FIELD OF INVENTION [0001] This invention relates to a formulation comprising p-type and n-type organic semiconductors and a solvent system, its use for manufacturing photosensitive organic electronic devices, and to photosensitive organic electronic devices, in particular organic photodetectors, prepared by using said formulations. BACKGROUND OF THE INVENTION

[0002] There is an increased interest in the development of novel organic photosensitive electronic devices as alternatives to inorganic photoelectronic devices because they provide high flexibility and may be manufactured and processed at relatively low costs by using low temperature vacuum deposition or solution processing techniques.

[0003] As examples of organic photosensitive electronic devices, organic photovoltaic devices (OPV), photocells and photodetectors may be mentioned. Usually, such an organic photosensitive electronic device includes as a photoactive layer a p-n junction which is prepared by film deposition of a donor/acceptor blend from solution and enables the device to convert incident radiation into electrical current.

[0004] US 2017/062725 discloses compositions comprising an organic semiconducting compound and their use as conducting inks for preparation of OPV and organic light- emitting devices.

[0005] WO 2016/027218 discloses preparation of a crystalline organic semiconductor material by evaporation of a solution.

[0006] WO 2013/190255 discloses preparation of a semiconducting layer of an organic electronic device from a solution comprising a polymeric semiconductor, a non-polymeric semiconductor, and first and second aromatic solvents wherein the boiling point of the second aromatic solvent is at least 15°C higher than that of the first aromatic solvent.

[0007] WO 2013/029733 discloses formulations comprising a p-type organic semiconductor, a n-type organic semiconductor and a solvent selected from the group consisting of alkylated tetralin, alkylated naphthalene and alkylated anisole.

[0008] In the case of an organic photodetector device, the current flowing through the device in the absence of any photons incident on the device, known as dark current, may affect the limit of detection of the device.

[0009] It is an object of the invention to provide formulations for forming high efficiency organic photosensitive electronic devices. [0010] It is a further object of the invention to provide formulations for forming organic photodetector devices with low dark current.

SUMMARY OF THE INVENTION

[0011] The present invention solves these objects with the subject matter of the claims as defined herein. The advantages of the present invention will be further explained in detail in the section below.

[0012] The present inventors have surprisingly found that the use of specific solvent mixtures in formulations comprising n-type and p-type organic semiconductors allows formation of organic photosensitive devices with excellent efficiency and, in the case of photodetector devices, low dark current.

[0013] In a first aspect, the invention provides a formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and a solvent mixture comprising a first solvent and a second solvent wherein the first solvent is an alkylated aromatic hydrocarbon and the second solvent is a benzene substituted with two or more substituents including at least two Ci-6 alkoxy groups. [0014] In a second aspect, the present invention relates to a method of manufacturing an organic photosensitive electronic device comprising an anode, a cathode and a photoactive layer between the cathode and the anode, the method comprising: applying the formulation of the first aspect over one of the anode and cathode to form a wet film; drying the wet film to provide the photoactive layer; and forming the other of the anode and cathode over the photoactive layer.

[0015] By formation of a layer "over" an underlying layer as used herein is meant that the layer is formed directly on the underlying layer or on an intervening layer between the layer so formed and the underlying layer. There may be no intervening layer or one or more intervening layers.

[0016] A third aspect of the present invention relates to an organic photosensitive electronic device comprising an anode, a cathode and a photoactive layer manufactured by the aforementioned method of the second aspect.

[0017] Preferred embodiments of the formulation according to the present invention and other aspects of the present invention are described in the following description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 schematically illustrates a photodetector device;

[0019] FIG. 2 A is a graph of graph of current density vs. voltage for a comparative device formed using a comparative formulation containing benzyl benzoate solvent and devices formed using formulations according to embodiments of the invention containing 1,2- dimethoxybenzene or 1,3-dimethoxybenzene solvent; and

[0020] FIG. 2B is a graph of external quantum efficiency vs. wavelength for the devices of FIG. 2A. DETAILED DESCRIPTION OF THE INVENTION

[0021] For a more complete understanding of the present invention, reference is now made to the following description of the illustrative embodiments thereof:

Semiconductor Formulation

[0022] In a first aspect, the present invention relates to a formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and a solvent mixture comprising a first solvent and a second solvent wherein the first solvent is an alkylated aromatic hydrocarbon and the second solvent is a benzene substituted with two or more substituents including at least two Ci _-6 alkoxy groups.

