GB2547460A - Solvent systems for tuning of the external quantum efficiency of organic photodiodes - Google Patents

Abstract

A formulation comprising p-type and n-type organic semiconductors and a solvent comprising indane, an indane derivative or a combination thereof in a total content of at least 50 vol % relative to the total solvent volume. This may be used to provide photoactive layers with a morphology optimized for photodetector applications, wherein the external quantum efficiency may be tuned so as to selectively enhance the response in particular wavelength regions and thereby improve the detector capability of the device. If a mixture of solvents is used, it is preferable that the indane content is at least 90 vol %, and furthermore, it is even more preferable to be at least 97 vol % relative to the total solvent volume. The p-type semiconductor material may be a conjugated organic polymer and the n-type semiconductor material may be a fullerene or fullerene derivative. The formulation may be deposited by solution deposition, such as spin coating before drying.

Description

SOLVENT SYSTEMS FOR TUNING OF THE EXTERNAL QUANTUM EFFICIENCY OF

ORGANIC PHOTODIODES

FIELD OF INVENTION

[0001] This invention relates to a formulation comprising p-type and n-type organic semiconductors and an organic solvent, its use for manufacturing organic electronic devices, and to organic electronic devices, in particular organic photodiodes, 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 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) may be mentioned. Usually, an OPV device includes as a photoactive layer a p-n junction which is prepared by film deposition of a donor/acceptor blend from solution (e.g. by blade coating or spin coating) 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, developments in solar cell research have shown that the choice of solvent 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 the OPV device. For instance, chlorinated solvents, such as chlorobenzene, dichlorobenzene or triochlorobenzene have shown to be excellent solvents for the above-mentioned donor/acceptor blends, leading to a favourable morphology and phase separation and thereby enabling to manufacture OPV devices with favourable power conversion efficiencies. In view of the environmental burden involved with the use of chlorinated solvents, particularly at industrial scale, alternative solvents have been sought and applied, however, often at the expense of OPV device performance and power conversion efficiency (PCE).

[0006] Recently, promising power conversion efficiencies have been achieved by using blends of at least two non-chlorinated solvents. For example, 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.

[0007] WO 2013/029733 A1 proposes a solvent selected from alkylated tetralin, alkylated naphthalene and alkylated anisole, optionally with a second, different solvent to further tune the morphology of the active layer and thereby maximize the PCE of the manufactured OPV device.

[0008] However, solar cells are generally optimized to have a maximum efficiency of the conversion of incident light to electrical energy, and photodiodes for photodetector applications have different requirements. Namely, in order to perform well as sensors such photodiodes are designed to maximize the photo current, to minimize the dark current and noise generated by the diode, and to respond quickly to incident light. In addition, solar cells are generally not operated by using an external voltage source but generate a current at a voltage created by the inherent properties of the organic materials used. In contrast, organic photodetector devices are usually provided with an external bias voltage, so that the conditions under which the photodiodes operate are substantially different.

[0009] In view of the above, it remains desirable to provide organic semiconductor formulations which may be used to provide photoactive layers with a morphology optimized for photodetector applications, which do not require the use of chlorinated solvents and allow manufacturing of organic photosensitive electronic devices having both reduced dark current and a favourably high external quantum efficiency (EQE).

[0010] Experiments with organic photodetector devices comprising photoactive layers deposited from 1,2-dichlorobenzene solutions have further shown that upon increasing the bias voltage, the EQE response of the device increases in a nearly-linear manner throughout the wavelength range of 400 to 800 nm.

[0011] It would be desirable for a large number of applications to provide an organic photosensitive electronic device, wherein the EQE may be tuned so as to selectively enhance the response in particular wavelength regions and thereby improve the detector capability of the device.

SUMMARY OF THE INVENTION

[0012] 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.

[0013] Achieving maximum generation of photocurrent from an organic photodetector device requires a layer with an optimized donor-acceptor morphology with a suitable exciton diffusion length from a donor/acceptor interface. The choice of solvent plays a decisive role in the control of the morphology and the intermixing of both donor and acceptor materials, since the relative solubility of donor and acceptor and the solvent boiling point contribute to the morphology of the dry film. It has been surprisingly found that the use of specific organic solvents in formulations comprising n-type and p-type organic semiconductors results in a non-linear response to the wavelength of illumination upon application of a bias voltage, which means that in certain wavelength regions the increase of the external quantum efficiency is disproportionally high. Advantageously, this effect allows to enhance the detector capability of the device.

[0014] Generally speaking, the present invention relates to a formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and a solvent; wherein the solvent comprises indane, an indane derivative or a combination thereof in a total content of at least 50 vol.-% relative to the total solvent volume.

