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Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 45– 51

Contents lists available at SciVerse ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h o me pa g e: www .e lsev ier .com/ locate / jphotochem

hite light emitting devices by doping polyfluorene with two red emitters

eferson F. de Deusa,e, Gregório C. Fariab, Roberto M. Fariab, Eduardo T. Iamazakic,eresa D.Z. Atvarsc, Ali Cirpand, Leni Akcelrude,∗

Multi Materials Laboratory (LPMM), Federal Technological University of Parana, CEP 80230-901, Curitiba, Paraná, BrazilPhysics Institute of São Carlos (IFSC), University of São Paulo, CP 369, CEP 13560-970, São Carlos, SP, BrazilChemistry Institute, State University of Campinas (Unicamp), CP 6154, CEP 13084-971, Campinas, SP, BrazilDepartment of Polymer Science and Engineering, University of Massachusetts, Amherst, MA, United StatesPaulo Scarpa Polymer Laboratory (LaPPS), Federal University of Parana, CP 19081, CEP 81531-990, Curitiba, Paraná, Brazil

r t i c l e i n f o

rticle history:eceived 31 July 2012eceived in revised form3 November 2012ccepted 6 December 2012

a b s t r a c t

The photo and electroluminescent properties of a single-layer multi-component blend composed of onlyblue and red emitters were studied. The blue emitter poly(9,9-dihexyl-2,7-fluorene)] (LaPPS10) wasused as the matrix, and the two red emitters, poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene viny-lene] (MEH-PPV) and 4-(dicyanomethylene)-2-methyl-6-(dimethylaminostyryl)-4H-pyrane (DCM), asthe dopants. A detailed study of this system was performed in solution and in the solid state, comparing

vailable online 31 December 2012

eywords:hite PLED

hotoluminescencelectroluminescence

the photophysics aspects of emission spectra of the blend and those of the matrix with each one of thecomponents. The white OLED device exhibited CIE chromaticity coordinates x = 0.30 and y = 0.31. Electroand photoluminescence emission profiles were discussed.

© 2012 Elsevier B.V. All rights reserved.

olymer blends

. Introduction

Organic light emitting diodes (OLEDs) have been developed asn important technology for general solid-state lighting and flatanel display backlighting. In the fabrication of full color displaysll three primary colors are important but white light emission hasarticular interest because any desired color range can be obtainedy the filtering of white light [1,2]. Several approaches to realizehite emission in organic LEDs have been reported including the

ollowing strategies: blending emissive polymers with differentmission colors; doping with phosphorescent complexes or fluo-escent dyes into small molecules or polymer hosts; formation ofxciplexes in bilayer structures; multiple component emissive lay-rs containing an appropriate ratio of red, green and blue (RGB)uminescent dopants; multilayer structure and single polymer withunctional groups [2–7]. Single layer devices are more attractiveue to the ease of fabrication, facile film formation, large scale pro-uction and cost effective features. According to published dataingle layer devices are more stable than those built with emitters

n separated layers [8,9].

In the host–guest system, the doping of a high energy emit-ing (broad band gap, host molecule) with low energy emitting

∗ Corresponding author. Tel.: +55 41 3361 3396; fax: +55 41 3361 3186.E-mail address: leni@leniak.net (L. Akcelrud).

010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2012.12.018

(narrower band gap, guest molecule), the excitation energies maybe transferred under certain conditions from the higher energyhost to lower energy guests [10]. In a white electroluminescentdevice of an active polymer doped with an active guest, the lightoutput usually is governed by the emissions of the host and guestcombined with eventual energy transfer from host to the guest[11–13]. The latter process depends on the relative energy of theHOMO and LUMO levels and on the distance between the donorand the acceptor chromophores. In this sense, the emissions fromboth host and guest in a particular ratio can result in white lightoutput, only if a partial energy transfer process is allowed. Thecontrol over the efficiency of the energy transfer process could inprinciple be achieved by adjusting the concentration of the guestinside a host, but this approach is not straightforward, since veryoften the photoluminescence (PL) and electroluminescence (EL)emission do not coincide in terms of profile and efficiency, due todifferent mechanism for the exciton formation.

