3,3′-Bicarbazole-Based Host Molecules for Solution-Processed Phosphorescent OLEDs

Solution-processed organic light-emitting diodes (OLEDs) are attractive due to their low-cost, large area displays, and lighting features. Small molecules as well as polymers can be used as host materials within the solution-processed emitting layer. Herein, we report two 3,3′-bicarbazole-based host small molecules, which possess a structural isomer relationship. 9,9′-Di-4-n-butylphenyl-9H,9′H-3,3′-bicarbazole (BCz-nBuPh) and 9,9′-di-4-t-butylphenyl-9H,9′H-3,3′-bicarbazole (BCz-tBuPh) exhibited similar optical properties within solutions but different photoluminescence within films. A solution-processed green phosphorescent OLED with the BCz-tBuPh host exhibited a high maximum current efficiency and power efficiency of 43.1 cd/A and 40.0 lm/W, respectively, compared to the device with the BCz-nBuPh host.


Introduction
Ever since organic light-emitting diodes (OLEDs) were first reported by Tang and VanSlyke with an organometal compound of 8-hydroxyquinoline aluminum and an aromatic diamine compound [1], they have comprised the core technology of mobile and flat-panel displays. During the development of OLEDs, one great breakthrough was the utilization of a triplet emission called phosphorescence [2]. Ir complexes, such as phosphorescent dyes, exhibit a high phosphorescence yield at room temperature [3,4]. The external quantum efficiency of phosphorescent OLEDs (PHOLEDs) has exceeded that of fluorescent OLEDs [5]. In PHOLEDs, many carbazole molecules have been employed as host materials in the emitting layers [6,7]. As 4,4 -bis(N-carbazolyl)-1,1 -biphenyl (CBP) is a famous phosphorescent host molecule from early PHOLED reports [3,8], it has been employed as a general host material in studies pertaining to PHOLED materials and devices [9,10].

Synthesis and Thermal Analysis
The synthetic routes and chemical structures for 3,3 -bicarbazole (BCz), BCz-nBuPh, and BCz-tBuPh can be seen in Scheme 1. Initially, 3,3 -bicarbazole was obtained via the chemical oxidative coupling of carbazoles in the presence of FeCl 3 according to a procedure described in the literature [20]. Additionally, a detailed reduction procedure using zinc powder can be found in the materials section. The designed new host molecules, BCz-nBuPh and BCz-tBuPh, were synthesized through an Ullmann coupling reaction between 3,3 -bicarbazole and two iodobenzenes with n-butyl and t-butyl groups, respectively. The two compounds with alkyl groups were much more soluble in common organic solvents, such as chloroform and toluene, compared to the intermediate compound, BCz. Scheme 1. The synthesis of host materials for solution processed organic light-emitting diodes (OLEDs). Reagents and conditions: (i) FeCl 3 , CHCl 3 , room temperature (RT); (ii) Cu, K 2 CO 3 , DMF, 145 • C.
Vacuum sublimation of the molecules yielded different aspects. BCz-nBuPh easily melted and then sublimed. BCz-tBuPh was sublimed as a powder and featured higher sublimation yields than BCz-nBuPh. The melting point and glass transition temperature of BCz-tBuPh (295 and 136 • C, respectively) was much higher than those of BCz-nBuPh (146 and 58 • C, respectively) as can be seen in the differential scanning calorimetry (DSC) curves (Figure 1a). This difference could be attributed to the stiffness of the t-butyl group compared to the n-butyl group. In contrast to the melting points, thermal degradation of the two isomers was similar as can be seen in the thermogravimetric analysis (TGA) curves ( Figure 1b). Both molecules were sufficiently stable up to 400 • C; their thermal decomposition temperatures (T d ) were 410 • C for BCz-nBuPh and 423 • C for BCz-tBuPh.  Figure 2 shows the UV-visible absorption and photoluminescence (PL) spectra of BCz-nBuPh and BCz-tBuPh in dilute chloroform and as films. In the solution spectra (Figure 2a), the photophysical properties of the two isomers were nearly unchanged by the different alkyl groups. This meant that the two host materials possessed nearly the same optical properties from the π-π* transition of the same conjugated bicarbazole core. On the other hand, with regard to the film PL spectra (Figure 2b), BCz-nBuPh exhibited a relatively broad shoulder emission peak over 450 nm compared to BCz-tBuPh. Because the n-butyl group in BCz-nBuPh exhibited less steric hindrance than the t-butyl group in BCz-tBuPh, molecules of BCz-nBuPh within the film exhibited a strong degree of molecular aggregation. t-Butyl groups have been introduced to blue emitting molecules to prevent molecular quenching [21,22]. BCz-tBuPh with t-butyl groups was expected to yield better host materials than BCz-nBuPh with regard to thermal and optical properties.

