JP7619425B2 - Photoelectric conversion element - Google Patents

Description

The present invention relates to a photoelectric conversion element useful as a solar cell.

Various proposals have been made for photoelectric conversion layers used in solar cell elements (also called photoelectric conversion elements), including silicon semiconductors. Among these, it is known that deterioration of the hole transport layer is a problem in photoelectric conversion elements that use perovskite-type materials (also called PVSK-type materials) as the photoelectric conversion substance. Specifically, this is deterioration of the hole transport layer that occurs after a test in which the element is left in a dark place at 85°C for 240 hours. Deterioration of the hole transport layer naturally leads to a decrease in photoelectric conversion efficiency, so suppressing the decrease in photoelectric conversion efficiency is known to be a challenge.

The most representative hole transport material contained in the hole transport layer is 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) (see, for example, Non-Patent Document 1). However, as described in Non-Patent Document 1, photoelectric conversion elements containing Spiro-OMeTAD as a hole transport material in the hole transport layer are known to have significant problems with their light resistance and heat resistance for various reasons.

Polytriarylamine (PTAA) is also known as another representative hole transport material (see, for example, Patent Document 1 and Non-Patent Document 2). Patent Document 1 discloses that solar cells using PTAA as a hole transport material have relatively good durability. Non-Patent Document 2 discloses that a hole transport layer having PTAA is used with a thickness of only 30-50 nm. The reason for this is that although this hole transport material contains an additive such as lithium bis(trifluoromethanesulfonyl)imide in addition to PTAA, the resistivity of this hole transport layer is very high. Therefore, when the film thickness is greater than 50 nm, the resistance of the hole transport layer significantly affects the characteristics of the solar cell element, and the photoelectric conversion efficiency decreases. It is not easy to easily and accurately obtain such a thin film using a coating process that is usually used in the case of industrially continuously producing large-area modules such as solar cell panels.

For this reason, when the thickness of the hole transport material exceeds 100 nm, an oxidizing agent called a dopant is generally added, and the dopant extracts electrons from the organic semiconductor, which is the main material, to give a positive charge, thereby increasing the electrical conductivity. In the case of PTAA, for example, as shown in Non-Patent Document 3, it is known to use 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB) as a dopant. However, sufficient knowledge has not been obtained so far about the heat resistance of solar cell elements having a hole transport layer using both PTAA and TPFB.

JP 2017-126731 A

Chemical Review, 2019, 1193036-3103 Science Vol. 354, Issue 6309, pp. 206-209 Sci. Adv. 2017;3: e1701293

The objective of the present invention is to suppress the deterioration of the hole transport layer that occurs after a durability test and to suppress the decrease in the photoelectric conversion efficiency of the photoelectric conversion element.

As a result of extensive investigations aimed at solving the above problems, the present inventors have found that there are two types of degradation patterns of hole transport materials.
That is, when the above-mentioned Spiro-OMeTAD is subjected to a durability test in a dark place at 60° C. and 90 RH as an environmental test, the degradation occurs as shown in Fig. 1. This type of degradation is "charge recombination enhancement type" degradation in which charge recombination occurs at the interface between the photoelectric conversion layer containing the PVSK type material and the hole transport layer.
The "charge recombination enhancement type" refers to a type of deterioration in which the fill factor is initially 0.65 or more, but as a result of the above-mentioned durability test (60°C, 90RH), the fill factor falls to less than 90% of the initial value within 240 hours, and during the same period the short-circuit current and open-circuit voltage remain at least 90% of their initial values, and the series resistance remains less than 10 Ωcm ^2. In this specification, hole transport materials that deteriorate in this manner are referred to as "charge recombination enhancement type" hole transport materials.

Yet another degradation is shown in FIG.
Another type of deterioration shown in Fig. 2 is a case where PTAA is used as the hole transport material, and although there is no problem at the interface between the photoelectric conversion layer containing a PVSK-type material and the hole transport layer, the resistance at the interface between the electrode and the hole transport layer is high, which is "charge injection resistance increase type" deterioration. This type of deterioration is deterioration in which the series resistance is initially less than 10 ^Ωcm2 , but as a result of the above-mentioned deterioration test (60°C, 90RH), it increases to 10 Ωcm2 or ^more within 240 hours, and during the same period, the short-circuit current and open circuit voltage maintain 90% or more of their initial values. In this specification, a hole transport material that deteriorates in this way is called a "charge injection resistance increase type" hole transport material.

Because there are two patterns of deterioration of the hole transport material as described above, it is conceivable to form a two-layer structure by placing a "charge injection resistance increasing type" hole transport material on the photoelectric conversion layer side of the hole transport layer containing a PVSK type material, and a "charge recombination increasing type" hole transport material on the electrode side of the hole transport layer. However, surprisingly, according to the inventors' investigations, by forming the hole transport layer into a mixed layer containing these two types of hole transport material, it was found that the deterioration of both was suppressed rather than intermediate performance between the two, and this led to the present invention. In other words, the gist of the present invention includes the following.