[0023] Optionally, the aromatic hydrocarbon of the first solvent is benzene, preferably a compound of formula (I):

(I)

wherein R ¹ in each occurrence independently represents a Ci-C ₆ alkyl group and n is at least 1 , optionally 1 -4. In the case where n is 2 or more, two groups R ¹ may be linked to form a ring which may be unsubstituted or substituted with one or more Ci _-5 alkyl groups. In the case where n is 2 or more it is preferred that none of the R ¹ groups are linked to form a ring. The or each R ¹ is preferably methyl, n is preferably 3.

[0024] As specific examples of compounds according to formula (I), 1 ,2-dialkyIbenzene (e.g. o-xylene), 1,2,4-trialkylbenzenes (e.g. 1,2,4-trimethylbenzene, 1,2,4-triethylbenzene, l,2-dimethyl-4-ethylbenzene), 1,2,3-trialkylbenzenes (e.g. 1,2,3-trimethylbenzene, 1,2,3- triethylbenzene), indane and its alkyl-substituted derivatives, and tetralin and its alkyl- substituted derivatives may be mentioned. [0025] Optionally, the second solvent is a compound of formula (II):

(II) wherein ² independently in each occurence represents a substituent with the proviso that at least two groups R ² are independently a Ci-C ₆ alkoxy group and m is 2-6. Any R ² groups other than Ci-C ₆ alkoxy, if present, are preferably selected from F and C1-C6 alkyl. Preferably, m is 2. Preferably, each R ² is methoxy. Preferably, the second solvent is selected from 1 ,2-dimethoxybenzene and 1,3-dimethoxybenzene.

[0026] Optionally, the second solvent has a boiling point in the range of 200-300°C, optionally 200-250°C.

[0027] Optionally, the boiling point of the second solvent is higher than the boiling point of the first solvent, optionally at least 10°C, at least 20°C or at least 30°C higher than the boiling point of the first solvent.

[0028] Optionally, the the First solvent is present in a content range of from 50 to 99.5 volume % based on the total volume of the solvent mixture, preferably 70-98 volume %,

[0029] Optionally, the second solvent is present in a content range of from 0.5 to 50 volume % based on the total volume of the solvent mixture, preferably 1-20 volume %, 1-10 volume % or 2-8 volume %. [0030] Optionally, the first solvent : second solvent volume ratio is within the range of from 70:30 to 99.5:0.5 or 70:30 to 99:1 or 70:30 to 98:2.

[0031] While it may be preferable that the solvent mixture consists of the first and second solvents, the solvent mixture may comprise one or more further solvents. The or each further solvents may be selected from materials that are liquid at 25°C and 1 atm and in which one or both of the n-type OSC and the p-type OSC or both are soluble at a solubility of 0.2 mg/ml or more. Such additional solvents are not particularly limited and may be appropriately selected by the skilled artisan. Exemplary further solvents include, without limitation, naphthalene, linear or cyclic ketones (e.g. cyclohexanone), aliphatic ethers, aliphatic or aromatic alcohols, optionally substituted thiophenes and chlorinated solvents (e.g. chlorobenzene, trichlorobenzene, dichlorobenzene or chloroform) and mixtures thereof. From the viewpoint of environment-friendliness it is, however, preferred that the solvent mixture does not comprise any chlorinated solvents. If present, the additional solvents are optionally comprised at a total content of less than 3 vol. %, more preferably less than 2 vol. % relative to the total solvent volume.

[0032] Optionally, the solvent mixture does not comprise an arylbenzoates, for example benzylbenzoate. Optionally, the solvent mixture does not comprise an alkylbenzoate. Optionally, the solvent mixture does not comprise a benzothiazole.

[0033] The p-type OSC is not particularly limited and may be appropriately selected from standard electron donating materials that are known to the person skilled in the art and are described in the literature, including organic polymers, oligomers and small molecules. In a preferred embodiment the p-type OSC comprises an organic conjugated polymer, which can be a homopolymer or copolymer including alternating, random or block copolymers. Preferred are non-crystalline or semi-crystalline conjugated organic polymers. Further preferably the p-type organic semiconductor is a conjugated organic polymer with a low bandgap, typically between 2.1 eV and 1.1 eV, preferably between 1.9 eV and 1.1 eV, and most preferably between 1.7 eV and 1.1 eV. As exemplary p-type OSC polymers, polymers selected from conjugated hydrocarbon or heterocyclic polymers including polyacene, polyaniline, polyazulene, polybenzofuran, polyfluorene, polyfuran, polyindenofluorene, polyindole, polypheny lene, polypyrazoline, polypyrene, polypyridazine, polypyridine, polytriarylamine, poly (pheny lene vinylene), poly(3 -substituted thiophene), poly(3,4- bisubstituted thiophene), poly selenophene, poly(3 -substituted selenophene), poly(3,4- bisubstituted selenophene), poly(bisthiophene), poly(terthiophene), poly(bisselenophene), poly(terselenophene), polythieno[2,3-b]thiophene, polythieno[3,2-b]thiophene, polybenzothiophene, polybenzo[l,2-b:4,5- b']dithiophene, polyisothianaphthene, poly(monosubstituted pyrrole), poly(3,4-bisubstituted pyrrole), poly-l _s 3,4-oxadiazoles, polyisothianaphthene, derivatives and co-polymers thereof may be mentioned. Preferred examples of p-type OSCs are copolymers of polyfluorenes and polythiophenes, each of which may be substituted, and polymers comprising benzothiadiazole-based and thiophene- based repeating units, each of which may be substituted. It is understood that the p-type OSC may also consist of a mixture of a plurality of electron donating materials.

