Ambipolar Phosphine Derivatives to Attain True Blue OLEDs with 6.5

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Ambipolar Phosphine Derivatives to Attain True Blue OLEDs with 6.5% EQE Ilya Kondrasenko,† Zheng-Hua Tsai,‡ Kun-you Chung,∥ Yi-Ting Chen,§ Yana Yu Ershova,† Antonio Doménech-Carbó,⊥ Wen-Yi Hung,*,‡ Pi-Tai Chou,*,∥ Antti J. Karttunen,# and Igor O. Koshevoy*,† †

University of Eastern Finland, Department of Chemistry, Joensuu 80101, Finland Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan § Soochow University, Department of Chemistry, Lin-shih Rd. 70, Shih-Lin Taipei, TW 111, Taiwan ∥ National Taiwan University, Department of Chemistry, Taipei 10617, Taiwan ⊥ Universidad de Valencia, Dr. Moliner 50, 46100, Burjassot, Valencia Spain # Aalto University, Department of Chemistry, FI-00076 Aalto, Finland ‡

S Supporting Information *

ABSTRACT: A family of new branched phosphine derivatives {Ph2N−(C6H4)n−}3P → E (E = O 1−3, n = 1−3; E = S 4−6, n = 1−3; E = Se 7−9, n = 1−3; E = AuC6F5 4−6, n = 1−3), which are the donor−acceptor type molecules, exhibit efficient deep blue room temperature fluorescence (λem = 403−483 nm in CH2Cl2 solution, λem = 400−469 nm in the solid state). Fine tuning the emission characteristics can be achieved varying the length of aromatic oligophenylene bridge −(C6H4)n−. The pyramidal geometry of central R3P → E fragment on the one hand disrupts π-conjugation between the branches to preserve blue luminescence and high triplet energy, while on the other hand provides amorphous materials to prevent excimer formation and fluorescence self-quenching. Hence, compounds 2, 3, 5, and 12 were used as emitters to fabricate nondoped and doped electroluminescent devices. The luminophore 2 (E = O, n = 2) demonstrates excellently balanced bipolar charge transport and good nondoped device performance with a maximum external quantum efficiency (EQEmax) of 3.3% at 250 cd/m2 and Commission International de L’Eclairage (CIE) coordinates of (0.15, 0.08). The doped device of 3 (E = O, n = 3) shows higher efficiency (EQEmax of 6.5, 6.0 at 100 cd/m2) and high color purity with CIE (0.15, 0.06) that matches the HDTV standard blue. The time-resolved electroluminescence measurement indicates that high efficiency of the device can be attributed to the triplet−triplet annihilation to enhance generation of singlet excitons. KEYWORDS: donor−acceptor molecules, phosphor-organic compounds, luminescent materials, electroluminescence, blue OLED

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INTRODUCTION The impressive progress in the area of optoelectronic materials and devices, observed over the past 20 years, has been largely achieved due to successful development of relatively facile and cost-effective approaches to numerous conjugated organic compounds, demonstrating a range of tunable physical characteristics.1−7 Understanding the origin and the ways of modulating these unique properties resulted in a rapid growth of the corresponding applications, which primarily involve stimuli-responsive compounds for various sensing devices, semiconductor photovoltaic and electroluminescent materials for the fabrication of organic solar cells, field-effect transistors, and light-emitting diodes (OLEDs). In the field of energyefficient light generation, both solid-state lightings and fullcolor displays generally require sources of three primary colors (red, green, blue) of high color purity and stability. So far, © XXXX American Chemical Society

performance of deep blue OLEDs, which meet the standards of high-definition television, HDTV (Commission International de L’Eclairage (CIE) coordinates 0.15, 0.06) or National Television System Committee, NTSC (CIE coordinates 0.14, 0.08), needs a considerable improvement as it very rarely exceeds 5% of an external quantum efficiency (EQE).8 The most efficient by now deep blue-emitting device is based on phenylanthracene-functionalized molecules and demonstrates an EQEmax higher than 10%, CIE of (0.15, 0.06).9 A similar EQEmax value of 9.9% with CIE (0.15, 0.07) was reached by an OLED, which employs thermally activated delayed fluorescence (TADF) of a donor−acceptor sulfone-carbazole derivative.10 Received: January 26, 2016 Accepted: April 13, 2016

