Influence of molecular configuration and functional substituents on excited state energy levels in two naphthyl-based phosphine oxide hosts

Organic Electronics 13 (2012) 1516–1525

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Organic Electronics

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Influence of molecular configuration and functional substituents onexcited state energy levels in two naphthyl-based phosphine oxide hosts

Hui Xu a,b,⇑, Guohua Xie c, Chunmiao Han a,b, Zhensong Zhang c, Zhaopeng Deng a,b, Yi Zhao c,⇑,Pengfei Yan a,b, Shiyong Liu c

a Key Laboratory of Functional Inorganic Material Chemistry, Heilongjiang University, Education of Ministry, Harbin 150080, PR Chinab School of Chemistry and Materials, Heilongjiang University, Harbin 150080, PR Chinac State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, 2699, Qianjin Street,Changchun 130012, PR China

a r t i c l e i n f o

Article history:Received 23 December 2011Received in revised form 10 April 2012Accepted 3 May 2012Available online 19 May 2012

Keywords:Phosphine oxideHostElectrophosphorescenceConfigurationTriplet energy level

1566-1199/$ - see front matter � 2012 Elsevier B.Vhttp://dx.doi.org/10.1016/j.orgel.2012.05.003

⇑ Corresponding authors. Address: Key Laboratorganic Material Chemistry, Heilongjiang University, EHarbin 150080, PR China. Tel.: +86 451 81773032; fa(H. Xu).

E-mail addresses: [email protected] (H. Xu),(Y. Zhao).

a b s t r a c t

Two phosphine oxide hosts, 1,8-bis(diphenylphosphoryl)naphthalene (NAPO) and 2,20-bis(diphenylphosphoryl)-1,10-binaphthyl (BiNAPO), were designed and synthesized toinvestigate the influence of molecular configuration and the density of functional substit-uents on the excited energy levels. With the much bigger conjugated area, BiNAPO has theT1 of 2.46 eV, which is only 0.17 eV less than that of naphthalene. This should be owing tothe orthogonal configuration of BiNAPO. Meanwhile, NAPO with the higher density offunctional substituents has the much lower T1 of 2.26 eV. It is showed that shorteningthe conjugated length by tuning molecular configuration and reducing the density of func-tional substituents can effectively remain T1 of the chromophore. Simultaneously, Gaussiansimulation showed that the larger conjugated area and the rational distribution of func-tional substituents endow BiNAPO with more balanced carrier injecting/transporting abil-ity, which was further proved by nominal singly-carrier-only devices. The low-voltagedriving and efficient green-emitting electrophosphorescent devices based on BiNAPO weredemonstrated.

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1. Introduction

Organic optoelectronic materials have achieved muchattention because of their wide applications in energyconversion (such as organic light-emitting diodes (OLEDs)[1–4], organic photovoltaic cells (OPV) [5,6], and up-con-version luminescence [7]) and information detecting anddisplay (such as chemical sensing [8] and biological imag-ing [9]). The intra- and intermolecular energy transfer pro-cesses are significant for these applications, which usually

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y of Functional Inor-ducation of Ministry,x: +86 451 86608042

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involve in singlet and triplet excited state energy levels(S1 and T1) [8]. For up-conversion luminescence based ontriplet–triplet annihilation (TTA), high S1 and low T1 ofthe emitters are expected with the aim to increase theanti-stocks shift [10]. Contrarily, for electrophosphores-cence, low S1 and high T1 of the hosts are superior inimproving the carrier injecting/transporting and increasingthe efficiencies of the energy transfer to light-emittingphosphors [11]. Therefore, for different applications differ-ent S1 and T1 are required. However, the relationshipbetween the molecular structures and the excited energylevels is still not clear. How to control or tune these energylevels purposely is still a major challenge.

Recently, the high-efficiency hosts for phosphorescentorganic light-emitting diodes (PHOLEDs) have been paidmuch attention [12]. Since the hosts used in these devicesshould have high enough T1 to facilitate the exothermal

Scheme 1. Structure of two naphthyl-based PO hosts.

