Wafer-scale lithography of silicone-integrated hole transporters for anti-pixel crosstalk organic light-emitting diodes

doi:10.21203/rs.3.rs-3729326/v1

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Wafer-scale lithography of silicone-integrated hole transporters for anti-pixel crosstalk organic light-emitting diodes

https://doi.org/10.21203/rs.3.rs-3729326/v1

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Ultrahigh-density displays are becoming increasingly prevalent in display technology for immersive digital interactive devices. However, the pursuit of higher pixel resolution has inadvertently led to the emergence of electrical pixel crosstalk, primarily due to the use of common hole transporting layers (HTLs). In this work, we present wafer-scale, anti-pixel crosstalk micro-lithography to mitigate electrical pixel crosstalk by incorporating a silicone-integrated small molecule HTL (SI-HTL), which not only enables ultrahigh-density pixelation but also enhances the functionality of the HTL itself. Leveraging the inherent silicon etching properties of SI-HTL, we successfully created high-fidelity micro-pattern arrays with a remarkable resolution of up to 10,062 pixels per inch on 6-inch wafer scales. Furthermore, SI-HTL effectively modulates charge balance within the emission layers, resulting in improved luminance characteristics in organic light-emitting diodes (OLEDs). Our comprehensive optical and quantitative assessment of electrical pixel crosstalk in OLEDs integrated with micro-patterned SI-HTL demonstrates the significant effectiveness of high pixelation of the HTL in alleviating the crosstalk issue.

Physical sciences/Materials science/Materials for devices/Electronic devices

Physical sciences/Engineering/Chemical engineering

With the rapid advancements in metaverse architectures utilizing augmented/virtual/extended reality (AR/VR/XR) technologies, high-density pixel microdisplays have emerged as cutting-edge devices enabling deeper human-digital interactions through immersive visual experiences^1–3. This shift in display technology has spurred significant efforts toward the development of high-resolution organic light-emitting diode (OLED) microdisplays, achieving pixel densities exceeding 3,000 pixels per inch (ppi)^2–5. However, as pixel resolution continues to increase, the unintended occurrence of electrical pixel crosstalk can lead to a deterioration in both color gamut and color purity^6,7. This issue arises from the use of a shared hole transport layer (HTL), creating a conductive pathway through which hole charges injected into active pixels can migrate to neighboring non-active pixels^6–9 (Fig. 1a and Supplementary Note). To fundamentally address this challenge, it is crucial to pixelate the HTL in alignment with the sub-pixel resolution of the emission layer (EML).

Patterning techniques for small-molecule organic semiconductors (s-OSCs), predominantly employed as HTL materials¹⁰, have undergone extensive investigation. Among these techniques, the fine metal mask (FMM) stands out as a widely adopted method for patterning s-OSCs, particularly in the context of light-emitting materials. Notably, the scalability of the FMM has advanced significantly, achieving resolutions of up to 3,000 ppi¹¹. Nevertheless, persistent challenges encompass manufacturing yield, throughput concerns, and mask deformation which impact reusability and pattern accuracy^12–14. In response to these challenges, alternative techniques such as transfer printing^15–17, inkjet printing^18,19, and template growth^20,21 have emerged (Supplementary Table 1). However, these methods are not exempt from their respective limitations, including issues related to insufficient resolution, pattern fidelity, and suboptimal yield. These shortcomings underscore the pressing need for the development of a highly efficient ultrahigh-resolution patterning method and the design of s-OSCs that are compatible with such a technique.

More importantly, besides patterning capability, it is crucial for the HTL materials to execute its intrinsic roles, which involve charge transport and injection into the EML, to maintain charge balance within the EML. To achieve this goal, significant efforts have been made to modulate the charge transport properties^22–24 and engineer the energy levels^25–27, such as shifting the highest occupied molecular orbital (HOMO) level or adopting multi-HTL structures. Despite the effectiveness of these approaches in improving HTL characteristics, they cannot simultaneously address the achievement of high-density pixelation and the enhancement of HTL functionalities. This implies the challenge in developing small-molecule HTL materials capable of both high-resolution patterning and improved HTL performance.