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DOI: 10.1021/acsami.6b01041 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of Compounds 1−12

a

THF, n-BuLi, − 78 °C, 1/3 PCl3. bE = H2O2 for 1−3, or S for 4−6, or KSeCN for 7−9, or Au(tht)C6F5 for 10−12, 25 °C.

functional D−A molecules in the past few years.23−29 In relation to electroluminescent devices, blue fluorescent P-based D−A compounds have been utilized as promising host or emitting materials.21,23,30,31 The R3P = E moiety allows for the construction of branched molecules, which have good solution processability and low degree of intermolecular aggregation. The latter provides uniform amorphous films and prevents emission quenching via excimer formation. Moreover, the insulating character of the P = E group excludes conjugation between the R branches and makes possible tuning the emission properties varying the length/electronic structure of R.21,30 In this contribution we present a series of branched intensely fluorescent molecules of a (D−π−)3A architecture. The central acceptor A was chosen as a R3P = E/R3P−AuX fragment (E = chalcogen, X = C6F5), while the diphenyl amine group serves as an electron donor D. Changing the length of the phenylene spacer −π− and the electronic characteristics of the A moiety, we systematically studied their influence on the photophysics of the target compounds. The blue emission of these materials could be well preserved with the extension of molecules due to the disrupted conjugation via phosphorus center. The introduction of the electron-withdrawing P → E group can also improve the electron transporting ability and high triplet energy,25,32−35 while diphenylamine is commonly used in holetransporting materials for its excellent hole-transporting property.36−39 Moreover, geometry of R3P → E fragment results in a triangular pyramidal structure, which could effectively hinder the close molecular packing in the solid state and prevent excimer formation and fluorescence quenching.21 As a result, nondoped devices of luminophore 2 achieved deep-blue EL with CIE coordinates of (0.15, 0.08) and ideally balanced carrier injection/transportation with EQEmax of 3.3% at 250 cd/m2. The promising luminophore 3 was selected as an emitter doped in CBP host for the evaporation-processed fabrication of the deep-blue OLEDs, which demonstrated the emission of HDTV standard CIE coordinates of (0.15, 0.06) and EQEmax of 6.5% (6.0% at 100 cd/m2) that is comparable with the best values reported for deep-blue electroluminescent devices in terms of high color purity and brightness. According to the time-resolved electroluminescence response results, such high EQE of the device is attributed to the triplet−triplet annihilation (TTA) in converting triplet excitons into additional singlet excitons.

However, a significant roll-off at a higher brightness was mentioned by the authors in both cases, leading to efficiencies of 5.3% and ca. 2.5% at 100 cd/m2, respectively. In this view, a notable example is a recently reported phenanthroimidazole− sulfone donor−acceptor molecule, which exhibits blue electrogenerated emission in the doped device at CIE (0.152, 0.077) and EQE of 6.8% with impressively small efficiency roll-off (EQE 6.63% at a brightness of 100 cd/m2, and 5.64% at 1000 cd/m2).11 A dramatic increase in performance with reduced roll-off was described for blue TADF OLED employing sulfone-dihydroacridine emitter (EQEmax 19.5%; 16.0% at 1000 cd/m2), though with somewhat insufficient color purity (CIE 0.16, 0.20).12 Despite that a large number of intensely blue fluorescent organic substances is known, only few of them are capable of demonstrating deep blue electroluminescence at a practically useful EQE and brightness.8,13 An important issue, which determines efficiency of a device, is a delicate balance of charge carrier (hole and electron) injection and mobility in the emitting material. The useful strategy to address this problem involves incorporation of the electron-accepting and electrondonating groups into the same π-conjugated molecular entity to produce bipolar donor−acceptor (D−A) structures. These species, which bear the p- and n-type fragments (i.e., electron rich and poor parts of the molecule), ideally could facilitate proper charge transport and provide their efficient recombination and subsequent light generation of high intensity.9,12,14−17 However, compounds possessing D and A functions linked through a π-conjugated bridge tend to undergo intramolecular charge transfer (ICT) that results in a considerable bathochromic shift of emission.18 Preventing a substantial D−A interaction and, thus, controlling the ICT process in order to retain blue luminescence are achieved via judicious adjustment of the emitter structure and composition. The rational molecular design implies decrease in (1) conjugation of the donor and acceptor units and (2) their donating ability, accepting ability, or both. Among a selection of electron-withdrawing moieties, those containing inorganic main group elements, such as B,19 S,10−12,20 and P,21−23 were found to be a remarkably attractive alternative to organic C,N,O blocks (e.g., carbonyl, cyano, imide, heterocyclic, fluorinated groups). In particular, a moderate and tunable electron-accepting nature of phosphine-oxide units R3P = O (R = organic substituents) was successfully employed in the preparation of a range of optically B


