The Improvement of Luminous Uniformity of Large-Area Organic Light-Emitting Diodes by Using Auxiliary Electrodes

Abstract

The study developed a large emission area of flexible blue organic light-emitting diodes (BOLED) on a polyethylene terephthalate/ Indium tin oxide (PET/ITO) substrate using a polycyclic skeleton ν-DABNA Thermally Activated Delayed Fluorescence (TADF) material. Initially, a 1 × 1 cm² blue OLED was fabricated to optimize the layer thickness. The blue OLED structure consisted of PET/ITO/HATCN/TAPC/UBH-21:ν-DABNA/TPBi/LiF/Al. However, as the emission area increased to 3.5 × 3.5 cm², the current density decreased due to the resistance of PET/ITO, leading to luminance non-uniformity. To address this issue, auxiliary Au lines were added to the ITO anode to enhance current injection. Despite this, when the Au lines reached a thickness of 30 nm, average light emission was disrupted. To improve the luminescence characteristics of large-area PET/ITO OLEDs, a capping and planarization layer of PEDOT:PSS was applied. Grid uniformity revealed a significant increase in overall luminance uniformity from 74.1% to 87.4% with the addition of auxiliary Au lines. Further increases in grid line density slightly reduced uniformity but enhanced brightness, resulting in brighter, flexible, large-area blue OLED lighting panels.

1. Introduction

Significant advancements have been made in the organic light-emitting diode (OLED) industry over the past decade, particularly in power efficiency and lifespan. OLEDs are now considered highly energy-efficient display technologies that offer vibrant colors with minimal power consumption. Compared to other display technologies, OLEDs also have a relatively long lifespan [1,2]. Their versatility comes from a wide range of organic light-emitting materials, which enable precise spectral customization for various applications. OLEDs are ideal for lighting applications due to their slim profile, self-emission, unrestricted viewing angles, and flexibility. Additionally, they contribute to energy conservation, which is a critical factor in addressing global energy challenges. The thin-film design of OLEDs facilitates the creation of large-area luminaires in flexible and transparent devices suitable for diverse applications, such as architectural lighting, automotive lighting, and wearable displays. As OLED lighting panels increase in size, maintaining uniform brightness becomes a concern. The transparent indium tin oxide (ITO) anodes have high resistance, leading to a concentration of current density near the electrode contacts. This causes reduced brightness at the panel’s center, a phenomenon known as IR drop. To address this issue, fine metal wires in auxiliary metal electrodes are used to distribute current evenly across the anode. Researchers often apply a vacuum-deposited insulating layer on the auxiliary electrode to prevent short circuits with the cathode. However, this insulating layer reduces the panel’s effective emitting area [3,4,5]. The article extensively studied the effects of the device architecture and processes on the performance of large-area organic light-emitting diodes with auxiliary electrodes. Jun Ho et al., using inkjet printing technology, created an Ag metal grid with a line width of 18 µm and a line spacing of 0.15 mm, reducing the sheet resistance from the original 9.06 to 2.37 Ω/sq. The Ag grid lines were applied to a green OLED with an emitting area of 7 × 7 cm², improving luminance uniformity by 10% [6]. In a recent study, Jung et al. fabricated Ag nanodot arrays on ITO anodes, demonstrating higher current and lower startup voltage compared to ITO anodes, and enhancing electroluminescence (EL) intensity through the coupling between the surfaces of Ag nanoarrays and organic materials [7]. Lee et al. introduced a striped thin metal layer between the ITO and the hole transport layer (HTL) to investigate the OLED microcavity effect. With the inserted metal layer, the microcavity effect was enhanced, the forward emission of the OLED becomes stronger, and the angular distribution becomes more forward-directed, resulting in a current efficiency (CE) nearly 1.45 times higher than that without the inserted metal layer [8]. Kang et al. developed external light extraction films with wrinkle-structured surfaces, applied to large transparent OLEDs with auxiliary metal buses. The wrinkle-structured film is formed on the outer surface of the glass substrate and has the same shape as the auxiliary metal electrodes on the ITO. This area-selective film enhances the external quantum efficiencies for both bottom and top emission OLEDs [9]. Hwang et al. demonstrated improved out-coupling efficiency and reduced haze in OLED lighting with an auxiliary electrode and selective microlens arrays (SMLAs). As a result of the SMLAs, the dark grid lines in the emission area become brighter, the haze was reduced, and the OLED spectrum remained unchanged [10]. Lenk et al. used nanometer-thickness metal layers as an anode substitute for ITO in white OLEDs. OLEDs using 4 and 6 nm Ag layers exhibited better external quantum efficiency (EQE) than those with ITO, particularly in light extraction from the glass substrate [11]. Park et al. investigated how an auxiliary metal electrode (chrome) in conjunction with a device configuration affects the luminance uniformity of a large-area (15 × 15 cm²) white OLED. The difference between the effective horizontal resistance of the anode (ITO embedded with the grid-patterned metal electrode) and the vertical resistance of the OLED device is crucial for determining luminance uniformity [12]. Park J et al. conducted a study to evaluate the impact of using 0.15 mm-wide grid-patterned auxiliary metal electrodes (Cr, Mo/Al/Mo, or Cu) on an ITO anode on the performance of OLED lighting panels. The results showed that Cu metal grids had the highest luminous efficacy and produced the least heat due to their lower sheet resistance, which helped achieve a more uniform current distribution. Moreover, Cu grids provided the most uniform light distribution compared to Mo/Al/Mo and Cr. Overall, using Cu metal grids improved the performance of both opaque and transparent white OLED lighting panels by reducing non-uniform current distribution and local heat generation. [13]. Despite their advantages, large-area OLED panels have drawbacks, such as short circuits, hot spots, heat generation, low luminous intensity, and non-uniform light emission.

