Low Work Function 2.81 eV Rb2CO3-Doped Polyethylenimine

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Organic Electronic Devices

Low Work Function 2.81 eV Rb2CO3 Doped Polyethylenimine Ethoxylated for Inverted Organic Light-Emitting Diodes Jeonggi Kim, Hyo-Min Kim, and Jin Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04760 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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ACS Applied Materials & Interfaces

Low Work Function 2.81 eV Rb2CO3 Doped Polyethylenimine Ethoxylated for Inverted Organic Light-Emitting Diodes Jeonggi Kim, Hyo-Min Kim and Jin Jang*

Department of Information Display, Advanced Display Research Center (ADRC), Kyung Hee University, Dongdaemoon-gu, Seoul, 130-701, Korea *E-mail: [email protected]

KEYWORDS: Rb2CO3 doped PEIE, interfacial layer, Mg doped ZnO, low work function, inverted OLED.

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ABSTRACT We report a low work function (2.81 eV), Rb2CO3 doped polyethyleneimine ethoxylated (PEIE) which is used for highly efficient and long lifetime, inverted organic light-emitting diodes (OLEDs). Doping Rb2CO3 into PEIE decreases the work function of Li doped ZnO (LZO) by 1.0 eV and thus significantly improve electron injection ability into emission layer (EML). The inverted OLED with PEIE:Rb2CO3 interfacial layer (IL) exhibits higher efficiency and longer operation lifetime than those of the device with a PEIE IL. It is found also that Mg doped ZnO (MZO) can be used instead of LZO as electron transporting layer (ETL). A MZO/PEIE:Rb2CO3 shows a low work function of 2.81 eV. The OLED with MZO/PEIE:Rb2CO3 exhibits low operating voltage of and low efficiency roll-off of 11.8% at high luminance of 10,000 cd m-2. The results are due to the suppressed exciton quenching at the MZO/organic EML interface.

INTRODUCTION Organic light-emitting diode (OLED) is currently the most promising light emitting device in display technology because of the advantages such as low power consumption, high frequency response, wide viewing angle, and vivid colors.1-6 Although active matrix OLEDs (AMOLEDs) are being commercialized in TVs and mobile phones, there exists still technical issues for applications in various displays.7 Most mobile AMOLED panels use low-temperature polycrystalline silicon thin-film transistor (LTPS-TFT) backplanes because of their high mobility and good stability. However, the poor LTPS material uniformity and high manufacturing cost are the main issues for application in large-area AMOLED.8-9 Oxide TFTs can be widely used because of their good performance and low cost.10-11 Since the oxide TFTs show n-type characteristics, inverted OLED structure is more suitable.12 Therefore, inverted OLED with


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bottom cathode can

be directly connected to the drain electrode of n-type oxide TFT. Note that

the inverted OLED with top anode can use the air-stable metals with high work function.7 Electron injection from the bottom cathode through the organic layers of electron transporting layer (ETL) into emission layer (EML) is a major issue in inverted OLEDs because of the high energy barrier between the bottom cathode and organic layers. Thus, electron injection layers (EILs) play an important role in the inverted OLEDs.13 Solution processed inorganic n-type metal oxides, such as ZnO,14 Li doped ZnO (LZO),15 Al doped ZnO (AZO),16 Mg doped ZnO (MZO),17 and Ga doped ZnO (GZO)18 have recently been proposed as effective ETLs of inverted OLEDs and inverted quantum-dot light-emitting diodes (QD-LEDs), due to their air stability, transparency and high electron mobilities. However, the work function of ZnO is about ~4.4 eV, which is much deeper than the lowest unoccupied molecular orbital (LUMO) level of the organic EML (2.8−3.0 eV). This large potential barrier reduces the electron injection into EML, resulting in the unbalanced electron-hole recombination and thus lower device performance.19 Recently, branched polyethyleneimine (PEI) and polyethyleneimine ethoxylated (PEIE) have been used as an effective interfacial layer (IL).20-26 The PEI and PEIE can be easily processed in air and dissolved in environmentally friend solvents such as deionized (DI) water or methoxyethanol. In addition, these polymers can cause the formation of strong interfacial dipole on ZnO. The PEI and PEIE coated ZnO layers has a work function of 3.3−3.6 eV which is much smaller compared to ZnO (~4.4 eV).27 This can induce efficient electron injection and blocking of exciton quenching in the inverted OLEDs. Zhou et al. report the air-stable PEI and PEIE without change in work function.27 Fukagawa et al. report the long-term storage stability of the inverted OLED with the ZnO/PEIE. However, the operational lifetime of the device with ZnO/PEIE is shorter than that of the device without ZnO/PEIE.28 Chiba et al. report the


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improved electron injection property and longer operational lifetime of the inverted OLEDs with the solution processed lithium 8-quinolate (Liq)-doped PEIE as an IL on ZnO. However, the device efficiency is much lower than those of state-of-the-art ZnO/PEIE-based OLEDs.19 Lowering the IL work function enhances the electron injection and device efficiency. Some alkali-metals have low work functions, such as Li (2.93 eV), Na (2.36 eV), K (2.29 eV), Rb (2.26 eV) and Cs (2.14 eV).