Research Article

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION Synthesis and Characterization. The intermediate phosphines (Ph2N(C6H4)n)3P, n = 1 (L1), 2 (L2), and 3 (L3), were prepared from bromo-diphenylamino-benzene,40 -biphenyl,41 and -terphenyl,41 respectively, using a lithiation protocol for haloaromatic compounds and subsequent coupling with stoichiometric amount of phosphorus trichloride (Scheme 1). The phosphines L1−L3 were then oxidized with hydrogen peroxide, elemental sulfur or potassium selenocyanide to give the corresponding series of oxides 1−3, sulfides 4−6 and selenides 7−9. The reactions of L1−L3 with labile gold(I) precursor Au(tht)C6F5 (tht = tetrahydrothiopene) afforded Auphosphine complexes in good yields. Details of synthesis are provided in the Supporting Information. The target compounds were characterized by 1H, 31P NMR spectroscopy and elemental analysis (Supporting Information). All of the species were isolated as amorphous solids except 10, which could be obtained as a crystalline material. Its crystal structure was estimated by X-ray diffraction analysis (Figure 1).

contrast to the related diphenylamine-functionalized oligophenylenes,41 the arylamino oxidation potential for 1−12 demonstrates a clear dependence on the length of phenylene spacers −(C6H4)n−, the increase of which leads to a visible decrease of the Emp value and, subsequently, raise of the HOMO level with the number of phenylene units n. The nature of electron-accepting P → E group as well has an influence on the midpeak potential, which is mostly pronounced for the species with the shortest spacers (n = 1) 1, 4, 7, and 10, the latter showing the highest oxidation potential (and lowest HOMO) within this series. The generally observed growth of the potential Emp (E = O) < Emp (E = S) < Emp (E = Se) ≈ Emp (E = Au) for the series with the same spacers −(C6H4)n− can be tentatively assigned to the stabilization of the HOMO-s upon increase of the metallic character of the electronaccepting elements E and their different contribution to the frontier orbitals. The −NPh2 localized redox couple is followed by relatively fast chemical reactions, as denoted by the appearance of a second cathodic signal at ca. 200 mV less positive potentials upon increasing the potential scan rate (Figure S1). Photophysical Properties. Figure 2 shows the absorption and emission spectra of the compounds 1−12 in aerated

Figure 1. Molecular view of complex 10. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity.

The molecule of 10 shows the expected bonding of the gold center to the phosphorus atom, the metal ion adopts virtually linear coordination geometry typical for this sort of species.42 Electrochemical Properties. Electrochemical behavior of 1−12 was investigated using cyclic voltammetry (CV) to estimate the HOMO/LUMO levels. All the compounds demonstrate a well-defined CV response in DMSO solution, which can be described in terms of the essentially reversible oxidation of the −NPh2 groups (Table 1, Figure S1). In

Figure 2. UV−vis absorption (solid lines) and emission spectra (solid line with symbols) of compounds 1−12 in CH2Cl2 solution, 298 K.

Table 1. Optical and Electrochemical Properties of 1−12, 298 K 1 2 3 4 5 6 7 8 9 10 11 12

