White Light from a Light-Emitting Electrochemical Cell: Controlling the

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White Light from a Light-Emitting Electrochemical Cell: Controlling the Energy-Transfer in a Conjugated Polymer/Triplet-Emitter Blend Shi Tang, Herwig A. Buchholz, and Ludvig Edman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09119 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 11, 2015

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ACS Applied Materials & Interfaces

White Light from a Light-Emitting Electrochemical Cell: Controlling the Energy-Transfer in a Conjugated Polymer/Triplet-Emitter Blend Shi Tang,1,2 Herwig A. Buchholz,3 and Ludvig Edman 1,2,* 1

The Organic Photonics and Electronics Group, Umeå University, SE-901 87 Umeå, Sweden 2

3

LunaLEC AB, Tvistevägen 47, SE-907 19 Umeå, Sweden

Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany

KEYWORDS: white luminance, light-emitting electrochemical cell, solid-state lighting, conjugated polymer, triplet emission, energy transfer

ABSTRACT: We report on the attainment of broadband white light emission from a host-guest light-emitting electrochemical cell, comprising a blue-emitting conjugated polymer as the majority host and a red-emitting small-molecule triplet emitter as the minority guest. An analysis of the energy structure reveals that host-to-guest energy transfer can be effectuated by both Förster and Dexter processes, and through a careful optimization of the active material composition partial energy transfer and white emission is accomplished at a low guest concentration of 0.5 %. By adding a small amount of a yellow-emitting conjugated polymer to

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the active material, white light emission with a high color rendering index of 79, and an efficiency of 4.3 cd/A at significant luminance (>200 cd/m2), is realized.

INTRODUCTION Emerging area-emitting devices, such as organic light-emitting diodes (OLEDs)1 and lightemitting electrochemical cells (LECs),2-6 promise novel application opportunities, including soft and glare-free white light illumination from large and conformable surfaces. With recent reports on an all-solution based and low-energy fabrication of LECs, it appears as such areal emitters also can carry a low price tag and be fabricated in an environmentally attractive manner.7-11 However, if these devices, in addition, are going to offer energy-efficient emission at high luminance, it is probably paramount that they are based on a host-guest active material, where the charge transport is being effectuated by a majority host, while parts or all of the emission should originate from an efficient triplet-emitting minority guest.12 Although white-emitting LECs are becoming more common in the scientific literature,13-25 and some examples on monochrome emission from host-guest LECs also do exist,26-28 reports on broadband-white LECs featuring efficient triplet emission from a host-guest system are still rather scarce. Wu and Wong introduced a white-emitting LEC comprising two cationic ionic transition metal complexes (iTMC) as the host-guest system, which featured a reasonably high external quantum efficiency (EQE) of 3.3 % at a modest luminance of 43 cd/m2.29 A similar concept has also been successfully employed by Qiu and co-workers.30-32 Finally, the group of Bolink reported a two-emission-layer device, comprising a blue-emitting host-guest layer and an orange-emitting iMTC layer, which exhibited white emission with a high quantum efficacy of 8.5 cd/A at a luminance of 845 cd/m2.33


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In order to allow for a more vibrant activity in this promising field, we recently initiated an effort towards high-performance host-guest LEC. In the first study, we focused on the properties of the host-compound, and were able to establish that a bipolar (i.e. both p- and n-type) electrochemical doping capability of the host compound(s) is an important criterion for the attainment of an efficient host-guest LEC.34 Here, we present a single-emission-layer LEC, comprising a blue-emitting conjugated polymer (CP) as the host and a triplet-emitting iTMC molecule as the guest, which features bipolar electrochemical doping and for which the energy transfer can be tuned to obtain white emission. By adding a small amount of a yellow-emitting CP to the active-material blend, we were able to achieve efficient and high-quality broadband white light emission at high luminance. EXPERIMENTAL SECTION Materials. All active-material compounds were used as received. The blue-emitting spirobifluorene-triphenylamine conjugated copolymer (blue-CP, trade name: SPB-02T, Merck; chemical structure presented in the inset of Fig. 1a) was the “host” compound, while the redemitting tris[1-(substituent-phenyl)-isoquinoline]iridium (III) (Ir(R-piq)3, Merck), and the yellow-emitting conjugated polymer Super Yellow (SY, trade name: PDY-132, Merck) were employed as the “guest” compounds; the chemical structures of (Ir(R-piq)3 and SY are presented in Figure S1 in the Supporting Information). The electrolyte comprised the salt LiCF3SO3 (Sigma-Aldrich) dissolved in hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH, Sigma-Aldrich). All compounds were dissolved separately in anhydrous tetrahydrofuran at a concentration of 10 mg/ml, and the mass stoichiometry of the active-material ink was invariably {host+guest:TMPE-OH:LiCF3SO3} = {1:0.1:0.03}.


