WO2013037404A1

WO2013037404A1 - Photorefractive composite

Info

Publication number: WO2013037404A1
Application number: PCT/EP2011/065876
Authority: WO
Inventors: Cornelia Denz; Katharina DITTE; Jianing Liu; Jiang Wei; Zhaohui Wang; Thomas SCHEMME
Original assignee: Institute of Chemistry CAS; Westfaelische Wilhelms Universitaet Muenster
Current assignee: Institute of Chemistry CAS; Westfaelische Wilhelms Universitaet Muenster
Priority date: 2011-09-13
Filing date: 2011-09-13
Publication date: 2013-03-21
Anticipated expiration: 2014-03-13
Also published as: CN103946922B; DE112011105620T5; CN103946922A

Abstract

The present invention relates to a photorefractive composite comprising a hole transporter, a nonlinear optical unit, and a sensitizer, wherein the sensitizer is C12-4C1 diperylene bisimide, and the use of a photorefractive composite comprising C12-4C1 diperylene bisimide in holography techniques, optics and laser optics, or in photo-voltaic cells, organic field effect transistors, and organic light emitting diodes.

Description

Photorefractive composite

The present invention relates to a photorefractive composite. In particular, the present invention relates to sensitizers for photorefractive composites.

Modern technologies, for example, lasers, LEDs, and optical fiber communication require and motivate the further elaboration of new materials exploring applicable matter-light interaction. The already achieved remarkable progress in the development of photosensitive and photoconductive materials enables the conversion of sunlight to electricity inside of solar cells. In addition, the production of electro-optic devices, such as flat-design LCD, based on the light modulating properties of liquid crystals is worth mentioning. A special kind of material interacting with light is photorefractive material. Combining the useful properties of photoconductivity and electro-optics photorefractive materials can change their refractive index under nonuniform light illumination. This specific mechanism makes them promising candidates for future applications such as high density holographic data storage, holographic imaging of living tissue, and 3D real time imaging displays. Thus, continuous data growth, medical diagnostics, and the entertainment industry have pressing need of innovative high- performance materials with index of refraction modulation approach. Furthermore,

UD 40329 / SAM:AL photorefractive materials are essential to realizing nanophotonic devices for optical information processing. But in spite of growing demand, insurmountable drawbacks like high-cost production and long-time fabrication have been inhibiting the definite advance since the first observation of the photorefractive phenomena by Ashkin in 1966 in an inorganic crystal. Nevertheless, considerable success became apparent by replacing the inorganic crystals with photosensitive and electro-optic organic materials. Their advantages include low-cost synthesis, easy modification, and fast fabrication.

In the last 20 years numerous organic photorefractive materials have been reported and approached potentially important applications but the anticipated breakthrough is still long in coming. The main reason for the absence of the predicted success is the insufficient photoconductivity at the desired wavelength resulting from ineffective charge carrier generation during illumination with light. Photoconductivity in organic materials for example can be achieved by doping a conductive polymer with a light absorbing molecule, called sensitizer.

Commonly this type of composites contain a polymeric hole transporter, a rod shape-like nonlinear optical unit, and a sensitizer. In these dc-field biased mixtures, the photorefractive effect is observed when following processes take place: The sensitizer absorbs optical radiation of a light pattern and thereby charge carriers are generated in the regions of high light intensity. Afterwards, while the sensitizer anions remain immobile, the mobile positive charge carriers are transported by the polymer to the dark regions, where they get trapped. This leads to the formation of a space-charge field E_sc, which rearranges the nonlinear optical molecules and thereby causes the refractive index change Δη. This induced index modulation is phase shifted with respect to the incident light pattern. According to the realignment of the nonlinear optical units in low glass-transition temperature materials the refractive index change of the material is strongly affected by the orientational enhancement effect. Although the content of the sensitizer in a photorefractive composite is very low, it plays a crucial role in the charge generation and hence the photoconductivity. It is well known that the photorefractive dynamics are strongly affected and even limited by the photoconductivity of the photorefractive composite. A well-known and widely used sensitizer is [6,6]-phenyl- C61 -butyric acid methyl ester (PCBM) which has been successfully applied as n-type material in organic field effect transistors, photodetectors, and photovoltaic cells. However, both devices need to be improved in photorefractive speed and light sensitivity. Thus, there is an ongoing demand for innovative sensitizers.

Therefore, the object underlying the present invention was to provide a sensitizer usable in photorefractive composites.

The problem is solved by a photorefractive composite comprising a hole transporter, a nonlinear optical unit, and a sensitizer, wherein the sensitizer is C12-4C1 diperylene bisimide according to the formula (1) as indicated below:

(i). As used herein, the term "C12-4C1 diperylene bisimide" refers to the compound according to the formula (1). The compound according to formula (1) is denoted C12-4C1 diperylene bisimide or C12-4C1 DiPBI. C12-4C1 diperylene bisimide also can be denoted 2,3, 13, 14- tetrachloro-6, 10,17,21-tetradodecylpyranthreno[6,7,8-def: 14, 15,16-d'e'f:3,4,5- d"e"f ^*g" : 11 , 12, 13 -d"'e"'f "g"']tetraisoquinolin-5,7,9, 11,16,18,20,22(6H, 1 OH, 17H,2 lH)-octaone according to the IUPAC nomenclature.

