GB2551585A - Solvent blends for improved jetting and ink stability for inkjet printing of photoactive layers - Google Patents

Abstract

Solvent blends for the preparation of semiconductor formulations with improved stability for ink jetting applications, the solvent blends comprising an alkyl benzoate or aryl benzoate; a first aromatic hydrocarbon. which is a dialkyl- or trialkylsubstituted aromatic hydrocarbon; and a second aromatic hydrocarbon different from the first aromatic hydrocarbon. An example blend is benzyl benzoate, trimethyl benzene and 1-methylnaphthalene or diphenylmethane. In addition, methods of manufacturing organic electronic devices with high efficiency and high-quality photoactive layers and films by using said formulations are disclosed.

Description

SOLVENT BLENDS FOR IMPROVED JETTING AND INK STABILITY FOR INKJET PRINTING OF PHOTOACTIVE LAYERS

FIELD OF INVENTION

[0001] This invention relates to a solvent blend comprising at least three solvents, to a formulation comprising the solvent blend as well as p-type and n-type organic semiconductors, and to a method of manufacturing organic electronic devices and photoactive layers and films 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 since they provide a 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] Typical examples of p-type materials are conjugated organic oligomers or polymers (e.g. oligomers or polymers of thiophenes, phenylenes, fluorenes, polyacetylenes, benzathiadiazoles and combinations thereof), whereas fullerene and fullerene derivatives (e.g. ΟβοΡΟΒΜ and C70PCBM) play an important role as n-type materials (see e.g. EP 1 447 860 A1).

[0005] In the recent years, it has been shown that the choice of solvents or solvent mixtures used for the preparation of the donor/acceptor solution plays an important role for the manipulation of the p-n junction morphology and allows to enhance the charge transport in organic photosensitive electronic devices.

[0006] Achieving maximum generation of photocurrent from an organic photosensitive electronic device is assumed to require an active layer with an optimised donor-acceptor distribution within the film with a suitable exciton diffusion length from a donor/acceptor interface.

[0007] In WO 2011/076324 A1, a range of suitable solvents used for organic semiconductor (OSC) compositions that may be used as inks for the preparation of organic electronic devices with improved efficiencies is disclosed, the solvents being selected from the group consisting of aromatic hydrocarbons, aromatic ethers, aromatic ketones, alkyl ketones, heteroaromatic solvents, halogenarylenes and anilin derivatives.

[0008] US 2010/0043876 A1 discloses organic semiconductor formulations comprising a blend of a first solvent comprising at least one alkylbenzene or benzocyclohexane, and the second solvent comprises at least one carbocyclic compound.

[0009] In WO 2013/029733 A1, a solvent selected from alkylated tetralin, alkylated naphthalene and alkylated anisole, optionally with a second, different solvent is proposed to further tune the morphology of the photoactive layer.

[0010] US 2011/0156018 A1 discloses that the use of blends comprising two or more solvents as solvent mixtures for benzothiadiazole- and benzooxadiazole-based polymer formulations is advantageous under aspects of film formability and device properties.

[0011] However, conventional solvent systems for organic semiconductor formulations optimized for the manufacture of organic photosensitive electronic devices with favourable photoresponse generally tend to exhibit a poor stability, which limits their application in a number of solution deposition techniques. For instance, a common phenomenon observed during inkjetting applications is nozzle clogging, which typically occurs during prolonged periods of jetting at reduced frequencies or during periods of non-jetting, for example during substrate alignment, so that extensive purging and cleaning steps at short intervals are required. In many cases, the clogging is irreversible and may even lead to a total blockage of nozzles which may not be recovered even after solvent purging.

[0012] In view of the above, there exists a need to provide a solvent blend which enables the preparation of a stable ink containing n- and p-type organic semiconductors which is suitable for inkjet printing and simultaneously allows manufacturing organic photosensitive electronic devices which achieve excellent photocurrent levels and efficiency.

SUMMARY OF THE INVENTION

[0013] 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 and further advantages will become apparent to the skilled artisan upon consideration of the invention disclosure.

[0014] The present inventors found that the use of specific solvent mixtures in formulations comprising n-type and p-type organic semiconductors provide for an excellent ink stability without imparting the photocurrent performance, so that organic photosensitive devices with excellent efficiency may be produced via inkjet printing in a simple manner, since nozzle clogging is effectively reduced and suppressed.