H. Xu et al. / Organic Electronics 13 (2012) 1516–1525 1517

energy transfer to the emitting phosphors (such as FIrpic orIr(ppy)2acac) [13], a common concept of the moleculardesign of hosts is controlling T1 through limitingconjugated area and functional substitution. Accordingly,carbarzole derivatives [14–24], silane derivatives [25–27],pyridine derivatives [28–36] and aryl phosphine oxide(PO) derivatives [37–54] with high T1 based on meso,twisted or insulating linkage are already developed. But,the small conjugated areas of these hosts often correspondto the large energy gaps between frontier molecular orbi-tals (FMOs). The limited p-conjugation and insufficientfunctionalization would be inferior in carrier injectingand transporting in the emitting layers (EMLs) [38]. Re-cently, PO hosts show the great potential as hosts withboth high T1 and carrier-transporting ability, in which C–P bond can effectively block the communication betweenthe aryl groups in the molecules and electron-withdrawingP@O bond has the effect of polarizing the molecules [40].However, it is noticeable that the electron-withdrawingPO moieties (such as diphenylphosphine oxide (DPPO))still slightly expend the conjugated length [37]. Therefore,only short chromophore units, including fluorene [38,55],biphenyl [37], carbazole [54,56] and dibenzofuran [40],

Scheme 2. Synthetic route of the

are utilized, which extremely restrains the systematicand comprehensive functional design of the highly effi-cient hosts. Our group also reported several PO hosts withhigh T1 and improved carrier injecting/transporting abilityon the basis of short-axis substitution, indirect linkage andmulti-insulating linkage [57–63]. It is obvious that thelinkage modes between each functional part have remark-able influences on excited states and carrier transport.However, for the subsequent work, we should make an ef-fort to figure out the relationship between molecular con-figuration and the properties.

In this work, we chose two naphthyl PO compoundscomposed of the same chromophore naphthyl and DPPOs,namely 1,8-bis(diphenylphosphoryl)naphthalene (NAPO)and 2,20-bis(diphenylphosphoryl)-1,10-binaphthyl (BiNAPO)(Schemes 1 and 2), to investigate the influence of molecu-lar configuration and the density of functional substituentson the excited energy levels. The photophysical, thermaland theoretical analysis of the PO compounds was per-formed to study the relationship between the molecularstructures and the properties. It indicated that based onthe appropriate molecular configuration BiNAPO with big-ger conjugated area could not only obtain the suitable ex-cited energy levels, but also achieve improved thermalstability and balanced carrier injecting/transporting abil-ity. As the result, the green-emitting PHOLEDs based onBiNAPO with much better electroluminescent (EL) perfor-mance of low-driving voltage, much higher brightness andefficiencies were achieved.

2. Experimental

2.1. Materials and instruments

All the reagents and solvents used for the synthesis ofNAPO and BiNAPO were purchased from Aldrich and Acroscompanies. NAPO was synthesized according to our previ-ous reports [64,65].

naphthyl-based PO hosts.

1518 H. Xu et al. / Organic Electronics 13 (2012) 1516–1525

1H NMR spectra were recorded using a Varian Mercuryplus 400NB spectrometer relative to tetramethylsilane(TMS) as internal standard. Molecular masses were deter-mined by a FINNIGAN LCQ Electro-Spraying Ionization-Mass Spectrometry (ESI-MS), or a MALDI-TOF-MS. Elemen-tal analyses were performed on a Vario EL III elementalanalyzer. Absorption and PL emission spectra of the targetcompounds were measured using a SHIMADZU UV-3150spectrophotometer and a SHIMADZU RF-5301PC spectro-photometer, respectively. Thermogravimetric analysis(TGA) and differential scanning calorimetry (DSC) wereperformed on Shimadzu DSC-60A and DTG-60A thermalanalyzers under nitrogen atmosphere at a heating rate of10 �C min�1. Phosphorescence spectra were measured indichloromethane using an Edinburgh FPLS 920 fluores-cence spectrophotometer at 77 K cooling by liquid nitrogenafter a delay of 2 ms. The crystal suitable for X-ray single-crystal diffraction analysis was obtained through the slowevaporation of the ethanol solution of the compound atroom temperature. All diffraction data were collected at295 K on a RIGAKU RAXIS-RAPID diffractometer withgraphite monochromatized Mo-Ka (k = 0.71073 Å) radia-tion in x scan mode. All structures were solved by directmethod and difference Fourier syntheses. Non-hydrogenatoms were refined by full-matrix least-squares tech-niques on F2 with anisotropic thermal parameters. Thehydrogen atoms attached to carbons were placed in calcu-lated positions with C–H = 0.93 Å and U (H) = 1.2Ueq (C) inthe riding model approximation. All calculations were car-ried out with the SHELXL97 program.

