GB2560725A - Ink formulation

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Abstract

Description

Claims

GB2560725A

Publication number: GB2560725A
Application number: GB1704488.4A
Authority: GB
Inventors: Fletcher Thomas; King Simon
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2017-03-21
Filing date: 2017-03-21
Publication date: 2018-09-26
Also published as: GB201704488D0

A formulation comprising a fullerene and an n-dopant dissolved or dispersed in a solvent mixture comprising a first aprotic solvent and a second aprotic solvent. The solvent mixture having a polar Hansen solubility parameter δP of at least 4.0. The formulation may be used in forming a layer of a thermoelectric device, for example by printing of the ink formulation into wells 107 defined by a patterned insulating layer over an electrode 103 to form the n-type leg 109 of a thermoelectric device. The n doped fullerene may be n-doped PCBM, and the dopant may be a 1-H-benzimidazole. The formulation may be dispersed or dissolved in a polymer binder. The first aprotic solvent may be selected from the group consisting of benzene substituted with one or more substituents. The second solvent may be selected form the group consisting of dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone or propylene carbonate.

(71) Applicant(s):

Sumitomo Chemical Company Limited (Incorporated in Japan)

27-1, Shinkawa 2-chome, Chuo-ku, Tokyo 104-8260, Japan (72) Inventor(s):

Thomas Fletcher Simon King (74) Agent and/or Address for Service:

Venner Shipley LLP

Byron House, Cambridge Business Park,

Cowley Road, Cambridge, CB4 0WZ, United Kingdom (51) INT CL:

H01L 51/00 (2006.01) H01L 35/24 (2006.01) (56) Documents Cited:

WO 2012/054504 A WO 2011/127075 A

Journal of Materials Chemistry A, NO. 4, 2016, Chang et al. A solution-processed n-doped fullerene cathode interfacial layer for efficient and stable largearea perovskite solar cells pg 640-648.

Solar Energy Materials & Solar Cells, Vol. 100, 2nd February 2012, Machui et al. Determination of the P3HT:PCBM solubility parameters via a binary solvent gradient method: Impact of solubility on the photovoltaic performance, pages 138-146. Nanotechnology, Vol. 24, 6th November 2013, Cheng et al. A DMF-assisted solution process boosts the efficiency in P3HT:PCBM solar cells up to 5.31%, pages 484008 (7pp) (58) Field of Search:

INT CL H01L

Other: WPI, EPODOC, INSPEC, Patent fulltext, Internet.

(54) Title of the Invention: Ink formulation

Abstract Title: Fullerene based ink formulation comprising two aprotic solvents (57) A formulation comprising a fullerene and an n-dopant dissolved or dispersed in a solvent mixture comprising a first aprotic solvent and a second aprotic solvent. The solvent mixture having a polar Hansen solubility parameter δΡ of at least 4.0. The formulation may be used in forming a layer of a thermoelectric device, for example by printing of the ink formulation into wells 107 defined by a patterned insulating layer over an electrode 103 to form the n-type leg 109 of a thermoelectric device. The n doped fullerene may be n-doped PCBM, and the dopant may be a 1-Hbenzimidazole. The formulation may be dispersed or dissolved in a polymer binder. The first aprotic solvent may be selected from the group consisting of benzene substituted with one or more substituents. The second solvent may be selected form the group consisting of dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone or propylene carbonate.

FIGURE 1

200 έτ

103

1/2

FIGURE 2

25a ^24a 25b ^24b

V

2/2

FIGURE 3

-250

-200

-150 •100

0x05-00 0θ/θ-0000,0255.ξθθ2θ0,0.2

Ink Formulation

Field of the Invention

The invention relates to ink formulations for use in formation of a layer of an organic electronic device, particularly organic thermoelectric devices, and methods of forming said devices.

Background

Organic electronic devices comprising one or more active layers of organic materials are known including organic light-emitting devices, organic photovoltaic devices, organic photosensors and organic thin film transistors.

Fullerenes are known for use in organic electronic devices.

WO 2011/127075 discloses transistors containing n-doped phenyl-C61-butyric acid methyl ester (PCBM) transistors.

