New Electroactive Polymers with Electronically Isolated 4,7-Diarylfluorene Chromophores as Positive Charge Transporting Layer Materials for OLEDs

A group of polyethers containing electroactive pendent 4,7-diarylfluorene chromophores have been prepared by the multi-step synthetic route. Full characterization of their structures has been presented. The polymeric materials represent derivatives of high thermal stability with initial thermal degradation temperatures in a range of 392–397 °C. Glass transition temperatures of the amorphous polymers range from 28 °C to 63 °C and depend on structures of the 4,7-diarylfluorene chromophores. Electron photoemission spectra of thin layers of the electroactive derivatives showed ionization potentials in the range of 5.8–6.0 eV. Hole injecting/transporting properties of the prepared polymeric materials were confirmed during formation of organic light-emitting diodes with tris(quinolin-8-olato)aluminium (Alq3) as a green emitter, which also serves as an electron transporting layer. The device using hole-transporting polymer with electronically isolated 2,7-di(4-biphenyl)fluorene chromophores demonstrated the best overall performance with low turn on voltage of 3 V, high current efficiency exceeding 1.7 cd/A, and with maximum brightness over 200 cd/m2. The organic light-emitting diode (OLED) characteristics were measured in non-optimized test devices. The efficiencies could be further improved by an optimization of device structure, formation conditions, and encapsulation of the devices.


Introduction
The advantages that organic light-emitting diode (OLED)-based technologies offer in terms of brightness, viewing angle, contrast ratio, production cost, opportunity for flexible displays, etc. are not rivaled by liquid crystal-based displays [1][2][3]. It is well established that multilayer devices comprising hole transport layer (HTL), electron transport layer, and emissive layer are necessary for efficient light emission [4][5][6].
One method that is very widely used to increase efficiencies of the organic light emitting devices is the incorporation of effective hole transporting layers in the structures of the OLEDs [1,7,8]. The charge transporting layers can be fabricated from low molecular weight compounds by vapor deposition or from polymeric materials by spin coating from prepared solutions. The solution-based route has the advantages, e.g., the production costs are lowered, large areas can be formed, and the molar mass of the materials is not limited [9,10].
The idea of attaching chromophoric fragments as the isolated side chain of a nonconjugated polymer was found to be of great advantage for the transformation of crystalline electroactive compounds to amorphous materials [11]. Advantageous aspects of this method are the availability of a great variety of functionalized amorphous polymers suitable for grafting reactions and high loading capacity without segregation of chromophores. Electro-optical properties could be also tuned by modification of molecular structure and by copolymerization [12].
Solution processed derivatives containing substituted carbazole fragments are among the most studied materials for organic electronics due to their good chemical and environmental stability as well as high positive charge mobility in their layers. In addition, derivatives containing electronically isolated carbazole chromophores have high triplet energies and are widely used as host materials for electro-phosphorescent devices (PhOLEDs) [13][14][15]. We have reported earlier that aryl substituted low-molar-mass carbazoles demonstrate suitable ionization potentials and positive charge transporting properties for application in OLED devices, however, the materials have evident tendency for crystallization [16]. Here, we report on polymeric hole transporting materials (HTM) containing diaryl-substituted fluorene fragments as electroactive chromophores. The materials are fully amorphous and were tested as hole transporting layers in OLEDs with Alq 3 emitter demonstrating preparation of large area devices by spin-coating from solutions.
All the newly synthesized derivatives were identified by IR and 1 H NMR spectroscopy as well as by elemental analysis. Low-molar-mass compounds were also confirmed by mass spectrometry. The data were found to be in good agreement with the proposed structures. The polymers 7-9 were soluble in common organic solvents at room temperature. Amorphous thin films of these materials could be prepared by using cheap spin coating technique.
