Efficient Solution-Processed Nondoped Deep-Blue Organic Light

ARTICLE pubs.acs.org/JPCC

Efficient Solution-Processed Nondoped Deep-Blue Organic Light-Emitting Diodes Based on Fluorene-Bridged Anthracene Derivatives Appended with Charge Transport Moieties Minrong Zhu,† Tengling Ye,‡ Chen-Ge Li,† Xiaosong Cao,† Cheng Zhong,† Dongge Ma,*,‡ Jingui Qin,† and Chuluo Yang*,† †

Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, People’s Republic of China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ABSTRACT: A series of fluorene-bridged anthracene derivatives appended with different charge transport moieties were synthesized, and their thermal, photophysical, and electrochemical properties were investigated. By the introduction of fluorene between two anthracene units as well as peripheral functional aryl substituents, the sophisticated compounds show a decreased tendency to crystallize and have high glass transition temperatures ranging from 165 to 229 °C. The theoretical calculations reveal that the four self-hosted blue emitters possess noncoplanar structure to suppress the intermolecular interaction in films. Solution-processed small-molecular organic light-emitting diodes featuring 1 as the emitter achieve a maximum current efficiency of 2.0 cd A
1 with Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.13), which are very close to the National Television Standards Committee’s blue standard. A facile strategy to design solution-processable highly emissive anthracene derivatives for nondoped deep-blue electroluminescence by incorporating π-conjugated bridge and bipolar charge transport periphery is demonstrated.

’ INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted considerable attention for potential application in next generation flat panel displays and solid-state light sources.1 Full-color displays require red, green, and blue emission with relatively equal stability, efficiency, and color purity. It is critical to develop highly efficient blue emission, which is defined as having a Commission Internationale de L’Eclairage (CIE) y coordinate value 450 °C). No obvious glass transition and endothermic melting transition were observed for 4 during the measurement. 1
3 show high glass transition temperature (Tg) ranging from 165 to 229 °C, which is much higher than that of common blue fluorescent materials 4,40 -bis(2,2-diphenylvinyl)biphenyl (DPVBi) (64 °C)14 and 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) (120 °C), indicating that the introduction of functional aryl groups greatly improved their morphological stability.15a 2 has the highest Tg value because of its largest molecular size and the more rigid nature of carbazole units. The slightly higher Tg value of 3 than that of 1 may be attributed to the different terminal substitution geometries.15b Besides, the results suggest that the alkyl side chains on the 9,90 -position of fluorene do not impair the thermal stabilities of the anthracene derivatives. The excellent thermal stability enables the preparation of homogeneous and stable amorphous thin films through solution processing, which is crucial for the operation of OLEDs. Density functional theory (DFT) calculations (B3LYP; 6-31G(d)) were carried out to obtain a better understanding about the molecular geometries of 1
4. As depicted in Figure 1, the peripheral substituents and fluorenyl bridge at the 9- and 10positions of the anthracene have twisted configuration, resulting in a noncoplanar structure in each molecule. According to our previous report, through the meta-linkage topology between the emissive core and charge transport antennas, the geometrical characteristics thereafter can sufficiently reduce crystallinity and intermolecular interactions of the π-conjugation systems and then improve morphological stability of thin film. Photophysical Properties. The UV
vis absorption spectra in toluene solution and PL spectra of compounds 1
4 recorded in both toluene solution and films are presented in Figure 2. The absorption band in the range from 290 to 320 nm can be assigned to the center fluorene unit and n
π* transition of the peripheral functional aryl groups.16 Similar characteristic vibronic patterns

