WO2014086778A1

WO2014086778A1 - Carbon nanotube material, devices and methods

Info

Publication number: WO2014086778A1
Application number: PCT/EP2013/075384
Authority: WO
Inventors: Dinesha DABERA; Imalka JAYAWARDENA; Damitha ADIKAARI; Ravi Silva
Original assignee: The University Of Surrey
Priority date: 2012-12-04
Filing date: 2013-12-03
Publication date: 2014-06-12

Abstract

A method for preparing hybrid carbon nanotube/polymer material is disclosed, the method comprising the steps of applying an ultrasonic process to a solution comprising carbon nanotubes (12) and a conjugated polymer (14) to substantially wrap or coat the carbon nanotubes with the polymer, and removing the excess polymer from the solution. Transistor devices incorporating the material exhibit narrow band gap, high mobility and p-type properties. Photovoltaic device results showing 30-40% improvement in efficiency over similar devices indicates the superior hole transport properties of the material.

Description

CARBON NANOTUBE MATERIAL, DEVICES AND METHODS TECHNICAL FIELD

This invention relates to carbon nanotube hybrid material systems and methods of preparation and use thereof in device structures. The invention has particular, but not exclusive application to devices in the field of organic electronics and optoelectronics.

BACKGROUND

The field of organic electronics, that is electronic devices that rely in whole or in part on carbon or carbon based compounds, has gathered much interest in the last few decades.

The applications of organic electronics include large area sensors and detectors, printable radio frequency identification (RF-ID) tags, labels and the like, large area displays, photovoltaic devices, organic light emitting devices, such as for example Organic Light Emitting Diodes (OLEDs), and transistors such as Organic Field-Effect Transistors (OFETs).

The applications for organic electronics, in contrast to conventional silicon- based or so-called compound semiconductor electronic technologies, are often characterised as being "large area" and "low operating frequency", the latter being limited by the typically low (when compared with crystalline silicon for example) carrier mobilities of the organic electronic layer or material employed.

A problem with many materials of interest in the field of organic electronics lies in the fact that such materials often degrade or are unstable when in contact with, or exposed to atmospheric conditions (oxygen) or moisture in particular. Therefore, the encapsulation of devices, and use of so-called "barrier layers" to prevent the degradation of the organic material of the device from such exposure is employed . This adds cost and complexity to the devices.

Another problem that organic electronic materials often suffer from involves the degradation of the aforementioned materials when in contact with other materials used in devices. For example, such degradation problems limit the useful lifetime of photovoltaic devices. One common system that suffers such problems comprises the transparent conductor Indium Tin Oxide (ITO) when in contact with hole transport layers such as PEDOT: PSS [poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)] .

There is therefore a desire for an organic semiconducting material that improves upon those currently known.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided a material comprising carbon nanotubes overlaid by or wrapped with a polymer forming a material with a low band gap and providing the material with inherent p-type semiconducting properties.

In a second aspect of the invention a method for producing a semiconducting material is provided, comprising the steps of applying an ultrasonic process to a solution comprising carbon nanotubes and a polymer to substantially wrap or coat a fraction of the carbon nanotubes with the polymer, and removing the excess polymer from the extracted solution.

The ultrasonic process used above is preferably carried out at a level that is not detrimental to the structure of the polymer chains, that is it does not break up the polymer chains leading to the formation of polymer nanocrystals or lumps on the carbon nanotube surface that can lead to inferior properties.

In yet another aspect of the invention a method for manufacturing an electronic backplane or substrate is provided, the method comprising providing a substrate, spin coating, spraying or printing a solution comprising semiconducting material according to the first aspect of the invention onto the substrate, and annealing the substrate.

In another aspect of the invention there is provided a device comprising the material according to the first aspect.

In yet a further aspect of the invention the use of the material of the first aspect as a hole transport layer in an electronic device is provided.

Advantageously, the invention provides a relatively narrow bandgap semiconducting hybrid material that exhibits intrinsic p-type behaviour, which demonstrates superior power conversion efficiency in photovoltaic structures due to modification of the absorption cross-section across the spectrum, as well as a high hole mobility. The overall hybrid material absorbs more photons thereby creating more excitons, separates the excitons and efficiently transports the carriers to the contacts or electrodes.

Owing to the invention, the relatively narrow bandgap of the material enables application to energy harvesting in the near infrared spectrum. Furthermore, the material is solution processable which lends itself to large area, and inexpensive electronic applications.

In an embodiment, the carbon nanotubes are semiconducting single walled carbon nanotubes (s-SWCNT). In other embodiments, the carbon nanotubes are metallic or semiconducting double walled, or multi-walled carbon nanotubes.

