1. INTRODUCTION

See-through displays, which enable users to view digital content while simultaneously seeing through the screen, have garnered significant attention due to their transformative applications in augmented reality (AR) systems, smart glasses, automotive windshield head-up displays (HUDs), retail shop window advertisements, and intelligent interactive displays [1–5]. These technologies enhance user experiences by seamlessly blending digital information with real-world environments, offering intuitive, immersive, and context-aware interactions. As a result, see-through displays are expected to generate substantial economic value, driven by their expanding market opportunities and the growing demand for next-generation display solutions. Among the various see-through display technologies [1,6–8], quantum-dot (QD) light-emitting diodes (QLEDs) have emerged as a particularly promising candidate due to their high brightness, vivid color reproduction, high transparency, and simple solution-processability [9–13]. To achieve high visual see-through performance, the QLED itself must be optically transparent. This is typically realized by sandwiching the QD light-emitting layer and the associated charge transport layers between transparent bottom and top electrodes [14,15]. These electrodes are commonly composed of transparent conductive oxides [16–18], silver (Ag) nanowires [19,20], or ultra-thin Ag films [21,22], which offer high optical transmittance while maintaining sufficient electrical conductivity. As a result, ambient light can pass through the entire device, while the electroluminescence (EL) generated by the QDs is emitted bidirectionally through both the top and bottom surfaces. Consequently, transparent (Tr) QLEDs inherently exhibit bidirectional emission. However, this bidirectional emission is not always desirable, as users typically view the display from a single direction. The intrinsically bidirectional emission leads to significant photon losses in the non-viewing direction, reducing luminous efficiency in the intended viewing path. Moreover, it can raise concerns related to personal information security by allowing unintended viewers to see the display content, and may even contribute to light pollution in ambient environments. For improved energy efficiency and user privacy, it is therefore crucial to tailor the emission directionality and guide most photons toward the viewer’s side. One common approach involves increasing the reflectance of either the bottom or top electrode to redirect more photons toward the desired direction [21,22]. However, this strategy inevitably compromises the optical transparency of the device. As a result, a fundamental trade-off arises between emission directionality and device transparency, making it challenging to simultaneously achieve both high directionality and high see-through performance in Tr-QLEDs. Furthermore, the use of highly transparent electrodes weakens the optical interference effects and thus reduces the outcoupling efficiency. Consequently, the overall EQE of many reported Tr-QLEDs remains around 15% [14,15,23,24], significantly lower than the 20%–40% typically achieved by their conventional counterparts [25–27]. Although the use of semi-transparent electrodes, such as ultra-thin Ag or Au films, can enhance the overall EQE to 25.5%, forming a continuous metallic film with adequate electrical conductivity typically requires a thickness greater than 20 nm. However, this significantly increases the film’s reflectance and absorption, dramatically decreasing the device’s averaged transparency to 30% [28]. Therefore, it is inherently difficult to simultaneously achieve high emission efficiency and high device transparency, which represents the second critical trade-off in Tr-QLEDs.

In this work, we address the fundamental trade-offs among emission directionality, EL efficiency, and device transparency in Tr-QLEDs to achieve optimal photon management for various see-through display applications. Theoretical analysis reveals that emission directionality, EL efficiency, and device transparency are strongly influenced by surface reflection at the electrode interfaces. Therefore, balancing surface reflection is key to optimizing Tr-QLED performance, and careful electrode design enables tunable trade-offs among directional emission, efficiency, and transparency. By strategically engineering the surface reflection, tunable emission directionality, ranging from symmetrically bidirectional to highly unidirectional, is demonstrated in Tr-QLEDs without much compromising the transparency. Specifically, by minimizing reflection at both the top and bottom electrodes, Tr-QLEDs with symmetric bottom and top emissions, high transparency (90%), and high EQE (11%) for each side emission are achieved, making them ideal candidates for see-through, dual-sided displays. Subsequently, the emission directionality is tailored to preferentially direct photons toward the viewer. By introducing a specially developed bottom electrode based on acid-treated poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), waveguided modes are effectively suppressed, resulting in enhanced bottom emission. Further incorporation of a ZnS/Ag/LiF/ZnS top electrode increases reflectivity at the top interface, leading to a pronounced unidirectional emission with a bottom-to-top luminance ratio of 10:1, a unidirectional EQE of 19.5%, and an averaged transparency of 40.7%, thereby enabling Tr-QLEDs for advanced applications in see-through, unidirectional displays such as ARs, HUDs, and smart windows.