In PL processes excitons are directly produced by optical excita-tion and their radiative decay efficiency depends on the radiativeand non-radiative rate processes [14–16]. Moreover, the decay effi-ciency is defined as a ratio between the radiative and non-radiativerate processes. These variables are intrinsically controlled by the

chemical structure of the macromolecule and extrinsically con-trolled by the solid state morphology, the temperature and thepresence of quenchers [11]. The PL quantum yield is defined bythe ratio between the rate constants [17,18].

4 and Photobiology A: Chemistry 253 (2013) 45– 51

de[rstoafeeqect

t[efeh12(ppugissTwdLwTtba

2

2

(naM(w

rsdatiada1

LaPPS10

MEH-PPV

DCM

n O

n

O

NCCN

H3C

N CH3

H3C

Fig. 1. Structures of the components used in emitting devices: LaPPS10 (host), MEH-

6 J.F. de Deus et al. / Journal of Photochemistry

In EL processes the formation of excitons by charge injectionepends, in addition to the PL quantum efficiency, on other prop-rties such as charge transport and charge recombination processes17]. The charge recombination is strongly dependent on the mate-ial’s morphology which in polymer blends and in host–guestystems is also influenced by the extension of the bulk heterojunc-ions [19]. Because the excitons generated by optical absorptionr by charge recombination may be different in terms of energynd chemical species, EL and PL are not necessarily the same. Apartrom the intrinsic photophysical properties of each component, themitted color in multi-component system is based in some gen-ral parameters: the relative concentration of the components, theuantum yield of the PL of each component, the efficiency of thenergy transfer among chromophores, and the efficiency of theharge recombination [17]. This complex set of variables makeshe control of the emitted color a very involved task.

Generally, white LED’s are produced by the mixing of thehree basic colors red, green and blue (RGB combination)3]. In the present study, only blue and red emitters weremployed. Single-layer multi-component devices were success-ully built by blending three electroluminescent materials: the bluemitter [poly(9,9-dihexyl-2,7-fluorene)] (LaPPS10) as the matrixost, and two red emitters, poly[2-methoxy-5-(2-ethylhexoxy)-,4-phenylene vinylene] (MEH-PPV) and 4-(dicyanomethylene)--methyl-6-(dimethylaminostyryl)-4H-pyrane (DCM), as guestsstructures in Fig. 1). LaPPS10 was chosen because it exhibits highhotoluminescence quantum yields in the blue, good charge trans-ort properties, good thermal stability and has been successfullysed in white emitting devices [3]. MEH-PPV is well known as aood hole transport electroluminescent material, whose emissionn solid state is located from 540 nm to 700 nm. Finally DCM is amall molecule with a characteristic red emission that can be dis-olved in the LaPPS10 host in a very uniform distribution [17,21].he composition of both guest components (MEH-PPV and DCM)as adjusted to get the white emission according to the CIE coor-inates. In the study of the binary blends of LaPPS10:DCM andaPPS:MEH-PPV a blue shifting of the red emitter was observed,hich varied according to its concentration in the LaPPS10 matrix.

his adjustment was performed following the PL emission, in ordero obtain in addition to the blue emission from the polyfluorene, aroad band spanning from the green to the red by controlling themounts of each the MEH-PPV and DCM dopants.

. Experimental

.1. Materials

The light emitting polymer poly(9,9-dihexyl-2,7-fluorene)LaPPS10 with weight average molecular weight (Mw) = 12000 andumber average molecular weight (Mn) = 8500) was synthesizednd characterized as described previously in detail in Ref. [22].EH-PPV (Mw = 181,400 and Mn 86,000) from (Aldrich) and DCM

Acros-organic) were used as purchased. The chloroform (Vetec)as used without further purification.

