Patent Application
20120067410
The present invention relates to a Schottky-barrier junction element having a Schottky-barrier junction formed between an inorganic semiconductor and an organic conductor, and a photoelectric conversion element and a solar cell using the same.
A Schottky-barrier junction between a metal and a semiconductor is known. This Schottky-barrier junction is used in Si integrated circuits in combination with bipolar transistors and field-effect transistors.
Non-patent Literature 1 discloses a Schottky-barrier photoelectric conversion element wherein a Schottky barrier is formed between an n-type semiconductor and a metallic thin film having work function of 5 eV or higher such as Au and Pd. With the conventional Schottky-barrier photoelectric conversion elements such as the one disclosed in Non-patent Literature 1, since significant attenuation of incident light occurs at metallic thin film electrodes, sufficient performance as a photoelectric conversion element cannot be ensured, which makes it difficult to put it into practical use.
Patent Literatures 1 and 2, and Non-patent Literatures 2, 3, and 4 disclose Schottky photoelectric conversion, elements forming a Schottky barrier with an organic conductor such as poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) and nickel phthalocyanine, a metallic thin film such as Au and Pd, and an oxide semiconductor such as TiO2 and SrTiO3. Since the light transmittance of organic conductors such as PEDOT:PSS and nickel phthalocyanine is higher than that of metallic thin films, the problem of occurrence of significant attenuation of incident light can be avoided.
However, since Shottky-barrier photoelectric conversion elements use oxides having a large optical band gap such as TiO2 and SrTiO3 as semiconductors, the wavelength region allowing the elements to have sensitivity as photoelectric conversion elements has been limited to shorter than 380 nm. This obstructive factor has prevented the elements from being used as a solar cell, which requires spectral sensitivity to visible light falling within the 400 nm to 800 nm region.
Patent Literature 1: JP2008-244006A
Non-patent Literature 1: K. M. Tracy et al., J. Appl. Physics Vol. 94, p. 3939 (2003)
The present invention intends to provide a Schootky-barrier junction element having a high Schottkey barrier, and a photoelectric conversion element and a solar cell using the Schottky-barrier junction element.
To achieve the above objectives, the Schottky-barrier junction element of the present invention has a Schottky junction between an inorganic semiconductor and an organic conductor, wherein the inorganic semiconductor is any one of the following: Nitride semiconductors, Si, GaAs, CdS, CdTe, CuInGaSe. InSb, PbTe, PbS, Ge, InN, GaSb, and SiC.
To achieve the above objectives, the solar cell according to the present invention uses the Schottky-barrier junction element of the present invention, wherein a photoelectric conversion section includes the Schottky-barrier junction.
To achieve the above objectives, the photoelectric conversion element of the present invention uses the Schottky-barrier junction element of the present invention, wherein a conversion section for interconverting light and electricity includes the Schottky -barrier junction.
According to the present invention, by providing an organic conductor on a specific inorganic semiconductor, a Schottky-barrier junction element having a high Schottky barrier can be provided. In particular, since the organic conductor has high light transmittance, the use as a photoelectric conversion element and solar cell exhibits good performance. By selecting an inorganic semiconductor having a specific band gap, absorption wavelength can be shifted from ultraviolet light to visible light, which ensures effective use of photoelectric effect in the visible light range.
The embodiment of the present invention will hereinafter be described by referring to the drawings.
As the substrate 2, a sapphire substrate, etc. may be used.
As the inorganic semiconductor 3, not only III-V semiconductors such as GaN, nitride semiconductors in particular, but also Si such as single-crystal Si, polycrystal Si, and amorphous Si, GaAs, CdS, CdTe, CuInGaSe, InSb, PbTe, PbS, Ge, InN, GaSb, and SiC can be used.
As the organic conductor 4, various polythiophene-series, polyaniline-series, polyacetylene-series, polyphenylene-series, and polypyrrole-series organic conductors can be used. Table 1 lists examples of organic conductors.
As polythiophene-series organic conductors, poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) expressed by chemical formula (1), poly(3,4-ethylenedioxythiophene)-block-poly (ethylene glycol) expressed by chemical formula (2), poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl) expressed by chemical formula (3), etc. can be used.
