JPH10125940A - Photoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a photoelectric conversion element having a more excellent conversion efficiency than the conventional one by forming a first and second amorphous or crystalline semiconductor layers; the second layer formed on the first one and contg. a rare earth metal element. SOLUTION: After depositing silicon dioxide on a glass substrate, a transparent indium-tin oxide ITO electrode is deposited. On this electrode a p-type silicon carbide layer is deposited and implanted with Nd or Er ions. An amorphous Si layer contg. little impurity is deposited on an amorphous silicon carbide layer and implanted with an n-type impurity ion to form an n-type amorphous Si layer to be an Ohmic region. On this layer an Al electrode is formed to complete a photoelectric conversion element. Since the amorphous Si carbide layer contains a rare earth metal element, the electron density in a space charge region is lowered to reduce the carrier recombination.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photoelectric conversion element having a laminated structure.

[0002]

2. Description of the Related Art A photoelectric conversion element for converting electromagnetic waves such as photons or ultraviolet rays and infrared rays into electric power is expected to play an important role in solving global warming and energy problems. It is indispensable to lower the cost per unit of generated power in order to spread it. For this purpose, it is necessary to reduce the manufacturing cost and increase the conversion efficiency. In order to reduce the manufacturing cost, it is desirable to use an inexpensive amorphous silicon thin film as the photoelectric conversion semiconductor material. Silicon is less toxic than compound semiconductor materials such as gallium and arsenic, and can be used for more general purposes.

When an amorphous material is used, it is important to increase the conversion efficiency. One of the major factors determining the conversion efficiency is the dependence of the absorption coefficient on the energy of the incident photons. Photon energy (h
ν) Energy band gap (Eg) of semiconductor material
In this case, the energy of hv-Eg is emitted as photons. Therefore, it is most efficient for light having a wavelength slightly shorter than the bandgap energy. For example, in the case of Si, the intensity increases when the wavelength is from 0.4 mm to 0.8 mm. For this reason, there is an upper limit to the conversion efficiency, when using natural sunlight that is black body radiation, the maximum is 20-25% for an element made of single crystal silicon, and 30% for a single crystal gallium arsenide Only a degree of efficiency has been obtained. In order to alleviate the wavelength selectivity in the absorption coefficient of such a semiconductor material, a photoelectric conversion element in which two types of semiconductors having different band gaps according to the wavelength are heterojuncted has been considered.

FIG. 1 shows an energy band diagram of a pn junction semiconductor comprising a hetero junction of silicon carbide (SiC) and silicon (Si) in a thermal equilibrium state. As can be seen from the figure, a space charge region is formed at the interface between silicon carbide having an energy band gap Eg1 and silicon having an energy band gap Eg2 smaller than that, forming a heterojunction. On the other hand, FIG. 2 shows an energy band diagram when the semiconductor is irradiated with photons. By using a heterostructure, in the region of silicon carbide near the surface, a short-wavelength photon (hν1) is absorbed and electrons are excited from the valence band to the conductor. The conversion efficiency can be increased by absorbing hν2) and also exciting electrons. Further, efficiency is improved by adopting a tandem type in which a plurality of these layer structures are stacked. On the other hand, another factor that determines the conversion efficiency is that electrons excited in the conduction band in the p-type region pass through the space-charge region and n
When approaching the mold region, holes are recombined with holes generated in the n-type region and cannot be taken out as a current. In particular, when an amorphous semiconductor material is used, an increase in conversion efficiency cannot be expected because of the presence of many recombination centers. For this reason, the conversion efficiency when light is applied to a photoelectric conversion element composed of a pn junction diode composed of amorphous silicon carbide and a silicon thin film is at most only about 10%, and the band gap itself is 1.6 to 2.0. e
Despite being wider than devices made of V and single crystal silicon (about 1.1 eV), the conversion efficiency is considerably lower.

[0005]

As described above, the conventional photoelectric conversion element using an amorphous semiconductor has a problem that the conversion efficiency is reduced due to the existence of many recombination centers. . The present invention has been made in consideration of the above problems, and has as its object to provide a photoelectric conversion element having a higher conversion efficiency than a conventional one.

[0006]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides an amorphous or crystalline first conductive semiconductor layer, and a rare earth metal element formed on the first conductive semiconductor layer. And an amorphous second conductivity type semiconductor layer containing the same.

A photoelectric conversion element according to the present invention is provided on an n-type or p-type first conductivity type semiconductor layer, for example, an amorphous silicon semiconductor layer.
A second p-type or n-type band-gap
It has a structure in which a conductive semiconductor layer, for example, an amorphous silicon carbide semiconductor layer is stacked. This second conductivity type semiconductor layer has Nd
Rare earth metal elements such as (atomic number 60) and Er (68) are added to form an electrode.

