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Abstract
Even though it is in high demand to introduce a nano-structure (NS) light extraction technology on a silicon nitride to be used as a thin film encapsulation material for an organic light-emitting diode (OLED), only an industry-incompatible wet method has been reported. This work demonstrates a double-layer NS fabrication on the silicon nitride using a two-step organic vapor phase deposition (OVPD) of an industry-compatible dry process. The NS showed a wrinkle-like shape caused by coalescence of the nano-lenses. The NS integrated top-emitting OLED revealed 40 percent enhancement of current efficiency and improvement of the luminance distribution and color change according to viewing angle.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, as the interest of virtual reality (VR) and augmented reality (AR) has been grown, near-eye displays are paid great attention for the VR/AR devices. The near-eye displays, also known as head mounted displays (HMDs) or wearable displays, are composed of illuminating image sources and optical system for giving us immersive experience [1]. A liquid crystal display on silicon (LCoS), a digital micro-mirror device (DMD), a light-emitting diode (LED) microdisplay, and an organic light-emitting diode (OLED) microdisplay are the representative illuminating image source. The LCoS and DMD are relatively cost effective but their volume is large due to separate illuminating light sources. Large volume of the image source is a crucial disadvantage for the near-eye display. The LED microdisplay shows high luminance and relatively low power consumption, but its price is too high due to its complicated structure and process. An OLED microdisplay has many advantages for the near-eye display system. It doesn’t need a backlight, so that it has light weight and very thin thickness besides excellent image quality. The OLED microdisplay supports the design freedom of optical system integration in the VR/AR devices. However, the OLED microdisplay also has disadvantages on limited luminance and relatively short lifetime [2]. Various methods to enhance the luminance of OLED microdisplays have been developed in order to overcome these disadvantages. Light extraction is one of the most important technologies to improve the luminance and efficiency of OLEDs, because 60 to 80 percent of the generated light is trapped and lost in the OLED as either a substrate confined mode, a waveguided mode, or a surface plasmon mode. The trapping light inside the OLED layers lowers the outcoupling efficiency, which results in low luminance. Furthermore, it requires a higher driving current, which affects the lifetime reduction [3,4]. Approximately half of the generated light is captured as waveguided mode, and 30 percent of the light is lost by the total internal reflection at the substrate surface in the bottom-emitting OLED. More than 60 percent of the generated light is lost by waveguided mode and surface plasmon mode in the top-emitting OLED. As a result, only about 20 percent of the generated light contributes to the light output without any light extraction structure [4–7]. A micro-lens array (MLA) with a lens diameter of several hundred micrometers is widely used as a light extraction technology to improve the efficiency of OLEDs [7,8]. However, the MLA is generally fabricated by a printing method, thus it is very difficult to apply the MLA to the top-emitting OLEDs. Furthermore, MLAs can cause image quality degradation such as pixel blurring of OLED microdisplays which have a pixel size of several micrometers [9].
The OLED microdisplay uses opaque Si complementary metal–oxide–semiconductor (CMOS) back-plane, thus it should have the top-emitting OLED structure. Most of published methods for light extraction of OLEDs are useless for the OLED microdisplay because they are concerning about the bottom-emitting OLED. The light extraction methods for the top-emitting OLED with manufacturing process compatibility are needed to enhance the outcoupling efficiency of OLED microdisplays. A top-emitting OLED device usually consists of a reflective bottom anode, a semi-transparent top cathode, and organic layers sandwiched in between. It is widely known that exciton dissipation is coupled into the air mode, absorption loss, trapped waveguided mode, and surface plasmon mode in the case of top-emitting OLED [4,10]. When the in-plane wave vector satisfies the surface plasmon resonance condition, the evanescent wave becomes a surface plasmon mode, which propagates in the direction parallel to the dielectric/metal interface without changing its profile of the electric field. Most of the surface plasmon modes do not escape to air and dissipate in the OLED layers due to several reasons such as absorption [11–13]. Besides the surface plasmon mode, there is another dissipation mode of the near-field absorption due to the interaction between a large in-plane wave vector and a planar metal layer in the vicinity of the radiation dipoles. In the case of top-emitting OLED, the surface plasmon mode and near-field mode hinder the external outcoupling efficiency significantly as the waveguided mode [14,15].