[0015] In a further 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 aforementioned formulation by a solution deposition method, preferably by spin coating, to form a wet film; and drying the wet film to provide the photoactive layer.

[0016] Another aspect of the present invention is an organic photosensitive electronic device comprising an anode, a cathode and a photoactive layer formed from the aforementioned formulation between the cathode and the anode.

[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 the general architecture of a conventional organic photodetector device.

[0019] FIG. 2 schematically illustrates an example of an encapsulated organic photodetector device.

[0020] FIG. 3 shows the results of dark J-V measurements of an exemplary device according to the present invention in comparison with a prior art device.

[0021] FIG. 4 shows the wavelength-dependent EQE response of a prior art device at different bias voltages.

[0022] FIG. 5 shows the wavelength-dependent EQE response of a device according to the present invention at different bias voltages.

DETAILED DESCRIPTION OF THE INVENTION

[0023] For a more complete understanding of the present invention, reference is now made to the following description of the illustrative embodiments thereof:

Semiconductor Formulation [0024] In a first embodiment, the present invention relates to a formulation comprising an n-type organic semiconductor (OSC), a p-type OSC and a solvent; wherein the solvent comprises indane, an indane derivative or a combination thereof in a total content of at least 50 vol.-% relative to the total solvent volume.

[0025] Preferably, the solvent comprises indane. If an indane derivative is used, the indane derivative is preferably indane substituted with one or more substituents selected from one or more C1-C6 alkyl groups or C1-C6 alkoxy groups.

[0026] It is preferred that the solvent comprises the indane, indane derivative or a combination thereof in a total content of at least 90 vol.-%, preferably at least 97 vol.-%, more preferably at least 98 vol.-%. The solvent may also consist of indane, indane derivative or a combination thereof, with the exception trace amounts of unavoidable impurities.

[0027] In a preferred embodiment, the boiling point of the solvent is lower than 200°C, more preferably lower than 180°C.

[0028] The solvent may be a mixture of the indane and/or indane derivative and 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, tetralin, naphthalene, alkylated benzene (e.g. toluene, xylene), alkoxylated benzenes, 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, aryl 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.

[0029] From the viewpoint of environment-friendliness it is, however, preferred that the composition does not contain chlorinated solvent(s).

[0030] In a further preferred embodiment, the solvent does not comprise alkyl benzoates or aryl benzoates.

[0031] The relative solubility of each of the n-type OSC and the p-type OSC in the solvent or solvent mixture is preferably such that the Hansen Relative Energy Difference is 1.0 or less, more preferably 0.9 or less. The Hansen Solubility Parameters of the solvent or solvent mixture are preferably in the following ranges: the dispersion contribution 5D is preferably within the range of 17 to 21 MPa0 5; the polar contribution δρ is preferably within the range of 0 to 7 MPa05, more preferably 0 to 6 MPa05; and the hydrogen bonding contribution 0h is preferably within the range of 0 to 6 MPa05, more preferably 0 to 5 MPa0 5. 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.

[0032] 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, 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 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.

[0033] 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, C96, PCBM-type fullerene derivatives (including phenyl-C61 -butyric acid methyl ester (CeoPCBM), 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). 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.

[0034] 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 5:1 to 1:5, more preferably 1:1 to 1:4, especially preferably 1:1 to 1:3.

[0035] The formulation may comprise further components in addition to the n-type organic semiconductor, the p-type organic semiconductor and the solvent. 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.

[0036] The above-defined formulations serve as a starting material for photoactive layers which exhibit an excellent donor-acceptor morphology especially for photodetection applications and which allow the manufacturing of devices having enhanced sensitivity in specific, tunable wavelength ranges. In particular, devices comprising photoactive layers that have been solution deposited from the aforementioned formulation comprising indane, an indane derivative or a combination thereof tend to exhibit lower external quantum efficiencies (EQEs) than the corresponding devices using common chlorinated solvents when no bias voltage is applied. By applying a negative bias, however, the response to incoming light ranges is increased in a non-linear manner throughout specific wavelength regions to a level comparable to that of the device using chlorinated solvents, while the dark current measured remains remarkaby lower. Thereby, the magnitude of on/off response to light is increased. In addition, by varying the components of the formulation according to the above description, the peak EQE in a specific wavelength range may be tuned via control of the active layer morphology.

Organic Photosensitive Electronic Devices and Methods for Manufacturing the Same [0037] 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 first embodiment by a solution deposition method, preferably by spin coating, to form a wet film; and drying the wet film to provide the photoactive layer.

[0038] 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 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, inkjet printing, letter-press printing, screen printing, doctor blade coating, roller printing, offset lithography printing, flexographic printing, or pad printing. Preferred are spin coating and blade coating methods. Spin coating is especially preferred.

