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Emissive Polyelectrolytes as Interlayer for Color-tuning and Electron Injection in Solution-processed Light-emitting Devices Serpil Tekoglu, Martin Petzoldt, Sebastian Stolz, Uwe H. F. Bunz, Uli Lemmer, Manuel Hamburger, and Gerardo Hernandez-Sosa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00665 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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ACS Applied Materials & Interfaces

Emissive Polyelectrolytes as Interlayer for Colortuning and Electron Injection in Solutionprocessed Light-emitting Devices Serpil Tekoglu1,2, Martin Petzoldt2,3, Sebastian Stolz1,2, Uwe H.F. Bunz3, Uli Lemmer1,4, Manuel Hamburger2,3,† and Gerardo Hernandez-Sosa1,2*

1

Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131

Karlsruhe, Germany, 2InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany, 3

Organisch-Chemisches Institut, Ruprecht-Karls-Universität, Im Neuenheimer Feld 270, D-

69120 Heidelberg, Germany, 4Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

ABSTRACT: Herein we present a solution-processed hybrid device architecture combining organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) in a bilayer architecture. The LEC interlayer promotes the charge injection from an air-stable Ag cathode as well as permits the color-tuning of the device emission. To this end, we used an alcohol-soluble anionic polyfluorene derivative, the properties of which, were investigated by absorption and photoluminescence spectroscopy as well as by cyclic voltammetry. The bilayer device exhibited operating voltages ~ 6V and a color-tuning of the emission spectrum 1 ACS Paragon Plus Environment


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dependent on the LEC interlayer thickness. The hybrid devices presented a color emission ranging from the yellow (x = 0.39, y = 0.47) towards the green region (x = 0.29, y = 0.4) of the Commission Internationale de I’Eclairage (CIE) 1931 chromaticity diagram. KEYWORDS: polyfluorene, conjugated polyelectrolyte, electron injection layer, OLEDs, LECs

INTRODUCTION Solution-processed organic light-emitting diodes (OLEDs) are promising devices whose highthroughput fabrication has potential applications in display and artificial lighting technologies by utilizing printing or coating techniques1-3. In the last years, different approaches and techniques have been employed to achieve highly efficient OLEDs offering white light or multi-color emission4-8. However, the most efficient approaches to date require a complex multilayer device architecture employing a sequential deposition of the active materials under high-vacuum conditions4-6. Despite the outstanding progress in the development of solutionprocessed active materials and multilayer device architectures compatible with printing processes, the injection of carriers from air-stable solution-processed cathodes still remains an important challenge to maximize device performance9.

Replacing air-sensitive cathode

materials will not only allow fully-printed devices but will also reduce the encapsulation requirements, which represent a significant fraction of the fabrication cost10-11.

Recent approaches to this problem rely on the use of polymeric interlayers, which reduce the work function of the metal cathode and improve injection of carriers into the active material1213

. Polyelectrolytes, such as ionically functionalized polyfluorenes (PFs) can be successfully

utilized as electron injection layers in OLEDs and organic solar cells with promising results142 ACS Paragon Plus Environment

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18

. PFs are soluble in polar solvents and can be easily deposited on top of organo-soluble

polymers, enabling multilayer device architectures. Interestingly, PFs have also found application as emitters in single layer light-emitting electrochemical cells (LECs)18-21. In LECs, the mobile ions within the film contribute to the electrochemical doping of the semiconductor, leading to doped regions that enable carrier injection to the semiconductor regardless of the work function of the electrodes. LECs have also been reported to produce white light emission and allow color-tuning by modifying the molecular structure of the emissive layer19, a voltage control

22

or via host-guest systems comprised of quantum dots

and/or polymers23-24. These characteristics have yielded LECs as an alternative to OLEDs for applications with lower performance requirements.3, 25, 26

In this work, we present a hybrid device architecture utilizing a LEC on top of an OLED. The LEC interlayer not only promotes electron injection from an air-stable cathode, but also allows for the color-tuning of the device emission. To this end, we used an alcohol-soluble cationic PF derivative (PFNCl), the properties of which, were investigated by absorption and photoluminescence spectroscopy as well as by cyclic voltammetry.. The bilayer device comprising an Ag cathode exhibited operating voltages ~ 6 V and a color-tuning of the emission spectrum dependent on the LEC interlayer thickness. The hybrid devices presented a color emission ranging from the yellow (x = 0.39, y = 0.47) towards the green region (x = 0.29, y = 0.4) of the Commission Internationale de I’Eclairage (CIE) 1931 chromaticity diagram. RESULTS AND DISCUSSION Photophysical and Electrochemical properties of PFNCl. Figure 1 presents the chemical structure, and the photophysical and electrochemical properties of PFNCl. The detailed synthesis route of PFNCl is presented in the supplemental 3 ACS Paragon Plus Environment


