Patent Application
20130264518
The present invention relates to the use of organic dyes as emitters in OLEDs (organic light-emitting diodes) and in other optoelectronic assemblies.
A dramatic change is currently on the horizon in the field of visual display and illumination technology. It will be possible to manufacture flat displays or illuminated surfaces having a thickness of less than 0.5 mm. These are notable for many fascinating properties. For example, it will be possible to achieve illuminated surfaces in the form of wallpaper with very low energy consumption. It is also of particular interest that color visual display units will be producible with hitherto unachievable colorfastness, brightness and viewing angle independence, with low weight and with very low power consumption. It will be possible to configure the visual display units as microdisplays or large visual display units of several square meters in area in rigid form or flexibly, or else as transmission or reflection displays. In addition, it will be possible to use simple and cost-saving production processes such as screen printing or inkjet printing or vacuum sublimation. This will enable very inexpensive manufacture compared to conventional flat visual display units. This new technology is based on the principle of the OLEDs, the organic light-emitting diodes. Furthermore, through the use of specific organic materials (molecules), many new optoelectronic applications are on the horizon, for example in the field of organic solar cells, organic field-effect transistors, organic photodiodes etc.
Particularly for the OLED sector, it is apparent that such assemblies are already now of economic significance, since mass production is expected shortly. Such OLEDs consist predominantly of organic layers which can also be manufactured flexibly and inexpensively. OLED components can also be configured with large areas as illumination bodies, but also in small form as pixels for displays.
Compared to conventional technologies, for instance liquid-crystal displays (LCDs), plasma displays or cathode ray tubes (CRTs), OLEDs have numerous advantages, such as a low operating voltage of a few volts, a thin structure of only a few hundred nm, high-efficiency self-illuminating pixels, high contrast and good resolution, and the possibility of representing all colors. In addition, in an OLED, light is produced directly on application of electrical voltage, rather than merely being modulated.
A review of the function of OLEDs can be found, for example, in H. Yersin, Top. Curr. Chem. 2004, 241, 1 and H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials”; Wiley-VCH, Weinheim, Germany, 2008.
Since the first reports regarding OLEDS (see, for example, Tang et al., Appl. Phys. Lett. 1987, 51, 913), these devices have been developed further particularly with regard to the emitter materials used, and particular interest has been attracted in the last few years by what are called triplet emitters or else phosphorescent emitters.
OLEDs are generally implemented in layer structures. For better understanding,
With a few exceptions, the electronic excited state, which can also be formed by energy transfer from a suitable precursor exciton, is either a singlet or triplet state. Since the two states are generally occupied in a ratio of 1:3 on the basis of spin statistics, the result is that the emission from the singlet state, which is referred to as fluorescence, according to the present state of the art, leads to maximum emission of only 25% of the excitons produced. In contrast, triplet emission, which is referred to as phosphorescence, exploits and converts all excitons and emits them as light (triplet harvesting), such that the internal quantum yield in this case can reach the value of 100%, provided that the additionally excited singlet state which is above the triplet state in terms of energy relaxes fully to the triplet state (intersystem crossing, ISC), and radiationless competing processes remain insignificant. Thus, triplet emitters, according to the current state of the art, are more efficient electroluminophores and have better suitability than purely organic singlet emitters for ensuring a high light yield in an organic light-emitting diode.
The triplet emitters suitable for triplet harvesting used are generally transition metal complexes in which the metal is selected from the third period of the transition metals. This predominantly involves very expensive noble metals such as iridium, platinum or else gold. (See also H. Yersin, Top. Curr. Chem. 2004, 241, 1 and M. A. Baldo, D. F. O'Brien, M. E. Thompson, S. R. Forrest, Phys. Rev. B 1999, 60, 14422).
