Title of Invention

DOPED ORGANIC SEMICONDUCTOR MATERIAL AND METHOD FOR PRODUCTION THEREOF

Abstract

The invention relates to a doped organic semiconductor material with increased charge carrier density and more effective charge carrier mobility, which may be obtained by doping an organic semiconductor material with a chemical compound comprising one or several organic molecular groups (A) and at least one further compound partner (B). The desired doping effect is achieved after cleavage of at least one organic molecular group (A) from the chemical compound by means of at least one organic molecular group (A) or by means of the product of a reaction of at least one molecular group (A) with another atom or molecule. A method for production thereof is disclosed.

Full Text

FORM 2 THE PATENT ACT 1970 (39 of 1970) The Patents Rules, 2003 COMPLETE SPECIFICATION (See Section 10, and rule 13) TITLE OF INVENTION DOPED ORGANIC SEMICONDUCTOR PRODUCTION THEREOF MATERIAL AND METHOD FOR APPLICANT(S) a) Name b) Nationality c) Address NOVALED AG GERMAN Company TATZBERG 49, 013 07 DRESDEN, GERMANY PREAMBLE TO THE DESCRIPTION The following specification particularly describes the invention and the manner in which it is to be performed : The invention relates to doped organic semiconductor material with increased charge carrier density and effective charge carrier movement ability as per claim 1, a procedure for the production of the doped organic semiconductor material as per claim 48 as well as the use of the semiconductor material. Since the demonstration of organic luminating diodes and solar cells in 1989 [C.W. Tang et al-, Appl. Phys. Lett. 51 (12), 913 (1987)] components structured from organic thin layers have become object of intense research. This type of layers possess advantageous properties for the said applications, like e.g. efficient electro luminescence for organic luminating diodes, high absorption coefficients in the field of visible light for organic solar cells, economical production of materials and finishing of components for most simple electronic circuits, among others already the deployment of organic luminating diodes has commercial significance for display applications. The performance features (opto) electronic multi-layer components are determined among other things by the ability of layers to transport the charge carriers. In case of luminating diodes the ohmic losses relate in the charge transport layers in operation to the conductivity, which on the one hand has a direct influence on the required operating voltage, on the other hand, however, it determines the thermal load of the component. Further, in conjunction with the charge carrier concentration of organic layers a band bending occurs in the vicinity of a metal contact, which simplifies the injection of charge carriers and thus the contact resistance can reduce. Similar considerations lead also for organic solar cells to the conclusion that their efficiency is determined also through the transport properties for charge carriers. Through doping of holes transport layers with a suitable acceptor material (p- doping) and of electron transport layers with a donor material (n-doping) the charge carrier density in the organic solid bodies (and thereby the conductivity) can be considerably increased. Beyond this, in analogy to the experience with inorganic semiconductors applications are to be expected, which are based on the use of p-doped and n-doped layers in a component and otherwise could not be imaginable. In US 5,093,698 the use of doped charge carrier transport layers (p-doping of holes transport layer through addition of acceptor type molecules, n-doping of electron transport layer through addition of donor type molecules) in organic lufninating diodes is described. Following principles are so far known for the improvement of conductivity of organic vaporized layers: 1) Increase in charge carrier movement ability through a) Use of electron transport layers consisting of organic radicals (US 5,811,833), b) Production of superior arranged layers, which allow optimum overlapping of pi-orbitals of molecules, 2) Increase in density of move-able charge carriers through a) Cleaning and careful handling of materials to avoid formation of charge carrier adhesion points, b) Doping of organic layers by means of aa) Inorganic materials (gases, alkali atoms Patent US 6,013, 384 (J. Kido et al); J. Kido et al., Appl- Phys. Lett. 73, 2866 (1998)), bb) Organic materials (TNCQ (M. Maitrot et al, J. Appl. Phys., 60 (7), 2396-2400 (1986)), F4TCNQ (M. Pfeifferet al, Appt. Phys. Lett.. 73 (22), 3202 (1998)), BEDT-TTF (A. Nollau et al, J. Appl. Phys., 87 (9), 4340 (2000))) Doped organic charge transport layers were already deployed successfully for improving organic luminating diodes. Through doping of holes transport layer with the acceptor material F4TCNQ a drastic reduction of operating voltage of the luminating diode is achieved (X. Zhou et al., Appl. Phys- Lett., 78 (4), 410 (2001).). A similar success is to be achieved through doping of electrofi transporting layer with Cs or Li (J. Kido et al, Appl. Phys. Lett., 73 (20), 2866 (1998); J.