[0015] Generally speaking, the present invention relates to a solvent blend comprising: (a) an alkyl or aryl benzoate; (b) a first aromatic hydrocarbon, which is a dialkyl- or trialkylsubstituted aromatic hydrocarbon; and (c) a second aromatic hydrocarbon different from the first aromatic hydrocarbon.

[0016] In a second aspect, the present invention relates to a formulation comprising the aforementioned solvent blend, an n-type organic semiconductor, and a p-type organic semiconductor.

[0017] In a further aspect, the present invention relates to a method of manufacturing an organic electronic device comprising an anode, a cathode and a photoactive layer between the cathode and the anode, the method comprising applying the aforementioned formulation by a solution deposition method to form the photoactive layer.

[0018] Another aspect of the present invention is the use of the aforementioned solvent blend in a solution deposition method, preferably an inkjet printing method, and the use of the aforementioned formulation as a coating or printing ink for the preparation of a photoactive thin film or photoactive layer.

[0019] 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

[0020] FIG. 1 schematically illustrates the general architecture of a conventional organic photodetector device.

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:

Solvent Blend and Semiconductor Formulation [0022] In a first embodiment, the present invention relates to a solvent blend comprising: (a) an alkyl benzoate or aryl benzoate; (b) a first aromatic hydrocarbon, which is a dialkyl-or trialkylsubstituted aromatic hydrocarbon; and (c) a second aromatic hydrocarbon different from the first aromatic hydrocarbon. The solvent blend has been shown to be useful in the preparation of semiconductor formulations with excellent ink stability.

[0023] The alkyl benzoate is preferably a C1-C18 alkyl benzoate, more preferably a C1-C12 alkyl benzoate, such as a CrCe alkyl benzoate.

[0024] The aryl benzoate is preferably a benzoic acid Ce-Cis aryl ester, preferably a benzoic acid C6-C10 aryl ester. The aryl group may be unsubstituted or substituted with an C1-C12 alkyl group or preferably a C1-C6 alkyl group.

[0025] In a preferred embodiment from the viewpoint of excellent n-type organic semiconductor solubility, especially when fullerenes and/or fullerene derivatives are used, the component (a), i.e. the alkyl or aryl benzoate is benzyl benzoate.

[0026] The alkyl or aryl benzoate is preferably present in a content range of from 0.5 to 50 vol.-%, more preferably from 0.5 to 30 vol.-%, further preferably from 1 to 10 vol.-% based on the total volume of the solvent blend.

[0027] The component (b), i.e. the first aromatic hydrocarbon is a dialkyl- or trialkylsubstituted aromatic hydrocarbon.

[0028] The dialkylsubstituted aromatic hydrocarbon preferably comprises alkyl groups which may be independently selected from a C1-C12 alkyl group or more preferably from a C1-C6 alkyl group. More preferably, the dialkylsubstituted aromatic hydrocarbon is a dialkylbenzene, more preferably an ortho-dialkylbenzene, wherein the alkyl groups may be the same or different and may be selected from a C1-C12 alkyl group or more preferably from a Ci-Ce alkyl group.

[0029] Preferably, the trialkylsubstituted aromatic hydrocarbon is a trialkyl benzene, wherein the alkyl groups may be the same or different and may be selected from a C1-C12 alkyl group or more preferably from a C1-C6 alkyl group. Further preferably, the trialkylsubstituted aromatic hydrocarbon is represented by the following general formula (I):

wherein R1 and R2 independently represent C1-C6 alkyl groups, which may be connected to each other to form a ring, and wherein R3 represents hydrogen or a C1-C6 alkyl group.

[0030] As specific examples of compounds according to general formula (I), 1,2- dialkylbenzene (e.g. o-xylene), 1,2,4-trialkylbenzenes (e.g. 1,2,4-trimethylbenzene, 1,2,4-triethylbenzene, 1,2-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. In a preferred embodiment the dialkyl- or trialkylsubstituted aromatic hydrocarbon is a trialkylbenzene, more preferably trimethylbenzene, further preferably 1,2,4-trimethylbenzene.

[0031] The first aromatic hydrocarbon is preferably employed in a content range of from 30 to 99.4 vol.-%, preferably from 60 to 99 vol.-%, more preferably from 70 to 98 vol.-% based on the total volume of the solvent blend.

[0032] The component (c), i.e. the second aromatic hydrocarbon preferably exhibits two benzene rings which may be fused, including naphtalene and alkylnaphthalene and diphenylalkanes, for example. More preferably, the second aromatic hydrocarbon is selected from any of 1-methylnaphthalene or diphenylmethane.