[0034] The p-type OSC preferably comprises a repeat unit of formula (III):

(III) wherein R ³ in each occurrence is independently H or a substituent. [0035] Optionally, each R ³ is independently selected from the group consisting of Ci-2o alkyl wherein one or more non-adjacent, non-terminal carbon atoms of the alkyl group may be replaced with O, S or C=0 and wherein one or more H atoms of the C 1-20 alkyl may be replaced with F; an aryl or heteroaryl group, preferably phenyl, which may be

unsubstituted or substituted with one or more substituents; and fluorine.

[0036] Substituents of an aryl or heteroaryl group are optionally selected from F, CN, NO2 and C 1-20 alkyl wherein one or more non-adjacent, non-terminal carbon atoms of the alkyl group may be replaced with O, S or C=0. [0037] By "non-terminal" as used herein is meant a carbon atom other than the methyl group of a linear alkyl (n-alkyl) chain and the methyl groups of a branched alkyl chain.

[0038] Preferably, each R ³ is fluorine. [0039] A polymer comprising a repeat unit of formula (I) is preferably a copolymer comprising one or more co-repeat units. The one or more co-repeat units may comprise or consist of one or more of C ₆ -20 monocyclic or polycyclic arylene repeat units which may be unsubstituted or substituted with one or more substituents; 5-20 membered monocyclic or polycyclic heteroarylene repeat units which may be unsubstituted or substituted with one or more substituents.

[0040] The one or more co-repeat units may have formula (IV):

— (Ar ¹ ) _x - I

(R ¹⁷ )y

(IV) wherein Ar ¹ in each occurrence is an arylene group or a heteroarylene group; x is at least 1 ; R ¹⁷ in each occurrence is independently a substituent; y independently in each occurence is 0 or a positive integer, optionally 0, 1, 2, 3 or 4; and two groups R ¹⁷ may be linked to form a ring.

[0041] Optionally, each R ¹⁷ is independently selected from the group consisting of a linear, branched or cyclic Ci-20 alkyl wherein one or more non-adjacent, non-terminal C atoms of the Ci-20 alkyl may be replaced with O, S, COO or CO.

[0042] Two groups R ¹⁷ may be linked to form a Ci-10 alkylene group wherein one or more non-adjacent C atoms of the alkylene group may be replaced with O, S, COO or CO.

[0043] Optionally, x is 2. [0044] Optionally, each Ar ^! is independently a 5 or 6 membered heteroarylene group, optionally a heteroarylene group selected from the group consisting of thiophene, furan, selenophene, pyrrole, diazole, triazole, pyridine, diazine and triazine, preferably thiophene. [0045] Optionally, the repeat unit of formula (IV) has formula (IVa):

(IVa)

[0046] Optionally, the groups R ¹⁷ are linked to form a 2-5 membered bridging group.

Optionally, the bridging group has formula -0-C(R ¹⁸ ) ₂ - wherein R ^ts in each occurrence is independently H or a substituent. Substituents R ¹⁸ are optionally selected from Ci-2o aIkyl. Preferably each R ^!8 is H.

[0047] A polymeric p-type OSC as described herein may have a polystyrene-equivalent number-average molecular weight (Mn) measured by gel permeation chromatography in the range of about lxlO ³ to 1x10 ^s , and preferably lxlO ³ to 5xl0 ⁶ . The polystyrene- equivalent weight-average molecular weight (Mw) of the polymers described herein may be lxlO ³ to 1x10 ^s , and preferably lxlO ⁴ to lxl O ⁷ [0048] The n-type OSC is also not particularly limited and may be suitably selected from electron accepting materials known to the skilled artisan and may consist of a mixture of a plurality of electron accepting materials. As examples thereof, n-type conjugated polymers, fullerenes and fullerene derivatives may be mentioned. Preferably, the n-type OSC is a single type or a mixture of fullerenes and/or fullerene derivatives, including C60, C70, C76, C78, C84, C96, PCBM-type fullerene derivatives (including phenyl-C61 -butyric acid methyl ester (C oPCBM), TCBM-type fullerene derivatives (e.g. tolyl-C61 -butyric acid methyl ester (CeoTCBM)), ThCBM-type fullerene derivatives (e.g. thienyl-C61 -butyric acid methyl ester (CeoThCBM). [0049] Fullerene derivatives may have formula (V):

(V)

[0050] 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.