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Material characterization. Thin films for the absorption (UV-3100 spectrophotometer, Shimadzu) and photoluminescence (PL) measurements (FP-6500 spectrofluorometer, JASCO) were spin-coated onto carefully cleaned quartz substrates. The surface morphology was measured with atomic force microscopy (AFM, MultiMode SPM microscope equipped with a Nanoscope IV Controller, Veeco Metrology). For the cyclic voltammetry (CV) measurements, the working electrode was prepared by spin-coating the material under study onto a Au-coated glass substrate, a Pt wire was used as the counter electrode, and a Ag wire was used as the pseudo-reference

electrode.

The

electrolyte

was

0.1

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tetrabutylammonium

hexafluorophosphate in CH3CN. The CV sweeps were driven and measured by an Autolab PGSTAT302 potentiostat. Directly after each CV scan, a calibration scan was run with a small amount of ferrocene added to the electrolyte. The onset potentials for oxidation and reduction were calculated as the intersection of the baseline with the tangent of the current at the halfmaximum of the peak. The energy level with respect to the vacuum level (VL) were established with the equation: EVL = − e⋅(4.8 V + VFc/Fc+). The sample preparation and the measurements were performed under inert atmosphere in a N2-filled glove box. Device Fabrication and Measurement. LEC sandwich cells were fabricated by sequentially spin-coating carefully cleaned indium-tin-oxide (ITO) coated glass substrates (20 Ω/sq., Thin Film Devices, US) with first poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS, Clevios P VP AI 4083, Heraeus) at 4000 rpm for 60 s and then the active material at 2000 rpm for 60 s. The dry thickness of the PEDOT:PSS and the active material was 40 nm and 130 nm, respectively, as established with a stylus profilometer (Dektak). A set of four Al cathodes was deposited on top of the active material by thermal evaporation at p < 5×10-4 Pa. The light-emission area, as defined by the size of one cathode, was 0.85×0.15 cm2. The LEC


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sandwich cells were characterized using a computer-controlled source-measure unit (Agilent U2722A) and a calibrated photodiode, equipped with an eye-response filter (Hamamatsu Photonics), connected to a data acquisition card (National Instruments USB-6009) via a currentto-voltage amplifier. The electroluminescence (EL) spectra were recorded using a calibrated fiber-optic spectrometer (USB2000+, Ocean Optics), and the Commission Internationale de l’Eclairage (CIE) coordinates and the Colour Rendering Index (CRI) were calculated using the SpectraWin software. All of the above procedures, except for the cleaning of the substrates, were carried out in two interconnected N2-filled glove boxes ([O2] < 1 ppm, [H2O] < 0.5 ppm). LEC surface cells were fabricated by evaporating two Au electrodes, with an inter-electrode gap of 300 µm as established by a shadow mask in the form of a fiber, onto carefully cleaned glass substrates. The bi-layer active material was fabricated by sequentially spin-coating first the blue-CP (10 mg/ml in tetrahydrofuran) at 2000 rpm for 60 s and then the {PEO+KCF3SO3} electrolyte (10 mg/ml in acetonitrile with a mass ratio of PEO:KCF3SO3 = 1.35:0.5) at 800 rpm for 60 s. The dry thickness of the blue-CP and the electrolyte layers was 130 nm and 180 nm, respectively. The LEC surface cells were characterized in an optical-access vacuum chamber at p < 5×10-5 Pa. A computer-controlled source-measure unit (Keithley 2400) sourced the voltage and measured the current. The photographs of the doping process and the light emission were recorded in a dark room under UV illumination (λpeak = 365 nm) through the optical window of the vacuum chamber, using a digital camera (Canon EOS 50D) equipped with a macro lens (65 mm, F/2.8).


