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Balancing charge-transporting characteristics in bipolar host materials
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Balancing charge-transporting characteristics in bipolar host materials Kyu Sung Kim , Dong Uk Kim , Kuk Soung Joung , Jae-Woong Yu PII: DOI: Reference:
S0040-6090(19)30806-5 https://doi.org/10.1016/j.tsf.2019.137781 TSF 137781
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
12 June 2019 27 November 2019 29 December 2019
Please cite this article as: Kyu Sung Kim , Dong Uk Kim , Kuk Soung Joung , Jae-Woong Yu , Balancing charge-transporting characteristics in bipolar host materials, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137781
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Highlights •
Bipolar host materials with balanced charge transporting property were synthesized.
•
The optimum bipolar host showed well enhanced device performances.
•
Charge balance is a function of alkyl chain length and internal compactness.
•
Morphology of the device with bipolar host materials was maintained.
Balancing charge-transporting characteristics in bipolar host materials
Kyu Sung Kima,b, Dong Uk Kima, Kuk Soung Joungb, Jae-Woong Yua,* a
Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1732 Deogyeong-daro, Giheung-gu, Yongin, Gyeonggi, 446-701, Korea b
R&D Center, Byucksan Paint & Coatings Co., Ltd, Incheon 404-310, Korea
Bipolar host materials with balanced charge transporting property were synthesized using a solvent-less green reaction method. The characteristics of these synthesized bipolar host materials were examined by thermal and spectroscopic analysis. The energy levels of these materials were estimated from cyclic voltammograms and absorption spectra. The optimized molecular geometries and spatial distributions were obtained from molecular simulations. Moreover, the current density vs. voltage features of single-carrier devices were investigated to assess the bipolar transport characteristics of the host materials. As the length of the alkyl chain increased, the electron-transporting capability and hole-transporting ability exhibited optimum values at the octyl chain attachment. The current efficiency, power efficiency and quantum efficiency values of white organic light-emitting diodes prepared by blending blue and yellow iridium phosphors with these bipolar hosts were high with decreasing alkyl chain length. The result was remarkably similar to the trend of current density characteristics of single-carrier devices. The power efficiency of the octyl chain attached bipolar host was approximately three times higher than that of typical blended hole and electron transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer by the bipolar host material inside an emissive layer. Moreover, the morphology of the device fabricated with a blend of charge transport materials changed due to deterioration, whereas that of the device fabricated with bipolar host materials did not change after 12 h of operation at 9 volts.
Key words: Bipolar host, balanced charge transport, white organic light emitting diodes, morphology change
Corresponding authors: [email protected] (J.-W. Yu)
1. Introduction The use of white organic light emitting diodes (WOLEDs) as a lighting source has received growing attention as the next-generation lighting [1-7]. The production cost of WOLEDs should be reduced to improve the price competitiveness against a conventional lighting device. OLED devices prepared via a conventional vacuum process show improved efficiency; however, the equipment for this vacuum process is very expensive. One of the most probable methods to reduce the production cost is utilization of a solution process. The process of forming a single organic active layer by mixing electrons and hole transporting materials combined with active materials is used in the soluble OLED process worldwide. The lifetime and efficiency of the OLED device with a single active layer are drastically reduced if the organic active layer containing various materials is phase separated by deterioration, resulting from its continuous usage [8,9], or if inhomogeneities occur due to the relative miscibility difference [10,11]. Thus, utilization of a bipolar host material both reduces the numbers of mixed material and helps maintain the device stability [8]. A bipolar host containing both hole- and electron-transporting moieties facilitates the charge balance in the emitting layer, resulting in broad charge recombination zones [12], which reduces the triplet-triplet annihilation and leads to high efficiency and decreased efficiency roll-off [13,14]. Recently, OLEDs using phosphorescence have been employed worldwide as a lighting application because of their excellent efficiency. Bipolar hosts for solutionprocessable phosphorescence OLEDs require large energy gaps, high charge carrier mobility for both electrons and holes, high thermal stability and high solubility. The hosts with carbazole moieties have been extensively used in phosphorescence OLEDs because of their good hole mobility and high intrinsic triplet energy (~ 3.02 eV) [15]. OLED lighting usually
employs white light, which is obtained by mixing blue and yellow emitters [16] or mixing blue, green and red emitters [17]. Thus, the bipolar host material must exhibit good transport properties for various band gaps [18-20]. In this study, solution-processable bipolar host materials were synthesized, and the charge-transporting properties for the hole and electron were controlled by attaching different sizes of alkyl groups, thus changing the molecular spatial geometries. WOLEDs were fabricated through a solution process. The device characteristics demonstrated that the host materials
with
balanced
charge-transporting
properties
exhibits
enhanced
electroluminescence properties.
2. Experimental details 2-1. Synthesis 2-1-1. Synthesis of 9H-thioxanthene-9-one-S,S-dioxide (1) Hydrogen peroxide (35% aqueous solution, 8 g, 235 mmol) was added to a solution of thioxanethene-9-one (25 g, 118 mmol) in acetic acid (400 mL) at room temperature. The resulting mixture was placed under reflux for 2 h and then cooled to room temperature to yield a precipitate, which was filtered and washed with n-hexane (400 mL) to produce yellow crystal. (Yield : 93 %) 1H-NMR (300 MHz, CDCl3, δ): 8.35(dd, J=7.5 Hz. J=1.5 Hz, 2H), 8.19 (dd, J=7.5 Hz. J=1.5 Hz, 2H), 7.88 (td, J=8 Hz, J=1.5 Hz, 2H), 7.78 (td, J=8 Hz, J=1.5 Hz, 2H) 2-1-2. Synthesis of 9-(2-ethylhexyl)-9H-carbazole (2)
Carbazole (5 g, 30 mmol) in THF (100 mL) was stirred in a two-necked flask. The reaction flask was cooled to 0 °C and sodium hydride (60 % dispersion in mineral oil, 2.4 g, 60 mmol) was added gradually. The entire solution was stirred at this temperature for 30 min, and then added with a solution of 2-ethylhexyl bromide (6.9 g, 36 mmol) in nitrogen atmosphere. The resulting reaction mixture was stirred at 80 °C for 12 h. When the reaction was completed, the reaction mixture was diluted with ethyl acetate and water. Then, the organic layer was extracted and washed with water and brine solution. Finally, the organic layer was dried under reduced pressure. The crude compound was purified by column chromatography on silica gel eluted with dichloromethane/n-hexane to achieve 2 as white solid. (Yield : 87 %) 1H-NMR (300 MHz, CDCl3, δ): 8.15 (td, J= 3 Hz, 2H), 7.53-7.42 (m, 4H), 7.30-7.24 (m, 2H), 4.21 (dd, J= 3 Hz, 2H), 2.16-2.08 (m, 1H), 1.49-1.28 (m, 8H), 0.98-0.89 (m, 6H) 9-(Octadecane)-9H-carbazole (3), 9-(octane)-9H-carbazole (4), and 9-(dodecane)-9Hcarbazole (5) were synthesized using a method similar to that used for 1-bromooctadecane, 1bromooctane and 1-bromododecane in the place of 2-ethylhexyl bromide. 3 : 1H-NMR (300 MHz, CDCl3, δ): 8.01 (d, J= 9 Hz, 2H), 7.42-7.30 (m, 4H), 7.17-7.11 (m, 2H), 4.21 (t, J= 9 Hz, 2H), 1.84-1.72 (m, 2H), 1.25-1.09 (m. 28H), 0.80 (t, J= 9 Hz, 3H). 4 : 1H-NMR (300 MHz, CDCl3, δ): 8.04 (d, J= 9 Hz, 2H), 7.42-7.31 (m, 4H), 7.18-7.12 (m, 2H), 4.22 (t, J= 9Hz, 2H), 1.80-1.77 (m, 2H), 1.34-1.12 (m, 10H), 0.78 (t, J= 9Hz, 3H). 5 : 1H-NMR (300 MHz, CDCl3, δ): 8.05 (d, J= 9Hz, 2H), 7.39-7.35 (m, 4H), 7.19-7.15 (m, 2H), 4.23 (t, J= 9 Hz, 2H), 1.86-1.74 (m, 2H), 1.26-1.16 (m, 16H), 0.81 (t, J= 9Hz, 3H). 2-1-3. Compounds BH1, BH2, BH3 and BH4 were synthesized using this general procedure.
9H-thioxanthene-9-one-S,S-dioxide (1): (3 g, 12 mmol) and 9-(2-ethylhexyl)-9H-carbazole (2) (7.5 g, 27 mmol) and dichloromethane (60 mL) were stirred in a three-necked flask in a nitrogen atmosphere for 1 h at 80 °C. Eaton’s reagent (8.39 mL, 61 mmol) was added dropwise to the resulting solution for a period of 30 min. The reaction mixture was stirred in a nitrogen atmosphere at 80 °C for 12 h. Then, the mixture was cooled, and methanol (500 mL) was added prior to the stirring of the mixture for another 2 h. The precipitate was collected and washed with a methanol. The crude compound was purified via column chromatography on silica gel eluted with dichloromethane/n-hexane to obtain the target compounds BH1, BH2, BH3 and BH4 with yields of 78 %, 76 %, 87 %, and 82 %, respectively. 2-1-3-1.
