Energy Transfer within Mixed Phase Polyfluorene Based Phosphorescent Electroluminescent Devices

Energy Transfer within Mixed Phase Polyfluorene BasedPhosphorescent Electroluminescent DevicesAraceli Gutierrez-Llorente,* B. Arredondo, and B. Romero

Departamento de Tecnología Electronica, ESCET; Universidad Rey Juan Carlos, Calle Tulipan, s/n 28933 Madrid, Spain

*S Supporting Information

ABSTRACT: The fraction of β chains is expected to have astrong influence on the energy transfer processes in organiclight-emitting diodes based on mixed phase polyfluorenedoped with transition metal phosphors. Here, we report on theelectroluminescence emission properties of devices based onpoly(9,9-dioctylfluorene), containing different fractions of βphase, doped with varying ratios of an iridium triplet emitter.We measure the electroluminescence spectra at constantcurrent density and the current density−voltage characteristicsof the diodes. We also investigate the temperature dependenceof electroluminescence emission within a temperature rangebetween 77 and 270 K in this system. For a given dopant concentration and β phase content, the electroluminescent emissioncovers a broad range of the spectrum. Trap-assisted recombination in β phase sites, which act as electron trapping sites, wouldcompete with direct charge recombination in dopant sites, which might act as a hole trap. Singlet energy transfer seems also tocontribute to the excitation of the phosphors in doped devices.

■ INTRODUCTIONAmong organic-based photonic devices, organic light-emittingdiodes (OLEDs) are the most developed.1 The true drivingelement of this exciting technology several years ago has beenthe enhancing of the internal quantum efficiency of devices.The introduction of transition-metal phosphors in OLEDs,2,3

which allow the harnessing of luminescence from tripletexcitons,4 was a major breakthrough in device efficiencyincrease.5 Phosphorescent OLEDs (PHOLEDs) offer thepotential of emitting a photon for every injected charge.6

The greatest asset of organic optoelectronic devices is theirlarge potential for developing low-cost large area devices bysimple solution processing technologies.7 In spite of that,commonly, high efficiency has taken priority over ease ofprocessing. Thus, state-of-the-art devices are generally thosebased on vacuum-deposited multilayer structures made bysequential layer deposition of small-molecule organic materials.This multilayer approach increases the complexity of thefabrication process, and it is not suitable for the fabrication oflarge size panels.Conjugated polymers (CPs) merge the ease of fabrication

and low production cost. Although they show an exceptionalcombination of optical and electronic properties,8 the numberof works that deal with the solution processable approach forpreparing phosphor doped polymer-based OLEDs is not highcompared to the volume of papers devoted to vacuumdeposition of multilayer small-molecule based PHOLEDs.Fortunately, most of the triplet emitters originally designed asphosphorescent dyes for vacuum-deposited devices can besuccessfully used in solution-processed diodes as well. Then,

efficient polymer-based PHOLEDs have been reported. Theyare frequently proposed as white emitters for solid-state lightingapplications where large size devices are demanded. Transition-metal phosphors are blended in a fluorescent host9 orcovalently bounded to the main polymer backbone.10 Inthese works, fluorene-based polymers or copolymers that emitin the blue range are widely used.Among fluorene-type polymers poly(9,9-di-n-octylfluorene)

(PF8) has been well studied due to the formation of aplanarized structure with narrow spectral features, called βphase. The domains of the well-ordered, longer conjugationlength β phase are embedded in the amorphous glassy phasewith a twisted chain arrangement. The great majority of theseworks have investigated PF8 thin films. These samples, whichcontain a mixture of β and amorphous phases, have beenanalyzed by means of time-dependent and temperature-dependent photoluminescence, photoinduced absorption, andelectroabsorption.11−18 As reported in these studies, β phaseacts as a low-energy trap for both singlet and triplet excitonsinitially created within the amorphous matrix. Also, theexciton−exciton annihilation rate17 as well as the nonradiativedecay rate19 were found to be higher in β phase rich films.However, electroluminescence emission in mixed-phase PF8based OLEDs has been much less studied.20−23 Among thoseworks, only two of them have estimated the content of the βphase in the PF8-based devices. Lu et al.20 have reported on the