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information. The resulting polymer PFNCl was designed to be soluble in polar solvents such as alcohols, DMSO and DMF. This solubility in polar solvents allows for sequential deposition of PFNCl onto organo-soluble materials to produce solution-processed polymer multilayer devices. The absorption and emission spectra of PFNCl and Super Yellow (a phenylene substituted poly(para-phenylenevinylene), Ph-PPV) were investigated in solution and are presented in Figure 1a. PFNCl showed a maximum absorption at 402 nm and an emission peak at 424 nm. The optical bandgap was found to be 2.92 eV from the onset of the absorption spectrum. The PPV derivative Super Yellow (SY) presented a maximum absorption at 446 nm and an emission at 512 nm, consistent with previous literature results. The large spectral overlap between the emission spectrum of PFNCl and the absorption spectrum of SY implies that reabsorption processes, as well as energy transfer, between PFNCl and SY could be present and potentially assist in the color-tuning of the emission of the final devices (see Figure S1 of the supporting information).

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Figure 1: (a) Normalized absorption and photoluminescence (PL) spectra of diluted PFNCl and SY solutions (10-8 M) in chloroform. (b) Cyclic voltammogram of PFNCl Inset: Molecule structure of PFNCl.

The electrochemical stability window of PFNCl plays an important role in defining the voltages at which undesired electrochemical reactions take place during device operation. The electrochemical behavior of PFNCl was investigated in solution by cyclic voltammetry (CV) using ferrocene as the internal reference redox system (Figure 1b). The CV measurements were employed to estimate the redox potentials and the energy levels of PFNCl. The energy of the highest-occupied molecular orbital (EHOMO) and the lowest-unoccupied molecular orbital (ELUMO) levels were calculated using the onset of the oxidation and reduction



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potentials vs ferrocene/ferrocenium (Fc/Fc+) from the CV data and assuming the energy level of Fc/Fc+ to be 5.1 eV below the vacuum level as follows27: EHOMO = -[(Eox)on+5.1] ELUMO = -[(Ered)on+5.1] The maximum peak potentials appeared at 0.73 V and -2.44 V, attributed to the oxidation and reduction potentials, respectively, for the polymer main chains. During the anodic scan, we observed an irreversible oxidation with the onset potential at 0.46 V. In the cathodic sweep, we detected a reversible reduction process with an onset potential of -2.25 V. The HOMO and LUMO levels (using the above equations and onset potentials) of PFNCl were determined to be -5.56 eV and -2.85 eV, respectively with a bandgap of 2.71 eV. SY is electrochemically oxidized and reduced with the onset potentials of 0.35 V and -2.15 V, respectively (Figure S2 of the Supporting Information). The HOMO and LUMO levels of SY were determined to be 5.45 eV and -2.95 eV, respectively and the band gap of SY is 2.5 eV. Device Fabrication and Characterization Single-layer PFNCl LECs: PFNCl was employed in a single-component polymer lightemitting electrochemical cell (PLEC). A single layer of PFNCl (65 nm) was spin cast on PEDOT:PSS covered ITO glass prior to the evaporation of 100 nm Ag as a top electrode. The current density-voltage-luminance (J-V-L) characteristics, electroluminescence (EL) spectra and the time-dependent voltage behavior of the device are shown in Figure 2. A turn-on voltage (Von ; defined as the voltage required to obtain a luminance of 1 cd/m2) of 3.8 V can be observed from Figure 2a as well as a maximum luminance of ~ 550 cd/m2 at 8 V. The inset of Figure 2a presents the device architecture and a photograph of the single component polyelectrolyte-based LEC in operation.


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Figure 2. (a) Current density–voltage–luminance characteristics and (b) electroluminescence spectra of single layer PFNCl LECs. Inset (a): Device architecture and photograph of PLEC in operation. Inset (b): Time-dependent voltage characteristics.

