Photoelectrochemical cell with a high response speed

Abstract: A highly sensitive light reception element includes a transparent electrode (1), an ion-conductive electrolyte (2), and a semiconductor electrode (3). In response to variation in quantity of light, the light reception element outputs a time-differentiated photoelectric response. The light reception element has high response speed and excellent stability.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a highly sensitive light reception element capable of converting variation in quantity of light into an electrical signal with high response speed. Description of the Related Art: Conventionally, a light reception element that exhibits differential responsiveness has been developed in the form of an electrochemical cell having a layered structure of transparent electrode/bacteriorhodpsin thin film/electrolyte/counter electrode (see Japanese Patent Application Laid-Open No. 3-205520 and Miyasaka, Koyama, and Itoh, Science 255, 342, 1992). Such a light reception element is known as a first element that exhibits differential responsiveness in the level of the material thereof. Although the light reception element has various advantages, its extremely low sensitivity is its weakest point. Further, there is anxiety about the reliability of the light reception element, because the light reception element utilizes a protein as a basic material. FIG. 1 shows a response pattern of the conventional light reception element (which will be described later for comparison with the present invention in terms of effect). SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a highly sensitive light reception element that has high response speed and excellent stability. To seek to achieve the above object, the present invention provides a highly sensitive light reception element which comprises a transparent electrode, an ion-conductive electrolyte, and a semiconductor electrode; and wherein in response to variation in quantity of light, a time differentiated photoelectric response is output and wherein said ion conductive electrolyte is an aqueous solution containing a salt selected from the group comprising KCl, NaCl, K 2 SO 4 , KNO 2 , LiCl, and NaClO 4 . Preferably, semiconductor electrode is formed of silicon. Preferably, the silicon is n-type silicon. Preferably, the silicon is p-type silicon. The present invention has the following advantageous effects. (a) There can be provided a highly sensitive light reception element having a high response speed and excellent stability. (b) Variation in quantity of light can be quickly converted into an electrical signal. At this time, the direction of output optical current can be changed through selection of an n-type silicon substrate or a p-type silicon. (c) The light reception element can be constructed in the form of technologically useful elements such as an optical sensor, an optical switch, an artificial retina, or the like. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a response pattern of a conventional light reception element; FIG. 2 is a view showing the structure of a highly sensitive light reception element according to an embodiment of the present invention; and FIG. 3 is a graph showing a response pattern of a highly sensitive light reception element according to the embodiment of the present invention in which an n-type silicon substrate is used. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will next be described in detail with reference to the drawings. FIG. 2 is a view showing the structure of a highly sensitive light reception element according to an embodiment of the present invention. As shown in FIG. 2, the highly sensitive light reception element has a structure such that an ion-conductive electrolyte 2 is sandwiched between a transparent electrode 1 and a silicon electrode (silicon substrate) 3 serving as an operative electrode. , Between external terminals of the highly sensitive light reception element is connected a resistor 4 that adjusts the response speed of the light reception element. The response speed of the light reception element increases as the resistance of the resistor 4 decreases. Further, one external terminal connected to the silicon substrate 3 is connected to ground 5. An oscilloscope 8 is connected to output ends 6 and 7 of the external terminals. When visible light was radiated onto the highly sensitive light reception element having the above-described structure such that the light reception element receives the light from the side of the transparent electrode 1, the external circuit detects a high-speed rising of photoelectric current, the level of which then returns to zero. When irradiation of light is stopped, the direction of the photoelectric current reverses, and the level of the photoelectric current then returns to zero. In other words, the light reception element exhibits a time-differentiated response. When an n-type silicon substrate is used as the silicon electrode 3, photoelectric current flows toward the anode upon irradiation of light. In other words, electrons move from the ion-conductive electrolyte 2 into the silicon electrode 3 upon irradiation of light, and move from the silicon electrode 3 into the ion-conductive electrolyte 2 when irradiation of light is stopped. When a p-type silicon substrate is used as the silicon electrode 3, the light reception element operates in a reversed manner. That is, photoelectric current flows toward the cathode upon irradiation of light. In other words, electrons move from the silicon electrode 3 into the ion-conductive electrolyte 2 upon irradiation of light, and move from the ion-conductive electrolyte 2 into the silicon electrode 3 when irradiation of light is stopped. Examples: A two-electrode type electrochemical cell shown in FIG. 2 was constructed through use of SnO 2 as the transparent electrode 1, 0.1 mol of KCl (NaCl or the like may alternatively be used) as the ion-conductive electrolyte 2, and an n-type silicon