LaPPS10, MEH-PPV and DCM were dissolved in chloroform sepa-ately, in 10−5 mol L−1 concentration. Films were obtained from theolutions filtered through 0.2 �m Millex-FGS Filters (Millipore Co.),eposited by spin coating at a speed of 4000 rpm onto quartz platesnd allowed to dry slowly in a controlled solvent ambient. The film’shickness was adjusted in order to assure optical behavior accord-ng to Beer’s Law. The solutions were mixed in different ratios,

ccording to the blend’s composition desired: the ternary blend,esignated as JF15 (LaPPS10:MEH-PPV:DCM = 100:0.05:0.02 wt%)nd the binary blends: LaPPS10:DCM (100:0.4; 100:0.8 and00:4.0) and LaPPS10:MEH-PPV (100:0.5; 100:1.5 and 100:10).

PPV and DCM dye (guests).

2.2. Device preparation

The EL device fabrication with the configurationITO/PEDOT:PSS/JF15/Al followed the procedure: the substrateswere cleaned with detergent, acetone and isopropyl alcohol andsubsequently underwent a process of hydrophylization withplasma ozone. Next a layer of PEDOT:PSS (Bayer) was depositedby spin coating at a speed of 4000 rpm, resulting in a 60 nmthick layer. The polymer blend was dissolved in chloroform,with 25 mg/mL concentration (the same proportion previouslyindicated). Films were obtained from the solutions filtered through0.2 �m Millex-FGS Filters (Millipore Co.), deposited by spin coat-ing using a rotation of 3000 rpm, forming films of 70 nm. Thealuminum cathode was vacuum-deposited onto the blend layerunder a pressure of about 10−6 Torr resulting in a layer 100 nmthick, completing the device architecture. The same procedurewas followed for the preparation of the binary blends with thecomposition: LaPPS10:DCM (100:0.4) and LaPPS10:MEH-PPV(100:0.5).

2.3. Optical analyses

UV–vis spectra were recorded on a Shimadzu model UV 2401PC spectrophotometer, single beam, in the range 250–750 nm.Steady-state emission spectra were acquired in a Shimadzu 5301PC spectrofluorimeter, in the visible range 390–780 nm. A 1.0 cmquartz square cuvette was used for solutions and a home-madeoptical support for film samples.

Fluorescence decays were recorded using time correlated sin-gle photon counting in an Edinburg Analytical Instruments FL 900spectrofluorimeter using a pulsed hydrogen lamp, in a frequencyrate of 40 kHz. Measurements were performed with wavelengthexcitation of �exc = 320 nm and the emission signals were collectedin �em = 440 nm, respectively for solutions and for films. The sam-ple cuvette was evacuated for 15 min and sealed under vacuum. Thesample decay signal was deconvoluted from the lamp signal usingthe scattering from a Ludox® sample. The experimental curves werefitted using the software F900 provided by Edinburg.

The best fits were obtained for �2 close to 1, and for that weused the multi-exponentials functions,

I(t) = B1 exp(−t

�1

)+ B2 exp

(−t

�2

)+ B3 exp

(−t

�3

)+ · · ·

and Photobiology A: Chemistry 253 (2013) 45– 51 47

wct

MoaM45

E

2

tmEtTnc

3

3

LfL�bwstsoama�orreftet

boahDsb

etMsf

300 350 400 450 500 550 600

0,0

0,5

1,0

440

492468416

378

No

rma

lize

d In

ten

sity

Wavelength (nm)

Absorption LaPPS10

Emission LaPPS10

Absorption DCM

Absorption MEH-PPV

(a)

300 350 400 450 500 550 600 6500,0

0,5

1,0

464

534

432

492468442390

No

rma

lize

d In

ten

sity

Wavelength (nm)

Absorption LaPPS10

Emission LaPPS10

Absorption DCM

Absorption MEH-PPV

(b)

J.F. de Deus et al. / Journal of Photochemistry

here the �i stands for the fluorescence lifetime and the Bi is theorresponding pre-exponential term and represents the contribu-ion of each decay time to the total curve.

Epifluorescence optical microscopy was performed using aodel DMIRB Leica microscope, with a mercury lamp HBO 103W/2,

perating with two excitation filters 340–380 nm for excitation ofll components (A) and 515–560 nm for excitation of DCM andEH-PPV (B) combining with the respective dichroic mirror for

00 nm (blocking only the excitation from 340 nm to 380 nm) and80 nm (blocking light down to 580 nm).