[1] A semiconductor device comprising a pair of electrodes, an electron transport layer, a photoelectric conversion layer, and a hole transport layer sandwiched between the electrodes,
The hole transport layer comprises a charge injection resistance increasing type hole transport material and a charge recombination increasing type hole transport material.
[2] The photoelectric conversion element according to [1], wherein the photoelectric conversion layer contains a PVSK type photoelectric conversion material.
[3] The photoelectric conversion element according to [1] or [2], wherein the hole transport layer contains a charge injection resistance increasing type hole transport material and a charge recombination increasing type hole transport material in a mass ratio of 1:9 to 9:1.
[4] The photoelectric conversion element according to any one of [1] to [3], wherein the hole transport layer has a mass content of a charge recombination enhancing hole transport material that is greater than the mass content of a charge injection resistance increasing hole transport material. [5] The photoelectric conversion element according to any one of [1] to [4], wherein the ionization energy of at least one of the hole transport materials is greater than 4.5 electron volts.
[6] The photoelectric conversion element according to any one of [1] to [5], wherein the hole transport material contained in the hole transport layer includes an organic compound.
[7] The photoelectric conversion element according to any one of [1] to [6], wherein the charge injection resistance increasing type hole transport material and the charge recombination increasing type hole transport material are both organic compounds.
[8] The photoelectric conversion element according to any one of [1] to [7], wherein the hole transport layer contains an additive. [9] The photoelectric conversion element according to [8], wherein the additive causes a charge transfer reaction with at least one hole transport material before or after the hole transport layer is formed.
[10] The photoelectric conversion element according to any one of [1] to [9], wherein the hole transport layer is formed by a coating method.
[11] The photoelectric conversion element according to any one of [1] to [10], wherein the hole transport layer contains a hole transport material having an aromatic amine skeleton.
[12] The photoelectric conversion element according to any one of [1] to [11], wherein the hole transport layer contains a hole transport material having a fluorene skeleton.
[13] The photoelectric conversion element according to [12], wherein the fluorene skeleton is a part of a spiro skeleton.
[14] The photoelectric conversion element according to any one of [8] to [13], wherein the additive contains a boron compound.
[15] The photoelectric conversion element according to [14], wherein the boron compound is tetrakis(pentafluorophenyl)borate.
[16] The photoelectric conversion element according to any one of claims 1 to 15, wherein the hole transport layer contains polytriarylamine and Spiro-OMeTAD.

The present invention provides a photoelectric conversion element that can suppress the deterioration of the hole transport layer that occurs after a durability test and suppress the decrease in photoelectric conversion efficiency.

1 is a graph showing the results of a durability test conducted in a dark place at 60° C. and 90 RH on a photoelectric conversion element having a hole transport layer containing Spiro-OMeTAD. 1 is a graph showing the results of a durability test carried out in a dark place at 60° C. and 90 RH on a photoelectric conversion element having a hole transport layer containing PTAA. FIG. 1 is a diagram illustrating a typical example of a photoelectric conversion element according to an embodiment of the present invention.

The present invention will be described in detail below. However, the following description of the components is an example (representative example) of an embodiment of the present invention, and the present invention is not limited to these contents, and can be practiced in various modifications within the scope of the gist.

The photoelectric conversion element according to one embodiment of the present invention includes a pair of electrodes, an electron transport layer, a photoelectric conversion layer, and a hole transport layer sandwiched between the electrodes. The hole transport layer includes a charge injection resistance increasing type hole transport material and a charge recombination increasing type hole transport material. The configuration of the photoelectric conversion element according to this embodiment will be described below with reference to the drawings.

Figure 3 is a cross-sectional view showing a schematic diagram of one embodiment of a photoelectric conversion element. The photoelectric conversion element shown in Figure 3 has a structure of a photoelectric conversion element used in a general thin-film solar cell, but the photoelectric conversion element according to this embodiment is not limited to the structure shown in Figure 3. In the photoelectric conversion element 100 shown in Figure 3, a photoelectric conversion layer 103 is located between a pair of electrodes consisting of a lower electrode 101 and an upper electrode 105. In the photoelectric conversion element 100, an electron transport layer 102 is disposed between the lower electrode 101 and the photoelectric conversion layer 103, and a hole transport layer 104 is disposed between the upper electrode 105 and the photoelectric conversion layer 103. Furthermore, as shown in Figure 3, the photoelectric conversion element 100 may have a substrate 106, and may have other layers such as an insulator layer and a work function tuning layer.

The photoelectric conversion layer 103 is a layer where photoelectric conversion takes place. When the photoelectric conversion element 100 receives light, the light is absorbed by the photoelectric conversion layer 103 and carriers are generated. The generated carriers are extracted from the lower electrode 101 and the upper electrode 105.

In this embodiment, the photoelectric conversion layer 103 may contain an organic-inorganic hybrid semiconductor material. An organic-inorganic hybrid semiconductor material is a material in which an organic component and an inorganic component are combined at the molecular level or nano level, and which exhibits semiconductor properties.

In this embodiment, the organic-inorganic hybrid semiconductor material may be a compound having a perovskite structure (hereinafter, sometimes referred to as a PVSK-type photoelectric conversion material). The PVSK-type photoelectric conversion material refers to a semiconductor compound having a perovskite structure. There are no particular limitations on the PVSK-type photoelectric conversion material, but for example, the PVSK-type photoelectric conversion material described in Galasso et al. Structure and
The PVSK type photoelectric conversion material can be selected from those listed in "Properties of Inorganic Solids, Chapter 7 - Perovskite type and related structures". For example, the PVSK type photoelectric conversion material can be an AMX ₃ type represented by the general formula AMX ₃ or an A ₂ MX ₄ type represented by the general formula A ₂ MX _4. Here, M represents a divalent cation, A represents a monovalent cation, and X represents a monovalent anion.

There is no particular restriction on the monovalent cation A, but those described in the above book by Galasso can be used. More specific examples include cations containing elements of Groups 1 and 13 to 16 of the periodic table. Among these, cesium ions, rubidium ions, ammonium ions which may have a substituent, or phosphonium ions which may have a substituent are preferred. Examples of ammonium ions which may have a substituent include primary ammonium ions and secondary ammonium ions. There are no particular restrictions on the substituent. Specific examples of ammonium ions which may have a substituent include alkylammonium ions and arylammonium ions. In particular, in order to avoid steric hindrance, monoalkylammonium ions which form a three-dimensional crystal structure are preferred, and from the viewpoint of improving stability, it is preferred to use alkylammonium ions substituted with one or more fluorine groups. In addition, a combination of two or more types of cations can be used as cation A.