As seen in Figure 2b, the device showed a maximum luminescence peak at 434 nm and a spectrum with CIE coordinates (x = 0.19, y = 0.16). There are only a few examples in literature related to blue emissive polymer LECs20-21,

28-29

.

The emission tail at longer

wavelengths, which was also observed in PL data (Figure 1a), is possibly caused by polymer aggregation favored by the ionic moieties on the side chains18. The inset of Figure 2b presents the time-dependent voltage characteristics of the device. The device showed typical LEC characteristics with a slow time response. The presented data was recorded under continuous operation at a current density of 50 mA/cm2. The initial slow response is indicative of the time required for the ionic groups to move towards the electrodes and dynamically form a p-i-n 7 ACS Paragon Plus Environment


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junction, as commonly observed in ionic transition metal complex-based LECs30. These doped regions at the contact interfaces allow for sufficient carrier injection independent of the work function of the electrodes.

LEC/OLED hybrid devices: The J-V-L and EL spectra of the bilayer hybrid devices are presented in Figure 3. PFNCl at various layer thicknesses was deposited on top of a SY layer and these two emissive layers were sandwiched between air-stable electrodes as depicted in Figure 3a. The HOMO and LUMO levels of PNFCl and SY, presented in the schematic energy level diagram, were calculated from the cyclic voltammetry measurements as described in the previous section. Figure 3b and 3c present the current-density and the luminance versus the applied voltage for all different PFNCl interlayer thicknesses. Von in the range of 5-8 V maximum luminance in the range of 500-550 cd/m2 was observed for the hybrid devices. The decrease in current density at higher thicknesses can be attributed to an additional series resistance. A comparison of the device characteristics for single and bilayer devices is presented in Table 1. The time-dependent voltage characteristics at a constant current density of 20 mA/cm2 are shown in Figure 3d. The observed initial decrease of the operational voltage is a typical signature of LEC behavior. During galvanostatic operation at a fixed current, a time-dependent decrease in operating voltage is typically addressed to the dynamic electrochemical doping of the emitting material31.


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Figure 3. Device characteristics of the hybrid devices: (a) Device architecture and schematic energy level diagram, (b) current density-voltage, (c) luminance-voltage characteristics and (d) time-dependent voltage characteristics at a constant current density of 20 mA/cm2. Figure 4a presents the EL spectra of the hybrid devices comprising different thicknesses of PFNCl on top of a SY layer as well as the single-layer reference devices. The emission spectra of the hybrid devices extend from 400 nm to 750 nm exhibiting spectral contributions from both emitting polymers. In figure 2 the single layer PFNCl LEC is shown to be able emit light which can only be consequence of exciton formation by injected electrons and holes from the electrodes. In this case, the formation and recombination of excitons in the SY layer is favored

by injected electrons from the PFNCl layer. The color change trend for all of the different devices can be better observed in the CIE diagram shown in Figure 4b. An increase in the PFNCl layer thickness contributed to a larger blue emission band, resulting in the color tuning 9 ACS Paragon Plus Environment


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demonstrated in Figure 4b. The change in layer thickness could potentially cause a change in thickness of the recombination zone. The spectral features of the pristine emission of PFNCl are observable in all devices, however we cannot rule out a partial contribution from energy transfer from PNFCl to SY (see SI Figure S1) or absorption-reemission processes assisting the emission color tuning. Interestingly, for all hybrid devices, a blue-shift between 500 and 550 nm was observed on the EL spectral features corresponding to SY. Such shifts have been reported in literature for the EL and PL spectra of SY in bilayer device architectures32 or when blended with different host materials33-34. In these systems, the electroluminescence was observed to shift from the bulk SY emission towards the blue due to material intermixing, with the emission shift dependent on the SY concentration. In our study, partial layer intermixing of SY with PFNCl might take place when depositing PFNCl from 2-methoxy ethanol, leading to a similar spectral shift as seen in literature32. A summary of CIE coordinates, color rendering index (CRI), color chromaticity temperature (CCT) and maximum EL wavelength of all bilayer devices is presented in Table 1. Figure S11 of the supporting information shows that the EL spectra of a device with 80nm PFNCl remained unchanged under applied bias, demonstrating the stability of the materials during device operation.


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Figure 4. (a) Electroluminescence spectra and (b) CIE coordinates of single and bilayer devices at a current density of 100 mA/cm2. The increase in thickness is depicted by the direction of the arrow.

