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Enhancement of electroluminescent green emission by far-field coupling of Au nanoparticles in organic light emitting diodes Jayaraman Jayabharathi, Palanisamy Sujatha, Venugopal Thanikachalam, Palanivel Jeeva, and Sekar Panimozhi Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Industrial & Engineering Chemistry Research

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Enhancement of electroluminescent green emission by far-field coupling of Au nanoparticles in organic light emitting diodes Jayaraman Jayabharathi*, Palanisamy Sujatha, Venugopal Thanikachalam, Palanivel Jeeva, Sekar Panimozhi Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India * Tel: +91 9443940735; E-mail address: [email protected].

Abstract Far-field surface plasmon-enhanced green electroluminescence in organic light-emitting devices (OLEDs) is harvested by tuning the gold nanoparticles (Au NPs) density at the interface of anode: hole transport layer (HTL) in OLEDs using Ir(DMSPI)2 (acac) as emissive layer. Au NPs increased

the

hole

injection

with

35/µm2 density at

ITO:N,N′-di-1-naphthyl-N,N′-

diphenylbenzidine (NPB) interface and leads to enhanced the emission intensity as a result of increased radiative rate (kr). The Au NPs modified anode in OLEDs injects holes effectively into NPB layer and stabilized its energy level which results increase of current density and the reduced hole injection barrier (HIB) was analyzed by using Richardson-Schottky equation. The far-field plasmonic coupling with hole injection ability of Au NPs at ITO: HTL interface enhanced the device efficiencies at low turn-on voltage in this work. However, the anode with 6.0/µm2 density of Au NPs shows poor hole injection ability into HTL due to trapping of holes at the interface.

Keywords: Far-field enhancement; Hole injection; HTL; Brightness; Current efficiency.

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1. Introduction The surface plasmon’s (SPS) of metallic nanostructures have been reported to tune the performances of organic optoelectronic devices [1–10]. The optical and electrical effects such as metal enhanced fluorescence [11–13], energy transfer [14] and interface effect [15] affects the efficiencies of OLEDs. The coupling of metal nanoparticles (NPs) with emissive layer, distance between them and morphology of metal NPs affect the performances of devices [13, 16]. The distance dependence between the metal NPs and emissive layer is key factor of metal enhanced fluorescence. The emission enhancement due to localized surface plasmon resonance (LSPR) is effective with the distance is 5-15 nm (between NPs and emitting materials) whereas far-field enhancement is possible with 60 nm distance in which radiative rate is increased [11, 17]. Kummerle et al [11] shown that the quantum efficiency (EQE) of the devices was controlled by the distance between the fluorescent dye and metal film. The increase of radiative rate with metal NPs was harvested from the distance (> l/10) of metal NPs with fluorophore results enhanced fluorescence lifetime and quantum yield. The plasmonic enhancement [18-22] with reducing HIB [23-26] by metal NPs coated OLEDs enhances the device performances. The newly formed electric double layer on modified ITO stabilized its Fermi energy which results an increase of carrier (hole) injection ability [23]. Localized surface plasmons (LSPR) of metal NPs enhanced the decay process to harvest increased number of photons from the fabricated OLEDs. The thickness of hole transport layer (HTL) was optimized to obtain the high performance OLEDs also to analyze the mechanism of device enhancement. In this manuscript, gold nanoparticles embedded anode and newly synthesized emissive layer,iridium(III)bis-1-(3,5-dimethyl)-2-styryl-1H-phenanthro[9,10-d]-imidazolato-


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N,C2)(acetylacetonate)[Ir(DMSPI)2 (acac)]:CBP has been used to analyze the device performances. The enhanced efficiencies is attributed to (i) hole injection abilities of Au NPs and (ii) plasmonic interaction of Ir(DMSPI)2 (acac) with Au NPs. The maximum luminous efficiency was harvested with 40 nm thickness of spacer NPB layer. Therefore, the far-field effect of Au NPs is likely to be the cause of luminance enhancement rather than localized surface plasma resonance. The mechanism for enhanced efficiency is discussed in detail with tuning Au NPs density and NPB thickness. 2. Experiment and Characterization Sigma-Aldrich supplied chemicals for synthesizing DMSPI, Ir( DMSPI)2 (acac) (Scheme S1) and gold nanomaterials. NMR and mass spectral data of DMSPI and Ir( DMSPI)2 (acac) were extracted from the measurements carried out on Bruker spectrometer (400 MHz) and Agilent LCMS VL SD, respectively to confirm the structure of the synthesized materials. The frontier energies (HOMO and LUMO) were determined with oxidation potential measured from CHI 630A potentiostat electrochemical analyzer (platinum electrode- working electrode; platinum wire- counter

