than the energy at the lower end of the conduction band (CB) can be taken out as a current.

Here, the intermediate level is formed by quantum wells, quantum wires, quantum dots, or impurity doping.
An intermediate level can be formed by forming a fine quantum nanostructure such as a quantum well (two-dimensional structure), a quantum wire (one-dimensional structure), or a quantum dot (0-dimensional structure) on the host semiconductor. In particular, according to the quantum dot superlattice, a large number of intermediate levels (minibands) can be formed.
The intermediate level formed by impurity doping means an in-band gap level due to impurities or an in-band gap level derived from a three-dimensional arrangement of impurities.

  More specifically, the solar cell of the present invention is a solar cell having a substrate, a p-type semiconductor layer, a light absorption layer, an n-type semiconductor layer, and an electrode, wherein the light absorption layer comprises quantum wells, quantum wires, or quantum dots. A compound semiconductor containing at least one element selected from Cu (copper), In (indium), Ga (gallium), and Al (aluminum), a metal nitride, a sulfur compound, a selenide, a metal arsenic compound, or It is comprised from the alloy semiconductor by a metal phosphorus compound or these combinations. The quantum well, quantum wire, quantum dot, or doped impurity forms an intermediate level between the energy level at the top of the valence band and the energy level at the bottom of the conduction band. Then, hot carriers having energy higher than the energy level at the lower end of the conduction band among the carriers at the intermediate level are extracted from the electrode as current.

In the above-described solar cell of the present invention, it is preferable that the hot carrier is one in which light having a wavelength smaller in energy than the band gap is absorbed and the temperature of the intermediate level carrier is increased.
More preferably, the increase in the temperature of the intermediate level carriers is due to light collection. This is because it is considered that the more the light is condensed, that is, the higher the light condensing magnification, the more likely it is to become a hot carrier.

Next, the preparation method of the solar cell of this invention is demonstrated.
The method for preparing a solar cell of the present invention comprises an intermediate level between the energy level at the upper end of the valence band and the energy level at the lower end of the conduction band, and among the carriers of the intermediate level, the energy level at the lower end of the conduction band. This is a method for preparing a solar cell in which hot carriers having an energy higher than that are taken out as current, and includes the following steps.
1) Band gap control step for adjusting the band gap by preparing the composition of the host semiconductor of the solar cell 2) Composition of the semiconductor to be the light absorption layer, size at which the quantum effect appears, quantum dimension, impurity to be doped, impurity doping amount, Alternatively, an intermediate level control step of preparing at least one impurity doping position to form an intermediate level, and controlling a difference between the energy level of the intermediate level and the conduction band to a predetermined value

In the band gap control of 1), for example, in the case of an In _x Ga _1-x As thin film on a gallium arsenide (GaAs) substrate, the composition of the thin film is controlled so that the band gap energy E _g is 0.36 to 1.42 eV. Is to control widely.

Controlling the composition of the semiconductor in the intermediate level control of 2) means controlling the composition of the In _x Ga _1-x As thin film forming the quantum well layer. Controlling the size at which the quantum effect appears is to control the width in the thickness direction of the quantum well layer. Controlling the quantum dimension means controlling the structure such as a quantum well (two-dimensional structure), a quantum wire (one-dimensional structure), and a quantum dot (0-dimensional structure). Controlling the impurity to be doped means controlling the type of the transition metal that is an impurity, and controlling the impurity doping position means controlling the three-dimensional position of doping.
Also, controlling the difference between the energy level of the intermediate level and the conduction band to a predetermined value means controlling the difference between the energy level of the intermediate level and the conduction band so that the conversion efficiency of the solar cell takes a maximum value. Is preferred.

  According to the solar cell of the present invention, the energy other than heat is used by utilizing the energy that has been heat loss for increasing the temperature of the electrons and taking out hot carriers having energy higher than the energy level at the bottom of the conduction band as current. The current can be increased. By absorbing the light that would otherwise be transmitted and lost in the intermediate band, and generating carriers in the intermediate band with high density, the carrier is pumped up to the conduction band, and the current generated in the solar cell is increased to increase the energy conversion efficiency. Can be improved.

