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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Effects of Energy-Level Alignment on Characteristics of Inverted Organic Light-Emitting Diodes Shizuka Kawamura,†,§ Kazuma Suzuki,†,§ Tsubasa Sasaki,‡,§ Taku Oono,‡,§ Takahisa Shimizu,‡ and Hirohiko Fukagawa*,†,‡ †

Tokyo University of Science, 1-3 Kagurazaka, Tokyo 162-8610, Japan Science & Technology Research Laboratories, Japan Broadcasting Corporation (NHK), 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan

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Downloaded by ALBRIGHT COLG at 07:47:04:047 on June 08, 2019 from https://pubs.acs.org/doi/10.1021/acsami.9b03895.

S Supporting Information *

ABSTRACT: Inverted organic light-emitting diodes (iOLEDs) without the use of alkali metals have attracted extensive attention owing to the demand for the realization of flexible OLEDs that do not require stringent encapsulation. In this paper, we discuss the correlation between the characteristics of iOLEDs and the energy-level alignment at cathode/organic layer interfaces examined by ultraviolet photoelectron spectroscopy. Two similar electron-transporting materials having different orbital energies, 2,8bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT) and 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene sulfone (PPT-S), are inserted between the cathode/polyethyleneimine and the emitting layer in the iOLED. The iOLED employing PPT-S exhibits a lower driving voltage and a higher efficiency than that employing PPT, which is consistent with the orbital energies of the two molecules. Although the stabilities of these two molecules are expected to be similar, the iOLED employing PPT-S exhibits an operational lifetime that is more than 100 times longer than that of the iOLED employing PPT. It was found that the difference in operational lifetime is caused by the difference in the energy-level alignment at the cathode/organic layer interfaces. Our results are expected to promote the development of promising materials and device configurations for fabricating efficient and operationally stable iOLEDs. KEYWORDS: organic light-emitting diodes, flexible electronics, energy-level alignment, electron injection, cathode/organic interfaces

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INTRODUCTION Organic light-emitting diodes (OLEDs) have been intensively studied as a promising technology for displays and solid-state lighting.1,2 In recent years, several products using OLEDs, such as smartphones and large displays, have been commercialized. In addition, OLEDs are suitable for realizing new-concept devices such as bendable/rollable displays,3 flexible lighting, and flexible sensors since they can be easily fabricated on flexible substrates such as plastic.4−6 The most important issue remaining for the practical application of flexible OLEDs is their typically short lifetimes due to the poor environmental stability of conventional OLEDs (cOLEDs). In cOLEDs, alkali metals such as Li, Ba, Ca, and Cs are essential for efficient electron injection since there is a large electron injection barrier between the cathode metal and the organic layer.7−9 However, flexible substrates such as plastic allow ambient oxygen and moisture to permeate into devices, which degrades the alkali metals in cOLEDs. The water vapor transmission rate necessary to prevent the degradation of alkali metals in cOLEDs is approximately 10−6 g m−2 per day.10,11 Thus, most of the flexible displays reported so far have employed high barrier layers to prevent the entry of oxygen and moisture.3,12 However, complicated encapsulation methods such as multi© XXXX American Chemical Society

pair organic/inorganic thin-film encapsulation are essential for realizing a sufficient barrier layer.3,13 Since the preparation of such a high barrier layer is not easy, it has been difficult to demonstrate long-lifetime flexible devices using cOLEDs with a plastic substrate.6 In recent years, inverted OLEDs (iOLEDs) with a bottom cathode, which have a much higher stability to oxygen/ moisture than cOLEDs, have been proposed.14−16 A longlifetime flexible display using an iOLED with simplified encapsulation has also been proposed.17 cOLEDs comprise a transient bottom anode, an emitting layer (EML) or multilayer (stack), and a top cathode, where the electron injection layer (EIL) is formed after the organic layers. Thus, the materials that can be used as the EIL are limited to vapor-depositable materials such as alkali metals. In the case of iOLEDs, on the other hand, the EIL can be prepared in advance of organic layers, increasing the range of choices for the EIL. Actually, some metal oxides that can be prepared by sputtering have been widely used as part of the EIL in iOLEDs since they can Received: March 3, 2019 Accepted: May 29, 2019

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DOI: 10.1021/acsami.9b03895 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Multilayer structure of an iOLED and chemical structures of the materials used in the interlayer.

