Organic light-emitting devices based on solution-processable small molecular emissive layers doped with interface-engineering additives

Abstract

In this study, we investigate small molecular organic light-emitting diodes (SM-OLEDs) consisting of emission layers (EMLs) fabricated using a solution-coating process of self-metered horizontal dip- (H-dip-) coating. The EML used was composed of a co-mixed small molecular host matrix of hole-transporting 4,4′,4′′-tris(9-carbazolyl)-triphenylamine (TcTa) and electron-transporting 2,7-bis (diphenyl phosphoryl)-9,9′-spirobifluorene (SPPO13) doped with blue-, green-, and/or red-emitting phosphorescent iridium complexes. To improve the electron-injecting and hole-blocking properties at the cathode interface and to enhance the film-forming capabilities, an interface-engineering additive of poly(oxyethylene tridecyl ether) (PTE) was mixed with the small molecular EMLs. Using PTE additives was shown to reduce dramatically the formation of film defects such as nano-pinholes in the EMLs, resulting in thin and homogeneous PTE-mixed EMLs with smooth surface morphologies, even when using a single H-dip-coating process. The use of simple H-dip-coated EMLs mixed with PTEs in SM-OLEDs resulted in good device performance, with maximum luminance levels of 29 200 cd m⁻², 115 000 cd m⁻², and 16 400 cd m⁻², with corresponding peak current efficiencies of 18.8 cd A⁻¹, 31.2 cd A⁻¹, and 10.0 cd A⁻¹, for blue, green, and red SM-OLEDs, respectively. Furthermore, we demonstrated the feasibility of fabricating large-area and high-performance solution-processable SM-OLEDs using H-dip-coated EMLs doped with PTEs. These results clearly indicate that H-dip-coated small molecular EMLs mixed with PTE can be used to yield simple, bright, and efficient solution-processable SM-OLEDs.

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Introduction

Numerous recent studies have focused on the development of organic semiconducting materials and related device structures for use in organic light-emitting devices (OLEDs), in an effort to realise cost-efficient, lightweight, flexible, and large-area flat panel displays and solid-state lighting applications.^1–5 In order to achieve such aims, the developments of particular interest to researchers are the improved efficiency, stability, and simplicity of the device fabrication process. With respect to the efficiency of OLEDs, for example, their internal quantum efficiency has already been significantly improved to nearly 100% by incorporating phosphorescent guest dopants into the emission layers (EMLs), leading to strong spin–orbit coupling and rapid intersystem crossing and resulting in an efficient radiative transition from triplet states to a ground state.^3–6 The use of these types of electrophosphorescent complexes has enabled the demonstration of phosphorescent OLEDs with a high peak luminescence of ca. 50 000–100 000 cd m⁻² together with a peak efficiency exceeding 25–60 cd A⁻¹.^4,5

Such electrophosphorescent small-molecular OLEDs (SM-OLEDs) can generally be divided into two categories: those prepared by vacuum-evaporation deposition,^3–12 and those fabricated by solution-processing methods such as spin-coating, ink-jet printing, and blade-coating.^13–22 Researchers have tended to use the more complex co-vacuum-evaporation deposition of multiple components to fabricate multi-layered SM-OLEDs, as vacuum-deposited SM-OLEDs have thus far generally outperformed solution-processed devices.^3–12 Nevertheless, in comparison with vacuum-evaporation, which can be used to create complex structures but involves the wasting of large amounts of organic materials and for which fabrication costs are relatively high, solution-processing techniques can overcome the difficulties of co-evaporation and accurate doping associated with vacuum deposition, having particular appeal due to the relative simplicity of the process and the potential advantages of large-area and cost-effective manufacturing.^13–16

Several important studies of phosphorescent SM-OLEDs fabricated by solution-processing methods have recently been reported.^16–22 In one study, researchers used mixed hosts consisting of hole- and electron-transporting small molecular materials to improve the charge balance and to increase the size of the recombination zone in solution-processed SM-OLEDs.^16–20 A SM-OLED containing a green phosphorescent EML fabricated by solution-coating and a carrier (hole) transporting layer deposited by vacuum-evaporation was studied, demonstrating good electroluminescent (EL) performance, with a peak efficiency of 56.9 cd A⁻¹.¹⁸ In most of these types of SM-OLEDs, solution-processed EMLs with precise ratios of multiple components of mixed host/doped guest emitters (host/guest) were typically fabricated using a spin-coating method. Spin-coating techniques are a relatively straightforward means of forming EMLs on substrates. However, limitations regarding the coating area and the inhomogeneous morphology of spin-coated EMLs may preclude solution-coating methods from being used in high-throughput manufacturing.^22–26 Hence, despite these successes, including recent developments in the solution-processing of SM-OLEDs, a simpler and more reliable solution-processing technique for small molecular EMLs in SM-OLEDs continues to attract the attention of researchers, as such a method can form flat and uniform layers over a large area, thus offering advantages when used for the production of cost-effective SM-OLEDs.

Most recently, using the horizontal dip- (H-dip-) coating solution process, multi-coated EMLs were developed in an attempt to avoid the generation of nano pinholes in solution-processed EMLs.²⁷ It was reported that multiple (double or triple) H-dip-coated EMLs in solution-processed OLEDs performed somewhat better than their single-coated counterparts. Nevertheless, in comparison with other solution processes, multiple coatings may be rather complex while also wasting large amounts of organic materials, thus leading to relatively high fabrication costs, precluding the use of solution-coating methods in high-throughput manufacturing. Further development of alternative simple solution-coating processes with which to fabricate flat, homogeneous EMLs together with balanced recombinations of charge carriers capable of providing highly efficient light emission is therefore necessary to achieve satisfactory solution-processed SM-OLEDs.