The phosphorescent organometallic triplet emitters known to date in OLEDs, however, have a disadvantage, which is that the emission lifetime, which is in the region of a few microseconds, is relatively long. This gives rise to saturation effects with increasing current densities and the resulting occupation of a majority of or all emitter molecules. Consequently, further charge carrier streams can no longer lead completely to the occupation of the excited and emitting states. The result is then merely unwanted ohmic losses. As a result, there is a distinct decline in efficiency of the OLED device with rising current density (called “roll-off” behavior). The effects of triplet-triplet annihilation and of self-quenching are similarly unfavorable (see, for example, H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials”, Wiley-VCH, Weinheim 2008 and S. R. Forrest et al., Phys. Rev. B 2008, 77, 235215). For instance, disadvantages are found particularly in the case of use of emitters with long emission lifetimes for OLED illuminations where a high luminance, for example of more than 1000 cd/m2, is required (cf.: J. Kido et al. Jap. J. Appl. Phys. 2007, 46, L10.). Furthermore, organometallic complexes in electronically excited states are frequently more chemically reactive than in the base states. This is generally caused by metal-ligand bond breakage. Therefore, the long-term stability of these emitter materials is inadequate in many cases. (T. Sajoto, P. I. Djurovich, A. B. Tamayo, J. Oxgaard, W. A. Goddard III, M. E. Thompson; J. Am. Chem. Soc. 2009, 131, 9813). As a result, efforts are being made to develop emitter molecules with no metal sites and with minimum emission lifetime, but nevertheless high emission quantum yield. OLEDs using such emitters then exhibit a much lesser extent of roll-off behavior in the efficiency, and additionally enable a longer lifetime of the optoelectronic device.
In summary, the prior art can be described such that the triplet emitters which are efficient per se and are known to date have the disadvantages that
Surprisingly, the problems described above can be significantly improved or solved by the present invention, using organic molecules (dyes, emitter molecules) which have particular electronic structures or singlet-triplet energy separations and which are modified in accordance with the invention by changes in the immediate environment of the emitters. This process of “singlet harvesting for organic emitters” which is proposed here for the first time is to be described briefly hereinafter using
a shows a (simplified) energy level scheme for a typical, purely organic molecule having a ΔE(S1−T1 value between the lowermost excited singlet state (S1) and the triplet state (T1) below it of greater than 2500 cm−1.
This scheme can be used to illustrate the photophysical electroluminescence properties of these molecules. Hole-electron recombination, as occurs, for example, in an optoelectronic component, leads, on statistical average, to 25% occupation of the singlet state and to 75% occupation of the three sub-states of the triplet state. Since the emission transition from the triplet state T1 to the singlet state S0 is strongly spin-forbidden in organic molecules due to the low level of spin-orbit coupling, the excitation energy which arrives at the triplet state is converted radiationlessly to heat and has thus been lost to the light production by electroluminescence. The occupied singlet state can, however, exhibit effective emission (fluorescence) because this is a spin-allowed singlet-singlet transition. In this context, it is important to mention that the radiationless relaxation process from the S1 state to the T1 state, called the intersystem crossing (ISC) process, is likewise strongly forbidden due to the low level of spin-orbit coupling. Otherwise, no fluorescence would be observable. For the time constants, this means that τ1(ISC) is several orders of magnitude longer than the fluorescence lifetime, which is in the region of one to a few nanoseconds for τ(S1).
According to the invention, the above-described disadvantages of the prior art can be avoided. This is possible by a combination of two steps:
Using these two strategies, which are to be used together—as illustrated by
Accordingly, the invention, in one aspect, provides a composition, especially for utilization in an optoelectronic device, which comprises
The terms “spin-orbit coupling constant” and “intersystem crossing time constant” are specialist terms which are commonly used in the photophysical literature and are therefore known to those skilled in the art.
