Diphenanthroline Electron Transport Materials for the Efficient Charge

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article pubs.acs.org/cm

Diphenanthroline Electron Transport Materials for the Efficient Charge Generation Unit in Tandem Organic Light-Emitting Diodes Gyeong Woo Kim,† Young Hoon Son,† Hye In Yang,† Jin Hwan Park,† Ik Jang Ko,† Raju Lampande,† Jeonghun Sakong,‡ Min-Jae Maeng,‡ Jong-Am Hong,‡ Ju Young Lee,† Yongsup Park,*,‡ and Jang Hyuk Kwon*,† †

Department of Information Display and ‡Department of Physics and Research Institute for Basic Sciences, Kyung Hee University, Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Korea S Supporting Information *

ABSTRACT: In this paper, we report two new phenanthroline-based compounds, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (pbPPhenB) and 1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (mbPPhenB), for the charge generation unit of tandem organic light-emitting diodes (OLEDs). These two compounds exhibited high electron mobility of (5.8−4.4) × 10−3 cm2/(V s), a very small injection barrier at the p−n junction interface, a high glass transition temperature of 123.9−182.1 °C, and exceptionally good operational stability. Because of such excellent characteristics, a single-stack red phosphorescent OLED (PhOLED) with pbPPhenB showed a low driving voltage (2.7 V) and significantly improved maximum power efficiency (56.8 lm/W), external quantum efficiency (30.8%), and device lifetime (LT95, 130 h) compared to those of the control device using bathophenanthroline (Bphen) (3.7 V, 39 lm/W, 27.1%, and 13 h). Furthermore, a two-stack (tandem) red PhOLED using p-bPPhenB in the charge generation unit exhibited superior charge generation as well as electron transport properties and excellent device performances (5.0 V, 54.0 lm/W, 56.1%) compared to those of the tandem device using Bphen (6.2 V, 45.2 lm/W, 53.3%).

1. INTRODUCTION Recently, tandem organic light-emitting diodes (OLEDs) have been regarded as the most promising technology for large-size displays and lighting applications because of their superior efficiency and operational stability compared to those of conventional single electroluminescent (EL) unit based OLEDs.1−5 Normally, in tandem OLEDs, individual EL units are vertically stacked and electrically connected via charge generation unit (CGU). Due to this series connection, the quantum efficiency of tandem OLEDs can be enhanced in proportion to the number of stacked EL units. Moreover, by separating the EL units through the CGU, it is possible to effectively suppress triplet−triplet and triplet−polaron quenching and reduce electrical stress compared to those of conventional OLEDs at the same luminance because of shared current flow in each EL unit. As a result, tandem OLEDs could have prolonged operational stability with enhanced quantum efficiency.6,17−19,21,31,74,75 Such dramatic improvement in the EL performances of tandem OLEDs is possible only if the CGU makes an ideal series connection between separated EL units and supplies charges to the EL units without electrical and optical losses. For the efficient charge generation in tandem OLEDs, generally the CGU requires an interface that is a p−n heterojunction between the p-doped hole-transporting layer (HTL) and n-doped electron-transporting layer (ETL). In this CGU structure, holes and electrons can be generated at the © 2017 American Chemical Society

highest occupied molecular orbital (HOMO) of the p-doped HTL and lowest unoccupied molecular orbital (LUMO) of the n-doped ETL, respectively, through external electric field assisted electron injection from the HOMO to the LUMO.7,8 However, this type of CGU structure showed inefficient charge generation because of the wide depletion region at the p−n heterojunction with a large energy barrier. To address this issue, an alternative approach, a p−d−n heterojunction type CGU structure (p-doped HTL/deep-lying LUMO material (DLM) layer/n-doped ETL), was developed and broadly used for recent research on tandem OLEDs.9,10,16,74 MoO3,11,12 WO,13 V2O3,14,15 ReO3,16 FeCl3,17 and F4-TCNQ18,19 were extensively used as p-dopant materials. On the other hand, Li,21,28,29 Mg,22,23 Ca,24 LiH,25 LiNH2,26 Li2CO3,27 Cs2CO3,28,29 CsN3,30 and Rb2CO316,31 were reported as n-dopants. Recently, HATCN was reported as a good DLM.20,25,26,74,75 The inserted DLM divides the charge generation process into two steps, charge generation and electron injection, by making two different heterojunctions, p−d (HTL/DLM layer) and d−n (DLM layer/n-doped ETL). Normally, the DLM layer makes a very small charge generation barrier at the interface of p−d due to the small energy difference between the HOMO of the HTL and LUMO of the DLM. However, a large electron injection Received: June 26, 2017 Revised: September 10, 2017 Published: September 11, 2017 8299

DOI: 10.1021/acs.chemmater.7b02655 Chem. Mater. 2017, 29, 8299−8312

Article

Chemistry of Materials

where k and ℏ represent the Boltzmann and Planck constants, respectively, T is the temperature, V is the electronic coupling matrix element between electron donor and acceptor molecules, and λe is the reorganization energy (RE) for electron transition.41−46 According to this equation, λe and V are key parameters to determine the electron mobility. In other words, to achieve high electron mobility in an amorphous organic matrix, the organic compound must have a small RE and a planar molecular shape, which are favorable for molecular packing and electronic coupling. Hence, these terms should be evaluated to estimate the electron-transporting property of the molecule. Finally, in terms of operational stability, the Tg of the Phen derivatives should be at least over 100 °C to prohibit Joule heating induced degradation and a molecular structure change.47−50 Therefore, new ETMs need to be more heavy and rigid than Bphen. 2.1.2. Molecular Design Based on DFT (Density Functional Theory) Calculation. To develop ETL materials with excellent ability to be doped n-type, high electron mobility, and good operational stability, several Phen derivatives were designed, and their nucleophilic nature and electron-transporting properties were estimated by calculating the molecular electrostatic potential (ESP), electron RE, and geometrically optimized three-dimensional (3D) molecular structure. It has been previously reported that the lower maximum negative ESP and RE can result in higher nucleophilicity and electron mobility.41−46,79,80 All related theoretical calculations were performed through DFT simulation by using the generalized gradient approximation with the Perdew−Burke−Ernzerhof (GGA-PBE) functional and the double numerical plus dfunctions (DND) basis set in DMol3 of the Materials Studio Package.51,52 To determine the influence of the phenyl substitution position of the Phen derivative on the n-doping and electron-transporting properties, we calculated the maximum negative ESPs and electron REs of the five Phenbased moieties, 4-DPPhen, 2-PPhen, 4,7-DPPhen (or Bphen), 2,4-DPPhen, and 2,9-DPPhen, with altered substitution positions and numbers of phenyl substituents, where only the 2-, 4-, 7-, and 9-positions are used for the phenyl substitution because the other positions are not active in the substitution reaction. As shown in Figure 1, the phenyl substitution position and the number of phenyl substituents not only significantly change

barrier at the interface of d−n can be generated because of the large energy gap between the two LUMO levels of the DLM and n-doped ETL. Hence, a function of the d−n heterojunction determines the electron injection barrier, and it mainly relies on the property of the n-doped ETL. Excellent n-doped ETL performance successfully leads to an abrupt downward shift in the LUMO level of the ETL with a narrow depletion region, which improves electron tunneling via a reduced energy barrier.31,58,59 Until now, several electron-transporting materials (ETMs) such as TmPyPB, B3PYMPM, TPBi, Alq3, BCP, and Bphen derivatives have been applied for the n-doped ETL in the CGU structure. Among them, Bphen has been reported as the best ndoped ETL material; however, Bphen has several issues such as low electron mobility (5.4 × 10−4 cm2/(V s)),34,35 weak thermal stability with a low glass transition temperature (Tg = 62 °C),36 and poor device stability. Indeed, there has been almost no research aimed to develop new ETMs for the CGU to date. Therefore, new ETMs are highly desired for highly efficient and stable tandem OLEDs. In this study, we report highly efficient ETMs through our useful molecular design approach to attain high electron mobility and high Tg as well as a good ability to be doped n-type for proficient electron injection/transport in the CGU.

2. RESULTS AND DISCUSSION 2.1. Molecular Design. 2.1.1. Molecular Design Strategy of the ETM for the CGU. For the efficient charge generation and electron transport/injection, ETL materials should satisfy the following key requirements: (i) good ability to be doped ntype for reducing the electron injection barrier; (ii) high electron mobility to adjust the charge balance in the EL units; (iii) good operational stability for use in real applications. To attain a proficient ability to be doped n-type, it is necessary to introduce a specific molecular structure to effectively interact with the metal-based n-dopants and easily accept free electrons from n-dopants. In this regard, 1,10-phenanthroline (Phen)based derivatives are very interesting because Phen is a highly reactive ligand to make metal complex compounds.37−39 These materials (for example, Bphen) generally show a good ability to be doped n-type. Two closely existing nitrogen atoms in the Phen moiety have highly nucleophilic character, which leads to a superior metal cation coordinating property compared to those of heteroaromatic-based ETL materials, for instance, TmPyPB and 1,3,5-tris(p-pyrid-3-ylphenyl)benzene.40 This indicates that the Phen moiety is favorable to form a complex with the cation of the metallic dopant, and it also promotes electron transfer from the n-dopant to the ETL, resulting in an excellent ability to be doped n-type. Therefore, the ability to be doped n-type should be considered as one of the appraisal criteria for designing new ETL materials. In addition, to improve the electron mobility of Phen-based materials, the molecule should be designed on the basis of the insight into the intermolecular charge transport mechanism in an amorphous organic matrix. In fact, the intermolecular charge transport in organic semiconductors follows the hopping process, and the electron transfer rate (κe) can be described by Marcus theory as shown in the following equation:

Figure 1. Comparison of the maximum negative ESP and electron RE of three 1,10-phenanthroline derivatives possessing different substitution positions and numbers of phenyl substituents. The maximum negative ESP and electron RE are related to the ability to be doped ntype and electron mobility, respectively, and the ability to be doped ntype and electron mobility are improved as the maximum negative ESP increases and electron RE decreases, respectively.