Specific examples of monovalent cation A include methylammonium ion, methylammonium monofluoride ion, methylammonium difluoride ion, methylammonium trifluoride ion, ethylammonium ion, isopropylammonium ion, n-propylammonium ion, isobutylammonium ion, n-butylammonium ion, t-butylammonium ion, dimethylammonium ion, diethylammonium ion, phenylammonium ion, benzylammonium ion, phenethylammonium ion, guanidinium ion, formamidinium ion, acetamidinium ion, and imidazolium ion.

There is no particular limitation on the divalent cation M, but it is preferably a divalent metal cation or semimetal cation. Specific examples include cations of group 14 elements of the periodic table, and more specific examples include lead cation (Pb ²⁺ ), tin cation (Sn ²⁺ ), and germanium cation (Ge ²⁺ ). In addition, a combination of two or more types of cations can be used as the cation M. In addition, from the viewpoint of obtaining a stable photoelectric conversion element, it is particularly preferable to use lead cations or two or more types of cations including lead cations.

Examples of monovalent anions X include halide ions, acetate ions, nitrate ions, sulfate ions, borate ions, acetylacetonate ions, carbonate ions, citrate ions, sulfur ions, tellurium ions, thiocyanate ions, titanate ions, zirconate ions, 2,4-pentanedionate ions, and silicofluorides. In order to adjust the band gap, X may be one type of anion or a combination of two or more types of anions. In one embodiment, X may be a halide ion, or a combination of a halide ion and another anion. Examples of halide ions X include chloride ions, bromide ions, and iodide ions. It is preferable to use iodide ions from the viewpoint of not widening the band gap of the semiconductor too much.

The photoelectric conversion layer 103 contains at least a halide-based PVSK-type photoelectric conversion material in which X is a halide ion or a combination of a halide ion and another anion, and may contain other perovskite semiconductor compounds. For example, the photoelectric conversion layer 103 may contain a PVSK-type photoelectric conversion material in which at least one of A, B, and X is different. The photoelectric conversion layer 103 may also have a layered structure formed of multiple layers containing different materials or having different components.

Specific examples of halide-based PVSK-type photoelectric conversion materials include CH ₃ NH ₃ PbI ₃ , CH ₃ NH ₃ PbBr ₃ , CH ₃ NH ₃ PbCl ₃ , CH ₃ NH ₃ SnI ₃ , CH ₃ NH ₃ SnBr ₃ , CH ₃ NH ₃ SnCl ₃ , CH ₃ NH ₃ PbI _(3-x) Cl _x , CH ₃ NH ₃ PbI _(3-x) Br _x , CH ₃ NH ₃ PbBr _(3-x) Cl _x , CH ₃ NH ₃ Pb _(1-y) Sn _y I ₃ , CH ₃ NH ₃ Pb _(1-y) Sn _y Br ₃ , and CH _3NH3Pb ( _1-y) Sn _y Cl ₃ , _CH3NH3Pb ( _{1 -y)} Sn _y I _(3-x) Cl _x , _CH3NH3Pb ( _{1 -y)} Sn _y I( _3-x) Br _x , and _{CH3NH3Pb (1-y)} Sn _y Br _{(3-x) Cl x} , as well _as the above compounds in which _CFH2NH3 , _CF2HNH3 , or _CF3NH3 is used instead of _CH3NH3 . Note that _x is an arbitrary value of 0 or more and ₃ or less, and y is _an arbitrary value of 0 or more _and 1 or less.

The amount of the PVSK-type photoelectric conversion material contained in the photoelectric conversion layer 103 is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more, so that good semiconductor characteristics can be obtained. There is no particular upper limit. The photoelectric conversion layer 103 may also contain additives in addition to the PVSK-type photoelectric conversion material. Examples of additives include inorganic compounds such as halides, oxides, or inorganic salts such as sulfides, sulfates, nitrates, or ammonium salts, or organic compounds.

There is no particular limit to the thickness of the photoelectric conversion layer 103. In terms of being able to absorb more light, the thickness of the photoelectric conversion layer 103 is 10 nm or more in one embodiment, 50 nm or more in another embodiment, 100 nm or more in yet another embodiment, and 120 nm or more in yet another embodiment. On the other hand, in terms of lowering the series resistance or increasing the charge extraction efficiency, the thickness of the photoelectric conversion layer 103 is 1500 nm or less in one embodiment, 1000 nm or less in another embodiment, and 600 nm or less in yet another embodiment.

The method for forming the photoelectric conversion layer 103 is not particularly limited, and any method can be used. Specific examples include a coating method and a vapor deposition method (or a co-deposition method). A coating method can be used because the photoelectric conversion layer 103 can be easily formed. For example, a method can be used in which a coating liquid containing a PVSK-type photoelectric conversion material or a precursor thereof is applied, and the photoelectric conversion layer 103 is formed by heating and drying as necessary. In addition, after applying such a coating liquid, the PVSK-type photoelectric conversion material can be precipitated by further applying a solvent in which the PVSK-type photoelectric conversion material has low solubility.

