WO2012000726A2

WO2012000726A2 - Film for preparing optoelectronic device, preparation process and optoelectronic device thereof

Info

Publication number: WO2012000726A2
Application number: PCT/EP2011/058613
Authority: WO
Inventors: Lei Cao; Guo Feng Chen
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-06-30
Filing date: 2011-05-26
Publication date: 2012-01-05
Also published as: CN102311615A; WO2012000726A3

Abstract

The present invention relates to a film for preparing an optoelectronic device, the preparation process and the optoelectronic device thereof. Aiming at the problem in the prior art that the transparency and the conductivity of the flexible electrodes cannot be met at the same time, the present invention provides a conductive polymer- carbon nanotube composite material. The composite material with high transparency and high conductivity is prepared by dispersing the carbon nanotubes uniformly in the conductive polymer matrix. The carbon nanotubes form a network in the conductive polymers to provide channels for electron transportation, thereby increasing the conductivity of the conductive polymers by 3 to 5 orders of magnitude. At the same time, a red shift occurs to the absorption band of the doped conductive polymers, so that its transparency in the visible region is also increased. Therefore, a flexible transparent electrode with both improved transparency and improved conductivity can be produced.

Description

Film for preparing optoelectronic device, preparation process and optoelectronic device thereof

Technical field

The present invention relates to a film for preparing an optoelectronic device, a preparation process thereof and an optoelectronic device produced by the film.

Background art

In the past few decades, organic electronics has

gradually become a highly attractive field. Recently, organic materials have been widely used in various optoelectronic display devices, such as mobile phones, notebook computers, high definition televisions and digital cameras. In order to reduce the costs of roll-to-roll printing production

processes, currently great efforts are being made to develop large-area, flexible, and low price optoelectronic devices, such as electrodes. In the prior art, the electrodes used for optoelectronic display devices are rigid glasses with indium tin oxides (ITO) (cf. Vandana Singh, C.K. Suman and Satyendra Kumar, Proc. of ASID '06, 8-12 Oct.) . However, in order to be suitable for roll-to-roll printing production processes, flexible transparent electrodes are needed. In order to obtain such a flexible electrode, currently one technique has been developed for sputtering ITO onto a plastic substrate. However, the vacuum deposition method is both expensive and time-consuming, and in addition it is still a challenge to achieve the object of the required conductivity and

transparency. Moreover, due to the limited resources, the price of ITO is increasing rapidly. Recently, alternatives to ITO have attracted great research interest. Some researchers have produced a flexible transparent electrode by using conductive polymers such as polyaniline, polythiophene, etc. (cf. Yang Y., Heeger A. J., Applied

Physics Letters, Volume 64, Issue 10, March 7, 1994, pp.1245- 1247) . However, there is a contradiction in the conductive polymers: either being transparent or conductive. Conductive polymers have highly conjugated properties, which have high light absorbance in their neutral, undopped state, and the case in the doped state is even more severe. In order to obtain a higher transparency, a thinner film can be prepared, which, however, would generally cause an increase in

resistance. In addition, the upper limit of the transparency is determined by the material itself, depending on the molecular absorption of different conductive polymers.

Furthermore, some researchers have also produced a flexible transparent electrode by depositing surfactant- stabilized carbon nanotubes on a plastic substrate (cf. G. B. Blanchet, et al . , Applied Physics Letters, 2003, 82, 1291). In such electrodes, the carbon nanotubes are adsorbed onto the plastic substrate by Van der Waals force, and the

adhesive force between the carbon nanotubes and the plastic substrate is relatively poor. Further, due to the disordered stacking of the carbon nanotubes, the roughness of the electrode is also made relatively high.

Neither of the abovementioned two types of electrodes can overcome all the problems regarding the performance

requirements of high conductivity, high transparency, low roughness, high mechanical strength, etc., while by only increasing any one of the abovementioned properties this cannot make the electrode meet the requirements on the electrodes for use in flexible electronic equipment.

Therefore, there is a need to develop a film which is low in price and has low UV and visible light absorption, so as to achieve the ideal combination of high transparency and low resistance . Contents of the invention

Aiming at the abovementioned problems, the object of the present invention is to provide a relatively low-price film, which can be used for preparing optoelectronic devices such as flexible transparent electrodes, display screens, field- effect transistors, or electronic printed circuits, etc. Both the conductivity and transparency of the film can meet the requirements by a flexible optoelectronic device.

