Hole-Buffer Material Derived from Pyrene , Schiff Base and Tris to Enhance Emission Efficiency of Polymer Light-Emitting Diodes

Inserting a hole-buffer layer is an effective way to enhance emission efficiency of electroluminescence devices. We have successfully synthesized a new hole-buffer material PSB composed of pyrene, Schiff base and trihydroxy tert-butyl groups by the Suzuki-coupling reaction. The HOMO and LUMO levels were −6.33 eV and −2.55 eV, respectively, as estimated from cyclic voltammograms. In addition, homogeneous films (rms roughness ~2 nm) were readily obtained by spin-coating process. Multilayer polymer light-emitting diodes, ITO/PEDOT:PSS/PSB/SY/LiF/Al, have been fabricated using PSB as hole-buffer layer (HBL). Inserting PSB as HBL significantly enhances the performance (maximum luminance: 26,439 cd/m, maximum current efficiency: 7.03 cd/A), compared with the one without PSB (9802 cd/m, 2.43 cd/A). It is also superior to the device with conventional BCP as hole-blocking layer (ITO/PEDOT:PSS/SY/BCP/LiF/Al: 15,496 cd/m, 5.56 cd/A). Current results strongly indicate that the PSB is a potential hole-buffer material for electroluminescent devices.


Introduction
Organic light-emitting diodes (OLEDs) and polymer light-emitting diodes (PLEDs) have attracted considerable attention from both academic and industrial points of view due to promising applications in flat panel displays and solid-state lightings [1] [2] [3].Device efficiency of OLEDs and PLEDs depend mainly on the recombination ratio of holes and electrons injected from anode and cathode, respectively.Balance in holes and electrons (high carriers' recombination ratio) is crucial in enhancing device emission efficiency [4] [5] [6].
However, hole mobility in most organic materials is higher than electron mobility [6]; moreover, the injection rate of holes for most OLED/PLED devices is greater than that of electrons.This leads to imbalanced carriers and reduced device efficiency.Carriers balance can be improved by multilayer device structure, i.e., inserting electron injection and transport layers between emission layer and the cathode [7] [8].In this way, the injection and transport of electrons are raised to a level comparable to those of holes, resulting in more balanced carriers.Another effective strategy is to inserta hole-blocking layer between emissive layer and electron injection layer, such as bathocuproine (BCP), 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)(TPBi) and 1,3,5-tri (m-pyridin-3-ylphenyl) benzene (TmPyPB).The hole-blocking layer effectively confines the holes and excitons within the emissive layer due to its low HOMO level [9] [10] [11] [12].Although this hole-blocking effect can accumulate holes within emissive layer to increase recombination ratio, the recombination region is usually a narrow zone close to the hole-blocking layer.The formation of excitons too near the interface usually leads to diminished color purity and emission efficiency [13].
Recently, hole-buffering materials have been developed to increase the recombination ratio and emission efficiency.The hole-buffering materials effectively reduce the mobility of holes, instead of increasing injection/transport of electrons, to raise the recombination ratio of carriers.For instance, copper (II) phthalocyanine (CuPC), and 4,4',4''-tris (N-carbazolyl)-triphenylamine (TATC) were deposited as hole-buffering layer which effectively reduced hole mobility [14] [15].We have designed a polymer 3P 5 O composed of alternating p-terphenyl and tetraethylene glycolether segments and successfully employed as hole-bufferlayer to improve carriers balance [16].The hole buffer layer was inserted between hole injection and emissive layers, it reduces the mobility of holes and thus recombine with electrons in the whole emissive layer, leading to enhanced device performance [16] [17].In 1999, Forsythe found that transport of holes from ITO anode to NPB hole-transporting layer was injection-limited and device performance depended on hole current [18].They inserted different thickness of CuPC (0 -30 nm) to reduce hole-injection into the emissive layer and effectively enhanced device performance.However, the CuPC layer should be formed by vacuum thermal deposition, whose disadvantages are high cost of fabrication processes and difficulty in fabricating large-area devices than solution process [19] [20].In solution process, polymers are usually used because of their excellent film-forming property.Although this advantage makes polymers very appropriate for PLEDs, some inherent difficulties are still existed for polymers applied in PLEDs, such as purification, control of batch-to-batch variations and wide distribution of molecular weight.On the contrary, the advantages of small Journal of Materials Science and Chemical Engineering molecules applicable in solution process are exact molecular structure and high purity.However, some problems may emerge when applied to device fabrication using solution processes, such as limited film-forming property and poor morphology in film state [20].
In this study, we have designed and successfully synthesized a new small molecular hole-buffering material PSB by the Suzuki-coupling reaction.The core of PSB is a pyrene symmetrically attached with two phenol groups, from which imine (Schiff base) and trihydroxy tert-butyl groups are linked.The pyrene core is introduced to raise hole transport ability [21]; however it aggregates readily due to its flat structure [22].The aggregation may cause aggregation or crystallization during device operation, which becomes as defect in PLEDs to reduce device efficiency and life times.To solve this problem, bulky trihydroxy tert-butyl groups are introduced to prevent aggregation by steric hindrance.Moreover, PSB is inserted with imine linkage (Schiff base) between phenol and trihydroxy tert-butyl groups.The electron-withdrawing propensity of imine moiety is expected to slow down hole-transporting rate.In addition, homogeneous film of PSB was readily obtained by solution process.Multilayer OLED devices were fabricated by using PSB as hole-buffering layer [ITO/PEDOT:PSS/PSB/SY/LiF/Al].
Apparently, the hole-buffering property of PSB is effective in enhancing emission efficiency of electroluminescent devices.

