Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated

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Organic Electronic Devices

Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated for Thickness-Insensitive Electron Injection Layers in Organic Light-Emitting Devices Satoru Ohisa, Tetsuya Kato, Tatsuya Takahashi, Michinori Suzuki, Yukihiro Hayashi, Tomoyuki Koganezawa, Christopher R. McNeill, Takayuki Chiba, Yong-Jin Pu, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00752 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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ACS Applied Materials & Interfaces

Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated for Thickness-Insensitive Electron Injection Layers in Organic Light-Emitting Devices Satoru Ohisa,1,2,3* Tetsuya Kato,1 Tatsuya Takahashi,1 Michinori Suzuki,1 Yukihiro Hayashi,1 Tomoyuki Koganezawa,4 Christopher R. McNeill,5 Takayuki Chiba,1,2,3 Yong-Jin Pu,1,2,3 and Junji Kido1,2,3* 1

Department of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa,

Yamagata 992-8510, Japan 2

Research Center for Organic Electronics, Yamagata University, 4-3-16 Jonan, Yonezawa,

Yamagata 992-8510, Japan 3

Frontier Center for Organic Materials, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata

992-8510, Japan 4

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-

5198, Japan 5

Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia

E-mail: [email protected]; [email protected] Keywords: polymer blend; electron injection layer; conjugated polyelectrolytes; polyethyleneimine; polymer light-emitting diodes

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Abstract Electron injection layers (EILs) based on a simple polymer blend of polyethyleneimine ethoxylated (PEIE) and poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)] (PFN-Br) can suppress the dependence of organic light-emitting device (OLED) performance on thickness variation compared with single PEIE or PFN-Br EILs. PEIE and PFN-Br were compatible with each other, and PFN-Br uniformly mixed in the PEIE matrix. PFN-Br in PEIE formed more fluorene–fluorene pairs than PFN-Br alone. In addition, PEIE:PFN-Br blends reduced the work function (WF) substantially compared with single PEIE or PFN-Br polymer. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices presented lower driving voltages and smaller dependences of device performance on EIL thickness than single PEIE or PFN-Br-based devices. These improvements were attributed to electron-transporting fluorene moieties, increased fluorene–fluorene pairs working as channels of electron transport, and the large WF reduction effect of PEIE:PFN-Br blends.


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Introduction In organic light-emitting devices (OLEDs), one of the most important factors for attaining highly efficient devices is achievement of high electron injection characteristics. Generally, electron injection to organic layers is more difficult than hole injection. Improvement of electron injection characteristics, thus, facilitates charge carrier balance in devices and increases luminous efficiency. Efficient electron injection also reduces the driving voltages of devices. The insertion of electron injection layers (EILs) is very effective in improving the electron injection characteristics in OLEDs.1-12 To date, various types of EIL materials, such as alkali or alkali-earth metals,2 inorganic or organic salts,1, 3, 9, 12 and polyamine compounds,6, 13-15 have been developed. Among them, polyamine compound derivatives have attracted great attention as highly efficient electron injection materials, which can be processed from solution. Compared with the conventional vacuum evaporation process, solution processing has an advantage in fabrication cost. Hence, the development of effective materials for solution processing is desired. Zhou et al. reported polyethyleneimine (PEI) and polyethyleneimine ethoxylated (PEIE) (Figure 1) as efficient EIL materials.6 The modification of electrodes by thin PEI and PEIE greatly reduced the work functions (WFs) of electrodes, significantly improving the electron injection characteristics. However, these aliphatic amine EIL materials are intrinsic insulators. The performance of OLEDs with these materials is sensitive to slight differences in EIL thickness. As EIL thickness increases, the driving voltages rapidly increase. This high sensitivity is an unfavorable characteristic because some degree of variation in layer thickness is inevitable during mass production. Therefore, EIL materials as thick films are desired.



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Figure 1. Chemical structures of (a) PEIE and (b) PFN-Br.

