Article
`
`pubs.acs.org/JACS
`
`⊥,∥
`
`Evelyn Fuchs,
`
`§
`
`Design Rules for Charge-Transport Efficient Host Materials for
`Phosphorescent Organic Light-Emitting Diodes
`†
`†,‡
`‡,§,⊥
`Björn Baumeier,
`Falk May,
`Mustapha Al-Helwi,
`Wolfgang Kowalsky,
`Christian Lennartz,*,‡,§
`and Denis Andrienko*,†
`†
`Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
`‡
`Innovation Lab Heidelberg, Speyerer Strasse 4, 69115 Heidelberg, Germany
`§BASF SE, B009, 67056 Ludwigshafen, Germany
`⊥
`Kirchhoff-Institut für Physik der Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
`Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Schleinitzstrasse 22, 38106 Braunschweig, Germany
`*S Supporting Information
`
`∥
`
`ABSTRACT: The use of blue phosphorescent emitters in
`organic light-emitting diodes (OLEDs) imposes demanding
`requirements on a host material. Among these are large triplet
`energies, the alignment of levels with respect to the emitter,
`the ability to form and sustain amorphous order, material
`processability, and an adequate charge carrier mobility. A
`possible design strategy is to choose a π-conjugated core with a
`high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents, however, induce large
`spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the
`charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an
`electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the
`frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for
`electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a
`high electron mobility and a small Poole−Frenkel slope, supporting our conclusions. By linking electronic structure, molecular
`packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.
`
`1. INTRODUCTION
`Organic light-emitting diodes (OLEDs) have recently entered
`the market of flat panel displays and lighting applications.1,2 In
`spite of this successful commercialization, the field still has a
`number of open issues, such as insufficient stability3 of OLEDs
`based on deep-blue (λ < 460 nm) emitters.4,5
`In a prototypical phosphorescent OLED, holes and electrons
`are injected from electrodes on opposite sides into transport
`and blocking layers that provide a balanced charge transport
`into an emission layer (EML). The EML itself consists of a
`charge-transporting organic semiconductor and an organo-
`metallic emitter that allows for triplet harvesting. To avoid
`triplet quenching and triplet−triplet annihilation, the charge-
`transporting material as the majority component (host) is
`doped with the emitter. An exciton can be formed on a host
`molecule with a subsequent energy transfer to the dopant.
`Alternatively, one of the charge carriers can be trapped on the
`emitter and form a neutral exciton on-site by attracting a charge
`of the opposite sign. This direct charge transfer to the emitter is
`argued to lead to more efficient OLEDs than excitation by
`energy transfer.6−9 The balance between the two routes is
`determined by the respective energy level alignment of the
`emitter and the host, and can therefore be rationally designed.
`However, due to the large band gap of the deep-blue emitter
`
`and the resulting high triplet energy, the number of promising
`compatible host materials is limited, since an even higher triplet
`energy is necessary for the host to ensure trapping of the
`exciton on the emitter.10,11 Among possible charge-transporting
`units fulfilling this criterion are dibenzofurans and N-phenyl-
`carbazoles.12 Substituents must be attached to these materials
`to prevent crystallization,13 suppress emitter aggregation, and
`optimize their molecular weight for vacuum deposition. The
`main role of the substituents, however, is to adjust the relative
`positions of electron- and hole-transporting levels as well as
`singlet and triplet excited states of the host to those of the
`emitter. The relative level alignment,
`together with the
`processes occurring in the EML, is shown in Figure 1a,b for
`a hole-conducting emitter. Synthetically, level adjustment can
`be achieved by inductive effects14 when strongly electronegative
`substituents (e.g.,
`trifluoromethyl) are used. An alternative
`approach is to exploit the mesomeric effect14 when the frontier
`orbital densities delocalize (by using, e.g.,
`triphenylsilyl
`substituents).
`Apart
`from a suitable level alignment, processability,
`amorphousness, and stability, an adequate charge carrier
`
`Received:
`Published:
`
`June 6, 2012
`July 30, 2012
`
`© 2012 American Chemical Society
`
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`Article
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`Figure 1. (a) Hole- (HT) and electron-transport (ET) levels of emitter and host in the EML. (b) Energy levels of singlets (S) and triplets (T) of the
`emitter emission spectrum and the host excitation spectrum. The following processes are shown assuming a hole (electron)-transporting emitter
`h. (2) Electron transfer is prevented by the barrier Δ
`(host): (1) Hole transfer from emitter to host is avoided by a large barrier, Δ
`e. In order to ensure
`
`h has to be larger than Δe and the Coulomb attraction between the hole on the emitter and the electron on the
`exciton formation on the emitter, Δ
`e, attracting the electron to the emitter cation. In practice Δ
`≈ 1 eV and Δ
`neighboring host should be capable of overcoming Δ
`≈ 0.3 eV. (3) Back-
`h
`e
`transfer of the exciton from the emitter to the host is avoided by the barrier Δ
`t for triplet excitons. This also prevents re-absorption of an emitted
`photon by the host. (4) Carrier recombination and emission of blue light. (c) Chemical structure of BTDF with eight “soft” dihedrals (red). (d)
`Isosurfaces of the LUMO. An isovalue of ±0.007 au allows to visualize small fractions of orbital density on the substituents.
