JP2014017440A - Photoelectric conversion element and image sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion element capable of obtaining a high S/N ratio by reducing a dark current, and an image sensor.SOLUTION: The photoelectric conversion element includes a first electrode layer, a photoelectric conversion layer laminated on the first electrode layer, a hole injection blocking layer constituted of gallium oxide and laminated on the photoelectric conversion layer, and a second electrode layer laminated on the hole injection blocking layer and constituted of a translucent electrode material.

Description

  The present invention relates to a photoelectric conversion element and an image sensor having a low voltage operation and a high S / N ratio.

Conventionally, a photoelectric conversion element that uses a chalcopyrite semiconductor CuIn _1-X Ga _X Se ₂ (Se-based CIGS film) or CuIn _1-X Ga _X S ₂ (S-based CIGS film) as a photoelectric conversion layer, It is mainly used as a solar cell, and has advantages such as a high light absorption coefficient, high quantum efficiency and energy conversion efficiency, and little deterioration due to light irradiation (for example, see Patent Document 1, Non-Patent Documents 1 and 2). ).

  In addition, conventionally, a photoelectric conversion element using a selenium-based amorphous semiconductor in a photoelectric conversion layer has a high light absorption coefficient and a low dark current characteristic, and thus has been mainly used as a high-sensitivity imaging element. The low sensitivity characteristic with respect to long wavelength light and the generation | occurrence | production of the white crack resulting from crystallization with time have been a problem.

  On the other hand, a selenium-based crystalline semiconductor (crystalline selenium (hexagonal selenium)) obtained by uniformly crystallizing a selenium-based amorphous semiconductor by heating after film formation is used for long wavelength light in a selenium-based amorphous semiconductor. It overcomes problems such as sensitivity characteristics and white scratches, and has high sensitivity characteristics in the entire visible light range and thermal stability of the element (see, for example, Non-Patent Document 3 and Patent Document 2).

Crystalline selenium (hexagonal selenium) has a band gap of about 1.85 eV, and has a high light absorption coefficient of 10 ⁵ cm ⁻¹ in the visible light region, which is considered useful as an image sensor or a solar cell ( For example, Non-Patent Documents 4 and 5).

JP 2007-123720 A JP 61-67279 A

Photoconductive Imaging Using CuInSe2 Film, Jpn.J.Appl.Phys.vol.32, Suppl.32-3, pp.113-115 (1993) Tokyo University of Agriculture and Technology Doctoral Dissertation "Application of ternary compound semiconductors to optical devices" Katsushi Tanaka, 1996 Efficient ITO / Se Heterojunction Solar Cells, Jpn.J.Appl.Phys.vol.23, No.8, pp.L587-L589 (1984) V.Prosser et al. The Optical Constants of Single Crystals of Hexagonal Selenium, Czech.J.Phys.B 10, p306 (1960) T. Nakada et al. Polycrystalline Thin-Film TiO2 / Se Solar Cells, J.J.App.Phys vol.24, No.7, pp.L536 (1985)

  However, a photoelectric conversion element using a chalcopyrite semiconductor for a photoelectric conversion layer has a large dark current when an electric field is applied, and a sufficient S / N ratio cannot be obtained. Further, if the crystalline selenium (Se) is also as it is (single layer), the dark current is large.

  As one of the causes of dark current, it is conceivable that the injection of charge from the electrode to the chalcopyrite semiconductor is not sufficiently suppressed.

  Further, in the photoelectric conversion element using the selenium-based crystalline semiconductor (crystal selenium (hexagonal selenium)) crystallized as described above for the photoelectric conversion layer, the dark current at the time of applying an electric field is large, and sufficient S / N ratio is not obtained.

  Therefore, the present invention provides a photoelectric conversion element and an image sensor that use a chalcopyrite type semiconductor or a selenium-based crystalline semiconductor (crystalline selenium (hexagonal selenium)) for a photoelectric conversion layer and greatly reduce dark current. The purpose is to provide.

  The photoelectric conversion element of one aspect of the present invention includes a first electrode layer, a photoelectric conversion layer stacked on the first electrode layer, a hole injection blocking layer formed of gallium oxide and stacked on the photoelectric conversion layer. And a second electrode layer laminated on the hole injection blocking layer and made of a translucent electrode material.

  The photoelectric conversion element and the image sensor that can reduce the dark current and obtain a high S / N ratio can be provided.

