White Polymer Light-Emitting Electrochemical Cells Fabricated Using

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White Polymer Light-Emitting Electrochemical Cells Fabricated Using Energy Donor and Acceptor Fluorescent #-Conjugated Polymers Based on Concepts of Band-Structure Engineering Yoshinori Nishikitani, Daisuke Takizawa, Hiroyuki Nishide, Soichi Uchida, and Suzushi Nishimura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08547 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 26, 2015

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The Journal of Physical Chemistry

White Polymer Light-Emitting Electrochemical Cells Fabricated Using Energy Donor and Acceptor Fluorescent π-Conjugated Polymers Based on Concepts of Band-Structure Engineering

Yoshinori Nishikitani,†,* Daisuke Takizawa,†Hiroyuki Nishide†, Soichi Uchida,‡ Suzushi Nishimura‡

†

Department of Advanced Science and Engineering, Waseda University, Ohkubo 3-4-1,

Shinjuku-ku, Tokyo 169-8555 JAPAN ‡

Central Technical Research Laboratory, JX Nippon Oil & Energy Corporation, 8, Chidoricho,

Naka-ku, Yokohama 231-0815 JAPAN

ACS Paragon Plus Environment

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ABSTRACT

The authors report on white polymer light-emitting electrochemical cells (PLECs) fabricated with a polymer blend film composed of a blue fluorescent π-conjugated polymer (blue FCP), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD); and a red-orange FCP, poly[2-methoxy-5-(2’ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), based on concepts of band-structure engineering. Polymer blending is one of the simplest and most promising methods for fabrication of van der Waals interfaces, which convert electricity to light in PLECs. By optimizing the composition of PFD, MEH-PPV, poly(ethylene oxide) (PEO) and salt (KCF3SO3) in the active layer, white-light emission with Commission Internationale de l’Eclairage (CIE) coordinates of (x = 0.33, y =0.31) can be achieved through light-mixing of blue exciton emission from PFD and red-orange exciton emission from MEH-PPV at an applied voltage higher than the threshold voltage, , which corresponds to / , where is the band gap of PFD

and e is the elemental charge. The white light produced by light-mixing of PFD and MEH-PPV emissions can be obtained at a low MEH-PPV concentration, while only red-orange emissions from MEH-PPV are obtained at high MEH-PPV concentrations. The emission color of FCPblend PLECs can be explained by Förster resonance energy transfer (FRET) from the excited PFD to the MEH-PPV because the photoluminescence (PL) spectrum of PFD overlaps with the UV-vis absorption spectrum of MEH-PPV. However, FRET was limited by the presence of PEO in the active layers of the FCP-blend PLECs. This meant it was much easier to tune the emission colors compared to FCP-blend polymer light-emitting diodes (PLEDs), in which FRET occurs predominantly. Utilization of a polymer blend film of blue and red-orange FCPs in PLECs is a very effective and promising method for fabrication of white light-emitting devices.


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1. Introduction Advances in organic electronic devices have promoted extensive research on low-cost, flexible, and energy-efficient novel lighting systems.1,2 Lighting systems that use white organic light-emitting diodes (OLEDs) have already hit the market, and intensive research is underway aimed at reducing their production costs and energy consumption.3-6 Alternative and promising devices for future lighting systems are white polymer light-emitting electrochemical cells (PLECs) based on fluorescent π-conjugated polymers (FCPs) and polymeric solid electrolytes (PSEs).7-11 PSEs made from polar polymers such as poly(ethylene oxide) (PEO) and salts such as LiClO4 are critical materials for fabricating solid-state electrochemical devices.12 PLECs have a p-i-n (p-doped region/insulating region/n-doped region) junction structure when an external voltage is applied that exceeds a certain threshold voltage, Vth ( = /, where Eg is the band gap of the FCP and e is the elementary charge), between the anode and cathode electrodes.13-28 PLECs having p-i-n junction structures are thought to start emitting light at voltages below Vth.29 This indicates that the doping of FCPs starts to occur at Vth. The FCPs are p-doped on the anode side and n-doped on the cathode side by in-situ electrochemical doping, forming the p-i-n junction structures that are desired for lighting. Light generation takes place in the i region by the recombination of electrons and holes, which are injected from the electrodes, without excitonpolaron quenching.30 PLECs have many advantages, such as a simple single-layer structure (electrode/active layer (containing FCPs and PSEs)/electrode), the possibility of utilizing airstable electrodes such as high-work-function metals, and a thicker active layer which is beneficial from the standpoint of device fabrication. Further, we can use simple and inexpensive solution processes such as spin-coating,13 printing,27 and roll-to-roll coating23 in their


