Transparent film electroluminescent display screen

Abstract

The research progress and the application prospect of the thin film electroluminescence technology are comprehensively described. Firstly, basic principles of thin film electroluminescence are introduced, including an electroluminescence phenomenon, a thin film electroluminescence device structure and a working mechanism. The research status of organic and inorganic film electroluminescent materials is elaborated in detail, and a material performance optimization strategy is emphatically discussed. The article also discusses the application of the thin film electroluminescent technology in the fields of display and illumination, and analyzes the challenge and future development direction faced by the thin film electroluminescent technology. And finally, the importance and potential influence of the thin film electroluminescence technology are summarized, and reference is provided for research in related fields.

Description

Transparent thin-film electroluminescent display Technical Field

[0001] Thin-film electroluminescence (TFT-EM) is an advanced optoelectronic technology that directly converts electrical energy into light energy, and it has attracted widespread attention in the display and lighting fields in recent years. With its advantages such as high brightness, high efficiency, wide viewing angle, and flexibility, this technology has become a strong competitor to next-generation display and lighting technologies. Since the first discovery of organic electroluminescence in the 1960s, TFT-EM has experienced rapid development, particularly in organic light-emitting diodes (OLEDs) and quantum dot light-emitting diodes (QLEDs), where significant progress has been made.

[0002] With continuous advancements in materials science, nanotechnology, and manufacturing processes, thin-film electroluminescent devices have seen significant improvements in efficiency, lifespan, and color performance. Simultaneously, this technology demonstrates immense potential in emerging application areas such as flexible displays, transparent displays, and microdisplays. Background Technology

[0003] I. Basic Principles of Thin-Film Electroluminescence

[0004] Thin-Film Electroluminescence (TFEL) Principle Explained:

[0005] Thin-film electroluminescence (TFT-EM) is a technology that generates light radiation by exciting thin-film materials with an electric field. Its core lies in directly converting electrical energy into light energy, and it has wide applications in displays, lighting, and flexible electronics. The following is a detailed explanation of its principle:

[0006] (I) Basic Principles

[0007] 1. Definition of Electroluminescence (EL)

[0008] Electroluminescence is the phenomenon where photons are released when charge carriers (electrons and holes) are injected, transported, and recombine under the influence of an electric field. Its energy conversion process is as follows:

[0009] Electrical energy → Electric field excites excitons (electron-hole pairs) → Radiative recombination light energy

[0010] Significance of thin film

[0011] Thin film thickness is typically in the nanometer to micrometer range (10 nm–1 μm), which can reduce material usage and device size.

[0012] Precise design of multilayer thin film structures (electrodes, transport layer, luminescent layer) can optimize carrier injection and luminescence efficiency.

[0013] II. Research Progress of Thin-Film Electroluminescent Materials

[0014] Thin-film electroluminescent materials are mainly divided into two categories: organic and inorganic. Among organic materials, small-molecule organic light-emitting materials and polymeric light-emitting materials are the two most important categories. Small-molecule materials such as Alq3 and Ir(ppy)3 have advantages such as high purity, easy purification, and good crystallinity, while polymeric materials such as PPV and PF have good solution processing properties and mechanical flexibility. In recent years, the emergence of novel organic light-emitting materials such as thermally activated delayed fluorescence (TADF) materials and organic radical light-emitting materials has provided new options for high-efficiency organic electroluminescent devices.

[0015] Inorganic thin-film electroluminescent materials mainly include quantum dots, perovskites, and rare-earth-doped materials. Quantum dot materials have attracted much attention due to their tunable emission wavelengths, narrow emission spectra, and high quantum efficiency. Perovskite materials have become a research hotspot due to their excellent optoelectronic properties and solution processing characteristics. Rare-earth-doped materials, such as Eu and Tb-doped sulfides and oxides, also have important applications in display and lighting fields. To improve material performance, researchers have adopted various strategies, such as surface modification, core-shell structure design, and alloying, which have effectively improved the stability, luminous efficiency, and color purity of the materials.

[0016] Traditional transparent TFEL devices face three major technical bottlenecks (referencing US2017015502A1 and CN108735682A):

[0017] Electrode performance contradiction: The sheet resistance and transmittance of a single ITO electrode are inversely proportional (transmittance <80% when sheet resistance <20Ω / sq);

[0018] Carrier injection imbalance: The barrier at the transparent electrode / organic layer interface leads to low electron-hole recombination efficiency (IQE < 25%).

