Photoelectric conversion element

Abstract

To provide a photoelectric conversion element capable of preventing specular deterioration of the photoelectric conversion element by causing a second transport layer to prevent evaporation of a solvent of an additive in a first transport layer and prevent coagulation of the additive.SOLUTION: In a photoelectric conversion element 1, a transport layer 50 disposed in an upper part of a photoelectric conversion layer 40, which absorbs received light and generates electric charge, and transporting the electric charge includes: a first transport layer 51 containing a transport material transporting the electric charge, an additive 511 and a solvent 512 of the additive for improving conductivity; and a second transport layer 52 disposed on an upper part of the first transport layer 51 and preventing evaporation of the solvent 512 of the additive. The second transport layer 52 is disposed on the first transport layer 51, thereby preventing the evaporation of the solvent 512 of the additive and preventing coagulation of the additive 511. Thus, specular deterioration of the photoelectric conversion element 1 can be prevented.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to the structure of a transport layer of a photoelectric conversion element.

Background Art

[0002] Spiro-OMeTAD (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamino)-9,9’-spirobifluorene) is used as a hole transport layer material of a photoelectric conversion element, and an ionic salt selected from Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide) as an example of an additive and doping with a base such as tBP (tertiary butylpyridine) as an example of a solvent of the additive are known in order to increase the hole concentration. (Patent Document 1)

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When using an additive and a solvent of the additive as described in Patent Document 1, especially when using a solvent that is easy to evaporate, as time passes, the solvent evaporates, and the additive may aggregate, leading to a decrease in power generation efficiency over time.

Means for Solving the Problems

[0005] A photoelectric conversion element according to an application example of the present invention is a photoelectric conversion element disposed above a photoelectric conversion layer that absorbs received light and generates charges, and includes a transport layer that transports charges, wherein the transport layer includes a first transport layer including a transport material that transports charges, an additive for improving conductivity, and a solvent of the additive, and a second transport layer disposed above the first transport layer and preventing evaporation of the solvent.

Advantages of the Invention

[0006] According to the present invention, since the second transport layer prevents the evaporation of the solvent of the additive in the first transport layer and prevents the aggregation of the additive, the deterioration of the photoelectric conversion element over time can be prevented.

Brief Description of the Drawings

[0007]

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Modes for Carrying Out the Invention

[0008] The mode (embodiment) for carrying out the present invention will be described in detail. The present invention is not limited by the content described in the following embodiments. Further, the constituent elements described below include those that can be easily assumed by those skilled in the art and substantially the same ones. Furthermore, in the drawings, the shapes, dimensions, etc. of the respective elements are schematically shown and do not indicate the actual shapes, dimensions, etc.

[0009] The photoelectric conversion element according to the present embodiment is a photoelectric conversion element disposed above the photoelectric conversion layer and including a transport layer for transporting charges, wherein the transport layer includes a first transport layer including a transport material for transporting charges, an additive for improving conductivity, and a solvent of the additive, and a second transport layer disposed above the first transport layer for preventing the evaporation of the solvent.

[0010] (Embodiment 1) FIG. 1 is a cross-sectional view schematically showing a photoelectric conversion element according to Embodiment 1. The photoelectric conversion element 1 has a lower electrode layer 20, an electron transport layer 30, a photoelectric conversion layer 40, a hole transport layer 50, and an upper electrode layer 60 laminated in this order on a substrate 10. The photoelectric conversion element 1 is a forward-structure photoelectric conversion element that generates a voltage between the lower electrode layer 20 and the upper electrode layer 60 and outputs a current from the upper electrode layer 60 in response to light being incident through the substrate 10 and the lower electrode layer 20.

[0011] Next, the lower electrode layer 20, the electron transport layer 30, the photoelectric conversion layer 40, the hole transport layer 50, and the upper electrode layer 60 provided on the substrate 10 included in the photoelectric conversion element will be described in detail.

[0012] (Substrate 10) As shown in FIG. 1, the substrate 10 is formed of a light-transmitting material that can support the components included in the photoelectric conversion element 1 and transmit the light incident on the photoelectric conversion element 1. The light-transmitting material is a material that transmits light having a wavelength absorbed by the photoelectric conversion layer 40, and for example, an insulating glass substrate, a resin substrate, or the like can be used.

