JP3444501B2 - Thermoelectric generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric generator, and more particularly to a thermoelectric generator capable of receiving solar heat and generating electromotive force.

[0002]

2. Description of the Related Art Conventionally, thermoelectric power generating elements based on the Seebeck effect have been put into practical use and are widely used in remote power sources and various other fields. However, the existing thermoelectric power generation element using the Seebeck effect has a maximum thermal efficiency of 18
Although it is as low as less than 10%, many improvement efforts have been made to improve the thermal efficiency, but it is still far from a fundamental solution. Further, because of this low thermal efficiency, when the thermoelectric power generation element is applied to a solar thermal power generation battery, the installation cost for obtaining the desired power becomes large, and attempts to put it to practical use in this aspect have hitherto been performed. The reality is that little has been done. However, the solar cell is a power generator that is extremely effective from the viewpoint of safety compared to other thermal power generation and nuclear power generation, whether its form is light utilization or heat utilization, and it is installed as an energy source for automobiles and the like. Even if you do, because fossil fuel is not used and its exhaust gas does not exist,
When the environment ecology has been advocated by the current global scale as of
In recent years , there is a strong demand for its large-scale spread in various aspects. If the thermal efficiency of the thermoelectric power generation element becomes higher than 30% due to such social demands, the use of the thermoelectric power generation method including solar heat utilization and its spread will be dramatically expanded.

[0003]

The maximum reason for the low thermal efficiency of the thermoelectric power generation element based on the Seebeck effect will be described in detail below. In a thermoelectric power generation element based on the Seebeck effect (hereinafter referred to as Seebeck element), a temperature difference is required between its two electrodes in order to generate a thermoelectromotive force.
That is, a difference in carrier concentration is caused by a temperature difference, and as a result, carrier diffusion movement and a Fermi level gradient are generated to generate an electromotive force. On the other hand, as a result of the existence of the temperature difference between the both ends of the element, a heat flow occurs from the high temperature side to the low temperature side in the element. Generally, this flow amount of heat, the heat generation amount and the intake amount of heat based on Peltier effects, or greater than the resistance heating value, has become the largest cause the presence of this for such throughflow heat to deteriorate the thermal efficiency of the thermoelectric power generating element.

For example, FIG. 1 shows a Seebeck element using a p-type semiconductor. In the drawing, E is the hole h ⁺ energy, E _F is the Fermi energy level, FHH the hole h ⁺ of the distribution function of the high temperature side, FHL the hole h ⁺ of the distribution function of the low temperature side, CB is the conduction band , VB is the valence band, H is the hole h ⁺ (carrier) concentration on the high temperature side, and L is the hole h ⁺ concentration on the low temperature side. As apparent from the figure, in the conventional Seebeck elements, in order to give rise to concentration difference of carriers in the device two electrodes, it is necessary to provide a temperature difference, which becomes a factor of up to you poor thermal efficiency There is.

In order to achieve the above technical object, the present invention has an object to provide a thermoelectric generator capable of generating electromotive force in a state where both electrodes are isothermal. Another object of the present invention is to provide a thermoelectric power generator capable of greatly improving thermal efficiency without causing heat flow-through in the semiconductor layer forming the power generator as a result.

[0006]

The present invention isDifferent work function
The semiconductor layer is sandwiched between the thin plates of metal A and B
Contact with metal A is Schottky barrier type, while contact with metal A
The contact with B was specified as ohmic contact type.
And the highest part of the conduction band is shown in the electron distribution function.
The energy level at which the probability of existing electrons becomes almost zero
It is located higher than the
Enel with almost zero probability of existence of holes, which is shown in distribution function
It is characterized by being located above the gee level.
Specifically, for thin plates of metals A and B having different work functions,
The semiconductor layer is sandwiched between them so that the contact with the metal A becomes holes.
Ohmic contact type, on the other hand, contact with metal B
Regarding the holes, the Schottky barrier type and the specified configuration
And set the work function relationship to satisfy the following formula
It is characterized by having done. φ _A -Φ _SC ≧ 0.1 eV and φ _SC -Φ _B ≧ 0.1 eV φ_A: Work function of metal A φ_B: Work function of metal B φ_SC: Work function of semiconductor