In another embodiment the polymer is a conjugated polymer. Example classes comprise regioregular thiophenes or derivatives of polyvinylenes such as poly(metaphenylenevinylene).

Other example polymer embodiments comprise polypyrrolidones such as poly(vinyl pyrrolidone), polystyrenes such as poly-(styrene sulfonate)), or polyethylenes such as (Poly(ethyleneoxide) dimethyl ether)).

Further example polymer embodiments comprise polyacrylates such as poly(methyl methacrylate), polyanilines, polycarbazoles such as Poly[[9- (l-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2, l,3- benzothiadiazole-4,7-diyl-2,5-thiophenediyl]), and polyfluorenes such as poly(9,9-dioctylfluorenyl-2,7-diyl).

In an embodiment, the wrapping polymer is poly(3-hexylthiophene-2,5- diyl), commonly known as P3HT, and the carbon nanotubes are semiconducting single walled carbon nanotubes (s-SWCNT).

In another embodiment, the material generated by the aforementioned method aspect, being solution processable, is spin coated and optionally annealed on a variety of substrates such as, for example, silicon dioxide coated crystalline silicon, glass, plastic or polymer/organic flexible substrates to provide an electronic structure in the form of a backplane or substrate.

In another embodiment, a device in the form of a thin film, field effect transistor is provided utilising the material, and exhibits p-type hole carrier properties and mobilities around 0.5cm²/Vs. In another embodiment, the material is incorporated in a photovoltaic device as a hole transport layer, and power conversion efficiencies in excess of 7% were recorded which represents somewhere in the region of a 30-40% improvement over devices not incorporating such hole transport layers. Similar improvements were obtained when the material according to an embodiment of the invention was compared, in the same device structure, with photovoltaic devices using the commonly used hole transport layer PEDOT: PSS rather than the material according embodiments of the present invention.

In another embodiment the material is incorporated in an organic solar cell as a semi-transparent hole collecting electrode. The material is used as a replacement for commonly used Indium Tin Oxide (ITO) electrodes.

Further optional features will be apparent from the following description and accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only with reference to the accompanying drawings, in which :

Figure 1 illustrates a schematic of the material according to an embodiment of the invention;

Figure 2 illustrates a method according to an embodiment of the invention;

Figure 3a is a flow chart of another method according to an embodiment of the invention; Figure 3b is a schematic representation of the backplane produced according to an embodiment of the invention;

Figure 4a shows the measured transfer characteristics of a transistor device according to an embodiment of the invention;

Figure 4b is a graph displaying the Raman spectroscopic characteristics of the material according to an embodiment of the invention;

Figure 5a illustrates an example optical photovoltaic device according to an embodiment of the invention;

Figure 5b is a graph of the measured characteristics of the device of Figure 5a;

Figure 6 is a is a flow chart of another method according to an

embodiment of the invention;

Figure 7a is a schematic representation of a solar cell according to an embodiment of the invention; and

Figure 7b is a graph of device performance characteristics of the device of Figure 7a .

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows a semi-conducting material 10 according to an embodiment of the invention. In this embodiment, the material 10 comprises a carbon nanotube (CNT) 12 wrapped, sheathed or overlaid with a polymer 14 as shown. In an embodiment the carbon nanotube comprises semiconducting singled walled carbon nanotubes (s-SWCNT) 12 and the polymer 14 is regioregular P3HT.

Figure 2 shows an embodiment of a method 20 for preparation of the material 10. In a first step 22 a solution comprising CNT 12 and polymer material 14 are mixed (M) in a mixing vessel. Subsequently, in step 24 the mixed solution is subjected to an ultrasonic (U) process which effectively wraps the polymer 14 around the carbon nanotubes 12.

A solvent (S) then may be added to the solution to dissolve excess polymer remaining in the solution, as shown in step 26.

The solution may then undergo further centrifugation at step 28 to extract the required fraction of material 10.

Steps 26 and 28 may be repeated several times as shown by the arrowed path 30 in Figure 2, in order to obtain and further refine the quantity of required material 10.

The following example embodiment further illustrates the method.

In an embodiment, a mass of 0.6mg of rr-P3HT (Rieke Metals Inc, weight average molecular weight, Mw = SOOOOgmol^"1 and regioregularity = 95%) was added to 1.00ml of chlorobenzene such that the concentration of rr-P3HT was 0.60mg ml^"1 and then sonicated for 60 minutes.