Synthesis
of
9,9-bis(9-(2-ethylhexyl)-9H-carbazol-3-yl)-9H-thioxanthene-S,S-
dioxide (BH1) White solid. Yield : 78 %. Mp : 146 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.28 (dd, J= 3 Hz, 2H), 7.80 (d, J= 9 Hz, 2H), 7.59-7.40 (m, 9H), 7.29 (d, J= 9 Hz, 3H), 7.22 (dd, J= 3H, 2H), 7.12 (td, J= 6 Hz, 2H), 6.95 (dd, J= 3 Hz, 2H), 4.16 (d, 6 Hz, 4H), 2.13-2.05 (m, 2H), 1.431.32 (m, 16H), 0.97-0.86 (m, 12H). 2-1-3-2. Synthesis of 9,9-bis(9-octyl-9H-carbazol-3-yl)-9H-thioxanthene-S,S-dioxide (BH2) White solid. Yield : 87 %. Mp : 153 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.33 (dd, J= 3 Hz, 2H), 7.85 (d, J= 9 Hz, 2H), 7.65-7.42 (m, 10H), 7.34 (d, J= 9 Hz, 2H), 7.28 (dd, J= 3 Hz, 2H), 7.17 (td, J= 6 Hz, 2H), 7.01 (dd, J= 3 Hz, 2H), 4.32 (t, 6 Hz, 4H), 1.98-1.89 (m, 4H), 1.451.30 (m, 20H), 0.94 (t, J= 6 Hz, 6H). 2-1-3-3. Synthesis of
9,9-bis(9-undecyl-9H-carbazol-3-yl)-9H-thioxanthene-S,S-dioxide
(BH3) White solid. Yield : 82 %. Mp : 122 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.30 (dd, J= 3 Hz, 2H), 7.80 (d, J= 9 Hz, 2H), 7.62-7.41 (m, 10H), 7.30 (d, J= 9 Hz, 2H), 7.22 (dd, J= 3 Hz), 7.13 (td, J= 4 Hz, 2H), 6.96 (dd, J= 3 Hz, 2H), 4.29 (t, 6 Hz, 4H), 1.95-1.85 (m, 4H), 1.431.27 (m, 20H), 0.90 (t, J= 6 Hz, 6H). 2-1-3-4. Synthesis of 9,9-bis(9-heptadecyl-9H-carbazol-3-yl)-9H-thioxanthene-S,S-dioxide (BH4) White solid. Yield : 76 %. Mp : 108 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.28 (dd, J= 3 Hz, 2H), 7.81 (d, J= 6 Hz, 2H), 7.62-7.39 (m, 10H), 7.30 (d, J= 9 Hz, 2H), 7.23 (dd, J= 3 Hz, 2H), 7.13 (td, J= 4 Hz, 2H), 6.96 (dd, J= 3 Hz, 2H), 4.29 (t, J= 9 Hz, 4H), 1.95-1.85 (m, 4H), 1.431.27 (m, 52H), 0.91 (t, J= 6 Hz, 6H).
2-2. Device Fabrication and Characterization A patterned indium-tin-oxide (ITO) cell with an insulator was prepared via photolithography and was cleaned sequentially using acetone, isopropyl alcohol, and deionized water. Then the cleaned ITO was treated with UV-ozone for better coating characteristics.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
[PEDOT:PSS]
(Clevios Al 4083) was spin coated on the ultraviolet (UV) ozone-treated ITO substrate for 40 s at 2,500 rpm, and then annealed in a vacuum oven for 20 min at 120 °C. The active layer
composed of 1.09 wt% of poly(N-vinyl carbazole) (PVK, MW = 25,000) and 4,4′-Bis(N-
carbazolyl)-1,1′-biphenyl (CBP) functioned as a host, and bis(3,5-difluoro-4-cyano-2-(2-
pyridyl)phenyl-(2-carboxypyridyl)
iridium(III)
(FCNIrPic)
and
Iridium(III)
bis(4-
phenylthieno[3,2-c]pyridinato-N,C2') acetylacetonate (PO-01) phosphor were dissolved in chlorobenzene to be used as emitting dopants. The blend ratio of the charge-transporting materials
(PVK:CBP)
was
1:3
(by
weight).
The
overall
blend
ratio
of
PVK:CBP:FCNIrPic:PO-01 was 19.98:59.95:19.98:0.0902. For the bipolar host system, the active layer (containing 1.09 wt% of a bipolar host based on carbazole/thioxanthene-S,S dioxide derivatives), FCNIrPic, and PO-01 phosphor as emitting dopants were dissolved in chlorobenzene. This solution was spin coated for 40 s at 1000 rpm and thermally annealed in a vacuum oven for 1 h at 100 °C. A 30nm thick 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi), 5-nm-thick lithium fluoride (LiF) and 100-nm-thick aluminum (Al) layers were sequentially deposited by thermal evaporation under high vacuum (4 ×10-3 Pa) as an electrode injection layer and cathode, respectively. The luminance characteristics (i.e., efficiency, external quantum yields, Commission Internationale de L'éclairage (CIE) coordinates, and current density) and electroluminescence spectra were measured using the Keithley 2400 source measurement unit and CS2000 spectrophotometer. All measurements were performed under ambient conditions at room temperature. The absorption spectra were obtained using a ultraviolet-visible (UV-Vis) spectrometer (SHIMADZU UV-2550). The photoluminescence spectra were determined at room temperature using the PerkinElmer LS55. The thickness of the coated film was measured with a surface profiler (TENCOR®, P-10 -step). The melting and glass transition temperatures were obtained using a differential scanning calorimeter (Perkin-Elmer DSC8000) at a heating rate of 10 °C/min-1. To measure the highest occupied molecular orbitals (HOMO) of the bipolar host materials, cyclic voltammetry was performed with the AMETECK Versa
STAT3. The HOMO value was obtained by a voltage sweep at a scan rate of 100 mV/s, where the reference electrode was Ag/AgCl and the counter electrode was a Pt wire, in an electrolyte of acetonitrile/0.1M tetrabutylammonium hexafluorophosphate (n-Bu4NPF6).
3. Results and discussion The bipolar hosts composed of 9,9-Bis(9-alkylcarbazole)thioxanthene-S,S-dioxide were synthesized by a solvent-less green reaction method [21]. Here, the alkyls were ethylhexyl (BH1), octyl (BH2), dodecyl (BH3), and heptadecane (BH4) and were synthesized by multi-step reactions, as shown in Fig. 1. The thioxanthene-S,S-dioxide moiety possessed good electron-transporting ability, whereas the bis-alkylcarbazole moiety has good holetransporting ability. The thermal properties of each compound were investigated by thermal gravimetric analysis (TGA) in a nitrogen (N2) atmosphere at a scanning rate of 10 °C min-1. The excellent thermal stability of the compounds was observed by their high decomposition temperatures (Td, corresponding to 5% weight loss) higher than 400 °C, as summarized in Table 1. TGA analysis of the host materials is given in Fig. 2. The high thermal stability of these host materials can prolong the lifetime of the devices an extended usage. All the four compounds exhibited quite similar thermal characteristics. The thermal analysis using differential scanning calorimetry (DSC) is shown in Fig. 3. The glass transition temperatures of the host materials with short aliphatic chains (BH1 and BH2) were higher than the possible highest device temperature (the temperature that could be reached by operating the OLED devices for extended time), while those for long aliphatic chains (BH3 and BH4) were lower
than the possible highest device temperature. The only differences were the dangling aliphatic chains, which modified the solubility of the compounds but not their basic characteristics. The UV-Vis absorption and fluorescence spectra of BH1 through BH4 at room temperature showed absorption peaks approximately 250, 270, and 300 nm, and emission peak was approximately 340 nm, as summarized in Table 1. Absorption and luminescence spectra of the host materials is shown in Fig. 4. These peaks originated from the benzene rings and carbazole-centered n−π* transition. To investigate the oxidative electrochemical properties of the host materials, cyclic voltammograms (CV) were obtained for the film deposited on ITO, as shown in Fig. 5. The oxidation peaks are located at about 1.35 V vs. silver/silver chloride (Ag/AgCl), which corresponds to the HOMO level of ~ 5.94 eV. The lowest unoccupied molecular orbitals (LUMO) levels were calculated by combining the oxidation potential from CV and the band gap estimated from the band edge of the absorption spectrum. Table 1 summarized all these properties. As observed in the table, the energy levels (HOMO, LUMO, and band gap) of these compounds were quite similar, indicating that the basic optoelectronic property of the compounds was not changed by substituent aliphatic chains. The molecular geometry of the bipolar host (i.e., molecular distance between the electron-donating and electron-withdrawing moieties) will determine the hole- and electrontransporting properties. Given that the bipolar host compound required a balanced hole- and electron-transporting property, the hole- and electron-transporting properties of these bipolar compounds were controlled by attaching different sizes of alkyl chains to the electrondonating moiety. Thus, the molecular distance between the electron-donating and electronwithdrawing moieties could be adjusted. The optimized molecular geometries and calculated spatial distributions of HOMOs and LUMOs of the host compounds were calculated using
the Gaussian 03 program. The HOMO surfaces of the host materials were mostly localized through the carbazole moiety, whereas the LUMO surfaces were localized over the thioxanthene-S,S-dioxide moiety as shown in Fig. 6. This was caused by the electrondonating property of carbazole moiety and the electron-withdrawing property of the thioxanthene-S,S-dioxide moiety, resulting in good hole- and electron-transporting properties of these bipolar hosts. The bipolar host with only thioxanthene-S,S-dioxide moiety and carbazole moiety (BH0) has an end-to-end distance of 1.67 nm. The end-to-end distance of the bipolar host material rapidly increased with increasing length of the alkyl chains. These long alkyl chains attached to the carbazole moieties reduced inter-chain interactions, minimized crystallization problems, and aided in the uniformity of the organic layer in the devices. When the alkyl chain length was from 8 to 12 and 17, the end-to-end distance of the bipolar host material shifted from 1.93 nm
to 3.30 and 3.71 nm. Both BH1 (ethylhexyl