In this study, we fabricated and investigated solution-processable blue, green, and red SM-OLEDs with small molecular EMLs using commercially available materials together with interface engineering additives, as shown in Fig. 1. For the solution-processable blue-emitting small molecular EMLs, we chose 4,4′,4′′-tris(9-carbazolyl)-triphenylamine (TcTa, with a first-triplet energy level (T₁) of 2.73 eV)¹⁸ as a hole-transporting host and 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13, lowest unoccupied molecular orbital (LUMO): ∼2.78 eV)¹⁸ as an electron-transporting host of co-mixed-host materials (TcTa:SPPO13) doped with phosphor bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (Firpic, LUMO: 2.9–3.2 eV, highest occupied molecular orbital (HOMO): 5.8 eV, T₁: 2.65 eV).^17,18 We also used green-emitting phosphor tris(2-phenylpyridinato)iridium (Ir(ppy)₃, LUMO: 2.92 eV, HOMO: 5.32 eV, T₁: 2.9 eV)¹³ and red-emitting phosphor bis(1-(phenyl)isoquinoline)iridium acetylacetonate (Ir(piq)₂acac, LUMO: 3.23 eV, HOMO: 5.17 eV, T₁: 2.0 eV)¹¹ as guests in the EMLs for the fabrication of the solution-processed SM-OLEDs. In order to improve the electron-transporting and hole-blocking properties of the EMLs at the cathode interface and to ensure excellent film-forming capabilities, we also introduced a wide-bandgap interface-engineering additive of poly(oxyethylene tridecyl ether) (PTE) as a surfactant^23,24 into the EMLs studied here. To fabricate the SM-OLEDs, we used a novel self-metered horizontal dip (H-dip) coating method,^25–30 which is an effective and advanced means of coating organic/polymeric or inorganic semiconducting layers. Applications of H-dip-coating include solution-coating for large OLEDs,^25–27 polymer solar cells,^26,28,29 and semiconducting layers used in organic thin-film transistors.³⁰ A simple single H-dip-coating method in which EL solutions were mixed with PTE additives resulted in highly homogeneous and defect-free small molecular EMLs. Using H-dip-coated EMLs with PTE additives resulted in SM-OLEDs that exhibited high device performance levels with maximum luminance levels of 29 200 cd m⁻², 115 000 cd m⁻², and 16 400 cd m⁻² with peak current efficiencies of 18.8 cd A⁻¹, 31.2 cd A⁻¹, and 10.0 cd A⁻¹ for blue (B), green (G), and red (R) SM-OLEDs, respectively. Moreover, we demonstrated the feasibility of the simple fabrication of H-dip-coated EMLs doped with PTE additives in SM-OLEDs, which can easily be extended for use with a mass-production alternative to vacuum-evaporated layers.

Fig. 1 (a) The device configurations of the solution-processable small molecular OLEDs studied together with the relevant energy level diagrams, and (b) chemical structures of the materials used in the EMLs.

Experimental

Materials

The hole-injection material of poly(styrene sulfonic acid) doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS, Clevios™ PVP AI 4083, H.C. Starck) was used as received from the manufacturer. For the solution-processable EMLs, the hole-transporting material TcTa (Lumtec), the electron-transporting material SPPO13 (Lumtec), the blue-emitting phosphor Firpic (Lumtec), the green-emitting phosphor Ir(ppy)₃ (Lumtec), and the red emitting phosphor Ir(piq)₂acac (Lumtec) were used as received without further purification. The interface-engineering additive of PTE (C₁₃H₂₇(OCH₂CH₂)₁₂OH, Sigma-Aldrich), the electron-injection material Cs₂CO₃ (Sigma-Aldrich), and the Al cathode (Sigma-Aldrich) were also used as received without further purification. The chemical structures of the materials used in the EMLs are shown in Fig. 1(b).

Horizontal-dip (H-dip) coating

The apparatus used for H-dip-coating had a maximum work space of 10 × 10 cm² (see Fig. 2(a)). A small volume of solution (∼5–10 μl) per unit coating area (1 × 1 cm²) was fed into the gap of the cylindrical H-dip-coating head (SUS steel) using a syringe pump (Pump Systems Inc. NE-1000). The height of the gap h₀ was adjusted vertically using micrometer positioners mounted at the end of the coating head, and the carrying speed U was controlled by a computer-controlled translation stage (SGSP26-200, Sigma Koki Co., Ltd). After a meniscus of coating solution had formed between the H-dip-coating head and the substrate, the substrate was transported horizontally such that the H-dip-coating head spread the solution evenly on the transporting substrate while maintaining the shape of the downstream meniscus of the solution. The transporting speed U was set to 1.5 cm s⁻¹, and it took approximately 1 min to prepare a film on a substrate with a length of about 1 m.

Fig. 2 (a) Coated film thickness data of the polymeric HILs of PEDOT:PSS as a function of the carrying speed U for the two gap heights h₀. The solid curves show the theoretically fitted predictions according to the Landau–Levich equation. The inset shows a photograph of the H-dip-coating process. (b) Coated film thickness data of the small molecular blue EML with PTE additives as a function of the carrying speed U for two gap heights h₀. The inset shows a schematic illustration of the H-dip-coating process with gap height h₀ and coating speed U.