substrate as the silicon electrode 3. The thickness of the ion-conductive electrolyte 2 was set to 1 mm. Xenon light (150 W) was caused to pass through an IR (infrared ray) cut filter and a yellow filter (HOYA Y48) and irradiated the cell for 0.25 seconds. FIG. 3 shows a response pattern of the highly sensitive light reception element according to the embodiment of the present invention in which an n-type silicon substrate is used. As is apparent from FIG. 3, the highly sensitive light reception element of the present invention has a response sensitivity that is several thousands times that of the conventional light reception element shown in FIG. 1 (the unit for the vertical axis in FIG. 3 is volt, whereas the unit for the vertical axis in FIG. 1 is millivolt). Further, since all the layers of the light reception element of the present invention are formed of inorganic materials, the light reception element has excellent stability. As described above, in the present invention, since the silicon electrode 3 is used as an operative electrode, the light reception element of the present invention has a sensitivity at least one-thousand times that of the conventional light reception element and a response speed at least ten times that of the conventional light reception element. A further detailed description will now be given of the electrode. Any of various kinds of precious metals (e.g., Au, Pt) and electrically conductive metallic oxides (e.g., SnO 2 , In 2 O 2 , RuO 2 ) are preferably used for the transparent electrode. Among them, thin film of Au or Pt (having a thickness of 1000 angstroms or less) or thin film of SnO 2 , In 2 O 2 , or composite material thereof (ITO) are more preferred from the viewpoint of light transmissiveness. Among them, SnO 2 and ITO are most preferably used because of their high chemical stability as electrode materials and high S/N ratio of light-response current, as well as high light transmissiveness. SnO 2 and ITO preferably have an electric conductivity of 10 2 Ω -1 cm -1 or greater, more preferably, 10 3 Ω -1 cm -1 or greater. These electrically conductive electrode materials are formed in the form of thin film on a transparent support formed of glass or resin through vacuum deposition or spattering. The thin film preferably has a thickness of 100 - 10000 angstroms, more preferably 500 - 6000 angstroms. Next, a description will be given of the electrolyte. Examples of the electrolyte used as an ion-conductive medium in the present invention include electrolytic solution The electrolytic solution is an aqueous solution containing supporting salt, which is selected from the group comprising KCl, NaCl, K 2 SO 4 , KNO 2 , LiCl, and NaClO 4 . The concentration of the supporting salt is generally in the range of 0.01 mol/l - 2 mol/l, and is preferably in the range of 0.05 mol/l - 1 mol/l. In the above-described embodiment, a two-electrode type is exemplified. However, the light reception element of the present invention may include a reference electrode as the third electrode element if needed. In this case, the reference electrode is inserted into the ion-conductive electrolyte. In this case, a voltage may be externally applied between the reference electrode and the operative electrode or between the reference electrode and the transparent electrode. For the three-electrode type cell, there is used an external circuit setup including a current measurement apparatus. A controlled potential electrolysis device (potentiostat) is one example of such a setup. When the third electrode is used, a silver/silver chloride electrode, mercury chloride electrode, or saturated calomel electrode is used as the third electrode. Among them, a silver/silver chloride electrode is preferably used in order to reduce the size of the element. The counter electrode and the reference electrode may have a shape of thin film, substrate, or a small probe. The response profile could be reproduced without any attenuation after a few thousand times of exposure to light. The spectrum of the response was measured by dispersing light from a light source by use of a band-pass filter. The results show that the light reception element of the present invention has a strong response throughout the entire visible light range of about 400 - 700 nm. The rising response speed of the light reception element of the present invention is very high, and rising time becomes 20 µs or less. The highly sensitive light reception element of the present invention has a function of quickly converting variation in quantity of light into an electrical signal and can be constructed in the form of technologically useful elements such as an optical sensor, an optical switch, an artificial retina, or the like. Needless to say, an amplifier, a waveform shaper, or the like may be added to the external circuit of the highly sensitive light reception element if needed. The present invention is not limited to the above-mentioned embodiment, being limited by the appended claims.

Claims: A highly sensitive light reception element characterised by comprising: (a) a transparent electrode (1); (b) an ion-conductive electrolyte (2), and (c) a semiconductor electrode (3), wherein (d) in response to variation in quantity of light, a time-differentiated photoelectric response is output; and wherein said ion conductive electrolyte (2) is an aqueous solution containing a salt selected from the group comprising KCl, NaCl, K 2 SO 4 , KNO 2 , LiCl, and NaClO 4 . The highly sensitive light reception element of Claim 1, characterised in that said semiconductor electrode (3) is formed of silicon. The highly sensitive light reception element of Claim 2, wherein the silicon is n-type silicon. The highly sensitive light reception element of Claim 2, wherein the silicon is p-type silicon.

Inventors: KOYAMA KOICHI, KOYAMA, KOICHI
Applicants: JAPAN SCIENCE & TECH AGENCY, JAPAN SCIENCE AND TECHNOLOGY AGENCY
Application year: 1998
Main classification: H01G9
IPC codes: H01G9, H01L31, G01J1