⎛ π ⎞1/2 V 2 ⎛ λ ⎞ exp⎜ − e ⎟ κe = ⎜ ⎟ ⎝ 4kT ⎠ ⎝ λekT ⎠ ℏ 8300


Article


Therefore, these two materials are not suitable for the basic building block. On the other hand, 2-PPhen and 2,4-DPPhen show moderate values for both of the properties as compared to 4,7-DPPhen and 2,9-DPPhen. Moreover, charge transfer interaction with the n-dopant is not fully interrupted by the shielding effect because only one of the two embedded nitrogen atoms is shielded by the phenyl unit. These results mean that a good ability to be doped n-type and high electron mobility could be achieved in these two moieties. For further enhancing the electron-transporting property and thermal stability, the Phen derivative needs a more extended πconjugation length with a heavier and more rigid molecular structure. Hence, we finally designed and synthesized two molecules, p-bPPhenB and m-bPPhenB, by linking 2-PPhen to the 4-phenyl of 2,4-DPPhen at the para- and meta-positions, respectively. The calculated maximum negative ESPs, electron REs, and measured electron mobilities of new designed molecules and conventional ETL materials (Bphen and TmPyPB) are compared in Figure 2b as well as Table 1, and

the RE but also alter the maximum negative ESP value. Phenyl substitution at the 2- and 9-positions of Phen tends to reduce both the RE and maximum negative ESP, while phenyl substitution at the 4- and 7-positions of Phen shows the opposite tendency. To clarify the exact reason behind changes in the RE and maximum negative ESP by the phenyl substitution position, the conformation changes between the neutral and anionic states of the Phen derivatives are compared in Figure S8 and Table S1 (Supporting Information). In general, the longer conjugation length and smaller conformational change between the neutral and charge states results in a smaller RE.82−85 The RE is more influenced by a change in the torsional angle than the bond length of the Phen core.86 Phenyl substitution in all the positions of Phen increases the change in the bond length of the Phen core, but the change in the torsional angle between the Phen and phenyl is dependent on the substitution position of the phenyl. Therefore, the torsional angle is a more important parameter than the bond length in anticipating the RE of molecules. The biggest difference between the phenyl substitution at the 2- and 9-positions and 4- and 7-positions is the presence or absence of an intramolecular hydrogen bond. In the case of phenyl substitution at the 2- and 9- positions, the neighboring nitrogen of Phen and hydrogen of phenyl generate an intramolecular hydrogen bond. Such an intramolecular interaction locks the conformational change and minimizes the dihedral angle between the Phen and phenyl, which results in extended πconjugation of Phen through a substituted phenyl and a reduced RE of 2-PPhen and 2,9-DPPhen.87−90 Moreover, the near nitrogen and hydrogen distance due to the intramolecular hydrogen bond lowers the maximum negative ESP of the molecules in spite of the weak electron-donating property of phenyl as shown in Figure S9 (Supporting Information). However, in the case of phenyl substitution at the 4- and 7positions, the phenyl substituents do not generate an intramolecular hydrogen bond and make a large dihedral angle with Phen owing to repulsion between the hydrogens of Phen and phenyl, which limits π-conjugation between the Phen and phenyl but slightly increases the maximum negative ESP of 4-PPhen and 4,7-PPhen due to the weak electron-donating property of the phenyl. The absence of a locking interaction (intramolecular hydrogen bond) allows the phenyl to rotate as the molecules accept an electron, resulting in an increased RE of the molecules. In the case of 2,4-DPPhen, which has phenyls at the 2- and 4-positions, the RE and maximum negative ESP are strongly affected by the phenyl at the 2-position. The RE and maximum negative ESP are slightly increased due to the phenyl at the 4-position. These results indicate that the phenyl substitution at the 2- and 9-positions improves the electrontransporting property but deteriorates the ability to be doped ntype, while the phenyl substitution at the 4- and 7-positions enhances the ability to be doped n-type but deteriorates the electron-transporting property. In terms of electron mobility, 2,9-DPPhen seems to be the best basic building block for good ETMs. However, 2,9-diphenyl substitution increases the maximum negative ESP by a large margin. In addition, it may further deteriorate the electron transfer interaction between Phen and n-dopants because the close proximity of the 2,9diphenyl substituents with the two embedded nitrogen atoms in Phen can limit the accessibility of the n-dopant to the embedded nitrogen atoms by the shielding effect of the two phenyl moieties. On the contrary, 4,7-DPPhen exhibits a lower maximum negative ESP value but a too high electron RE.

Figure 2. (a) Molecular structures, 3D molecular structures (side view), LUMO distributions, and electrostatic potential map (EPM) of the four ETL materials. (b) Comparison of the electron RE, electron mobility, and maximum negative ESP of four ETL materials, Bphen, pbPPhenB, m-bPPhenB, and TmPyPB.

their optimized 3D molecular conformations, LUMO distributions, and ESP maps are shown in Figure 2a. As expected, by linking the 2-PPhen and 2,4-DPPhen moieties, the electron REs and maximum negative ESPs are significantly reduced compared to those of 2-PPhen and 2,4-DPPhen. As compared to m-bPPhenB, p-bPPhenB exhibits a more reduced RE, resulting from a smaller change in the torsional angles of the phenyls at the 2-position as shown in Figure S10 and Table S2 (Supporting Information). Herein, both m-bPPhenB and pbPPhenB show lower electron REs than TmPyPB (high electron mobility, 10−3 cm/(V s)) as well as Bphen and higher maximum negative ESPs compared with Bphen but lower maximum negative ESPs than TmPyPB. Hence, a high electron 8301


Article

Chemistry of Materials Table 1. Maximum Negative ESP, Electron RE, and Electron Mobility of ETL Materials

max negative ESPa (a.u.) electron REa (eV) electron mobility [cm2/(V s)]

Phen

4-PPhen

2-PPhen

2,9-DPPhen

2,4-DPPhen

−0.1050 0.109

−0.1058 0.172

−0.0894 0.106

−0.0722 0.084

−0.0926 0.107

4,7-DPPhen (Bphen)

p-bPPhenB

m-bPPhenB

TmPyPB

−0.1089 0.120 5.5 × 10−4b

−0.0918 0.056 5.8 × 10−3b

−0.0946 0.069 4.4 × 10−3b

−0.0690 0.071 1.0 × 10−3c

5.4 × 10−4c a

b

Calculated value using DFT simulation. Electron mobility measured using the SCLC method. cReported values from refs 34 and 35.

relative photophysical properties are presented in Figure 3a and Table 2, respectively. The UV−vis absorption spectra of Bphen, m-bPPhenB, and p-bPPhenB are almost identical in shape, and weak absorption peaks at around 310−320 nm are assigned to the n−π* transition from the phenanthroline moiety, while absorption peaks at shorter wavelength (270−280 nm) are attributed to the π−π* transition from the phenyl and phenanthroline moieties.53,54 Due to the increased conjugation length, the absorption band edges of m-bPPhenB and p-bPPhenB are observed in a longer wavelength region than that of Bphen, which indicates that the optical Eg values of these two compounds are reduced compared to that of Bphen. Similarly, the room- and low-temperature PL peaks of m-bPPhenB and pbPPhenB are also red-shifted compared to that of Bphen owing to the increased conjugation length. The ET values of mbPPhenB and p-bPPhenB are determined to be 2.50 and 2.51 eV, respectively, which are high enough to confine triplet excitons in the emissive layer (EML) of red and green phosphorescent OLEDs (PhOLEDs). It is interesting to note that both m-bPPhenB and pbPPhenB show a huge difference between the solution and film state PL spectra, while Bphen exhibits similar solution and film state PL spectra. Similarly, a well-known ETL material (TmPyPB), possessing high electron mobility, also exhibits a large red-shifted PL spectrum in the film state. These kinds of large red shifts in the film state PL generally occurred in an organic matrix with close intermolecular interaction between the neighboring molecules.55−57 As shown in Figure 2a, the 3D molecular structures of m-bPPhenB, p-bPPhenB, and TmPyPB have wide and planar molecular shapes, which are favorable for a horizontal molecular orientation and molecular packing. The molecular planarity of the three molecules is in the order pbPPhenB > m-bPPhenB > TmPyPB, and the magnitude of the red shift shows the same tendency as the planarity. On the other hand, a large red shift in the film state PL of Bphen was not observed, even though Bphen has a very planar molecular shape. This is attributed to its compact molecular size. Normally, molecules with a compact size are unfavorable for a horizontal molecular orientation and have weak molecular interaction due to the poor π−π stacking between the molecules, resulting in low charge mobility.76−78 These PL and DFT simulation results indicate that the molecules in mbPPhenB and p-bPPhenB films are strongly connected to each other through intermolecular π−π stacking. This feature is one of the mobility-enhancing factors related to the V term of the Marcus theory. Hence, high electron mobilites are expected in m-bPPhenB and p-bPPhenB films. 2.3.2. Electrochemical Properties. To estimate the HOMO levels of new molecules, cyclic voltammetry (CV) measurements were performed. From the cyclic voltammograms (Figure S7, Supporting Information), the onset oxidation

mobility and good ability to be doped n-type are anticipated for m-bPPhenB and p-bPPhenB. Additionally, both new compounds (m-bPPhenB and p-bPPhenB) have planar molecular shapes, which can be helpful to achieve a good electroncoupling matrix element V between two molecules, thus expecting high electron mobility. Moreover, both new molecules have two Phen moieties instead of one, which can be beneficial for the good thermal stability owing to their high molecular weight. 2.2. Synthesis of m-bPPhenB and p-bPPhenB. The synthetic routes of m-bPPhenB and p-bPPhenB are shown in Scheme 1. 2-Phenyl-1,10-phenanthrolin-4-ol was synthesized Scheme 1. Synthetic Schemes of m-bPPhenB and p-bPPhenB

by using an acid-catalyzed coupling reaction and cyclization. Chlorination of the resulting compound gave 4-chloro-2phenyl-1,10-phenanthroline, which was then reacted with 1,3and 1,4-diboronic esters to produce m-bPPhenB and pbPPhenB, respectively. The detailed procedure and structural characterization of m-bPPhenB and p-bPPhenB are described in the Supporting Information. 2.3. Material Properties of m-bPPhenB and pbPPhenB. 2.3.1. Photophysical Properties. To determine the photophysical properties of four ETL materials, Bphen, mbPPhenB, p-bPPhenB, and TmPyPB, UV−vis absorption and photoluminescence (PL) spectra were measured in a 2methyltetrahydrofuran (2-MeTHF) solution and solid film state at 278 K. The optical band gap energies (Eg) were estimated from the absorption band edges of the UV−vis absorption spectra. The triplet energies (ET) were determined from the highest energy vibronic peak coming from the T1−S0 electronic transition of the phosphorescent spectra in the 2MeTHF matrix at 77 K. All the measured optical spectra and 8302