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RESULTS AND DISCUSSION

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Figure 1. (a) Cyclic voltammetry traces of the blue-CP film; the chemical structure of the blueCP is displayed in the inset. (b) The current as a function of time for a bilayer-surface cell, and (c) optical micrographs of the same bilayer-surface cell during operations, as probed from its top surface. This bilayer-surface cell comprised a bottom layer of blue-CP, a top layer of PEO:KCF3SO3 (1.35:0.5) electrolyte, and two charge-injecting Au electrodes separated by a 300 µm inter-electrode gap. The device was driven by 5 V at 360 K. The dashed lines indicate the electrode interfaces. The employed host material was a blue-emitting spirobifluorene-triphenylamine conjugated copolymer termed blue-CP, and its chemical structure is presented in the inset of Figure 1(a).


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The electrochemical doping capacity of the blue-CP was established with cyclic voltammetry (CV) and through the operation of a bilayer-surface cell, using a procedure presented in Ref. 34. The CV scan displayed in Figure 1(a) reveals a relatively well-behaved and reversible oxidation (p-type doping) and reduction (n-type doping) of a blue-CP film positioned in an electrolyte solution. A further confirmation of the doping capacity of the blue-CP is provided by the operation of an LEC bilayer-surface cell.35 Such “open” devices were fabricated by evaporating two Au electrodes side-by-side with an interelectrode gap of 300 µm on a glass substrate, and then sequentially spin-coating layers of the blue-CP and a {poly(ethylene oxide)+KCF3SO3} electrolyte on top the electrodes. Fig. 1(b) presents the current vs. time evolution for such a device when driven by 5 V at 360 K. The initially decreasing current (see inset in Figure 1b) is assigned to the formation of electric double layers at the two electrode interfaces, whereas the subsequently increasing current implies that electrochemical doping of the blue-CP, with a concomitant increase in electronic conductivity, takes place. Electrochemical doping of CPs is commonly accompanied by not only an increase in conductivity, but also by a quenching of a UV-excited photoluminescence (PL).36-38 Accordingly, by observing the device under UV-excitation, we are able to collect further evidence for an ongoing electrochemical doping of the blue-CP via the emergence and growth of dark regions (see Figure 1c). A p-type doping region is observed to grow from the positive anode, and an n-type doping regions grows from the negative cathode; and after a turn-on time a light-emitting p-n junction forms at their meeting point, as shown in the last photograph in Figure 1(c). The overall conclusion must then that the blue-CP indeed features p-type and n-type


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doping capacity, and thereby qualifies as an appropriate candidate for the host material in a hostguest LEC.

(a)

Ir(R-piq)3 Blue-CP Ir(R-piq)3

Normalized PL

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Figure 2. (a) PL spectra of the blue-CP (solid blue squares) and Ir(R-piq)3 (open red circles); and the absorption spectrum of Ir(R-piq)3 (solid black line). (b) The electron-energy levels (left) and the exciton-energy diagram (right) of the blue-CP (solid lines) and Ir(R-piq)3 (dashed lines), respectively. We now shift our attention to the identification of a matching guest material. Figure 2(a) presents the absorption spectrum of a tris[1-(substituent-phenyl)-isoquinoline]iridium (III) (Ir(Rpiq)3) film (solid black line), which displays a significant and broad absorption band in the visible region (peak = 455 nm) and an even stronger absorptions in the near-UV range (data not shown); the former is assigned to a metal-to-ligand charge-transfer transition, whereas the latter is a ligand-to-ligand π-π* transition.39 It is interesting to note that the PL spectrum of the blue-CP film (solid blue squares) exhibits a significant overlap with the absorption of Ir(R-piq)3, as this a requirement for efficient Förster resonance energy transfer from the blue-CP to the Ir(R-piq)3. It is further shown that the pristine Ir(R-piq)3 film features red PL with a peak emission at 615 nm (open red circles).


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The triplet level of the blue-CP is reported to be 2.2 eV,21 while the triplet level of Ir(R-piq)3 is estimated to be 2.0 eV, using the PL data in Fig. 2(a). This implies that Dexter transfer from the blue-CP to the Ir(R-piq)3 also could be a viable energy-transfer route, as indicated by the data presented in the right part of Fig. 2(b). The electron-energy levels of the two compounds (see left part of Fig. 2b) were gleaned from CV measurements, and we find that the LUMO level of Ir(Rpiq)3 is positioned 0.7 eV below the LUMO of the blue-CP, while the HOMO level is positioned at essentially the same energy for both compounds. This implies that electron trapping, but not hole trapping, will take place on the Ir(R-piq)3 compound in a blend.