λabs (nm)a

λem (nm)a

τ (ns)a

Φfa

λem (nm)b

τ (ns)b

Emp (V)c

EHOMO (eV)d

ELUMO (eV)e

Eg (eV)f

329 353 353 330 356 353 335 356 354 336 360 355

404 448 472 403 450 477 406 450 477 410 457 483

2.28 2.72 2.10 1.83 2.66 2.15 2.54 0.38 1.10 0.44 1.16 1.92

0.42 0.95 0.20 0.32 0.95 0.87 0.004 0.091 0.50 0.014 0.14 0.27

400 433 460 407 447 460 412 445 465 395 448 468

1.52 1.71 2.05 1.49 1.82 1.09 1.27 0.44 0.91 0.47 1.04 1.53

0.620 0.580 0.520 0.650 0.590 0.530 0.640 0.610 0.540 0.715 0.610 0.540

−5.42 −5.38 −5.32 −5.45 −5.39 −5.33 −5.44 −5.51 −5.34 −5.52 −5.41 −5.34

−2.12 −2.37 −2.37 −2.25 −2.47 −2.42 −2.27 −2.62 −2.48 −2.32 −2.47 −2.52

3.30 3.01 2.95 3.20 2.92 2.91 3.17 2.89 2.86 3.20 2.94 2.82

a

CH2Cl2 solution. bSolid. cEmp = midpeak potential vs Fc+/Fc, 0.10 M Bu4NPF6/DMSO solution. dEHOMO = highest occupied molecular orbitals calculated from the onset oxidation potential in the vacuum scale taking ENHE = −4.4 eV44 and EFc/Fc+ = +0.40 V vs NHE.45 eELUMO = lowest unoccupied molecular orbitals, ELUMO = EHOMO + Eg. fEg = energy band gap calculated from the onset of the absorption spectrum. C


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ACS Applied Materials & Interfaces CH2Cl2. All pertinent photophysical data in solution are listed in Table 1. Obviously, similar absorption and emission spectral features were observed when the titled compounds have the same phenylene spacers units, despite of different electron acceptor moiety (E = O, S, Se, AuC6F5). Conversely, increasing the phenylene spacers units from n = 1 to n = 3 results in significant red shift in both absorption and emission. For example, the absorption is progressively red-shifted in the order of 3 (353 nm, n = 3) > 2 (∼350 nm, n = 2) > 1 (329 nm, n = 1), accompanied by energy decrease of the emission peak wavelength of 3 (472 nm) > 2 (448 nm) > 1 (λem = 404 nm). The results can be rationalized by the increase of π-conjugation upon increasing the number of phenylene units from n = 1 to n = 3, such that the corresponding HOMO (LUMO) energy is increased (decreased), resulting in the decrease of the energy gap. On the other hand, the electron accepting strength directly correlates with the phosphorus atom whereas the substituent plays a minor role, showing its rather small influence toward the spectral alternation.43 The quantum yields, on the contrary, substantially depend on the nature of R3P → E group and are the lowest for Se derivatives 7 (n = 1) and 8 (n = 2). However, compound 9 having the longest phenylene spacer (n = 3), demonstrates quantum efficiency of the same order as those of O and S congeners. Compared to photoluminescence in solution, similar emission profile but with a slight shift was observed for all titled compounds in the solid state (Figure S2). The emission relaxation dynamics obtained for degassed and aerated solutions and the solids indicate that the emission of 1−12 can be fitted well by single-exponential decay kinetics with a lifetime in the nanoseconds range. This together with good quantum yield clearly concludes its fluorescence origin without an appreciable contribution of the delayed fluorescence. The experimental photophysical results are further supported by our quantum chemical calculations carried out using timedependent density-functional theory (TDDFT-PBE0/TZVP; see the Supporting Information for computational details). The predicted S0 → S1 excitation wavelengths are in agreement with the experimental absorption spectra (Table S2). The electron density difference plots for the S0 → S1 excitation illustrate the electron-donation from the diphenylamine part to the electronaccepting R3P → E unit (Figure 3 and Figure S3). The predicted S1 → S0 emission wavelengths are generally underestimated in comparison to experiment, but the overall trends are reproduced reasonably well (Table S2). The only exceptions are compounds 7 and 8, for which the predicted emission wavelength is clearly overestimated in comparison to the experiment. Theoretical investigation of these species, which show the lowest quantum yields in experiments, reveals that the Se atoms show a major contribution to the excited state, in particular for the S1 → S0 emission (Figure 3 and Figure S3). This is very different from the other compounds, where the S1 → S0 emission is clearly related to the S0 → S1 excitation and the E entity does not play a noticeable role (shown in Figure S1 for the compound 1). Device Fabrication and Properties. For the fabrication of electroluminescent devices compounds 2, 3, 5, 12 were selected as representative and promising luminophores based on their decent quantum yields and lifetimes. Though Se derivative 9 also demonstrates reasonable quantum efficiency, it is not included in the discussion due to its noticeably lower stability in comparison with other congeners. Figure 4 depicts the UV−vis absorption and PL spectra of selected compounds 2, 3, 5, 12

Figure 3. Electron density difference plots for the lowest energy singlet excitation of 3, 8, and 12 (isovalue 0.002 au). In addition, the lowest energy singlet emission of 8 is shown. During the electronic transition, the electron density increases in the blue areas and decreases in the red areas.