Furthermore, the slope of the energy band is due to charge separation.
To ensure that you are
For electrons, the energy part with a high conduction band is
The energy at which the electron existence probability is almost zero
Be higher than the level, and
The high-energy part of the valence band becomes the hole distribution function.
Energy level at which the probability of hole existence is almost zero
It is necessary to be positioned higher. Sensually
Determines the region where excited electrons and holes can exist in the assumption.
High energy in the conduction band and the valence band, which is similar to a
The conditions under which the luge section is exposed from each carrier pool
It is necessary to make. Such exposure is φ_A-Φ_SC≧ 0.1 eV(In case of n-type semiconductor) φ_SC-Φ_B≧ 0.1 eV(In the case of p-type semiconductor) Article It is realized by satisfying the conditions. Furthermore, thermal excitation
To ensure, BaBand gap E_gBut, 1 ． 2> E_g≧ 0.2 eVMust be in the range Is.

[0008]

Now, in order to generate a Fermi energy level gradient in a thin semiconductor layer and generate a potential difference between both end faces, I. Electrons e-are excited in the conduction band and move in one direction. II. Holes h ⁺ are excited in the valence band and move in one direction. III. As a result, it is necessary to create an excess or deficiency of carriers from the thermal equilibrium state on both ends or poles. Here, the above I and II may occur independently, but it is desirable that I and II occur together and in opposite directions. Seeback
In the case of a device, the driving force that causes carrier movement is generally the difference in carrier concentration between the end faces of each energy band. If this concentration difference is large enough, carrier migration can occur against the slope of the energy band. This is also clear from FIG.

Further, the second driving force of carrier movement is the inclination of the energy band, and more specifically, the driving force which forms the basis of charge separation in the photovoltaic device. The specific action is shown in FIG. 2. Generation of electrons e ⁻ · hole h ⁺ pair and generation of light having energy larger than the band gap of the pn junction semiconductor are made incident from the p-type semiconductor side. The separation state is shown, W is the electromotive force generated on the n-type semiconductor side, and FB is the Fermi energy level.
Note that electrons e ⁻ from the valence band to the conduction band as shown in this figure.
Excitation also occurs due to heat. When the energy band has an inclination, the electron e ⁻ that has climbed to the conduction band and the holes h ⁺ h generated in the valence band move toward the counter electrode due to the action of the energy band inclination. , A Fermi level gradient, that is, an electromotive force is generated. Here big problem, the how is both poles or cause inclination to the energy band in a state of isothermal.

[0010] The thermal excitation, electrons e ^- · holes h ⁺ to generate a pair semiconductor bandgap intends to use the shall small kuna <br/>. This is because, as shown in FIG. 3, under the condition of the temperature T, in a semiconductor having a large band gap, the carrier concentration in the conduction band and the valence band becomes small and the electric conductivity becomes insufficient. .

On the other hand, as is well known, in a semiconductor having a small band gap, it is not possible to create an energy band gradient by a pn junction. Accordingly, in the present invention, the ohmic contact及beauty shea Yottoki barrier formed in the semiconductor layer C when contacting the semiconductor surface and the metal surface, the resolution of the problem by creating a gradient of energy band in the semiconductor layer C I am measuring. Moreover, the shape of the inclination of the energy band 1 must be monotonous as shown in FIG. It has been found that the present invention needs to satisfy the following equation as a condition for constructing the energy band 1 having this monotonous slope. φ _A ＞ φ _SC ＞ φ _B … 1) φ _A : Work function of metal A φ _B : Work function of metal B φ _SC : Work function of semiconductor layer When an element having this energy band 1 is heated, conduction band is generated by thermal excitation. The electron e ⁻ climbs up.