Figure 3. AFM surface morphology of (10×10 µm2) regions of the {Blue-CP:Ir(Rpiq)3(100:x):TMPE-OH:LiCF3SO3} active layer with different values of x: 0 (left), 0.5 (middle), 5 (right). In order for the above outlined processes, i.e. Förster, Dexter and electron trapping, to be efficient in a solid-state device a common criterion is that the two compounds should mix well in a blend film. An indication on the mixing capacity of a {blue-CP:Ir(R-piq)3} blend in a solidstate thin film is provided by the atomic force microscopy investigation presented in Figure 3. For all investigated stoichometries, we find no signs of phase separations and the surface morphology of the blend film is highly smooth, with a root-mean-square (RMS) surface


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roughness of ~0.5 nm, which indicates that the capacity for efficient energy transfer from a blueCP host to an Ir(R-piq)3 guest in a blend film should be good.

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Figure 4. The temporal evolution of ITO/PEDOT:PSS/Blue-CP:Ir(R-piq)3 (100:x) :TMPEOH:LiCF3SO3/Al sandwich cells, with (a) x = 5 and (b) x = 0.5. (c) The EL spectra for such sandwich cells with x = 0 (solid blue squares), x = 0.5 (open black circles), and x = 5 (solid red triangles). (d) The temporal evolution of the EL spectra for a sandwich cell with x = 0.5, with the different spectra recorded at 10 min (‘turn-on’), 100 min (‘peak’) and 12 h (‘decay’). All devices were driven with j = 5.8 mA/cm2. The true test whether this host-guest combination is functional in an LEC is provided through the testing of actual devices. LEC sandwich cells were fabricated by sandwiching an active


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material, comprising the blue-CP:Ir(R-piq)3 host-guest system blended with LiCF3SO3 dissolved in hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH) as the electrolyte, between a bottom

indium-tin-oxide

(ITO)/poly(3,4-ethylenedioxythiophene)-poly(styrene

sulfonate)

(PEDOT:PSS) anode and a top Al cathode. The mass percentage of the Ir(R-piq)3 guest in the active material with respect to the blue-CP host was varied and is provided for by the parameter x. Figure 4(a) presents the initial temporal evolution of the luminance and voltage for an x = 5 device as driven by a constant current density of j = 5.8 mA/cm2. It is clear that it exhibits the characteristic signs of functional LEC operation in the form of an increasing luminance and a decreasing voltage during the turn-on phase. In Figure 4(c) it is further shown that the host-toguest energy transfer is effectively complete for this device, as it features solely red emission from the Ir(R-piq)3 guest, with a CIE value of (0.65, 0.33) and an efficiency of 2.9 cd/A (1.4 lm/W) at a luminance of 170 cd/m2. It is thus confirmed that the blue-CP:Ir(R-piq)3 combination indeed represents a functional host-guest system for LEC operation. We have also tested devices with lower values for x. We find that these also feature electrochemical doping and that the general trend is that the maximum luminance increases and the drive voltage decreases with decreasing guest concentration. Interestingly, we find that the emission color can be controlled in a facile manner (see Fig. 4c), and for a device with x = 0.5 we are able to attain white emission with a high efficiency of 3.6 cd/A (1.7 lm/W) at a significant luminance of 208 cd/m2, as shown in Figure 4(b). The white emission is quantified by CIE coordinates of (0.35, 0.36), a correlated color temperature (CCT) of 4800 K, and a color rendering index (CRI) of 60. We find that the emission color for both the white and red LECs is rather stabile over time (as shown in Figs. 4d and S2a), and we consider this a manifestation of


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that both the host and guest compounds are electrochemically stabile and that the active material features a robust morphology so that the energy-transfer process is maintained. It is interesting to compare this performance with that of an electrolyte-free OLED, comprising ITO/PEDOT:PSS as the anode and air-sensitive Ca as the cathode and with an identical host:guest ratio. For such a device we find that the quality of the white light is similar, but that the emission efficiency is notably lower at 1.6 cd/A (0.7 lm/W) at 250 cd/m2; see Figure S3 in the Supporting Information. We attribute the lower performance of the OLED to that the Ca cathode, despite being a low-work function material, does not provide for perfect electron injection into the blue-CP host, whereas the in-situ formed p-n junction structure in the LEC allows efficient and balanced electron and hole injection.