The precursor of the PVSK type photoelectric conversion material refers to a material that can be converted into a PVSK type photoelectric conversion material after the coating liquid is applied. As a specific example, a PVSK type photoelectric conversion material precursor that can be converted into a PVSK type photoelectric conversion material by heating can be used. For example, a coating liquid can be prepared by mixing a compound represented by the general formula AX, a compound represented by the general formula _MX2 , and a solvent, and heating and stirring the mixture. By applying this coating liquid and drying it by heating, a photoelectric conversion layer 103 containing a PVSK type photoelectric conversion material represented by the general formula _AMX3 can be prepared. The solvent is not particularly limited as long as it dissolves the PVSK type photoelectric conversion material and the additives, and examples thereof include organic solvents such as N,N-dimethylformamide.

Any method can be used to apply the coating solution, including, for example, spin coating, inkjet coating, doctor blade coating, drop casting, reverse roll coating, gravure coating, kiss coating, roll brush coating, spray coating, air knife coating, wire barber coating, pipe doctor coating, impregnation/coating, and curtain coating.

[1.2 Electrodes]
The electrode has a function of collecting holes and electrons generated by light absorption in the photoelectric conversion layer 103. The photoelectric conversion element 100 according to one embodiment has a pair of electrodes, one of which is called an upper electrode and the other is called a lower electrode. When the photoelectric conversion element 100 has a substrate or is provided on a substrate, the electrode closer to the substrate can be called a lower electrode, and the electrode farther from the substrate can be called an upper electrode. In addition, a transparent electrode can be called a lower electrode, and an electrode less transparent than the lower electrode can be called an upper electrode. The photoelectric conversion element 100 shown in FIG. 3 has a lower electrode 101 and an upper electrode 105.

The pair of electrodes can be an anode suitable for collecting holes and a cathode suitable for collecting electrons. In this case, the photoelectric conversion element 100 can have a forward configuration in which the lower electrode 101 is an anode and the upper electrode 105 is a cathode, or a reverse configuration in which the lower electrode 101 is a cathode and the upper electrode 105 is an anode.

Either one of the pair of electrodes needs to be translucent, and both may be translucent. Translucent means that 40% or more of sunlight passes through. In addition, it is preferable for the sunlight transmittance of the transparent electrode to be 70% or more, so that more light passes through the transparent electrode and reaches the active layer 103. The light transmittance can be measured with a spectrophotometer (e.g., Hitachi High-Tech U-4100).

When the lower electrode and/or the upper electrode are transparent electrodes, the lower electrode and/or the upper electrode may be formed as a single layer of a transparent conductive layer or a metal layer, or may be formed by laminating a transparent conductive layer and a metal layer, as long as it has the above-mentioned visible light transmittance. However, if the transparent electrode is formed only from a transparent conductive layer, the resistance is high and the conversion efficiency may decrease because it tends not to show good conductivity. In addition, if the transparent electrode is formed only from a thin metal layer, the metal layer is easily corroded and the photoelectric conversion element tends to deteriorate over time, so that the electrode to be the transparent electrode is preferably formed by laminating a transparent conductive layer and a metal layer.

The material used for the transparent conductive layer is not particularly limited, but may be tin-doped indium oxide (ITO), zinc-doped indium oxide (IZO), tungsten-doped indium oxide (IWO), zinc-aluminum oxide (AZO), indium oxide (In ₂ O ₃ ), etc. Among these, it is preferable to use amorphous oxides such as tin-doped indium oxide (ITO), zinc-doped indium oxide (IZO), tungsten-doped indium oxide (IWO), and zinc-tin composite oxide (ZTO).

The transparent conductive layer preferably has a sheet resistance of 100 Ω/□ or less, more preferably 50 Ω/□ or less, and more preferably 0.1 Ω/□ or more.

There are no particular limitations on the material of the metal layer, and examples of the material include metals such as gold, platinum, silver, aluminum, nickel, titanium, magnesium, calcium, barium, sodium, chromium, copper, and cobalt, or alloys thereof. Among these, the material forming the metal layer is preferably silver or a silver alloy, which exhibits high electrical conductivity and has a high visible light transmittance in a thin film. Examples of silver alloys include silver-gold alloys, silver-copper alloys, silver-palladium alloys, silver-copper-palladium alloys, and silver-platinum alloys, which are less susceptible to sulfurization or chlorination and improve the stability of the thin film.

There are no particular limitations on the thickness of the metal layer, so long as it can maintain a visible light transmittance of 70% or more as a transparent electrode, but in order to obtain good electrical conductivity, it is preferably 1 nm or more, and more preferably 5 nm or more, while in order to prevent a decrease in the amount of light incident on the active layer due to a decrease in light transmittance, it is preferably 15 nm or less, and more preferably 10 nm or less.

As described above, when one electrode of a pair of electrodes is a transparent electrode, the other electrode does not necessarily have to be a transparent electrode and may be a non-transparent electrode. When a non-transparent electrode is used, there are no particular restrictions, but for example, a non-transparent electrode can be formed by forming a thick metal layer as described above. Note that when both the lower electrode and the upper electrode are transparent electrodes, it is preferable that both the lower electrode and the upper electrode have a laminated structure of a metal layer and a transparent conductive layer.

There are no particular limitations on the overall thickness of the lower electrode and the upper electrode, and it may be selected as desired, taking into consideration the optical and electrical properties. In particular, in order to suppress sheet resistance, the film thickness of each of the lower electrode and the upper electrode is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 50 nm or more, while in order to maintain high transmittance, it is preferably 10 μm or less, more preferably 1 μm or less, and even more preferably 500 nm or less.

There are no particular limitations on the method for forming the lower electrode and the upper electrode, and they can be formed by a known method according to the material used. Examples of the film formation step in coating include vacuum methods such as deposition and sputtering, or wet methods in which an ink containing nanoparticles or precursors is applied. The lower electrode and the upper electrode may be subjected to surface treatment to improve electrical properties, wetting properties, etc.