Conjugated polyelectrolytes (CPEs) possess a conjugated main-chain backbone and side chains containing ionic functionalities. CPEs have been used as water- or alcohol-soluble light-emitting materials or carrier injection layers in organic electronic devices.4,

16-22

The electron-transporting

conjugating backbone can suppress the increase in driving voltage in thick CPE-based devices. Huang et al. synthesized poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7(9,9-dioctylfluorene)] (PFN-Br) (Figure 1) and applied it as an emitting layer in a device structure with [ITO/anode buffer layer/PFN-Br (5 nm)/cathode].4 This device with PFN-Br was operated at a much lower driving voltage than that with polyfluorene-based compounds as emitting materials in the same device structure, which did not possess ionic side chains. The low driving voltage of the PFNBr-based device suggested that the ionic side chain efficiently reduced the driving voltage. However, the PFN-Br thickness of 5 nm was still thin. Unfortunately, to the best of our knowledge, applications of a thick PFN-Br layer in organic electronic devices have rarely been reported. Other thick CPEs have been applied in organic electronic devices.21-23 Woo and Lee et al. synthesized a new series of poly(fluorene-phenylene)-based CPEs with different numbers of ionic functionalities containing ammonium cations and bromide anions.21 Among them, a CPE named P3 was applied to an EIL in OLEDs with a device structure of [ITO/ poly(3,4-ethylenedioxy-thiophene):poly(4-styrenesulfonate) (PEDOT:PSS) (30 nm)/poly[2-methoxy-5-(2´-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) (80 4 ACS Paragon Plus Environment

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nm)/P3 (32 nm)/Al]. The device can be operated at much lower driving voltage than that without P3. Despite the great thickness of the P3 layer, the device showed a very low driving voltage. Moreover, the driving voltage of the P3/Al-based device was almost similarly low to that of a Ca/Al-based device, in which Ca was the low WF metal and reduced the driving voltage. P3 presented excellent effectivity in thick EILs. However, the complexity of the synthesis scheme of CPEs is unfavorable. A simple method is needed to suppress the sensitivity of device performance to the thickness variation in EILs. We report that a simple polymer blend EIL of PEIE and PFN-Br can suppress sensitivity of device performances to its thickness variation. PEIE and PFN-Br were compatible with each other, and PFNBr uniformly mixed with PEIE matrix. PFN-Br in PEIE formed more fluorene–fluorene pairs than PFN-Br only. In addition, PEIE:PFN-Br blend showed larger WF reduction effects than each PEIE and PFN-Br polymers. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices showed lower driving voltages and smaller dependences of device performances on EIL thicknesses than PEIE- and PFN-Br based devices. These improvements were attributed to electron-transporting fluorene moieties, more formed fluorene– fluorene pairs working as channels of electron transporting and the large WF reduction effect of PEIE:PFN-Br blends.

Results and Discussion PEIE and PFN-Br were dissolved in methanol separately. The solutions were mixed to obtain PEIE:PFN-Br solutions and stirred for several hours. The thermal properties of the PEIE, PFN-Br, and PEIE:PFN-Br blends were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. As shown in the TGA curves (Figure S1), PEIE showed almost one-step decomposition behavior, and the decomposition temperature (Td) was 278 °C. On the 5 ACS Paragon Plus Environment


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other hand, PFN-Br showed two-step decomposition behavior. The first step occurred at around 199 °C, corresponding to the decomposition of the ionic side chains. The second step occurred at around 411 °C, which was assigned to the decomposition of the polyfluorene main chain. Figure S2 shows the DSC measurement results, and the effect of blend ratio on the glass transition temperature (Tg) of PEIE is shown in Figure 2. The Tg of pristine PEIE was −25.3 °C. On the other hand, the Tg of PFNBr was not observed, which to the best of our knowledge has never been reported. All the DSC curves of PEIE:PFN-Br (10, 30, and 50 wt%) blends showed a single glass transition, and the Tg values of PEIE were −22.8, −19.0, and −3.7 °C, respectively. These results suggested that the two types of polymers were compatible.24-25 If PEIE and PFN-Br were incompatible, the Tg of PEIE in the polymer blends would not depend on blend ratio but would be the same as that of neat PEIE. The ionic side chains of PFN-Br were compatible with polar PEIE, contributing to the compatibility of these two polymers.