Illustration of transmission loss and heat loss Graph showing the output of each solar cell and the ratio of each loss to the incident light energy in the Shockley-Queisser model Conceptual diagram of hot carrier solar cell Conceptual diagram of a hot carrier solar cell disclosed in FIG. 1 of Non-Patent Document 1. Conceptual diagram of an intermediate band solar cell Explanatory drawing of the electric current in the solar cell of this invention Illustration of current in a conventional intermediate band solar cell Graph showing the relationship between electron Fermi distribution function and electron temperature Schematic drawing to extract current using hot carriers in solar cell of the present invention Explanatory drawing of pseudo-Fermi level of electron-hole in solar cell of the present invention Comparison graph of the number of photons that can be absorbed between VIs when E _VC = 1.42 eV Graph of concentration factor and carrier temperature for quantum well solar cells Conceptual diagram of E _VC model and E _VI model Graph showing X and E _IC dependence of conversion efficiency η in E _VC model Graph showing the trade-off relationship between the number of photons and the occupation probability of electrons Graph showing X and E _VC dependence of conversion efficiency η in E _VI model Graph (AM1.5, AM0) showing the theoretical value of conversion efficiency at maximum light concentration Photoluminescence characteristics of InGaAs quantum well and InAs / GaAs quantum dots Explanatory drawing of energy bands of InGaAs quantum well and InAs / GaAs quantum dots Diagram showing temperature characteristics of carrier Graph showing the theoretical value of energy conversion efficiency of single-junction solar cells with intermediate levels formed by quantum dots Graph showing theoretical values of energy conversion efficiency of single-junction solar cells with intermediate levels formed by quantum wells

  Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. The scope of the present invention is not limited to the following examples and illustrated examples, and many changes and modifications can be made.

  The state of current in the solar cell of the present invention is shown in FIG. For reference, FIG. 7 shows a current state in a conventional intermediate band solar cell. The excitation of electrons in a conventional intermediate band type solar cell is performed between the valence band and the conduction band (between VC), between the valence band and the intermediate band (between VI), and between the intermediate band and the conduction band (between ICs). On the other hand, in the solar cell of the present invention, the excitation of electrons between VC and VI is caused by light as in the conventional intermediate band solar cell, Unlike conventional ones, it is assumed that electrons are not excited by light between ICs.

In the solar cell of the present invention, the voltage is determined by the difference between the electron pseudo-Fermi level E _fe and the hole pseudo-Fermi level E _fh , as in the conventional intermediate band solar cell. In addition to E _fe and E _fh , there is an intermediate band pseudo-Fermi level E _fi as in the conventional intermediate band solar cell, and it can be assumed that E _fi exists in IB. Therefore, in the solar cell of the present invention, the difference in pseudo-Fermi level between VI and IC can be expressed as E _fi -E _fh and E _fe -E _fi , respectively, and their sum is E _fe -E _fh . . Further, since E _fe -E _fh coincides with the difference in pseudo-Fermi level between VCs, the sum of the voltage between VI and the voltage between ICs becomes equal to the voltage between VCs.
Here, the electrons are Fermi particles, and the occupation probability of the electrons that are Fermi particles follows the Fermi distribution function. The Fermi distribution function is given by the following formula (1). T _e in the equation represent the electron temperature. FIG. 8 shows the relationship between the electron Fermi distribution function and the electron temperature.

FIGS. 8A and 8B show the state where T _e = 300K and T _e = 1000K are substituted into the above equation (1), respectively. The horizontal axis represents the occupancy probability of the electrons, and the vertical axis represents the difference (EE−E _fe ) between a certain energy E and the quasi-Fermi level E _fe of the electrons. When E = E _fe , the electron occupation probability is 0.5. In f _e (E) of the above formula (1), as shown in FIG. 8, the occupation probability of electrons on the high energy side increases as T _e increases. Electrons present in the excited by IB in light absorption between the VI, occupancy probability at higher energy side increases with increasing T _e, electrons in the IB is greater than the energy E _c of the conduction band minimum Can exist in energy stochastically.

