Surface‐Engineered ZnO Nanoparticles via Acetone Immersion for Charge‐Balanced High Resolution Full‐Color Organic Light Emitting Diode Displays

ABSTRACT

In solution‐processed organic light‐emitting diodes (OLEDs), carrier injection imbalance and interfacial quenching persist as major hurdles, primarily due to the hole‐dominated transport in polymer‐based emitters. Here, we introduce a simple but highly effective post‐fabrication strategy for enhancing charge balance and device performance by chemically modifying ZnO nanoparticles (ZnO‐NPs) through acetone immersion. This treatment selectively passivates oxygen vacancy–related trap states at the ZnO NPs surface, resulting in improved electron injection and restored hole‐blocking behavior, without disrupting the device structure or layer interfaces. Comprehensive analyses, including X‐ray photoelectron spectroscopy (XPS), photoluminescence (PL), and transient electroluminescence (EL) measurements, confirm the defect passivation mechanism and its impact on charge injection dynamics. Acetone‐treated OLEDs exhibit reduced turn‐on voltage, significantly higher luminance (exceeding 44 000 cd/m²), and enhanced efficiency across all RGB subpixels, while maintaining spectral purity. Furthermore, this method enables scalable fabrication of ultra‐high‐resolution RGB pixel arrays on a large scale without requiring additional functional layers.

Acetone post‐immersion of OLEDs effectively passivates oxygen vacancies in the ZnO electron transport layer, reducing charge trapping and enhancing electron injection. This improves hole‐blocking at the interface and decreases exciton quenching in the emission layer, resulting in significantly enhanced electrical and optical device performance.

1. Introduction

Organic light‐emitting diodes (OLEDs) have already achieved widespread commercialization and are emerging as an essential platform for next‐generation display technologies, including foldable, transparent, and wearable devices [1, 2, 3]. Their intrinsic advantages include outstanding self‐emissive optoelectronic properties, superior contrast ratios, and wide viewing angles [4, 5, 6]. OLEDs can be fabricated using either thermal vacuum evaporation or solution‐processed processes. Compared to vacuum depositions, solution processes such as inkjet printing and roll‐to‐roll (R2R) printing methods offer significant advantages, including lower material consumption and greater potential for large‐area [7, 8, 9].

However, these advantages are often undermined by charge imbalance and interfacial quenching, which limit device efficiency and operational lifetime [10, 11]. In particular, solution‐processed OLEDs are generally hole‐dominated due to the electronic characteristics of widely used p‐type emissive polymers such as poly(9,9‐dioctylfluorene‐alt‐bithiophene) (F8T2) and poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) [10, 12]. As a result, the recombination zone tends to form near the emission layer/electron transport layer (EML/ETL) interface, leading to exciton and polaron accumulation [13, 14, 15]. This, in turn, facilitates non‐radiative quenching and charge leakage, ultimately degrading device performance and stability.

Accordingly, the optimization of the ETL and electron injection layer (EIL) is crucial for achieving balanced charge injection and high‐efficiency operation. Among various candidate materials, solution‐processable metal oxides such as ZnO nanoparticles (ZnO NPs) have attracted considerable attention due to their wide bandgap, high electron mobility, and chemical robustness [16, 17, 18]. Nevertheless, ZnO NPs inherently possess a high density of defect states, particularly oxygen vacancies (V_o) that act as charge traps, hinder electron injection, and exacerbate interfacial exciton quenching [19, 20]. Prior efforts to address these limitations in quantum‐dot light‐emitting diodes (QLEDs) have employed strategies such as inserting polymeric interlayers like polyethylenimine ethoxylated (PEIE) [21] or doping ZnO NPs with metal ions (e.g., Mg, Al) [22], effectively reducing trap density but often at the cost of reduced electron injection capability. However, these approaches are not directly transferable to hole‐dominated OLEDs, which require strategies that concurrently enhance electron injection and suppress interfacial trap states.

Inspired by acetone gas sensor studies where ZnO‐based materials exhibited reduced oxygen vacancies and enhanced conductivity upon acetone exposure, we applied a similar concept to OLEDs to develop a simple, post‐deposition acetone immersion process that selectively modulates the surface defect states of ZnO NPs after full device fabrication [23, 24]. This treatment, compatible with standard OLED processing, enables rapid passivation of oxygen vacancies and substantially enhances carrier transport at the ZnO NPs/Al interface. In this study, we demonstrate that this acetone‐based strategy leads to remarkable improvements in optoelectronic characteristics of solution‐processed RGB OLEDs, including increased luminance, reduced turn‐on voltage, enhanced electron injection, and improved emission uniformity. Mechanistic analyses using X‐ray photoelectron spectroscopy (XPS), transient electroluminescence (EL), and single‐layer diode measurements reveal that these enhancements originate from effective suppression of ZnO NPs surface trap states and restored hole‐blocking behavior. Furthermore, the treatment is shown to be scalable and compatible with high‐resolution photolithographic patterning, enabling full‐color OLED displays with excellent color purity and pixel uniformity. Unlike previously reported “positive aging” approaches in QLEDs, this is the first demonstration of ZnO–acetone interface chemistry specifically engineered for OLEDs, providing a rapid, universal, and compatible pathway as well as the manufacturability toward ultra‐high resolution solution‐processed OLED displays.