electrode; Ag/Ag+ electrode-reference electrode;scan rate-100mVs-1; 0.1M

tetrabutylammoniumperchlorate in CH2Cl2 - supporting electrolyte). The Lambda 35 Perkin Elmer instrument was used to measure absorption and diffused reflectance spectra (DRS) measurements to known the band gap were carried out using Lambda 35 spectrophotometer with RSA-PE-20 integrating sphere. Emission wavelength (λemi) was measured using Perkin Elmer LS55 fluorescence spectrometer. The PL quantum yield (QY) was calculated with 0.5 M H2SO4 solution of quinine (0.54) as reference using the following equation: φunk = φstd  I unk  I  std

   

 Astd  A  unk

   

 η unk  η  std

   

2

[φunk - unknown QY ; φstd – standard QY ; Iunk - unknown emission

intensity ; Istd - standard emission intensity ; Aunk - unknown absorbance ; Astd - standard



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absorbance ; ηunk- unknown refractive index ; ηstd - standard refractive index ]. The film QY was measured with integrating sphere (quartz plate). Decomposition temperature (Td) was measured with Perkin Elmer thermal analysis system (10° C min-1; nitrogen flow rate - 100 mL min-1). Glass transition temperature (Tg) was recorded with NETZSCH (DSC-204) (10° C min-1 under nitrogen atmosphere). The morphology and size of synthesized nanomaterials was recorded with JEOL JEM 2100 HR-TEM (200 kV - resolution 0.1 nm). The chemical composition of nanomaterials was recorded with XPS (X-ray photoelectron spectra: ESCA-3 Mark II spectrometer-VG - Al Kα (1486.6 eV) radiation). Energy dispersive X-ray spectra (EDS) were recorded with a FEI Quanta FEG. The TEM (transmission electron microscopy) images was recorded on Philips TEM with 200 kV electron beam and the SAED (selected area electron diffraction) pattern was obtained from Philips TEM with CCD camera (200 kV). The powder XRD (X-ray diffractograms) was obtained with Eqinox 1000 diffractometer using Cu Kα rays (1.5406 Å; current - 30 mA; 40 kV). 2.1. Fabrication of OLED A series of devices with configuration of ITO/Au NPs [with AuNPs (6/µm2 (II); 35/µm2 (III)] or without Au NPs (I) /NPB (4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl) (40 nm)/ Ir( DMSPI)2 (acac):CBP (7%) (50 nm)//LiF (1 nm)/ Al (100 nm) have been fabricated by vacuum deposition (5 x 10-6 torr) onto ITO with resistance of 20 Ω/square. For comparison purpose the hole only device with configuration of ITO (with and without Au nanoparticles)/NPB/Al also fabricated. Deposition of organic materials onto ITO at a rate of 0.1 nm s-1 and LiF was thermally evaporated on organic layer surface. The thickness of all layers was measured using a quartz crystal thickness monitor. Measurements of current density (J)-voltage (V)-Luminescence (L) were made simultaneously with Keithley 2400 sourcemeter.