In the solar cell of the present invention, it is defined that the number of electrons that exist stochastically at a certain energy is given by the product of the number of electrons excited between VI and the distribution function. Of the excited electrons between VI, electrons present stochastically at a higher energy than E _c is assumed to be extracted as a current. The current taken out by this process is called a current that can be taken out using hot carriers. In FIG. 9, the conceptual diagram which takes out an electric current using the hot carrier in the solar cell of this invention is shown.

In the solar cell of the present invention, as shown in FIG. 10, it is defined that the electron-hole pseudo-Fermi level between VC and the electron-hole pseudo-Fermi level between VI open separately. In other words, in the solar cell of the present invention, it is defined that the pseudo-Fermi level of electrons between VC and VI does not match, and the entire voltage is determined by the difference of the pseudo-Fermi level of electron-hole between VCs. Hereinafter, the structure of the solar cell of the present invention is referred to as a hot carrier intermediate band solar cell model (HCIBSC).
In HCIBSC, the number of electrons n _Te (E) that can be extracted using hot carriers at a certain energy E can be expressed by the product of the electron distribution function and the number of electrons generated between VIs. Later method of setting the _{E fe} and _{T e} of the formula (1) in HCIBSC. After that, how to obtain the current that can be extracted using hot carriers will be described.

First, how E _fe is set in HCIBSC will be described. In HCIBSC, E _fe in the above formula (1) represents the pseudo-Fermi level of electrons between VI. E _fe was assumed to be present in the IB. This is the same assumption as the model of the intermediate band solar cell proposed by Luque and Marti, and means that electrons are buried halfway through IB. Assuming that E _fe exists in the IB, even if electrons in the IB escape, the escaped electrons are compensated by excitation from the VB. Therefore, the electrons in the IB may become empty and are not satisfied, and the transition of electrons between VI and IC is always possible.

Next, in HCIBSC, describe what has been set how the _{T e.} Since the number of electrons excited between VIs is equal to the number of photons absorbed between _VIs , it varies with _EVI . FIG. 11 shows a comparison of the number of photons (F _sVI ) that can be absorbed between VIs when E _VC = 1.42 eV under AM1.5 non-condensing. The number of photons (F _sVI ) when E _VI = 0.80 eV shown in FIG. 11A is the number of photons having an energy of 0.80 eV to 1.42 eV. On the other hand, the number of photons (F _sVI ) when E _VI = 1.10 eV shown in FIG. _11B is the number of photons having energy from 1.10 eV to 1.42 eV. If is less _{E VI} from FIG. 11 to increase the number of photons _{(F SVI)} is the number of electrons excited between VI it can be seen that increased.

In addition, the number of electrons excited between VIs also varies depending on the light collection magnification. When the condensing magnification is X, the number of electrons excited between VIs can be expressed as XF _sVI .
FIG. 12 shows the relationship between the condensing magnification of the InGaAs / GaAsP quantum well solar cell and the carrier temperature (T _eh ). Here, the horizontal axis of FIG. 12 represents the light collection magnification (Suns equivalent), and the vertical axis represents the carrier temperature T _eh . “Deep well” is a value estimated by measuring a sample of In _0.2 Ga _0.8 As, and “shallow well” is a value estimated by measuring a sample of In _0.11 Ga _0.89 As. T _eh of “deep well” is experimentally estimated by observing light emission of 1.30 eV.

In the solar cell of the present invention, in the case of “deep well” from FIG. 12, it is assumed that the carrier temperature increases linearly by about 1 ° C. by increasing the condensing magnification by 300. Note that this is a simple modeling, and does not limit the physical change in the carrier temperature. Depending on the energy distribution of the density of states of the quantum structure, the carrier temperature may respond nonlinearly to the focusing factor.
Since the observed emission 1.30EV, number of carriers in the non-condensing light when the number of photons contained in the energy 1.42eV from 1.30EV and _{F ref, 300F ref} when the condensing magnification 300 It becomes. In HCIBSC From _here, the _{XF SVI} increases _{300F ref,} the electron temperature _{T e} is assumed to rise 1 ° C.. The following formula (2) shows the relationship between _{T e} and _{XF SVI} at HCIBSC. In the equation (2), T ₀ is the temperature of the electrons when the F _ref electrons are excited between VIs during non-condensing.