Device fabrication

As shown in Fig. 1(a), the solution-processed SM-OLEDs were fabricated on glass substrates pre-coated with indium tin oxide (ITO) anodes (thickness: 80 nm, sheet resistance: 30 Ω per square). The ITO substrates were ultrasonically cleaned with detergent, deionised water, acetone and isopropanol and then dried by blowing nitrogen over them in sequence, followed by ultraviolet ozone cleaning for 15 min. A 40 nm hole-injecting layer (HIL) of PEDOT:PSS was H-dip-coated on the pre-cleaned ITO substrates and then baked at 120 °C in a vacuum oven for 10 min to extract the residual water. To fabricate the blue SM-OLEDs, a well-mixed EL solution was prepared for the EMLs using the mixed solvents 1,2-dichloroethane and chloroform (at a mixing weight ratio of 3 : 1) by dissolving co-mixed hosts consisting of TcTa and SPPO13 and the guest of phosphorescent blue-emitter of Firpic with an interface-engineering additive of PTE. The mixed host was TcTa:SPPO13 with a mixing ratio of 35 : 48 wt%, and the concentration of Firpic was maintained at 17 wt% (optimised for blue emission). Next, the EMLs were H-dip-coated on top of the PEDOT:PSS HIL and then annealed at 110 °C in a vacuum oven for 5 min to remove any residual solvent; the thickness of the EMLs was approximately 80 nm. Then, an electron-injecting interfacial layer of Cs₂CO₃ (2 nm) and an Al cathode layer (50 nm) were formed sequentially on top of the EML via thermal deposition at a rate of 0.2 nm s⁻¹ under a base pressure of 2 × 10⁻⁶ Torr. The active emissive area between the ITO and the Al electrodes of the devices was 3 mm × 3 mm. To make green and red SM-OLEDs, green and red guests of Ir(ppy)₃ and Ir(piq)₂acac, respectively, were also introduced into the mixed blue EL solutions. The concentrations of Ir(ppy)₃ and Ir(piq)₂acac were 0.15 and 0.07 wt% in the green and red SM-OLEDs, respectively. For comparison, reference devices that did not contain any PTE additive were also fabricated to investigate the EL performance of the devices studied. The concentrations of the PTE additive were then 0.0 and 0.1 wt% in the blue SM-OLEDs without (device B1) and with (B2) PTE, respectively, and their corresponding concentrations were also 0.0 and 0.1 wt% in the green or red devices without (device G1 or R1) and with (G2 or R2) PTE, respectively. The optical properties of the fabricated functional layers were investigated using a UV-visible spectroscopy system (8453, Agilent). In order to investigate the surface morphologies of the fabricated layers, variations in the surface roughness levels of the layers were monitored using an atomic force microscope (AFM, Nanosurf easyscan 2 AFM, Nanosurf AG Switzerland Inc.). The current density–luminance–voltage (J–L–V) characteristics and the Commission Internationale d'Èclairage (CIE) chromatic coordinates were measured using a Keithley source measurement unit (Keithley 2400) and a luminance meter (Chroma Meter CS-200, Konica Minolta Sensing, Inc.). The EL spectra were recorded using a spectrometer (Ocean's Optics). The characterization of the device was carried out at room temperature under ambient conditions, without encapsulation.

Results and discussion

H-dip-coated HIL and EML for solution-processable SM-OLEDs

Fig. 2 shows a photograph and its schematic illustration of the H-dip-coating method,^25–30 which was used in this study as a self-metered coating process for solution-processable SM-OLEDs. Here, we are interested in the self-metered coating process, in which the coating thickness h increases with an increase in the capillary number (C_a = μUσ⁻¹) of the coating solution, where μ and σ represent the viscosity and surface tension of the coating solution, respectively, and U is the coating speed. The advantage of self-metered coating is that the coating thickness can be controlled easily and precisely by external parameters of the viscosity, the surface tension of the coating solution, and the coating speed, in contrast to more typical pre-metered coating methods. The film thickness (h) of the self-metered H-dip-coated layer can be described by the associated drag-out problem as proposed by Landau and Levich for C_a ≪ 1; h = kC_a^2/3R_d, where R_d represents the radius of the associated downstream meniscus and k is a constant of proportionality.^25–31 We initially investigated the thickness of the H-dip-coated HIL and EML as functions of the coating speed U and the gap height h₀ (Fig. 2(a) and (b)). As shown in the figure, for an h₀ value of 0.5 mm, the film thickness of the H-dip-coated PEDOT:PSS layer showed a continuous increase from ca. 18 nm to ca. 58 nm when U was increased from 0.25 to 2.0 cm s⁻¹. Furthermore, when h₀ was increased to 0.6 mm, the thickness of the H-dip-coated PEDOT:PSS film also increased with additional increases in U. These results are in good agreement with the theoretical description of the associated drag-out problem (solid curves in Fig. 2(a)). Moreover, similar to the polymeric PEDOT:PSS HIL, the small molecular EMLs doped with PTE additives show similar dependences of the thickness on U and h₀ in the observed ranges. It is thus clear that self-metered H-dip-coating allows the precise control of the thicknesses of the functional layers of both small organic molecules and polymers in solution-processable OLEDs, with U and h₀ as controlling parameters.