2. Result and Discussion

2.1. Chemical Passivation of Oxygen Vacancies and Optoelectronic Enhancement in OLEDs via Acetone Immersion

As illustrated in Figure 1a, the pristine ZnO NPs inherently contain oxygen vacancies, which act as carrier trapping sites [19]. Upon acid treatment, these vacancies are passivated, which suppresses carrier trapping and promotes enhanced charge flow across the interface. Previous studies have reported this reaction mechanism referred to as “positive aging,” in which the device performance gradually improves over time when acidic resin materials are used for encapsulation [25, 26]. More recently, acid gas leakages have been proven to induce similar effects in ZnO NPs layers, albeit with prolonged reaction times often exceeding five days [25]. This behavior is consistent with vacancy‐mediated conduction mechanisms previously observed in oxide‐based electronic systems, but its application in OLEDs has not been reported yet. Previously reported results on positive aging were compiled in Table S1 and compared with our study.

FIGURE 1.

Schematic concept of acetone immersion process and experimental results of enhanced optoelectrical characteristics of OLEDs. (a) Theoretical illustration of the ZnO‐NPs/Al interface before (left) and after (right) acetone reaction. (b) Step‐by‐step fabrication process of the ZnO‐NPs‐based OLED device incorporating acetone immersion. (c–f) Optoelectrical property improvements induced by acetone immersion: c) EL spectrum, d) current density–voltage–luminance (J–V–L) characteristics, e) luminous efficiency as a function of luminance while the inset showing the luminance efficiency plotted on a logarithmic luminance scale up to 5000 cd m^{− 2}, f) power efficiency as a function of luminance of the devices with insets presenting OLED emission images captured under identical bias conditions. The untreated OLED and the treated OLED are denoted as None and Ace, respectively.

In acetone gas‐sensor research, acetone is suggested to reach the ZnO NPs interface and bind to oxygen‐vacancy–related defect sites: the carbonyl oxygen coordinates to under‐coordinated Zn, and surface O species can interact with hydrogen from acetone. Such defect‐site binding effectively passivates oxygen vacancies and suppresses mid‐gap trap states [27]. In QLED studies, the electron dynamics of ZnO NP/Al layers can be modified by operando chemical reactions, where metal‐ion diffusion under acidic conditions induces recrystallization, reduces defect density, and improves device performance [25]. Herein, this phenomenon is extended to solution‐processed OLEDs to investigate enhanced charge transport and interfacial stability in this study. We introduce a novel post‐treatment strategy based on acetone immersion, which enables rapid modification of the surface defect states in ZnO NPs and significantly improves device performance. The detailed fabrication sequence of the acetone‐immersed OLEDs is presented in Figure 1b. Following standard substrate cleaning, the functional layers such as hole injection layer (HIL), hole transport layer (HTL), EML, and ETL are deposited on the ITO by spin‐coating. Then, the Al cathode is deposited by thermal evaporation. Finally, the OLED is immersed in an acetone‐filled vessel, initiating a surface chemical reaction at the ZnO NPs interface as shown in Figure S1.

The optoelectronic characteristics of the devices before (None) and after acetone immersion (Ace) are systematically compared in Figure 1c–f. Through systematic experiments, we identified an optimal acetone‐immersion time of 1 h, as summarized in Table S2 and Figure S2. As shown in Figure 1c, the EL spectra remain virtually unchanged, with the emission peak maintained at 542 nm, indicating that the acetone immersion does not influence spectral integrity and color purity. After acetone immersion, the turn‐on voltage is reduced from 4.5 to 4.0 V (Figure 1d), accompanied by a substantial increase in both overall current density and luminance. The maximum current density and luminance increase markedly, more than doubling to 924.8 mA cm^{− 2} and 44 018 cd m^{− 2}, respectively. As shown in Figure 1d, the acetone‐treated OLEDs exhibit enhanced L–V characteristics, operating at approximately 1 V lower voltage than the untreated OLEDs, except at low luminance levels. Notably, the acetone‐treated OLED exhibits consistently higher luminous efficiency than the untreated OLED across the entire luminance range (Figure 1e), while maintaining the pronounced improvements even under high‐brightness bias conditions to achieve the luminance of 5000 cd m^{− 2}. As shown in Figure 1f, power efficiency is similarly enhanced by the acetone treatment, maintaining higher efficiency across the entire voltage range. Accordingly, optical images of the inset show markedly brighter and more uniform emission from the acetone‐immersion pixels compared to the untreated pixels under the same bias conditions. To verify the reproducibility of our strategy, we fabricated 20 devices and statistically analyzed the efficiency and max luminance distribution (Figure S3). The results confirm a clear increase in efficiency and max luminance after acetone immersion.