In HCIBSC, it was assumed that the lattice temperature of the solar cell was the same, and T ₀ = 300K. If dT _e / d _{(XF sVI)} is _{XF SVI} increases _{300F ref,} the electron temperature _{T e} is because it is assumed that the rise 1 ° _C., and a _{dT e / d (XF sVI)} = 1 / 300F ref. Therefore, to determine the _{T e} by using the following equation in HCIBSC the (3).

In HCIBSC, the number of electrons n _Te (E) that can be extracted using hot carriers at a certain energy can be expressed by the product of the electron distribution function f _e (E) and the number of electrons generated between VI. The number of electrons generated between VIs is equal to the number of photons XF _sVI that can be absorbed between VIs. Therefore, n _Te (E) is represented by the following formula (4). From equation (1) described above, where, although the _f e (E) = 0.5 when E _{= F fe,} since be written as _n Te (E) _{= XF sVI} when E _{= F fe,} equation (4 ) Is multiplied by 2.

Further, the number of electrons Δn _Te (E) existing per unit energy is given by the following formula (5) obtained by differentiating n _Te (E).

The current I _Te that can be extracted by using hot carriers is assumed to extract electrons that are present stochastically at an energy higher than E _c . Therefore, I _Te can be expressed by the following equation (6), assuming that the value obtained by integrating Δn _Te (E) from E _c to a sufficiently large energy E _{∞ is} multiplied by the elementary charge _q .

The current I _total that can be taken out as the entire system is the sum of the current I _VC that can be taken out between _VC and the current that can be taken out through IB, that is, the current I _IC that can be taken out between _ICs, and can be expressed as I _total = I _VC + I _IC. . Further, since the voltage V _total is a difference in the electron-hole pseudo-Fermi level between _VCs, it can be expressed as V _total = V _VC . Further, the current I _IC that can be extracted between the _ICs is the current extracted from IB to CB, and is therefore equal to I _{Te in the} above formula (6), so that I _IC = I _Te can be expressed. Here, the relationship between I _VC and V _VC can be expressed by the following formula (7) representing the current-voltage characteristics of the solar cell. Here, X is the light collection magnification, k is the Boltzmann constant, and _Tc is the lattice temperature of the solar cell.

Further, F _c0 shown in the following equation (8) is the number of electron excitations due to the temperature of the solar cell itself, and is obtained by integrating the Planck's black body radiation equation. In the equation, ν represents the frequency, ν _g represents the frequency of a photon having an energy of E _g , and 2π represents the solid angle of the photon emitted from the solar cell.

As described above, since the current I _total that can be taken out as the entire system can be expressed as I _total = I _VC + I _IC , the following expression (9) is obtained by substituting the above expressions (6) and (8). Then, when the expression (9) is rewritten as V _VC = V _total , the following expression (10) is obtained.

From the above formula (10), when the current and voltage at which the product of I _total and V _total is maximized are expressed as I _max and V _max , the output P _max of the HCIBSC can be expressed as P _max = I _max × V _max . The conversion efficiency eta, the incident light energy as _{P in,} the output _{P max} can be calculated by the following formula divided by _{P in} (11).

In the following examples, as an example of a hot carrier intermediate band solar cell (HCIBSC), a numerical simulation is performed using a model in which one intermediate band is provided in the band gap of a single junction solar cell, and a specific conversion efficiency is obtained. The calculated result will be described.
The conversion efficiency η was calculated using the energy E _IC between _ICs and the light condensing magnification X as variables. The incident light used AM1.5 spectrum. As shown in FIG. 13, the numerical simulation was performed by changing the _EIC by the following two different methods. _Hereinafter, it will be called a model for changing the _{E IC} in terms of fixing the _{E VC,} a model of changing the _{E IC} in terms of fixing the _{E VI,} respectively _{E VC model,} and _{E VI} model.