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Figure 5. (a) The temporal evolution, and (b) the EL spectrum of ITO/PEDOT:PSS/BlueCP:SY:Ir(R-piq)3:TMPE-OH:LiCF3SO3/Al sandwich cell. The device was driven with j = 5.8 mA/cm2. A drawback with the above white-emitting LEC (and OLED) is that its CRI value is insufficient for higher-value applications, such as indoor illumination, and an inspection of the EL spectrum yields that it is lacking in the yellow-green region. In order to address this issue, we have opted to add a small amount of a conjugated polymer termed “Super Yellow” (SY) to the


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active material. This choice was motivated by that the absorption of SY features a good overlap with the PL of the blue-CP thus allowing for efficient Förster energy transfer, that the LUMO level of SY is lower than that of the blue-CP thus allowing for electron trapping, and most importantly that SY emits in the yellow-green region (the optical and electronic properties of SY are presented in Figure S4). After a careful optimization procedure, we find that an LEC device with a {blue-CP:SY:Ir(R-piq)3} mass ratio of {100:1.5:1} features a lowered drive voltage, a faster turn-on, and a higher efficiency of 4.7 cd/A (2.8 lm/W) at 250 cd/m2 (see Fig. 5a). Even more importantly, the emission spectrum (see Fig. 5b) turned more broadband, and the CRI value was accordingly notably improved to 79, while the CIE coordinates (0.37, 0.45) and the CCT (4500 K) were retained, which is a positive in the context of functional white-light emission. The temporal stability of this broadband white LEC was also rather stabile over time, as disclosed in Fig. S2(b). CONCLUSION To summarize, by selecting the constituent host and guest materials with rational LEC-specific and energy-transfer requirements in mind, we are able to demonstrate host-guest LECs with airstabile electrodes that emit efficient white light at significant luminance. The best performing device comprised a blue-emitting conjugated polymer featuring balanced electrochemical doping as the host, and a yellow-emitting conjugated polymer and a red-emitting small-molecule triplet emitters as the guest compounds. Such an optimized device delivered broadband white light emission with an efficiency of 4.7 cd/A at 250 cd/m2 and featured a color rendering index of 79. ASSOCIATED CONTENT


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Supporting Information. Chemical structures of employed guest materials, optoelectronic characterization of the OLED device, and optical properties and electronic structure of SY. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support from the Swedish Foundation for Strategic Research, the Swedish Research Council, the Swedish Energy Agency, Kempestiftelserna and the Knut and Alice Wallenberg Foundations. REFERENCES 1.

Tang, C. W.; VanSlyke, S. A., Organic Electroluminescent Diodes. Appl. Phys. Lett.

1987, 51, 913-915. 2.

Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J., Polymer Light-Emitting

Electrochemical-Cells. Science 1995, 269, 1086-1088. 3.

Weber, M. D.; Adam, M.; Tykwinski, R. R.; Costa, R. D., Controlling the Chromaticity

of Small-Molecule Light-Emitting Electrochemical Cells Based on Tips-Pentacene. Adv. Funct. Mater. 2015, 25, 5066-5074.


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Page 15 of 20

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


4.

Norell Bader, A. J.; Ilkevich, A. A.; Kosilkin, I. V.; Leger, J. M., Precise Color Tuning

Via Hybrid Light-Emitting Electrochemical Cells. Nano Lett. 2010, 11, 461-465. 5.

Shanmugasundaram, K.; Subeesh, M. S.; Sunesh, C. D.; Chitumalla, R. K.; Jang, J.;

Choe, Y., Synthesis and Photophysical Characterization of an Ionic Fluorene Derivative for Blue Light-Emitting Electrochemical Cells. Org. Electron. 2015, 24, 297-302. 6.

Ayguler, M. F.; Weber, M. D.; Puscher, B. M. D.; Medina, D. D.; Docampo, P.; Costa, R.

D., Light-Emitting Electrochemical Cells Based on Hybrid Lead Halide Perovskite Nanoparticles. J. Phys. Chem. C 2015, 119, 12047-12054. 7.

Zhang, Z.; Guo, K.; Li, Y.; Li, X.; Guan, G.; Li, H.; Luo, Y.; Zhao, F.; Zhang, Q; Wei,

B.; Pei, Q.; Peng, H., A Colour-Tunable, Weavable Fibre-Shaped Polymer Light-Emitting Electrochemical Cell. Nat. Photonics 2015, 9, 233-238. 8.

