A method for improving performance of copper iodine cluster electroluminescent diode by using methoxy-substituted carbazole phosphonic acid group self-assembled hole transport layer

Abstract

The present application belongs to the technical field of photoelectric materials and devices, and relates to a method for improving the performance of copper-iodine cluster electroluminescent diodes by using a carbazole phosphonic acid-based self-assembled monolayer with methoxy substitution and conjugate extension. In view of the problems of electrode corrosion, interface exciton quenching and poor running stability caused by the acidity of traditional hole transport layer materials, the present application introduces a self-assembled monolayer with a π-conjugated skeleton and strong electron-donating methoxy substitution between the anode and the copper-iodine cluster light-emitting layer, thereby realizing multi-dimensional regulation of the interface characteristics of the device. The key of the method lies in the use of the strong p-pi conjugation effect between the methoxy group and the benzene ring in the introduced molecule and the extended π-conjugated skeleton, so as to realize the effective improvement of the anode work function through a more significant interface dipole effect, thereby optimizing the interface energy level arrangement and reducing the hole injection barrier. At the same time, the introduction of the methoxy group in the self-assembled interface layer improves the wettability of the anode surface and reduces the surface roughness, which is helpful to obtain a copper-iodine cluster film with low defect state density, thereby inhibiting the non-radiative recombination loss at the interface. The copper-iodine cluster electroluminescent diode prepared based on the interface regulation strategy improves the external quantum efficiency and brightness of the device, significantly improves the turn-on characteristics and prolongs the running life of the device. The present application provides an effective solution for constructing an efficient, stable and environmentally friendly copper-iodine cluster electroluminescent diode device by introducing an interface layer with a specific functional structure.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic technology, and more specifically, relates to a method for improving the performance of copper-iodine cluster electroluminescent diodes by utilizing a methoxy-substituted carbazole phosphate self-assembled hole transport layer. Background Technology

[0002] In recent years, with the deepening of global environmental protection trends, research on environmentally friendly luminescent materials has gradually gained attention. Copper-iodine clusters, as a type of hybrid luminescent material formed by the coordination of inorganic cuprous iodide structural units with organic ligands, have advantages such as abundant resources and tunable structure, making them a highly promising alternative material for constructing environmentally friendly and stable light-emitting diode (LED) devices.

[0003] However, the performance improvement and practical application of copper-iodine cluster LEDs still face multiple challenges in terms of interface matching and charge injection. On the one hand, in common device structures, traditional poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT:PSS) is typically used as the hole transport layer. Although it can assist hole injection to some extent, PEDOT:PSS is highly acidic and easily corrodes the bottom indium tin oxide (ITO) transparent electrode. At the same time, its physicochemical properties easily cause interface quenching of excitons in the copper-iodine cluster light-emitting layer, severely limiting the luminous efficiency and long-term operational stability of the device. On the other hand, due to the solvent orthogonality problem between the copper-iodine cluster and the lower transport layer, some existing technologies attempt to use multilayer transport layer stacked structures (such as introducing poly-TPD, PVK, etc.) to achieve a stepped energy level arrangement in order to improve energy level matching. However, this complex stacking process not only significantly increases the fabrication difficulty and manufacturing cost of the device, but also easily introduces additional interface defect states at the multilayer junctions, thereby hindering the effective transport of charge carriers. Therefore, there is an urgent need in this field to develop a hole transport layer material or interface modification strategy that is simple to process, has good interface compatibility, and can simultaneously solve the problems of strong acid corrosion, exciton quenching and energy level mismatch, so as to break through the performance bottleneck of existing copper-iodine cluster LEDs. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies, such as easy corrosion of the anode substrate by the hole transport layer, leading to exciton interface quenching and poor device stability. It also addresses the problems of complex fabrication processes and the easy introduction of interface defects in multilayer transport structures. This invention provides a scheme for controlling the performance of copper-iodine cluster light-emitting diodes (LEDs) using a phosphonate-carbazole-based self-assembled monolayer. This invention constructs a phosphonate-carbazole-based self-assembled monolayer (SAMs) with a π-extended conjugated framework and strong electron-donating group substitution on the anode substrate surface. Utilizing its strong interfacial dipole effect, it effectively enhances the anode work function, thereby achieving precise matching with the energy levels of the copper-iodine cluster light-emitting layer. This scheme aims to control the interfacial energy level arrangement through molecular structure design, reduce the charge injection barrier, improve the wettability of the substrate surface, and reduce surface roughness to suppress interfacial defect states and non-radiative recombination losses. This simplifies the device structure while achieving a copper-iodine cluster electroluminescent device with low turn-on voltage, high luminous efficiency, and high operational stability.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] The first aspect of this invention provides the application of a phosphonate-carbazole-based self-assembled monolayer in a copper-iodine cluster electroluminescent device. The phosphonate-carbazole-based self-assembled monolayer serves as a hole transport interface layer, used to modulate the anolyte work function and passivate interface defects.

