Two-Dimensional Ca2Nb3O10 Perovskite Nanosheets for Electron

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Organic Electronic Devices

Two-Dimensional Ca2Nb3O10 Perovskite Nanosheets for Electron Injection Layers in Organic Light-Emitting Devices Satoru Ohisa, Tatsuya Hikichi, Yong-Jin Pu, Takayuki Chiba, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05759 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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ACS Applied Materials & Interfaces

Two-Dimensional Ca2Nb3O10 Perovskite Nanosheets for Electron Injection Layers in Organic Light-Emitting Devices Satoru Ohisa,1,2,3 Tatsuya Hikichi,1 Yong-Jin Pu,1,2,3*, Takayuki Chiba,1,2,3and Junji Kido1,2,3* 1

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

Yamagata, Japan 2

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

Japan 3

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

Japan E-mail: [email protected]; [email protected] Keywords: calcium niobate, ferroelectric, dielectric, nanosheet, OLED

Abstract We report in this article the application of calcium niobate (CNO) perovskite nanosheets for electron injection layers (EILs) in organic light-emitting devices (OLEDs). Four kinds of tetraalkylammonium hydroxides having different alkyl lengths were utilized as the exfoliation agents of a layered compound precursor HCa2Nb3O10 to synthesize CNO nanosheets, including tetramethylammonium

hydroxide

(TMAOH),

tetraethylammonium

hydroxide

(TEAOH),

tetrapropylammonium hydroxide (TPAOH), and tetrabutylammonium hydroxide (TBAOH). CNO nanosheet EILs were applied in fluorescent poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]

(F8BT)

organic

light-emitting polymer-based devices. The effects of dispersion concentrations and alkyl chain length on the devices’ performances were investigated. The results demonstrated that OLEDs’ performances were related to the coverage ratio of the CNO nanosheets, their thicknesses, and their workfunction values. Among the four exfoliation agents, the device with CNO nanosheets exfoliated by TPAOH showed the lowest driving voltage. The OLEDs with the CNO nanosheet EILs showed lower driving 1 ACS Paragon Plus Environment

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voltages compared with the devices with conventional EIL material lithium 8-quinolate.

1. Introduction Recently, two-dimensional (2D) nanosheets have attracted the attention of many investigators.1-15 Such materials show an unprecedented functionality and high reactivity owing to their distinctive thickness (less than a few nanometers) and extremely high anisotropy. Therefore, their catalytic, magnetic, dielectric, electrically conductive, and photoluminescent properties were intensively studied. It is also a big challenge to create of composite materials and ultrathin devices with novel functions from various types of 2D nanosheets as building blocks. At present, various types of 2D nanosheets of graphenes,1,

3, 5

graphene oxides,4,

7, 16

boron nitrides,6,

17

transition metal

dichalcogenides such as MoS2,10, 11, 13, 14 carbides,12 hydroxides,8, 15 and oxides8, 15, 18 are investigated. Among them, graphene has attracted great attention since the discovery because of the appealing characteristics such as high mechanical strength, thermal and electronic conductivities. Recently, transition metal dichalcogenides such as MoS2 have attracted great attention, because monolayer MoS2 is a semiconductor possessing a band gap of 1.8 eV and shows high carrier mobility up to 200 cm2/Vs. This characteristic can compensate the character of gapless graphene in creating devices with novel functions. Oxide perovskite materials are also uniquely attractive because of their specific properties. Calcium niobate (CNO) is one kind of oxide perovskites with wide band gaps of over 3.5 eV.18-23 CNO nanosheets were reported by Treacy et al. in 1990.20 Using a bulky base such as tetrabutylammonium hydroxide (TBAOH), CNO nanosheets were obtained by exfoliation of layered perovskite HCa2Nb3O10, which was the ion-exchange product of KCa2Nb3O10 synthesized via solid-state calcination. CNO nanosheets are one attractive building block, and so far, they have been utilized in several applications,24 such as dielectric layers for nanocapacitors,18 ferroelectric layers 2 ACS Paragon Plus Environment