To address this issue, a study has shown that adding auxiliary Au lines to ITO can improve current injection in PET/ITO OLEDs, resulting in more uniform luminance for blue OLEDs on large-area panels. The work function of Au is about 5.1 eV, which is similar to that of ITO, at 4.9 eV. This compatibility makes them suitable for hole injection into the organic material (HATCH/TAPC). Additionally, gold can be easily deposited using a thermal evaporator. In a nine-square grid uniformity test, adding six lines with a spacing of 5 mm led to an overall uniformity of 87.4%. Increasing the number of lines to 16 with a spacing of 2 mm slightly decreased uniformity (to 84.6%) but increased brightness from 24.68 to 35.49 cd/m². The study recommends using auxiliary Au lines, capping, and planarization layers to enhance the luminance uniformity and brightness of large-area flexible blue OLED lighting panels on PET/ITO substrates. Even with OLED technology, device configuration can affect luminance uniformity. The structure and materials employed in the device can impact the distribution of current and light emissions. Factors such as the thickness and composition of the various layers and the placement and size of electrodes can all influence the overall uniformity of light output. Therefore, when designing OLED devices for uniform illumination, it is crucial to carefully consider device structure and materials to optimize luminance uniformity.

2. Experiments

This study used the PET/ITO flexible substrate with a substrate thickness of 0.05 mm and resistance of 30 Ω/sq. A laser engraver was used to define the pattern for the anode. The luminous areas are 1.0 × 1.0 cm² and 3.5 × 3.5 cm², respectively. In sequence, the engraved substrate was immersed in acetone, isopropanol, and deionized water and washed with ultrasonic vibration for 20 min each. After that, the substrates were blown dry with a nitrogen (N₂) gun and baked in an oven at 80 °C for 10 min. The substrate was placed into a thermal evaporation system. An Au auxiliary electrode (with a thin line pattern) was evaporated over the ITO anode surface with a deposition rate of 0.5 Å/s under a vacuum of 8 × 10⁻⁶ torr through a slit metal mask. After that, the organic layers were deposited by vacuum thermal evaporation at a vacuum pressure of 6 × 10⁻⁶ torr. Digital crystal monitor sensors are used to manipulate layer thickness and deposition rate. On the PET/ITO substrate, the organic layers were evaporated in the following sequence: hole injection layer (HIL) HATCN, hole transport layer (HTL) TAPC, emitting layer (EML) UBH-21 doped with ν-DABNA, and electron transport and hole-blocking layer TPBi. ν-DABNA is N7,N7, N13, N13,5,9,11,15-octaphenyl-5,9,11,15-tetrahydro-5,9, 11,15-tetraaza-19b,20b-diboradinaphtho [3,2,1-de:1′, 2′,3′-jk] pentacene-7,13-diamine. In the next step, LiF and Al were deposited onto the organic layer by a thermal evaporation system under a vacuum of 8 × 10⁻⁶ torr. LiF deposition rate and thickness were 0.1 Å/s and 0.8 nm, respectively, and Al deposition rate and thickness were 10 Å/s and 150 nm, respectively. The fabrication of blue OLEDs was complete. Finally, OLED current–voltage characteristics were measured with a Keithley 2400, and luminance and spectrum were measured with a Spectra Scan PR 650 Spectra Colorimeter. The current–luminance–voltage (I–L–V) characteristic curve was obtained. An illustration of the fabricated multilayer device structure is shown in Figure 1.