Film quality of the H-dip-coated small molecular EMLs doped with interface-engineering additives

Next, in order to investigate the film quality of the fabricated small molecular EMLs, we monitored the variation of the surface roughness of the solution-coated EMLs using AFM. Fig. 3 shows the AFM morphologies of spin-coated and H-dip-coated EMLs (thickness: 80 nm) of the blue-emitting EMLs, TcTa:SPPO13:Firpic EML and TcTa:SPPO13:Firpic:PTE EML, deposited on top of flat substrates. As shown in the figure, because the small molecular materials had poor film-forming abilities for the solvents used, low-quality and inhomogeneous films could be formed by both spin-coating and single H-dip-coating processes, as reported previously;²⁷ the root-mean-square (RMS) surface roughness of an EML fabricated by spin-coating was ca. 0.55 nm, and the RMS surface roughness of EMLs fabricated by single H-dip-coating was ca. 0.33 nm. In particular, the small molecular EMLs fabricated by spin-coating and H-dip-coating both showed sub-micron-sized pinhole defects down to the nano-pore level, which could deteriorate device performance capabilities. Similarly, the spin-coated EMLs with PTE were of poor quality with many nano-sized pinhole defects despite the fact that the EMLs were mixed with PTE as an additive (lower panel in Fig. 3(a)). In marked contrast to the spin-coated EMLs with PTE, we note that no such film defect or phase separation was observed in the single H-dip-coated small molecular EMLs, even when the same solvents and PTE additives were used (lower panel in Fig. 3(b)). These investigations demonstrate that the topographies of the EMLs fabricated by H-dip-coating became more homogeneous following the introduction of PTE additives into the EMLs; the RMS surface roughness levels of PTE-doped EMLs fabricated by H-dip-coating were relatively low (ca. 0.27 nm), and the surface roughness levels at different locations on the EML were identical for the layers investigated. This uniformity was achieved because the conditions under which the EMLs were formed involved the surfactant action of the PTE additives. Thus, the resulting smooth and uniform EMLs of TcTa:SPPO13:Firpic:PTE fabricated by simple single H-dip-coating may be suitable for the fabrication of solution-processed SM-OLEDs.

Fig. 3 10 μm × 10 μm AFM topography images of the spin-coated (a) and H-dip-coated (b) blue small molecular EMLs: upper: TcTa:SPPO13:Firpic EML, lower: TcTa:SPPO13:Firpic EML doped with PTE additives.

SM-OLEDs based on the H-dip-coated host:guest EML of TcTa:SPPO13:Firpic:PTE

Given the impressive film qualities of the small molecular EMLs fabricated using H-dip-coating, we fabricated blue SM-OLEDs including EMLs of TcTa:SPPO13:Firpic with PTE interface-engineering additives. To investigate the effect of the PTE additives on the device performance capabilities, blue devices of [ITO/PEDOT:PSS HIL/TcTa:SPPO13:Firpic EML doped with PTE/Cs₂CO₃/Al] (B2) were fabricated, with devices without PTE (B1) also fabricated for comparison. To estimate device performance capabilities, we investigated the J–L–V characteristics of H-dip-coated blue SM-OLEDs, denoted here as B1 and B2. As shown in Fig. 4(a), the slopes of the J–V curves between 0 and 19.0 V indicated excellent diodic behaviour for both of the blue SM-OLEDs. However, it was also noted that the J–V curve of the blue SM-OLED with PTE (B2) is slightly but clearly higher than that of the blue SM-OLED without PTE (B1), indicating that the wide-bandgap PTE additive introduced to the EMLs may enhance carrier (electron) injection at the interface between the EML and the cathode electrode.^23,24 It is also clear from the J–L–V curves (Fig. 4(a) and (b)) that the charge injections are below 4.0–4.5 V, with sharp increases in the J–L–V curves above this range. For example, the operating voltage of the reference blue OLEDs without PTE (B1) was approximately 6.5 V at a brightness level of 100 cd m⁻², 8.5 V at 1000 cd m⁻², and 13.3 V at 10 000 cd m⁻². In addition, the luminescence reached a maximum of ca. 22 800 cd m⁻² (at 19.0 V). The figure also shows that the blue EL brightness is significantly higher in the B2 device with PTE additives; the operating voltage of the B2 device is about 6.0 V at a brightness of 100 cd m⁻², 8.0 V at 1000 cd m⁻², and 12.0 V at 10 000 cd m⁻². The luminescence reached a maximum of ca. 29 200 cd m⁻² (at 19.0 V) in this case. The increased EL brightness of B2 clearly indicates that excitons are generated efficiently following the introduction of the H-dip-coated EMLs doped with PTE, which may cause an increase in the number of radiative exciton recombinations via the electron–hole balance in the EMLs.^16–20 The onset voltage (V_ON, defined as the voltage at 1 cd m⁻²) in the device is also below 4.0 V (B2), which is lower than the devices without PTE, (B1, about 4.5 V). The reduced V_ON indicates that the electron-injection barrier is reduced considerably by the introduction of PTE additives, which may cause not only a reduction in the number of film defects but also a decrease in the work function and a lower contact resistance of the Al cathode. A possible mechanism for the reduction of the barrier is the formation of an interfacial dipole layer at the cathode interface caused by the interaction between the Al cathode and the PTE.^23,24 We also note, as shown in the inset in Fig. 4(b), that the normalized EL spectra (at 1000 cd m⁻²) obtained from the blue SM-OLEDs showed only single dominant peaks at 475 nm for the two blue devices B1 (without PTE) and B2 (with PTE), corresponding to the emission from Firpic¹⁸ and implying that the simple introduction of PTE additives into the small molecular blue EMLs does not influence the characteristics of the emission spectra of the blue SM-OLEDs.