[0007] Preferably, the phosphonate carbazole-based self-assembled monolayer material includes at least one of [4-(3,6-bis(2,4-dimethoxyphenyl)-9H-carbazole-9-yl)butyl]phosphonic acid (XS4), [2-(9H-carbazole-9-yl)ethyl]phosphonic acid (2PACz), and [2-(3,6-dimethoxy-9H-carbazole-9-yl)ethyl]phosphonic acid (MeO-2PACz).

[0008] A second aspect of the present invention provides a copper-iodine cluster electroluminescent device, the device comprising a positive electrode, a hole transport interface layer, a copper-iodine cluster light-emitting layer, an electron transport layer and a negative electrode stacked sequentially; wherein the hole transport interface layer is a phosphonic carbazole-based self-assembled monolayer as described above.

[0009] Preferably, the copper-iodine cluster luminescent layer is composed of copper-iodine clusters (such as TMeOPP-CuI, etc.) and has a thickness of 50-100 nm.

[0010] Preferably, the electron transport layer is composed of 1,3,5-tris[(3-pyridyl)-3-phenyl]benzene (TmPyPB) with a thickness of 50-60 nm.

[0011] Preferably, the positive electrode is indium tin oxide (ITO) transparent glass, and the negative electrode is a layer of metallic aluminum (Al).

[0012] A third aspect of the present invention provides a method for preparing the above-mentioned copper-iodine cluster electroluminescent device, comprising the following steps:

[0013] (1) The phosphonate carbazole-based self-assembled monomolecule material is prepared in a solvent to obtain a self-assembled solution; the solvent is preferably at least one of anhydrous ethanol and ethylene glycol methyl ether; the concentration of the self-assembled monomolecule material is preferably 1-4 mmol·L⁻¹. -1 ;

[0014] (2) Spin-coat the self-assembly solution onto the cleaned ITO substrate surface; the spin-coating speed is preferably 2000-3000 rpm and the time is 15-40 s;

[0015] (3) Anneal the spin-coated substrate to covalently anchor the phosphonic acid groups to the hydroxyl groups on the ITO surface, forming a dense monolayer on the ITO surface; the annealing temperature is preferably 70-80℃ and the time is 5-15min.

[0016] (4) A copper-iodine cluster light-emitting layer, TmPyPB and Al negative electrode are sequentially prepared on the obtained monomolecular interface layer to obtain the copper-iodine cluster electroluminescent device.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects:

[0018] (1) Significantly optimizes energy level matching and improves carrier injection efficiency:

[0019] This invention achieves precise control of the energy levels of the SAM molecule through aryl functionalization and methoxy substitution of the carbazole skeleton. XS4 possesses a HOMO energy level that closely matches the luminescent layer of the copper-iodine cluster, effectively reducing the hole injection barrier and thus promoting balanced carrier injection, significantly improving the exciton radiative recombination efficiency. The device improved by this invention achieves a maximum external quantum efficiency (EQE) of 9.52% and a maximum luminance of 1.38 × 10⁻⁶. 4 cd·m -2 It significantly outperforms the exciton radiative recombination efficiency of traditional devices.

[0020] (2) Eliminate interfacial acid corrosion and significantly improve device stability:

[0021] This invention utilizes a neutral self-assembled monolayer (SAM)XS4 to replace the traditional acidic PEDOT:PSS as the hole transport layer, fundamentally avoiding the electrochemical corrosion of the indium tin oxide (ITO) anode and its interface by acidic hole transport materials. By constructing a chemically stable organic / inorganic interface, performance degradation caused by interface degradation is significantly suppressed, thus significantly extending the device's lifespan. Continuous operation tests show that the device's operating lifetime (T50) is within 100 cd·m.-2 Under the initial conditions, it reached 10.87h.

[0022] (3) Construct a flat and dense interface to suppress nonradiative recombination losses:

[0023] This invention utilizes the high surface coverage and ordered assembly characteristics of XS4 molecules to construct a dense and flat self-assembled monolayer on the anode surface. Experiments demonstrate that the root mean square roughness (RMS) of the copper-iodine cluster film on XS4 is as low as 0.095 nm, and the defect state density of the XS4-based single-hole device is as low as 2.44 × 10⁻⁶. 17 cm -3 This indicates that XS4 effectively suppresses nonradiative recombination quenching of charges at the interface, thereby improving the fluorescence lifetime of the luminescent layer and the luminescence intensity of the device.