Here, we present wafer-scale anti-pixel crosstalk micro-lithography in s-OSCs, designed to mitigate pixel crosstalk in high-density micro-OLEDs. This method introduces a silicone-integrated hole transport layer (SI-HTL) by incorporating silicone blocks (Si-O-Si) into a matrix of s-OSCs. The incorporation of silicone molecules enables SI-HTL to undergo micro-patterning using conventional photolithography coupled with reactive ion etching (RIE), similar to what is conventionally done for silicon (Si)-based materials. This facilitates the creation of anisotropic, finely defined SI-HTL patterns with resolutions as low as 1 µm. Simultaneously, the charge transport capability and energy levels of SI-HTL can be controlled by adjusting the concentration of the silicone blocks. This results in improved light-emitting performance in OLEDs based on SI-HTL. Furthermore, we confirm the prevention of pixel crosstalk between the OLED subpixels based on the micro-patterned SI-HTL. Our evaluations encompass both optical and quantitative assessments, revealing a notable reduction in electrical pixel crosstalk. These findings underscore the effectiveness of pixelated SI-HTL in mitigating pixel crosstalk, irrespective of pixel pitch sizes. This suggests that the conceptual design and patterning methodology of SI-HTL hold significant promise for applications in the realm of microdisplays.

Material design and photolithographic micro-patterns of SI-HTL

To realize the proposed SI-HTL, we employed the simultaneous crosslinking reaction between oxetane-functionalized, crosslinkable small-molecule OSC (QUPD, N,N′-bis(4-(6-((3-ethyloxetan-3-yl)-methoxy)-hexyloxy)phenyl)-N,N′-bis(4-methoxyphenyl)biphenyl-4,4′-diamine) and bi-functionalized silicone crosslinkers (Fig. 1b and see Methods for the detailed crosslinking procedures). The oxetane ring-opening and sol-gel reactions facilitated the covalent crosslinking of QUPD and the silicone crosslinkers, successfully leading to the in-situ formation of a silicone network within SI-HTL matrix (detailed structural analysis will be discussed later). The integrated silicone network plays a crucial role in imparting anisotropic etching nature of silicon (Si) materials into the s-OSCs by a generation of non-volatile etch inhibitor (Si_xO_y) at the sidewall of the etched trench during the RIE (Fig. 1c). Intrinsically, s-OSCs exhibits isotropic etching behavior, which is attributed to omni-directional chemical etching reactions that produce volatile reactants. However, the Si_xO_y etch inhibitor can reduce the lateral etch rate (\({r}_{l}\)) by alleviating the horizontal chemical etching reactions, allowing the vertical etch rate (\({r}_{v}\))—accelerated by ion bombardments—to become predominant. This orthogonal etching direction leads to a development of anisotropic etching profiles (\({r}_{v}\gg {r}_{l}\)). The cross-sectional transmission electron microscopy (TEM) analysis revealed that the \({r}_{l}\)of SI-HTL was effectively mitigated compared to that of the crosslinked QUPD (X-HTL), resulting in the clear anisotropic etching behavior of SI-HTL (Fig. 1d and Supplementary Fig. 1). This photolithography-driven anisotropic patterning strategy is beneficial to implement precise micro-patterns irrespective of dimensional form factors. Figure 1e exhibited wafer-scale, high-resolution patterning of SI-HTL, achieving the ultra-high density pixelation corresponding to 10,062 ppi. The demonstrated resolution surpassed those of previously developed patterning methods for s-OSCs (Fig. 1f and Supplementary Table 1). This indicates that the SI-HTL design, compatible with photolithography and RIE processes, proves its capability to simultaneously achieve high-resolution and scalable fabrications.

Molecular and etching characteristics of SI-HTL

To ensure the outstanding patternability of SI-HTL, the aforementioned crosslinking reactions between QUPD and the silicone crosslinkers should be successfully executed. The Fourier transform infrared (FTIR) spectrum of SI-HTL showed a distinct disappearance of the peak corresponding to oxetane groups at 978 cm^-1, accompanied by an increase in the intensity of the ether stretching peaks at 1,107 cm^-1 (Fig. 2a)²⁸. This provided clear evidence that the oxetane ring-opening reactions of both QUPD and the silicone crosslinkers were completely carried out. Concurrently, the silicone stretching vibrations corresponding to 1,060 cm^-1 and 1,039 cm^-1 (lateral and vertical stretching, respectively) became notably prominent²⁹, indicating that the silicone network in SI-HTL matrix was well-developed. The distribution of the silicone network within SI-HTL films was confirmed using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The NH^- and Si^-/SiO^- signals corresponding to QUPD and elements of the silicon network, respectively, were determined as a function of ion etching time for both X-HTL and SI-HTL. As shown in Fig. 2b, the signal distributions of Si^- and SiO^- were identical to that of NH^- across the entire films. This implies that the silicone network was uniformly distributed within SI-HTL, so that the film morphology of SI-HTL was analogous to that of X-HTL without noticeable increase in surface roughness (Supplementary Fig. 2). Thanks to the integration of silicone network, SI-HTL can simultaneously possess chemical and dry etching resistances which are a prerequisite to utilize the photolithography and RIE processes, even in a small-molecule system (Fig. 2c and Supplementary Figs. 3, 4).