Fig. 4 (a) Current density–voltage (J–V), (b) luminance–voltage (L–V), (c) current efficiency–voltage (LE–V), and (d) power efficiency–voltage (PE–V) characteristics of the H-dip-coated blue SM-OLEDs with blue-emitting co-mixed EMLs of TcTa:SPPO13:Firpic with and without PTE interface-engineering additives. The inset in (b) shows the normalised EL spectra obtained from the H-dip-coated blue-emissive SM-OLEDs.

Interestingly, it was also noted that the efficiencies were significantly higher when the PTE additives were used; excellent overall performance was observed in the co-mixed host:guest B2 device, where a peak luminous efficiency (LE) of 18.8 cd A⁻¹ and a peak power efficiency (PE) of 7.9 lm W⁻¹ were achieved, mainly as a result of homogeneous and defect-free small molecular EMLs doped with PTE and of the electron-injecting and hole-blocking nature of the PTE at the cathode interface.^23,24 Even at a luminance of 10 000 cd m⁻², the LE of B2 with PTE is still 16.7 cd A⁻¹, which is higher than that (10.2 cd A⁻¹) of B1 without PTE, clearly indicating significantly improved device performance capabilities upon the use of PTE additives. All of these results clearly indicate that device performance levels are significantly higher when H-dip-coated EMLs doped with PTE additives are used. Note that the blue SM-OLEDs showed optimum performance for an EML with a thickness of 80 nm and a PTE concentration of 0.1 wt%. When the thickness and the PTE concentration of the EML were changed, the device performance began to deteriorate. The performances of the blue devices are summarised in Table 1.

Table 1 Summary of device performance levels for the co-host (SPPO13:TcTa):guest-based SM-OLEDs with and without interface-engineering PTE additives^a

Type	PTE additive	LMAX cd m^−2	LEMAX cd A^−1	PEMAX lm W^−1	At 10 000 cd m^−2		CIE (x, y) at 10 000 cd m^−2
					LE cd A^−1	PE lm W^−1
a OLED device structure: ITO/PEDOT:PSS HIL/EML/Cs2CO3/Al.
Blue	w/o (B1)	22 800	12.1	5.0	10.2	2.5	0.17, 0.40
	w (B2)	29 200	18.8	8.0	16.7	4.4	0.17, 0.40
Green	w/o (G1)	99 300	25.3	7.4	25.1	6.3	0.30, 0.62
	w (G2)	115 000	31.2	11.0	31.0	8.5	0.30, 0.63
Red	w/o (R1)	12 100	5.5	2.0	3.3	0.6	0.65, 0.34
	w (R2)	16 400	10.0	4.1	7.3	1.6	0.64, 0.34

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In order to investigate the charge-carrier flows at the interface between the blue-emitting co-mixed EMLs and the Al cathode, the flowing current density versus the applied electric field (J–E) characteristics for hole-only and electron-only devices were observed, as shown in Fig. 5(a) and (b), respectively. To study the hole-flowing characteristics, we tested hole-only devices with the structures of [ITO/PEDOT:PSS (40 nm)/EML (150 nm)/Ag (60 nm)] and [ITO/PEDOT:PSS/EML-doped PTE (EML:PTE) (150 nm)/Ag], where the EML used is a blue-emitting small molecular TcTa:SPPO13:Firpic layer. Fig. 5(a) shows that the hole-only device with EML:PTE exhibited a reduced hole-current flow, which indicates that the PTE molecules in the EML, including the PTEs at the EML/Al cathode interfaces, can block the hole current efficiently. For confirmation, we also compared the hole-flowing characteristics of both devices with those of a comparative hole-only device with PTEs on the EML (EML/PTE), i.e., [ITO/PEDOT:PSS/EML (150 nm)/PTE (∼5 nm)/Ag]. As clearly shown in the figure, the hole-only device with EML/PTE has a lower hole-current flow than that of the device without PTE but exhibits a higher hole-current flow than that of the device with EML:PTE. This result confirms that the PTE molecules in the EML, including PTEs at the EML/Al cathode interfaces, can induce not only a reduction of the number of sub-micron-sized pinhole defects but can also block the hole current efficiently due to the low HOMO level of the PTE.²⁴ Next, to estimate the electron transfer, we tested electron-only devices with the structures of [ITO/ZnO (120 nm)/EML (150 nm)/Cs₂CO₃ (2 nm)/Al (50 nm)] and [ITO/ZnO/EML:PTE (150 nm)/Cs₂CO₃/Al]. As is evident in Fig. 5(b), the electron-only device with EML:PTE shows a higher electron-current flow than the device without PTE, indicating that the PTE molecules in the EML layer, including those at the EML/Al cathode, can increase the electron current fairly effectively. We also compared the electron-flow characteristics of both devices with those of a similar electron-only device with PTEs on the EML (EML/PTE), i.e., [ITO/ZnO/EML (150 nm)/PTE (∼5 nm)/Cs₂CO₃/Al]. This comparison demonstrated that the electron-only device with the EML/PTE bilayer has a higher electron-current flow than the device without PTE but a lower electron-current flow than the device with EML:PTE, also confirming that the PTE molecules on the EML, including those at the EML/Al cathode interfaces, can inject electron current efficiently owing to the reduced electron injection barrier caused by the dipole effect of PTE at the Al cathode interface.²⁴ These results indicate that charge-carrier flows can take place selectively in PTE-doped EMLs. Therefore, the improved device performance of the blue SM-OLED with PTEs can be attributed to the balancing of holes and electrons in EMLs through the increased electron injection and blocking of holes with fewer film defects.