[0033] It is preferable that the second aromatic hydrocarbon is present in a content range of from 0.1 to 50 vol.-%, preferably from 0.5 to 30 vol.-%, more preferably from 1 to 20 vol.-% based on the total volume of the solvent blend.

[0034] Preferably, the first aromatic hydrocarbon exhibits a melting point of less than -30°C, e.g. between -80°C to -40°C, and/or the second aromatic hydrocarbon exhibits a melting point of -30°C or higher, e.g. between -25°C to +20°C. With respect to boiling point characteristics, the first aromatic hydrocarbon may have a boiling point lower than 200°C. It is preferable that the second aromatic hydrocarbon has a boiling point of 200°C or higher, more preferably between 200°C and 300°C, further preferably in combination with a relative evaporation rate (determined according to DIN 53170:2009-08; butyl acetate = 100) of 160 or less, such as e.g. 120 or less, or 80 or less, which ensures that evaporation losses of the first aromatic hydrocarbon during jetting do not cause the semiconductor (typically the p-type organic semiconductor) to precipitate and thereby further reduces nozzle clogging.

[0035] While it may be preferable that the solvent blend consists of components (a), (b) and (c), the solvent mixture may comprise one or more further solvents. Said further solvents may be liquid components, wherein either 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. As examples thereof, linear or cyclic ketones (e.g. cyclohexanone), aromatic and/or aliphatic ethers (e.g. anisole), aromatic alcohols, optionally substituted thiophenes, benzothiophenes, alkoxylated naphthalene, substituted benzothiazoles, alkyl benzoates, chlorinated solvents (e.g. chlorobenzene, trichlorobenzene, dichlorobenzene or chloroform) and mixtures thereof may be mentioned. In another preferred embodiment, the additional solvents are comprised at a total content of less than 3 vol.-%, more preferably less than 2 vol.-% relative to the total solvent volume. From the viewpoint of environment-friendliness it is, however, preferred that the composition does not contain chlorinated solvents.

[0036] In a second embodiment, the present invention relates to a formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and the solvent blend in accordance with the above-described first embodiment.

[0037] The p-type organic semiconductor 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 organic semiconductor comprises a conjugated organic polymer, which can be a homopolymer or copolymer including alternating, random or block copolymers. Preferred are noncrystalline or semi-crystalline conjugated organic polymers. As exemplary p-type OSC polymers, polymers selected from conjugated hydrocarbon or heterocyclic polymers including polyacene, polyaniline, polyazulene, polybenzofuran, polyfluorene, polyfuran, polyindenofluorene, polyindole, polyphenylene, polypyrazoline, polypyrene, polypyridazine, polypyridine, polytriarylamine, poly(phenylene vinylene), poly(3-substituted thiophene), poly(3,4-bisubstituted thiophene), polyselenophene, 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[1,2-b:4,5-b']dithiophene, polyisothianaphthene, poly(monosubstituted pyrrole), poly(3,4-bisubstituted pyrrole), poly-1,3,4-oxadiazoles, polyisothianaphthene, derivatives and copolymers thereof may be mentioned. Preferred examples of p-type organic semiconductors 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 organic semiconductor may also consist of a mixture of a plurality of electron donating materials.

[0038] The n-type organic semiconductor 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 organic semiconductor is a single type or a mixture of fullerenes and/or fullerene derivatives, including C60, C70, C96, PCBM-type fullerene derivatives (including CeoPCBM and C70PCBM), TCBM-type fullerene derivatives (e.g. tolyl-C61-butyric acid methyl ester (C60TCBM)), ThCBM-type fullerene derivatives (e.g. thienyl-C61-butyric acid methyl ester (CeoThCBM). As further examples of fullerene derivatives, those disclosed in WO 2004/073082 A1, US 2011/0132439 A1, WO 2015/036075 A1, and US 2011/0132439 A1 may be mentioned.

[0039] The ratio of p-type material to n-type material present in the formulation may be routinely determined by the skilled artisan. Preferably, the ratio is 10:1 to 1:10, more preferably 4:1 to 1:4, especially preferably 1:1 to 1:3.