[0013] (Lower Electrode Layer 20) The lower electrode layer 20 functions as a cathode suitable for collecting electrons and is formed of a light-transmitting material for allowing light to be incident on the photoelectric conversion layer. Examples of the material for forming the lower conductive layer 20 include conductive transparent materials such as fluorine-doped tin oxide (FTO), indium tin oxide (ITO), tin oxide (SnO2), aluminum-doped zinc oxide (AZO), indium zinc oxide (IZO), gallium-doped zinc oxide (GZO), and niobium-doped titanium oxide (NTO).

[0014] (Electron Transport Layer 30) The electron transport layer 30 is responsible for the function of transporting the electrons generated by the photoexcitation of the photoelectric conversion layer 40. Examples of the material for forming the electron transport layer 30 include not only inorganic materials such as titanium oxide (TiO2), tungsten oxide (WO2, WO3, W2O3, etc.), zinc oxide (ZnO), niobium oxide (Nb2O5, etc.), tantalum oxide (Ta2O5, etc.), yttrium oxide (Y2O3, etc.), strontium titanate (SrTiO3, etc.), tin oxide (SnO2), etc., but also organic materials such as fullerenes (C60, C70, etc.) and their derivatives (PC60BM, PC70BM, ICBA, hydrogenated C60, hydroxylated C60, etc.). The above materials may be used alone or as a mixture of two or more kinds.

[0015] (Photoelectric conversion layer 40) The photoelectric conversion layer 40 absorbs light and causes photoexcitation. When the photoelectric conversion element 1 receives light, the light is absorbed by the photoelectric conversion layer 40, generating electrons and holes. The generated electrons are taken out to the electron transport layer 30, and the holes are taken out to the hole transport layer 50. Examples of the material for forming the photoelectric conversion layer 40 include compounds having ionic crystallinity, such as perovskite-type compounds. The perovskite-type compound is, for example, a halide containing at least two kinds of cations selected from the group consisting of one or more anions selected from halogen elements and cations of at least two kinds of elements selected from Pb, Ag, Na, Mg, Al, K, Ca, Sc, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, In, Sn, Sb, Cs, Ba, Bi and lanthanoid elements. In addition, the cations of the perovskite-type compound may partially contain cations other than metal elements, such as ammonium ions. Note that the photoelectric conversion layer 40 may have luminescence properties. When it has luminescence properties, it can become a light-emitting element that emits a predetermined wavelength from the photoelectric conversion layer 40 by applying a voltage to the photoelectric conversion element.

[0016] (Hole transport layer 50) The hole transport layer 50 is responsible for the function of transporting holes generated by photoexcitation of the photoelectric conversion layer 40. The hole transport layer 50 includes a first transport layer 51 and a second transport layer 52, and is arranged in the order of the first transport layer 51 and the second transport layer 52 from the side of the photoelectric conversion layer 40.

[0017] (The first transport layer 51 The first transport layer 51 is intended to efficiently extract holes from the photoelectric conversion layer 40 to the anode (upper electrode layer 60), and includes a hole transport material 510 as the main material, an additive 511, and a solvent 512 for the additive 511. Examples of materials for forming the hole transport include Spiro-OMeTAD (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamino)-9,9’-spirobifluorene), P3HT (poly(3-hexylthiophene-2,5-diyl)), PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)), CuSCN, CsSnI3, or PTAA (polytriallylamine semiconductor), etc.

[0018] (Additive 511) The additive 511 is intended to facilitate the movement of holes in the first transport layer 51 and improve the conductivity. Examples of materials for forming the additive 511 include M(TFSI)n. M is a metal cation, such as Li, Zn, Ca, Cu, Sc, Co, etc. n is the valence, and takes values such as 1, 2, 3, for example.

[0019] (Solvent 512 for the additive) The solvent 512 for the additive prevents phase separation between the additive 511 and the hole transport material 510 of the first transport layer 51, and has the effect of making the distribution of the hole transport layer material and the additive 511 in the first transport layer 51 uniform. Examples of the material of the solvent 512 for the additive include tBP (tertiary butyl pyridine).