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constituting superlattices,23 and seed layers for the oriented growth of crystal films.25-28 Recently, applications of 2D nanosheets in multilayered organic and perovskite electronic devices have been reported.16, 29-33 As an example, graphene is a good transparent electrode material in organic electronics.29 Graphene oxides are an efficient solution-processed hole-transporting layer in polymer solar cells (PSCs).16 Ultraviolet- (UV-) ozone processed transition metal dichalcogenides MoS2, WS2, and TaS2 are efficient hole-injection layers in organic light-emitting devices (OLEDs).31 The application of CNO nanosheets in organic electronics is also reported. Chang et al. reported that CNO nanosheets functioned as an electron-transporting material for solution-processed tandem PSCs,30 where they were applied in electron-transporting layers (ETLs) within the interconnecting recombination layers. Compared with devices having the conventional TiOx ETL, devices with CNO nanosheets based ETL showed a higher photon-to-current efficiency (PCE). Moreover, devices with an ETL comprised of a combination of TiOx and CNO nanosheets showed a further increased PCE value compared with devices with only CNO nanosheet ETL. As seen in these reports, 2D nanosheets have high potential for application in organic electronic devices. In OLEDs, electron injection layers (EILs) are indispensable to achieve high performance devices.34-48 Generally, there is a large electron injection barrier between a cathode such as Al or Ag and an organic functional layer, making it difficult for electron to inject from the cathode to the organic layer. Inserting EILs between a cathode and an organic functional layer, the electron injection can be greatly improved. To date, various types of EIL materials, such as alkali or alkali-earth metals,35 metal complexes,45, 46 inorganic or organic salts including poly(ionic liquids),34, 36, 44, 47, 48

polyamine compounds,39-41, 45, 48 and metal oxide semiconductors,43 have been developed.

Among them, polyamine compounds such as polyethyleneimine derivatives have attracted great attentions as highly efficient EIL materials forming large electric dipole moment between a cathode 3 ACS Paragon Plus Environment


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and an organic functional layer.39, 45, 48 The electric dipole moment is the sum of the permanent dipole moment of its own molecular framework and induced dipole moment resulting from charge transfer between the polyamine and the cathode. The formed electric dipole moment can virtually lower the WF of the cathode and enhance the electron injection. Here, CNO nanosheet are dielectric materials generating large electric polarization under application of electric field owing to their high dielectric constant of over 200.24 Hence, we expect that CNO nanosheets can also function as efficient EIL materials. In this work, we report the application of CNO nanosheets for electron injection layers (EILs) in OLEDs. CNO nanosheets were synthesized according to the reported procedure. Four kinds of tetraalkylammonium hydroxides with different alkyl lengths—tetramethylammonium hydroxide (TMAOH), tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), and tetrabutylammonium hydroxide (TBAOH)—were utilized for exfoliation. CNO nanosheet EILs were formed by spin-coating onto a fluorescent poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) light-emitting polymer. The effects of dispersion concentrations and alkyl chain length on the devices’ performances were investigated.

2. Results and Discussion CNO nanosheets were synthesized according to the established procedure;19 the details are described in the Experimental section. Figure 1 shows the synthetic route to TBA-CNO. First, the calcination reaction of KCa2Nb3O10 was performed. Then, the obtained KCa2Nb3O10 was ion-exchanged from K+ to H+ to obtain the layered HCa2Nb3O10, which was exfoliated to CNO nanosheets by the intercalation of the tetraalkylammonium hydroxides (TMAOH, TEAOH, TPAOH, 4 ACS Paragon Plus Environment

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and TBAOH). In this work, the CNO nanosheets that were exfoliated by TMAOH, TEAOH, TPAOH, and TBAOH are denoted as TMA-CNO, TEA-CNO, TPA-CNO, and TBA-CNO, respectively. The obtained CNO nanosheets were dispersed in 2-propanol (IPA) at the desired concentrations. Identification of KCa2Nb3O10 and HCa2Nb3O10 was performed using X-ray diffraction (XRD) analysis (Figure S1). The obtained XRD profiles of KCa2Nb3O10 and HCa2Nb3O10 agreed well with previously reported data.27

Figure 1: The synthetic route to calcium niobate nanosheets.