Article


following formula: EHOMO = −4.8 − (Eox − EFc), where Eox and EFc represent the measured onset oxidation potentials of the material and ferrocene, respectively. The increased conjugation lengths of p-bPPhenB and m-bPPhenB raise their HOMO levels by 0.37−0.39 eV compared to that of Bphen. The LUMO levels of these three materials are calculated by subtracting Eg from the HOMO energy level (even though this method has an error as high as the exciton binding energy81). Despite relatively large HOMO energy differences between Bphen and the two new ETMs, the LUMO energy level differences are not significantly large due to the reduced Eg of the new ETMs. All the estimated energy levels are summarized in Table 2. 2.3.3. Thermal and Morphological Properties of mbPPhenB and p-bPPhenB. The thermal properties of pbPPhenB and m-bPPhenB were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (Table 2). The decomposition temperature (Td), melting point (Tm), and Tg were remarkably improved compared to those of Bphen. Particularly, the Tg values of pbPPhenB and m-bPPhenB are found to be 123.9 and 182.1 °C, respectively, and these values are significantly higher than that of Bphen (62 °C). The higher Tg of m-bPPhenB than that of pbPPhenB could result from different linking positions between two PPhen moieties and central phenyl linker of the compounds. The meta-linkage is normally more effective than the para-linkage in restricting rotation and rotational motion of the central phenyl linker, thereby causing enhanced rigidity of the molecule. The improvements in Tg values are attributed to the higher molecular weight and more rigid structure and will be beneficial to improve the operational stability of OLEDs. The morphology of the organic film is one of the important properties which affects the mobility and stability of the OLED device. To confirm the film morphology of p-bPPhenB and mbPPhenB, the topologies of the surface of 100 nm thick Bphen, p-bPPhenB, and m-bPPhenB were investigated by using atomic force microscopy (AFM). As shown in Figure S12 (Supporting Information), all the films show a very smooth surface. The RMS roughness of the films is in the order Bphen (0.852 nm) > m-bPPhenB (0.695 nm) > p-bPPhenB (0.525 nm). The planar and wide molecular structures of p-bPPhenB and m-bPPhenB might assist in the formation of such smooth films, and the smooth-film-forming property of p-bPPhenB and m-bPPhenB is beneficial to attain high charge mobility and better stability. 2.3.4. Electron-Transporting Properties. To compare the electron-transporting properties of Bphen, p-bPPhenB, and mbPPhenB, electron-only devices (EODs) were fabricated with the following configuration: Al (100 nm)/Cs2CO3 (1 nm)/ Bphen, p-bPPhenB, or m-bPPhenB (100 nm)/Cs2CO3 (1 nm)/Al (100 nm). From the electron dominant current density versus voltage (J−V) characteristics in Figure S11 (Supporting Information), well-fitted space charge limited current (SCLC) regions (J ∝ V2) were confirmed, and then electric-fielddependent mobilities were evaluated by using the SCLC model.69−71 As presented in Figure 3e and Table 1, p-bPPhenB and m-bPPhenB exhibit very high electron mobilities (5.8 and 4.4 × 10−3 cm2/(V s) at 0.3 MV/cm, respectively). These values are almost 1 order higher than that of Bphen (5.5 × 10−4 cm2/(V s)) and considerably higher than the reported electron mobility of TmPyPB (1.0 × 10−3 cm2/(V s)).34,35 According to the Marcus theory, charge mobility is inversely proportional to the RE but directly proportional to the electronic coupling term. The electron mobilities of these four ETMs are in the order p-bPPhenB > m-bPPhenB > TmPyPB > Bphen. This

Figure 3. (a) UV−vis absorption in the solution state and PL spectra in the solution and solid film states at room temperature (RT; 300 K) and low temperature (LT; 77 K) for Bphen, p-bPPhenB, m-bPPhenB, and TmPyPB. (b) DSC and TGA traces of p-bPPhenB and mbPPhenB. (c) Electron mobility of Bphen, p-bPPhenB, and mbPPhenB derived from the current density versus voltage characteristics of EODs by using the SCLC method.

potentials versus Ag/AgNO3 reference potential of Bphen, pbPPhenB, and m-bPPhenB are observed at 1.85, 1.42, and 1.44 eV, respectively. The HOMO levels are derived by using the 8303


Article

Chemistry of Materials Table 2. Photophysical, Electrochemical, and Thermal Properties of ETL Materials Bphen p-bPPhenB m-bPPhenB TmPyPB

Ega (eV)

EHOMO (eV)

ELUMO (eV)

3.60 3.36 3.37 3.95i

−6.65 −6.22 −6.24 −6.68i

−3.01 −2.88 −2.87 −2.73i

ETb (eV)

solution PL λmaxc (nm)

2.60 2.50 2.51 2.81

381 390 387 352

film PL λmaxd (nm) 387 460 452 408

Δλmaxe (nm) 6 70 65 56

Tgf (°C) i

62 123.9 182.1 79i

Tmg (°C) i

220 395.3 368.6 181i

Tdh (°C) 240i 442.6 433.5 457i

Optical band gap. bTriplet energy. cWavelength of the maximum peak of solution PL. dWavelength of the maximum peak of film PL. eDifference between λmax(solution PL) and λmax(film PL). fGlass transition temperature. gMelting point. hDecomposition temperature. iReported value from refs 34−36. a

order is inversely proportional to the order of the calculated REs of these materials (Figure 2b) and directly proportional to the electronic coupling properties deduced from the planarity of the 3D molecular shape (Figure 2a) and the magnitude of the red-shifted PL (film state) spectra of the four molecules (Figure 3a). The overall tendencies between measured electron mobilities and calculated electron REs or electronic coupling properties of the four molecules are in good agreement with the Marcus theory. Therefore, it can be concluded that the high electron mobilities of p-bPPhenB and m-bPPhenB are ascribed to their small REs and planar molecular shape enhancing molecular packing and electronic coupling properties in the solid matrix. 2.4. Charge Generation Performance. 2.4.1. Charge Generation Abilities of CGU Devices. To evaluate the effect of ETL materials on the charge generation property of the CGU, we fabricated four CGU devices, CGU-(p), CGU-(m), CGU(B), and CGU-(T), with different ETL materials, p-bPPhenB, m-bPPhenB, Bphen, and TmPyPB, respectively. Figure 4 presents the energy level diagram with the current flow mechanism (a) and J−V characteristics (b) of the four CGU devices. As shown in Figure 4a, under forward bias, current flow in the CGU device is only attributed to generated charge carriers at the HATCN/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) interface because charge carrier injections from indium tin oxide (ITO) and Al are prevented due to the large energy barrier at the ITO/ETL and TAPC/Al interfaces. The difference in current densities of the devices at the same applied voltage is only affected by the electron mobility of the ETL material and electron injection barrier at the 5% Li-doped ETL (n-ETL)/HATCN interface because all CGU devices have the same charge generation contact (HATCN/TAPC). Before comparing the charge generation properties of the CGU devices, we fabricated one more CGU device without the ndoped ETL, CGU-(b), with the following structure: ITO (150 nm)/Bphen (40 nm)/HATCN (7 nm)/TAPC (25 nm). To confirm the influence of the n-doped ETL on the CGU performances, the J−V characteristics of the CGU-(B) and CGU-(b) devices are compared. In the case of CGU-(B), the current flow is turned on at 3.5 V and the current density reaches 10 mA/cm2 at 4.9 V, while CGU-(b) is not turned on even at 7 V, which indicates that the large electron injection barrier is formed at the Bphen/HATCN interface of CGU-(b) and such a high energy barrier blocks electron flow from HATCN to Bphen. This result apparently shows the role of the n-doped ETL as an electron injection layer and its importance in the CGU. In addition to the electron injection barrier, electron mobility of the ETL material is also a crucial and performancedetermining factor in the CGU device. Indeed, the J−V characteristics of the CGU devices are significantly changed according to the applied ETL materials as shown in Figure 4b.

Figure 4. (a) Device structure with the approximate energy level diagram of the CGU device. The figure schematically shows current flow in the CGU device under forward bias. (b) Current density versus voltage (J−V) characteristics of the CGU devices based on p-bPPhenB (CGU-(p)), m-bPPhenB (CGU-(m)), TmPyPB (CGU-(T)), Bphen (CGU-(B)), and Bphen without a Li-doped Bphen layer (CGU-(b)).