As used herein, the term "photorefractivity" refers to the reversible change of the refractive index during inhomogeneous illumination with light. Photorefractivity requires beside the photoconductivity an electro-optic response.

Surprisingly, it was found that C12-4C1 diperylene bisimide (C12-4C1 DiPBI) provides excellent processability of photorefractive materials in the whole range of visible light. The absorption of a photorefractive composite containing C12-4C1 diperylene bisimide (C12-4C1 DiPBI) covers the whole range of visible light showing maxima in the blue, green, and red region and is especially advantageous, as the known sensitizer [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) is hardly absorbing and even mono perylene bisimide only absorbs blue and green light preferably. Advantageously, it was found that C12-4C1 diperylene bisimide (C12-4C1 DiPBI) increases the photogeneration efficiency over known sensitizers such as [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) and thereby reduces the time required to write a hologram by a factor of 39. Such an improvement can counteract the need of stability of the experimental

configuration against mechanical environment disturbances and can even allow 3D imaging of moving objects. Further advantageously, C12-4C1 diperylene bisimide can provide a photorefractive performance double that high compared with PCBM at only one quarter of sensitizer concentration. Furthermore, C12-4C1 diperylene bisimide (C12-4C1 DiPBI) provides strong absorption of visible light, high fluorescence quantum yields, and excellent photostability, high electron affinity and charge carrier mobility. Moreover, C12-4C1 diperylene bisimide provides extraordinary thermal, chemical, and physical stability. Beside this, C12-4C1 diperylene bisimide is highly soluble in common solvents like toluene, tetrahydrofurane, thiophene, and cyclohexanone.

The photorefractive composite can provide a high amplification, and a fast photorefractive reaction. Further, the photorefractive composite advantageously can be resistant against high electric fields. Advantageously, the photorefractive composite shows no phase separation, and no crystallization. Further, the glass transition temperature of the photorefractive composite is close to ambient temperature.

In a preferred embodiment, the composite comprises C12-4C1 diperylene bisimide in an amount in the range of > 0.001 wt% to < 1 wt%, preferably in the range of > 0.002 wt% to < 0.5 wt%, more preferably in the range of > 0.01 wt% to < 0.348 wt%, even more preferably in the range of > 0.01 wt% to < 0.122 wt%, referring to a total amount of the composite of 100 wt%. Weight percent, weight-% or wt% are synonyms that refer to the concentration of a component as the weight of the component divided by the weight of the composition and multiplied by 100. The weight-% (wt%) of the components are calculated based on the total weight amount of the composition, if not otherwise stated. Photorefractive composites comprising low concentration of C12-4C1 diperylene bisimide (C12-4C1 DiPBI) have proved to be remarkably effective. For example, photorefractive composites containing C12-4C1 diperylene bisimide can provide a photorefractive

performance double that high compared with PCBM at only one quarter sensitizer concentration. It is especially advantageous that the amount of sensitizer can be decreased markedly in photorefractive composites.

The photorefractive composite, besides a sensitizer, contains a hole transporter and a nonlinear optical unit.

Preferably, the hole transporter is selected from the group comprising polymeric carbazole derivatives, poly(p-phenylene vinylene) derivatives, N,N'-Bis(3-methylphenyl)-N,N'- diphenylbenzidine, polymeric derivatives of N,N'-Bis(3-methylphenyl)-N,N'- diphenylbenzidine, polythiophenes, p-type conducting rylene dyes, tri-p-tolylamine, pentacene and/or anthracene.

Non polymer hole transporters such as p-type conducting rylene dyes, tri-p-tolylamine, pentacene and anthracene are usable in non polymer form. Preferably, these hole transporters are usable in polymer form, for example in a mixture with a polymer such as polystyrene, or as functional group of a polymer. Further preferred are conductive derivatives of pentacene and anthracene, especially 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacen).

Preferred are polymeric hole transporters. Preferably, the photorefractive composite is a photorefractive polymer composite. Preferred polymeric hole transporter are selected from the group comprising polymeric carbazole derivatives, poly(p-phenylene- vinylene) derivatives, polymeric derivatives of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine, and

polythiophenes. Preferred polymeric carbazole derivatives are selected from the group comprising poly(N- vinylcarbazole) (PVK), poly[methyl(3-carbazol-9-ylpropyl)siloxane] (PSX-Cz), and poly(p- phenyleneterephthalate) with pendent carbazole groups (PPT-Cz). Preferred poly(p- phenylene- vinylene) derivatives are selected from the group comprising poly[l,4-phenylene- l,2-di(4-benzyloxyphenyl)vinylene] (DBOP-PPV), poly[o(p)-phenylenevinylene-alt-2- methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), and poly[o(p)- phenylenevinylene-alt-2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (p-PMEH- PPV). Preferred polymeric derivatives of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) are selected from the group comprising poly(acrylic tetraphenyldiaminobiphenol) (PATPD), and Poly [2 -Methyl- l,4-phenylen-phenylimino-4,4'-biphenylen-phenylimino-3- methyl- 1 ,4-phenylen- 1 ,2-vinylen-2, 5-dioctyloxy- 1 ,4-phenylen- 1 ,2-vinylen] (TPD-PPV). A preferred polythiophene is poly(3-hexylthiophene-2,5-diyl). Advantageously, such polymers can provide highly ordered crystalline thin films.