Four kinds of CNO nanosheet dispersions were spin-coated onto glass substrates and baked at 100°C for 10 min in N2. The CNO nanosheets were observed using atomic force microscopy (AFM). Figures S2 shows the AFM images of the CNO films. In respective samples, nanosheets with lateral sizes of up to hundreds of nanometers were observed. Single CNO nanosheet thicknesses were also obtained by the AFM measurements (Figure S2 inset and Table S1). The averaged thicknesses of five times measurements for respective samples were 1.9 nm for TMA-CNO, 2.6 nm for TEA-CNO, 5 ACS Paragon Plus Environment


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2.6 nm for TPA-CNO, and 2.7 nm for TBA-CNO, respectively. Then, the ultraviolet-visible (UV-Vis) absorption spectra of the CNO nanosheet films were measured in the transmission manner. CNO nanosheets were deposited on quartz substrates by spin-coating followed by baking at 100°C for 10 min. Figure 2 shows the UV-Vis absorption spectra. The four kinds of CNO nanosheets showed similar spectra, indicating that exfoliation agents did not influence the optical transition. It is difficult to determine the optical band gaps because the absorption edges had long tails in all films. The optical band gap values of TBA-CNO nanosheets were reported by Virdi et al. to be 3.6 ± 0.1 and 3.8 ± 0.2 eV, using Tauc plot obtained from diffuse reflectance spectra and valence electron energy loss spectroscopy, respectively.49 Work functions (WFs) of the CNO nanosheets were measured using ultraviolet photoelectron spectroscopy (UPS). The CNO nanosheets were deposited onto ITO-coated glass substrates. Figure S3(a) shows the UPS spectra of the secondary-electron cut-off region. The WF value was estimated to be 3.1 eV for TMA-CNO, 3.9 eV for TEA-CNO, 4.0 eV for TPA-CNO, and 4.5 eV for TBA-CNO. The WFs increased by increasing the alkyl chain lengths, indicating the effects of the tetraalkylammonium hydroxide on the WF values. Chang et al. reported a WF value of 3.5 eV for TBA-CNO nanosheets by Kelvin probe measurements.49 This value is much lower than that obtained in this work. The difference in measurements and sample preparation methods probably influenced the WF values. Figure S3(b) shows the UPS spectra of the Fermi energy region. The estimated valence band edge level was 6.4 eV for TMA-CNO, 6.5 eV for TEA-CNO, 7.2 eV for TPA-CNO, and 7.3 eV for TBA-CNO.

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Figure 2: UV-Vis absorption spectra of CNO nanosheet films.

OLEDs with a structure of [ITO (130 nm)/poly(3,4-ethylenedioxy-thiophene):poly(4styrenesulfonate)

(PEDOT:PSS)

(30

nm)/poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-

diphenylamine) (TFB) (20 nm)/F8BT (80 nm)/EIL/Al or Ag (100 nm)] were fabricated. PEDOT:PSS, TFB, F8BT, and EILs were formed by spin-coating. The Al and Ag cathodes were formed by evaporation under high vacuum. The optoelectronic properties of the materials are summarized in Table S2. The electron affinity of F8BT is 3.5 eV, and so there are electron injection barrier of 0.7 eV for Al, and 1.2 eV for Ag electrodes. First, TBA-CNO and lithium 8-quinolate (Liq) (3 nm) were applied as EILs, whereas Al and Ag were applied as electrodes. A 4 mg/mL TBA-CNO dispersed in IPA was spin-coated onto an F8BT layer. Figure 3 shows the current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the fabricated OLEDs. The other characteristics are shown in Figure S4. In the Al-electrode-based devices, the device with TBACNO showed a slightly lower driving voltage in the J–V characteristics than that with Liq. In the current density region up to 5 mA/cm2, the EQE of the device with TBA-CNO was higher than that of the device with Liq. This EQE enhancement probably resulted from the influences of changes in 7 ACS Paragon Plus Environment