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manufacture, which are already in use in the chemical industry, the pulp and paper industry, the printing industry and other mass-production industrial fields. Several different types of white PLECs have been reported. The first white PLEC was fabricated with a single blue-FCP, poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl], and white emission is thought to have been achieved through light-mixing of blue exciton and red excimer emissions originating from segments of the blue-FCP and its dimer, respectively.7 White PLECs have also been produced by light-mixing of blue, green, and red emissions from trichromatic FCPs.8 A PLEC based on mixed layers of these trichromatic FCPs was reported to emit warm white light. Further, white PLECs fabricated with a FCP having trichromatic segments, namely blue-, green- and red-segments, in a main-chain have been reported.9 To achieve white emission, a suitable phase-separation structure must be produced by optimizing the device fabrication conditions. Recently, the van der Waals interfaces,31 which convert light to electricity (organic and hybrid solar cells such as dye-sensitized solar cells (DSCs) based on hole-conductors or π-conjugated polymers instead of liquid electrolytes,32 perovskite solar cells (PSCs),

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and organic thin-film

solar cells)34 and vice versa (OLEDs),5 have become a central and crucial research area in the field of organic electronics. The characteristics of the van der Waals interfaces are a key factor affecting the device properties, including efficiency. The van der Waals interfaces of DSCs and PSCs are made up of inorganic semiconductors and π-conjugated organic semiconductors (small molecules, oligomers, and polymers), whereas those of organic thin film solar cells and OLEDs are formed by bringing donors and acceptors of π-conjugated organic semiconductors into contact with one another. For organic thin film solar cells and OLEDs, a blend of two πconjugated organic semiconductors is commonly used because blending is one of the simplest


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and most promising methods for fabricating van der Waals interfaces. Consequently, the use of blend films of two FCPs having different band gaps is a promising means of enhancing PLEC performance. In such cases, the phase-separated morphology of the film, which affects efficiency and emission color, can be easily optimized through the proper choice of FCP and PSE concentrations. The first PLEC based on a polymer blend film composed of two different FCPs (FCP-blend PLECs) was fabricated with a p-type FCP, poly[9-(3,6,9-trioxadecyl)-carbazole-3,6diyl] (blue-FCP), and an n-type FCP, poly[2,3-di(p-tolyl)-quinoxaline-5,8-diyl] (blue-green FCP).35 The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the p-type FCP are higher than those of the n-type FCP (Figure 1a). The large energy gaps between the HOMOs and LUMOs of the p-type and n-type FCPs prevent hole and electron transfers between these FCPs. As a result, the PLEC emitted a bright orange-yellow light, which corresponds to exciplex emission36,37 and/or direct intermolecular electron transfer emission from the LUMO of the n-type FCP to the HOMO of the p-type FCP. However, no reports on white PLECs fabricated with a polymer blend film of two monochromatic FCPs have yet been published. In this paper, we report on a simple and promising approach to realize white PLECs using a blend film of blue and red-orange FCPs having the band structures shown in Figure 1b. Our research clearly shows that electrons and holes are injected simultaneously to the HOMO and LUMO levels of the blue-FCP and those of the red-orange-FCPs with the application of an external voltage higher than , which equals / ( being the band

gap of the blue-FCP). It can therefore be presumed that, at an applied voltage higher than , white-light emission can be obtained by light-mixing of the blue exciton emission

from the blue-FCP and the red-orange exciton emission from the red-orange-FCP. We used


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poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) as the blue-FCP38 and poly[2-methoxy-5-(2’ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the red-orange-FCP. 39 Figure 1b shows the possibility for charge transfer (CT), namely hole and electron transfer, between these FCPs due to the fact that the LUMO level of PFD is very close to that of MEH-PPV and the HOMO level of PFD is lower than that of MEH-PPV. It is also possible that energy transfer (ET) could occur from the excited PFD to MEH-PPV by the Förster resonance energy transfer (FRET) mechanism,40-43 wherein PFD becomes an energy donor (D) and MEH-PPV becomes an energy acceptor (A). The PLEC performance characteristics, such as emission color and brightness, depend on the efficiency of the CT and ET, hence the effects of the salt concentration and the MEH-PPV and PFD concentrations in the active layers were studied using concepts of bandstructure engineering.

Figure 1. Strategy to realize white PLECs fabricated with a blend film of two different fluorescent π-conjugated polymers. (a) Orange PLECs based on p-type and n-type FCPs, (b) White PLECs based on blue and red-orange FCPs.


