Efficient Deep Blue Electroluminescence with an External Quantum

Oct 6, 2015 - Tremendous efforts have been devoted to develop efficient deep blue organic light-emitting diodes (OLEDs) materials with CIEy < 0.10 (Co...
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Efficient Deep Blue Electroluminescence with an External Quantum Efficiency of 6.8% and CIEy < 0.08 Based on a Phenanthroimidazole− Sulfone Hybrid Donor−Acceptor Molecule Xiangyang Tang,†,¶ Qing Bai,†,¶ Qiming Peng,† Yu Gao,† Jinyu Li,† Yulong Liu,† Liang Yao,† Ping Lu,*,† Bing Yang,† and Yuguang Ma†,‡ †

State Key Lab of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun, Jilin 130012, P. R. China ‡ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, P. R. China S Supporting Information *

ABSTRACT: Tremendous efforts have been devoted to develop efficient deep blue organic light-emitting diodes (OLEDs) materials with CIEy < 0.10 (Commission International de L’Eclairage (CIE)) and match the National Television System Committee (NTSC) standard blue CIE (x, y) coordinates of (0.14, 0.08) for display applications. However, deep blue fluorescent materials with an external quantum efficiency (EQE) over 5% are still rare. Herein, we report a phenanthroimidazole− sulfone hybrid donor−acceptor (D−A) molecule with efficient deep blue emission. D−A structure molecular design has been proven to be an effective strategy to obtain high electroluminescence (EL) efficiency. In general, charge transfer (CT) exciton formed between donor and acceptor is a weak coulomb bonded hole−electron pair and is favorable for the spin flip that can turn triplet excitons into singlet ones. However, the photoluminescence quantum yield (PLQY) of CT exciton is usually very low. On the other hand, a locally excited (LE) state normally possesses high PLQY owing to the almost overlapped orbital distributions. Hence, a highly mixed or hybrid local and charge transfer (HLCT) excited state would be ideal to simultaneously achieve both a large fraction of singlet formation and a high PLQY and eventually achieve high EL efficiency. On the basis of such concept, phenanthroimidazole is chosen as a weak donor and sulfone as a moderate acceptor to construct a D−A type molecule named as PMSO. The PMSO exhibits HLCT excited state properties. The doped device shows deep blue electroluminescence with an emission peak of 445 nm and CIE (0.152, 0.077). The maximum external quantum efficiency (EQE) is 6.8% with small efficiency roll-off. The device performance is among the best results of deep blue OLEDs reported so far.

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INTRODUCTION Efficient deep blue light-emitting materials, which have a Commission International de L’Eclairage (CIE) coordinate of y < 0.10 and match the National Television System Committee (NTSC) standard blue CIE (x, y) coordinates of (0.14, 0.08) for display applications, are of great significance for high resolution full-color displays and solid state lightings, because they not only can act as excitation sources to generate other visible emissions and white lightings via energy cascade to lower energy dopants but also increase the color gamut and reduce power consumption in full-color applications.1,2 While blue phosphorescent materials based on transition metal complexes, such as iridium (Ir) complexes, have already realized external quantum efficiency (EQE) over 20%,3 they suffer from sharp efficiency roll-off at high brightness.4 Furthermore, performance of deep blue phosphors still remains unsatisfactory because the nonradiative process via metal d-orbitals becomes competitive when © 2015 American Chemical Society

elevating the radiative metal−ligand charge transfer (MLCT) excited state into the deep blue region.5 In fact, the most widely used blue phosphor FIrpic is a sky-blue emitter; hence, the color purity is compromised, and there are few deep blue alternatives which are comparable to FIrpic in terms of device efficiency.6,7 Therefore, developing efficient deep blue fluorescent materials based on pure aromatic structures should still be a pressing concern. Actually, great efforts have been made to develop a wide variety of deep blue fluorescent materials such as carbazole,8,9 imidazole,10,11 pyrene,12,13 anthracene,14−16 and fluorene17−19 derivatives. For example, Lin et al. systematically synthesized several carbazole−dimesitylborane donor−acceptor (D−A) molecules and one of them revealed deep blue emission with Received: July 14, 2015 Revised: September 30, 2015 Published: October 6, 2015 7050

