JP7685804B2 - Hole transport layer materials for perovskite solar cells - Google Patents

Description

This application relates to hole transport layer materials for perovskite solar cells.

In recent years, perovskite solar cells, whose photoelectric conversion layer is a perovskite crystal layer, have been attracting attention. To improve the photoelectric conversion efficiency of perovskite solar cells, additives are used in the hole transport layer. Non-Patent Document 1 describes a hole transport layer in which LiTFSI (lithium bis(trifluoromethane)sulfonimide) is added to Spiro-OMeTAD (2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene). However, perovskite solar cells with a hole transport layer containing LiTFSI have poor thermal stability. There is a demand for additives to the hole transport layer that contribute to the thermal stability of perovskite solar cells.

Adv. Energy Mater., 2019, 9, 1901519.

This application has been made in consideration of these circumstances, and aims to provide a hole transport layer material that contributes to the thermal stability of perovskite solar cells.

The hole transport layer material of the perovskite solar cell of the present application contains a hole transport substance and an additive substance represented by the following general formula (1): In the following general formula (1), R ¹ , R ² , and R ³ are independently an alkyl group having 1 to 2 carbon atoms, and R ⁴ is an ether group having 2 to 5 carbon atoms.

The perovskite solar cell of the present application has a transparent electrode layer, an electron transport layer, a perovskite crystal layer, a hole transport layer composed of the hole transport layer material of the present application, and an electrode layer.

The hole transport layer material of the perovskite solar cell of the present application contains the additive substance represented by the above general formula (1). Therefore, the perovskite solar cell using the hole transport layer material of the present application has excellent thermal stability.

FIG. 1 is a schematic cross-sectional view of a perovskite solar cell according to an embodiment. FIG. 1 is a schematic cross-sectional view of a perovskite solar cell according to an embodiment. Graph showing the thermal stability of the perovskite solar cells of the examples.

Hereinafter, the hole transport layer material and perovskite solar cell of the present application will be described based on embodiments and examples with reference to the drawings. The hole transport layer material of the present application will be described as the material of the hole transport layer, which is a constituent member of the perovskite solar cell of the present application. FIG. 1 shows a schematic cross section of a perovskite solar cell 10 of an embodiment of the present application. The perovskite solar cell 10 includes a substrate 12, a transparent electrode layer 14, an electron transport layer 16, a perovskite crystal layer 18, a hole transport layer 20, and an electrode layer 22.

In addition to these components, the perovskite solar cell 10 may also include (a) a diffusion prevention film provided on the light incident side of the substrate 12, (b) an interface modification film or auxiliary layer provided at the interface between the electron transport layer 16 and the perovskite crystal layer 18, (c) an interface modification film or auxiliary layer provided at the interface between the perovskite crystal layer 18 and the hole transport layer 20, or (d) a sealant or moisture getter material that protects the perovskite solar cell 10 from moisture in the air. The perovskite solar cell may also be an inverted structure solar cell in which the electron transport layer 16 and the hole transport layer 20 in the perovskite solar cell 10 are interchanged.

In this embodiment, the substrate 12 is a glass substrate, the transparent electrode layer 14 is a fluorine-doped tin oxide (FTO) layer, the electron transport layer 16 is a tin oxide ( _SnO2 ) layer composed of nanoparticles, the perovskite crystal layer 18 is a _Cs0.05 ( _FA0.89MA0.11 ) _{0.95Pb ( I0.89Br0.11} ) _triple layer (FA is formamidinium, MA _is methylamine (same below)), and the electrode layer 22 is a gold layer. The _perovskite crystal layer 18 may be _CH3NH3PbI3 , CH( _NH2 ) _{2PbI3 ,} or _the like.

The hole transport layer 20 is composed of a hole transport layer material. The hole transport layer material includes Spiro-OMeTAD, which is a hole transport material, and an additive material represented by the following general formula (1). In the following general formula (1), R ¹ , R ² , and R ³ are independently an alkyl group having 1 to 2 carbon atoms, and R ⁴ is an ether group having 2 to 5 carbon atoms. Since R ¹ , R ² , and R ³ are alkyl groups having 1 to 2 carbon atoms, and R ⁴ is an ether group having 2 to 5 carbon atoms, which has a small number of carbon atoms, phase separation of the hole transport material and the additive material in the hole transport layer due to increased lipophilicity of the additive material is suppressed. In addition, since the number of carbon atoms of R ¹ , R ² , R ³ , and R ⁴ is small, deterioration of the electrical characteristics of the perovskite solar cell due to an increase in the electrical resistance of the additive material is suppressed.

The hole transport layer material may contain substances other than the hole transport material and the additive, such as a solvent that dissolves the hole transport material and the additive. In addition, it is preferable that the hole transport layer material does not substantially contain Li. This is to prevent Li from diffusing in the perovskite solar cell 10 and reducing the thermal stability of the perovskite solar cell 10. The hole transport layer material does not substantially contain Li means that the mass content of Li in the hole transport layer material is 2% or less. The mass content of Li in the hole transport layer material is preferably 1% or less, and more preferably 0.01% or less. Furthermore, it is most preferable that Li is not contained in the hole transport layer material, except when it is unavoidably contained. The mass content of Li in the hole transport layer material can be measured by mass spectrometry.

