Hole transport material with low refractive index and application thereof

Abstract

The invention belongs to the field of organic photoelectricity, and discloses a hole transport material with low refractive index and a preparation method and application thereof. The chemical structure of the hole transport small molecule is shown in the specification, wherein M is a large steric hindrance group unit. The hole transport material with the low refractive index is of an asymmetric structure and is an aromatic amine compound containing silicon fluorene, the overall polarization degree of molecules can be reduced due to existence of silicon atoms, meanwhile, the molecular density can be reduced due to large-steric-hindrance groups, the low polarization rate and the low density form a synergistic effect, and the refractive index of the molecules is reduced. The hole transport material with low refractive index can be used for preparing an OLED device, and forms a refractive index difference with a hole transport material with high refractive index, so that the loss of light in the device is reduced, the luminous efficiency of the OLED device is improved, and the hole transport material has huge development potential and prospect in the field of organic electronic display.

Description

Technical Field

[0001] This invention belongs to the field of organic optoelectronics, specifically relating to a class of hole transport materials with low refractive index, their preparation methods, and applications. Background Technology

[0002] Organic light-emitting diodes (OLEDs) have successfully become a crucial light source in high-end mobile displays and televisions, thanks to advancements across a wide range of technologies, from materials, devices, and processes to hermetic packaging and systems. The core challenge currently facing OLED display technology lies in the luminous efficiency of blue phosphors, with low light out-coupling efficiency being the root cause. Traditional OLEDs employ a single-layer hole transport layer (HTL), and due to the mismatch in refractive index (n) with adjacent layers, a large number of photons are trapped in waveguide modes and suffer from surface plasmon resonance loss (SPP loss). Dual HTL structures, by establishing a stepped refractive index gradient, can significantly suppress light loss.

[0003] • The high n-layer (HTL1, n>1.9) is closely attached to the anode to reduce ITO interface reflection;

[0004] • The lower n-layer (HTL2, n < 1.9) is adjacent to the light-emitting layer, reducing light leakage to the organic layer / metal electrode. Summary of the Invention

[0005] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a type of hole transport material with a low refractive index (n) between 1.7 and 1.9.

[0006] Another object of the present invention is to provide a method for preparing the above-mentioned hole transport material with low refractive index.

[0007] Another object of the present invention is to provide applications of the aforementioned hole transport materials with low refractive index.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A class of hole transport materials with low refractive index has the chemical structure shown in Formula II below:

[0010]

[0011] Wherein, M is one of the following sterically hindered functional groups:

[0012]

[0013] Among them, R is the same or different of H, aryl, triphenylamine, straight-chain or branched alkyl with 1 to 20 carbon atoms, cycloalkyl with 3 to 20 carbon atoms, aromatic hydrocarbon group with 5 to 20 carbon atoms, or aromatic heterocyclic group with 3 to 20 carbon atoms.

[0014] Preferably, the hole transport material with low refractive index has a chemical structure of one of the following:

[0015]

[0016] The refractive index of the hole transport material with low refractive index is 1.72-1.85.

[0017] A method for preparing the above-mentioned hole transport material with low refractive index mainly includes the following steps:

[0018] (1) Under nitrogen or inert atmosphere, aniline and 2-bromo-9,9-dimethyl-9H-silicon fluorene compound were dissolved in an organic solvent, and under the action of base and catalyst, Buchwald-Hartwig coupling was generated by heating. The compound A1 was obtained by column chromatography separation and purification.

[0019] (2) Under nitrogen or inert atmosphere, compound A1 and 4'-bromo-4-biphenylboronic acid were added to an organic solvent and heated under alkaline and catalytic conditions to undergo Buchwald-Hartwig coupling. The mixture was purified by column chromatography to obtain the compound shown in Formula I.

[0020] (3) Under the protection of nitrogen or inert gas, the compound shown in Formula I, the monomer MBr containing a large sterically hindered group unit M, the base and the catalyst are added to an organic solvent to undergo a Suzuki coupling reaction. After the reaction stops, the reaction solution is purified to obtain a hole transport material SiNC with a low refractive index.