In the _EVC model, the host semiconductor is assumed to be GaAs, and the model is assumed to change the IB formation method. Since the _{E g} of GaAs is a about _1.42eV, it was fixed with the E VC = 1.42eV. On the other hand, the _EVC model is a model that assumes that the material used as the host semiconductor is changed while the IB formation method is fixed. Typical _{E VI} in IB formation method of the intermediate band solar cell using InAs / GaAs quantum dots is about 1.00 eV. Therefore, the E _VC model was fixed at E _VI = 1.00 eV.

In the first embodiment, energy conversion efficiency when the light condensing magnification in the _EVC model and the energy between the ICs are changed will be described.
FIG. 14 shows the dependence of the conversion efficiency η in the E _VC model on X and E _IC . In the figure, the vertical axis represents E _IC and the horizontal axis represents X. The conversion efficiency η is represented by contour lines and plots. When E _IC = 0 eV, it is equal to the case of a single junction solar cell with E _g of 1.42 eV.
FIG. 14 shows that the conversion efficiency η increases monotonically as X increases. What the above equation _(7), solving for _{V VC} expressed by the following equation (12).

From the above equation (12), it can be seen that V _VC increases as X increases. The electron temperature _{T e} rises when _{F SVI} increases from the above equation (3), when _{T e} from the equation (1) above is increased high _f in energy side e (E) is understood to be increased. The above equation (4), (5), I Te increases when _f e (E) increases from (6). From the above, it can be seen that as X increases, V _VC increases and f _e (E) increases on the high energy side, resulting in an increase in conversion efficiency η.

Further, FIG. 14 shows that when X is fixed and E _IC is changed, the conversion efficiency takes the maximum value when E _IC = 0.03 eV. When E _IC = 0.03 eV under non-condensing, η is about 33.1%, and an increase in efficiency of 0.5% is estimated compared to about 32.6% of a single-junction solar cell. . When X = 45000 and E _IC = 0.03 eV, the conversion efficiency η is about 42.1%, which is compared with about 41.1% of a single-junction solar cell having a band gap energy of 1.42 eV. An increase in efficiency of 1.0% was estimated. In the E _{VC model,} consider the effect of changes in the _{E IC} give.

As shown in FIG. _11, when _{E IC} is _small, the number of photons _{(F SVI)} is reduced _{to E VI} is increased. Further, since the difference between E _fe and E _C becomes small, the probability of occupying electrons with energy larger than E _C increases. _Conversely, if _{E IC} is _large, the number of photons _{(F SVI)} is increased _{to E VI} is reduced. Further, since the difference between E _fe and E _C becomes large, the probability of occupying electrons with energy larger than E _C decreases. Thus in accordance with a change in E _IC, it can be seen that there is a tradeoff between the occupancy probability of the number of photons and (F _SVI) electrons.

15 shows a trade-off relationship between the electron occupancy probability and number of photons and (F _SVI) when changing the E _IC. It was found that I _Te was maximized when E _IC = 0.03 eV.
From the above, it can be seen that HCIBSC can achieve high efficiency by forming an intermediate band (IB) so that E _IC = 0.03 eV when GaAs is used as a host semiconductor.

In the second embodiment, energy conversion efficiency when the magnification between the _EVI model and the energy between the ICs is changed will be described.
FIG. 16 shows the dependence of the conversion efficiency η in the _EVI model on X and _EVC . In the figure, the vertical axis represents E _VC and the horizontal axis represents X. The conversion efficiency η is represented by contour lines and plots.
As in the _EVC model, it can be seen that the conversion efficiency η increases monotonically as X increases. In addition, when X is fixed and E _VC is changed, it can be seen that the conversion efficiency η has a maximum value when E _VC is approximately 1.05 eV, 1.16 eV, and 1.34 eV. _{E IC} is about 0.05eV next when the E _VC is about 1.05 _eV, roughly match the _{conditions I Te} as estimated by _{E VC} model is maximized. Thus since the current using hot carriers is increased when E _VC is about 1.05 eV, the conversion efficiency η can be expected to take a maximum value.