Particularly, the present invention provides a film for preparing an optoelectronic device, which has improved transparency and conductivity and has a relatively low surface roughness. Furthermore, it is relatively low in price, which complies with the requirements of cost saving.

The film provided by the present invention for preparing an optoelectronic device comprises carbon nanotubes and a conductive polymer. In this case, the conductive polymer can comprise at least one polymer selected from the following general formulae (1) to (4) :

wherein, R and R' , independently from each other, are respectively C1-C18 alkyl or C1-C18 alkoxy; and n is an integer in the range of 10 to 10⁵. The conductive polymer can contain but is not limited to polythiophene, poly (phenylene vinylene) , polyaniline, polypyrrole and the derivatives of the abovementioned conductive polymers, for example, the compounds having the following general formula (5) . In this case, the carbon nanotubes can be single-walled, double- walled or multi-walled carbon nanotubes,

General Formula

(5) .

The present invention also provides an optoelectronic device produced by the film as described above, such as a flexible transparent electrode, a display screen, a field- effect transistor, an electronic printed circuit, etc. The film of the present invention ensures at the same time transparency and conductivity, and avoids the defect of high roughness, thereby complying better with the requirements for the production of optoelectronic devices.

Furthermore, the present invention provides a process for preparing a film for preparing an optoelectronic device, comprising the following steps of:

(1) dispersing uniformly carbon nanotubes into a solution of a conductive polymer;

(2) coating the mixture dispersion so obtained onto a substrate; and

(3) curing the same at room temperature or by heating so as to form the film;

or

(1) dispersing uniformly carbon nanotubes into the monomers of the conductive polymer;

(2) coating the mixture dispersion so obtained onto a substrate; and

(3) polymerizing said monomers at room temperature or by heating, so as to form the film. The conductive polymer described above can comprise least one polymer selected form the following general formulae (1) to (4) :

wherein, R and R' , independently from each other, are respectively C1-C18 alkyl or C1-C18 alkoxy; and n is an integer within the range of 10 to 10⁵. The carbon nanotubes can be single-walled, double-walled or multi-walled carbon nanotubes. In terms of conductivity and transparency, the single-walled and double-walled carbon nanotubes are better than the multi-walled carbon nanotubes.

By controlling the amount to be coated, the thickness of the film for preparing an optoelectronic device is made to be 0.01-10 micrometers, preferably 0.01-1 micrometer, so as to ensure that the material meets the requirements of

transparency .

A transparent conductive film can be produced by the abovementioned process, which film can be used in the

production of the abovementioned optoelectronic devices such as electrodes, display screens, field-effect transistors, electronic printed circuits, etc. The process for preparing composite materials used for electronic devices such as electrodes and so on by a solution film-forming process has low costs, and the advantages of this are obvious compared with the process of ITO vacuum deposition in the prior art. Furthermore, since the carbon nanotubes are premixed

uniformly before forming a film, the roughness thereof is also reduced greatly. Due to the addition of the carbon nanotubes, the film, even when it is very thin, for example 0.01-1.0 micrometers, still possesses the required

conductivity, which complies with the requirements by an optoelectronic device. Description of the drawings

Fig. 1 is a schematic flow chart of the roll-to-roll printing production process for preparing electronic devices.

Exemplary embodiments Currently, the production of optoelectronic devices is mostly by using the roll-to-roll printing production process as shown in Fig. 1, which is convenient and efficient but requires that the electrode possesses some flexibility. One embodiment of the present invention provides a film

comprising conductive polymers and carbon nanotubes for preparing optoelectronic devices, which film can be used for preparing optoelectronic devices such as flexible transparent electrodes, etc.

In a preferred embodiment of the present invention, the conductive polymer in the film comprises at least one polymer selected from the following general formulae (1) to (4) :

General Formula

(4) wherein, R and R' , independently from each other, are respectively C1-C18 alkyl or C1-C18 alkoxy; and n is an integer within the range of 10 to 10⁵.