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the charge carrier balance. In the Ag-electrode-based devices, the driving voltage and the EQE value of the device with TBA-CNO were greatly improved compared with those of the device with Liq. The driving voltages of the Ag-electrode-based devices were much higher than those of the Alelectrode-based devices, because of the higher WF value of Ag (4.7 eV) compared with that of Al (4.1 eV). The moderately low driving voltage of the TBA-CNO device confirmed the high electron injection property of the TBA-CNO nanosheets. It is noteworthy that the EQE value of the TBACNO device with Ag-electrode was similar to that of the TBA-CNO device with Al-electrode, nevertheless the driving voltage of the former was significantly higher than that of the latter. In this device system, electron current is very larger than hole current in the F8BT layer because of the high hole injection barrier between TFB and F8BT (0.6 eV). Hence, oversupply of hole is hard to occur and all of the injected holes can recombine with injected electrons in F8BT layer. Then, electron injection from the Ag cathode is more difficult than that from the Al cathode, resulting in a shift of carrier recombination zone. The career recombination zones in the Ag-based devices should be closer to the cathodes than those in the Al-based devices. Thus, the Ag-based devices suffer from more quenching effect by the EIL materials than the Al-based devices. Figure S5 shows the photoluminescence (PL) decay curves of F8BT and F8BT/EIL (Liq, TMA-CNO, TEA-CNO, TPACNO, and TBA-CNO) films on quartz substrates. The PL decay lifetime of the F8BT/Liq film was significantly shorter than that of the F8BT film, indicating that the excitons in F8BT were quenched by Liq. On the other hand, the PL decay lifetimes of the F8BT/CNO films were near the same as that of the F8BT film, indicating that the excitons in F8BT were not quenched by CNO. Probably, these high exciton confinement effects of the CNO nanosheets brought similarly high quantum efficiency in the TBA-CNO-device with Ag cathode to the TBA-CNO-device with Al cathode.


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Figure 3: (a) Current density–voltage and (b) luminance–voltage characteristics of devices with Liq or TBA-CNO EIL, using Al or Ag electrodes.

Then, ZnO nanoparticles (NPs) and polyethyleneimine ethoxylated (PEIE)-modified ZnO NPs (ZnO NPs/PEIE) were applied to the EILs in the abovementioned Al-electrode-based OLEDs to compare with the TBA-CNO EIL. Here, ZnO NPs is widely used EIL, and ZnO NPs/PEIE is known as highly efficient EIL.39,

43, 45, 48

Figures S6(a)-(e) show the J–V–L and the other OLED

characteristics. The ZnO NPs-based device showed much higher driving voltage than the TBACNO-based device. On the other hand, the ZnO NPs/PEIE-based device showed slightly lower driving voltage than the TBA-CNO-based device. The devices with TBA-CNO and ZnO/PEIE showed similar device efficiencies and greatly higher device efficiencies than that with ZnO NPs only. The operational lifetime of the devices with TBA-CNO and ZnO NPs/PEIE were tested at constant current density of 12.5 mA/cm2, corresponding to initial luminances of 1220 cd/m2 and 1056 cd/m2, respectively. Figure S6(f) shows plots of relative luminance versus operational time. Operational lifetimes to 80% luminances were 54.8 h for TBA-CNO and 4.6 h for ZnO NPs/PEIE, respectively. This result suggests that the TBA-CNO was significantly more stable EIL than ZnO NPs/PEIE.



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Then, Ag-electrode-based OLEDs with TBA-CNO EILs formed from dispersions with different concentrations (0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 mg/mL) were fabricated. The other layers were the same as those of the abovementioned OLEDs. Figures 4(a) and 4(b) show the J–V–L and Figure S7 presents the other OLED characteristics. There was no correlation between the driving voltages at 75 mA/cm2 and the dispersion concentrations. Figure 5 shows AFM images of glass/F8BT (80 nm)/TBA-CNO nanosheets. The AFM image of the 0.5 mg/mL TBA-CNO covered F8BT was nearly similar to that of the bare F8BT (0 mg/mL). Thus, at 0.5 mg/mL, F8BT was almost uncovered by nanosheets. At 1.0, 2.0, and 4.0 mg/mL, most of the F8BT was covered by nanosheets. At 4.0 mg/mL, however, small amounts of aggregates were observed. At 6.0 and 8.0 mg/mL, there were many aggregates on the F8BT films. Figure 4(c) shows light-emitting images of OLEDs at 75 mA/cm2. At 0.5 mg/mL, nonuniform light emission was observed, resulting from the low coverage ratio of nanosheets. At 1.0, 2.0, and 4.0 mg/mL, almost uniform light emission was observed. However, there were some dark spots in the 4.0 mg/mL TBA-CNO based device. At 6.0 and 8.0 mg/mL, large flower-shaped non-light-emitting areas were observed, resulting from the many aggregates of the TBA-CNO nanosheets. Figure 4(d) shows a plot of the driving voltages at 75 mA/cm2 versus the ratio of non-light-emitting parts to the total device area. The increase in the nonlight-emitting area ratios increased the driving voltages. Thus, the uniform coverage of nanosheets is a key factor for realizing low-driving-voltage devices.