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reduce the surface work function of the cathode.18−20 However, a large electron injection barrier remains between the metal oxide and the lowest unoccupied molecular orbital (LUMO) level of the EML.21 Thus, an interlayer that can improve the electron injection efficiency has been inserted between the metal oxide and the EML in iOLEDs, as typified by amine-based molecules such as polyethyleneimine (PEI).15,16,22−27 iOLEDs employing PEI on zinc oxide (ZnO) as the EIL can inject electrons easily by inducing a strong interfacial dipole and then decreasing the work function.22 It has already been reported that the driving voltage of an iOLED combining ZnO and a PEI layer as the EIL is improved by decreasing the energy barrier from the cathode to the EML.16,23 In addition, both the efficiency and the operational stability of iOLEDs have been demonstrated to strongly depend on the interlayer.17 However, the effects of the energy-level alignment between the interlayer and the EML on the characteristics of iOLEDs, which are essential knowledge for improving the efficiency and stability of iOLEDs, have seldom been discussed quantitatively. In this work, we investigated the effects of the energy-level alignment on the device characteristics of iOLEDs using the following two similar electron-transporting materials (ETMs) as the interlayer: 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT),28 the ionization potential (IP)/optical gap (Egopt) of which is 6.1/3.6 eV, and 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene sulfone (PPTS), the IP/Egopt of which is 7.1/3.4 eV. The molecular structures of the two ETMs are similar, but their energy levels are completely different. We evaluated the characteristics of iOLEDs using four types of interlayer, PPT, PPT-S, PPT on PEI, and PPT-S on PEI, as shown in Figure 1. The iOLED employing PPT-S exhibits a lower driving voltage and a higher external quantum efficiency (EQE) than the iOLED employing PPT, regardless of the existence of PEI. Moreover, a clear difference was observed in the operational lifetime. Although the operational lifetimes of the two iOLEDs without PEI are similar regardless of the interlayer, the operational lifetimes of the two iOLEDs with PEI strongly depend on the interlayer. The iOLED employing PPT-S on PEI exhibits an operational lifetime that is more than 100 times longer than that of the iOLED employing PPT on PEI. The origin of the difference in operational lifetime has successfully been clarified by comparing the characteristics of the four iOLEDs and the energy diagrams of the four interlayers. The effect of energylevel alignment around the interlayer in the iOLEDs was successfully observed by using similar ETMs that have different energy levels.

EXPERIMENTAL SECTION

OLED Fabrication and Measurements. The iOLEDs were fabricated on a glass substrate coated with a 150 nm thick indium tin oxide (ITO) layer, as shown in Figure 1. Prior to the fabrication of the organic layers, the substrate was cleaned with ultrapurified water, organic solvents, and UV−ozone treatment. ZnO was deposited using a Mirror Tron sputtering system (Choshu Industry Co., Ltd.). The device configuration of the fabricated iOLED is ITO/ZnO (5 nm)/ interlayer/Zn(BTZ)2/Ir(piq)3 (6 wt %, 35 nm)/α-NPD (40 nm)/ HAT-CN (10 nm)/Al (100 nm), where Zn(BTZ)2 is bis[2-(2hydroxyphenyl)benzothiazolato]zinc(II),29 Ir(piq)3 is tris[1-phenylisoquinolinato-C2,N]iridium(III),30 α-NPD is 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl, and HAT-CN is 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile. Four types of interlayer were formed by using PPT, PPT-S, and PEI, as shown in Figure 1. PPT and PPT-S were deposited to form 10 nm thick films. PEI (P-1000, Nippon Shokubai Co., Ltd.) was spin-cast from an ethanol solution (0.5 wt %, 2000 rpm, 30 s) to give an ultrathin layer. The film thickness of the PEI layer was estimated to be about 1−2 nm by using X-ray photoelectron spectroscopy.31 After spin casting, the substrate was annealed in ambient atmosphere (5 min, 150 °C). After the interlayer formation, the other organic layers were sequentially deposited on the substrate, without breaking the vacuum, at a pressure of about 10−5 Pa. After the organic layers were formed, a 100 nm thick Al layer was deposited as the anode. The devices were encapsulated using a UV− epoxy resin and a glass cover in nitrogen atmosphere after cathode formation. The electroluminescence (EL) spectra and luminance were measured using a spectroradiometer (Minolta CS-1000). A digital source meter (Keithley 2400) and a desktop computer were used to operate the devices. We assumed that the emission from the OLEDs was isotropic so that the luminance was Lambertian; thus, we calculated the EQE from the luminance, current density, and EL spectra. Measurement of Energy Levels. The energy diagrams of various interfaces were obtained by measurement using ultraviolet photoelectron spectroscopy (UPS) and UV−vis−near-IR spectroscopy. UPS images of interlayers on glass/ITO and glass/ITO/ZnO were measured using a concentric hemispherical analyzer with a 128channel multichannel detector, where the excitation source was a HeI (21.22 eV) discharge lamp. A bias of −8.0 V was applied to each sample to separate the sample and the secondary edge for the analyzer. The thickness of the PPT and PPT-S films prepared on the substrates by thermal evaporation was 10 nm in the UPS measurement. The absorption spectra of 50 nm thick PPT and PPT-S films were also measured at room temperature. Synthesis. PPT-S was synthesized by oxidizing PPT, as shown in Scheme 1. In an amber vial, PPT (0.499 g, 0.855 mmol, 1.00 equiv) was dissolved in dichloromethane (8.5 mL) and cooled to 0 °C. To this solution was added 70% m-chloroperoxybenzoic acid (0.527 g, 2.139 mmol, 2.50 equiv); then, the reaction solution was warmed to room temperature, stirred for 4 h, and diluted with saturated aqueous sodium bicarbonate. The water layer was extracted twice with B


