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Abstract
A moth-eye nanopatterned hole-transporting layer (ME-HTL) is proposed to enhance the device efficiency of organic light-emitting diodes (OLEDs), which is fabricated via spontaneous phase separation during spin-coating between poly(N-vinylcarbazole) (PVK) and poly (9,9-dioctylfluorene) (PFO) induced by their surface energy difference. Meanwhile, film morphology characteristics confirm the conformal deposition of the following organic layers and metal electrode on the ME-HTL, indicating the extension of ME nanostructure over all layers in OLEDs. Finally, owning to the disruption of the internal waveguide light at the organic layer/anode interface and the suppression of surface plasmonic loss at organic layer/cathode interface, this device architecture obtained a current efficiency of 78.9 cd/A, with an enhancement factor of 40%. This approach takes the advantage of manufacturing compatibility on behalf of solution-process and thus can be a promising strategy to reduce the production cost of OLEDs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic light-emitting diodes (OLEDs) are developing rapidly in the past 3 decades, and have become a mainstream display technology in consumer electronics [1,2]. In order to obtain high-efficient OLEDs, numerous researches have been performed. As a consequence, a 100% internal quantum efficiency (IQE) has been achieved by using phosphorescent or thermally activated delayed fluorescent (TADF) emitters [3–6]. Besides, the device structure optimization and the molecular orientation control strategies have also proved to be effective for reducing charge injection barrier and improving charge transport ability [7–10]. State-of-the-art power efficiency (PE) of white OLEDs has reached 100 lm/W, which is beyond the fluorescent tube efficiency [11,12]. However, the corresponding external quantum efficiency (EQE) value of 20∼25% is far from the theoretical efficiency limit, due to the low light out-coupling efficiency of 20-30% in the conventional OLED architecture [13]. Generally, 70-80% of the photons are trapped in the substrate (23%) and waveguide modes (WG) (15%) due to the index mismatch of multiple layers, as well as losses to absorption (4%) and surface plasmon polaritons (SPP) (40%) of the metal electrode [14]. Therefore, extensive researches have been proposed to develop light extraction technologies for enhancing the overall device performance of OLEDs [14–16].
Various techniques have been developed, including microlens arrays [17,18], scattering layers [19–21], and corrugated structures [22–25]. For instance, photons in the substrate mode can be outcoupled effectively by using microlens arrays on the substrates’ backside, the scattering layers embedded with nanoparticles or low index grids, quasi-periodic corrugated substrates, or appropriate microcavity structures [26,27]. The light in the WG mode can be extracted by using high-refractive-index substrates (n > 1.8) [28–30]. The periodic or random corrugated structures at the metal/organic interface are effective to outcouple the light in SPP losses [31–35]. Typically, the microlens array and scattering layer are benefit for lighting applications because of their wavelength dependent property. For display applications, reducing the SPP mode is dominant and the corrugated structure is necessary. However, periodic corrugated structures act as a diffraction grating, not only resulting in strong angular and viewing angle-dependent emission spectra, but also requiring complicated and expensive fabricating procedures, such as laser-interference lithography, nanoimprint lithography and holographic lithography [36]. Quasiperiodic and random corrugated structures can be fabricated by Langmuir-Blodgett, spontaneous bucking, phase separation and self-aggregation methods, which can offer the unique advantages of low cost without photolithography and compatibility with roll-to-roll large area process [37–39].
Tang et al. prepared a deterministic aperiodic nanostructure of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) layer by using a soft nanoimprint lithography technique, which significantly reduced the optical confinement without spectra distortion of phosphorescent OLEDs [39,40]. Koo et al. produced the quasi-periodic buckling structure induced by the thermal expansion coefficient difference of Al and poly(dimethylsiloxane) (PDMS) films. OLEDs based on the buckles showed an improved device efficiency by a factor of two [31,38,41]. Similarly, Feng et al. reported spontaneously formed random corrugations on the modified surface of SU-8 film due to the mismatch of thermal expansion coefficients between the surface and bulk of SU-8. Finally, the corrugated OLEDs show the enhancements of 34% in luminance and 15% in current efficiency (CE) [22]. Hippola et al. fabricated OLEDs on a novel corrugated polycarbonate substrate with dome-shaped covex nanopatterns, which was fabricated by direct room-temperature molding [37]. Huang et al. reported a flexible PDMS substrate with periodic wrinkles through prestretch and relaxation, which can outcouple the photons in WG and substrate mode in OLEDs [32]. Jiao et at. fabricated the quasi-periodic corrugated microcavity OLEDs based on a spontaneous phase separation of polystyrene and poly(methyl methacrylate) induced by their solubility difference in tetrahydrofuran, leading to an enhancement of 41% in EQE owing to the scattering effect that reduced the SPP and WG mode [42].