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2. Experimental Section Materials: Poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phyenylene vinylene] (MEH-PPV) and poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) were purchased from Sigma-Aldrich and used as received without further purification. The number-average molecular weights (Mns), weightaverage molecular weights (Mws), and molecular-weight distributions (Mw/Mns) of the MEHPPV and PFD used in this work were determined to be, respectively, Mn = 17,000, Mw = 46,700, Mw/Mn = 2.7 (MEH-PPV); Mn = 6,000, Mw = 16,700, Mw/Mn = 2.8 (PFD). Poly(ethylene oxide) (PEO) (with a viscosity-average molecular weight (Mv) of around 600,000) was also purchased from Sigma-Aldrich and used as received without further purification. All other materials and chemicals were purchased from Tokyo Chemical Industry, Kanto Chemical Co. Ltd., Dai-ichi Kogyo Seiyaku, Co. Ltd., and Sigma-Aldrich, and were used as supplied. Material Characterization: Cyclic voltammetry (CV) measurements of MEH-PPV and PFD, spin-coated on fluorine-doped tin oxide (FTO) coated glass substrates, were performed on a BAS ALS660DX electrochemical analyzer using a conventional three-electrode cell with Pt wire as the counter electrode and Ag/AgCl as the reference electrode. 0.1 M (CH3CH2CH2CH2)4NClO4 in acetonitrile was used as the electrolyte. The experiments were calibrated with the standard ferrocene/ferrocenium (Fc/Fc+) system. The thickness of the polymer films was measured with a KLA-Tencor P6 profilometer. The Mn, Mw, and Mw/Mn were determined using a Tosoh, HLC8220 gel permeation chromatography (GPC) system. Tetrahydrofuran was used as the eluent, and the molecular weight was calibrated with polystyrene standards. The photoluminescence (PL) spectra of MEH-PPV and PFD in chloroform solution were measured using a Hitachi HighTechnologies F-7000 spectro-fluorometer. The PL spectra of the active layers, PFD:MEHPPV:PEO:KCF3SO3, were also measured using the Hitachi spectro-fluorometer. Active layers


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without KCF3SO3 were spin-coated onto glass substrates from chloroform/cyclohexanone solvent mixtures containing appropriate concentrations of PFD, MEH-PPV and PEO, allowing us to study PL quenching as a function of the MEH-PPV doping density. For PL quenching experiments of PFD as a function of MEH-PPV doping density, active layers without KCF3SO3 were formed on glass substrates by spin-coating from chloroform/cyclohexanone solvent mixtures containing appropriate concentrations of PFD, MEH-PPV, and PEO. The PL spectra of these active layers were also measured using the Hitachi High-Technologies F-7000 spectrofluorometer. Device Fabrication and Characterization: Three types of PLECs having typical sandwich structures of ITO (indium-tin oxide)/MEH-PPV:PEO:KCF3SO3/Al, ITO/PFD:PEO:KCF3SO3/Al, and ITO/PFD:MEH-PPV:PEO:KCF3SO3/Al, were fabricated by sequentially spin-coating polymer layers onto ITO glass substrates and vacuum deposition of Al on top of the polymer layers. The ITO glass substrates were cleaned ultrasonically with detergent, deionized water, acetone and 2-propanol, and treated with UV/O3 for 5 minutes before use. PLECs were fabricated by first spin-coating active films from chloroform/cyclohexanone solvent mixtures containing appropriate concentrations of MEH-PPV, PFD, PEO and KCF3SO3 (800 rpm for 60 s, then 1000 rpm for 10 s) onto a cleaned ITO glass substrate. The obtained polymer films were dried at room temperature for 12 hours before deposition of the Al cathode. The active layer thicknesses were around 100 nm. A 100 nm-thick Al layer was thermally evaporated at 6 × 10-4 Pa to complete device fabrication. The active size of each device was 3 mm × 3 mm. All fabrication processes were performed in an argon-filled glove-box. The current-voltage (I-V) characteristics, electroluminescence (EL) spectra, luminance (L), Commission Internationale de l’Eclairage (CIE) coordinates, and color rendering index (CRI) values were measured using an


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Otsuka Electronics MCPD 9800 spectral photo detector equipped with an ADCMT 6241A DC voltage current source monitor.

3. Results and Discussion 3.1 Working Mechanism of PLECs Using a Blend Film of PFD and MEH-PPV Figure 2 shows the energy levels of the HOMOs and LUMOs of PFD and MEH-PPV. Since PFD and MEH-PPV are π-conjugated polymers which have continuous energy bands, their HOMO levels (E ) were determined from the onset oxidation potential as measured by cyclic voltammetry (CV) (Figure S1) and the LUMO levels (E ) were calculated from the optical absorption onset ( ) of UV-vis absorption spectra in chloroform solution (Figure 3) using Eq. 1. 44

= +

(1)


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-2.8 eV (-3.2 eV)

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-3.0 eV (-3.4 eV)

LUMO

LUMO

HOMO HOMO

-5.1 eV (-5.5 eV) -5.7 eV (-6.1 eV) OCH3

R

R

n

n

O

CH3

R=CH2(CH2)10CH3

PFD

MEH-PPV

CH3

Figure 2. HOMO and LUMO levels and chemical structures of PFD and MEH-PPV. The HOMO levels were calculated using two different work function values of -4.8 eV and -5.23 eV for the Fc/Fc+ redox couple. The energy values in parentheses were calculated using a work function value of -5.23 eV for the Fc/Fc+ redox couple.