[0039] After the formulation has been solution deposited to form the wet film, the solvent or solvent mixture is dried, i.e. preferably removed by evaporation, for example by exposing the deposited wet film to high temperature and/or reduced pressure. Preferably, drying is carried out in a vacuum oven at a temperature of 50 to 100°C and at a pressure of 0.01 to 0.1 mbar, more preferably 0.03 to 0.07 mbar. Alternatively, if pure indane is used as solvent, drying may be carried out at room temperature under athmospheric pressure within a time frame of 3 to 10 min, typically about 4 to 6 min. These drying conditions contribute to a favourable photoactive layer morphology.

[0040] The thickness of the photoactive layer produced after removal of solvent is preferably from 10 nm to 2 pm, more preferably from 10 nm to 1 pm, most preferably from 50 nm to 500 nm.

[0041] The photoactive layer 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.

[0042] It is to be noted that since the solvent used in the formulation of the present invention decisively influences the donor-acceptor morphology of the resulting photoactive layer, the latter exhibits a characteristic texture and/or phase distribution.

[0043] In a third embodiment, 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 embodiment, respectively.

[0044] A typical general architecture of an organic photosensitive electronic device according to the present invention 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.

[0045] A more specific illustrative example is depicted in Fig. 2. Herein, a hole-injecting layer 16 is provided between the anode 12 and the photoactive layer 13. The contact 15 connects the anode 12 with the cathode 14, and an encapsulation layer 77 is provided on the layer stack to encapsulate the device in combination with the substrate layer 11.

[0046] It is to be understood that Figs. 1 and 2 serve 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 and hole-blocking layers (HBL).

[0047] The organic photosensitive electronic device may be a photovoltaic device or a solar cell, a photoconductor cell or a photodetector, for example. In order to take full advantage of the benefits of the present invention, 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 bias voltage is applied) and thereby functions as a photoconductor cell or a photodetector, as opposed to a solar cell, which is operated in photovoltaic mode (with zero bias voltage). It is further preferred that the photodiode is reverse biased (with the cathode being driven positive with respect to the anode).

[0048] Therefore, the organic photosensitive electronic device comprises a means for applying a fixed or variable bias voltage across the device structure. More preferably, the fixed or variable bias voltage is a reverse bias voltage. As a fixed reverse bias voltage, a negative voltage of at least 1.5 V, more preferably 2.5 V or greater is advantageous. EXAMPLES Solubility properties [0049] In order to assess the solubility of typical p-and n-type organic semiconductors in a solvent system as used in the present invention, the Hansen solubility parameters of indane with respect to the n-type organic semiconductor CeoPCBM and a p-type organic semiconductor according to structural formula (A) were determined in accordance with C. M. Hansen, “Hansen Solubility Parameters: A User’s Handbook’, 2nd Ed. 2007, Taylor and Francis Group LLC and compared to that of 1,2-dichlorobenzene.

(A) [0050] The results of the measurements are given in the following Table 1:

Tab.1: Hansen solubility data for polymer (A) and CeoPCBM with indane and 1,2-dichlorobenzene [0051] In the table above the Hansen Relative Energy Difference (RED) values for each solvent indicate the solubility of a material, with a value of less than 1 indicating a material is soluble. Thus, it is shown that the solubility of p-type polymer (A) in indane is higher than in 1,2-dichlorobenzene. Moreover, the difference in RED values in comparison to 1,2-dichlorobenzene indicates that the precipitation rates of the two materials are different and that therefore also the resulting morphology of the photoactive layer is different.

Device Testing [0052] An exemplary device having the configuration illustrated in Fig. 2 has been prepared, comprising the following layer stack (in order of deposition): • photopatterned 45 nm ITO film (commercially available from Geomatec Co., Ltd) as anode on a glass substrate • 40 nm Plextronics CA2004 hole injection layer (HIL) • 150 nm spin-coated photoactive layer • 200 nm Ag evaporated cathode • glass encapsulation with getter [0053] Spin coating was used as the process for depositing the organic layers in the OPD device stack including the photoactive layer.

[0054] The active layer was formulated by combining a 1:2 weight ratio of the p-type semiconductive polymer (A) and C70PCBM dry and adding indane solvent 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, and then allowed to cool for 8 minutes before immediately drawing the solution into a syringe for spin coating. The spin coating was performed using a glass syringe, 2 pm glass filter and a needle. In particular, the substrate was flooded with the solution before spin coating using a single spin phase (with lid and gyroset cover) at a speed of 530 rpm, an acceleration of 1000 rpm and a duration of 6 seconds. The wet film was then held flat and allowed to dry in an extraction hood for 5 minutes. After drying, the film was transferred into a nitrogen filled glovebox for loading into a deposition tool for the cathode process. During this transfer the film were briefly placed in a chamber under vacuum. The cathode is then deposited followed by encapsulation of the device in a nitrogen environment. The device was then scribed and pinned in preparation for testing.