Received: October 3, 2011Revised: January 10, 2012Published: January 20, 2012

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increase in device efficiency for PF8-based diodes with lowcontents of β phase (ca. 1%) compared to those with no βphase (3.85 cd A−1 relative to 1.26 cd A−1). They boosted theformation of β phase by dipping PFO films spin-coated onindium tin oxide substrate in mixed solvent/nonsolventmixtures for various periods of time (1.32% was the highest βphase content that could be obtained by this dipping process).Peet et al.21 have compared the device performance of PF8-based diodes with no β phase to devices with a β phase contentof 45%. They found that the enhancement of luminanceefficiency for the β phase rich devices was not as meaningful asit could be expected. It varied from 1 cd A−1 to 1.6 cd A−1.Bansal et al.,19 after having studied the fluorescence quenchersin mixed phase PF8 films, predicted that the best efficiency oflight emitting devices is expected to be achieved in materialswith a low amount of β phase (lower than 5%).In view of these results, the fraction of β phase is expected to

have a strong influence on the energy transfer processes inhost−guest systems of PHOLEDs. Recently, the effects oftemperature and solvent on the β phase formation and energytransfer in thin films and solutions of an Ir(III) complexcontaining polyfluorene have been investigated by a group atthe Nanjing University.24 The authors report on the improvedenergy transfer from polyfluorene to iridium(III) complex forthe highest β phase contents. Because of the introduction of theiridium(III) complex into the PF8 main chain, the formation ofthe β phase became more difficult with the increase ofiridium(III) complex content. Despite the interest of thissubject, as far as we are aware, the role of β phase on thetransfer processes in Ir complexes/PF8 blends and its effect onthe efficiency of devices based on those blends have not beenstudied yet.Here, we report on the emission properties of polymer based

phosphorescent devices whose active layer contains a cyclo-metalated iridium(III) complex, [Ir(dhfpy)2acac],

25,26 blendedin a polyfluorene host: PF8 containing varying proportions of βphase. Another polyfluorene, PF2/6, which never adopts the βphase, has also been employed in order to compare certainresults. The β phase concentration of PF8 matrices was variedby changing the spin coating solvent and/or temperature asdetailed in the Experimental Methods section, albeit this workis not a systematic analysis of the effects of experimentalparameters on the β phase formation. The system PF8:Ir-(dhfpy)2acac, where PF8 contains mixed phases, is appealingfor studying singlet and triplet transfer processes in host−guestsystems. As a matter of fact, the exciton energy level of bothphases of the host differs, and thus, the emission properties ofthe system can be varied as function of the relative amount of βphase. We have found that singlet energy transfer as well ascharge trapping seem to contribute to the excitation ofIr(dhfpy)2acac in doped devices.

■ EXPERIMENTAL METHODS

Materials. We have used as phosphor a bis−cyclometalatediridium complex with acetylacetonate (acac) as ancillary ligand:Ir((dhfpy)2(acac), bis(2-(9,9-dihexylfluorenyl)-1-pyridine)-(acetylacetonate) iridium(III) (from American Dye Source).Two polyfluorenes have been used as host: PF2/6 (Poly[9,9-di-(2′-ethyl-hexyl)fluorenyl-2,7-dyil], from Sigma-Aldrich) andPF8 (Poly(9,9-dioctylfluoren-2,7-diyl) end-capped with DMP,from H.W Sands). No electron transport materials wereincluded into the host matrix.

Device Fabrication. Phosphorescent light-emitting diodeswere fabricated in a glovebox under N2 atmosphere (back-ground levels of water and oxygen lower than 0.1 ppm). ITOcoated glass substrates were cleaned in an ultrasonic bath andexposed to UV−ozone for 15 min before use. PEDOT:PSS(Clevios from H.C Starck) film (thickness ≈ 40 nm) was spin-coated from an aqueous solution at 4000 rpm and baked at 110°C under inert atmosphere for 1 h to remove any residualwater. The green emitter [Ir(dhfpy)2acac] was used as dopantinto the polyfluorene host. The emitting layer was spin-coatedfrom a 2 wt % solution at 6000 rpm. This solution was a blendof Ir−phosphor doped polymer host at different doping ratios(99.5:0.5; 98:2; 95:5; and 90:10). PF2/6 based solutions wereprepared in ortho-xylene. Incorporation of different amounts ofβ phase in PF8 films was achieved by using different solvents:toluene, ortho-xylene, or toluene/ortho-xylene mixture (volumeratio 1:1). All polymer solutions were passed through a 0.45 μmsyringe filter before spin-coating to remove any residualparticulates. A Ba (30 nm)/Al (100 nm) bilayer cathode wasthermally evaporated on top of the organic layer surfacethrough a shadow mask in an atmosphere of 10−6 Torr. Thedevice area was 2 mm2. Finally, devices were encapsulated bymeans of a glass cover attached by a bead of epoxy adhesive.Film thickness was typically 70 nm.Once the devices were encapsulated, they were taken out of