10

2.5×104 4

2×10

ε (M-1 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5×104 1×104

1.2 UV-vis 1 PFD MEH-PPV 0.8 PL 0.6 PFD MEH-PPV 0.4

5000 0 300

0.2 400 500 600 700 Wavelength (nm)

Normalized PL Intensity (a.u.)

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0 800

Figure 3. UV-vis absorption and PL emission spectra of PFD and MEH-PPV in chloroform solution.

In order to convert the HOMO levels to the vacuum reference (vs. eV), we have to use the work function value of the reference electrode. However, as C. M. Cardona et al. have pointed out, there are inconsistencies in the HOMO levels of π-conjugated molecules and polymers (determined by CV methods) reported in different papers due to inconsistent work function values for the reference electrode.45 For instance, several work function values of the Fc/Fc+ reference electrode were used: -4.8 eV, -5.1 eV, -5.39 eV, and -5.23 eV.44,45 Therefore, the HOMO levels relative to vacuum calculated by different groups using different work function values cannot be directly compared. Although there are several options for the work function value of the Fc/Fc+ redox couple, it is apparent that the relative positions of the HOMO and LUMO levels of PFD and MEH-PPV which are fundamental to the working mechanism of the FCP-blend PLECs do not change, irrespective of the values used for calculation. Hence, for this discussion, the HOMOs and LUMOs of PFD and MEH-PPV were calculated using -4.8 eV, which is the most common value in the field of organic electronics. Further, with regard to the


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HOMO-LUMO gaps of π-conjugated polymers, two analytical methods, namely UV-vis absorption spectroscopy (which we used) and CV, are normally used to determine the HOMOLUMO gaps. The HOMO-LUMO gap obtained by UV-vis absorption spectroscopy is called the optical band gap, and that obtained by CV is called the electrochemical band gap.46 It should be emphasized that the optical and electrochemical band gaps are two fundamentally different quantities and there are discrepancies between them. However, because there is a linear relationship between these gaps,46 the relative positions of the HOMO and LUMO levels of PFD and MEH-PPV, as determined by UV-vis absorption spectroscopy and CV, do not change. Therefore, the optical band gap will be used to elucidate the working mechanism. The band diagrams indicate clearly that as the applied voltage (V) to the PLECs increases, the electrochemical doping of MEH-PPV occurs first when V ≥ 2.1 V, and then the electrochemical doping of PFD takes place when V ≥ 2.9 V. Hence, an applied voltage higher than 2.9 V is required to achieve white-light emission by light-mixing of the red-orange emission from MEHPPV and the blue emission from PFD. FRET and/or CT from PFD to MEH-PPV might quench the blue exciton fluorescence from PFD when excitons are formed in PFD with the application of voltage higher than 2.9 V, since the HOMO level of MEH-PPV is higher than that of PFD by 0.6 eV and the LUMO level of MEH-PPV is lower than that of PFD by 0.2 eV. Further, in the low concentration regime of MEH-PPV, the electrochemical doping of PFD probably occurs before that of MEH-PPV, because MEH-PPV produces isolated regions in the blend film and electrons and holes cannot be directly injected from the electrodes into the LUMO and HOMO of MEHPPV, respectively. In this case, MEH-PPV emits red-orange fluorescence via FRET and/or CT from PFD.


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Q. Xu, Y. Yang and their colleagues reported that the working mechanism of white polymer light-emitting diodes (PLEDs) fabricated with blend films composed of MEH-PPV and poly(9,9di-n-octylfluorenyl-2,7-diyl) (PFO) can be summarized as follows: FRET from the PFO to the MEH-PPV leads to the fluorescent quenching of PFO and emission from MEH-PPV.47 In that case, the device structure was ITO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid))/MEH-PPV + PFO/Cs2CO3/Al. Because PFO and PFD have the same πconjugated back-bone, PFD has the same band structure as that of PFO. Therefore, we also assume that FRET is a dominant process in the fluorescent quenching in our discussion of the working mechanism of the PLECs fabricated with PFD and MEH-PPV. The overlap between the PL spectrum of PFD and the UV-vis absorption spectrum of MEHPPV in the range of about 400nm-570nm clearly suggests the possible occurrence of FRET from the excited PFD to the MEH-PPV (Figure 3).40 The Förster radius ( R0 ), which determines the rate of energy transfer from the excited PFD to the MEH-PPV (kET), is a very important parameter vis-à-vis optimization of the emission color of FCP-blend PLECs. kET increases with increasing R0 , resulting in improved fluorescent quenching of PFD. Therefore, the emission color of the PLECs is red-shifted as R0 gets larger because FRET causes the emission intensity of the PFD to decrease and increases that of MEH-PPV. To gain an insight into the working mechanism of FCP-blend PLECs, R0 was calculated by Eq. 2.40-43 R06 =

∫

∞

0

9000(ln 10)κ 2 Φ 0D 128π 5 N A n 4

f D (λ )dλ = 1

∫

∞

0

f D (λ )ε A (λ )λ4 dλ

(2)