group attached) and BH2 (octyl group attached) had equal carbon number in their alkyl group. However, the ethylhexyl group is a branched alkyl chain, whereas an octyl group is a linear alky chain. Therefore, the end-to-end distance of BH1 (2.34 nm)is the greater than that of BH2 (1.93 nm). To evaluate the charge-transporting properties of the hole and electron of these synthesized bipolar host compounds, single-carrier devices were fabricated. Fig. 7 shows the J-V characteristics of the hole-only and electron-only devices for all bipolar host compounds. The structures of the hole-only and electron-only devices were as follows: ITO/PEDOT:PSS (35 nm)/NPB (10 nm)/host (40 nm)/NPB (10 nm)/Al (100 nm) and ITO/PEDOT:PSS (35 nm)/4,40-bis(carbazole-9-yl)biphenyl (BCP) (10 nm)/host (40 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm), respectively. The current density-voltage (J-V) characteristics of these devices showed continuously increasing current density, which increases with an increase in the voltage of both holes and electrons. Therefore, these host compounds possessed good
bipolar carrier-transport ability. Notice that the BH2-based devices demonstrated the highest balanced bipolar carrier transportation characteristics among the other host materials. Based on the molecular orbital simulation results, BH2 (hexyl chain attached) showed the most compact molecular structure. Therefore, this host material would have better electrontransporting properties than the other host materials because inter-molecular interactions were predicted to be the strongest among the synthesized host materials. As the number of substituted alkyl chains increases (8 carbons for BH1 and BH2, 12 carbons for BH3, 17 carbons for BH4), the absolute value of the current density decreases in electron-only devices. The differences in the current density between the hole- only and the electron-only devices for all the host materials were also observed to increase with an increasing number of alkyl chains. These single-carrier experimental results showed that the electron-transporting capability decreases and the hole-transporting ability improves as the alkyl chain length is increased. When the number of alkyl chains is equal (8 carbons for BH1 and BH2), the linear structure (BH2 : octyl chain substituted) showed a higher current density in the electron-only device than the iso-structure (BH1 : ethylhexyl chain substituted). No significant difference was observed in the current density of the hole-only device between the two devices. The molecular structure of the linear alkyl chain substituted host molecule was more compact than that of the iso-structure with same carbon number, resulting in superior electron transport capability. The estimated end-to-end distance using the Gaussian 03 program was determined to be 2.34 nm for BH1 (iso-structure), whereas a considerably compacted geometry was observed for BH2 (1.93 nm, linear structure). Based on the single carrier device current density data only, we expected that BH2 (possessing the shortest linear alkyl chain) would have the best bipolar host characteristics. To confirm further the bipolar character of these host materials, the carrier mobilities were calculated by fitting the J-V curve of the single carrier devices to the space charge limited
current model using following equation [22,23]: 9 𝑉2 𝐽 = ( ) ε0 𝜀𝑟 𝜇 3 8 𝐿 where J is current density, ε0 is the permittivity in free space and εr is the relative permittivity of the material, is the mobility, V is applied voltage, and L is thickness of the organic material. The procedures of the calculations are as follows: Fist, plot J vs V in its each log scale, and then find a voltage value where a slope is rapidly changing. Use the corresponding applied voltage minus 0.5 V (the built-in potential between ITO and Al) to V. Calculate the mobility by substituting the current density value of that point to J. In this calculation, the value for vacuum permittivity was 8.85 x 10-14 A·sec/cm·V and the value for relative permittivity was 3 (which widely used as a value for the typical polymer). The electron and hole mobilities of BH0 were approximately 2.3 x 10-6 cm2/V•s and 3.4 x 10-6 cm2/ V•s, respectively. The electron mobilities of the host materials were approximately 1.5 x 10-6 cm2/V•s for BH1, 1.0 x 10-5 cm2/ V•s for BH2, 2.9 x 10-6 cm2/ V•s for BH3, 2.9 x 10-7 cm2/ V•s for BH4. Meanwhile, the hole mobilities were approximately 7.2 x 10-6 cm2/ V•s for BH1, 3.6 x 10-5 cm2/ V•s for BH2, 2.9 x 10-5 cm2/ V•s for BH3, and 1.4 x 10-5 cm2/ V•s for BH4. These carrier mobility data demonstrated that BH2 exhibits a superior and good balance of charge transport properties, which was the same result obtained from a molecular simulation. From these single carrier device results, the charge mobilities of the bipolar host could be controlled evidently by adjusting the molecular distance between the electron-donating and electron-withdrawing moieties. In WOLED application, many components are blended together to produce white light, indicating that phase morphology control is crucial. If a bipolar host is used in this
WOLED, than the host components that used to fabricate this WOLED can be minimized. To verify the usefulness of these synthesized bipolar hosts, the WOLEDs were prepared. The synthesized bipolar host contains the carbazole moiety as a hole-transporting moiety. Considering that PVK is a polymerized carbazole, using it as a hole-transporting host is feasible for better comparison. Generally, blending CBP with the PVK host improves the quantum efficiency of the device by moderate electron transport properties of the CBP host (despite CBP is hole-transport-type host material). For these reasons, PVK and CBP were used as hole-transporting hosts for the preparation of WOLED. Fig. 8 shows the energy diagrams of all the materials used in this study. Efficient energy transfer and exciton blocking were obtained because of the suitable energy levels of the synthesized host materials. The HOMO and LUMO of the host materials were quite matched for PEDOT and TPBi; thus, efficient charge carrier injection into the emitting layer was attained. A reference OLED was fabricated with a device structure of ITO/PEDOT:PSS (40 nm)/active layer (40 nm)/TPBI (30 nm)/LiF (5 nm)/Al (100 nm). Here the ratio of PVK:CBP was 1:3 (by weight) and the overall blend ratio of the active layer (PVK:CBP:FCNIrPic:PO01) was 19.98:59.95:19.98:0.0902. Meanwhile, OLED devices with bipolar hosts were fabricated with a device structure of ITO/PEDOT:PSS (40 nm)/active layer (35 nm)/TPBi (30 nm)/LiF (5 nm)/Al (100 nm). Here the overall blend ratio of the active layer (bipolar host:FCNIrPic:PO-01) was 79.92:19.98:0.0902. PEDOT:PSS and LiF were employed as hole- and electron-injecting layers, respectively. TPBi was used as an electron-transporting layer. To construct a similar luminescent environment, the factors were maintained equal except for the charge transfer materials. Fig. 9 shows the white EL properties of the devices: luminescence versus voltage and current density versus voltage. Fig. 10 shows the external quantum efficiency versus luminance (a), the current efficiency versus luminance (b), and the power efficiency versus luminance (c). Table 2 summarizes the electroluminescence
characteristics of the white OLED devices. The devices based on PVK:CBP exhibited a maximum current efficiency (CEmax) of 5.24 cd/A, a maximum power efficiency (PEmax) of 2.49 lm/W, and a maximum quantum efficiency (EQEmax) of 2.07 % with CIE coordinates of (0.30, 0.38). BH0 obtained CEmax of 8.66 cd/A, PEmax of 3.26 lm/W, and EQEmax of 2.63 % with CIE coordinates of (0.36, 0.44). BH1 showed CEmax of 5.23 cd/A, PEmax of 2.16 lm/W, and EQEmax of 2.52 % with CIE coordinates of (0.23, 0.33). BH2 exhibited CEmax of 16.04 cd/A, PEmax of 8.69 lm/W, and EQEmax of 6.52 % with CIE coordinates of (0.29, 0.36), BH3 showed CEmax of 13.95 cd/A, PEmax of 7.30 lm/W, and EQEmax of 5.61 % with CIE coordinates of (0.29, 0.35). Meanwhile,
BH4 obtained CEmax of 7.17 cd/A, PEmax of 3.13
lm/W, and EQEmax of 3.90 % with CIE coordinates of (0.22, 0.33). We studied the influence of using bipolar host in WOLEDs using solution process. Blue and yellow iridium complexes were used as the emissive materials. Since this study aiming for lighting application, the solution process was used. The obtained WOLED devices performances were slightly lower than the other reporting using multi-layer process [24], but better than the performance of printable OLED [25]. All of the WOLED devices prepared with bipolar host demonstrated better performance than that with the PVK:CBP blend. Given that the synthesized bipolar hosts contain a carbazole moiety as a hole transporting property, PVK (polymerized carbazole) should be employed as the hole-transporting material for comparison. CBP was combined with PVK to enhance the quantum efficiency of the device [26,27]. The bipolar host with thioxanthene-S,S-dioxide moiety only and the carbazole moiety (BH0) exhibited an approximately 20 % enhancement in the quantum efficiency. However, other bipolar hosts with the bis-alkylcarbazole moiety showed further enhancement. These results prove the superiority of the synthesized bipolar hosts. When the same host ratio was used, the color coordinates of BH2 (0.29, 0.36) and BH3 (0.29, 0.36) were similar to that of the reference
