EP2329539A1

EP2329539A1 - Use of dibenzotetraphenylperiflanthene in organic solar cells

Info

Publication number: EP2329539A1
Application number: EP09783163A
Authority: EP
Inventors: Martin KÖNEMANN; Jae Hyung Hwang; Gabriele Mattern; Christian DÖRR; Peter Erk
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2008-09-19
Filing date: 2009-09-18
Publication date: 2011-06-08
Also published as: CN102160207A; US20110168248A1; KR20110074522A; JP2012503320A; WO2010031833A1

Abstract

The present invention relates to the use of dibenzotetraphenylperiflanthene of formula (I) as an electron donor material in an organic solar cell comprising photoactive donor-acceptor transitions in the form of a bulk heterojunction.

Description

Use of dibenzotetraphenylperiflanthen in organic solar cells

description

STATE OF THE ART

The present invention relates to the use of dibenzoetraphenylperiflanthene as electron donor material in an organic solar cell.

The synthesis of dibenzotetraphenylperiflanthene is described by J.D. Debad, J.C. Morris, V. Lynch, P. Magnus, and A.J. Bard in J. Am. Chem. Soc. 1996, 1 18, pages 2374-2379.

Due to dwindling fossil raw materials and the CO2 produced by the combustion of these raw materials and acting as a greenhouse gas, the direct generation of energy from sunlight plays an increasing role. Photovoltaic means the direct conversion of radiant energy, mainly solar energy, into electrical energy. For solar cells, as with any voltage source, the voltage at idle, ie when the current is zero, is highest. The more current that is drawn, the lower the voltage and reaches the value 0 in the short circuit. The solar cell does not emit any power either in idle or short circuit. Between idle and short circuit exists a point on the characteristic curve (current as a function of voltage) of the solar cell, in which the output power is maximum (mpp, maximum power point). The characterization of a solar cell is generally carried out with the aid of three parameters: the open-circuit voltage Voc, the short-circuit current Isc and the filling factor FF (FF = (V _mp p lm _PP / Voc Isc).) Also of interest are the internal and external quantum efficiency internal quantum efficiency is the ratio of the number of charge carriers extracted at the contacts to the number of absorbed photons, the external quantum efficiency is the ratio of the number of charge carriers extracted to the contacts to the number of injected photons maximum photovoltaically generated power to the corresponding incident light power (Pucht):

η = (Vmpp I mpp / Pücht) = FF VoC I SC / Pücht

By the demonstration of the first organic solar cell with a percentage efficiency by Tang et al. In 1986 (CW Tang et al., Appl. Phys. Lett. 48, 183 (1986)), intensive development was initiated. Organic solar cells consist of a sequence of thin layers, which typically have a thickness between 1 nm to 1 μm, and which consist at least partly of organic materials, which are preferably vapor-deposited in vacuo or from a solution. to be brought. The electrical contacting is usually carried out by metal layers and / or transparent conductive oxides (TCOs).

In contrast to inorganic solar cells, organic solar cells do not directly generate free charge carriers by the light, but excitons are initially formed, ie. electrically neutral excitation states in the form of electron-hole pairs. These excitons can only be separated by very high electric fields or at suitable interfaces. In organic solar cells sufficiently high fields are not available, so that all previous concepts for organic solar cells based on the exciton separation at photoactive interfaces

(organic donor-acceptor interfaces or inorganic semiconductor interfaces). This requires that excitons generated in the bulk of the organic material can diffuse to this photoactive interface.

The diffusion of excitons to the active interface thus plays a critical role in organic solar cells. In order to make a contribution to the photocurrent, the exciton diffusion length in a good organic solar cell must be at least of the order of magnitude of the typical penetration depth of the light, so that the greater part of the light can be used. Structurally and with regard to chemical purity, perfect organic crystals or thin films definitely fulfill this criterion. For large scale applications, however, the use of high purity, monocrystalline organic materials is not possible and the production of multiple layers with sufficient structural perfection is still very difficult.

There has been no lack of attempts to improve the effectiveness of organic solar cells. Some approaches for realizing or improving the properties of organic solar cells are listed below:

One of the contact metals used has a large and the other a small work function, so that with the organic layer, a Schottky barrier is formed.

A layer contains two or more types of organic pigments that have different spectral characteristics.

Various dopings are used i.a. to improve the transport properties.

Arrangement of a plurality of individual solar cells, so that a so-called tandem cell is formed, which can be further improved, for example by using pin structures with doped transport layers large band gap. Alternatively, instead of increasing the exciton diffusion length, one can also reduce the mean distance to the nearest interface. For this purpose, mixed layers of donors and acceptors can be used, forming an interpenetrating network in which internal donor-acceptor heterojunctions are possible. Organic solar cells with photoactive donor-acceptor transitions in the form of a bulk heterojunction are z. B of G. Yu, J. Gao, JC Hummelen, F. Wudl, AJ Heeger in Science, Vol. 270, December 1995, pages 1789-1791.