Fig. 5 (a) The J–E characteristics of the hole-only devices (a) and electron-only devices (b) of H-dip-coated blue emitting co-mixed EMLs of TcTa:SPPO13:Firpic with and without PTE interface-engineering additives.

Next, we fabricated and investigated green SM-OLEDs including EMLs of the co-mixed TcTa:SPPO13 host and co-mixed guest phosphors of Firpic:Ir(ppy)₃ doped with and without the PTE additives. When we observed the AFM morphologies of the spin-coated EMLs (thickness: 80 nm) of green-emitting TcTa:SPPO13:Firpic:Ir(ppy)₃ with and without the PTE additives, we found that the spin-coated small molecular TcTa:SPPO13:Firpic:Ir(ppy)₃ EMLs doped with and without PTE showed surface morphologies containing many sub-micron-sized pinhole defects, similar to the spin-coated blue-emitting EMLs (Fig. 3(a)), with relatively high levels of surface roughness (ca. 0.30 nm). In contrast to the spin-coated green EMLs, H-dip-coated TcTc:SPPO13:Firpic:Ir(ppy)₃:PTE EMLs exhibited good film-forming abilities, while H-dip-coated TcTc:SPPO13:Firpic:Ir(ppy)₃ EMLs still exhibited a few sub-micron-sized defects, as shown in Fig. 6(a). The observed levels of RMS surface roughness of the H-dip-coated green EMLs doped with the PTE additives were ca. 0.26 nm.

Fig. 6 (a) 6 μm × 10 μm AFM topography images of the H-dip-coated green small molecular EMLs: left: TcTa:SPPO13:Firpic:Ir(ppy)₃ EML, right: TcTa:SPPO13:Firpic:Ir(ppy)₃:PTE EML. J–L–V (b) and LE–PE–V (c) characteristics of the H-dip-coated green SM-OLEDs. (d) 6 μm × 10 μm AFM topography images of the H-dip-coated red small molecular EMLs: left: TcTa:SPPO13:Firpic:Ir(ppy)₃:Ir(piq)₂acac EML, right: TcTa:SPPO13:Firpic:Ir(ppy)₃:Ir(piq)₂acac:PTE EML. J–L–V (e) and LE–PE–V (f) characteristics of the H-dip-coated red SM-OLEDs. The insets in (b) and (e) show the normalised EL spectra obtained from the green- and red-emissive SM-OLEDs, respectively.

Next, we investigated the J–L–V characteristics of the H-dip-coated green SM-OLEDs, as shown in Fig. 6(b). Similar to the blue SM-OLEDs (Fig. 4), the slopes of the J–L–V curves of both the green SM-OLEDs indicate the excellent diodic behaviour of the H-dip-coated EMLs. It is also clear from the J–L–V curves that the charge injections are less than 4.5–5.0 V, with sharp increases in the J–L–V curves above this range. For example, the operating voltage of the green SM-OLED G1 without PTE was approximately 6.5 V at a brightness of 100 cd m⁻², 8.3 V at 1000 cd m⁻², and 11.3 V at 10 000 cd m⁻². The maximum luminescence was ca. 99 300 cd m⁻² (at 20.0 V). We note that the green EL brightness levels were significantly higher in the device (G2) with the H-dip-coated EMLs when the PTE additives were used. The peak luminance of 115 000 cd m⁻² achieved in the H-dip-coated G2 device is much higher than that of the device without PTE (G1). The inset in Fig. 6(b) shows the normalised EL spectra (at 1000 cd m⁻²) obtained from G1 and G2, showing identical single dominant peaks at 518 nm, corresponding to the emission from Ir(ppy)₃.¹³ Moreover, as shown in Fig. 6(c), the efficiency levels were significantly higher when H-dip-coated EMLs with PTEs were used. Improved overall performance was obtained in the G2 device, which showed a peak LE of 31.2 cd A⁻¹ and a peak PE of 11.0 lm W⁻¹. Even at a luminance level of 10 000 cd m⁻², the LE of the G2 device with the PTEs reached 31.0 cd A⁻¹, which was much higher than the value (25.1 cd A⁻¹) for the G1 device. The performance levels of the green devices are summarised in Table 1.

We also fabricated and investigated red SM-OLEDs using single H-dip-coated EMLs of the co-mixed TcTa:SPPO13 host and co-mixed guest phosphors of Firpic:Ir(ppy)₃:Ir(piq)₂acac with PTE additives. We observed the AFM morphologies of the spin-coated and single H-dip-coated EMLs (thickness: 80 nm) with and without the PTE additives. Similar to the blue- and green-emitting EMLs, both the small molecular EMLs of TcTa:SPPO13:Firpic:Ir(ppy)₃:Ir(piq)₂acac doped with and without PTE fabricated by spin-coating also showed surface morphologies with many sub-micron-sized pinhole defects and relatively high surface roughness levels (ca. 0.33 nm). In contrast to these red EMLs, single H-dip-coated red EMLs doped with PTE exhibited good film-forming abilities, while H-dip-coated EMLs without PTE still exhibited many sub-micron-sized defects, as shown Fig. 6(d). The RMS surface roughness of the red EMLs with PTE fabricated by means of H-dip-coating was ca. 0.27 nm, also showing that the topographies of the EMLs fabricated by single H-dip-coating became more homogeneous following the introduction of PTE additives into the small molecular EMLs.

