JPS5816308B2 - semiconductor electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor electrode in a photovoltaic cell or photoelectrolysis that utilizes a photoelectrode reaction using a semiconductor, and particularly the present invention relates to a semiconductor electrode that is sensitive to the wavelength region of sunlight. .

A photoelectrode reaction is when a substance is irradiated with light at a wavelength in its specific absorption range, electrons in the valence band become excited, and electrode reactions involving this excited state are called photoelectrode reactions in electrochemical systems. .

The photoelectrode reaction is described in “Industrial Chemistry Magazine J Vol 72, A
I (1969), P, 108 or rNATURE.

This was reported by Kobe et al. in Vol. 1238 (1972) P, 37.

Furthermore, a photovoltaic cell or photoelectrolysis method applying the same principle was filed by Koshibe et al. in Japanese Patent Publication No. 46-20182.48-
13825.

Kobe et al. reported that rutile (TiO2), an n-type semiconductor, is particularly effective as a semiconductor electrode in photoelectrolysis.

When the TiO□ electrolytic surface is irradiated with light, the band gap of TiO2 increases. Excitation by light with a wavelength shorter than 415 mμ, which corresponds to OeV, significantly increases the anode current.

Further, this anode current depends almost linearly on the light irradiation intensity.

Furthermore, the photoelectrode reaction of TiO2 is thought to occur due to photoexcitation occurring near the semiconductor surface due to short-wavelength light irradiation t, holes being accumulated in the valence band, and electrons being accumulated in the conduction band.

On the other hand, one of the requirements for the material that becomes the electrode plate of a semiconductor electrode is to have high chemical stability.
iO2 is sufficiently chemically stable.

If a photovoltaic cell or photoelectrolysis is realized using TiO□, which has the above characteristics, as a semiconductor electrode, the utilization rate of sunlight will be very low at 2% because T io 2 will only be excited by light with a wavelength shorter than 415 mμ, and the light This is not ideal from the point of view of energy utilization.

Therefore, the present invention aims to obtain a semiconductor electrode that can be used for photovoltaic cells or photoelectrolysis by photoelectrode reaction using a semiconductor that absorbs light in the wavelength range of sunlight and generates photovoltaic force.

Typical semiconductors that generate photovoltaic force by absorbing light in the sunlight wavelength range include GaP, GaAs, and GaAlAs.
, GaAsP.

■-V group compound semiconductors such as InP or CdS etc...
- Group compound semiconductors, etc.

However, these compound semiconductors cannot be said to be chemically stable enough to be used as semiconductor electrodes.

For example, when photoelectrolysis of water is carried out using the above compound semiconductor as an electrode, the electrolyte dissolves into the electrolyte solution, and a dissolution current flows, resulting in erosion of the electrode.

Therefore, the present invention aims to coat the surface of the above-mentioned semiconductor, which generates photovoltaic force by absorbing sunlight, with a chemically stable thin film to form a semiconductor electrode.

An embodiment of the present invention will be described in detail below.

FIG. 1 shows a cross-sectional view of an embodiment of the semiconductor electrode of the present invention.

In FIG. 1, 1 is a semiconductor that generates photovoltaic force by absorbing light in the sunlight wavelength range, and 2 is a chemically stable thin film that coats the surface of the semiconductor 1, preferably transparent.
Further, the thickness must be such that the electrons generated in the semiconductor 1 can pass through. 3 is a lead wire, and the part that comes into contact with the solution is covered with a stable substance.

The thin film 2 must be transparent to sunlight, but any conductor, semiconductor, or insulator can be used as long as it is chemically stable.

Further, the thickness of the thin film 2 is set so that electrons and holes generated in the semiconductor 1 can pass through.

For example, when an insulator is used as the thin film 2, the thickness is set so that electrons or holes can pass through the thin film 2 due to the tunnel effect.

In addition, to explain in detail the case where GaP is used as the semiconductor 1, the lead wire 3 has a band gap of 2
．． It is excited by light having a wavelength of 550 mμ or less, which corresponds to 25 eV, and an anode current of 7.19 mA/ffl flows.

At this time, the energy recovery rate for sunlight, that is, the ratio of energy absorbed to generate photovoltaic force among the energy of irradiated sunlight, is 19%, which is about ten times that of TiO2.

However, as the thin film 2 covering the GaP semiconductor, Ga2O3
When using TiO2, the light absorption rate decreases depending on the light transmittance of Ga2O3 or the thickness of the film, and therefore, it goes without saying that the energy recovery rate decreases, but a much larger electromotive force can be obtained than with TiO2. .

It goes without saying that energy loss can be reduced by using a material for the thin film 2 that has a light refractive index close to that of the semiconductor 1.

Table 1 shows the characteristics of semiconductor materials that can be used in the present invention.
This is shown in comparison with 2.

As shown in Table 1, the conventionally used TlO2 is 41
While Ga only absorbs light with a short wavelength of 3 mμ or less,
P, CdS absorbs light in the visible light range of 550 mμ and 520 mμ and converts it into photocurrent, and GaAs,
Since InP absorbs light in longer wavelength regions such as 885 mμ and 932 mμ, it is possible to increase the utilization rate of sunlight.

Furthermore, in the GaAsP mixed crystal, by changing the composition ratio of the mixed crystal, light in a wavelength range of 550 mμ to 885 mμ can be absorbed and converted into photocurrent.

In addition, in the semiconductor electrode of the present invention, as the thin film provided on the surface of the semiconductor that generates photovoltaic force upon irradiation with light, it is possible to use a film that is transparent to sunlight, is chemically stable, and has a strong bond with the semiconductor. For example, SiO2
, a thin film of oxides such as Al2O3 or GaN,
In addition to nitrides such as AAN, thin films of metals or metal oxides can be considered.

FIG. 2 shows an example in which photoelectrolysis of water is carried out using the semiconductor electrode of the present invention.

In FIG. 2, n-type GaP is used as the semiconductor 1, a Ga 203 thin film 2 is formed on the surface thereof to serve as an anode electrode, and one cathode electrode 4 is made of, for example, platinum black.

An alkaline aqueous solution is used as the electrolyte 5 on the anode electrode side,
Glass or quartz containers 7 and 7' are respectively filled with an acidic aqueous solution as the electrolyte 6 on the cathode side.

The containers 7, 7' are connected via a diaphragm 8.

The anode electrode and the cathode electrode are connected via a load 9.

When the anode electrode is irradiated with sunlight, a photovoltaic force is generated in the semiconductor electrode, and oxygen gas is generated from the anode electrode side and hydrogen gas is generated from the cathode electrode side.

As described above, the present invention uses a semiconductor that generates photovoltaic force when irradiated with sunlight as an electrode substrate, and chemically stabilizes the semiconductor surface to eliminate the chemical instability of the semiconductor that occurs at this time. It is a semiconductor electrode coated with a thin film that is large and transparent to sunlight.

By relying on the present invention ζ, a semiconductor electrode that can effectively utilize solar energy can be obtained.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a semiconductor electrode of the present invention, and FIG. 2 is a schematic diagram of an apparatus for photolyzing water using the electrode of FIG. 1. 1... Semiconductor, 2... Chemically stable thin film.

Claims (1)

[Claims] 1. A photoelectrode reaction device characterized by coating the part of a semiconductor that comes into contact with an electrolyte solution with a thin film that is transparent to sunlight and chemically stable to the electrolyte solution and generates a photovoltaic force when irradiated with sunlight. semiconductor electrode.