Figure 4. UV−vis absorption and emission spectra of compounds 2, 3, 5, 12 in a neat film, 298 K.

D


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ACS Applied Materials & Interfaces recorded from a neat film on a quartz substrate. All the species exhibit a similar absorption features and the maxima of low energy absorption peaks around 357 nm, which can be attributed to the intramolecular charge transfer (ICT) transition from the electron-donating diphenylamine to the electron-accepting R3P → E fragment, respectively. The neat film photoluminescence (PL) spectra display deep-blue emissions, which show maxima PL peak at 450 nm for 2, 5 and 456 nm for 3, 12 with slight red-shift upon increasing phenyl unit. Fluorescence quantum yields (Φf) of the titled compounds in solid state measured on the quartz plate using an integrating sphere are 0.53 (2), 0.55 (3), 0.15 (5), and 0.36 (12), showing significantly enhanced Φf for the phosphine oxide derivatives (Figure S4). In order to obtain the HOMO energy level, which better represents the solid state environment in a device, we used photoelectron yield spectroscopy (Figure S5). The LUMO levels were calculated using the equation of LUMO = HOMO + Eg, in which Eg is the optical energy gap determined from the onset wavelength of the film absorption band. Thus, the HOMO and LUMO energy levels of 2, 3, 5, 12 in neat films were estimated to be −5.54/−2.42, − 5.45/−2.41, − 5.52/−2.52, and −5.45/−2.35 eV, respectively. The charge-carrier properties of the compounds were investigated in hole-only and electron-only devices. The holeonly device had the following structure: ITO/pedot:pss/TCTA (20 nm)/compound (60 nm)/TCTA (20 nm)/Al (100 nm) and the electron-only device had the structure of ITO/BCP (20 nm)/ compound (60 nm)/BCP (20 nm)/LiF (1 nm)/Al (100 nm). The 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA)46 and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)47 layers are used to prevent electron- and hole-injection from the cathode and anode, respectively. The current density versus voltage curves shown in Figure 5 indicates the bipolar charge

dicarbazolyl-1,1′-biphenyl (CBP)48 devices with the following configuration: indium tin oxide (ITO)/HAT-CN (10 nm)/ NPB (60 nm)/TCTA (5 nm)/emitter (25 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al (100 nm). To improve the hole injection from the anode, we used 4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN)49 as a hole injection layer. Two hole transporting layers (HTLs), which consisted of a 20 nm thick layer of 4,4′-bis[N-(1-naphthyl)-N-phenyl]biphenyldiamine (NPB)50 and a 5 nm thick layer of 4,4′,4″tri(N-carbazolyl)triphenylamine (TCTA),51 were implemented. To further confine the holes or the excitons generated within the emissive region, 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI)52 with a high-energy gap is selected as the electron-transporting layer (ETL). LiF and Al served as electron-injecting layer and cathode, respectively. Figure 6 depicts the current density−voltage−luminance (J−V−L) characteristics, device efficiencies, and EL spectra of the device. The pertinent data are summarized in Table 2. The turn-on voltages of all nondoped devices were extremely low at 2.4 eV owing to the good matching of HOMO and LUMO energy levels between hole and electron transporting materials and emitter. Brightness of 100 cd m−2 for displays was achieved at 4.1, 4.4, 5.6, and 3.8 V for devices employing 2, 3, 5, and 12, respectively (Figure 6a). It is clear that the device based on 5 has a higher driving voltage compared to others, which can be ascribed to the unbalanced carriers recombined in the EML. Device using 2 as an emitter revealed maximum brightness (Lmax) of 13 370 cd m−2 at 10.4 V (2330 mA cm−2) with CIE coordinates of (0.15, 0.08), which is very close to the NTSC’s blue standard (0.14, 0.08). The maximum external quantum (EQE), current (CE), and power efficiencies (PE) were 3.3%, 2.2 cd A−1, and 1.8 lm W1−. Such high efficiencies may result from the high Φf and more balanced carrier transport of compound 2. The EQEs of the devices 3, 5, and 12 were 3.1, 0.4, and 2.0%, respectively, that follows the same trend as the Φf value (Figure S4). Furthermore, we also evaluated the practical use of these materials doped in CBP host (Figure 7). The EL spectra of the doped devices display visible hypsochromic shift compared with the nondoped ones. This change in emission energies can be attributed to the difference in polarity of the environment (i.e., of the host) in the solid films. The highest efficiency was achieved for device using 3 as a dopant, giving EQE, CE, and PE of up to 6.5%, 3.4 cd A−1, and 3.1 lm W1−, respectively. In addition, the roll-off of efficiency is extremely low with EQE of 6.0% at 100 cd m−2 (4.8 V). In the doped devices, a higher performance of 3 could arise from a better match of the electronic properties of the host and the dopant that facilitates charge injection and hence their recombination on the emitter. The effect of the host polarity and variation of local environment might be also responsible for the enhancement of quantum efficiency of the doped device employing 5 that could arise from the improved electron transporting ability. The EL spectrum with CIE coordinates of (0.15, 0.06) attains the standard of high-definition television (HDTV) ITU-R BT.709. To the best of our knowledge, these performance parameters are comparable with the highest values reported for deep-blue OLEDs with CIEy below 0.06.9,23,53 Considering a singlet ratio around 25%, and a common optical out-coupling efficiency of 20% for typical fluorescence OLEDs, one would only expect an upper limit EQE of ca. 5%. Careful analysis indicates that the superior efficiency of the CBP:5 wt % 3 device (EQE ∼ 6.5%) is attributed to the