2. RESULTS

A. Influence of Surface Reflection on the Emission Directionality, EL Efficiency, and Device Transparency

Figure  1 (a) schematically illustrates the importance of tailoring emission directionality for various see-through display applications. To quantify emission directionality, a directional emission ratio (DER), defined as the ratio of bottom-to-top luminance, is used. A DER of 1 indicates symmetric emission from both surfaces, while values significantly deviate from 1 correspond to strong unidirectional emission. For bidirectional viewing applications such as public signage, interactive kiosks, or dual-sided advertising displays, a symmetric emission with a DER of 1 is essential to ensure consistent information delivery to viewers on both sides. Additionally, high optical transmittance is crucial in these scenarios to enhance the sense of visual immersion by allowing clear visibility of objects behind the display. In contrast, unidirectional applications such as AR glasses, HUDs, and smart windows require emission predominantly directed toward the viewer. In these cases, a DER that deviates substantially from 1 is preferred, as it maximizes optical efficiency in the viewing direction while minimizing photon loss through the opposite side. This not only improves energy efficiency but also helps prevent information leakage and reduce undesired light pollution.

Fig. 1. Influence of surface reflection on emission directionality, efficiency, and transparency. (a) The requirements of DER and DT in different see-through display application scenarios. (b) Tr-QLED structure and internal light propagation schematics. The variation of (c) DER, (d) DT, and (e) OCE with the refractive indices of the top and bottom TCEs. The yellow dashed lines in (c)–(e) represent a contour with a DER of 1. The yellow stars in (c)–(e) mark the optimal design points for Tr-QLEDs that simultaneously achieve symmetric emission (DER = 1), high transparency, and high efficiency. The red stars in (c)–(e) mark the DER, DT, and OCE of typical Tr-QLEDs with both IZO TCEs. The blue stars in (c) mark the optimal design points for Tr-QLEDs with high unidirectional emission.

The emission directionality, efficiency, and transparency of Tr-QLEDs are intrinsically governed by the surface reflection of their top and bottom transparent conductive electrodes (TCEs), as schematically illustrated in Fig.  1 (b). Surface reflection, primarily dictated by the refractive indices of the TCEs, determines how photons are distributed and emitted from the device. To quantitatively examine the influence of surface reflection on emission directionality, efficiency, and transparency, the refractive indices of the TCEs were systematically varied, and the corresponding DER, outcoupling efficiency (OCE), and device transparency (DT) were calculated using the classical dipole emission model coupled with the transfer matrix method (Supplement 1, Note 1) [29–31]. The refractive index of all the layers used in the optical simulation is shown in Fig. S3. As demonstrated in Fig.  1 (c), when the refractive index of the bottom TCE is fixed, the DER increases with increasing refractive index of the top TCE. This trend arises from enhanced reflection at the top interface, which redirects more photons toward the bottom side, resulting in stronger bottom emission and weaker top emission. Conversely, when the refractive index of the top TCE is fixed and the refractive index of the bottom TCE is increased, the DER decreases, indicating enhanced top emission due to stronger reflection at the bottom interface. However, as shown in Fig.  1 (d), the DT exhibits a roughly inverse trend relative to DER. This inverse relationship indicates that stronger directional emission typically comes at the expense of reduced optical transparency. Additionally, as shown in Fig.  1 (f), higher refractive indices of both TCEs lead to enhanced OCE, primarily due to increased interfacial reflection that promotes constructive interference via the microcavity effect. These simulated results highlight the critical role of surface reflection in modulating photon distribution within Tr-QLEDs and underscore an inherent trade-off among DER, DT, and OCE. Therefore, simultaneous optimization of these parameters requires rational electrode design and precise optical engineering. These findings provide a fundamental basis for tailoring Tr-QLED performance to meet the specific requirements of various see-through display applications through controlled manipulation of electrode refractive indices. For example, to achieve Tr-QLEDs with symmetric emission (DER = 1), multiple refractive index combinations are possible, as indicated by the contour line (yellow dashed line) in Fig.  1 (c). However, only one specific combination, highlighted by the yellow star in Figs.  1 (c)– 1 (e), enables the simultaneous realization of symmetric emission, high transparency, and high efficiency, representing the optimal design point for balanced Tr-QLED performance, as will be discussed in the following section.