In order to extract light from the cathode side of the top-emitting OLED, the cathode should be transparent and electrically conductive. A dielectric material with a high refractive index on the cathode (capping layer) can enhance the transparency of the top cathode. The capping layer made with organic materials, inorganic dielectric materials, or semi-conducting materials has been utilized as an outcoupling enhancing layer, and a significant enhancement on light output has been achieved by using the capping layer [16–20]. However, the improvement of outcoupling efficiency by the capping layer is limited due to its planar surface. The capping layer requires a prism, lenses, or corrugated structure on it in order to increase the outcoupling efficiency sufficiently [15,21]. We reported that the nano-lens fabrication using an OVPD technique on the transparent conductive oxide layer of top-emitting OLED was effective to increase the outcoupling efficiency. The nano-lens formation was assisted by crystallization and dewetting of the organic material consisting of the nano-lens. The nano-lenses with a diameter of several hundred nanometers increased the outcoupling efficiency by a factor of approximately 1.5 [9]. The nano-lenses act as an excellent light extraction structure, as well as which improves the angular color stability in a wide viewing angle range, and provides less image blurring. We verified those effects of the nano-lens with a finite difference time domain (FDTD) calculations and experiments in the previous study [22].
In the OLED industry, a silicon nitride is widely used as a thin film encapsulation layer; thus, it is generally understood that the introduction of the light extraction technology on the silicon nitride encapsulation film is one of the most effective methods to achieve a highly efficient top-emitting OLED. However, it is very difficult to make the NS with a size of several hundred nanometers on the silicon nitride due to its high surface energy. Only the process-incompatible wet surface treatment has been proposed to make it [23]. In this study, the industry-compatible dry methods are suggested to fabricate the NS on the silicon nitride. The NS with a size of several hundred nanometers was successfully fabricated using the two-step dry process. Furthermore, the NS integrated top-emitting OLED devices were fabricated and showed good electro-optical performances.
2. Experimental procedure
The NS was fabricated by OVPD equipment, in which an evaporator of organic material was separately installed from a main deposition chamber including a sample holder and a shower head. The sample holder was chilled with −10 °C refrigerant, and base pressure of the main chamber 1×10−3 Torr. Stainless steel tubing connected the main chamber and the evaporator, and the vapors were transferred by means of nitrogen gas through the tubing. We used tetra-N-phenylbenzidine (TPB) with 97% purity and PNH-8017 as materials for the NS formation without further purification. TPB was purchased from Merck and PNH-8017 was supplied from P&H Tech. PNH-8017 was developed by P&H Tech and LG Display. The top-emitting OLED device (reference device) in this study was fabricated using the following configuration: (ITO/Ag/ITO) / HTL (140 nm) / Blue EML (20 nm) / ETL (30 nm) / Mg:LiF (1 nm) / Mg:Ag (15 nm) / capping layer (80 nm) / aluminum oxide (60 nm) / silicon nitride (1000 nm), where HTL is a hole transporting layer, EML an emission material layer, ETL an electron transporting layer. The schematic diagrams of the OLED devices used in this study are shown in Fig. 1. The OLED-stack and cathode were deposited on an ITO/Ag/ITO coated glass substrate by a thermal evaporation method in a high vacuum chamber below 5×10−7 Torr. The aluminum oxide thin films were fabricated by an atomic layer deposition (ALD) process and the silicon nitride thin films by a plasma enhanced chemical vapor deposition (PECVD) process. The NS integrated OLED devices were fabricated by means of the deposition of the organic NS by the OVPD process and silicon oxide protective layer by the PECVD process on the silicon nitride encapsulated top-emitting OLED (reference device). The current density – voltage – luminance (J–V–L) characteristics of the OLED devices were measured using a Keithley 2400 programmable source meter. The luminance angular distribution and electroluminescence (EL) spectra were measured with a goniometer equipped spectroradiometer (Minolta CS-2000), at a constant current density level of 0.02 A/cm2.