3 is a diagram showing a cross section of the photoelectric conversion element of Embodiment 1. FIG. 6 is a diagram illustrating an analysis result of a band structure of the photoelectric conversion element of Embodiment 1. FIG. It is a figure which shows the characteristic of the dark current measured with the photoelectric conversion element which does not contain the positive hole injection blocking layer 4 for the comparison with Embodiment 1. FIG. FIG. 3 is a diagram illustrating characteristics of dark current measured by the photoelectric conversion element 10 according to the first embodiment. 1 is a diagram illustrating a cross-sectional structure of an image sensor using a photoelectric conversion element 10 according to Embodiment 1. FIG. 6 is a diagram illustrating a cross section of a photoelectric conversion element according to Embodiment 2. FIG. It is a figure which shows the characteristic of the dark current measured with the photoelectric conversion element which does not contain the positive hole injection blocking layer 4 for the comparison with Embodiment 2. FIG. It is a figure which shows the characteristic of the dark current measured with the photoelectric conversion element 20 of Embodiment 2. FIG. 6 is a diagram illustrating a cross section of a photoelectric conversion element of Embodiment 3. FIG. It is a figure which shows the characteristic of the dark current of the photoelectric conversion element 30 of Embodiment 3. It is a figure which shows the cross section of the photoelectric conversion element of the modification of Embodiment 1 thru | or 3.

  Hereinafter, embodiments in which the photoelectric conversion element and the image sensor of the present invention are applied will be described.

<Embodiment 1>
FIG. 1 is a diagram illustrating a cross section of the photoelectric conversion element of the first embodiment.

  The photoelectric conversion element 10 of Embodiment 1 includes a substrate 1, an electrode 2, a photoelectric conversion layer 3, a hole injection blocking layer 4, and an electrode 5.

  As the substrate 1, for example, a glass substrate can be used. The photoelectric conversion element 10 may receive light from either the upper side or the lower side in FIG. For example, when light is incident from above, the electrode 5 is required to be a light-transmitting electrode, but the substrate 1 may not be transparent. In this case, for example, a silicon substrate or the like may be used as the substrate 1.

  When light is incident from the lower side, the substrate 1 and the electrode 2 are required to be light-transmitting substrates and electrodes, but the electrode 5 may not be transparent. In addition, you may be comprised so that all the board | substrate 1, electrode 2, and electrode 5 may have translucency.

  As the electrode 2, for example, a thin film electrode made of gold or titanium nitride can be used. The electrode 2 is used as a negative electrode. The electrode 2 is an example of a first electrode layer.

  For example, the gold thin film may be formed on the surface of the substrate 1 by a vapor deposition method, and the titanium nitride thin film may be formed on the surface of the substrate 1 by a sputtering method or the like. When it is desired to provide a light-transmitting property to the gold thin film or the titanium nitride thin film, the film thickness should be thin enough to ensure the light-transmitting property. Further, as the electrode 2, an ITO (Indium Tin Oxide) film formed by vapor deposition or the like may be used. In addition, since the electrode 2 should just be comprised with the electrically conductive film, it is not limited to the thing of the material described here.

The photoelectric conversion layer 3 is composed of chalcopyrite semiconductor _{(e.g., CuIn 1-X Ga X Se} 2 (Se -based CIGS film), _{or, CuIn 1-X Ga X S} 2 (S based CIGS film), etc.) . _{Here, CuIn 1-X Ga X Se} 2 (Se -based CIGS film) and _{CuIn 1-X} Ga _{X S} x in 2 (S based CIGS film) is 0~1 (0 ≦ x ≦ 1) .

  The photoelectric conversion layer 3 made of a chalcopyrite semiconductor can be formed on the electrode 2 by, for example, a multi-source deposition method, a three-stage method, a sputtering method, or the like, and the film thickness is, for example, about 0.5 μm to 3 μm. .

  A DC voltage is applied between the electrode 2 and the electrode 5 to the photoelectric conversion layer 3. The DC voltage can be applied by connecting a DC power source between the electrode 2 and the electrode 5 in a direction in which the electrode 2 is a negative electrode and the electrode 5 is a positive electrode.

  It has been confirmed that an avalanche multiplication occurs in the photoelectric conversion layer 3 made of a chalcopyrite semiconductor by applying a predetermined voltage (for example, about 7 V).

The hole injection blocking layer 4 is a layer that blocks (suppresses) injection of holes from the electrode 5 to the photoelectric conversion layer 3, and includes a gallium oxide (Ga ₂ O ₃ ) layer. The gallium oxide (Ga ₂ O ₃ ) layer used as the hole injection blocking layer 4 can be formed on the photoelectric conversion layer 3 by, for example, a vacuum deposition method, a sputtering method, a pulse laser deposition method, or the like. The film thickness of the hole injection blocking layer 4 is, for example, about 0.01 μm to 1 μm.