[0040] From the viewpoint of an excellent balance between jetting stability and efficiency, the p-type organic semiconductor is preferably selected so as to exhibit a dynamic viscosity at 25°C in tetralin solution within a range of 1 to 30 mPas, more preferably to a range of 3 to 6 mPas, further preferably to a range of 3.6 to 5.4 mPa-s, such as from 3.7 to 4.1 mPa-s. The dynamic viscosity at 25°C may be measured by dissolving the p-type organic semiconductor in tetralin (tetrahydronaphthalene) so as to provide a solution having a p-type organic semiconductor concentration of about 0.6 wt.-%, stirring the solution for a period of 12 to 20 hours at a temperature of between 45°C and 85°C, keeping the solution at 25°C for a period of at least 1 hour after heating, and measuring the dynamic viscosity at 25°C using a viscosimeter (e.g. a Brookfield viscosimeter). If the p-type organic semiconductor is an organic polymer, its weight-average or number-average molecular weight may be appropriately selected to obtain a formulation having the desired dynamic viscosity. Preferably, the weight-average molecular weight Mw of the p-type polymer is less than 100,000, more preferably in the range of from 30,000 to 95,000, further preferably in the range of from 50,000 to 85,000. The dynamic viscosity at 25°C of the final ink formulation, i.e. the formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and the solvent blend is not particularly limited and is typically in a range of from 0.1 to 10 mPa-s, often in a range of from 0.5 to 2 mPa-s.

[0041] Preferably, the total concentration of n- and p-type material in the solvent blend is in the range of from 0.1 to 2.5 w/v%, more preferably from 0.5 to 1.8 w/v%.

[0042] In general, the relative solubility of donor and acceptor and the solvent boiling point are considered as important parameters when designing an ink formulation for achieving high efficiencies.

[0043] The efficiency and photocurrent is believed to be controlled by inclusion of a main solvent which is typically good for solubilising both donor and acceptor materials and a higher boiling point additive which usually achieves good solubility for either the acceptor or the donor.

[0044] The selection of suitable solvents according to their relative solubility for the p-type and n-type organic semiconductors can be achieved for example through the use of known parameters like the Hansen Solubility Parameters (HSP). The 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”, 2nd Ed. 2007, Taylor and Francis Group LLC. While not being limited thereto, the Hansen Solubility Parameters of each of the organic semiconductors of the formulation of the present invention are preferably in the following ranges: the dispersion contribution 5d(P) of the p-type semiconductor is preferably within the range of 17 to 22 MPa05; the polar contribution δρ(Ρ) of the p-type semiconductor is preferably within the range of 0 to 7 MPa05, more preferably 0 to 3 MPa05; and the hydrogen bonding contribution 6h(P) of the p-type semiconductor is preferably within the range of 0 to 6 MPa05, more preferably 0 to 3 MPa05. The dispersion contribution 5d(N) of the n-type semiconductor is preferably within the range of 17 to 21 MPa05; the polar contribution δρ(Ν) of the n-type semiconductor is preferably within the range of 0 to 7 MPa05, more preferably 3 to 7 MPa05; the hydrogen bonding contribution 6h(N) of the n-type semiconductor is preferably within the range of 0 to 8 MPa05, more preferably 3 to 6 MPa0·5.

[0045] The alkyl or aryl benzoate, the first aromatic hydrocarbon, and the second aromatic hydrocarbon are preferably chosen so that their Hansen Solubility Parameters match at least one of the p-type or the n-type organic semiconductor so that the difference between their dispersion, polar and hydrogen bonding contributions 6d, δρ and 6h is 5 MPa05 or less, more preferably 3 MPa05 or less, even more preferably 2 MPa05 or less.

[0046] In a preferred embodiment, the Hansen Solubility Parameters of the second aromatic hydrocarbon and the p-type organic semiconductor satisfy the following relationships:

or more preferably satisfy the following relationships:

wherein 6d(SAH), 6p(SAH) and 6h(SAH) denote the dispersion, polar and hydrogen bonding Hansen Solubility Parameters of the second aromatic hydrocarbon and 6d(P), δρ(Ρ) and 6h(P) denote the dispersion, polar and hydrogen bonding Hansen Solubility Parameters of the p-type organic semiconductor.

[0047] The formulation may comprise further components in addition to the n-type organic semiconductor, the p-type organic semiconductor and the solvent blend. As examples for 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.

[0048] It will be appreciated that the preferred features of the first and second embodiments specified above may be combined in any combination, except for combinations where at least some of the features are mutually exclusive.