3. Results and discussion
Encapsulations that act as a gas barrier especially for O2 and H2O are essential for the top-emitting OLED to improve the reliability of organic semiconductor layers and top metal electrode. The water vapor transmission rate (WVTR) required is 1×10−6 g/(m2day) or less for the commercial top-emitting OLEDs [24–26]. Glass plates can be used for the encapsulations of the top-emitting OLEDs, but they cannot be used in flexible and stretchable OLEDs with flexible substrates [27–29]. A thin film encapsulation is widely applied in flexible OLED displays for maintaining flexibility, and it is generally used in OLED microdisplays for maintaining thin and light form factor. The NS is formed on the OLED pixels, thus the OVPD process should be a low-temperature process and the thin film encapsulation layer between the NS and OLED pixel is helpful to make the fabrication process safe. In the previous study, we used aluminum oxide thin film encapsulation (thickness: less than 30 nm) fabricated by the ALD process, and then deposited the nano-lenses on it. However, it is difficult to obtain thick encapsulation layer by ALD due to its low deposition rate. Moreover, the aluminum oxide layer is easily cracked even by small strains because of the high Young’s modulus and low flexural strength. The aluminum oxide encapsulation is not thought to be suitable for mass production [30–32]. Recently inorganic/organic multi-layers are usually applied to the thin film encapsulation of OLEDs, and a silicon nitride thin film is the most favorite inorganic layer of the thin film encapsulation. Therefore, the NS for the light extraction of the top-emitting OLED is favorable to be formed on the silicon nitride film. However, the NS such as organic nano-lens is difficult to be formed on the silicon nitride film due to high surface energy of the silicon nitride as mentioned in the previous study. If the surface energy of the substrate (γs) is smaller than the sum of those of the thin film (deposited material) and the interface of the thin film-substrate (γt + γst), we can make the nano-lens because the thin film grows with an island morphology (Volmer-Weber growth mode). If γs is larger than γt + γst, the thin film grows with a planar morphology (Frank-van der Merwe growth mode). We should use an organic material with high surface energy in order to make the nano-lens on the silicon nitride film unless we lower the surface energy of silicon nitride film [9,23,33,34].
We tested the TPB for the organic NS material because it has a low meting point (230–234 °C). TPB have a low vaporization temperature for the OVPD process compared to N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)−4,4′- diamine (NPB) which is used as the NS material in the previous study. Clogging of the gas line and showerhead can be avoided if we use TPB. Additionally, particle contamination on devices due to deposited particles in the line can be reduced. Figure 2 shows TPB NS which was fabricated on the aluminum oxide and the silicon nitride thin film by the OVPD process. The crucible temperature for vaporization was 310 °C, carrier gas flow rate 800 sccm, and deposition time 300 sec. The nano-lenses were formed well in the case of aluminum oxide, the diameters of the nano-lenses were 300–500 nm, and heights 80–110 nm. However, a planar thin film was formed on the silicon nitride film.
Fig. 2. SEM images of 45° projected surface of the TPB NS on a) aluminum oxide and b) silicon nitride.
Molecules in a substance are bound by cohesive forces (binding energy). Surface tension (surface energy) is generated by the cohesive force which is created to remove the asymmetry due to the exposure of the surface to air, thus the surface tension is proportional to the binding energy of the cohesive force in a condensed matter. Accordingly, the surface energy of silicon nitride is larger than that of aluminum oxide because the binding energy of covalent Si-N bond is larger than that of ionic Al-O bond [23,33]. The TPB films on the silicon nitride was likely to be grown by Frank-van der Merwe growth mode to be a planar shape. TPB is not considered to be a suitable NS material on the silicon nitride film.