[0020] (The second transport layer 52) The second transport layer 52 undertakes the function of transporting holes, similar to the first transport layer 51, and functions as an evaporation prevention layer that prevents the evaporation of the solvent 512 of the additive contained in the first transport layer 51. As the material of the second transport layer 52, inorganic compounds or organic compounds can be used. Examples of the inorganic compound forming the second transport layer 52 include salts of alkali metals such as lithium, sodium, potassium, or cesium, and metal oxides such as zinc oxide, titanium oxide, aluminum oxide, indium oxide, or tin oxide. Examples of the organic compound forming the second transport layer 52 include Spiro-OMeTAD (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamino)-9,9’-spirobifluorene), P3HT (poly(3-hexylthiophene-2,5-diyl)), PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)), CuSCN, CsSnI3, or PTAA (polytriallylamine semiconductor), etc. In particular, it is more preferable to make the hole transporting material used in the first transport layer 51 the same as the hole transporting material of the second transport layer 52 in order to facilitate the alignment of energy levels.

[0021] (Upper electrode layer 60) The upper electrode layer 60 serves as an anode suitable for hole collection. Examples of the material forming the upper electrode layer 60 include Au, Ag, Cu, Al, C, Ti, Pt, Ni, W, Mo, etc. The above materials may be used alone or as a mixture of two or more.

[0022] (Description of the manufacturing method of Embodiment 1) FIG. 2 is a flowchart illustrating the manufacturing method of the photoelectric conversion element 1 according to Embodiment 1. The manufacturing method of the photoelectric conversion element according to Embodiment 1 includes a substrate preparation step (step S101), an electron transport layer formation step (step S102), a photoelectric conversion layer formation step (step S103), a hole transport layer formation step (step S104), and an upper electrode formation step (step S105).

[0023] In the first embodiment, a glass substrate is used for the substrate 10, FTO is used for the lower electrode 20, titanium oxide is used for the electron transport layer 30, Spiro-OMeTAD is used for the hole transport material 510, Li-TFSI is used for the additive 511, tBP is used for the solvent 512, a perovskite layer made of cesium silver bismuth bromide is used for the photoelectric conversion layer 40, and Au is used for the upper electrode 60. An example will be described below.

[0024] First, in the substrate preparation step, the substrate 10 is prepared (step S101). The substrate 10 has the lower electrode 20 formed on one surface thereof, and is, for example, a flat glass substrate on which FTO is formed. FTO is formed by depositing a film on the glass substrate using a dry process such as chemical vapor deposition (CVD) in the atmosphere or spray pyrolysis deposition (SPD). It is preferable that the substrate 10 is subjected to a cleaning process such as UV ozone cleaning before performing the next step.

[0025] Next, in the electron transport layer formation step, the electron transport layer 30 is deposited on the lower electrode 20 (step S102). The electron transport layer 30 can be formed by a dry process such as vapor deposition, sputtering, CVD, or spray pyrolysis. When the electron transport layer 30 is titanium oxide, after depositing the first layer on the substrate 10 by the spray pyrolysis method, the substrate is immersed in an aqueous solution of titanium tetrachloride and then baked for a predetermined time to deposit the second layer on the first layer.

[0026] In the process of step S102, in the spray pyrolysis method for depositing the first layer, the substrate 10 prepared in the substrate preparation step is heated to a predetermined temperature, a solution containing a titanium chelate compound is sprayed onto the FTO, and then baked at a predetermined temperature for a predetermined time. The titanium chelate compound sprayed onto the substrate in the spray pyrolysis method is, for example, titanium(IV)·acetylacetonate tetrakis(2,4-pentanedionate) titanium(IV) and diisopropoxytitanium(IV) bis(acetylacetonate).

[0027] Thereafter, in the photoelectric conversion layer forming step, a photoelectric conversion layer 40 is formed on the electron transport layer 30 (step S103). Specifically, a precursor solution containing a precursor capable of forming crystals and a solvent containing an organic substance is applied to the surface of the electron transport layer 30 and dried (fired), whereby the photoelectric conversion layer 40 is formed. As the solution application method, for example, a spin coating method, a screen printing method, a dip coating method, etc. can be applied. Specifically, for example, a solution obtained by dissolving cesium bromide, silver bromide, and bismuth bromide in dimethyl sulfoxide (DMSO) is heated and stirred, and then spin-coated onto titanium oxide under a nitrogen atmosphere and fired at a predetermined temperature for a predetermined time to form it. By setting the molar ratio of cesium bromide, silver bromide, and bismuth bromide in the DMSO solution to, for example, 2:1:1, a photoelectric conversion layer of Cs2AgBiBr6 can be obtained. The molar ratio is represented by the molar ratio of cesium bromide, silver bromide, and bismuth bromide in the DMSO solution. The solvent containing an organic substance contained in the precursor solution may be dimethylformamide (DMF) or n-butylamine instead of the above-mentioned DMSO.