[0021] The synthetic route for the compound represented by Formula I of this invention is shown below:

[0022]

[0023] In step (1), the base is sodium tert-butoxide, preferably a sodium tert-butoxide tetrahydrofuran solution with a mass fraction of 21%.

[0024] In step (1), the catalyst is tributylphosphine and palladium acetate, and the molar ratio of aniline, 2-bromo-9,9-dimethyl-9H-silicon fluorene compound, sodium tert-butoxide, tributylphosphine, and palladium acetate is 1:(0.4~1):(0.1~0.5):(1×10⁻⁶). -3 ~5×10 -3 ): (1×10 -3 ~5×10-3 The preferred ratio is 1:0.6:1.5:2.5×10. -3 2.5×10 -3 ;

[0025] The organic solvent mentioned in step (1) includes one or more of toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide, and dimethyl sulfoxide;

[0026] The coupling reaction mentioned in step (1) refers to the reaction at 80-120℃ for 5-20 hours.

[0027] In step (2), the base is sodium tert-butoxide, preferably a sodium tert-butoxide tetrahydrofuran solution with a mass fraction of 21%; the catalyst is palladium acetate and tributylphosphine.

[0028] In step (2), the molar ratio of compound A1, 4'-bromo-4-biphenylboronic acid, palladium acetate, tributylphosphine, and sodium tert-butoxide is 1:(0.2~2):(1×10 -3 ~10×10 -3 ):(1×10 -3 ~10×10 -3 ): 1~10, preferably 1.4:1.4:5×10 -3 5×10 -3 :3.

[0029] The organic solvent mentioned in step (2) includes one or more of toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide, and dimethyl sulfoxide;

[0030] The coupling reaction described in step (2) refers to a reaction at 100–150 °C for 5–40 h.

[0031] The catalyst mentioned in step (3) is palladium acetate and X-Phos; the base is tetraethylammonium hydroxide; and the organic solvent includes one or more of toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide, and dimethyl sulfoxide.

[0032] In step (3), the molar ratio of the compound shown in Formula I, the monomer containing the sterically hindered group unit M, palladium acetate, X-Phos, and tetraethylammonium hydroxide is 1:(1-1.5):(0.01-0.05):(0.01-0.1):(2-20), preferably 1:1:0.03:0.06:5;

[0033] The suzuki coupling reaction described in step (3) refers to heating under reflux for 2-72 hours.

[0034] The purification described in step (3) refers to the process of separating the aqueous phase after cooling to room temperature, extracting the aqueous phase with an organic solvent, drying it with anhydrous sodium sulfate, evaporating the solvent under reduced pressure, and then separating it by column chromatography. The organic solvent used for extraction is one or more of dichloromethane, toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide, and dimethyl sulfoxide. The eluent used is two of petroleum ether, cyclohexane, toluene, dichloromethane, ethyl acetate, acetone, methanol, and water.

[0035] The application of the aforementioned low-refractive-index hole transport material in the fabrication of multilayer hole transport layers for devices such as organic low-refractive-index hole transport diodes, organic field-effect transistors, organic solar cells, or organic laser diodes is preferred, with application in the fabrication of multilayer hole transport materials for light-emitting diode devices.

[0036] The multilayer hole transport layer includes hole transport layer 1 made of hole transport material with low refractive index and hole transport layer 2 made of hole transport material with high refractive index.

[0037] An organic light-emitting device comprises, from bottom to top, an ITO electrode, a hole injection layer, a high-refractive-index hole transport layer, a low-refractive-index hole transport layer, an electron blocking layer, a light-emitting layer, a hole blocking layer, an electron transport layer, an electron injection layer, and a cathode. The low-refractive-index hole transport layer is prepared from the low-refractive-index hole transport material described above.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] 1. The hole transport material with low refractive index of the present invention has an asymmetric structure and large steric hindrance groups, which can reduce molecular density and thus reduce the refractive index of the molecules.