the glovebox. All of the measurements were performed underambient atmosphere. Current−voltage characteristics wererecorded using an Agilent 4155C semiconductor parameteranalyzer. Electroluminescence spectra were obtained with aMinolta CS 2000 spectroradiometer. Devices were drivenunder dc conditions. Absorption spectra of the films weremeasured with a Varian Cary 500 UV−vis spectrophotometer.Films for absorption analysis were deposited by spin coating onquartz substrates (Suprasil2 grade B, from Heraeus). The glassyphase film (0% β phase) of PF8 was spin coated from achloroform solution. It was used as reference for the estimationof the β phase fraction in other films since it did not containany appreciable amount of β phase chromophores. Temper-ature-dependent measurements were made in an open cyclecryostat operated with liquid nitrogen connected to atemperature controller.

■ RESULTS AND DISCUSSIONEnergy transfer processes play an essential role in the emissionproperties of PHOLEDs. One of these processes, a long-rangeresonance mechanism known as Forster energy transfer,requires the overlap of the emission of the matrix and theabsorption of the guest. Also, differences in triplet energybetween host and guest molecules are of primary importancesince host-to-guest triplet energy transfer (short-range processknown as Dexter transfer) can be endothermic or exothermic.Figure 1 shows the overlap of the emission of the hosts and theabsorption of the guest. It shows the absorption of the guestand the EL spectra at room temperature of OLEDs based onthe materials employed in this work. Figure 1a displays a UV−vis absorption spectrum of spin-coated thin films of the dye,Ir((dhfpy)2(acac). An absorption band around 475 nm can beassigned to transitions to 1MLCT(1dπ*) excited states.Absorption bands below 400 nm mainly correspond totransitions from the singlet ground state, S0, to

1LC state ofthe cyclometallating ligand. Spin-forbidden transitions 3MLCTand 3LC, although enhanced due to strong spin−orbit couplingof the Ir atom, are seldom distinguised. They would appear at

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lower energies (1LC > 1MLCT > 3MLCT > 3LC). It is worthmentioning that phosphorescence emission from Ir-((dhfpy)2(acac) shows a vibronic structure. It is generallyaccepted that the quantum mechanical induced mixing between3LC, 1LC, 3MLCT, and 1MLCT states determines theproperties of the lowest triplet state, T1, (due to the spin−orbit coupling between 1MLCT and 3MLCT and electron−electron interaction between 1LC and 3LC).5,27,28 A highMLCT perturbation on the emitting triplet wave functionwould result in structureless phosphorescence, which has beenrelated to a higher OLED performance. In contrast, theIr((dhfpy)2(acac) triplet state exhibits a LC-based emission,which determines the vibronic structure of phosphorescencespectrum. The EL spectrum of Ir(dhfpy)2(acac) has itsmaximum at 558 nm (Figure 1a), which corresponds to thelowest triplet state energy of 2.22 eV, in accordance withprevious published values.25

EL spectra at room temperature of PF8 for glassy and βphase emission are shown in Figure 1b. EL spectrum from PF8exhibits the characteristic narrowing of β phase emission. Itshows peaks at 2.83, 2.66, and 2.49 eV, which correspond to thevibronic structure of β phase. The vibronic progression for theamorphous emission of PF8 appears at 2.93, 2.77, and 2.61 eV.Thus, we observe a singlet shift of 100 meV between bothphases, in accordance with reported values.15

EL spectrum of PF2/6 is shown in Figure 1c. This ELspectrum also exhibits a vibronic structure. The two main peaksappear at 2.95 and 2.81 eV. These features correspond to the