General Procedures
Thermal degradation and transition behaviors of the PSB were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively.The TGA was conducted under a nitrogen atmosphere, heated from ambient temperature to 580˚C at a rate of 10˚C/min using a Perki-nElmer TGA-7 thermal analyzer.The thermal decomposition temperature (T d ) was determined at 5% weight loss of the sample.The thermal transitional properties were measured from −100˚C to 260˚C under a nitrogen atmosphere, using a DSC 2010 from TA Instruments.The glass transition temperature (T g ) was determined from the second heating scan.Cyclic voltammograms (CVs) were measured with a voltammetric apparatus (model CV-50W from BAS) equipped with a three-electrode cell.The cell was made up of a glassy carbon as the working electrode, an Ag/AgNO 3 electrode as the reference electrode, and a platinum wire as the auxiliary electrode.The electrodes were immersed in acetonitrile containing 0.1 M (n-Bu) 4 NClO 4 as the electrolyte.The energy levels were calculated using the ferrocene (FOC) value of −4.8 eV with respect to vacuum level, Journal of Materials Science and Chemical Engineering which is defined as zero.Absorption and photoluminescence (PL) spectra were measured with a Jasco V-550 spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively.The surface morphologies of deposited films were measured with an atomic force microscope (AFM), a Dimension Icon from Bruker.The film thickness was measured with a surface profilometer (Surfcorder ET4000 M).

Devices Fabrication and Measurements
ITO glass substrates were cleansed successively, in an ultrasonic cleaner, with neutral detergent, deionized water, acetone, and isopropyl alcohol at 45˚C, each for 20 min.Then the substrates were further treated in a UV-ozone chamber for 20 min to remove trace organic contaminants.A solution of PEDOT:PSS was spin-coated onto the ITO surface to deposit the hole-injection layer, followed with baking at 140˚C for 20 min.Next, a PSB solution in N, N-dimethylformamide (DMF) was spin-coated on top of the PEDOT:PSS layer to form as hole-buffering Scheme 1. Synthetic procedures of pyrene-Schiff base derivative PSB.Methanol PSB Journal of Materials Science and Chemical Engineering layer, followed by annealing at 70˚C for 20 min.The emission layer was spin-coated from a toluene solution of Super Yellow (SY, Merck), followed with annealing at 70˚C for 20 min.Finally, the substrate was transferred into a vacuum chamber to deposit LiF (1 nm) as an electron injection layer and then aluminum (90 nm) as the cathode, at a pressure less than 2 × 10 −6 Torr.Finally, current density-voltage-brightness (J-V-L) characteristics of the devices were recorded using a combination of a Keithley power source (model 2400) and an Ocean Optics usb2000 fluorescence spectrophotometer.