[0040] 2. It is an aromatic amine compound containing silylfluorene. The silicon atoms have suitable polarizability, which can reduce the overall polarizability of the molecule.

[0041] 3. The preparation method of the hole transport material with low refractive index of the present invention is simple, the raw materials are readily available, the synthesis conditions are mild, and the purification is convenient. Attached Figure Description

[0042] Figure 1 For SiNC2 1 H NMR spectrum.

[0043] Figure 2 For SiNC3 1 H NMR spectrum.

[0044] Figure 3 This is the structure of the electroluminescent device of the present invention.

[0045] Figure 4 The electroluminescence spectra of devices 1-4 of the present invention are shown. Detailed Implementation

[0046] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0047] Unless otherwise specified, all reagents used in the examples are commercially available.

[0048] In this embodiment, the refractive index was obtained using the following method: Ellipsometry was used for testing. The material whose refractive index was to be determined was introduced into a chamber of a vacuum vapor deposition apparatus, and the pressure in the chamber was then controlled to 10⁻⁶ Torr. Subsequently, an electric current was applied to the chamber to evaporate the introduced material, thereby producing a sample with a thickness of 30 nm on a silicon wafer substrate. The refractive index was measured using an ellipsometry. Specifically, a UVSEL from Horiba Ltd. was used to measure the refractive index at wavelengths from 350 to 800 nm with an incident angle of 60 degrees, and the result of the 460 nm test was defined as the refractive index (n).

[0049] Example 1: Preparation of the hole transport monomer shown in Formula I

[0050]

[0051] (1) Synthesis of 9,9-dimethyl-9H-9-silazine-N-aniline (intermediate A1)

[0052] Under an inert atmosphere, a mixture of aniline (1.86 g, 0.020 mol) and 2-bromo-9,9-dimethyl-9H-siliconfluorene compound (3.46 g, 0.012 mol) dissolved in toluene (150 ml), sodium tert-butoxide (15 ml, 2 mol / L), and palladium acetate (11.2 mg, 5 × 10⁻⁶ mg / L) were prepared. -5 mol), tributylphosphine (10.1 mg, 5 × 10⁻⁶) -5 The mixture was heated to 105°C under a nitrogen atmosphere and reacted for 8 hours. After cooling to room temperature, the mixture was separated into two phases. The aqueous phase was extracted with toluene, and the organic phases were combined. The mixture was dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure. The mixture was then purified by column chromatography (eluent: petroleum ether:DCM = 10:1 to 1:1) and recrystallized to obtain the intermediate (compound A1).

[0053] (2) Synthesis of 9-(dimethylphenylsilyl)-N-phenyl-4-(4-bromophenyl)triphenylamine (Formula I)

[0054] In a 250 ml three-necked flask under nitrogen protection, intermediate A1 (4.22 g, 0.014 mol), 4'-bromo-4-biphenylboronic acid (4.34 g, 0.014 mol), sodium tert-butoxide (15 ml, 2 mol / L), and palladium acetate (11.2 mg, 5 × 10⁻⁶ mg / L) were added. -5 mol), tributylphosphine (10.1 mg, 5 × 10⁻⁶) -5 The mixture was heated to 115°C and refluxed with stirring for 20 hours. The reaction endpoint was monitored by TLC. After cooling to room temperature, the aqueous phase was extracted with toluene, and the organic phases were combined, dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure. The mixture was then purified by column chromatography (eluent: petroleum ether:DCM = 10:1).

[0055] Example 2: Synthesis of SiNC1, a hole transport material with low refractive index

[0056]