3. Results and Discussion

In this study, HATCN was selected as the hole injection layer. HATCN is a derivative of hexaazatriphenylene, and its molecules enhance the hole injection efficiency of p-type organic materials. The HATCN material contains six acetonitrile groups with strong electron affinity, which assist in penetrating the adjacent TAPC LUMO energy levels and extracting electrons [14]. ITO has a significant energy barrier between its work function and the HOMO of the HTL TAPC, as shown in Figure 2. By inserting HATCN at the interface between ITO and HTL TAPC, the energy barrier is lowered, thereby improving the hole injection efficiency [15]. The charge transfer complexes (dipoles) formed at the HATCN/TAPC interface readily facilitate charge transport. The interfacial dipole layer on the HTL (TAPC) surface, created by the insertion of HATCN, helps prevent electron diffusion and improves hole extraction [16].

The thicknesses of HIL HATCN are adjusted to 10, 7, 5, and 3 nm, respectively, to analyze the effects of HIL thickness on device characteristics. The device structure used was ITO/HATCN (X nm)/TAPC (30 nm)/UBH21:20 wt% v-DABNA (30 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al (150 nm). Figure 3a,b shows the current density–voltage (J–V) and luminance–voltage (L–V) characteristics curves of blue OLED with different HATCN thicknesses. The HATCN layers with a thickness of 3–5 nm exhibited higher current density and brightness compared to the 7 nm layer. The OLED with a 5 nm HATCN layer achieved a current density of 16.41 mA/cm², a luminance of 208 cd/m², and an efficiency of 1.27 cd/A. However, as the HATCN thickness increased to 7–10 nm, a decrease in current density and brightness was observed. As shown in Figure 3, the OLED with a 10 nm HATCN layer produces a lower current density of 9.14 mA/cm² and a brightness of 107 cd/m² at 9 V, due to the bulk characteristics of HATCN. Excessive thickness in the HATCN layer hinder the injection of holes into TAPC, resulting in poor carrier (hole) transport characteristics. A thinner HATCN layer (3–5 nm) generally facilitates better energy alignment, allowing for easier injection of holes from the anode (ITO) into the organic layer. A thicker HATCN layer (7–10 nm) introduces additional resistance to hole injection. The energy barrier becomes more significant, resulting in a decrease in hole current. As the HATCN layer thickness increases (7–10 nm), it may exhibit more bulk characteristics, which can lead to less efficient charge transport and increased recombination losses. Excessive thickness (10 nm or more) may hinder efficient transport of holes into the subsequent TAPC layer, resulting in reduced overall carrier mobility and lower current density. Therefore, a 5 nm HATCN layer is selected for subsequent experiments.

TAPC is deposited on HATCN as the hole transport layer. The LUMO of HATCN is 5.4 eV, while the highest occupied molecular orbital (HOMO) of TAPC is 5.5 eV. As shown in Figure 1, TAPC’s HOMO is only 0.1 eV higher than HATCN’s LUMO. This small energy difference allows electrons to be easily transferred from the HOMO level of TAPC to the LUMO level of HATCN, thereby generating electron-hole pairs at this organic–organic interface. The small HOMO–LUMO gap enables the generation and separation of electrons and holes at the interface when an external bias is applied, a process sometimes referred to as charge carrier generation (CCG) [17].