Table 1. Summary of the Device Characteristics for Single and Bilayer Devices.

PFNCl Thickness

Turn on Voltage

Max.

Luminance*

Max. EL λ

CIE 1931*

CRI

CCT [K]

Current Density

(cd/m2)

(nm)

(x:y)

(V)

(mA/cm2)

80

7.8

183

294

521

0.288 : 0.407

67.49

6980

70

6.2

178

227

540

0.34 : 0.44

70.24

5316

65

5.5

149

158

550

0.382 : 0.442

69.09

4343

60

5.3

168

179

544

0.385 : 0.451

68.01

4322

55

5.8

144

141

546

0.39 : 0.472

67.35

4310

(nm)



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65 (PFNCl LEC)

3.8

342

164

434

0.192 : 0.167

59.17

-

0 (SY OLED)

4.7

350

915

556

0.483 : 0.508

45.84

-

* at a current density of ~ 100 mA/cm2

CONCLUSIONS In this study, we demonstrated a solution processed light-emitting device architecture comprising an LEC interlayer on top of a conventional OLED. The poly electrolyte-based interlayer allows for electron injection from an air-stable cathode as well as for color-tuning of the device emission. By choosing the interlayer thickness we showed that the CIE color coordinates of the device emission can be moved from the yellow towards the green region of the CIE diagram. We envisage that by further engineering of the LEC interlayer such device architecture can be used for the fabrication of fully-printed multilayer devices with improved performance and precisely tuned color emission.

Experimental Section Methods and Instrumentation: All reactions requiring exclusion of oxygen and moisture were carried out in dry glassware under a dry and oxygen free argon or nitrogen atmosphere. Solvents were distilled prior to use if necessary. All absolute solvents were dried by a MB SPS-800 using drying columns. Chemicals were supplied from the chemical store at the Organic Chemistry Institute of the University of Heidelberg or purchased. Reagents were used without any further purification unless otherwise stated. IUPAC names and atom numberings of the compounds were determined with the program ACD/ChemScetch 2012 of Advanced Chemistry Development Inc. Formulas depicted in the experimental section were drawn with 12 ACS Paragon Plus Environment

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ChemBioDraw Ultra 12.0 and 14.0. 1H NMR and

13

C NMR spectra were recorded at room

temperature on Bruker Avance DRX 300 (300 MHz), Bruker Avance III 300 (300 MHz), Bruker Avance III 400 (400 MHz), Bruker Avance III 500 (500 MHz) and Bruker Avance III 600 (600 MHz). Chemical shifts (δ) are reported in parts per million (ppm) relative to residual undeuterated solvent peak35-36. The following abbreviations are used to indicate the signal multiplicity: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublet). All NMR spectra were integrated and processed using ACD/Spectrus Processor 2012 of Advanced Chemistry Development Inc. Ultra performance liquid chromatography with tandem mass spectrometer (UPLC-MS) were carried out on a Waters® Acquity UPLC-MS System with SQD2 detector and APCI ionization source. High resolution mass spectra (HR-MS) were determined at the Organic Chemistry Institute of the University of Heidelberg under the direction of Dr. J. Gross. All methods were recorded on Vakuum Generators ZAB-2F (EI+), Finnigan MAT TSQ 700 (ESI+), IonSense Saugus DART-SVP-OS (DART) or JEOL JMS-700 (FAB+) spectrometer. Infrared (IR) spectra were recorded on a JASCO FT/IR-4100 spectrometer as neat oil or powder. Preparative gel permeation chromatography (Prep GPC) was performed on Bio-BeadsTM (S-X1 Beads, 200-400 Mesh, crosslinked polystyrene), purchased from Bio-Rad Laboratories, Inc. Gel permeation chromatography (GPC) was performed with JASCO intelligent RI- and UV/Vis-detectors (RI-2031Plus, UV-2075plus). Number- (Mn) and weight average (Mw) molecular weights and polydispersities (PDI) were determined by GPC versus polystyrene standards. Measurements were carried out at room temperature in chloroform (by a flow rate of 1 mL/min) with PSSSDV columns (8.0⨉30.0 mm, 5 µm particles, 102-, 103- and 105- Ǻ pore size). All GPCspectra were analyzed and processed by PSS WinGPC Unity Build 9350. Cyclic Voltammetry (CV) was performed by using a VERSASTAT 3 potentiostat (Princeton Applied Research). A 3-electrode system consisting of a Pt working electrode, a Pt/Ti counter electrode, and an Ag wire pseudo-reference electrode placed in a glass vessel was employed 13 ACS Paragon Plus Environment