The driving voltages (observed at a current density of 10 mA/ cm2) of the CGU devices are in the order CGU-(p) (4.2 V) < CGU-(m) (4.5 V) < CGU-(B) (4.9 V) < CGU-(T) (6.5 V). Here, it is noteworthy that the driving voltages of CGU-(p) and CGU-(m) are significantly lower than those of CGU-(B) and CGU-(T), while the turn-on voltages of all three devices except CGU-(T) are similar at about 3.5 V. By considering the tendency of the turn-on voltages, the CGU devices seem to have similar electron injection barriers at the n-doped ETL/ HATCN interfaces. However, the lower driving voltages of CGU-(p) and CGU-(m) compared to CGU-(B) are ascribed to the higher electron mobilities of p-bPPhenB and m-bPPhenB. This result is very meaningful, given the fact that Bphen has been the best ETL material among reported ETLs in the CGU.58,59,74 Meanwhile, an important reason for the inferior charge generation property of CGU-(T) can be attributed to the ability of TmPyPB to be doped n-type. As mentioned in 8304


Article


Figure 5. UPS spectra of the (a) 5% Li-doped Bphen/Bphen, (b) HATCN/5% Li-doped Bphen, and (c) TAPC/HATCN interfaces of CGU-(B) and the (d) 5% Li-doped p-bPPhenB/p-bPPhenB, (e) HATCN/5% Li-doped p-bPPhenB, and (f) TAPC/HATCN interfaces of CGU-(p) as a function of incremental deposited layers. The left and right panels of each figure display UPS spectra near the photoemission onset and HOMO region, respectively. The insets of (a) and (d) show enlarged figures of photoemission onset of the HOMO region of the 5% Li-doped Bphen/Bphen and 5% Li-doped p-bPPhenB/p-bPPhenB interfaces, respectively. Schematic energy level diagrams: (g) 5% Li-doped Bphen/HATCN/TAPC and (h) 5% Li-doped p-bPPhenB/HATCN/TAPC interfaces.

Bphen, whereas TmPyPB possesses a much weaker nucleophilic nature compared to the three ETL materials. These simulation results are in good agreement with the turn-on voltage of the CGU devices. Accordingly, it can be concluded that the weak n-doped ability of TmPyPB resulted from its weak nucleophilic nature, leading to a relatively higher electron injection barrier at the n-doped ETL/HATCN interfaces of CGU-(T). 2.4.2. Electronic Structures and Charge Generation and Electron Injection Mechanism of CGUs. To completely understand the real origin of the excellent charge generation

section 2.1.2, the n-doping abilities of ETL materials can be approximately estimated from the maximum negative ESP values of the materials. The maximum negative ESP values of the four ETL materials are in the order Bphen (−0.10890 a.u. ) < m-bPPhenB (−0.09464 a.u.) < p-bPPhenB (−0.09181 a.u.) < TmPyPB (−0.06899 a.u.) as shown in Figure 2. Here, the maximum negative ESP values of m-bPPhenB and p-bPPhenB are higher than but comparable with that of Bphen, while the difference between the maximum negative ESPs of these three materials and TmPyPB is quite large; i.e., the nucleophilicities of m-bPPhenB and p-bPPhenB are comparable with that of 8305


Article


gressively moves toward the low BE side, and the HOMO level shift is saturated to 1.55 eV after deposition of 4 nm thick HATCN, which means that a 4 nm thick depletion region is generated in the n-Bphen layer due to electron transfer from nBphen to HATCN, resulting in downward band bending of 1.55 eV within the depletion region away from the interface with HATCN. On the other hand, the HOMO peak of HATCN starts to appear from the deposition of 2 nm thick HATCN, and apparently, it can be distinguished from the HOMO peak of Li-doped Bphen. The HOMO peak moves to lower BE as the HATCN thickness increases, indicating upward band bending of 0.25 eV away from the interface with n-Bphen. The secondary electron cutoff point also progressively shifts to lower BE, indicating an upward vacuum level shift of 2.70 eV due to band bending (1.55 eV at n-Bphen and 0.25 eV at HATCN) and an interface dipole (0.9 eV) as depicted in Figure 5g. Generally, this kind of electronic structure was observed in the previously reported CGU structures using an nETL/n-type organic layer (HATCN and F4-TCNQ) or metal oxide (WO3 and MoO3).28−31,58,59 Likewise, the UPS spectra of the Li-doped p-bPPhenB (n-p-bPPhenB)/HATCN heterojunction in CGU-(p) exhibit a similar behavior as shown in Figure 5e. In the case of the n-p-bPPhenB/HATCN heterojunction, a 3 nm thick depletion region is generated in the n-p-bPPhenB layer, and downward band bending of 1.50 eV in the depletion region, upward band bending of 0.20 eV at the HATCN layer, an upward vacuum level shift of 2.60 eV due to band bending (1.50 eV at n-p-bPPhenB and 0.20 eV at HATCN), and an interface dipole of 0.90 eV are observed as depicted in Figure 5h. Accordingly, strong n-doped ETLs induce sharp triangular band bending within a very narrow region at the n−d heterojunction of CGU-(B) and CGU-(p), which facilitates tunneling of electrons generated in the LUMO of HATCN.32,33 The HATCN/TAPC interfaces of CGU-(B) and CGU-(p) show general features of a charge-generating p−n heterojunction. As depicted in Figure 5g,h, the HOMO levels of TAPC in CGU-(B) and CGU-(p) are formed 0.9 and 0.8 eV below the LUMO level of HATCN. Because of these small energy gaps between the LUMO level of HATCN and HOMO level of TAPC (ΔELUMO−HOMO), it is possible that the electrons in the HOMO of TAPC are injected into the LUMO of HATCN by an external electric field. As a result, electrons and holes are generated in the LUMO of HATCN and HOMO of TAPC, respectively. The above results indicate that the charge generation and electron injection properties of CGU-(B) and CGU-(p) are similar, but the difference in CGU performances between the two CGU devices is attributed to their electrontransporting abilities; i.e., the higher electron mobility of pbPPhenB leads to better CGU performance. 2.5. Red Phosphorescence OLED (PhOLED) Performances. 2.5.1. Device Characteristics of Single-Stack Red PhOLEDs. Prior to fabrication of the tandem device, we fabricated single-stack red PhOLEDs to confirm the electron injection and transport abilities of ETL materials (p-bPPhenB and m-bPPhenB). For each device structure, three devices were fabricated, and all the device data, average values, and standard deviations are presented in the Supporting Information (Figures S13−S19 and Table S3). The device structures with different ETLs, p-bPPhenB for device R-1S-(p), m-bPPhenB for device R-1S-(m), and Bphen for device R-1S-(B), are presented in Figure 6a. The optimum distances from the anode and cathode to the EML (the first field antinode condition) were

and electron injection properties of CGU-(p), the electronic structures of CGU-(p) were investigated and compared with those of CGU-(B) by using ultraviolet photoelectron spectroscopy (UPS) measurement. Figure 5 shows the UPS spectra of incremental deposition of n-doped ETL on ETL (parts a and d), HATCN on n-doped ETL (parts b and e), and TAPC on HATCN (parts c and f) for CGU-(B) and CGU-(p). It also shows the energy level diagrams (parts g and h) of ETL/ndoped ETL/HATCN/TAPC interfaces for both CGU devices (the detailed method for determining the energy level diagram is explained in the Supporting Information). As shown in Figure 5a,d, the UPS spectra of the secondary electron cutoff (left panels of the figures) and HOMO edge (right panels of the figures) regions of both ETLs abruptly shift toward higher binding energies (BEs) with just 0.1 nm thick deposition of the n-doped ETL. This shift indicates that 5% doped Li atoms elevate the Fermi level close to the LUMO level of the ETL (0.05 and 0.08 eV below the LUMO levels of Bphen and pbPPhenB, respectively) with a concomitant downward shift of the vacuum level. In addition, the deposition of the n-doped ETL has generated new peaks corresponding to gap states in the low BE region between the HOMO edge and Fermi level as shown in the insets of Figure 5 a,d. This also indicated that the chemical or charge transfer interaction happened between doped Li atoms and embedded nitrogen atoms in the Phen moieties of the ETLs.30,31,60,61 These results demonstrate that the n-Bphen and n-p-bPPhenB layers are highly n-doped by only 5% Li doping. The excellent ability of these materials to be doped n-type is attributed to the chelating ability of the Phen moiety. The Phen unit has not only an advantageous molecular structure to coordinate Li atoms but also highly nucleophilic nitrogen atoms reactive with Li atoms, leading to an efficient electron transfer reaction between Li and the Phen moiety, and consequently, the materials are strongly doped n-type. Because Bphen has a higher maximum negative ESP (−0.1089 a.u.) compared to p-bPPhenB (−0.0918 a.u. ), the Phen unit of Bphen reacts more strongly with the Li atom and is doped compared to that of p-bPPhenB. Hence, the n-Bphen layer exhibits a smaller difference between the LUMO and Fermi levels (0.05 eV) than p-bPPhenB (0.08 eV). The abrupt energy level shift with emerging gap states at the ETL/n-doped ETL interface was previously observed.22,23,30 Most interestingly, deposition of just 0.1 nm of n-doped-ETL abruptly shifted the HOMO peak of the underlayer (pristine ETL), and a further shift was not observed in spite of additional n-doped ETL deposition. Since, the UPS spectrum is usually obtained by electrons being ejected from approximately 1 nm of the sample surface, this observation indicates that an at least 1 nm thick underlayer was doped by diffused n-dopants during n-doped ETL deposition. The diffusion length of Li in Bphen is known to be 5−10 nm.79,80 Therefore, the real electronic structure at the ETL/n-doped ETL interface shows a gradual energy level bending without any electron injection barrier as depicted in Figure 5g,h due to slow n-doping of the ETL by diffused ndopants. To clarify changes in the energy levels of adjoining layers of the n−d heterojunction, the HOMO peaks of the n-ETL and HATCN are traced from the UPS spectra of HATCN subsequently deposited on the n-ETL. Parts b and e of Figure 5 show the UPS spectra of the n-doped ETL (Li:Bphen and Li:p-bPPhenB)/n-type HATCN layer (n−d heterojunction). As shown in Figure 5b, as the HATCN thickness increases, the HOMO onset point of Li-doped Bphen (n-Bphen) pro8306