2.2. Synthesis of 2,20-bis(diphenylphosphoryl)-1,10-binaphthyl(BiNAPO)

2,20-Dibromo-1,10-binaphthyl (8.25 g, 20 mmol) in drytetrahydrofuran (150 mL) was stirred under argon andcooled to �78 �C. n-Butyl lithium (25.6 mL, 2.5 mol/L,64 mmol) was slowly added with vigorous stirring. Stirringwas continued for 2 h at the same temperature. Chlorodi-phenylphosphine (18 mL, 95.3 mmol) was slowly addedand the mixture allowed stirring for 30 min at the sametemperature. Subsequently the mixture was stirring at roomtemperature for 12 h. The reaction mixture was quenchedwith water and the organic contents were extracted withmethylene dichloride. The extract phase was dried withanhydrous sodium sulfate and the solvent was removed bydistillation in a vacuum.at 45 �C. Then the crude intermedi-ate (0.622 g, 1 mmol) in 10 mL dichloromethane were stir-red in a round-bottom flask, and then H2O2 (1.2 mL,10 mmol, 30%) was slowly added into the mixture whilestirring at room temperature. The organic layer was sepa-rated and washed with water. The extract was evaporatedto dryness affording a white solid, which was further puri-fied by column chromatography (silica, ethyl acetate).BiNAPO was obtained as white powder with the total yieldof 62%. 1H NMR (400 MHz, CDCl3): d (ppm): 7.858 (dd,J = 8.8, 2.4 Hz, 2H), 7.828 (d, J = 8.4 Hz, 2H), 7.736 (d,J = 6.8 Hz, 2H), 7.705 (d, J = 7.2 Hz, 2H), 7.510–7.339 (m,12H), 7.315–7.228 (m, 8H), 6.825 (d, J = 4 Hz, 4H); ESI-MS(m/z,%): 654 (M+, 100); elemental analysis (%): for calculatedC 80.72, H 4.93, O 4.89, found C 81.01, H 4.96, O 4.94.

2.3. Theoretical calculation

Computations on the electronic ground state of the com-pounds were performed using Becke’s three-parameterdensity functional in combination with the nonlocal corre-lation functional of Lee, Yang, and Parr (B3LYP) [66,67]. 6-.31G(d) basis sets were employed. The ground-state geom-etries were fully optimized at the B3LYP level. All computa-tions were performed using the Gaussian 03 package [68].

2.4. Fabrication and testing of OLEDs

Prior to the device fabrication, the patterned ITO-coatedglass substrates were scrubbed and sonicated consecu-tively with acetone, ethanol, and de-ionized water, respec-tively. All the organic layers were thermally deposited invacuum (�4.0 � 10�4 Pa) at a rate of 1–2 Å/s monitoredin situ with the quartz oscillator. In order to reduce the oh-mic loss, a heavily p-doped layer with MoOx, consideringthe low doping efficiency in amorphous organic matrixwith transition-metal-oxide-based acceptors, was directlydeposited onto the ITO substrate for each sample. Afterthe deposition of LiF, the samples were transferred to me-tal chamber, and suffered from a vacuum break due to thechange of the shadow masks to determine the active area.The current–voltage–luminance characteristics were mea-sured with a PR650 spectrascan spectrometer and aKeithley 2400 programmable voltage–current source. Allthe samples were measured directly after fabricationwithout encapsulation in ambient atmosphere at roomtemperature.