`
`Figure 2. (a) Distributions of differences in electrostatic energies including polarization, ΔEe(h)
`el
`(from a neighbor list), conformational energies,
`ΔEe(h)
`, and reorganization energies, λ
`e(h), for electrons (holes). Mean and variance σ given in eV. Inset shows the distribution of the dihedral angle δ
`cf
`introduced in Figure 1c. (b) Distributions of the logarithm of the transfer integral J for electron and hole transport constructed from diabatic states
`based on dibenzofuran (DBF) core or using the whole molecule (BTDF). The same neighbor list and morphology of 4096 BTDF molecules was
`used for all distributions. Inset: Radial distribution functions g(r) of the centers of mass of the DBF core (DBF) and of phenyl rings of the
`triphenylsilyl groups (PHE).
`
`mobility of the host is required in order to prevent ohmic
`losses. Due to its complexity,
`the effect of
`the attached
`substituents on charge carrier mobility has
`rarely been
`addressed. It is, however, obvious that bulky substituents can
`lead to large spatial separations of π-conjugated systems of
`neighboring molecules. Since electronic couplings decrease
`exponentially with intermolecular separations, one might expect
`very poor charge carrier mobility of the host. The aim of this
`study is to show that the use of the mesomeric effect can
`
`remedy the situation by delocalizing the frontier orbitals over
`the substituents. To do this, we perform a combined
`experimental (admittance spectroscopy) and computer simu-
`lation study of charge transport
`in 2,8-bis(triphenylsilyl)-
`dibenzofuran (BTDF), a typical electron-conducting host
`used in combination with hole-conducting deep-blue emitters.5
`The maximal external quantum efficiencies of such OLEDs are
`above 17% (see the Supporting Information for the device
`charateristics).
`
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`Article
`
`DBF = 0.27 eV) and hence
`e(h)
`
`energy (the dibenzofuran core has λ
`increases electron-hopping rates.
`The distributions of conformational energy differences,
`ΔEe(h)
`, are of Gaussian type with a moderate variance of σ
`cf
`cf
`e(h)
`= 0.04 (0.05) eV and are uncorrelated in space, which allows us
`to draw them from such a distribution in the larger box of 4096
`molecules. Simulations are then additionally averaged over two
`realizations of this disorder.
`Electrostatic contributions to site energy differences are
`calculated using partial charges for charged and neutral
`molecules in the ground state obtained from DFT. Polarization
`contributions are taken into account self-consistently using the
`Thole model33,34 with a cutoff of 3.5 nm between molecular
`centers of mass. This results in polarized electrostatic site
`energy differences, ΔEe(h)
`el
`, that are Gaussian distributed with a
`variance of σ
`el
`= 0.12 (0.11) eV when computed from the
`e(h)
`neighbor list. Rather small energetic disorder and weak spatial
`correlations are due to small variations of atomic partial charges
`(local dipole moments) as well as the total dipole moment of
`BTDF of less than 1 D. Note that the attachment of the
`substituents does not affect the molecular dipole moment. All
`distributions are shown in Figure 2a.