B. Realization of Tr-QLEDs with Symmetric Emission, High Transmittance, and High Efficiency

Typical Tr-QLEDs are often fabricated using indium-zinc-oxide (IZO) as both the top and bottom TCEs [16,32]. However, this design cannot achieve symmetric emission due to the inherently higher reflection at the top IZO interface. As a result, the bottom-side emission is stronger, with an EQE of 15.8%, compared to 10.1% from the top side, as shown in Fig. S4. This imbalance yields a DER of 1.56, which closely matches the simulated value of 1.61 [marked by the red star in Fig.  1 (c)], confirming the accuracy of the optical model.

Based on the simulation results, to design a Tr-QLED with a DER of 1, the refractive indices of the TCEs have to be tuned to satisfy the contour-defined parameters in Fig.  1 (c). However, few available TCE materials precisely satisfy the required refractive index combinations. To address this limitation, composite electrodes are introduced to tune the effective refractive index () of TCEs, enabling more flexible control over surface reflection. According to effective medium theory [33], for periodically stacked bilayers composed of homogeneous materials, when the individual layer thicknesses are significantly smaller than the incident wavelength and under the condition that the electric field is oriented perpendicular to the layer interfaces (transverse magnetic polarization), the effective refractive index of the composite electrode can be determined by the following formula [34]:

where  and  are the thickness fractions of the two dielectric layers, and  and  are their respective refractive indices. To achieve a balanced trade-off among DER, OCE, and DT, the optimal design point, marked by the yellow star in Fig.  1 (c), corresponds to bottom and top TCEs with refractive indices of approximately 2.2 and 1.7, respectively. Taking the bottom TCE design as an example, a composite electrode structure consisting of ZnS/IZO was used. As shown in Fig. S3, ZnS exhibits a high refractive index, and by fixing the IZO thickness at 60 nm, the ZnS thickness is calculated to be approximately 30 nm to achieve a composite electrode with a  of 2.2. Similarly, a top composite electrode comprising ITO (120 nm)/LiF (60 nm) yields a  of about 1.7. A schematic device structure is shown in Fig.  2 (a), with detailed layer thickness provided in the device structure section. The simulation results shown in Fig.  2 (b) indicate that with a bottom IZO layer of 60 nm and a top ITO layer of 120 nm, the device achieves a DER of 1.06, which is very close to the symmetric emission, validating the effectiveness and reliability of the composite electrode optimization strategy. The simulated electromagnetic field distributions within the Tr-QLED further confirm the realization of symmetric emission. As shown in Fig. S5a, the field intensities at the two emission interfaces (bottom TCE/glass and top TCE/air) are nearly identical. The subsequent Figs. S5b–S5d likewise provide theoretical support for the Tr-QLEDs exhibiting different DERs achieved in later sections. Under these conditions, the device also achieves a near-maximum OCE of 32.5%, as shown in Fig. S6. Based on this optimized design, the resultant red Tr-QLEDs exhibit highly symmetric emission (DER of 1.06), with bottom-side luminance, EQE, and EL spectra closely matching those of the top side, as shown in Figs.  2 (c)– 2 (e). The perceived brightness from both viewing directions is nearly identical, as further demonstrated by the uniform and similar EL images shown in the inset of Fig.  2 (f) and Visualization 1. In addition to the symmetric emission, the Tr-QLEDs demonstrate a high EQE of 11% for both top and bottom emissions, along with excellent optical transparency of 91.7% at the emission wavelength of 630 nm and a luminous transmittance () of over 88% (detailed calculations can be found in Supplement 1, Note 2 [35,36]), as shown in Fig.  2 (f). The simultaneous achievement of symmetric emission, high efficiency, and high transparency is in excellent agreement with the simulation results. Tr-QLEDs with ultra-high transparency offer a clear view of the background, as depicted in the inset of Fig.  2 (f), where the regions visible through the Tr-QLED maintain brightness and color purity comparable to the unoccluded areas. By applying the same optimization strategy, similar results can be obtained in green and blue Tr-QLEDs, as shown in Fig. S7, demonstrating the universality and effectiveness of our surface reflection engineering approach across different emission colors. The developed Tr-QLEDs, featuring symmetric emission, high transparency, and high efficiency, are well-suited for see-through dual-sided display applications such as public signage and advertising window displays. Moreover, their high transparency broadens their application scope in ambient-integrated displays by enabling visual information to be seamlessly overlaid onto physical objects without compromising the objects’ original appearance or functionality. This capability enables immersive visual experiences in scenarios such as real-world navigation systems, intelligent service terminals, and context-aware digital signage. As a proof-of-concept demonstration [Fig.  2 (g) and Visualization 1], the device was placed in front of a background. In the off-state, the perceived image exhibits luminance nearly identical to that of the unobstructed background. Upon activation of the emissive regions, the inherently high transmittance allows the emitting areas to remain visually integrated with the background elements, thereby enhancing the vividness and clarity of the displayed patterns.