b shows an energy level diagram for an organic molecule having a small energy difference ΔE(S1−T1)<2500 cm−1. This energy difference is small enough to enable thermal repopulation of the S1 state from the T1 state according to a Boltzmann distribution, or according to the thermal energy kBT, and hence thermally activated light emission from the S1 state. This process is controlled by equation (1):
Int(S1→S0)/Int(T1→S0)=k(S1)/k(T1)exp(−ΔE/kBT) (1)
In this equation, Int(S1→S0)/Int(T1→S0) is the intensity ratio of the emissions from the S1 state and the T1 state. kB is the Boltzmann constant and T the absolute temperature. k(S1)/k(T1) is the rate ratio of the conversion processes from the singlet S1 and from the triplet T1 to the electronic ground state S0. For organic molecules, this ratio is between 107 and 1010. Preference is given in accordance with the invention to molecules having a rate ratio of about 108, better of about 109, more preferably of about 1010. ΔE represents the energy difference ΔE2(S1−T1) according to
The process of thermal repopulation described opens up an emission channel via the singlet state S1 from the populated triplet. Since the transition from the S1 to the S0 state is strongly allowed, the triplet excitation energy, which is otherwise lost is obtained virtually completely as light emission via the singlet state. At a given temperature, for example at room temperature, the smaller the energy difference ΔE, the more marked this effect is. Preference is therefore given to organic molecules having a ΔE=ΔE(S1−T1) value between the lowermost excited singlet state and the triplet state below it of less than 2500 cm−1, better less than 1500 cm−1, preferably of less than 1000 cm−1.
This effect is to be illustrated by a numerical example. Given an energy difference of ΔE=1300 cm−1, for room temperature applications (T=300 K) with kBT=210 cm−1 and a rate ratio of 108, an intensity ratio of the singlet to triplet emission according to equation (1) of approx. 2·105 is obtained. This means that the singlet emission process is dominant to an extreme degree for a molecule having these example values.
The emission lifetime of this example molecule also changes as a result. The thermal repopulation results in a mean lifetime τav. This can be described by equation (2)
τav≈τ(S1)·exp(ΔE/kBT) (2)
In this equation, τ(S1) is the fluorescence lifetime without repopulation and τav the emission lifetime which is determined on opening of the repopulation channel by the two states T1 and S1 (see
Equation (2) is again to be illustrated by a numerical example. For the assumed energy difference of ΔE=1300 cm−1 and a decay time of the fluorescing S1 state of 1 ns, an emission decay time (of the two states, i.e. of the S1 state thermally repopulated from the T1 state) of τav≈500 ns is obtained. This decay time is already much shorter than those of very good triplet emitters, the decay times of which are in the range from approx. 1.5 μs to 10 μs.
The applicability of equation (2) and the validity of the above-described calculation of the decay time τav, in accordance with the invention, require the use of additives which increase spin-orbit coupling (for detailed arguments see, for example, below). These additives, i.e. optically inert atoms or molecules of the composition, interact with the organic emitter molecules such that the mean (av, thermalized) emission lifetime of the two states S1 and T1 of the organic molecule is reduced to about 500 ns. Preference is given to compositions of such a kind that the emission lifetime is reduced to less than 1 μs, preferably to less than 600 ns and more preferably to less than 200 ns.
In summary, using this “singlet harvesting process for organic molecules”, it is thus possible in the ideal case to capture virtually all, i.e. a maximum of 100%, of the excitons and convert them to light via singlet emission. In addition, it is possible to shorten the emission decay time well below the value for triplet emitters, which is a few microseconds. Therefore, the inventive composition is particularly suitable for optoelectronic components.
Organic molecules having the above-described properties, i.e. having a small singlet-triplet energy difference ΔE (S1−T1), are preferably organic molecules having the following general formulae I to III:
In these formulae, D is a chemical group or a substituent with electron-donating effect (D, donor effect). Substituents of this kind may be present once, twice or more than twice. They may be the same or different.
A is a chemical group or a substituent with electron-withdrawing propensity (A, acceptor effect). Substituents of this kind may be present once, twice or more than twice. They may be the same or different.
The base structure B is formed from conjugated organic groups which consist of aromatic, heteroaromatic and/or conjugated double bonds.