The advantage of such a mixed layer is that the generated excitons only have to travel a very short distance until they reach a domain boundary where they are separated. Since the materials in the mixed layer are in contact with each other everywhere, it is crucial in this concept that the separate charges have a long service life on the respective material and that closed percolation paths for each type of charge carrier are present from each location to the respective contact. Efficiencies of up to 2.5% have been achieved with this approach (S.E. Shaheen et al., Appl. Phys. Lett, Vol. 78, No. 6, pp. 841-843).

Despite the advantages described above, one critical point of bulk heterojunction (BHJ) is to find suitable materials and fabrication processes that result in mixed layers that have closed transport paths for both electrons and holes to their respective contacts. In addition, since the individual materials each fill only a part of the mixed layer, in many cases the transport properties for the charge carriers deteriorate markedly in comparison to the pure layers. In addition, there are classes of substances, such as certain oligothiophenes, which are surprisingly not suitable for use in BHJ cells. A possible cause could be that these molecules mix too well with the second semiconductor material used for producing the mixed layer and therefore do not form percolation paths. A verifiable explanation is not yet available. It is therefore virtually unpredictable whether a particular electron or hole conductor material, and certainly not whether it is advantageous for use in an organic solar cell with photoactive donor-acceptor transitions in the form of a bulk heterojunction.

JP 2008-135540 describes the use of perylene derivatives of the general formula

wherein R ¹ and R ^{2 are} condensed rings which may be substituted by alkyl, alkenyl, aryl, aralkyl or heterocyclyl and AR ¹ - AR ⁸ may be alkyl, alkenyl, aryl, aralkyl or heterocyclyl, as electron donor material for the production of organic solar cells. Dibenzotetraphenylperiflanthen is also mentioned here. However, this document does not teach the use of this compound to prepare an organic solar cell with photoactive donor-acceptor transitions in the form of a bulk heterojunction.

The object of the invention is to provide an organic solar cell in which the efficiency of energy conversion is improved.

Surprisingly, it has now been found that dibenzotetraphenylperiflanthene are particularly advantageously suitable as electron donor material for the production of organic solar cells with photoactive donor-acceptor transitions in the form of a bulk heterojunction.

SUMMARY OF THE INVENTION

A first subject of the invention is therefore the use of Dibenzotetraphenylperiflanthen (DBP) of the formula

as electron donor material in an organic solar cell with photoactive donor-acceptor transitions in the form of a bulk heterojunction. Another object of the invention is an organic solar cell comprising at least one photoactive donor-acceptor transition in the form of a bulk heterojunction, wherein Dibenzotetraphenylperiflanthen is used as electron donor material.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a solar cell with a normal structure which is suitable for the use of dibenzoetraphenylperiflanthene.

FIG. 2 shows a solar cell with inverse structure.

FIG. 3 shows the structure of a tandem cell.

Figure 4 shows a bulk heterojunction with large donor-acceptor interface and uninterrupted transport paths to the electrodes.

DESCRIPTION OF THE INVENTION

Organic solar cells are generally layered and typically include at least the following layers: anode, photoactive layer and cathode. It is an essential feature of the invention that the organic solar cell has a mixed layer containing dibenzotetraphenylperiflanthene as electron donor material and at least one electron acceptor material. According to the invention, the mixed layer has donor-acceptor transitions in the form of a bulk heterojunction.

Dibenzotetraphenylperiflanthene is prepared by standard methods known to those skilled in the art (e.g., J.D. Debad, J.C. Morris, V. Lynch, P. Magnus and A.J. Bard J. Am. Chem. Soc., 1996, 118, pp. 2374-2379).

Prior to use in an organic solar cell, the dibenzotetraphenylperiflanthene may be subjected to purification. The purification can be carried out by customary methods known to those skilled in the art, such as separation on suitable stationary phases, sublimation, extraction, distillation, recrystallization or a combination of at least two of these measures. Each cleaning can be configured in one or more stages.

In a specific embodiment, the purification comprises a column chromatographic method. For this purpose, the starting material present in a solvent or solvent mixture can be subjected to separation or filtration on silica gel. Finally, the solvent is removed, for example by evaporation under reduced pressure. Suitable solvents are aromatics such as benzene, toluene, xylene, mesitylene, chlorobenzene or dichlorobenzene, hydrocarbons and hydrocarbon mixtures such as pentane, hexane, ligroin and petroleum ether, halogenated hydrocarbons such as chloroform or dichloromethane, and mixtures of the solvents mentioned. For chromatography, it is also possible to use a gradient of at least two different solvents, for example a toluene / petroleum ether gradient.