In this work, a moth-eye nanopatterned hole-transporting layer (ME-HTL) is proposed to enhance the device efficiency of OLEDs. A ME-HTL is fabricated via spontaneous phase separation during spin-coating between poly(N-vinylcarbazole) (PVK) and poly (9,9-dioctylfluorene) (PFO) induced by their surface energy difference. Furthermore, optical property and film morphology are characterized to evaluate its application potential. Finally, OLEDs are fabricated based on the ME-HTLs, which obtain a CE of 78.9 cd/A with an enhancement of 80%, due to the disruption of the internal waveguide light at the organic layer/anode interface and the suppression of surface plasmonic loss at organic layer/cathode interface. This self-assembled ME-HTL method is fully solution-processed and compatible with roll-to-roll large-area process, which can reduce the production cost of OLEDs without photolithography.
2. Experimental methods
2.1 Materials
PVK (Mw > 100,000), PFO (Mw = 50,000 ∼ 150,000), 1,10-bis(di-4-tolylaminophenyl) cyclohexane (TAPC), 1,3-bis(carbazol-9-yl)benzene (mCP), tris(2-phenylpyridine) iridium (Ir(ppy)3) and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were purchased from Lumtec Co., Ltd. Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS, CLEVIOS AI P 4083) were purchased from Heraeus.
2.2 Preparation and characterization of moth-eye nanostructure
PVK, PFO and PVK:PFO (with various blend ratios) were firstly dissolved in chlorobenzene at the concentration of 10 mg/mL, and then spin-coated at 2000rpm for 60 s, followed by baking at 80 °C for 30 min. The surface morphology was characterized by optical microscopy (OM, Zeiss Primotech MAT cod.), atomic force microscopy (AFM, Bruker, Dimension icon) and transmission electron microscopy (TEM, Hitachi, HT7700). The thicknesses were determined by Bruker DektakXT Stylus Profiler. The crystalline characteristics were determined by X-ray diffraction (XRD, Bruker D8 ADVANCE). Transmittance was measured with ultraviolet-visible spectrophotometer (Shimadzu, DV-3600).
2.3 Device fabrication and characterization
The OLEDs were fabricated according to the device structure of ITO/PEDOT:PSS (35 nm)/HTL/TAPC (20 nm)/mCP:Ir(ppy)3 (10 wt%, 25 nm)/TmPyPB (40 nm)/LiF (0.5 nm)/Al (100 nm). Firstly, PEDOT:PSS was spin-coated at 2500 rpm for 60 s onto the ITO surfaces, followed by annealing at 120 °C for 15 min. After the flat or ME-HTLs were prepared, these samples were transferred into a vacuum chamber to deposit the layers of TAPC, mCP:Ir(ppy)3, TmPyPB, LiF and Al at a pressure of 4×10−4 Pa in sequence. The current density (J)-voltage (V)-luminance (L) characteristics and electroluminescence (EL) spectra of the OLEDs were measured with a Keithley 2400 sourcemeter and a coupled PR-655 spectroradiometer.
3. Results and discussion
PVK and PFO are widely used hole-transporting polymers, owning to their highest occupied molecular orbital (HOMO) level of 5.8 eV [43]. Besides, they have good solubility in nonpolar solvents, such as chloroform, chlorobenzene, toluene and tetrahydrofuran, which enables them to be compatible with low-cost solution-process. Here, moth-eye nanostructures are produced spontaneously via phase separation between PFO and PVK induced by the difference of surface energy as presented in Table 1. Figure 1 presents the AFM and TEM images of PFO and the blends with different weight ratios of PVK. As shown in Figs. 1(a) and 1(f), we can find that there are nanoclusters in PFO film, indicating PFO aggregates which usually present in pure PFO phase as reported [44,45]. Meanwhile, the PVK film is featureless amorphous and smooth [Figs. 1(e) and 1(j)]. In the case of blend films, isolated pillars are surrounded by the matrix. As increasing the PVK content [Figs. 1(b) and 1(c)], the scale of dispersed pillars ranges from 60-100 nm (PVK is 25 wt.%) to 300-750 nm (PVK is 50 wt.%), and the height of the PVK pillars varies from 80-100 nm (PVK in 25 wt.%) to 50-70 nm (PVK in 50 wt.%). In the case of 75 wt.%-PVK, a two-phase bicontinuous network was observed [Figs. 1(d) and 1(i)]. The diameter of PVK pillars increases as increasing the composition of PVK, which confirms that the pillars are PVK-dominated phase and the matrix is PFO-dominated phase.