`The remaining ingredient entering the rate expression, eq 1,
`is the transfer integral J, which relies on the definition of
`diabatic states of a pair of molecules. The latter are usually
`constructed from representative orbitals of the π-conjugated
`parts, since the effect of attached substituents on the diabatic
`states is rather small (e.g.,
`in case of alkyl or glycol side
`chains20,24,26). Following this approach, the diabatic states are
`evaluated by substituting triphenylsilyl by a hydrogen (without
`modifying the rest of the morphology). Reorganization energies
`of the dibenzofuran core are used, and transfer integrals are
`then calculated on DFT level with the PBE functional and a
`TZVP basis set using the dimer projection method.35,36 The
`distribution of the logarithm of transfer integrals J (see Figure
`2b) for pairs of the neighbor list is very broad. This can be
`rationalized in terms of morphology, as the transfer integral
`depends exponentially on the intermolecular separation. The
`distance between the dibenzofuran cores is large due to the
`attached bulky substituents, as
`illustrated by the radial
`distribution function for centers of mass of dibenzofuran
`cores, shown in the inset of Figure 2b. The onset of this
`function is at ca. 0.5 nm and has a peak g(r) > 1 at a separation
`larger
`than 1 nm, which eventually leads to the broad
`distribution of J. The small number of high transfer integrals
`due to a few close-lying cores is apparently not sufficient for
`charge percolation. As a consequence, simulations predict low
`e(h) < 4 ×
`mobilities at experimentally relevant electric fields, μ
`10−7 (3 × 10−8) cm2/V·s, which would lead to ohmic losses and
`poor device performance.
`The above assumptions on the nature of the diabatic states
`seem logical but are ultimately invalid. Indeed, if the diabatic
`states are constructed using the frontier orbitals of the entire
`BTDF molecule, the distributions of transfer integrals become
`significantly less broad and peak at much larger values, as
`shown in Figure 2b. As a result, predicted mobilities are much
`≈ 5 × 10−4 (10−5) cm2/V·s, which is in agreement
`higher, μ
`e(h)
`with experiments performed by admittance spectroscopy (AS).
`2.2. Admittance Spectroscopy. AS allows to extract
`mobilities
`in an organic film sandwiched between two
`electrodes by applying dc and ac voltages and finding the
`transit time of the carriers in the film from a maximum in the
`negative differential susceptance −ΔB = ω(C − C0), where C
`dx.doi.org/10.1021/ja305310r | J. Am. Chem. Soc. 2012, 134, 13818−13822
`
`Journal of the American Chemical Society
`
`The chemical structure of BTDF is shown in Figure 1c. The
`two triphenylsilyl groups are attached to a dibenzofuran core,
`which lowers its electron-transport level below that of the
`emitter. The details of the synthesis are described in the
`Supporting Information.
`
`2. RESULTS
`2.1. Computer Simulations. To relate charge carrier
`mobility to the chemical
`structure, atomistic molecular
`dynamics (MD) is used to simulate material morphologies.
`Then the high-temperature limit of Marcus theory15,16 is
`employed to evaluate charge-transfer rates between molecules i
`and j according to
`2
`J
`π
`ij
`ℏ
`k T
`B
`
`ω
`ij
`
`=
`
`λ
`
`ij
`
`(1)
`
`⎤ ⎦⎥⎥
`
`2
`)
`
`ij
`
`−
`
`λ
`Δ −
`E
`(
`ij
`λ
`k T
`4
`B
`
`ij
`
`⎡ ⎣⎢⎢
`
`exp
`
`is the electronic coupling
`where T is the temperature, Jij
`element, or transfer integral, and ΔEij
`is the site energy
`difference which has contributions due to an applied electric
`field, electrostatics including polarization, ΔEel, and internal
`energy differences due to molecular conformations, ΔEcf.
`Finally, λ
`ij is the reorganization energy which is dominated by
`intramolecular contributions due to a small Pekar factor.17
`More information about the transport parameters can be found
`in the Supporting Information.
`The rates and molecular centers of mass are used in kinetic
`Monte Carlo simulations to solve the master equation for a
`charge drift-diffusing in a box with periodic boundary
`conditions in an applied electric field F. The charge carrier
`mobility is then determined as μ = ⟨v⟩/F, where ⟨v⟩ is the
`averaged projection of the carrier velocity on the direction of
`the field.
`Simulated mobilities are averaged over two MD snapshots,
`ten injection points, and six different spatial directions of the
`field. More details are given in the Supporting Information.
`Simulations are performed using the VOTCA package.17,18
`This approach has been used to calculate mobility in columnar
`systems,25−28
`discotic mesophases,19−24 amorphous
`self-
`assembled monoloayers,29 and conjugated polymers.30,31
`An amorphous morphology of 4096 BTDF molecules is
`obtained by first annealing the system at 700 K, well above the
`followed by fast
`glass transition temperature, Tg = 380 K,
`quenching to room temperature. The final length of the cubic
`box is L = 16 nm. To determine intermolecular charge-hopping
`rates in this morphology, a neighbor list based on the closest
`approach of centers of mass between phenyl
`rings or
`dibenzofuran cores is constructed using a cutoff of 0.7 nm.
`The parameters entering the rate expression eq 1 are then
`calculated for each molecular pair from the neighbor list.