We also investigated the J–L–V characteristics of the H-dip-coated red SM-OLEDs, as shown in Fig. 6(e). Similar to the blue and green SM-OLEDs, the slopes of the J–V curves of the red SM-OLEDs indicate the excellent diodic behaviour of the H-dip-coated EMLs. It is also clear from the J–L–V curves that the charge injections are less than 5.0–5.5 V, with sharp increases in the J–L–V curves above this range. The figure also shows that the red EL brightness levels were considerably higher in device R2 when the PTE additives were used. A peak luminance of 16 400 cd m⁻² (at 17.5 V) was achieved in R2 with the PTEs, a level higher than that (12 100 cd m⁻² at 17.5 V) of the R1 device without the PTEs. Note that, as shown in the inset in Fig. 6(e), the normalised EL spectra (at 1000 cd m⁻²) obtained from R1 and R2 show identical single dominant peaks at 622 nm, corresponding to the emission from Ir(piq)₂acac.¹¹ Furthermore, the efficiency levels were higher when H-dip-coated EMLs with PTEs were used (Fig. 6(f)). Good overall performance was noted in the R2 device, with a peak LE of 10.0 cd A⁻¹ and a peak PE of 4.1 lm W⁻¹. Even at a luminance of 10 000 cd m⁻², the LE of R2 with PTEs was still 7.3 cd A⁻¹, which is higher than that of the R1 device without PTE additives (3.3 cd A⁻¹). The performance levels of the red devices are also summarised in Table 1.

Considering the foregoing device performance (J–L–V) data of the H-dip-coated SM-OLEDs with PTEs (Fig. 4 and 6), it is apparent that the single H-dip-coated EMLs of the co-host:guest system with the interface-engineering PTE additive promote not only smooth and uniform EMLs but also efficient EL properties for all blue, green, and red EL emissions. In addition, the simple single H-dip-coated EMLs with PTE in solution-processed OLEDs performed slightly better than the previous triple H-dip-coated EMLs²⁷ without any additive. It is therefore clear that the use of self-metered H-dip-coated EMLs with PTE in solution-processable SM-OLEDs is a sensible option for R, G, and B colour emission due to its simplicity and the high luminance/efficiency levels offered.

Large-area H-dip-coated R, G, and B SM-OLEDs

Finally, encouraged by the above results obtained from the R, G, and B colour SM-OLEDs based on the H-dip-coated EMLs with the PTE, we fabricated large-area SM-OLEDs on 5 cm × 5 cm ITO-coated glass substrates using the H-dip-coating method in order to assess the processability of large-area solution-processed SM-OLEDs. A photograph of the fabricated device is shown in Fig. 7(a). Blue, green, and red EMLs were deposited in sequence laterally on a glass substrate using the H-dip-coating method with an emission-area of approximately 1 cm × 1 cm. Although the solution-processed small molecular EMLs with PTE additives in the SM-OLEDs were fully fabricated in air, the figure clearly shows that the SM-OLEDs produced in this manner were fairly luminous. Moreover, the low variation of the emission intensity in the emission areas implies a small amount of variation in the thickness of the solution-coated EMLs. Furthermore, this low variation in the EL intensity over the active area provides evidence of the suitability of this method for large-scale fabrication. The EL spectra, collected from the pixels on the substrate, were nearly identical to those shown in Fig. 4 and 6. As shown in Fig. 7(b), the blue-, green-, and red-emissive SM-OLEDs, which respectively exhibited CIE coordinates of (0.17, 0.40), (0.30, 0.63), and (0.64, 0.34) at 10 000 cd m⁻² clearly illustrate how the introduction of H-dip-coated EMLs with PTEs into OLEDs can readily be used to produce full-colour SM-OLEDs. The results above clearly demonstrate that the single H-dip-coating method with PTE additives for EMLs offers an effective fabrication process, with easy scaling to larger sizes at lower costs compared with other processes.

Fig. 7 (a) Photograph of solution-processable blue, green, and red SM-OLEDs in operation at ca. 10.0 V on a 5 cm × 5 cm substrate, demonstrating the efficient EL properties of the H-dip-coated EMLs with PTE additives, and (b) the 1931 CIE chromaticity diagram of the R, G, and B EL emissions from the SM-OLEDs studied here.

In light of the above observations, it is clear that the self-metered H-dip-coating process with the PTE additives shows considerable promise for the simple production of flat, uniform, and large-area small molecular EMLs, allowing the realization of rapidly processable, low-cost, bright, and efficient full-colour SM-OLEDs. It is also important to note that the solution-process approach using the self-metered H-dip-coating mode is more convenient than conventional vacuum-evaporation for fabricating multi-component EMLs, as the desired compositions of the EMLs can easily be achieved using the appropriate weighting of components, in preference to the more complicated co-evaporation process. We note that the performance of solution-processable full-colour SM-OLEDs could be further improved by optimizing the selection of the host/guest materials, solvents, solution concentrations, viscosities, gap height, and other parameters. Furthermore, the formation of other functional layers by the self-metered H-dip-coating method could also be applied for a useful advantage during the fabrication of new organic electrical devices.

Conclusions

In summary, we successfully fabricated highly luminous, efficient, cost-efficient, and large-area solution-processed small molecular OLEDs incorporating mixed host:guest EMLs doped with interface-engineering PTE additives. Blue, green, and red EL emissions from the single self-metered H-dip-coated EMLs were demonstrated. Maximum luminous efficiency levels of 18.8, 31.2, and 10.0 cd A⁻¹ were achieved for blue, green, and red EL emissions, respectively, which decreased only slightly to the corresponding values of 16.7, 31.0, and 7.3 cd A⁻¹ at a high luminance level of 10 000 cd m⁻² for the H-dip-coated small molecular OLEDs containing B, G, and R EMLs with PTE additives. These high luminance and efficiency levels provide evidence of a high-quality film with no nano-hole defects and with efficient EL properties, produced in an H-dip-coating process with PTE additives. Moreover, blue, green, and red SM-OLEDs with PTE additives on 5 cm × 5 cm substrates with high uniformity were successfully fabricated using H-dip-coated EMLs with PTE additives. We therefore showed how the introduction of emission layers fabricated using self-metered H-dip-coating methods with a co-mixed bipolar host:guest system doped with PTE could be used to create high-performance full-colour SM-OLEDs. Furthermore, this technique can be adapted for use in roll-to-roll coatings, which would then render the use of such devices acceptable in many applications, such as lighting, display, and/or optoelectronic devices.