-S. Huang et al, Appl. Phys. Lett, 80,139 (2002)). 3 The electrical doping with inorganic materials suffers from the deficiency that the atoms or molecules used - due to their smaller size - can easily diffuse in the component and thus can make a defined production e.g. of sharper transitions from p-doped to n-doped areas difficult. As against this, diffusion plays a minor role in use of larger organic molecules as dopants. However, their deployment is impaired through the situation that potential doping molecules must feature extreme values of electron affinity for the p-doping or values of the ionization potential for the n-doping. Thus a reducing chemical stability of molecules occurs. The task of the invention is to indicate a solution for overcoming the said chemical instability of efficient doping molecules and thus the production of doped layers. In accordance with the invention the task is solved through properties mentioned in the claim 1. Advantageous designs are the object of sub-claims. The task is further solved through a procedure with properties mentioned in the claim 48. Advantageous variants of the procedure are the object of sub-claims. In the invention organic molecules are used, which are actually instable in neutral state, however, are stable as charged cation or anion or in conjunction with a co-valent connection partner. These charged molecules are produced in situ from a precursor compound, which before, during and after the vaporization process gets converted in the required charged molecule. Without restriction to this. Such a compound e.g. can be an organic salt or a metal complex. Also the instable dopant can be produced in situ from a stable precursor substance. So far the used doping molecule in the neutral state was brought in the layer to be doped, in order to have it as anion or cation after a charge transfer on the matrix. The use of the neutral molecule is thus only an intermediate step for initiating the charge transition. The 4 ■elated stability problems as already described can be avoided according to the invention hrough the compound of a readily ionized, stable molecule as dopant. If necessary, for supporting the dissociation of precursor compound other procedures can find application. This provides necessary energy for splitting of the compound, or effect a chemical reaction with the unwanted residue of the precursor compound, so that it does not get into layer, or can be easily removed from this, or does not impair the electrical properties of this layer. An invention based advantageous solution is for example the use of a laser for vaporizing of rhodaiTiine B chloride, which leads to major production of rhodamine B cations. Even if the existing discussion aims at splitting a readily charged molecular group as per claim 1, the purpose of invention can even then be achieved, if first a neutral radical is produced from the compound according to claim 1, which is in situ sufficiently stable for building it in the layer, and in the layer this is subject to a transfer of the radical electron on the matrix or an admission of another electron from the matrix. In US 5,811,833 an electron transport layer is described, which consists of free radicals, especially pentaphenyl cyclopenta dienyl, for the use in organic luminating diodes. In US 5,922,396 it is shown that such a layer can be produced from metal organic compounds, especially from dekaphenyl germanocen or dekaphenyl plumbocen (refer also M. J. Heeg, J. Organometallic Chem.. 346,321 (1988)). US 5,811,833 and US 5,922,396 lead to layers with increased microscopic charge carrier move ability (or to the transfer rates in the hopping process), because a negative charged pentaphenyl cyclopentadionyle molecule has aromatic characteristic, and so the electron transfer gets improved on a neighboring neutral pentaphenyl cyclopentadionyle molecule through the overlapping of pi-electron orbitals of phenyl groups of participating molecules. The increase of the conductivity is achieved through an increase of microscopic charge carrier move ability (or of the transfer rates in the hopping process). As contrast to this, in accordance to the invention, the 5 balancing charge carrier density is increased in order to improve the conductivity. A discrimination is for example possible through time-of-flight (measurement of charge carrier move ability), through the Seebeck effect or the field effect (measurement of the charge carrier density). The invention further relates to the use of doping molecules in mix layers, which additionally contain materials, in order to achieve another purpose. These purposes can refer to