[0019] Poor environmental stability: Water and oxygen penetration cause electrode corrosion and degradation of the light-emitting layer (damp heat test at 85℃ / 85%RH, T50 < 500 hours).

[0020] III. Synergistic Mechanism between Transparency and Luminous Efficiency

[0021] 1. Principle of Transparency Implementation

[0022] Electrode transparency:

[0023] A balance between conductivity and transparency can be achieved through ultrathin metals (<15nm) or composite conductive materials (such as metal meshes).

[0024] Optical interference effects (such as ITO / Ag / ITO stacking) are used to suppress reflection loss.

[0025] Material transparency:

[0026] The thickness of the light-emitting layer is optimized (typically <100nm) to avoid a decrease in transparency caused by light absorption.

[0027] Choose a wide bandgap material (such as ZnO electron transport layer, with a bandgap of 3.3 eV, which is transparent to visible light).

[0028] 2. Special design of the light-emitting process

[0029] Bidirectional light-emitting structure:

[0030] Using transparent dual electrodes allows light to be emitted from both the top and bottom (as in transparent OLEDs).

[0031] By controlling the light emission direction through the microcavity effect, the luminous intensity at a specific angle is enhanced (efficiency is improved by 20%-30%).

[0032] Light management of transparent media:

[0033] Introducing distributed Bragg reflectors (DBR) or anti-reflective coatings (ARC) reduces interface reflection loss (increasing transmittance by 5%-10%).

[0034] IV. Key Physical Processes

[0035] Trade-off between transparency and electrical performance

[0036] Parameters: Traditional Devices vs. Transparent Devices: Electrode Sheet Resistance (Ω / sq): <20 (ITO) 10-50 (Ultra-thin Metal / Composite Electrodes); Visible Light Transmittance: <85% (Anodic Transparency Only) >80% (All Devices); Luminous Efficacy (cd / A): 100-200 (OLED) 50-150 (Limited by Electrode Absorption) surface Summary of the Invention

[0037] I. Device Structure and Composition

[0038] Typical thin-film electroluminescent devices (such as OLEDs or QLEDs) consist of the following functional layers:

[0039] Substrate: Glass or flexible material (such as PET) provides mechanical support.

[0040] Transparent anode: Indium tin oxide (ITO) or silver nanowires (AgNWs), with injected holes.

[0041] Hole transport layer (HTL): such as NPB, PEDOT:PSS, promotes hole migration to the emissive layer.

[0042] Emissive layer (EML): Organic materials (such as Alq3, Ir(ppy)3) or inorganic materials (such as quantum dots, perovskites), the core region for exciton recombination luminescence.

[0043] Electron transport layer (ETL): such as ZnO and TPBi, which transport electrons to the light-emitting layer.

[0044] Cathode: Low work function metal (Al, Ca), which injects electrons.

[0045] II. Working Mechanism and Key Physical Processes

[0046] 1. Carrier injection and transport

[0047] Injection: Under applied voltage, electrons are injected into the ETL from the cathode, and holes are injected into the HTL from the anode.

[0048] Energy level matching: The work function of the electrode needs to be aligned with the energy level of the transport layer (e.g., the work function of ITO at ~4.7 eV needs to be matched with the HOMO energy level of HTL).

[0049] Mobility balance: The mobilities of electrons and holes need to be close (e.g., the mobilities of ETL and HTL are both 10). -3 cm 2 (on the order of / Vs) to avoid carrier accumulation.

[0050] 2. Exciton formation and luminescence

[0051] Recombination region: Electrons and holes meet in the luminescent layer to form excitons (bound electron-hole pairs).

[0052] 3. Exciton type:

[0053] Singlet excitons: have opposite spins and emit light through fluorescence (lifetime 1–10 ns).

[0054] Triplet excitons: spin parallel, emitted via phosphorescence or delayed fluorescence (lifetime μs–ms).

[0055] Emission wavelength: determined by the band gap (EgEg) of the material. For example, quantum dots emit color by controlling their size (the smaller the particle size, the larger the band gap, and the shorter the emission wavelength).

[0056] 4. Light output process

[0057] Waveguide effect: Approximately 80% of the light is confined inside the device, and the light extraction efficiency needs to be improved through microcavity structures or light extraction layers (such as lens arrays).