As polyaniline-series organic conductors, polyaniline expressed by chemical formula (4) can be used, for example.
As polyacetylene-series organic conductors, poly[1,2-bis(ethylthio)acetylene] expressed by chemical formula (5) can be used, for example.
As polyphenylene-series organic conductors, poly(1,4-phenylene sulfide) expressed by chemical formula (6) can be used.
As polypyrrole-series organic conductors, polypyrrole expressed by chemical formula (7) may be used, for example.
In the embodiment of the present invention, a Schottky-barrier junction is formed between the inorganic semiconductor 3 and the organic conductor 4. If the inorganic semiconductor 3 is an n-type semiconductor, a hole-conduction-type organic conductor 4 can be used to form a Schottky-barrier junction. In this case, an inorganic semiconductor 3 having the electron affinity of less than 5.0 eV must be used. Theoretically, if the electron affinity of the inorganic semiconductor 3 is smaller than the work function of p-type organic semiconductor, a Schottky barrier can be formed. In practice, however, Schottky characteristics cannot be obtained unless there is a difference of approximately 1 eV. It is therefore preferable that the electron affinity of the organic semiconductor 3 be smaller than the work function of the p-type organic semiconductor by 1 eV or more. In Examples 1 to 3 to be described below, the work function of the organic conductor 4 is approximately 5 eV, and the electron affinity of the inorganic semiconductor 3 is approximately 3.5±0.3 eV. Since the difference between the work function of the organic conductor 4 and the electron affinity of the inorganic semiconductor 3 is higher than 1 eV, a good Schottky junction can be achieved.
The embodiment of the present invention is the Schottky-barrier junction element 1. However, the embodiment can be applied to various photoelectric conversion elements such as ultraviolet sensor, infrared sensor, solar cell, diode element for voltage control, and variable-capacity diode element.
Namely, the solar cell in the embodiment of the present invention uses the Schottky-barrier junction element 1, and the conversion section for converting light into electricity includes a Schottky-barrier junction.
The photoelectric conversion element in the embodiment of the present invention uses the Schottky-barrier junction element 1, and the conversion section for converting light into electricity, or vice versa, includes a Schottky-barrier junction.
In the embodiment of the present invention to be described below, highly-conductive polyaniline-series organic solvent solution (ORMECON) was used as the organic conductor 4, and gallium nitride was used as the nitride semiconductor. As the highly-conductive polyaniline-series organic solvent solution, the one containing water as solvent and having viscosity of 16 mPa·s, ph of 1.8, and the conductivity found by spin-coating deposition of 180 S/cm under the measurement environment of 25° C. was used. However, it is easy to imagine that similar Schottky-barrier junctions can be achieved by using other hole conduction-type organic materials such as PEDOT:PSS as the organic conductor 4, and various inorganic semiconductors such as Si including single-crystal Si, polycrystal Si, and amorphous Si, GaAs CdS, CdTe, and CuInGaSe as the inorganic semiconductor 3. The work function of ORMECON and that of PEDOT:PSS are both assumed to be 5.0 eV. Inorganic n-type semiconductors capable of forming a Schottky-barrier junction with these materials are those whose electron affinity is less than 5.0 eV. Namely, the electron affinity of CdS, CdTe, GaAs, Si, and CuInGaSe is 4.8 eV, 4.3 eV, 4.07 eV, 4.05 eV, and 4.0 eV respectively, it is easy to imagine based on general knowledge on semiconductor physics that the use of these n-type inorganic semiconductors forms a Schottky-barrier junction.
A solar cell having the same structure as the one shown in
In step ST1, a sapphire (0001) substrate 2 was prepared. In step ST2, using trimethyl gallium, ammonia, and hydrogen as raw materials, epitaxial growth of gallium nitride (GaN) was promoted by the organic metal vapor phase growth method until a thickness of 3 μm was obtained to form a GaN film 3 on the sapphire (0001) substrate 2. In Example 1, a commercially available sapphire substrate 2 having a GaN film 3 on its surface was used. This sapphire substrate 2 was n-GaN epitaxial wafer (wafer No. PT01AB04H26491121) (POWDEC K.K.) with an undoped layer having the thickness of 1 μm and a doped layer having the thickness of 2 μm laminated on the sapphire substrate (0001) in that order, and with the total film thickness measuring 3 μm.