Figure 2. Tg values of PEIE in PEIE:PFN-Br blends.


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The surface topology of films was investigated by atomic force microscopy (AFM). PEIE, PFNBr, or PEIE:PFN-Br blend layers with a thickness of 20 nm were spin coated on a [glass/ITO/poly(9,9dioctylfluorene-alt-benzothiadiazole) (F8BT) (80 nm)/ZnO nanoparticles (NPs) (10 nm)] film, which is part of the OLED structure mentioned below. As shown in Figure 3, PEIE- and PFN-Br-coated films showed surface roughness (Ra) values of 0.62 and 0.68 nm, respectively, whereas the bare ZnO NPs gave an Ra value of 2.19 nm. Thus, these polymers covered and planarized the surface roughness of the ZnO NP layer. PEIE:PFN-Br-coated ZnO NPs also showed smaller Ra than pristine ZnO NPs. The Ra values of blended films with PFN-Br concentration weight ratios from 10 to 50 wt% were larger than those of neat PEIE and PFN-Br films. At a PFN-Br weight ratio of 30 wt%, the largest Ra of 1.40 nm was obtained. The slight increase in Ra also suggested that these two types of polymers should be compatible.

Figure 3. AFM images of PEIE:xwt% PFN-Br blends (20 nm) on ITO/F8BT (80 nm)/ZnO NPs (10 nm), where x = (a) 0 (PEIE), (b) 10, (c) 30, (d) 50, (e) 70, (f) 90, and (g) 100 (PFN-Br).



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The molecular packing of PFN-Br was investigated by two-dimensional grazing-incidence wideangle X-ray diffraction (GIWAXD). Figure S3(a) and (b) shows the GIWAXD results of PFN-Br films. The PFN-Br film baked at 100 °C presented no diffraction peaks. After baking the film at 200 °C, a broad diffraction peak occurred at 14.6 nm−1 along the qz direction, corresponding to a d-spacing of 4.3 Å. Generally, the p–p spacing is between 3.4 and 4.2 Å. Although this observed peak was slightly out of this range, it is probably assigned to fluorine–fluorene p–p spacing. The anisotropic peak suggests the face-on rich orientation of the fluorene moiety. Figure S3(c) and (d) shows the measurement results of a 50 wt% PEIE:PFN-Br blend film baked at 100 and 200 °C. No diffraction peaks were observed in these films, suggesting that noticeable packing among fluorenes was not observed in the blend films. Subsequently, PFN-Br molecular orientation was investigated by nearedge X-ray absorption fine-structure (NEXAFS) measurements.26-27 Figure S4(a) shows the NEXAFS spectra of PEIE, PFN-Br, and PEIE:PFN-Br (50 wt%) blend films baked at 100 °C. The spectra of PFN-Br and PEIE:PFN-Br presented an absorption peak at 285 eV, corresponding to the transition from carbon-1s to π* antibonding states. This peak can be used as a probe of fluorene orientation in the blend film. Subsequently, Figure S4(b)–(d) shows angle-resolved NEXAFS spectra of PEIE, PFNBr, and PEIE:PFN-Br blend films baked at 100 °C. All the spectra showed no dichroism, indicating no prominent orientation of these polymers. Both measurements suggested that PFN-Br uniformly mixed in the PEIE matrix. The ultraviolet–visible (UV–Vis) absorption spectra of PEIE, PFN-Br, and PEIE:PFN-Br blend films on quartz substrates were measured (Figure S5). PEIE showed a main absorption band in shorter wavelengths than 250 nm. In addition, there was a weak absorption band at around 400 nm, derived from components of yellow discolored PEIE. PFN-Br showed a strong absorption band at around 400 nm, derived from conjugated polyfluorene chains. The spectrum of the PEIE:PFN-Br blend film was 8 ACS Paragon Plus Environment

