KR20200069608A - Method of manufacturing a hole transporting layer for orgnaic photodioe and method of manufacturing a organic photodiode - Google Patents

Abstract

The present specification provides a method for manufacturing a hole transport layer for an organic photodiode and a method for manufacturing the organic photodiode. The method for manufacturing a hole transport layer for an organic photodiode includes the steps of: depositing a hole transport material in a vacuum condition; and exposing the deposited hole transport material to an atmospheric pressure condition.

Description

Method for manufacturing hole transport layer for organic photodiodes and method for manufacturing organic photodiodes {METHOD OF MANUFACTURING A HOLE TRANSPORTING LAYER FOR ORGNAIC PHOTODIOE AND METHOD OF MANUFACTURING A ORGANIC PHOTODIODE}

The present specification relates to a method for manufacturing a hole transport layer for an organic photodiode and a method for manufacturing an organic photodiode.

With the development of smart devices, image sensors including photodiodes are demanding high resolution. However, in the case of a photodiode using silicon, which is currently mainly used, it is difficult to be thinned, and there is a limit in increasing absorbance.

Accordingly, an organic photodiode having a high absorbance and a variety of absorption wavelengths is receiving attention as a material to replace the silicon diode, but the existing organic photodiode has a problem in that performance is lower than that of the silicon diode. In order to solve this, research on a method of manufacturing an organic photodiode is required.

Korean Patent Publication No. 10-2013-0047367

The present specification relates to a method for manufacturing a hole transport layer for an organic photodiode and a method for manufacturing an organic photodiode.

An exemplary embodiment of the present specification is a step of depositing a hole transport material in a vacuum condition; And

It provides a method for manufacturing a hole transport layer for an organic photodiode comprising the step of exposing the deposited hole transport material to an atmospheric pressure condition.

Another exemplary embodiment of the present specification includes preparing a first electrode;

Forming a photoactive layer on the first electrode; And

A method of manufacturing an organic photodiode comprising forming a second electrode on the photoactive layer,

The method of manufacturing an organic photodiode includes forming a hole transport layer by the method of manufacturing a hole transport layer for the organic photodiode,

The forming of the hole transport layer may include forming a hole transport layer by depositing a hole transport material on the first electrode under vacuum conditions before forming the photoactive layer and then exposing it to atmospheric pressure; Or depositing a hole transport material on the photoactive layer in a vacuum condition before forming the second electrode, and then exposing it to atmospheric pressure to form a hole transport layer.

The manufacturing method according to the exemplary embodiment of the present specification has an effect of improving the performance of the organic photodiode.

1 and 2 illustrate an organic photodiode manufactured according to an exemplary embodiment of the present specification.
3 is a view showing a photocurrent (EQE) value according to a wavelength of an organic photodiode according to an exemplary embodiment of the present specification.
4 is a diagram showing the current density according to the voltage of the organic photodiode according to the exemplary embodiment of the present specification.

Hereinafter, the present specification will be described in detail.

An exemplary embodiment of the present specification is a step of depositing a hole transport material in a vacuum condition; And

It provides a method for manufacturing a hole transport layer for an organic photodiode comprising the step of exposing the deposited hole transport material to an atmospheric pressure condition.

Generally, a hole transport layer for an organic photodiode is manufactured through a solution process or a deposition process. In the case of a deposition process, a hole transport layer is formed only by depositing a hole transport material under vacuum conditions.

On the other hand, an exemplary embodiment of the present specification exhibits an effect of improving the performance of the organic photodiode by additionally performing a step of exposing the hole transport material in a vacuum condition and then exposing it to a normal pressure condition.

In one embodiment of the present specification, the organic photodiode has an inverted structure or a conventional structure.

In the present specification, the inverted structure includes a structure in which a substrate, a first electrode, a photoactive layer, a hole transport layer and a second electrode are sequentially stacked; Alternatively, it means a structure in which a substrate, a first electrode, an electron transport layer, a photoactive layer, a hole transport layer and a second electrode are laminated sequentially. 1 shows an organic photodiode having an inverted structure manufactured according to an exemplary embodiment of the present specification.