It has been reported that trap sites in ZnO NPs can become more pronounced after Al deposition. This is because when Al diffuses into ZnO NPs and forms Al─O bonds, it can extract oxygen from the ZnO NPs lattice, thereby generating oxygen vacancies [28]. To examine this effect, we performed acetone treatment both before and after Al deposition using full EL devices and ZnO NP only stacks. Notably, only the devices treated with acetone after Al deposit exhibited increased current density and efficiency, indicating that the key modification occurs at the ZnO NPs/Al interface (Figures S4 and S5). In addition, to elucidate the solvent effect, we evaluated various organic solvents (toluene, ethanol, acetone, 2‐propanol, and dichloromethane). As shown in Figure S6, the device integrity was preserved after immersion except for toluene. Device performance after 2‐propanol immersion was comparable to that of the untreated Ref, whereas all other solvents except acetone degraded performance; consequently, acetone was the only solvent that improved device characteristics.

This improvement is not limited to green OLEDs: OLEDs incorporating MEH‐PPV (red) or PFO:PVK:FIr6 (blue) EML likewise show marked performance enhancement (Figure S7). Then, Table 1 summarizes the luminance, EQE, and emission peak wavelengths of red and blue EML materials before and after acetone immersion. The device architecture and energy‐level alignment are summarized in Figure S8. The electron injection barrier is ∼0.8 eV at the ZnO NPs/Al interface, whereas the barrier from ZnO NPs to the EML is ∼0.4 eV. Since barriers of ∼0.3 eV can typically be overcome via thermally assisted processes, electron injections are primarily limited by the ZnO NPs/Al interface [29]. Therefore, once electron injection is improved at this interface, subsequent electron transport from ZnO NPs into the EML becomes comparatively facile.

TABLE 1.

Optoelectronic properties of MEH‐PPV, F8T2, and PFO:PVK:FIr6 EML unpatterned OLEDs without and with acetone immersion (denoted as None and Ace), including maximum luminance (L_max), maximum external quantum efficiency (EQE_max), and peak emission wavelength.

Name	Condition	Lmax[cd/m2]	EQEmax	λmax [nm]
MEH‐PPV	None	4371	0.257	589
	Ace	5972	0.446	590
F8T2	None	8914	0.348	541
	Ace	44 018	0.512	540
PFO: PVK: FIr6	None	2903	0.298	410
	Ace	3614	0.537	412

Compared to the previous research, our proposed acetone immersion method offers a fast and effective alternative, expecting to enable significant modification of the ZnO NPs surface components within only a few hours. Those results confirm that acetone immersion enhances electrical performance while maintaining visual fidelity and emission uniformity, underscoring its applicability to full color OLED display technologies.

2.2. Structural and Electrical Stability of Acetone‐Immersed ZnO‐NPs in Multilayer OLEDs

Organic semiconductors are inherently susceptible to oxygen, moisture, and other environmental factors, which can induce both physical and chemical degradation in OLEDs [30]. Prior to evaluating the potential benefits of acetone immersion, we first investigated whether the treatment induces any adverse physical effects, such as delamination or film peeling. To assess the physical integrity of the OLED structure following acetone immersion, cross‐sectional transmission electron microscopy (TEM) images were obtained for devices before and after immersion, as shown in Figure 2a,b, respectively. The multilayered structure, including ITO, PEDOT: PSS, QUPD, F8T2, ZnO NPs, and Al, remains intact after acetone immersion, with no evidence of layer delamination, morphological collapse, or notable thickness variation across the stack. While organic layers are generally indistinct in conventional TEM due to low atomic contrast, we further verified the physical stability of each layer using energy‐dispersive X‐ray spectroscopy (EDS) elemental mapping. The elemental distributions of indium (ITO), sulfur (PEDOT: PSS and QUPD), zinc (ZnO NPs), and aluminum (Al) clearly remain sharply confined to their respective layers in both None and Ace OLEDs. The detailed thicknesses of each functional layer, estimated using a calibrated accumulation method, are provided in Figure S9. Notably, no evidence of elemental intermixing or diffusion across the interfaces was observed, confirming that acetone immersion does not compromise the chemical composition or boundary definition of the multilayer stack.

FIGURE 2.