29,30

The alkali-metal carbonate compounds (Li2CO3, K2CO3,

Rb2CO3 and Cs2CO3) are widely used as effective n-type dopants to reduce the electron injection barrier.31-33 In addition, these metal carbonates are suitable n-type dopants for solution-processed IL because of their good solubility in DI water or methoxyethanol. Here, we report an inverted OLEDs with the solution-processed, Rb2CO3 doped PEIE ILs. The LZO and MZO are used as ETL because of their better electron transport ability than ZnO in inverted devices.15,17 The Rb2CO3 doping in PEIE lowers its work function and thus device performance is enhanced. The work function decreases as Rb2CO3 doping concentration in PEIE increases. The inverted OLED with PEIE:Rb2CO3 shows higher efficiency and longer operation lifetime than those of the device with PEIE. We also employed MZO as an ETL instead of LZO. The MZO/PEIE:Rb2CO3 shows much lower work function, 2.81 eV, compared with other ZnO/PEIE based films (Table S1). Its electron transport properties are significantly improved, resulting in lower operating voltage and lower efficiency roll-off at high luminance region. This improvement is due to the suppressed exciton quenching as well as non-radiative recombination at MZO/EML interface.

RESULTS AND DISCUSSION


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Figure 1a shows the chemical structures of PEIE and Rb2CO3. The PEIE:Rb2CO3 can be formed by hydrogen bonding between hydroxyl groups (-OH) in PEIE and carbonyl groups (C=O) in Rb2CO3. Figure 1b–d shows the atomic force microscopy (AFM) surface images and line profiles of the LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 layers on ITO substrate. As shown in Figure 1b, the LZO film shows a smooth surface with root mean square (RMS) roughness of 1.04 nm and peak-to-valley (Rpv) of 2.96 nm, which indicates that the LZO is uniformly coated on ITO substrate. However, some aggregations can be observed in the LZO/PEIE film, with the RMS roughness of 1.55 nm and Rpv of 8.68 nm (Figure 1c). This is due to the intramolecular hydrogen bonding of PEIE.34 Note that with doping of Rb2CO3 into the PEIE, large spherical grains with the RMS roughness of 3.55 nm and Rpv of 14.31 nm are found as shown in Figure 1d. The grains are formed by the strong hydrogen bonding between PEIE and Rb2CO3, resulting in the aggregation of PEIE and Rb2CO3. The transmittance spectra of the LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 films in the wavelength range 250–700 nm are shown in Figure 1e. The transmittances are over 95% in the visible region. Figure 1f shows the UV−vis absorption spectra of LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 films. The onset of absorption in the LZO/PEIE:Rb2CO3 exhibits a slightly red shift. The inset in Figure 1f shows the plots of (αhν)2−photon energy (hν), where α is the absorption coefficient. The optical band gaps of the thin-films could be determined by the absorption onset of the linear regions,35 and found to be 3.52, 3.52 and 3.42 for the LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 films, respectively. Figure 1g shows the X-ray diffraction (XRD) intensities of LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 films. The broad peak at 2θ = 22o and the absence of sharp peak confirm that all samples are amorphous.36


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The effect of Rb2CO3 doping in PEIE on the work function shift was investigated. The LZO and MZO solutions were spin-coated to form 70 nm films on ITO substrates, then the PEIE:Rb2CO3 solutions were spin-coated on the LZO or MZO to form a 10 nm layer. He I (21.2 eV) Ultraviolet photoelectron spectroscopy (UPS) was performed on LZO, LZO/PEIE, LZO/PEIE:Rb2CO3 and MZO/PEIE:Rb2CO3 films as shown in Figure 2a. According to their secondary electron cut-off regions, the cut-off position of the LZO film shifts to higher binding energy when PEIE is coated on the LZO layer. Figure 2b shows the work functions from the UPS spectra. It decreases from 4.03 eV for the LZO to 3.58 eV for LZO/PEIE. This is due to the strong interfacial dipole induced by the large number of amine groups in backbone and side chains of PEIE (Figure 1a). The work function of LZO/PEIE is 3.58 eV which is almost same as the ZnO/PEIE bilayer (3.55 eV).37 The large interfacial dipole formation results in significant work function shift and subsequent reduction of the electron injection barrier between the LZO ETL and organic EML. The Rb2CO3 doping into PEIE further decreases the work function. The work function of LZO/PEIE:Rb2CO3 gradually shifts from 3.58 eV for LZO/PEIE (1:0) to 3.04 eV for LZO/PEIE:Rb2CO3 (1:1) as shown in Figure S1. For the LZO/PEIE:Rb2CO3 1:2, the work function increases to 3.16 eV. Note that Rb2CO3 (0:1) coated LZO layer shows a small work function shift of 0.16 eV, from 4.03 eV for LZO to 3.87 eV for LZO/Rb2CO3. The MZO, known as effective ETL in a QD-LED,17,38 can function well in OLED. To investigate the band structure of MZO, UPS analysis was performed as shown in Figure S2a. Its work function is 3.81 eV, which is lower than that of the LZO (4.03 eV). The lower work function of MZO can shift the Fermi level more, which is consistent with previous report.17 The work function of MZO/PEIE:Rb2CO3 is found to be 2.81 eV, shifted from 3.81 eV of MZO. Therefore, the electron injection barrier between MZO/PEIE:Rb2CO3 (2.81 eV) and LUMO level