A mass of 0.50 mg of s-SWNTs (NanointegrisIsoNanotubes-s (90% semiconducting)) was added to the solution (O.Smgml^"1) and was ultrasonically treated at a frequency of 32 - 38 kHz using an ultrasonic bath for a further 60 minutes. This gentle sonication appears to minimise any polymer chain scission and hence the formation of polymer nanocrystals or nano-lumps that decorate the nanotubes and adversely affect the properties achieved . It is thought that this sonication process provides less damage to the material leading to a good interface between the nanotubes and the polymer chains, which in turn gives rise to the observed p-type mobility and low bandgap of the hybrid material .

In a next step, a volume of 1.00ml of toluene was added to the solution mixture, sonicated for 15 min and centrifuged for 8 minutes at 16000g in order to remove the excess rr-P3HT in the medium .

The precipitate was collected while the supernatant (dark yellow) was discarded.

The above sonication and centrifugation procedures were then repeated twice more and the precipitate was collected while the supernatant (pale yellow) was discarded .

The precipitate obtained was subsequently dissolved in 1.0ml of 1,2- dichlorobenzene by ultra-sonication for 60 minutes (rr-P3HT/s-SWNT) such that the final concentration of s-SWNTs in rr-P3HT/s-SWNTs was approximately 0.5 mg ml^"1.

In an embodiment, the material 10 may subsequently, or during the processes described above, be treated with metallic oxides such as molybdenum, vanadium, nickel or graphene oxides to further hole dope the material 10 as well as to form an efficient hole transport layer for application to, for example, organic light emitting diodes and organic photovoltaic cells. Such oxide layers can be deposited onto films of material 10 on a substrate through thermal evaporation of the respective metal oxide powders or through spin coating of a metal containing precursor solution (for example ammonium heptamolybdate, vanadyl acetylacetonate) and subsequent thermal treatment in air. This improves the hole transport properties and provides the basis for a substitute transparent conducting material that is solution processable or sprayable on to large area substrates. It is the basis of a substitute for ITO, providing a p-type conductor that is substantially transparent over the visible spectrum .

The material 10 obtained from the embodiments of methods of preparation as described previously, has been found to exhibit interesting and new properties. The material clearly lends itself to solution processability on large area and inexpensive substrates as shall now be described.

Figure 3a and Figure 3b illustrate a method for producing an electronic structure such as a substrate or a backplane for device fabrication.

In Figure 3a, at step 34 a solution 42 (S) comprising polymer 14 and carbon nanotubes 12 is applied to a substrate 40. The substrate is then spun at step 36 to spin coat the solution 42 evenly across the substrate 40 in thin film form . The resulting substrate and thin film is then annealed at step 38 to produce the electronic backplane 48. Although spin coating is used here, alternative coating techniques such as spray deposition and printing that are used on an industrial scale can be used to achieve the same effect. Such coated substrates or backplanes can then be supplied to device manufacturers for device fabrication and use.

In embodiments, the substrate is a silicon dioxide coated silicon wafer or glass. In other embodiments, the substrate may be an organic or plastic thin film since the annealing step is at relatively low temperatures. For example, in an embodiment the substrate was annealed at 120°C for 10 minutes.

In another embodiment, the backplane may also comprise a conductive layer 54 depending on the application that the backplane is intended for.

In the embodiment above, spin coating was utilised . Due to the solution processable nature of the material, those skilled in the art will appreciate that other methods of deposition for coating a solution 42 comprising the material 10 onto the substrate 40 at step 36may be used in other embodiments. By way of example, other suitable methods that may be used comprise spray coating, roller coating or printing (including ink-jet, gravure printing, tampon printing, pad printing, transfer printing or screen printing), doctor blade coating, dip coating, Langmuir-Blodgett coating, or electrophoretic deposition.

In an embodiment for photovoltaic applications, a transparent conductor such as ITO is deposited on the substrate using conventional deposition and/or patterning techniques, followed by the provision of a layer of material 44 as described above. This embodiment provides a mass producible backplane 48 suitable for supply to photovoltaic device manufacturers or foundries.

Devices were then produced comprising the material 10, 44 as a semiconducting layer as follows, with reference to Figure 4a and Figure 4b.

In one embodiment, a field effect transistor structure was fabricated using the semi-conducting material 10 produced as described previously as an active semi-conducting layer. In this embodiment an n-type crystalline silicon substrate was provided with a 230nm dielectric layer of silicon dioxide. Gold/Tin metal contacts were subsequently deposited in an interdigitated pattern (fabricated using conventional photolithographic techniques with a channel width of 2000pm and channel length of lOprn) in order to form drain and source electrodes. Subsequently, a solution 42 containing the carbon nanotubes 12 and polymer 14 was spin coated and annealed at 110°C for 10 minutes to produce a layer of semiconducting material 10, 44 comprising s-SWCNT 12 wrapped in rr-P3HT 14.