Structural and electrical stability analysis of OLED devices before and after acetone immersion. (a,b) Cross‐sectional TEM images of OLEDs without (a, None) and with (b, Ace) acetone immersion. The corresponding elemental distributions of In (ITO), S (PEDOT: PSS and QUPD), Zn (ZnO NPs), and Al (cathode) are analyzed using energy‐dispersive spectroscopy (EDS) mapping. (c–f) J–V characteristics of respective functionalized layer, such as c) PEDOT: PSS, d) QUPD, e) F8T2, and f) ZnO NPs are investigated with ITO/X/Al structure. Only for PEDOT: PSS, we additionally included a condition in which acetone treatment was performed before Al deposition. Each graph compares untreated (open circles) and acetone‐immersed (filled circles) conditions, revealing the role of acetone immersion in modulating the charge transport behavior across individual functional layers.

To determine whether acetone immersion alters the electronic characteristics of individual layers, we fabricated a series of simplified “only‐devices” (ODs) by introducing PEDOT: PSS, QUPD, F8T2, or ZnO NPs as the sole functional layer between the ITO anode and Al cathode. The J–V characteristics of each OD were measured and are presented in Figure 2c–f. For PEDOT: PSS (Figure 2c), QUPD (Figure 2d), and F8T2 (Figure 2e) ODs, the J–V curves of acetone‐immersed samples closely well matched with those of the untreated references, indicating that their electronic properties remain unaffected by acetone immersion. In this study, although PEDOT: PSS is known to exhibit increased conductivity after acetone treatment, no measurable change was observed in our device configuration (Figure 2c) [31]. Once Al is deposited, it effectively encapsulates the PEDOT: PSS layer, hindering PSS leaching and suppressing structural reorganization. In sharp contrast, the ZnO NPs‐based OD after Al deposition (Figure 2f) exhibited a remarkable increase in current density upon acetone immersion, suggesting enhanced electron transport. In Figure S5, the ZnO NPs‐based OD before Al deposition has a similar current density to None device. This sequence dependence is consistent with reported ZnO/Al interfacial redox chemistry: during Al thermal evaporation, Al extracts oxygen from ZnO, creating oxygen vacancies near the ZnO/Al interface that act as electron traps and hinder injection and transport [28].

Collectively, these results indicate that acetone immersion preserves the structural and electronic integrity of all functional layers except ZnO NPs. The selective enhancement observed exclusively in the ZnO NPs layer strongly supports the notion that the performance improvements in the complete OLED originate primarily from defect healing within the ZnO NPs, rather than from degradation or modification of other constituent layers.

2.3. Electrochemical Analysis of ZnO NPs After Acetone Immersion

The absence of physical degradation after acetone immersion suggests that the changes in electrical behavior arise from chemically induced modifications in the ZnO NPs. Accordingly, to deeply understand the mechanism of the OLED performance enhancement by acetone immersion, the electrochemistry change of the ZnO NPs layer is investigated. First, XPS was employed to investigate the chemical states of zinc in ZnO NPs, as shown in Figure 3a. The Zn 2p core‐level spectrum exhibits two distinct peaks centered at approximately 1021.5 and 1044.6 eV, corresponding to Zn 2p₃⁄₂ and Zn 2p₁⁄₂, respectively [32]. The energy separation of 23.1 eV between the two peaks is in good agreement with the spin–orbit splitting characteristic of Zn^{2 +} in a ZnO NPs lattice [32], confirming the presence of divalent zinc without detectable metallic Zn or suboxide phases. The symmetric peak shapes and absence of satellite features further indicate a single chemical environment, consistent with stoichiometric ZnO NPs. These results confirm that the ZnO NPs keep their high purity and phase uniformity after acetone immersion.

FIGURE 3.

Electrochemical analysis of ZnO NPs before and after acetone immersion. (a) Zn 2p and (b) O 1s core level XPS spectra of the ZnO NPs films. Purple and brown hollow dots show the XPS raw data, while the black solid line corresponds to the envelope‐fitting curve, and the colored solid lines to each component. (c) Spectral PL intensity measured for the F8T2 EML under 365 nm UV light. The sample structure is glass/F8T2/ZnO NPs. (d,e) Transient EL profiles from normal OLED structure. The corresponding EL mechanism is drawn in the transient EL graph. (f) J–V characteristics of normal OLEDs employing an Au cathode instead of Al. Inset images show the energy level diagrams of the OLED. J‐V curves are separated into three regions: (1) leakage, (2) blocking, (3) tunneling regions. g) J–V characteristics of EOD and HOD. h) Transient EL traces at the rising edge showing the turn‐on delay time (t_d) for the normal OLED. i) UPS analysis of the ZnO NPs layer. All the comparisons are shown without (None) and with acetone immersion (Ace).