The CIE coordinates were calculated from data taken from theL or PL emission, using the software cie 31 xyz.xls.

.4. Electroluminescence spectra, J × V and L × V measurements

An ITO/PEDOT:PSS/JF15/Al diode structure was analyzed inerms of current and luminance. The current–voltage measure-

ents (J × V) were performed using a 2400 Keithley Source. TheL spectra were acquired using a Labsphere Diode Array Spec-rometer 2100 connected with Labsphere System Control 5500.he luminance–voltage (L × V) was measured by 238 Keithley con-ected with a photodiode. The samples were kept in a sealed Janishamber with high vacuum.

. Results and discussion

.1. Photoluminescence properties

Fig. 2(a) shows the absorption and emission spectra ofaPPS10 and absorption spectra of DCM and MEH-PPV, recordedrom the respective chloroform solutions (10−5 mol L−1). TheaPPS10 absorption band in solution is broad and centered atabs(0–0) = 378 nm. Differently of the absorption, which shows aroad band its emission has a well resolved vibronic structure,ith the 0–0 band at �em(0–0) = 416 nm. Absorption and emission

pectra were not mirror images, indicating that intrachain energyransfer or energy migration processes take place before the emis-ion [23–25]. Fig. 2(b) presents the absorption and emission spectraf LaPPS10 film. The relative intensity of the 0–1 band (464 nm)nd the presence of the emission at 534 nm indicated the for-ation of interchain aggregates in the solid state. The LaPPS10

bsorption band in the solid state was broad and centered atabs = 390 nm, with a sharp peak at 432 nm indicating the presencef some amount of the �-phase [26–28]. Its emission displays a wellesolved vibronic structure, with the 0–0 band at �em(0–0) = 442 nm,ed-shifted compared with the spectrum acquired in solution. Thismission may be attributed to the �-phase, which works as a trapor the excitation migration from the polyfluorene matrix. In addi-ion to the emission at 442 nm and its vibronic progression, anmission band at �em = 534 nm was also observed, which is charac-eristic of aggregates of this polymer [20,26–28].

The MEH-PPV electronic absorption band in solution was alsoroader and centered at �abs = 492 nm, with the blue-edge partiallyverlapped with the LaPPS10 emission. In solid state is centeredt �abs = 492 nm, broader than in solution, with a tail extending toigher wavelengths due to the presence of aggregates [27–29]. TheCM electronic absorption spectrum in solution and in solid state

howed a broad band centered at �abs = 468 nm, but this is a broaderand in solid with a tail extending to higher wavelengths [30–32].

Absorption spectra of LaPPS10, MEH-PPV and DCM cover thentire region of the visible spectrum, from 350 nm to 600 nm. Also,

here is a strong spectral overlap between the absorption spectra of

EH-PPV (�abs = 492 nm) and DCM (�abs = 468 nm) and the emis-ion spectrum of LaPPS10 which satisfies one of the requirementsor energy transfer processes from the LaPPS10 (donor) in the

Fig. 2. Electronic absorption spectra of LaPPS10, DCM and MEH-PPV in (a)10−5 mol L−1 chloroform solutions and (b) solid state. LaPSS10 emission spectrumwas included for comparison. �exc(LaPPS10) = 380 nm.

electronic excited state to the DCM or to the MEH-PPV (acceptors)in the electronic ground state, either by non-radiative resonantenergy process (FRET) or by radiative resonant process (trivial)[11]. The other requirements such as the relative distance betweendonor–acceptor centers and the relative orientation of the dipolemoments (for FRET) depend on the donor–acceptor distance whichis defined by the polymer blend morphology [11,12,33,34].