We investigated another organic material of PNH-8017 (P&H Tech and LG Display co-developed) which shows relatively high evaporation temperature and high surface energy. Figure 3 shows contact angles of water droplets on the silicon nitride, PNH-8017, and TPB surface, respectively. The contact angles were 10.3, 60.5, and 73.4 degrees for silicon nitride, PNH-8017, and TPB, respectively. The magnitude of the surface energy should be as following: γsn > γpnh > γtpb, where γsn is the surface energy of the silicon nitride, γpnh that of PNH-8017, and γtpb that of TPB.
We can assume that the interfacial energy between water and the condensed matter is proportional to the surface energy between air and the condensed matter [33,35,36]. The surface energy relations of the PNH-8017 and TPB can be expressed as follow,
where γw is the surface energy of water, A the proportional factor for PNH-8017 interface energy, and B the proportional factor for TPB interface energy. Thus,If A has similar value to B, the surface energy of the PNH-8017 is roughly twice of that of TPB. Therefore, PNH-8017 is more advantageous for the nano-lens formation by Volmer-Weber growth mode on the silicon nitride encapsulation than TPB.
Figure 4 shows PNH-8017 nano-structures (NSs) which were fabricated on the silicon nitride film by the OVPD process. The crucible temperature for vaporization was 380 °C, and carrier gas flow rate 800 sccm. The deposition times were 10 minutes (Fig. 4(a) and 4(c)), and 20 minutes (Fig. 4(b) and 4(d)), respectively. Although the NS was formed, the diameters of the NSs were quite small. The diameters were less than 150 nm for 10 minute deposition and less than 300 nm for 20 minute deposition. The heights were also quite small (10 minutes: lower than 30 nm; 20 minutes: lower and 50 nm). The height/diameter ratio looks like small. Although the size (that is, the diameter and the height) of the PNH-8017 NS was increased with evaporation time, the PNH-8017 NS on the silicon nitride film was too small compared to the TPB nano-lens on the aluminum oxide in spite of high evaporation temperature and long deposition time. PNH-8017 was not considered as a good organic material for the NS formation on the silicon nitride encapsulation.
Fig. 4. SEM images of cross-section for a) 10 minute, b) 20 minute deposition, and 45° projected surface for c) 10 minute, d) 20 minute deposition of the PNH-8017 NS.
Therefore, we tried to make the double-layer NS which was fabricated by a two-step OVPD process. Firstly PNH-8017 was deposited on the silicon nitride film, and then TPB was deposited on the PNH-8017 layer. It is difficult to make a large NS by using a one-step OVPD process on a silicon nitride film with a single-layer of TPB or PNH-8017 as mentioned above. However, the PNH-8017 can make the small NS on the silicon nitride film and it is considered to act as a seed-layer for TPB NS in the two-step process, because PNH-8017 is an organic material like TPB and the surface energy of PNH-8017 is much lower than that of silicon nitride as shown in the result of the contact angle measurement (Fig. 3). Figure 5 shows the double-layer (TPB on PNH-8017) NS manufactured by the two-step OVPD process on the silicon nitride film. The shape of the NS (Fig. 5) was similar to the TPB nano-lens on the Al2O3 film (Fig. 2(a)), but the size was larger than that of TPB nano-lens. The number of the double-layer NS per unit area (0.2/µm2) was decreased significantly compared to that of the single-layer TPB nano-lens on the Al2O3 film (5.1/µm2). The temperature for evaporation was 380 °C, carrier gas flow rate 800 sccm, deposition time 10 minutes for PNH-8017, and the temperature 310 °C, the flow rate 800 sccm, deposition time 5 minutes for TPB deposition. The size of the double-layer NS (Fig. 5) was increased significantly compared to the single-layer PNH-8017 NS (Fig. 4). The diameters of the double-layer NS were 300–800 nm and the heights were 60–120 nm. TPB could be formed into a relatively large NS on the PNH-8017 layer, thus the PNH-8017 layer is considered to act as the seed-layer successfully. TPB is considered to be deposited on the surface of the PNH-8017 NS preferentially. The coalescence, which is mergence of several nano-lenses into one, and Ostwald ripening should be occurred during the OVPD process. We suppose that Ostwald ripening was dominant to consider the large distribution of the NS size and significant reduction of NS number per unit area compared to the single-layer PNH-8017 NS. (The number of the NS per unit area for the single-layer PNH-8017 NS was 24/µm2.)