To reduce the recombination of carriers in the space charge region, the density of electrons or holes in this region may be reduced. To this end, photons of a short wavelength absorbed as excitation of electrons from the valence band to the conduction band in the second conductivity type semiconductor layer such as an amorphous silicon carbide semiconductor layer having a wide band gap are again emitted as photons. Do it. However, if the photon has the same wavelength as the incident photon, the absorption coefficient in the first conductivity type semiconductor layer such as the amorphous silicon semiconductor layer becomes low. Therefore, it is necessary to make the wavelength longer than the incident photon. is there.

Here, when a rare earth metal element is implanted into a semiconductor layer such as silicon or silicon carbide, a sharp emission peak is observed in the wavelength region of 0.5 to 1.5 mm (2.0 to 0.8 eV) near the band gap of these semiconductors. It is well known.

Therefore, it is considered that a rare earth metal element, for example, Nd or Er is added to the amorphous silicon carbide layer. In order to explain the principle of the photoelectric conversion element of the present invention, an energy band diagram at this time is shown in FIG.

Here, the band gap energy (Eg1) of amorphous silicon is about 1.6 eV, and that of amorphous silicon carbide (Eg2) is about 2 eV. On the other hand, the wavelength of the photon emitted from the 4f _9/2 level of the rare earth metal Nd or Er atom is about 0.7 mm, and the energy difference (E) between the ground level and the excited level is Eg1 <E < It can be seen that the relationship of Eg2 is satisfied.

The electrons excited from the valence band of silicon carbide to the conduction band by the incident photon hν1 can excite the 4f shell electrons of these rare earth atoms to an excited level, and the photon hν1 is directly converted to the rare earth element. It can also excite the electrons of an atom. Then, when the excited electrons transition to the ground state, a photon hν1 ′ having a wavelength corresponding to E
Release. This is a wavelength longer than the wavelength of the incident photon hν1 and corresponding to sufficient energy to excite electrons from the valence band to the conduction band in the silicon layer. After passing through the space charge region, the photon hν1 'excites electrons from the valence band to the conduction band in the silicon layer.
In the silicon layer, electrons are also excited from the valence band to the conduction band by the incidence of a photon hν2 having a longer wavelength than the photons hν1 and hν1 '.

Thus, some of the electrons excited in the amorphous silicon carbide layer pass through the space charge region as photons, not as electrons. Therefore, the density of electrons in the space charge region decreases, and recombination of carriers decreases,
It is possible to improve photoelectric conversion efficiency. In addition, since the material used for photoelectric conversion is amorphous, cost can be significantly reduced.

Among the rare earth metal elements, those having a 4f shell electron are those of atomic number 58 belonging to the lanthanoid series.
Is an element having an atomic number larger than that of Ce,
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Y
b, Lu.

The first conductive type semiconductor layer may be made of a crystalline material such as a single crystal instead of being amorphous.
In this case, there is an advantage that carrier recombination can be further reduced.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 4 is a cross-sectional view schematically showing a structure of a photoelectric conversion element as a first embodiment according to the present invention. Hereinafter, this will be described according to the manufacturing process.

First, silicon dioxide (SiO 2) is placed on a glass substrate.
₂ ) After depositing by the CVD method, a transparent ITO electrode made of indium tin oxide (ITO) is deposited. A p-type amorphous silicon carbide layer (p-type a-SiC) having a thickness of 1 μm or less is deposited on the ITO electrode, and Nd is deposited on this amorphous silicon carbide layer at a high dose of 10 ¹³ or more.
Alternatively, Er ions are implanted. Instead of ion implantation, sputtering or EB may be used. Further, an amorphous silicon layer containing less impurities (i-type a
-Si). From the surface of this amorphous silicon layer, n
An n-type amorphous silicon layer (n-type a-Si), which is an ohmic region, is formed by ion implantation of a type impurity, and an Al electrode is formed on the n-type amorphous silicon layer to form a photoelectric conversion element. Complete.

The total thickness of the p-type amorphous silicon carbide layer, the amorphous silicon carbide layer, and the n-type amorphous silicon carbide layer is on the order of several tens of μm. Light enters the photoelectric conversion element from the glass substrate side and excites electrons as described above. This gives I
When the TO electrode and the Al electrode are short-circuited, a current flows, and when the TO electrode and the Al electrode are opened, a voltage is generated between the electrodes.