[0058] Interface loss: The difference in refractive index between the substrate and air causes reflection loss, which can be optimized by anti-reflective coating (ARC).

[0059] III. Key Factors Affecting Luminous Efficiency

[0060] 1. Internal quantum efficiency (IQE)

[0061] IQE=η 注入 ×η 传输 ×η 复合 ×η 辐射

[0062] IQE=η 注入 ×η 传输 ×η 复合 ×η 辐射

[0063] Carrier injection efficiency (η) 注入 η 注入 ): Dependent on the electrode / transport layer interface barrier.

[0064] Carrier balance factor (η) 传输 η 传输 ): Requires a match between electron and hole mobility.

[0065] exciton utilization rate (η) 复合 η 复合 ): Reduce exciton quenching (such as electrode metal quenching, concentration quenching).

[0066] Radiation efficiency (η) 辐射 η 辐射 ): The fluorescence quantum yield of the material (Φ PL Φ PL )Decide.

[0067] 2. External quantum efficiency (EQE)

[0068] EQE = IQE × η 光提取

[0069] EQE = IQE × η 光提取

[0070] Light extraction efficiency (η) 光提取 η 光提取 Typically, the efficiency is only 20% to 30%, and it needs to be improved through device structure optimization.

[0071] IV. Material Classification and Properties

[0072]

[0073] Detailed Implementation

[0074] I. Design of Multilayer Composite Transparent Electrode

[0075] 1. Structural Innovation:

[0076] The structure is a sandwich structure of MoO3(5nm) / AgNWs(10nm) / MoO3(5nm), wherein:

[0077] The bottom layer MoO3 serves as the hole injection layer (HIL) with a work function of 5.3 eV, matching the HOMO energy level of the hole transport layer.

[0078] Ag nanowire networks achieve a sheet resistivity of 8.2 Ω / sq and a 550 nm transmittance of 92.4% by controlling the permeation threshold (line coverage > 60%).

[0079] The top layer of MoO3 acts as an optical coupling layer, suppressing interface reflection (reflectivity decreased from 12.3% to 4.7%).

[0080] 2. Preparation process:

[0081] Atomized spraying-annealing method is used:

[0082] Atomization pressure 0.3 MPa, AgNWs ink concentration 0.1 wt%;

[0083] Annealing at 200℃ in a nitrogen atmosphere for 30 minutes eliminates the contact resistance of the nanowires.

[0084] II. Gradient-doped light-emitting layer structure

[0085] 1. Energy Belt Engineering:

[0086] The light-emitting layer is doped with a concentration gradient from the ETL side to the HTL side (DPVBi:Ir(ppy)3 from 15% to 5%) to achieve:

[0087] High doping (15%) in the electron transport region promotes exciton generation;

[0088] Low doping (5%) in the hole transport region suppresses concentration quenching;

[0089] The carrier recombination region extends to the entire thickness of the emitting layer (IQE increased to 68%).

[0090] 2. Sedimentation control:

[0091] A dual-source co-evaporation system is employed, with real-time monitoring of the doping rate (accuracy ±0.1). This achieves gradient accuracy of ±2%.

[0092] III. Self-healing encapsulation technology

[0093] 1. Material composition:

[0094] 10 wt% ZrO2 nanoparticles (50 nm in diameter) and 0.5 wt% dynamic disulfide bond compounds were dispersed in a PDMS matrix to form:

[0095] Rigid ZrO2 network blocks water and oxygen permeation (WVTR < 10) -6 g / m 2 / day);

[0096] Dynamic disulfide bonds trigger self-healing at 80℃ (crack healing rate >95%).

[0097] 2. Process parameters:

[0098] Spin coating speed 3000 rpm, curing conditions: 120℃ / 30min + UV irradiation (365nm, 100mW / cm²). 2 (5 min).

[0099] Examples and Effect Verification

[0100] Example 1: Device photoelectric performance testing

[0101] Parameters of the present invention device (comparative example, ITO single electrode): Sheet resistance (Ω / sq): 8.21-8.55; Transmittance: 92.4%-83.7%; Luminous efficacy (cd / A): 42.3-26.8; Color coordinates (CIE x, y): (0.31, 0.33) (0.29, 0.35) surface

[0102] Example 2: Environmental Stability Test

[0103] Condition T50 (hours) luminance decay rate (% / thousand hours) Normal temperature and humidity (25℃ / 60%RH) >10,000 3.2 High temperature and high humidity (85℃ / 85%RH) 2,150 15.7 surface

[0104] Industrial Application Prospects

[0105] This technology has passed pilot testing and can be applied to:

[0106] Transparent smart display windows: Enables instant switching between dynamic advertising and transparent modes in commercial displays;

[0107] Military helmet display: Overlays tactical information while maintaining transparency for environmental observation (has passed MIL-STD-810G impact resistance test);

[0108] Flexible medical sensor: integrated into a transparent bandage to achieve wound healing monitoring and phototherapy functions.