Fig. 2. Realization of Tr-QLEDs with symmetric emission, high transmittance, and high efficiency. (a) The schematic diagram of Tr-QLEDs with composite electrodes. (b) The variation of DER with the thickness of the top and bottom TCEs. The (c) luminance-voltage, (d) EQE-current density, (e) EL spectra, and (f) transmittance of Tr-QLEDs. The inset of (d) shows photographs of the Tr-QLED taken from both the top and bottom viewing sides, while the inset of (f) presents photographs of the Tr-QLED placed in front of a background. (g) Demonstrations of ambient-integrated displays with red, green, and blue Tr-QLEDs integrated with a background object.

C. Realization of Tr-QLEDs with Unidirectional Emission, High Transmittance, and High Efficiency

For unidirectional displays such as AR smart glasses and HUDs, Tr-QLEDs with a high DER, favoring photon emission toward the viewer, are highly desirable. Typical Tr-QLEDs with IZO as both the top and bottom TCEs already exhibit weakly unidirectional emission with a DER of 1.56, as demonstrated in Fig. S4. However, this relatively low DER indicates that a significant portion of photons are still emitted in the undesired direction, resulting in considerable energy loss, potential information leakage, and unwanted light pollution. Guided by the theoretical framework [blue star in Fig.  1 (c)], enhancing the DER requires lowering the refractive index of the bottom TCE to approach that of the glass substrate (). This eliminates total internal reflection at the bottom TCE/glass interface, allowing more photons to be coupled into the glass substrate and thereby enhancing bottom-side emission.

One promising TCE that has a low refractive index is the PEDOT:PSS due to its polymeric nature. Previous studies have demonstrated that specially engineered bottom electrodes based on acid-treated poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) can effectively convert waveguide modes into substrate modes, thereby enhancing the OCE when combined with light-extraction structures [37]. In this present work, employing acid-treated PEDOT:PSS as the electrode in Tr-QLEDs simultaneously enhances bottom emission while suppressing top emission, resulting in a markedly improved unidirectional emission performance. Typical PEDOT:PSS exhibits a relatively high refractive index [, as shown in Fig.  3 (b)] and low electrical conductivity. To tailor its refractive index and conductivity, a methanesulfonic acid () treatment method was developed. The schematic of the device structure and the treatment process are shown in Fig.  3 (a). During the acid treatment,  ions from the acid solution readily combine with  chains, forming neutral PSSH, which then separates from the PEDOT:PSS matrix, [38] as illustrated in Fig. S8. Finally, residual  is removed by sequential rinsing with ethanol and isopropanol. This treatment process also improves surface wettability, as evidenced by a significant reduction in contact angle (Fig. S9), which facilitates better spreading of the subsequent solution and thereby promotes the formation of a uniform and compact upper layer. Moreover, due to the removal of high refractive index PSS, the refractive index of the acid-treated PEDOT:PSS is effectively reduced to 1.51 at 630 nm, which is comparable to that of the glass, as shown in Fig.  3 (b). Concurrently, the reduction of non-conductive PSS increases the relative proportion of conductive PEDOT, and enhances  stacking interactions within the PEDOT molecular chains [5]. These synergistic effects contribute to a substantial improvement in electrical conductivity, as demonstrated in Fig.  3 (c). After a 20 min treatment, the conductivity of PEDOT:PSS greatly increases from 20 to , and the sheet resistance of a 100 nm film is reduced to , comparable to that of ITO electrodes, thereby indicating its promising potential as a pixel electrode. The reduced refractive index and smoother surface also led to a further enhancement of optical transmittance, which surpasses that of both ITO and IZO electrodes in the visible range, as shown in Fig. S10. Figure S11 shows the energy levels of the components of the ITO films, and PEDOT:PSS before and after acid treatment. It can be observed that, compared to the ITO electrode, the acid-treated PEDOT:PSS exhibits a more aligned energy level with hole injection layer PEDOT:PSS 4083 [39]. These significant enhancements in both optical and electrical properties make acid-treated PEDOT:PSS a highly promising TCE for Tr-QLEDs targeted with high DER.