[0028] Next, in the first transport layer forming step in the hole transport layer forming step (step S104), a first transport layer 51 is formed on the photoelectric conversion layer 40 (step S1041). The formation process of the first transport layer can be formed by a wet process or a dry process, but a wet process is preferable because an additive 511 can be easily added and a relatively high power generation efficiency can be obtained. Examples of the wet process include a spin coating method, an inkjet method, a die coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush method, a spray coating method, an air knife coating method, a wire bar coating method, a pipe doctor method, an impregnation coating method, a curtain coating method, etc.

[0029] The coating solution used for forming the first transport layer 51 is a chlorobenzene solution containing Spiro-OMeTAD which is a hole transport material 510, to which a solution of Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide) which is an additive 511 (520 mg of Li-TFSI in 1 ml of acetonitrile) and tBP (tertiary butylpyridine) which is a solvent 512 of the additive are added. The first transport layer forming step shown in step S1041 is executed, for example, by applying the above-described coating solution onto the photoelectric conversion layer 40 by a spin coating method. The film thickness of the first transport layer is preferably 0.01 μm or more and 1 μm or less, more preferably 10 nm or more and 300 nm or less. When the film thickness of the first transport layer is within the above range, the photoelectric conversion efficiency tends to improve.

[0030] The concentration of the additive 511 and the solvent 512 containing the additive is preferably 0.1 or more and less than 1 in terms of the concentration ratio with the matrix component which is the material of the first transport layer 51. The concentration ratio is represented by the molar ratio of the solvent 512 and the matrix component. When the concentration ratio of the solvent 512 becomes 1 or more, the progress of the decomposition of the photoelectric conversion layer 40 is accelerated and the power generation efficiency is reduced. When the concentration ratio of the solvent 512 is less than 0.1, the effect of the additive 511 cannot be obtained. Further, the concentration ratio is more preferably 0.1 or more and less than 0.3. This is because there is a possibility that aggregation of the additive 511 which does not depend on the evaporation of the solvent 512 of the additive may partially occur in the range where the concentration ratio is 0.3 or more and 1 or less.

[0031] Next, in the second transport layer formation step in the hole transport layer formation step (step S104), the second transport layer 52 is formed on the first transport layer 51 (step S1042). The second transport layer 52 can be formed by a dry process or a wet process, but in order to prevent evaporation of the solvent 512 of the additive, a dry process that can form a relatively dense film is desirable. Further, even when minute pinholes or coating defects occur in the first transport layer 51, it can be covered by forming by a dry process. As the film formation method of the dry process, a vacuum evaporation method, a sputtering method, an ion plating method, a CVD method, an ALD method, or the like can be used. The film thickness of the second transport layer 52 is preferably 5 nm or more and preferably less than 100 nm. If the film thickness is less than 5 nm, there is a possibility that it cannot be uniformly covered when irregularities are present on the surface of the first transport layer 51 which is the lower layer, and if the film thickness is 100 nm or more, the series resistance component increases and the power generation efficiency tends to decrease. Note that it is not easy to determine whether the second transport layer 52 is formed by either a dry process or a wet process by cross-sectional observation using a scanning electron microscope (SEM) or the like.

[0032] Finally, in the upper electrode formation step, the upper electrode 60 is formed on the second transport layer 52 (step S105). The upper electrode 60 can be formed by a dry process such as a vacuum evaporation method or a sputtering method using a metal as the electrode material. Thus, the manufacturing process of the photoelectric conversion element 1 is completed.

[0033] FIG. 3 is a diagram showing the action of the second transport layer 52. FIG. 3(A) shows the state of the additive 511 and the solvent 512 of the additive in a conventional laminated structure formed only of the first transport layer 51 without forming the second transport layer 52, and FIG. 3(B) shows the state of the additive 511 and the solvent 512 of the additive in the hole transport layer 50 including the second transport layer 52.