In preferred embodiments, the hole transporter is a polymeric carbazole derivative selected from the group comprising poly(N-vinylcarbazole), poly[methyl(3-carbazol-9- ylpropyl)siloxane] and/or poly(p-phenyleneterephthalate) comprising pendent carbazole groups. In a preferred embodiment, the hole transporter is poly(9-vinylcarbazole).

In preferred embodiments, the composite comprises the hole transporter in an amount in the range of > 1 wt% to < 84 wt%, preferably in the range of > 10 wt% to < 70 wt%, more preferably in the range of > 59.601 wt% to < 59.807 wt%, referring to a total amount of the composite of 100 wt%.

The nonlinear optical units also are referred to as "nonlinear optical moieties" or nonlinear optical chromophores. As the qualities as a dye are not relevant for the photorefractive effect, these elements of the photorefractive composite are referred to as nonlinear optical units (NLO).

In preferred embodiments, the nonlinear optical unit is selected from the group comprising cyano-biphenyles, dicyanostyrene derivatives, l-alkyl-5-[2-(5- dialkylaminothienyl)methylene]-4-alkyl-[2,6-dioxo-l,2,5,6-tetrahydropyridine]-3- carbonitrile, 2-dicyanomethylen-3-cyano-5,5-dimethyl-4-(4'-dihexylaminophenyl)-2,5- dihydrofuran, 4-N,N-diethylamino-P-nitrostyrene, 3-fluoro-4-(N,N-diethylamino)-P - nitrostyrene, 2,5-dimethyl-(4-p-nitrophenylazo)anisole, 3-methoxy-(4-p- nitrophenylazo)anisole, 2-N,N-dihexylamino-7-dicyanomethylidenyl-3,4,5,6, 10- pentahydronaphthalene, and/or 3-(N,N-di-n-butylaniline-4-yl)- 1 -dicyanomethylidene-2- cyclohexene.

Preferred are rod shape-like nonlinear optical units. In preferred embodiments, the nonlinear optical unit is a cyano-biphenyle selected from the group comprising 4'-(n-octyloxy)-4- cyanobiphenyl (80CB) and/or 4'-(n-pentyl)-4-cyanobiphenyl (5CB). Preferably, the cyano- biphenyle is 4'-(n-pentyl)-4-cyanobiphenyl. 4'-(n-pentyl)-4-cyanobiphenyl also is denoted 4- pentyl-4'-cyanobiphenyl, 4'-Pentyl-4-biphenylcarbonitrile, or 4-cyano-4-n-pentylbiphenyl. Preferred dicyanostyrene derivatives are selected from the group comprising 2-[4-bis(2- methoxyethyl)amino-benzylidene]-malononitrile (AODCST), 4-piperidinobenzylidene malononitrile (PDCST), and 2-(4-azepan-l-yl-benzylidene) malononitrile (7-DCST).

Advantageously, rod shape-like nonlinear optical units upon formation of a space-charge field can be more easily rearranged in the polymer.

In preferred embodiments, the composite comprises the nonlinear optical unit in an amount in the range of > 5 wt% to < 85 wt%, preferably in the range of > 20 wt% to < 65 wt%, more preferably in the range of > 40.052 wt% to < 40.187 wt%, referring to a total amount of the composite of 100 wt%.

In a preferred embodiment, the composite comprises poly(9-vinylcarbazole), 4'-(n-pentyl)-4- cyanobiphenyl, and C12-4C1 diperylene bisimide at a ratio in the range of 59.802:40.187:0.010 wt% to 59.736:40.142:0.122 wt%, preferably at a ratio of 59.601 :40.052:0.348 wt%.

Advantageously, even low concentrations of C12-4C1 diperylene bisimide (C12-4C1 DiPBI) are remarkably effective, and the amount of photosensitizer therefore can be decreased in the composite.

Preferably, the hole transporter is a polymeric hole transporter. Referring to polymers having a weight in the range of 25000 to 50000 g/mol, especially having a medium weight of 37500 g/mol, the composite preferably comprises C12-4C1 diperylene bisimide (C12-4C1 DiPBI) in an amount in the range of > 0.001 mol% to < 1 mol%, preferably in the range of > 0.002 mol% to < 0.2 mol%, more preferably in the range of > 4.25 mmol% to < 136 mmol%; the hole transporter in an amount in the range of > 0.5 mol% to < 1.5 mol%, preferably in the range of > 0.9 mol% to < 1 mol%, more preferably in the range of > 0.983 mol% to < 0.984 mol%; and the nonlinear optical unit in an amount in the range of > 97.5 mol% to < 99.5 mol%, preferably in the range of > 98.5 mol% to < 99.2 mol%, more preferably in the range of > 98.881 mol% to < 99.016 mol%, referring to a total amount of the composite of 100 mol%. Preferably, the ratio of poly(9-vinylcarbazole), 4'-(n-pentyl)-4-cyanobiphenyl, and C12-4C1 diperylene bisimide in the photorefractive composite is in the range of

0.979:98.974:0.047 mol% to 0.979:99.017:0.004 mol%, preferably at a ratio of

0.983 :98.881 :0.136 mol%.