[0057] The hole transport monomer (0.984 g, 2.00 mmol) prepared in Example 1, 1-bromoadamantane (0.428 g, 2.00 mmol), and tetraethylammonium hydroxide aqueous solution (5.9 ml, 25% aqueous solution) were added to 20 ml of toluene solution. After three gas exchanges with an oil pump, Pd(OAc)2 (13.46 mg, 0.06 mmol) and X-Phos (57.21 mg, 0.12 mmol) were added, and the mixture was heated under reflux for 5 h. The reaction endpoint was monitored by TLC. After cooling to room temperature, the mixture was separated, and the aqueous phase was extracted with toluene. The organic phases were combined and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure, and the mixture was purified by column chromatography (eluent: petroleum ether:DCM = 8:1). The compound SiNC1 was obtained by filtration. The refractive index is 1.72. 1 HNMR (400MHz, DMSO-d6) δ0.74 (s, 6H), 1.70 (m, 12H, J = 7.0Hz), 1.90 (m, 3H, J = 7.0Hz) ),6.85(dd,1H,J=11.3,1.6Hz),7.08-7.81(m,18H),7.75(dd,1H,J=11.3,0.5Hz).

[0058] Example 3: Synthesis of SiNC2, a hole transport material with low refractive index

[0059]

[0060] The hole transport monomer compound of Formula I shown in Example 1 (0.984 g, 2.00 mmol), 2-bromo-1,1':2',1”:2”,1”'-tetraphenyl (0.768 g, 2.00 mmol), and tetraethylammonium hydroxide aqueous solution (5.9 ml, 25% aqueous solution) were added to 20 ml of toluene solution. After three gas exchanges with an oil pump, Pd(OAc)2 (13.46 mg, 0.06 mmol) and X-Phos (57.21 mg, 0.12 mmol) were added, and the mixture was heated under reflux for 5 h. The reaction endpoint was monitored by TLC. After cooling to room temperature, the mixture was separated, and the aqueous phase was extracted with toluene. The organic phases were combined and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure, and the mixture was purified by column chromatography (eluent: petroleum ether:DCM = 8:1). SiNC2 was obtained with a refractive index of 1.85. 1 H NMR(400MHz,DMSO-d6)δ7.96(d,6H),7.87(d,1H),7.83(d,1H),7.79(d,2H),7.65(d,1H),7.60(m,7H),7.55(d,2H),7.51(d,1H),7 .47(t,2H),7.46(t,1H),7.45(t,1H),7.41(t,1H),7.37(d,2H),7.25(dd,4H),7.24(t,2H),7.08(d,2H),7.00(t,1H),0.66(s,6H).

[0061] Example 4: Synthesis of SiNC3, a hole transport material with low refractive index

[0062]

[0063] The hole transport monomer compound of Formula I shown in Example 1 (0.984 g, 2.00 mmol), 3-bromo-9,9'-spirodifluorene (0.788 g, 2.00 mmol), and tetraethylammonium hydroxide aqueous solution (5.9 ml, 25% aqueous solution) were added to 20 ml of toluene solution. After three gas exchanges with an oil pump, Pd(OAc)2 (13.46 mg, 0.06 mmol) and X-Phos (57.21 mg, 0.12 mmol) were added, and the mixture was heated to reflux for 5 h. The reaction endpoint was monitored by TLC. After cooling to room temperature, the mixture was separated, and the aqueous phase was extracted with toluene. The organic phases were combined and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure, and the mixture was purified by column chromatography (eluent: petroleum ether:DCM = 8:1). SiNC3 was obtained with a refractive index of 1.76. 1H NMR(400MHz,DMSO-d6)δ8.18(s,1H),7.90(d,1H),7.89(d,2H),7.87(d,1H),7 .83(d,1H),7.74(d,1H),7.68(d,1H),7.65(d,1H),7.60(t,1H),7.55(d,3H),7 .51(s,1H),7.47(t,2H),7.45(d,2H),7.38(t,1H),7.37(d,2H),7.28(t,3H),7 .27(t,2H),7.25(dd,4H),7.24(t,2H),7.08(d,2H),7.00(t,1H),0.66(s,6H).