In step ST3, coating of the organic conductor 4 by spin coating and baking were conducted. As spin coating, 2 mL of a stock solution of organic conductor (p-type conductive polymeric polyaniline, ORMECON) was applied to the GaN film 3 evenly using a pipet, revolution was accelerated to 1000 rpm in 10 seconds, 1000 rpm was maintained for 10 seconds, revolution was further accelerated to 4000 rpm in another 10 seconds, 4000 rpm was maintained for 30 minutes, and then decelerated to 0 rpm in 10 seconds. Regarding the above operations as one set, four sets were repeated. The spin-coated item was then left on a hot plate heated to the temperature setting of 150° C. for 10 minutes for drying/baking. The above operations were all conducted in the atmosphere. After baking, film thickness of the organic conductor 4 was measured by a surface profiler, and the average of film thickness was found to be 173 nm.
In step ST4, unnecessary parts of the organic conductor 4 were peeled off. Namely, the organic conductor 4 covering the GaN film 3 evenly was peeled using a pair of stainless steel tweezers, exposing the surface of the GaN film 3, except for the area of the element of 2.7 mm×3.1 mm.
In step ST5, an indium electrode 5 was formed. Namely, on a part of the surface of the GaN film 3 exposed in ST4, indium metal was soldered to make the indium electrode 5 in ohmic contact with the GaN film 3.
It was found by the light transmittance measurement of the organic conductor 4 that the organic conductor 4 had the light transmittance of 75% to 85% in the wavelength region from 250 nm to 280 nm, and approximately 90% in the wavelength region of 280 nm and longer.
As shown by the measurement result of the spectral sensitivity of the solar cell 1, the spectral sensitivity increased sharply in the short wavelength side centered around 360 nm, which is the wavelength of optical band edge of GaN, and external quantum efficiency of organic/n-GaN solar cell reached 0.3 at 300 nm.
In step ST6, a sapphire substrate 2 was provided, in step 7, a GaN film was formed on the sapphire substrate 2 as the inorganic semiconductor 3, and in step 8, an organic conductor 4 was formed on the GaN film as the inorganic semiconductor 3, all of which are the same as steps ST1, ST2, and ST3 in Example 1. Detailed description is therefore omitted.
In step ST9, tin-doped indium oxide was deposited as the transparent conductive oxide 7 by the magnetron sputtering method. Sputtering deposition was conducted in a state where a stainless steel mask having an opening of 0.75 mm in diameter was adhered to the sample obtained in step ST8 to obtain a circular deposition area of 0.75 mm in diameter. The sputtering conditions were as follows: Target material; tin-doped indium oxide, argon flow rate; 19.2 sccm, oxygen flow rate; 0.8 sccm, and radiofrequency power; 200 W. The reaction pressure at that time was 0.29 Pa. After the deposition, film thickness of the transparent conductive oxide 7 was measured by a surface profiler, and the average of film thickness was found to be 124 nm.
In step ST10, unnecessary area of the organic conductor 4 was peeled off. Namely, the organic conductor 4 covering the GaN film 3 evenly was peeled off using a pair of stainless steel tweezers to expose the surface of the GaN film 3 except a rectangular area of 1.6 mm×2.0 mm.
In step ST11, an indium electrode 5 was formed. Namely, on a part of the surface of the GaN film 3 exposed in step ST10, indium metal was soldered to create the indium electrode 5 in ohmic contact with the GaN film 3.
Voltage and current were measured while the light 13 of the xenon lamp was irradiated from above the solar cell 6, and current density/voltage characteristics were calculated. The area of the element of the solar cell 6 was 0.032 cm2. To make the effect of photoelectric conversion more visible, the positive and negative current values were reversed, with a part enlarged. The, voltage at an open end VOC, short-circuit current density JSC, maximum output density Pmax, and fill factor FF were 0.69 V, 0.70 mA/cm2, 0.238 mW/cm2, and 0.49 respectively.
As Example 3, an element was made following the same procedure as Example 1, with a non-doped GaN film having the thickness of 1 μm used as the inorganic semiconductor 3, PEDOT:PSS having the thickness of 10 μm as the organic conductor 4, and an Ag film having the thickness of 100 μm as the electrodes 5.