[1.3 Hole transport layer]
The hole transport layer 104 is a layer located between the photoelectric conversion layer 103 and the electrode 105. The hole transport layer 104 can be used, for example, to improve the efficiency of carrier movement from the photoelectric conversion layer 103 to the upper electrode 105.

In this embodiment, the hole transport layer includes a charge injection resistance increasing type hole transport material and a charge recombination increasing type hole transport material.
The charge injection resistance increasing type hole transport material is a hole transport material that causes the following degradation:
This degradation occurs when the series resistance is initially less than 10 Ω ^cm2 , but as a result of a degradation test at 60°C and 90 RH, it increases to 10 Ω cm2 or ^more within 240 hours, and during the same period the short-circuit current and open-circuit voltage maintain 90% or more of their initial values.

Charge recombination enhanced hole transport materials are hole transport materials that exhibit the following degradation:
This deterioration is such that the fill factor is initially 0.65 or more, but as a result of a durability test at 60°C and 90RH, the fill factor falls to less than 90% of the initial value within 240 hours, and during the same period the short-circuit current and open-circuit voltage remain at least 90% of their initial values, and the series resistance remains less than 10 Ω ^cm2 .

The charge injection resistance increasing type hole transport material is not particularly limited as long as it causes the above-mentioned deterioration, and may be an organic compound, preferably an organic compound having an aromatic amine skeleton, and typically includes polytriarylamine (PTAA).
The charge recombination enhancing hole transport material is not particularly limited as long as it causes the above-mentioned deterioration, and may be an organic compound, preferably an organic compound having a fluorene skeleton, and more preferably the fluorene skeleton is a part of a spiro skeleton, and a typical example is 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD).

The content ratio (mass ratio) of the charge injection resistance increasing type hole transport material to the charge recombination increasing type hole transport material in the hole transport layer is preferably within a range of 9:1 to 1:9, more preferably within a range of 8:2 to 2:8, and the content by mass of the charge recombination increasing type hole transport material is preferably greater than that of the charge injection resistance increasing type hole transport material.
The content of the hole transport material in the hole transport layer, in terms of the total of the charge injection resistance increasing type hole transport material and the charge recombination increasing type hole transport material, is usually 30% by mass or more, preferably 50% by mass or more, and usually 100% by mass or less. By setting the content of the hole transport material within the above range, the conductivity of the entire hole transport layer can be kept good, and the initial series resistance and FF can be improved.
The ionization energy of the hole transport material contained in the hole transport layer is preferably 4.5 electron volts or more.

The hole transport layer may contain additives in addition to the hole transport material described above, such as boron compounds such as tetrakis(pentafluorophenyl)borate, molybdenum compounds such as tris[1-(methoxycarbonyl)-2-(trifluoromethyl)-ethane-1,2-dithiolene]molybdenum, and organic compounds having a tetracyanoquinodimethane skeleton such as 2,3,4,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane.
The additive preferably undergoes a charge transfer reaction with at least one hole transport material before or after the formation of the hole transport layer.
The amount of the additive added in the hole transport layer is usually 0% by mass or more, preferably 0.01% by mass or more, and usually 70% by mass or less, preferably 50% by mass or less.
The additive is preferably used when the thickness of the hole transport layer is thick, and is not particularly limited, but is preferably added when the thickness of the hole transport layer is 50 nm or more, particularly 100 nm or more. When the thickness of the hole transport layer is 100 nm or more, the conductivity of the film tends to be insufficient if no additive is contained, so the conductivity of the film can be improved by containing the additive.

The hole transport layer is preferably formed by a coating method similar to the photoelectric conversion layer, the details of which are as described above in the description of the photoelectric conversion layer.
The thickness of the hole transport layer is not particularly limited, but is usually 10 nm or more, preferably 20 nm or more, and is usually 1000 nm or less, preferably 50 nm or less.

[1.4 Electron transport layer]
The electron transport layer 102 is a layer located between the photoelectric conversion layer 103 and the electrode 101. The electron transport layer 102 is used, for example, to improve the efficiency of carrier movement from the photoelectric conversion layer 103 to the lower electrode 101. It is possible.

In this embodiment, there are no particular limitations on the constituent members of the electron transport layer 102 and the manufacturing method thereof, and well-known techniques can be used. For example, the members and manufacturing methods described in known documents such as WO 2013/171517, WO 2013/180230, or JP 2012-191194 can be used.

[1.4 Substrate]
The photoelectric conversion element 100 usually has a substrate 106 that serves as a support. The material of the substrate 106 is not particularly limited as long as it does not significantly impair the effects of the present invention, and for example, materials described in publicly known documents such as WO 2013/171517, WO 2013/180230, or JP 2012-191194 A can be used.

[1.5 Other Layers]
The photoelectric conversion element 100 may have other layers. For example, the photoelectric conversion element 100 may have a work function tuning layer for adjusting the work function of the electrode between the lower electrode 101 and the electron transport layer 102, or between the upper electrode 105 and the hole transport layer 104. The photoelectric conversion element 100 may also have a thin insulator layer between the lower electrode 101 and the photoelectric conversion layer 103, or between the upper electrode 105 and the photoelectric conversion layer 103, for suppressing moisture and the like from reaching the photoelectric conversion layer 103. In addition, the photoelectric conversion element 100 may be further sealed to improve durability. For example, the photoelectric conversion element 100 can be sealed by further laminating a sealing plate on the upper electrode 105 and fixing the substrate 106 and the sealing plate with an adhesive.