The possibility of energy transfer processes in this systemwas firstly addressed through the analysis of the photophysicalproperties of blends of LaPPS10 with each one of the other twocomponents. LaPPS10:DCM mixtures in three compositions werestudied (LaPPS10:DCM = 100:0.4, 100:0.8 and 100:4.0 wt%). Forlower DCM concentrations the PL emission spectra were composedby two regions, the higher energy emission band in the 400–520 nmwhich can be attributed to the LaPPS10 host and the lower energyband in the 500–600 nm from the DCM guest (Fig. 3(a)). The higherenergy emission has the 0–0 band at 427 nm blue-shifted com-pared to the �-phase emission [24–26]. A pronounced change ofthe LaPPS10 emission intensity due to the presence of DCM wasobserved: its intensity decreased to four times with the increaseof the dye concentration from 0.4 wt% to 0.8 wt% in the mixture,while the increase of the DCM intensity band at 537 nm was twice,which is in accordance with the Lambert–Beer law. With furtherincrease in DCM concentration to 4 wt%, the LaPPS10 emission iscompletely suppressed. This set of data strongly suggests that DCM

is quenching the LaPPS10 excited state without generation of addi-tional radiative emission. Therefore, considering exclusively thephotophysical properties of this system, in order to maintain theblue, the green and the red emissions with reasonable intensities,

48 J.F. de Deus et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 45– 51

400 450 500 550 600 650 7000,0

0,5

1,0562537

527

474

447426

447427

No

rma

lize

d In

ten

sity

Wavelength (nm)

LaPPS10:DCM - 100:0.4

LaPPS10:DCM - 100:0.8

LaPPS10:DCM - 100:4.0

(a)

400 450 500 550 600 650 7000,0

0,5

1,0

635

589

562

555

447

447429

No

rma

lize

d In

ten

sity

Wavelength (nm)

LaPPS10:MEH-PPV - 100:0.5

LaPPS10:MEH-PPV - 100:1.5

LaPPS10:MEH-PPV - 100:10

(b)

Ffi

t1

sseoiPtoadwtPbofEc([tw

ocit

300 350 400 450 500 550 600 6500,0

0,5

1,0

410

394

538498

465442

No

rma

lize

d I

nte

nsity

Wavelength (nm)

Absorption LaPPS10

Absorption JF15

Emission LaPPS10

Emission JF15

interactions and to the formation of the �-phase [26–28]. For thepolymer mixture JF15 in solution, there is a very small decreaseof both the faster (�PL1 = 0.42 ± 0.10 ns) and of the longer lifetime

Table 1Lifetimes of LaPPS10 and of the blend (JF15), in degassed chloroform solution(10−5 mol L−1) and in film, using �exc = 320 nm, �em = 440 nm; B is the contribution ofevery lifetime to the entire decay, �2 measures the quality of the exponential fitting.

Materials �PL1 (ns) B1 (%) � PL2 (ns) B2 (%) �2

ig. 3. Photoluminescence spectra of LaPPS10/DCM (a), LaPPS10/MEHPPV (b) inlms (�exc = 380 nm).

he DCM concentration in the LaPPS10 host should be less than wt%.

Results for blends of LaPPS10 and MEHPPV (Fig. 3(b)), showedimilar trends: there are two PL bands, one at higher energy emis-ion of LaPPS10 (peaks at 429 nm and 447 nm) and another at lowernergy band (peak around 560 nm) of MEH-PPV. With the increasef the MEH-PPV, there is a remarkable relative decrease of thentensity of the LaPPS10 0–0 band and an increase of the MEH-PV emission proportional to the increase of concentration. Whenhe amount of MEH-PPV increases, there is a significant red-shiftf its emission which can be explained by the polymer aggregations well as by the inner filter effect due to the increase of the opticalensity. Also there is a decrease of the LaPPS10 emission intensityhen the MEH-PPV content increases from 0.5 wt% to 1.5 wt% and

he LaPPS10 emission is completely suppressed for 10 wt% of MEH-PV. Again, the decrease of the LaPPS10 intensity is also followedy a change of the band profile; it is more effective for the red-edgef the 0–0 band indicating that there is some kind of energy trans-er associated not only to the resonance non-radiative processes.nergy transfer processes from LaPPS10 and equivalent commer-ial polyfluorene by both Förster non-radiative resonant processFRET) and trivial mechanisms to have already been documented13]. Some reports also claimed for the exciplex formation betweenhe polyfluorene and MEH-PPV, but no evidences for this processere obtained with the data reported here [35].