—O(—), —N-alkyl group, —N-(alkyl group)2 —NH2, —OH, —O-alkyl group, —NH(CO)— alkyl group, —O(CO), -alkyl group, -alkyl group, -phenyl group, —(CH)═C—(alkyl group)2
-halogen, —(CO)H, —(CO)-alkyl group, —(CO)O-alkyl group, —(CO)OH, —(CO)Cl, —CF3, —CN, —S(O)2OH, —NH3(+), —N(alkyl group)3(+), N(O)2
B is formed from conjugated organic groups which consist of aromatic, heteroaromatic and/or conjugated double bonds. Preference is given to molecular base structures B having aromatic or heteroaromatic rings smaller than 15, more preferably smaller than 10, most preferably smaller than seven. The aromatic or heteroaromatic rings are chemically joined directly or chemically bonded via alkenyl groups having conjugated double bonds smaller than 10, more preferably smaller than six and most preferably smaller than 3.
Preference is given to chemically bonded alkenyl groups having a number of conjugated double bonds of less than 10, more preferably less than 6 and most preferably less than 3.
The organic molecules described by formulae I to III have ΔE(S1−T1) values between the lowermost excited singlet state and the triplet state below it of less than 2500 cm−1, preferably less than 1500 cm−1 and more preferably less than 1000 cm−1. Processes for measurement or calculation of the ΔE(S1−T1) values are discussed below.
Preference is given to organic molecules which, without use of additives, have a high fluorescence quantum yield from the S1 state of greater than 50%, preferably greater than 70%, more preferably greater than 90% (determination with commercial measuring instruments for emission quantum yield) and for which the absorption intensities, i.e. the molar decadic extinction coefficients, of the transitions between the ground state S0 and the excited state S1 are greater than 104 l/mol cm, preferably greater than 2×104 l/mol cm, more preferably greater than 5×104 l/mol cm (determination with commercial absorption spectrometers).
The invention relates, in a further aspect, to a process for selecting organic molecules for which the ΔE(S1−T1) value between the lowermost excited singlet state (S1) and the triplet state (T1) below it is less than 2500 cm−1, preferably less than 1500 cm−1, more preferably less than 1000 cm−1.
The determination of the ΔE(S1−T1) value can either be performed by quantum-mechanical calculations using computer programs known in the prior art (for example using Turbomole programs executing TDDFT calculations with reference to CC2 calculations) or determined experimentally, as explained below.
The energy difference ΔE(S1−T1), more particularly of the organic molecules described by formulae I to III, can be described as an approximation by quantum-mechanical means via the exchange integral multiplied by a factor of 2. The value of the latter depends directly on the overlap of the molecular orbitals in the area on the D side of B with the molecular orbitals in the area on the A side of B. Due to the properties of D and A described above, these molecular orbitals are distributed over different spatial areas (partly delocalized over n or n* molecular orbitals). This means that an electronic transition between the different molecular orbitals represents a charge transfer (CT) process. The smaller the overlap of the above-described molecular orbitals, the more marked is the electronic charge transfer character. This is then associated with a decrease in the exchange integral and hence a decrease in the energy difference ΔE(S1−T1). In other words, ΔE(S1−T1) can be varied via the strengths of the electron-donating and -withdrawing substituents/groups of the organic molecule. Due to these photophysical (quantum-mechanical) properties, it is possible to achieve the inventive energy differences with ΔE(S1−T1) of less than 2500 cm−1 or less than 1500 cm−1 or less than 1000 cm−1.