[2. Method for producing photoelectric conversion element]
According to the above-mentioned method, each layer constituting the photoelectric conversion element 100 is formed, thereby manufacturing the photoelectric conversion element 100. There is no particular limitation on the method for forming each layer constituting the photoelectric conversion element 100, and the layers can be formed by a sheet-to-sheet method or a roll-to-roll method.

The roll-to-roll method is a method in which a flexible substrate wound in a roll is unwound and processed while being transported intermittently or continuously until it is wound up by a take-up roll. The roll-to-roll method makes it possible to process long substrates on the order of kilometers in length all at once, making it more suitable for mass production than the sheet-to-sheet method. However, when attempting to deposit each layer using the roll-to-roll method, due to its structure, the film may be scratched or partially peeled off due to contact between the deposition surface and the roll.

After laminating the upper electrode 105, the photoelectric conversion element 100 can be heated at a temperature range of 50°C or higher in one embodiment, 80°C or higher in another embodiment, 300°C or lower in one embodiment, 280°C or lower in another embodiment, and 250°C or lower in yet another embodiment (this process may be referred to as an annealing process). By performing the annealing process at a temperature of 50°C or higher, the adhesion between each layer of the photoelectric conversion element 100, for example, between the electron transport layer 102 and the lower electrode 101, between the electron transport layer 102 and the photoelectric conversion layer 103, etc., can be improved. By improving the adhesion between each layer, the thermal stability and durability of the photoelectric conversion element can be improved. Setting the temperature of the annealing process to 300°C or lower reduces the possibility of thermal decomposition of the organic compound contained in the photoelectric conversion element 100. In the annealing process, stepwise heating using different temperatures within the above temperature range may be performed.

The heating time is 1 minute or more in one embodiment, 3 minutes or more in another embodiment, and 180 minutes or less in one embodiment, and 60 minutes or less in another embodiment, in order to improve adhesion while suppressing thermal decomposition. The annealing process can be terminated when the open circuit voltage, short circuit current, and fill factor, which are parameters of the solar cell performance, reach a certain value. In addition, the annealing process can be performed under normal pressure and in an inert gas atmosphere in order to prevent thermal oxidation of the constituent materials. As a heating method, the photoelectric conversion element may be placed on a heat source such as a hot plate, or the photoelectric conversion element may be placed in a heating atmosphere such as an oven. In addition, heating may be performed in a batch or continuous manner.

[3. Photoelectric Conversion Characteristics]
The photoelectric conversion characteristics of the photoelectric conversion element 100 can be determined for each light source used as follows. Light is irradiated onto the photoelectric conversion element 100, and the current-voltage characteristics are measured. From the obtained current-voltage curve, photoelectric conversion characteristics such as photoelectric conversion efficiency (PCE), short-circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), series resistance, and shunt resistance can be determined.

The light source used to determine the photoelectric conversion characteristics can be sunlight or an artificial light source. Artificial light sources include halogen lamps, xenon lamps, metal halide lamps, fluorescent lamps, white LED lamps, warm white LED lamps, mercury lamps, sodium lamps, and combinations of these lamps.

Generally, a xenon lamp or a metal halide lamp is used as a light source for artificial sunlight, and light with conditions approximating the spectrum of AM1.5G is used as a measurement light source for obtaining photoelectric conversion characteristics at an irradiation intensity of 100 mW/cm ^2. Although there is no particular limitation as a low-illuminance light source, a white LED lamp is used as a measurement light source for obtaining photoelectric conversion characteristics at an illuminance of 1 to 5000 Lx.

The photoelectric conversion efficiency of the photoelectric conversion element 100 is not particularly limited, but in one embodiment it is 3% or more, in another embodiment it is 5% or more, and in yet another embodiment it is 8% or more. On the other hand, there is no particular limit on the upper limit, and the higher the better. In addition, the fill factor of the photoelectric conversion element 100 is not particularly limited, but in one embodiment it is 0.6 or more, in another embodiment it is 0.7 or more, and in yet another embodiment it is 0.8 or more. On the other hand, there is no particular limit on the upper limit, and the higher the better.

The photoelectric conversion element according to this embodiment has a maximum output (Pmax) retention rate of 60% or more, 80% or more in one embodiment, 90% or more in another embodiment, and 95% or more in still another embodiment after being placed in a test environment at a temperature of 85° C. for one day. For example, the Pmax retention rate can be calculated based on the initial Pmax immediately after the photoelectric conversion element is produced and the Pmax after the photoelectric conversion element is placed in the test environment. The Pmax retention rate can also be measured after sealing the photoelectric conversion element as described above. Here, the Pmax retention rate can be calculated as follows based on the Pmax before and after being placed in the test environment.
Pmax maintenance rate (%) = ((Pmax after being placed in the test environment)/(Pmax before being placed in the test environment)) x 100

Furthermore, after being placed in a test environment at 85°C for a long period of time (e.g., 60 hours or more), the Pmax retention rate is 40% or more, in one embodiment 60% or more, in another embodiment 80% or more, in yet another embodiment 90% or more, and in yet another embodiment 95% or more.

The photoelectric conversion element 100 can be suitably used as a solar cell, particularly as a solar cell element for a thin-film solar cell. When used as a thin-film solar cell, a known configuration can be applied.

The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to the following examples.

[Preparation of Coating Solution]
(Preparation of Coating Solution for Electron Transport Layer)
Ultrapure water was added to a 15% aqueous dispersion of tin (IV) oxide (manufactured by Alfa Aesar) to prepare a 7.5% aqueous dispersion of tin oxide.

(Preparation of Coating Solution for Photoelectric Conversion Layer)
Lead iodide (II) was weighed into a vial and introduced into a glove box. N,N-dimethylformamide was added as a solvent so that the concentration of lead iodide (II) was 1.3 mol/L, and then the mixture was heated and stirred at 100° C. for 1 hour to prepare coating solution 1 for photoelectric conversion layer.