From the steady-state PL emission either for the LaPPS10:DCMr the LaPPS10:MEH-PPV systems it can be concluded that lower

oncentrations of both guests are required to maintain emissionsn a broader spectral range, without a significant disappearance ofhe blue component.

Fig. 4. Absorption and photoluminescence spectra of LaPPS10 and of the blend JF15(LaPPS10:MEH-PPV:DCM =100:0.05:0.02 wt%) �exc = 380 nm.

The absorption and emission spectra were also studied for amixture of the three components in JF15. LaPPS10 is the only com-ponent observed in the electronic absorption spectrum of the blendbecause the amount of the other components is very low (Fig. 4).The presence of the LaPPS10 �-phase is confirmed in solid state bythe sharp peak at 410 nm in the absorption spectrum, in addition tothe amorphous solid state phase. The PL spectrum is composed byseveral bands: a shoulder at 442 nm (0–0 band of the LaPPS10 �-phase), a higher intensity band at 465 nm (0–1 band of the LaPPS10�-phase) and two other red-edge bands that are relatively intensi-fied when compared to the LaPPS10 emission. These bands may beassigned to the well dispersed DCM molecules (537 nm in Fig. 3(a))and well dispersed MEH-PPV emitting at 560 nm (Fig. 3(b)).

To gain further insight about the emission mechanisms ofenergy transfer in the JF15 sample, fluorescence decays were mea-sured in the characteristic emission wavelength range of LaPPS10(�exc = 320 nm, �em = 440 nm). Values for the three components insolution and in film were compared with that of the LaPPS10 insolution and film. The analysis was performed by non-linear least-squares routines minimizing the �2. The results are depicted inTable 1.

The LaPPS10 photoluminescence decays in degassed chloro-form solution (10−5 mol L−1) (�exc = 320 nm, �em = 440 nm) werebi-exponential with � PL1 = 0.48 ns (54%) and �PL2 = 0.79 ns (46%)(Table 1) and faster decays are observed for films. The excitationwith pulsed hydrogen lamp is limited in terms of spectral rangeand this lamp does not emit around the maximum of the absorp-tion peak. The excitation wavelength of 320 nm gave the best decaysignal as a result of the combination of pulsed lamp intensity andsample absorption. The decrease of the lifetimes in the solid statecompared to the solution may be attributed to the inter-chain

LaPPS10 (solution) 0.48 ± 0.08 54 0.79 ± 0.06 46 1.041LaPPS10 (film) 0.22 ± 0.02 84 0.72 ± 0.01 16 1.170JF15 (solution) 0.42 ± 0.10 40 0.73 ± 0.04 60 1.226JF15 (film) 0.27 ± 0.04 100 – – 1.083

J.F. de Deus et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 45– 51 49

Fig. 5. Epifluorescence microscopy of JF15 blend (LaPPS10:MEH-PPV:DCM = 100:0.05:0.02 wt%) using (A) 340–380 nm excitation filter with 400 nm dichroic mirror and (B)e

csPboa

wLtDLcqt(ctd

titdwamctbb

mW�e�aodidd

MEH-PPV. A shoulder in the 621 nm region was also seen in theLaPPS10:MEH-PPV blend emission. It is noteworthy that in spite ofthe low dopant concentration (0.40% DCM) and (0.50% MEH-PPV),their emissions were quite strong compared to that of the blue

400 450 500 550 600 650 7000,0

0,5

1,0528

621

562

451

No

rma

lize

d In

ten

sity LaPPS10:DCM - 100:0.40

LaPPS10:MEHPPV - 100:0.50

xcitation filter 515–560 nm with 580 nm dichroic mirror. Amplification 22×.

omponents (�PL2 = 0.73 ± 0.04 ns). Differences are almost of theame order of the fitting deviation. Solutions of LaPPS10 with MEH-PV showed a very inefficient non-radiative energy transfer processy Forster mechanism in several solvents and this is also beenbserved here for solutions containing simultaneously MEH-PPVnd DCM [13].