Article


devices show pure red emissions originating from the phosphorescent red dopant Ir(mphmq)2(tmd) without any emissions from the adjacent materials such as the HTL (TAPC), host (Bebq2), and ETLs. This indicates effective exciton and carrier confinement within the EML due to higher triplet energies as well as the LUMO level of TAPC and lower HOMO levels of ETLs compared to that of the red dopant.62,63 Except EL spectra, other device characteristics are drastically changed according to applied ETL materials. R-1S-(p) and R1S-(m) exhibit much higher current density and luminance values at the same voltage as compared to R-1S-(B). In particular, R-1S-(p) and R-1S-(m) show very low driving voltages of 2.7 and 2.9 V, respectively, at a luminance of 1000 cd/m2, while a much higher driving voltage of 3.7 V is observed in R-1S-(B). The drastic reduction of driving voltages in R-1S(p) and R-1S-(m) results from higher electron mobilities of pbPPhenB (5.8 × 10−3 cm2/(V s)) and m-bPPhenB (4.4 × 10−3 cm2/(V s)) compared to that of Bphen (5.5 × 10−4 cm2/(V s)). Moreover, these two new ETL materials enhance charge balance in the EML by matching the electron mobility of ETL with the hole mobility of TAPC (10−2−10−3 cm2/(V s)),72,73 leading to improved external quantum efficiencies (EQEs) of 30.8% for R-1S-(p) and 28.4% for R-1S-(m). The very high EQE of the devices exceeding the theoretical EQE limitation (∼25%) is attributed to enhanced out-coupling efficiency due to the horizontally oriented emitting dipole of Ir(mphmq)2(tmd).63,94 Because of significantly decreased driving voltage with increased EQE, R-1S-(p) and R-1S-(m) exhibited extremely high maximum power efficiencies of 56.8 and 51.0 lm/W, respectively. These values are approximately 1.5 and 1.3 times higher than that of R-1S-(B) (39.0 lm/W). 2.5.2. Electrical Stability of m-bPPhenB and p-bPPhenB. Phen derivatives such as Bphen and BCP are well-known unstable ETL materials, and OLEDs with these materials normally show very low lifetimes. Indeed, the low stability of such devices is attributed to the low Tg and electrochemical reactions during device operation. In section 2.3.3, we already revealed significantly improved Tg values of p-bPPhenB and mbPPhenB compared to that of Bphen. To further demonstrate the stabilities of these materials, the electrical stabilities of pbPPhenB and m-bPPhenB were confirmed by comparing the operational lifetimes of R-1S-(p) and R-1S-(m) with that of R1S-(B). The lifetimes of the devices were estimated by tracing the EL intensity change as a function of the operation time under the applied condition of constant current density corresponding to an initial brightness of 1000 cd/m2. The time required for 95% luminance (LT95) from its initial brightness was observed to be 130, 95, and 13 h for devices R1S-(p), R-1S-(m), and R-1S-(B), respectively. It is important to note that the lifetimes of R-1S-(p) and R-1S-(m) are 10 times and 7 times higher than that of R-1S-(B), which demonstrates significantly improved electrical stabilities of p-bPPhenB and mbPPhenB compared to that of Bphen. Generally, it is well-known that Bphen is chemically degraded by dimerization of the molecule itself or metal complex formation between Bphen and excited Ir-based molecules such as phosphorescent dopants in the EML during device operation.64−68 In Bphen, two phenyl substituents in the 4and 7-positions of the Phen moiety are spatially away from the nitrogen atoms; hence, there is no steric hindrance in the reactants approaching the nitrogen atoms, which may make Bphen prone to chemical reactions. Moreover, the highly nucleophilic nature of nitrogen atoms promotes nucleophilic

Figure 6. (a, b) Device structures of single-stack and two-stack (tandem) red PhOLEDs, respectively. The double arrows represent the distance from the EML to the cathode (Al) and to the anode (ITO). The distances are decided by following the optical simulation results. Optical simulation results: (c) contour plots for simulated radiance intensity as a function of the distance from the EML to the cathode (Al) and the distance from the EML to the anode (ITO); (d) simulated radiance intensity versus the distance from the EML to Al (line 1) and the distance from the EML to ITO (line 2).

decided by calculating emitted radiance intensities from the devices as a function of the distances using the optical simulation model (SimOLED)91−93 as shown in Figure 6c. The device characteristics are presented in Figure 7, and their performances are summarized in Table 3. The fabricated 8307


Article


Figure 7. (a) Current density and luminance versus voltage characteristics, (b) power efficiency and EQE versus luminance characteristics, (c) normalized EL spectra, and (d) normalized EL intensity versus operation time characteristics (operational lifetime of the device) of the single-stack red PhOLEDs using Bphen (R-1S-(B)), p-bPPhenB (R-1S-(p)), and m-bPPhenB (R-1S-(m)) as the ETL.

Table 3. OLED Device Characteristics single-stack red PhOLED VDb

a

(V)

p-bPPhenB

2.7

m-bPPhenB

2.9

Bphen

3.7

PE (lm/W) a

56.8 45.9b 51.0a 40.2b 39.0a 32.0b

EQE (%) a

30.8 28.0b 28.4a 26.4b 27.1a 25.5b

two-stack (tandem) red PhOLED b

CIE (x, y)

LT95 (h)

(0.646, 0.351)

130

(0.644, 0.350)

95

(0.646, 0.353)

13

VDb

(V)

PE (lm/W)

EQE (%)

CIEb (x, y)

5.0

a

54.0 49.3b

a

56.1 53.1b

(0.646, 0.345)

6.2

45.2a 39.2b

53.3a 51.1b

(0.647, 0.351)

Maximum value. bMeasured value at 1000 cd/m2.

molecule during the charge-transporting process.95 Hence, the lower RE is another possible reason for the superior stability of p-bPPhenB. 2.5.3. Device Characteristics of Two-Stack (Tandem) Red PhOLEDs. Finally, two-stack (tandem) red PhOLEDs R-2S-(B) and R-2S-(p) were fabricated to clearly elucidate the charge generation ability of CGU-(p). The detailed device structure is depicted in Figure 6b, wherein Bphen and p-bPPhenB are used as the ETL for R-2S-(B) and R-2S-(p), respectively. In the device structure, the positions of the two EMLs were decided to be those in the second field antinode condition as depicted in Figure 6. Parts a and b and parts c and d of Figure 8 present the performances of single- and two-stack red PhOLED devices comprising Bphen or p-bPPhenB, respectively. As shown in Figure 8, both tandem devices with Bphen and p-bPPhenB exhibit a driving voltage lower than twice the driving voltages of the single-stack devices. This result indicates that the CGUs comprising Bphen and p-bPPhenB effectively restrain the voltage increase coming from the series connection between individual EL units in the tandem structure. Due to the effective

addition reactions. However, p-bPPhenB and m-bPPhenB have not only a weaker nucleophilic nature than Bphen but also only one phenyl substituent (2-position of 1,10-phenanthroline) right next to the nitrogen, which may impose a constraint on the approach of reactants to the nitrogen atoms. These electrical and geometrical features may reduce the reactivities of p-bPPhenB and m-bPPhenB and thereby drastically improve the operational lifetimes of R-1S-(p) and R-1S-(m) devices. In comparison between p-bPPhenB and m-bPPhenB, the better electrical stability of p-bPPhenB can be explained by two factors, the nucleophilic nature and reorganization energy. As mentioned above, a weaker nucleophilic nature is helpful to prevent dimerization during device operation. Hence, the lower maximum negative ESP of p-bPPhenB (−0.0918 a.u.) is the possible reason for its better stability compared to that of mbPPhenB with a higher maximum negative ESP (−0.0946 a.u.). Moreover, as shown in section 2.1.2, p-bPPhenB has a lower RE (0.056 eV) than m-bPPhenB (0.069 eV), which means pbPPhenB is more stable in terms of electrical current because a lower RE indicates a smaller conformational change of the 8308


Article


Figure 8. Comparison of device characteristics between single-stack red PhOLED (R-1S) and two-stack (tandem) red PhOLED (R-2S) devices using Bphen (R-1S-(B), R-2S-(B)) and p-bPPhenB (R-1S-(p), R-2S-(p)) as the ETL: (a, c) Current density and luminance versus voltage characteristics and (b, d) power efficiency versus EQE characteristics of the Bphen (a, b) and p-bPPhenB (c, d) based single- and two-stack red PhOLEDs.

3. CONCLUSION In summary, we have developed two efficient electrontransporting materials, p-bPPhenB and m-bPPhenB, for application in CGUs in tandem OLEDs through a novel molecular design strategy. Theoretical calculation of the electron RE and ESP was performed to anticipate the electron mobility and n-doped ability of the designed molecules. From the calculation, we found that a phenyl substitution at the 2position of the Phen unit successfully reduces the electron RE while minimizing the reduction of the ESP of embedded nitrogen atoms in Phen, thereby increasing the electron mobility and nucleophilic nature. The new molecules designed by linking two Phen-based moieties have a planar and rigid molecular structure with a low electron RE and a high nucleophilicity. These new ETMs possess high electron mobilities of (5.8−4.4) × 10−3 cm2/(V s) as well as high glass transition temperatures of 123.9−182.1 °C. Furthermore, they also exhibited an excellent electron-injecting property at the CGU structure. The electronic structures of the CGU estimated by UPS measurements revealed that the highly nucleophilic nature of these materials make them strongly ndoped due to electron transfer interaction between p-bPPhenB and the n-dopant (Li). This induces a sharp triangular band bending with a very narrow depletion region (∼3 nm) at the ndoped ETL/HATCN interface, facilitating tunneling of electrons from HATCN to the ETL. Due to these excellent ETL characteristics, single-stack red PhOLEDs using new ETL materials exhibit much lower driving voltages of 2.7−2.9 V, higher power efficiencies of 56.8−51.0 lm/W, higher EQEs of 30.8−28.4%, and longer device lifetimes of 130−95 h compared