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of tris(2-phenylpyridinato-C2,N)iridium(III) (Ir(ppy)3) emitting material (2.80 eV) is negligible (Figure 2c). In this work, Ir(ppy)3 green dopant with LUMO level of 2.80 eV had been selected because of the well matching with the MZO/PEIE:Rb2CO3 ETL having 2.81 eV work function. To investigate the work function shift of ETL by PEIE:Rb2CO3, the inverted OLEDs were fabricated using the following structure : ITO (200 nm)/ETL (70 nm)/PEIE:Rb2CO3 (10 nm)/ 2,2 ′,2″-(1,3,5-benzenetriyl)-tris-[1-phenyl-1-H-benzimidazole] (TPBi):12% Ir(ppy)3 (15 nm)/ 4,4′,4 ″ -tris(carbazol-9-yl)-triphenylamine (TCTA):12% Ir(ppy)3 (15 nm)/TCTA (5 nm)/ N,N ′ di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) (20 nm)/ 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) (20 nm)/Al (100 nm). The schematic energy diagram of the inverted OLEDs with different PEIE:Rb2CO3 ratio is shown in Figure 2c. The ETLs (LZO or MZO) and PEIE:Rb2CO3 layers were spin-coated from solution, and the other subsequent organic layers and Al electrode were deposited by thermal evaporation as shown in Figure 2d. To optimize the PEIE:Rb2CO3 ratio, different Rb2CO3 doping ratio in PEIE are employed as the IL for inverted OLEDs. Figure S3 and Table S2 compare the performances of the devices with different PEIE:Rb2CO3 ratio. Current density−voltage (J−V) and luminance−voltage (L−V) characteristics of the devices are shown in Figure S3a,b. The device with PEIE:Rb2CO3 1:1 exhibits the highest current density among them. The J−V characteristics can be seen in log scale in the inset of Figure S3a. The leakage currents at -5 V decrease as Rb2CO3 concentration increases, and saturate at the PEIE:Rb2CO3 ratio of 1:1. At PEIE:Rb2CO3 1:2, the leakage current at -5 V increases slightly. As shown in Table S2, the devices with different PEIE:Rb2CO3 ratio show similar turn-on voltage (VT) and driving voltage (VD). The VT and VD are defined as voltages which the luminance reaches at 1 and 1000 cd m-2, respectively. Note that the PEIE:Rb2CO3 1:1 exhibits the highest luminance at 8 V, which is 1.5~1.8 times higher than those of the other


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devices. Note that the device with PEIE:Rb2CO3 1:1 exhibits higher current and power efficiencies. These results are consistent with work function shifts of LZO/PEIE:Rb2CO3 observed by UPS measurement (Figure S1). We also fabricated the devices with LZO or LZO/Rb2CO3 (without PEIE) as shown in Figure S4 and Table S3. The devices with a LZO/Rb2CO3 exhibit higher VD and lower efficiency due to a large electron injection barrier between LZO/Rb2CO3 (3.67eV) and LUMO level of Ir(ppy)3 emitting material (2.8 eV). Figure 3 shows the performances of the inverted OLEDs with various ETLs (LZO and MZO) and ILs (PEIE and PEIE:Rb2CO3). The VT, VD and efficiency values of the devices are summarized in Table 1. J−V and L−V characteristics of the devices are shown in Figure 3a,b. The device with LZO/PEIE (without Rb2CO3) exhibits a current density of 38 mA cm-2 and luminance of 17,250 cd m-2 at 8V, while the device with PEIE:Rb2CO3 exhibits significantly improved current density of 83 mA cm-2 and luminance of 31,540 cd m-2 at 8V. Moreover, the device with LZO/PEIE:Rb2CO3 exhibits a VT of 3.62 V and VD of 5.10 V, which are lower than those of the device with LZO/PEIE (VT of 3.85 V and VD of 5.12 V). It clearly demonstrates that the Rb2CO3 doping in PEIE improves electron injection from ETL to EML. The efficiencies−luminance characteristics of devices are shown in Figure 3c,d. The maximum current efficiency of the device with LZO/PEIE:Rb2CO3 is much higher than those the device with LZO/PEIE. The device with LZO/PEIE:Rb2CO3 exhibits the current efficiency of 53.37 cd A-1 and power efficiency of 24.65 lm W-1 at 10,000 cd m-2, whereas the device with LZO/PEIE exhibits 39.98 cd A-1 and 17.99 lm W-1 at 10,000 cd m-2. The device with LZO/PEIE shows higher efficiency roll-off at high luminance region compared to the device with LZO/PEIE:Rb2CO3. It is due to the imbalance of electrons and holes in the device with