In a further specific embodiment, the cleaning comprises a sublimation. This may preferably be a fractional sublimation. For fractionated sublimation, a temperature gradient can be used in the sublimation and / or deposition of the dibenzotrapraphenylperiflanthene. Furthermore, the cleaning can be done by sublimation using a carrier gas stream. Suitable carrier gases are inert gases, e.g. As nitrogen, argon or helium. The loaded with the compound gas stream can then be passed into a separation chamber. Suitable separation chambers may have several separation zones that can be operated at different temperatures. Preferred is e.g. a so-called three-zone inflating device. Another method and apparatus for fractional sublimation is described in US 4,036,594.

An organic solar cell according to the invention usually comprises a substrate. The substrate is often coated with a transparent, conductive layer as an electrode.

Suitable substrates for organic solar cells are z. Oxidic materials (such as glass, ceramics, SiO 2, quartz, etc.), polymers (eg, polyethylene terephthalate, polyolefins such as polyethylene and polypropylene, polyesters, fluoropolymers, polyamides, polyurethanes, polyalkyl (meth) acrylates, polystyrene, polyvinyl chloride and mixtures and composites thereof) and combinations thereof.

As electrodes (cathode, anode) are in principle metals (preferably the

Groups 2, 8, 9, 10, 11 or 13 of the Periodic Table, e.g. Pt, Au, Ag, Cu, Al, In, Mg, Ca), semiconductors (eg, doped Si, doped Ge, indium tin oxide (ITO), fluorinated tin oxide (FTO), gallium indium). Tin oxide (GITO), zinc indium tin oxide (ZITO), etc.), metal alloys (eg based on Pt, Au, Ag, Cu, etc., especially Mg / Ag alloys), semiconductor alloys , Etc.

Preferably, the material used for the electrode facing the light (the anode in the case of a normal structure, the cathode in the case of the inverse structure) is a material which is at least partially transparent to the incident light. These include, in particular, glass and transparent polymers, such as polyethylene terephthalate. The electrical contact is usually done by metal layers and / or transparent conductive oxides (TCOs). This preferably includes ITO, FTO, ZnO, TiO ₂ , Ag, Au, Pt.

The light-facing layer is made to be sufficiently thin to cause only minimal light absorption but thick enough to allow good charge transport of the extracted charge carriers. The thickness of the layer is preferably in a range of 20 to 200 nm.

In a special embodiment, the material used for the electrode remote from the light (the cathode in the case of a normal structure, the anode in the case of an inverse structure) is a material which at least partially reflects the incident light. These include metal films, preferably of Ag, Au, Al, Ca, Mg, In, and mixtures thereof. The thickness of the layer is preferably in a range of 50 to 300 nm.

The photoactive layer comprises dibenzotetraphenylperiflanthene as electron donor material (p-type semiconductor). In a specific embodiment, dibenzotetraphenyl periflanthene is used as the sole electron donor material.

According to the invention, the photoactive layer is configured as a mixed layer and, in addition to DBP, comprises at least one electron acceptor material (n-type semiconductor).

For the combination with DBP (donor), the following semiconductor materials are suitable in principle as acceptors for use in the solar cells according to the invention:

Fullerenes and fullerene derivatives, preferably selected from Cβo, C70, Cs _4, phenyl Cβi-Butyrsäuremethylester ([6O] PCBM), phenyl-C7i-Butyrsäuremethylester ([7I] PCBM), phenyl-C ₈ 4-Butyrsäuremethylester ([84JPCBM) Phenyl ceric butyrylbutyl ester ([6O] PCBB), phenyl C _6- i-butyryl octoyl ester ([6O] PCBO), methyl thiocyanate ([6O] ThCBM) and mixtures thereof. Especially preferred are Cβo, [6O] PCBM and mixtures thereof.

Phthalocyanines, e.g. are suitable as acceptors due to their substituents. These include hexadecachlorophthalocyanines and hexadecafluorophthalocyanines, such as hexadecachloro copper phthalocyanine, hexadecachlorozinc phthalocyanine, metal-free hexadecachlorophthalocyanine, hexadecafluoro copper phthalocyanine, hexadecafluoro zinc phthalocyanine or metal-free hexadecafluorophthalocyanine.