CGUs, the tandem devices show enhanced power efficiencies compared with the single-stack devices. However, R-2S-(p) shows a 1.9 times higher EQE (53.1%) than R-1S-(p) (28.0%), which is slightly lower than the sum of the EQEs (56.0%) of R1S-(p). This results from the difference in the EML positions in the single-stack and tandem devices. As previously explained, the EMLs of the single-stack and tandem devices are placed in the first and second field antinode positions, respectively. As shown in Figure 6c,d, there are two second field antinode positions (1 and 2), and their radiances are 90% and 95% of those of the first field antinode. This means that the EQE of the tandem device should be 1.85 times higher than the EQE of the single-stack device using first field antinode condition. In reality, the fabricated tandem device (R-2S-(p)) shows a 1.9 times higher EQE. The additionally increased EQE is attributed to enhanced charge balance and reduced exciton quenching because the separated EMLs of the tandem device require a smaller number of excitons in emitting the same intensity of light as the EML of the single-stack device. Likewise, the efficiency roll-off of the tandem device is slightly lower than that of the single-stack device as shown in Figure 8b,d. In addition , thick organic layers of tandem devices induce slightly narrower EL spectra compared to the single-stack device because of the second field antinode condition (weak microcavity effect) as shown in Figure S20 (Supporting Information). Furthermore, R-2S-(p) exhibits a much lower driving voltage (5.0 V), a higher power efficiency (49.3 lm/W), and a higher EQE (53.1%) at 1000 cd/m2 compared to R-2S(B) (6.2 V, 39.2 lm/W, and 51.1%), which demonstrates the superiority of p-bPPhenB as the ETL material for the CGU. 8309


Article


(2) D’Andrade, B. W.; Forrest, S. R. White Organic Light-Emitting Devices for Solid-State Lighting. Adv. Mater. 2004, 16, 1585−1595. (3) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459, 234−238. (4) Fung, M.-K.; Li, Y.-Q.; Liao, L.-S. Tandem Organic LightEmitting Diodes. Adv. Mater. 2016, 28, 10381−10408. (5) Han, C.-W.; Park, J.-S.; Shin, Y.-H.; Lim, M.-J.; Kim, B.-C.; Tak, Y.-H.; Ahn, B.-C. Advanced Technologies for Large-sized OLED TV. Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 2014, 45, 770−773. (6) Chiba, T.; Pu, Y.-J.; Miyazaki, R.; Nakayama, K.-I.; Sasabe, H.; Kido, J. Ultra-high efficiency by multiple emission from stacked organic light-emitting devices. Org. Electron. 2011, 12, 710−715. (7) Kröger, M.; Hamwi, S.; Meyer, J.; Dobbertin, T.; Riedl, T.; Kowalsky, W.; Johannes, H.-H. Temperature-independent fieldinduced charge separation at doped organic/organic interfaces: Experimental modeling of electrical properties. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 235321. (8) Law, C. W.; Lau, K. M.; Fung, M. K.; Chan, M. Y.; Wong, F. L.; Lee, C. S.; Lee, S. T. Effective organic-based connection unit for stacked organic light-emitting devices. Appl. Phys. Lett. 2006, 89, 133511. (9) Liu, J.; Huang, S. J.; Shi, X. D.; Wu, X. K.; Wang, J.; He, G. F. Charge Separation Process in an Ultrathin Electron-Injecting BilayerAssisted Charge Generation Unit for Tandem Organic Light-Emitting Diodes. J. Phys. Chem. C 2013, 117, 13887−13893. (10) Scholz, S.; Lüssem, B.; Leo, K. Chemical changes on the green emitter tris(8-hydroxy-quinolinato)aluminum during device aging of pi-n-structured organic light emitting diodes. Appl. Phys. Lett. 2009, 95, 183309. (11) Lee, T.-W.; Noh, T.; Choi, B.-K.; Kim, M.-S.; Shin, D. W.; Kido, J. High-efficiency stacked white organic light-emitting diodes. Appl. Phys. Lett. 2008, 92, 043301. (12) Yook, K. S.; Jeon, S. O.; Min, S. Y.; Lee, J. Y.; Yang, H. J.; Noh, T.; Kang, S. K.; Lee, T. W. Highly Efficient p-i-n and Tandem Organic Light-Emitting Devices Using an Air-Stable and Low-TemperatureEvaporable Metal Azide as an n-Dopant. Adv. Funct. Mater. 2010, 20, 1797−1802. (13) Chang, C. C.; Chen, J. F.; Hwang, S. W.; Chen, C. H. Highly efficient white organic electroluminescent devices based on tandem architecture. Appl. Phys. Lett. 2005, 87, 253501. (14) Tsutsui, T.; Terai, M. Electric field-assisted bipolar charge spouting in organic thin-film diodes. Appl. Phys. Lett. 2004, 84, 440− 442. (15) Guo, F. W.; Ma, D. G. White organic light-emitting diodes based on tandem structures. Appl. Phys. Lett. 2005, 87, 173510. (16) Leem, D. S.; Lee, J. H.; Kim, J. J.; Kang, J. W. Highly efficient tandem p-i-n organic light-emitting diodes adopting a low temperature evaporated rhenium oxide interconnecting layer. Appl. Phys. Lett. 2008, 93, 103304. (17) Liao, L. S.; Klubek, K. P.; Tang, C. W. High-efficiency tandem organic light-emitting diodes. Appl. Phys. Lett. 2004, 84, 167−169. (18) Liao, L. S.; Slusarek, W. K.; Hatwar, T. K.; Ricks, M. L.; Comfort, D. L. Tandem Organic Light-Emitting Diode using Hexaazatriphenylene Hexacarbonitrile in the Intermediate Connector. Adv. Mater. 2008, 20, 324−329. (19) Yan, F.; Chen, R.; Sun, H. D.; Sun, X. W. Silver nanoparticle facilitated charge generation in tandem organic light-emitting devices. Appl. Phys. Lett. 2013, 102, 203303. (20) Sun, H. D.; Guo, Q. X.; Yang, D. Z.; Chen, Y. H.; Chen, J. S.; Ma, D. G. High Efficiency Tandem Organic Light Emitting Diode Using an Organic Heterojunction as the Charge Generation Layer: An Investigation into the Charge Generation Model and Device Performance. ACS Photonics 2015, 2, 271−279. (21) Kanno, H.; Holmes, R. J.; Sun, Y.; Kena-Cohen, S.; Forrest, S. R. White Stacked Electrophosphorescent Organic Light-Emitting Devices Employing MoO3 as a Charge-Generation Layer. Adv. Mater. 2006, 18, 339−342.

to the control device with Bphen (3.7 V, 39 lm/W, 27.1%, and 13 h, respectively). Likewise, tandem red PhOLEDs comprising CGUs with p-bPPhenB also show much improved performances (5.0 V, 54.0 lm/W, 56.1%) compared to tandem devices with Bphen (6.2 V, 45.2 lm/W, 53.3%). This work provides crucial information regarding the development of highperformance ETMs and CGU structures for low power consumption and highly efficient as well as stable tandem OLEDs. We strongly believe that this study will help in the further improvement of tandem OLED technology.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02655. Experimental details, synthesis procedures, device fabrication and characterization, and figures showing the NMR and HRMS spectra, cyclic voltammograms, and electroluminescence spectra (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jang Hyuk Kwon: 0000-0002-1743-1486 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Grant No. NRF2016R1A6A3A11930666 and the Human Resources Development Program (Grant Nos. 20134030200250 and 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Government, Ministry of Trade, Industry and Energy.

■

ABBREVIATIONS F4-TCNQ, tetrafluorotetracyanoquinodimethane; HATCN, hexaazatriphenylenehexacarbonitrile; TmPyPB, 1,3,5-tris[(3pyridyl)phen-3-yl]benzene; B3PYMPM, 4,6-bis(3,5-di-3-pyridylphenyl)-2-methylpyrimidine; TPBi, 1,3,5-tris(1-phenyl-1Hbenzo[d]imidazol-2-yl)benzene; Alq3, tris(8hydroxyquinolinato)aluminum; BCP, 1,10-phenanthroline; Phen, 4,7-diphenyl-1,10-phenanthroline; Bphen, bathophenanthroline; 4-DPPhen, 4-diphenyl[1,10]phenanthroline; 2-PPhen, 2-phenyl[1,10]phenanthroline; 4,7-DPPhen, 4,7-diphenyl[1,10]phenanthroline; 2,4-DPPhen, 2,4-diphenyl[1,10]phenanth roline; 2,9-DPPhen, 2,9-diphenyl[1,10]phenanthroline; p-bPPhenB, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene; m-bPPhenB, 1,3-bis(2-phenyl-1,10phenanthrolin-4-yl)benzene; Ir(mphmq)2(tmd), (bis(4-methyl-2-(3,5-dimethylphenyl)quinoline))iridium(III) tetramethylheptadionate; Bebq2, bis(10-hydroxybenzo[h]quinolinato)beryllium

■

REFERENCES

(1) Shen, Z.; Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Thompson, M. E. Three-Color, Tunable, Organic Light-Emitting Devices. Science 1997, 276, 2009−2011. 8310