It is to be noted that the thickness of the amorphous silicon carbide layer is set to 1 μm or less. If the thickness is larger than 1 μm, recombination of carriers is liable to occur and the depletion layer spreads undesirably. In the present embodiment, the amorphous silicon carbide layer contains Nd or Er, which is a rare earth metal element, so that the electron density in the space charge region decreases and the recombination of carriers decreases. The conversion efficiency is improved as compared with. Further, the cost can be significantly reduced by using an amorphous semiconductor.

(Second Embodiment) FIG. 5 is a sectional view schematically showing the structure of a photoelectric conversion element according to a second embodiment of the present invention. Hereinafter, the description will be given in accordance with the manufacturing process as in the first embodiment.

First, an electrode such as Al is formed on a glass substrate. An n-type amorphous silicon layer (n-type a-S
i) to form an amorphous silicon layer (i-type a
-Si). Further, an amorphous silicon carbide layer is deposited. The amorphous silicon carbide layer (p-type a-SiC)
Nd or Er ions are implanted at a high dose of at least ¹³ orders. Further, a transparent electrode made of indium tin oxide (ITO) is deposited, and Ta ₂ O ₅ , S
The photoelectric conversion element is completed by coating a surface anti-reflection film such as iO ₂ or Si ₃ N ₄ .

The present embodiment is different from the first embodiment in that light is incident from the antireflection film side, but the other is the same as the first embodiment, and is similar to the first embodiment. Conversion efficiency is improved, and costs can be significantly reduced.

(Third Embodiment) FIG. 6 is a sectional view schematically showing a structure of a photoelectric conversion element according to a third embodiment of the present invention. This will also be described according to the manufacturing process.

First, a substrate (not shown) made of glass or the like
An electrode is formed. An n-type single-crystal silicon layer (n-type single-crystal Si) is deposited on this electrode, and an amorphous silicon layer with few impurities (i-type single-crystal Si) is deposited. Further, an amorphous silicon carbide layer (p-type a-SiC) is deposited. Nd or Er ions are implanted into this amorphous silicon carbide layer at a high dose. Furthermore, indium tin oxide (IT
O), a transparent electrode made of Ta ₂ O ₅ , SiO
₂ , a photoelectric conversion element is completed by coating a surface anti-reflection film such as Si ₃ N ₄ .

This embodiment is different from the first embodiment in that light is incident from the antireflection film side, as in the second embodiment, but the mechanism for exciting electrons is the first embodiment. As in the first embodiment, the conversion efficiency is improved as in the first embodiment. In addition, since the silicon layer is a single crystal, there is an advantage that carrier recombination is further reduced as compared with the case where the silicon layer is amorphous. As described above, the embodiment of the present invention has been described, but the present invention is not limited to the above-described embodiment,
Various modifications are possible within a range not exceeding the gist.

[0026]

According to the present invention as described above,
It is possible to realize a photoelectric conversion element having higher conversion efficiency than before, and this photoelectric conversion element can be usefully used for a solar cell, an optical sensor, and the like.

[Brief description of the drawings]

FIG. 1 is an energy band diagram of a pn junction semiconductor made of silicon carbide and silicon.

FIG. 2 is an energy band diagram of a pn junction semiconductor made of silicon carbide and silicon.

FIG. 3 is an energy band diagram showing the principle of the photoelectric conversion element according to the present invention.

FIG. 4 is a schematic sectional view of the photoelectric conversion element according to the first embodiment of the present invention.

FIG. 5 is a schematic sectional view of a photoelectric conversion element according to a second embodiment of the present invention.

FIG. 6 is a schematic sectional view of a photoelectric conversion element according to a third embodiment of the present invention.

Claims (4)

[Claims] An amorphous or crystalline first conductive type semiconductor layer, and an amorphous second conductive type semiconductor layer formed on the first conductive type semiconductor layer and containing a rare earth metal element are provided. A photoelectric conversion element characterized in that: 2. The energy of the second conductivity type semiconductor layer.
2. The semiconductor device according to claim 1, wherein a band gap is wider than an energy band gap of the first conductivity type semiconductor layer.
The photoelectric conversion device according to any one of the preceding claims. 3. An energy difference between a ground level and an excited level of electrons of the rare earth metal element is smaller than an energy band gap of the second conductivity type semiconductor layer, and an energy energy of the first conductivity type semiconductor layer is smaller. The photoelectric conversion element according to claim 1, wherein the photoelectric conversion element is wider than a band gap. 4. The photoelectric conversion element according to claim 1, wherein a main component of the first conductivity type semiconductor layer is silicon, and a main component of the second conductivity type semiconductor layer is silicon carbide.