0−0 and 0−1 singlet exciton decays, respectively. The spectralposition of S1 is comparable to that previously reported for aPF2/6 film at 80 K, 2.93 eV,29 and at 11 K, 2.89 eV,15 since asmall change (order of tens of meV) to lower energies uponcooling is expected.11

For conjugated polymers, the exchange energy (S1−T1) isalmost constant and close to 0.7 eV32 and the exciton bindingenergy, about 0.1−0.3 eV. T1 has been reported to appear at 2.1eV for a film of PF2/6 measured at 77 K33 and at 2.18 eV at 80K34 with a singlet−triplet energy gap of 0.75 eV, thoughphosphorescence spectrum depends heavily on film thicknessand hence on preparation conditions. Hoffmann et al.35 havefound that the phosphorescence spectrum of PF2/6 shiftstoward the blue spectral range upon lowering the temperature(hypsochromic shift). They found a shift of ca. 80 meVbetween 10 and 100 K. T1 for low temperature β phase PF8emission has been measured at 2.08 eV and the redshift of the βphase phosphorescence compared to the amorphous one, 70meV.15 The same authors have found a red shift of 10 meV ofthe PF8 amorphous triplet compared to PF2/6. Thesedifferences in triplet levels are depicted in Figure 2b.

The guest might also act as a carrier trap and recombinationsite. Thus, charge transport levels are fundamental parametersto understand the emissions properties of PHOLEDs. HOMOand LUMO values for the dye are 5.1 and 2.4 eV,respectively;31 and 5.45 and 2.41 eV, respectively, for PF2/6.30 HOMO levels of amorphous and β phase of PF8 have beenmeasured to be the same.20 Both phases differ in their LUMOlevel. On the basis of these values from the literature, Figure 2adisplays the energy level diagram of the host and the guest andthe work functions of the electrode materials used.In the first series of experiments, we have studied the relative

contribution of either phase (β and glassy) in the EL emissionfrom ITO/PEDOT:PSS/PF8/Ba/Al devices as function of thedriven current. Previously, we have estimated the fraction of βphase in each emission layer from the absorption spectra in theUV−vis range after normalization of the mixed phasesspectrum to the amorphous phase spectrum at 354 nm,21

assuming that both phases have similar oscillator strengths.Figure 3 shows the UV−vis absorption spectra for filmsprepared onto quartz substrates from various solutions.

Figure 1. UV−vis absorption spectrum (a, black) of a spin-coatedIr(dhfpy)2(acac) film and normalized EL spectra of the materialsemployed. For all figures, EL spectra were obtained from devices withITO/PEDOT:PSS/EML/Ba/Al structure at constant current drive(corresponding to a current density of 50 mA/cm2). The EML was (a,gray), PF2/6:Ir(dhfpy)2(acac) ratio of 85:15, the emission from thepolymer is completely quenched; (b) PF8 with 0% β phase (black)and PF8 with 20% β phase (gray); (c) PF2/6.

Figure 2. (a) HOMO−LUMO diagram for the materials used in thiswork. The energies are measured in eV with respect to the vacuumlevel. The values are taken from the literature.20,30,31(b) Triplet levelsof the hosts and the guest.15



Absorption spectrum of an amorphous PF8 film prepared froma chloroform solution is also shown. It has been used asreference for the calculation of the β phase content. Thehighest content of β phase becomes apparent as a peak at 432nm. This peak appears only as a shoulder in the spectra of filmswith lower β content. Different fractions of β phasechromophores have been estimated. The film spin-coatedfrom an ortho-xylene solution contains ca. 20% of β phase. Filmspun from the same solution at, approximately, 50 °C contains5% of β phase and that prepared from a mixture of toluene−ortho-xylene (volume ratio 1:1) incorporates 0.7% of β chains.In all cases, the evaporation of the solvent took place at a lowrate, in N2 atmosphere, since there was no thermal annealingafter deposition.Figure 4 shows normalized EL spectra PF-based diodes with