[0034] As shown in FIG. 3(A), in the case of a configuration without the second transport layer 52, the solvent 512 of the additive evaporates over time, so the dispersed additive 511 gradually aggregates over time, causing aggregation of the entire hole transport layer. As a result, it may prevent optimal electrical contact in the vicinity of the interface between the photoelectric conversion layer 40 and the first transport layer 51, leading to a decrease in power generation efficiency over time.

[0035] As shown in FIG. 3(B), by adopting a configuration having the first transport layer 51 and the second transport layer 52, the second transport layer 52 prevents the evaporation of the solvent 512 of the additive, so that the additive 511 for improving conductivity can be maintained to be uniformly distributed. Thereby, it is possible to prevent a decrease in the photoelectric conversion efficiency of the photoelectric conversion element 1 over time.

[0036] Note that the photoelectric conversion element according to Embodiment 1 may include components other than those described above. For example, it may have a plurality of layers having the same function such as a tandem structure, or may be an inverted-structure photoelectric conversion element. The inverted-structure photoelectric conversion element has a structure in which a lower conductive layer, a hole transport layer, a photoelectric conversion layer, an electron transport layer, and an upper electrode are formed in order from the substrate side. In the inverted-structure photoelectric conversion element, the arrangement of the electron transport layer and the hole transport layer is opposite to that of the forward-structure photoelectric conversion element, and the electron transport layer has the first transport layer and the second transport layer. Similarly, in the manufacturing method, the electron transport layer forming step (S102) described in FIG. 2 becomes the hole transport layer forming step, and the hole transport layer forming step (S104) becomes the electron transport layer forming step. Therefore, the electron transport layer forming step includes the first transport layer forming step and the second transport layer forming step. Further, in the inverted-structure photoelectric conversion element, the first transport layer is preferably formed by a wet process.

[0037] (Embodiment 2) FIG. 4 shows a photoelectric conversion element 2 according to Embodiment 2. The difference from the photoelectric conversion element 1 of Embodiment 1 is that an oxide layer 53 is formed between the first transport layer 51 and the second transport layer 52 in the hole transport layer 55. Since the components with other reference numerals are the same as those in Embodiment 1, the description thereof is omitted.

[0038] (Oxidation layer 53) The oxidation layer 53 formed on the hole transport layer 55 is a layer in which a part of the surface side of the first transport layer 51 is oxidized. By appropriately oxidizing the surface side of the first transport layer 51, the transport material of the first transport layer 51 can easily react with the additive 511, and the carrier density of the first transport layer 51 can be further improved.

[0039] FIG. 5 is a flowchart illustrating a method for manufacturing the photoelectric conversion element 2 according to Embodiment 2. In the method for manufacturing the photoelectric conversion element according to Embodiment 2, in the hole transport layer formation step (step S204), an oxidation layer formation step (step S2043) is added after the first transport layer formation step (step S2041) and before the second transport layer formation step (step S2042). Since the other steps (steps S201 to S203, step S205) are the same as the corresponding steps (steps S101 to S103, step S105) of Embodiment 1, the description thereof is omitted.

[0040] In the oxidation layer formation step, the surface of the first transport layer 51 is oxidized to form the oxidation layer 53 (step S2043). Examples of the method for forming the oxidation layer include, for example, oxidation treatment in an atmospheric environment, oxygen plasma treatment, addition treatment of an oxidizing agent, and the like. For example, when oxidizing in an atmospheric environment, after forming the first transport layer 51 in the first transport layer formation step (step S2041), it is exposed in an atmospheric atmosphere for 72 hours or more. After forming the oxidation layer 53, the second transport layer formation step (step S2042) is performed.

[0041] Also in the photoelectric conversion element of the present Embodiment 2, it may be an inverted structure photoelectric conversion element. In this case, in the electron transport layer formation step, an oxidation layer formation step is performed after the first transport layer formation step and before the second transport layer formation step.

Example

[0042] Examples will be described with reference to FIG. 6. In this example, Examples 1 to 3 in which the film thicknesses of the first transport layer 51 and the second transport layer 52 are different as samples, and Comparative Example 1 in which the second transport layer 52 is not formed for comparison were prepared.