[0051] Exemplary fullerene derivatives include formulae (Va), (Vb) and (Vc):

(Va) (Vb) (Vc) wherein ⁴ -R ¹⁵ are each independently H or a substituent. [0052] Substituents R -R ¹⁵ 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-20 alkyl wherein one or more non-adjacent, non-terminal C atoms may be replaced with O, S, CO or COO and one or more H atoms may be replaced with F. [0053] 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 O, S, CO or COO and one or more H atoms may be replaced with F.

[0054] As further examples of fullerene derivatives, those disclosed in WO 2004/073082 Al, US 2011/0132439 Al, WO 2015/036075 Al, and US 201 1/0132439 Al may be mentioned, the contents of which are incorporated herein by reference. [0055] The weight ratio of p-type material to n-type material present in the formulation is optionally in the range of 10: 1 to 1 : 10, more preferably 4: 1 to 1 :4, especially preferably 1 : 1 to 1 :3.

[0056] Preferably, the ink concentration, i.e. the concentration of n- and p-type material in the solvent mixture, is in the range of from 0.1 to 10.0 w/v%, more preferably from 1.0 to 6 w/v%.

[0057] The formulation may comprise further components in addition to the n-type organic semiconductor, the p-type organic semiconductor and the solvent mixture. As examples of such components, adhesive agents, defoaming agents, deaerators, viscosity enhancers, diluents, auxiliaries, flow improvers colourants, dyes or pigments, sensitizers, stabilizers, nanoparticles, surface-active compounds, lubricating agents, wetting agents, dispersing agents and inhibitors may be mentioned. [0058] It will be appreciated that the preferred features specified above may be combined in any combination, except for combinations where at least some of the features are mutually exclusive.

[0059] The above-defined formulations serve as a starting material for the solution deposition of photoactive layers of organic photosensitive devices, in particular photodetectors. [0060] In a second embodiment, the present invention relates to a method of manufacturing an organic photosensitive electronic device comprising an anode, a cathode and a photoactive layer between the cathode and the anode, the method comprising: applying the formulation according to the above-described first aspect over one of the anode and cathode to form a wet film; drying the wet film to provide the photoactive layer; and forming the other of the anode and cathode over the photoactive layer.

[0061] A typical general architecture of an organic photosensitive electronic device according to the present invention is schematically depicted in Fig.l . Herein, an anode 2, which may consist of a high work-function material, is deposited onto a substrate 1 made of a material transparent to visible light. Exemplary anode materials include conductive metal oxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO), aluminum zinc oxide (AlZnO), and metals (e.g. gold). Glass or a plastic may be used as a substrate material. Between the anode 2 and the cathode 4, which may comprise or consist of a metal or a metal alloy (e.g. Ag, Ag:Mg) or metal oxides, the photoactive layer 5 comprising a bulk heteroj unction comprising or consisting of a mixture of the n-type OSC and the p-type OSC formed by solution deposition of the formulation according to the first aspect. A contact 5 is provided between the anode 2 and cathode 4, which may include a voltage source for applying a reverse bias to the device and a detector (e.g. current meter or readout device, wired in series with the reverse bias voltage source, as detection circuit), for example, to measure the generated photocurrent.

[0062] It is to be understood that Fig. 1 serves illustrative purposes only and one or more further layers may be present in the device, such as e.g. electron blocking layers (EBL), hole- injecting layers (HIL), electron-injecting layers (EIL), exciton-blocking layer (XBL), spacer layers, connecting layers, hole-blocking layers (HBL) and electrode-modification layers for modifying the work function of an electrode.

[0063] It will be appreciated that the anode may be supported on a substrate, as illustrated in Fig. 1 , and the formulation may be deposited over the anode to form the photoactive layer followed by formation of the cathode, or the cathode may be supported on a substrate in which case the formulation may be deposited over the cathode to form the photoactive layer followed by formation of the anode.