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In general, EA has been estimated by subtracting Egopt from IP. However, it has been recently proposed that the actual EA is about 1 eV smaller than the value estimated from Egopt and IP.34,35 Although the actual EA values of PPT and PPT-S cannot be measured owing to the lack of a suitable measurement system, it is reasonable to suppose that the EA of PPT-S is larger than that of PPT by considering both the larger IP and the similar Egopt of PPT. Thus, PPT-S is expected to be more suitable than PPT for the interlayer in iOLEDs since the larger EA and larger IP are effective for efficient electron injection and hole blocking, respectively. Characteristics of iOLEDs Using PPT or PPT-S as Interlayer. Figure 3a shows the current density (J)−voltage

Scheme 1. Synthesis of PPT-S

chloroform, and the combined organic layer was dried over magnesium sulfate and filtrated, and the filtrate was evaporated under reduced pressure. Then, the residue was isolated by column chromatography over silica gel (hexane/ethyl acetate (2:1 → 0:1) → chloroform/methanol = (9:1)) as the eluent to give PPT (0.280 mg, 0.453 mmol, 53% yield). We see from the results of differential scanning calorimetry that the glass-transition temperature of PPT-S is similar to that of PPT (Figure S1 in the Supporting Information). 1 H NMR (500 MHz DMSO-d6): δ 8.60 (d, J = 11.0 Hz, 2H), 8.19 (dd, J = 8.0, 2.0 Hz, 2H), 7.88 (ddd, J = 11.0, 7.5, 0.5 Hz, 2H), 7.72− 7.64 (m, 12H), 7.60−7.56 (m, 8H).

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RESULTS AND DISCUSSION Energy Levels of PPT and PPT-S. To discuss the effects of the energy-level alignment on iOLED characteristics, we used PPT and PPT-S as the interlayer. Since PPT has been reported to be a good ETM,28 PPT-S is also expected to have sufficient electron transportability.32,33 We performed density functional theory calculations to characterize the frontier molecular orbital energy levels of PPT and PPT-S at the B3LYP/6-31G(d,p) level by using the Gaussian 09 program. Although the calculated highest occupied molecular orbital (HOMO)/LUMO distributions are similar, as shown in Figure 2a, the IP and electron affinity (EA) of these molecules are

Figure 3. (a) Current density (left, open symbols) and luminance (right, filled symbols)−voltage characteristics of iOLEDs employing PPT or PPT-S as the interlayer. (b) EQE−current density curves of iOLEDs. (c) Luminance−time characteristics for devices under a constant direct current (dc) with an initial luminance of 1000 cd m−2. Energy diagrams of (d) ITO/ZnO/PPT/Zn(BTZ)2 and (e) ITO/ ZnO/PPT-S/Zn(BTZ)2 interfaces estimated from UPS results and absorption spectra.