DOI: 10.1021/acs.chemmater.5b02685 Chem. Mater. 2015, 27, 7050−7057

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Chemistry of Materials

Phenanthroimidazole (PI) has a rather rigid planar structure and is an efficient deep blue/violet chromophore.40,41 Moreover, phenanthroimidazole is endowed with bipolar nature arising from the different bonding modes of the two nitrogen atoms in the imidazole ring: it is a weak electron acceptor when connected to a stronger donor and a weak donor otherwise.42,43 Whereas in most cases it is used as the acceptor part and seldom as donor moiety,44,45 herein, phenanthroimidazole is selected as the weak donor and sulfone is used as the moderate acceptor. The D/A moieties are linked by a phenyl π bridge, and the corresponding symmetric D−π−A−π−D structure molecule PMSO is constructed. In such case, we hope that the CT state between phenanthroimidazole and sulfone can enhance the proportion of singlet exciton yield while the phenyl π bridge between the donor phenanthroimidazole and the acceptor sulfone can promote the formation of LE state and improve fluorescence efficiency. The resultant molecule PMSO exhibits HLCT excited property. The doped OLED shows deep blue emission with an EL peak at 445 nm and CIE (0.152, 0.077) which is very close to the NTSC standard blue CIE (x, y) coordinates of (0.14, 0.08). The highest EQE of doped OLED can reach 6.80% (corresponding to a current efficiency of 4.64 cd/A) and is among the best device performances of deep blue OLEDs with CIEy < 0.08 reported so far. PPIBr and PPIB were synthesized according to the literature.36,42 Described below is the synthesis and purification procedure of the final product PMSO.

CIE coordinates of (0.15, 0.07), but the maximum EQE was only 2.4%.20 Chou et al. achieved a maximum EQE of 10.2% at high brightness by using triplet−triplet annihilation (TTA) molecules, but the emission was not deep enough with CIE coordinates of (0.14, 0.14).21 Kim et al. realized very deep blue emission with CIEy = 0.049 and a relatively high EQE value of 4.62% based on an asymmetric anthracene derivative, but there is still room for further efficiency improvement.22 Lee and co-workers also reported a deep blue molecule consisting of carbazole and sulfone showing a very small CIEy value of 0.055 and an EQE of 4.21%.23 Kido and co-workers recently reported a bisanthracene based D−A molecule with deep blue CIE coordinates of (0.15, 0.06) and high EQE of 12%; this may be the highest device efficiency of deep blue organic light-emitting diodes (OLEDs) with CIEy < 0.08 up to now.24 Very recently, Adachi’s group developed a series of extremely efficient blue fluorescent materials on the basis of thermally activated delayed fluorescence (TADF); however, most of them exhibited sky blue or bluish green emission as a result of the strong charge transfer (CT) effect.25,26 Among them, only one molecule which is constructed by connecting t-butyl substituted carbazole with sulfone achieved deep blue emission, but the efficiency roll-off was serious at high current density.27 Overall, though a wide range of blue fluorescent materials have been developed, only a few can simultaneously realize NTSC standard deep blue emission as well as an EQE exceeding 5% which is generally considered as the EQE upper limit of fluorescent materials.24,27−29 Hence, the development of efficient deep blue materials does not progress well. As a result, more efforts should be devoted and deliberate molecular design should be adopted to carry out efficient deep blue emission. D−A structure molecular design has been proven to be an effective strategy to obtain highly efficient blue electroluminescence (EL) probably due to the low charge injection barrier and broad carrier recombination zone in the emissive layer.30 Besides, the CT exciton formed between the donor and acceptor is a weak coulomb bounded hole−electron pair, and the electron flip can thus reasonably take place which would facilitate a large fraction of singlet exciton formation under electrical charge injection.31−33 However, the CT state is detrimental for acquiring high photoluminescence quantum efficiency (PLQY) because of the forbidden electronic transition between the spatially separated orbitals.34,35 In contrast, the locally excited (LE) state usually shows high PLQY resulting from the large transition moment with almost complete orbital overlap,36 but the strongly coulomb bounded hole−electron pair of LE exciton inhibits the spin flip from triplet state to singlet state and results in a low singlet exciton yield.37 A high PLQY as well as a large yield of singlet exciton formation are two prerequisites for acquiring high device efficiency. To exert the advantages of both LE and CT excitons, a highly mixed or hybrid local and charge transfer (HLCT) excited state was recently proposed by our group. Some blue and red fluorophores have simultaneously obtained a high PLQY as well as a large yield of singlet exciton formation and, as a result, excellent device efficiency was achieved.38,39 Therefore, it would be promising to attain efficient deep blue device performance by adopting HLCT molecular design. In order to construct a deep blue HLCT molecule in a D−A system, the donor and acceptor segments should be really carefully chosen to regulate the CT energy close or even identical to the LE energy and into the deep blue region as well. Consequently, neither a strong donor nor acceptor should be adopted as the D−A component to avoid longer wavelength emission.