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near the sum of those of the PEIE and PFN-Br films. No new absorption bands were observed in the blend film. The energy gap (Eg) of PFN-Br was obtained from the absorption edge as 2.81 eV. The ionization potential (Ip) of PFN-Br was obtained as 5.72 eV by photoelectron yield spectroscopy. The electron affinity value of PFN-Br was calculated to be 2.89 eV by subtracting the Eg value from the Ip value. Subsequently, the photoluminescence spectra of PFN-Br blended with PEIE at PFN-Br concentrations from 1 to 100 wt% were measured (Figure 4). All the spectra showed a strong emission peak at around 420 nm, assigned to emission from fluorene moieties. At the weight ratio of 1 wt%, this peak was located at 416 nm. By increasing the PFN-Br blend ratio to 50 wt%, a red shift was observed. At the weight ratio of 50 wt%, the peak position was 426 nm. This red shift was attributed to elongation of the p-conjugation length in the polyfluorene backbone. At blend ratios higher than 50 wt%, the peak position changed negligibly. In the long-wavelength region, a new broad emission band was generated in the 480–650 nm region. In polyfluorene compounds, such long-wavelength light emission is well known, and the origin of light emission results from the existence of keto defect sites,28-30 b phase,31-32 and fluorene–fluorene excimers.33-34 In this work, the light emission should result from fluorene–fluorene excimers. Figure S6 shows the PL decay lifetimes of these films. The PL decay lifetime of the 500 nm emission was much longer than that of the 430 nm emission, supporting this fact. The intensity of this emission increased on increasing the PFN-Br blend ratio up to 50 wt%, possibly because the excimer formation probability increased at a high blend ratio. However, a further increase in the blend ratio above 50 wt% adversely decreased the excimer emission intensity. A repulsive force resulted from the increased charge density, inhibiting the formation of the fluorene–fluorene pair. As seen in GIWAXD, regular fluorene–fluorene packing was not observed. However, there were some fluorene–fluorene pairs. 9 ACS Paragon Plus Environment


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Figure 4. PL spectra of PFN-Br and PEIE:PFN-Br blend films [(a) 1 wt%, 10 wt%, 20 wt%, 30 wt%, and 50 wt% as well as (b) 50 wt%, 70 wt%, 80 wt%, 90 wt%, and 100 wt% (PFN-Br)].

WF was investigated by ultraviolet photoelectron spectroscopy (UPS) measurements. PEIE, PFNBr, and PEIE:PFN-Br blends coated on ITO/ZnO NP films were measured. Figure 5 shows the UPS measurement results of unmodified and modified ZnO NPs. The unmodified ZnO NPs showed a WF value of 4.21 eV, whereas the PEIE- and PFN-Br-modified ZnO NPs showed WF values of 3.65 and 3.64 eV, respectively. These polymers reduced the WF of ZnO NPs, and these reduction effects have been reported previously.6, 20 Surprisingly, the PEIE:PFN-Br blend film further reduced the WF values of ZnO NPs to 3.58 eV for 30 wt% and to 3.46 eV for 50 wt%, respectively. To investigate the ionic side-chain effect on WF reduction, the WF values of ZnO NPs modified by tetrabutylammonium bromide (TBABr) and PEIE:TBABr (10 wt%) were evaluated (Figure S7). The TBABr-modified ZnO NPs showed a WF value of 3.70 eV, whereas the PEIE:TBABr-modified ZnO NPs showed 3.48 eV. Similar to PEIE and PFN-Br, a synergetic effect of PEIE and ammonium bromide on the WF value was observed. It is well known that PEIE and ionic functionalities form dipole moments on the ZnO


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surfaces, contributing to WF reduction. The origin of synergetic WF reduction effects has not been clarified. It is possible that the synergetic effect might result from the WF reduction effects by both PEIE and ionic functionalities.

Figure 5. UPS spectra of ZnO NPs, ZnO NPs/PEIE (20 nm), ZnO NPs/PFN-Br (20 nm), and ZnO NPs/PEIE:50 wt% PFN-Br blend (20 nm) films.