Figure 5. Current density versus voltage curves of the hole-only and electron-only devices for the compounds 2, 3, 5, and 12.

transport nature of the probed species 2, 3, 12 but not of the compound 5, for which no electron-only current was detected. Apparently, the introduction of a sulfide group efficiently suppresses the electron transport properties. As can be seen from Figure 5, the biphenylene bridge between the D and A groups in 2 provides a better hole and electron-transporting behavior as compared to that of the triphenylene spacer of 3. Compound 2 with the strongest electron transporting ability possesses the most balanced carrier injecting/transporting ability among the studied molecules. To assess the utility of compounds 2, 3, 5, and 12 as blue emitters, we have fabricated nondoped and doped in 4,4′-N,N′E


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Figure 6. (a) Current density−voltage−luminance (J−V−L) characteristics of nondoped devices. (b) External quantum (EQE) and power efficiencies (PE) as a function of brightness. (c) EL spectra for nondoped device.

Table 2. EL Performances of the Fabricated Devices Von (V)a

EML 2 3 5 12 CBP: CBP: CBP: CBP: a

5 5 5 5

wt wt wt wt

% % % %

2 3 5 12

2.4 2.4 2.4 2.4 2.8 2.8 2.8 2.8

Lmax (cd m−2) 13370 41470 2100 9990 7250 18580 8420 16220

(10.4 (11.2 (11.2 (12.4 (11.6 (12.6 (10.8 (12.6

V) V) V) V) V) V) V) V)

Imax (mA cm−2)

EQE (%)

CE (cd A−1)

PE (lm W1−)

2330 1880 2850 2460 2320 1330 2720 2060

3.3 3.1 0.4 2.0 4.4 6.5 4.1 3.6

2.2 4.3 0.3 2.1 1.2 3.4 1.2 1.9

1.8 3.0 0.2 1.9 1.0 3.1 1.0 1.4

EQE100 (%, V)b 3.2, 2.8, 0.4, 2.0, 4.3, 6.0, 4.0, 3.4,

4.1 4.4 5.6 3.8 5.1 4.8 5.1 5.4

CIE (x,y) 0.15, 0.16, 0.17, 0.16, 0.16, 0.15, 0.16, 0.15,

0.08 0.19 0.10 0.13 0.04 0.06 0.04 0.07

Turn-on voltage at which emission became detectable. bThe values of driving voltage and EQE of device at 100 cd m−2 are depicted in parentheses.

Figure 7. (a) Current density−voltage−luminance (J−V−L) characteristics of doped devices. (b) External quantum (EQE) and power efficiencies (PE) as a function of brightness. (c) EL spectra for doped in CBP device. (d) CIE coordinates for the fabricated devices.