Fig. 3. Engineering the bottom TCE by tailoring the refractive index and conductivity of the PEDOT:PSS to achieve Tr-QLEDs with unidirectional emission, high transmittance, and high efficiency. (a) The schematic device structure of Tr-QLEDs based on PEDOT:PSS bottom TCE, and the acid treatment process for PEDOT:PSS. (b) The refractive index of PEDOT:PSS with and without acid treatment as a function of wavelength. (c) The sheet resistance and conductivity of PEDOT:PSS as a function of acid treatment time. (d) The J-V characteristics of Tr-QLEDs based on PEDOT:PSS bottom TCE with and without acid treatment. The (e) L-V and (f) EQE-J characteristics of Tr-QLEDs from the bottom and top sides.

]

Fig. 4. Engineering the top TCE by tailoring the transmittance and conductivity of the Ag films to achieve Tr-QLEDs with strong unidirectional emission. (a) The schematic device structure of Tr-QLED with strongly reflective Ag/ZnS/LiF top TCE. (b) The sheet resistance of Ag films with and without ZnS seed layer as a function of thickness. (c) The transmittance as a function of wavelength for Ag of different thicknesses with and without ZnS seed layer. The inset shows the photographs of the Ag films placed over a background image. (d) The DER and averaged transmittance of Tr-QLEDs as a function of Ag thickness. (e) The L-J-EQE characteristics of Tr-QLEDs from the bottom and top sides. (f) The demonstration of a large-area Tr-QLED with a DER of close to 10. (g) The application of Tr-QLED in automotive window displays. (h) Radar chart summarizing the Tr-QLEDs performance of this work and references.

Benefiting from the smoother surface, enhanced optical transmittance, and improved electrical conductivity, Tr-QLEDs incorporating acid-treated PEDOT:PSS as the bottom TCE exhibit significantly lower leakage current, along with higher recombination current and luminance, as shown in Fig.  3 (d) and Fig. S12. For example, the luminance at 6 V is boosted from 38,151 to  by using the acid-treated PEDOT:PSS as the bottom TCE. More importantly, this specially designed TCE remarkably enhances bottom-side emission, achieving a peak luminance of  and an EQE of 15.3%, which are approximately three-fold higher than those of the top emission, as demonstrated in Figs.  3 (e) and 3 (f). Based on the power dissipation spectra of the devices, we quantitatively analyzed the relative contributions of various optical modes, further verifying the intrinsic mechanism by which replacing ITO with acid-treated PEDOT:PSS enhances the DER. As shown in Fig. S13, the introduction of acid-treated PEDOT:PSS with a more closely matched refractive index effectively suppresses the waveguide mode at the bottom TCE/substrate interface. This results in an increased OCE in the bottom side and a reduced OCE in the top side, ultimately leading to an enhanced DER. The resulting Tr-QLEDs also feature a high averaged DT of 77.2%, as shown in Fig. S14. The simultaneous achievement of high emission directionality (DER = 3), high efficiency (total EQE of 20.8%), and high averaged DT (77.2%) enables the strategic implementation of these Tr-QLEDs in advertising window displays. This design is particularly well-suited to meet the stringent requirements of outdoor environments with high ambient illumination, where enhanced pixel luminance on the viewer-facing side is essential to maintain display legibility.