Fig. 5. SEM image of 45° projected surface of the double-layer NS on silicon nitride with 10 minute PNH-8017 and 5 minute TPB deposition.
Figure 6 shows the double-layer (TPB on PNH-8017) NS, in which the deposition time 20 minutes for PNH-8017 and 5 minutes for TPB deposition. Other process parameters were same to above. A complicated and dense wrinkle-like structure was formed by the double-layer NS. The density of the double-layer NS of 20 min PNH-8017 and 5 min TPB was significantly increased compared to the double-layer NS of 10 min PNH-8017 and 5 min TPB as shown in Table 1. The NS density was calculated by means of dividing the NS area by total area. The width of the wrinkle-like structure was about 310 nm, which was considered to be similar to individual island lens. Thus, Ostwald ripening was not thought to be dominant in this case but the coalescence among single nano-lenses was considered to make the wrinkle-like structure dominantly. It is observed that the distance between the NSs was relatively close in the case of the 20 min PNH-8017 single-layer NS (Fig. 4(d)) compared to the 10 min PNH-8017 single-layer NS (Fig. 4(c)). Therefore, the coalescence between adjacent NS was easily occurred in the case of the 20 min PNH-8017 and 5 min TPB deposition compared to the 10 min PNH-8017 and 5 min TPB deposition. This indicates that the coalescence between close nano-lenses occurred earlier, then the coalescence was suspended before Ostwald ripening occurred actively, and finally such wrinkle-like structure was formed. This wrinkle-like structure has advantages for light extraction due to its high density and proper size for strong scattering power because the scattering power by particles increases with the size as well as density of the particles [37–41].
Fig. 6. SEM images of a) 45° projected surface and b) cross-section of the double-layer NS on silicon nitride with 20 minute PNH-8017 and 5 minute TPB deposition.
Table 1. Size, length/width ratio (L/W), height/width ratio (H/W), and density of the NS
Figure 7 shows the double-layer (TPB on PNH-8017) NS, in which the deposition time 20 minutes for PNH-8017 and 8 minutes for TPB deposition. The size of the wrinkle-like structure increased significantly, but the NS density decreased as shown in Table 1. Ostwald ripening between the wrinkle-like structures and the island lenses was considered to occur dominantly in the end stage of the TPB deposition. The mass transfer of TPB is supposed to be generated easily in this deposition condition to reduce total surface area. The increased size would be helpful but the sparse density should be harmful for the light extraction, thus the excessive deposition of TPB should be avoided in the double-layer NS fabrication process.
Fig. 7. SEM image of 45° projected surface of the double-layer NS with 20 minute PNH-8017 and 8 minute TPB deposition.
The data related to size (that is, length, width and height), shape (that is, length/width ratio and height/width ratio), and density of the NS depending on materials and process parameters were summarized in Table 1. The NSs are classified into two groups such as single-layer (SL, Figs. 2 and 4) and double-layer (DL, Figs. 5–7) based on the number of NS layer. The size values are average values calculated based on the SEM measurements. Very interestingly, the size of the DL is much larger than that of the SL, indicating that TPB in the DL exhibits higher growth speed; thus, it is supposed that PNH-8017 in the DL acts as an effective seed layer for the growth of the TPB. Meanwhile, the density value shows a little different behavior. All DL density values are higher than all SL density values. However, the density of DL is increased in the order of DL1 < DL3 < DL 2 (65%), in spite that the length of DL is increased in the order of DL1 < DL2 < DL 3.