The dry etching resistance and anisotropic etching behavior, originated from the integrated silicone network, enables SI-HTL to achieve more accurate fine micro-patterns through the photolithography and RIE processes. The pattern fidelity of photolithographic micro-patterns of SI-HTL was statistically evaluated in terms of the line width variation (LWV) and the line edge roughness (LER)³⁰ (Fig. 2d and Supplementary Fig. 5). The LWV and LER of patterns derived from SI-HTL were significantly reduced compared to those of the X-HTL, resulting in the well-defined micro-patterns with high clarity. This result can be attributed to anisotropic etching mechanism of SI-HTL driven by silicone-based etch inhibitors. The X-ray photoelectron spectroscopy (XPS) analysis was conducted to confirm the formation of the etch inhibitors in the small-molecule SI-HTL depending on exposure to the RIE. After RIE treatment on SI-HTL film, the intensities of Si 2p peaks at 102.65 eV (Si³⁺, Si₂O₃) and 103.55 eV (Si⁴⁺, SiO₂) relatively increased compared to those at 100.75 eV (Si⁺, Si-O-Si) and 101.85 eV (Si²⁺, Si-O), causing a shift of the prominent main peak toward higher binding energy regions^31,32 (Fig. 2e and Supplementary Fig. 6). This means that the reactive gases (O*) effectively combined with the elements of the silicone network via chemical reactions, resulting in the formation of non-volatile Si suboxide reactants. Attributed to the etch inhibitor, chemical etching reactions (i.e. decomposition of organic compounds) on the surface of SI-HTL can be effectively suppressed (Supplementary Fig. 7). This retards the chemical etching-induced reduction in molecular weight, which could affect the film modulus^14,33. As shown in Fig. 2f, the decrease in the surface modulus for the SI-HTL film was less pronounced than that of the X-HTL (the reduction rates of surface modulus were 165 MPa/s and 238 MPa/s, respectively) as RIE exposure time increased (Supplementary Fig. 8). Consequently, these findings indicate that the etch inhibitor-based mechanism inherent to silicon materials is achievable in our small-molecule design of SI-HTL.

Luminance characteristics of OLEDs based on SI-HTL

With the dry-etching robustness and high-resolution patternability of SI-HTL, it is essential to retain its intrinsic HTL functionalities for implementation of high-performance light-emitting devices. To explore the impact of the SI-HTL on the electroluminescence (EL) characteristics, we fabricated OLED devices in the following configuration: ITO/PEDOT:PSS (40 nm)/X- or SI-HTL (30 nm)/CBP:Ir(ppy)₃ (4,4'-Bis(carbazol-9-yl)biphenyl : Tris(2-phenylpyridine)iridium(III), 30 nm)/TPBi (1,3,5-Tris(1-phenyl-1Hbenzimidazol-2-yl)benzene, 40 nm)/LiF (1.5 nm)/Al (150 nm) (Fig. 3a). Here, PEDOT:PSS, CBP:Ir(ppy)₃ and TPBi are hole injection layer (HIL), EML, and electron transport layer (ETL), respectively. The incorporation of X-HTL resulted in improved OLED performances, as shown in the current density-voltage-luminance (J-V-L) curves, external quantum efficiency (EQE), and power efficiency (Supplementary Fig. 9). Figure 3b, c showed the J-V-L characteristics and EQE, as well as, the current efficiency of SI-HTL-based OLEDs with varying concentrations of the silicone crosslinker. The detailed light-emitting performances were summarized in Supplementary Table 2. Notably, at the silicone crosslinker concentration of 4 mol%, the OLED exhibited the lowest turn-on voltage of 3.4 V while concurrently achieving the highest luminance of 14,320 cd/m² at 8 V. This contrasted with the luminance of 9,670 cd/m² from the pristine device (based on X-HTL) under equivalent voltage conditions (Fig. 3b). Moreover, the OLED with 4 mol% SI-HTL exhibited the highest maximum EQE of 6.87% compared to the other OLEDs, indicating a significant improvement in luminance performance. (Fig. 3c).