  The electrode 5 is made of an ITO film, for example. The ITO film is formed on the photoelectric conversion layer 3 by vapor deposition or the like. The electrode 5 is used as a positive electrode. The electrode 5 is an example of a second electrode layer.

  As the electrode 5, a gold thin film or a titanium nitride thin film may be used as in the case of the electrode 2. In addition, since the electrode 2 should just be comprised with the electrically conductive film, it is not limited to the thing of the material described here.

FIG. 2 is a diagram showing the analysis result of the band structure of the photoelectric conversion element of the first embodiment. This analysis result is obtained by measuring the positions of the Fermi level (E _F ) and the valence band (E _V ) with XPS (X-ray photoemission spectroscopy). The band gap (Eg) is calculated from optical absorption measurement.

Analysis of Figure 2, a gold thin film used as an electrode 2, using a CuGaSe ₂ in which the x = 1 in _CuIn 1-X _Ga X Se ₂ as a photoelectric conversion layer 3, photoelectric manufactured using the ITO film as the electrode 5 Obtained with the conversion element 10.

In FIG. 2, the electrode 2 (Au), the photoelectric conversion layer 3 (CuGaSe ₂ ), the hole injection blocking layer 4 (Ga ₂ O ₃ ), and the electrode 5 (ITO) are shown from right to left.

  As shown in FIG. 2, it was found that a high hole injection barrier of about 3.4 eV was obtained between the hole injection blocking layer 4 and the electrode 5. If such a high hole injection barrier is obtained, injection of holes from the electrode 5 into the hole injection blocking layer 4 is considered to be blocked (suppressed).

  FIG. 3 is a diagram showing characteristics of dark current measured by a photoelectric conversion element not including the hole injection blocking layer 4 for comparison. FIG. 4 is a diagram illustrating the characteristics of dark current measured by the photoelectric conversion element 10 of the first embodiment. The comparative photoelectric conversion element in which the dark current characteristics shown in FIG. 3 are measured has a configuration in which the hole injection blocking layer 4 is removed from the photoelectric conversion element 10 shown in FIG. The characteristics of dark current were measured by applying a voltage between the electrode 2 (negative voltage) and the electrode 5 (positive voltage).

As shown in FIG. 3, in the comparative photoelectric conversion element, for example, when a DC voltage of 2 V was applied, the dark current was a very high value of 40 μA / cm ² .

On the other hand, in the photoelectric conversion element 10 of Embodiment 1, as shown in FIG. 4, for example, when a DC voltage of 2 V was applied, the dark current was 1 nA / cm ² . That is, the photoelectric conversion element 10 of Embodiment 1 has a dark current of about 1/10000 compared with the photoelectric conversion element for comparison, which can be greatly reduced (4 digits or more).

  In addition, since the dark current can be reduced in this manner, the S / N ratio of the photoelectric conversion element 10 can be improved.

  FIG. 5 is a diagram illustrating a cross-sectional structure of an image sensor using the photoelectric conversion element 10 according to the first embodiment.

  An image sensor 100 shown in FIG. 5 includes a signal reading unit 101 instead of the substrate 1 shown in FIG. The signal reading unit 101 extracts a signal obtained by photoelectric conversion in the photoelectric conversion layer 3 through the electrode 2 and functions as a substrate.

  In FIG. 5, the signal reading unit 100 is shown in a simplified manner, but as the signal reading unit 100, for example, a signal reading unit using a CCD (Charge Coupled Device) may be used. The signal reading unit 100 may be provided for each pixel of the image sensor 100.

As described above, according to Embodiment 1, the electrode 5 is provided by providing the hole injection blocking layer 4 made of gallium oxide (Ga ₂ O ₃ ) between the photoelectric conversion layer 3 and the electrode 5 (positive electrode). And a photoelectric conversion layer 3 can form a hole injection barrier.

As a result, according to Embodiment 1, it is possible to provide a photoelectric conversion element and an image sensor that can reduce dark current and obtain a high S / N ratio. Although the form to use was demonstrated, you may use the photoelectric conversion element 10 of Embodiment 1 for a solar cell, for example.

<Embodiment 2>
FIG. 6 is a cross-sectional view of the photoelectric conversion element of the second embodiment. Below, the same code | symbol is attached | subjected to the component similar to the photoelectric conversion element 10 of Embodiment 1, and the description is abbreviate | omitted or simplified.