It is preferable that R ¹ , R ² , and R ³ are linear alkyl groups, and R ⁴ is linear ether group. Among these, it is more preferable that R ¹ and R ² are methyl groups, R ³ is ethyl group, and R ⁴ is methoxyethyl group. The perovskite solar cell 10 is manufactured, for example, as follows. Using a spin coating method, a sputtering method, a vacuum deposition method, a spray film forming method, a die coating method, a gravure printing method, a screen printing method, or the like, a transparent electrode layer 14 is formed on a substrate 12, an electron transport layer 16 is formed on the transparent electrode layer 14, a perovskite crystal layer 18 is formed on the electron transport layer 16, a hole transport layer 20 is formed on the perovskite crystal layer 18, and an electrode layer 22 is formed on the hole transport layer 20.

[Fabrication of perovskite solar cells]
Example 1
A 15% by mass aqueous dispersion of tin (IV) oxide (Alfa Aesar) was dropped and spin-coated onto the FTO surface of glass with FTO (NSG TEC 10, Nippon Sheet Glass Co., Ltd.), and dried at 150° C. for 1 hour to obtain a substrate having a glass substrate, an FTO layer serving as a transparent electrode layer, and a tin oxide layer serving as an electron transport layer laminated in this order. The tin oxide surface of this substrate was then treated with oxygen ions in oxygen plasma using the following procedure.

First, in the processing vessel of a plasma processing apparatus (Diener, FEMTO (frequency of high frequency power supply: 40 kHz, maximum power: 100 W)), the substrate was placed on the lower electrode so that the tin oxide layer faced the upper electrode. Next, in order to remove moisture and nitrogen from the atmosphere, the pressure inside the processing vessel was reduced to 20 Pa or less by vacuum evacuation. Then, to generate plasma, oxygen gas was introduced into the processing vessel, the pressure inside the processing vessel was kept at 100 Pa, and 100 W of high frequency power was supplied between the upper and lower electrodes to perform plasma cleaning for 30 seconds.

Next, in order to remove the solvent contained in the substrate, the substrate was heated at 150°C for 1 hour. Then, oxygen gas was introduced into the treatment vessel, the pressure inside the treatment vessel was kept at 100 Pa, 100 W of high-frequency power was supplied between the upper electrode and the lower electrode, and the oxygen gas was converted into plasma to generate oxygen plasma. The surface of the tin oxide layer was treated for 30 seconds using oxygen ions in the oxygen plasma.

Next, 123 mg of FAI, 382 mg _of PbI2, 14 mg of MABr, 36 _mg of PbBr2, and 29 μL of a DMSO solution of CsI (1.5 M) were dissolved in a mixture of 560 μL of DMF and 140 μL of DMSO to prepare a precursor solution of _{Cs0.05 ( FA0.89MA0.11} ) _{0.95Pb ( I0.89Br0.11} ) ₃ . Then, this precursor solution was spin-coated on the tin oxide layer surface-treated as above at 1000 rpm for 10 seconds, and then a small amount of chlorobenzene was further spin-coated at 6000 rpm for 20 seconds to obtain a uniform perovskite precursor thin film.

Next, the substrate was heated at 100°C for 1 hour on a hot plate to form _three layers of _{Cs0.05 ( FA0.89MA0.11} ) _{0.95Pb ( I0.89Br0.11} ), and a laminate including a substrate, a transparent electrode layer, an electron transport layer, and a perovskite crystal layer was obtained. Then, 31mg of Spiro-OMeTAD and 2μL of N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium=bis(trifluoromethanesulfonyl)imide (FUJIFILM Wako Pure Chemical Industries, Ltd.) represented by the following chemical formula (2) were dissolved in 0.35mL of chlorobenzene, and 11μL of 4-tert-butylpyridine was added to obtain a hole transport layer precursor solution, which is a hole transport layer material.

Next, the hole transport layer precursor solution was spin-coated at 3000 rpm for 30 seconds on the surface of _{the three} layers of _{Cs0.05 ( FA0.89MA0.11} ) _0.95Pb ( _I0.89Br0.11 ) of the laminate obtained above. Then, it was dried at 65° _C for 10 minutes to form a hole transport layer. A gold layer having a thickness of 50 nm was deposited on the surface of this hole transport layer using a vacuum deposition machine to obtain a perovskite solar cell member. Note that this perovskite solar cell member corresponds to the perovskite solar cell described in the above embodiment. In the examples, the perovskite solar cell is a combination of the perovskite solar cell member and the following housing member.

Next, a housing member was added to this perovskite solar cell member to form a perovskite solar cell. That is, calcium oxide 52 was supported in the center of the surface of a glass plate 50, and a UV-curable adhesive 54 containing glass spheres with a diameter of 10 μm was applied to the periphery to a thickness of 0.05 mm and a width of 0.2 mm as a spacer to obtain a sealing member. Then, in a nitrogen atmosphere, this sealing member was placed on the perovskite solar cell member obtained above, and UV rays were irradiated to cure the adhesive 54, thereby producing the perovskite solar cell of Example 1. Figure 2 shows a schematic cross section of this perovskite solar cell.