To comprehend the underlying factors contributing to the enhancement in EL performance of the SI-HTL integrated OLEDs, we conducted space charge-limited current (SCLC) measurement to elucidate the hole mobility of the SI-HTL (Fig. 3d). A hole-only device (HOD) was fabricated with the configuration of ITO/PEDOT:PSS/SI-HTL/Al. The resulting J-V curve exhibited the characteristic slope \(\left[\frac{d\text{log}\left(J\right)}{d\text{log}\left(V\right)}\right]\) of 1 under a low voltage regime (Ohmic), followed by a regime with a high slope (> 3) indicative of trap-filled limit (TFL), and lastly a SCLC regime characterized by a slope of 2. The hole mobility for the HOD was calculated using the Mott-Gurney equation; \(J=\frac{9}{8}\epsilon {\epsilon }_{0}\mu \frac{{\left(V-{V}_{bi}\right)}^{2}}{{L}^{3}}\) where J, ε, ε₀, V, V_bi, L, and µ is a current density, a dielectric constant, a permittivity of free space, an applied voltage, a built-in voltage, a thickness of HTL films, and a hole mobility, respectively. As a result, the hole mobility values for pristine, 4 mol%, 8 mol%, and 16 mol% of the silicone crosslinkers in SI-HTLs were extracted to be 4.4 × 10⁻⁶, 4.1 × 10^− 6, 3.8 × 10^− 6, and 3.6 × 10^− 6 cm²V⁻¹s⁻¹, respectively (Supplementary Fig. 10). The observed decrease in the hole mobility can be attributed to the presence of insulating silicone blocks, as evidenced by the increased the trap densities and photoluminescence quenching effects in SI-HTLs (Supplementary Figs. 10, 11). Depending on the electrical characteristics of the host material of the EML, however, the decreased mobility of HTL can adversely enhance the hole-electron charge balance in the EML. Considering that the hole mobility (2 × 10⁻³ cm²V⁻¹s⁻¹) of the CBP (i.e. the host material of the EML) is higher than its electron mobility (3 × 10⁻⁴ cm²V⁻¹s⁻¹)³⁴, the reduction in the hole mobility of SI-HTL inversely contributed to balancing the hole-electron transport in the EML^10,35,36, thereby improving the luminance performance.

In conjunction with the charge transporting capability of SI-HTL, its energy level structure needs to be explored to fully understand its influence on the luminance performance in OLEDs. To characterize the band gaps and electronic structures of the SI-HTL upon introduction of the silicone crosslinker, ultraviolet photoelectron spectroscopy (UPS) was conducted (Fig. 3e). The calculated HOMO energy level for X-HTL, 4 mol% SI-HTL, 8 mol% SI-HTL, and 16 mol% SI-HTL were 5.17, 5.25, 5.37, and 5.50 eV, respectively. Combined with the optical bandgap obtained from the tau plot of ultraviolet-visible absorption spectra (Supplementary Fig. 12), the energy band diagrams of these films were determined as shown in Fig. 3f. We conjecture that the downshift of the HOMO may be attributed to the presence of silanol groups (Si-OH) in the bulk of SI-HTL film which did not participate in the sol-gel reactions^29,37 (Supplementary Fig. 13); the electron-withdrawing property of the silanol groups can induce a stabilization of the HOMO energy^38,39. The deeper HOMO level of SI-HTL improved the energy level alignment with the EML, facilitating efficient hole injections into the EML film. Note that HTLs possessing a low injection barrier along with moderately reduced charge mobility can effectively improve the charge balance within EMLs^40,41. Consequently, the controlled charge mobility and well-aligned HOMO levels resulted in enhancement of the luminance performance in the OLED integrated with SI-HTL films (4, 8, and 16 mol%), compared to that with X-HTL (Fig. 3b, c and Supplementary Table 2). The optimal ratio of the silicone crosslinker was determined to be 4 mol% of the silicone crosslinker because the higher concentrations led to an immoderate reduction in charge mobility and a downshift of the LUMO levels in SI-HTL, both of which would be potentially detrimental to the luminance performance of OLEDs.