  The photoelectric conversion element 20 of Embodiment 2 includes a substrate 1, an electrode 2, a photoelectric conversion layer 23, a hole injection blocking layer 4, and an electrode 5.

  The photoelectric conversion layer 23 is composed of crystallized selenium (hexagonal selenium (Se)). The photoelectric conversion layer 23 forms, for example, amorphous selenium (a-Se) having a film thickness of 0.5 μm to 4 μm on the electrode 2 by a vacuum deposition method, and then at a temperature of 100 ° C. to 220 ° C. for 1 minute. It is formed by performing heat treatment for ˜30 minutes.

In the second embodiment, gallium oxide (Ga ₂ O ₃ ) constituting the hole injection blocking layer 4 is formed on the photoelectric conversion layer 23 by a vacuum deposition method, a sputtering method, a pulse laser deposition method, or the like. The film thickness of the hole injection blocking layer 4 is, for example, 0.01 μm to 1 μm.

  FIG. 7 is a diagram showing characteristics of dark current measured by a photoelectric conversion element not including the hole injection blocking layer 4 for comparison. FIG. 8 is a diagram showing the characteristics of dark current measured by the photoelectric conversion element 20 of the second embodiment. The comparative photoelectric conversion element for measuring the dark current characteristics shown in FIG. 7 has a configuration in which the hole injection blocking layer 4 is removed from the photoelectric conversion element 20 shown in FIG. The characteristics of dark current were measured by applying a voltage between the electrode 2 (negative voltage) and the electrode 5 (positive voltage).

As shown in FIG. 7, in the comparative photoelectric conversion element, for example, when a DC voltage of 2 V was applied, the dark current was a very high value of 70 μA / cm ² .

On the other hand, in the photoelectric conversion element 20 of Embodiment 2, as shown in FIG. 8, for example, when a DC voltage of 2 V was applied, the dark current was 100 nA / cm ² . That is, the photoelectric conversion element 20 of Embodiment 2 has a dark current of about 1/100 compared to the comparative photoelectric conversion element, and can be greatly reduced (about two digits).

  In addition, since the dark current can be reduced in this way, the S / N ratio of the photoelectric conversion element 20 can be improved.

<Embodiment 3>
FIG. 9 is a diagram illustrating a cross section of the photoelectric conversion element of the third embodiment. Below, the same code | symbol is attached | subjected to the component similar to the photoelectric conversion elements 10 and 20 of Embodiment 1, 2, and the description is abbreviate | omitted or simplified.

  The photoelectric conversion element 30 of Embodiment 3 includes a substrate 1, an electrode 2, an electron injection blocking layer 32, a photoelectric conversion layer 23, and an electrode 5. The photoelectric conversion element 30 of Embodiment 3 has a configuration in which the hole injection blocking layer is removed from the photoelectric conversion element 20 of Embodiment 2 and an electron injection blocking layer 32 is provided between the electrode 2 and the photoelectric conversion layer 23. Have.

  The electron injection blocking layer 32 is a layer that blocks (suppresses) injection of electrons from the electrode 2 to the photoelectric conversion layer 3, and is composed of a nickel oxide (NiO) layer. The electron injection blocking layer 32 composed of a nickel oxide (NiO) layer is formed on the electrode 2 by, for example, vapor deposition or sputtering. The film thickness of the electron injection blocking layer 32 is, for example, 10 nm to 500 nm.

  In Embodiment 3, the photoelectric conversion layer 23 is formed on the electron injection blocking layer 32. In Embodiment 3, the film thickness of the photoelectric conversion layer 23 is, for example, 0.01 μm to 20 μm.

  FIG. 10 is a diagram illustrating the dark current characteristics of the photoelectric conversion element 30 according to the third embodiment. FIG. 10 also shows dark current characteristics of a photoelectric conversion element that does not include the electron injection blocking layer 32 for comparison. This comparative photoelectric conversion element has a configuration in which the electron injection blocking layer 32 is removed from the photoelectric conversion element 30 of the third embodiment.

  In FIG. 10, the dark current characteristics of the photoelectric conversion element 30 of the embodiment are indicated by a round plot (●), and the dark current characteristics of the comparative photoelectric conversion element are indicated by a triangular plot (▲).

As shown in FIG. 10, in the comparative photoelectric conversion element, for example, when a DC voltage of 10 V was applied, the dark current was a very high value of about 500 μA / cm ² .