3. Results and discussions

3.1. Design and synthesis

How to tune the excited energy levels purposefully isalways one of the most important issues for organic func-tional materials because there are still very few clear anddirect conclusions of the relationship between the molecu-lar structures and the properties. Since it is believed thatthe exothermic energy transfer from host to the emittersis the basis of the high EL efficiencies [27] and commonlythe bigger conjugated areas correspond to both of thelow S1 and T1, the misgiving of converse energy transferobviously limits the choices of the chromophores andfunctional groups. Many potential hosts are abandonedbecause of their big conjugated areas. According to ourprevious works, we found that the molecular configurationis one of the key factors influencing the molecular excitedenergy levels [59,61,62]. In this sense, if organizing themolecular structure carefully, the high T1 might also beachieved through chromophores with large conjugatedarea. If we can find the direct examples, this result willwidely expand the choice of the hosts and greatly improvethe relative studies. With the aim to investigate the influenceof molecular configuration and density of functional sub-stituents on the properties of the hosts, no other redundantmodification and functionalization can be introduced.Therefore, NAPO and BiNAPO were chosen and composed

Fig. 1. Single-crystal structure and packing diagram of NAPO.

Fig. 2. Absorption and PL spectra of NAPO and BiNAPO in CH2Cl2

(10�6 mol L�1).

H. Xu et al. / Organic Electronics 13 (2012) 1516–1525 1519

of the same chromophore naphthyl and the same func-tional DPPO moieties (Scheme 1). Simultaneously, DPPOsare boned with naphthyls along their short-axis. The differ-ences between NAPO and BiNAPO are that there are twonaphthyls in BiNAPO while only one in NAPO, simulta-neously, in BiNAPO each naphthyl is substituted withone DPPO while in NAPO both two DPPO are bonded tothe single naphthyl. Therefore, BiNAPO has the bigger con-jugated area, but its density of functional substituents isless than that of NAPO.

NAPO and BiNAPO were conveniently preparedthrough a three-step procedure of lithiation, phospho-rization and oxidation, with good total yield over 50%(Scheme 2). The structure characterization was establishedon the basis of mass spectrometry, NMR spectroscopy andelemental analysis. The molecular structure of NAPO wasfurther confirmed by single-crystal X-ray diffraction analy-sis (Fig. 1a). Weak aromatic edge-to-face p–p stackinginteractions between adjacent NAPO molecules can be ob-served, which give rise to a one-dimensional chain struc-ture as shown in Fig. 1b. This kind of weak p–pinteraction contributes to the stability of the solid filmsand improves the thermal properties.

3.2. Optical properties

UV–Vis absorption spectra and photoluminescent (PL)spectra of the PO compounds in solution (10�6 mol L�1 inCH2Cl2) were measured (Fig. 2 and Table 1). The absorptionspectrum of NAPO consists of two main absorption peaks at310 and 245 nm. BiNAPO has nearly the same absorptioncharacteristics with the peaks at 333, 289 and 238 nm.The weaker absorption peaks around 300 nm are attributedto n ? p⁄ transition from P@O to naphthyls, while thestronger peaks around 240 nm are ascribed to p ? p⁄

transition from naphthyls. Since there are two equivalentnaphthyls and DPPOs in BiNAPO, the relative absorptionat long wavelength is divided into two peaks. The molarextinction coefficient of BiNAPO is much bigger than thatof NAPO, which should be attributed to the larger absorp-tion cross-section of BiNAPO. Nevertheless, the absorptionedges of NAPO and BiNAPO are the same at 351 nm, corre-sponding to S1 of 3.53 eV. Therefore, the much larger area ofBiNAPO does not reduce its single-excited energy level.

The PL spectra of NAPO and BiNAPO in dilute solutionare thoroughly the same with the peak at 375 nm. This alsoindicated the similar p⁄? n transition from naphthyl toDPPO in NAPO and BiNAPO. In film, due to the aggregation,the emissions of the PO compounds remarkably shift redand become broader. Nevertheless, the bathochromic shiftof BiNAPO is 14 nm, which is much <25 nm of that of NAPO.Furthermore, the emission of monomer in the solid spectraof NAPO is remarkably weaker than that of its dimmer (at486 nm). It is showed that compared with BiNAPO the com-pact structure of NAPO makes the aggregation much easier,which is in accord with the result of single-crystal analysis.The relative PL quantum yield (PLQY) of BiNAPO is 17.5%measured with 9,10-diphenylanthracene as the standard,which is bigger than that of NAPO (14.6%). Consideringthe bigger extinction coefficient of BiNAPO, the more effi-cient energy transfer from BiNAPO to the phosphorescentguests can be expected. For a suitable host of PHOLEDs, highT1 is one of the essential conditions. The phosphorescencespectra of NAPO and BiNAPO at 77 K was measured todetermine their T1 as 2.26 and 2.46 eV, respectively, whichwere estimated from the t0,0 transitions identified as thehighest-energy bands (549 and 504 nm). It is interestingthat T1 of BiNAPO with the larger conjugated area is asmuch as 0.2 eV higher than that of NAPO with much smallerconjugated area. Compared with T1 of naphthalene(2.63 eV), T1 of BiNAPO is only reduced for 0.17 eV. There-fore, the energy gap between S1 and T1 (DEST) of BiNAPOis 1.07 eV, which is 0.2 eV less than that of NAPO. The smallDEST is beneficial to the carrier injection and transporting inPHOLEDs [69]. It is showed that the orthogonal configura-tion of BiNAPO endows itself with the same S1 to that ofNAPO. Since the same functional units and the similar