β phase fractions estimated above. They have been measured

under dc conditions at different constant current densities: 12.5mA/cm2, 50 mA/cm2, 150 mA/cm2, and 250 mA/cm2. βemission predominates in the 20% β phase based diodealthough the spectra showing a shoulder at ca. 2.9 eV, which

corresponds to the emission of the amorphous phase. Thisresult is not surprising since β phase acts as a low-energy trapfor both singlet and triplet excitons initially created in theamorphous domains. It has been previously observed that thephotoluminescence spectrum is also dominated by β emissioneven for a low content of this phase (less than 10%).12,16 TheEL spectrum of Figure 4b corresponds to pure amorphousphase since any trace of β emission, which would appear ataround 2.83 eV, is observed at any current density. For a βphase fraction of 20%, the differences between the EL spectraobtained at diverse current densities are scarcely noticeable. Incontrast, EL emission from diodes with 5% of β phase showsdistinct features as a function of current density (Figure 4c).The spectrum measured at 12.5 mA/cm2 exhibits two mainpeaks almost equally intense at 2.83 and 2.93 eV, whichcorrespond to the S1 → S0 transition for β and amorphousphases, respectively. As current density increases, the relativeintensity of the amorphous emission decreases. At the highestcurrent density (250 mA/cm2), emission from β phasedominates the spectrum, and the peak at 2.93 eV appearsonly as a shoulder. Even for a fraction of β phase chains as lowas 0.7%, the EL spectrum measured at 250 mA/cm2 includesemission from β phase (Figure 4d). Thus, high currentdensities make β phase emission distinguishable even forcontents of this phase as low as 0.7%.Since the LUMO level of the β phase is 120 meV lower than

LUMO of the amorphous phase (Figure 2a), β phasechromophores are expected to trap electrons. We proposethat β phase EL emission at high current density for lowfractions of β phase (<5%) is due to radiative recombination ofan electron that is trapped on the chromophore with a freehole. This process, called Shockley−Read−Hall (SRH)recombination, is linearly dependent on the carrier density.36

Then, it explains the current density dependence observed inthe EL spectra of Figure 4.Luminance of these mixed-phase devices is shown in Figure 5

as function of current density. The highest luminance

corresponds to the higest luminous efficiency since the activearea is the same for all devices. Each curve has been obtainedafter the average of measurements corresponding to differentdiodes of the same matrix. The highest luminance is obtainedfor diodes that do not contain β phase chromophores. Thisresult is surprising since an improvement in device performanceis expected for low contents of β phase. However, other

Figure 3. Absorption spectra of PF8 films prepared from differentsolutions. The percentage of β phase content is indicated in each plot.In the first figure, the absorption spectrum of a PF8 film with 0% of βchromophores (100% amorphous glassy phase) is shown (dashedline). Absorption spectrum of the β phase in each figure (light gray)has been obtained after normalization of the mixed phases spectrum tothe amorphous phase spectrum at 354 nm.

Figure 4. Normalized EL spectra from ITO/PEDOT:PSS/PF8/Ba/Aldevices with different contents of β phase: 0, 0.7, 5, and 20%. For eachplot, spectra obtained at 12.5, 50, 150, and 250 mA/cm2 are shown.Inset in panel d enlarges the 2.7−3 eV range of its reference figure.

Figure 5. Luminance (cd/m2) as function of current density for ITO/PEDOT:PSS/PF8/Ba/Al devices with diverse β phase content.



parameters have also to be taken into account, namely, theinfluence of the solvent on the nanoscale structure of the activelayer. As a matter of fact, the formation of β phase is favored bythe use of a high boiling point, poor solvent. In this study, thehighest β fraction has been obtained for films prepared fromortho-xylene solutions. The solubility of PF8 in ortho-xylenewas, at first glance, clearly lower than in toluene, even if thesolubility parameter of both solvents is quite similar. Thisdifference in solubility could determine a quite differenthomogeneity in the distribution of both phases in the activelayer. This spatial distribution, in turn, has a strong influence onthe device performance since an inhomogeneous distributionmight hinder exciton transfer to the lower energy β phase.More detailed analyses are required to make this point clear.Nonetheless, in view of these results, we can state thatperformance of PF8-based devices with diverse β phasefractions introduced by the use of different solvents does notdepend exclusively on the relative amount of β phase.In another series of experiments, we have investigated the EL