On the other hand, in the photoelectric conversion element 30 of Embodiment 3, as shown in FIG. 10, for example, when a DC voltage of 10 V was applied, the dark current was reduced to about 80 μA / cm ² . That is, the photoelectric conversion element 30 of Embodiment 3 has a dark current of about 1/6 compared to the comparative photoelectric conversion element, and can be greatly reduced.

  In addition, since the dark current can be reduced in this way, the S / N ratio of the photoelectric conversion element 30 can be improved.

  Note that, in the first and second embodiments, the photoelectric conversion elements 10 and 20 including the hole injection blocking layer 4 have been described above. In the third embodiment, the photoelectric conversion element 30 including the electron injection blocking layer 32 has been described.

  In the photoelectric conversion elements 10 and 20 of Embodiments 1 and 2, an electron injection blocking layer 32 may be provided between the electrode 2 and the photoelectric conversion layer 3 or 23. In Embodiment 3, the hole injection blocking layer 4 may be provided between the photoelectric conversion layer 23 and the electrode 5. The configuration of the photoelectric conversion element in this case is as shown in FIG.

  FIG. 11 is a diagram illustrating a cross section of a photoelectric conversion element according to a modification of the first to third embodiments.

  A photoelectric conversion element 40 illustrated in FIG. 11 includes a substrate 1, an electrode 2, an electron injection blocking layer 32, a photoelectric conversion layer 3, a hole injection blocking layer 4, and an electrode 5. The photoelectric conversion element 40 has a configuration in which an electron injection blocking layer 32 is provided between the electrode 2 of the photoelectric conversion element 10 of Embodiment 1 and the photoelectric conversion layer 3.

  Thus, in the photoelectric conversion element 40 including both the hole injection blocking layer 4 and the electron injection blocking layer 32, holes are blocked from being injected from the electrode 5 into the photoelectric conversion layer 3 by the hole injection blocking layer 4. (Suppressed), and the electron injection blocking layer 32 prevents (suppresses) electrons from being injected from the electrode 2 into the photoelectric conversion layer 3.

  For this reason, according to the photoelectric conversion element 40, dark current can be reduced and S / N ratio can be improved.

  Note that the photoelectric conversion element 40 may include a photoelectric conversion layer 23 made of crystallized selenium (hexagonal selenium (Se)) instead of the photoelectric conversion layer 3 made of chalcopyrite semiconductor.

  The photoelectric conversion element and the image sensor according to the exemplary embodiment of the present invention have been described above, but the present invention is not limited to the specifically disclosed embodiment, and is not limited to the claims. Various modifications and changes can be made without departing from the above.

10, 20, 30, 40 Photoelectric conversion element 1 Substrate 2 Electrode 3, 23 Photoelectric conversion layer 4 Hole injection blocking layer 5 Electrode 32 Electron injection blocking layer 100 Image sensor

Claims (7)

A first electrode layer;
A photoelectric conversion layer laminated on the first electrode layer;
A hole injection blocking layer composed of gallium oxide and stacked on the photoelectric conversion layer;
A photoelectric conversion element comprising: a second electrode layer laminated on the hole injection blocking layer and made of a translucent electrode material. An electron injection blocking layer made of nickel oxide and stacked on the first electrode layer;
The photoelectric conversion element according to claim 1, wherein the photoelectric conversion layer is stacked on the first electrode layer via the electron injection blocking layer. A first electrode layer;
An electron injection blocking layer made of nickel oxide and stacked on the first electrode layer;
A photoelectric conversion layer laminated on the electron injection blocking layer;
A photoelectric conversion element comprising: a second electrode layer laminated on the photoelectric conversion layer and made of a translucent electrode material. Further comprising a hole injection blocking layer made of gallium oxide and stacked on the photoelectric conversion layer;
The photoelectric conversion element according to claim 3, wherein the second electrode layer is stacked on the photoelectric conversion layer via the hole injection blocking layer.   The photoelectric conversion element according to claim 1, wherein the photoelectric conversion layer is formed of a chalcopyrite semiconductor layer.   The photoelectric conversion element according to any one of claims 1 to 4, wherein the photoelectric conversion layer is formed of a crystalline semiconductor layer mainly containing selenium. The photoelectric conversion element according to any one of claims 1 to 6,
An image sensor comprising: a signal reading unit that is disposed on a surface of the electrode layer opposite to a surface on which the photoelectric conversion layer is laminated and reads a signal generated by photoelectric conversion of the photoelectric conversion layer.

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