[0055] As a comparative example, a device has been prepared in the same manner as stated above, with the exception that 1,2-dichlorobenzene has been used as a solvent instead of indane.

[0056] Both devices have been subjected to dark current measurements in dependence of bias voltage, using a custom IV test box scanning from -3 to 3V under dark conditions with a 10 mA current compliance limit. The results of the measurements are shown in Fig. 3.

[0057] As Fig. 3 demonstrates, the morphology change introduced to generate a nonlinear EQE response does not have a negative impact on the dark current. In particular, the JV data in Fig. 3 shows that the dark current is reduced with indane, which shows that the magnitude of on/off response to light may be increased.

[0058] Figs. 4 and 5 display the results of EQE measurements for the device fabricated with 1,2-dichlorobenzene as a solvent (Fig. 4) and with indane as a solvent (Fig. 5) within the visible and near-infrared wavelength region of from 400 to 800 nm. Said measurements have been carried out by using a xenon lamp conjunction with a 74100 Oriel Cornerstone™ Monochromator, scanning in 5 nm steps over the 800-400 nm wavelength range. An illumination area of approximately 1 mm x 4 mm was applied to the active region of the device and the illumination intensity was calibrated against a silicon photodiode.

[0059] Fig. 4 shows that upon increasing the negative bias voltage from 0 V to -5 V, the EQE increases uniformly in a nearly linear manner throughout the entire recorded wavelength spectrum.

[0060] In contrast, Fig. 5 demonstrates that by applying a negative bias to the device fabricated using indane as the solvent the response to incoming light in the red/near-infra red region is remarkably increase to about 80%, while the increase in the wavelength range of from 400 to 600 nm is significantly lower.

[0061] Accordingly, it has been shown that organic semiconductor formulations have been provided which may be used to manufacture photoactive layers with a morphology optimized for photodetector applications, which do not require the use of chlorinated solvents and allow manufacturing of organic photosensitive electronic devices having both reduced dark current and a favourably high external quantum efficiency (EQE). Moreover, it is shown that by using a formulation according to the present invention, a non-linear EQE response with a peak in a specific wavelength region is achieved, which enables improved detector capabilities in a device that would otherwise operate less effectively in said wavelength region.

[0062] Once given the above disclosure, many other features, modifications, and improvements will become apparent to the skilled artisan.

[0063] Reference numerals: 1 /11: substrate layer 2/12:anode 3/13: photoactive layer 4/14: cathode 5/15: contact 16: hole-injecting layer 17: encapsulation layer

Claims (13)

1. A formulation comprising an n-type organic semiconductor, a p-type organic semiconductor and a solvent; wherein the solvent comprises indane, an indane derivative or a combination thereof in a total content of at least 50 vol.-% relative to the total solvent volume.

2. The formulation according to claim 1, wherein the indane derivative is a substituted indane, the substituent being selected from one or more C1-C6 alkyl or C1-C6 alkoxy groups.

3. The formulation according to any of claims 1 or 2, wherein the boiling point of the solvent is lower than 200°C, preferably lower than 180°C.

4. The formulation according to any of claims 1 to 3, wherein the indane or indane derivative is comprised in the solvent in a content of at least 90 vol.-%, preferably at least 97 vol.-% relative to the total solvent volume.

5. The formulation according to any of claims 1 to 4, wherein the solvent comprises indane.

6. The formulation according to any of claims 1 to 5, wherein the solvent consists of indane.

7. The formulation according to any of claims 1 to 6, wherein the p-type organic semiconductor is a conjugated organic polymer.

8. The formulation according to any of claims 1 to 7, wherein the n-type organic semiconductor is fullerene or a fullerene derivative.

9. 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 any of claims 1 to 8 by a solution deposition method, preferably by spin coating, to form a wet film; and drying the wet film to provide the photoactive layer.

10. An organic photosensitive electronic device comprising an anode, a cathode and a photoactive layer formed from a formulation according to claims 1 to 8 between the cathode and the anode.

11. The organic photosensitive electronic device according to claim 10, wherein the organic photosensitive electronic device is an organic photodiode.

12. The organic photosensitive electronic device according to claim 11, wherein the organic photodiode comprises a means for applying a reverse bias across the device structure.

13. The organic optoelectronic device according to claim 12, wherein the reverse bias is a negative voltage of 2.5 V or greater.