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Figure 4: (a) Current density–voltage and (b) luminance–voltage characteristics and (c) light-emitting images of devices with TBA-CNO EILs formed from different dispersion concentrations of 0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 mg/mL. (d) A plot of driving voltages at 75 mA/cm2 versus the non-light-emitting area ratio of the devices.



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Figure 5: AFM images of glass/F8BT/TBA-CNO nanosheets. The nanosheets were formed from different dispersion concentrations of (a) 0, (b) 0.5, (c) 1.0, (d) 2.0, (e) 4.0, (f) 6.0, and (g) 8.0 mg/mL.

Finally, Al-electrode-based OLEDs with TMA-CNO, TEA-CNO, TPA-CNO, and TBA-CNO nanosheet EILs were fabricated. The effects of the alkyl chain length on the devices’ performances were investigated. All CNO nanosheet films were formed from 2 mg/mL dispersion in IPA solvents. The other layers were the same as those of the abovementioned OLEDs. Figures 6(a) and 6(b) show the J–V–L characteristics, and the other OLED characteristics are shown in Figure S8. The driving voltages increased in the order TPA-CNO < TBA-CNO < TMA-CNO < TEA-CNO. Among the four devices, the TPA-CNO based device showed the lowest driving voltage. Figures 6(c) and 6(d) show light-emitting images of OLEDs at 75 mA/cm2 and a plot of the driving voltages at 75 mA/cm2 versus the ratio of non-light-emitting parts to the total device area. The devices with TMA-CNO and TEA-CNO showed rugged light-emitting images and almost similar non-light-emitting ratios, suggesting the aggregation of the CNO nanosheets. The device with TBA-CNO showed slightly rugged light-emitting image and the second smallest non-light-emitting 12 ACS Paragon Plus Environment

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area ratio. The device with TPA-CNO showed the most good-looking light-emitting image and the smallest non-light-emitting area ratio among them. These qualities of light-emitting images should relate with the stability of each dispersion in IPA solvent. CNO has negative charges on the sheet surface, and bulky cations such as tetraalkylammonium cations stabilize the single CNO nanosheets. Here, small cations such as potassium cations cannot stabilize the single CNO nanosheets because such small ions attract another nanosheet and stabilize multiple nanosheets form. The aggregation in the TMA-CNO- and TEA-CNO-based devices suggested TMA and TEA were too small to stabilize the single CNO nanosheets. The TPA-CNO-based device showed the best appearance among the devices. TPAOH stabilized the CNO single nanosheets efficiently. The small amount of aggregations in the TBA-CNO-based device suggested that relatively instability of the nanosheets in the dispersion. Longer alkyl length weakens electronic interaction between tetraalkylammonium cations and the negative charges on the CNO nanosheets. Hence, we considered that TBAOH with longer alkyl lengths more instabilized the single CNO nanosheets than TPAOH. It is noteworthy that the light-emitting image of the device with TBA-CNO in Figure 6 looks slightly more rugged than that with 2.0 mg/mL TBA-CNO in Figure 4. The light-emitting image qualities were sensitive to the light intensity of device. It seems that the light intensity of the device in Figure 4 is higher than that of the device in Figure 6. Under high light intensity, the light-emitting-images look more uniform. It was difficult to set the light intensities of all devices to one value because of the difference of the lightemitting area ratio for all devices, and each device requires different light intensity for the best lightemitting image quality. Hence, we quantified the light-emitting images as the non-light-emitting area ratios. In fact, both of the devices showed similarly small non-light-emitting area ratio values. The non-light-emitting area ratio also related with the device driving voltage and was a good indicator for evaluation of the quality of devices. 13 ACS Paragon Plus Environment