Rylenes, ie in general compounds having a molecular skeleton of naphthalene units linked in the peri-position. Depending on the number of naphthalene units, it may, for. For example, perylenes (m = 2), terylenes (m = 3), quaterrylenes (m = 4) or higher rylenes act. Accordingly, they may be perylenes, terylenes or quaterrylenes of the following formulas

(m = 4)

wherein

the radicals R ⁿ¹ , R ⁿ² , R ⁿ³ and R ⁿ⁴ for n = 1 to 4 independently of one another may stand for hydrogen, halogen or groups other ^than halogen,

Y ^{1 is} O or NR ^a , where R ^{a is} hydrogen or an organyl radical,

Y ^{2 is} O or NR ^b , where R ^{b is} hydrogen or an organyl radical,

Z ¹ , Z ² , Z ³ and Z ^{4 are} O,

where, in the case where Y ^{1 is} NR ^a , one of the radicals Z ¹ and Z ^{2 may also represent} NR ^C , wherein the radicals R ^a and R ^c together represent a bridging group having 2 to 5 atoms between the flanking bonds stand, and

where, in the case where Y ^{2 is} NR ^b , one of the radicals Z ³ and Z ^{4 may also represent} NR ^d , where the radicals R ^b and R ^d together represent a bridging group having 2 to 5 atoms between the flanking bonds stand. Suitable rylenes are z. In WO2007 / 074137, WO2007 / 093643 and WO2007 / 1 16001 (PCT / EP2007 / 053330), to which reference is hereby made.

Also suitable are the following donor semiconductor materials, e.g. in a tandem cell, as described below, can be used in a further subcell instead of DBP:

Phthalocyanines which are non-halogenated or halogenated. These include metal-free or divalent metals or phthalocyanines containing metal atom-containing groups, in particular those of titanyloxy, vanadyloxy, iron, copper, zinc, chloroaluminum etc. Suitable phthalocyanines are in particular copper phthalocyanine, zinc phthalocyanine, chloroaluminum phthalocyanine and metal-free phthalocyanine. In a specific embodiment, a halogenated phthalocyanine is used. These include:

2,6,10,14-tetrafluorophthalocyanines, e.g. Chloroaluminum-2,6,10,14-tetrafluorophthalocyanine, 2,6,10,14-tetrafluoro copper phthalocyanine and 2,6,10,14-

Tetrafluorzinkphthalocyanin; 1, 5,9,13-Tetrafluorphthalocyanine, z. Chloroaluminum-1, 5,9,13-tetrafluorophthalocyanine, 1,5,9,13-tetrafluoro copper phthalocyanines and 1,5,9,13-

Tetrafluorzinkphthalocyanine;

2,3,6,7,10,1,14,15-octafluorophthalocyanine, e.g. Chloroaluminum

2,3,6,7,10,1,1,14,15-octafluorophthalocyanine, 2,3,6,7,10,1,1,14,15-octafluorophoric phthalocyanine and 2,3,6,7,10, 1 1, 14, 15-octafluorozinc phthalocyanine;

Porphyrins, such as. For example, 5,10,15,20-tetra (3-pyridyl) porphyrin (TpyP), or tetrabenzoporphyrins, such as metal-free tetrabenzoporphyrin, Kupferfertetrabenzo- porphyrin or Zinktetrabenzoporphyrin. In particular, preference is given to tetrabenzoporphyrins which, like the compound dibenzotetraphenylpran- flanthene used in accordance with the invention, are processed as soluble precursors from solution and are converted on the substrate by thermolysis into the pigmentary photoactive component.

Acenes, such as anthracene, tetracene, pentacene, which may each be unsubstituted or substituted. Substituted acenes preferably comprise at least one substituent selected from electron-donating substituents (eg, alkyl, alkoxy, ester, carboxylate or thioalkoxy), electron-withdrawing substituents (eg, halogen, nitro, or cyano), and combinations thereof. These include 2,9-dialkylpentacenes and 2,10-dialkylpentacenes, 2,10-dialkoxypentacenes, 1, 4,8,11-tetraalkoxypentanes and rubrene (5,6,11,12-tetraphenylnaphthacene). Suitable substituted Pentacenes are described in US 2003/0100779 and US 6,864,396, which is incorporated herein by reference. A preferred acen is rubrene.

Liquid-crystalline (LC) materials, for example coronene, such as hexabenzocoronene (HBC-PhCl 2), coronene diimides, or triphenylenes, such as 2,3,6,7,10,1-hexahexylthiouriphenylene (HTTβ), 2,3,6 , 7,10,1 1-hexakis (4-n-nonylphenyl) -triphenylene (PTPg) or 2,3,6,7,10,1-hexakis- (undecyloxy) -triphenylene (HATu). Particularly preferred are liquid crystalline materials that are discotic.

Thiophenes, oligothiophenes and substituted derivatives thereof; suitable oligothiophenes are quaterthiophenes, quinquethiophenes, sexithiophenes, α, ω-di (Ci-C8) -alkyloligothiophenes, such as α, ω-Dihexylquaterthiophene, α, ω-Dihexylquinquethiophene and α, ω-Dihexylsexithiophene, poly (alkylthiophene), such as poly (3 -hexylthiophene), bis (dithienothiophenes), anthradithiophenes and dialkylanthra- dithiophenes such as dihexylanthradithiophene, phenylene-thiophene (PT) -oligomers and derivatives thereof, especially α, ω-alkyl-substituted phenylene-thiophene oligomers.