`Since BTDF has soft degrees of freedom, such as dihedrals δ
`in Figure 1c, molecules in the amorphous phase have different
`conformations. The distribution of this dihedral angle is shown
`in the inset of Figure 2a. These conformations are frozen on the
`time scale of charge transport (see the Supporting Information
`for details). Reorganization energies λ
`ij and internal energy
`differences ΔEij
`cf are therefore computed from potential energy
`surfaces of 512 molecules in neutral and charged states making
`use of density functional theory (DFT). We find a small
`variance in reorganization energies which does not affect the
`mobility. Hence, the mean values of λ
`e(h) = 0.19 (0.27) eV for
`electrons (holes) are used. Due to delocalization effects,32
`attaching the triphenylsilyl groups decreases the reorganization
`
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`
`Journal of the American Chemical Society
`
`and C0 are the ω-dependent capacitance of the organic layer
`and its zero-frequency analogue37,38 (see the Supporting
`Information for details of device preparation and mobility
`extraction). Both experimental and simulated mobilities for
`electrons and holes are shown in Figure 3. The agreement is
`
`Article
`
`beneficial, since it practically does not change the charge
`distribution of the conjugated core. One might argue that the
`mesomeric effect
`leads
`to an additional, conformational
`disorder of site and reorganization energies due to soft
`molecular degrees of
`freedom frozen in an amorphous
`morphology. This disorder is, however, small compared to
`the electrostatic disorder, is uncorrelated in space, and therefore
`has a minor effect on charge transport.17
`To summarize, we suggest using the mesomeric effect to
`adjust
`the energy levels via side group attachments
`to
`conjugated cores. It substantially improves electronic couplings
`in the host by delocalizing the frontier orbitals,
`reduces
`reorganization energy, and does not
`lead to significant
`additional energetic disorder.
`
`■ ASSOCIATED CONTENT
`*S Supporting Information
`Details of synthesis, MD simulations, force-field parameters,
`charge-transport simulations, and admittance spectroscopy.
`This material
`is available free of charge via the Internet at
`http://pubs.acs.org.
`
`■ AUTHOR INFORMATION
`
`Figure 3. Simulated electron (black) and hole (red) mobility μ as
`function of applied field F, with diabatic states based on the whole
`BTDF molecule (solid) or the DBF core (dashed), respectively. For
`BTDF, error bars computed from a bootstrap analysis are smaller than
`the size of
`the data points. Experimental data obtained from
`admittance spectroscopy at room temperature on films of thickness
`d are shown for comparison.
`
`the relative electron/hole mobilities, and
`for
`excellent
`experimentally measured values are closer to the scenario
`where the substituents are incorporated in the diabatic states.
`Both simulations and experiments have small Poole−Frenkel
`slopes, indicating small energetic disorder. Simulated electron
`mobilities are higher due to stronger delocalization leading to λ
`e
`≈ σ
`
`h, and Je2 ≫ Jh
`< λ
`2, while σ
`h.
`e
`3. DISCUSSION AND CONCLUSIONS
`We now discuss the discrepancy between the two approaches,
`that is, including/excluding the substituents in the definition of
`diabatic states. The reason for the much higher mobility in the
`first case is the mesomeric effect, which delocalizes the frontier
`orbitals over the silicon atom to the substituents, as shown in
`Figure 1d. A Mulliken population analysis39 indicates that a
`fraction of only 10 (4)% of LUMO (HOMO) populates the
`substituents (see the Supporting Information). Although such
`small delocalization can be easily overlooked on a single-
`molecule level, the effect on electronic couplings is much more
`pronounced since the substituents are in a closer contact than
`the cores. This is illustrated in the inset of Figure 2b, where the
`radial distribution function for centers of mass of phenyl rings is
`shown. Smaller
`separations boost electronic couplings
`exponentially and dramatically increase charge mobility.
`The charge-transfer
`rate also depends on site energy
`differences. It is therefore essential to minimize variations of
`local dipole moments which lead to spatially correlated
`energetic disorder.40,41 Here the mesomeric effect
`is also
`
`Corresponding Author
`christian.lennartz@basf.com; denis.andrienko@mpip-mainz.
`mpg.de
`Notes
`The authors declare no competing financial interest.
`
`■ ACKNOWLEDGMENTS
`
`This work was partially supported by the DFG program IRTG
`1404, DFG grant SPP 1355, and BMBF grant MESOMERIE.
`We are grateful
`to Mara Jochum, Kostas Daoulas, Carl
`Poelking, Pascal Kordt, and Nicolle Langer for critical reading
`of the manuscript.
`
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`Duk-San v. Idemitsu Kosan