Since the barrier between the HOMO level of TAPC and the LUMO level of HATCN is below 0.5 eV, the carrier transport (exchange) between the two materials occurs with minimal effort. By utilizing this small energy ladder effect and the carrier transport (exchange) mechanism facilitated by HATCN, this OLED structure can exhibit excellent current–voltage characteristics [18,19].

The thicknesses of the HTL TAPC are adjusted to 30, 40, and 50 nm, respectively, to study the effects of TAPC thickness on device characteristics. The device structure used was ITO/HATCN (5 nm)/TAPC (X nm)/EML (30 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al (150 nm). Figure 4a–c show the current density–voltage (J–V), luminance–voltage (L–V), and yield voltage (Y-V) characteristic curves of OLED with different TAPC thicknesses.

At 9 V, increasing the TAPC thickness from 30 to 40 nm results in a decrease in current density decreases from 16.41 mA/cm² to 16.01 mA/cm², while the brightness increases from 208 cd/m² to 230.1 cd/m², and the efficiency improves from 1.26 cd/A to 1.43 cd/A. This improvement in efficiency is attributed to a better electron-hole balance. For the OLED with a TAPC thickness of 30 nm, the efficiency–voltage curve increases linearly with voltage above 3 V. When the TAPC thickness is increased from 30 to 40 nm and the voltage exceeds 4.5 V, the efficiency–voltage curve rises rapidly and tends to stabilize, showing no significant change with further increases in voltage, as shown in Figure 4c. According to the principle of voltage division, the increased thickness of TAPC, leads to a higher partial voltage at the anode terminal, which facilitates the injection of more holes into the emitting layer and thereby provides more hole current to the device. Consequently, the partial voltage at the cathode terminal decreases, reducing the electron current and allowing the electron and hole currents to approach equilibrium. Upon thickening the TAPC to 50 nm, even higher efficiency is achieved, yielding a current density of 15.38 mA/cm², a luminance of 232.2 cd/m², and an efficiency of 1.51 cd/A at 9 V. When the TAPC thickness is increased to 40–50 nm, the hole current density increases more significantly than the electron current density, leading to a better balance of carriers and thus improved efficiency.

A TPBi layer with high electron mobility serves as an electron transport layer (ETL) at the cathode terminal. Due to its high HOMO level of 6.7 eV, TPBi exhibits excellent hole-blocking capabilities, allowing it to function as both an electron transport and hole-blocking material. This effectively confines triplet excitons within the emitting layer. The thickness of the TPBi layer was varied between 30, 35, and 40 nm to study its effects on device characteristics. The device structure is as follows: ITO/HATCN (5 nm)/TAPC (50 nm)/UBH21:20 wt% v-DABNA (30 nm)/TPBi (X nm)/LiF (0.8 nm)/Al (150 nm). Figure 5a,b shows the current density–voltage (J–V) and luminance–voltage (L–V) characteristics curves, respectively, for different TPBi thicknesses. When the TPBi thickness is increased from 30 to 40 nm, the current density rises from 15.38 to 23.17 mA/cm² at 9 V, while the luminance increases from 232.6 to 323.8 cd/m². As the TPBi thickness increases, resistance increases, which reduces electron injection current from the cathode terminal but increases hole current at the anode terminal, thereby enhancing luminance. Consequently, a TPBi thickness of 40 nm was selected for subsequent OLED fabrication.

This study utilized a novel polycyclic skeleton ν-DABNA material, which contains no precious metal and exhibits Thermally Activated Delayed Fluorescence (TADF) characteristics, to fabricate blue OLEDs. These OLEDs emitted a blue spectrum with a narrow full width at half maximum (FWHM) of 28 nm [20]. The OLED emitting area is adjusted in this section, and the effects of different emitting areas on OLED characteristics are discussed. The structure of the blue OLED is as follows: ITO/HATCN (10 nm)/TAPC (50 nm)/UBH-21: 20 wt% v-DABNA (30 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (150 nm). Figure 6a–c show the current density-voltage (J–V), luminance–voltage (L–V), and yield-voltage (Y-V) characteristic curves for different emitting areas. An OLED with an emitting area of 1 × 1 cm² has a current of 23.14 mA and a current density of 23.14 mA/cm² at 9 V. Meanwhile, an OLED with an emitting area of 3.5 × 3.5 cm² at 9 V has a current of 51.74 mA and a current density of 4.19 mA/cm². As the emitting area increases, the current density decreases, which leads to a reduction in luminance, as shown in Figure 6.