OLEDs with a structure of [ITO(+)/phosphomolybdic acid (PMA)35-36 (10 nm)/poly((9,9dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine) (TFB) (20 nm)/F8BT (80 nm)/EIL (20 nm)/Al(–) (100 nm)) were fabricated. Except for cathode Al, all the layers were formed by spin coating. The applied EILs were PEIE, PEIE:50wt% PFN-Br blend, PFN-Br films. The energy level diagram is shown in Figure 6. Figure 7 shows the current density (J)–voltage (V)–luminance (L) characteristics. In the J–V characteristics, the PEIE:50wt% PFN-Br -based device clearly showed lower driving voltages than those of the devices with PEIE and PFN-Br. In the L–V characteristics, the driving voltages at 1 and 100 cd/m2 were 3.4 and 6.1 V for PEIE, 5.3 and 6.3 V for the 50 wt% blend, and 5.4 and 9.6 V for PFN-Br, respectively. At 100 cd/m2, the PEIE:50wt% PFN-Br -based device showed similar driving voltage to that with PEIE. However, the behavior of the PEIE:PFN-Br-based 11 ACS Paragon Plus Environment


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device was instable. We attributed the instability to the low compatibility between hydrophobic F8BT and hydrophilic PEIE:50wt% PFN-Br blend films. Therefore, we inserted ZnO NPs layer between F8BT and EILs.

Figure 6. Energy level diagram of OLEDs.

Figure 7. (a) Current density–voltage, and (b) luminance–voltage characteristics of OLEDs with 20 nm EILs of PEIE, PEIE:50wt% PFN-Br blend, and PFN-Br.


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OLEDs with a structure of [ITO(+)/ PMA (10 nm)/TFB (20 nm)/F8BT (80 nm)/ZnO NPs (10 nm)/EIL (20 nm)/Al(–) (100 nm)] were fabricated. The applied EILs were PEIE, PFN-Br, and PEIE:PFN-Br (10, 30, and 50 wt%) blend films. Figure 8 shows the J–V–L characteristics. The other characteristics are shown in Figure S8. The driving voltages at 1 and 100 cd/m2 were 4.3 and 6.0 V for PEIE, 4.5 and 6.4 V for the 10 wt% blend, 3.6 and 5.1 V for the 30 wt% blend, 3.2 and 4.8 V for the 50 wt% blend, and 6.4 and 9.3 V for PFN-Br, respectively. The PFN-Br-based device showed the largest driving voltage. Devices based on the 30 and 50 wt% PFN-Br blend showed much lower driving voltages than those of PEIE- and PFN-Br-based devices. The reduction of driving voltages originated from WF reduction effects of ZnO by polymer blending. At 100 cd/m2, the current efficiencies of these devices were 0.50 cd/A for PEIE, 0.39 cd/A for PEIE:10wt% PFN-Br, 0.58 cd/A for PEIE:30wt% PFN-Br, 0.68 cd/A for PEIE:50wt% PFN-Br, and 0.82 cd/A for PFN-Br, respectively. Unfortunately, the current efficiencies of these devices were very low. Probably, the appropriate charge carrier balance was not achieved in this device system. It is noteworthy to observe the order of layer stacking of ZnO and PEIE because the order in this work was opposite to common device configurations.6 Unfortunately, we could not fabricate such common device configurations because ZnO coating solvent dissolved PEIE. Therefore, devices with inverted layer stacking order were created. It is considered that electron injection is difficult in such devices with the inverted layer stack-in order. However, interspaces in ZnO NP films existed, and EIL materials could reach the F8BT/ZnO interface through the interspaces during EIL coating.37 The EIL materials at the F8BT/ZnO interface facilitated electron injection from ZnO to F8BT. Then, we tested the driving stabilities of the devices with ZnO NPs/PEIE, ZnO NPs/PEIE:30wt% PFN-Br, and ZnO/NPs PEIE:50wt% PFN-Br at a constant current density of 25 mA/cm2. Figure S9 shows the results. As a result, the PEIE:30wt% PFN-Br-based device showed comparable device lifetime to the PEIE-based device. Moreover, the PEIE:30wt% PFN-Br13 ACS Paragon Plus Environment


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based device showed suppressed driving voltage increase compared with the PEIE-based device. On the other hand, the PEIE:50wt% PFN-Br-based device showed extremely short device lifetime and considerable large driving voltage. We assume that the stability of the blend film related with the device lifetime.