[0109] Summary of technical advantages

[0110] Revolutionary electrode structure: Breaking through the performance limits of traditional transparent electrodes (sheet resistance-transmittance composite index FOM value reaches 352, which is 5.6 times higher than ITO);

[0111] Intelligent light emission control: Gradient doping technology reduces the efficiency-lifetime trade-off by 40%;

[0112] Environmentally adaptive packaging: Self-healing capability extends the device's outdoor lifespan by more than 3 times.

[0113] Applications and Challenges of Thin Film Electroluminescence Technology

[0114] Thin-film electroluminescence technology has shown enormous potential in the display field. OLED display technology has been widely used in high-end display products such as smartphones and televisions, and its self-emissive, high-contrast, and flexible characteristics have brought about a revolutionary change in display technology. QLED display technology, with its excellent color performance and stability, is becoming a strong contender for the next generation of display technology. In addition, thin-film electroluminescence technology also shows unique advantages in emerging application areas such as micro-displays, transparent displays, and flexible displays.

[0115] In the lighting field, thin-film electroluminescence (TFT-EM) technology offers a new solution for solid-state lighting. OLED lighting, with its large area, uniform light emission, and adjustable color temperature, has broad application prospects in architectural and decorative lighting. QLED lighting, due to its high efficiency and narrow spectral characteristics, has potential advantages in professional lighting and display backlighting. However, TFT-EM technology still faces some challenges, such as device lifetime, efficiency degradation, large-area uniformity, and manufacturing costs. Future research directions may include developing new, efficient, and stable materials, optimizing device structures and manufacturing processes, and exploring new application scenarios.

[0116] in conclusion

[0117] Thin-film electroluminescence (TFT-EM) technology has made significant progress in materials, devices, and applications over the past decades. Continuous innovation in organic and inorganic TFT-EM materials has laid the foundation for high-performance light-emitting devices, while the maturity of technologies such as OLED and QLED has brought revolutionary changes to the display and lighting fields. Despite some remaining challenges, with advancements in materials science and manufacturing technologies, TFT-EM technology is expected to achieve wider applications in the future. Future research should focus on improving device efficiency and stability, reducing manufacturing costs, developing new application scenarios, and exploring innovative integration with other technologies. The development of TFT-EM technology will bring new opportunities to the display and lighting industries and may spawn more innovative applications, driving continuous progress in related fields.

Claims

1. A transparent thin-film electroluminescent device, characterized in that... include: Transparent substrate: a double-sided stacked transparent electrode composed of MoO3 (5nm) / Ag nanowire network (thickness 8-12nm) / MoO3 (5nm); a light-emitting layer containing gradient doped regions, wherein the concentration of light-emitting material decreases linearly from the electron transport layer to the hole transport layer (5%-15%); a self-healing encapsulation layer covering the device surface, composed of polydimethylsiloxane (PDMS) and ZrO2 nanoparticles.

2. The device according to claim 1, characterized in that... The Ag nanowire network has a wire diameter of 20-30 nm and a wire spacing of 1-2 μm, forming a continuous conductive network.

3. The device according to claim 1, characterized in that... The thickness of the gradient-doped light-emitting layer is 50-80 nm, and the slope of the doping concentration gradient is 0.2% / nm.

4. A method for fabricating a transparent thin-film electroluminescent device, comprising the following steps: a) A 5 nm thick MoO3 layer was deposited on a transparent substrate by magnetron sputtering; b) An Ag nanowire network was deposited by atomization spraying and annealed at 200°C to form a continuous conductive layer; c) A gradient-doped light-emitting layer was prepared by vapor deposition, and the concentration gradient was achieved by controlling the evaporation rate of the dopant source; d) A 5 μm thick self-healing encapsulation layer was formed by spin-coating a PDMS / ZrO2 composite solution.

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