Photophyscial and Electrochemical Properties
Optical band gaps were calculated from an absorption edge of 374 nm in the films, yielding values of 3.32 eV for both BCz-nBuPh and BCz-tBuPh. This value was the same as that of BCzPh (3.32 eV) [11] and enough to cover OLED dopants within visible light as the host materials.
The electrochemical properties of the two host molecules and N,N -bis(3-methyl-phenyl)-[l,l -biphenyl]-4,4 -diamine (TPD) were measured via cyclic voltammetry (CV) measurements ( Figure 3). The ionization potential (IP) energy levels of the molecules were estimated according to an empirical relationship: E(IP) = E [ox, onset vs. Fc/Fc+] + 5.1 (eV) [23][24][25], where E [ox, onset vs. Fc/Fc+] (V) is the onset potential of oxidation versus the redox potential of ferrocene/ferrocenium. The oxidation potentials of the two host molecules were almost the same. The IP energy levels were estimated to be 5.55 eV for both BCz-nBuPh and BCz-tBuPh. The thermal, optical, and electrochemical properties of the molecules are summarized in Table 1.
The device configuration was indium tin oxide (ITO) (150 nm)/poly(3,4-ethylenedioxy thiophene):poly(4-styrenesulfonate (PEDOT:PSS) (35 nm)/spin-coated EML (20 nm)/1,3,-bis(3,5dipyrid-3-yl-phenyl)benzene (BmPyPB) (50 nm)/LiF (0.7 nm)/Al (70nm). As shown in Figure 4a, PHOLEDs using BCz-nBuPh and BCz-tBuPh exhibited green emission of Ir(mppy) 3 with an emission maximum at 510 nm. Figure 4b-d present OLED characteristics, such as the current density-voltage-luminance (J-V-L) curves and efficiency-luminance curves. Table 2 summarizes the OLED performance. Devices with BCz-tBuPh exhibited maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) values of 43.1 cd/A, 40.0 lm/W, and 12.5%, respectively; devices with BCz-nBuPh exhibited values of 30.8 cd/A, 28.1 lm/W, and 8.8%, respectively. Moreover, devices with BCz-tBuPh exhibited a high current efficiency of 42.8 cd/A and 38.8 cd/A at 100 and 1000 cd/m 2 of the display-relevant luminance range, respectively. The efficiency of BCz-tBuPh was lower than the highest reported efficiency in solution-processed green PHOLEDs, but comparable to the efficiency of devices based on several newly synthesized host materials [18,27].  As can be seen in Figure 4c,d, BCz-tBuPh exhibited superior efficiency compared to BCz-nBuPh at all degrees of luminance. This result could be attributed to a difference in effective transfer from the host to dopant. As BCz-tBuPh with t-butyl groups did not exhibit aggregation emission in the film PL spectrum, the material served a better host role in OLED devices compared to BCz-nBuPh. Further device works, such as the reports of other host materials [28,29], will be reported elsewhere.
NMR spectra were recorded on a Bruker AVANCE II 400 spectrometer (Billerica, MA, USA). UV-vis spectra were obtained using a UV-Vis spectrophotometer (UV-3150, Shimadzu, Kyoto, Japan). Photoluminescence (PL) spectra were obtained using a F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan). Films of small molecules used for the UV-vis and PL measurements were prepared via solution spin-coating. Optical energy band gaps (E g ) were estimated from the absorption onset wavelengths (E g (eV) = 1240/λ onset ) of the small molecule films. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed with a differential scanning calorimeter (DSC 1, Mettler Toledo, Columbus, OH, USA) and TGA Q500 (TA instruments, New Castle, DE, USA), respectively. Cyclic voltammetry (CV) data was obtained with a BAS 100B electrochemical analyzer (Bioanalytical Systems, West Lafayette, IN, USA).