device that utilizes the PVK:CBP blend (0.30, 0.38), whereas the power efficiency of bipolar hosts was approximately three times higher. EL spectra of the host materials are shown in Fig. 11. BH2 showed the balanced blue and yellow emission at lowest external bias among all host materials. In addition, the maximum current efficiency, power efficiency, and quantum efficiency values were high with decreasing alkyl chain length, which was remarkably similar to the trend of the current density data of single-carrier devices. The performance of the devices using the bipolar host with an ethylhexyl group attached (BH1) exhibited a minimum value. BH1 (ethylhexyl group attached) and BH2 (octyl group attached) contain the same number of carbon (i.e., 8) in their alkyl group, whereas BH3 and BH4 comprise dodecyl (11 carbons) and heptadecane (17 carbons) chain, respectively. The calculated end-to-end distance of BH1 is 2.34 nm which is smaller than those of BH3 (3.30 nm) and BH4 (3.71 nm). According to the molecular geometries of bis-alkylcarbazole moiety in their spatial distributions, the iso-structure ethylhexyl group in BH1 requires a large internal spacing to ensure that it causes a tilting of two alkylcarbazole moiety each other. Therefore, the device performance was not a simple function of the alkyl chain length nor the end-to-end distance. Interestingly, BH2 shows outstanding device efficiency among the four bipolar host materials, as shown in Fig. 7. As observed in the figure, CEmax is 16.04 cd/A, PEmax is 8.69 lm/W, and EQEmax is 6.52 % with CIE coordinates of (0.29, 0.36). This higher efficiency could be attributed to the well-balanced charge (based on the calculated mobilities from single carrier devices, the electron and hole mobilities of BH2 were 1.0 x 10-5 cm2/V•s and 3.6 x 10-5 cm2/ V•s, respectively) and better charge transfer inside the emissive layer. This would help to broaden the exciton formation zone. Since the energy levels of bipolar hosts contains energy levels of blue and yellow emitting phosphors, the hole and electron that
injected from the injection layer were confined within the bipolar host so that the recombination zone of the hole and electron will be enlarged. It can also be attributed to the balanced charge transfer and improved carrier recombination ratio in the emissive layer. The high triplet energies of these hosts efficiently suppress the energy back transfer from the dopants to the host, resulting in the enhanced performance of these devices. All bipolar hosts have good solubility in organic solvents due to their alkyl groups attached, which can help form films with excellent uniformity through solution processes; this formed film possesses good morphological stability. To assess the deterioration caused by continuous usage, the device morphologies were investigated using atomic force microscopy (AFM). The AFM images were captured for the reference device and the device with a BH2 bipolar host (the best performing bipolar host material) after burn-out by applying an external bias of 9 V for 12 hours. Fig. 8 shows the morphological change of both the devices as prepared and after burn-out. AFM images of the other host materials are also shown in supplementary data (see SFig. 5). All other host materials also showed similar trend as that for BH2. The root mean square roughness of the reference devices changed from 0.3 nm (as prepared) to 0.7 nm (after burn-out), whereas the devices with BH2 remained approximately 0.3 nm for both as prepared and after burn-out. From these morphological results, we concluded that the morphology of the device fabricated with the blend of charge transport materials (PVK and CBP) changes due to deterioration, whereas that of the device fabricated with bipolar host materials did not change despite the deterioration of the device. These experiments demonstrated the effect of using bipolar host materials, which both reduce the types of mixed material in the active layer and help maintain the stability of the device.
4. Conclusions The bipolar host materials demonstrated excellent thermal stability based on the TGA and DSC measurements. All four host materials showed quite similar thermal characteristics. The HOMO level of the host materials was obtained from the oxidation potential measured using CVs, whereas the LUMO level was calculated from the band gap estimated from the band edge of the absorption spectrum. The optimized molecular geometries and spatial distributions were obtained using the Gaussian 03 program. The HOMO surfaces were mostly localized through the electron-donating carbazole moiety. Meanwhile, the LUMO surfaces were localized over the electron-withdrawing thioxantheneS,S-dioxide moiety. The end-to-end distance of the host materials increased with increasing alkyl chain length. Single-carrier devices were fabricated, and then the J-V characteristics were evaluated to estimate the bipolar transporting characteristics of the host materials. The current density rose smoothly with the bias increase for both the holes and electrons. As the length of the alkyl chain increased, the electron-transporting capability decreased, but the hole-transporting ability improved. The hexyl chain attached host showed an excellent balance of the hole- and electron-transporting capability. By evaluating the molecular geometries, it can be concluded that the device performance was not a simple function of the alkyl chain length nor the end-to-end distance. The current efficiency, power efficiency, and quantum efficiency values of the WOLEDs were high with decreasing alkyl chain length, which was remarkably similar to the trend of current density data of single-carrier devices. Moreover, the power efficiency of the bipolar host (BH2) was approximately three times higher than that of the blended transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer inside the emissive layer, which would aid in widening the exciton formation zone. Investigation of the morphology changes after a
prolonged continuous usage was conducted using AFM. The morphology of the device fabricated with the blend of charge transport materials was changed by deterioration, whereas that of the device fabricated with bipolar host materials remained constant. These results prove the advantages of using bipolar host materials over a blend of transporting materials to maintain the stability of the device morphology.
Acknowledgments This work was supported by the Industrial Technology Innovation Program (Grant No. 10063277, Development of pattern deposition system based on roll to roll processing under low temperature and atmospheric pressure condition for smart thin film device fabrication) funded by Ministry of Trade, Industry & Energy.
Author statement
Kyu Sung Kim : Conceptualization, Methodology, Investigation, Validation, Investigation, Writing - Original Draft
Dong Uk Kim : Conceptualization, Methodology, Investigation, Validation, Investigation, Writing Original Draft
Kuk Soung Joung : Resources, Data Curation
Jae-Woong Yu : Conceptualization, Methodology, Writing - Review & Editing, Supervision, Project administration, Funding acquisition
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Figure captions
Figure 1. Schematics of the synthesis.
Figure 2. TGA analysis of the host materials.
Figure 3. DSC diagrams of the host materials.
Figure 4. Absorption and Luminescence spectra of the host materials.
Figure 5. Cyclic voltammograms of the host materials.
Figure 6. Optimized molecular geometries and calculated spatial distributions of the HOMOs and LUMOs of the host materials using Gaussian 03 program.
Figure 7. J-V characteristics of the hole-only and electron-only devices for the host materials.
Figure 8. Energy diagram of the materials used in this study.
Figure 9. Luminescence-voltage and Current density-voltage characteristics of the white OLED devices.
Figure 10. (a) External Quantum efficiency vs Luminescence, (b)Current Efficiency vs Luminance, and (c) Power Efficiency vs Luminance characteristics of the white OLED devices.
Figure 11. EL and normalized EL spectra of the host materials.
Figure 12. AFM images for the reference device and bipolar host devices.
Table 1. Optical properties of bipolar host materials
UVmax (nm) BH1 BH2 BH3 BH4
303,339, 353 302,338, 353 303,338, 352 303,339, 352
PLmax (nm)
HOMO (eV)
LUMO (eV)
Band gap (eV)
TD (oC)
Tg (oC)
378
5.67
2.27
3.40
418
95
377
5.71
2.31
3.40
406
81
377
5.70
2.28
3.42
434
48
377
5.67
2.23
3.44
430
-
Table 2. Electroluminescence characteristics of the white OLED devices
PVK:CBP (1:3) BH0 BH1 BH2 BH3 BH4
Luminance (cd/m2)
Current Efficiency (cd/A)
Power Efficiency (lm/W)
Quantum Efficiency (%)
CIE
1158.2
5.24
2.49
2.07
(0.30, 0.38)
395.0 567.31 1437.9 1388.7 557.75
8.66 5.23 16.04 13.95 7.17
3.26 2.16 8.69 7.30 3.13
2.63 2.52 6.52 5.61 3.90
(0.36, 0.40) (0.23, 0.33) (0.29, 0.36) (0.29, 0.35) (0.22, 0.33)
Balancing charge-transporting characteristics in bipolar host materials Kyu Sung Kim , Dong Uk Kim , Kuk Soung Joung , Jae-Woong Yu PII: DOI: Reference:
S0040-6090(19)30806-5 https://doi.org/10.1016/j.tsf.2019.137781 TSF 137781
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
12 June 2019 27 November 2019 29 December 2019
Please cite this article as: Kyu Sung Kim , Dong Uk Kim , Kuk Soung Joung , Jae-Woong Yu , Balancing charge-transporting characteristics in bipolar host materials, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137781
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Highlights •
Bipolar host materials with balanced charge transporting property were synthesized.