In the present specification, the conventional structure includes a structure in which a substrate, a first electrode, a hole transport layer, a photoactive layer and a second electrode are sequentially stacked; Alternatively, it means a structure in which a substrate, a first electrode, a hole transport layer, a photoactive layer, an electron transport layer, and a second electrode are sequentially stacked. 2 shows an organic photodiode having a conventional structure manufactured according to an exemplary embodiment of the present specification.

In an exemplary embodiment of the present specification, a position in which a hole transport material is deposited differs according to the structure of the organic photodiode. For example, when manufacturing a conventional organic photodiode, the hole transport material is deposited on the first electrode. As another example, when manufacturing an inverted structured organic photodiode, the hole transport material is deposited on the photoactive layer.

In one embodiment of the present specification, the step of exposing the atmospheric pressure condition is performed in an atmospheric atmosphere or an inert gas atmosphere.

In the present specification, the atmospheric atmosphere means general air conditions in which the type of gas is not controlled. For example, the atmospheric atmosphere may be a gas atmosphere in which nitrogen, oxygen, argon, carbon dioxide, and the like exist.

In the present specification, the inert gas is a gas that is not chemically active to make a compound, for example, nitrogen, argon, helium, and the like, but is not limited thereto.

In one embodiment of the present specification, the step of exposing the atmospheric pressure condition is performed in an atmospheric atmosphere or an inert gas atmosphere.

In one embodiment of the present specification, the step of exposing the atmospheric pressure condition may be performed in an inert gas atmosphere to improve stability.

In one embodiment of the present specification, the inert gas is nitrogen.

In one embodiment of the present specification, the hole transport material is deposited using heat under vacuum conditions.

In one embodiment of the present specification, the step of depositing the hole transport material is performed at 300°C to 2,400°C. More specifically, the step of depositing the hole transport material is performed at 300 ℃ to 1,500 ℃.

In the present specification, the vacuum condition means a low pressure of 1 x 10 ^-3 mmHg or less.

In the present specification, the atmospheric pressure condition is 0.8 atm to 1.2atm. Specifically, the atmospheric pressure condition is 1 atm.

In this specification, 1atm = 760mmHg.

In one embodiment of the present specification, the step of exposing to the atmospheric pressure condition is performed for 1 minute to 60 minutes. When exposed to more than 60 minutes under normal pressure conditions, there is a high possibility that foreign matter adheres to the surface, and when exposed to less than 1 minute, there is no effect of exposing to normal pressure, and thus there is a problem of not exhibiting an effect of improving performance.

In one embodiment of the present specification, the hole transport material includes a metal oxide. The metal oxide may include molybdenum oxide (MoO _x ), vanadium oxide (V ₂ O ₅ ), nickel oxide (NiO), tungsten oxide (WO _x ), copper oxide (CuO _x ); Copper aluminum oxide (CuAlO ₂ ), germanium oxide (GeO ₂ ) and chromium oxide (CrO _x ). Specifically, the hole transport material includes one of the aforementioned metal oxides.

In one embodiment of the present specification, the hole transport layer is deposited to a thickness of 5nm to 100nm. When the thickness of the hole transport layer is more than 100 nm, there is a problem in that the resistance in the hole transport layer increases to decrease the current flow in the entire device of the organic photodiode, and thus the efficiency (EQE) decreases. In addition, when the thickness of the hole transport layer is less than 5 nm, there is a problem in that the leakage current increases and the dark current value of the organic photodiode increases.

An exemplary embodiment of the present specification includes preparing a first electrode;

Forming a photoactive layer on the first electrode; And

A method of manufacturing an organic photodiode comprising forming a second electrode on the photoactive layer,

The method of manufacturing an organic photodiode includes forming a hole transport layer by the method of manufacturing a hole transport layer for the organic photodiode,

The forming of the hole transport layer may include forming a hole transport layer by depositing a hole transport material on the first electrode under vacuum conditions before forming the photoactive layer and then exposing it to atmospheric pressure; Or depositing a hole transport material on the photoactive layer in a vacuum condition before forming the second electrode, and then exposing it to atmospheric pressure to form a hole transport layer.