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for the CV measurements. The Anhydrous DMSO and Acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphote (>99%, Sigma–Aldrich) was used as the supporting electrolyte to define the redox potentials of PFNCl and SY, respectively, at the scan rate of 0.1 V/s. A small amount of ferrocene (Di(cyclopentadienyl)iron) (>98%, Sigma– Aldrich) was used as an internal reference redox system for each sample. All the CV measurements were performed under Ar atmosphere. UV-Vis Absorption spectra were carried out on a JASCO UV-VIS V-660 or JASCO UV-VIS V-670 in solution. Emission spectra were recorded on a JASCO FP-6500 in solution. Fabrication and Characterization of devices: Super Yellow (Merck PDY-132) and PFNCl were dissolved in toluene and 2-methoxy ethanol, respectively. The PFNCl solution was filtered through a 0.45 µm PTFE filter. ITO coated glass substrates were cleaned with acetone and IPA in an ultrasonic bath. For the single component LECs, the PFNCl solution was spincoated on top of ITO glass or PEDOT:PSS coated ITO glass. For the bilayer hybrid and single layer devices, the substrates first were covered with a 40 nm thick poly-3,4-ethylene dioxythiophene:poly-styrene sulfonate (PEDOT:PSS, Heraeus Clevios AI4083) layer by spincoating at 5000 rpm for 60 s and then baked at 120°C for 15 min in air. A 50 nm layer of SY layer was spin-coated at 1750 rpm, 60 s and dried at 120°C for 15 min. PFNCl was coated on top of SY layer by spin-coating using different spin-rates yielding thicknesses between 55-80 nm. Finally, Ag top contacts (100 nm) were thermally evaporated through a shadow mask to define the active area of the device (0.24 cm2). The electrical and the optical characterization of all devices were performed using a calibrated BOTEST characterization system at a measuring speed of 0.1 V s −1. The electroluminescence spectra were recorded using an Ocean Optics spectrophotometer. The calculations and figures related to CIE chromaticity coordinates were done with the help of SpectrAsis (spectrasis.lti.kit.edu). Thickness measurements were performed using a Veeco profilometer.


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ASSOCIATED CONTENT Supporting Information. Detailed synthesis procedure and characterization (1H NMR, 13

C NMR, FT-IR, and mass spectra) of the materials, FRET data, CV of SY and PFNCl, J-V-

L of time-dependent luminance characteristics of single-layer PFNCl LEC. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT The authors thank S. Valouch for useful discussions, J. Mescher and S. Wendel from KIT for the SpectrAsis software. The work was supported by the German Federal Ministry of Education and Research (BMBF) via the projects FKZ: 13N12794 and FKZ: 03X5526.

AUTHOR INFORMATION Corresponding Author *Dr. Gerardo Hernandez-Sosa, [email protected] Present Address †Dr. Manuel Hamburger, Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany.

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GRAPHICAL TABLE OF CONTENTS


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Figure 1: (a) Normalized absorption and photoluminescence (PL) spectra of diluted PFNCl and SY solutions (10-8 M) in choloroform. (b) Cyclic voltammogram of PFNCl Inset: Molecule structure of PFNCl. 98x151mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2. (a) Current density–voltage–luminance characteristics and (b) electroluminescence spectra of single layer PFNCl LECs. Inset (a): Device architecture and photograph of PLEC in operation. Inset (b): Time-dependent voltage characteristics. 84x111mm (300 x 300 DPI)


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Figure 3. Device characteristics of the hybrid devices: (a) Device architecture and schematic energy level diagram, (b) current density-voltage, (c) luminance-voltage characteristics and (d) time-dependent voltage characteristics at a constant current density of 20 mA/cm2. 148x113mm (300 x 300 DPI)



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Figure 4. (a) Electroluminescence spectra and (b) CIE coordinates of single and bilayer devices at a current density of 100 mA/cm2. The increase in thickness is depicted by the direction of the arrow. 155x255mm (300 x 300 DPI)


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Emissive Polyelectrolytes As Interlayer for Color ... - ACS Publications

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