(3)


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where λ is the wavelength, κ is the factor of orientation of the PFD and MEH-PPV dipoles,

Φ 0D is the PL quantum yield of PFD, NA is Avogadro’s number, n is the refractive index of the medium, f' (λ) is the normalized fluorescent intensity of PFD as defined by Eq. 3, and ε+ (λ) is the molar absorption coefficient of MEH-PPV. We obtained R0 of 7.3 nm, using κ 2 = 2 3 (assuming a random orientation of the dipoles), Φ 0D = 0.55 ,42 and n = 2.041 for the FCPs. This value is close to the R0 of 6.5 nm calculated for a polyfluorene copolymer (PFO-DBT5) containing 0.05 mol% of 4,7-bithienyl-2,1,3-benzothiadiazole (DBT5) in a polyfluorene mainchain in which the PFO segment (blue emitter) functions as an energy donor and DBT (orange emitter) functions as an energy acceptor in intra-molecular FRET in the polymer chain.42 The calculation method employed here is the most-widely used for determining R0 , but the effects of exciton diffusion and the existence of many acceptors are not taken into account. In our case, the FCP-blend PLECs fabricated with PFD and MEH-PPV have multiple energy acceptors in the active layers which act as energy sinks, and exciton diffusion occurs in the FCP. Therefore, to take these effects into consideration, R0 was calculated based on the PL quenching ratio using Eq. 4, which was developed by S. van Reenen, and M. Kemerink et al.41 ,,

?

=

1

=

- √)> .3 /0 (2 456788 9 - :∙4√596788 3 ?

1

.3 /0 (2 )> 3 6B

@AA =0.676(C 39)D/4 3

(4)

(5)

where PDA and PD are the PL intensities of PEO films containing PFD and MEH-PPV and those containing only PFD, respectively; EF is the lifetime of the energy donor, PFD, in the absence of an electron acceptor, MEH-PPV; G is the diffusion coefficient of PFD exciton, and


14

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HI is the density of the MEH-PPV. Here, we used the effective density of MEH-PPV, as defined by Eq. 6, to calculate CA. HI =

J-

(6)

×LMNO

where M is the weight per unit volume of MEH-PPV, LP Q − SSO is the molecular weight of the MEH-PPV repeating unit, and T is the average conjugation length of MEH-PPV, which determines the absorption onset energy.48-51 That is to say, we assume that FCPs are composed of repeating segments, having average conjugation lengths, for light absorption. We can solve Eq. 4 analytically, and Eq. 7 can be obtained. ,,

= DU45C

D

3 6788 9 -

V1 − √WXYZ(X : )[1 − \](X)^_

D

: γ = 4√WG@AA HI (C + 4W@AA GHI )D/: 3

(7) (8)

Based on Eq. 7 and Figure 4, an R0 of 3.0 nm was obtained when we took l = 7,50,51 EF = 0.5 ns,52 and D = 1. 0 × 10-4 cm2/s.53,54 This conjugation length is much shorter than that calculated from Mn (l = 94), but is somewhat longer than the polaron delocalization length, lp, of 4.5 units in poly(9,9-dihexylfluorene-2,7-diyl) (lp = 4.5).55 That the polaron delocalization lengths are shorter than the exciton delocalization lengths is due to the fact that polarons are coulombically trapped by oppositely charged ions. Based on the obtained R0 , the working mechanism of FCPblend PLECs will be discussed later in greater detail.


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1.2 Normalized PL Intensity (-)

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1 0.8 0.6 0.4 0.2 0

0

2×1018 4×1018 6×1018 8×1018 Doping Density (cm-3)

Figure 4. PL quenching of PFD as a function of MEH-PPV doping density.

3.2 Characteristics of PLECs fabricated with a blend of PFD and MEH-PPV Figure 5 shows the EL emission spectra and CIE coordinates of reference PLECs fabricated with PFD and MEH-PPV, and operated at 5.0 V. These PLECs have the typical PLEC structures of ITO/PFD:PEO:KCF3SO3/Al and ITO/MEH-PPV:PEO:KCF3SO3/Al, using ITO as the anode and Al as the cathode. The compositions of the active layers were PFD:PEO:KCF3SO3 = 1:0.79:0.037 and MEH-PPV:PEO:KCF3SO3 = 1:1.5:0.072 (mass ratio), respectively, and the average thickness of the active layers was about 100 nm. The EL emission spectra show fluorescence originating from blue exciton emission from the PFD and red-orange exciton emission from the MEH-PPV. The CIE coordinates of the PFD-based PLEC are (0.18, 0.16), corresponding to blue emission, and those of the MEH-PPV-based PLEC are (0.57, 0.43), corresponding to red-orange emission. White light emission can be obtained by optimizing the proportions of the PFD’s blue and MEH-PPV’s red-orange emissions.