In the method of manufacturing the organic photodiode, the order in which the hole transport material is deposited varies depending on the structure of the organic photodiode.

For example, an exemplary embodiment of the present specification includes preparing a first electrode;

Depositing a hole transport layer material on the first electrode and then exposing it to atmospheric pressure to form a hole transport layer;

Forming a photoactive layer on the hole transport layer; And

It provides a method of manufacturing an organic photodiode having a conventional structure comprising the step of forming a second electrode on the photoactive layer.

As another example, an exemplary embodiment of the present specification includes preparing a first electrode;

Forming a photoactive layer on the first electrode;

Forming a hole transport layer by depositing a hole transport material in a vacuum condition on a photoactive layer and then exposing it to normal pressure; And

It provides a method for manufacturing an organic photodiode of an inverted structure comprising the step of forming a second electrode on the hole transport layer.

In one embodiment of the present specification, the step of forming the photoactive layer is performed under normal pressure conditions or vacuum conditions.

In one embodiment of the present specification, when the photoactive layer is formed under atmospheric pressure, the photoactive layer is formed in a solution process.

In one embodiment of the present specification, the solution process refers to a method of spin coating a solution, dip coating, screen printing, spray coating, doctor blade or brush painting.

In one embodiment of the present specification, the photoactive layer may be formed through a solution process after dissolving the photoactive material in a solvent.

In one embodiment of the present specification, when the photoactive layer is formed under vacuum conditions, the photoactive layer is formed by a deposition method.

In one embodiment of the present specification, the organic photodiode may be used without limiting materials and/or methods in the art except for a method for manufacturing a hole transport layer.

In one embodiment of the present specification, the first electrode is provided on the substrate.

In one embodiment of the present specification, the substrate may be a glass substrate or a transparent plastic substrate excellent in transparency, surface smoothness, handling ease, and waterproofness, but is not limited thereto, and is limited to substrates commonly used in organic electronic devices. Does not work. Specifically, there are glass or PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PP (polypropylene), PI (polyimide), TAC (triacetyl cellulose). It is not limited to this.

The first electrode may be a material that is transparent and has excellent conductivity, but is not limited thereto. Metals such as vanadium, chromium, copper, zinc, gold or alloys thereof; Metal oxides such as zinc oxide, indium oxide, indium tin oxide (ITO), and indium zinc oxide (IZO); ZnO:Al or SnO ₂ : Combination of metal and oxide such as Sb; And conductive polymers such as poly(3-methylthiophene), poly[3,4-(ethylene-1,2-dioxy)thiophene] (PEDOT), polypyrrole and polyaniline, but is not limited thereto. .

The method for forming the first electrode is not particularly limited, but is applied to one surface of the substrate or in the form of a film using, for example, sputtering, E-beam, thermal vapor deposition, spin coating, screen printing, inkjet printing, doctor blade or gravure printing. It can be formed by coating.

When the first electrode is formed on a substrate, it may undergo cleaning, water removal, and hydrophilic modification processes.

For example, the patterned ITO substrate is sequentially washed with a cleaning agent, acetone, and isopropyl alcohol (IPA), and then heated to 60° C. to 200° C. for 1 minute to 30 minutes, preferably 120° C. for 10 minutes to remove moisture. When it is dried and the substrate is completely cleaned, the substrate surface is modified to be hydrophilic.

Through the surface modification as described above, the bonding surface potential can be maintained at a level suitable for the surface potential of the photoactive layer. In addition, it is easy to form a polymer thin film on the first electrode during modification, and the quality of the thin film may be improved.

Pretreatment techniques for the first electrode include: a) a surface oxidation method using a parallel plate discharge, b) a method of oxidizing the surface through ozone generated by using UV ultraviolet rays in a vacuum, and c) oxygen generated by plasma And oxidation using a radical.

One of the above methods can be selected according to the state of the first electrode or the substrate. However, regardless of which method is used, it is preferable to prevent oxygen desorption on the surface of the first electrode or the substrate in common and to suppress the residual of moisture and organic matter as much as possible. At this time, the practical effect of pretreatment can be maximized.