Active organic layers of organic electronic devices may be formed by solution deposition methods. However, in the case of doped materials, stability of an ink formulation containing an n-dopant in a solvent may be low.

It is an object to provide a stable formulation for forming an n-doped fullerene layer.

It is a further object of the invention to provide a formulation suitable for use in formation of a thermoelectric device.

Summary of the Invention

The present inventors have found that a combination of solvents having a polar Hansen solubility parameter δΡ of at least 4.0 can be used to form a stable formulation containing materials for forming an n-doped fullerene film.

Accordingly, in a first aspect the invention provides a formulation comprising a fullerene and an n-dopant thereof dissolved or dispersed in a solvent mixture comprising a first aprotic solvent and a second aprotic solvent, the solvent mixture having a polar Hansen solubility parameter δΡ of at least 4.0.

In a second aspect, the invention provides a method of forming a layer of an organic electronic device, the method comprising the step of depositing a formulation according to the first aspect onto a surface and evaporating the solvents.

Description of the Drawings

The invention will now be described in more detail with reference to the Figures, in which:

Figure 1 is a schematic illustration of a process of printing a formulation according to an embodiment of the invention in formation of a thermoelectric device;

Figure 2 schematically illustrates the device configuration used for Seebeck coefficient determination; and

Figure 3 is a graph of thermopower vs. Hanson solubility parameter δΡ for a range of solvents.

Detailed Description of the Invention

The ink formulations described herein may contain a first aprotic solvent having a dielectric constant at 20°C of less than 5 (referred to hereinafter a non-polar solvent) and / or a second aprotic solvent having a dielectric constant at 20°C greater than 25 (referred to hereinafter a polar solvent).

Preferably, the non-polar solvent is a benzene substituted with one or more substituents selected from Ch2 alkyl, Ch2 alkoxy and halogen, optionally chlorine, preferably Cn2 alkyl, Cm2 alkoxy, wherein two or more substituents may be linked to form a ring which may be unsubstituted or substituted with one or more Ci_6 alkyl groups. Preferably, the non-polar solvent is selected from toluene, xylenes, trimethylbenzenes, tetramethylbenzenes, anisole, 1,3-benzodioxole which may be unsubstituted or substituted with one or more Ci-12 alkyl groups, indane which may be unsubstituted or substituted with one or more Ci-12 alkyl groups, and tetralin which may be unsubstituted or substituted with one or more Ci-12 alkyl groups

The polar solvent has a dielectric constant at 20°C greater than 25, preferably greater than

30.

Preferably, the or each polar solvent is selected from dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone, y-butrylactone or propylene carbonate.

The ink formulation may contain only one non-polar solvent or more than one non-polar solvent. Preferably, the ink formulation contains only one non-polar solvent.

The ink formulation may contain only one polar solvent or more than one polar solvent. Preferably, the ink formulation contains only one polar solvent or two polar solvents.

Preferably, each solvent of the formulation has a boiling point of no more than 210°C.

Preferably, the Hansen parameter δρ of the solvent mixture is at least 4. Optionally, δρ is less than 10.

Preferably, the Hansen parameter hd of the solvent mixture is greater than 16.

Hansen Solubility Parameters can be determined according to the HSPiP program (Versions 4.1 or 5.0) as supplied by Hansen and Abbot et al. Values of Hansen parameters and details regarding their calculation can be found in C. M. Hansen, “Hansen Solubility Parameters: A User’s Handbook”, 2^nd Ed. 2007, Taylor and Francis Group LLC.

The ink formulation may be a solution.

The ink formulation may be a dispersion in which one or more components of the formulation are dispersed. The dispersion may have a solids content of 1-50 wt %, optionally 3-30 wt %. By “solids content” as used herein is meant the materials which are solid at 20°C which are dissolved or dispersed in the ink formulation.

In the case of a dispersion it will be understood that the formulation may comprise both dissolved and dispersed components.

It will be understood that some or all of the n-doped fullerene may be dissolved in the solvent mixture, any remaining n-doped fullerene being dispersed therein.

A formulation as described herein in the form of a dispersion may be sonically treated, optionally ultrasonically treated, to break up any aggregates present in the formulation.

