Organic Electronics 15 (2014) 500–508

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Organic Electronics

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A new blue light emitting and electrochromic polyfluorenederivative for display applications

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.12.003

⇑ Corresponding authors. Tel./fax: +90 3122103188.E-mail addresses: rhf10@cam.ac.uk (R.H. Friend), aonal@metu.edu.tr

(A.M. Önal).

Buket Bezgin Carbas a, Demet Asil b, Richard H. Friend b,⇑, Ahmet M. Önal a,⇑a Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkeyb Cavendish Laboratory, University of Cambridge, JJ Thomson Ave, Cambridge CB3 0HE, UK

a r t i c l e i n f o

Article history:Received 25 October 2013Accepted 2 December 2013Available online 15 December 2013

Keywords:PolyfluoreneElectrochromismLight emitting diodeConjugated polymer

a b s t r a c t

A novel blue emitting and electro-chromic conjugated copolymer based on 9,90dioctylflu-orene (F8) and spiro(fluorene-9,90-xanthene) (SFX) has been prepared. Optical, photophys-ical and electrochemical characterizations are given for the synthesized polymer;poly[spirofluorene-co-9,90dioctylfluorene] P(F8-SFX). Switching of the corresponding poly-mer between yellow and purple states is demonstrated, and blue emission with Commis-sion Internationale de L’Eclairage (CIE) coordinate at (0.19, 0.15) is obtained from PLEDdevice. The addition of a thin polymer interlayer ((F8)0.5–(TFB)0.5) is further investigatedto maintain color purity. Furthermore, all polymer electrochromic device (ECD) based onP(F8-SFX) and poly(ethylene dioxythiophene) (PEDOT) was constructed. Spectroelectro-chemistry and switching ability of the device were also investigated by UV–visspectroscopy.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Display applications have been in progress ever sincethe discovery of electrochromic and light emitting technol-ogies. Although many organic and inorganic materials exhi-bit the property of electrochromism, especially, conductingpolymers have been visualized as one of the most favoredelectrochromes in devices, smart windows, optical displays,mirrors and camouflage materials [1–4] due to their prom-ising advantages, such as processability over large surfaces,high optical contrast ratio, multicolors with the same mate-rial, high redox stability and long cycle life with low re-sponse time, when compared to the inorganicelectrochromes [5–13]. Thus many research groups havebeen working hard attempting to design and synthesize ofpolymeric electrochromics based on conjugated polymers[14]. In the case of light-emitting devices, modifying thecomposition of the polymers at the molecular level is also

very important for color-tuning purposes. It is possible tomake large area displays with polymeric materials by sim-ple solution processable methods such as spin coating andlarge area printing, e.g. ink-jet printing. The key advantagesof this technology are low-cost and environmentally-friendly manufacture together with the possibility to up-scale to very large substrates. Nonetheless, that kind ofapplications (ECDs and LEDs) possesses some intrinsic lim-itations during daily use depending on the light conditions.For example, the usage of back or front light source to visu-alize the image in dark conditions is necessary for ECDs andthis causes the consumption of extra power [15]. Con-versely, the visibility of the image gets difficult in brightambient conditions for light emitting diodes (LEDs) [16].Therefore, the materials with dual electrochromic and elec-troluminescent properties have been attracting a greatamount of attention [15,17] in order to pave the way fordisplays which operates at reflective and emissive statessimultaneously. Dual type displays however requires care-ful design of materials and engineering of the devices.Therefore, it is very important to pick up the electrochro-mism and light emission in the same material. However,