emission from devices with a blended active layer. Figure 6

shows normalized EL spectra for a doping ratio of 98:2 anddiverse fractions of β-phase in the PF8 matrix. The relativeamount of β phase in each blended layer has been estimatedfrom the UV−vis absorption spectra of spin-coated films onquartz substrates as described above. This estimation leads tothe following β phase fractions: 0% for the film spun at,approximately, 50 °C from a mixture of ortho-xylene−toluene(volume ratio 1:1); 2% for the film deposited at the sametemperature from ortho-xylene solution; 12% and 17% for filmsprepared from a toluene solution without any annealingtreatment or annealed at 70 °C for 1 h in oven, respectively.Samples were driven under constant current density. For a pureglassy PF8 host (Figure 6a), the EL emission from the polymeris almost completely quenched. In this case, there is almost nodifference between the spectra obtained at diverse currentdensities. The maximum intensity in each spectrum corre-sponds to the phosphorescence emission of the Ir compound.EL spectra of PF8-2% β based diodes are shown in Figure 6b.The dominant emission still comes from the triplet decay of the

guest, but fluorescence from the polymer is more important,relative to the dye emission, than in PF8-0% β diodes. Bothphases, glassy and β, are perceptible (at 2.93 and 2.83 eV,respectively) with a similar intensity even if the relative amountof β regions is as low as 2%. When the relative amount of βphase reaches 12%, the emission from the polymer isdominated by the β chains, as it could be expected, butemission from the amorphous phase is also present (Figure 6c).Finally, EL spectra from devices containing 17% of βchromophores (Figure 6d) are dominated by the emission ofthis phase at the highest current density although itscontribution diminishes, relative to that of the dye, for lowercurrent intensities. In summary, for a doping ratio of 98:2, theEL emission from the polymer increases, relative to thephosphorescence from the Ir compound, as the fraction of βphase in the host increases. The spectrum from a 17%-β phasedevice measured at 250 mA/cm2 is dominated by the emissionfrom this phase, whereas, at the same current density, ELemission from the polymer is almost completely quenched for adevice based on pure amorphous PF8 host.The increase of β phase fluorescence relative to the guest

phosphorescence for increasing β fractions could be explainedby the trap-assisted recombination in β phase sites, which act aselectron trapping sites, as we have discussed above. Thismechanism would compete with trap-assisted recombination indopant sites, which might act as a hole trap due to the offset ofits HOMO level compared to that of the PF8 (Figure 2). Thus,upon increasing of the β phase concentration, the trap-assistedrecombination in β phase sites would exceed the recombinationin dopant sites. It is expected that this effect be more significantupon increasing current density through the device as it is,indeed, observed. Figure 7a visualizes this reasoning.

The trap-assisted recombination hypothesis is furthersupported by the fact that electron-trapping in the β phasechromophores seems to exist in undoped diodes of mixedphase PF8, as we have discussed above (Figure 4). Then, wehave measured the J(V) characteristics of the devices atdifferent doping concentrations in the search for someexperimental evidence that could support the occurrence ofhole trapping at the guest molecules. Figure 8 shows the J(V)characteristics of diodes prepared at a doping ratio of 99.5:0.5or 98:2 in PF8 matrices with different β phase content. For thediodes whose J(V) curves show variations in the doping levelmean also a change in the β phase content of the polymer

Figure 6. Normalized EL spectra from ITO/PEDOT:PSS/PF8:Ir(98:2)/Ba/Al devices with different contents of β phase inthe host: 0, 2, 12, and 17%. Spectra have been obtained at constantcurrent density: 12.5, 50, and 250 mA/cm2. Spectra in panel d arenormalized at the emission peak of β phase (2.83 eV) for all the threecurrent intensities studied.

Figure 7. (a) Schematic diagram depicting the trap-assistedrecombination hypothesis to explain the EL emission in dopeddevices based on mixed phases of PF8. (b) Singlet energy transferprocesses in the devices. The energy positions of PF8 and PF8−βtriplet levels are located in accordance with the literature data.15,32

Abbreviations are isc, intersystem crossing, and ST, sinlget transfer.