Thermal, Photo-Physical and Electrochemical Properties
Thermal decomposition behavior of PSB was investigated by thermogravimetric analysis under nitrogen atmosphere, between room temperature and 580˚C.As shown in Figure 1 The absorption and photoluminescence spectra of PSB in DMF solution (10 −5 M) and as thin film spin-coated from DMF solution (10 mg/mL) are illustrated in Figure 2, with characteristic data summarized in Table 1.In DMF solution, PSB revealed three absorption peaks at 267, 307 and 342 nm, attributable to the hydroxyl groups n -π* transition, pyrene and aryl groups π -π* transition [24], and imine groups π -π* transition [25], respectively.In film state, two absorptions at 294 nm and 430 nm were observed; the former is ascribed to blue shiftedabsorption of pyrene and aryl groups, while the latter is caused by the aggregation of PSB [26].In DMF solution, PSB shows a weak emission peak at 515 nm (normalized), attributed to pyrene and aryl groups.However, no emission was detectable in film state, due probably to complete fluorescence quenching caused by the two terminal Schiff base (imine) groups.The fluorescence of excited pyrene groups is prohibited by photo-induced electron transfer (PET) from imine moieties [27].Non-fluorescence of PSB is a great merit for electroluminescent devices, because its will not degrade color purity of the emissive layer when used as hole-buffer layer.Table 1.Optical properties of PSB.

E
are the onset oxidation and onset reduction potentials respectively, relative to the ferrocene/ferrocenium couple whose energy level is already known (−4.8 eV).The estimated LUMO and HOMO energy levels are −2.55 eV and −6.33 eV, respectively.Therefore, the electrochemically determined band-gap (E g el ) is 3.78 eV.The estimated LUMO and HOMO levels of PSB are much lower than those of pyrene (−1.48 eV, −5.44 eV) [28], due to the presence of two terminal Schiff base-modified phenol groups.
Low HOMO level is effective in reducing hole mobility, which in turn raise carriers' recombination ratio in the emissive layer (Super Yellow) to enhance emission efficiency.Density functional theory (DFT) theory was employed to investigate electron distribution in PSB, using an electronic structure-modeling program Gaussian 09.As shown in Figure 4, the HOMO is exclusively localized on pyrene core, while the LUMO is mainly located on the Schiff base groups and partially on the pyrene core.Therefore, the two Schiff base groups in PSB should be effective in reducing hole mobility due to their electron-withdrawing characteristics.

Surface Morphology
Surface morphology of deposited film during device fabrication was observed by an atomic force microscopy (AFM); the root-mean-square (rms) surface roughness was estimated from the AFM topography images.At first, the effect of concentration was investigated using PSB solutions in DMF (5, 10, 15 and 20 mg/mL).The hole-injecting PEDOT:PSS was spin-coated onto ITO substrate at 3000 rpm followed by annealing at 140˚C for 20 min.The PSB was then spin-coated on top of the PEDOT:PSS layer first at 2000 rpm for 15 sec and then at 6000 rpm for 1 min.The spin-coated PSB film was annealed at 70˚C for 20 min under nitrogen atmosphere.Figure 6 shows the AFM topography images.
The root-mean-square (rms) surface roughness of the PEDOT:PSS layer was 1.18 nm which is comparable to that reported in literature.PSB films revealed smooth morphology when spin-coated from lower concentrations (5 and 10 mg/mL), with surface roughness of 1.88 nm and 2.08 nm, respectively.However, the surface became much rougher when spin-coated from higher concentrations (15 and 20 mg/mL), with the roughness of 10.70 nm and 9.77 nm, respectively.