Furthermore, the chemical robustness and the ability to control the HOMO level of SI-HTL are advantageous for realizing multi-layered HTL-based OLEDs that exhibit enhanced energy level alignment. To this end, we fabricated the bilayered SI-HTLs using a solution process, consisting of 4 and 16 mol% (each layer with the identical thickness), to promote gradient hole injections into the EML (Fig. 3g). Notably, the OLED with the bilayered SI-HTL demonstrated more improved luminous characteristics compared to both the X-HTL and 4 mol% SI-HTL configurations (Fig. 3h, i and Supplementary Fig. 14), indicating that SI-HTL can effectively modulate charge transport and injection into EML for enhancement of OLED performance.

Evaluation of electrical pixel crosstalk effect

The demonstrated ultrafine pixelation and improved HTL functionalities of SI-HTL can possess a potential to realize highly efficient, anti-pixel crosstalk OLEDs. To evaluate the electrical pixel crosstalk depending on pixelation of HTLs, we designed the OLED configurations with varying ITO pixel pitches (5, 10, 15, and 20 µm), in which either a common SI-HTL was employed (Fig. 4a) or SI-HTL was pixelated corresponding to the size of ITO pixel pitches (Fig. 4b). The other OLED layer configurations were identical to those in Fig. 3. In the both device configurations, a voltage bias was swept from 0 V to 15 V between the anode of the active pixel and the shared cathode, while ensuring the anodes of the adjacent pixels were remained open-circuited. In all pixel pitches within the common SI-HTL case, light emissions from the adjacent pixels were observed, especially for the test devices with 5 µm pixel pitches (Fig. 4a). This light emission originated from electric currents travelling from the active anode to the common SI-HTL and finally to the cathode layer in neighboring pixels. Conversely, the pixel crosstalk phenomenon was significantly reduced in the patterned SI-HTL device at all pixel pitches (Fig. 4b and Supplementary Movie 1). Particularly, even in the 5 µm pixel pitch configuration with the pixelated SI-HTL, the intensity of the green pixel n the adjacent pixels was significantly diminished compared to the that in the devices based on the common SI-HTL (Fig. 4c and Supplementary Fig. 15). Figure 4d showed that the voltage-dependent green pixel intensities of the active pixel and the adjacent pixel (referred as Adjacent 2 in the figure) of both OLEDs employing common SI-HTL and pixelated SI-HTL, further validating the effectiveness of SI-HTL pixelation in reducing the crosstalk emission.

This mitigation in the pixel crosstalk effect can be attributed to the micro-patterned SI-HTL obstructing the electrical path across the adjacent pixels. To prove this, we measured the lateral leakage currents in the adjacent pixels during operation of the active pixel⁶. The currents of both the active pixel and the adjacent pixel (referred as Adjacent 1 in the figure) upon sweeping the voltage bias of the active pixel—for testbed devices in common and patterned SI-HTL configurations—were shown in Supplementary Fig. 16a, b. In both configurations, the active pixels exhibited a current profile typical of OLED operations. Two operational regimes were identified; an initial regime where the dark current was observed at lower voltages, followed by a regime where current increased after the turn-on voltage, indicating the carrier recombination. It can be noted that the current in the adjacent pixel with patterned SI-HTL was significantly lower, nearly by an order of magnitude, than that with the common SI-HTL. Supplementary Fig. 16c exhibited the lateral leakage current of the adjacent pixel 1 for each pixel pitch configuration at a constant voltage bias of 8 V. The lateral leakage currents in OLEDs with the patterned SI-HTL were substantially decreased across all pixel pitches, compared to those with the common SI-HTL. Consequently, these results indicate that the efficacy of SI-HTL patterning as an effective strategy to mitigate the crosstalk effect, a long-standing challenge in OLED engineering.