To further enhance the DER, the refractive index of the top TCE should be increased, as pointed out by the simulation results [marked by the dark cyan star in Fig.  1 (c)]. However, this objective faces a fundamental materials science limitation, as there are no suitable transparent materials or composite architectures capable of achieving such high refractive indices while simultaneously preserving the essential optoelectronic properties required for device performance. Ag thin films, with their lowest optical absorption among all metals, tunable reflectance and transmittance depending on thickness, and excellent electrical conductivity, represent a promising candidate for use as the top TCE. To achieve high device transparency, the Ag TCE should be as thin as possible. However, due to its intrinsically high surface energy relative to that of the substrate, Ag tends to nucleate and grow into island-like microstructures during deposition. This distinct growth behavior fundamentally impedes the formation of continuous ultra-thin Ag films. To address this issue, a 1 nm ZnS layer, characterized by its high surface energy, was introduced as a seed layer [Fig.  4 (a)] to suppress the Volmer–Weber growth mode of Ag [40,41], thereby promoting the formation of continuous ultra-thin Ag films. As shown in Fig.  4 (b), the incorporation of the ZnS seed layer enables the formation of continuous Ag films with significantly reduced sheet resistance, allowing for the realization of ultra-thin films down to 8 nm in thickness. More importantly, the introduction of ZnS suppresses island growth and the associated surface plasmon resonance (SPR) effects [42], thereby enabling 10–15 nm Ag films to achieve significantly higher averaged transmittance () compared to their counterparts () without the ZnS seed layer, as shown in Fig.  4 (c). Unlike previous studies [43], ZnS in this work serves not only as a seed layer but also as an insulating interfacial layer, and an excessively thick ZnS layer can deteriorate the device’s charge injection performance. Therefore, the electrical performances of devices with and without 1 nm ZnS were evaluated. It is worth noting that the introduction of the 1 nm ZnS seed layer does not adversely affect device performance (Figs. S15a and S15b). On the contrary, it contributes to reduced contact resistance due to the reduced Ag sheet resistance, as shown in Fig. S15c. Consequently, enabling by the ZnS seed layer, Tr-QLEDs incorporating a 10 nm Ag/IZO directly as the top TCE were successfully fabricated, exhibiting high emission directionality with a DER of about 5, along with a  of 68.68%, as shown in Fig. S16. While the DER can be further enhanced by increasing the thickness of the Ag TCE, this approach significantly compromises device  due to increased optical absorption, as shown in Fig.  4 (d). To simultaneously achieve high DER and high transmittance, a ternary composite electrode composed of Ag/LiF/ZnS was introduced. In this design, the Ag thickness is fixed at 12 nm, while a LiF/ZnS double-layer dielectric stack, featuring low and high refractive indices, respectively, is employed to boost the reflectance of the top TCE without substantially increasing absorption. With this top TCE, the Tr-QLEDs exhibit a pronounced bottom emission, demonstrating a peak luminance of  and an EQE of 19.5%, which are approximately 9.45 times higher than those of the top emission, indicating that over 90% of the generated photons are directed toward the bottom side, as shown in Fig.  4 (e). As demonstrated in Fig.  4 (f), the Tr-QLED operating at 2.2 V shows a strong bottom emission, while the top emission, as can be observed from the mirror, is relatively weak. Featuring high emission directionality (DER of 9.45), high efficiency (EQE of 19.5%), and moderate averaged DT (40.7%, Fig. S17), the developed Tr-QLEDs can find potential applications in AR smart glasses and automotive window displays. The moderate device transmittance helps enhance display contrast by reducing background light interference, thereby improving visual clarity in ambient environments. As a proof-of-concept demonstration, Fig.  4 (g) and Visualization 1 showcase the application of the Tr-QLED in an automotive window display. The Tr-QLED was positioned over a background image to simulate real-world integration. In the off state, the background remains clearly visible through the device, demonstrating its high transparency. When switched on, the Tr-QLED emits a simple navigation pattern, which appears superimposed on the background. This enables the viewer to simultaneously perceive both the emitted content and the underlying scene, highlighting the device’s suitability for transparent, context-aware display applications. Moreover, the ultra-high DER significantly enhances display efficiency and luminance in the viewing direction, while simultaneously minimizing information leakage and mitigating potential light pollution.

Table 1. Detailed Structure and Performance of the Developed Tr-QLEDs

D. Summary

The TCE structures, associated Tr-QLED performance metrics, and corresponding application scenarios are summarized in Table  1 . The operational stability of Tr-QLEDs with different structures is shown in Fig. S18, all of which demonstrate superior lifetime compared to typical Tr-QLED. Guided by theoretical simulations, the inherent trade-offs among emission directionality, transparency, and efficiency are effectively balanced, leading to record-high performance metrics with an DER of 10, averaged DT of 90%, and EQE of 25.9%, which surpass those of the state-of-the-art Tr-QLEDs reported in recent years, as compared in Fig.  4 (h) and detailed in Table S1 [14,15,19,21–23,28,44–46].