•
The optimum bipolar host showed well enhanced device performances.
•
Charge balance is a function of alkyl chain length and internal compactness.
•
Morphology of the device with bipolar host materials was maintained.
Balancing charge-transporting characteristics in bipolar host materials
Kyu Sung Kima,b, Dong Uk Kima, Kuk Soung Joungb, Jae-Woong Yua,* a
Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1732 Deogyeong-daro, Giheung-gu, Yongin, Gyeonggi, 446-701, Korea b
R&D Center, Byucksan Paint & Coatings Co., Ltd, Incheon 404-310, Korea
Bipolar host materials with balanced charge transporting property were synthesized using a solvent-less green reaction method. The characteristics of these synthesized bipolar host materials were examined by thermal and spectroscopic analysis. The energy levels of these materials were estimated from cyclic voltammograms and absorption spectra. The optimized molecular geometries and spatial distributions were obtained from molecular simulations. Moreover, the current density vs. voltage features of single-carrier devices were investigated to assess the bipolar transport characteristics of the host materials. As the length of the alkyl chain increased, the electron-transporting capability and hole-transporting ability exhibited optimum values at the octyl chain attachment. The current efficiency, power efficiency and quantum efficiency values of white organic light-emitting diodes prepared by blending blue and yellow iridium phosphors with these bipolar hosts were high with decreasing alkyl chain length. The result was remarkably similar to the trend of current density characteristics of single-carrier devices. The power efficiency of the octyl chain attached bipolar host was approximately three times higher than that of typical blended hole and electron transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer by the bipolar host material inside an emissive layer. Moreover, the morphology of the device fabricated with a blend of charge transport materials changed due to deterioration, whereas that of the device fabricated with bipolar host materials did not change after 12 h of operation at 9 volts.
Key words: Bipolar host, balanced charge transport, white organic light emitting diodes, morphology change
Corresponding authors: [email protected] (J.-W. Yu)
1. Introduction The use of white organic light emitting diodes (WOLEDs) as a lighting source has received growing attention as the next-generation lighting [1-7]. The production cost of WOLEDs should be reduced to improve the price competitiveness against a conventional lighting device. OLED devices prepared via a conventional vacuum process show improved efficiency; however, the equipment for this vacuum process is very expensive. One of the most probable methods to reduce the production cost is utilization of a solution process. The process of forming a single organic active layer by mixing electrons and hole transporting materials combined with active materials is used in the soluble OLED process worldwide. The lifetime and efficiency of the OLED device with a single active layer are drastically reduced if the organic active layer containing various materials is phase separated by deterioration, resulting from its continuous usage [8,9], or if inhomogeneities occur due to the relative miscibility difference [10,11]. Thus, utilization of a bipolar host material both reduces the numbers of mixed material and helps maintain the device stability [8]. A bipolar host containing both hole- and electron-transporting moieties facilitates the charge balance in the emitting layer, resulting in broad charge recombination zones [12], which reduces the triplet-triplet annihilation and leads to high efficiency and decreased efficiency roll-off [13,14]. Recently, OLEDs using phosphorescence have been employed worldwide as a lighting application because of their excellent efficiency. Bipolar hosts for solutionprocessable phosphorescence OLEDs require large energy gaps, high charge carrier mobility for both electrons and holes, high thermal stability and high solubility. The hosts with carbazole moieties have been extensively used in phosphorescence OLEDs because of their good hole mobility and high intrinsic triplet energy (~ 3.02 eV) [15]. OLED lighting usually
employs white light, which is obtained by mixing blue and yellow emitters [16] or mixing blue, green and red emitters [17]. Thus, the bipolar host material must exhibit good transport properties for various band gaps [18-20]. In this study, solution-processable bipolar host materials were synthesized, and the charge-transporting properties for the hole and electron were controlled by attaching different sizes of alkyl groups, thus changing the molecular spatial geometries. WOLEDs were fabricated through a solution process. The device characteristics demonstrated that the host materials
with
balanced
charge-transporting
properties
exhibits
enhanced
electroluminescence properties.
2. Experimental details 2-1. Synthesis 2-1-1. Synthesis of 9H-thioxanthene-9-one-S,S-dioxide (1) Hydrogen peroxide (35% aqueous solution, 8 g, 235 mmol) was added to a solution of thioxanethene-9-one (25 g, 118 mmol) in acetic acid (400 mL) at room temperature. The resulting mixture was placed under reflux for 2 h and then cooled to room temperature to yield a precipitate, which was filtered and washed with n-hexane (400 mL) to produce yellow crystal. (Yield : 93 %) 1H-NMR (300 MHz, CDCl3, δ): 8.35(dd, J=7.5 Hz. J=1.5 Hz, 2H), 8.19 (dd, J=7.5 Hz. J=1.5 Hz, 2H), 7.88 (td, J=8 Hz, J=1.5 Hz, 2H), 7.78 (td, J=8 Hz, J=1.5 Hz, 2H) 2-1-2. Synthesis of 9-(2-ethylhexyl)-9H-carbazole (2)
Carbazole (5 g, 30 mmol) in THF (100 mL) was stirred in a two-necked flask. The reaction flask was cooled to 0 °C and sodium hydride (60 % dispersion in mineral oil, 2.4 g, 60 mmol) was added gradually. The entire solution was stirred at this temperature for 30 min, and then added with a solution of 2-ethylhexyl bromide (6.9 g, 36 mmol) in nitrogen atmosphere. The resulting reaction mixture was stirred at 80 °C for 12 h. When the reaction was completed, the reaction mixture was diluted with ethyl acetate and water. Then, the organic layer was extracted and washed with water and brine solution. Finally, the organic layer was dried under reduced pressure. The crude compound was purified by column chromatography on silica gel eluted with dichloromethane/n-hexane to achieve 2 as white solid. (Yield : 87 %) 1H-NMR (300 MHz, CDCl3, δ): 8.15 (td, J= 3 Hz, 2H), 7.53-7.42 (m, 4H), 7.30-7.24 (m, 2H), 4.21 (dd, J= 3 Hz, 2H), 2.16-2.08 (m, 1H), 1.49-1.28 (m, 8H), 0.98-0.89 (m, 6H) 9-(Octadecane)-9H-carbazole (3), 9-(octane)-9H-carbazole (4), and 9-(dodecane)-9Hcarbazole (5) were synthesized using a method similar to that used for 1-bromooctadecane, 1bromooctane and 1-bromododecane in the place of 2-ethylhexyl bromide. 3 : 1H-NMR (300 MHz, CDCl3, δ): 8.01 (d, J= 9 Hz, 2H), 7.42-7.30 (m, 4H), 7.17-7.11 (m, 2H), 4.21 (t, J= 9 Hz, 2H), 1.84-1.72 (m, 2H), 1.25-1.09 (m. 28H), 0.80 (t, J= 9 Hz, 3H). 4 : 1H-NMR (300 MHz, CDCl3, δ): 8.04 (d, J= 9 Hz, 2H), 7.42-7.31 (m, 4H), 7.18-7.12 (m, 2H), 4.22 (t, J= 9Hz, 2H), 1.80-1.77 (m, 2H), 1.34-1.12 (m, 10H), 0.78 (t, J= 9Hz, 3H). 5 : 1H-NMR (300 MHz, CDCl3, δ): 8.05 (d, J= 9Hz, 2H), 7.39-7.35 (m, 4H), 7.19-7.15 (m, 2H), 4.23 (t, J= 9 Hz, 2H), 1.86-1.74 (m, 2H), 1.26-1.16 (m, 16H), 0.81 (t, J= 9Hz, 3H). 2-1-3. Compounds BH1, BH2, BH3 and BH4 were synthesized using this general procedure.
9H-thioxanthene-9-one-S,S-dioxide (1): (3 g, 12 mmol) and 9-(2-ethylhexyl)-9H-carbazole (2) (7.5 g, 27 mmol) and dichloromethane (60 mL) were stirred in a three-necked flask in a nitrogen atmosphere for 1 h at 80 °C. Eaton’s reagent (8.39 mL, 61 mmol) was added dropwise to the resulting solution for a period of 30 min. The reaction mixture was stirred in a nitrogen atmosphere at 80 °C for 12 h. Then, the mixture was cooled, and methanol (500 mL) was added prior to the stirring of the mixture for another 2 h. The precipitate was collected and washed with a methanol. The crude compound was purified via column chromatography on silica gel eluted with dichloromethane/n-hexane to obtain the target compounds BH1, BH2, BH3 and BH4 with yields of 78 %, 76 %, 87 %, and 82 %, respectively. 2-1-3-1.