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Unlike the case of the aforementioned Ag-electrode-based devices, it seems that there was no correlation between the driving voltages and the ratio of non-light-emitting parts to the total device area. We considered that multiple factors involved the operation mechanism of these devices. CNO nanosheets are considered as dielectrics.24 Thus, under the application of voltages, CNO nanosheets should be electrically polarized. Hence, the electrical polarization of the CNO dielectrics caused WF shift of Al-electrodes and induced positive charges in the F8BT layer at the F8BT/CNO interface, enhancing tunneling electron injection through these dielectrics layers as shown in Figure 7. For achieving efficient tunneling injection, we need to consider effects of CNO nanosheet thicknesses, their WF values, and uniform coverage of them onto the F8BT layer on the electron injection efficiency as shown in Figures 8(a)-(c). First, tunneling injection works efficiently through thinner dielectrics than thicker dielectrics,50 and the dependence of tunneling injection on dielectrics thickness shows exponential behavior. In this work, the TMA-CNO showed thinner thickness than the others, and they showed almost similar thicknesses each other despite the difference of alkyl chain lengths. Second, large WF values of dielectrics cause electron injection to the trap sites of dielectrics themselves from a cathode, hindering charge accumulation at the interface to form efficient electrical polarization of the dielectrics. Hence, dielectrics with smaller WF value are desired for achieving more efficient electrical polarization. It is noteworthy that, in typical EIL materials, larger WF value or electron affinity value is more favorable for efficient electron injection.46 However, the WF values of 3.9 eV for TEA-CNO, 4.0 eV for TPA-CNO, and 4.5 eV for TBA-CNO were too large to inject electron to the F8BT with electron affinity of 3.5 eV. Hence, smaller WF value is more favorable in the point of efficient electron accumulation at the interface in this work. Third, the uniform coverage of CNO is favorable for efficient electron injection, thus aggregation of CNO nanosheets is not favorable. Electrical polarization should probably be the 14 ACS Paragon Plus Environment

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maximum when single CNO nanosheets are deposited on the F8BT layer to be parallel to the 2D plane of nanosheets in the interface direction. There are major two types of aggregates. One type is formation of aggregates growing along film interface direction, for example, the flower-shaped aggregates shown in Figure 5(c). The other type is locally formed stacked CNO nanosheets along the film thickness direction. Of course, the former aggregates are not favorable for efficient electric polarization. On the other hand, the influence of the latter aggregates means dependence of the nanosheet thickness on the electric polarization. The degree of electric polarization is proportional to the dielectric constant of dielectrics and inversely proportional to their thickness. It is reported that the dielectric constants of the CNO nanosheets do almost not show dependence on their thickness range from 4.5 nm to 20 nm.24 Hence, thinner thickness is more favorable than thicker thickness for efficient electric polarization in this region. We do not know the dependence of the dielectric constant on the CNO thickness less than 4.5 nm. Overlapped double CNO nanosheets may show dielectric constants which are at least twice as large as single CNO nanosheets. However, considering the dependence of tunneling injection on the film thickness, we considered that single CNO nanosheets are more efficient than overlapped double CNO nanosheets for efficient electron injection. Based on these considerations, we discuss about the experimental results regarding the devices with the different exfoliation agents. The device with TMA-CNO showed lower driving voltage than the device with TEA-CNO although these showed similar light-emitting area ratios. TMA-CNO showed thinner single nanosheet thickness of 1.9 nm than that of TEA-CNO (2.6 nm). Moreover, the WF value of TMA-CNO (3.1 eV) was greatly smaller than that of TEA-CNO (3.9 eV). Hence, more charges were accumulated at the interface at the TMA-CNO/Al interface, enabling more efficient electrical polarization. The device with TPA-CNO showed lower driving voltage than the 15 ACS Paragon Plus Environment