[0049] The above-defined formulations serve as a starting material for the solution deposition of photoactive layers and films having an advantageously high stability for solution deposition applications and allow faster and more precise manufacturing of organic photosensitive devices, in particular photodetectors with excellent efficiency.

Organic Photosensitive Electronic Devices and Methods for Manufacturing the Same [0050] In a third embodiment, the present invention relates to the use of the solvent blend according to the first embodiment in a solution deposition method, including but not limited to coating or printing or microdispensing methods like for example spin coating, spray coating, web printing, brush coating, dip coating, slot-die printing, ink jet printing, letter-press printing, screen printing, doctor blade coating, roller printing, offset lithography printing, flexographic printing, or pad printing. Preferably, the solution deposition method is an inkjet printing method, as will be discussed in further detail hereinbelow with reference to the fourth embodiment of the present invention.

[0051] Namely, in a fourth embodiment, the present invention relates to a method of manufacturing an organic 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 second embodiment by a solution deposition method to form a photoactive layer.

[0052] The organic electronic device may be any organic electronic device or component which requires a layer comprising an p-n organic semiconductor junction, such as e.g. an organic photodiode. Typically, the organic electronic device is an organic photosensitive electronic device, which may be a photovoltaic device or a solar cell, a photoconductor 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.

[0053] A typical general architecture of an organic photosensitive electronic device is schematically depicted in Fig. 1. Herein, an anode 2 usually consisting of a high work-function material is deposited onto a substrate 1 made of a material transparent to visible light. Typical anode materials include conductive metal oxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO), aluminum zinc oxide (AIZnO), and metals (e.g. gold), while glass or plastics are conventionally used as substrate materials. Between the anode 2 and the cathode 4, which may be made of metals (e.g. Ag, Ag:Mg) or metal oxides, the photoactive layer 3 comprising the so-called bulk heterojunction is formed by solution deposition of the formulation according to the first embodiment. A contact 5 is provided between the anode 2 and cathode 4, which may include a bias voltage source and a detector (e.g. current meter or readout device, wired in series with the bias voltage source as detection circuit), for example, to measure the generated photo response.

[0054] While the formulation according to the present invention can be applied onto a substrate or a component of the organic photosensitive electronic device by any suitable selective solution deposition method, including but not limited to coating or printing or microdispensing methods like for example spin coating, spray coating, web printing, brush coating, dip coating, slot-die printing, ink jet printing, dispense printing, letter-press printing, screen printing, doctor blade coating, roller printing, offset lithography printing, flexographic printing, or pad printing, it is preferable that the selective solution deposition method is an ink jet printing method (including continuous inkjet printing or Drop-on-Demand (DOD) inkjet printing methods) in order to take full advantage of the present invention.

[0055] Inkjet printing generally involves the ejection of a fixed quantity of a liquid phase, i.e. ink, in form of droplets from a chamber through a nozzle. The ejected drops are provided onto a substrate to form a pattern. While solidification of the liquid drops may be brought about through chemical changes or crystallization, solvent evaporation is commonly used, in some cases by exposing the deposited wet film to high temperature and/or reduced pressure, preferably immediately upon printing.

[0056] In conventional methods jetting intervals have to be set short in order to avoid the risk of nozzle clogging and blockage. The method of the present invention has the advantage that the risk of clogging is significantly reduced, which enables stable operation at idle state frequency, i.e. frequencies of lower than 50 Hz, typically even lower than 20 Hz, such as in the range of 5 to 10 Hz. Moreover, the latency/decap time (i.e. the time the ink can be left non-jetting without having to purge the head before getting the nozzles jetting again) may be advantageously increased when compared to using conventional organic semiconductor formulations.

[0057] The thickness of the dried photoactive layer or film is preferably from 10 nm to 3 pm, more preferably from 20 nm to 2 pm, such as from 50 nm to 600 nm. In a further preferred embodiment, the thickness of the dried layer film is in the range of from 80 to 250 nm.

[0058] The photoactive layer or film may be homogenous or phase-separated and contain different phases which phases may differ in the ratio of p-type to n-type material. The photoactive layer may have a more or less 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.