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the examples of organic polymers and oligomers havingboth electrochromism and photo- or electroluminescenceare scarce in the literature [18–21]. Many electroactive con-jugated polymers generally show excellent electrochromicproperties for instance, fast switching capability, high con-trast ratio, high coloration efficiency (CE), and good long-term stability. However, most of them were prepared byelectrochemical polymerization and become insoluble afterdeposition on the electrode surface or low molecularweight. Furthermore, they are not thermally stable, whichlimit their applications for large area devices. For that rea-son, it is essential to develop novel processable and ther-mally stable polymeric materials with electrochromicproperties [22]. It is known that, among large number ofconjugated polymers, polyfluorene homopolymers andcopolymers exhibit encouraging performance in electro-chromic and electroluminescent devices on an individualbasis [23–38]. The 2,7 positions in fluorene structure, whichare the most reactive sites towards electrophilic attack,provides construction of a fully conjugated rigid-rod poly-mer chain by substitution reactions [39]. Furthermore,many electron donor or acceptor groups are introducedalong this backbone to form an alternative copolymer totune the band-gap and electronic properties and enhancedstabilities [27–30]. The methylene bridge in fluorene struc-ture also offers an opportunity to modify the processabilityor functionality of the polymer by substituents. For in-stance, replacement of labile H atom at C-9 position of flu-orenes with alkyl groups improves the solubility ofpolyfluorenes [40]. Nevertheless, excimer formation in thesolid state and high-energy barrier for hole injection con-fine their application in PLED’s [41–43]. Many researchgroups have been tested extensively to figure out thephotophysical behavior of this kind of polyfluorene deriva-tives [44–48]. Recently, Tseng et al. synthesized and charac-terized a stable organic blue light emitting device preparedfrom a fluorene-based homopolymer containing spiro(fluo-rene-9,90-xanthene) skeleton with octyl alkyl chains for sol-ubility aspect and compared thermal, optical andluminescence properties of the polymer with previousdialkylfluorene based polymers [49]. Xie et al. proposed amodel compound namely (2,7-bis(spiro[fluorene-9,90-xan-thene]-2-yl]-9,9-dioctylfluorene) with no excimer and ketodefect and high thermal stability with respect to polyfluo-renes [50].

In the light of above information, we attempted to syn-thesize a novel P(F8-SFX) polymer by Suzuki–Miyauracoupling method as a multipurpose material. The polymerexhibits high thermal stability (5% weight loss at 300 �C)and glass transition temperature (Tg = 205 �C) with a mod-erate molecular weight (104 k, PDI = 1.04). The polymeralso has good solubility in chloroform, toluene and 1,2-dichlorobenzene. An electrochromic device, switchingfrom �0.5 V at neutral state to 2.0 V at oxidized statewas prepared and fully characterized, utilizing P(F8-SFX).Finally, PLEDs optimized by inserting a hole transportinglayer of (F8)0.5–(TFB)0.5 were also prepared. To the best ofour knowledge P(F8-SFX) is the first example of both elec-troluminescent and electrochromic material containing

only fluorene units along the polymer main chain back-bone in the literature.

2. Experimental

2.1. Instrumentation

1H NMR spectra were recorded on a Bruker NMR Spec-trometer (DPX-400) in CDCl3. FTIR spectra were obtainedusing a Bruker Vertex 70 spectrophotometer. GPC analysisof polymer was carried out with Polymer Laboratories PL-GPC 220 instrument. Standard polystyrene samples wereused for calibration. Differential scanning calorimetry(DSC) and thermal gravimetric analysis (TGA) were per-formed using a Perkin Elmer Diamond and Perkin ElmerPyris 1 TGA under nitrogen atmosphere with 10 �C/minheating rate, respectively. A Hewlett–Packard 8453A diodearray spectrometer and a Gamry PCI4/300 potentiostat–galvanostat were used for in situ spectroelectrochemicalstudies and electroanalytical measurements, respectively.Current density and voltage were measured by Keithley2400 source measurement unit. Electroluminescence (EL)spectra were recorded with a multimode optical fiber(diameter = 600 lm) attached to an intensity calibratedOcean Optics USB2000 spectrometer. Colorimetry mea-surements for electrochromic device were done via Minol-ta CS-100 spectrophotometer.

2.2. Materials

All chemicals purchased from Aldrich and Fluka wereused without further purification. 2,7-dibromospiro[fluo-rene-9,90-(2070-di-n-octyloxyxanthene)] was preparedfrom previously published procedure [49].