In conclusion, we address the fundamental trade-offs among emission directionality, device transparency, and luminous efficiency in Tr-QLEDs. Theoretical analysis reveals that these trade-offs are strongly governed by surface reflections at the TCE interfaces. Guided by this theoretical framework, we systematically design TCE structures to tailor interfacial reflections, which enables the realization of Tr-QLEDs with tunable emission directionality, transparency, and efficiency to meet diverse application scenarios. Specifically, by modulating the surface reflection using specially designed ZnS/IZO bottom TCE and ITO/LiF top TCE, Tr-QLEDs with symmetric emission, high transparency (), and high EQE (11%) for both top and bottom emissions are achieved, which have great potential application in see-through dual-sided displays. To realize strong directional emission, a low-refractive-index, high-conductivity bottom TCE is developed via acid-treated PEDOT:PSS, resulting in Tr-QLEDs that simultaneously achieve strong bottom emission (DER = 3), high efficiency (EQE of 15.3% for bottom emission), and high averaged transparency (77.2%), making them suitable for advertising window displays. Further enhancement in emission directionality is achieved by optimizing the reflection and transmittance of the top TCE. With a specially designed Ag/LiF/ZnS top TCE, the Tr-QLED demonstrates a highly unidirectional emission with a DER of 10, a bottom-side EQE of 19.5%, and a moderate averaged transparency of 40.7%, enabling their applications in AR glasses and automotive window displays. This work provides both theoretical insight and practical guidance for designing Tr-QLEDs with tunable emission directionality, laying a solid foundation for their implementation across a wide spectrum of see-through display applications.

3. METHODS

A. Materials

All materials are commercially available. CdSe-based colloidal red QDs were purchased from Suzhou Xingshuo Nanotech Co., Ltd. ZnMgO nanoparticles were purchased from Guangdong Poly Optoelectronics Co., Ltd. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(-(N-(pbutylphenyl))diphenylamine)] (TFB) was purchased from VOLT-AMP Optoelectronic Technology Co., Ltd. PEDOT:PSS AI4083 (CLEVIOS) was purchased from Luminescence Technology Corp. PEDOT:PSS PH1000 (CLEVIOS) was purchased from OPV Tech Co., Ltd. ZnS, IZO, and ITO materials were purchased from Hebei Gaocheng New Materials Technology Co., Ltd. LiF was purchased from Shanghai Macklin Biochemical Co., Ltd. Chlorobenzene, octane, and  were purchased from Aladdin Industrial Corp. Absolute ethanol was purchased from Shanghai LingFeng Chemical Reagent Co., Ltd.

B. Device Structures

Red Tr-QLEDs with a structure of glass/IZO (120 nm)/PEDOT:PSS (35 nm)/TFB (25 nm)/R-QDs (20 nm)/ZnMgO (70 nm)/ultra-thin Al (2 nm)/IZO (100 nm) were fabricated with IZO functioning as both bottom and top TCEs.

Red Tr-QLEDs with DER of 1 were fabricated with a structure of glass/ZnS (30 nm)/IZO (60 nm)/PEDOT:PSS (35 nm)/TFB (25 nm)/R-QDs (20 nm)/ZnMgO (70 nm)/ultra-thin Al (2 nm)/ITO (120 nm)/LiF (60 nm).

Green Tr-QLEDs with DER of 1 were fabricated with a structure of glass/ZnS (30 nm)/IZO (60 nm)/PEDOT:PSS (35 nm)/TFB (25 nm)/G-QDs (15 nm)/ZnMgO (70 nm)/ultra-thin Al (2 nm)/ITO (70 nm)/LiF (60 nm).

Blue Tr-QLEDs with DER of 1 were fabricated with a structure of glass/ZnS (10 nm)/IZO (60 nm)/PEDOT:PSS (35 nm)/TFB (25 nm)/B-QDs (15 nm)/ZnMgO (70 nm)/ultra-thin Al (2 nm)/ITO (60 nm)/LiF (60 nm).

Red Tr-QLEDs with DER approaching 3:1 were fabricated with a structure of glass/PEDOT:PSS PH1000 (70 nm)/PEDOT:PSS 4083 (35 nm)/TFB (25 nm)/R-QDs (20 nm)/ZnMgO (70 nm)/ultra-thin Al (2 nm)/IZO (70 nm).