matrix. Thus, the improvement on the J(V) characteristics upondoping might be due to morphological changes in the blendfilm produced by the process employed to increase the β phasecontent. Indeed, we observe some differences between the SEMimages of 98:2 films as-spun or annealed at 70 °C (SupportingInformation). Nevertheless, some evidence of hole-trappinghave been found in doped PF2/6 based diodes. Figure 8 showsthe J(V) characteristics at different dopant concentrations ofIr(dhfpy)2acac:PF2/6 devices as well. It seems that charge(hole) trapping plays a major role in the EL emission from theIr complex in the case of PF2/6 matrix, which never adopts theβ phase: the bias required to achieve a fixed current densityincreases as the dopant concentration increases. Since ELemission spectra from diodes based on PF26:Ir or PF8(0.7%β):Ir with the same doping ratio (95:5) are practicallyindistinguishable (Supporting Information), it could beexpected that hole-trapping should also play a significant rolein doped PF8−amorphous matrix whose HOMO level does notdiffer from the PF8−β matrix (Figure 2). Hole-only devices atdifferent doping levels based on PF8 blends with the same βphase content would have to be electrically characterized toestablish the hole trapping on the Ir:PF8 diodes, though.Figure 7b depicts singlet energy transfer processes in this

system. Although the majority of excitons are initially generatedin the glassy phase, a rapid energy transfer to the β phaseoccurs, in accordance with previous reports.13,14 Moreover,effective transfer of the PF8 amorphous singlet excitation to theexcited singlet state of the Ir complex, which is a dipole−dipoleinteraction, is expected to occur since there is a considerablespectral overlap of the PF8−glassy fluorescence and the Ircompound absorption (Figure 1). Then, the lowest triplet state

of Ir is populated due to efficient intersystem crossing (isc)enhanced by spin−orbit coupling of the Ir center. Consistentwith the reported literature data, we consider that the energyposition of the Ir triplet level (2.22 eV, measured from ELspectra) should be higher than the energy of the triplet level forthe β phase of PF8 (see Figure 2b and related discussionabove). However, triplet−triplet transfer from the guest to theβ phase chains of the host is unlikely to occur. Indeed, theDexter-type transfer rate of a triplet excitation depends on thespectral overlap between the donor emission and acceptorabsorption (as it is for Forster-type transfer).4 In the blendsunder study, this would require spectral overlap of the Ircompound phosphorescence and triplet absorption of β phasedomains (β triplet absorption). Nevertheless, photoinducedabsorption spectra of PF8−β films previously reported aredominated by a sharp transition centered at 1.43 eV, which hasbeen attributed to triplet−triplet absorption from β phasechains,11,15,16 thus far from the EL of the Ir compound. Furtherinvestigations, namely, the study of the effect of reverse biasingon the delayed EL in this system, would allow to explore thecontribution of singlet excitons generated via triplet−tripletannihilation in PF8−β phase to the delayed fluorescence.37

We have studied the EL emission of 0.5% Ir doped devices.At such low dopant concentration, Dexter energy transfer (if itwould exist in this system) is insignificant since it is a short-range process and the distance between host−guest is not shortenough for triplet transfer to happen. Figure 9 shows the EL

spectra, at 50 mA/cm2 and 250 mA/cm2, from diodes with thesame doping ratio (99.5:0.5) and different β phase content, 5%or 7%. Both devices have been prepared from a toluenesolution and subsequently annealed at 70 °C (7% β) or not

Figure 8. J(V) characteristics of PF8 and PF2/6 based diodes withdifferent doping ratios. PF8 devices contain various β phase fractions.Each J−V characteristic was obtained after the average of measure-ments corresponding to different diodes of the same matrix. In eachcase, only devices prepared with the same solvent are compared(ortho-xylene and toluene for PF2/6 and PF8 based devices,respectively).

Figure 9. Normalized EL spectra for PF8:Ir based diodes with adoping ratio of 99.5:0.5 and different β phase content, 5% (upper plot)or 7% (lower plot). Spectra have been obtained at constant currentdensity: 12.5 mA/cm2 and 250 mA/cm2. Spectrum from an undopedPF8 diode with a β phase content of 20% is also shown in the lowergraph as a reference.