(V)−luminance (L) characteristics of iOLEDs employing PPT or PPT-S as the interlayer. The driving voltage of the iOLED using PPT-S is lower than that of the iOLED using PPT. Although the difference in the J−V characteristics is relatively small, a clear difference was observed in the L−V characteristics, as shown in Figure 3a. The observed difference in the L− V characteristics may originate from the difference in EQE, as shown in Figure 3b. The J−current efficiency characteristics are also shown in Figure S2 in the Supporting Information for comparison. Although the operational lifetimes of the two iOLEDs are relatively short, their behavior is opposite to that of the J−V−L characteristics. As shown in Figure 3c, the lifetimes LT50 of the iOLEDs using PPT and PPT-S, defined as the time for the luminance to decay to 50% of the initial luminance, were 150 and 29 h, respectively. It is likely that the lower driving voltage and higher EQE of the iOLED using PPT-S are caused by the larger IP/EA of PPT-S than that of PPT. To discuss the effect of the energy-level alignment on the characteristics of the iOLEDs in detail, the energy diagrams at

Figure 2. (a) Calculated molecular orbitals of PPT and PPT-S. (b) UV−vis−near-IR spectra of PPT and PPT-S thin films at 298 K. HeI ultraviolet photoelectron spectroscopy (UPS) images of each film in the (c) secondary and (d) highest occupied molecular orbital (HOMO) regions.

expected to differ significantly owing to the strong accepting property of sulfonic oxide in PPT-S. The calculated IP/EA of PPT is 6.1/1.3 eV, whereas that of PPT-S is 6.7/2.1 eV. Here, we discuss the actual IP and EA from the UPS results and optical gap (Egopt). We see from Figure 2b that the Egopt of PPT is similar to that of PPT-S, that is, the Egopt of PPT is 3.6 eV and that of PPT-S is 3.4 eV. IP was measured to be 6.1 and 7.1 eV for PPT and PPT-S, respectively, as shown in Figure 2c,d. C


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ACS Applied Materials & Interfaces ITO/ZnO/interlayer interfaces were investigated by UPS (Figure S3 in the Supporting Information). The schematics of the energy diagrams at the ITO/ZnO/ PPT and ITO/ZnO/PPT-S interfaces estimated by UPS are summarized in Figure 3d,e, respectively. Although we did not measure the UPS image of Zn(BTZ)2 on each interlayer, the expected diagram for Zn(BTZ)2 is also shown on the assumption that the vacuum level (VL) is aligned at the interlayer/Zn(BTZ)2 interface to discuss the carrier injection/ blocking mechanisms.36 ZnO, which is prepared on ITO, can significantly reduce the surface work function (ϕsurf) [Figures 2c andS3 in the Supporting Information]. Thus, ZnO plays a key role in electron injection in iOLEDs.20 Upon the formation of a 10 nm thick interlayer (PPT or PPT-S), a vacuum-level (VL) shift (ΔVL), which may have been caused by the formation of electric dipoles in the film,37 was observed in both systems. The surface work functions (ϕsurf) of the ITO/ZnO/ PPT and ITO/ZnO/PPT-S systems are 2.4 and 2.8 eV, respectively. Although the observed values of ΔVL seem to be effective for enhancing the electron injection from the cathode to the EML, the existence of a relatively large electron injection barrier between ZnO and the interlayer (PPT or PPT-S) is expected since the driving voltages of the two iOLEDs are relatively high compared to that of a cOLED using the same emitting layer.29 It is reasonable to suppose that the difference in EQE between the two iOLEDs originates from the difference in the electron injection barrier (Δe), which is defined as the energy difference between the EF of the ITO and the EA of the interlayer, and the difference in the hole blocking property (Δhb), which is defined as the energy difference between the IP of the interlayer and that of Zn(BTZ)2, as shown in Figure 3d,e. We see from Figure 3e that the actual band gap of PPT-S is approximately 1 eV larger than Egopt because the LUMO of PPT-S must be located above the Fermi level (EF) of ITO.34,35 Although the actual EA values of PPT and PPT-S cannot be measured, Δe in ITO/ZnO/PPT-S is expected to be lower than that in ITO/ZnO/PPT, as shown in Figure 3d,e. In addition, we see from Figure 3d,e that Δhb in ITO/ZnO/PPTS is higher than that in ITO/ZnO/PPT. The lower Δe and higher Δhb in ITO/ZnO/PPT-S may lead to the better L−V characteristics and higher EQE of the iOLED using PPT-S. Actually, the leakage current in the iOLED using PPT is higher, which may have been caused by the lower Δhb in ITO/ ZnO/PPT, as shown in Figure 3d (Figure S4 in the Supporting Information). The L−V characteristics and EQE of the iOLED using PPT-S are better than those of the iOLED using PPT; however, the iOLED using PPT-S exhibits a shorter operational lifetime, as shown in Figure 3c. In recent years, exciton− polaron annihilation has been identified as one of the major degradation factors in OLEDs.38−40 Thus, this shorter operational lifetime may originate from the accumulation of a large number of holes at the PPT-S/Zn(BTZ)2 interface, as shown in Figure 3e. This is further discussed in the next subsection by considering the characteristics of the iOLED employing PEI. Characteristics of iOLEDs Using PEI/PPT or PEI/PPT-S as Interlayer. Figure 4a shows the current density (J)− voltage (V)−luminance (L) characteristics of the iOLEDs employing PEI/PPT or PEI/PPT-S as the interlayer. Since PEI is effective for improving the electron injection efficiency, the driving voltage of each iOLED becomes lower than that of the iOLED without PEI.22 In particular, the J−V characteristics of