Figure 8. (a) Current density–voltage, and (b) luminance–voltage characteristics of OLEDs with EILs of ZnO NPs (10 nm)/PEIE:xwt% PFN-Br blends (20 nm), where x = 0 (PEIE), 10, 30, 50, and 100 (PFN-Br).

The dependence of EIL thickness on device characteristics was then investigated. The thicknesses of PEIE, PFN-Br, and PEIE:PFN-Br blend layers were varied to 12, 16, 30, or 40 nm. The PFN-Br blend ratio was fixed at 50 wt%. Figures 9 and S10 show the J–V–L characteristics and plots of voltages at 1 cd/m2. The driving voltage of the PEIE- and PFN-Br-based devices largely increased with increasing thickness, whereas the driving voltage of the PEIE:PFN-Br blend-based devices changed negligibly. Probably, the electron-transporting fluorene moiety and the fluorene–fluorene contact points in PFN-Br contributed to the maintenance of driving voltage in thick EIL-based devices.


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Figure 9. Luminance–voltage characteristics of OLEDs with different EIL thicknesses of (a) PEIE, (b) PEIE:PFN-Br (50 wt%) blend, and (c) PFN-Br. (d) Plot of voltages at 1 cd/m2.

These EILs were also applied in tris[2-(4-n-hexylphenyl)quinoline]iridium(III) [Hex-Ir(phq)3]phosphorescent emitter-based OLEDs. Details of the devices are described in Figure S11. In the J–V characteristics, at blend ratios of 10 and 30 wt%, the driving voltages of PEIE:PFN-Br-based devices were lower than that of the PEIE-based device and much lower than that of the PFN-Br-based device, demonstrating that PEIE:PFN-Br is applicable to phosphorescent emitter-based devices. Finally, these EILs were also applied in devices with an inverted device structure. we fabricated devices with a structure of ITO(–)/ZnO NPs (10 nm)/EIL(30 nm)/F8BT(80 nm)/4,4’-bis[N-(115 ACS Paragon Plus Environment


naphthyl)-N-phenyl-amino]biphenyl

(α-NPD)

(20

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nm)/1,4,5,8,9,11-hexaazatriphenylene-

hexacarbonitrile (HAT-CN)38-40 (5 nm)/Al(+) (100 nm), where the EILs were PEIE, PEIE:30wt% PFN-Br, PEIE:50wt% PFN-Br, and PFN-Br. Here, the ZnO NPs, EILs, and F8BT layers were formed by spin-coating. On the other hand, the α-NPD and HAT-CN films were formed by evaporation under vacuum. In this device structure, charge pairs generate at the HAT-CN/α-NPD interface, and the generated hole transports through the α-NPD layer, and it is injected to the F8BT layer. Figure 10 shows the device characteristics. At 1 cd/m2, the driving voltages were 8.0 V for PEIE, 2.6 V for PEIE:30wt% PFN-Br, 3.5 V for PEIE:50wt% PFN-Br, and 8.3 V for PFN-Br, respectively. At 100 cd/m2, the driving voltages were 12.7 V for PEIE, 7.0 V for PEIE:30wt% PFN-Br, and 8.2 V for PEIE:50wt% PFN-Br, respectively. The maximum luminance of the PFN-Br-based device did not achieve 100 cd/m2. In the EL spectra at 1 mA/cm2, the PEIE-, PEIE:30wt% PFN-Br-, and PEIE:50wt% PFN-Br-based devices showed similar spectra shapes derived from F8BT. On the other hand, the PFNBr-based device showed a different spectrum shape. Emission derived from PFN-Br was observed in addition to the emission derived from F8BT due to the poor electron injection property. Like the cases of the devices with the regular structure, The devices with PEIE:PFN-Br blend devices showed much lower driving voltages than those of the PEIE and PFN-Br-based devices. At 100 cd/m2, the current efficiencies were 1.7 cd/A for PEIE, 2.6 cd/A for PEIE:30wt% PFN-Br, 3.4 cd/A for PEIE:50wt% PFN-Br, respectively. These blend films evidently enhanced the device efficiencies owing to the improvement of charge carrier balance by polymer blend.