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2.2. Computational Details All calculations were performed using density functional theory (DFT) with Guassian-09 with B3LYP/ LANL2DZ /6-31G (d, p) [27]. 2.3. Synthesis of Gold Nanoparticles (Au NPs) About one gram of dried powder of G. sylvestre leaf in 40 mL deionised water was digested in a water bath at 90 oC for 30 min., the extract was cooled and filtered using cellulose nitrate membrane filter paper (0.22 µm) and stored in refrigerator until use. Aqueous HAuCl4 (0.001 M, 10 mL) was added to 20 mL fruit extract and the resultant solution was kept at 60 oC for 1 h under vigorous stirring. Formation of Au NPs was visualized by colour change from colourless to ruby red and stored at 5 oC before use. 2.4. 1-(3 , 5-dimethyl)-2-styryl-1H-phenanthro[9,10-d]-imidazole (DMSPI) DMSPI was synthesised by refluxing 9,10–phenanthrenequinone (5 mmol), cinnamaldehyde (5 mmol), 3,5-dimethylaniline (6 mmol) and ammonium acetate (61 mmol) in ethanol (20 mL) for 12 h under N2 atmosphere. The reaction mixture was extracted with dichloromethane and purified by column chromatography (petroleum ether: CH2Cl2, 1:1) and the compound is pale yellow in colour. M. P. 250º. Anal. calcd. for C31H24N2: C, 87.70; H, 5.70; N, 6.60. Found: C, 87.62; H, 5.68; N, 6.54. 1H NMR (400 MHz, CDCl3): δ 2.65 (s, 6H), 6.74 (s, 3H), 6.82 (d, J=15.2 Hz, 2H), 7.24-7.31 (m, 5H), 7.62-7.71 (m, 4H), 8.41 (s, 2H), 8.70 (d, J=16.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): 40.29, 40.44, 112.20, 112.50, 125.19, 125.43, 125.69, 126.85, 127.01, 127.26, 127.53, 128.24, 128.33, 129.59, 134.25, 140.07, 141.30, 144.98, 147.74, 156.01. MALDI–TOF MS: m/z.424.3 [M+]. calcd. 424.5. 2.5. Iridium(III)bis-1-(3,5-dimethyl)-2-styryl-1H-phenanthro[9,10-d]-imidazolatoN,C2)(acetylacetonate)[Ir( DMSPI)2 (acac)]



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The DMSPI (2.2 mmol) and iridium(III) chloride trihydrate (1 mmol) in 2-ethoxyethanol:H2O (3:1) was refluxed at 120 ºC under N2 atmosphere. The formed dimer was washed with hexane [28]. The cyclometalated iridium(III) complex have been synthesized by refluxing the dimer (1 mmol), acetylacetone (2.2 mmol) and potassium carbonate (2.5 mmol) in

2-ethoxyethanol (5

mL) at 120 °C under N2 atmosphere. The obtained acetylacetonate iridium(III) complex was washed with hexane and petroleum ether and characterized by various spectral techniques. Anal. calcd. for C67H56IrN4O2: C, 59.61; H, 4.75; N, 7.13. Found: C, 59.53; H, 4.42; N, 7.08. 1H NMR (400 MHz, CDCl3): δ 1.43 (s, 6H), 1.71 (s, 6H), 2.61 (s, 6H), 5.28 (s, 1H), 6.57 (s, 7H), 6.65 (s, J=8.6 Hz, 1H), 6.81 (s, 3H), 7.08 (d, J=10.0 Hz, 2H), 7.58-7.69 (m, 17H), 8.28 (d, J=8.2 Hz, 2H), 8.45 (d, J=8.8 Hz, 2H), 8.86 (t, 3H).

13

C NMR (100 MHz, CDCl3): 26.82, 27.08, 40.34, 40.48,

68.53, 112.16, 112.36, 123.06, 125.05, 125.38, 125.52, 126.27, 126.41, 126.58, 126.82, 127.18, 127.32, 127.43, 128.06, 128.27 128.39, 128.85, 129.64, 129.73 134.65, 139.93, 140.11, 141.28, 144.51, 147.28, 155.98. MALDI–TOF MS: m/z.1140.7 [M+]. 3. Results and discussion 3.1. Characterisation of Gold nanoparticles (Au NPs) Gold nanoparticles (Au NPs) are formed by the addition of G. sylvestre leaf extract to HAuCl4 solution and its crystallinity was confirmed by XRD analysis (Figure 1) which shows that the peaks at 2θ of 38.18, 44.38, 64.34, 77.61 and 81.84 corresponds to (111), (200), (220), (311) and (222) interplanar reflections of face centered cubic (FCC) crystal, respectively (JCPDS No. 65-2870). The highly intensified peak at 38.18º corresponds to (111) plane indicating its predominant orientation and the broadening of Bragg’s peak confirm the small crystallite size. The average crystallite size deduced from Debye–Scherrer equation [D=kλ/βCosθ, D - average