Molecular weights and polydispersity indices (PDI) of the prepared were estimated by GPC. The number-average molecular weights (M n ), weight-average molecular weights (M w ), and PDI of these polymers are presented in the Table 1. It was observed that rather low-molecular-weight polymeric materials were obtained after the cationic polymerization. It could be mentioned that the values of M n and M w of the polymers depend on the electronic structure of chromophores attached to the polymerizable unite. The monomer 4 containing 4,7-diphenylfluorene chromophores yielded polymer 7 with the lowest molecular weight and small PDI. It could be considered that polymer 7 has much better solubility due to smaller phenyl fragments at fluorene core and bigger amount of higher molecular weight fractions were removed during the Soxhlet extraction. The values of molecular weights of polyethers 8 and 9 containing 4,7-dinaphthylfluorene and 4,7-di(4-biphenyl)fluorene fragments were considerably higher. The polymers also have broad molecular weight distribution and big PDI values. It could be considered that the polymers having higher PDI values should demonstrate better film forming properties for formation of homogenous amorphous films. It could be also mentioned that molecular weights of polymers could be controlled by using other amount of initiator, by using other solvent for polymerization as well as by using other solvents for Soxhlet extraction of row polymer. Scheme 1. Ar = phenyl (monomer 4), 1-naphtyl (monomer 5), and 4-biphenyl (monomer 6). The behavior under heating of the polymeric materials 7-9 was tested under a nitrogen atmosphere by DSC and TGA methods. The synthesized polymers have very high thermal stabilities.
DSC measurements have demonstrated that the synthesized polymers 7-9 are amorphous materials with clearly expressed glass-transition temperatures (T g ). The curves of second heating of DSC measurements, which show the most exact T g values of the polymers, are presented in Figure 1. It could be seen that the glass-transitions of the polymeric materials were observed at temperature of 28 • C for 7, at 58 • C for 8, and at 63 • C for 9, and no peaks due to melting or crystallisation were obtained during all the measurements, i.e., the polymers are amorphous and suitable for preparation of thin homogeneous layers. It could be stated that T g values of the materials depend on chemical compositions of chromophores of the polymers. The polymeric materials 8 and 9 with bulky biphenyl of naphtyl fragments have considerably higher T g than that of polymer 7 having di-phenyl substituted fluorine core. The low T g of the polymer 7 is a result of the aromatic chro-mophore of the material. Small mass and volume of the chromophore lead to low T g value of the polymer. Thin amorphous layers of the polymers 7-9 were used for measurements of ionization potentials (I p ) of these materials having various chromophores. The values of I p were established from electron photoemission spectra of the thin films. The spectra and values of the I p are shown in Figure 2. It could be observed that I p of the materials range from 5.8 to 6.0 eV and depend on chemical compositions of the chromophores. The layer of polymer 8 having biphenyl-substituted fluorine fragments demonstrated the lowest I p of 5.8 eV due to longer conjugated system in the aromatic fragments. I p of the polymer 7 with biphenyl-substituted fluorine chromophores had the highest value, which reached 6.0 eV. The obtained characteristics show that the thin films of the prepared polymers should be suitable for hole injection and transport in OLED devices using emitters with high ionization potentials, i.e., deep highest occupied molecular orbital (HOMO) energy levels [17]. Positive charge transporting properties of layers of the prepared polymers 8-9 were tested in electroluminescent organic light-emitting diodes, which have been formed on glass substrates covered with ITO anode. It should be mentioned that polymer 7 could not compose homogenous thin layers suitable for the devices, probably due to its low glass transition temperature. Hole transporting layers of the polymeric materials 8 and 9 were prepared by spin-coating from their solution. Commercially available Alq 3 was used as an emitter and also electron transporting layer in the OLEDs. Aluminium layer was used as a cathode in composition with a thin electron injection layer of LiF. When a positive voltage was applied for such OLEDs, a bright green electroluminescence originating from Alq 3 emitting layer was obtained with an emission maximum at around 520 nm in all the devices. As an example, Figure 3 shows the electroluminescent spectra of OLED using polymer 9 as hole transporting layer (HTL) material at different voltages (7-10 V). This observation confirmed that the polymeric materials 8 and 9 only served as hole transporting layers without an exciplex formation at the interface with Alq 3 emitter. The data also show that hole injection and mobility in the thin hole transporting films of the polymers was fully sufficient for an effective charge carrier recombination occurring within the Alq 3 emitter layer. The current density-voltage (I-V) curves of the devices depicted in Figure 4 show a typical OLED's behavior with rather low turn-on voltages of 3.0 V for diode using HTL of polymer 9 and of 5.6 V for device with HTL of 8. The luminance-voltage and current efficiency-luminance characteristics for the OLEDs are also presented in the Figure 4. Both the devices could be characterized by maximum brightness of 113-204 cd/m 2 at 12.5-13.5 V with current densities of 38-43 mA/cm 2 and luminous efficiency of about 1.7 cd/A. The highest brightness exceeding 200 cd/m 2 with efficiency of about 1.7 cd/A with was measured in the diode containing HTL of polymer 9 containing electro-active 2,7-di(4-biphenyl)fluorine chromophores. It was also observed from electron photoemission spectra that the materials were characterized by the lowest ionization potential, i.e., the best hole injecting properties. It should be pointed out that these OLED data were measured in non-optimized test devices. The efficiencies may be further improved by an optimization of device formation conditions, layer thicknesses as well as by encapsulation of the devices [18].