In conclusion, we developed a silicone-integrated small-molecule HTL capable of ultrahigh-density pixelation with resolutions up to 10,062 ppi on wafer scales through photolithography and RIE processes. This exceptional patternability is a result of the embedded silicone network within the s-OSC matrix, allowing SI-HTL to be micro-patterned with remarkable fidelity by emulating the anisotropic etching characteristics of silicon materials. Furthermore, SI-HTL exhibited the enhanced luminance efficiency in OLED, which is attributed to the modulation of charge transporting capability and the optimized alignment of the HOMO level relative to the energy level of the EML. This emphasizes that SI-HTL can possess high-resolution patternability and enhancement of HTL functionalities, simultaneously. Building on this, we fabricated patterned SI-HTL-based OLEDs to optically and quantitatively evaluate pixel crosstalk effects. The results revealed that the accurate pixelation of SI-HTL effectively alleviated the electrical pixel crosstalk phenomenon. Our material approach not only offers inspiration in the realm of ultrahigh-density microdisplay technology but also has broad applicability in the field of high-efficiency light-emitting optoelectronics, including quantum dots and perovskites.

Materials.

Processing solvents including chlorobenzene (CB) were purchased from Sigma-Aldrich and were used as received. The QUPD and bi-functionalized silicone crosslinker (triethoxy[3-[(3-ethyl-3-oxetanyl)methoxy]propyl]silane) were provided by Lumtec and JSI Silicone, respectively. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, AI4083) was purchased from Ossila. Photoinitiator, (4-(Octyloxy)phenyl)(phenyl)iodonium hexafluorostibate(V) (OPPI) was purchased from AmBeed. The CBP and Ir(ppy) ₃ were purchased from TCI Chemicals. The TPBi and lithium fluoride (LiF) were purchased from Sigma-Aldrich. Al pellets (\(\ge\) 99.999%) were purchased from Taewon Science. Glass substrate with pre-patterned ITO electrode (~20 Ohm/sq) was purchased from AMG (Korea). Photoresist (KL5302) and developer (AZ 300MIF) were purchased from NM Tech and AZ Electronic Materials, respectively.

Preparation and characterization of SI-HTL film.

The PEDOT:PSS was spin-coated at 4,000 rpm for 1 min onto SiO ₂ or ITO/glass substrate and was annealed at 120 ℃ for 1 h. A solution of SI-HTL was prepared by dissolving the QUPD (10 mg/ml in CB) and the silicone crosslinkers (4, 8, 16 mol% relative to QUPD), and was mixed at 100 ℃ for 2 h. After mixing, the solution was spin-coated at 2,000 rpm for 1 min onto the prepared PEDOT:PSS coated substrate, and then the resulting film was annealed at 180 \(℃\) for 3 h in N₂ environment. During annealing process, the oxetane ring-opening reaction was activated by protonation from the underlying PEDOT:PSS or OPPI^42,43, and concurrently, the sol-gel reaction between the silicone crosslinkers was driven by the thermal energy^14,29. The thickness of the films was measured by a surface profilometer (Bruker, Dektak XT-E) and AFM (Park systems, NX10). The oxetane ring-opening reaction and formation of silicone network were confirmed by FTIR spectroscopy (Thermo Fisher Scientific, Nicolet iS50). ToF-SIMS analysis was performed with ToF SIMS-5 (ION-ToF, Germany) instrument. The surface morphology and height profile of the patterned SI-HTL films were characterized using the AFM in noncontact mode under ambient condition. The cross-sectional TEM samples were obtained by Focused ion beam (FIB, Helios 650), and the TEM analysis was conducted with Cs-corrected scanning transmission electron microscope (Cs-STEM, JEM-ARM200F) at National Center of Inter-University Research Facilities (Seoul). The pattern fidelity of the micro-patterned SI-HTL films was evaluated by statistically analyzing the AFM images with ImageJ software. The surface modulus and mapping images were obtained with probes from Park Systems (PPP-FMR, resonant frequency = ~ 75 kHz, k (spring constant) = ~ 2.8 N/m, tip radius = 8 µm). XPS were performed by theta probe base system (Thermo Fisher Scientific). UV absorption and PL spectra of SI-HTL films were measured by a UV-Vis-NIR spectrophotometer (Jasco V-770) and fluorescence spectrometer (Scinco, FS-2), respectively. UPS measurements were performed using an XPS system (Thermo Fisher Scientific, XPS-Theta Probe). A Negative bias voltage (-10 eV) was applied to the samples using He(Ⅰ) (21.2 eV) as the excitation source. The SCLC measurements on hole-only devices were conducted using Keithley 4200A-SCS instrument. The voltage was incrementally swept from 0.1 V to 9 V on the ITO, with the step size of 0.1 V.

Fabrication and performance measurement of OLED.