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Figure 10. Characteristics of the inverted structured OLEDs with PEIE, PEIE:30wt% PFN-Br, PEIE:50wt% PFN-Br, and PFN-Br EILs. (a) Current density (J)–voltage (V), (b) luminance–L, (c) current efficiency–J, and (d) power efficiency–J characteristics. The Inset shows the EL spectra at 1 mA/cm2.

Conclusions We succeeded in suppressing the dependence of OLED performances on EIL thicknesses. PEIE and the PFN-Br blend formed uniform films. Prominent fluorene packing and orientation of PFN-Br in the polymer blends were not observed, but some degree of formation of fluorene–fluorene pairs was observed. Probably, these pairs contributed to the maintenance of driving voltage in the thick EILbased devices. Electrons were transported through the electron-transporting polyfluorene backbone 17 ACS Paragon Plus Environment


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and fluorene–fluorene pair points. Synergetic WF reduction effects were observed in polymer blend films, resulting from the WF reduction effects by both PEIE and ionic functionalities. However, detailed elucidation of the synergetic effects needs further investigation. We believe that the obtained knowledge in this work can contribute to advances in further developing high-performance devices.

Experimental Materials ITO-coated glass substrates were purchased from Asahi Glass Co., Ltd., and electrodes were patterned by photolithography. PMA was purchased from Kanto Chemical Co., Inc. TFB was purchased from American Dye Source, Inc. F8BT was provided by Sumitomo Chemical Co., Ltd. Spherical-shaped ZnO NP solutions with particle diameters of 6–8 nm were synthesized according to an established procedure.8 PEIE (Product No. 423475) (Mw = 70,000) was purchased from SigmaAldrich Co. LLC. PFN-Br (LT-N878) (Mw > 10,000) was purchased from Lumtec Corp. and used after careful purification. HAT-CN and α-NPD were purchased from e-Ray Optoelectronics Technology Co., Ltd.

Characterization methods TGA measurements under a nitrogen flow were performed using a SEIKO EXSTAR 6000 TG/DTA 6200 unit at a heating rate of 10 °C min−1, and TG measurements under vacuum were performed using an ULVAC RIKO VAP-9000 unit below 10−4 Pa. DSC measurements were performed using a PerkinElmer Diamond DSC Pyris instrument under a nitrogen flow at a heating rate of 10 °C min–1. Polymer blend samples were formed by drying PEIE:PFN-Br solutions under vacuum at room temperature. AFM measurements were performed using a Veeco Dimension Icon atomic force 18 ACS Paragon Plus Environment

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microscope with a scanning area of 2.0 µm × 2.0 µm. GIWAXD measurements were performed at BL46XU in Spring-8. Samples were coated onto Si substrates. NEXAFS spectroscopy was performed at the soft X-ray beamline at the Australian Synchrotron.41 Partial Electron Yield (PEY) spectra were measured, with the spectra double normalized via the 'stable monitor' method. Spectra were analysed with QANT,42 with further experimental details provided elsewhere.27 Samples were coated onto highly doped Si substrates. Ip values were determined by PYS under vacuum (-10−3 Pa). UV–Vis spectra were measured using a Shimadzu UV-3150 UV–vis–NIR spectrophotometer. Eg values were determined from the absorption band edges. EA values were calculated by subtracting Eg values from Ip values. PL spectra were measured using a HORIBA JOBIN YVON Fluoromax-4 fluorometer. The WF values of the films were measured using a Thermo Fisher Scientific Theta Probe X-ray and UV photoelectron spectrometer system. For all characterizations, PEIE, PFN-Br, and PEIE:PFN-Br blend layers with a thickness of 20 nm were formed by spin coating followed by baking at 100 °C for 10 min. Each solution concentration was 4–5 mg/mL, and the spin coating condition was varied from 2000 to 5000 rpm to give a film of 20 nm thickness.