2.3. Synthesis of poly[spirofluorene-co-9,90dioctylfluorene]P(F8-SFX)

About 100.0 mg (0.12 mmol) of 2,7-dibromospiro[fluorene-9,90-(2070-di-n-octyloxyxanthene)], 88.0 mg (0.12mmol) of 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-pro-panediol) ester, 5.0 mg (4.3 lmol) of Pd(PPh3)4, and 1.1 mLof 2 M K2CO3 were combined in 2.2 mL of toluene, and themixture was degassed via freeze–pump–thaw (3 cycles),backfilling with N2. The reaction was heated to reflux undernitrogen atmosphere for 3 days, the end groups were cappedby heating the mixture under reflux for 12 h with benzeneb-oronic acid (28.0 mg, 0.24 mmol) and then for 12 h with bro-mobenzene (37.0 mg, 0.24 mmol). The reaction mixture wascooled to room temperature and then precipitated with meth-anol. The precipitate was collected via centrifugation, and theexcess solvent was decanted off. The polymer was dried undervacuum, and polymer was reprecipitated three times by dis-solving in 50 mL of THF, concentrating to ca. 10 mL, precipitat-ing with methanol, centrifuging, decanting and then dryingunder vacuum. The polymer was dried under vacuum to give105.0 mg of purified material (yield 91.08%). 1H NMR(400 MHz, CDCl3): d 7.66–7.80 (m, 8H), 7.40–7.56 (m, 8H),

502 B.B. Carbas et al. / Organic Electronics 15 (2014) 500–508

7.24–7.36 (m, 8H), 6.64–6.71 (m, 2H), 6.23–6.34 (m, 4H), 3.92(t, J = 8.0 Hz, 4H), 1.71–1.94 (m, 6H) 1.27–1.43 (m, 40 H), 0.87(b, 6H).

2.4. Electrochemical characterization of P(F8-SFX)

For electrochemical characterization, polymer was dis-solved in 1,2-dichlorobenzene to a concentration of20 mg/mL and then drop-casted onto glassy carbon elec-trode. The electrochemical behavior of the polymer wasstudied in 0.1 M tetrabutylammonium hexafluorophos-phate (TBAPF6) dissolved in acetonitrile via differentialpulse voltammetry. A glassy carbon disc (0.07 cm2) as aworking electrode, a platinum wire as a counter electrodeand a Ag-wire as a pseudo-reference electrode were used.The pseudo-reference electrode was calibrated externallyusing a 5 mM solution of ferrocene (EpðFc=Fþc Þ) = 0.41 V vsAg-wire and 0.45 V vs Ag/AgCl). The changes in the elec-tronic absorption spectra of the P(F8-SFX) film on indiumtin oxide electrode (ITO, Delta Tech. 8–12 X, 0.7 � 5 cm)at different potentials from neutral to oxidized states wererecorded on a Hewlett Packard 8453A diode array ultravi-olet–visible (UV–Vis) spectrophotometer.

2.5. Fabrication of electrochromic device

For a dual type electrochromic device (ECD) fabrication,PEDOT as a cathodically coloring polymer was electropoly-merized onto a 1.25 cm2 ITO glass surface from anacetonitrile (ACN) solution containing 2 � 10�3 M 3,4-eth-ylenedioxythiophene and 0.1 M TBAPF6 via constant po-tential electrolysis at 1.2 V. After coating PEDOT onto theITO-glass surface, the film was rinsed with ACN to removeall the unreacted monomers on the electrode surface. P(F8-SFX) (20 mg/mL) was obtained by drop casting from its1,2-dichlorobenzene solution with a concentration of 2%w/v onto the ITO-glass substrate. The device was con-structed using the electrochromic electrodes stackedface-to-face separated by gel electrolyte (TBABF4; acetoni-trile; poly(methyl methacrylate); polycarbonate in theratio of 3:70:7:20). The electrochromic device was allowedto dry for 48 h at room temperature under atmosphericpressure. The electro-optical properties of the device wererecorded in situ under various applied potentials. Finally,square wave potential method was used to performswitching between the colored states.

2.6. PLED device preparation

Indium-tin oxide (ITO) substrates were cleaned sepa-rately in acetone and isopropanol in an ultrasonic bath

Scheme 1. Synthesis route of P(F8-SF

for 15 min each, sequentially. Substrates were treated withoxygen plasma and spin-coated with PEDOT-PSS followedby annealing at 230 �C under nitrogen for 30 min. The A–B type copolymer, (F8)0.5–(TFB)0.5, produced by CambridgeDisplay Technology (Mn = 186 k, PDI = 3.15 with HOMO–LUMO levels �5.1 and �2.1 eV) was dissolved in p-xylenewith the concentration of 4 mg/mL and spin-coated onPEDOT;PSS (see chemical structure of (F8)0.5–(TFB)0.5 inESI Scheme 1). After annealing at 200 �C for 15 min, thesubstrates were washed with xylene and annealed again.P(F8-SFX) was spin coated from 1,2-dichlorobenzene(15 g/L). The substrates then annealed at 170 �C for15 min to get rid of solvent. Finally, all the samples weretransferred to a thermal evaporation chamber for Ca(20 nm) and Al (100 nm) deposition under high vacuumof 1 � 10�6 mbar with shadow mask to give an active areaof 4.5 mm2. The devices were then encapsulated under in-ert conditions.