[0043] (Fabrication of the Photoelectric Conversion Element) First, a glass substrate with FTO formed as the bottom electrode, i.e., FTO-coated glass, is subjected to UV ozone cleaning for 20 minutes. Next, an electron transport layer is formed on the FTO of the UV ozone-cleaned FTO-coated glass by spray pyrolysis. A solution containing titanium(IV) tetrakis(2,4-pentanedionato) heated to 450 °C on a hot plate is sprayed to form titanium oxide as the electron transport layer. Also, the FTO-coated glass on which titanium(IV) tetrakis(2,4-pentanedionato) has been sprayed is fired at 500 °C for 1 hour. The FTO-coated glass fired at 500 °C is immersed in an aqueous titanium tetrachloride solution heated to 70 °C for 30 minutes and then fired at 500 °C for 1 hour to form the electron transport layer on the FTO. The FTO-coated glass with the electron transport layer formed on the FTO is subjected to UV ozone cleaning for 20 minutes.

[0044] Next, on the FTO-coated glass with the electron transport layer formed, the photoelectric conversion layer is formed by spin coating inside a glove box filled with nitrogen. To form Cs2AgBiBr6 as the photoelectric conversion layer, as the spin coating solution, each of the precursors CsBr, AgBr, and BiBr3 was used in a ratio of 2:1:1, and a precursor solution was prepared using dimethyl sulfoxide (DMSO) as the solvent. The FTO-coated glass is rotated at 4000 rpm by a spin coater to apply the precursor solution. After the precursor solution is applied, the FTO-coated glass is left at room temperature for 2 minutes, fired at 70 °C for 5 minutes, and then fired at 250 °C for 1 hour to form the photoelectric conversion layer.

[0045] The FTO-coated glass with the photoelectric conversion layer formed thereon is taken out from inside the glove box, and the first transport layer serving as the hole transport layer is formed by spin coating. By means of a spin coater, the FTO-coated glass is rotated at 3000 rpm, and a chlorobenzene solution containing Spiro-OMeTAD added with a solution of tBP (tert-butylpyridine) and Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide) (520 mg of Li-TFSI in 1 ml of acetonitrile) is applied as a coating solution. In Comparative Example 1, it was prepared to have a film thickness of 260 nm, in Example 1, a film thickness of 250 nm, in Example 2, a film thickness of 210 nm, and in Example 3, a film thickness of 160 nm.

[0046] Then, in Examples 1 to 3, the Spiro-OMeTAD layer corresponding to the second transport layer was formed by a resistance heating method using a vacuum evaporation apparatus (EX-200 manufactured by ULVAC, Inc.). As the material used for forming the second transport layer, Spiro-OMeTAD powder was used. The FTO-coated glass with the first transport layer formed thereon was placed on a metal mask with holes of 25 mm × 15 mm, the crucible was filled with Spiro-OMeTAD powder, and the inside of the vacuum chamber was evacuated to 4×10-5 Torr or less. Then, after heating the Spiro-OMeTAD sample with a resistance heating heater, the shutter was opened and it was deposited on the surface of the first transport layer. In Comparative Example 1, the formation of the second transport layer was not carried out, and it was carried out only in Examples 1 to 3. In Example 1, it was prepared to have a film thickness of 10 nm, in Example 2, a film thickness of 50 nm, and in Example 3, a film thickness of 100 nm.

[0047] Next, on the FTO-coated glass with the hole transport layer formed thereon in Comparative Example 1 and Examples 1 to 3, Au (upper electrode) was formed by vacuum evaporation. Through the above steps, photoelectric conversion elements were obtained respectively. The period until the power generation efficiency of the photoelectric conversion element was measured was stored in an environment with a humidity of 50% and room temperature.

[0048] The power generation efficiencies of Examples 1 to 3 and Comparative Example 1 were measured under the following measurement conditions. As the light source, BLD-100 (manufactured by Spectroscopic Instruments Co., Ltd.) was used, and the evaluation illuminance was set to 200 Lux. The light-receiving area was 6.25 mm 2 2.