For ink formulations to be used in formation of a thermoelectric device, the ink formulation preferably further comprises a polymeric binder. The binder may make layers formed from the formulation more resilient and less likely to crack than films in which no binder is present.

The polymeric binder may be a conjugated or non-conjugated polymer.

Exemplary non-conjugated binders include, without limitation, polyvinylpyrrolidone, PVDF, polyacrylates, preferably PMMA, and polystyrene.

By “conjugated polymer” as used herein is meant a polymer in which conjugated groups of adjacent repeat units of the polymer, preferably arylene, heteroarylene, arylenevinylene or heteroarylenevinylene groups, are bound directly to form a conjugated chain along some or all of the polymer backbone. Preferred conjugated polymers comprise one or more repeat units selected from phenylene, naphthalene, anthracene, fluorene, thiophene and benzothiadiazole repeat units.

Exemplary conjugated polymer binders include, without limitation, P(NDI2OD-T2), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-a/t-(benzo[2,l,3]thiadiazol-4,8-diyl)j (F8BT) and benzodifurandione-based PPV (BDPPV).

The ink formulation may contain only one polymeric binder or may contain more than one polymeric binder.

The polymeric binder or binders is optionally present in an amount in the range of 1 -50 mol % or 1 -25 wt % relative to the fullerene.

The fullerene may be a Ceo, C70, C76, C78 or (fo fullerene or a derivative thereof including, without limitation, PCBM-type fullerene derivatives (including phenyS-C61butyric acid methyl ester (CgoPCBM), TCBM-type fullerene derivatives (e.g. tolyl-C61 butyric acid methyl ester (C60TCBM)), and ThCBM-type fullerene derivatives (e.g. thienyl-C61-butyric acid methyl ester (CeoThCBM).

Fullerene derivatives may have formula (I):

wherein A, together with the C-C group of the fullerene, forms a monocyclic or fused ring group which may be unsubstituted or substituted with one or more substituents.

Exemplary fullerene derivatives include formulae (la), (lb) and (Ic):

15 wherein R -R are each independently H or a substituent.

Substituents R -R are optionally and independently in each occurrence selected from the group consisting of aryl or heteroaryl, optionally phenyl, which may be unsubstituted or substituted with one or more substituents; and Ci_2o alkyl wherein one or more nonadjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F.

Substituents of aryl or heteroaryl, where present, are optionally selected from Ci-12 alkyl wherein one or more non-adjacent, non-terminal C atoms may be replaced with 0, S, CO or COO and one or more H atoms may be replaced with F.

By “non-terminal C atom” of an alkyl group as used herein is meant a carbon atom other than the carbon atom of the methyl group of a n-alkyl group or the carbon atoms of the methyl groups of a branched alkyl group.

The ink formulation may contain only one fullerene compound or may contain two or more different fullerenes.

Preferably, the fullerene undergoes little or no n-doping by the n-dopant in the ink formulation. Preferably, n-doping occurs upon activation treatment of a film formed from the ink composition.

Preferably, the n-dopant has a HOMO level that is the same as or, preferably, deeper (further from vacuum) than the LUMO level of the fullerene as measured by square wave voltammetry, optionally at least 1 eV or 1.5 eV deeper to limit or completely prevent spontaneous doping upon mixing of the n-dopant and fullerent at room temperature.

Preferably, n-doping is effected after formation of a device comprising the layer formed from the formulation, and optionally after encapsulation. Activation may be by excitation of the n-dopant and / or the fullerene.

Exemplary activation methods are thermal treatment and irradiation.

Optionally, thermal treatment is at a temperature in the range 80°C to 170°C, preferably 120°C to 170°C or 140°C to 170°C.

Lor activation by irradiation, any wavelength of light may be used, for example a wavelength having a peak in the range of about 200-700 nm.

Preferably, activation to cause n-doping takes place after the device has been formed and encapsulated. The device may be manufactured in an environment in which limited or no spontaneous doping occurs, for example a room temperature environment wherein the device is exposed to little or no wavelengths of light that induce n-doping until after encapsulation of the device.

The n-dopant may be a hydride donor. The n-dopant may be a material that is capable of converting to a radical that can donate an electron from a SOMO level.