Device fabrication and characterization ITO-coated glass substrates were sequentially cleaned with ultrapure water, acetone, and 2propanol using an ultrasonic bath sonicator and then dry-cleaned using a UV/ozone cleaner. The substrates were then transferred into a nitrogen-purged glove box. In the regular structured OLEDs, PMA solutions in acetonitrile at 10 mg/mL were spin coated onto the ITO electrodes and baked at 100 °C for 10 min. TFB solutions in p-xylene at 7 mg/mL were spin coated onto the PMA layers and baked at 200 °C for 10 min. F8BT solutions in p-xylene at 15 mg/mL were spin coated onto the TFB layers and baked at 130 °C for 10 min. ZnO NP solutions at 5 mg/mL were spin coated onto the F8BT 19 ACS Paragon Plus Environment


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layers and baked at 100 °C for 10 min. PEIE, PFN-Br, and PEIE:PFN-Br blend solutions in methanol were spin coated and baked at 100 °C for 10 min. Al cathodes were deposited onto the EILs using a shadow mask with patterned openings under vacuum (10–5 Pa). In the inverted structured OLEDs, ZnO NP solutions at 5 mg/mL were spin coated onto the ITO electrodes and baked at 100 °C for 10 min. PEIE, PFN-Br, and PEIE:PFN-Br blend solutions in methanol were spin coated onto the ZnO NPs layers and baked at 100 °C for 10 min. F8BT solutions in p-xylene at 15 mg/mL were spin coated onto the TFB layers and baked at 130 °C for 10 min. The α-NPD and HAT-CN layers were formed by evaporation under vacuum. Al cathodes were deposited onto the EILs using a shadow mask with patterned openings under vacuum (10–5 Pa). The emitting area of the devices was 2 mm × 2 mm. J– V–L characteristics were measured using a current source meter, Keithley 2400, and a luminance meter, Konica Minolta CS-200. EL spectra were measured using a photonic multichannel analyzer, Hamamatsu PMA-11. Quantum efficiencies were calculated based on the Lambertian assumption.

Supporting Information TGA curves, DSC curves, GIWAXD profiles, NEXAFS spectra, UV–Vis absorption spectra, PL decay curves, UPS spectra, and OLED characteristics are available from the supporting information. Acknowledgments Two-dimensional GIWAXD experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, proposal no. 2017A1767), and we thank Seijiro Fukuta and Prof. Tomoya Higashihara for support with the GIWAXD measurements. Part of this research was undertaken on the soft X-ray beamline at the Australian Synchrotron, part of ANSTO. We thank the Strategic Promotion of Innovative R&D Program and the Center of Innovation (COI) Program from the Japan Science and Technology Agency, JST, for financial support. References 1. Hung, L. S.; Tang, C. W.; Mason, M. G., Enhanced Electron Injection in Organic Electroluminescence Devices Using an Al/LiF Electrode. Appl. Phys. Lett. 1997, 70, 152-154. 20 ACS Paragon Plus Environment

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42. Gann, E.; McNeill, C. R.; Tadich, A.; Cowie, B. C. C.; Thomsen, L., Quick as NEXAFS Tool (QANT): A Program for NEXAFS Loading and Analysis Developed at the Australian Synchrotron. J Synchrotron Radiat 2016, 23, 374-380.


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for Table of Contents use only OH

N

N x HO

N

Br N

N Hz

y

N Br

OH

N

C8H17 C8H17

n

OH

Electron-transporting property

High electron-injection property

PEIE only

PEIE:PFN-Br blend

Thickness-Sensitive

Thickness-Insensitive


Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated

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