Figure 4. (a) Current density (left, open symbols) and luminance (right, filled symbols)−voltage characteristics of iOLEDs employing PEI/PPT or PEI/PPT-S as the interlayer. (b) EQE−current density curves of iOLEDs. (c) Luminance−time characteristics for devices under a constant dc with an initial luminance of 1000 cd m−2. Energy diagrams of (d) ITO/ZnO/PEI/PPT/Zn(BTZ)2 and (e) ITO/ZnO/ PEI/PPT-S/Zn(BTZ)2 interfaces estimated from UPS results and absorption spectra.

the iOLED employing PEI/PPT-S are improved significantly by the insertion of PEI. A similar tendency was observed in the EQE, as shown in Figure 4b: the EQE of the iOLED employing PEI/PPT-S is improved significantly by the insertion of PEI. The maximum EQE of the iOLED employing PEI/PPT-S was comparable to that of cOLEDs using the same emitter.29,30 Since the quantum yield of Ir(piq)3 is about 0.45, it is reasonable to conclude that almost all carriers injected into the iOLED employing PEI/PPT-S are converted to photons.41,42 The J−current efficiency characteristics are also shown in Figure S5 in the Supporting Information for comparison. On the other hand, the behavior of the lifetime is completely different, as shown in Figure 4c. The LT50 of the iOLED employing PEI/PPT-S was improved to about 1000 h by the insertion of PEI,43 whereas that of the iOLED employing PEI/PPT was shortened to only 2 h by the insertion of PEI. This is the first observation of a significant improvement in the operational lifetime of an iOLED employing amine-based molecule, which is one of the most promising electron injection materials.15,16,22−27 We investigated the energy diagram of each interface, as shown in Figure 4d,e, to discuss the effects of PEI on the device characteristics of the iOLEDs (Figure S6 in the Supporting Information). ΔVL > 1 eV was observed by preparing the ultrathin PEI layer on ZnO, resulting in an extremely small ϕsurf of 2.2 eV.22 This small ϕsurf improved the L−V characteristics of each iOLED. Note the difference in characteristics between the iOLED using PPT (Figure 3) and that using PEI/PPT (Figure 4). With the insertion of the PEI layer, not only were the L−V characteristics improved but also the leakage current, which was clearly observed in the iOLED using PPT, was suppressed (Figure S4 in the Supporting Information). It is reasonable to suppose that many holes are blocked at the PEI/PPT interface in the iOLED using PEI/ D


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both the organic layer and the cathode.53 In the case of iOLEDs fabricated without the use of alkali metals, on the other hand, cathode/EIL/organic layer interfaces can be realized in the physisorbed system. Thus, studying the correlation between the characteristics of iOLEDs without the use of alkali metals and the energy-level alignment at cathode/EIL/organic layer interfaces will contribute to a comprehensive understanding of the electron injection mechanism in organic electronics.