As a specific example, a method of oxidizing the surface through ozone generated using UV may be used. At this time, after the ultrasonic cleaning, the patterned ITO substrate is baked on a hot plate, dried well, and then put into a chamber, and by operating a UV lamp, the oxygen gas reacts with UV light to generate ozone. The patterned ITO substrate can be cleaned.

However, the method of surface modification of the patterned ITO substrate in the present specification is not particularly limited, and any method may be used as long as it is a method of oxidizing the substrate.

The second electrode may be a metal having a small work function, but is not limited thereto. Metals such as magnesium, calcium, sodium, potassium, titanium, indium, yttrium, lithium, gadolinium, aluminum, silver, tin and lead, or alloys thereof; Alternatively, a material having a multilayer structure such as LiF/Al, LiO ₂ /Al, LiF/Fe, Al:Li, Al:BaF ₂ or Al:BaF ₂ :Ba may be used, but is not limited thereto.

The second electrode is 1x10 ^- may be formed is deposited on the internal heat evaporator showing a vacuum degree of less than ⁶ torr, not limited to this method.

The hole transport layer and/or the electron transport layer serves to efficiently transfer electrons and holes separated from the photoactive layer to the electrode.

In one embodiment of the present specification, the electron transport layer includes an electron transport material. The electron transporting material may be bathocuproine (BCP) or electron-extracting metal oxides (BCP), specifically, a metal complex of bathocuproine (BCP) and 8-hydroxyquinoline; Complexes including Alq ₃ ; Metal complexes including Liq; LiF; Ca; Titanium oxide (TiO _x ); Zinc oxide (ZnO); And cesium carbonate (Cs ₂ CO ₃ ), but is not limited thereto.

In one embodiment of the present specification, the photoactive layer includes a photoactive material.

In one embodiment of the present specification, the photoactive material includes an electron donor and an electron acceptor. The electron donor and the electron acceptor may be used without limitation as long as they are materials used in the art.

For example, the electron acceptor may be selected from the group consisting of fullerene, fullerene derivatives, vasocuproin, semiconducting elements, semiconducting compounds, non-fullerene, and combinations thereof. Specifically, fullerenes such as C ₆₀ ; Fullerene derivatives such as PCBM ((6,6)-phenyl-C ₆₁ -butyric acid-methylester) and PCBCR ((6,6)-phenyl-C ₆₁ -butyric acid-cholesteryl ester); Perylene; Polybenzimidazole (PBI); And PTCBI (3,4,9,10-perylene-tetracarboxylic bis-benzimidazole).

For example, the electron donor is Subron (Subron), Boron subphthalocyanine chloride (ZnPc), ZnPc (Zinc phthalocyanine), DMAQ (5,12-Dihydro-5,12-dimethylquino[2,3-b]acridine-7,14-dione), P3HT ( poly 3-hexyl thiophene), PCDTBT(poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4'-7'-di-2-thienyl-2',1',3 '-benzothiadiazole)]), PCPDTBT(poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt- 4,7-(2,1,3-benzothiadiazole)], PFO-DBT(poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4,7-bis(thiophene-2) -yl)benzo-2,1,3-thiadiazole)]), PTB7(Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene -2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]), PSiF-DBT(Poly[2,7-(9,9- dioctyl-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole]), PTB7-Th(Poly[4,8-bis(5-(2-ethylhexyl)) thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b] thiophene-)-2-carboxylate-2-6-diyl)]), PBDB-T(poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo) [1,2-b:4,5-b']dithiophene) )-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'- c']dithiophene-4,8-dione))), PCE10(Poly([2,6'-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene ]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) and PBDBT (poly(benzodithiophene-benzotriazole)). .