The photorefractive composite further can comprise plasticizers and/or sensitizers. Preferred plasticizers are selected from the group comprising N-ethylcarbazole, butyl benzyl phthalate, diphenyl phthalate, diisooctylphthalate, and N-(2-ethylhexyl)-N-(3-methylphenyl)-aniline. Preferred sensitizers are selected from the group comprising fullerenes and/or derivatives of 2,4,7-trinitro-9-fluorenone. Preferred fullerenes are selected from the group comprising fullerenes in the form of (C60-Ih)[5,6]fullerene and C7o-D_5h(6)-fullerene or [6,6]-phenyl-C61- butyric acid methyl ester. A preferred derivative of 2,4,7-trinitro-9-fluorenone is (2,4,7- trinitro-9-fluorenylidene)malononitrile.

The photorefractive composite further can comprise Tris(8-hydroxyquinolinato)aluminium, polystyrene, nano particles, quantum dots, and/or carbon nanotubes. Tris(8- hydroxyquinolinato)aluminium for example is usable as dopant for condition traps.

Polystyrene is usable for stabilizing the composite.

Another aspect of the invention refers to the use of C12-4C1 diperylene bisimide as a sensitizer in a photorefractive composite comprising a hole transporter, and a nonlinear optical unit.

Advantageously, it was found that C12-4C1 diperylene bisimide (C12-4C1 DiPBI) is usable as a broadband photosensitizer with outstanding optical and physical properties. Especially the broad absorption spectrum of C12-4C1 diperylene bisimide (C12-4C1 DiPBI) can provide a basic prerequisite for excellent processability of photorefractive materials in the whole range of visible light.

The photorefractive composite is usable for applications such as high density holographic data storage, holographic imaging of living tissue, and 3D real time imaging displays. The photorefractive composite is especially advantageous as a high-performance material with index of refraction modulation approach in data storage, medical diagnostics, and for the entertainment industry. Further, the photorefractive composite is usable for realizing nanophotonic devices for optical information processing.

Another aspect of the invention refers to the use a photorefractive composite according to the invention in holography techniques, especially in holographic data storage devices and holographic displays. The photorefractive composite is usable in the field of holography, especially holographic data storage, and medical diagnostics. The photorefractive composite even is usable for realistic holographic projection of people and non-living objects in virtual reality. Especially the photorefractive composite is usable for holographic displays, for example for holographic imaging of living tissue. Fast writing and deleting times of the photorefractive composite enable a use in 3-D colour displays.

Further, the photorefractive composite is usable in dynamic holographic lattices, for example solar collimators. Moreover, the photorefractive composite is usable in implementations such as updateable 3-D displays, or data storage devices. Further, the photorefractive composite is usable for example as a switch or beam splitter in optical computers.

Further, the photorefractive composite is usable in optics and laser optics. Especially the photorefractive composite is usable as an optical switch, a beam splitter, or intensity control. The photorefractive composite also is usable as a wave plate.

Further, the photorefractive composite is usable in photo-voltaic cells, organic field effect transistors (OFETs), and organic light emitting diodes (OLEDs). Especially, the

photorefractive composite is usable in optical fiber communication. Further, the

photorefractive composite is usable instead of anorganic crystals in microscopy and measuring techniques.

Another aspect of the invention refers to a holographic data storage device or a holographic display comprising a photorefractive composite according to the invention.

Another aspect of the invention refers to an optical switch, especially a beam splitter, or an intensity control, comprising a photorefractive composite according to the invention. Further, another aspect of the invention refers to an organic semiconductor, especially a photo-voltaic cell, an organic field effect transistor, or an organic light emitting diode, comprising a photorefractive composite according to the invention.

Unless otherwise defined, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The examples which follow serve to illustrate the invention in more detail but do not constitute a limitation thereof. While at least one exemplary embodiment is presented, it should be appreciated that a vast number of variations exist.

In the figures show:

Figure 1 Absorption spectra of an unsensitized photorefractive composite and

photorefractive composites comprising a molecular amount of 136 mmol% of sensitizers PCBM or C12-4C1 DiPBI.

Figure 2 Photoconductivity I_Ph of the photorefractive composites comprising a molecular amount of 136 mmol% of the sensitizers PCBM or C12-4C1 DiPBI as a function of the electric field.

Figure 3 Double refraction of the photorefractive composites comprising a molecular amount of 136 mmol% of sensitizers PCBM or 34 mmol% or 136 mmol% of C12-4C1

DiPBI as a function of the electrical field.

Figure 4 Gain coefficient Γ of the photorefractive composites comprising a molecular

amount of 136 mmol% of sensitizers PCBM or 34 mmol% or 136 mmol% of CI 2-

4C1 DiPBI as a function of the electrical field. Figure 5 Kinetics of the energy transfer between two laser beams at 70 V/μιη using photorefractive composites comprising a molecular amount of 136 mmol% of sensitizers PCBM or 34 mmol% or 136 mmol% of C12-4C1 DiPBI.