In FIG. 4B, energy band 1
The electron e-excited in the conduction band of the region (Z region) where is not buried in the electron pool ep moves into the electron pool ep on the right side by the action of the inclination of the energy band 1, and on the other hand, similarly The pool hp of h ⁺ also becomes large, so that a Fermi level gradient and an electromotive force are generated inside the semiconductor layer C. Regarding the relationship between work functions,
Another condition is necessary. That is, the high energy portion 1h in the energy band 1 is exposed without being buried in the electron pool ep, and similarly, the high energy portion 2h in the energy band 2 is exposed without being buried in the hole pool hp. It should be. Incidentally, FIG. 4 (B)
As shown in, the maximum value of the energy band 1 of the conduction band is E ₀ .

The behavior of the electron e-in the solid is in accordance with Fermi Dirac statistics, and its distribution function fe (E)
Is Can be expressed as

Now, at the condition T ₀ of the temperature T, fe
(E) Let E ^* be the value of E that gives = 0. T ₀ = 400
As K, it is an E ^* -E _F = 0.25eV. That is, 4
In the thermoelectric generator according to the present invention operating at 00K, since the high energy portion 1h of the energy band of the conduction band is exposed from the electron pool ep, the maximum value of the energy band 1 is set to E ₀ , and E ₀ -E _F > 0.25 eV. Furthermore , in the case of an n -type semiconductor, E ₀ −
E _F = φ _A −φ _SC + E _g −E _F (E standard is valence band)
When the condition of E _g −E _F <0.2 eV is added for commonization, φ _A −φ _SC > 0.05 eV, and preferably φ _A −φ _SC ≧ 0.1 eV. Similarly, for p-type semiconductors φ _SC −φ _B ≧
It is necessary to set it in the range of 0.1 eV.

[0015]

EXAMPLES Examples of the present invention will now be illustratively described in detail. However, unless otherwise specified, the materials, shapes, relative positions, etc. of the components described in this embodiment are not intended to limit the scope of the present invention thereto, but are merely illustrative examples. . FIG. 5 is a schematic perspective view of a thermoelectric power generation element according to an embodiment of the present invention, in which the semiconductor layer C is held in contact with the metal thin layers A and B having different work function values, and the metal thin layer is Lead wire 2 is led out to A and B.

In the device having the above structure, the metal thin layer A is a copper Cu thin layer (work function φ _Cu : 4.65 eV), and the metal thin layer B is an aluminum Al thin layer (work function φ _Al :
4.28 eV), the semiconductor layer C is further formed with an intrinsic silicon wafer (work function φ _{I · Si} = 4.55 eV), and it is confirmed whether or not the inclination of the energy band 1 occurs in the semiconductor layer C. In order to do so, sunlight was irradiated in the direction of the arrow and the electromotive force was measured. As a result, an electromotive force with a voltage of 25 to 28 mV and a current of 1.7 to 2.1 μA was generated between the terminals of the lead wire. From this result, it was confirmed that there is an energy band gradient in the semiconductor layer of the device, and that the charge pairs generated by solar irradiation are separated to generate an electromotive force. The size of the device of this example is 30 mm × 25 mm.

Next, in order to confirm that the above-mentioned solar cell element generates power in an isothermal state, it was immersed in an oil bath at 130 ° C. and the electromotive force generated was measured, and the voltage was 150.
The values of mV, current: 33 μA and resistance: 4.5 × 10 ³ Ω were obtained, and it was confirmed that power generation was performed in an isothermal state.
(Example 1) The reason why the current value generated in Example 1 is small is that the semiconductor layer C used is silicon Si and the hand gap is relatively large at Eg = 1.1 eV.
This is because the concentration of electrons e − climbing to the conduction band and the concentration of holes h ^{+ in the} valence band are still low in the heating state.