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LZO/PEIE, resulting in degradation of the luminance and efficiencies. These results demonstrate that the addition of Rb2CO3 into PEIE is highly effective to improve the device performance. To evaluate the electron transport property of MZO/PEIE:Rb2CO3, the inverted OLED with MZO/PEIE:Rb2CO3 was also fabricated. The device with MZO/PEIE:Rb2CO3 shows better performance than the devices with LZO/PEIE:Rb2CO3 (Figure 3 and Table 1). The device with MZO/PEIE:Rb2CO3 exhibits the VD of 5.00 eV, which is lower than that of the device with LZO/PEIE:Rb2CO3 (5.10 eV). Although, the device with MZO/PEIE:Rb2CO3 exhibits lower the maximum efficiencies of 65.49 cd A-1 and 43.00 lm W-1, the device maintains high efficiencies of 57.75 cd A-1 and 27.82 lm W-1 at 10,000 cd m-2. This may be attributed the balanced electron– hole balance in EML. We have characterized 24 devices with and without Rb2CO3 doping. The histograms of the maximum current and power efficiencies of 24 OLED devices are shown in Figure S5. Standard deviations of OLEDs with LZO/PEIE are ± 0.13 cd A-1 for current efficiency and ±0.14 lm W-1 for power efficiency. And, the standard deviations of OLEDs with LZO/PEIE:Rb2CO3 and MZO/PEIE:Rb2CO3 are ±0.27 cd A-1 and ±0.39 cd A-1 for current efficiency and ±0.34 lm W-1 and ±0.30 lm W-1 for power efficiency, respectively. The devices with PEIE:Rb2CO3 show higher standard deviation than the device with PEIE, which may be related with the higher roughness shown in Figure 1b to 1d. The single carrier devices, electron and hole only devices are investigated, and the results are shown in Figure S6. The structure of electron-only device (EOD) is ITO/ETL (70 nm)/PEIE:Rb2CO3 (10 nm)/TPBi:Ir(ppy)3 (15 nm)/LiF (1 nm)/Al (100 nm), and hole-only device (HOD) is ITO/PEDOT:PSS (20 nm)/TCTA:Ir(ppy)3 (15 nm)/TCTA (5 nm)/NPB (20 nm)/HATCN (20 nm)/Al (100 nm). Significant increase in current density is observed for EODs when MZO is used as ETL, which reaches 2.2-fold higher current than that of LZO. Note that the


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current density of EOD with MZO/PEIE:Rb2CO3 matches well with that of the HOD. This result demonstrates that the MZO/PEIE:Rb2CO3 can balance the injection of charge carriers into the EML. The effect of Cs2CO3 doping into PEIE was also investigated for comparison. The PEIE:Cs2CO3 ratio is fixed at 1:1. The PEIE:Cs2CO3 has also been demonstrated as a surface modification material to EML for OLEDs.38 The work function of LZO/PEIE:Cs2CO3 significantly shifts from 3.58 eV for LZO/PEIE to 3.16 eV as shown in Figure S2b, but the shift is smaller than the LZO/PEIE:Rb2CO3 (3.01 eV). Therefore, the election injection barrier is larger than that between LZO/PEIE:Rb2CO3 and EML. The maximum efficiencies of the device with PEIE:Cs2CO3 are comparable to those of the device with PEIE:Rb2CO3 as shown in Figure S7 and Table S4. However, the device with PEIE:Cs2CO3 shows higher operating voltages and higher efficiency roll-off at high luminance region. The device with PEIE:Cs2CO3 exhibits the current efficiency of 49.48 cd A-1 and power efficiency of 20.43 lm W-1 at 10,000 cd m-2, whereas the device with PEIE:Rb2CO3 exhibits 53.37 cd A-1 and 24.65 lm W-1 at 10,000 cd m-2. These results are consistent with the difference in work function shift between LZO/PEIE:Rb2CO3 and LZO/PEIE:Cs2CO3 (Figure S2b), and clearly demonstrate that electron injection property of the PEIE:Rb2CO3 is better than that of the PEIE:Cs2CO3. To investigate the optical characteristics of the LZO and MZO films, the UV-vis absorption measurement was performed. Figure 4a shows the absorption onset of MZO film which is slightly blue-shifted from LZO and the inset shows the plots of (αhν)2−hν. The optical band gaps for LZO and MZO are found to be 3.52 and 3.60 eV, respectively. The thermal stabilities of LZO and MZO were measured by thermogravimetric analysis (TGA). The samples were heated from room temperature to 500 oC. Figure 4b shows that the decomposition temperature (T50,