Figure 6 Writing rate depicted as reciprocal time constant as a function of the concentration of C12-4C1 DiPBI in the photorefractive composites.

Example 1

Preparation of indium-tin oxide electrodes comprising a photorefractive composite 1.1. Structuring of the indium-tin oxide electrode

As the photorefractive composite is an amorphous solid, high voltage has to be applied to the sample to induce a symmetry axis. A one side indium-tin oxide (ITO) coated float glass (Praezisions Glas and Optik GmbH) was used as transparent electrodes. This type of electrodes is used for the photoconductivity measurements. The size of the glass was 30 mm x 30 mm x 0.7 mm and the 100 nm thick ITO layer coverd a 26 nm Si0₂ passivation layer. The electrode was structured by etching with HC1. The electrode was structured to provide a well defined circular area of 38.5 mm² and a diameter of 7 mm, which was structured to match the diameter of the power sensor. The electrode could be illuminated homogeneously with an expanded beam.

1.2. Composite preparation

C12-4C1 diperylene bisimide was synthesized according to the reaction conditions as reported by H. Qian, Z. Wang, W. Yue, D. Zhu, J. Am. Chem. Soc. 2007, 129, 10664. The polymere poly-9-vinylcarbazole (PVK), the liquid crystal 4'-(n-pentyl)-4-cyanobiphenyl (5CB) and the sensitizer [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) used for comparison were bought from Sigma Aldrich. The polymere poly-9-vinylcarbazole (PVK) had a weight in the range of 25000 to 50000 g/mol. For the calculation of the concentrations in mol% a medium weight of 37500 g/mol was used. 30 mg of poly-9-vinylcarbazole (PVK), 20 μΐ of 4'-(n-pentyl)-4-cyanobiphenyl (5CB) and the desired amount of sensitizer were solved in 200 μΐ chloroform under ultra sonification in a bottle to a viscous substance.

The concentration of the sensitizer to yield 136 mmol% of [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) was added in form of 10 μΐ of a 10 mg/ml solution in chloroform. The concentration of the sensitizer to yield 136, 102, 68, 54.4, 40.8, 34, 17, 8.5, and 4.25 mmol% of C12-4C1 DiPBI was added in form of 100 μΐ, 75 μΐ, 50 μΐ, 40 μΐ, 30 μΐ, 25 μΐ, 12.5 μΐ, 6.25 μΐ, and 3.125 μΐ of a 1.75 mg/ml solution of C12-4C1 DiPBI in chloroform.

1.3. Melt-pressing

Melt-pressing is a common procedure for the preparation of photorefractive samples. The prepared composites comprising C12-4C1 DiPBI or PCBM were pipetted onto an ITO electrode bubble-free and dried at ambient air for 20 minutes. As the boiling point of chloroform is 62°C it evaporates even at room temperature very quickly. Afterwards, the almost dried composite was annealed at 55°C for 1 hour in the oven to remove the rest of the solvent. These steps were repeated four times until the desired thickness was achieved. Then the composite was melted at 90°C and covered with the second glass plate to be pressed by its weight to the thickness of the spacer. The spacer was a transparent foil with a defined thickness of 50 μπι and the pressing took one hour. Finally the sample was cooled down to room temperature slowly.

Example 2

Measurement of absorption spectra

For the measurement of absorption spectra indium-tin oxide electrodes containing

photorefractive composites having a molecular amount of 136 mmol% of C12-4C1 diperylene bisimide or the sensitizers for comparison [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) or prepared according to example 1 were used.

The photorefractive composite with C12-4C1 diperylene bisimide comprised poly(9- vinylcarbazole) (PVK), 4-cyano-4-n-pentylbiphenyl (5CB), and C12-4C1 DiPBI at a ratio of 0.983 :98.881 :0.136 mol%. The photorefractive composite for comparison comprised poly(9- vinylcarbazole) (PVK), 4-cyano-4-n-pentylbiphenyl (5CB), and PCBM also at a ratio of 0.983 :98.881 :0.136 mol%. The absorption spectra of the photorefractive composites were measured by the spectrometer JascoV-530 UV/VIS. Using the relation A = d x a and assuming the glass of the samples nonabsorbing, the absorption coefficient of 9 cm^-1 for the PCBM-sample was calculated from two samples, which had a thickness of d = 50 μπι and d = 100 μπι. The results for the composite preparations comprising C12-4C1 DiPBI, which are presented in Figure 1, were achieved by measuring the intensity of the transmitted light behind the sample with a photodiode and relating these values to the sample containing PCBM.

As can be seem from Figure 1, the absorption of the photorefractive composite containing C12-4C1 DiPBI covered the whole range of visible light showing maxima in the blue, green, and red region. This shows that C12-4C1 DiPBI as a sensitizer is particularly advantageous compared with the hardly absorbing PCBM composite.

The photorefractive composite comprising C12-4C1 DiPBI had a dielectric constant of 2.9 and a glass transition temperature in the range of room temperature. The photorefractive composite remained free of crystallization for at least six months and was able to endure electrical fields of 80 ν/μπι.