Also suitable are compounds of the type α, α'-bis (2,2-dicyanovinyl) quinquethiophene (DCV5T), (3- (4-octylphenyl) -2,2'-bithiophene) (PTOPT), poly3- (4 ' - (1, 4,7-trioxaoctyl) phenyl) thiophene (PEOPT), (poly (3- (2'-methoxy-5'-octylphenyl) thiophene)) (POMeOPT), poly (3-octylthiophene) (P ₃ OT), poly (pyridopyrazine-vinylene) -polythiophene blends such as EHH-PpyPz, copolymers PTPTB, BBL, F ₈ BT, PFMO, see Brabec C, Adv. Mater., 2996, 18, 2884, (PCPDTBT) Poly [2,6- (4,4-bis (2-ethylhexyl) -4 H -cyclo-penta [2.1b; 3,4b '] -dithiophene) -4,7- (2,1,3 -benzothiadiazol).

Paraphenylenevinylene and paraphenylenevinylene-containing oligomers or polymers, such as. Polyparaphenylenevinylene, MEH-PPV (poly (2-methoxy-5- (2'-ethylhexyloxy) -1,4-phenylenevinylene, MDMO-PPV (poly (2-methoxy-5- (3 ', 7'-dimethyloctyloxy) - 1, 4-phenylene-vinylene)), PPV, CN-PPV (with various alkoxy derivatives).

Phenyleneethynylene / phenylenevinylene hybrid polymers (PPE-PPV).

Polyfluorene and alternating polyfluorene copolymers, such as. For example, with 4,7-dithien-2'-yl-2,1,3-benzothiadiazole, poly (9,9'-dioctylfluorene-co-benzothio-diazole) (F ₈ BT), poly (9,9'-dioctylfluorene-co-bis-N, N '- (4-butyl-phenyl) -bis-N, N'-phenyl-1,4-phenylenediamine (PFB).

Polycarbazoles, d. H. Carbazole containing oligomers and polymers.

Polyanilines, ie aniline-containing oligomers and polymers. Triarylamines, polytriarylamines, polycyclopentadienes, polypyrroles, polyfurans, polysiols, polyphospholes, TPD, CBP, spiro-MeOTAD.

DBP and at least one fullerene or fullerene derivative are particularly preferably used in the organic solar cells according to the invention in the photoactive layer. In a particularly preferred embodiment, the semiconductor mixture used in the photoactive layer consists of DBP and Cβo.

The content of Dibenzotetraphenylperiflanthen in the photoactive layer is preferably 10 to 90 wt .-%, particularly preferably 25 to 75 wt .-%, based on the total weight of the semiconductor material (p- and n-type semiconductor) in the photoactive layer.

The photoactive layer is made to be sufficiently thick to cause maximum light absorption but thin enough to efficiently extract the generated charge carriers. The thickness of the layer is preferably in a range of 5 to 200 nm, more preferably 10 to 80 nm.

In addition to the photoactive layer, the organic solar cell may have one or more further layers.

These include z. B.

Layers with hole transporting properties (HTL),

Electron conductive layer (ETL), exciton (and possibly hole) blocking layers (exciton blocking layers, EBL).

Suitable layers with hole-conducting properties preferably comprise at least one material with a low ionization energy in relation to the vacuum level, ie the layer with hole-conducting properties has a lower ionization energy and a lower electron affinity, based on the vacuum level, than the layer with electron-conducting properties. The materials may be organic or inorganic materials. For use in a layer with hole-conducting properties suitable organic materials are preferably selected from poly (3,4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT-PSS), Ir DPBIC (Tris-N, N ^{'-Diphenylbenzimidazol-2-} yliden-iridium (III)), N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1, 1'-diphenyl-4,4'-diamine (α-NPD), 2,2 ' , 7,7'-tetrakis (N, N-di-p-methoxyphenylamine) -9,9'-spirobifluorene (spiro-MeOTAD), etc., and mixtures thereof. If desired, the organic materials may be doped with a p-type dopant which has a LUMO which is in the same range or lower than the HOMO of the hole-conducting material. Suitable dopants are, for example, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F ₄ TCNQ), WO 3, MoO 3, etc. Suitable inorganic materials for use in a layer with hole-conducting properties preferably selected from WO3, Moθ3, etc.

If present, the thickness of the layers with hole-conducting properties is preferably in a range of 5 to 200 nm, more preferably 10 to 100 nm.