The conductive polymers can contain but are not limited to polythiophene, poly (phenylene vinylene) , polyaniline, polypyrrole and the derivatives of the abovementioned

conductive polymers, such as the compounds having the

abovementioned general formula (5) . In the final film, said conductive polymers can be doped or can also be undoped. The conductive polymer imparts flexibility and transparency to the film, thus making it capable of being used in the

production of devices of optoelectronic equipment such as flexible transparent electrodes, etc.

In said film, the highly conductive carbon nanotubes form channels for transporting electrons in the conductive polymer matrix, which can increase the conductivity of the conductive polymers by 3 to 5 orders of magnitude. The higher the content of the carbon nanotubes, the higher the conductivity thereof, but the transparency thereof will become poorer. By taking both the two factors into comprehensive consideration, in the preferred embodiments, the content of the carbon nanotubes in the film is 0.01-20 wt% by weight, and more preferably 0.1-10 wt%. The carbon nanotubes used can be single-walled, double-walled or multi-walled carbon nanotubes. The single-walled or double-walled carbon

nanotubes are preferred. Composite materials using single- walled or double-walled carbon nanotubes have higher

conductivity and transparency.

The flexible electrode produced by the film of the present invention is not only capable of increasing the conductivity of the electrode but also improving the adhesion of the carbon nanotubes to the plastic substrate, thus avoiding weak adhesion and high roughness resulting from the direct adhesion of the carbon nanotubes onto the plastic substrate .

When the film of the present invention is too thick, the transparency thereof will be affected, while when it is too thin, the resistance will be increased. In the present invention, a preferred thickness of film is 0.01-10

micrometers. 0.01-1 micrometer is further preferred. In the case of such a thickness, to some extent, consideration can be given to both the transparency and conductivity thereof at the same time.

The present invention also provides optoelectronic devices produced by the film as described above, which comprise but are not limited to, for example, flexible transparent electrodes, display screens, field-effect

transistors or electronic printed circuits.

The present invention also provides a process for

preparing the abovementioned film, with the main raw

materials being conductive polymers and carbon nanotubes. In this case, the conductive polymer comprises at least one polymer selected from the following general formulae (1) to (4) :

General Formula

(1)

General Formula

(2)

General Formula (3) and

General Formula (4) wherein, R and R' , independently from each other, are respectively C1-C18 alkyl or C1-C18 alkoxy; and n is an integer within the range of 10 to 10⁵. In a preferred embodiment, the content of the carbon nanotubes is 0.01-20 wt%, and more preferably 0.1-10 wt%. The carbon nanotubes used can be single-walled, double-walled or multi-walled carbon nanotubes. For the consideration of transparency and conductivity, single-walled or double-walled carbon nanotubes are preferred.

Depending on the time when the carbon nanotubes are added, the abovementioned process can be divided into two types: adding the carbon nanotubes before the polymerization of the conductive polymer and adding the carbon nanotubes after the polymerization of the conductive polymer. Both processes are able to obtain a composite material with increased transparency and conductivity. It is preferable to add the carbon nanotubes before the polymerization of the conductive polymer, so that the carbon nanotubes can be dispersed more uniformly.

When adding the carbon nanotubes, an ultrasonic method can be used to make them dispersed uniformly in the monomers of the conductive polymer or a solution of the conductive polymer. Other methods commonly used in the art can also be employed as long as they can make them dispersed uniformly. In the method for adding the carbon nanotubes before the polymerization of the conductive polymers, this can comprise the following steps of:

(1) stabilizing the carbon nanotubes in a solution of conductive polymer monomers by an ultrasound method, and dispersing said carbon nanotubes into the conductive polymer monomers uniformly;

(2) coating the mixture dispersion so obtained onto a substrate; and (3) polymerizing the monomers at room temperature or by heating, so as to form the film.

The abovementioned polymerization process can be

determined by the monomers used for the polymer, and a polymerization process well known to those skilled in the art can be used. When the monomers used are polythiophene, the oxidant used in the abovementioned polymerization step is iron toluene sulfonate Fe(OTs)3, while imidazole is used as a base for weakening the oxidation potential of Fe(OTs)3. After the polymerization, it can comprise a doping step to dope the same and it is also possible not to comprise such a doping step. Doping methods and dopants that are well known to those skilled in the art can be used for the doping.