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temperature at 50% weight loss) of MZO is ~346 oC, whereas it is 334 oC for LZO. This indicates that the thermal stabilities for both oxides are good. In the inverted OLED structure (ITO/ETL/PEIE:Rb2CO3/EML/HTL/Al), a recombination zone can form at the ETL/EML interface due to higher hole current density of HTL than the electron current density of ETL/PEIE:Rb2CO3 (Figure S6). Thus, the excitons can be easily quenched by the non-radiative recombination at the ETL/EML interface. Kim et al. report that PEIE can block holes and thus reduce the exciton quenching at the ZnO/EML interface.39 To investigate the effect of PEIE:Rb2CO3 on exciton quenching at the LZO and MZO surfaces, we conducted the time-resolved photoluminescence (TRPL) for LZO and MZO ETLs. Figure 4c shows the TRPL spectra of the LZO/EML, LZO/PEIE:Rb2CO3/EML and MZO/PEIE:Rb2CO3/EML. The LZO/PEIE:Rb2CO3/EML shows a shorter PL decay time than LZO/EML (without PEIE:Rb2CO3). The decrease of PL decay time by inserting PEIE:Rb2CO3 indicates that the excitons quenching is reduced. The measured PL decay times of the LZO/PEIE:Rb2CO3/EML and MZO/PEIE:Rb2CO3/EML are 1.21 and 1.08 µs, respectively. The MZO exhibits shorter PL decay time than the LZO. Thus, by using a MZO as ETL, exciton quenching and non-radiative recombination processes can be more suppressed. Furthermore, MZO can efficiently block holes than LZO due to the reduced defect concentration in MZO.38 To evaluate the stability of the inverted OLEDs, we measured luminance over time at room temperature and constant current at the initial luminance of ~5000 cd m-2. LT90 lifetime is defined as the time for the luminance drop to 90% of initial luminance under continuous operation. The device with LZO/PEIE:Rb2CO3 shows longer operational lifetime of LT90 = 56 h than the device with LZO/PEIE (LT90 = 17 h). This demonstrates that the addition of Rb2CO3 in PEIE can effectively improve device lifetime. In the device with MZO/PEIE:Rb2CO3, the


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operational lifetime further improves to LT90 = 96 h, which is 5.6-fold longer compared to the device with LZO/PEIE. Figure 5 shows the X-ray photoelectron spectroscopy (XPS) spectra of the LZO, LZO/PEIE, LZO/PEIE:Rb2CO3 and MZO/PEIE:Rb2CO3 films. In the Zn 2p XPS spectra (Figure 5a), the LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 show the two peaks at 1044.1 and 1021.1 eV corresponding to Zn 2p1/2 and Zn 2p3/2, respectively, due to Zn−O bonds. Interestingly, MZO/PEIE:Rb2CO3 exhibits a shift toward higher binding energies from 1044.1 to 1044.5 eV for Zn 2p1/2 and from 1021.1 to 1021.5 eV for Zn 2p3/2, which confirms that the Mg dopants are incorporated into ZnO. A PEIE is a polymer composed of amine groups in their backbone and side chains (Figure 1a), which can cause strong dipoles by Zn−N chemical bonding. As shown in Figure 5b, the samples with PEIE show clearly N 1s peak at 339.7 eV, corresponding to nitrogen atoms in Zn−N bonding.40 However, N 1s peak is not shown in the XPS spectrum of the LZO. These results reveal the formation of the PEIE on the LZO and MZO layers. Figure 5c–f shows O 1s XPS spectra of the LZO, LZO/PEIE, LZO/PEIE:Rb2CO3 and MZO/PEIE:Rb2CO3. Here, the O 1s spectra are fitted with two main peaks at 531.9 ± 0.3 and 530.5 ± 0.3 eV, which correspond to zinc-hydroxide (Zn−OH) and oxygen atoms in ZnO, respectively. As shown in Figure 5c, LZO film shows Zn−OH groups of 67% and Zn-O bonding of 33%. However, LZO/PEIE film exhibits the decreased Zn−OH from 67 to 62% as shown in Figure 5d. This indicates that some of Zn−OH converts to Zn-N when PEIE is coated on the LZO, which is consistent with the formation of Zn−N with peak at 339.7 eV in the N 1s XPS spectrum of LZO/PEIE. Interestingly, unlike the LZO/PEIE, the LZO/PEIE:Rb2CO3 exhibits the increase in Zn−OH from 67 to 76% and shift of main peak from 531.9 to 531.6 eV as shown in Figure 5e, which is attributed to the similar locations of O 1s peaks of Zn−OH (531.9 eV) and Rb2CO3