The extreme value structure of the conversion efficiency η seen at 1.16 eV and 1.34 eV is due to the fine structure of AM1.5 spectrum. FIG. 17 shows a theoretical value of conversion efficiency at the time of maximum condensing (45900) in AM1.5. FIG. 17 shows that when the band gap is about 1.16 eV and 1.34 eV, the conversion efficiency of the single-junction solar cell takes a maximum value depending on the spectral structure of AM1.5. Along with this, it can be seen that the maximum values when E _VC is approximately 1.13 eV and 1.34 eV are due to the increased influence of optical transition of carriers between VCs.

As a result, HCIBSC the host semiconductor _{E g} is 1.05eV, 1.13eV, the condition of 1.34 eV, but corresponds approximately, and it is necessary to use a material capable of forming an intermediate band (IB) I understand that. As a material corresponding to this condition, E _g is at 1.42 eV, include GaAs, which is often used in the intermediate band solar cell using quantum dots.
Moreover, the _{E VC} model described in Example 1, when using a GaAs host semiconductor, for high _efficiency, like the E IC = 0.03 eV, should the intermediate band (IB) I know that. Typical E _VC and E _IC of an intermediate band solar cell using InAs / GaAs quantum dots are E _VC = 1.42 eV and E _IC = 0.40 eV, respectively, so that E _IC = 0 It can be seen that 0.03 eV is much lower energy than the conventional intermediate band solar cell.
From the two types of numerical simulation results of the E _VC model and the E _VI model, in order to realize a high-efficiency intermediate band solar cell using hot carriers, the material used as the host semiconductor is E _g of 1.05 eV, It is necessary to use a band approximately corresponding to the conditions of 1.13 eV and 1.34 eV, and to form the intermediate band (IB) so that E _IC = 0.03 eV.

In Example 3, in a single-junction solar cell using GaAs as the host semiconductor of the light absorption layer, an intermediate level is provided by an InGaAs quantum well or InAs / GaAs quantum dots, with reference to FIGS. explain.
FIG. 18 shows the photoluminescence characteristics of InGaAs quantum wells and InAs / GaAs quantum dots. In the case of an InGaAs quantum well, an intermediate level is formed at 1.35 ev. In the case of InAs / GaAs quantum dots, an intermediate level is formed at 1.05 ev.
Since the _{E g} of GaAs is about 1.42 eV, the difference between the intermediate level formed by InGaAs quantum well, _{i.e., E IC} is about 0.07 eV (see FIG. 19 (1)). On the other hand, the E _IC that is the difference from the intermediate level formed by InAs / GaAs quantum dots is about 0.37 eV (see FIG. 19 (2)).

FIG. 20 shows the temperature characteristics of the carrier. As shown in FIG. 20, in the case of InGaAs quantum wells, the carrier temperature is 100 to 200 (K), and in the case of InAs / GaAs quantum dots, the carrier temperature is 800 to 2000 (K), and InAs / GaAs quantum dots. It can be seen that the carrier temperature is about 8 to 10 times higher than that of the InGaAs quantum well. Further, it can be seen that when the number of incident photons is increased from 10 ²¹ to 10 ²² , the carrier temperature is approximately doubled.
That is, since the carrier temperature of InAs / GaAs quantum dots is higher than that of InGaAs quantum wells, carriers are likely to become hot carriers, and the occupation probability of carriers with high energy increases.

FIG. 21 shows a theoretical value of energy conversion efficiency of a single junction solar cell in which an intermediate level is formed by quantum dots. FIG. 22 shows a theoretical value of energy conversion efficiency of a single junction solar cell in which an intermediate level is formed by a quantum well. The plots of the light collection magnification in AM1.5 are 1 ×, 10 ×, 100 ×, 1000 ×, 10000 ×, and 45900 ×, respectively. The increment (%) appended to the plot represents the difference between the maximum value of the conversion efficiency in each plot and the value of the energy efficiency when E _IC = 0 eV.
As shown in FIG. 21, the energy conversion efficiency of a single-junction solar cell in which an intermediate level is formed by quantum dots reaches a maximum value near E _IC = 0.3 eV when the condensing magnification is 10 times or more. Further, the maximum value is obtained near E _IC = 0.5 eV when the condensing magnification is 10,000 times or more. In the case of InAs / GaAs quantum dots (E _IC = 0.37 eV), it can be seen that the energy conversion efficiency is almost close to the maximum value when the condensing magnification is 10 times or more. In particular, in the case of InAs / GaAs quantum dots, it can be seen that the conversion efficiency at the time of maximum focusing (45900) in AM1.5 reaches 50%.
Further, as shown in FIG. 22, the energy conversion efficiency of the single-junction solar cell in which the intermediate level is formed by the quantum well varies in the E _IC that becomes the maximum value depending on the concentration ratio, but is generally E _IC = 0. A maximum value is reached at 0.04 to 0.08 eV. In the case of an InGaAs quantum well (E _IC = 0.07 eV), it can be seen that the energy conversion efficiency approaches the maximum value as the concentration factor increases.