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 describes the synthesis route of PMSO. To begin with, PPIBr was synthesized in a one-pot reaction. The mixtures of phenanthrenequinone, phenylamine, ammonium acetate, and 4-bromobenzaldehyde were reacted in acetic acid at 120 °C under N2 atmosphere for 2 h. The crude product was washed with acetic acid and then purified through chromatography to give PPIBr as light yellow powder with good yield. Subsequently, the PPIBr was treated with n-BuLi at −78 °C under N2 atmosphere for 3 h, and then, isopropoxyboronic acid pinacol ester (M1) was added and stirred at room temperature under N2 atmosphere overnight. The crude mixture was purified by chromatography to give PPIB with good yield. Finally, PPIB was reacted with 4,4′-sulfonylbis(chlorobenzene) via the Suzuki coupling reaction to give target compound PMSO. PMSO was fully characterized by NMR, MS, and elemental analysis and corresponded well with its expected structure. Thermal Properties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out under nitrogen atmosphere to investigate the thermal properties of PMSO. PMSO exhibited a very high decomposition temperature (Td, corresponding to 5% weight loss) of 455 °C (Figure S4), indicating its excellent thermal stability. The good morphological stability, which is an important factor to get appreciable device performance, was demonstrated by the rather smooth DSC curve without any glass transition peak or melting point. Electrochemical Properties. Cyclic voltammetry (CV) was employed to calculate the HOMO/LUMO levels of PMSO. The oxidation and reduction potentials were determined to be 0.77 and −2.07 V against the ferrocenium/ferrocene (Fc+/Fc) redox couple (Figure S7), corresponding to a HOMO level of −5.57 eV and a LUMO level of −2.73 eV, respectively. The oxidation potential of PMSO was similar to that of the donor 7051

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Chemistry of Materials Scheme 1. Synthesis Route of PMSOa

a: CH3COOH, CH3COONH4, 120 °C, reflux for 2 h under N2 atm; b: n-BuLi, −78 °C, stirring for 3 h under N2 atm, then adding M1, and stirring overnight under N2 atm; c: Suzuki coupling, Pd(PPh3)4, Na2CO3 (2 M aq), toluene, 90 °C, 48 h under N2 atm. a