Suitable layers with electron-conducting properties preferably contain at least one material whose LUMO, based on the vacuum level, is higher in energy than the LUMO of the material with hole-conducting properties. The materials may be organic or inorganic materials. Suitable organic materials for use in a layer having electron-conducting properties are preferably selected from the aforementioned fullerenes and fullerene derivatives, 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP), 4,7-diphenyl- 1, 10-phenanthroline (Bphen), 1, 3-bis [2- (2,2-bipyridin-6-yl) -1, 3,4-oxadiazo-5-yl] benzene (BPY-OXD), etc. The If desired, organic materials may be doped with an n-type dopant having a HOMO that is in the same range or lower than the LUMO of the electron-conductive material. Suitable dopants are e.g. CS2CO3, pyronin B (PyB), rhodamine B, cobaltocenes, etc. Suitable inorganic materials for use in a layer having electron-conducting properties are preferably selected from ZnO, etc. Particularly preferably, the layer having electron-conducting properties contains Cβo.

If present, the thickness of the layers with electron-conducting properties is preferably in a range of 5 to 200 nm, particularly preferably 10 to 100 nm.

Suitable excitons and holes blocking layers are for. As described in US 6,451, 415. Suitable materials for Excitonenblockerschichten are z. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), 1,3-bis [2- (2,2 -bipyridin-6-yl) -1, 3,4-oxadiazo-5-yl] benzene (BPY-OXD), polyethylene dioxythiophene (PEDOT), etc. It is preferred to use a material which at the same time is well suited for electron transport , Preferred are BCP, Bphen and BPY-OXD.

If present, the thickness of the layers having exciton blocking properties is preferably in the range of 1 to 50 nm, more preferably 2 to 20 nm. According to the invention, the heterojunction is embodied as a bulk heterojunction or interpenetrated donor-acceptor network (compare, for example, BCJ Brabec, NS Sariciftci, JC Hummelen, Adv. Funct. Mater., 11 (1), 15 (2001). .). The solar cells according to the invention thus obtained surprisingly have advantageous properties compared with solar cells in which the heterojunction is flat (smooth). For the construction of solar cells with flat heterojunctions, for example, CW Tang, Appl. Phys. Lett. 48 (2), 183-185 (1986) or N. Karl, A. Bauer, J. Holzäpfel, J. Marktanner, M. Möbus, F. Stölzle, Mol. Cryst. Liq. Cryst., 252, 243-258 (1994).

In a preferred embodiment, the preparation of the photoactive donor-acceptor transitions in the form of a bulk heterojunction by a vapor deposition process (Physical Vapor Deposition, PVD). Suitable methods are e.g. in US 2005/0227406, to which reference is hereby made. For this purpose, dibenzotetraphenylperiflanthen and at least one electron acceptor material can be subjected to vapor deposition in the sense of cosublimation. PVD processes are performed under high vacuum conditions and include the following steps: evaporation, transport, deposition.

The deposition is preferably carried out at a pressure in the range of about 10 " ⁵ to 10" ⁷ mbar.

The deposition rate is preferably in a range of about 0.01 to 10 nm / sec.

The temperature of the substrate in the deposition is preferably in a range of about -100 to 300 ^° C, more preferably -50 to 250 ^° C. The deposition can be carried out under an inert atmosphere, for. B. under nitrogen, argon or helium.

The production of the other, the solar cell-forming layers can be carried out by conventional methods known in the art. These include vapor deposition in a vacuum or inert gas atmosphere, laser ablation or solution or dispersion processing methods such as spin coating, doctoring, casting, spraying, dip coating or printing (eg inkjet, flexo, offset, gravure, gravure, nanoimprint). The production of the entire solar cell is preferably carried out by a gas phase deposition process.

The photoactive layer (mixed layer) can be subjected to a thermal treatment directly after its production or after the production of further layers forming the solar cell. Such a tempering can often improve the morphology of the photoactive layer. The temperature is preferably in a range of about 60 ^° C. to 300 ^° C. The treatment time is preferably in a range of 1 minute to 3 hours. In addition or as an alternative to a thermal After mixing, the photoactive layer (mixed layer) can be subjected to a treatment with a solvent-containing gas directly after it has been prepared or after the preparation of further layers forming the solar cell. In a suitable embodiment, saturated solvent vapors are used in air at ambient temperature. Suitable solvents are toluene, xylene, chloroform, N-

Methyl pyrrolidone, dimethylformamide, ethyl acetate, chlorobenzene, dichloromethane and mixtures thereof. The treatment time is preferably in a range of 1 minute to 3 hours.