(Other examples)
(1) In the above embodiment, GaAs is used as the host semiconductor of the light absorption layer of the solar cell, but compound semiconductors, metal nitrides, sulfur compounds containing Cu (copper), In (indium), and Al (aluminum) are used. Further, it may be a selenide, a metal arsenic compound, a metal phosphorus compound, or an alloy semiconductor made of a combination thereof. For example, AlGaAs, InGaAs, InGaN, InAlGaN, GaInP, or the like may be used.
(2) In the above embodiment, the quantum well and the quantum dot are used to form the intermediate band. However, the quantum band and the host semiconductor may be formed by doping a transition metal.
(3) In the above embodiments, InGaAs quantum wells are used, but multiple quantum wells may be used.

The present invention is useful for single-junction solar cells. The energy conversion efficiency of single-junction solar cells is limited to about 30%, but in principle, the energy conversion efficiency can be increased by 10% or more at the maximum. High-efficiency solar cells can reduce the unit price of power generation, and there is a possibility that the unit price of solar cells, which is currently said to be 40 yen / KWh, may be reduced to 30 yen / KWh or less, which is equivalent to oil-fired power generation. .

Claims (7)

  It has an intermediate level between the energy level at the upper end of the valence band and the energy level at the lower end of the conduction band. Among the carriers in the intermediate level, the hot carriers having energy higher than the energy level at the lower end of the conduction band A solar cell characterized by being taken out as   The solar cell according to claim 1, wherein the intermediate level is formed by a quantum well, a quantum wire, a quantum dot, or impurity doping. In a solar cell having a substrate, a p-type semiconductor layer, a light absorption layer, an n-type semiconductor layer, and an electrode,
The light absorption layer contains a quantum well, a quantum wire or a quantum dot, and includes at least one element selected from Cu (copper), In (indium), Ga (gallium), and Al (aluminum). A metal nitride, a sulfur compound, a selenide, a metal arsenic compound or a metal phosphorus compound, or a combination thereof, and an alloy semiconductor,
The quantum well, quantum wire, quantum dot or doped impurity forms an intermediate level between the energy level at the top of the valence band and the energy level at the bottom of the conduction band;
A solar cell characterized in that hot carriers having energy higher than the energy level at the lower end of the conduction band among the intermediate level carriers are taken out as current from the electrode.   The solar cell according to any one of claims 1 to 3, wherein the hot carrier is one in which light having a wavelength smaller in energy than a band gap is absorbed, and a temperature of a carrier at an intermediate level is increased.   The solar cell according to claim 4, wherein the increase in the temperature of the intermediate level carriers is caused by light collection. It has an intermediate level between the energy level at the upper end of the valence band and the energy level at the lower end of the conduction band. Among the carriers in the intermediate level, the hot carriers having energy higher than the energy level at the lower end of the conduction band A method for preparing a solar cell to be taken out as
A band gap control step of adjusting the band gap by preparing a composition of the host semiconductor of the solar cell;
The intermediate level is formed by preparing at least one of the composition of the semiconductor to be the light absorption layer, the size at which the quantum effect appears, the quantum dimension, the impurity to be doped, the impurity doping amount, or the impurity doping position. An intermediate level control step for controlling the difference between the energy level of the level and the conduction band to a predetermined value;
A method for preparing a solar cell, comprising:   The said predetermined value is a value from which the conversion efficiency of a solar cell becomes a maximum value, The preparation method of the solar cell of Claim 6 characterized by the above-mentioned.