[0064] The formulation according to the present invention can be applied by any suitable solution deposition method, including but not limited to, spin-coating, dip-coating, roll- coating, spray coating, doctor blade coating, wire bar coating, slit coating, ink jet printing, letter-press printing, dispense printing screen printing, gravure printing, offset lithography printing and fiexographic printing [0065] Dispense printing of the active layers of an OPD is a step towards simple additive fabrication process for integrating OPDs with other components. Herein, a continuous flow of ink is deposited from a nozzle positioned in a defined distance from the substrate. The desired pattern is created by a relative movement of the nozzle and the substrate. [0066] By controlling the nozzle dispense rate (solution flow rate), the pattern density (line spacing), the nozzle movement speed (line speed) as well as the ink concentration, the uniformity of the photoactive layer film may be tuned.

[0067] After the formulation has been solution deposited to form the wet film, the solvent mixture is dried. Solvent may be evaporated from the wet film by drying at ambient temperature and pressure, by heating the deposited wet film, optionally to a temperature in the range of 50-200°C, and/or by application of reduced pressure, optionally a pressure in the range of 0.01 to 0.1 mbar, preferably immediately upon formation of the wet film. [0068] The thickness of the photoactive layer produced after removal of solvent is preferably from 10 nm to 3 μηι, more preferably from 20 nm to 2 μπι.

[0069] The photoactive layer may be homogenous or phase-separated and may contain different phases which phases may differ in the ratio of p-type to n-type material. The photoactive layer may have a substantially uniform ratio throughout the thickness of the photoactive layer or the ratio of p-type to n-type material may vary gradually or stepwise throughout the thickness of the photoactive layer. [0070] In a third aspect, the present invention relates to an organic photosensitive electronic device comprising an anode, a cathode and a photoactive layer formed from a formulation according to the first embodiment between the cathode and the anode, or formed by a method according to the second aspect, respectively.

[0071] The organic photosensitive electronic device may be a photovoltaic device or a solar cell, a photo conductor cell or a photodetector, for example, each of which may be operated as a single device or in an array depending on the desired purpose. In the case of a photodetector, an array of photodetectors may be used, each photodetector being associated with one or more light sources. The organic photosensitive electronic device of the present invention is preferably an organic photodiode, which is operated in photoconductive mode (wherein external voltage, the so-called reverse bias voltage is applied) and thereby functions as a photo conductor cell or a photodetector, as opposed to a solar cell, which is operated in photovoltaic mode (with zero bias voltage).

[0072] An organic photodetector as described herein may be used in a wide range of applications including, without limitation, detecting the presence and / or brightness of ambient light and in a sensor comprising at least one organic photodetector as described herein and at least one light source. The photodetector may be configured such that light emitted from a light source is incident on the photodetector and changes in wavelength and / or brightness of the light may be detected. An array of photodetectors as described herein may be configured to detect light emitted from a single light source or from two or more light sources. The sensor may be, without limitation, a gas sensor, a biosensor, X-ray imaging or a motion sensor, for example a motion sensor used in security applications, a proximity sensor or a fingerprint sensor.

EXAMPLES

Formulation Examples

[0073] Formulations were prepared comprising the fullerene C70-IPH as the n-type organic semiconductor (63 wt %) and a p-type organic semiconductor polymer having structural formula (1) (37 wt %) in a solvent mixture of 1,2,4-trimethylbenzene (first solvent) and a second solvent set out in Table 1.

C70-IPH: R = hexyl

Table 1

Formulation 1 , 2- dimethoxy benzene 90 : 10 207 Example 2

Formulation 1 ,2-diniethoxybenzene 95 : 5 207

Example 3

Formulation 1 ,3-dimethoxybenzene 90 : 10 217

Example 4

Device Examples

[0074] A device having the following structure was prepared:

Cathode / Donor : Acceptor layer / Anode

[0075] Device Examples 1-4 were formed by coating Formulation Examples 1-4 respectively by wire bar coating onto a glass substrate coated with a layer of indium-tin oxide (ITO) and a layer of poly(ethyleneimine) (PEIE) to form a photoactive layer having a thickness of about 900 nm. The anode was formed over the photoactive layer by spin- coating a hole-transporting material available from Heraeus, Inc.

Comparative Device 1

[0076] A device was prepared as described with reference to Device Examples 1-4 except that Comparative Formulation 1 was used in place of Formulation Examples 1-4.

[0077] With reference to Figure 2A, dark currents of Device Examples 1-4 are similar to that of Comparative Device 1 at a similar volume ratio of the second solvent. [0078] With reference to Figure 2B, external quantum efficiencies of Device Examples 1 -4 at 2V are considerably higher across the measured wavelengths from about 350-850 nm than that of Comparative Device 1.

[0079] 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.

REFERENCE NUMERALS

1 : substrate layer

2: anode

3 : photoactive layer

4: cathode

5: contact