The solar cells according to the invention can be present as a single cell with a normal structure. In a specific embodiment, such a cell has the following layer structure

an at least partially transparent substrate, a first electrode (front electrode, anode), a hole-conducting layer, a mixed layer of DBP and at least one electron acceptor in the form of a bulk heterojunction, electron-conducting layer, an exciton-blocking / electron-conducting layer, a second electrode (back electrode, Cathode)

FIG. 1 shows a solar cell according to the invention with a normal structure.

The solar cells according to the invention can also be present as a single cell with inverse structure. In a specific embodiment, such a cell has the following layer structure

an at least partially transparent substrate, a first electrode (front electrode, cathode), an exciton-blocking / electron-conducting layer, electron-conducting layer, a mixed layer of DBP and at least one electron acceptor in the form of a bulk heterojunction, a hole-conducting layer, a second electrode (back electrode, Anode)

FIG. 2 shows a solar cell according to the invention with inverse structure.

The solar cells according to the invention can also be configured as a tandem cell. The basic structure of tandem cell is described, for example, by P. Peumans, A. Yakimov, SR Forrest in J. Appl. Phys. 93 (7), 3693-3723 (2003) and US 4,461,922, US 6,198,091 and US 6,198,092.

A tandem cell consists of two or more than two (e.g., 3, 4, 5, etc.) sub-cells. In this case, a single subcell, a subset of cells or all subcells may have photoactive donor-acceptor transitions in the form of a bulk heterojunction based on dibenzotetraphenylperiflanthene. Preferably, at least one of the sub-cells contains DBP and at least one fullerene or fullerene derivative. Particularly preferably, the semiconductor mixture used in the photoactive layer of at least one subcell consists of DBP and Cβo.

The sub-cells forming the tandem cell may be connected in parallel or in series. Preferably, the sub-cells forming the tandem cell are connected in series. Preferably, an additional recombination layer is in each case located between the individual subcells. The individual subcells have the same polarity, i. usually only cells with normal structure or only cells with inverse structure are combined with each other.

FIG. 3 shows the basic structure of a tandem cell according to the invention. Layer 21 is a transparent conductive layer. Suitable materials are those previously mentioned for the individual cells.

Layers 22 and 24 represent sub-cells. Sub-cell refers to a cell, as previously defined, without cathode and anode. The subcells may be e.g. either all have DBP-C60 bulk heterojunction or other combinations of semiconductor materials, e.g. B C60 with Zn phthalocyanine, C60 with oligothiophene (such as DCV5T). In addition, individual sub-cells can also be designed as a dye-sensitized solar cell or polymer cell. In all cases, a combination of materials is preferred which cover different regions of the spectrum of incident radiation, e.g. of natural sunlight. Thus, e.g. DBP-C60 mainly in the range of 400 nm to 600 nm. Zn phthalocyanine C60 cells absorb mainly in the range of 600 nm to 800 nm. Thus, a tandem cell from a combination of these subcells should have radiation in the range of 400 nm to 800 nm nm absorb. By suitable combination of subcells, the used spectral range should expand. For optimum performance properties, the optical interference should be considered. Thus, sub-cells that absorb at shorter wavelengths should be located closer to the metal top contact than sub-cells with longer-wavelength absorption.

It is also possible that a tandem cell has at least one subcell in which the photoactive donor-acceptor transition is in the form of a flat heterojunction is present. Then, the aforementioned semiconductor materials can be used, which may additionally be doped. Suitable dopants are z. B. pyronin B and rhodamine derivatives.

Layer 23 is a recombination layer. Recombination layers make it possible to recombine the charge carriers from a subcell with those of an adjacent subcell. Suitable are small metal clusters, such as Ag, Au or combinations of highly n- and p-doped layers. In the case of metal clusters, the layer thickness is preferably in a range of 0.5-5 nm. In the case of highly n- and p-doped layers, the layer thickness is preferably in a range of 5 - 40 nm. The recombination layer usually connects the Electron-conducting layer of a subcell with the hole-conducting layer of an adjacent subcell. In this way, additional cells can be combined into a tandem cell.

Layer 26 is the top electrode. The material depends on the polarity of the sub-cells. For normal structure subcells, low work function metals such as Ag, Al, Mg, Ca, etc. are preferably used. For inverse structure subcells, high workfunction metals, such as Au or Pt, or PEDOT-PSS are preferably used.

For subcells connected in series, the total voltage corresponds to the sum of the individual voltages of all subcells. The total current, however, is limited by the lowest current of a subcell. For this reason, the thickness of each subcell should be optimized so that all subcells have substantially the same current intensity.

The invention will be further illustrated by the following non-limiting examples.

EXAMPLES

Example 1 :

Making a mixture of

50 ml of xylene, 5.0 g (18.5 mmol) of 1,3-diphenylisobenzofuran and 4.0 g (22 mmol) of acetonaphthalene (80% purity) were heated at reflux for 7 hours. After cooling the reaction mixture to room temperature, about 40 ml of ethanol were added and the resulting precipitate was filtered off. This gave 6.8 g (87%) of a reaction product which, according to ¹³ C-NMR spectroscopy, is present as a 1: 1 mixture of the compounds described above.