Next, formamidine hydroiodide (FAI), methylamine hydrobromide (MABr), and methylamine hydrochloride (MACl) were weighed out in a mass ratio of 10:1:1.5 into another vial and introduced into the glove box. Isopropyl alcohol was added as a solvent to prepare coating solution 2 for photoelectric conversion layer, in which the total concentration of FAI, MABr, and MACl was 0.49 mol/L.

(Preparation of Coating Solution 1 for Hole Transport Layer)
15 mg of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA, manufactured by our company), 5 mg of [2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene] (Spiro-OMeTAD, manufactured by Aldrich), and 2.25 mg of 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB, manufactured by TCI) were weighed into a vial and introduced into a glove box. 0.5 mL of orthodichlorobenzene was added thereto as a solvent. Next, the resulting mixture was heated and stirred at 150°C for 1 hour to prepare hole transport layer coating solution 1.

[Example 1]
A glass substrate (manufactured by Geomatec Co., Ltd.) having a patterned indium tin oxide (ITO) transparent conductive film was subjected to ultrasonic cleaning using ultrapure water, drying by nitrogen blowing, and UV-ozone treatment.

Next, the coating solution for the electron transport layer prepared as described above was spin-coated onto the substrate at room temperature at a speed of 2000 rpm to form an electron transport layer with a thickness of approximately 35 nm. The substrate was then heated on a hot plate at 150°C for 10 minutes.

Next, the substrate was introduced into a glove box, and 150 μL of photoelectric conversion layer coating solution 1 heated to 100°C was dropped onto the electron transport layer and spin-coated at a speed of 2000 rpm. The substrate was then heated and annealed on a hot plate at 100°C for 10 minutes to form a lead iodide layer. After the substrate returned to room temperature, photoelectric conversion layer coating solution 2 (120 μL) was spin-coated onto the lead iodide layer at a speed of 2000 rpm and heated at 150°C for 20 minutes to form an organic-inorganic perovskite active layer (thickness 650 nm).

Next, after the substrate was returned to room temperature, the hole transport layer coating solution (200 μL) was applied to the photoelectric conversion layer in 100 μL.
The solution was spin-coated at a speed of 1,000 rpm and then heated on a hot plate at 90° C. for 5 minutes to form a hole transport layer (170 nm).

Next, on the hole transport layer, a film of IZO was formed to a thickness of about 30 nm on the metal layer by sputtering, forming a metal oxide layer. Furthermore, on the metal oxide layer, silver was deposited to a thickness of about 100 nm by resistance heating vacuum deposition, forming a metal layer, which served as the upper electrode. In this manner, a photoelectric conversion element was produced.

[Example 2]
(Preparation of Coating Solution 2 for Hole Transport Layer)
10 mg of PTAA (manufactured by our company), 10 mg of Spiro-OMeTAD (manufactured by Aldrich), and 2.25 mg of TPFB (manufactured by TCI) were weighed into a vial and introduced into a glove box. 0.5 mL of orthodichlorobenzene was added as a solvent. Next, the resulting mixture was heated and stirred at 150° C. for 1 hour to prepare coating solution 2 for hole transport layer.

(Creation of photoelectric conversion element)
A photoelectric conversion element was prepared in the same manner as in Example 1, except that the hole transport layer was formed using the above-mentioned coating liquid 2 for hole transport layer.

[Example 3]
(Preparation of Coating Solution 3 for Hole Transport Layer)
5 mg of PTAA (manufactured by our company), 15 mg of Spiro-OMeTAD (manufactured by Aldrich), and 2.25 mg of TPFB (manufactured by TCI) were weighed into a vial and introduced into a glove box. 0.5 mL of orthodichlorobenzene was added as a solvent thereto. Next, the resulting mixture was heated and stirred at 150° C. for 1 hour to prepare coating solution 3 for hole transport layer.

(Creation of photoelectric conversion element)
A photoelectric conversion element was prepared in the same manner as in Example 1, except that the hole transport layer was formed using the above-mentioned Coating Liquid 3 for Hole Transport Layer.

[Comparative Example 1]
(Preparation of Coating Solution 4 for Hole Transport Layer)
64 mg of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and 7.2 mg of 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB, manufactured by TCI) were weighed into a vial and introduced into a glove box. 1.6 mL of orthodichlorobenzene was added as a solvent to the vial. Next, the resulting mixture was heated and stirred at 150°C for 1 hour to prepare hole transport layer coating solution 4.

(Creation of photoelectric conversion element)
A photoelectric conversion element was prepared in the same manner as in Example 1, except that the hole transport layer was formed using Coating Liquid 4 for Hole Transport Layer.

[Comparative Example 2]
(Preparation of Coating Solution 5 for Hole Transport Layer)
20 mg of [2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene] (Spiro-OMeTAD, manufactured by Aldrich) and 2 mg of TPFB were weighed into a vial and introduced into a glove box.
0.5 mL of orthodichlorobenzene was added as a solvent to the mixture, and the mixture was heated and stirred at 150° C. for 1 hour to prepare a coating solution 5 for a hole transport layer.

(Creation of photoelectric conversion element)
A photoelectric conversion element was prepared in the same manner as in Example 1, except that the hole transport layer was formed using Coating Liquid 5 for Hole Transport Layer.