Sandström, A.; Asadpoordarvish, A.; Enevold, J.; Edman, L., Spraying Light: Ambient-

Air Fabrication of Large-Area Emissive Devices on Complex-Shaped Surfaces. Adv. Mater. 2014, 26, 4975-4980. 9.

Hernandez-Sosa, G.; Tekoglu, S.; Stolz, S.; Eckstein, R.; Teusch, C.; Trapp, J.; Lemmer,

U.; Hamburger, M.; Mechau, N., The Compromises of Printing Organic Electronics: A Case Study of Gravure-Printed Light-Emitting Electrochemical Cells. Adv. Mater. 2014, 26, 32353240. 10. Sandström, A.; Dam, H. F.; Krebs, F. C.; Edman, L., Ambient Fabrication of Flexible and Large-Area Organic Light-Emitting Devices Using Slot-Die Coating. Nat. Commun. 2012, 3, 1002.


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11. Filiatrault, H. L.; Porteous, G. C.; Carmichael, R. S.; Davidson, G. J. E.; Carmichael, T. B., Stretchable Light-Emitting Electrochemical Cells Using an Elastomeric Emissive Material. Adv. Mater. 2012, 24, 2673-2678. 12. Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R., Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151-154. 13. Yang, Y.; Pei, Q., Efficient Blue-Green and White Light-Emitting Electrochemical Cells Based on Poly[9,9-Bis(3,6-Dioxaheptyl)-Fluorene-2,7-Diyl]. J. Appl. Phys. 1997, 81, 3294-3298. 14. Sun, M.; Zhong, C.; Li, F.; Cao, Y.; Pei, Q., A Fluorene-Oxadiazole Copolymer for White Light-Emitting Electrochemical Cells. Macromolecules 2010, 43, 1714-1718. 15. Tang, S.; Pan, J.; Buchholz, H. A.; Edman, L., White Light from a Single-Emitter LightEmitting Electrochemical Cell. J. Am. Chem. Soc. 2013, 135, 3647-3652. 16. Tsai, C.-S.; Yang, S.-H.; Liu, B.-C.; Su, H.-C., Single-Component Polyfluorene Electrolytes Bearing Different Counterions for White Light-Emitting Electrochemical Cells. Org. Electron. 2013, 14, 488-499. 17. Xiong, Y.; Li, L.; Liang, J.; Gao, H.; Chou, S.; Pei, Q., Efficient White Polymer LightEmitting Electrochemical Cells. Mater. Horiz. 2015, 2, 338-343. 18. Fernández-Hernández, J. M.; De Cola, L.; Bolink, H. J.; Clemente-León, M.; Coronado, E.; Forment-Aliaga, A.; López-Muñoz, A.; Repetto, D., White Light-Emitting Electrochemical Cells Based on the Langmuir–Blodgett Technique. Langmuir 2014, 30, 14021-14029.


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Page 17 of 20

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


19. Tang, S.; Pan, J.; Buchholz, H.; Edman, L., White Light-Emitting Electrochemical Cell. ACS Appl. Mater. Interfaces 2011, 3, 3384-3388. 20. Wang, J.; Tang, S.; Sandström, A.; Edman, L., Combining an Ionic Transition Metal Complex with a Conjugated Polymer for Wide-Range Voltage-Controlled Light-Emission Color. ACS Appl. Mater. Interfaces 2015, 7, 2784-2789. 21. Sessolo, M.; Tordera, D.; Bolink, H. J., Ionic Iridium Complex and Conjugated Polymer Used to Solution-Process a Bilayer White Light-Emitting Diode. ACS Appl. Mater. Interfaces 2013, 5, 630-634. 22. Jhang, Y.-P.; Chen, H.-F.; Wu, H.-B.; Yeh, Y.-S.; Su, H.-C.; Wong, K.-T., Improving Device Efficiencies of Solid-State White Light-Emitting Electrochemical Cells by Adjusting the Emissive-Layer Thickness. Org. Electron. 2013, 14, 2424-2430. 23. Su, H.-C.; Chen, H.-F.; Chen, P.-H.; Lin, S.-W.; Liao, C.-T.; Wong, K.-T., Efficient Solid-State White Light-Emitting Electrochemical Cells Based on Phosphorescent Sensitization. J. Mater. Chem. 2012, 22, 22998-23004. 24. Su, H.-C.; Chen, H.-F.; Shen, Y.-C.; Liao, C.-T.; Wong, K.-T., Highly Efficient DoubleDoped Solid-State White Light-Emitting Electrochemical Cells. J. Mater. Chem. 2011, 21, 96539660. 25. Su, H.-C.; Cheng, C.-Y., Recent Advances in Solid-State White Light-Emitting Electrochemical Cells. Isr. J. Chem. 2014, 54, 855-866. 26. Chen, F.-C.; Yang, Y.; Pei, Q., Phosphorescent Light-Emitting Electrochemical Cell. Appl. Phys. Lett. 2002, 81, 4278-4280.