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Figure 5. (a) EL emission spectra of the PFD-based PLEC (ITO/PFD:PEO:KCF3SO3/Al) and MEH-PPV-based PLEC (ITO/MEH-PPV:PEO:KCF3SO3/Al). (b) Plots of CIE emission coordinates of the PFD-based PLEC (ITO/PFD: PEO:KCF3SO3/Al) and MEH-PPV-based PLEC (ITO/MEH-PPV:PEO:KCF3SO3/Al).

Having the proper concentration of salt (KCF3SO3) is very important for achieving high PLEC performance. Table 1 shows the characteristics of PLECs (Device A, B, C, D) having different concentrations of KCF3SO3 with the same mass ratio of PFD:MEH-PPV:PEO. M. Kemerink et al. have recently suggested that LECs would be better suited for applications that demand highbrightness rather than high-efficiency because increasing the salt concentration to increase brightness has the effect of reducing efficiency due to exciton-polaron quenching.56 Thus, we focused our research on the development of high-brightness PLECs. The highest maximum brightness (Lmax), 233 cd/m2, was achieved in Device B [PFD:MEH-PPV:PEO:KCF3SO3; 1:0:52:0.79:0.037 (mass ratio), 1:1:8.6:0.1 (molar ratio using molar mass of repeating unit of


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polymers)]. The doping concentration (x %) in the p-doped and n-doped regions, which is defined by the molar ratio of [ion]/[FCP repeating unit], can be calculated by Eq. 9.19 Y=

:> defg1 ⁄efg1 >>c dijk ⁄ijk

(9)

where d is the thickness of the active layer, di is the thickness of the i-region, msalt is the mass of salt in the active layer, mFCP is the mass of FCPs in the active layer, and Msalt and MFCP are the molar mass of the salt and repeating unit of the FCPs, respectively. The average thickness of the active layers of Devices A and B is about 100 nm. We used di = 20nm,19 and MFCP = MPFD + MMEH-PPV, where MPFD is the molar mass of the repeating unit of PFD and MMEH-PPV is that of MEH-PPV, based on the assumption that the doping concentrations in the p-type and n-type regions of PFD and MEH-PPV are nearly identical at the maximum brightness at 5.5 V (> 2.9 V). In this case, the doping concentration in the p-type and n-type regions is around 10%. Here, the assumption mentioned above means that the emission colors of Devices A through D (redorange to orange) are determined largely by FRET from the PFD to the MEH-PPV. This result for doping concentration is identical with that reported by Edman,57 in which a doping concentration of around 10% is desired for maximum device performance. When x < 10%, p-i-n junctions cannot be formed effectively due to insufficient ion concentration. Meanwhile, when x > 10%, exciton-polaron quenching and micro short-circuiting occur, decreasing brightness dramatically, because of excess doping in the p-type and n-type regions of the FCPs. Figure 6 shows the dependence of the current density on the applied voltage (J vs. V) and the EL spectra of Devices B and D. It clearly indicates that only emission from MEH-PPV was observed in Device B even at an applied voltage of 6.5 V (> 2.9 V), whereas emissions from both PFD and MEH-PPV were observed in Device D, although the PFD emission was of very low intensity. These results can be explained as follows: Device B has a high MEH-PPV content


18

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and low ionic concentration in the active layer, hence the EL emission from PFD is quenched efficiently by FRET from the PFD to the MEH-PPV. Meanwhile, Device D has a high MEHPPV content but a high ionic concentration in the active layer, hence a very high concentration of exctions in the PFD is achieved by increasing the applied voltage and PFD emission is observed because it is not completely quenched by the MEH-PPV. This result suggests that the ionic concentration, which determines the doping concentration in the p-type and n-type regions, plays a very important role in controlling the emission characteristics of FCP-blend PLECs.

(b)

(a) 1500

1.2 Normalized EL Intensity (a.u.)

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Device B Device D

1000

500

0

0

1

2

3 4 Voltage (V)

5

6

1

Device B Device D

0.8 0.6 0.4 0.2 0 400

500 600 700 Wavelength (nm)

800

Figure 6. (a) Current density-applied voltage characteristics (J vs. V) of Device B and D. (b) EL emission spectra of Device B and D at 6.5 V. The arrow indicates the EL emission from PFD.

We selected a salt concentration of PEO:KCF3SO3 = 0.79:0.037 (mass ratio), resulting in a doping concentration of around 10%, in order to achieve high brightness in our PLECs. However, using the same salt concentration, Device B showed only red-orange emission from


19


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Page 20 of 38

the MEH-PPV. Therefore, we examined the emission characteristics of PLECs with lower MEHPPV concentrations with the aim of obtaining PFD emission by inhibiting FRET from the PFD to the MEH-PPV. Table 1 shows the characteristics of FCP-blend PLECs having different concentrations of MEH-PPV (Device E, F, G) and Figure 7 shows their PL and EL spectra.