Example 3 Photoconductivity measurement

Indium-tin oxide electrodes containing photorefractive composites having a molecular amount of 136 mmol% of the sensitizers [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) or C12-4C1 diperylene bisimide prepared according to example 1 were used for the photoconductivity measurement.

To measure the photoconductivity the defined electrode area of 38.5 mm² of the indium-tin oxide electrodes was illuminated homogeneously with an expanded Gaussian beam. The setup for the conductivity measurement was the following: a frequency doubled cw Nd: YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al₅0i2) laser beam (Compass 315m

100, Coherent) of a wave length of 532 nm was varied in its intensity using a half-wave plate and a polarizing beam splitter. The light, that passed the opened shutter, was expanded by a concave lens which provided the homogeneous illumination of the sample. The intensity used for the measurement was 16 mW/cm² High voltage was applied to the sample by a high voltage source (Heinzinger LNC 10000-5 neg) and controlled via a NI Box (USB-6009). A picoammeter (Keithley 6485 Picoammeter) was incorporated into the electric circuit including the indium-tin oxide electrodes comprising the photorefractive composite and the high voltage source to measure the dark current and the photocurrent. The photocurrent I_Ph arises from the subtraction of the dark current I_d from the total current I_tot.

The procedure of the measurement was as follows: to provide enough time for electric relaxation processes in the circuit, a stepwise application of the electric field of 40 ν/μπι to the sample within 2 minutes was applied. The measurement was started at a time defined as t = 0 s. The shutter was opened at t = 10 s and closed at t = 120 s. The measurement was stopped at t = 150 s. These steps were repeated increasing the electric field from 40 ν/μπι in 2 V/μπι steps to 70 ν/μπι. After the application of every next voltage a pause of 30 seconds was kept for the dark current to reach a constant value. The time resolution was 60 ms. The current resolution of the picoammeter was 10 fA. The photoconductivity I_Ph of the photorefractive composites comprising a molecular amount of 136 mmol% of the sensitizers PCBM or C12-4C1 DiPBI as a function of the electric field is shown in Figure 2. As can be taken from Figure 2, the composite comprising C12-4C1 DiPBI showed an absorption at 532 nm above the composite comprising PCBM, and therefore generated a higher photocurrent. This means that to the composite comprising C12-4C1 DiPBI less voltage needs to be applied to yield a comparable photocurrent. For example if 50 V at a thickness of 50 μπι corresponding to ΐν/μπι are applied, the composite comprising C12-4C1 DiPBI yields a photocurrent of 4.2 nA, while the composite comprising PCBM yields a photocurrent of 0.1 nA.

Example 4

Transmission ellipsometry

In order to investigate the contribution of the orientation of the liquid crystal to the photorefractive properties, ellipsometric measurement was performed.

Indium-tin oxide electrodes containing photorefractive composites having a molecular amount of 136 mmol% of [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) or 34 mmol% or 136 mmol% of C12-4C1 diperylene bisimide prepared according to example 1 were used.

In transmission ellipsometry measurement, the electrodes were placed between two +45° and -45° polarizers and were rotated by 45°. During milliseconds a high electric field strength between 40 ν/μπι and 70 ν/μπι, corresponding to a voltage between 2 kV and 3.5 kV at a thickness of the sample 50 μπι, was applied and the liquid crystals orientated parallel to the electric field and provided a birefringent characteristic to the composite. The 45° polarized laser beam was rotated by the composite and was detected by a photodiode. The setup and the physical processes of the transmission ellipsometry measurement was the following: A half-wave plate and a +45° polarizer produced a laser beam with the direction of the electric field vector consisting of a component parallel (p) and perpendicular (s) to the plane of light incidence. This light entered the sample electrode, which was 45° rotated with respect to the incoming beam. As the refractive index of the sample was assumed to be 1.7, the angle inside the composite, calculated using Snell's law, was 25°. The voltage that was applied to the sample by a high voltage source (TREK 609E-6) was controlled by an arbitrary waveform generator (hp 33120A) and broke the centrosymmetry. A photodiode after a -45° polarizer detected the intensity of the light that passed the -45° polarizer. The signal was amplified by a current amplifier (FEMTO DFIPCA-100), which converted the photocurrent of the diode into voltage, which was measured with a NI Box (USB-6009). The intensity I was thus proportional to the achieved voltage U.

The ellipsometric measurement was done at a wavelength of 532 nm and the laser beam power was kept at 100 μ\Υ. The procedure of the measurement was as follows: I₀ was measured with the -45° polarizer adjusted parallel to the first +45° polarizer and without any electric field applied to the sample. The second polarizer was adjusted perpendicular to the first one and the measurement was started at a time defined as t = 0 s. An electric field E = 40 V/μπι was applied within a few milliseconds to the sample at t = 10 s. The electric field was switched off within a few milliseconds at t = 40 s and the measurement was stopped at t = 60 s. These steps were repeated increasing the electric field from 40 ν/μπι in 2 ν/μπι steps to 70 ν/μπι. The time and voltage resolution were 1 ms and 0.5 mV, respectively. The steady state value was used to calculate the electro-optic response The birefringence of the photorefractive composites comprising a molecular amount of 136 mmol% of PCBM or of 34 mmol% or 136 mmol% of C12-4C1 DiPBI as a function of the electrical field is shown in Figure 3. As can be seem from Figure 3, the composite comprising only 34 mmol% of C12-4C1 DiPBI still showed an absorption still twice as high as that of the composite comprising 136 mmol% of PCBM.