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crystal size; k - Scherer coefficient; λ - X-ray wave length; θ - Braggs angle; β - full width at half maximum intensity] is 11.4 nm and the calculated surface area is 93.05 m2/g. The absorption spectrum of Au NPs shows the surface plasmon absorption (SPR) at 538 nm and its monodispersity reveal by the appearance of sharp peak (Figure 1). Morphology and shape of Au NPs was determined by TEM analysis (Figure 2). The TEM image shows the spherical nature of particles with 11.6 nm size (Figure 2) and the d-spacing (2.355 Å) in the lattice fringes of the gold nanocrystals corresponds to the 111-plane of face centered cubic (FCC) gold which is in alignment with the XRD d- spacing (2.35 Å) between (111) plane of FCC gold crystal (JCPDS No. 65-2870). The diffraction rings with bright circular spots in SAED pattern are indexed to (111), (200), (220), (311) and (222) planes, respectively of FCC (Figure 2). The average size of 11.45 nm is obtained from DLS analysis (Figure 3) which is well matches with XRD and TEM results. The repulsive forces exist with the electrical charge of the Au NPs surface accounts the negative ζ potential (-26.3 mV) which increases its stability. 3.2. Characterisation of emissive layer [Ir(DMSPI)2 (acac)] 3.2.1. Photophysical properties The absorption (λabs) of Ir(DMSPI)2 (acac) show an intense band at 245 nm and weaker intensity bands at 295 and 301 nm in the lower part of energy are due to ligand -centered (π-π*) and MLCT transitions, respectively [29] (Figure 4) . The nature (position, shape and extinction coefficient) of the bands at 295 and 301nm suggest that these absorptions are due to 1MLCT ← S0 and 3MLCT ← S0 transitions, respectively [30-35]. The intensity of 3MLCT ← S0 transition is almost match with 1MLCT ← S0 transition, suggesting that 3MLCT ← S0 transition is allowed by S-T mixing of spin-orbit coupling of iridium(III) [36, 37] (Figure. 4). The observed short



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lifetime, broad luminescence spectra and absorption peaks reveal that the triplet state of Ir(DMSPI)2 (acac) possess predominantly 3MLCT character. Phosphorescence spectra (PL) of mononuclear iridium(III) complexes

are accounted by

ligand-centered excited state and MLCT excited state transition to S0. The wave function of excited triplet state (ΦT) of cyclometalated iridium (III) complex is responsible for phosphorescence and is a mixture of ΦT (π- π*) and ΦT (MLCT) [38], which is expressed as, ΦT = a ΦT (π- π*) + b ΦT (MLCT) [a and b are normalized co-efficients, ΦT (π- π*) and ΦT (MLCT) are the wave function of 3(π- π*) and 3(MLCT) excited states, respectively]: (i) when a > b, the triplet state is due mainly with 3π- π* state and (ii) when b > a the triplet state is attributed with 3MLCT state majorly. The phosphorescence spectra of Ir(DMSPI)2(acac) show green emission (λemi) at 501 and 520 nm with broad shape and the PL quantum yields (Φ) was measured to be 0.62 (Table 1:Figure 4). The emission peak from ligand-centered 3π-π* state show vibronic progressions, while those from 3MLCT state is broad in shape [32-35]. The broad shape of the luminescence spectra of Ir(DMSPI)2 (acac) confirm the large contribution of 3

MLCT in the excited state. The observed emissive peaks at peak 501 and 520 nm of the

iridium(III) complex is corresponds to the electronic transitions between the vibrational levels of 3

MLCT / 3π- π* triplet state and ground state (S0). The peak with dominant intensity stemmed

from 0 to 0 electronic transition between 3MLCT / 3π- π* and S0 and a shoulder with lower intensity derived from 0 to 1 electronic transition [39, 40]. The representative Franck - Condon electronic transitions are displayed in Figure 5.