[Evaluation of photoelectric conversion element]
A 1 mm square metal mask was attached to the photoelectric conversion elements obtained in Examples 1 to 3 and Comparative Examples 1 and 2, and the current-voltage characteristics between the ITO electrode and the upper electrode were measured. A source meter (Keithley, Model 2400) was used for the measurement, and a solar simulator with an air mass (AM) of 1.5 G and an irradiance of 100 mW/cm ² was used as the irradiation light source. From the measurement results, the open circuit voltage Voc (V), short circuit current density Jsc (mA/cm ² ), form factor FF, and photoelectric conversion efficiency PCE (%) were calculated. These values calculated based on the measurement results immediately after the photoelectric conversion elements were produced are shown in Table 1.

Here, the open circuit voltage Voc is the voltage value when the current value is 0 (mA/cm ² ), and the short circuit current density Jsc is the current density when the voltage value is 0 (V). The form factor FF is a factor representing the internal resistance, and is expressed by the following equation when the maximum output is Pmax.
FF=Pmax/(Voc×Jsc)
Furthermore, the photoelectric conversion efficiency PCE is given by the following equation, where Pin is the incident energy.
PCE = (Pmax/Pin)×100
= (Voc×Jsc×FF/Pin)×100

The photoelectric conversion elements of Comparative Examples 1 and 2 were sealed by bonding the glass with the hollowed-out center to the glass substrate of the photoelectric conversion element with a sealant (photocurable resin) and left to stand in a dark place at 60°C for 240 hours. The current-voltage characteristics were measured in the same manner to calculate the photoelectric conversion efficiency PCE, and the photoelectric conversion efficiency retention rate was calculated from the photoelectric conversion efficiency before and after the test. The photoelectric conversion efficiency retention rate is shown in Table 2.

The elements of Examples 1 to 3 and Comparative Example 1 were left in a dark place at 85°C for 240 hours, and the current-voltage characteristics were measured in the same manner to calculate the photoelectric conversion efficiency PCE. The photoelectric conversion efficiency retention rate was calculated from the photoelectric conversion efficiency before and after the test. The photoelectric conversion efficiency retention rate is shown in Table 3.

As shown in Table 1, there was no significant difference in the initial conversion efficiency between Examples 1 to 3 and Comparative Examples 1 and 2. Also, as shown in Table 2, it can be seen that the maintenance rate of the photoelectric conversion efficiency in the heat resistance test at 60 degrees is almost the same between Comparative Example 1 and Comparative Example 2. However, as shown in Table 3, it can be seen that the maintenance rate of the photoelectric conversion efficiency of Examples 1 to 3 having a hole transport layer in which PTAA and Spiro-OMeTAD are mixed is significantly improved compared to Comparative Example 1 in the heat resistance test at 85 degrees. Among these examples, Example 3, in which the weight mixing ratio of PTAA and Spiro-OMeTAD is 1:3, has the highest maintenance rate of 0.93, followed by Example 2, in which the weight mixing ratio is 1:1, with a maintenance rate of 0.87, and Example 1, in which the weight mixing ratio is 3:1, with a maintenance rate of 0.80.

The reason for this is thought to be that the thermal degradation mechanisms of PTAA and Spiro-OMeTAD are different, and by mixing the two materials, they complement each other's degraded areas. For example, it is thought that PTAA deteriorates more in areas close to the top electrode, while Spiro-OMeTAD deteriorates more in different areas in the active layer.

These results show that by using at least two different semiconductor materials in the hole transport layer, i.e., a hole transport material with increased charge injection resistance and a hole transport material with increased charge recombination, the photoelectric conversion element has a high maintenance rate of photoelectric conversion efficiency (heat resistance).

100 Photoelectric conversion element 101 Lower electrode 102 Electron transport layer 103 Active layer 104 Hole transport layer 105 Upper electrode 106 Substrate

Claims (12)

The device includes a pair of electrodes, an electron transport layer, a photoelectric conversion layer, and a hole transport layer sandwiched between the electrodes,
the hole transport layer includes a layer in which an organic compound having an aromatic amine skeleton and an organic compound having a fluorene skeleton are mixed,
The organic compound having an aromatic amine skeleton includes a polytriarylamine . The photoelectric conversion element according to claim 1, wherein the photoelectric conversion layer includes a PVSK type photoelectric conversion material. The photoelectric conversion element according to claim 1 or 2, wherein the hole transport layer contains an organic compound having an aromatic amine skeleton and an organic compound having a fluorene skeleton in a mass ratio of 1:9 to 9:1. 4. The photoelectric conversion element according to claim 1, wherein at least one of the organic compound having an aromatic amine skeleton and the organic compound having a fluorene skeleton has an ionization energy of 4.5 electron volts or more. 5. The photoelectric conversion element according to claim 1, wherein the organic compound having a fluorene skeleton includes an organic compound having a fluorene skeleton as a part of a spiro skeleton. 6. The photoelectric conversion element according to claim 1 , wherein a mass ratio of the organic compound having an aromatic amine skeleton to the organic compound having a fluorene skeleton in the hole transport layer is within a range of 5:5 to 1:9. The photoelectric conversion element according to claim 1 , wherein the hole transport layer contains an additive. The photoelectric conversion element according to claim 7 , wherein the additive causes a charge transfer reaction between at least one of the organic compound having an aromatic amine skeleton and the organic compound having a fluorene skeleton before or after formation of a hole transport layer. The photoelectric conversion element according to any one of claims 1 to 8 , wherein the hole transport layer is formed by a coating method. The photoelectric conversion element according to claim 7 , wherein the additive comprises a boron compound. The photoelectric conversion element according to claim 10 , wherein the boron compound is tetrakis(pentafluorophenyl)borate. The photoelectric conversion element according to claim 1 , wherein the hole transport layer contains polytriarylamine and Spiro-OMeTAD.

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