The electron transport layer may be formed by a method used in the art. For example, after dissolving each material in an organic solvent, the solution can be formed by spin coating, dip coating, screen printing, spray coating, doctor blade, brush painting, or the like, and the compound is produced in a film form through a deposition process. It can be formed by determining after, but is not limited to these methods

In one embodiment of the present specification, the electron transport layer is a metal complex of 8-hydroxyquinoline; Complexes including Alq ₃ ; Metal complexes including Liq; LiF; Ca; Titanium oxide (TiO _x ); Zinc oxide (ZnO); Polyethyleneimine (PEI); Polyethylenimine ethoxylated (PEIE); And cesium carbonate (Cs ₂ CO ₃ ), but is not limited thereto.

The method for manufacturing the hole transport layer for the organic photodiode and the organic photodiode is described in detail in the following Production Examples and Examples. However, the following examples are for illustrative purposes only, and the scope of the present specification is not limited by them.

Example 1.

The patterned ITO substrate was sequentially washed with a detergent, acetone, and isopropyl alcohol (IPA), and then dried for 10 minutes on a 120° C. heating plate to remove moisture.

The ITO substrate 5x10 ^- were moved into the vacuum chamber ⁸ torr vacuum. After depositing a hole transport material MoO _x (0<x≤3) on the ITO thin film at 30 nm at 560°C, it was moved to a nitrogen atmosphere at atmospheric pressure (1.0atm) in a vacuum chamber and exposed to atmospheric nitrogen atmosphere for 10 minutes to form a hole transport layer. .

The substrate again 5x10 ^- go into ⁸ torr of degree of vacuum of the vacuum chamber by depositing the electron donor material (SubPc) and an electron acceptor material (fullerene, C ₆₀₎ on the hole transport layer to form a photoactive layer.

An electron transport layer (BCP) was deposited on the photoactive layer in the same vacuum chamber to form an electron transport layer.

A second electrode layer was formed by depositing a second electrode material (Al) on the electron transport layer in the same vacuum chamber to complete the organic photodiode device.

Example 2.

An organic photodiode was manufactured in the same manner as in Example 1, except that a hole transport layer was formed by depositing a hole transport material in the manufacturing method of Example 1 and then exposing it to an atmospheric pressure (1.0 atm) atmosphere.

Comparative Example 1.

In the same manner as in Example 1, except in ⁸ torr vacuum, followed by the depositing a photoactive ^{layer, said} embodiment in which one-deposit of a hole transport material of the process for producing a post without being exposed to atmospheric pressure to form a hole transport layer, and 5x10 An organic photodiode was prepared.

3 shows EQE values according to wavelengths of the organic photodiodes prepared in Examples 1 and 2 and Comparative Example 1.

4 shows the current density according to the voltage of the organic photodiodes prepared in Examples 1 and 2 and Comparative Example 1.

The performances of the organic photodiodes prepared in Examples 1 and 2 and Comparative Example 1 are compared and shown in Table 1 below.

Both the photocurrent and dark current were measured in the dark box, and the current was measured in a 3V reverse voltage state using a source meter. The photocurrent represents the current value of the device when the 12000 lux white LED light source is illuminated on the organic photodiode device, and the dark current represents the current value of the device when there is no light.

Relative current value Dark current relative value Example 1 1.03 0.05 Example 2 1.03 0.05 Comparative Example 1 1.00 1.00

In Table 1, the relative value of the photocurrent was obtained through the following equation (1).

Equation (1): Photocurrent value at -3V reverse bias of the organic photodiode prepared in Example X / Photocurrent value at -3V reverse bias of the organic photodiode prepared in Comparative Example 1

In Table 1, the relative value of dark current was obtained through the following equation (2).

Equation (2): Dark current value at -3V reverse bias of the organic photodiode prepared in Example X / Dark current value at -3V reverse bias of the organic photodiode prepared in Comparative Example 1

In the formulas (1) and (2), Example X is Example 1, Example 2 or Comparative Example 1.

When the light is shined onto the organic photodiode, the current generated by the incident light must be greater than the intrinsic dark current of the device to detect the light. In other words, the signal-to-noise ratio (SNR) must be greater than 1 to detect the light, and the lower the dark current corresponding to N in the SNR, the important parameters of the light sensor, Noise Equivalent Power (NEP), Dynamic Range ( DR), and Detectivity are both likely to improve. That is, as the dark current value of the organic photodiode decreases, the ability to sense light improves.