Example 5

Measurement of photorefractive performance by two-beam coupling

Two-beam coupling is used to probe the steady-state properties and the kinetics of photorefractive materials. The process referred to as two-beam coupling is the interaction of two coherent laser beams with the simultaneously induced index grating. The energy exchange between the beams depends not only on the amplitude of the space-charge field but is strongly affected by the value of the phase shift between the index modulation and the interference pattern. Thus if this phase shift is 0, which is the case for, e.g., thick absorption gratings, no energy transfer occurs. That is how a confusion of other physical mechanisms with the photorefractive effect can be avoided. The energy exchange is characterized by the gain coefficient Γ that is given in units of cm^-1.

The setup of the two-beam coupling was the following: A laser beam was divided into two beams by a polarizing beam splitter, which could be varied in the intensity by a first half wave plate placed before the polarizing beam splitter. The wavelength of the laser was 532 nm and the intensity of each beam was 8 mW/cm² measured in front of the sample. As the

polarization of the two beams was different (beam 1 was p-polarized while beam 2 was s- polarized), a second half wave plate was inserted into beam 2 to achieve a 90° rotation of the polarization. Hence, both beams were p-polarized. The beams 1 and 2 intersected inside the sample, which was poled with a high voltage source (Heizinger LNC 10000-5 neg). It is important to apply a high negative voltage to the first electrode on purpose to avoid beam fanning. In this approach the energy transfer was from beam 2 to beam 1. The sample was tilted with respect to beam 1 by a = 40° and to beam 2 by α + β = 60°. This geometry is advantageous to provide a high projection value of the grating vector on E. The grating spacing was already calculated yielding ca. 2 μιη.

The energy transfer between the two beams was observed by the detection of the light intensity via two photodiodes. A fast photodiode detected the intensity of beam 1 and a following current amplifier (FEMTO DHPCA-100) converted the photocurrent into voltage U that was received by a NI Box (USB-6009). The intensity of beam 2 was monitored with a light power sensor (Coherent Fieldmaster) after a shutter which was placed after the second half wave plate was opened.

The procedure of the measurement was as follows: The electric field was slowly increased to E = 40 ν/μπι. This field remained applied for 30 seconds before the measurement of the light intensity started and the shutter was opened. This time is necessary for the liquid crystal to align parallel to the electric field as was concluded from the ellipsometry measurements. That is why the kinetics of the energy transfer can be attributed to photoconductive processes. The measurement of I of beam 1 was started at a time defined as t = 0 s keeping the shutter closed. The shutter was opened at t = 5 s and closed after the steady-state was reached at t = 85 s for the PCBM-samples, and at t = 45 s for the C12-4C1 DiPBI-samples. The measurement was stopped at t = 100 s for PCBM-sample, and at t = 60 s for the C12-4C1 DiPBI-samples. These steps were repeated increasing the electric field from 40 ν/μπι in 2 ν/μπι steps to 70 ν/μπι. The time and voltage resolution were 1 ms and 0.5 mV, respectively.

The gain coefficient Γ of photorefractive composites comprising a molecular amount of 136 mmol% of sensitizers PCBM or 34 mmol% or 136 mmol% of C12-4C1 DiPBI as a function of the electrical field is shown in Figure 4. As can be seem from Figure 4, the photorefractive composites with 34 mmol% of C12-4C1 DiPBI shows a gain coefficient Γ more than twice that high compared to the gain coefficient Γ of the photorefractive composites with 136 mmol% of PCBM. The photorefractive composites with 136 mmol% of C12-4C1 DiPBI shows a lesser gain coefficient Γ, but reached it faster.

Figure 5 demonstrates the writing rate of the lattice. In Figure 5 is depicted the kinetics of the energy transfer as the gain γ as a function of time between the two beams at 70 ν/μπι of the photorefractive composites comprising a molecular amount of 136 mmol% of sensitizers PCBM or 34 mmol% or 136 mmol% of C12-4C1 DiPBI. Five seconds after the start of the measurement the shutter for beam 2 was opened and the writing of the lattice started. This is effected by an energy transfer between the beams 1 and 2, and Figure 5 shows the energy yield of the beam. It is assumed that the fast writing rates of the composites containing C12- 4C1 DiPBI is based on the higher absorption and resulting photoconductivity.

Using a double exponential fit to the writing graph, two time constants can be determined. A fast constant ti is influenced by the process of charge generation and transport, while the orientation of the liquid crystals contributes to the second slower constant t₂.

Figure 6 demonstrates the writing rate and depicts the reciprocal time constant ti as a function of the concentration of C12-4C1 DiPBI in the photorefractive composites. Photorefractive composites comprising concentrations of 4.25, 8.5, 17, 34, 40.8, 54.4, 68, 102, and 136 mmol% of C12-4C1 DiPBI were used.

For the PCBM composite the reciprocal of the fast time constant was 0.14 s^"1. The slowest C12-4C1 DiPBI photorefractive composite was 10 times faster, while the fastest C12-4C1 DiPBI photorefractive composite was 100 times faster.