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3.3. Electrochemical and thermal stability of Ir(DMSPI)2 (acac) The CV curve of Ir(DMSPI)2(acac) complex displayed reversible one-electron oxidation at Eox1/2 = 0.30 V vs. Fc/Fc+ couple, indicating the stability (electrochemical) of Ir(DMSPI)2(acac) complex (Figure 5). The oxidation process is usually considered as a metalcentered couple, i.e. IrIII/IrIV and the highest occupied molecular orbital (HOMO) energy can be estimated from half-wave oxidation potential and energy level of the ferrocenium/ferrocene redox couple (4.8 eV negative to the vacuum level) (EHOMO = Eox + 4.8 eV). The energy gap between HOMO and LUMO (ELUMO = EHOMO– 1239/λonset) was obtained from the red edge of the absorption spectra (Figure S1). The determined HOMO and LUMO energies of Ir(DMSPI)2(acac) are −5.10 eV and −2.63 eV, respectively. The HOMOs are composed of d(Ir) and π(C^N), while the LUMOs are localized on C^N ligand of the iridium complex. The formula employed

to

calculate

the

radiative

(kr)

and

non-radiative

(knr)

rate

constants

is,

Φ = ΦISC {KT/(kr + knr)}; kr = Φ/τ; knr = (1/τ) - (Φ/τ); τ = (kr + knr)-1: the intersystem-crossing yield (ΦISC) for iridium complexes is taken as 1.0 because of strong spin-orbit coupling [41]. The calculated radiative (kr) and non-radiative (knr) rate constants of Ir(DMSPI)2(acac) confirmed that the radiative emission is larger than non-radiative transitions (kr > knr). The glass transition temperature (Tg) and thermal decomposition temperature (Td) of Ir(DMSPI)2(acac) have been analyzed to understand the device stability (Figure 6) and the Tg and Td were measured as 141 and 398 °C, respectively and the melting point (Tm) measured by DSC examination is 371 °C. The higher thermal stability (high Tm, Td and Tg) of Ir(DMSPI)2(acac) is attributed to the stronger rigidity which will be in favour of OLED stability and could be utilized as emissive material in EL devices.



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3.4. Description

of

the

structure

of

Iridium(III)bis-1-(3,5-dimethyl)-2-styryl-1H-

phenanthro[9,10-d]-imidazolato-N,C2)(acetylacetonate)[Ir( DMSPI)2 (acac)] The optimized geometry of Ir(DMSPI)(acac) was obtained using Gaussian -09 [DFT/LANL2DZ/6-31G (d,p)] (Figure S1)

and the optimized parameters are displayed in

Table 2. This complex shows an octahedral geometry with cis –C,C and trans-N,N chelate disposition rather than trans- C,C and trans-N,N chelate. The richer electron density phenyl rings normally exhibit very strong influence and expected to show trans effect. The obtained trans-C, C geometry is thermodynamically higher in energy and kinetically more labile referred as “transphobia” [36]. The Ir-C bond length of complex Ir(DMSPI)(acac), i.e., Ir-Cav = 2.02 Å is shorter than Ir-N bond length i.e., Ir-Nav =2.06 Å. The Ir−O bond length [2.13 Å] is longer than mean Ir−O bond length (2.09 Å), reported [42]. 4. Electroluminescent Properties X-ray photoelectron spectroscopy (XPS) was recorded for both ITO and Au NPs dispersed ITO to analyze the hole injection (HI) efficiency of Au NPs. The hole injection barrier (HIB) at the interface of anode:HTL junction deduced from, ɸB = IE– ɸanode, [IE - ionization energy of NPB; ɸ anode - work function of anode material]. The shift of Au NPs indicate that the injection barrier was reduced results ɸanode energy level is stabilized. The reduced hole injection barrier (HIB) at ITO:NPB interface can be determined using Richardson–Schottky thermionic emission model, JRS = A × T2 exp − m*= m0; = /4 ; ɸB

-

∅ √

[Richardson’s constant A* = 120 A / (cm2 K2) at

zero-field injection barrier; KB - Boltzmann constant; T-

absolute temperature; E - electric field strength]. The device parameter of hole only devices with ITO(bare) is added in the equation and the zero field injection barrier was calculated to be -0.92 eV and the reduced hole injection barrier for Au coated ITO is calculated as 0.82 eV. The