PPT. Since the EQE is almost the same for these two iOLEDs, the blocked holes are not expected to recombine with electrons; however, they are closely related to the extremely short operational lifetime of the iOLED using PEI/PPT. This is because it has already been demonstrated that the accumulation of holes at the emitting layer/PEI interface accelerates the degradation of cOLEDs using PEI.44 A similar degradation of PEI is expected even in the iOLED using PEI/ PPT. We next discuss the characteristics of the iOLED using PEI/ PPT-S. The effect of the insertion of the PEI layer on the characteristics of the iOLED is different from that for the iOLEDs using PPT. The L−V characteristics were considerably improved, the EQE was increased, and the operational lifetime was extended significantly. A unique phenomenon was also observed in the energy diagram of ITO/ZnO/PEI/PPT-S, as shown in Figure 4e: ϕsurf was increased by the deposition of PPT-S on PEI. This ΔVL of 0.4 eV is considered to be causally related to the pinning of the LUMO level of PPT-S to the EF of ITO, as shown in Figure 4e.36,45−49 On the basis of the Fermilevel pinning, electrons can reasonably be assumed to be transferred effectively from ITO to PPT-S for thermal equilibrium since the EA of PPT-S is large and the ϕsurf of ITO/ZnO/PEI is extremely small. This effective electron transfer plays a key role in improving not only the J−V−L characteristics and EQE, but also the operational lifetime of the iOLED using PEI/PPT-S. The longest LT50 of the iOLED using PEI/PPT-S among the four iOLEDs originates from the reduced accumulation of holes. The accumulation of holes at the PEI/PPT-S/Zn(BTZ)2 interface is expected to be much less than that at the PPT-S/Zn(BTZ)2 interface, as shown in Figure 3e. In addition, the number of holes that reach the PEI at the PEI/PPT-S interface must be reduced significantly by the Fermi-level pinning compared to that at the PEI/PPT interface, as can be seen from Figure 4d,e. Note the energylevel alignment at the PPT or PPT-S/Zn(BTZ)2 interface. The energy barrier at the PPT/Zn(BTZ)2 interface seems to be smaller than that at the PPT-S/Zn(BTZ)2 interface. The results of our experiment suggest that the energy-level alignment at the cathode/organic interface may play a key role in improving the performances of iOLEDs rather than that at the organic/organic interface. Although there are many interfaces such as inorganic/organic and organic/organic interfaces in organic devices, little is known about which interface dominates device performances. These results and discussion of iOLEDs are considered to be important for future understanding of the relation between device performances and the energetics at interfaces. We succeeded in correlating the characteristics of iOLEDs such as driving voltage, efficiency, and operational stability and the energy-level alignment at cathode/interlayer/emitting layer interfaces. Since the carrier injection/transport property at the electrode/organic layer interfaces is one of the dominant factors determining the characteristics of organic optoelectronic devices, the energy-level alignment in cOLEDs has also been intensively studied.50 There are many reports on the correlation between the hole injection mechanism in cOLEDs and the energy-level alignment at anode/hole injection layer interfaces;51,52 however, it has been difficult to understand the correlation between electronic structures at the organic layer/ EIL/cathode interfaces and the device characteristics of cOLEDs. This is because the main material used as the EIL in cOLEDs has been alkali metals, which chemically react with

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CONCLUSIONS We have succeeded in observing the effects of the energy-level alignment on the driving voltage, efficiency, and operational stability of iOLEDs. To discuss the effects of orbital energies on the characteristics of iOLEDs, we newly synthesized a novel ETM named PPT-S, the chemical structure of which is similar to PPT but its orbital energies are different. Both the electronic structure at the cathode/ZnO/interlayer/emitting layer interfaces and the characteristics of the iOLEDs were evaluated using four types of interlayer, PPT, PPT-S, PPT on PEI, and PPT-S on PEI. The iOLED employing PPT-S exhibits a lower driving voltage and a higher EQE than the iOLED employing PPT, which is consistent with the electron injection/hole blocking property expected from UPS results. The operational lifetime (LT50) of the iOLED employing PPT is 150 h, which is about 5 times longer than that of the iOLED employing PPT-S. In the iOLED employing PEI as part of the interlayer, on the other hand, a 500-fold difference in operational lifetime was observed: the LT50 values of the iOLEDs employing PEI/ PPT and PEI/PPT-S are 2 and 1000 h, respectively. On the basis of the energy-level alignment, it was concluded that the observed difference in operational lifetime originates from the difference in the number of accumulated holes in each iOLED. These findings will contribute not only to the further improvement of iOLED performance but also to the design of novel interlayers suitable for iOLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03895.

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Thermal behavior of materials; J−current efficiency characteristics of iOLEDs; J−V characteristics of iOLEDs; and UPS images of various interfaces (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hirohiko Fukagawa: 0000-0001-5531-6510 Author Contributions §

S.K., K.S., T.S., and T.O. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913−915. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-emitting Diodes Based on Conjugated Polymers. Nature 1990, 347, 539−541.

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