Exemplary n-dopants comprise a 2,3-dihydro-benzoimidazole group, optionally a 2,3dihydro-1 //-benzoimidazole group.

The n-dopant is preferably a compound of formula (II):

wherein:

each R is independently a Ci_2ohydrocarbyl group, optionally a Cno alkyl group;

R is H or a Ci_2ohydrocarbyl group, optionally H, Cno alkyl or Cno alkylphenyl;

each R is independently a Ci_2ohydrocarbyl group, optionally Cno alkyl, phenyl or phenyl substituted with one or more Cno alkyl groups;

each R is independently a substituent and k is 0 or a positive integer; and each R is independently a substituent and 1 is 0 or a positive integer.

Optionally, at least one of k and 1 is at least 1 and R and / or R is an ionic substituent.

Exemplary n-dopants include the following:

N-DMBI

N-DMBI is disclosed in Adv. Mater. 2014, 26, 4268 - 4272, the contents of which are incorporated herein by reference.

Other exemplary n-dopants are leuco crystal violet disclosed in J. Phys. Chem. B, 2004, 108 (44), pp 17076-17082, the contents of which are incorporated herein by reference, and NADH.

Optionally, the fullerene : n-dopant molar ratio is in the range of 1:1 - 10:1, optionally 2:1 - 10:1.

Formulations may be deposited by any coating or printing method including, without limitation, spin-coating, dip-coating, roll-coating, spray coating, doctor blade coating, slit coating, inkjet printing, screen printing, dispense printing, gravure printing and flexographic printing.

Dispense printing is particularly preferred. In dispense prining a continuous flow of ink is deposited from a nozzle positioned above the substrate.

Figure 1 schematically illustrates dispense printing in formation of a thermoelectric device array. A substrate 101 carries a patterned first electrode 103. An insulating layer 105 over the patterned electrode is patterned to expose the underlying patterned electrode.

The ink formulation is dispense printed into wells 107 defined by the insulating layer to form the n-type leg 109 of a thermoelectric device.

The ink formulation may be deposited from a dispense printer 200 into alternate wells 107n, as illustrated in Figure 1. An ink formulation for forming the p-type leg may be deposited (before or after deposition of the n-type legs) into the wells 107p not filled with the n-type ink formulation to give an alternating pattern of n-type and p-type legs.

The ink formulation may be dispensed into each well only once, or the ink formulation may be dispensed multiple times in multiple passes of the dispense printer.

The substrate may be heated during deposition of the ink. The temperature of the substrate may be selected so as to control rate of evaporation of the solvents from the ink, for example to avoid non-uniform evaporation.

Pairs of n-type and p-type legs may be connected by a second electrode (not shown) deposited over the p-type and n-type legs to form a thermoelectric device array.

In the embodiment of Figure 1, a n-type well and a p-type well has a common patterned first electrode region, although the skilled person will be aware of other arrangments.

The first and second electrodes are in electrical communication with the n-type and ptype legs. Preferably, the first and second electrodes are in direct contact with the n-type and p-type legs. The first and second electrodes are preferably metals, more preferably transition metals.

The substrate may be any insulating material, optionally glass, plastic (e.g. PEN) or a ceramic. Optionally, the patterned insulating layer 105 is a layer of a negative or positive photoresist or a printed insulating layer.

The device may be used in thermoelectric applications known to the skilled person including use as a thermoelectric generator and as a thermoelectric cooler.

Examples

Example 1

Powdered PCBM (41.92 mg) and polystyrene obtained from Alfa Aesar (4.66 mg, 650,000 g / mol, Mw/Mn 1.06) was mixed with a solvent or solvent mixture (0.932 ml) as set out in Table 1 in a nitrogen filled glovebox to give a formulation having a 5 % w/v ratio. This first partial ink formulation was sealed and heated to 80C for 1 hour and then sonicated for 1 hour at 37 kHz.

A second partial ink formulation was formed by dissolving N-DMBI (10 mg) in 0.2 ml of the same solvent or solvent blend as the first partial ink formulation to form a 5 % w/v ratio solution.

The ink formulation was formed by adding a 0.068 ml aliquot of the second partial ink formulation to the first partial ink formulation which was sealed and sonicated for 30 minutes at 37 kHz.