[0059] It is to be noted that since the solvent blend used in the formulation of the present invention decisively influences the uniformity and morphology of the resulting photoactive layer, the latter exhibits characteristic properties (e.g. texture and/or phase distribution). Also, residual amounts of solvents having high boiling points (e.g. 200°C or higher) may be found in the active layer even after extensive drying and hence contribute to distinct structural properties. EXAMPLES Ink stability test [0060] The ink jetting stability of formulations comprising C70PCBM as the n-type organic semiconductor and a p-type organic semiconductor according to structural formula (1) (in a ratio of 2:1, concentration of w/v 1% in solvent) has been studied by using different solvent blends:

[0061] The materials constituting the tested ink formulations are shown in Tab.1:

TABLE 1: Overview of used materials and Hansen Solubility Parameters Comparative Example 1 [0062] Initially, an OPD-ink comprising the above-identified n- and p-type organic conductors (1% w/v) in a solvent blend consisting of trim ethyl benzene and benzylbenzoate (9:1) was prepared and evaluated for its jetting stability using 8pL SX3 printheads (27 pm nozzle diameter) and 35pL SE3 printheads (42 pm nozzle diameter) available from Fujifilm Dimatix, Inc.

Example 1 [0063] An OPD-ink has been prepared and evaluated in the same manner as in Comparative Example 1, with the exception that a solvent blend comprising trimethylbenzene, benzylbenzoate and diphenylmethane in a volume ratio of 90:5:5 has been used.

[0064] Upon jetting the composition of Comparative Example 1 at a frequency of 50 Hz using both types of printheads, clogging of the nozzles within hours or up to a couple of days has been observed, with more than one nozzle out of the 128 nozzles available on the printhead not jetting any more, after which there was an increased risk of irreversible nozzle blockage. However, when diphenylmethane was added as a 3-component solvent blend, no clogging was observed when left jetting (even at down to 5-10Hz jetting frequency) for up to more than three days. The differences in jetting stability for the two types of inks are summarized in Table 2 below.

TABLE 2: Overview of jetting stability observations.

[0065] The time for stable idle jetting (in “standby” mode) at low frequency was found to be substantially larger with the 3-component solvent blend than with the ink according to Comparative Example 1. The latency/decap time, which is the time the ink can be left nonjetting without having to purge the head before getting the nozzles jetting again, was also found to be significantly better for the composition of Example 1.

Device Performance Tests [0066] In a further series of experiments, organic semiconductor formulations comprising different solvent systems were used in the preparation of device stacks by spin coating.

[0067] In particular, device stacks with the following configuration have been prepared: - Photopatterned Geomatic 45 nm ITO as anode on glass with 100 nm MoCr in tracking regions - 50 nm Nissan NDHIL layer - 100 nm spin-coated photoactive layer - 200 nm evaporated Ag cathode - glass encapsulation with getter [0068] The photoactive layer was formulated by combining a 1:2 weight ratio of a p-type organic semiconductor according to structural formula (1) and C70PCBM dry and adding a solvent blend in a nitrogen environment with a concentration of 24 mg/ml. This solution was then heated with stirring at 80°C over a period of 14 hours. The solution was then allowed to cool for 8 minutes before immediately being drawn into a syringe for spin coating. The spin coating was performed with a glass syringe, 2 pm glass filter and a needle.

[0069] The substrate was flooded with the solution before applying a spin process using a single spin phase (with lid + gyroset cover) at a speed of 500 to 1000 rpm (1000 rpm acceleration) and a duration of 6 seconds.

[0070] The coated substrate was then dried in a Vacucel vacuum oven for 5 minutes at a temperature of 80°C with a vacuum pressure of < 5-10'2 mbar.

[0071] After drying, the films were transferred into a nitrogen filled glovebox for loading into a deposition tool for the cathode process. During this transfer the films were briefly placed in a chamber under vacuum.

[0072] The cathode was then deposited followed by encapsulation of the device in a nitrogen environment. The devices were then scribed and pinned in preparation for testing.

[0073] The testing procedure involved illumination of each OPD element with a calibrated LED (λ = 525 nm) inside a sealed test box and the measurement of mean photocurrent (active device area = 1 mm2). The wavelength selected is considered useful for biosensor applications. The external quantum efficiency (EQE) of the photodetector has been calculated based on the equation:

wherein / is the mean photocurrent, P is the power of monochromatic light with wavelength λ falling on the photodetector, h is Planck’s constant, e is the electron charge, and c is the speed of light in vacuum.

Comparative Example 2 [0074] In Comparative Example 2, a blend of 1,2,4-trimethylbenzene and benzyl benzoate (9:1) has been used as a solvent system. As a result, a mean photocurrent of 49.2 nA/mm2 was measured (EQE = 44.1%). A second measurement yielded a value of 48.3 nA/mm2 (EQE = 43.3%).