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reducted hole injection barrier (HIB) with optimized coverage of Au NPs provide an idea for hole enhancement in OLEDs. Figure 7 shows XPS of bare ITO and Au NPs coated ITO [3d indium (3d3/2 and 3d5/2) and 3d tin (3d3/2 and 3d5/2)] and the red- shifted peaks of both In and Sn (0.9 eV- In; 0.92-eV Sn) was observed for Au coated ITO compared to bare ITO. This kind of peak shift was observed when the carrier (hole) injection material was deposited ITO [43, 44] results the stabilized energy level which is due to the formation of electric double layer by Au NPs. A series of devices with double-layer configuration of ITO/Au NPs with [6.0/µm2 (II); 35/µm2 (III)]/ without Au NPs (I) / NPB (40 nm)/ Ir( DMSPI)2 (acac):CBP (7%) (50 nm) /LiF (1 nm)/ Al (100 nm) have been fabricated (Figure 7; Table 1). The J-V-L curve shows that that the brightness (L, cd/m2) and current efficiency (ηc, cd A-1) increased by high Au NPs density. The brightness increased by 57 % and 89 % and the current efficiency enhanced by 33 % and 73 % corresponding to Au NPs density of 6/µm2 and 35/µm2, respectively. The devices exhibit green emission at 501nm which corresponds to the energy gap of emissive material Ir(DMSPI)2 (acac). The surface analysis of devices II and III, Au NPs and ITO anode were investigated by atomic force microscopy (AFM) topography images (Figure S1) which shows the uniform distribution of Au NPs on ITO anode, which is the requirement for far- field plasmonic enhancement. The root mean square (RMS) values calculated for Au NPs coated ITO is higher than Au NPs [0.48 nm-Au coated ITO (35/µm2): 0.42 (6/µm2) & bare-0.33nm], which indicate the higher density of Au NPs on ITO results high efficiency [45]. The surface plasmon resonance peaks of gold nanostructures can be tuned from the visible to the near infrared region by controlling the shape and structure [46, 47]. Au NCs show size-dependent tunable fluorescence from visible region to near-infrared region, even to 800 nm wavelength [48-50]. Au NPs with surface plasmonic absorption in the visible region is beneficial for optoelectronic devices otherwise the plasmonic



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enhancement is not possible. Hence, the highly homogenous or monodispersed Au NPs shows plasmon absorption around 500 nm is necessary to achieve plasmonic enhancement. If there is an agglomeration between the nanoparticles, absorption moves away from visible region make it unworthy. Lifetime measurements show that the Au NPs coated ITO enhances the decay time of devices II and III when compared with reference device. The emission due to the effective electron-hole recombination with the applied voltage may pass via the plasmons present in the Au NPs boundary of the bottom-emissive device. The tri-exponential decay parameters are observed for devices I-III (Table 1) and the lifetime for the reference device I is τ1 = 0.91, τ2 = 2.50 and τ3 = 11.05 ns. Among them, τ2 and τ3 are attributed to the two geometrical isomers of DMSPI ligand orientation in Ir(DMSPI)2 (acac) (coplanar and perpendicular). The coplanar isomer appears with 30 % relative amplitude whereas the perpendicular isomer appears with 60 % relative amplitude. The lifetime 0.91 ns (τ1) with amplitude of 10 % is due to the scattering of photons from NPB or ITO layers. Therefore τ2 and τ3 is taken to analyze the luminescent character of Ir(DMSPI)2 (acac) in the hetero junction pattern. The plasmon – exciton coupling (LSPR enhancement) is effective with the 5–15 nm distance between the emitter and Au NPs which reduces the lifetime with increasing the decay rate without affecting the charge transport property [45]. The distance is tuned to 40 nm (NPB thickness -40 nm) in the fabricated devices, thus, the Au NPs on ITO helped to liberate photons via the hetero junction results far-field plasmon enhancement (FFPE). Therefore, the optically strengthened pathway (hetero junction) increases the lifetime with increasing emission intensity [47]. This can be connected to retardation effect exerted by light wave in front of a reflecting boundary and it depends on the distance (higher than LSPE limit) between nanoparticles to emitter and the refractive index of the