The ΔE(S1−T1) value can be determined experimentally as follows:
For a given organic molecule, the energy separation ΔE(S1−T1)=ΔE can be determined in a simple manner using the above-specified equation (1). A rearrangement gives:
ln{Int(S1→S0)/Int(T1→S0)}=ln{k(S1)/k(T1)}−(ΔE/kB)(1/T (3)
For the measurement of the intensities Int(S1→S0) and Int(T1→S0), it is possible to use any commercial spectrophotometer. A graphic plot of the (logarithmized) intensity ratios ln{Int(S1→S0)/Int(T1→S0)} measured at different temperatures against the reciprocal of the absolute temperature T generally gives a straight line. The measurement is conducted within a temperature range from room temperature (300 K) to 77 K or to 4.2 K, the temperature being established by means of a cryostat. The intensities are determined from the (corrected) spectra, Int(S1→S0) and Int(T1→S0) representing, respectively, the integrated fluorescence and phosphorescence band intensities, which can be determined by means of the programs provided with the spectrophotometer. The respective transitions (band intensities) can be identified easily since the triplet band is at lower energy than the singlet band and gains intensity with falling temperature. The measurements are conducted in oxygen-free dilute solutions (approx. 10−2 mol L−1) or on thin films of the corresponding molecule or on films doped with the corresponding molecules. If the sample used is a solution, it is advisable to use a solvent or solvent mixture which forms glasses at low temperatures, such as 2-methyl-THF, THF (tetrahydrofuran) or aliphatic hydrocarbons. If the sample used as a film, the use of a matrix having a much greater singlet and triplet energy than that of the organic emitter molecules, for example PMMA (polymethyl methacrylate), is suitable. This film can be applied from solution. It is particularly important that, as described below, the molecules to be analyzed are used with the respective additives.
The slope of the straight line is −ΔE/kB. With kB=1.380·10−23 JK−1=0.695 cm−1 K−1, it is possible to determine the energy separation directly.
Viewing this in an equivalent manner, it is found that it is also possible to determine the ΔE(S1−T1) value by means of the temperature dependence of the emission decay time.
A simple, approximate estimation of the ΔE(S1−T1) value can also be made by recording the fluorescence and phosphorescence spectra at low temperature (e.g. 77 K or 4.2 K using a cryostat). The ΔE(S1−T1) value then corresponds approximately to the energy difference between the high-energy slope flanks of the fluorescence and phosphorescence bands.
The more marked the CT character of an organic molecule, the greater the variation in the electronic transition energies as a function of solvent polarity. For instance, a marked polarity dependence of the emission energies already gives a pointer to the presence of small ΔE(S1−T1) values.
Preferred organic molecules consist exclusively of light atoms such as C, H, N, O, F, S, K, Na. For such organic molecules, the electronic singlet and triplet states of which result essentially from transitions between τ and τ* molecular orbitals, as already mentioned, the effective spin-orbit coupling (SOC) is so small that the relaxation transitions from the S1 to the energetically lower T1 state and in the reverse direction from the T1 state to the S1 state barely occur (are strongly forbidden).
According to the invention, this is no longer forbidden: the organic molecules (emitter molecules), especially those of the formulae I, II and III, may be doped, for example, into optoelectronic devices, or into matrix materials, for example in an OLED emission layer. According to the invention, optically inert atoms or molecules (called “additives”) are added to this matrix to reduce the intersystem crossing time constant of the organic molecule. These optically inert atoms or molecules are notable for high spin-orbit coupling (SOC) (SOC constant of the atoms or molecular units greater than 1000 cm−; see the explanations given below). These additives are introduced, for example, in a concentration corresponding to or higher than that of the emitter molecules. These additives can, for example, also be used in a concentration twice to five times as high as that of the organic emitter molecules. In general, the numeric ratio between organic emitter molecules and optically inert atoms or molecules is 1:0.1 to 1:5 or 1:10, preferably 1:0.2 to 1:5, more preferably 1:1. This gives rise to such a distribution probability that at least one additive particle/additive molecule having high SOC is present in the immediate environment of an emitter molecule. This induces external SOC which accelerates the process of intersystem crossing by several orders of magnitude. This brings about very rapid relaxation from the S1 to the T1 state and likewise very rapid thermal repopulation according to equations (1) and (2). This enables the singlet harvesting effect for organic molecules.
In a further aspect of the invention, the composition described here is used in an emitter layer in an optoelectronic (organic electronic) device, especially an OLED.
The OLED devices can be produced by processes known from the prior art (cf. H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials”, Wiley-VCH, Weinheim, Germany 2008).
In a preferred configuration of an organic light-emitting diode (OLED), the proportion of the composition (organic emitter and additive) in the emitter layer is between 2% by weight and 100% by weight, preferably between 6% by weight and 30% by weight.