[0064] Example 5: Synthesis of SiNC4, a hole transport material with low refractive index

[0065]

[0066] The hole transport monomer compound of Formula I shown in Example 1 (0.984 g, 2.00 mmol), 2-bromobicyclo[2.2.1]heptane (0.348 g, 2.00 mmol), and tetraethylammonium hydroxide aqueous solution (5.9 ml, 25% aqueous solution) were added to 20 ml of toluene solution. After three gas exchanges with an oil pump, Pd(OAc)2 (13.46 mg, 0.06 mmol) and X-Phos (57.21 mg, 0.12 mmol) were added, and the mixture was heated to reflux for 5 h. The reaction endpoint was monitored by TLC. After cooling to room temperature, the mixture was separated, and the aqueous phase was extracted with toluene. The organic phases were combined and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure, and the mixture was purified by column chromatography (eluent: petroleum ether:DCM = 8:1). SiNC4 was obtained with a refractive index of 1.82. 1 H NMR (400MHz, DMSO-d6) δ0.74 (s, 6H), 1.22 (m, 2H, J=12.4, 7.0Hz), 1.35 (m, 4H, J=12.4, 7.0Hz), 1.50 (m, 3H, J= 12.4,7.0Hz),2.25(m,2H,J=7.0Hz),6.85(dd,1H,J=11.3),7.08-7.81(m,18H),7.75(dd,1H,J=11.3,0.5Hz).

[0067] Example 6: Fabrication of Organic Light-Emitting Devices

[0068] A pre-made indium tin oxide (ITO) glass with a sheet resistance of 15Ω was ultrasonically cleaned sequentially with acetone, detergent, deionized water, and isopropanol, followed by plasma treatment for 10 minutes. Then, in a vacuum evaporation apparatus, a hole injection layer (HI:PD mass ratio of 97:3) was deposited on the ITO surface. Next, a hole transport layer HT1 (a high refractive index material with a refractive index of 2.02) and a hole transport layer HT2 (the material used in this patent application) were deposited sequentially. An electron blocking layer (BP) was then deposited on top of the hole transport layer HT2. Finally, a light-emitting layer was formed by vacuum evaporation of the BH host and dopant BD (host-guest mass ratio of 99:1). Finally, a hole blocking layer HBL, an electron transport layer ET:Liq (mass ratio: 1:1), an electron injection layer YB, and a cathode Ag:Mg (mass ratio: 10:1) were deposited on the light-emitting layer. The device structure is: ITO / HI:PD(3%) / HT1 / HT2 / BP / BH:BD(1%) / HBL / ET:Liq / Yb / Ag:Mg.

[0069]

[0070] Data for the fabricated organic light-emitting devices are listed in Table 1 below. Comparative Examples 1 and 2 differ from Device 1 only in the material of the hole transport layer; all other parameters, such as thickness, are the same. For example, Comparative Example 1 uses a single SiNC4 layer as the hole transport layer, while Comparative Example 2 uses a single HT1 layer. The organic light-emitting devices achieve an efficiency of 10 mA / cm². 2 The driving voltage and luminous efficiency were measured at a current density of 1000 kJ / m², and it was found that all the devices in the embodiments had low turn-on voltages. In addition, the devices based on the embodiments all showed higher external quantum efficiency than the control, indicating that these hole transport materials with low refractive index can be used to construct high-efficiency blue fluorescent organic light-emitting diodes.

[0071] Table 1 Performance of Organic Light-Emitting Devices

[0072]

[0073] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A hole transporting material having a low refractive index, characterized by The chemical structure is shown in formula II as follows: M is one of the following bulky group units: R is H, aryl, triphenylamine, straight-chain or branched alkyl with 1-20 carbon atoms, cycloalkyl with 3-20 carbon atoms, aromatic hydrocarbon group with 5-20 carbon atoms, or aromatic heterocyclic group with 3-20 carbon atoms.

2. The hole transporting material having a low refractive index according to claim 1, wherein The structure is one of the following structural formulas:

3. The hole transport material with low refractive index according to claim 1, characterized in that: The refractive index of the hole transport material with low refractive index is 1.72-1.

85.