In Table 1, TMB is 1,2,4-trimethylbenzene; NMP is N-methylpyrrolidone; DMF is dimethylformamide; PC is propylene carbonate; and DMSO is dimethylsulfoxide.

Hansen solubility parameters of Table 1 were obtained as described above.

Inks were left standing in a nitrogen filled glovebox. If no precipitation was observed after 24 hours then the ink was deemed stable.

Table 1

Ink formulation	Solvent(s) (vol %)	Hansen solubility parameters	Stable
δϋ	δΡ	δΗ
Comparative Ink 1	TMB (100)	18.0	1	1	No
	TMB:NMP 80:20	18.0	3.3	2.2	No
Comparative Ink 2
Comparative Ink 3	TMB:DMF 80:20	17.9	3.5	3.1	No
Ink Example 1	TMB:DMSO 80:20	18.1	4.1	2.8	Yes
Ink Example 2	TMB:PC 80:20	18.4	4.4	1.6	Yes
Ink Example 3	TMB:NMP 60:40	18.0	5.5	3.5	Yes
Ink Example 4	TMB:DMF 60:40	17.8	6.1	5.1	Yes
Ink Example 5	TMB:DMSO 60:40	18.2	7.2	4.7	Yes
Ink Example 6	TMB:DMF 40:60	17.6	8.6	7.2	Yes
Ink Example 7	TMB:NMP:DMF 60:30:10	17.9	5.7	3.9	Yes
Ink Example 8	TMB:NMP:DMSO 60:30:10	18.0	5.9	2.2	Yes

A further ink example, Ink Example 9, was formed as described for Ink Example 3 except that the solids content of the ink was increased to 20%.

The effect of the ink stability on the resistance of a film formed from the ink was determined by dispense printing inks of Table 1 immediately after formation of the ink formulation and after 24 hours storage under nitrogen.

With reference to Table 2, the increase in resistance of the film formed from Comparative Ink Formulation 1 is much greater than that for either of the ink formulation examples of Table 2.

Table 2

Ink formulation	Resistance of film formed from fresh ink (kQ)	Resistance of film formed from stored ink (kQ)
Comparative Ink 1	6.4	85.2
Ink Example 1	3.5	7.7
Ink Example 3	1.2	1.1

A structure as illustrated in Figure 3 was formed to determine the effect of the solvent δΡ value on Seebeck coefficient ink was dropcast and dried on glass substrates (21) prepatterned by photolithography defining two electrode contacts with leadouts (23a and 23b) separated by 95 mm to form the thermoelectric material layer (22). Additionally, an insulating photoresist ‘bank’ of thickness 1 pm was defined to limit the open area of the contacts to 95 mm, thereby producing an open square between the bank and electrodes that the material can cover. Additionally two snake-like resistor features with leadouts (herein referred to as thermistors) were patterned from the metal in line with the electrodes, maintaining the same spacing of 95 mm.The glass substrate (21) was placed upon two Peltier units (26a and 26b) which were subsequently driven in opposite polarity, one producing a hot surface and the other a cold surface. The electrical contact was made to the leadouts on the topside of the substrate using pogo pins (24a/24b) attached to a breadboard circuit and clamped to the baseplate. The circuit created allowed for a) measurement of voltage generated by sample; b) application of a small current to the thermistors; c) measurement of voltage dropped across the thermistors. Voltages were sensed using a Pico Technology TC-08 unit. Additionally, a thermocouple (25a/25b) was placed in contact (held in place by the spring force of the thermocouple) with the substrate above each thermistor and the temperature was monitored with the same TC-08 unit.

The generated voltage (VI), thermistor voltages (V2, V3) and temperature (ΤΙ, T2) were logged using Picolog software. The peltier units (26a/26b) were switched off and the temperature difference was allowed to decay to equilibrium during the logging phase. In subsequent data processing the substrate temperature at each thermistor was calculated from the change in V2 and V3 in relation to the final voltage and temperature at equilibrium (Vo, To) according to the equation

P - l·/

T =-- + A where a is the predetermined constant of resistivity for the metal in question. The temperature difference dT was then calculated from the two thermistor temperatures. The Seebeck coefficient was then determined from the slope from the linear regression of VI and dT.