The growth of the individual nano-lens as well as coalescence and Ostwald ripening occur during the deposition time. The individual growth and coalescence raise the density but Ostwald ripening does not raise the density. The individual growth is thought to be dominant in the early stage of the second deposition in the two-step process. The coalescence becomes active in the intermediate stage and finally Ostwald ripening is considered to be dominant in the end stage. Our supposition for each double-layer NS forming mechanism is as follows. The coalescence was not active probably due to its relatively long distance between seed layer NSs, thus the individual growth and Ostwald ripening were dominant in the case of the DL1. (The NS density of SL2 was less than half of SL3.) The relatively vigorous Ostwald ripening was considered to decrease the NS density of DL3 in the end stage of deposition. Fortunately, the coalescence between adjacent nano-lenses occurred dominantly and was suspended before Ostwald ripening became dominant in the case of DL2.
The light extraction by the NS has been reported to be generated by the scattering of the confined light in the OLED layers [9,22], and the scattering power increases with the size as well as the density of the particles as mentioned above discussion. The scattering power in Rayleigh scattering is proportional to d6/λ4 and inversely proportional to R2, where d is diameter of the scattering particle, λ wavelength of the incident light, and R distance between particles [39,40]. Therefore, the light extraction by the scattering of the NS integrated OLEDs should increase with the NS size and density. We studied the light extraction effect of the double-layer wrinkle-like structure with very high NS density by fabricating the top-emitting blue OLED devices integrated with the DL2- and the DL3-type double-layer NS and measuring the electro-optical characteristics of them. The silicon nitride encapsulation was carried out by the same low-temperature PECVD process to that of silicon nitride film of the substrates for the NS fabrication. We deposited the double-layer NS on the top-emitting OLED devices, and coated 180 nm-silicon oxide thin film to protect the organic NS with a low-temperature PECVD process. Figure 8 shows J–V–L characteristics of the double-layer NS integrated OLED devices. The J–V curves and leakage current were scarcely changed, indicating that the fabrication process of the NS and the silicon oxide protection layer did not degrade the electric characteristics of the OLED devices. On the other hand, the luminance of the NS integrated devices at the same voltage was significantly increased compared to the reference device. The reference device had the same OLED structure to the NS integrated devices except for the NS and the silicon oxide layer. The current efficiencies depending on current density were shown in Fig. 9, where we can see that the current efficiencies were stable with change of current density. The current efficiencies of the NS integrated devices were higher compared to the reference device, and those of the DL2 device were higher compared to the DL3 device. (The TPB deposition time of the DL2 device was 5 minutes and that of the DL3 device was 8 minutes.) The current efficiencies of the reference device, the DL2 device, and the DL3 device at 3.8 V were 5.22 cd/A, 7.31 cd/A, and 6.17 cd/A, respectively. The enhancements of the current efficiency were 40% and 18% in the DL2 device and the DL3 device, respectively. The enhancement of the current efficiency exhibits the luminance increase at the same current density. It also indicates the increase of the outcoupling efficiency (light extraction) because other parameters and OLED structure were fixed except for the NS integration. The outcoupling efficiencies were considered to increase by almost the same amount with the enhancements. It is generally understood that the extraction of light confined in the encapsulation and the capping layers is an effective method to increase the current efficiency of the top-emitting OLEDs. The cavity effects were also supposed to be scarcely changed by the NS due to the sufficiently thick encapsulation layer [9,22]. Therefore, the light extraction of the DL2 device should be larger than that of the DL3 device.
Fig. 9. Current efficiencies depending on current density for the reference, the DL2, and the DL3 device.