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spacer. Therefore, the optically improved pathway in hole junction leads to increase of lifetime with enhanced emission intensity [45]. The measured higher relative amplitude of perpendicular isomer (τ3) is responsible for the light generating property of emissive material on combined with the Au NPS, thus supports the coupling of far-field plasmons with Ir( DMSPI)2 (acac) emission. The plasmonic coupling enhancement in OLEDs was supported by: (i) increase of life time with relative amplitude: decay lifetime of excitons generated in the emissive layers [51]. Energy transferred from metal nanoparticles to emissive layer reduces the life period of the generated excitons and results increase of decay rate with the enhanced light emission for localized plasmonic coupling whereas for far-field plasmonic enhancement, the distance is higher than the limit necessary for localized plasmons to react with excitons. So, the probability of energy transfer between them is ruled out. However, the light generated in the emissive layer has to pass via nanoparticles layer on anode. Metal nanoparticles are having high scattering cross section when compared to conventional emissive molecules. So light generated from the emitter undergo retardation effect with metal NPs which ease the way for light emission from the OLEDs [51, 52]. As a result, the decay time of the excitons increases with enhanced light emission (higher relative amplitude) from the OLEDs (Table 1); (ii) increase of luminance and current efficiency: The 40 nm thickness of NPB is responsible for the increased luminance and current efficiency [18, 53, 54]. The brightness and current efficiency increases by high Au NPs density and brightness increased by 57 % and 89 % and the current efficiency enhanced by 33 % and 73 % corresponding to Au NPs density of 6/µm2 and 35/µm2, respectively: the brightness and current efficiency enhancement in our study is due to far-field effect enhancement and (iii) increased quantum yield (QY): increase of QY is also attributed to far-field effect [11, 18].



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The device III with 35/µm2 Au NPs coverage show intense emission than the reference device I, the HIB is reduced by stabilizing the Fermi state of Au NPs in device III results voltage reduction than the reference device. Au NPs play key role in electrical parameters, such as carrier injection and transport and dopant into semiconductor which may be due to poor Au NPs density [NPB layer thickness (40 nm) > Au NPs size (11.4 nm)]. The larger interface area between the emitter and HTL layer enhanced the charge injection make effective electron-hole recombination results enhanced device performances [7, 11, 54]. The anode with 6.0/µm2 density Au NPs exhibit poor hole injection results poor device performances due to imbalance charge recombination. Besides of Au NPs density, the thickness of NPB layer also affects device performances [11]. The enhanced brightness (L, cd/m2) and current efficiency (ηc, cd A-1) of device III can be obtained with the 40 nm thickness of NPB and 35/µm2 density of Au NPs [5, 11]. The schematic metal enhanced emission with far-field enhancement is displayed in Figure 8. The uniformly dispersed Au NPs on ITO stabilized the work function of ITO by the formation of electric double layer which results reduction in HIB. The enhanced device efficiencies can be explained by the merging of metal enhancement region with light emitting profile CBP: (Ir(DMSPI)2 (acac) [6, 12, 55-57] which is nearer to cathode. Therefore, the efficient OLEDs performance is not accounted by LSPR but from far-field enhancement. Jeganathan etal., [51] reported the maximum luminance from the device: ITO/ (with and without Au NPs)/ α-NPD (30 nm) / Alq3/ LiF/ Al as 9000 cd/m2 and Ma etal., reported the maximum luminance of 15909 cd/m2 with 60/µm2 gold nanoparticles thickness [45]. The maximum luminance of 16008 cd/m2 obtained from our device with double-layer configuration of ITO/Au NPs with [6.0/µm2 (II); 35/µm2 (III)]/ without Au NPs (I) / NPB (40 nm)/Ir(DMSPI)2 (acac):CBP (7%) (50 nm) /LiF (1 nm)/ Al


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(100 nm) is higher than those reported by literature [45, 51]. Hence it is possible to improve the fabricated device performances through modification of thickness of the nanomaterials, hole transport layer and emissive layer. Effort will be made to modify the thickness of the various hole transport layer, density of noble metal NPs and varying emissive layer to enhance the device efficiency in future of our studies. 5. Conclusion The gold nanoparticles with 35/µm2 density at the junction of anode: HTL in OLEDs using CBP:Ir( DMSPI)2 (acac) as emissive layer improve the

hole injection into HTL, leads to

enhanced emission intensity attributed to increasing the radiative rate of OLEDs. The distance between the emissive layer and gold NPs influence the enhancement effect and the gold NPs incorporated hole only devices reduced the hole injection barrier and analyzed by Richardson– Schottky model. The plasmonic nanoparticles on ITO generates dipoles which stabilized the work function of anode results reduction in hole injection barrier. The Au nanoparticles dispersed anode enhances the luminescence intensity through far-field plasmonic effect, thereby the lifetime has been improved with reduced turn-on voltage. The J-V-L curve shows that the brightness and current efficiency increases by high Au NPs density and brightness increased by 57 % and 89 % and current efficiency enhanced by 33 % and 73 % corresponding to Au NPs density of 6/µm2 and 35/µm2, respectively attributed to the far-field effect of Au NPs in visible spectral range.