The O 1s core‐level XPS spectra provides further insight into the surface chemistry and defect states of the ZnO NPs. The spectrum can be deconvoluted into three main components: the primary peak at 530.2 eV is attributed to lattice oxygen (O^{2 −} in Zn─O), while the component at 531.4 eV corresponds to oxygen vacancies, and the higher binding energy peak near 532.3 eV is assigned to surface‐adsorbed oxygen species, such as hydroxyl groups or molecular O₂ [33]. As shown in Figure 3b, a noticeable decrease in the intensity of the higher binding energy component from 49% to 42% is observed upon acetone immersion, indicating the effective removal or passivation of oxygen vacancy–related states. In contrast, the lattice oxygen peak has increased from 42% to 51%. This result suggests that acetone immersion leads to surface reconstruction or defect healing, thereby reducing non‐stoichiometric oxygen species and improving the structural integrity of the ZnO NPs surface. Mechanistically, carbonyl oxygen atoms of the acetone molecules enable the effective passivation of surface oxygen vacancies during the treatment process by restoring the ZnO NPs lattice (Note S1), resulting in significantly reduced electronic trap states [27]. Thus, acetone immersion effectively passivates such defects, as confirmed by the reduction in the oxygen vacancy in O 1s XPS spectra in Figure 3b, leading to suppressed interfacial quenching pathways.

Defect healing in ZnO NPs can also be assessed by UV irradiation of F8T2/ZnO NPs interface and tracking the ensuing rise in F8T2 photoluminescence (PL) intensity. Normally, the decrease of PL is attributed to the exciton quenching with surface defects on the EML/ZnO NPs interface, which are known to act as non‐radiative recombination. In Figure S10a, the PL of pristine F8T2 is higher than the heterostructure ZnO NPs/F8T2 and F8T2/ZnO NPs. Prior to investigating the impact of ZnO NPs surface defects, we quantified the intrinsic acetone tolerance of F8T2. Pristine F8T2 films exhibited a pronounced loss of PL after acetone immersion (Figure S10b), a signature of partial polymer dissolution or degradation. To analyze solvent effects from interfacial quenching, we compared heterostructures that locate F8T2 either above or below a ZnO NP layer. In glass/ZnO NP/F8T2 stacks, acetone induced the same PL degradation observed for bare F8T2 (Figure S10c), confirming that an exposed F8T2 surface remains susceptible to dissolution. On the other hand, encapsulating the polymer beneath ZnO NPs (glass/F8T2/ZnO NP) showed a substantial PL enhancement after immersion (Figure 3c). This improvement is attributed to the passivation of non‐radiative surface traps on ZnO NPs, which otherwise quench excitons at the polymer/oxide interface, while the ZnO NPs overlayer simultaneously shields F8T2 from solvent effect, thereby isolating the role of defect suppression in boosting emissive efficiency. Notably, we attempted to verify whether PEDOT: PSS and QUPD exhibit a PL enhancement similar to that observed for the F8T2/ZnO NP stack. As a result, we confirmed that a similar effect also occurs for PEDOT: PSS and QUPD, demonstrating that this ZnO NP–interfacing strategy is broadly applicable to various adjacent layers. The detailed results are summarized in Figure S11.

To investigate the impact of ZnO NPs defect states on charge dynamics, transient EL measurements were conducted on a normal OLED without and with acetone immersion of the device as shown in Figure 3d,e. In this device structure, QUPD has a deeper highest occupied molecular orbitals (HOMO) level than F8T2, forming a hole potential well that can store injected holes. In the transient EL rising edge, it is indicative of transient carrier accumulation and subsequent rapid radiative recombination, typically arising from stored holes within the EML, and overshoot behavior can be detected [34]. As shown in Figure 3d, the overshoot as the rising of the EL signal was not observed in the untreated OLED, whereas a pronounced overshoot at the rising edge of the EL signal was observed only in the acetone‐immersed OLED in Figure 3e. In Figure 3d, the energy band diagram, untreated ZnO NPs with abundant defect states facilitate leakage of stored holes toward the cathode, suppressing charge accumulation and thereby eliminating the overshoot. In contrast, in the presence of well‐passivated ZnO NPs after acetone immersion, these holes remain confined, allowing transient buildup and burst‐like recombination upon voltage application as shown in Figure 3e energy band diagram. Consistent with the transient EL rising‐edge behavior, acetone‐immersed passivation of ZnO NPs surface traps lengthens the EL decay tail (Figure S12), a sign of diminished exciton quenching at the EML/oxide interface. The time that requires transient EL to decay onto 36% of its peak value increased from 0.513 µs in the untreated device to 0.580 µs in the acetone‐treated device. This 13% extension signifies that acetone immersion effectively suppresses hole and exciton leakage across the EML/ZnO NPs interface, thereby enhancing the hole blocking barrier and sustaining radiative recombination within the EML.