The effect of the solvent δΡ value on Seebeck coefficient is shown in Figure 3.

Table 3 shows the effect of the solvents on internal resistance.

Table 3

Ink formulation	Internal resistance (Ω)
Ink Example 1	300-600
Ink Example 7	-200
Ink Example 8	-100
Ink Example 9	-80-100

DC resistance was measured in a 4 point probe configuration using a Keithley Sourcemeter 2400 by contacting electrode leadouts provided on the respective conductive layer; the sourcemeter was configured to automatically select appropriate current source and measurement range (2k0hm, 1mA). The layer thicknesses were then measured by removing a portion of the film and measuring the step height of the surface profile (Dektak V400-Si). The conductivity was then calculated from the formula:

wi ft iwherein σ is conductivity, w is the length of the electrode formed by the conductive layer (10 mm), t is the thickness, R is the resistance and I is the spacing (14mm).

The higher solids content of Ink Example 9 was found to result in improved containment and reduced print time of the ink compared to lower solids content inks.

Example 2

Ink formulations and devices were prepared as described in Example 1 except that the binder, provided in an amount of 10 wt % relative to PCBM, was varied as set out in Table 4.

Table 4

Binder	Conductivity (S/cm)	Thermopower (pV/K)
None	2	-200
Polystyrene	1.1	-209
PVC	0.05	-192
P(NDI2OD-T2)	1.1	-200
F8BT	0.9	-198

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.

0= JWF« /0¼ 00' ' -- - .- . , ..

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1. A formulation comprising a fullerene and an n-dopant thereof dissolved or dispersed in a solvent mixture comprising a first aprotic solvent and a second aprotic solvent, the solvent mixture having a polar Hansen solubility parameter δΡ of at least 4.0.

2. A formulation according to claim 1 wherein the first aprotic solvent has a dielectric constant at 20°C of less than 5.

3. A formulation according to claim 1 or 2 wherein the formulation comprises only one first aprotic solvent.

4. A formulation according to any of claims 1-3 wherein the first aprotic solvent is selected from the group consisting of benzene substituted with one or more substituents selected from Cm2 alkyl and Cm2 alkoxy wherein two substituents may be linked to form a ring which may be unsubstituted or substituted with one or more Ci-6 alkyl groups.

5. A formulation according to any one of the preceding claims wherein the solvent mixture consists of 30-90 vol % of the first aprotic solvent or solvents.

6. A formulation according to claim 1 wherein the second aprotic solvent has a dielectric constant at 20°C of greater than 25.

7. A formulation according to any one of the preceding claims wherein the formulation comprises only one second aprotic solvent.

8. A formulation according to any one of claims 1-6 wherein the formulation comprises at least two second aprotic solvents having a dielectric constant at 20°C greater than

25.

9. A formulation according to any one of the preceding claims wherein the or each second aprotic solvent has a dielectric constant at 20°C greater than 30.

10. A formulation according to any one of the preceding claims wherein the second aprotic solvent is selected from the group consisting of dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone or propylene carbonate.

11. A formulation according to any one of the preceding claims wherein the solvent mixture consists of 10-70 vol % of the second aprotic solvent or solvents.

12. A formulation according to any one of the preceding claims wherein the formulation comprises a dissolved or dispersed polymer binder.

13. A formulation according to any one of the preceding claims wherein the n-doped fullerene is n-doped PCBM.

14. A formulation according to any one of the preceding claims wherein the n-dopant is a 1-H-benzimidazole dopant.

15. A formulation according to claim 14 wherein the n-dopant is N-DMBI.

16. A method of forming a layer of an organic electronic device, the method comprising the step of depositing a formulation according to any one of the preceding claims onto a surface and evaporating the solvents.

17. A method according to claim 16 wherein the formulation is deposited by dispense printing.

18. A method according to claim 16 or 17 wherein the deposited formulation is heated.

19. A method according to any one of claims 16-18 wherein the organic electronic device is an organic thermoelectric device.

Intellectual

Property

Office

Application No: Claims searched:

GB1704488.4

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