Therefore, the hand gap is small and
Under the condition that the shape of energy band 1 should be desirable
is there, φ_Cu> Φ_SC> Φ_{A l} Tellurium Te is selected as the semiconductor layer C that satisfies
A solar cell element having the same structure as in Example 1 was produced. still,
The electronic properties of Te are E_g* 0.32eV (300K), φ
_SC= 4.3 eV. Furthermore, in this embodiment, the electric
Another factor that increases resistance is the resistance of each layer contact surface.
In order to reduce the resistance, this time Cu and Al were added to the Te melt.
The element was manufactured by the method of immersing the thin plate of. (Implementation
Example 2) The size of the device of Example 2 is 10 mm × 2
It is 0 mm. Then, the device according to the third embodiment is
The electromotive force generated was measured by immersing in an oil bath at ℃
Location, voltage: 100 mV, current: 18 mA, resistance: 5.5
The value of Ω is obtained, which allows the device of this structure to be charged under isothermal conditions.
It was confirmed that it has a practical power generation capacity.

[0019]

As described above, according to the present invention, it is possible to convert thermal energy into electric power with high thermal efficiency. For example, when the thermoelectric generator of the present invention receives sunlight and is housed in a heat box in a temperature rising state to generate power,
It is possible to exert a power generation capacity that exceeds that of ordinary solar cells. Furthermore, in order to avoid the hassle of putting the thermoelectric generator in a heat box, a solar thermoelectric generator that operates at room temperature,
It is also possible to configure by using the semiconductor layer C having a small Eg, whereby the location and the mounting device are not limited.
Moreover, a new type solar cell can be provided at a low installation cost. It has various remarkable effects.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the principle of generation of electromotive force in a Seebeck element and its problems.

FIG. 2 is an explanatory diagram showing a state of charge separation based on an inclination of an energy band in a photovoltaic element (pn junction semiconductor).

FIG. 3 is a distribution diagram of carrier concentration when the Fermi energy level gradient is 0, distribution function when the temperature is constant, density of states, and the like.

FIG. 4 is an operation diagram showing the principle of the present invention.

FIG. 5 shows a schematic perspective view of a solar cell element according to an embodiment of the present invention.

Claims (5)

(57) [Claims] 1. A contact sandwich the semiconductor layer between the thin plate having different metals A and B the number of work function, sucrose <br/> Ttoki barrier-type contact with the metal A, while the metal B Ohmic contact
Contact type and the specified configurations respectively,
In the high part, the electron existence probability shown in the electron distribution function is almost
It is located above the energy level where it becomes zero, and
At the top of the valence band, the hole distribution shown by the hole distribution function
Position above the energy level at which the existing probability is almost zero
A thermoelectric generator characterized in that 2.Thin plates of metals A and B with different work functions
The semiconductor layer is sandwiched between the two, and the contact with the metal A is made into holes.
Regarding ohmic contact type, on the other hand, contact with metal B
Specified as Schottky barrier type for holes
And the work function relationship is set to satisfy the following equation.
The thermoelectric generator according to claim 1, wherein the thermoelectric generator is defined. φ_A-Φ_SC≧ 0.1 eV and φ_SC-Φ_B≧ 0.1 eV φ_A: Work function of metal A φ_B: Work function of metal B φ_SC: Work function of semiconductor 3. The thermoelectric generator according to claim 1, wherein the band gap E _{g of the} semiconductor is set in the range of 1.2 eV> E _g ≧ 0.2 eV. 4. A wherein when the semiconductor is an n-type semiconductor, the Fermi level as EF, thermoelectric generator of claim 3, wherein a set in a range of Eg-E _F ≦ 0.2eV. 5. The thermoelectric generator according to claim 3, wherein when the semiconductor is a p-type semiconductor, the Fermi level is set to EF and set to a range of E _F ≦ 0.2 eV.