Example 2: Diphenylfluoranthene

6.0 g (14.2 mmol) of the reaction product from Example 1 were heated to reflux temperature in a mixture of 40 ml of acetic acid and 4.1 ml of 48% hydrobromic acid for two hours. The resulting residue was filtered off and washed with ethanol. This gave 5.7 g (98%) of a strongly fluorescent compound (melting point: 272 ° C).

Example 3: Dibenzotetraphenylperiflanthen

A mixture of 60 ml of trifluoroacetic acid, 5.0 g of diphenylfluoranthene (12 mmol) and

7.6 g (65 mmol) of cobalt trifluoride was heated at reflux temperature (72 ° C) for 20 hours. The reaction mixture was added to 400 ml of water and the product was extracted by triplicate extraction with 400 ml of dichloromethane. The combined organic phases were washed in triplicate with 400 ml of water, dried over MgSO ₄ and the solvent removed in vacuo. The residue was purified by chromatography on silica gel with a gradient of petroleum ether and toluene and subsequent crystallization from the corresponding solvent mixture. nigt. 0.92 g (18%) of a high purity fraction was obtained. The remaining slightly contaminated fractions yielded another 1.14 g (22%).

The high purity crystalline fraction is subjected to three-zone sublimation to produce solar cells.

Sublimation of DBP:

DBP was purified by Dreizonensublimation at 2-3 x10 ^"6 mbar, wherein the first zone 450 ⁰ C was obtained. The sublimed at 250 ± 50 ⁰ C the product was used. From 821 mg loading was obtained after 48 hours of sublimation of 553 mg (67 %) sublimated product.

Materials:

DBP: purified by three-zone sublimation (once) as previously described. C60: from Alfa Aesar, grade (purity + 99.92%, sublimed), used without further purification. Bphen: used by Alfa Aesar, without further purification.

Substrate: ITO was sputtered onto the glass substrate. The thickness of the ITO film was 140 nm, the resistivity was 200 μΩ cm, and the root mean square roughness (RMS roughness) was less than 5 nm. The substrate was allowed to stand for 20 minutes before deposition of the organic material UV irradiation "ozonized" (UV ozone cleaning).

Production of the cells:

Bilayer cells (bilayer cells) and bulk heterojunction cells (BHJ cells) were prepared under high vacuum (pressure <10 " ⁶ mbar.) In the bilayer cell, the donor-acceptor transition is flat (smooth), in contrast In the BuIk heterojunction cell, there is an interpenetrated donor-acceptor network.

To prepare the bilayer cell (ITO / DBP / C60 / Bphen / Ag), DBP and C60 were vapor-deposited successively on the ITO substrate. The deposition rate for both layers was 0.2 nm / sec. The deposition temperature was 410 ⁰ C and 400 ⁰ C. Subsequently, Bphen and then 100 nm Ag was evaporated as a top contact. The arrangement had an area of 0.03 cm ² .

To prepare the BHJ cell (ITO / DBP: C60 (1: 1) / C60 / Bphen / Ag), DBP and C60 were deposited on the ITO substrate by coevaporation at the same rate (0.1 nm / sec) that the DBP-C60 volume ratio in the mixed th layer was 1: 1. The deposition of the Bphen and Ag layer was carried out as described for the bilayer cell.

Measurement:

The solar simulator used was an AM 1.5 simulator from Solar Light, USA, with a xenon lamp (Model 16S-150 V3). The UV region below 415 nm was filtered and current-voltage measurements were made at ambient conditions. The intensity of the solar simulator was calibrated with a monocrystalline FZ (float zone) silicon solar cell (Fraunhofer ISE). By calculation, the mismatch factor was approximately 1.0.

Results:

Bilayerzelle

Claims

claims

1. Use of dibenzoetraphenylperiflanthene of the formula

as electron donor material in an organic solar cell with photoactive donor-acceptor transitions in the form of a bulk heterojunction.

2. Use according to claim 1, wherein at least one fullerene or a fullerene derivative is used as electron acceptor material.

3. Use according to one of the preceding claims in the form of a single cell or in the form of a tandem cell.

4. Use according to one of the preceding claims, wherein a gas phase deposition method is used to produce the photoactive donor-acceptor transitions in the form of a bulk heterojunction.

5. Organic solar cell comprising at least one photoactive donor

Acceptor transition in the form of a bulk heterojunction, wherein Dibenzotetraphenylperiflanthen is used as electron donor material.

6. Solar cell according to claim 5 in the form of a single cell or in the form of a tandem cell.