Another aspect of the invention is the use of the inventive composition composed of organic molecule and optically inert atom or optically inert molecule for use in light-emitting electrochemical cells (LEECs), OLED sensors, especially in a gas and vapor sensor not hermetically sealed from the outside, optical temperature sensors, organic solar cells (OSCs; organic photovoltaics, OPVs), organic field-effect transistors, organic diodes, organic photodiodes and “downconversion” systems.
Generally, the proportion of the composition in an emitter layer of an optoelectronic device may be 2 to 100% by weight, preferably 6 to 30% by weight, based on the total weight of the emitter layer.
In a further aspect, the invention relates to a process for reducing the emission lifetime and to a process for increasing the electroluminescence quantum yield of an organic molecule as an emitter in an optoelectronic device. In this case, an organic molecule which has a ΔE(S1−T1) value between the lowermost excited singlet state (S1) and the triplet state (T1) below it of less than 2500 cm−1 is introduced into the vicinity of an optically inert atom or molecule (optionally via a chemical bond), such that the organic molecule can interact with the optically inert atom or molecule. Due to a spin-orbit coupling constant of greater than 1000 cm−1 for the optically inert atom or molecule or for parts of the optically inert molecule, a short mean emission lifetime (from the singlet S1 and the triplet T1 states) of the organic molecule and an increase in the emission quantum yield are achieved.
The invention further relates to a process for converting the triplet excitation energy of an organic molecule generated in the course of electroluminescence to fluorescent energy. This involves interaction of an organic molecule having a ΔE(S1−T1) value between the lowermost excited singlet state (S1) and the triplet state (T1) below it of less than 2500 cm−1 with an optically inert atom or molecule such that triplet excitation energy of the organic molecule is converted via a singlet state of the organic molecule to fluorescent energy.
The invention also relates to a process for selecting organic molecules for which the ΔE(S1−T1) value between the lowermost excited singlet state (S1) and the triplet state (T1) below it is less than 2500 cm−1, preferably less than 1500 cm−1, more preferably less than 1000 cm−1. The process comprises at least two steps, namely: firstly the determination of the ΔE(S1−T1) value of organic molecules by means of a) an ab initio molecular calculation, b) measurement of the temperature dependence of the fluorescence and phosphorescence intensities, or c) measurement of the temperature dependence of the emission decay time, and secondly the finding of organic molecules for which the ΔE(S1−T1) value is less than 2500 cm−1, preferably less than 1500 cm−1, more preferably less than 1000 cm−1. The organic molecules thus found have a ΔE(S1−T1) value between the lowermost excited singlet state (S1) and the triplet state (T1) below it of less than 2500 cm−1, preferably less than 1500 cm−1, more preferably less than 1000 cm−1.
From the multitude of realizable organic molecules having a small singlet S1-triplet T1 energy difference, using the example of the emitters of the formulae I to III, some examples are given, these having the following properties:
More particularly, the introduction of above-listed inert additives shows that the intersystem crossing (ISC) times of the transitions between the singlet S1 and the triplet T1 of the organic emitter molecules can be drastically reduced or, correspondingly, the ISC rates can be drastically increased. For example, various organic emitter molecules selected from the examples shown in
Various examples contain charged organic molecules and counterions. These emitter molecules can preferably be used in light-emitting electrochemical cells (LEECs or LECs), the basic structure of which is known to those skilled in the art. In the case of use of these charged organic molecules in OLEDs, it may be advisable to replace the small counterions with larger counterions of the same charge, such as (PF6)−, (BF4)−, [CF3SO2]−, singly negatively charged hexaphenylphosphate, singly negatively charged tetraphenylborate, etc.
Some example molecules are apparently of symmetric structure and therefore do not appear to contain any separate D or A groups. These molecules, however, are polarized in solution and/or by the action of the counterions so as to result in corresponding donor or acceptor effects.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 025 547.5 | Jun 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/060834 | 6/28/2011 | WO | 00 | 12/26/2012 |