Photophysical Properties. To get a better insight into the ground state and the excited state properties of PMSO, the solvation effects on UV/vis absorption as well as photoluminescence (PL) were measured in various solvents ranging from nonpolar solvent n-hexane to high polar solvent acetonitrile. As presented in Figure 2, upon increasing solvent polarity, the absorption

part PhPPI indicating that the HOMO level was determined by the donor PhPPI. Whereas the reduction potential of PMSO was much higher than that of the donor PhPPI, such difference can be attributed to the introduction of electron-withdraw group sulfone which is beneficial to improve electron injecting ability. The CV measurement demonstrates the bipolar injection characteristic of PMSO which is in favor of enhancing device efficiency. Theoretical Calculations. For the purpose of examining the nature of excited state, natural transition orbitals (NTOs) of S1 → S0 were calculated using the TD-M062X/6-31G(d, p) method based on the optimized lowest singlet excited state configuration. As presented in Figure 1, the NTOs of S1 → S0

Figure 1. Natural transition orbitals (NTOs) of S1 → S0 of PMSO.

clearly reveal an HLCT transition character: the hole was mainly distributed on the donor PI and adjacent biphenyl rings with no distribution on S,S′-dioxide, while the particle was mostly spread over the acceptor sulfone and extended to the imidazole ring with little residual on the phenanthrene part. Thus, the hole and particle distributions were well overlapped from the imidazole ring of the donor PI to the phenyl ring of the acceptor sulfone while separated on the S,S′-dioxide and phenanthrene, respectively. The overlap indicates a LE like transition of S1 → S0 and is beneficial for improving the PLQY. Whereas the separation of hole and particle distributions demonstrates a CT featured transition of S1 → S0, accordingly, the electronic transition of S1 → S0 should be comprised of both LE and CT like transitions and should possess HLCT property. The phenyl π bridge and the phenyl ring of acceptor sulfone together can be regarded as biphenyl rings, and the steric hindrance between the biphenyl rings should not be very large. Hence, it is most probable that the small dihedral angle between the phenyl π bridge and the phenyl ring of acceptor sulfone (17.9° as shown in Table S2) in the S1 state renders strong interstate coupling of LE and CT states and consequently results in the HLCT transition character of the S1 → S0. The transition characters of higher singlet and triplet excited states and the corresponding energy diagram were given in Tables S3 and S4 and Figure S15.

Figure 2. Solvation effects on UV/vis absorption of PMSO.

spectra of PMSO barely changed in terms of absorption shape, peak position, and onset, indicating little dipolar variations of the ground state in different solvents. The absorption spectra of PMSO was similar to that of the biphenyl rings substituted PI donor PhPPI (Figure S8); however, the absorption onset of PMSO (403 nm) slightly red-shifted compared with that of PhPPI (383 nm). Since the conjugation between the two donors PhPPI in PMSO is interrupted by the sp3 S atom, the red-shifted absorption onset of PMSO should not be caused by the prolonged conjugation of PhPPI. We speculate that it is the weak CT transition of the ground state between donor PI and acceptor sulfone that results in the red-shift. Hence, the 365 nm absorption band of PMSO arises from a mixture of CT and LE transition indicating that the lowest LE excitation energy and the CT excitation energy are very close. The 347 nm absorption band can be assigned to the π−π* transition of phenanthroimidazole moiety, and the 261 nm absorption band is caused by the π−π* transition of benzene ring connected to the N atom in the imidazole ring.46 The absorption onset of PMSO in THF is 403 nm, and the corresponding optical gap is thus calculated to be 3.08 eV. The electrochemical gap (i.e., the HOMO−LUMO gap) of PMSO is 7052

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Chemistry of Materials 2.84 eV and is a little narrower than the optical gap. As presented in Figures S13 and S14, the excitation NTOs of S0 to S1 is different from the HOMO/LUMO distributions: the HOMO/LUMO orbitals are well separated which indicates a CT like transition while the NTOs of S0 to S1 possess HLCT character. This demonstrates that the optical excitation from S0 to S1 involves not only the transition from HOMO to LUMO but also transitions between other molecular orbitals which give rise to an optical gap larger than the electrochemical gap. As shown in Figure 3, the photoluminescence (PL) spectra of PMSO in low polar solvents such as n-hexane and triethylamine