Table 1. Device characteristics of PLECs, ITO/PFD:MEH-PPV:PEO:KCF3SO3/Al, with different mass ratios of PFD:MEH-PPV:PEO:KCF3SO3. Composition a)

Lmax

Device

CIE

PFD

MEH-PPV

PEO

KCF3SO3

cd/m2

x

y

A

1

0.52

0.79

0.019

202

0.57

0.43

B

1

0.52

0.79

0.037

213

0.55

0.44

C

1

0.52

0.79

0.056

137

0.53

0.45

D

1

0.52

0.79

0.074

90

0.49

0.46

E

1

0.052

0.79

0.037

710

0.33

0.31

F

1

0.16

0.79

0.037

743

0.46

0.42

G

1

0.26

0.79

0.037

906

0.53

0.45

a) Mass Ratio


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(b)

(a)

1.4

2 Device E Device F Device G

1.5

Normalized EL Intensity (a.u.)

Normalized PL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.5

0 400

500 600 Wavelength (nm)

700

1.2 1

Device E Device F Device G

0.8 0.6 0.4 0.2 0 400

500 600 700 Wavelength (nm)

800

Figure 7. (a) PL emission spectra of blend films (PFD:MEH-PPV:PEO:KCF3SO3) on glass substrates, corresponding to the active layers of Device E, F, and G. (b) EL emission spectra of Device E, F, and G.

All three devices show PFD and MEH-PPV emissions simultaneously because the efficiency of the FRET from the PFD to the MEH-PPV decreases as the MEH-PPV concentration decreases. Figure 8 shows the dependence of the emission-intensity ratio of the energy acceptor (A), MEHPPV, to the energy donor (D), PFD, at the peak wavelength (IA/ID) on the mass ratio of MEHPPV to PFD (WA/WD).


21


(b)

(a)

40

15

PLEC PLED

PLEC PLED

35 30

10

IAIA// IIDD

25

IA /I ID IA/ D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

20 15 10 5

0

0

5

10

15 20 W WD WA / WAD /x 100

25

30

0

0

5

10

15

20

25

30

WA /x 100 WD WA / W D

Figure 8. (a) Dependence of the PL emission intensity ratio of MEH-PPV:PFD at peak wavelength (IMEH-PPV/IPFD) on the mass ratio of MEH-PPV:PFD (WMEH-PPV/WPFD). (b) Dependence of the EL emission intensity ratio of MEH-PPV:PFD at peak wavelength (IMEHPPV/IPFD)

on the mass ratio of MEH-PPV:PFD (WMEH-PPV/WPFD).

The emission-intensity of MEH-PPV is dramatically enhanced and that of PFD is dramatically reduced by increasing the MEH-PPV concentration. For comparison, the IA/ID dependences of PL and EL on WA/WD in PLEDs fabricated with a blend film of MEH-PPV and PFO are also plotted in Figure 8.47 Here, MEH-PPV is the energy acceptor (A) and PFO, which has a πconjugated system identical to that of PFD, is the energy donor (D). These dependences in the FCPs-blend PLEDs were calculated by us using the data shown in the manuscript. It can be clearly seen that the quenching efficiency by FRET in the FCP-blend PLECs is much lower than that in the FCP-blend PLEDs, corresponding to smaller values of IA/ID at a given value of WA/WD, in both PL and EL. Since the active layers of the PLECs contain wide-band-gap


22

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insulating PEO, which does not contribute to the FRET process (the Eg of PEO being much larger than that of PFD); and separate PFD and MEH-PPV main-chains, as shown in Figure 9, FRET from the PFD to the MEH-PPV is limited. Here, the Förster radius ( R0 = 3.0 nm) is smaller than the thickness of the insulating region, i.e., the light-emission region, of the PLECs (di = 20 nm). If the active layer did not contain PEO, the emission from PFD would be reduced dramatically and white emission would not be generated because the concentration of exciton is at its highest at the center of the insulating region and the distance between the center and the doped region/insulating region interface is about 10 nm. These results indicate that tuning the emission color of FCP-blend PLEDs is very difficult due to the very sharp increase of IA/ID as WA/WD increases. On the other hand, tuning the emission color of FCP-blend PLECs is very easy because the FRET from the PFD to the MEH-PPV is inhibited by PEO.

Figure 9. (a) Schematic of FRET between PFD and MEH-PPV in PLECs. (b) Schematic of FRET between PFD and MEH-PPV in PLEDs.