To further investigate charge dynamics, particularly the hole‐blocking capability of ZnO NPs without and with acetone immersion, we fabricated OLED devices identical to the normal structure but incorporating an Au cathode in place of Al in order to limit the electron injection. The deep work function of Au (∼5.2 eV) suppresses electron injection, ensuring hole‐only transport. Hole blocking ability were analyzed using Fowler–Nordheim tunnelling, a protocol previously employed to evaluate charge modulation layers in QLEDs [35]. Three well‐defined conduction regimes emerge in the acetone‐treated device, as shown in Figure 3f. At low bias (V <4 V), a leakage current dominates. Between 4 and 8.5 V, the trace flattens, reflecting efficient hole blocking by the ZnO NPs layer. Once the bias exceeds 8.5 V, the current steeply rises, consistent with Fowler–Nordheim tunnelling of holes through the barrier. By contrast, the untreated ZnO‐NP device exhibits a featureless, monotonically rising J–V trace: the blocking plateau observed in the acetone‐treated devices is entirely absent. The large density of surface defect states provides additional transport pathways that undermine the barrier to hole flow, so leakage dominates across the entire bias range. This direct correlation between defect density and the loss of the blocking regime highlights the critical role of surface passivation in preserving the hole‐blocking functionality of ZnO NPs.

Finally, to evaluate the charge transport properties without and with acetone immersion, hole‐only and electron‐only devices (HODs and EODs) were fabricated with structures of ITO/PEDOT: PSS/QUPD/F8T2/QUPD/Al and ITO/ZnO NPs/F8T2/ZnO NPs/Al, respectively. As shown in Figure 3g, the J–V characteristics of the HODs exhibit higher current densities than the EODs, indicating an inherent charge imbalance in the EML of the untreated OLEDs. This imbalance leads to non‐radiative recombination and Joule heating, ultimately shortening device lifetime [36]. Notably, the J–V behavior of the HOD remains unchanged after acetone immersion, consistent with the earlier observations in Figure 2c–e. In contrast, EOD exhibits a substantial increase in current density, approaching that of the HOD, suggesting improved electron transport consistent with the earlier observations in Figure 2f. Although ZnO NPs may not offer perfectly ideal hole blocking in the EOD configuration, acetone immersion reduces hole leakage while still increasing the overall current. This behavior indicates that electron injection/transport is significantly improved by the acetone immersion.

In principle, oxygen vacancies in ZnO NPs are donor‐like, so their reduction could lower conductivity; however, our devices show increased electron conductivity after acetone immersion. Because σ = neµ, an increase in mobility can compensate for, or even outweigh, a reduction in carrier density. To quantify the role of electron mobility, we evaluated the electron mobility of the ZnO NP layers using transient EL. In typical OLEDs, hole transport is faster than electron transport, such that holes reach the EML first and EL is initiated when electrons arrive and form excitons. Therefore, the delay time (t_d ) between the electrical pulse and the TrEL onset provides an estimate of electron mobility. As shown in Figure 3h, the acetone‐treated device shows a shorter electron‐injection delay (0.29 µs) than the untreated device (0.46 µs), indicating more efficient electron transport through the ZnO NP layer. We further quantified the mobility using an independent method based on space‐charge‐limited current (SCLC) incorporating Poole‐Frenkel‐type field‐dependent mobility analysis and found that the electron mobility increases by ∼67% after acetone treatment (Figure S13) [18].

To better understand the mechanism, we performed UV–Vis and UPS measurements to probe changes in the energy‐level alignment. UV–Vis absorption shows no change in the optical bandgap before and after acetone treatment (Figure S14). Thus, as illustrated in Figure 3i, a downshift of the HOMO level in the ultraviolet photoelectron spectroscopy (UPS) results implies a corresponding lowering of the lowest unoccupied molecular orbitals (LUMO) level, indicating that acetone‐induced oxygen‐vacancy modulation slightly alters the energy‐level structure by ∼0.1 eV. Therefore, this LUMO downshift leads to a more favorable stepwise alignment between Al and the EML, thereby facilitating electron injection.

Comprehensively, these results confirm that acetone‐induced defect passivation in ZnO NPs not only suppresses hole leakage current but also enhances electron injections, thereby establishing a more spatially uniform recombination zone and resolving the intrinsic charge imbalance leading to significantly improved OLED performance. Furthermore, the positive impact of acetone immersion on RGB OLED stability is corroborated by extended lifetime measurements shown in Figure S15.

2.4. High Resolution Full‐Color OLED Display Scalability Using Acetone‐Immersed ZnO‐NPs

To evaluate the applicability of our approach to practical display systems, we fabricated a fully micropatterned, high‐resolution RGB OLED pixel array. The fabrication process for the micropatterned OLEDs is well organized in Figure S16. Figure 4a–c shows monochromic red, green, and blue OLED arrays with pixel densities exceeding 1000 ppi, comparing their EL characteristics with and without acetone immersion. In all cases, the emissive area of a single pixel was confined to 7 µm × 7 µm by a photolithographically defined pixel define layer (PDL). Consistent with previous results from green OLED test cells, the acetone‐immersed OLEDs exhibit increased luminance at a constant driving voltage of 9 V, confirming the compatibility of this method with micropatterned architectures. Figure 4d shows EL spectra of the RGB subpixels without and with acetone immersion, with peak maxim at 587 nm (red), 542 nm (green), and 433 nm (blue). The absence of any spectral shift or broadening indicates that the emission color purity is well preserved following treatment. This clearly demonstrates the universal effectiveness of acetone‐induced oxygen vacancy removal in ZnO NPs across all RGB subpixels.