For JF15 in solid state the decay becomes mono-exponential,ith a complete suppression of the longer component of the

aPPS10 PL. The faster component (�PL1 = 0.27 ± 0.04 ns) is, withinhe fitting deviation, comparable to the faster LaPPS10 component.isappearance of the longer component has also been observed foraPPS10 blended with MEH-PPV but in greater concentration, indi-ating that DCM is also playing an important role in the LaPPS10uenching process. Further, since there is a decrease of the PL life-ime, in addition to the decrease of the PL intensity (Fig. 3(a) andb)), it may be concluded that FRET by Forster has an importantontribution to the quenching process but the contribution to therivial process for the decrease of the emission intensity cannot beisregarded [13].

Steady-state PL and lifetime measurements may give impor-ant information for the interactions between the species presentn the media and also when FRET by Forster is involved, abouthe intermolecular distances. As it is well known, FRET by Forsterepends strongly on the donor and acceptor distance. Since FRETas observed in JF15, we may conclude that the DCM molecules

nd MEH-PPV segments must be close to those LaPPS10 seg-ents where part of the excitons is located. In terms of polymeric

hains, this close proximity requires a partial interpenetration ofhe chain segments [36]. Although this important information maye obtained, no further information about the film morphology cane drawn from these data.

The film morphology was analyzed by epifluorescence opticalicroscopy (Fig. 5) that gave images of the emitting domains.hen the polyfluorene is excited (filter A, �exc = 340–380 nm,

em > 400 nm), a blue emission is observed with some domainsmitting yellow. When DCM and MEH-PPV were excited (filter B,exc = 515–560 nm, �em > 580 nm), only the red emission of DCMnd MEH-PPV can be observed and now a red uniform emissionf the entire film is observed with some strong spots of isolated

omains. Therefore, using the PL data and the epifluorescence

mages, it may be concluded that DCM guests are preferentiallyissolved into the polymer matrix host and MEH-PPV is partiallyistributed inside the matrix and partially distributed as isolate

domains. Due to the uniform distribution of the guests in thematrix, part of the blue emission of the polyfluorene is absorbed byinner filter effect. Thus the lifetime of the donor remains as in theabsence of the acceptors. In addition, the polymer matrix itself is notuniform: � phase is distributed in a matrix of amorphous and crys-talline phases of the polyfluorene sample. The coexistence of phasesand composition distribution encompasses complex variables thatwill determine the photophysical processes leading to the PLefficiency. It will probably play an important role on the EL results,because of the presence of several types of bulk heterojunctions.

3.2. Electroluminescent properties

Fig. 6 shows the Electroluminescence spectra of the blends withtwo components LaPPS10:DCM (100:0.40) and LaPPS10:MEH-PPV(100:0.50), whose PL spectra are depicted in Fig. 3. In both binarysystems the LaPPS10 emission was seen with low intensity around451 nm along with another one at 528 nm for DCM and 562 nm for

Wavelength (nm)

Fig. 6. EL spectra of the of the devices of the binary blendsITO/PEDOT:PSS/LaPPS10:DCM/Al and ITO/PEDOT:PSS/LaPPS10:MEH-PPV/Al.

50 J.F. de Deus et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 45– 51

400 450 500 550 600 650 7000,0

0,5

1,0

571

515

478

452

No

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d In

ten

sity

Wavelength (nm)

LaPPS10

LaPPS10:MEH-PPV:dcm (100:0.05:0.02)

Fig. 7. EL spectra of the of the ITO/PEDOT:PSS/LaPPS10/Al and of theI

mfttce

IrFft5mibmtcwseDMP

stac

crfic

TW

TO/PEDOT:PSS/JF15/Al devices.

atrix. This result indicated that excitons were preferentiallyormed in the lower bandgap components, that functioned asraps for the charge carriers. This finding was used to controlhe emission color, as done in the PL study. Devices with higheroncentrations of the red emitters were not built since the redmission was predominant in those conditions.