The high resistance of the PET/ITO anode in a large-area flexible OLED hinders the uniform flow of current across the entire light-emitting area, especially in regions far from the power contact point, thereby causing a significant decrease in brightness.

Therefore, in this study, Au auxiliary electrodes are deposited on top of the ITO film to conduct more current to areas distant from the contacts and enhance the luminance uniformity of the large-area OLED. This section examined the transmittance of Au films with varying thicknesses deposited on the entire PET/ITO substrate, as shown in Figure 7.

Figure 7 illustrates the electroluminescence (EL) spectrum of the blue OLED in this study, showing a peak at 486 nm. The transmittance of the bare PET/ITO substrate without Au deposition, was 75.0% at wavelength 486 nm. As the Au film thickness increased, the transmittance gradually decreased. When the Au thickness reached 15 nm, the transmittance of PET/ITO/Au at 486 nm dropped to 63.5%, as shown in Figure 7 and as detailed in Table 1.

The blue OLED in this study emits light over an area of 1225 mm², while the Au auxiliary lines (comprising six lines) occupy a total area of 19.8 mm². The gold auxiliary line accounts for approximately 1.62% of the light-emitting area, thus having a minimal impact on the effective light-emitting area. The sheet resistance of PET/ITO with varying thicknesses of Au was measured using a four-point probe method. The driving current (I) flows through the outer probes, and the voltage difference (V) is measured across the inner probes. The sheet resistance value ρ_s is calculated using Formula (1), where 4.532 is the correction factor:

(1)${\mathsf{\rho }}_{\mathrm{s}}={\displaystyle \frac{\mathsf{\rho }}{\mathrm{t}}}=4.532{\displaystyle \frac{\mathrm{V}}{\mathrm{I}}}$

where ρ is resistance, t is film thickness, V is applied voltage, and I is current [21].

The effect of Au thickness on sheet resistance was studied, followed by a discussion on the impact of Au auxiliary lines on the uniformity of injected current flow. As shown in Table 2, a 5 nm thick ITO/Au layer (fully covering the ITO) exhibits a higher sheet resistance of 55.53 Ω/sq. When the Au thickness on ITO is increased to 10 nm, the sheet resistance decreases significantly to 31.47 Ω/sq. This change is attributed to the increased island-like structure of the Au layer on ITO [22]. When the thickness of ITO/Au is further increased to 15 nm, the sheet resistance decreases to 28.11 Ω/sq.

High resistance of PET/ITO and poor current conduction can deteriorate the uniformity of light emission as the light-emitting area increases. To address the issue of non-uniform light emission, Au auxiliary lines were deposited on top of the ITO anode to enhance anode current transport. A schematic diagram of auxiliary Au line electrodes is shown in Figure 8. The line width is 0.1 mm, with a spacing of 5 mm, and there are six lines located within the emitting area.

In this study, the effects of the thickness of auxiliary Au lines on OLED characteristics were investigated to determine the optimal thickness for large-area blue OLED. The structure of blue OLED is ITO/Au lines (X nm)/HATCN (10 nm)/TAPC (50 nm)/UBH-21:20 wt% ν-DABNA (30 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (150 nm). Figure 9a–c shows the current density-voltage (J–V), luminance–voltage (L–V), and yield-voltage (Y-V) curves, respectively, for the blue OLEDs with different auxiliary Au line thicknesses. The current density increases with the thickness of Au lines. Compared to the OLED without Au auxiliary lines, the OLED with 15 nm thick auxiliary Au lines on ITO anode showed an increase in current density from 4.19 to 4.33 mA/cm², an increase in luminance from 26.87 to 37.07 cd/m², and an increase in efficiency from 0.64 to 0.86 cd/A at 9 V, as shown in Figure 9.

As observed, the Au auxiliary lines effectively promote current injection. Therefore, there are plans to increase their thickness to 30 nm.