Those skilled in the art will also appreciate that other configurations such as top gate architectures, top electrode architectures, different electrode materials, and channel width and/or length considerations as common in thin film transistor architecture may be utilised .

Measurements were then performed in a nitrogen glove box using an electrometer. The c-Si substrate was negatively biased so as to behave as a global back gate.

Figure 4a shows the measured transfer characteristics (I_ds versus V_gs) of the transistors obtained at varying drain-source bias.

The devices display on/off ratios of 10³- 10⁵ and hole mobilities of 0.01 - 1 cmV's^"1. Other measurements indicate that the bandgap appears to be in the region of 1 to 1.2eV.

Of significance is the fact that the characteristic of the thin film transistor clearly exhibits inherent p-type behaviour, evidenced by the increase in I_DS current when the gate is biased negatively. This dominant hole transport behaviour is attributed to charge transfer from the polymer to the nanotubes which results in a compound hybrid material semiconducting system comprising hole doped nanotubes and an electron rich polymer.

The inventors postulate that as the polymer rr-P3HT used in this embodiment is mainly a hole transporting system, the electron transport is weakened in the device which leads to the lack of ambipolar (or dual carrier) nature observed in this system, in contradistinction to carbon nanotubes devices prepared using surfactants.

To investigate further, Raman spectroscopic measurement on the material 10 were undertaken. Figure 4b illustrates the results lending credence to the above theory, as indicated by the red shift of the G peak of the carbon nanotube as shown in the Figure 4b.

In another embodiment, photovoltaic devices 50 were fabricated to assess the material 10, 44 further. Figure 5a shows such a device having a transparent glass substrate 52 onto which a transparent electrode 54 in the form of ITO was deposited . Onto this material 10; 44 was provided as previously described to form a hole transport layer 56. The material 10; 44 in this embodiment comprised s-SWCNT wrapped in rr-P3HT. Onto this a photoactive donor/ acceptor blend layer 58 was then spun coated.

In this embodiment, the polymer used for layer 58 was the highly efficient low bandgap polymer commonly referred to as PTB7 [Poly[[4,8-bis[(2- ethylhexyl)oxy]benzo[l,2-b :4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2- ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] which is known to yield power conversion efficiencies above 7%. The device 50 was then provided with an electron transport layer 60, which in this embodiment was formed of Bathocuproine (BCP) and finally an aluminium electrode 62 was provided to bias the device 50.

Figure 5b illustrates the photovoltaic performance 64 of device(s) 50 measured under standard testing conditions of 100 mWcm^"2 illumination using AM 1.5G standardised solar spectrum equipment. The devices 50 incorporating the polymer wrapped carbon nanotube material 10, 44 as the hole transport layer 56 (the curve labelled rr-P3HT/s-SWNT in the figure) displayed a 35% enhancement in the power conversion efficiency from 5.6% to 7.6% compared to similar devices fabricated without the hole transport layer.

Devices using the commonly used PEDOT: PSS as the hole transport layer 66 were also fabricated for comparison and, as shown in the figure, found to be inferior in comparison to rr-P3HT/s-SWNT material 10, 44 as prepared according to embodiments of the invention previously described .

In another embodiment, the material 10 was used as a hole collecting semi-transparent electrode in an organic solar cell .

Figure 6 illustrates a method for producing such an electrode.

Step 1. Material 10 (0.5 mg ml^"1) was drop cast onto a glass substrate and slow dried at 40°C for 10 min to form a uniform film.

Step 2. The dried film on the substrate was heated at 300°C for 5min.

Step 3. A second film was drop cast onto the film obtained after step 2 and was slow dried at 40°C for 10 min.

Step 4. The dried film on the substrate was heated at 300°C for 5min. Step 5. The annealed film was soaked in 70% HN0₃ for 2 hrs.

Heating the electrode film at 300°C burns out the polymer and the 70% HN0₃ soaking process increases the conductivity of the electrode film through hole doping the CNTs.

The sheet resistance of the film after step 1 is 8748 Ohm/sq and the final sheet resistance after step 5 is 187 Ohm/sq .