FIGURE 4.

Enhanced optoelectronic performance and high‐resolution display scalability of OLEDs with acetone‐immersed ZnO NPs. (a–c) Optical and microphotographic images of a) red, b) green, and c) blue OLED pixel arrays (10 µm × 10 µm) before and after acetone immersion. (d) Normalized EL spectra of RGB subpixels with stable peak positions before and after acetone immersion. (e) Luminance comparison for red, green, and blue OLED pixels without and with acetone immersion. (f) J–V–L curves without and with acetone immersion. (g) Simultaneous RGB emission arrays without and with acetone immersion. (h) Photograph of a 40 mm × 40 mm full color micropatterned OLED display featuring an eave of traditional Korean building design, with 1000 ppi resolution, demonstrating high‐fidelity emission and scalability of the fabrication process. The micropatterned OLEDs are operated under the same voltage bias (9 V).

In particular, Figure 4e shows that the red, green, and blue subpixels exhibit luminance enhancements of 93%, 76%, and 52%, respectively, after acetone immersion. These improvements are attributed to enhanced carrier recombination probability and suppressed exciton quenching at the ZnO NPs/organic interface, enabled by the selective passivation of trap‐rich oxygen vacancies. As the trap density decreases, carrier balance within the device improves, leading to more efficient radiative recombination. Detailed optical and electrical characterizations of the individual red, green, and blue micropatterned subpixels are presented in Figure S17, while Table S3 summarizes the luminance, EQE, and emission peak wavelengths of OLEDs with each EML material before and after acetone treatment.

Based on the improved performance observed in micropatterned monochromic single‐color OLEDs, we further evaluated the effect of acetone immersion in micropatterned full‐color (RGB) OLEDs. Figure 4f compares the J–V–L characteristics of full‐color OLEDs incorporating untreated versus acetone‐immersed ZnO NPs. The acetone‐immersed OLEDs display markedly lower turn‐on voltages, steeper current rise, and significantly enhanced luminance across the voltage range. At 14 V, for example, the luminance increases from 662 to over 1530 cd m^{− 2}, accompanied by a substantial rise in current density. Under an operating voltage of 10 V, the acetone‐immersed OLEDs emit visibly brighter light, as shown in Figure 4g. In addition to improved electrical and optical performance, the devices also exhibit clean, sharply defined emission regions without the phenomenon of delamination or morphological defects.

Finally, Figure 4h presents a photographic image of the eave of a traditional Korean building displayed on a full‐color OLED panel (40 mm × 40 mm) incorporating acetone‐immersed ZnO NPs. To efficiently reproduce the colors of the eave image, the subpixel layout was carefully designed based on the luminance characteristics of each RGB subpixel, as illustrated in Figure S18a. As a result, this treated display exhibits substantially higher brightness and more faithful color representation than the untreated counterpart shown in the optical microscope (OM) image in Figure S18b. The enhanced emission from the treated panel confirms both the scalability of the acetone immersion protocol and its compatibility with high‐resolution, solution‐processed micropatterning. The successful reproduction of a complex, culturally significant image with high visual fidelity underscores the practical potential of this approach for next‐generation full‐color OLED displays. Taken together, the results in Figure 4 establish that acetone immersion effectively passivates interfacial defects in ZnO NPs, thereby enhancing electron injections, suppressing exciton quenching, and improving both luminance and emission uniformity across RGB subpixels without compromising the intrinsic photophysical properties of the EMLs or the fidelity of micropatterned structures. This study demonstrates that acetone‐induced surface passivation offers a simple yet powerful route toward scalable, high‐performance solution‐processed OLED displays, with immediate relevance to next‐generation display technologies.

3. Conclusions

In this study, we present a facile yet highly effective post‐fabrication strategy to enhance the optoelectronic performance of OLEDs by chemically modulating the surface of ZnO‐NPs through acetone immersion. As confirmed by XPS, PL, and transient EL analyses, acetone immersion rapidly and selectively passivates oxygen vacancy–related surface traps in ZnO NPs, thereby reducing trap density, suppressing interfacial quenching, and restoring hole‐blocking functionality, all of which contribute to improved charge balance and enhanced electron injection. As a result, the acetone‐treated green OLEDs exhibit a reduced turn‐on voltage (from 4.5 to 4.0 V), substantially higher current density and luminance (up to ∼44 000 cd/m²), and enhanced EQE and power efficiency across the full driving range. Performance enhancements were observed not only in green OLEDs but also in red and blue devices.