Supporting Information:

Contents 1. Scheme S1 2. Figure S1 This material is available free of charge via the Internet at http://pubs.acs.org.



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Acknowledgments

One of the author Dr. J. Jayabharathi thank Department of Science and Technology (EMR/2014/000094), Defence Research and Development Organization (213/MAT/10-11), Council of Scientific and Industrial Research [No. 01/ (2707)/13EMR-II], University Grant Commission (36-21/2008) and Nano Mission (SR/NM/NS-1001/2016) for financial support.


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Figure 1: (a) X-ray diffraction pattern of Au NPs; (b) UV-vis spectrum of Au NPs



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Figure 2: HR-TEM images and SAED pattern of Au NPs


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Figure 3: Dynamic light scattering (DLS) and Zeta potential of Au NPs



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Figure 4: (a) UV-vis; (b) Emission spectra of DMSPI and Ir(DMSPI)2 (acac); (c) Representative Franck - Condon electronic transitions of Ir(DMSPI)2 (acac)


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Figure 5: (a) Spin-orbit coupling of heavy-metal facilitated triplet emission; (b) Cyclic voltamogram of Ir(DMSPI)2 (acac)



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Figure 6: DSC and TGA graphs of Ir(DMSPI)2 (acac)


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Figure 7: (a) XPS core level spectra of indium 3d and tin 3d along with bare ITO and ITO/AuNPs.; (b) Luminance-Voltage; (c) Current efficiency-Current density of devices I, II and III:



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Figure 8: (a) Life time spectra of DMSPI and Ir(DMSPI)2acac; b) Life time spectra of devices IIII; (c) Overlapping of Ir(DMSPI)2acac light emitting profile with far-field region of Au NPs


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Table 1: Device performances and Decay parameters τ1: τ2: τ3

A1: A2: A3

τave / χ 2

(ns)

(%)

(ns)

Ir(DMSPI)2(acac)

0.46:13.01

10:90

2.46/1.00

Device I

0.91:2.50:11.05

10:30:60

Device II

1.10:3.01:11.54

Device III

0.85:2.88:11.10

ɸ

Kr

Knr

×106 s- ×106 s-

a

V1000

L

ZB

b

ηc

ZLE

1

1

(V)

(cd/m2)

0.42

0.41

0.25

-

-

-

-

-

5.25/1.00

0.44

0.08

0.11

5.4

8428

-

7.5

-

6:24:70

8.65/1.00

0.59

0.12

0.05

5.0

13281 (57%)

1.57

11.2 (33% )

1.49

7:30:63

8.95/1.00

0.60

0.11

0.04

3.4

16008 (89 %)

1.90

13.0 (73 %)

1.73

a

(cd A-1)

ZB = Bd/B, Bd – maximum brightness with Au NPs; B – maximum brightness without Au NPs: bZLE = LEd/LE, LEd – maximum luminous efficiency with Au NPs; L – maximum luminous efficiency without Au NPs: L – Brightness; ηc - Luminous efficiency; τ – Life time; A – Amplitude.



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Table 2: Selected bond lengths (Å) and bond angles (°) of Ir(DMSPI)(acac) by DFT/LANL2DZ/6-31G (d,p) Connectivity

Bond length (Å)

Connectivity

Bond angle (°°)

Ir(1)–C(2)

2.01

C(2)–Ir(1)–C(9)

92.00

Ir(1)–C(9)

2.02

C(3)–C(7)–F(85)

116.98

Ir(1)–O(39))

2.13

C(10)–C(13)–F(86)

117.92

C(13)–F(86)

1.41

N(34)–C(32)–N(36)

110.01

C(7)–F(85)

1.41

O(39)–C(37)–O(38)

127.12

C(37)–O(39)

1.32

C(37)–C(19)–N(18)

118.4

Ir(1)–N(34)

2.04

-

-

Ir(1)–N(36)

2.08

-

-


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Enhancement of Electroluminescent Green ... - ACS Publications

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