Table 1The PL properties and frontier energy levels of NAPO and BiNAPO.

Compound kexa (nm) kem (nm) Lifetime (ns) PL Q.E.c (%) S1/T1

d (eV) HOMO/LUMO (eV)

NAPO 310, 245 375a

400, 486b1.97 14.6 3.53/2.26 �6.095/�1.687e

�6.04/�2.51f

BiNAPO 333, 289, 238 375a

389b1.76 17.5 3.53/2.46 �5.687/�1.143e

�5.91/�2.38f

a 10�6 mol L�1 in CH2Cl2.b In film.c Using 9,10-diphenylanthracene as standard.d S1 levels were estimated according to the absorption edges, and T1 levels were estimated according to the phosphorescent peaks 77 K.e Estimated by Gaussian calculation.f Calculated by the turn-on potentials of the first oxidation peaks and optical energy gaps.

1520 H. Xu et al. / Organic Electronics 13 (2012) 1516–1525

conjugation lengths, the much lower T1 of NAPO should beascribed to its higher density of DPPO moieties on naphthyl.

3.3. DFT Calculation

To understand the nature of the optical properties ofNAPO and BiNAPO and the influence of the molecularstructures on FMOs and exited energy levels, density func-tion theory (DFT) calculations were conducted employingthe Gaussian 03 package at the level of B3LYP 6-31G⁄.

The calculated highest occupied molecular orbitals(HOMO) of BiNAPO and NAPO are �5.687 and �6.095 eV,respectively, while the calculated lowest unoccupiedmolecular orbital (LUMO) of BiNAPO and NAPO are�1.143 and �1.687 eV, respectively (Fig. 3). The HOMO–LUMO energy gap (Eg) of BiNAPO is 4.544 eV, which is even0.136 eV bigger than that of NAPO. It is noticed that com-pared with FMOs of BiNAPO, both of HOMO and LUMO ofNAPO remarkably fall down for as big as 0.4 eV, whichshould be attributed to the bigger density of electron-

Fig. 3. DFT simulation of

withdrawing P@O moieties in NAPO. Different with NAPO,FMOs of BiNAPO are mainly contributed by the binaphthylgroup, which are also owing to its larger conjugated area.Although the higher density of DPPO in NAPO supportsthe lower LUMO, its hole-injecting ability is also sacrificedsince the meantime declining of its HOMO. Therefore, theenergy levels of FMOs of BiNAPO are much more appropri-ate for balanced carrier injecting. Furthermore, its FMOs arevery close that the energy gap (Eg) of LUMO and LUMO-1 isonly 0.027 eV, meanwhile, HOMO and HOMO-1 are equiva-lent. However, the corresponding Egs of NAPO are muchbigger as 0.816 eV and 0.109 eV. It is believed that the smallintramolecular potential gaps facilitate the intramolecularcharge transfer [70]. Thus, balanced carrier injecting andtransporting in BiNAPO can be achieved. It is noticed thatthe dihedral angle of two naphthyls of 1,1’-binaphthyl isonly 60o (Supplementary Fig. SI1). Therefore, there shouldbe effective p–p interaction between two naphthyls. Simul-taneously, its Eg is 4.517 eV, which is even small than that ofBiNAPO. However, Eg of naphthalene is as high as 4.817 eV.

NAPO and BiNAPO.