3,3 -bicarbazole (BCz)
Carbazole (12.0 g, 72.0 mmol) and anhydrous FeCl 3 (47.0 g, 288 mmol) were dissolved in 240 mL of chloroform and stirred for 30 min under nitrogen. The solution was subsequently poured into a larger quantity of methyl alcohol. The precipitated green powder and zinc powder (18.7 g, 288 mmol) were dissolved in a solution of acetic acid and ethyl acetate (180 mL, 1:1) under nitrogen. The mixture was heated at 50 • C overnight. The reaction mixture was filtered and washed with hot ethyl acetate. The filtered solution was poured into water and extracted with ethyl acetate. The organics were combined, washed with brine, and dried with sodium sulfate. After solvent removal, 5.94 g of a brown powder was obtained. This crude product was purified via vacuum sublimation. Obtained as a while solid was 2.53 g (8.0 mmol, 22%) of BCz. 1  3.2.2. 9,9 -di-4-n-butylphenyl-9H,9 H-3,3 -bicarbazole (BCz-nBuPh) 3,3 -Bicarbazole (1.5 g, 4.5 mmol), 1-n-butyl-4-iodobenzene (2.5 mL, 18 mmol), copper powder (1.70 g, 27.1 mmol), and K 2 CO 3 (3.73 g, 27.1 mmol) were dissolved in 10 mL of anhydrous DMF under nitrogen. After heating the mixture at 145 • C overnight, the reaction solution was filtered through a plug of celite. The filtered solution was poured into water and extracted with chloroform. The organics were combined, washed with brine, and dried with sodium sulfate. After solvent removal, the product was purified via column chromatography with 1:1 (v/v) dichloromethane-hexane as the eluent to yield 1.41 g (2.36 mmol, 52%) of BCz-nBuPh as a white solid. For OLED devices, the product was further purified twice via vacuum sublimation. The sublimation yield was 33%. Mp 146 • C. 1  3,3 -Bicarbazole (1.0 g, 3 mmol), 1-t-butyl-4-iodobenzene (2.06 mL, 11.5 mmol), copper powder (1.1 g, 17 mmol), and K 2 CO 3 (2.4 g, 17 mmol) were dissolved in 10 mL of anhydrous DMF under nitrogen. After heating the mixture at 145 • C overnight, the reaction solution was filtered through a plug of celite. The filtered solution was poured into water and extracted with chloroform. The organics were combined, washed with brine, and dried with sodium sulfate. After solvent removal, the product was purified via column chromatography with 1:2 (v/v) chloroform-hexane as the eluent to yield 1.23 g (2.01 mmol, 69%) of BCz-tBuPh as a white solid. For OLED devices, the product was further purified twice via vacuum sublimation. The sublimation yield was 74%. Mp 295 • C. 1

Device Fabrication and Characterization
Pre-patterned indium tin oxide (ITO) glass substrates were cleaned via ultrasonication with acetone, isopropyl alcohol, and deionized water. The substrates were treated with ultraviolet-ozone for 20 min using a UV-ozone cleaner (AH1700, AHTECH LTS Co., Ltd., Anyang, Gyeonggi, Korea). The poly(3,4-ethylenedioxy thiophene):poly(4-styrenesulfonate) (PEDOT:PSS, AI 4083, Heraeus, Hanau, Germany) solution was filtered through a 0.45 µm PVDF syringe filter and spin-coated at 4000 rpm for 40 s to form the hole-injection layer (HIL) onto ITO substrates. The PEDOT:PSS-coated substrates were placed onto a hotplate at 150 • C for 30 min under air. Prior to depositing the emissive layer (EML), a 1 wt. % chlorobenzene solution was prepared in the weight ratio of 61:9:24:6 for BCz-nBuPh:TPD:PBD:Ir(mppy) 3 and BCz-tBuPh:TPD:PBD:Ir(mppy) 3 and stirred at 70 • C for 1 h. The solutions were subsequently filtered through a 0.45 µm PTFE syringe filter; the EML was spin-coated onto the HIL-coated substrate at 2500 rpm for 60 s under nitrogen. Thereafter, the electron-transporting layer of BmPyPB, the electron injection layer of LiF, and the Al cathode were deposited via thermal evaporation in a vacuum chamber under 5 × 10 −7 torr. The OLED devices were encapsulated with a glass lid, a moisture getter, and epoxy glue in a glove box. The current-voltage-luminance characteristics of the OLEDs were obtained with a Keithley 2400 source meter (Cleveland, OH, USA) and a Konica Minola CS-2000 spectroradiometer (Tokyo, Japan) in air.

Conclusions
We synthesized 3,3 -bicarbazole-based molecules, BCz-nBuPh and BCz-tBuPh, for phosphorescent OLED host materials through an Ullmann coupling reaction between 3,3 -bicarbazole and iodobenzne with n-butyl and t-butyl groups. BCz-nBuPh exhibited a low melting point of 146 • C compared to 295 • C for BCz-tBuPh. Molecular aggregation emission was observed in the PL spectra of the BCz-nBuPh films. BCz-tBuPh did not exhibit aggregation emission in the solid state and was more suitable as a host material in solution-processed EMLs compared to BCz-nBuPh, exhibiting a maximum current efficiency of 43.1 cd/A with a high current efficiency of 38.8 cd/A at 1000 cd/m 2 for green PHOLEDs.