Synthesis
of
9,9-bis(9-(2-ethylhexyl)-9H-carbazol-3-yl)-9H-thioxanthene-S,S-
dioxide (BH1) White solid. Yield : 78 %. Mp : 146 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.28 (dd, J= 3 Hz, 2H), 7.80 (d, J= 9 Hz, 2H), 7.59-7.40 (m, 9H), 7.29 (d, J= 9 Hz, 3H), 7.22 (dd, J= 3H, 2H), 7.12 (td, J= 6 Hz, 2H), 6.95 (dd, J= 3 Hz, 2H), 4.16 (d, 6 Hz, 4H), 2.13-2.05 (m, 2H), 1.431.32 (m, 16H), 0.97-0.86 (m, 12H). 2-1-3-2. Synthesis of 9,9-bis(9-octyl-9H-carbazol-3-yl)-9H-thioxanthene-S,S-dioxide (BH2) White solid. Yield : 87 %. Mp : 153 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.33 (dd, J= 3 Hz, 2H), 7.85 (d, J= 9 Hz, 2H), 7.65-7.42 (m, 10H), 7.34 (d, J= 9 Hz, 2H), 7.28 (dd, J= 3 Hz, 2H), 7.17 (td, J= 6 Hz, 2H), 7.01 (dd, J= 3 Hz, 2H), 4.32 (t, 6 Hz, 4H), 1.98-1.89 (m, 4H), 1.451.30 (m, 20H), 0.94 (t, J= 6 Hz, 6H). 2-1-3-3. Synthesis of
9,9-bis(9-undecyl-9H-carbazol-3-yl)-9H-thioxanthene-S,S-dioxide
(BH3) White solid. Yield : 82 %. Mp : 122 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.30 (dd, J= 3 Hz, 2H), 7.80 (d, J= 9 Hz, 2H), 7.62-7.41 (m, 10H), 7.30 (d, J= 9 Hz, 2H), 7.22 (dd, J= 3 Hz), 7.13 (td, J= 4 Hz, 2H), 6.96 (dd, J= 3 Hz, 2H), 4.29 (t, 6 Hz, 4H), 1.95-1.85 (m, 4H), 1.431.27 (m, 20H), 0.90 (t, J= 6 Hz, 6H). 2-1-3-4. Synthesis of 9,9-bis(9-heptadecyl-9H-carbazol-3-yl)-9H-thioxanthene-S,S-dioxide (BH4) White solid. Yield : 76 %. Mp : 108 °C. 1H-NMR (300 MHz, CDCl3, δ): 8.28 (dd, J= 3 Hz, 2H), 7.81 (d, J= 6 Hz, 2H), 7.62-7.39 (m, 10H), 7.30 (d, J= 9 Hz, 2H), 7.23 (dd, J= 3 Hz, 2H), 7.13 (td, J= 4 Hz, 2H), 6.96 (dd, J= 3 Hz, 2H), 4.29 (t, J= 9 Hz, 4H), 1.95-1.85 (m, 4H), 1.431.27 (m, 52H), 0.91 (t, J= 6 Hz, 6H).
2-2. Device Fabrication and Characterization A patterned indium-tin-oxide (ITO) cell with an insulator was prepared via photolithography and was cleaned sequentially using acetone, isopropyl alcohol, and deionized water. Then the cleaned ITO was treated with UV-ozone for better coating characteristics.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
[PEDOT:PSS]
(Clevios Al 4083) was spin coated on the ultraviolet (UV) ozone-treated ITO substrate for 40 s at 2,500 rpm, and then annealed in a vacuum oven for 20 min at 120 °C. The active layer
composed of 1.09 wt% of poly(N-vinyl carbazole) (PVK, MW = 25,000) and 4,4′-Bis(N-
carbazolyl)-1,1′-biphenyl (CBP) functioned as a host, and bis(3,5-difluoro-4-cyano-2-(2-
pyridyl)phenyl-(2-carboxypyridyl)
iridium(III)
(FCNIrPic)
and
Iridium(III)
bis(4-
phenylthieno[3,2-c]pyridinato-N,C2') acetylacetonate (PO-01) phosphor were dissolved in chlorobenzene to be used as emitting dopants. The blend ratio of the charge-transporting materials
(PVK:CBP)
was
1:3
(by
weight).
The
overall
blend
ratio
of
PVK:CBP:FCNIrPic:PO-01 was 19.98:59.95:19.98:0.0902. For the bipolar host system, the active layer (containing 1.09 wt% of a bipolar host based on carbazole/thioxanthene-S,S dioxide derivatives), FCNIrPic, and PO-01 phosphor as emitting dopants were dissolved in chlorobenzene. This solution was spin coated for 40 s at 1000 rpm and thermally annealed in a vacuum oven for 1 h at 100 °C. A 30nm thick 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi), 5-nm-thick lithium fluoride (LiF) and 100-nm-thick aluminum (Al) layers were sequentially deposited by thermal evaporation under high vacuum (4 ×10-3 Pa) as an electrode injection layer and cathode, respectively. The luminance characteristics (i.e., efficiency, external quantum yields, Commission Internationale de L'éclairage (CIE) coordinates, and current density) and electroluminescence spectra were measured using the Keithley 2400 source measurement unit and CS2000 spectrophotometer. All measurements were performed under ambient conditions at room temperature. The absorption spectra were obtained using a ultraviolet-visible (UV-Vis) spectrometer (SHIMADZU UV-2550). The photoluminescence spectra were determined at room temperature using the PerkinElmer LS55. The thickness of the coated film was measured with a surface profiler (TENCOR®, P-10 -step). The melting and glass transition temperatures were obtained using a differential scanning calorimeter (Perkin-Elmer DSC8000) at a heating rate of 10 °C/min-1. To measure the highest occupied molecular orbitals (HOMO) of the bipolar host materials, cyclic voltammetry was performed with the AMETECK Versa
STAT3. The HOMO value was obtained by a voltage sweep at a scan rate of 100 mV/s, where the reference electrode was Ag/AgCl and the counter electrode was a Pt wire, in an electrolyte of acetonitrile/0.1M tetrabutylammonium hexafluorophosphate (n-Bu4NPF6).
3. Results and discussion The bipolar hosts composed of 9,9-Bis(9-alkylcarbazole)thioxanthene-S,S-dioxide were synthesized by a solvent-less green reaction method [21]. Here, the alkyls were ethylhexyl (BH1), octyl (BH2), dodecyl (BH3), and heptadecane (BH4) and were synthesized by multi-step reactions, as shown in Fig. 1. The thioxanthene-S,S-dioxide moiety possessed good electron-transporting ability, whereas the bis-alkylcarbazole moiety has good holetransporting ability. The thermal properties of each compound were investigated by thermal gravimetric analysis (TGA) in a nitrogen (N2) atmosphere at a scanning rate of 10 °C min-1. The excellent thermal stability of the compounds was observed by their high decomposition temperatures (Td, corresponding to 5% weight loss) higher than 400 °C, as summarized in Table 1. TGA analysis of the host materials is given in Fig. 2. The high thermal stability of these host materials can prolong the lifetime of the devices an extended usage. All the four compounds exhibited quite similar thermal characteristics. The thermal analysis using differential scanning calorimetry (DSC) is shown in Fig. 3. The glass transition temperatures of the host materials with short aliphatic chains (BH1 and BH2) were higher than the possible highest device temperature (the temperature that could be reached by operating the OLED devices for extended time), while those for long aliphatic chains (BH3 and BH4) were lower
than the possible highest device temperature. The only differences were the dangling aliphatic chains, which modified the solubility of the compounds but not their basic characteristics. The UV-Vis absorption and fluorescence spectra of BH1 through BH4 at room temperature showed absorption peaks approximately 250, 270, and 300 nm, and emission peak was approximately 340 nm, as summarized in Table 1. Absorption and luminescence spectra of the host materials is shown in Fig. 4. These peaks originated from the benzene rings and carbazole-centered n−π* transition. To investigate the oxidative electrochemical properties of the host materials, cyclic voltammograms (CV) were obtained for the film deposited on ITO, as shown in Fig. 5. The oxidation peaks are located at about 1.35 V vs. silver/silver chloride (Ag/AgCl), which corresponds to the HOMO level of ~ 5.94 eV. The lowest unoccupied molecular orbitals (LUMO) levels were calculated by combining the oxidation potential from CV and the band gap estimated from the band edge of the absorption spectrum. Table 1 summarized all these properties. As observed in the table, the energy levels (HOMO, LUMO, and band gap) of these compounds were quite similar, indicating that the basic optoelectronic property of the compounds was not changed by substituent aliphatic chains. The molecular geometry of the bipolar host (i.e., molecular distance between the electron-donating and electron-withdrawing moieties) will determine the hole- and electrontransporting properties. Given that the bipolar host compound required a balanced hole- and electron-transporting property, the hole- and electron-transporting properties of these bipolar compounds were controlled by attaching different sizes of alkyl chains to the electrondonating moiety. Thus, the molecular distance between the electron-donating and electronwithdrawing moieties could be adjusted. The optimized molecular geometries and calculated spatial distributions of HOMOs and LUMOs of the host compounds were calculated using
the Gaussian 03 program. The HOMO surfaces of the host materials were mostly localized through the carbazole moiety, whereas the LUMO surfaces were localized over the thioxanthene-S,S-dioxide moiety as shown in Fig. 6. This was caused by the electrondonating property of carbazole moiety and the electron-withdrawing property of the thioxanthene-S,S-dioxide moiety, resulting in good hole- and electron-transporting properties of these bipolar hosts. The bipolar host with only thioxanthene-S,S-dioxide moiety and carbazole moiety (BH0) has an end-to-end distance of 1.67 nm. The end-to-end distance of the bipolar host material rapidly increased with increasing length of the alkyl chains. These long alkyl chains attached to the carbazole moieties reduced inter-chain interactions, minimized crystallization problems, and aided in the uniformity of the organic layer in the devices. When the alkyl chain length was from 8 to 12 and 17, the end-to-end distance of the bipolar host material shifted from 1.93 nm
to 3.30 and 3.71 nm. Both BH1 (ethylhexyl