3. Result and discussion

3.1. Synthesis and properties of P(F8-SFX)

P(F8-SFX) was prepared by Suzuki–Miyaura couplingpolymerization of 9,9-dioctylfluorene-2,7 diboronic acidbis(1,3-propanediol) ester and (2,7-dibromospiro[fluo-rene-9,90-(2070-di-n-octyloxyxanthene)]) with tetrakispal-ladium catalyst Pd(PPh3)4 in a toluene/2 M potassiumcarbonate solution (Scheme 1). After purification by repre-cipitation into methanol, P(F8-SFX) was obtained as anash-colored powder. The chemical structure of the polymerwas verified by 1H NMR and FT-IR spectra (see ESI Figs. S1and S2). All aromatic and aliphatic protons of P(F8-SFX)resonated in the region of 6.23–7.80 ppm and 0.87–1.94 ppm, respectively. The peak at 3.92 ppm also belongsto the ether protons of octyl chain (Fig. SI 1). In the case ofFTIR spectrum, polymer exhibits characteristic peaks at3063–3033 cm�1 (aromatic C–H stretching), 2850–2930 cm�1 (C–H stretching for octyl chains) and 1184,1240 cm�1 (C–O stretching of ether linkage). The absorp-tion peaks at 1404, 1450 and 1498 cm�1 can be assignedto the vibration of aromatic ring and one at 1610 cm�1 be-longs to asymmetrically substituted benzene. The810 cm�1 peak in the spectrum of the polymer belongs tothe 1,2,4-trisubstituted benzene rings (see ESI Fig. S2).

GPC measurements of P(F8-SFX) was conducted to pro-vide the relative molecular weight to the polystyrene stan-dards and polydispersity index. The results of thesynthesized copolymer were compared with those ofhomopolymers poly(9,9-dioctylfluorene) (PF8) and(Poly[20,70-di-octyloxyspiro(fluorene-9,90-xanthene)-2,7-

X) by Suzuki–Miyaura coupling.

Table 1Molecular weight and thermal data for spirofluorene derivatives.

Polymer Mna Mw

a DPI Tg (�C)b Td (�C)c

PF8 [51] 23,200 61,176 2.68 67 458⁄

PSFX [49] 11,000 16,000 1.46 149 411, 433⁄⁄

P(F8-SFX) 104,480 108,305 1.04 205 300, 398⁄⁄

a Molecular weights were determined by GPC using polystyrenestandards.

b Glass transition temperature.c ‘‘*’’ – Onset decomposition temperature and ‘‘**’’ – the decomposition

temperature at 5% and 10% weight loses measured by TGA undernitrogen.

Fig. 1. Normalized UV–vis absorption and PL spectra of P(F8-SFX) inCHCl3.

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diyl](PSFX)) in Table 1 [49,51]. The synthesized copolymerwas highly soluble in most of the organic solvents such aschloroform, THF, 1,2-dichlorobenzene and toluene. Themolecular weight of the polymer was determined in THF.P(F8-SFX) has a number average molecular weight (Mn)of 1.04 � 105 g/mol and a weight average molecular weight(Mw) of 1.08 � 105 g/mol. These values are extremely highthan those of PSFX, due to solubility problem of PSFX inTHF [49].

The thermal properties of the synthesized polymerwere evaluated by means of DSC and TGA under nitrogenatmosphere. P(F8-SFX) has a glass transition temperature(Tg) at around 205 �C as confirmed by DSC. The high Tg va-lue of P(F8-SFX) is most probably arising from reducedsegmental mobility due to rigid xanthane units along thepolymer backbone. The copolymer also exhibits good ther-mal stability, showing only 5% and 10% weight losses at300 and 398 �C, respectively (see ESI Figs. S3 and S4).