(Comparative Example 1)
A hole transport layer precursor solution was obtained by dissolving 61 mg of Spiro-OMeTAD and 10 mg of LiTFSI (Sigma-Aldrich) in 0.7 mL of chlorobenzene and adding 22 μL of 4-tert-butylpyridine. A perovskite solar cell of Comparative Example 1 was obtained in the same manner as in Example 1 except for this.

(Comparative Example 2)
A perovskite solar cell of Comparative Example 2 was obtained in the same manner as in Example 1, except that 1 μL of 1-butyl-1-methylpyrrolidinium=bis(trifluoromethylsulfonyl)imide (FUJIFILM Wako Pure Chemical Industries, Ltd.) represented by the following chemical formula (3) was used instead of 2 μL of N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium=bis(trifluoromethanesulfonyl)imide in Example 1.

(Comparative Example 3)
A perovskite solar cell of Comparative Example 3 was obtained in the same manner as in Example 1, except that 3 μL of tributylmethylammonium=bis(trifluoromethanesulfonyl)imide (FUJIFILM Wako Pure Chemical Industries, Ltd.) represented by the following chemical formula (4) was used instead of 2 μL of N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium=bis(trifluoromethanesulfonyl)imide in Example 1.

(Comparative Example 4)
A perovskite solar cell of Comparative Example 4 was obtained in the same manner as in Example 1, except that 5 μL of 1-allyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (FUJIFILM Wako Pure Chemical Industries, Ltd.) represented by the following chemical formula (5) was used instead of 2 μL of N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium bis(trifluoromethanesulfonyl)imide in Example 1.

[Evaluation of Perovskite Solar Cells]
(Initial characteristics)
Using a solar simulator (OTENTO-SUN, manufactured by Bunkoukeiki Co., Ltd.), the perovskite solar cells of Example 1 and Comparative Examples 1 to 4 were irradiated with AM1.5 simulated sunlight (intensity 1000 W/m ² ). The short-circuit current density, open circuit voltage, fill factor, and photoelectric conversion efficiency were determined from curves plotting the current-voltage relationship using a source meter (Keithley 2400, manufactured by Keithley Instruments, Inc.). The results are shown in Table 1.

As shown in Table 1, the short-circuit current density, open-circuit voltage, fill factor, and photoelectric conversion efficiency of the perovskite solar cell of Example 1 were equivalent to the short-circuit current density, open-circuit voltage, fill factor, and photoelectric conversion efficiency of the perovskite solar cell of Comparative Example 1. In other words, the perovskite solar cell having a hole transport layer containing the additive substance N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium=bis(trifluoromethanesulfonyl)imide (chemical formula (2) above) exhibited initial characteristics equivalent to those of the perovskite solar cell having a hole transport layer containing the additive substance LiTFSI.

In addition, the short-circuit current density, open-circuit voltage, fill factor, and photoelectric conversion efficiency of the perovskite solar cells of Comparative Example 2 to Comparative Example 4 were smaller than the short-circuit current density, open-circuit voltage, fill factor, and photoelectric conversion efficiency of the perovskite solar cell of Example 1. In other words, simply adding a substance containing bis(trifluoromethanesulfonyl)imide anion to the hole transport layer is insufficient. By adding the additive substance represented by the above general formula (1) to the hole transport layer, good initial characteristics of the perovskite solar cell were obtained.

(thermal stability)
Using the perovskite solar cells of Example 1, Comparative Example 1, and Comparative Example 2, the change in photoelectric conversion efficiency over time was measured in a dark place at 85°C in the same manner as in determining the photoelectric conversion efficiency with the initial characteristics described above. The results are shown in Figure 3. As shown in Figure 3, in the perovskite solar cell of Example 1, the photoelectric conversion efficiency was 72.5% of the initial value even after 960 hours. In contrast, in the perovskite solar cell of Comparative Example 1, the photoelectric conversion efficiency decreased to 51.6% of the initial value after 960 hours.

That is, a perovskite solar cell having a hole transport layer containing N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium=bis(trifluoromethanesulfonyl)imide (chemical formula (2) above) as an additive had better thermal stability than a perovskite solar cell having a hole transport layer containing LiTFSI as an additive. Note that in the perovskite solar cell of Comparative Example 2, the photoelectric conversion efficiency decreased to 16.4% of the initial value after 456 hours.

10 Perovskite solar cell 12 Substrate 14 Transparent electrode layer 16 Electron transport layer 18 Perovskite crystal layer 20 Hole transport layer 22 Electrode layer

Claims (2)

A hole transport layer material for a perovskite solar cell, comprising a hole transport substance and an additive substance represented by the following formula (1):

The R ¹ and R ² are methyl groups, the R ³ is an ethyl group, and the R ⁴ is a methoxyethyl group . 13. A perovskite solar cell comprising: a transparent electrode layer; an electron transport layer; a perovskite crystal layer; a hole transport layer composed of the hole transport layer material according to claim 1 ; and an electrode layer.

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