[0024] (4) Synergistically improve surface wettability to ensure high-quality film formation of the luminescent layer:

[0025] This invention significantly enhances the surface energy of the self-assembled monolayer by introducing methoxy functional groups at the molecular ends, thereby greatly improving its wettability to the precursor solution of the luminescent layer (contact angle as low as 3.23°). This excellent wettability ensures that the subsequent luminescent layer can form a uniform, continuous, and pinhole-free high-quality film, significantly reducing non-radiative recombination channels caused by poor film formation quality.

[0026] (5) The process is simple and the cost is controllable, with good prospects for industrialization:

[0027] The self-assembled monolayer described in this invention has a simple molecular structure, a mature synthesis route, and low preparation cost. In the device fabrication process, both the self-assembled monolayer and the subsequent light-emitting layer are prepared using a solution method. This process not only significantly simplifies the production process of the core functional layer and reduces operating energy consumption, but also demonstrates excellent prospects for industrial application. Attached Figure Description

[0028] Figure 1 This is a device structure diagram of the copper-iodine cluster light-emitting diode in this invention;

[0029] Figure 2 The ultraviolet photoelectron spectra of Examples 2-4 deposited on indium tin oxide glass substrates;

[0030] Figure 3 The diagram shows the energy level arrangement of each functional layer in the fabricated copper-iodine cluster light-emitting diode device.

[0031] Figure 4 Contact angle diagrams of DMF solutions with copper-iodine clusters on the substrates of Examples 1-4;

[0032] Figure 5AFM images and three-dimensional AFM images of copper-iodine cluster films on substrates of Examples 1-4;

[0033] Figure 6 The graph shows the brightness of the copper-iodine cluster light-emitting diodes using the hole transport layer in Examples 1-4 as a function of voltage.

[0034] Figure 7 The graph shows the external quantum efficiency of the copper-iodine cluster light-emitting diodes using the hole transport layer in Examples 1-4 as a function of current density.

[0035] Figure 8 The copper-iodine cluster light-emitting diodes using Examples 1-4 as hole transport layers have an initial luminance of approximately 100 cd·m. -2 The device's operational stability diagram at that time. Detailed Implementation

[0036] It should be noted that the descriptions of these embodiments are for the purpose of helping to understand the present invention and do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments described below can be combined with each other as long as they do not conflict with each other. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods; unless otherwise specified, the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0037] The following analysis of the fabrication process and performance testing of copper-iodine cluster light-emitting diode devices A1 to A4 prepared in Examples 1 to 4 of this invention further details the application and effects of the self-assembled monolayer of this invention. A schematic diagram of the light-emitting diode device structure of this invention is shown below. Figure 1 As shown.

[0038] Example 1

[0039] 1. Preparation of the mixture:

[0040] Hole transport layer material: poly(3,4-ethylenedioxythiophene): polystyrene sulfonate aqueous solution;

[0041] 2. Spin coating:

[0042] Mixture volume: 80 μL;

[0043] Spin coating speed: 3000 rpm;

[0044] Spin coating time: 45s.

[0045] 3. Annealing:

[0046] Annealing temperature: 150℃

[0047] Annealing time: 15 min

[0048] Example 2

[0049] 1. Preparation of the mixture:

[0050] Hole transport layer material: [2-(9H-carbazole-9-yl)ethyl]phosphonic acid, concentration 2 mmol·L⁻¹ -1 ;

[0051] Solvent: Anhydrous ethanol;

[0052] 2. Spin coating:

[0053] Mixture volume: 80 μL;

[0054] Spin coating speed: 3000 rpm;

[0055] Spin coating time: 30s.

[0056] 3. Annealing:

[0057] Annealing temperature: 80℃

[0058] Annealing time: 10 min

[0059] Example 3

[0060] 1. Preparation of the mixture:

[0061] Hole transport layer material: [2-(3,6-dimethoxy-9H-carbazole-9-yl)ethyl]phosphonic acid, concentration 2 mmol·L⁻¹ -1 ;

[0062] Solvent: Anhydrous ethanol;

[0063] 2. Spin coating:

[0064] Mixture volume: 80 μL;

[0065] Spin coating speed: 3000 rpm;

[0066] Spin coating time: 30s.

[0067] 3. Annealing:

[0068] Annealing temperature: 80℃

[0069] Annealing time: 10 min

[0070] Example 4

[0071] 1. Preparation of the mixture:

[0072] Hole transport layer material: [4-(3,6-bis(2,4-dimethoxyphenyl)-9H-carbazole-9-yl)butyl]phosphonic acid, concentration 2 mmol·L-1 ;

[0073] Solvent: Anhydrous ethanol and ethylene glycol methyl ether (1:1);

[0074] 2. Spin coating:

[0075] Mixture volume: 80 μL;

[0076] Spin coating speed: 3000 rpm;

[0077] Spin coating time: 30s.