H. Xu et al. / Organic Electronics 13 (2012) 1516–1525 1521

Therefore, two DPPOs in NAPO induce the reduction of Eg

for 0.4 eV.The strongly contrastive T1 of NAPO and BiNAPO can be

explained by their different configurations. For NAPO, dueto the high density of P@O moieties on the ring, naphthyl istwisted slightly. However, the strong electron-withdrawingeffect of DPPOs still remarkably decreases its Eg and T1.For BiNAPO, its two naphthyls are orthogonal attributedto the steric DPPOs at ortho- positions. Therefore, the inter-action between two naphthyls is blocked. Actually BiNAPOcan be imaged as a combination of two separated 1-(diphe-nylphosphinoyl)-naphthalene molecules. Thus, the lowerdensity of functional moieties and broken conjugatedconjugation are two main reasons for the highly reservedT1 of BiNAPO. It can be concluded that the excited energylevels of the larger conjugated systems can be also well con-trolled by the rational configuration design through tuningthe position and density of the substituents.

3.4. Electrochemical properties

The redox behaviours of BiNAPO and NAPO were inves-tigated by cyclic voltammetry (CV) at room temperature(in CH2Cl2 for oxidation and tetrahydrofuran for reduction)measured against an Ag/AgCl (0.1 M) electrode (Fig. 4) andthe data were listed in Table 1 for comparison. Both ofthese two compounds displayed irreversible one-electronoxidation waves at 1.77 V for BiNAPO and 1.90 V for NAPO,respectively, which originate from naphthyls. The irrevers-ible oxidation peaks at high potentials can be ascribed tophenyls of their DPPO moieties. Their onset oxidationpotentials were 1.64 and 1.51 V, respectively. The naph-thyl-contributed reversible reduction peaks of BiNAPOand NAPO were at �2.23 and �1.18 V with the onsetpotentials of 1.97 and 1.66 V, respectively. According tothe equation reported by de Leeuw et al. [71], EHOMO ¼

EOxyonset þ 4:4 eV

� �and ELUMO ¼ ERed

onset þ 4:4 eV� �

, EHOMO of

BiNAPO and NAPO are about �5.91 and �6.04 eV, whileELOMO are �2.74 and �2.43 eV, respectively, which are inaccordance with optical analysis and DFT calculation. It isobvious that their FMO orbitals are mainly contributed

Fig. 4. CV curves of BiNAPO and NAPO.

by naphthyls. Considering the equivalence of two naphth-yls in BiNAPO, the doubled density of DPPOs in NAPOshould be the main factor inducing its lower HOMO andLUMO. The overlapped induction effect of two DPPOs onone naphthyl remarkably enhances the electron injectingability of NAPO. The electrochemical analysis indicatedthe same conclusion as theoretical investigation that thehigher density of DPPOs on the chromophore can supportstronger electron injecting ability and reduce the holeinjection simultaneously.

3.5. Thermal properties

The thermal and phase stability of the hosts is impor-tant since the decomposition and phase transition duringthe device fabrication and operation is one of the mostimportant factors influencing the EL performances of thecorresponding devices. The thermal properties of NAPOand BiNAPO were investigated by the thermogravimetricanalysis (TGA) and the differential scanning calorimetry(DSC) (Fig. 5). The temperature of decomposition (Td) ofBiNAPO is as high as 372 �C, which is 45 �C higher thanthat of NAPO and makes the device fabrication more feasi-ble through vacuum evaporation. The worse thermal sta-bility of NAPO is due to its rigid and twisted structure.The smaller intramolecular stress in BiNAPO makes themolecule flexible and adjustable under heating.

The melting point (Tm) of NAPO is 319 �C, which is 23 �Chigher than that of BiNAPO. Simultaneously, NAPO alsohas a high temperature of glass transition (Tg) of 88 �C,while Tg of BiNAPO is 64 �C. The improved phase stabilityof NAPO should be attributed to its rigid and compactstructure and the stronger p–p intermolecular interaction.

3.6. EL performances of OLEDs

The green-emitting PHOLEDs based on NAPO andBiNAPO were fabricated to investigate their role as the hostmaterials, whose configuration is ITO/MoOx (2 nm)/4,40,400-tri(N-3-methylphenyl-N-phenylamino)triphenylamine(m-MTDATA):MoOx (15 wt.%, 30 nm)/m-MTDATA (10 nm)/tris(phenylpyrazole)iridium (Ir(ppz)3) (10 nm)/Ir(ppy)2

Fig. 5. TGA and DSC (inserted) curves of NAPO and BiNAPO.