Instrumentation
1 H nuclear magnetic resonance (NMR) spectra were obtained using a Bruker Avance III (400 MHz) apparatus (Bruker, Berlin, Germany). The data are given as chemical shifts (δ) in ppm against trimethylsilane (in parenthesis: multiplicity, integration, coupling constant). Mass spectra were obtained on a Waters ZQ 2000 mass spectrometer (Agilent, Santa Clara, CA, USA). Infrared (IR) spectra are measured using a Vertex 70 Bruker spectrophotometer (Bruker, Berlin, Germany).
The molecular weights of the polymers were determined by a gel permeation chromatography (GPC) system Malvern/Viscotek (Malvern Panalytical, Westborough, MA, USA). Polystyrene standards were used for calibration of the columns and tetrahydrofuran (THF) was chosen as an eluent.
Differential scanning calorimetry (DSC) measurements were carried out using a Bruker Reflex II thermosystem (Bruker, Berlin, Germany). Thermogravimetric analysis (TGA) was performed on a TGAQ50 aparatus ((Bruker, Berlin, Germany)). The TGA and DSC curves were recorded in a nitrogen atmosphere at a heating rate of 10 • C/min.
The electron photoemission method for measurement of ionization potentials (I p ) of the solid-state layers of the studied materials was exploited in air [19]. The spin-coated layers of the polymers onto commercial indium tin oxide (ITO)-coated glass substrates were utilized as the samples for the electron photoemission measurements. Thickness of the films for ionization potential measurements was 100 nm. The used experimental setup was the same as it was previously described [20].
The electroluminescent OLED devices were fabricated on glass substrates. The electroactive organic layers were sandwiched between a bottom indium tin oxide (ITO) anode and a top metal cathode. The ITO-coated substrates (Sigma-Aldrich) were carefully cleaned and treated with UV/ozone right before deposition of the organic layers. The substrates with a sheet resistance of 10 Ω/sq. and with ITO thickness of 150 nm were used. Hole transporting (HT) layers from the polymers were prepared by spin-coating from chloroform solutions (5 mg/mL). Thickness of the layers was controlled by changing speed (1100-1500 rpm for 60 s) of the substrate in order to obtain thin films of 40 nm. The thickness of the deposited polymer layer was measured by using a profilometer. Evaporation of the emitting/electrontransporting tris(quinolin-8-olato)aluminium (Alq 3 ) layer (40 nm) and a LiF/Al electrode (1/150 nm) was done at the pressure of 1 × 10 −5 torr in vacuum evaporation equipment. The final configuration of the devices was ITO/HT polymer/Alq 3 /LiF/Al. Size of the emitting areas was 0.10 cm 2 .
The luminance of the fabricated devices was measured using a Minolta CS-100 luminance-meter. A Keithley 2400 electrometer was used to measure the current-voltage characteristics of the devices. All the measurements were performed at ambient conditions in air.

Conclusions
In conclusion, new electro-active polymers having 4,7-diarylfluorene chromophores were prepared by cationic polymerization of the corresponding oxetane-based monomers. The amorphous materials demonstrated very high thermal stability (392-397 • C) and glass transition temperatures in a range of 28-63 • C. Electron photoemission spectra of thin films of the polymers confirmed that ionization potentials of the materials are depending on their chromophores and are in a range from 5.8 eV to 6.0 eV. The potentials could be suitable for hole injection and transport into emitting layers having high ionization potentials. The polymers have been tested as positive charges transporting films in bilayer organic light-emitting diodes with tris(quinolin-8-olato)aluminium (Alq 3 ) as an emitter as well as electron transporting layer. An OLED device with polymer having electro-active 2,7-di(4-biphenyl)fluorine chromophores exhibited the best overall performance with low turn on voltage of 3 V, maximum brightness exceeding 200 cd/m 2 , and current efficiency of 1.7 cd/A. These OLED properties are rather promising among Alq 3 -based two-layer devices having polymeric hole transporting layers.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.