Figure 4. Linear correlation of orientation polarization ( f) of solvent media with the Stokes shift (υa − υf; a: absorbed light; f: fluorescence) for PMSO (see Tabel S1 for the solvents and corresponding data).

different solvents is very high. For example, the PLQY of PMSO in moderate polar solvent THF was as high as 85%. Detailed information was summarized in Table S1. The very high PLQYs of PMSO in various solvents demonstrate that the HLCT state is a highly emissive excited state which is beneficial for achieving good device performance. The emission peak of neat film was at 470 nm (Figure S9), and the corresponding PLQY was 45%. In contrast, the doped film using 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) as host and 10% weight ratio of PMSO as dopant exhibited deep blue emission peaking at 443 nm. The PLQY of doped film was improved to 72%. The PL difference between neat film and doped film may be caused by the aggregation effect.48,49 To further investigate the excited state properties, transient PL decay of 10−5 M THF solution at 450 nm was performed using the time-correlated single photon counting method under the excitation of a 375 nm laser source with a 68.9 ps pulse width. As presented in Figure 5, the result revealed a single-exponential

Figure 3. Solvatochromic PL of PMSO in different solvents.

exhibited fine vibrational structure revealing the LE feature of the excited state in low polar solvents. With increasing solvent polarity, the emission spectra gradually broadened and became structureless and red-shifted from 404 nm in n-hexane to 479 nm in acetonitrile. Such solvatochromic behavior demonstrates the existence of CT moiety in the excited state. As presented in Figure 4, the linear correlation between Stokes shift and solvent polarity manifests that the attribute of S1 state does not change in either low polar solvents or high polar ones.47 Hence, the S1 state of PMSO may consist of a highly mixed or hybrid local and charge transfer (HLCT) state. In low polar solvents, the LE energy of the HLCT state is a little lower than the CT energy; therefore, the molecule has LE dominated emission with fine vibrational structure. In moderate and high polar solvents, the LE energy does not change much while the CT energy decreases below the LE energy; consequently, the molecule has CT dominated emission with an obvious solvatochromic effect. Specifically, whether in low or high polar solvents, the HLCT nature of the S1 state does not change according to the linear fitted slope of the Stokes shift vs solvents polarity plot; it is only a matter of which ingredient, i.e., LE or CT, is more dominant in the HLCT state. The solvatochromism behavior of PMSO agrees well with the NTOs calculations as shown in Figure 1 and also in accordance with our recent report.37 The PLQY of PMSO in

Figure 5. Decay profile of PMSO in THF.

lifetime of 1.84 ns in THF. The one lifetime decay profile indicates that the LE and CT states in the S1 state are highly mixed or hybridized, suggesting the HLCT attribute of the S1 state. The fluorescence and phosphorescence at 78 K were measured in THF as shown in Figure 6. The prompt emission was the fluorescence of PMSO located in the deep blue region with an emission peak of 438 nm. The 1 ms delayed emission can be assigned to the phosphorescence of PMSO and exhibited fine vibrational structure proving that the phosphorescence 7053

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Figure 6. Fluorescence and phosphorescence of PMSO in THF at 78 K.