[0075] The high efficiency is believed to be controlled by inclusion of a main solvent (in this case 1,2,4-trimethylbenzene) which is good for solubilising both donor and acceptor materials and a higher boiling point additive (in this case benzyl benzoate) which achieves good solubility for the acceptor but is a non-solvent of the donor.

Example 2 [0076] A blend of 1,2,4-trimethylbenzene, benzyl benzoate and diphenylmethane (85:5:10) has been used as a solvent system in Example 2. A mean photocurrent of 49.8 nA/mm2 (EQE = 44.7%) was measured, which demonstrates that the addition of diphenylmethane (component (c)) does not negatively affect the device efficiency. To the contrary, the external quantum efficiency is even slightly higher than that measured with Comparative Example 2.

Examples 3 to 6 [0077] In another series of experiments, different compositions according to the present invention were tested in inkjet-printed devices having the following configuration: photopatterned Geomatic 45 nm ITO as anode on glass with 100 nm MoCr in tracking regions inkjet-printed 50 nm Nissan PN7-B1 layer inkjet-printed photoactive layer 200 nm evaporated Ag cathode glass encapsulation with getter [0078] The photoactive layer was formulated by combining a 1:2 weight ratio of a p-type organic semiconductor according to structural formula (1) and C70PCBM dry and adding a solvent blend in a nitrogen environment with a concentration of 9.8 mg/ml. This solution was then heated with stirring at 80°C over a period of 14 hours. The solution was then degassed for 1 hour using a sonication process under vacuum before being loaded onto a Litrex inkjet printer. The printer was left to jet continuously at 10-100 Hz jetting frequency for several hours before printing the devices.

[0079] The mean photocurrent of the ink-jet printed devices (active device area = 1 mm2) was measured and the EQE has been calculated in the same manner as outlined above with respect to Comparative Example 2 and Example 2.

[0080] The results of the measurements for each of the tested compositions are shown in Table 3.

TABLE 3: Photocurrent data for inkjet-printed OPDs.

[0081] The external quantum efficiencies of the devices prepared by using the compositions of the present invention can be seen to achieve comparable levels to the device according to Comparative Example 2, wherein the photoactive layer has been spin coated using the HSPiP-optimized blend of the two solvents 1,2,4-trimethylbenzene and benzyl benzoate.

[0082] Accordingly, it has been demonstrated that the organic semiconductor formulations according to the present invention exhibit improved stability for ink jetting applications, even at low jetting frequencies, may be used to provide high-quality photoactive thin films or photoactive layers, and simultaneously allow manufacturing of organic photosensitive electronic devices which achieve excellent photocurrent levels.

Measurements in dependence of p-type semiconductor [0083] In a final series of experiments, ink formulations have been prepared and tested, comprising a p-type semiconductor having different dynamic viscosities in tetralin solution, using the p-type polymer according to Structural Formula (1) in different weight-average molecular weights. The dynamic viscosity at 25°C has been measured by dissolving the p-type organic semiconductors in tetralin (tetrahydronaphthalene) so as to provide a solution having a p-type organic semiconductor concentration of 0.6 wt.-%, stirring the solution for 16 hours at 50°C, keeping the solution at 25°C for about 4 hours after heating, and measuring the dynamic viscosity at 25°C using a Brookfield viscosimeter (Model DV2TLV CP; spindle CPA-40Z; rotating speed of 30 rpm; 0.70 ml_ liquid; three measurements). The different p-type polymers were then combined with C70PCBM in a 1:2 weight ratio and a solvent blend consisting of trimethylbenzene, benzylbenzoate and diphenylmethane in a volume ratio of 90:5:5 (0.98 w/v%) to provide the ink formulations according to Examples 7 to 15.

[0084] The ink formulations have been tested with respect to filterability and jetting stability (in accordance with Example 1) and OPD devices (each having an active device area of 1 mm2) using the ink formulations have been manufactured in accordance with Example 2. Thereafter, the relative efficiency has been compared on the basis of the photocurrent performance (cf. Example 2). The results of the evaluation are shown in Table 3, wherein “++” denotes a very good, “+” a good and “0“ an acceptable performance.

TABLE 4: Ink performance in dependence of p-type semiconductor viscosity in formalin solution.

[0085] As is shown above, a favourable balance between jetting stability and photocurrent performance is observed in Examples 9 to 13. Ideal results are obtained in Examples 9 and 10, wherein the p-type organic semiconductor exhibits a dynamic viscosity at 25°C in tetralin solution of 3.9 and 4.1 mPa-s.