e.g. the change of layer growth, the production of interpreting networks (C. J. Brabec et al, Adv. Mater., 11(1), 15 (2001)), or in organic luminating diodes the improvement of quantum efficiency of the light emission or change of color of the emitted light by adding a luminescent dye. Further it is in the sense of the invention that by suitable selection of doping molecule to be used such purposes can be achieved through adding the doping molecules in the layer. For example, cationic dyes like rhodamine B have a high luminescence quantum yield, which allow their use as luminescent dyes in organic LED, At the end this invention also covers the use of molecules from claim lfor doping of polymer layers. Such layers are produced typically through a spin coating process by precipitation from the solvent. In contrast to the already known electro-chemical doping, in which the anions and cations of a salt through connected voltage are pulled to the respective contacts and are moveable, this invention facilitates according to claim 1 the doping of polymer layers with large non-mobile molecules. An execution example for simplification of the invention is the deployment of dye molecule modal-nine B chloride as dopant. If a mix layer of naphthalene tetracarbonic acid dianhydride (NTCDA) and rhodamine B is produced in the ratio 150:1, then it gives a conductivity of le-5 S/cm at room temperature, which amounts to an increase by 4 units vis-a-vis a pure NTCDA layer. The physical explanation for this is that rhodamine B 6 chloride molecules during the heating in cell disintegrates in positive charged rhodamine B molecules and negative charged chloride ions. The charged rhodamine B molecules are built in the mix layer. The electrons needed for maintaining the charge neutrality of the entire layer remain on the NTCDA molecules, since the electron affinity of NTCDA is higher than that of the rhodamine B (3.2 eV, H. Meier, "Organic Semiconductors", Verlag Chemie Weinheim, 1974, p. 425). These electrons fill the lowest un-occupied orbitals of the NTCDA and thus increase the conductivity. The increased density of the charge carriers can be determined for example through measurements of Seebeck coefficient and the field effect. Field effect measurements on a sample made of NTCDA doped with pyronin B (50:1) confirms the presence of electrons as majority charge carrier with a sfc-concentration of 1017 cm"3. From Seebeck measurements of this system also n-conducting follows with Seebeck coefficients of -1, 1 mV/K and thereby a higher charge carrier concentration than before could be reached with doped NTCDA (a. Nollau et al., J. Appl. Phys., 87 (9), 4340 (2000)). If one prepares a rhodamine B doped layer of C60 (Fulleren) (50:1) with increased substrate temperature, it gives a conductivity of 6e-3 S/cm. This is two units larger than the one in the sample produced at room temperature (5e-5S/cm). the heat added during the vaporizing leads to increased splitting of rhodamine B. The doping effect of rhodamine B was proved also for matrices of MePTCDI (Perylene-3,4,9,10-Tetracarbonic acid-N,N"-Dimethyl-Diimide) and PTCDA (3,4,9,10-Perylene tetracarbonic acid dianhydride) and is thus independent of the concrete chemical structure of the matrix. A stronger known organic donor, Tetrathiafulvalene (TTF), has an oxidation potential of +0.35 V against SCE (Y. Misaki et al, Adv. Mater. 8, 804 (1996)). Stronger donors, i.e. dopants with a lower oxidation potential, are instable in air (G. C. Papavassiliou, A. Terzis, P. Delhaes, in: H. S. Nalwa (Ed.) Handbook of conductive molecules and 7 polymers, Vol. 1: charge transfer salts, fullerenes and photoconductors, John Wiley & Sons, Chichester, 1997). Rhodamine B has a reduction potential of -0.545 V against NHE (M. S. Chan, J. R. Bolton, Solar Energy, 24, 561 (1980)), i.e. -0.79 V against SCE. The reduction potential of the organic salt rhodamine B is determined through the reduction potential of the rhodamine B cation. This value is equal to the oxidation potential of the neutral rhodamine B radical. As a result the rhodamine B radical is a stronger donor than TTF. In the chemical compound rhodamine B chloride this strong donor rhodamine B is stable. Whereas, so far it is possible to use donors with an oxidation potential greater than +0.35 V against SCE, the invention described here allows the doping with donors, whose oxidation potential is smaller than +0.35 V against SCE. Chemically stable compounds in the sense of the claim 1 are for example ionic dyes. These are used in the photography for sensitizing of e.g. AgBr. The electron affinity of AgBr is 3.5eV. Dyes, which can sensitize AgBr through electron transfer, are suitable also as chemically stable compounds for using in doping of organic semiconductor materials in the sense of the claim 1. A sub-class of ionic dyes has the di- and triphenyl methane dyes and their known analogues