This work represents the first demonstration of ZnO–acetone interface chemistry specifically applied to OLEDs, clearly suggesting that the so‐called ‘positive aging’ effect observed in QLEDs and OLEDs originates from the healing of oxygen vacancies in the ZnO NPs layer. While prior ‘positive aging’ strategies in QLEDs require prolonged exposure to acidic media, our method enables comparable or superior performance improvements within hours, directly applied to fully fabricated OLED structures. This OLED‐specific adaptation of nanoparticle‐level chemical engineering marks a clear departure from QLED‐centered approaches, offering a new paradigm for interface control in hole‐dominated devices. Furthermore, the acetone‐treated ZnO‐NP strategy proves highly versatile and scalable, as demonstrated by its successful integration into high‐resolution RGB OLED subpixels and large‐area display panels. Therefore, these findings suggest this strategy as a versatile and scalable solution for advancing next‐generation OLED displays, with implications spanning fundamental research and practical device engineering.

4. Experimental Section

4.1. Device Fabrication

The device structure is anode/hole injection layer (HIL)/hole transport layer (HTL)/emission layer (EML)/electron transport layer (ETL) or electron injection layer (EIL)/cathode. Pre‐patterned indium tin oxide (ITO) glass substrates were sequentially cleaned using an ultrasonic bath in acetone, isopropanol, and deionized water to eliminate organic and inorganic contaminants. The cleaned substrates were then treated with UV/ozone (UV/O₃) to enhance surface energy and promote better adhesion of subsequent layers. First, the HIL of (poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate)) (PEDOT: PSS Al 4083, Sigma Aldrich) was spin‐coated (4000 rpm, 30 s) and annealed (120°C, 20 min). The HTL of (N4, N4'‐Bis(4‐(6‐(3‐ethyloxetan‐3‐yl)methoxy)hexyloxy)phenyl)‐N4, N4'‐bis(4‐methoxyphenyl)biphenyl‐4,4'‐diamine) (QUPD, Luminescence Technology) was spin‐coated (1000 rpm, 60 s) and annealed (180°C, 2 h). The EML of (poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2, Ossila) was spin‐coated (1000 rpm, 60 s) and annealed (180°C, 1 hr). Consequently, a ZnO NPs ETL/EIL was spin‐coated (6000 rpm, 30 s) and annealed (140°C, 20 min). Finally, an Al cathode was thermally evaporated in a high‐vacuum chamber (pressure <10^{− 6} Torr) at a deposition rate of 2–3 Å/s.

For patterned devices, there are additional processes required. A silicon dioxide (SiO₂) layer was deposited onto the pre‐patterned ITO glass substrate using plasma‐enhanced chemical vapor deposition (PECVD) to serve as an insulating barrier for pixel separation. The substrate was then patterned using photolithography, followed by reactive ion etching (RIE) to selectively remove the SiO₂ layer and define distinct pixel areas. After patterning, the HIL, the HTL, and the EML were sequentially deposited and annealed under the same conditions described previously. A second photolithography step was conducted to refine the active pixel regions and ensure accurate alignment of the organic layers. Afterward, the EIL/ETL and the Al cathode were deposited following the same procedures as for non‐patterned devices.

All functional layers were spin‐coated and thermally annealed in a nitrogen‐filled glovebox (O₂ <1 ppm, H₂O <1 ppm) to avoid exposure to ambient oxygen and moisture. The acetone immersion of the OLED devices was also carried out in the same inert atmosphere, and the immersion was performed at room temperature for 1 h. QUPD and F8T2 were dissolved in chlorobenzene and filtered through a 0.45 µm PVDF syringe filter prior to use.

4.2. Device Measurement

For measuring the current density, voltage, and luminance characteristics (J‐V‐L) of the OLED, a spectroradiometer (CS‐2000, Konica Minolta, Japan) and an electrometer (Keithley 2400, Tektronix, USA) were used. The cross‐sectional view of the multilayered OLED and the elemental analysis were conducted by using a JEM‐2100F high‐resolution transmission electron microscope (HRTEM; JEOL, Tokyo, Japan). The PL intensity of the OLED EML was measured using an FP‐8300 spectrofluorometer (JASCO, Japan). The images of PL emitted by the EML were obtained under 365 nm UV illumination conditions. To measure the transient EL analysis, a transient EL testing system consisting of a function‐signal generator, a photomultiplier tube, and a digital oscilloscope is used. PL spectra of the films were performed by a fluorescence spectroscopy (PE LS‐55). X‐ray photoelectron spectroscopy (XPS) measurements were performed using Bruker D8 ADVANCE equipment.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73275‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.