[0049] Figure 6 is a graph showing the power generation efficiency ratios for each photoelectric conversion element of the examples. The symbols a (a-1, a-2) represent Comparative Example 1, which is a photoelectric conversion element having a first transport layer with a film thickness of 260 nm and no second transport layer. The symbols b (b-1, b-2) represent Example 1, which is a photoelectric conversion element having a first transport layer with a film thickness of 250 nm and a second transport layer with a film thickness of 10 nm. The symbols c (c-1, c-2) represent Example 2, which is a photoelectric conversion element having a first transport layer with a film thickness of 210 nm and a second transport layer with a film thickness of 50 nm. The symbols d (d-1, d-2) represent Example 3, which is a photoelectric conversion element having a first transport layer with a film thickness of 160 nm and a second transport layer with a film thickness of 100 nm, and show the power generation efficiency ratios. Also, a-1, b-1, c-1, d-1 are based on the values obtained by measuring the power generation efficiency 3 days after fabricating the photoelectric conversion elements, and a-2, b-2, c-2, d-2 are based on the values obtained by measuring the power generation efficiency 41 days after fabricating the photoelectric conversion elements. Here, each measured value in Figure 6 is normalized with the power generation efficiency 3 days after (a-1) of Comparative Example 1 being set to 1.

[0050] In the results of Comparative Example 1, the power generation efficiency after 41 days (a-2) decreased by about 10% compared to the power generation efficiency after 3 days (a-1). For Example 1 (b) and Example 2 (c), the power generation efficiencies after 3 days (b-1, c-1) are substantially the same, but the power generation efficiencies after 41 days (b-2, c-2) are slightly improved compared to the power generation efficiencies after 3 days (b-1, c-1). This is considered to be because by laminating the deposited Spiro-OMeTAD film, the evaporation of tBP was prevented and the deterioration of the photoelectric conversion element was prevented. Regarding the slightly improved power generation efficiency, it is considered that the carrier density increased due to the promotion of the oxidation of Spiro-OMeTAD, and the power generation efficiency also improved due to the improvement of the short-circuit current value. In Comparative Example 1, it is considered that the power generation efficiency decreased because the demerit due to the evaporation of tBP exceeded the merit due to the promotion of the oxidation of Spiro-OMeTAD. Also, in Example 3 (d), the power generation efficiency after 3 days (d-1) was less than half compared to Comparative Example 1, but the deterioration over time after 41 days (d-2) could be reduced. In Example 3 (d), the deposited Spiro-OMeTAD film was 100 nm thick, and the series resistance component increased, so it is considered that the electrical characteristics of the entire photoelectric conversion element deteriorated. As described above, for a configuration having an additive and a solvent of the additive in the transport material, by forming a second transport layer that functions as a prevention layer for preventing the evaporation of the solvent of the additive on the transport layer, the evaporation of the solvent of the additive can be prevented and the aggregation of the additive can be suppressed. Also, in this example, the film thickness of the deposited Spiro-OMeTAD film as the second transport layer is preferably 10 nm or more and less than 100 nm.

Explanation of Signs

[0051] 1, 2 Photoelectric conversion element 10 Substrate 20 Lower electrode layer 30 Electron transport layer 40 Photoelectric conversion layer 50, 55 Hole transport layer 51 First transport layer 52 Second transport layer 53 Oxidation layer 60 Upper electrode layer 510 Hole transport material 511 Additive 512 Solvent for Additive

Claims

1. A photoelectric conversion element disposed above a photoelectric conversion layer that absorbs received light and generates charges, and includes a transport layer that transports charges, wherein the transport layer includes a first transport layer containing a transport material for transporting charges, an additive for improving conductivity, and a solvent for the additive, and a second transport layer disposed above the first transport layer for preventing evaporation of the solvent. The photoelectric conversion element.

2. The transport material of the second transport layer is the same as the transport material of the first transport layer. The photoelectric conversion element according to Claim 1.

3. The film thickness of the second transport layer is 5 nm or more and less than 100 nm. The photoelectric conversion element according to Claim 1.

4. The additive is Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide), and the solvent is tBP (tertiary butyl pyridine). The photoelectric conversion element according to Claim 1.

5. The second transport layer is formed by a vapor phase process. The photoelectric conversion element according to Claim 1.

6. An oxide layer formed between the first transport layer and the second transport layer is provided. The photoelectric conversion element according to Claim 1.

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