4. A method for producing a hole transporting material having a low refractive index according to any one of claims 1 to 3, characterized by The method comprises the following steps: (1) Under nitrogen or inert atmosphere, aniline and 2-bromo-9,9-dimethyl-9H-silole compound are dissolved in an organic solvent, heated to undergo Buchwald-Hartwig coupling under the action of a base and a catalyst, and separated and purified by column chromatography to obtain compound A1; (2) Under nitrogen or inert atmosphere, compound A1 and 4'-bromo-4-biphenylboronic acid are added to an organic solvent, heated to undergo Buchwald-Hartwig coupling under the action of a base and a catalyst, and separated and purified by column chromatography to obtain the compound shown in formula I; (3) Under nitrogen or inert atmosphere, the compound shown in formula I, a monomer MBr containing a bulky group unit M, a base and a catalyst are added to an organic solvent to undergo Suzuki coupling reaction, and after the reaction is stopped, the reaction liquid is purified to obtain the hole transport material SiNC with low refractive index.

5. The preparation method of the hole transport material with low refractive index according to claim 4, characterized in that: In step (1), the base is sodium tert-butoxide; the catalyst is tributylphosphine and palladium acetate, and the molar ratio of the used aniline, 2-bromo-9,9-dimethyl-9H-silole compound, sodium tert-butoxide, tributylphosphine and palladium acetate is 1: (0.4-1): (0.1-0.5): (1x10 -3 ~5x10 -3 ): (1x10 -3 ~5x10 -3 ). In step (1), the base is sodium tert-butoxide; the catalyst is tributylphosphine and palladium acetate, and the molar ratio of the used aniline, 2-bromo-9,9-dimethyl-9H-silole compound, sodium tert-butoxide, tributylphosphine and palladium acetate is 1: (0.4-1): (0.1-0.5): (1x10 -3 ~5x10 -3 ): (1x10 -3 ~5x10 -3 ). In step (1), the organic solvent comprises one or more of toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide and dimethyl sulfoxide; In step (1), the coupling reaction refers to a reaction at 80-120℃ for 5-20h.

6. The preparation method of the hole transport material with low refractive index according to claim 4, characterized in that: In step (2), the base is sodium tert-butoxide, and the catalyst is palladium acetate and tributylphosphine; In step (2), the molar ratio of the compounds A1, 4'-bromo-4-biphenylboronic acid, palladium acetate, tributylphosphine and sodium tert-butoxide used is 1 : (0.2-2) : (1 x 10 -3 ~10 x 10 -3 ) : (1 x 10 -3 ~10 x 10 -3 ) : 1-10; In step (2), the organic solvent comprises one or more of toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide and dimethyl sulfoxide; In step (2), the coupling reaction refers to a reaction at 100-150℃ for 5-40h.

7. The preparation method of the hole transport material with low refractive index according to claim 4, characterized in that: In step (3), the catalyst is palladium acetate and X-Phos, the base is tetraethylammonium hydroxide, and the organic solvent comprises one or more of toluene, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide and dimethyl sulfoxide; In step (3), the molar ratio of the compound shown in formula I, the monomer containing a bulky group unit M, palladium acetate, X-Phos and tetraethylammonium hydroxide is 1:(1-1.5):(0.01-0.05):(0.01-0.1):(2-20). The Suzuki coupling reaction in step (3) is heated to reflux for 2-72 hours.

8. Use of a hole transport material having a low refractive index according to any one of claims 1 to 3 as a multi-layer hole transport layer material, characterized in that: The multi-layer hole transport layer comprises a hole transport layer 1 prepared from the hole transport material with low refractive index according to any one of claims 1-3 and a hole transport layer 2 prepared from a hole transport material with high refractive index.

9. Use of the hole transport material with low refractive index according to claim 8 for the production of a multilayer hole transport layer, characterized in that The multi-layer hole transport layer is one of an organic light emitting diode, an organic field effect transistor, an organic solar cell or an organic laser diode.

10. An organic light-emitting device, characterized in that... From bottom to top, it comprises an ITO electrode, a hole injection layer, a high refractive index hole transport layer, a low refractive index hole transport layer, an electron blocking layer, a light emitting layer, a hole blocking layer, an electron transport layer, an electron injection layer and a cathode; wherein the low refractive index hole transport layer is prepared from the hole transport material with low refractive index according to any one of claims 1-3.

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