Article


layer as cathode and interconnecting layer. Org. Electron. 2014, 15, 675−679. (41) Marcus, R. A. On the Theory of Electron-Transfer Reactions. VI. Unified Treatment for Homogeneous and Electrode Reactions. J. Chem. Phys. 1965, 43, 679−701. (42) Marcus, R. A. Electron transfer reactions in chemistry. Theory and experiment. Rev. Mod. Phys. 1993, 65, 599−610. (43) Yin, J.; Chen, R.-F.; Zhang, S.-L.; Ling, Q.-D.; Huang, W. Theoretical Studies of the Structural, Electronic, and Optical Properties of Phosphafluorenes. J. Phys. Chem. A 2010, 114, 3655− 3667. (44) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. Hopping Transport in Conductive Heterocyclic Oligomers: Reorganization Energies and Substituent Effects. J. Am. Chem. Soc. 2005, 127, 2339− 2350. (45) Berlin, Y. A.; Hutchison, G. R.; Rempala, P.; Ratner, M. A.; Michl, J. Charge Hopping in Molecular Wires as a Sequence of Electron-Transfer Reactions. J. Phys. Chem. A 2003, 107, 3970−3980. (46) Zhuo, M.; Sun, W.; Liu, G.; Wang, J.; Guo, L.; Liu, C.; Mi, B.; Song, J.; Gao, Z. Pure aromatic hydrocarbons with rigid and bulky substituents as bipolar hosts for blue phosphorescent OLEDs. J. Mater. Chem. C 2015, 3, 9137−9144. (47) Tyagi, P.; Kumar, A.; Giri, L. I.; Dalai, M. K.; Tuli, S.; Kamalasanan, M. N.; Srivastava, R. Exciton quenching by diffusion of 2,3,5,6-tetrafluoro-7,7′,8,8′-tetra cyano quino dimethane and its consequences on joule heating and lifetime of organic light-emitting diodes. Opt. Lett. 2013, 38, 3854−3857. (48) Gong, J.-R.; Wan, L.-J.; Lei, S.-B.; Bai, C.-L.; Zhang, X.-H.; Lee, S.-T. Direct Evidence of Molecular Aggregation and Degradation Mechanism of Organic Light-Emitting Diodes under Joule Heating: an STM and Photoluminescence Study. J. Phys. Chem. B 2005, 109, 1675−1682. (49) Tokito, S.; Tanaka, H.; Noda, K.; Okada, A.; Taga, Y. Thermal stability in oligomeric triphenylamine/tris(8-quinolinolato) aluminum electroluminescent devices. Appl. Phys. Lett. 1997, 70, 1929−1931. (50) Li, W.; Li, J.; Liu, D.; Li, D.; Zhang, D. Dual n-type units including pyridine and diphenylphosphine oxide: effective design strategy of host materials for high-performance organic light-emitting diodes. Chem. Sci. 2016, 7, 6706−6714. (51) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (52) Kim, G. W.; Yang, D. R.; Kim, Y. C.; Yang, H. I.; Fan, J. G.; Lee, C.-H.; Chai, K. Y.; Kwon, J. H. Di(biphenyl)silane and carbazole based bipolar host materials for highly efficient blue phosphorescent OLEDs. Dyes Pigm. 2017, 136, 8−16. (53) Accorsi, G.; Listorti, A.; Yoosaf, K.; Armaroli, N. 1,10Phenanthrolines: versatile building blocks for luminescent molecules, materials and metal complexes. Chem. Soc. Rev. 2009, 38, 1690−1700. (54) Mazumdar, P.; Das, D.; Sahoo, G. P.; Salgado-Moran, G.; Misra, A. Aggregation induced emission enhancement from Bathophenanthroline microstructures and its potential use as sensor of mercury ions in water. Phys. Chem. Chem. Phys. 2014, 16, 6283−6293. (55) Mo, H.-W.; Tsuchiya, Y.; Geng, Y.; Sagawa, T.; Kikuchi, C.; Nakanotani, H.; Ito, F.; Adachi, C. Color Tuning of Avobenzone Boron Difluoride as an Emitter to Achieve Full-Color Emission. Adv. Funct. Mater. 2016, 26, 6703−6710. (56) Wang, R.-Y.; Jia, W.-L.; Aziz, H.; Vamvounis, G.; Wang, S.; Hu, N.-X.; Popović, Z. D.; Coggan, J. A. 1-Methyl-2-(anthryl)-imidazo[4,5f][1,10]-phenanthroline: a highly efficient electron-transport compound and a bright blue-light emitter for electroluminescent devices. Adv. Funct. Mater. 2005, 15, 1483−1487. (57) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. Correlation between Molecular Packing and Optical Properties in Different Crystalline Polymorphs and Amorphous Thin Films of mer-Tris(8-hydroxyquinoline)aluminum(III). J. Am. Chem. Soc. 2000, 122, 5147−5157. (58) Lee, S.; Lee, J.-H.; Lee, J.-H.; Kim, J.-J. The Mechanism of Charge Generation in Charge-Generation Units Composed of p-

(22) Chan, M. Y.; Lai, S. L.; Lau, K. M.; Fung, M. K.; Lee, C. S.; Lee, S. T. Influences of Connecting Unit Architecture on the Performance of Tandem Organic Light-Emitting Devices. Adv. Funct. Mater. 2007, 17, 2509−2514. (23) Bao, Q. Y.; Yang, J. P.; Li, Y. Q.; Tang, J. X. Electronic structures of MoO3-based charge generation layer for tandem organic lightemitting diodes. Appl. Phys. Lett. 2010, 97, 063303. (24) Hong, K.; Lee, J. L. Charge Generation Mechanism of Metal Oxide Interconnection in Tandem Organic Light Emitting Diodes. J. Phys. Chem. C 2012, 116, 6427−6433. (25) Zhou, D. Y.; Shi, X. B.; Liu, Y.; Gao, C. H.; Wang, K.; Liao, L. S. Role of hole injection layer in intermediate connector of tandem organic light-emitting devices. Org. Electron. 2014, 15, 3694−3701. (26) Ding, L.; Tang, X.; Xu, M. F.; Shi, X. B.; Wang, Z. K.; Liao, L. S. Lithium Hydride Doped Intermediate Connector for High-Efficiency and Long-Term Stable Tandem Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2014, 6, 18228−18232. (27) Liu, J.; Chen, Y.; Qin, D.; Cheng, C.; Quan, W.; Chen, L.; Li, G. Improved interconnecting structure for a tandem organic light emitting diode. Semicond. Sci. Technol. 2011, 26, 095011. (28) Hamwi, S.; Meyer, J.; Kroger, M.; Winkler, T.; Witte, M.; Riedl, T.; Kahn, A.; Kowalsky, A. The Role of Transition Metal Oxides in Charge-Generation Layers for Stacked Organic Light-Emitting Diodes. Adv. Funct. Mater. 2010, 20, 1762−1766. (29) Chen, C. W.; Lu, Y. J.; Wu, C. C.; Wu, E. H. E.; Chu, C. W.; Yang, Y. Effective connecting architecture for tandem organic lightemitting devices. Appl. Phys. Lett. 2005, 87, 241121. (30) Yang, J.-P.; Xiao, Y.; Deng, Y.-H.; Duhm, S.; Ueno, N.; Lee, S.T.; Li, Y.-Q.; Tang, J.-X. Electric-Field-Assisted Charge Generation and Separation Process in Transition Metal Oxide-Based Interconnectors for Tandem Organic Light-Emitting Diodes. Adv. Funct. Mater. 2012, 22, 600−608. (31) Lee, S.; Shin, H.; Kim, J. J. High-Efficiency Orange and Tandem White Organic Light-Emitting Diodes Using Phosphorescent Dyes with Horizontally Oriented Emitting Dipoles. Adv. Mater. 2014, 26, 5864−5868. (32) Wei, H.-X.; Zu, F.-S.; Li, Y.-Q.; Chen, W.-C.; Yuan, Y.; Tang, J.X.; Fung, M.-K.; Lee, C.-S.; Noh, Y.-Y. Charge transport dependent high open circuit voltage tandem organic photovoltaic cells with low temperature deposited HATCN-based charge recombination layers. Phys. Chem. Chem. Phys. 2016, 18, 4045−4050. (33) Chen, Y.-H.; Ma, D.-G.; Sun, H.-D.; Chen, J.-S.; Guo, Q.-X.; Wang, Q.; Zhao, Y.-B. Organic semiconductor heterojunctions: electrode-independent charge injectors for high-performance organic light-emitting diodes. Light: Sci. Appl. 2016, 5, e16042. (34) Su, S.-J.; Chiba, T.; Takeda, T.; Kido, J. Pyridine-Containing Triphenylbenzene Derivatives with High Electron Mobility for Highly Efficient Phosphorescent OLEDs. Adv. Mater. 2008, 20, 2125−2130. (35) Khan, M. A.; Xu, W.; Khizar-ul-Haq; Bai, Y.; Jiang, X. Y.; Zhang, Z. L.; Zhu, W. Q.; Zhang, Z. L.; Zhu, W. Q. Electron mobility of 4,7diphyenyl-1,10-phenanthroline estimated by using space-chargelimited currents. J. Appl. Phys. 2008, 103, 014509. (36) Choi, Y. H.; Jeon, Y. P.; Choo, D. C.; Kim, T. W. Enhancement of out-coupling efficiency due to an organic scattering layer in organic light-emitting devices. Org. Electron. 2015, 22, 197−201. (37) Takeda, H.; Ohashi, M.; Goto, Y.; Ohsuna, T.; Tani, T.; Inagaki, S. A Versatile Solid Photosensitizer: Periodic Mesoporous Organosilicas with Ruthenium Tris(bipyridine) Complexes Embedded in the Pore Walls. Adv. Funct. Mater. 2016, 26, 5068−5077. (38) Tesfaldet, Z. O.; van Staden, J. F.; Stefan, R. I. Sequential injection spectrophotometric determination of iron as Fe(II) in multivitamin preparations using 1,10-phenanthroline as complexing agent. Talanta 2004, 64, 1189−1195. (39) Ali, A.; Yin, X.; Shen, H.; Ye, Y.; Gu, X. 1, 10-Phenanthroline as a complexing agent for on-line sorbent extraction/preconcentration for flow injection−flame atomic absorption spectrometry. Anal. Chim. Acta 1999, 392, 283−289. (40) Wen, X.; Yin, Y.; Li, Y.; Liu, S.; Zhang, L.; Ma, N.; Shan, G.; Xie, W. Tandem white organic light-emitting device using non-modified Ag 8311