In the method for adding the carbon nanotubes after the polymerization of the conductive polymers, this comprises the following steps of:

(1) stabilizing carbon nanotubes in a solution of conductive polymers by ultrasound, and dispersing uniformly said carbon nanotubes into the solution of the conductive polymer;

(2) coating the mixture dispersion so obtained onto a substrate; curing the same at room temperature or by heating to form a film; and

(3) an optional step of: carrying out a doping treatment on the conductive polymer on the substrate. The dopant and doping method used can be those well known in the art. Those skilled in the art can choose by themselves depending on the needs.

When the conductive polymer used is polythiophene, it can be performed by using a polyalcohol as the dopant, for example, ethylene glycol and glycerol and so on. The doped conductive polymer displays a relatively high conductivity. The mechanical performances and adhesion property can also be improved by post treatments, for example, by a thermal treatment .

The substrate used in the abovementioned methods can be a plastic substrate, including a plastic substrate made from polycarbonate or poly (ethylene terephthalate) .

A schematic process flowchart for preparing an electronic device by this process is shown in Fig. 1, which can, for example, be used for preparing a flexible transparent

electrode. The preparation process mainly comprises three steps :

Step one: depositing the abovementioned raw material, i.e. the composite material of carbon nanotubes and

conductive polymer, on a plastic substrate;

Step two: patterning, depending on the electronic device that needs to be produced, such as the shape of an electrode, patterning the deposited composite material, so as to form a suitable pattern;

Step three: packaging, carrying out an appropriate treatment on the product obtained in step two, packaging the same appropriately, so as to obtain the finished electronic device, such as a flexible transparent electrode.

The present invention is described below particularly in conjunction with the embodiments, which are only for

illustration, rather than limiting the scope of the present invention .

Embodiment 1

In order to produce a flexible electrode to increase the conductivity of the electrode and to improve the weak adhesion of the carbon nanotubes on the plastic substrate, a carbon nanotube-polythiophene composite material film was prepared, and this film was used for preparing the electrode. The abovementioned flexible transparent electrode could also be prepared by an in situ polymerization process. This preparation process can be summarized as follows.

In this embodiment, thiophene monomers were selected as starting materials, and the thiophene monomers can be

oxidatively polymerized. In this process, iron toluene sulfonate (Fe(OTs)3) was used as a chemical equivalent oxidant, and imidazole was used as a base for weakening the oxidation potential of Fe(OTs)3 to perform an oxidative polymerization of the thiophene monomers.

Firstly, 20 wt% of single-walled carbon nanotubes were dispersed into thiophene monomers at a concentration of 0.1- 50% by ultrasound. The obtained mixture was coated onto a plastic substrate. Then, the abovementioned thiophene

monomers were oxidatively polymerized by using Fe(OTs)3 as an oxidant, and imidazole as a base for weakening the oxidation potential of Fe(OTs)3. The polymerization was carried out under a protective atmosphere of nitrogen. The polymerization temperature was 20-150°C, and the time was 0.5-24 hours. It was preferably 0.5-12 hours, and most preferably 3-5 hours.

In the composite material film so obtained, polythiophene acts as the polymer matrix, and the network structure of the carbon nanotubes makes them highly conductive.

The measurement for conductivity can be carried out by a 4-probe method. The transparency can be measured by UV- visible absorption spectrum. In this embodiment, the thickness of the composite material film so obtained was 0.01-10 micrometers, and in the case of a thickness of 0.01-10 micrometers, the transparency of the composite material film was ensured. At the same time, conductivity was increased by the carbon nanotubes having a network structure therein, thereby remedying the defect of a resistance increase caused by the thinner film. Embodiment 2

Multi-walled carbon nanotubes with a content of 10 wt% were dispersed in a solution of 0.1-50% polythiophene by ultrasound, and the mixture obtained was coated onto a plastic substrate, with glycerol being used as a dopant to increase the conductivity.

The doped polythiophene exhibited a relatively high conductivity. The carbon nanotubes formed a network in the polythiophene matrix, which provided channels for electron transportation. The mechanical performances and adhesion property can also be improved by post treatments, for example by annealing.

Said plastic substrate comprised polycarbonate or

poly (ethylene terephthalate) .

The measurement for conductivity can be carried out by a 4-probe method. The transparency can be measured by UV- visible absorption spectrum.