Scheme 3. Molecular structures of the materials and schematic energy level diagram of the devices.

Fig. 6. Brightness–current density–voltage (B–J–V) curves of devices A–D.

1522 H. Xu et al. / Organic Electronics 13 (2012) 1516–1525

acac:Host (6%, y nm)/4,7-diphenyl-1,10-phenanthroline(BPhen) (50 � y nm)/LiF (1 nm)/Al (y = 10 nm for A and Cand 20 nm for B and D) were fabricated, where MoOx andLiF served as hole- and electron-injecting layers, m-MTDA-TA and BPhen served as hole- and electron-transportinglayers (HTL and ETL), Ir(ppz)3 was used as hole-transportingand electron-blocking material, respectively. Device A andB were based on BiNAPO, and Device C and D were basedon NAPO, respectively (Scheme 3).

The Brightness–Current density–Voltage (B–J–V) curvesof the devices were measured and shown in Fig. 6. Theturn-on voltage of device A was the lowest as 2.8 V at1 cd m�2, and its highest brightness of 12,040 cd m�2 wasachieved at as low as 5 V. The practical luminance of1000 cd m�2 was achieved at less than 3.6 V. Device Bhad a turn-on voltage at 3.0 V. Its maximum brightnessof 7942 cd m�2 was also achieved at 5 V. Simultaneously,at the same driving voltages, the current density of DeviceA was more than twice of that of Device B. It is obvious that

the operating voltage is relative to the carrier injecting andtransporting ability of each layer. Since the total thick-nesses of Device A and B were the same, the low driving

Fig. 7. Efficiency-J curves of devices A–D.

Fig. 8. Current density–voltage curves of the single-carrier only devicesof BiNAPO and NAPO.

H. Xu et al. / Organic Electronics 13 (2012) 1516–1525 1523

voltage and high current density of Device A were origi-nated from the differences in the electron injecting/trans-porting ability between BiNAPO and Bphen. Nevertheless,the J–B curves of Device A and B showed that at the sameJ, the brightness of B was bigger than that of A. Since theexciton concentration is in direct proportion to the currentdensity, it means that although the recombination zones inDevice A and B were at the interfaces between EMLs andthe electron-blocking layers, the thicker EML was stilladvantageous in diluting the exciton concentration andincreasing the luminance. The turn-on voltages of DeviceC and D were 3.5 V. However, their brightness was lessthan 100 cd m�2. Compared with C and D, the much lowerdriving voltages of Device A and B were attributed to themuch more balanced carrier injecting/transporting abilityof BiNAPO than that of NAPO. The much weaker brightnessof C and D was due to the quenching effect originated fromthe lower T1 of NAPO than that of Ir(ppy)2acac (2.36 eV).

Because of its higher luminance at the same current den-sities, Device B realized more stable current efficiency (C.E.)that the maximum C.E. of 28.9 cd A�1 was achieved atthe high brightness of 475 cd m�2 and the high J of1.64 mA cm�2 (Fig. 7a). At the practical luminance of1614 cd m�2, its C.E. remained to be 28.0 cd A�1 with theextremely small efficiency roll-off of 3.2%. The maximumC.E. of Device A was 29.8 cd A�1, which was achieved atmuch lower brightness of 77.1 cd m�2 and low J of0.26 mA cm�2. Furthermore, at 1423 cd m�2, C.E. of DeviceA was reduced to 26.2 cd A�1 with the worse efficiencyroll-off of 12.1%. Contrarily, owing to the low drivingvoltages of Device A, its maximum power efficiency(P.E.) of 31.2 lm W�1 was achieved at 77.1 cd m�2 and0.26 mA cm�2. At 1423 cd m�2, P.E. of Device A remainedto be 22.9 lm W�1 with the efficiency roll-off of 26.6%(Fig. 7b). The maximum P.E. of Device B was 26.5 lm W�1

at 185 cd m�2 and 0.65 mA cm�2. At 1614 cd m�2, P.E. ofDevice B was reduced to 22.0 lm W�1 with the efficiencyroll-off of 17%. The more stable efficiencies of Device B werealso mainly ascribed to the effective exciton dilution by itsthicker EML.