Fig. 1. AFM and TEM images for (a, f) PFO, (b, g) PFO:PVK (25 wt.%), (c, h) PFO:PVK (50 wt.%), (d, i) PFO:PVK (75 wt.%), and (e, j) PVK.
Table 1. Surface energy and interfacial energy for PEDOT:PSS, PFO and PVK.
Next, in order to verify the phase components of PVK pillars and PFO matrix, the crystalline property of the PFO:PVK blend films is characterized [Fig. 2(a)]. The XRD pattern of PFO homopolymer film performs two peaks at 2θ = 6.8° and 20.8°, corresponding to the (200) and (600) reflections with d200 = 1.30 nm and d600 = 0.43 nm of the PFO α′ crystalline phase [44,45]. The PVK homopolymer film is featureless. As increasing the PVK fraction in the blend films, the peak intensity of PFO α′ crystalline phase reduced, which was consistent with the fraction decrease of PFO in the blend films. Therefore, the nanocluster aggregate in the matrix is attributed to the crystalline PFO phase, and the isolated pillars belong to the amorphous PVK phase.
Fig. 2. (a) XRD patterns for PFO blended with various content of PVK. (b) Chemical structure and schematic diagram for spontaneous moth-eye nanostructure formation process.
Furthermore, the moth-eye nanostructure formation mechanism is proposed. The Hildebrand solubility parameters (δ) of PVK and PFO are 9.97 and 9.30 (cal/cm3)1/2, respectively [46]. It is known that if the polymer’s Hildebrand solubility parameter approaches that of the solvent, the polymer prefers to dissolve into this solvent. In this work, Δδ between PVK or PFO and chlorobenzene solvent (9.67 (cal/cm3)1/2) are similar, indicating good solubility of PVK and PFO in chlorobenzene [47]. Furthermore, the solubility values of PVK and PFO in chlorobenzene are tested. It is found that 35 mg of PVK and PFO can dissolve in 1 mL chlorobenzene at room temperature without heating, confirming their good solubility. Thus, the phase separation mechanism can’t be attributed to the solubility difference. The surface tension of PEDOT:PSS film, PFO and PVK are 70.9 [48], 17.3 [49], 63.8 mJ/m2 [50] as displayed in Table 1, respectively. The interfacial energy between PFO or PVK and PEDOT:PSS substrate surface was calculated following the equation:
As reported, the nanopatterns with scale of ∼750-800 nm were found to be optimal for light-scattering and outcoupling in OLEDs [37]. Thus, the ME-HTL of PFO:PVK (50 wt.%) was further investigated for application in OLEDs in this work. The optical transmittance of the flat and ME-HTLs are illustrated in Fig. 3(a). Compared with the flat PFO- or PVK-HTL, ME-HTL obtains a comparable transmittance over the entire visible spectrum. Besides, the amount of light scattering is determined by transmittance haze, which shows the percentage of light diffusely scattered compared to the light transmitted. As shown in Fig. 3(b), the average haze value for the ME-HTL is 8.2% over the visible spectra as a comparison to 4.1% of the flat-HTLs. Thus, this ME-HTL not only enhances the total haze but also keeps high total transmittance, which is beneficial for realizing high light outcoupling efficiency.
Fig. 3. (a) Optical transmittance (Inset: Optical microscopy image of the corresponding films), and (b) haze of Glass/ITO/PEDOT:PSS substrate with different HTLs.
In order to evaluate the capability of ME-HTL for light extraction, flat- and ME-OLEDs were fabricated according to the device structure in Figs. 4(a) and 4(b). Initially, surface morphologies of ME-HTL and the sequentially deposited organic layers are tested, and their AFM images are presented in Figs. 4(c)–4(e) and 4(f)-4(h). The average diameter and height of pillars are 733 and 63 nm for ME-HTL, which are 736 and 75 nm for the Glass/ITO/PEDOT:PSS/ME-HTL/TAPC/mCP:Ir(ppy)3/TmPyPB stacks. The increasing height results from the additional organic layers of TAPC/mCP:Ir(ppy)3/TmPyPB evaporated on the ME-HTL. This result confirms the conformal deposition of the following organic layers on the ME-HTL, indicating he extension of ME nanostructure over all layers in OLEDs.
Fig. 4. Device configuration of (a) flat and (b) ME-OLEDs. AFM topology morphology and the corresponding diameter and height statistics for (c, d, e) Glass/ITO/PEDOT:PSS/ME-HTL and (f, g, h) the Glass/ITO/PEDOT:PSS/ME-HTL/TAPC/mCP:Ir(ppy)3/TmPyPB stacks.