ranging from 350 to 400 nm can be ascribed to the π
π* transitions of the anthracene units of the compounds.17 The compounds are highly emissive in toluene solution with emission maxima falling in the range of 434
442 nm. Remarkably, the PL spectra of 1
3 in the film state still in the deep-blue region show a slight bathochromic shift (∼10 nm) with respect to those of the solution, which implies that significant intermolecular interactions do not occur in the ground state. However, 4 with four oxadiazole moieties attached onto the core exhibits a red-shifted maximum emission at 461 nm owing to the strong charge-transfer effect. Additionally, the full widths at halfmaximum (fwhm) are almost the same in films compared with that in solution.18 The insertion of fluorene into two anthracene units as well as introduction of aryl substituents through metalinkage at the 9- and 10-positions of the anthracenes effectively suppress close-packing of constituent molecules and shield the intermolecular interactions.19 Due to the presence of the anthracene moiety, all of the blue materials show relatively high photoluminescent quantum yield (PLQY) in dilute toluene solution when using 9,10-diphenylanthracene (Φ = 0.90) as the calibration standard.20 2 with rigid cabazole units in periphery has the highest quantum yield since the most twisted configuration could reduce the nonradiative decay. In addition, the compounds show high quantum yields (Φ = 0.66
0.41) in thin films, which can be attributed to that the steric hindrance suppresses molecular close-packing in the solid state.4c Electrochemical Properties. Cyclic voltammetry (CV) was carried out to characterize the electrochemical behaviors of these materials as revealed in Figure 3. All reduction and oxidation potentials recorded were relative to the redox couple of ferrocene/ferrocenium (Fc/Fc+). In each case, the anodic scan was performed in CH2Cl2, while the cathodic scan was conducted in dry degassed THF. The highest occupied molecular orbital (HOMO)/lowest occupied molecular orbital (LUMO) energy levels were estimated from the onset of oxidation/reduction potentials, respectively, as summarized in Table 1. The HOMO level for 1 with a peripheral triphenylamine group is higher than that of the other anthracene derivatives. It could be rationalized that the strong-donating substituents might reduce oxidation potentials of the molecule and raise the HOMO energy level.21 The low energy barrier between 1 and the anode will facilitate hole injection into the emission layer. Electroluminescence Properties. 1
3 were chosen to evaluate the EL properties due to their deep-blue-emitting nature in films. Device I was then fabricated with the following configuration: 17969

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Table 1. Photophysical, Thermal, and Electrochemical Data of 1
4 λabsa (nm)

λema (nm)

fwhma (nm)

λemb (nm)

fwhmb (nm)

ΦFLc

1

358/378/397

438

46

449

52

0.90 (0.62)

165/-

5.30/2.40

2

358/377/398

434

47

444

50

0.98 (0.66)

229/378

5.46/2.45

3

359/377/398

442

45

442

53

0.84 (0.41)

189/376

5.45/2.46

4

359/378/398

441

47

461

54

0.83 (0.54)


/


5.48/2.53

Tg/Tm (°C)

HOMOd/LUMOd (eV)

a Measured in toluene. b Measured in films. c Fluorescence quantum yields in solution and films (in parentheses). The fluorescence quantum yields in toluene solution were measured using 9,10-diphenylanthracence (Φ = 0.9) as a standard, and the solid state fluorescence quantum yields were measured on the quartz plate using an integrating sphere. d Estimated from the onset of oxidation and reduction potentials.

Figure 4. Energy diagram of the materials used in devices.

ITO/PEDOT4083 (50 nm)/emission layer (EML, 80 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). The PEDOT4083 is used as an anode buffer layer for ITO, and TPBI and LiF act as electrontransporting and electron-injecting layers (ETL and EIL), respectively. The device energy level diagram was present in Figure 4. Figure 5 shows current density
voltage
brightness (J
V
L) characteristics, current efficiency versus current density curves, and EL spectra for device I. The devices based on 1
3 display turn-on voltages around 5.5 V, which are relatively low for smallmolecular solution-processed OLEDs. The emission shoulders around 380 nm for 2 can be ascribed to the emission from TPBI. Distinct long-wavelength emission at 520 nm is observed for 1, which might be generated by an exciplex formed at the interface. Accordingly, we can assume that the recombination zones for the bilayer devices are located near the EML/ETL interface.22 The luminescence efficiencies vary in accordance with the changes in the end-capping groups though these compounds share the same emissive core. The best EL performance is achieved by 1 with a maximum current efficiency of 0.6 cd A
1 (Table 2), and the value remains at 0.48 cd A
1 upon increasing to the practical brightness of 100 cd m
2, though its color purity is not ideal yet. Surprisingly, 2 with more elegant photophysical and thermal properties exhibits poorer device efficiency. As evidenced by the lowest current density, the EL performance might suffer from the diminished conductivity by electrically inserting the tert-butyl group terminated to C3 and C6 of carbazole, which was supposed to enhance the electrochemical stability of the compound.23