The EL emission spectra of the ITO/PEDOT:PSS/JF15/Al andTO/PEDOT:PSS/LaPPS10/Al devices (Fig. 7) exhibit a considerableed-shift in comparison with its corresponding PL emissions (Fig. 4).or the device with LaPPS10, the maximum of 0–0 band shiftsrom 442 nm (PL) to 452 nm, that of 0–1 from 465 nm to 478 nm,he 0–2 from 498 nm to 515 nm, and the 0–3 from 538 nm to61 nm. Similar effect occurred for the JF15 device, its 0–0 bandatches perfectly with that of the LaPPS10 one, with an additional

ntense and broad band at 571 nm. The red-shift can be explainedy morphological differences between the films used in PL and ELeasurements. The first was made by a simple cast procedure;

he EL film was considerably thinner and was processed by spin-oating. Probably, the formation of a more compact spin coated film,hich increases aggregation, is responsible for the observed red-

hift in the EL spectrum. The broad band at 571 nm can be explainedither for a direct cross transition of electrons on the LUMO ofCM and holes of MEH-PPV (electroplex emission), or due to anEH-PPV emission activated in the EL emission (not present in the

L) [37].The values of the work-functions, HOMO and LUMO levels

hown in Table 2, facilitate the injection of carriers from the elec-rodes into the active layer and its transport across the device,nd are also compatible with photo-physical processes related toharge transfer and recombination mechanisms [38,39].

Fig. 8(a) shows current density–voltage–luminance (J–V–L)haracteristic curves of the devices ITO/PEDOT:PSS/JF15/Al. The

esults show that the turn-on voltage is about 6 V, the device exhibitor voltage above 10 V a maximum luminance 36 cd/m2. JF15 diodes a potential structure for white OLED, since its emission in the CIEoordinates confirms the white emission, Fig. 8(b).

able 2ork functions for ITO and Al and HOMO and LUMO for LaPPS 10, MEH-PPV and DCM (re

ITO Al PEDOT:PSS

Work-function 5.0 4.2 5.1HOMOLUMO

Fig. 8. (a) Current density × voltage and luminance × voltage ofITO/PEDOT:PSS/JF15/Al devices; (b) CIE coordinates (x = 0.30 and y = 0.31).

4. Conclusion

It was shown that a film-composite made with only one blue andtwo red emitters is capable of white light output. The blue emitterwas a polyfluorene derivative [poly(9,9-dihexyl-2,7-fluorene)](LaPPS10) as the matrix, and the red emitters, poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) and4-(dicyanomethylene)-2-methyl-6-(dimethylaminostyryl)-4H-pyrane (DCM) were used as dopants. A detailed photoluminescencestudy was performed involving mixtures of the matrix with eachone of two components, and of the complete blend, either in solu-tion or in films, providing information about the photophysicalmechanisms operating in PL and EL emissions. In the study of

the binary blends of LaPPS10:DCM and LaPPS:MEH-PPV a blueshifting of the red emitter was observed, which varied accordingto its concentration in the LaPPS10 matrix. This adjustment wasperformed following the PL emission, in order to obtain in addition

ferences in brackets).

LaPPS10 [22] MEH-PPV [20] DCM [40]

5.97 4.9 5.072.6 2.8 3.04

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Chen, Q.H. Gong, Journal of Luminescence 131 (2011) 2252.

J.F. de Deus et al. / Journal of Photochemistry

o the blue emission from the polyfluorene, a broad band spanningrom the green to the red by controlling the amounts of each the

EH-PPV and DCM dopants. The white OLED device exhibitedIE chromaticity coordinates x = 0.30 and y = 0.31. Electroplexmission is one possible explanation for a broad band at 571 thatas observed only in the electroluminescence spectrum.

cknowledgments

Authors thank to CNPq (Conselho Nacional de Pesquisas –razil), FAPESP (Fundac ão de Amparo à Pesquisa do Estado deão Paulo – Brazil) and National Institute on Organic ElectronicsINEO/CNPq/FAPESP/CAPES) for financial support and fellowships.

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