Figure 5 and Table 2 displays the performances of OLEDs fabricated on flat-PVK, PFO and ME-PVK:PFO HTLs. The thicknesses for neat PVK and PFO films are 45 and 52 nm, respectively. For the ME-HTL, the thickness of the PFO matrix is ∼ 20 nm, and the height of the PVK nanopillar is ∼ 63 nm as mentioned above. The OLEDs on the flat-PVK HTL yield the lower current density and higher driving voltage in comparison with the flat-PFO HTL, which can be attributed to its lower hole mobility of 2.5 × 10−6 cm2/V·s than the crystalline PFO film (8.0 × 10−3 cm2/V·s) [51], instead of the HOMO level (5.8 eV) and thickness difference between PVK and PFO. Meanwhile, we find that the current density for OLED on the ME-PVK:PFO HTL is higher than flat-PVK, but lower than flat-PFO. Thus, the turn-on voltage is much lower for ME-OLEDs than flat-PFO OLEDs. This situation can be addressed by the ME-nanostructure composed of the PVK-pillars and thinner PFO-matrix. Finally, the device performance of ME-OLED is significantly enhanced in comparison to that of the flat-OLEDs, e.g. maximum CE and PE of ME-OLED are 78.9 cd/A and 38.1 lm/W, which are ≈1.8 and 2.2 times higher than those of the flat-PFO OLEDs, respectively. The device performance enhancement can be attributed to the ME nanostructure, which disrupts the internal waveguide light in the organic and anode interface and reduces the surface plasmonic loss at the metal cathode [33,39,52].
Fig. 5. (a) Current density-voltage, (b) luminance-voltage, (c) current efficiency-luminance, and (d) power efficiency-luminance curves for flat- and ME-OLEDs.
Figures 6(a) and (b) display the voltage-dependent emission spectra of the flat-OLEDs. The shoulder of the main peak near 540 nm is significantly increased with higher driving voltages. This ME-OLED shows almost identical spectra over all driving voltage ranges as shown in Fig. 6(c). Correspondingly, the voltage dependence of Commission Internationate de L’Eclairage (CIE) coordinates of OLEDs based on flat- and ME-HTLs are compared in Fig. 6(d), in which the ME-OLED obtains higher stability with increasing voltage. Inset image in Fig. 6(d) compares the EL spectra for flat and ME-OLEDs at the same luminance of 1000 cd/m2. The emission intensity at 540 nm of ME-OLED is smaller than PVK- and PFO-OLEDs, which can be attributed to the cavity effect. In this work, Ir(ppy)3 was the only emissive component in the light-emitting layer, the recombination zone in the ME-OLED located nearer to the cathode than the flat-OLED, because the hole-transporting nanopillars help the hole carriers transport in the light-emitting layer, leading to a broader recombination zone. Besides, the broad recombination zone can suppress the shift of CIE (x, y) coordinates and results in the stable spectra [43]. The results indicate that the OLEDs fabricated on ME-HTL can meet the stable spectra and high-color-purity requirement for display application.
Fig. 6. Electroluminescent (EL) spectra drove by increasing voltages for OLEDs based on (a) flat-PVK, (b) flat-PFO, and (c) ME-PVK:PFO HTLs, and (d) CIE coordinates as a function of luminance on various HTLs (Inset: EL spectra at 1000 cd/m2).
Table 2. Summarized device performances of OLEDs prepared on different HTLs.
3. Conclusions
In summary, we proposed a facile and effective ME-HTL structure produced via spontaneous phase separation between PFO and PVK induced by their surface energy difference. Meanwhile, film morphology characteristics confirmed the conformal deposition of the following organic layers and metal electrode on the ME-HTL, indicating the extension of ME nanostructure over all layers in OLEDs. Owning to the disruption of the internal waveguide light at the organic layer/anode interface and the suppression of surface plasmonic loss at organic layer/cathode interface, this device architecture obtained a CE of 78.9 cd/A, with an enhancement factor of 80%. This approach takes the advantage of manufacturing compatibility on behalf of solution-process and thus can be a promising strategy to reduce the production cost of OLEDs.
Funding
National Key Research and Development Program of China (2017YFB0404501); Major State Basic Research Development Program of China (91833306); National Natural Science Foundation of China (61704091, 61705111, 62005131, 62074083); Science Fund for Distinguished Young Scholars of Jiangsu Province of China (BK20160039); Priority Academic Program Development of Jiangsu Higher Education Institutions (YX030003); Synergetic Innovation Center for Organic Electronics and Information Displays; Nanjing University of Posts and Telecommunications (NY217010).
Disclosures
The authors declare no conflicts of interest.
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