Electroluminescent Properties
To investigate the feasibility of PBS as a hole-buffering material, multilayer was also prepared to evaluate the effectiveness of hole-buffering PSB. Figure 7 demonstrates luminance and current density versus voltage and efficiency versus current density of the multilayer devices, with the characteristic data summarized in Table 3.The hole-buffer layer was inserted between hole-injecting PEDOT:PSS and emissive SY layers by spin-coating PSB solutions with different concentrations (5, 7, 10, 15, 20 mg/mL).The thickness of deposited PSB films was increased gradually from 7 nm to 38 nm with the concentration increasing from 5 mg/mL to 20 mg/mL (Table 3).
As expected, insertion of hole-blocking BCP layer (15 nm) decreases current Journal of Materials Science and Chemical Engineering density moderately under the same operating voltages (Figure 7(b)); for instance, the current density of blank device was about 200 mA/cm 2 at 8 V which decreased to ca. 100 mA/cm 2 after the insertion of BCP.The BCP layer effectively blocks the holes at the interface with the emissive SY layer.However, hole-buffering PSB layer reduced the current density dramatically, with the reduction increased with thickness.In addition, the maximum luminance of PSB-based device increased significantly from 9060 cd/m 2 to 26,439 cd/m 2 as the thickness increased from 7 nm to 15 nm (Table 3).Apparently, the maximum luminance (26,439 cd/m 2 ) at 15 nm PSB is much higher than 9802 cd/m 2 and 15,496 cd/m 2 of the blank and BCP-based devices, respectively.However, the maximum luminance dropped dramatically to 8103 cd/m 2 and 5812 cd/m 2 when the thickness of PSB further increased to 24 nm and 38 nm respectively.Clearly, an optimal PSB thickness exists at about 15 nm, at which the balance of carriers is most effective.Similar trends are observed for current efficiency and power efficiency as shown in Figure 7(c) and Figure 7(d) and Table 3.The maximum current efficiency first increased from 3.69 cd/A to 7.03 cd/A, followed with dramatic drop to 5.26 and 2.89 cd/A at 24 nm and 38 nm, respectively.However, these current efficiencies are still higher than 2.43 cd/A of the blank device.Furthermore, hole-buffering PSB is more effective than hole-blocking BCP in enhancingefficiency.Under the same thickness (15 nm), the current efficiency of PSB-based device was 7.03 cd/A, compared with 5.56 cd/A of BCP-based one.But the PSB layer raised turn-on voltage gradually from 3.3 V to 4.9 V when its thickness was increased from 7 nm to 38 nm.Apparently, inserting hole-buffering PSB results in significant performance enhancement, in terms of maximum luminance and current efficiency, compared with blank device (Table 3).The performance is even superior to the device with conventional BCP as the hole-blocking layer (maximum luminance: 15,496 cd/m 2 , maximum current efficiency: 5.56 cd/A).These results indicate that hole-buffering PSB outperforms hole-blocking BCP when applied to balance carriers' recombination.Single-carrier transporting devices are thought effective in elucidating underlying mechanisms, whose discussion is in the next section.
Single-carrier devices (hole-only and electron-only) with hole-buffering (PSB) or hole-blocking (BCP) layers were fabricated to investigate charge-transporting properties.The hole-only device used Au as cathode to replace LiF/Al, whereas the electron-only one employed ZnO to replace hole-injecting PEDOT:PSS.For hole-only device, large barrier height between Au (work function: 5.1 eV) and emissive SY layer (LUMO: −2.8 eV) obstructs electron injection, resulting in exclusive holes' contribution to current density.Similarly, for electron-only device, the barrier height between ZnO (−7.6 eV) and ITO anode (−4.8 eV) is very high (2.8 eV), leading to exclusive contribution of electrons.Therefore, the current density of hole-only and electron-only devices can be seen as hole density and electron density.For hole-only devices (Figure 8(a)), current density reduced dramatically with 7 nm PSB or 15 nm BCP as hole-bufferingor hole-blocking layer.Moreover, the reduction increased gradually with increasing thickness of PSB (from 7 nm to 38 nm).
On the contrary, for electron-only devices the current density increased only slightly with PSB thickness (from 7 nm to 15 nm) as shown in Figure 8(b).Further increase to 24 nm and 38 nm the current density became lower than the blank device.However, the BCP layer (15 nm) raised the current density significantly.This is because BCP, inserted between emissive SY and Al cathode, is also an effective electron injection/transport material due to its low LUMO level (−3.2 eV), in addition to its hole-blocking capability (low HOMO level).The PSB layer, inserted between emissive SY and ZnO, promotes electron transport slightly at low thickness (7 -15 nm) due probably to polar and electron-withdrawing imine and trihydroxy tert-butyl groups.Interfacial polarization in carrier-transporting layer contributes greatly to the promotion of carriers' injection.However, the contribution gradually diminishes with increasing thickness due to decreased polarity gradient. The

Conclusion
We have successfully synthesized a new compound PSB composed of pyrene core linked with trihydroxy tert-butyl groups through imine moiety by the Suzuki-coupling reaction and applied as hole-buffer layer in multilayer PLEDs.