3.2. Electrochemical characterization of P(F8-SFX)

Differential pulse voltammetry (DPV) is one of the com-mon electroanalytical methods to determine the electro-chemical properties of polymers due to the elimination ofcapacitive current during the process [52,53]. The polymerbehavior was investigated in 0.1 M TBAPF6/ACN mediumafter the polymer film was drop-casted onto a glassy car-bon electrode. The onset potentials for oxidation andreduction were measured as 0.89 and �2.10 V, respec-tively. On the basis of this data, the highest molecular orbi-tal (5.69 eV) and lowest unoccupied molecular orbital level(2.70 eV) with respect to ferrocene/ferrocenium (4.8 eV)were calculated. The band gap was calculated as 2.99 eVwhich is in good agreement with optical measurements(see ESI Fig. S5).

Table 2Summary of optical properties of P(F8-SFX).

Polymer Abs. kmax (nm) PLbkmax (nm)

Solutiona Film Solutiona Film

PF8 [51] 389 390 418 (443) 424 (448PSFX [49] (280) 390 (281) 393 418 (442) 426 (450P(F8-SFX) (281) 372 (283) 377 415 (437) 422 (450

a Evaluated in chloroform.b Excited at 375 nm.c Calculated from ‘‘*’’ – cyclic voltammetry and ‘‘**’’ – differential pulse voltam

3.3. Optical characterization of P(F8-SFX)

The absorption of P(F8-SFX) consists of a strong fea-tureless p–p� transition that peaks around 372 nm(3.3 eV) and a weak p–p� transition at 281 nm which canbe attributed to the spiro SFX unit. The optical band gapwas determined from the absorption onset as 2.99 eV.The optical behavior of the polymer was summarized andcompared with PF8 and PSFX in Table 2. As seen in Table 2,the band gaps of the polymers are close to each other andtheir HOMO and LUMO levels show slight variationdepending on type of the method used duringmeasurement.

Fig. 1 shows a typical absorption and PL spectra in di-lute 1,2-dichlorobenzene solution and in the solid state.As expected, the absorption is dominated by the well-known UV band of polyfluorene. The emission of the poly-mer P(F8-SFX) in solution shows a well resolved broademission with peaks at 415, and at 437 nm as a shoulderassigned to the 0–0 and 0–1 intra-chain singlet transitions.In contrast to the emission in solution, the polymer filmshows vibronic structures assigned to 0–0, 0–1, and 0–2transitions at 422, 450 and 486 nm as well as additionaltransitions at long wavelength region [54]. In comparisonwith the solution, the absorption spectrum of the thin filmslightly broadened and 7 nm red shifted most probably dueto stronger interchain interactions in the solid state [55].The PL spectrum on the other hand showed a much morenarrow emission with 7 nm red-shift. Additional broad

HOMOc (eV) LUMOc (eV) Band gap (eV)

) 5.80⁄ 2.85⁄ 2.95) 5.79⁄ 2.87⁄ 2.96, 486, 520) 5.69⁄⁄ 2.70⁄⁄ 2.99

metry with regard to the energy level of ferrocene (4.8 eV below vacuum).

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green contribution at 520 nm is assigned to interchainexcimers (Fig. 1) [56,57].

3.4. Electrochromic properties of P(F8-SFX) and it is switchingproperty

The changes in the electronic absorption spectrum ofP(F8-SFX) upon changing the applied potentials representthe electro-optical properties of the polymer film. In orderto evaluate the electrochromic features and get informa-tion about charge carriers, P(F8-SFX) film was drop-castedonto ITO (20 mg/mL). Following coating process of theelectrode, the spectroelectrochemical behavior of the poly-mer film was monitored during anodic oxidation. The elec-tronic absorption spectra of neutral forms of the filmsexhibit absorption bands at around 380 nm, which is dueto p–p� transitions for P(F8-SFX)(Fig. 2). The electronicband gap defined as the onset energy for the p–p� transi-tion and was found to be 2.80 eV. This value is lower thanthe value obtained from electroanalytical measurements ofthe polymer. The evolution of the spectra during dopingprocess shows simultaneous increases in the absorbanceintensities, corresponding to the formation of the newcharge carriers (polarons and bipolarons). In the electronicabsorption spectra of P(F8-SFX) film, a new increasingabsorption band was observed at 527 nm in the potentialrange of 0.0–0.40 V and upon further applied potential,another absorption band appears about 1000 nm in thenear-IR region with a clear isosbestic points at 440 nmindicating that polymer film was being interconvertedbetween its neutral and oxidized states. As a result of thesevariations in the absorption spectrum of P(F8-SFX), thecolor of the polymer changes from yellow in the neutralstate to pale purple upon doping.