annealed (5% β). Green emission should mainly come fromForster transfer of host singlet excitons. β phase based diodes of5% show EL emission from both components, host and guest.Emission from the host is basically due to β phase domains,although amorphous emission is also noticeable as a shoulder.Besides, contrary to what we have observed for 98:2 devices(Figure 6), the EL spectra of 99.5:0.5 diodes with a β phasecontent of 5% do not depend on current density. Indeed, greenemission from Ir:PF8−5% β for a doping level of 99.5:0.5 cannot be generated exclusively by charge trapping at the dopantsites since spectra are independent of the driving conditions asit is expected when the dye emission is preceded by singlet−singlet energy transfer from the amorphous PF8 matrix to theguest. Spectra from 7% β phase based diodes do not showemission from the guest, and emission from the host iscompletely due to β phase. This could be due to themorphology of the film (induced by the annealing treatment),which would hamper the singlet exciton transfer from theamorphous phase of the host to the guest. It is worth notingthat Forster transfer from β singlets to guest singlets seems notto exist, although singlet transfer from amorphous to β phaseshould occur.We have also studied the temperature dependence of EL

emission from ITO/PEDOT:PSS/PF8:Ir(98:2)/Ba/Al deviceswith β phase content of 17% at a range of temperaturesbetween 77 and 270 K. EL spectra are shown in Figure 10.

They are normalized at the Ir complex phosphorescence peak.Devices were dc driven at a constant current density of 50 mA/cm2 and 250 mA/cm2. For both current densities, there are nodifferences between the spectra obtained in the temperaturerange 270−120 K. However, the spectra measured at 77 Kshow a lower emission from the host relative to the emission

from the guest. This behavior is observed for both currentdensities, 50 and 250 mA/cm2. Temperature dependent datacould be explained in the framework of the trap-asistedrecombination hypothesis. Detrapping of carriers from traps tothe effective transport level will be stimulated by thermalenergy upon increasing temperature. The increase of ELemission in the blue region relative to the green emission fromthe dopant that we observe in Figure 10 could be caused byholes that are detrapped from the dopant sites which wouldsubsequently recombine with electrons trapped in the β phasesites, though detrapping from the deep hole traps shouldrequire higher energy than release of electrons from the shallowtraps (Figure 2). Since singlet transfer should not depend ontemperature, back triplet transfer from the β phase to the Ircomplex, whose energy barrier seems to be temperaturedependent,35 could be also envisaged to explain the temper-ature dependent data of Figure 10.

■ CONCLUSIONSWe have investigated the EL emission from mixed phase PF8based diodes undoped or doped with varying ratios of an Irtriplet emitter complex. We have observed that EL spectra ofthe undoped devices depend on current density. In effect, ELspectrum taken at high current density (250 mA cm−2) of theundoped devices shows β phase emission even for contents ofthis phase lower than 1%. This is explained as due to radiativerecombination of an electron trapped at the β phasechromophore with a free hole. This process (calledShockley−Read−Hall recombination) is linearly dependenton the carrier density. For diodes based on mixed-phasePF8:Ir(dhfpy)2acac blends, the competition between trapped-assisted recombination at β phase strands and dopant sitesplays a major role in EL emission. Finally, we have studied theEL emission of doped devices in a range of temperaturebetween 77 and 270 K. We have observed a decrease of the ELfrom the β phase of the host relative to that of the guest in thespectra taken at 77 K. This could be explained by thedetrapping of holes from the dopant sites upon increasingtemperature, which would subsequently recombine withelectrons trapped in the β phase sites. Back triplet transferfrom the β phase to the Ir complex, whose energy barrier seemsto be temperature dependent, could be envisaged as well.

■ ASSOCIATED CONTENT*S Supporting InformationEL spectra of PF2/6 and PF8 doped devices; scanning electronmicroscopy (SEM) images of blends. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +34 91 4888522. E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe measurements of UV−vis absorption were taken at thelaboratory of the Chemical and Environmental EngineeringGroup at Universidad Rey Juan Carlos (Madrid, Spain). Thiswork was supported by MICINN-National R&D&I Plan,MICINN-RYC Program (to A.G.L), and Community ofMadrid-URJC.

Figure 10. EL spectra from a ITO/PEDOT:PSS/PF8:Ir(98:2)/Ba/Aldevice with β phase content of 17% taken at a range of temperaturesbetween 77 and 270 K. Spectra are normalized at the Ir complex peak.They have been obtained at constant current density: 50 mA/cm2

(upper plot) and 250 mA/cm2 (lower plot).



http://pubs.acs.org

mailto:[email protected]

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Energy Transfer within Mixed Phase Polyfluorene Based Phosphorescent Electroluminescent Devices

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