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Similar results were obtained by M. Granström et al. in FCP-blend PLEDs fabricated with a blend film of three FCPs (red, yellow-green, and green-blue FCPs) and an insulating polymer.58 In that case, the addition of an insulating polymer inhibited FRET from a wide-band-gap FCP to a low-band-gap one and the desired emission could be easily obtained because the content of the low-band-gap FCP can be increased in FCP-blend PLEDs with insulating polymers. By optimizing the polymer contents, white light emission was achieved in PLEDs fabricated with blend films of polythiophene derivatives: poly[3-(4-octylphenyl)-2,2’-bithiophene] (PTOPT) as a red light emitter, poly(3-cyclohexylthiophene) (PCHT) as a yellow-green light emitter, poly(3methyl-4-octylthiophene) (PMOT) as a green-blue light emitter, and poly(methyl methacrylate) (PMMA) as an insulator. However, although the emission color could be easily tuned through the use of the insulating polymer, this FCP-blend PLED needed a very high applied voltage of about 20 V to achieve white emission. Such high driving voltages cannot be used in actual application in organic light-emitting devices because of issues of durability. Figure 8 also indicates that the IA/ID of PL is much smaller than that of EL at the same WA/WD in both the PLEC and the PLED. This result can be explained by the fact that the PL process is instantaneous and does not include the CT process, whereas the EL process does include the CT process.47 Figure 10 shows a CIE chromaticity diagram for Device E when operated at 6 V, showing white-light emission with CIE coordinates of (x = 0.33, y =0.31), and a photograph of Device E emitting white light. When the applied voltage was increased from 6 V to 8 V, corresponding to a four-fold increase in the current density, a slight blue-shift color change (CIE coordinates of (x = 0.28, y =0.30) at 8 V) occurred because the numbers of electrons and holes injected into the


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PFD’s LUMO and HOMO from the electrodes increased, respectively. The color rendering index (CRI) values were 74 at 6V and 80 at 8V, respectively. Figure 11 summarizes the working mechanism of the FCP-blend PLECs. The emissions from PFD and MEH-PPV can be simultaneously obtained at low MEH-PPV concentrations, while the emissions from MEH-PPV are only obtained at high MEH-PPV concentrations because of FRET from the PFD to the MEHPPV. White PLECs using a blend of a blue FCP and a red-orange one can be realized by optimizing the salt concentration and the MEH-PPV and PFD concentrations in the active layer.

Figure 10. (a) Plots of CIE emission coordinates of Device E when operated at 6 V and 8 V. (b) Photograph of the white-light-emitting cell (Device E), indicated by the arrow. The white light is reflected by the Al electrode of the cell on the right cell, which is not being driven.


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Figure 11. Working mechanism of PLECs with different concentrations of MEH-PPV. Figure 12 shows the temporal evolution of luminance, current density, and efficacy of Device E when driven at a constant voltage of 5 V. The luminance and current density increased with time, whereas the efficacy increased initially to a peak value and then decreased, shows a roll-off effect related to the relationship between the luminance and efficacy. We obtained a maximum efficacy of 0.14 cd/A, power conversion efficiency (PCE) of 0.085 lm/W, and turn-on time (defined as the time needed to reach maximum luminance after a constant voltage is applied) of 100 s. The universal transients in Device E clearly indicate that the PLECs being studied operate according to the typical working mechanism of LECs.59


26

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80 Current

0.1

60 Efficacy

40

60

30

Luminance (cd/m2)

Luminance

Efficacy (cd/A)

0.15

100

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 20 0

0 0

20

40

60

80

0

100 120

Time (s)

Figure 12. Temporal evolution of luminance, current density, and efficacy of Device E at 5 V The lines are presented as a guide for the eye.

4. Conclusion In summary, we reported on white PLECs fabricated using a blend film of blue FCP (PFD) and red-orange FCP (MEH-PPV), based on the concept of band-structure engineering. By optimizing the composition of PFD, MEH-PPV and salt in the active layer, white-light emission can be achieved by light-mixing of the blue exciton emission from PFD and the red-orange exciton emission from MEH-PPV at an applied voltage higher than the threshold voltage, ,

which corresponds to the band gap of PFD. That is to say, the white light resulting from lightmixing of PFD and MEH-PPV emissions can be obtained at a low MEH-PPV concentration, while the red-orange emission from MEH-PPV is only obtained at high MEH-PPV concentrations because of FRET from the PFD to the MEH-PPV. Since the band gap of PFD is


27


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Page 28 of 38

larger than that of MEH-PPV and the PL spectrum of PFD overlaps the UV-vis absorption spectrum of MEH-PPV, FRET occurs from the PFD to the MEH-PPV, which affects the emission color. However, FRET was inhibited by the presence of PEO in the FCP-blend PLECs, which made it much easier to tune the emission color compared to FCP-blend PLEDs. Utilization of a blend film of a blue FCP and a red-orange one is a very simple and promising method for fabrication of white light-emitting devices.

ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (No. 24225003).


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ASSOCIATED CONTENT Supporting Information Available This information is available free of charge via the Internet at http://pubs.acs.org. Cyclic voltammogram of PFD and MEH-PPV; Complete reference list (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected] (Y.N.).

*TEL:

+81-3-5286-3370

Notes The authors declare no competing financial interest.


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Table of Contents Graphic


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White Polymer Light-Emitting Electrochemical Cells Fabricated Using

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