Highly sensitive light reception element

A highly sensitive light reception element includes a transparent electrode, an ion-conductive electrolyte, and a semiconductor electrode. 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.

BACKGROUND OF THE INVENTION 
1. 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. 
2. 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 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, wherein in 
response to variation in quantity of light, a time-differentiated 
photoelectric response is output. 
Preferably, the semiconductor electrode is formed of silicon. The silicon 
may be n-type silicon or p-type silicon. 
Preferably, the ion-conductive electrolyte is a solid electrolyte. 
Preferably, the highly sensitive light reception element has a rising 
response speed of 20 .mu.s or faster. 
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 substrate as an operative electrode. 
(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.

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 the reverse 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.sub.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.sub.2, In.sub.2 O.sub.2, RuO.sub.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.sub.2, In.sub.2 O.sub.2, or composite material thereof (ITO) are more 
preferred from the viewpoint of light transmissiveness. Among them, 
SnO.sub.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.sub.2 and ITO preferably have an electric conductivity of 10.sup.2 
.OMEGA..sup.-1 cm.sup.-1 or greater, more preferably, 10.sup.3 
.OMEGA..sup.-1 cm.sup.-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 and solid electrolyte formed of an inorganic 
material or a organic polymer material. The electrolytic solution is an 
aqueous solution containing supporting salt. Examples of the supporting 
salt include KCl, NaCl, K.sub.2 SO.sub.4, KNO.sub.2, LiCl, and 
NaClO.sub.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. A 
polymer electrolyte containing an organic polymer material as a medium is 
preferably used as the solid electrolyte. For example, as the solid 
electrolyte there is used a polymer electrolyte whose medium is formed of 
gelatin, agar, polyacrylamide, polyvinyl alcohol, general-purpose cation 
or anion exchange resin, or a mixture of these materials, and which 
contains a supporting salt as an ion carrier and water if needed. 
Also, in addition to H.sup.+ --WO.sub.3 system, Na.sup.+ --.beta.--Al.sub.2 
O.sub.3 system, K.sub.2 --ZnO system, and non-oxide materials such as 
PbCl.sub.2 /KCl and SnCl.sub.2 can be used as the solid electrolytem, 
there can be used a polymer electrolyte whose medium is formed of gelatin, 
agar, polyacrylamide, polyvinyl alcohol, or general-purpose cation or 
anion exchange resin and which contains a salt as an ion carrier. 
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 .mu.s 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. 
Numerous modifications and variations of the present invention are 
possible in light of the spirit of the present invention, and they are not 
excluded from the scope of the present invention.