Acknowledgements

This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean Government (MEST) (2014R1A2A1A10054643) and by a grant from the Innopolis Foundation funded by the Korean government (MSIP) through Kwangwoon University (Grant No. 15DDI825). The present Research has been also conducted by the Research Grant of Kwangwoon University in 2016.

Notes and references

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature, 1990, 347, 539–541.
3. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1988, 395, 151–154.
4. M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4–6.
5. G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich and J. Salbeck, Appl. Phys. Lett., 2004, 85, 3911–3913.
6. Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing and F. Wang, Adv. Funct. Mater., 2009, 19, 84–95.
7. G. He, O. Schneider, D. Qin, X. Zhou, M. Pfeiffer and K. Leo, J. Appl. Phys., 2004, 95, 5773–5777.
8. N. Chopra, J. Lee, Y. Zheng, S.-H. Eom, J. Xue and F. So, Appl. Phys. Lett., 2008, 93, 143307.
9. Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and D. Ma, Angew. Chem., 2008, 47, 8104–8107.
10. L. Xiao, S.-J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater., 2009, 21, 1271–1274.
11. Y.-Y. Lyu, J. Kwak, W. S. Jeon, Y. Byun, H. S. Lee, D. Kim, C. Lee and K. Char, Adv. Funct. Mater., 2009, 19, 420–427.
12. S.-Y. Kim, W.-I. Jeong, C. Mayr, Y.-S. Park, K.-H. Kim, J.-H. Lee, C.-K. Moon, W. Brütting and J.-J. Kim, Adv. Funct. Mater., 2013, 23, 3896–3900.
13. Y. Hino, H. Kajii and Y. Ohmori, Org. Electron., 2004, 5, 265–270.
14. N. Rehmann, D. Hertel, K. Meerholz, H. Becker and S. Heun, Appl. Phys. Lett., 2007, 91, 103507.
15. L. Hou, L. Duan, J. Qiao, W. Li, D. Zhang and Y. Qiu, Appl. Phys. Lett., 2008, 92, 263301.
16. L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, L. Wang and Y. Qiu, J. Mater. Chem., 2010, 20, 6392–6407.
17. Q. Fu, J. Chen, C. Shi and D. Ma, ACS Appl. Mater. Interfaces, 2012, 4, 6579–6586.
18. J. Chen, C. Shi, Q. Fu, F. Zhao, Y. Hu, Y. Feng and D. Ma, J. Mater. Chem., 2012, 22, 5164–5170.
19. Q. Fu, J. Chen, H. Zhang, C. Shi and D. Ma, Opt. Express, 2013, 21, 11078–11085.
20. K. S. Yook and J. Y. Lee, Adv. Mater., 2014, 26, 4218–4233.
21. H. Liu, Q. Bai, L. Yao, D. Hu, X. Tang, F. Shen, H. Zhang, Y. Gao, P. Lu, B. Yang and Y. Ma, Adv. Funct. Mater., 2014, 24, 5881–5888.
22. H.-W. Chang, Y.-T. Lee, M.-R. Tseng, M.-H. Jang, H.-C. Yeh, F.-T. Luo, H.-F. Meng, C.-T. Chen, Y. Chi, Y. Qiu, L. Duan, H.-W. Lin, S.-F. Horng and H.-W. Zan, Synth. Met., 2014, 196, 99–109.
23. Y.-H. Niu, H. Ma, Q. Xu and A. K.-Y. Jen, Appl. Phys. Lett., 2005, 86, 083504.
24. J. H. Park, S. S. Oh, S. W. Kim, E. H. Choi, B. H. Hong, Y. H. Seo, G. S. Cho, B. Park, J. Lim, S. C. Yoon and C. Lee, Appl. Phys. Lett., 2007, 90, 153508; B. Park, Y. H. Huh and M. Kim, J. Mater. Chem., 2010, 20, 10862–10868.
25. B. Park and M. Y. Han, Opt. Express, 2009, 17, 21362–21369.
26. H. G. Jeon, Y. H. Huh, S. H. Yun, K. W. Kim, S. S. Lee, J. Lim, K. S. An and B. Park, J. Mater. Chem. C, 2014, 2, 2622–2634.
27. H. G. Jeon and B. Park, J. Mater. Chem. C, 2015, 3, 2389–2398.
28. B. Park and M.-Y. Han, Opt. Express, 2009, 17, 13830–13840.
29. H. G. Jeon, C. Y. Cho, J. C. Shin and B. Park, J. Mater. Chem., 2012, 22, 23022–23029.
30. B. Park, H. G. Jeon, J. Choi, Y. K. Kim, J. Lim, J. Jung, S. Y. Cho and C. Lee, J. Mater. Chem., 2012, 22, 5641–5646.
31. L. D. Landau and V. G. Levich, Acta Physicochimica URSS, 1942, 17, 42–54.

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