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(531.5 eV). The MZO/PEIE:Rb2CO3 shows the similar trend with LZO/PEIE:Rb2CO3 (i.e., increased proportion of Zn-OH groups from 67 to 71% and shifted main peak from 531.9 to 531.5 eV). These results confirm that the Rb2CO3 is successfully doped into PEIE, and PEIE:Rb2CO3 is formed on LZO and MZO layer. Based on the XPS analysis, we propose that the interface between ETLs (LZO and MZO) and ILs (PEIE and PEIE:Rb2CO3) can be represented as shown in Figure 6. The LZO film has a more Zn−OH bonds compared to Zn−O bonds, and the Li dopants are incorporated in ZnO as shown in Figure 6a. PEIE is composed of amine groups in their backbone and side chains, which leads to the saturation of Zn−N bonds. Therefore, the interface between LZO and PEIE is formed by the combination of Zn and N atoms as shown in Figure 6b. The interface between LZO and PEIE:Rb2CO3 shows a similar structure with LZO/PEIE as shown in Figure 6c, forming by the combination of Zn and N atoms. However, LZO/PEIE:Rb2CO3 film shows significantly reduced Zn−N bonds compared to the LZO/PEIE (Figure S8e) because of the reduced amount of PEIE as Rb2CO3 doping into PEIE. The hydrogen bonding between the hydroxyl groups (-OH) of PEIE and the carbonyl groups (C=O) of Rb2CO3 can lead to the aggregation with PEIE and Rb2CO3 to form the large spherical grains in the film, explaining the grains shown in AFM image (Figure 1d). The MZO/PEIE:Rb2CO3 interface seems to be similar to that of LZO/PEIE:Rb2CO3. The role of metal cations and doping mechanism of alkali metal can be found in the article.30 The carbonyl group (C=O) of Rb2CO3 can bond with hydroxyl group (-OH) of PEIE in PEIE:Rb2CO3 solution and the generated Rb cations (Rb+) remain in the PEIE polymer chain. When the PEIE:Rb2CO3 solution is coated on LZO (or MZO), the Rb+ in PEIE increases the dipole moment at the interface. This leads to shift more the work function.


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CONCLUSION In this work, we demonstrate that addition of Rb2CO3 into PEIE as an IL of inverted OLED is highly effective in enhancing electron injection from LZO ETL into organic EML, and improves the device performance. The MZO/PEIE:Rb2CO3 shows a low work function of 2.81 eV, which significantly reduces an electron injection barrier into EML. Furthermore, the device with PEIE:Rb2CO3 shows higher efficiency and higher luminance because of its enhanced electron injection property. MZO is also used as an ETL instead of LZO and it is found its excellent electron transport ability. The device with MZO/PEIE:Rb2CO3 shows a lower efficiency roll-off at high luminance region compared to the device with LZO/PEIE:Rb2CO3, and exhibits 5.6-fold longer operational lifetime compared to the device with LZO/PEIE. The significant improvement of device performance could be ascribed to the following reasons: 1) enhanced electron injection property of PEIE:Rb2CO3; 2) balanced electron–hole and recombination; 3) suppressed exciton quenching and non-radiative recombination processes at ETL/EML interface; 4) efficient holes blocking of MZO ETL. From the XPS analysis, it is found that the Rb2CO3 is doped into PEIE, and PEIE:Rb2CO3 is formed on the MZO layer. Therefore, PEIE:Rb2CO3 can be utilized as an effective IL in the inverted OLEDs. EXPERIMENTAL Materials. PEIE solution (Mw = 110 000) was purchased from Sigma-Aldrich and diluted with 2-methoxyethanol to 0.5 wt%. Rb2CO3 powder was also purchased from Sigma-Aldrich and dissolved in 2-methoxyethanol with concentration of 0.5 wt%. The PEIE solution was mixed with Rb2CO3 solution at different ratio (PEIE:Rb2CO3 2:1, 1:1 and 1:2). For comparison, Cs2CO3 solution dissolved in 2-methoxyethanol with 0.5 wt% was also prepared. The LZO and MZO


14

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solutions dissolved in 2-methoxyethanol were prepared according the previously reported method.15,16,41 Characterization. The transmittance and absorbance data were measured using a SCINCO S4100 spectrophotometer at room temperature. The work functions determined by UPS with photon source of He I (21.2 eV). The KONICA MINOLTA CS100A luminance meter and Keithley 2635A source meter were used to measure the J–V, L–V and efficiency–L characteristics. TGA was carried out using a TA instrument Q600 under nitrogen atmosphere at a heating rate of 10

o

C min-1. The TRPL spectra were measured using a FP-8300

spectrofluorometer. Device fabrication of inverted OLED. Inverted OLEDs were fabricated on ITO glass substrates with the sheet resistance of 9 Ω sq-1. The ITO substrates were first cleaned by ultrasonication with acetone, methanol, and isopropanol sequentially for 15 min each. The LZO and MZO as ETL were spin-coated onto the ITO substrate at 2000 rpm for 30 s, and annealed at 190 o

C for 30 min in a nitrogen-filled glove box. After that, the prepared PEIE:Rb2CO3 solutions

were spin-coated onto the ETL at 4000 rpm for 50 s, and then annealed at 100 oC for 10 min in ambient atmosphere. Other functional layers of TPBi, Ir(ppy)3, TCTA, NPB, HAT-CN and Al anode were deposited by thermal evaporation under high vacuum (~10-7 Pa). Active area of the devices was 4 mm2. Finally, the inverted OLEDs were encapsulated with epoxy glue and glass cover in a nitrogen-filled glove box. ASSOCIATED CONTENT Supporting Information Available: Work function of PEIE and doped PEIE on different under layers, UPS measurement, device performance of inverted OLEDs using different ETLs and ILs,


15


Page 16 of 32

structures and J-V curves of EODs and HOD and XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Prof. Jin Jang. (E-mail: [email protected]), Tel: +82-2-961-0688, Fax: +82-2-961-9154 Author Contributions All authors contributed to this work and wrote the manuscript equally. ACKNOWLEDGMENT This work was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program(10080454,Development of High-resolutions OLED MicroDisplay and Controller SoC for AR/VR Device ) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea).