Figure 1 .
Figure 1.Thermogravimetric and differential scanning calorimetric (inset) traces of PSB at a heating rate of 10˚C/min under nitrogen atmosphere.

Figure 2 .
Figure 2. Normalized UV/Vis absorption and photoluminescence spectra of PSB in DMF solution and at film state.

5 M
DMF solution.bThe film prepared by spin-coating 10 mg/mL in DMF solution on quartz substrate.c Excitation wavelength was 307 nm.d The value was estimated by the λ onset of the UV/Vis spectrum.Cyclic voltammetric analysis is a powerful method to evaluate electrochemical properties of organic and polymeric materials, especially the estimation of oxidation and reduction potentials.Usually the sample is coated onto the working electrode or dissolved in the electrolyte.The propensity to lose or obtain electron is readily estimated from the oxidation and reduction potentials.The PSB film was deposited onto the glassy carbon electrode by dropping its chloroform solution.The cyclic voltammogram of PSB is shown in Figure3and the corresponding electrochemical data are summarized in

3
Journal of Materials Science and Chemical Engineering

Figure 5
Figure5shows the energy level diagrams of hole-buffering and hole-blocking devices.The HOMO level of BCP (−6.7 eV) is much lower than −5.0 eV of the emissive SY layer.Accordingly, BCP is effective in blocking the holes when inserted between emissive SY layer and the cathode.On the other hand, the HOMO level of PSB (−6.33 eV) is also much lower than −5.0 eV of the hole-injecting PEDOT:PSS layer.By the same way, PSB can effectively obstruct the holes when inserted between PEDOT:PSS and emissive SY layers.This will reduce transport rate of the holes to raise carrier recombination ratio and emission efficiency, because for most conjugated emissive materials holes transport much faster than electrons[29].

Figure 5 .Figure 6 .
Figure 5. Energy level diagrams of the PLEDs using (a) PSB as hole-buffering layer and (b) BCP as hole-blocking layer.

Figure 9 .
Figure 9. Proposed illustrations for recombination region of holes and electrons in the PLEDs using (a) hole-blocking layer (BCP) and (b) hole-buffer layer (PSB).
Thermal decomposition temperature (T d ) at 5% weight loss was observed at 300˚C.The HOMO and LUMO levels of PSB were −6.33 and −2.55 eV respectively, estimated from onset oxidation and reduction potentials determined by cyclic voltammetry.Multilayer PLEDs have been successfully fabricated (ITO/PEDOT:PSS/PSB/SY/LiF/Al), in which PEDOT:PSS, PSB and SY layers were deposited by spin-coating process.The thickness of PSB layer increased gradually from 7 nm to 38 nm with PSB concentration increased from 5 mg/mL to 20 mg/mL in DMF.Inserting PSB as hole-buffer layer effectively raised luminance and emission efficiency, with optimal PSB thickness being 15 nm.The maximum luminance and maximum current efficiency were 26,439 cd/m 2 and 7.03 cd/A, respectively, which were superior to 9802 cd/m 2 and 2.43 cd/A of the blank device.Under the same thickness (15 nm), the PSB-based device also outperformed the one with the conventional BCP as hole-blocking layer [ITO/PEDOT:PSS/SY/BCP/LiF/Al],in terms of maximum luminance and maximum current efficiency (15,496 cd/m 2 , 5.56 cd/A).Current results indicate that the PSB composed of pyrene, polar imine and trihydroxy tert-butyl groups is highly effective in buffering holes' transport to enhance performance of electroluminescent devices.

Table 2
Journal of Materials Science and Chemical Engineering trolyte ions was still not established.The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were esti- . The CV curve was not a closed loop, probably attributed to the equilibrium between PSB and elec-

Table 2 .
Electrochemical properties of PSB and pyrene.

Table 3 .
Electroluminescent properties of multilayer PLEDs.a Thickness was determined by alpha-step measurement.c Turn-on voltage at 10 cd/m 2 .d Maximum luminance.e Maximum current efficiency.f Maximum power efficiency.g The 1931 CIE coordinate at maximum current efficiency.h Concentration of PSB in DMF. b