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device with TEA-CNO although these showed similar single nanosheet thicknesses and WF values. In this case, more uniform nanosheet coverage of TPA-CNO than TEA-CNO resulted in more efficient electric polarization in TPA-CNO. The device with TPA-CNO showed lower driving voltage than the device with TBA-CNO. The TBA-CNO showed higher WF value of 4.5 eV than that of TPA-CNO (4.0 eV). The WF value of TBA-CNO was also larger than that of Al-electrode (4.2 eV). Hence, electron was efficiently injected to the TBA-CNO, and so, charge accumulation was hindered at the TBA-CNO/Al interface to form the efficient electric polarization. Moreover, the light-emitting area of the TPA-CNO-based device showed larger than that of the TBA-CNO-based device. These are probably the factors determining the device performances.


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Figure 6: (a) Current density–voltage and (b) luminance–voltage characteristics and (c) light-emitting images of devices with TMA-CNO, TEA-CNO, TPA-CNO, and TBA-CNO EILs. (d) A plot of driving voltages at 75 mA/cm2 versus the non-light-emitting area ratio of the devices.

Figure 7: Electrical polarization induced tunneling electron injection mechanism. Energy level diagram (a) without electric field, and (b) under electric field.



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Figure 8: Factors determining electron tunneling efficiency. (a) CNO thickness effect, (b) electron trapping effect in the CNO, and (c) CNO film uniformity effect. 3. Conclusions In this study, we reported the application of CNO nanosheets for EILs in OLEDs. Devices with CNO nanosheets showed higher performances than those with conventional Liq EILs. We also investigated the influences of the dispersion concentrations and the exfoliation agents of the CNO nanosheets on the devices’ performances, and we found that the uniform coverage of nanosheets on the light-emitting polymer of F8BT, their thicknesses, and their WF values were the important factors for realizing high-performance devices. The results of this work proved that 2D CNO nanosheets can be candidates for EILs in high-performance OLEDs owing to their distinctive functionalities. We believe that their functionalities are useful in developing other high-performance devices.

4. Experimental 4.1. Materials


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PEDOT:PSS (Clevios P VP CH 8000) was purchased from Heraeus Materials Technology. TFB was purchased from American Dye Source, Inc. F8BT was provided by Sumitomo Chemical Co., Ltd. ZnO NPs were synthesized according to established procedure.43 PEIE was purchased from Sigma-Aldrich, Inc. Liq was purchased from e-Ray Optoelectronics Technology Co., Ltd. K2CO3 and CaCO3 were purchased from Wako Pure Chemical Industries, Ltd. Nb2O5 was purchased from Kishida Chemical Co., Ltd. TMAOH, TEAOH, TPAOH, and TBAOH were purchased from commercial sources.

4.2. Preparation of CNO Nanosheet Dispersion19

K2CO3, CaCO3, and Nb2O5 were mixed at a molar ratio of 1 : 4 : 3 and ground for 10 min. Then, the ground mixture was placed in an electric furnace and calcined at 1,200°C for 12 h in air. A white KCa2NbO3 powder was obtained after cooling down the mixture to room temperature. The identification of KCa2Nb3O10 powder was performed using XRD analysis. Then, KCa2Nb3O10 was ion-exchanged to obtain HCa2Nb3O10. 1 g of KCa2Nb3O10 powder and 40 mL of nitric acid were added in a conical flask, and the mixture was stirred for 3 days. Then, the reaction mixture was filtered using a paper filter and washed with water to sufficiently remove potassium ions. The filtered residue powder was dried at 60°C in a vacuum oven overnight. HCa2Nb3O10 was obtained as the dried powder and was then identified using XRD analysis. Then, in order to exfoliate HCa2Nb3O10 to CNO nanosheets, two neutralization equivalents of TMAOH, TEAOH, TPAOH, and TBAOH solutions to the amount of protons in 0.4 g of HCa2Nb3O10 were added in the conical flask. Ultrapure water was added to adjust the total volume of the solution to 100 mL. The solution was stirred for 3 19 ACS Paragon Plus Environment