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Figures

Figure 1. (a) Chemical structure of the used materials for IL. AFM surface images and surface line profiles for (b) LZO, (c) LZO/PEIE and (d) LZO/PEIE:Rb2CO3. (e) Transmittance spectra, (f) absorption spectra and (g) XRD pattern for LZO, LZO/PEIE and LZO/PEIE:Rb2CO3 thinfilms. The inset in Figure 1f shows the plots of (αhν)2−hν converted from the absorption spectra of the thin-films, where α is the absorption coefficient.


23

(a)

Intensity (a.u)

MZO/PEIE:Rb2CO3 LZO/PEIE:Rb2CO3 LZO/PEIE LZO 20

19

18

2.8

Page 24 of 32

MZO/PEIE:Rb2CO3

(b)

LZO/PEIE:Rb2CO3

3.2

LZO/PEIE 3.6

LZO 4.0

17

ETL / lL

Binding energy (eV)

(c)

MZO/PEIE:Rb2 CO3 – 2.81 eV LZO/PEIE:Rb 2CO3 – 3.01 eV 2.7 eV LZO/PEIE – 3.58 eV ITO

4.03 eV

4.2 eV MZO LZO 7.4 eV 7.5 eV

2.2 eV

2.8 eV

3.81 eV Interfacial layer

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Work function (eV)


TCTA: TPBi: Ir(ppy)3 Ir(ppy) 3

T C T A

Al (100 nm) HAT-CN (20 nm)

NPB

5.3 eV 5.6 eV

(d)

2.3 eV

5.4 eV

Al

NPB (20 nm)

4.3 eV

TCTA (10 nm) TCTA:Ir(ppy)3 (15 nm) TPBi:Ir(ppy)3 (15 nm) PEIE:Rb2CO3

5.7 eV

6.2 eV HATCN

LZO or MZO (70 nm)

9.5 eV

ITO

Thermalevaporated

Spin-coated

Figure 2. (a) The secondary electron cut-off region of UPS measurement for ETL/IL thin-films on ITO substrate. (b) The work function of ILs on ETLs determined from UPS measurement. (c) Energy level diagram of inverted OLED using various ETLs and ILs. (d) Device structure of green inverted OLED with PEIE:Rb2CO3 IL.


24

Page 25 of 32

LZO / PEIE LZO / PEIE:Rb2CO3

80 Log current density (mA cm-2)

MZO / PEIE:Rb2CO3

60

40

3


2 1

MZO / PEIE:Rb2CO3

0 -1 -2 -3 -4 -5 -6 -7 -4

20

-2

0

2

4

6

(b)

104

Luminance (cd m-2)

(a)

-2

Current density (mA cm )

100

8

Voltage (V)

103

102

101


100

MZO / PEIE:Rb2CO3 10-1

0 3

4

5

6

7

3

8

4

5

80

7

8

8000

10000

60

(c)

-1

70

Power efficiency (lm W )

-1

6

Voltage (V)

Voltage (V)

Current efficiency (cd A )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20


10

MZO / PEIE:Rb2CO3

0

(d)

50 40 30 20


10

MZO / PEIE:Rb2CO3 0

0

2000

4000

6000

8000 -2

Luminance (cd m )

10000

0

2000

4000

6000

-2

Luminance (cd m )

Figure 3. Device performance of the inverted OLEDs with various ETLs (LZO and MZO) and ILs (PEIE and PEIE:Rb2CO3 1:1); (a) J−V characteristics (Inset image shows J–V characteristics plotted in log scale), (b) L−V characteristics, (c) current efficiency−luminance, and (d) power efficiency−luminance characteristics.


25


2.5

3.0

3.5

4.0

Weight (%)

2.0

(b)

100

LZO MZO

2 (α α hv)

Absorbance (a.u.)

(a)

4.5

hv (eV)

80

LZO MZO

T50 ~ 346 oC

60 40

T50 ~ 334 oC

20 0

300

400

500

600

100

700

200

300

400

500

o

Temperature ( C)

Wavelength (nm) 100

LZO/EML LZO/PEIE:Rb2CO3/EML

Luminance (%)

PL intesity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

MZO/PEIE:Rb2CO3/EML

τ ~ 1.21 μs τ ~ 1.25 μs

(c)

τ ~ 1.08 μs

(d)

98

LZO / PEIE LZO / PEIE:Rb2CO3 MZO / PEIE:Rb2CO3

96

@ 5000 cd m-2 94

92

90

2

3

4

5

6

7

8

0

20

40

60

80

100

Time (hours)

Time (µ µs)