Figure 10 shows luminance distributions of the OLED devices, which were measured at 0.02 A/cm2. Those of the NS integrated devices were closer to Lambertian distribution compared to the reference device, and that of the DL2 device was closer to Lambertian distribution than the DL3 device. Figure 11 shows EL spectrum changes depending on viewing angle. It is a little difficult to analyze the scattering effects by the EL spectrum change due to the cavity effect, but the peak wavelength difference between vertical direction (0 °) and 60 degree from the vertical direction (60 °) is decreased in order of reference device > the DL3 device > the DL2 device. Furthermore, the color differences of Δ(u′ v′) according to viewing angle (0-60 degree) were 0.105, 0.093, and 0.091 for the reference device, the DL3 device, and the DL3 device, respectively. The color difference depending on viewing angle decreased in the NS integrated devices compared to the reference device. The reduction of the color difference is thought to be originated by the light scattering of the NS. The enhancement of the luminous efficacy along with improvements of luminance distribution and color stability depending on viewing angle implies that the light extraction was enhanced by the scattering of the NS layer. Additionally, the scattering was larger in the DL2 device than the DL3 device. As mentioned above, the light extraction by the scattering of the NS increase with the NS size and density, however the NS density effect was supposed to be stronger and more important than the size effect in these cases. We didn’t test the NS effects on the optical characteristics of other OLED colors in this study, however, reported the nano-lens effects depending on the OLED color variation in the previous manuscripts [9,22,41]. The enhancements of the outcoupling efficiencies and the color stabilities were similar in red, green, and blue OLEDs. Thus, the NS effects depending on OLED color in this study are not supposed to be seriously changed.
Fig. 11. EL spectra depending on viewing angle of a) the reference, b) the DL2, and c) the DL3 device.
We analyzed the NS morphology on the devices after the measurements of electro-optical characteristics to check if the difference between the NSs on the device and the substrate is observed. Figure 12 shows surface morphology of the DL2 and the DL3 devices. The silicon oxide protective layer covered the double-layer NS in both devices, and the silicon oxide surface morphology was well followed with the NS surface. The dense and wrinkle-like NS shape of the DL2 device was very similar with that of DL2 NS on the substrate. For the case of DL3 device, the surface morphology was somewhat different from the DL3 NS on the substrate. The widths were about 360 nm and about 1 µm for the DL2 device and DL3 device, respectively, and the NS densities were about 75% and 65% for the DL2 device and DL3 device, respectively. The values for the width and density were not accurate because the SEM images of the device surface were not clear, but the size and density of the NS on the device definitely increased compared to the NS on the substrate in the case of DL2. The silicon oxide cover layer is considered to raise the size and density. The size was significantly larger and the density was lower in the case of the DL3 device compared to the DL2 device as in the case of NS on the substrate. The DL3 device morphology looks like the intermediate stage proceeding to the DL3 NS on the substrate considering that the size and the space between the wrinkle-like structure. It is supposed that Ostwald ripening were vigorously generated within relatively short time in the end stage of TPB deposition, so the shape of the NS was changed fast in the end stage. Thus the size and density could be changed even in small difference of the deposition time as the case of the DL3 device. Lowering density of the NS due to excessive Ostwald ripening is considered to be harmful for the light scattering and light extraction, thus long TPB deposition should be avoided to fabricate the optimum double-layer NS for the light extraction.
4. Summary
We fabricated the organic NSs on the silicon nitride thin films by the dry OVPD process. The organic material of PNH-8017 having relatively high evaporation temperature and high surface energy could be deposited in the form of NS on the silicon nitride, but the size was too small for sufficient light extraction of OLED. Thus, we used the two-step OVPD process and fabricated the double-layer NS with PNH-8017 and TPB to make proper sized NS for the light extraction. The double-layer NS was formed in the shape of wrinkle-like structure by the coalescence of the adjacent nano-lenses, thus the NS density was significantly increased compared to the island type nano-lenses. However, an excessively long time deposition caused Ostwald ripening to make a large but sparse NS. The top-emitting blue OLED integrated with the double-layer NS showed 40% enhancement of current efficiency and improvement of the luminance distribution and color stability depending on viewing angle compared to the reference device without the NS. These double-layer NS can be applied to the top-emitting OLEDs as well as OLED microdisplays due to their applicability on the silicon nitride thin film encapsulation without any damage on the OLED layers.
Funding
Electronics and Telecommunications Research Institute (ETRI); Development of the Technologies for ICT Materials, Components and Equipment (21ZB1200).
Acknowledgments
The authors appreciate P&H Tech and LG Display for the supply of PNH-8017.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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