Due to its importance in electrochromic applications,switching times and optical contrast of the P(F8-SFX) filmon ITO were also investigated under square wave input of0.0 to 1.5 V in 10 s intervals by monitoring the visibletransmittance and the kinetic responses of the film at

Fig. 2. Optical characterization of P(F8-SFX) by applying differentpotentials between oxidized and neutral states with inset of theirelectrochromic photographs.

527 nm. P(F8-SFX) film shows a reversible response withinthe range of applied potential pulses with a response timeof 1.62 s for oxidation and 2.2 s for reduction at 95% of themaximum transmittance and the optical contrasts (%T) are14.0% at 527 nm and 33% at 1000 nm. Besides responsetime and optical contrast, coloration efficiency (CE), a mea-sure of power efficiency, is also important for electrochro-mic materials. The CE of P(F8-SFX) film is calculated fromEq. (1) as described previously [58,59].

CE ¼ DOD=Q d ð1Þ

where DOD is the change in optical density and Qd is thecharge (C/cm2) passed during this process. DOD is deter-mined from the percent transmittance (%T) before andafter a full switch and is calculated using Eq. (2).

DOD ¼ log ð%T of bleached state=%T of colored stateÞð2Þ

On the basis of these equations, CE of P(F8-SFX) wasfound to be 120 cm2/C (527 nm) and 266 cm2/C at(1000 nm), indicating that it is a promising candidate forelectrochromic device applications.

3.5. Optical properties of the P(F8-SFX)/PEDOT electrochromicdevice

All-polymer electrochromic device consisting of P(F8-SFX) and PEDOT was constructed as shown in the Fig. 3aand its spectroelectrochemical properties investigated bymonitoring the optical absorbance spectra at different ap-plied potentials. Electrochromic device showed a revers-ible response in the potential range of –0.5 V (neutral)and +2.0 V (oxidized). At �0.5 V, the colors of polymerand PEDOT were yellow and transparent, respectively (In-set of Fig. 3b). Therefore the polymer film was yellow col-ored in its neutral state with a kmax at 386 nm. As thedevice was oxidized two new bands at 527 nm and600 nm started to appear which are attributed to P(F8-SFX) and PEDOT, respectively. As the device further oxi-dized PEDOT band intensified and the color of the devicebecame dark purple due to the mixing of dominating darkblue color of the PEDOT and pale purple of P(F8-SFX).

The colors of the electrochromic material and it is de-vice were defined by performing colorimetric measure-ments. The CIE system was used as a quantitative scaleto define and compare colors. Three attributes of color,hue (a), saturation (b) and luminance (L), were measured,and these were recorded in Table 3.

The long cycle lifetime is also another importantparameter in the electrochromic devices. For this purpose;P(F8-SFX)/PEDOT device was switched between �0.5 Vand +2.0 V by applying potential intervals of 1 s. Spectro-electrochemical and electrochemical behavior of the de-vice were also investigated after switching steps and itsswitching time was found to be 1.4 s at 562 nm. After2000 times switching steps, the device still keeps its redoxstability, retaining 98.64% of its optical activity (Fig. 4a–c).The stability of the device was also investigated in the sol-vent electrolyte solution via cyclic voltammetry method.The results indicate that the polymer film is quite stable

Fig. 3. (a) Illustration of electrochromic device construction, (b) optical characterization of P(F8-SFX)/PEDOT electrochromic device by applying differentpotentials between oxidized and neutral states (Inset: the colors of oxidized and neutral states).

Table 3Electronic and electrochromic properties of the P(F8-SFX) and it is electrochromic device.

Polymer/device kmax (nm) Redox state La aa ba Color CE cm2/C (k)

P(F8-SFX) film 380 Oxidized 52.17 15.7 �15.8 Pale purple 120 (527 nm)5271000 Neutral 88.92 �8.62 7.44 Yellow 266 (1000 nm)

P(F8-SFX)/PEDOT device 386 Oxidizedb 82.91 24.64 50.99 Dark Yellow –561 Neutralb 68.75 �3.70 �4.94 Dark purple

a CIE L � a � b system: luminance (L), hue (a) and saturation (b).b Redox states for PEDOT layer.