group attached) and BH2 (octyl group attached) had equal carbon number in their alkyl group. However, the ethylhexyl group is a branched alkyl chain, whereas an octyl group is a linear alky chain. Therefore, the end-to-end distance of BH1 (2.34 nm)is the greater than that of BH2 (1.93 nm). To evaluate the charge-transporting properties of the hole and electron of these synthesized bipolar host compounds, single-carrier devices were fabricated. Fig. 7 shows the J-V characteristics of the hole-only and electron-only devices for all bipolar host compounds. The structures of the hole-only and electron-only devices were as follows: ITO/PEDOT:PSS (35 nm)/NPB (10 nm)/host (40 nm)/NPB (10 nm)/Al (100 nm) and ITO/PEDOT:PSS (35 nm)/4,40-bis(carbazole-9-yl)biphenyl (BCP) (10 nm)/host (40 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm), respectively. The current density-voltage (J-V) characteristics of these devices showed continuously increasing current density, which increases with an increase in the voltage of both holes and electrons. Therefore, these host compounds possessed good
bipolar carrier-transport ability. Notice that the BH2-based devices demonstrated the highest balanced bipolar carrier transportation characteristics among the other host materials. Based on the molecular orbital simulation results, BH2 (hexyl chain attached) showed the most compact molecular structure. Therefore, this host material would have better electrontransporting properties than the other host materials because inter-molecular interactions were predicted to be the strongest among the synthesized host materials. As the number of substituted alkyl chains increases (8 carbons for BH1 and BH2, 12 carbons for BH3, 17 carbons for BH4), the absolute value of the current density decreases in electron-only devices. The differences in the current density between the hole- only and the electron-only devices for all the host materials were also observed to increase with an increasing number of alkyl chains. These single-carrier experimental results showed that the electron-transporting capability decreases and the hole-transporting ability improves as the alkyl chain length is increased. When the number of alkyl chains is equal (8 carbons for BH1 and BH2), the linear structure (BH2 : octyl chain substituted) showed a higher current density in the electron-only device than the iso-structure (BH1 : ethylhexyl chain substituted). No significant difference was observed in the current density of the hole-only device between the two devices. The molecular structure of the linear alkyl chain substituted host molecule was more compact than that of the iso-structure with same carbon number, resulting in superior electron transport capability. The estimated end-to-end distance using the Gaussian 03 program was determined to be 2.34 nm for BH1 (iso-structure), whereas a considerably compacted geometry was observed for BH2 (1.93 nm, linear structure). Based on the single carrier device current density data only, we expected that BH2 (possessing the shortest linear alkyl chain) would have the best bipolar host characteristics. To confirm further the bipolar character of these host materials, the carrier mobilities were calculated by fitting the J-V curve of the single carrier devices to the space charge limited
current model using following equation [22,23]: 9 𝑉2 𝐽 = ( ) ε0 𝜀𝑟 𝜇 3 8 𝐿 where J is current density, ε0 is the permittivity in free space and εr is the relative permittivity of the material, is the mobility, V is applied voltage, and L is thickness of the organic material. The procedures of the calculations are as follows: Fist, plot J vs V in its each log scale, and then find a voltage value where a slope is rapidly changing. Use the corresponding applied voltage minus 0.5 V (the built-in potential between ITO and Al) to V. Calculate the mobility by substituting the current density value of that point to J. In this calculation, the value for vacuum permittivity was 8.85 x 10-14 A·sec/cm·V and the value for relative permittivity was 3 (which widely used as a value for the typical polymer). The electron and hole mobilities of BH0 were approximately 2.3 x 10-6 cm2/V•s and 3.4 x 10-6 cm2/ V•s, respectively. The electron mobilities of the host materials were approximately 1.5 x 10-6 cm2/V•s for BH1, 1.0 x 10-5 cm2/ V•s for BH2, 2.9 x 10-6 cm2/ V•s for BH3, 2.9 x 10-7 cm2/ V•s for BH4. Meanwhile, the hole mobilities were approximately 7.2 x 10-6 cm2/ V•s for BH1, 3.6 x 10-5 cm2/ V•s for BH2, 2.9 x 10-5 cm2/ V•s for BH3, and 1.4 x 10-5 cm2/ V•s for BH4. These carrier mobility data demonstrated that BH2 exhibits a superior and good balance of charge transport properties, which was the same result obtained from a molecular simulation. From these single carrier device results, the charge mobilities of the bipolar host could be controlled evidently by adjusting the molecular distance between the electron-donating and electron-withdrawing moieties. In WOLED application, many components are blended together to produce white light, indicating that phase morphology control is crucial. If a bipolar host is used in this
WOLED, than the host components that used to fabricate this WOLED can be minimized. To verify the usefulness of these synthesized bipolar hosts, the WOLEDs were prepared. The synthesized bipolar host contains the carbazole moiety as a hole-transporting moiety. Considering that PVK is a polymerized carbazole, using it as a hole-transporting host is feasible for better comparison. Generally, blending CBP with the PVK host improves the quantum efficiency of the device by moderate electron transport properties of the CBP host (despite CBP is hole-transport-type host material). For these reasons, PVK and CBP were used as hole-transporting hosts for the preparation of WOLED. Fig. 8 shows the energy diagrams of all the materials used in this study. Efficient energy transfer and exciton blocking were obtained because of the suitable energy levels of the synthesized host materials. The HOMO and LUMO of the host materials were quite matched for PEDOT and TPBi; thus, efficient charge carrier injection into the emitting layer was attained. A reference OLED was fabricated with a device structure of ITO/PEDOT:PSS (40 nm)/active layer (40 nm)/TPBI (30 nm)/LiF (5 nm)/Al (100 nm). Here the ratio of PVK:CBP was 1:3 (by weight) and the overall blend ratio of the active layer (PVK:CBP:FCNIrPic:PO01) was 19.98:59.95:19.98:0.0902. Meanwhile, OLED devices with bipolar hosts were fabricated with a device structure of ITO/PEDOT:PSS (40 nm)/active layer (35 nm)/TPBi (30 nm)/LiF (5 nm)/Al (100 nm). Here the overall blend ratio of the active layer (bipolar host:FCNIrPic:PO-01) was 79.92:19.98:0.0902. PEDOT:PSS and LiF were employed as hole- and electron-injecting layers, respectively. TPBi was used as an electron-transporting layer. To construct a similar luminescent environment, the factors were maintained equal except for the charge transfer materials. Fig. 9 shows the white EL properties of the devices: luminescence versus voltage and current density versus voltage. Fig. 10 shows the external quantum efficiency versus luminance (a), the current efficiency versus luminance (b), and the power efficiency versus luminance (c). Table 2 summarizes the electroluminescence
characteristics of the white OLED devices. The devices based on PVK:CBP exhibited a maximum current efficiency (CEmax) of 5.24 cd/A, a maximum power efficiency (PEmax) of 2.49 lm/W, and a maximum quantum efficiency (EQEmax) of 2.07 % with CIE coordinates of (0.30, 0.38). BH0 obtained CEmax of 8.66 cd/A, PEmax of 3.26 lm/W, and EQEmax of 2.63 % with CIE coordinates of (0.36, 0.44). BH1 showed CEmax of 5.23 cd/A, PEmax of 2.16 lm/W, and EQEmax of 2.52 % with CIE coordinates of (0.23, 0.33). BH2 exhibited CEmax of 16.04 cd/A, PEmax of 8.69 lm/W, and EQEmax of 6.52 % with CIE coordinates of (0.29, 0.36), BH3 showed CEmax of 13.95 cd/A, PEmax of 7.30 lm/W, and EQEmax of 5.61 % with CIE coordinates of (0.29, 0.35). Meanwhile,
BH4 obtained CEmax of 7.17 cd/A, PEmax of 3.13
lm/W, and EQEmax of 3.90 % with CIE coordinates of (0.22, 0.33). We studied the influence of using bipolar host in WOLEDs using solution process. Blue and yellow iridium complexes were used as the emissive materials. Since this study aiming for lighting application, the solution process was used. The obtained WOLED devices performances were slightly lower than the other reporting using multi-layer process [24], but better than the performance of printable OLED [25]. All of the WOLED devices prepared with bipolar host demonstrated better performance than that with the PVK:CBP blend. Given that the synthesized bipolar hosts contain a carbazole moiety as a hole transporting property, PVK (polymerized carbazole) should be employed as the hole-transporting material for comparison. CBP was combined with PVK to enhance the quantum efficiency of the device [26,27]. The bipolar host with thioxanthene-S,S-dioxide moiety only and the carbazole moiety (BH0) exhibited an approximately 20 % enhancement in the quantum efficiency. However, other bipolar hosts with the bis-alkylcarbazole moiety showed further enhancement. These results prove the superiority of the synthesized bipolar hosts. When the same host ratio was used, the color coordinates of BH2 (0.29, 0.36) and BH3 (0.29, 0.36) were similar to that of the reference