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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

Page 18 of 20

27. Liao, C.-T.; Chen, H.-F.; Su, H.-C.; Wong, K.-T., Improving the Balance of Carrier Mobilities of Host-Guest Solid-State Light-Emitting Electrochemical Cells. Phys. Chem. Chem. Phys. 2012, 14, 1262-1269. 28. Pertegas, A.; Shavaleev, N. M.; Tordera, D.; Orti, E.; Nazeeruddin, M. K.; Bolink, H. J., Host-Guest Blue Light-Emitting Electrochemical Cells. J. Mater. Chem. C 2014, 2, 1605-1611. 29. Su, H.-C.; Chen, H.-F.; Fang, F.-C.; Liu, C.-C.; Wu, C.-C.; Wong, K.-T.; Liu, Y.-H.; Peng, S.-M., Solid-State White Light-Emitting Electrochemical Cells Using Iridium-Based Cationic Transition Metal Complexes. J. Am. Chem. Soc. 2008, 130, 3413-3419. 30. He, L.; Qiao, J.; Duan, L.; Dong, G.; Zhang, D.; Wang, L.; Qiu, Y., Toward Highly Efficient Solid-State White Light-Emitting Electrochemical Cells: Blue-Green to Red Emitting Cationic Iridium Complexes with Imidazole-Type Ancillary Ligands. Adv. Funct. Mater. 2009, 19, 2950-2960. 31. He, L.; Duan, L.; Qiao, J.; Dong, G.; Wang, L.; Qiu, Y., Highly Efficient Blue-Green and White Light-Emitting Electrochemical Cells Based on a Cationic Iridium Complex with a Bulky Side Group. Chem. Mater. 2010, 22, 3535-3542. 32. He, L.; Duan, L.; Qiao, J.; Zhang, D.; Wang, L.; Qiu, Y., Highly Efficient SolutionProcessed Blue-Green to Red and White Light-Emitting Diodes Using Cationic Iridium Complexes as Dopants. Org. Electron. 2010, 11, 1185-1191. 33. Akatsuka, T.; Roldán-Carmona, C.; Ortí, E.; Bolink, H. J., Dynamically Doped White Light Emitting Tandem Devices. Adv. Mater. 2014, 26, 770-774.


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Page 19 of 20

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


34. Tang, S.; Buchholz, H. A.; Edman, L., On the Selection of a Host Compound for Efficient Host-Guest Light-Emitting Electrochemical Cells. J. Mater. Chem. C 2015, 3, 81148120. 35. Sandstrom, A.; Matyba, P.; Inganas, O.; Edman, L., Separating Ion and Electron Transport: The Bilayer Light-Emitting Electrochemical Cell. J. Am. Chem. Soc. 2010, 132, 66466647. 36. Gao, J.; Dane, J., Visualization of Electrochemical Doping and Light-Emitting Junction Formation in Conjugated Polymer Films. Appl. Phys. Lett. 2004, 84, 2778-2780. 37. Hu, Y.; Gao, J., Direct Imaging and Probing of the P−N Junction in a Planar Polymer Light-Emitting Electrochemical Cell. J. Am. Chem. Soc. 2011, 133, 2227-2231. 38. Fang, J.; Matyba, P.; Robinson, N. D.; Edman, L., Identifying and Alleviating Electrochemical Side-Reactions in Light-Emitting Electrochemical Cells. J. Am. Chem. Soc. 2008, 130, 4562-4568. 39. Cleave, V.; Yahioglu, G.; Barny, P. L.; Friend, R. H.; Tessler, N., Harvesting Singlet and Triplet Energy in Polymer Leds. Adv. Mater. 1999, 11, 285-288.


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Table of Contents Graphic


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White Light from a Light-Emitting Electrochemical Cell: Controlling the

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