EL response measurements at 4 V electrical pulse (Figure 8). To exclude the charges remaining after the excitation, we applied a reverse bias of 2 V during the measurement following the excitation. We also performed transient EL measurement on a typical fluorescence material Alq3 as the reference to identify the different EL mechanisms. As a result, in the CBP:5% 3 device, the normal fluorescence rapidly diminished within the nanosecond decay time of a singlet exciton generated right after the pulse excitation, but there appeared

triplet−triplet annihilation, which assists in converting triplet excitons into additional singlet excitons, thus appreciably improving to the light output of OLEDs.54−57 Despite that all the studied luminophores demonstrate photoluminescence only of a singlet origin (i.e., fluorescence), triplet−triplet annihilation could be significant in electroluminescent devices because of spin statistics, according to which 75% of the charge recombination occurs in the triplet state. To support this viewpoint, the fabricated devices were examined using transient F


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efficiency of deep blue electroluminescence is assigned to a contribution of triplet−triplet annihilation into the singletexciton generation, which results in enhanced device output.

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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/acsami.6b01041. Experimental procedures. (PDF) Optimized Cartesian coordinates of the studied systems. (XYZ) X-ray crystallographic data for 10. (CIF)

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Figure 8. Time-resolved electroluminescence responses of the devices at 4 V electrical pulse.

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected]. * E-mail: [email protected]. * E-mail: igor.koshevoy@uef.fi.

to be a delayed fluorescence, of which the intensity decreased much slowly with a decay time of milliseconds. For a fair comparison, except for the observation of prompt fluorescence the Alq3 device lacked any delayed fluorescence (Figure 8). Although the initial intensity of the delayed fluorescence versus that of the prompt fluorescence is small, the integration of the intensity over millisecond is still appreciable, leading to the enhancement of the singlet-exciton generation efficiency via TTA mechanism.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Academy of Finland (grant 268993, I.O.K.), the Alfred Kordelin Foundation (A.J.K.), and the Russian Science Foundation (grant 16-13-10064) is gratefully acknowledged. Computational resources were provided by CSC−the Finnish IT Center for Science (A.J.K.).

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CONCLUSIONS To sum up, we prepared a family of novel donor−acceptor molecules of a (D−(C6H4)n−)3A (n = 1, 2, 3) architecture. These compounds comprise phosphine derivatives R3P → E (E = O, S, Se, AuC6F5) as moderate acceptors A, which are separated from the terminal diphenylamine fragments (electron donor D) by oligophenylene spacers of a variable length. All the species exhibit blue photoluminescence at room temperature (λem = 403−483 nm in CH2Cl2 solution, λem = 400−469 nm in the solid state). The nanosecond range of the lifetimes, radiation decay dynamics, and good quantum yields indicate fluorescence origin of emission. The length of the π-conjugated −(C6H4)n− bridge has a considerable effect on the photophysical characteristics, resulting in a significant red shift in both absorption and emission upon elongation of the spacer from n = 1 to n = 3. Nevertheless, blue emission is preserved even for the extended molecules. The nature of an A moiety shows a very minor influence on the emission energy, but affects quantum efficiency, which is the lowest for Se-containing compounds (n = 1, 2). The selected promising luminophores were used as emitters in nondoped and doped electroluminescent devices. Among the nondoped OLEDs the one employing luminophore 2 (E = O, n = 2) demonstrates nearly true blue EL (CIE 0.15, 0.08). Investigation of the chargecarrier properties revealed that the phosphine oxides and gold complexes are capable of a bipolar charge transport, while the sulfide suppresses the electron transport ability. The effectiveness and balance of carrier injecting/transporting is dependent on the −(C6H4)n− spacer being the highest for the biphenylene bridge. The doped device of 3 (E = O, n = 3) in CBP host achieved highest efficiency (EQE, CE, and PE of up to 6.5%, 3.4 cd A−1, and 3.1 lm W1−) and small roll-off (EQE of 6.0% at 100 cd m−2 and 4.8 V). The EL spectrum with CIE coordinates of (0.15, 0.06) meets the HDTV standard ITU-R BT.709, and the attained performance is comparable with the best reported examples for deep-blue OLEDs with CIEy ≤ 0.06. The high

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