Article

Chemistry of Materials Doped Hole-Transporting Layer/HATCN/n-Doped Electron-Transporting Layers. Adv. Funct. Mater. 2012, 22, 855−860. (59) Wu, Y.-L.; Chen, C.-Y.; Huang, Y.-H.; Lu, Y.-J.; Chou, C.-H.; Wu, C.-C. Highly efficient tandem organic light-emitting devices utilizing the connecting structure based on n-doped electron-transport layer/HATCN/hole-transport layer. Appl. Opt. 2014, 53, E1−E6. (60) Sakurai, T.; Toyoshima, S.; Kitazume, H.; Masuda, S.; Kato, H.; Akimoto, K. Influence of gap states on electrical properties at interface between bathocuproine and various types of metals. J. Appl. Phys. 2010, 107, 043707. (61) Chen, M.-H.; Chen, Y.-H.; Lin, C.-T.; Lee, G.-R.; Wu, C.-I.; Leem, D.-S.; Kim, J.-J.; Pi, T.-W. Electronic and chemical properties of cathode structures using 4,7-diphenyl-1,10-phenanthroline doped with rubidium carbonate as electron injection layers. J. Appl. Phys. 2009, 105, 113714. (62) Kim, D. H.; Cho, N. S.; Oh, H.-Y.; Yang, J. H.; Jeon, W. S.; Park, J. S.; Suh, M. C.; Kwon, J. H. Highly Efficient Red Phosphorescent Dopants in Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 2721−2726. (63) Kim, K.-H.; Lee, S.; Moon, C.-K.; Kim, S.-Y.; Park, Y.-S.; Lee, J.H.; Lee, J. W.; Huh, J.; You, Y.; Kim, J.-J. Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes. Nat. Commun. 2014, 5, 4769. (64) Scholz, S.; Kondakov, D.; Lüssem, B.; Leo, K. Degradation Mechanisms and Reactions in Organic Light-Emitting Devices. Chem. Rev. 2015, 115, 8449−8503. (65) Scholz, S.; Corten, C.; Walzer, K.; Kuckling, D.; Leo, K. Photochemical reactions in organic semiconductor thin films. Org. Electron. 2007, 8, 709−717. (66) de Moraes, I. R.; Scholz, S.; Lüssem, B.; Leo, K. Chemical degradation processes of highly stable red phosphorescent organic light emitting diodes. Org. Electron. 2012, 13, 1900−1907. (67) Meerheim, R.; Scholz, S.; Olthof, S.; Schwartz, G.; Reineke, S.; Walzer, K.; Leo, K. Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices. J. Appl. Phys. 2008, 104, 014510. (68) Salem, A. A. Fluorimetric determinations of nucleic acids using iron, osmium and samarium complexes of 4,7-diphenyl-1,10phenanthroline. Spectrochim. Acta, Part A 2006, 65, 235−248. (69) Yasuda, T.; Yamaguchi, Y.; Zou, D.-C.; Tsutsui, T. Carrier Mobilities in Organic Electron Transport Materials Determined from Space Charge Limited Current. Jpn. J. Appl. Phys. 2002, 41, 5626− 5629. (70) Chu, T.-Y.; Song, O.-K. Hole mobility of N,N′-bis(naphthalen1-yl)-N,N′-bis(phenyl) benzidine investigated by using space-chargelimited currents. Appl. Phys. Lett. 2007, 90, 203512. (71) Murgatroyd, P. N. Theory of space-charge-limited current enhanced by Frenkel effect. J. Phys. D: Appl. Phys. 1970, 3, 151−156. (72) Strohriegl, P.; Grazulevicius, J. V. Charge-Transporting Molecular Glasses. Adv. Mater. 2002, 14, 1439−1452. (73) Sasabe, H.; Gonmori, E.; Chiba, T.; Li, Y. J.; Tanaka, D.; Su, S. J.; Takeda, T.; Pu, Y. J.; Nakayama, K. I.; Kido, J. Wide-Energy-Gap Electron-Transport Materials Containing 3,5-Dipyridylphenyl Moieties for an Ultra High Efficiency Blue Organic Light-Emitting Device. Chem. Mater. 2008, 20, 5951−5953. (74) Liao, L. S.; Klubek, K. P. Power efficiency improvement in a tandem organic light-emitting diode. Appl. Phys. Lett. 2008, 92, 223311. (75) Zhang, Y.; Lee, J.; Forrest, S. R. Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes. Nat. Commun. 2014, 5, 5008. (76) Yokoyama, D. Molecular orientation in small-molecule organic light-emitting diodes. J. Mater. Chem. 2011, 21, 19187−19202. (77) Yokoyama, D.; Sakaguchi, A.; Suzuki, M.; Adachi, C. Enhancement of electron transport by horizontal molecular orientation of oxadiazole planar molecules in organic amorphous films. Appl. Phys. Lett. 2009, 95, 243303. (78) Yokoyama, D.; Setoguchi, Y.; Sakaguchi, A.; Suzuki, M.; Adachi, C. Orientation Control of Linear-Shaped Molecules in Vacuum-

Deposited Organic Amorphous Films and Its Effect on Carrier Mobilities. Adv. Funct. Mater. 2010, 20, 386−391. (79) Parthasarathy, G.; Shen, C.; Kahn, A.; Forrest, S. R. Lithium doping of semiconducting organic charge transport materials. J. Appl. Phys. 2001, 89, 4986−4992. (80) Angel, F. A.; Wallace, J. U.; Tang, C. W. Effect of lithium and silver diffusion in single-stack and tandem OLED devices. Org. Electron. 2017, 42, 102−106. (81) Bredas, J.-L. Mind the gap! Mater. Horiz. 2014, 1, 17. (82) Low, P. J.; Paterson, M. A. J.; Yufit, D. S.; Howard, J. A. K.; Cherryman, J. C.; Tackley, D. R.; Brook, R.; Brown, B. Towards an understanding of structure−property relationships in hole-transport materials: The influence of molecular conformation on oxidation potential in poly(aryl)amines. J. Mater. Chem. 2005, 15, 2304. (83) Pan, J.-H.; Chou, Y.-M.; Chiu, H.-L.; Wang, B.-C. Theoretical investigations of the molecular conformation and reorganization energies in the organic diamines as hole-transporting materials. J. Phys. Org. Chem. 2007, 20, 743. (84) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. Hopping Transport in Conductive Heterocyclic Oligomers: Reorganization Energies and Substituent Effects. J. Am. Chem. Soc. 2005, 127, 2339. (85) Zade, S. S.; Bendikov, M. Study of Hopping Transport in Long Oligothiophenes and Oligoselenophenes: Dependence of Reorganization Energy on Chain Length. Chem. - Eur. J. 2008, 14, 6734. (86) Berlin, Y. A.; Hutchison, G. R.; Rempala, P.; Ratner, M. A.; Michl, J. Charge Hopping in Molecular Wires as a Sequence of Electron-Transfer Reactions. J. Phys. Chem. A 2003, 107, 3970. (87) Jackson, N. E.; Savoie, B. M.; Kohlstedt, K. L.; Olvera de la Cruz, M.; Schatz, G. C.; Chen, L. X.; Ratner, M. A. Controlling Conformations of Conjugated Polymers and Small Molecules: The Role of Nonbonding Interactions. J. Am. Chem. Soc. 2013, 135, 10475−10483. (88) Huang, H.; Chen, Z.; Ortiz, R. P.; Newman, C.; Usta, H.; Lou, S.; Youn, J.; Noh, Y.-Y.; Baeg, K.-J.; Chen, L. X.; Facchetti, A.; Marks, T. Combining Electron-Neutral Building Blocks with Intramolecular “Conformational Locks” Affords Stable, High-Mobility P- and NChannel Polymer Semiconductors. J. Am. Chem. Soc. 2012, 134, 10966. (89) Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ponce Ortiz, R.; Butler, M. R.; Boudreault, P.-L. T.; Strzalka, J.; Morin, P.-O.; Leclerc, M.; López Navarrete, J. T.; Ratner, M. A.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Bithiopheneimide−Dithienosilole/ Dithienogermole Copolymers for Efficient Solar Cells: Information from Structure−Property−Device Performance Correlations and Comparison to Thieno[3,4-c]pyrrole-4,6-dione Analogues. J. Am. Chem. Soc. 2012, 134, 18427. (90) Guo, X.; Quinn, J.; Chen, Z.; Usta, H.; Zheng, Y.; Xia, Y.; Hennek, J. W.; Ortiz, R. P.; Marks, T. J.; Facchetti, A. Dialkoxybithiazole: A New Building Block for Head-to-Head Polymer Semiconductors. J. Am. Chem. Soc. 2013, 135, 1986. (91) Sim4tec GmbH. SimOLED, http://www.sim4tec.com. (92) Staudigel, J.; Stoessel, M.; Steuber, F.; Simmerer, J. A quantitative numerical model of multilayer vapor-deposited organic light emitting diodes. J. Appl. Phys. 1999, 86, 3895. (93) Meerheim, R.; Nitsche, R.; Leo, K. High-efficiency monochrome organic light emitting diodes employing enhanced microcavities. Appl. Phys. Lett. 2008, 93, 043310. (94) Jurow, M. J.; Mayr, C.; Schmidt, T. D.; Lampe, T.; Djurovich, P. I.; Brütting, W.; Thompson, M. E. Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials. Nat. Mater. 2016, 15, 85. (95) Yang, B.; Kim, S.-K.; Xu, H.; Park, Y.-I.; Zhang, H.; Gu, C.; Shen, F.; Wang, C.; Liu, D.; Liu, X.; Hanif, M.; Tang, S.; Li, W.; Li, F.; Shen, J.; Park, J.-W.; Ma, Y. The Origin of the Improved Efficiency and Stability of Triphenylamine-Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation. ChemPhysChem 2008, 9, 2601.

8312


Diphenanthroline Electron Transport Materials for the Efficient Charge

Recommend Documents