The band gap of the doped polythiophene was very low, and the absorption band thereof was red-shifted to the near- infrared region. Therefore, it was possible to prepare a highly conductive transparent film. Furthermore, the method itself has inherent compatibility with the process of patterning deposition for the deposition region simulating the electrode shape, which makes it easier for it to be integrated with the prior art to prepare new structures for apparatuses. The flexible electrode can be used in the roll- to-roll printing production process for preparing electronic equipment of organic and polymer types at low costs.

Although the embodiments of the present invention are described above in detail by way of example, it will be appreciated by those skilled in the art that the present invention is not limited to these embodiments. Appropriate modifications of the embodiments should also fall into the protective scope of the present invention. The protective scope of the present invention is defined according to the appended claims.

Claims

1. A film for preparing an optoelectronic device, wherein it comprises carbon nanotubes and a conductive polymer .

2. The film as claimed in claim 1, wherein said conductive polymers comprise at least one polymer selected from the general formulae (1) to (4) :

General Formula

(1)

General Formula

General Formula (3) and

General Formula (4) wherein, R and R' , independently from each other, are respectively C1-C18 alkyl or C1-C18 alkoxy; and n is an integer in the range of 10 to 10⁵.

3. The film as claimed in claim 1, wherein the content of said carbon nanotubes is 0.01-20 wt%.

4. The film as claimed in claim 3, wherein the content of said carbon nanotubes is 0.1-10 wt%.

5. The film as claimed in claim 1, wherein said carbon nanotubes are of single-walled, double-walled or multi-walled carbon nanotubes.

6. The film as claimed in claim 1, wherein the thickness of said film is 0.01-10 micrometers.

7. The film as claimed in claim 6, wherein the thickness of said film is 0.01-1 micrometer.

8. An optoelectronic device produced by the film as claimed in any one of claims 1 to 7.

9. The optoelectronic device as claimed in claim 8, wherein said optoelectronic device is a flexible transparent electrode, a display screen, a field-effect transistor or an electronic printed circuit.

10. A process for preparing the film for preparing an optoelectronic device as claimed in any one of claims 1 to 7, comprising the following steps of:

(1) dispersing uniformly the carbon nanotubes into the monomers of the conductive polymer;

(2) coating the mixture dispersion so obtained onto a substrate; and

(3) polymerizing said monomers at room temperature or by heating, so as to form the film.

11. The process as claimed in claim 10, wherein said conductive polymer comprises at least one polymer selected from the following general formulae (1) to (4) :

General Formula

General Formula

General Formula (3) and

respectively C1-C18 alkyl or C1-C18 alkoxy; and n is an integer in the range of 10 to 10⁵.

12. The process as claimed in claim 10, wherein the content of said carbon nanotubes is 0.01-20 wt%.

13. The process as claimed in claim 12, wherein the content of said carbon nanotubes is 0.1-10 wt%.

14. The process as claimed in claim 10, wherein said carbon nanotubes are of single-walled, double-walled or multi-walled carbon nanotubes.

15. The process as claimed in claim 10, wherein said carbon nanotubes are dispersed uniformly into the monomers of the conductive polymer by an ultrasonic method.

16. The process as claimed in claim 10, wherein, the oxidant used in said monomer polymerizing step is iron toluene sulfonate, and at the same time imidazole is used as a base for weakening the oxidation potential of iron toluene sulfonate .

17. The process as claimed in claim 10, wherein, said coating step is adjusted for the thickness of the film finally obtained to be 0.01-10 micrometers.

18. A process for preparing the film for preparing an optoelectronic device as claimed in any one of claims 1 to 7, comprising the following steps of:

(1) dispersing uniformly the carbon nanotubes into a solution of the conductive polymer;

(2) coating the mixture dispersion so obtained onto a substrate; and

(3) curing the same at room temperature or by heating to form the film.

19. The process as claimed in claim 18, wherein said carbon nanotubes are dispersed uniformly into the solution of the conductive polymer by an ultrasonic method.

20. The process as claimed in claim 18, wherein, it further comprises a doping step after said dispersing

uniformly the carbon nanotubes into the solution of the conductive polymer and coating the mixture dispersion so obtained onto the substrate.

21. The process as claimed in claim 18, wherein said doping step is performed by using a polyalcohol as a dopant.