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days at 500 rpm. The solution was left to stand, and then the supernatant was filtered three times in sequence using paper filters with pore sizes of 7, 4, and 1 µm. The obtained filtrate solution was centrifuged at 10,000 rpm for 15 min and the supernatant was removed. Subsequently, the precipitates were dried in vacuum at 60°C. After drying, IPA was added to the dried precipitates and dispersed by an ultrasonic wave for 20 min. In order to improve the dispersion stability, the dispersion was filtered three times in sequence using paper filters with pore sizes of 7, 4, and 1 µm to remove aggregates. After filtration, parts of the dispersion were evaporated and dried at 150°C for 30 min in order to know the concentration of the dispersion. Finally, IPA was added to the dispersion to obtain CNO nanosheet dispersions with the desired concentrations.

4.3. Characterization Methods for Material Properties

CNO nanosheets for all measurement samples were spin-coated at 2,000 rpm for 20 s followed by thermal annealing at 100°C for 10 min in N2. XRD measurements of KCa2Nb3O10 and HCa2Nb3O10 were performed using a Rigaku SmartLab diffractometer equipped with a rotating anode (Cu Ka radiation, λ = 1.5418 Å). AFM measurements were performed using a Veeco Dimension Icon atomic force microscope with scanning areas of 2.0 µm × 2.0 µm. The samples for the measurements of single nanosheet thickness were formed from 2 mg/mL dispersions, and the dispersions were spin-coated onto glass substrates. The CNO nanosheets for the F8BT/CNO nanosheets samples were formed from dispersions with concentrations of 0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 mg/mL. UV-Vis spectra were measured using a Shimadzu UV-3150 UV-Vis-NIR spectrophotometer. The CNO nanosheets for UV-Vis measurement samples were formed from 2.0 mg/mL. WF values and valence band energy levels were determined using UPS under vacuum. UPS spectra were measured using a 20 ACS Paragon Plus Environment

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Thermo Fisher Scientific Theta Probe X-ray and UV photoelectron spectrometer system. The CNO nanosheets on ITO for UPS measurement samples were formed by spin-coating for three times from 2.0 mg/mL. PL decay curves were measured using a Hamamatsu Photonics Quantaurus-Tau.

4.4. Device Fabrication and Characterization

Glass substrates with ITO electrodes were sequentially cleaned by scrub washing using a neutral detergent dissolved in ultrapure water and rinsed in ultrapure water using an ultrasonic bath sonicator, before dry-cleaning using a UV-ozone cleaner. PEDOT:PSS was spin-coated onto the substrates in air, followed by thermal annealing at 200°C for 10 min. The substrates were then transferred into a N2-purged glove box. TFB dissolved in p-xylene was spin-coated onto PEDOT:PSS, followed by thermal annealing at 180°C for 60 min. F8BT dissolved in p-xylene was spin-coated onto TFB, followed by thermal annealing at 130°C for 10 min. Liq dissolved in 2-ethoxyethanol was spin-coated onto F8BT. ZnO NPs dispersed in alcohol was spin-coated onto F8BT, followed by thermal annealing at 130°C for 10 min. PEIE dissolved in alcohol was spin-coated onto ZnO NPs, followed by thermal annealing at 130°C for 10 min. CNO nanosheets dispersed in IPA were spin-coated onto F8BT. CNO nanosheets for all devices were spin-coated at 2,000 rpm for 20 s, followed by thermal annealing at 100°C for 10 min. Subsequently, the substrates were transferred to a vacuum chamber, and Al or Ag cathodes were deposited using a shadow mask with patterned openings. 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 on the basis of the Lambertian assumption. 21 ACS Paragon Plus Environment


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Supporting Information The optoelectronic properties of OLED materials, XRD data, AFM images, UPS spectra, and OLED characteristics are available from supporting information.

Acknowledgments We would like to thank the “Strategic Promotion of Innovative R&D Program” and “the Center of Innovation (COI) Program” of Japan Science and Technology Agency (JST) for financial support.

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Two-Dimensional Ca2Nb3O10 Perovskite Nanosheets for Electron

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