may arise from the LE state. The phosphorescence of PMSO and PhPPI was very alike (Figure S12). Thus, the phosphorescence of PMSO may come from the LE state of donor part PhPPI. The first vibrational peak of phosphorescence was 550 nm. Using the emission peak of fluorescence and phosphorescence at 78 K, the respective energy levels of S1 and T1 were thus calculated to be 2.83 and 2.25 eV. The energy gap between S1 and T1 was determined to be 0.58 eV, and reverse intersystem crossing from T1 to S1 should not effectively take place. Detailed photophysical information was summarized in Table 1. Electroluminescence Properties. To verify the applicable potential of PMSO as blue solid emitter, we initially fabricated a nondoped device with a multilayer structure: ITO/MoO3 (6 nm)/NPB (35 nm)/TCTA (10 nm)/PMSO (20 nm)/TPBi (50 nm)/LiF (0.8 nm)/Al (100 nm), in which ITO (indium tin oxide) was used as anode, MoO3 was used as hole injecting layer, NPB (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine) was used as hole transporting layer, TCTA (tris(4-carbazoyl-9-ylphenyl)amine) was used as buffer layer, PMSO was the emitting layer, TPBi (1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene) was used as electron transporting layer, LiF was used as electron injecting layer, and Al was used as cathode. The nondoped device exhibited sky-blue electroluminescence (EL) with emission peak at 465 nm and CIE coordinates of (0.157, 0.206). The EL spectra were very stable at various voltages from 4 to 9 V as shown in Figure 7b. The turn-on voltage of the device was only 2.8 V. As shown in Figure S16, the EL spectrum matched well with the PL of neat film indicating that the EL emission was from the emissive layer, and excimer or exciplex species were effectively suppressed. As can be seen in Figure 7a, the maximum external quantum efficiency (EQE) and current efficiency (CE) were 4.95% and 7.31 cd A−1, respectively. The EQE of the device reached its maximum value at a practical brightness of about 1800 cd m−2; even at the brightness of 10 000 cd m−2, the EQE can still remain 4.65%. The efficiency roll-off is rather small, and the device can continue to have a relatively high efficiency at very high brightness.

Figure 7. (a) EQE−brightness and CE−brightness curves of the nondoped device; (b) normalized EL spectra of the nondoped device at different voltages.

To further improve the device efficiency and color purity, we also constructed the doped OLED device with the structure of ITO/MoO3 (6 nm)/TCTA (40 nm)/PMSO: CBP (wt. 8−10% 20 nm)/TPBi (50 nm)/LiF (0.8 nm)/Al (100 nm) where CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl) was selected as the host. The doped device showed deep blue EL with emission peak at 445 nm and CIE coordinates of (0.152, 0.077), which is very close to the NTSC standard blue CIE coordinates of (0.14, 0.08). The EL spectra barely changed upon increasing voltages from 4 to 9 V (Figure 8). The turn-on voltage of the device was as low as 3.2 V. The maximum EQE and CE were up to 6.80% and 4.64 cd A−1, respectively. The EQE was 6.63% at a brightness of 100 cd m−2, 5.64% at 1000 cd m−2, and 3.66% at 10 000 cd m−2, respectively. The nondoped and doped device data were summarized in Table 2. A comparison between our results and some recently reported deep blue OLEDs with CIEy < 0.1 were also listed in Table S6. The efficiency roll-off of the PMSO based doped device is relatively small, and comprehensively speaking, the device performance of PMSO is among the best of deep blue OLEDs with CIEy < 0.1.

Table 1. Photophysical Properties of PMSO λPL (nm) Tg/Tda b

(°C)

n.o. /455

λAbs (nm) (log ε) c

d

365(4.77), 347(4.75), 261(5.04)

Φfe (%)

doped film

neat film

doped film

neat film

Egf (eV)

HOMO/LUMOg (eV)

ES1/ET1h (eV)

443

470

72

45

3.08

−5.57/−2.73

2.83/2.25

a Tg = glass transition temperature, Td = decomposition temperature. bn.o. = not observed. cAbsorption: measured in 10−5 M THF. dε is the excitation coefficient (M−1 cm−1). ePhotoluminescent quantum yield measured by integrating sphere. fOptical gap calculated from the absorption onset in THF. gMeasured by cyclic voltammetry. hCalculated from the first vibrational peak at 78 K.