From Table 1, it can be seen that the organic photodiodes prepared in Examples 1 and 2 have improved photocurrent values and reduced dark current values compared to the organic photodiodes produced in Comparative Examples. Through this, it can be seen that the organic photodiodes prepared in Examples 1 and 2 have improved ability to sense light compared to the organic photodiodes prepared in Comparative Example 1.

Example 3.

The organic photodiode prepared in Example 1 was stored at 80°C for 40 minutes.

Example 4.

The organic photodiode prepared in Example 2 was stored at 80°C for 40 minutes.

Comparative Example 2.

The organic photodiode prepared in Comparative Example 1 was stored at 80°C for 40 minutes.

The performances of the organic photodiodes prepared in Examples 3, 4 and Comparative Example 2 are compared and shown in Table 2 below.

Relative current value
(Before heat treatment / After heat treatment) Dark current relative value
(Before/after heat treatment) Example 3 1.00 1.88 Example 4 1.00 10.32 Comparative Example 2 1.00 13.16

Example 5.

The organic photodiode prepared in Example 1 was stored at 120°C for 10 minutes.

Example 6.

The organic photodiode prepared in Example 2 was stored at 120°C for 10 minutes.

Comparative Example 3.

The organic photodiode prepared in Comparative Example 1 was stored at 120°C for 10 minutes.

The performances of the organic photodiodes prepared in Examples 5, 6 and Comparative Example 3 are compared and shown in Table 3 below.

Relative current value
(Before heat treatment / After heat treatment) Dark current relative value
(Before/after heat treatment) Example 5 0.98 15.44 Example 6 0.97 50.00 Comparative Example 3 0.98 88.75

From Tables 2 and 3, it can be seen that the organic photodiodes prepared in Examples 1 and 2 are superior in performance retention at high temperature conditions compared to the organic photodiodes prepared in Comparative Example 1.

10: substrate
20: first electrode
30: electron transport layer
40: photoactive layer
50: hole transport layer
60: second electrode

Claims (10)

Depositing a hole transport material in a vacuum condition; And
A method of manufacturing a hole transport layer for an organic photodiode comprising exposing the deposited hole transport material to an atmospheric pressure condition. The method according to claim 1,
The hole transport material is a method for manufacturing a hole transport layer for an organic photodiode that is deposited on a photoactive layer. The method according to claim 1,
The hole transport material is a method for manufacturing a hole transport layer for an organic photodiode that is deposited on an electrode. The method according to claim 1,
Method of manufacturing a hole transport layer for an organic photodiode in which the step of exposing to the atmospheric pressure condition is performed in an inert gas atmosphere. The method according to claim 1,
The hole transport material is a method for manufacturing a hole transport layer for an organic photodiode comprising a metal oxide. The method according to claim 5,
The metal oxide comprises a molybdenum oxide, vanadium oxide, nickel oxide, tungsten oxide, copper oxide, copper aluminum oxide, germanium oxide and one or more of the chromium oxide method for manufacturing a hole transport layer for an organic photodiode. The method according to claim 1,
The atmospheric pressure condition is 0.8 atm to 1.2atm method for manufacturing a hole transport layer for an organic photodiode. The method according to claim 1,
The method of depositing a hole transport material in the vacuum condition is performed at 300°C to 2,400°C. The method according to claim 1,
Method of manufacturing a hole transport layer for an organic photodiode is that the step of exposing to the normal pressure condition is performed for 1 to 60 minutes. Preparing a first electrode;
Forming a photoactive layer on the first electrode; And
A method of manufacturing an organic photodiode comprising forming a second electrode on the photoactive layer,
The organic photodiode manufacturing method comprises the steps of forming a hole transport layer by a method according to any one of claims 1 and 4 to 9,
The forming of the hole transport layer may include forming a hole transport layer by depositing a hole transport material on the first electrode under vacuum conditions before forming the photoactive layer and then exposing it to atmospheric pressure; Or depositing a hole transport material on the photoactive layer in a vacuum condition before forming the second electrode, and then exposing it to atmospheric pressure to form a hole transport layer.

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