Red Tr-QLEDs with DER approaching 10:1 were fabricated with a structure of glass/PEDOT:PSS PH1000 (70 nm)/PEDOT:PSS 4083 (35 nm)/TFB (25 nm)/R-QDs (20 nm)/ZnMgO (70 nm)/ultra-thin Al (2 nm)/ZnS (1 nm)/Ag (12 nm)/LiF (80 nm)/ZnS (60 nm).

C. Device Fabrication

For the red Tr-QLEDs with IZO as TCEs, the cleaned glass substrates were transferred to a magnetron sputtering system to deposit the IZO (120 nm) as anode at a working pressure of 0.30 Pa, a power of 50 W, and an Ar flow of 20 sccm. The IZO target used here is composed of 90 wt%  and 10 wt% ZnO. First, the HILs were formed by spin-casting PEDOT:PSS 4083 solution at 3000 rpm and baked at 130°C for 20 min in the atmosphere after treating the glass/IZO with  plasma for 5 min. Subsequently, the samples were transferred into a nitrogen-filled glove box to prepare the subsequent functional layers. The TFB HTLs ( in chlorobenzene) were spun-cast at 3000 rpm for 45 s and baked at 100°C for 15 min. Then, the red QDs were deposited by spin-casting the QDs solution (CdSe/ZnS QDs, dissolved in -octane with a concentration of ) at 3000 rpm and baked at 100°C for 5 min. Afterward, ZnMgO nanoparticles ( for 70 nm) were spun-cast on EMLs as ETLs at 3000 rpm and baked at 100°C for 10 min. Next, the samples were transferred to a high-vacuum evaporation chamber to deposit a buffer layer Al of 2 nm with an evaporation rate of 5 ÅÅ s^-1 at a base pressure of . Afterward, the samples were transferred to a magnetron sputtering system again to deposit the top IZO electrode (100 nm) at an environment, which is mentioned previously. In the end, the Tr-QLEDs were encapsulated with UV resin and cover glass.

For Tr-QLEDs with a DER of 1, the bottom ZnS layer is deposited at a rate of 1 Å, while the top LiF layer is deposited at  Å, and the rest of basic process flow can be referred to above.

For Tr-QLEDs with a DER approaching 3:1, the bottom PEDOT:PSS PH1000 electrode was filtered through a 0.45 µm aqueous filter and was spin-coated on glass at 1500 rpm and baked at 120°C for 15 min. The samples were then placed in a glass container and acid was dropped on the surface to gently treat for 10 min. The sample surface was then rinsed several times with ethanol and isopropanol to remove excess acid and improve surface wettability. The samples were annealed at 120°C for 10 min before proceeding to the solution method for the preparation of the subsequent film layers, and the rest of basic process flow can be referred to above.

For Tr-QLEDs with a DER approaching 10:1, the basic steps before preparing the top TCE is identical to the process flow of Tr-QLEDs with a DER of 5:1. Finally, the samples were transferred to a magnetron sputtering system to deposit the Ag/LiF/ZnS cathode at a working pressure of 0.30 Pa, a power of 50 W, an Ar flow of 20 sccm.

D. Characterizations

The thicknesses of the functional layers were measured through a Bruker DektakXT stylus profiler. The evaporation rates and the thicknesses of the Al electrode were in situ monitored by a quartz crystal microbalance. The optical simulation was performed using our developed MATLAB code which is based on the classical dipole emission model [27]. The J-V-L curve, EL spectrum, and EQE-J curve of QLEDs were measured simultaneously by a commercialized system (XPQY-EQE, Guangzhou Xi Pu Optoelectronics Technology Co., Ltd) that was equipped with integrating sphere (GPS-4P-SL, Labsphere), fiber-optic spectrometer (XPFS1000-TEC), and a photodetector array (S7031-1006, Hamamatsu Photonics). The J-V characteristics of QLEDs were characterized by a single-channel Keithley 2400B programmable source meter under ambient conditions. The droplet contact angle was measured with a static droplet contact angle meter. The refractive index of all materials was measured by ellipsometry (Horiba Tf-UVISEL). Thin-film sheet resistance measurement was performed by a four-probe resistance tester.

Funding

National Key Research and Development Program of China (2024YFB3612400); National Natural Science Foundation of China (62174075); Shenzhen Science and Technology Innovation Program (JCYJ20241202125804006, JCYJ20230807093604009).