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promote a large ratio of singlet exciton formation. Since the energy gap of S1 and T1 is as large as 0.58 eV (Figure 6), we believe that the TADF process cannot occur. Further, the EL intensity had a linear relationship with current density (Figure S18), demonstrating the contribution from TTA is insignificant. We tentatively explained the high ηr as a result of a hot exciton channel, i.e., reverse intersystem crossing from upper excited states as we reported before.38,50

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CONCLUSIONS In conclusion, by using phenanthroimidazole as a weak donor and sulfone as a moderate acceptor, a phenanthroimidazole− sulfone hybrid D−A molecule named as PMSO has been successfully synthesized. The PMSO exhibits LE like emission in low polar solvents and CT featured emission in moderate and high polar solvents. Time-resolved PL decay indicates a singleexponential lifetime of several nanoseconds. Such properties manifest a HLCT excited state character. The doped device of PMSO shows deep blue EL with a emission peak of 445 nm and CIE of (0.152, 0.077) which is very close to the NTSC standard blue CIE coordinates of (0.14, 0.08). The maximum luminescence, EQE, LE, and PE of doped device are 15599 cd m−2, 6.80%, 4.64 cd A−1, and 4.0 lm W1−, respectively. The device results are among the best deep blue OLEDs performance with CIEy < 0.1. Our experimental results prove that the HLCT molecular design may provide a new pathway for further molecular design to realize efficient deep blue electroluminescence.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02685. Experimental details, 1H NMR identification, TGA and DSC curves, cyclic voltammogram, detailed photophysical data in different solvents, optimized configuration of PMSO in ground state and lowest excited state, NTOs of singlet and triplet excited states, PL of neat film, and EL of nondoped device. (PDF)

Figure 8. (a) EQE−brightness and CE−brightness curves of the doped device; (b) normalized EL spectra of the doped device at different voltages.

The radiative exciton yield can be calculated according to the following equation: EQE = γ × ΦPL × ηr × ηout

where EQE is the maximum external quantum efficiency, γ is the carrier recombination efficiency, which in the ideal case is supposed to be unity if the injected holes and electrons are fully recombined and degrade to excitons in the emissive layer, ΦPL is the PLQY of the emission layer, ηr is the radiative exciton yield, and ηout is light out-coupling efficiency, which is 20% if there are not any out-coupling enhancing structures in the device. For the nondoped device, the maximum EQE is 4.95% and ΦPL of the PMSO thin film is 45%; thus, the calculated radiative exciton yield is 55% provided that γ is unity and ηout is 20%. Similarly, the radiative exciton yield for the doped device is calculated to be 47%. The estimated radiative exciton yield of PMSO far exceeds the 25% branching ratio of singlet exciton yield, indicating that our molecular design principle can break the theoretical limit of spin statistics for fluorescent OLEDs and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ¶

Dr. Xiangyang Tang and Dr. Qing Bai contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support of the National Basic Research Program of China (973 Programs, 2013CB834701,

Table 2. EL Performance of Nondoped and Doped Devices Using PMSO as Blue Emitter EQE (%)e device nondoped doped

Vona

(V)

2.8 3.2

b

−2

−1

c

Lmax (cd m )

PEmax (lm W )

35 350 15 598

6.23 4.00

CEmaxd

−1

(cd A )

7.31 4.64

maximum

at 1000 cd m−2

at 10 000 cd m−2

λELf (nm)

CIE (x, y)

4.95 6.80

4.86 5.64

4.65 3.66

465 445

0.157, 0.206 0.152, 0.077

Turn on voltage at a brightness of 1 cd m−2. bMaximum luminance. cMaximum power efficiency. dMaximum current efficiency. eExternal quantum efficiency. fElectroluminescence peak. a

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Chemistry of Materials

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2015CB655003) and National Science Foundation of China (Grant Nos. 21374038, 91233113).

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DOI: 10.1021/acs.chemmater.5b02685 Chem. Mater. 2015, 27, 7050−7057