ITO-patterned glass substrate was sonicated with acetone, isopropyl alcohol, and de-ionized water sequentially for 15 min each. The cleaned substrate was treated with UV/O ₃ for 20 min. PEDOT:PSS (Al4083) was spin-coated onto the ITO-patterned glass substrate at 4,000 rpm for 1 min and the resulting film was annealed at 150 ℃ for 30 min in ambient condition. The prepared QUPD or SI-HTL solutions were spin-coated onto the PEDOT:PSS film in a N₂-filled glove box at 2,000 rpm for 60 s, followed by thermal annealing at 180 ℃ for 3 h. Next, the emission layer (CBP:Ir(ppy)₃) was spin-coated onto the HTL films, followed by thermal annealing at 60 ℃ for 10 min. Then, 40 nm of TPBi was thermally evaporated through a shadow mask at a high vacuum (2.0×10^− 6 Torr). Lastly, 1.5 nm of LiF and 150 nm of Al as the cathode electrode were thermally evaporated through a shadow mask at high vacuum condition (2.0×10^− 6 Torr). The EL performance of the OLED was evaluated by a spectroradiometer (CS-2000, Konica Minolta) while the input voltage was swept from 0 V to 12 V using Keithley 2400 SCS.

Patterning of the SI-HTL film and evaluation pixel crosstalk.

A glass substrate was cleaned as previously described. Photoresist (KL 5302) was spin-coated at 4,000 rpm for 1 min onto the substrate, and then soft-baking was conducted at 105 ℃ for 1 min. The photoresist-coated substrate was exposed to the UV source (365 nm, 25 mW/cm ² ) for 2 s with a photomask using a mask aligner (Pro Win M-150), and post-baking was followed at 115 ℃ for 1 min. The photoresist pattern was developed using the developer for 20 s in room temperature. After the conventional photolithography process, a 50-nm-thick ITO layer was sputtered onto the photoresist pattern layer using radio frequency magnetron sputtering. Finally, the photoresist pattern layer was removed by sonication with acetone for 5 min, resulting in ITO patterns. Both the PEDOT:PSS and SI-HTL were then sequentially deposited as the previously described process onto the patterned ITO glass. For micro-patterning of the SI-HTL films, the photoresist was patterned by the above described photolithography process. The photoresist-patterned SI-HTL films were dry-etched with RIE (JVAC JVRIE-8AT). The films were exposed to Ar/O ₂ (40 sccm/20 sccm) etching gas mixture for 20 ~ 30 s, using a 50 W radio-frequency power at a pressure bellow 0.1 mTorr. Subsequently, the patterned photoresist was stripped using a sonication process with acetone and isopropyl alcohol, resulting in the formation of SI-HTL micropatterns. Pixel crosstalk were observed by an optical microscope (ECLIPSE LV100ND, Nikon) while applying voltages though a DC power supply. The crosstalk currents were measured by Keithley 4200A-SCS instrument.

Data availability

All data are included in this articles and its Supplementary Information files. All data are available from the corresponding authors upon reasonable request.

Acknowledgments

This work was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2021M3D1A2049315). This work was also supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2020R1A2C3014237).

Author contributions

M.S.K., J.H.C., and D.H.K. supervised this project. H.K., S.K., B.H., M.S.K., J.H.C, and D.H.K. conceived the concept. H.K., S.K., and B.H. designed and carried out experiments. H.K., B.H., S.L., J.H., and D.H.K. analyzed the molecular structure and etching behavior of SI-HTL. H.K. and B.H. fabricated high-resolution patterns of SI-HTL. S.K., S.L, M.K., S.H.R, M.S.K., and J.H.C. evaluated and interpreted optoelectronic characteristics of SI-HTL and OLEDs. H.K., S.K., and B.H. evaluated pixel crosstalk of patterned SI-HTL-based OLEDs. All authors discussed the results and commented on the manuscript. H.K., S.K., B.H., M.S.K., J.H.C., and D.H.K. co-wrote the manuscript.

Competing interests

The authors declare that they have no competing interests.

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Wafer-scale lithography of silicone-integrated hole transporters for anti-pixel crosstalk organic light-emitting diodes

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Material design and photolithographic micro-patterns of SI-HTL

Molecular and etching characteristics of SI-HTL

Luminance characteristics of OLEDs based on SI-HTL

Evaluation of electrical pixel crosstalk effect

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