Figure 5. (a) J
V
L characteristics. (b) Current efficiency versus current density curves for device I. (c) EL spectra recorded at 10 V.

To further improve the EL efficiency and color purity of 1 with high-lying HOMO level, we fabricate device II in the same structure except replacing PEDOT4083 with PEDOT8000, the typical electrical conductivity of which is ∼10
3 and ∼10
5 S cm
1, respectively. It was reported that the device with PEDOT8000 presented more efficient attenuation of hole flux and control over electrical leakage.24 The J
V
L characteristic, current efficiency versus current density curve, and EL spectrum are shown in Figure 6. The current density is much lower than that of the PEDOT4083 device, which can be attributed to the 17970

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Table 2. Electroluminescence Performances of the Devices

a

device

host emitter

Vona (V)

Lmax (cd m
2)b, voltages (V)

ηc.maxc (cd A
1)

ηp.maxd (lm W1
)

λem, fwhm (nm)

CIE (x, y)e

I

1

5.9

1174, 13.5

0.52

0.26

442, 64

0.18, 0.20

II

1

5.5

615, 10.0

1.96

1.08

442, 60

0.15, 0.13

I

2

5.7

902, 15.9

0.38

0.19

455, 63

0.16, 0.10

I

3

5.7

132, 15.5

0.10

0.05

442, 42

0.19, 0.22

Turn-on voltages at 1 cd m
2. b Maximum luminance. c Maximum current efficiency. d Maximum power efficiency. e Measured at 10 V.

quantum yields. Moreover, the functional moieties appended to the anthracenes facilitate the charge injection and transportation. Simple bilayer devices based on the new blue materials show moderate EL efficiency with maximum current efficiency to 0.6 cd A
1 for 1. In particular, through using a low-conductivity anode buffer layer to restrict the hole flux, the EL efficiency was significantly improved to 2.0 cd A
1 with deep-blue color chromaticity. We believe that the rational molecular strategy can be applied to generate deeper blue fluorescent materials for use in OLED displays.

’ AUTHOR INFORMATION Corresponding Authors

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

’ ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (Nos. 90922020, 50773057), the National Basic Research Program of China (973 Program 2009CB623602), the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and the Fundamental Research Funds for the Central Universities of China for financial support. Figure 6. (a) J
V
L characteristic for device II with 1 as EML. Inset: Current efficiency plus power efficiency versus current density curves. (b) EL spectrum of device II at 10 V.

decreased leakage current and balanced charge flux, and simultaneously the current/power efficiencies are elevated to a higher level. The maximum current efficiency of device II reaches nearly 2.0 cd A
1, and the maximum power efficiency is five times higher than that of device I at the CIE coordinates of (0.15, 0.13), representing deep blue emission in the 1931 CIE diagram, which is comparable with the best device performances in the literature for the solution-processed small-molecular nondoped deep-blue OLEDs.25 This drastic improvement could be rationalized from the fact that excess hole carriers have been retarded to transport into the EML, and thus the recombination zone shifts away from the interface of EML/ETL.

’ CONCLUSIONS In summary, we have designed and synthesized four bluelight-emitting fluorene-cored anthracene derivatives for solutionprocessed nondoped blue OLEDs. Theoretical calculations reveal that the blue emitters have noncoplanar structures resulting from the central fluorenyl bridge and charge-transporting groups substituted on the anthracene units. The steric hindrance suppresses molecular close-packing in the solid state and also enables us to form stable amorphous films and pronounced PL

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