Figure 4. (a) UV-vis absorption spectra of LZO and MZO thin-films. Inset shows the (αhν)2−hν plots converted from the absorption spectra of LZO and MZO. The optical band gaps of LZO and MZO are calculated to be 3.52 and 3.60 eV, respectively. (b) TGA for LZO and MZO ETLs. The T50 of LZO and MZO are around 334 and 346 oC, respectively. (c) TRPL spectra of the LZO/EML, LZO/PEIE:Rb2CO3/EML and MZO/PEIE:Rb2CO3/EML. The PL decay times are 1.25

µs

for

LZO/EML,

1.21

µs

for

LZO/PEIE:Rb2CO3/EML

and

1.08

µs

for

MZO/PEIE:Rb2CO3/EML. (d) Lifetime measurement of inverted OLEDs with various ETLs and ILs at initial luminance of 5000 cd m-2.


26

Page 27 of 32

(a)

(b)

N1s

Intensity (a.u.)

Intensity (a.u.)

Zn2p

MZO / PEIE:Rb2CO3 LZO / PEIE:Rb2CO3 LZO / PEIE

MZO / PEIE:Rb2CO3 LZO / PEIE:Rb2CO3 LZO / PEIE

LZO 1050

1040

LZO 1030

1020

390

395

Original peak Fitted peak Zn-OH peak Zn-O peak

67%

535

534

533

33%

532

531

530

529

528

62%

535

Intensity (a.u.)

O1s

Zn-O peak

535

534

533

24% 532

531

530

Binding energy (eV)

O1s

534

38%

533

532

531

530

529

528

Binding energy (eV)

Fitted peak Zn-OH + Rb2CO3 peak

76%

410

Original peak Fitted peak Zn-OH peak Zn-O peak

Binding energy (eV)

(e) Original LZO/PEIE:Rb2CO3 peak

405

(d) LZO/PEIE

O1s

Intensity (a.u.)

Intensity (a.u.)

(c) LZO

400

Binding energy (eV)

Binding energy (eV)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


529

528

MZO/PEIE:Rb2CO3 (f) Original peak

O1s

Fitted peak Zn-OH + Rb2CO3 peak Zn-O peak

71% 535

534

533

29% 532

531

530

529

528

Binding energy (eV)

Figure 5. XPS spectra of LZO, LZO/PEIE, LZO/PEIE:Rb2CO3 and MZO/PEIE:Rb2CO3 thinfilms on glass; (a) Zn 2p, (b) N 1s, (c) O 1s for LZO, (d) O 1s for LZO/PEIE, (e) O 1s for LZO/PEIE:Rb2CO3 and (f) O 1s for MZO/PEIE:Rb2CO3. The peaks at 1044.7 and 1021.5 eV in the Zn 2p spectra of the four samples are assigned to Zn 2p1/2 and Zn 2p3/2, respectively, suggesting that they are characteristic of Zn−O bonds. The binding energy peaked at around 400


27


Page 28 of 32

eV is assigned to the characteristic peak of N 1s, which reveals the formation of the PEIE and PEIE:Rb2CO3 atop the LZO layer. O 1s spectra have two components corresponding to Zn−OH (531.9 eV) and Zn-O (530.4 eV).


28

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Figure 6. Schematic illustration of proposed interface structure between ETL (LZO and MZO) and IL (PEIE and PEIE:Rb2CO3). (a) LZO layer with Li dopant into the ZnO. The Li dopants were successfully incorporated into ZnO. (b) LZO/PEIE interface. PEIE is polymer composed of


29


Page 30 of 32

amine groups in their backbone and side chains, which can cause strong molecules dipoles by Zn−N chemical bonding. (c) LZO/PEIE:Rb2CO3 interface. The PEIE:Rb2CO3 film can be formed by hydrogen bonding between the hydroxyl groups (-OH) of PEIE and the carbonyl groups (C=O) of Rb2CO3. (d) MZO/PEIE:Rb2CO3 interface. Mg dopants were successfully incorporated into ZnO.


30

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Tables Table 1. Device performance of inverted OLEDs with various ETLs and ILs. Maximum

ETL/IL

VTa) [V]

VDb) [V]

LZO/PEIE

3.85

5.12

59.19

LZO/PEIE:Rb2CO3

3.62

5.10

MZO/PEIE:Rb2CO3

3.64

5.00

a)

CEc) [cd A-1]

PEd) [lm W-1]

@ 1000 cd m-2

@ 10,000 cd m-2

CE [cd A-1]

PE [lm W-1]

CE [cd A-1]

PE [lm W-1]

43.04

53.62

33.40

39.98

17.99

67.48

45.36

65.73

38.81

53.37

24.65

65.49

43.00

64.48

40.55

57.75

27.82

-2

The voltage at luminance of 1 cd m . The voltage at luminance of 1000 cd m-2. Current efficiency. d) Power efficiency.

b) c)


31


Page 32 of 32

ToC figure


32

Low Work Function 2.81 eV Rb2CO3-Doped Polyethylenimine

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