[0086] Once given the above disclosure, many other features, modifications, and improvements will become apparent to the skilled artisan.

REFERENCE NUMERALS 1: substrate layer 2: anode 3: photoactive layer 4: cathode 5: contact

Claims (15)

1. A solvent blend comprising: (a) an alkyl benzoate or aryl benzoate; (b) a first aromatic hydrocarbon, which is a dialkyl- or trialkylsubstituted aromatic hydrocarbon; and (c) a second aromatic hydrocarbon different from the first aromatic hydrocarbon.

2. The solvent blend according to claim 1, wherein component (a) is selected from a C1-C18 alkyl benzoate or a benzoic acid Cs-Cis aryl ester, wherein the C6-C18 aryl group may be unsubstituted or substituted with an C1-C12 alkyl group; wherein component (b) is selected from a dialkylbenzene comprising alkyl groups which may be independently selected from a C1-C12 alkyl group or a trialkylbenzene, wherein the alkyl groups may be the same or different and may be selected from a C1-C12 alkyl group; and/or wherein component (c) is an aromatic hydrocarbon comprising two benzene rings which may be fused.

3. The solvent blend according to any of claims 1 or 2, wherein the first aromatic hydrocarbon has a boiling point lower than 200°C and the second aromatic hydrocarbon has a boiling point of 200°C or higher, preferably between 200°C and 300°C.

4. The solvent blend according to any of claims 1 to 3, wherein component (a) is benzyl benzoate; wherein component (b) is a trialkylbenzene, preferably trimethylbenzene; and/or wherein component (c) is selected from any of 1-methylnaphthalene or diphenylmethane.

5. The solvent blend according to any of claims 1 to 4, wherein the alkyl or aryl benzoate is present in a content range of from 0.5 to 50 vol.-%, preferably from 0.5 to 30 vol.-%, more preferably from 1 to 10 vol.-% based on the total volume of the solvent blend.

6. The solvent blend according to any of claims 1 to 5, wherein the first aromatic hydrocarbon is present in a content range of from 30 to 99.4 vol.-%, preferably from 60 to 99 vol.-%, more preferably from 70 to 98 vol.-% based on the total volume of the solvent blend.

7. The solvent blend according to any of claims 1 to 6, wherein the second aromatic hydrocarbon is present in a content range of from 0.1 to 50 vol.-%, preferably from 0.5 to 30 vol.-%, more preferably from 1 to 20 vol.-% based on the total volume of the solvent blend.

8. A formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and the solvent blend according to any of claims 1 to 7.

9. The formulation according to claim 8, wherein the Hansen Solubility Parameters of the second aromatic hydrocarbon and the p-type organic semiconductor satisfy the following relationships:

wherein 5DSah, 5PSah and 5Hsah denote the dispersion, polar and hydrogen bonding Hansen Solubility Parameters of the second aromatic hydrocarbon and 5DP, 5Pp and δΗρ denote the dispersion, polar and hydrogen bonding Hansen Solubility Parameters of the p-type organic semiconductor.

10. The formulation according to claim 9, wherein the Hansen Solubility Parameters of the second aromatic hydrocarbon and the p-type organic semiconductor satisfy the following relationships:

wherein 5DSah, 5PSah and 5HSah denote the dispersion, polar and hydrogen bonding Hansen Solubility Parameters of the second aromatic hydrocarbon and 5DP, δΡρ and δΗρ denote the dispersion, polar and hydrogen bonding Hansen Solubility Parameters of the p-type organic semiconductor.

11. The formulation according to any of claims 8 to 10, wherein the p-type organic semiconductor is a conjugated organic polymer and/or wherein the n-type organic semiconductor is fullerene or a fullerene derivative.

12. The formulation according to any of claims 8 to 11, wherein the dynamic viscosity of the p-type organic semiconductor in tetralin solution at 25° C is in the range of from 3.6 to 5.4 mPa-s.

13. Use of the solvent blend according to claims 1 to 7 in a solution deposition method, preferably an inkjet printing method.

14. Method of manufacturing an organic 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 any of claims 8 to 12 by a solution deposition method to form the photoactive layer, wherein the solution deposition method is preferably an inkjet printing method.

15. Use of the formulation according to any of claims 8 to 12 as coating or printing ink for the preparation of a photoactive thin film or photoactive layer.