The HATCN/TAPC was evaporated for blue OLED fabrication as hole injection/transport layer on the PET/ITO/Au auxiliary lines. As the thickness of the Au auxiliary lines increased to 30 nm, the HATCN, being only 5 nm thick, could not completely cover the Au auxiliary lines. Consequently, the Au lines directly contacted TAPC instead of HATCN, preventing the OLED from emitting the light typically. To address this, PEDOT:PSS were spin-coated three times on the ITO/Au auxiliary lines as a planarization layer (flat layer) before HATCN evaporation to achieve adequate thickness. With the addition of three-layer PEDOT:PSS, the current density decreased due to the increased resistance. The decrease in current density is attributed to the reduced hole current, as the extra PEDOT:PSS layer hinders hole injection from the ITO anode, resulting in a higher onset voltage.

However, the luminescence characteristics improved, and efficiency increased up to 1.01 cd/A at 9 V, with a thicker auxiliary Au line thickness of 30 nm, as shown in Figure 9. For the ITO/PEDOT:PSS/auxiliary Au line configuration (with a thickness of 30 nm), the luminance characteristics improved, resulting in the lowest turn-on voltage. This enhancement is attributed to the thicker auxiliary Au line, which improves the uniformity of hole current flow across the entire emitting area on the anode side, as shown in Figure 9b.

As shown in Figure 9b, the OLED with Au auxiliary lines emits light at 4 V, whereas the OLED without Au auxiliary lines begins to emit at 5.5 V. Without the Au auxiliary lines, the PET/ITO anode electrode in the OLED exhibits a non-uniform current distribution at a low voltage of 4 V, as in the emitting photos shown in Figure 10. The brightness is greater near the positive electrode clip side (left side in the photos in Figure 10) compared to the negative electrode clip side and, even at 9 V, non-uniform luminescence persists, as shown in Figure 10.

In this study, the encapsulated blue OLED with an emitting area of 3.5 × 3.5 cm² was divided into three columns by three rows (creating nine-square-blocks). The luminance of the nine-square-blocks was measured under a fixed voltage. The lifetime of the large-area OLED with encapsulation is only approximately 4 h. The measurement procedures were completed as quickly as possible. The initial luminance was recorded at the beginning of the tests and repeated at the end to ensure that the uniformity assessments of the nine-square-blocks were not affected by any deterioration of the OLEDs.

In Equation (2), x_i represents the luminance of any square-block within the nine-square-blocks. n is equal to 9. The $\overline{\mathrm{x}}$ arithmetic mean $\overline{\mathrm{x}}$ of the luminance values from nine-square-blocks was then calculated. Using Equation (2), the standard deviation (s) was calculated. The standard deviation can be understood as the absolute value of the degree of dispersion:

(2)$\mathrm{s}=\sqrt{{\displaystyle \frac{\mathsf{\Sigma }{\left({\mathrm{x}}_{\mathrm{i}}-\overline{\mathrm{x}}\right)}^2}{\mathrm{n}-1}}}$

The coefficient of variation (V) is defined as the ratio of the standard deviation to the average value (s/$\overline{\mathrm{x}}$). It is frequently used in engineering quality control to represent the non-uniformity of engineering quality, with larger values indicating greater non-uniformity. For the purposes of this study, we define the uniformity judgment value as 100% minus the coefficient of variation, as shown in Equation (3).

(3)$100\mathrm{\%}-V=100\mathrm{\%}-\left({\displaystyle \frac{\mathrm{s}}{\overline{\mathrm{x}}}}\times 100\mathrm{\%}\right)$

The PET/ITO substrate was purchased from a commercial company, whose ITO film is very uniform. The Au lines were deposited with a thermal evaporator at 8 × 10⁻⁶ torr, which are also uniform. Therefore, the uniformity of the transmittance of ITO/auxiliary Au lines in nine area is good enough.

We had nine pieces of 4 cm × 4 cm paper cardboard, which functioned as a mask. Each piece of cardboard had a single hole for the measuring block, measuring 11.6 mm × 11.6 mm area. During the measurement process, this mask was placed on top of the OLED panel. This arrangement ensured that only the brightness of one block in the nine-square-blocks was measured, preventing any influence from other blocks. The measurement focused on the center of each block.