Fig. 4. (a) Chronoabsorptometry, (b) current density experiments, (c) chronocoloumetry for P(F8-SFX) electrochromic device at kmax (562 nm) under anapplied square voltage signal between �0.5 V and 2.0 V, and (d) electrochemical stability of P(F8-SFX)/PEDOT electrochromic device.

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and retains 93.0% of it is electroactivity after 2500 cycles(Fig. 4d).

3.6. PLED characterization

Color purity is the main problem for polyfluorenederivatives [60] and they usually suffer from green emis-sion due to keto-defect formation under applied bias[61,62]. This formation is believed to be increasing the ra-pid energy transfer from high energy sites to lower en-ergy sites before radiative decay of the excited speciesand causing a decrease the color purity [63,64]. The con-tribution from the long wavelength emission can be over-come either by doping the active material with holetrapping molecules or using hole transporting layers.[65,66]. Although single layer devices showed poor deviceperformance, inserting a thin layer of A–B type (F8)0.5–(TFB)0.5 copolymer significantly improved the brightnessand color purity of P(F8-SFX). The device that has a thinlayer of (F8)0.5–(TFB)0.5 between PEDOT:PSS and P(F8-SFX) showed a 4-fold increase in luminescence (73 cd/m2) and had a color purity with CIE (0.194, 0.1561)(Fig. 5b and c) (see movie 1. ESI).

Fig. 5a illustrates the device fabricated with the inter-layer. As the total current of the two structures with(Fig. 5b) and without (ESI-Fig. S6) interlayer are similar,

Fig. 5. (a) Schematic illustration of ITO/PEDOT/(F8)0.5–(TFB)0.5/P(F8-SFX)/Ca(20performance of the device, (c) electroluminescence spectrum of the device, and ((F8)0.5–(TFB)0.5 layer. (For interpretation of the references to colour in this figur

the interlayer is believed to act as an electron blockinglayer rather than hole transporting. This can further beexplained by the relative energy levels as shown inFig. 5a. While the HOMO level of the (F8)0.5–(TFB)0.5

matches with the PEDOT: PSS (�5.1), it is relatively lowerLUMO level (�2.2) compared to P(F8-SFX) (�2.7) formsan energy barrier for electrons. Therefore, electrons in-jected from cathode to P(F8-SFX) layer are blocked by(F8)0.5–(TFB)0.5 from reaching to PEDOT:PSS and ITO,which maximizes hole-electron recombinations withinthe active layer and improve the device performance.Fig. 5d shows the normalized electroluminescence spec-trum of the devices with and without the interlayer.Insertion of a thin electron-blocking layer diminishedthe predominant long wavelength emission, which wasobserved for the single layer devices. No noticeablechange in electroluminescence is observed even undervery high voltages (Fig. 5c).

4. Conclusions

We have synthesized and characterized a new process-able polyfluorene derivative, based on spiro functionalunits, having dual electrochromic and electroluminescenceproperties. The multifunctional polymer, P(F8-SFX)exhibits yellow to purple electrochromism upon oxidation

nm)/Al(100 nm) PLED device and it is energy band diagram, (b) I–V–Ld) normalized ECL spectrum of the devices with (blue) and without (black)e legend, the reader is referred to the web version of this article.)

B.B. Carbas et al. / Organic Electronics 15 (2014) 500–508 507

with relatively high coloration efficiency (120 cm2/C at527 nm and 266 cm2/C at 1000 nm). We also successfullyestablished the utilization of dual-type complementarycolored polymer electrochromic devices using P(F8-SFX)/(PEDOT) in sandwich configuration. The switching abilityand spectroelectrochemical properties of the electrochro-mic device were investigated utilizing UV–vis spectropho-tometry and cyclic Voltammetry. The results obtainedindicated a high switching ability and redox stability. Inthe second part of the study, blue emitting (CIE coordinate;(0.19, 0.15)) PLED device using P(F8-SFX) was constructedand device performance was optimized utilizing an elec-tron blocking layer. Significant improvement in the colorpurity and 4-fold increase in the brightness was observedfor the devices with the electron blocking layer as com-pared to the single layer devices.

Acknowledgements

Research for this study was conducted with fundingfrom the Council of Higher Education in Turkey, MiddleEast Technical University and Schlumberger FoundationFaculty for the Future (D.A).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.orgel.2013.12.003.

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