Figure 7a represents the device architecture used to fabricate the solar cell. The device contains a transparent glass substrate onto which a semi- transparent electrode film (steps 1 to 5) was deposited. A film of

PEDOT: PSS (Ethanol/EtOH) 66 was spin coated on top of the electrode 68 to reduce the roughness of the CNT electrode.

Onto this a photoactive donor/ acceptor blend layer 70 was then spun coated.

In this embodiment, the electron donating polymer used for the active layer was PTB7 [Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[l,2-b:4,5- b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophenediyl]] and the electron accepeting fullerene was PC₇₀BM.

The device was then provided with an electron transport layer 72, which in this embodiment was formed of Bathocuproine (BCP) and finally an aluminium electrode 74 was provided to bias the device.

Figure 7b illustrates the photovoltaic performance of the device measured under standard testing conditions of 100 mWcm^"2 illumination using AM 1.5G standard solar spectrum equipment. The devices incorporating the conductivity enhanced carbon nanotube material as the hole collecting semi-transparent electrode displayed a 4.4% power conversion efficiency.

Those skilled in the art will also appreciate that other configurations and architectures regarding thin film transistor, light emitting diodes or photovoltaic devices such as solar cells may be utilised due to the processable, and organic nature of the material and methods described herein.

Finally, it will be understood that the present invention has been described in its preferred embodiment and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.

Claims

1. A material comprising carbon nanotubes overlaid by or wrapped with a polymer forming a material with a low band gap and providing the material with inherent p-type semiconducting properties.

2. A material according to claim 1, wherein the carbon nanotubes comprise semiconducting or metallic single walled carbon nanotubes, double or multi-walled carbon nanotubes.

3. A material according to claim 1 or claim 2 wherein the polymer is a conjugated polymer.

4. A material according to claim 3 wherein the conjugated polymer is a polythiophene derivative.

5. A material according to claim 3 wherein the polymer is selected from the group consisting of poly (3-hexylthiophene-2,5-diyl), polyvinylene derivatives, a polypyrrolidone, polystyrenes, polyethylene, polyacrylate, a polyaniline, a polycarbazole, a polyfluorene and combinations thereof.

6. A material according to any of claims 1 to 5, wherein the carrier mobility is in the range 0.01 cm²/Vs to 10 cm²/Vs, preferably 0.01 cm²/Vs to 1 cm²/Vs.

7. A material according to claim 6, wherein the carrier mobility is 0.1 cm²/ Vs.

8. A material according to claim 1 or 2, wherein the bandgap of the material is in the range of 0.5 - 2.5eV, preferably 0.5 - 1.5eV.

9. A material according to claim 8, wherein the bandgap has a value around leV.

10. A material according to any of claims 1 to 9, wherein the material is either dispersed or coated with a metal oxide.

11. A material according to claim 10, wherein the metal oxide is selected from the group consisting of molybdenum oxide, vanadium oxide, tungsten oxide, nickel oxide or graphene oxide.

12. A method for producing a semiconducting material, comprising the steps of:

i) applying an ultrasonic process to a solution or dispersion comprising carbon nanotubes and a polymer to substantially wrap or coat a fraction of the carbon nanotubes with the polymer, and

ii) removing the excess polymer from the solution or dispersion.

13. A method according to claim 12, wherein the ultrasonic process comprises treating the solution or dispersion in an ultra sonic bath operating in the frequency range of 32-38 kHz.

14. A method according to claim 12 or claim 13, wherein the method comprises the further step of heating and acid soaking in order to form a thin transparent electrode.

15. An electronic device comprising semiconducting material according to any one of claims 1 to 11.

16. An electronic device according to claim 15, wherein the device is: i) a thin film transistor, or

ii) an organic photovoltaic cell or

iii) a detector/sensor or

iv) a touch screen or

v) a display or

vi) an electronic device such as a tag/label or

vii) an organic light emitting diode.

17. A method of manufacturing an electronic structure comprising :

i) providing a substrate, and

ii) spin coating, or optionally: spray coating, roller coating or printing (including ink-jet, gravure, tampon printing, pad printing, transfer printing or screen printing), or doctor blade coating, dip coating, Langmuir-Blodgett coating, or electrophoretic coating a solution comprising the material according to any one of claims 1 to 11 onto the substrate.

18. A method according to claim 17, wherein the solution comprises thixotropic agents, levelling agents or carrier solvents to aid deposition.

19. A method according to claim 17 or claim 18, further comprising depositing a first electrode layer in contact with the semiconducting material .

20. A method according to claim 19, wherein the first electrode layer is Indium Tin Oxide.

21. Use of the semiconducting material according to any one of claims 1 to 11 as a hole transport layer in an electronic device.