Figure 11a shows the measured luminance distribution of nine-square-blocks for the PET/ITO OLED without auxiliary Au lines. Figure 11b shows the luminance measurement for the square-block (column 1, row 2) in the nine-square-blocks. The average luminance of the nine-square-blocks is 26.13 ± 6.75 cd/m² at 9 V, and the overall uniformity, calculated using Equation (3), is 74.1%. From Figure 11a, it is evident that the luminance near the positive electrode side (left side) is higher. The leftmost column reaches its peak brightness due to its proximity to the positive electrode clip. However, as we move away from the positive electrode, the brightness begins to diminish. In the second column from the left, a noticeable decrease in brightness is already apparent compared to the top row. The luminance gradually decreases from the left column to the right. Specifically, the luminance uniformity in the left two columns is 81.2%, which dropped to 74.1% when considering all three columns in the nine-square-blocks.

The glass/ITO with lower resistance was utilized for the nine-square uniformity measurement to determine whether the conductivity of the ITO is the primary factor influencing uniformity. Figure 12a shows that the mean luminance and standard deviation were 70.85 ± 2.20 cd/m². The overall uniformity, calculated using Equation (3), is 96.9%, which is significantly better than that of PET/ITO OLEDs. Figure 12b shows the full area emission characteristics of the glass/ITO/OLED without the use of auxiliary Au lines.

Based on the measured luminance distribution of nine-square-blocks of glass/ITO/OLED without auxiliary Au lines, it is evident that low-resistance ITO contributes to enhanced luminous uniformity when compared to PET/ITO/OLED.

Subsequently, auxiliary Au lines were deposited on top of the ITO to investigate whether the auxiliary electrode could improve current transportation to positions farther from the anode electrode contact point and thus enhance luminance uniformity. Figure 13a shows the luminance measurement of nine-square PET/ITO OLED blocks equipped with auxiliary Au lines (15 nm thickness, 5 mm spacing, 6 lines) on the ITO substrate. The average luminance recorded is 29.68 ± 3.73 cd/m², and the overall uniformity, calculated using Equation (3), is 87.4%. This represents an improvement in uniformity compared to the 74.1% observed without auxiliary Au lines (Figure 11).

Next, the spacing of auxiliary Au lines was decreased from 5 to 2 mm to investigate its effect on luminance uniformity. Figure 13c shows the luminance measurement of the nine-square blocks of PET/ITO OLED with auxiliary Au lines (15 nm thickness, 2 mm spacing, 6 lines) deposited on the ITO substrate. The average luminance recorded is 35.49 ± 5.48 cd/m², and the overall uniformity, calculated using Equation (3) is 84.6 %. This is lower than the uniformity observed with auxiliary lines spaced 5 mm apart. However, the average luminance has been improved from 29.68 to 35.49 cd/m².

4. Conclusions

A large-area blue OLED using TADF material ν-DABNA was successfully fabricated on a PET/ITO substrate. The optimal OLED structure comprised PET/ITO/HATCN (5 nm)/TAPC (50 nm)/UBH-21: 20 wt% ν-DABNA (30 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al (150 nm). For a 1.0 × 1.0 cm² emitting area, and the resulting current density, luminance, and current efficiency were 23.17 mA/cm², 323.8 cd/m², and 1.39 cd/A, respectively, when operated at 9 V. However, when the emitting area was increased to 3.5 × 3.5 cm², the current density decreased from 23.17 to 4.19 mA/cm² at the same voltage of 9 V. The reduction in luminance was primarily due to the low current density caused by the high resistance of PET/ITO anode.

To enhance the conductivity of the anode for the 3.5 × 3.5 cm² area, an auxiliary electrode consisting of Au wire with a line width of 0.1 mm and a spacing of 5 mm was evaporated onto the ITO anode, with a total of six lines located within the emitting area. Compared to an OLED without auxiliary Au lines, the OLED with 15 nm thick auxiliary Au lines on ITO anode showed an increase in current density from 4.19 to 4.33 mA/cm², an enhancement in luminance from 26.87 to 37.07 cd/m² and an improvement in efficiency from 0.64 to 0.86 cd/A at 9 V.