The carrier transporting characteristics of BiNAPO andNAPO were investigated through their nominal hole-onlyand electron-only devices with the configurations of

ITO/MoOx (2 nm)/m-MTDATA:MoOx (30 nm)/m-MTDATA(10 nm)/Ir(ppz)3 (10 nm)/PO Host (40 nm)/Ir(ppz)3

(10 nm)/m-MTDATA (10 nm)/m-MTDATA:MoOx (30 nm)/MoOx (2 nm)/Al (hole-only) and ITO/LiF (1 nm)/BPhen(40 nm)/PO Host (40 nm)/BPhen (40 nm)/LiF (1 nm)/Al(electron-only) (Fig. 8). It indicated that the hole trans-porting abilities of BiNAPO was much stronger than thatof NAPO, which is in accordance with the results of DFTcalculation. The substitution with electron-withdrawingDPPO can reduce the ionizability of chromophores, andfurther decrease the hole transporting ability. Contrarily,NAPO revealed stronger electron transporting ability.Furthermore, the discrepancy between the electrontransporting ability of these two hosts is more remark-able than that between their hole transporting ability.This can be ascribed to the compact structure and stron-ger intermolecular interaction of NAPO, which facilitatethe carrier transporting and therefore magnify the differ-ence of their electron transporting ability. It can be con-cluded that the density of functional substituents is moreeffective on the carrier transport than the sheer numberin the hosts. The high density of DPPO induces thestrong tendency of electron-transporting for NAPO. How-ever, the dispersive DPPOs in BiNAPO not only provideimproved electron-transporting ability, but also maintainthe enough hole-transporting characteristics of naphth-yls. Therefore, BiNAPO showed the balanced carriertransporting ability.

EL spectra of Device A and B only consisted of the char-acteristic emissions of Ir(ppy)2acac (Fig. 9). It is showedthat along with the voltage increasing, the spectra of Aand B kept stable, which demonstrate the efficient energytransfer from BiNAPO to Ir(ppy)2acac. Contrarily, the ELspectra of Device C and D were remarkably changed alongwith the increasing of current density. At very low currentdensity less than 5 mA cm�2, EL spectra of C and D mainlycontained the emission of Ir(ppy)2acac. However, anotheremission at 640 nm became stronger at higher currentdensity, which should be attributed to lower T1 of NAPOand the interaction between NAPO and Ir(ppy)2acac.

Fig. 9. EL spectra of devices A–D at different current densities.

1524 H. Xu et al. / Organic Electronics 13 (2012) 1516–1525

4. Conclusion

Two PO compounds NAPO and BiNAPO, composed ofthe same chromophore naphthyl and DPPOs, were chosento investigate the influence of molecular configurationand the density of functional substituents on the excitedenergy levels. Although the compact and rigid structureof NAPO is superior in phase stability, the absorption andPL spectra of NAPO and BiNAPO showed that both of thesetwo PO compounds have the same S1. The much biggerconjugated area of BiNAPO does not decrease its S1. Fur-thermore, T1 of BiNAPO is 0.2 eV higher than that of NAPO.According to Gaussian simulation, the much higher T1 ofBiNAPO should be attributed to its orthogonal naphthylsand smaller density of DPPOs. Simultaneously, the biggerconjugated area endows BiNAPO with the balanced carrierinjecting and transporting ability. Obviously, shorteningthe conjugated length by tuning molecular configurationand reducing the density of functional substituents caneffectively remain T1 of the chromophore. With thebalanced carrier injecting and transporting ability andthe higher T1, the green-emitting PHOLEDs based onBiNAPO realized much better EL performance includingthe low-driving voltage, much higher brightness and effi-ciencies than that of the devices based on NAPO.

Acknowledgments

This work was financially supported by the NationalKey Basic Research and Development Program of China

under Grant (2010CB327701), NSFC (50903028,61176020, 60977024 and 21102039), Key Project of Minis-try of Education (212039), New Century Talents Develop-ing Program of Heilongjiang Province (1252-NCET-005),Education Bureau of Heilongjiang Province (10td03),the Supporting Program of High-level Talents of HLJU(2010hdtd08) and Heilongjiang University Funds forDistinguished Young Teachers.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.orgel.2012.05.003.

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