Similar to Example 1, the current/voltage characteristics were measured to calculate the current density/voltage characteristics. With the element manufactured in Example 3, the ideal diode value n was 1.8, ideal saturated current density J0 was 6.5×10−12 A, and the Schottky barrier height fB was 1.8 eV.
Similar to Example 1, current/voltage measurement was conducted while the light of the xenon lamp was irradiated, and the voltage at open end VOC, short-circuit current Isc, maximum output Pmax, and fill factor FF were found to be 0.44 V, 3.84 nA, 0.64 nW, and 0.38 respectively.
Table 2 summarizes the results of Example 1 to Example 3.
A Schottky-barrier junction element formed by the junction between the polythiophene-series organic conductor 4 and the GaN film 3 was shown in Examples 1 and 2, and a Schottkey-barrier junction element formed by the junction between polyaniline-series organic conductor and the GaN film was shown in Example 3, using the solar cell as a model. Organic conductors in the embodiments of the present invention are not limited to polythiophene-series or polyaniline-series organic conductors, but various organic conductors such as those shown in Table 1 may be used. Inorganic semiconductors are not limited to GaN, but various inorganic semiconductors shown in Table 3 can be used. Consequently, as shown in Table 4, Schottky-barrier junction elements can be achieved by the combination of any one of the organic materials A to E and any one of semiconductor materials.
With the embodiment of the present invention, as described in Examples, a conductive polymeric coat was applied to the GaN film used as the inorganic semiconductor 3 to form a high Schottky barrier exceeding 1.2 eV between the inorganic semiconductor 3 and the organic conductor 4. This Schottky-barrier junction formed between the inorganic semiconductor 3 and the organic conductor 4 has high light transmittance. Consequently, if this Schottky-barrier junction is used for photoelectric conversion elements or a photoelectric conversion unit of a solar cell, good performance will be produced.
In addition, by controlling the band gap of the inorganic semiconductor 3, the absorption wavelength can be shifted from ultraviolet light to visible light, which allows photoelectric effect in the visible light range to be used. For example, if In is mixed with GaN as crystal to have InxGa1−XN, the band gap decreases, and when x is made to be equal to 1 eventually, the band gap becomes 0.7 eV. By changing compositions as described above, the band gap can be continuously controlled between 3.4 eV to 0.7 eV.
With the embodiment of the present invention, as shown in Examples 1 to 3, a device can be manufactured by an extremely simple method without using processes such as photolithography and dry etching.
Since devices can be configured using electrode materials easily available compared to rare and noble metals such as Au and Pd conventionally considered to be essential to form a Schottky barrier, high serviceability is ensured.
In addition to the use as a solar cell, the photoelectric conversion element of the present invention can be used for the following devices.
The first application is an ultraviolet (intensity) sensor. Namely, the photoelectric conversion element can be used as a sensor for outputting current, without applying bias, in proportion to the intensity of ultraviolet light, thus measuring the intensity of ultraviolet light in the environment. Possible applications include an outdoor sunburn watch detector and a sensor used with a UV bactericidal lamp for checking that the amount of environmental UV light falls within the proper range.
The second application is an infrared ray sensor. By using a semiconductor having a small band gap, application as an infrared ray sensor is made possible. As such semiconductors, InSb, PbTe, PbS, Ge, InN, and GaSb are available. The band gap of InSb, PbTe, PbS, Ge, InN, and GaSb is 0.17 eV, 0.31 eV, 0.41 eV, 0.66 eV, 0.7 eV, and 0.72 eV respectively. Since they all have a small band gap, they are ideal fore an infrared ray sensor, and possible applications include a radiation thermometer and a human presence sensor.
The third application is a diode having various startup voltages. The Schottky-barrier height varies depending on the electron affinity of a semiconductor to be used. By selecting semiconductor materials having different electron affinities, the startup voltage of diodes can be changed, which is effective when using diodes for voltage control.
The fourth application is a variable-capacity diode. Since the width of a depletion layer changes in response to the application of voltage in reverse direction as with conventional diodes, the use as a variable-capacity diode is possible.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-077948 | Mar 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/055574 | 3/29/2010 | WO | 00 | 12/1/2011 |