device that utilizes the PVK:CBP blend (0.30, 0.38), whereas the power efficiency of bipolar hosts was approximately three times higher. EL spectra of the host materials are shown in Fig. 11. BH2 showed the balanced blue and yellow emission at lowest external bias among all host materials. In addition, the maximum current efficiency, power efficiency, and quantum efficiency values were high with decreasing alkyl chain length, which was remarkably similar to the trend of the current density data of single-carrier devices. The performance of the devices using the bipolar host with an ethylhexyl group attached (BH1) exhibited a minimum value. BH1 (ethylhexyl group attached) and BH2 (octyl group attached) contain the same number of carbon (i.e., 8) in their alkyl group, whereas BH3 and BH4 comprise dodecyl (11 carbons) and heptadecane (17 carbons) chain, respectively. The calculated end-to-end distance of BH1 is 2.34 nm which is smaller than those of BH3 (3.30 nm) and BH4 (3.71 nm). According to the molecular geometries of bis-alkylcarbazole moiety in their spatial distributions, the iso-structure ethylhexyl group in BH1 requires a large internal spacing to ensure that it causes a tilting of two alkylcarbazole moiety each other. Therefore, the device performance was not a simple function of the alkyl chain length nor the end-to-end distance. Interestingly, BH2 shows outstanding device efficiency among the four bipolar host materials, as shown in Fig. 7. As observed in the figure, CEmax is 16.04 cd/A, PEmax is 8.69 lm/W, and EQEmax is 6.52 % with CIE coordinates of (0.29, 0.36). This higher efficiency could be attributed to the well-balanced charge (based on the calculated mobilities from single carrier devices, the electron and hole mobilities of BH2 were 1.0 x 10-5 cm2/V•s and 3.6 x 10-5 cm2/ V•s, respectively) and better charge transfer inside the emissive layer. This would help to broaden the exciton formation zone. Since the energy levels of bipolar hosts contains energy levels of blue and yellow emitting phosphors, the hole and electron that
injected from the injection layer were confined within the bipolar host so that the recombination zone of the hole and electron will be enlarged. It can also be attributed to the balanced charge transfer and improved carrier recombination ratio in the emissive layer. The high triplet energies of these hosts efficiently suppress the energy back transfer from the dopants to the host, resulting in the enhanced performance of these devices. All bipolar hosts have good solubility in organic solvents due to their alkyl groups attached, which can help form films with excellent uniformity through solution processes; this formed film possesses good morphological stability. To assess the deterioration caused by continuous usage, the device morphologies were investigated using atomic force microscopy (AFM). The AFM images were captured for the reference device and the device with a BH2 bipolar host (the best performing bipolar host material) after burn-out by applying an external bias of 9 V for 12 hours. Fig. 8 shows the morphological change of both the devices as prepared and after burn-out. AFM images of the other host materials are also shown in supplementary data (see SFig. 5). All other host materials also showed similar trend as that for BH2. The root mean square roughness of the reference devices changed from 0.3 nm (as prepared) to 0.7 nm (after burn-out), whereas the devices with BH2 remained approximately 0.3 nm for both as prepared and after burn-out. From these morphological results, we concluded that the morphology of the device fabricated with the blend of charge transport materials (PVK and CBP) changes due to deterioration, whereas that of the device fabricated with bipolar host materials did not change despite the deterioration of the device. These experiments demonstrated the effect of using bipolar host materials, which both reduce the types of mixed material in the active layer and help maintain the stability of the device.
4. Conclusions The bipolar host materials demonstrated excellent thermal stability based on the TGA and DSC measurements. All four host materials showed quite similar thermal characteristics. The HOMO level of the host materials was obtained from the oxidation potential measured using CVs, whereas the LUMO level was calculated from the band gap estimated from the band edge of the absorption spectrum. The optimized molecular geometries and spatial distributions were obtained using the Gaussian 03 program. The HOMO surfaces were mostly localized through the electron-donating carbazole moiety. Meanwhile, the LUMO surfaces were localized over the electron-withdrawing thioxantheneS,S-dioxide moiety. The end-to-end distance of the host materials increased with increasing alkyl chain length. Single-carrier devices were fabricated, and then the J-V characteristics were evaluated to estimate the bipolar transporting characteristics of the host materials. The current density rose smoothly with the bias increase for both the holes and electrons. As the length of the alkyl chain increased, the electron-transporting capability decreased, but the hole-transporting ability improved. The hexyl chain attached host showed an excellent balance of the hole- and electron-transporting capability. By evaluating the molecular geometries, it can be concluded that the device performance was not a simple function of the alkyl chain length nor the end-to-end distance. The current efficiency, power efficiency, and quantum efficiency values of the WOLEDs were high with decreasing alkyl chain length, which was remarkably similar to the trend of current density data of single-carrier devices. Moreover, the power efficiency of the bipolar host (BH2) was approximately three times higher than that of the blended transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer inside the emissive layer, which would aid in widening the exciton formation zone. Investigation of the morphology changes after a
prolonged continuous usage was conducted using AFM. The morphology of the device fabricated with the blend of charge transport materials was changed by deterioration, whereas that of the device fabricated with bipolar host materials remained constant. These results prove the advantages of using bipolar host materials over a blend of transporting materials to maintain the stability of the device morphology.
Acknowledgments This work was supported by the Industrial Technology Innovation Program (Grant No. 10063277, Development of pattern deposition system based on roll to roll processing under low temperature and atmospheric pressure condition for smart thin film device fabrication) funded by Ministry of Trade, Industry & Energy.
Author statement
Kyu Sung Kim : Conceptualization, Methodology, Investigation, Validation, Investigation, Writing - Original Draft
Dong Uk Kim : Conceptualization, Methodology, Investigation, Validation, Investigation, Writing Original Draft
Kuk Soung Joung : Resources, Data Curation
Jae-Woong Yu : Conceptualization, Methodology, Writing - Review & Editing, Supervision, Project administration, Funding acquisition
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Figure captions
Figure 1. Schematics of the synthesis.
Figure 2. TGA analysis of the host materials.
Figure 3. DSC diagrams of the host materials.
Figure 4. Absorption and Luminescence spectra of the host materials.
Figure 5. Cyclic voltammograms of the host materials.
Figure 6. Optimized molecular geometries and calculated spatial distributions of the HOMOs and LUMOs of the host materials using Gaussian 03 program.
Figure 7. J-V characteristics of the hole-only and electron-only devices for the host materials.
Figure 8. Energy diagram of the materials used in this study.
Figure 9. Luminescence-voltage and Current density-voltage characteristics of the white OLED devices.
Figure 10. (a) External Quantum efficiency vs Luminescence, (b)Current Efficiency vs Luminance, and (c) Power Efficiency vs Luminance characteristics of the white OLED devices.
Figure 11. EL and normalized EL spectra of the host materials.
Figure 12. AFM images for the reference device and bipolar host devices.
Table 1. Optical properties of bipolar host materials
UVmax (nm) BH1 BH2 BH3 BH4
303,339, 353 302,338, 353 303,338, 352 303,339, 352
PLmax (nm)
HOMO (eV)
LUMO (eV)
Band gap (eV)
TD (oC)
Tg (oC)
378
5.67
2.27
3.40
418
95
377
5.71
2.31
3.40
406
81
377
5.70
2.28
3.42
434
48
377
5.67
2.23
3.44
430
-
Table 2. Electroluminescence characteristics of the white OLED devices
PVK:CBP (1:3) BH0 BH1 BH2 BH3 BH4
Luminance (cd/m2)
Current Efficiency (cd/A)
Power Efficiency (lm/W)
Quantum Efficiency (%)
CIE
1158.2
5.24
2.49
2.07
(0.30, 0.38)
395.0 567.31 1437.9 1388.7 557.75
8.66 5.23 16.04 13.95 7.17
3.26 2.16 8.69 7.30 3.13
2.63 2.52 6.52 5.61 3.90
(0.36, 0.40) (0.23, 0.33) (0.29, 0.36) (0.29, 0.35) (0.22, 0.33)