For the PCBM composite comprising 136 mmol% of PCBM the reciprocal of the second time constant was 80 s. Even the slowest C12-4C1 DiPBI photorefractive composite was nearly twice that fast compared to the PCBM composite. This shows that the C12-4C1 DiPBI photorefractive composites were much faster than the PCBM composite.

These experiments show that the absorption of the photorefractive composites containing C12-4C1 DiPBI covered the whole range of visible light and therefore grant the ability of photoconductivity to the composites. The rod-shape like molecules are able to orientate in electrical field. These abilities can effect a strong and fast photorefractive effect of the composites even at low concentrations of C12-4C1 DiPBI.

Claims

1. A photorefractive composite comprising a hole transporter, a nonlinear optical unit, and a sensitizer, wherein the sensitizer is C12-4C1 diperylene bisimide according to the formula (1) as indicated below:

2. The composite according to claim 1, wherein the composite comprises C12-4C1 diperylene bisimide in an amount in the range of > 0.001 wt% to < 1 wt%, preferably in the range of > 0.002 wt% to < 0.5 wt%, more preferably in the range of > 0.01 wt% to < 0.348 wt%, even more preferably in the range of > 0.01 wt% to < 0.122 wt%, referring to a total amount of the composite of 100 wt%.

3. The composite according to claim 1 or 2, wherein the hole transporter is selected from the group comprising polymeric carbazole derivatives, poly(p-phenylene vinylene) derivatives, N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine, polymeric derivatives of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine, polythiophenes, p-type conducting rylene dyes, tri-p-tolylamine, pentacene and/or anthracene.

4. The composite according to any of the preceding claims, wherein the hole transporter is a polymeric carbazole derivative selected from the group comprising poly(N- vinylcarbazole), poly[methyl(3-carbazol-9-ylpropyl)siloxane] and/or poly(p- phenyleneterephthalate) comprising pendent carbazole groups, preferably poly(9- vinylcarbazole). 5. The composite according to any of the preceding claims, wherein the composite comprises the hole transporter in an amount in the range of > 1 wt% to < 84 wt%, preferably in the range of > 10 wt% to < 70 wt%, more preferably in the range of > 59.601 wt% to < 59.807 wt%, referring to a total amount of the composite of 100 wt%. 6. The composite according to any of the preceding claims, wherein the nonlinear optical unit is selected from the group comprising cyano-biphenyles, dicyanostyrene derivatives, 1- alkyl-5-[2-(5-dialkylaminothienyl)methylene]-4-alkyl-[2,6-dioxo-l,2,5,6-tetrahydropyridine]- 3-carbonitrile, 2-dicyanomethylen-3-cyano-5,5-dimethyl-4-(4'-dihexylaminophenyl)-2,5- dihydrofuran, 4-N,N-diethylamino-P-nitrostyrene, 3-fluoro-4-(N,N-diethylamino)-P - nitrostyrene, 2,5-dimethyl-(4-p-nitrophenylazo)anisole, 3-methoxy-(4-p- nitrophenylazo)anisole, 2-N,N-dihexylamino-7-dicyanomethylidenyl-3,4,

5,

6, 10- pentahydronaphthalene and/or 3-(N,N-di-n-butylaniline-4-yl)-l-dicyanomethylidene-2- cyclohexene.

7. The composite according to any of the preceding claims, wherein the nonlinear optical unit is a cyano-biphenyle selected from the group comprising 4'-(n-octyloxy)-4- cyanobiphenyl and/or 4'-(n-pentyl)-4-cyanobiphenyl, preferably 4'-(n-pentyl)-4- cyanobiphenyl.

8. The composite according to any of the preceding claims, wherein the composite comprises the nonlinear optical unit in an amount in the range of > 5 wt% to < 85 wt%, preferably in the range of > 20 wt% to < 65 wt%, more preferably in the range of > 40.052 wt% to < 40.187 wt%, referring to a total amount of the composite of 100 wt%.

9. The composite according to any of the preceding claims, wherein the composite comprises poly(9-vinylcarbazole), 4'-(n-pentyl)-4-cyanobiphenyl, and C12-4C1 diperylene bisimide at a ratio in the range of 59.802:40.187:0.010 wt% to 59.736:40.142:0.122 wt%, preferably at a ratio of 59.601 :40.052:0.348 wt%.

10. Use of C12-4C1 diperylene bisimide as a sensitizer in a photorefractive composite comprising a hole transporter, and a nonlinear optical unit.

11. Use of a photorefractive composite according to any of the preceding claims in holography techniques, especially in holographic data storage devices and holographic displays, in optics and laser optics especially as an optical switch, a beam splitter, or intensity control, or in photo-voltaic cells, organic field effect transistors, and organic light emitting diodes.

12. A holographic data storage device or a holographic display comprising a

photorefractive composite according to any of claims 1 to 9.

13. An optical switch, especially a beam splitter, or an intensity control, comprising a photorefractive composite according to any of claims 1 to 9.

14. An organic semiconductor, especially a photo-voltaic cell, an organic field effect transistor, or an organic light emitting diode, comprising a photorefractive composite according to any of claims 1 to 9.