Therefore, it is evident that the Au auxiliary lines positively enhance the current injection in PET/ITO OLED. However, when the thickness of the auxiliary Au lines is increased to 30 nm, the OLED fails to emit light effectively due to poor step coverage of the HATCN film on the PET/ITO/Au lines. This inadequate coverage prevents HATCN/TAPC from properly facilitating hole injection and transportation, resulting in inefficient OLED performance. To address this issue, PEDOT:PSS was then spin-coated onto the 30 nm thick auxiliary Au lines as a capping and planarization layer, followed by HATCN/TAPC deposition.

In the nine-square-blocks uniformity test, PET/ITO/OLEDs without auxiliary Au lines exhibited an overall luminance uniformity of only 74.1%. It was observed that the luminance decreases as the testing point moves away from the positive electrode contact clip. However, by incorporating auxiliary Au lines with a thickness of 15 nm, a line width of 0.1 mm, and a spacing of 5 mm (6 lines in total within the emitting area) on PET/ITO substrate, the overall uniformity in the nine-square block uniformity test improved significantly to 87.4%. When the density of the auxiliary Au lines was increased to a spacing of 2 mm (resulting in 16 lines), the overall uniformity slightly decreased to 84.6%. Nonetheless, the overall brightness of the blue OLED increased from 24.68 to 35.49 cd/m², making the flexible large-area blue OLED lighting panels appear noticeably brighter.

Author Contributions

Conceptualization, F.-S.J.; methodology, J.-Y.C.; formal analysis, W.-K.K.; writing—review and editing, K.D.P. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the idea suggestions from Peter Yeh and experimental supports from Day-Shan Liu.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Multilayer structural diagram of blue OLED includes three layers of PEDOT:PSS spin-coated on the auxiliary Au lines. In the absence of PEDOT:PSS layers, HATCN would be directly deposited onto the auxiliary Au lines.

Figure 2. Energy band diagram of blue OLED.

Figure 3. Comparisons of (a) J–V and (b) L–V of blue OLEDs with different HATCN thicknesses.

Figure 4. Comparisons of (a) J–V, (b) L–V, and (c) Y-V of blue OLEDs with different TAPC thicknesses.

Figure 5. Comparisons of (a) J–V and (b) L–V of blue OLEDs with different TPBi thicknesses.

Figure 6. Comparisons of (a) J–V, (b) L–V, and (c) Y-V of blue OLEDs with different emitting areas.

Figure 7. Transmission curves of different Au thicknesses and EL spectrum (relative to the right Y-axis, as indicated by the arrow) of blue OLED in this study.

Figure 8. Schematic diagram of auxiliary electrode.

Figure 9. Comparisons of (a) J–V, (b) L–V (c) Y-V of blue OLEDs with different Au auxiliary line thickness.

Figure 10. Without the Au auxiliary lines, the emission photos of PET/ITO OLEDs at (a) 4 V, (b) and 9 V, respectively.

Figure 11. (a) The measured luminance distribution of nine-square grids, (b) schematic photograph of measuring situation in one of the nine square grids of PET/ITO OLED without auxiliary Au lines.

Figure 12. (a) The measured luminance distribution of nine-square grids of glass/ITO/OLED, (b) full area emission situation of glass/ITO/OLED without auxiliary Au lines.

Figure 13. (a) Luminance measurement of nine-square blocks and (b) full panel emission of PET/ITO OLED with auxiliary Au lines (15 nm thickness, 5 mm spacing, 6 lines). (c) Luminance measurement of nine-square blocks and (d) full panel emission of PET/ITO/Au lines (15 nm thickness, 2 mm spacing, 16 lines).

Table 1

Transmittance data at 486 nm of different Au thicknesses.

Device	Transmittance @486 nm (Au Full Area)
Au 5 nm	68.4%
Au 10 nm	65.4%
Au 15 nm	63.5%

Table 2

Measurement data of sheet resistance with different Au thicknesses.

Au Thickness (Line Space 5 nm)	Au Fully Covered (Ω/sq.)	Au Grid Lines (Ω/sq.)
No grid lines	29.07
5 nm	55.53	33.53
10 nm	31.47	32.68
15 nm	28.11	31.67

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