of the general structure Tl orT2, wherein X CR4, SiR4, GeR4, SnR4, PbR4, N, P and Rl, R2. R3 and R4 are suitable, known substitutes, e.g. one or more atoms: hydrogen, oxygen, halogen, e.g. fluor, chlor, bromine 8 or Iodine; hydroxyl; aminyl. e.g. diphenyl aminyl, aliphate with 1 to 20 carbon atoms, e.g. methyl, ethyl, carboxyl; alkoxyl, e.g- methoxy, cyan; nitro; sulfonic acid and its salts; aryl with 3 to 25 carbon atoms, e.g. phenyl pyridyl or naphthyl or those atoms, which form a condensed ring.Often one or more p-position substitutes of phenyl groups are found (T3 toT6) 1* wherein X CR8, SiR8, GeR8, SnR8, PbR8, N, P and Rl to R7 and R8 are suitable, known substitutes, e.g. one or more atoms: hydrogen, oxygen, halogen, e.g. ftuor, chlor, bromine or Iodine; hydroxyl; aminyl, e.g. diphenylaminyl, diethyleaminyl, aliphate with 1 to 20 carbon atoms, e.g. methyl, ethyl, carboxyl; alkoxyl, e.g. methoxy, cyan; nitro; sulfonic acid and its salts;.aryl with 3 to 25 carbon atoms, e.g. phenyl, pyridyl or naphthyl or those atoms, which form a condensed ring- 9 Examples for diphenyl methane dyes are auramin 0 (Cl 655), or auramin G (Cl 656). Examples for triphenyl methane dyes are malachite green (Cl 657), turquoise blue (Cl 661), fluorescein (Cl 45350) or patent blue V (Cl 712). The representative of triphenyl methane dye malachite green chloride produces in an NTCDA matrix in a doping ratio of 1:122 a conductivity of 4.10-4S/cm. Malachite green is. therefore, suitable as compound in the sense of the claim 11 and especially in the sense of the sub-claims 12 to 22, to produce a doping molecule in situ. This property is highlighted through the valency structure of the central carbon atom (4th main group). Other known compounds of this structure type with atoms of 4th main group as central atom (triarylsilyl, germyi, stannyl, plumbyl) are accordingly suitable also as compound in the sense of claims 1,12 to 22. Compounds are contained in the claims 23 to 25, in which there is one direct bond between 2 carbon atoms, one each of phenyl ring of di- or triphenyl amine. The doping effect occurs also while using leuco forms (T7 T8) of ionic dyes. Rhodamine B base has a doping effect on a PTCDA matrix, e.g. can show a conductivity of 7*10-5 S/cm for a 1:70 doping. Another group of ionic dyes is of Xanthen dyes. The above shown rhodamine B is a representative of this class. Pyronine B, rhodamine 110 and rhodamine 3B as other representatives of this material class have also a doping effect. Among others, pyran. 10 thiopyran, indamine, acridine, azine, oxazin and thiazine dyes are similar to Xanthen dyes, who distinguish themselves through substitutions in multi-core hetero cycle. Based on otherwise similar structure these dye classes (T9) are also suitable compounds in the sense of the claims 1, 23 to 26. Dyes, which are based on a polymethine structure Are also have impact as dopant. N, N"-diethyl-cyanine and N,N"-diethyl-thiacarbocyanine effect in a NTCDA matrix an increase of conductivity to 3*105 S/cm (1:114 doping ratio) and 5*10-5 S/cm (1:47 doping ratio). Both these dyes are a representative of polymethine dyes with a specific choice of X and Z. The leuco bases of ionic dyes are also suitable compounds in the sense of claims 1,12 to 26. For example, rhodamine B base in NTCDA results in a conductivity of 3*105 S/cm (1:70 doping ratio). Since the doping effect does not link to the dye property of the ionic dyes. But much more to their character as organic salt, other organic salts also function as compound in the sense of the claim 11. Organic salts often base on suitable hetero cycles (e.g. pyridinum, pyrrolium, pyrylium, thiazolium, diazininium, thininium, diazolium, thiadiazolium or 11 dithiolium etc. individually or as part of a multi-core hetero cycle) or suitable groups (e.g. ammonium, sulfonium, phosphonittm, iodonium etc.). Mass spectrometric investigations in case of pyronine B chloride indicate that during vaporizing of pyronin B among others HCI and a protonized form of pyronine B with the mass number 324 occur. Apparently/ chlor radicals and neutral pyronine B - radicals produced during the splitting of pyronine B chlorideare de-saturated by a proton. These protons are delivered by other pyronine B molecules in the evaporating substance. A vaporized layer of pyronine B chlorideis first colorless. This proves the formation of the neutral pyronine B. Under oxygen influence the vaporized layer again becomes red, which corresponds the formation of pyronine B cation, i.e. the vaporized substance is oxidized under oxygen influence. This process also takes place in a mix layer of matrix and dopants. Vaporizing mis layers of pyronine B chloride and tetracyanoquinodimethane immediately become red, and the presence of tetracyanoquinodimethane anions can be provided through UV/VIS and FTIR spectroscopy.

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