[0078] 3. Annealing:

[0079] Annealing temperature: 80℃

[0080] Annealing time: 10 min

[0081] Application examples

[0082] The fabrication method for device A1, i.e., the copper-iodine cluster light-emitting diode, is as follows:

[0083] 1) Indium tin oxide (ITO) transparent anode: The ITO glass substrate with a coating thickness of 150nm was wiped with ethanol, then cleaned twice with acetone, ultrasonically cleaned for 20min, and then transferred to an isothermal stage for drying. After baking, it was cooled and transferred to UV-O3 treatment for 15min.

[0084] 2) Preparation of the hole transport layer: as described in Example 1;

[0085] 3) Copper-iodine clusters were spin-coated onto the hole transport layer at a spin-coating parameter of 2000 rpm / 45 s and an acceleration of 1000. A 45 nm TmPyPB layer was deposited as the electron transport layer using a vacuum evaporation apparatus. A 100 nm Al layer was then vacuum-deposited on the electron transport layer as the back electrode.

[0086] The fabrication method of device A2 is the same as that of device A1, except that the hole transport layer on the ITO is fabricated using Example 2.

[0087] The fabrication method of device A3 is the same as that of device A1, except that the hole transport layer on the ITO is fabricated using Example 3.

[0088] The fabrication method of device A4 is the same as that of device A1, except that the hole transport layer on the ITO is fabricated using Example 4.

[0089] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0090] This invention relates to a method for improving the performance of copper-iodine cluster electroluminescent diodes by utilizing a methoxy-substituted carbazole phosphate self-assembled hole transport layer. To further clarify the technical content of this invention, performance tests were conducted using Examples 1-4, and the results are illustrated below:

[0091] (1) Energy level structure characterization and analysis: Ultraviolet photoelectron spectroscopy was used to test different substrates. Figure 2 ), and based on this, draw the energy level arrangement diagram of each functional layer of the device ( Figure 3 The results show that Examples 2-4 achieved effective control of the work function through the interfacial dipole effect. Among all self-assembled monolayers, the HOMO level (-5.28 eV) of Example 1 best matched the emitting layer (-4.90 eV), significantly reducing the hole injection barrier. This optimized energy level arrangement promoted balanced carrier injection and recombination, explaining the physical mechanism behind the lowest turn-on voltage (2.6 V) and optimal photoelectric performance of the device in Example 4.

[0092] (2) Surface wettability test: The affinity of the interface was characterized by measuring the contact angle of the copper-iodine cluster luminescent layer solution on different hole transport layer substrates. The results are as follows: Figure 4 As shown in the figure, the contact angles of the solution on Examples 1-4 were 4.69°, 3.23°, 4.46°, and 5.55°, respectively. The results indicate that all substrates exhibited good wettability (contact angles all below 6°), with Example 4 (XS4) showing the smallest contact angle. This is attributed to the high surface energy of the molecules in Example 4 and their optimal compatibility with the cluster solution, which greatly promoted the uniform spreading of the luminescent layer solution at the interface, laying the foundation for the formation of a high-quality, dense luminescent layer film.

[0093] (3) Characterization of film morphology and smoothness: The morphology of copper-iodine cluster films spin-coated on different substrates was characterized using atomic force microscopy. The results are as follows: Figure 5As shown, all films exhibit a uniform and dense surface, but with significant differences in flatness: the copper-iodine cluster film based on Example 1 has the highest roughness at 0.183 nm; while the copper-iodine cluster films based on Examples 2-4 are generally flatter, with root mean square roughness of 0.128 nm and 0.115 nm for Examples 2 and 3, respectively. Among them, the copper-iodine cluster film on the substrate of Example 4 has the lowest roughness at only 0.095 nm, exhibiting the best surface flatness. Lower surface roughness can significantly reduce the defect state density at the interface, effectively suppress nonradiative recombination losses, thereby promoting balanced transport and efficient recombination of charge carriers at the interface.

[0094] (4) Device photoelectric performance and efficiency characterization: The copper-iodine cluster light-emitting diodes prepared in Examples 1-4 were subjected to device performance testing, and the results are as follows: Figures 6-7 As shown, compared to device A1, device A4 exhibits superior energy level matching and balanced carrier injection. Its on-time voltage is significantly reduced to 2.6V (compared to 3.5V for device A1), and its maximum luminance reaches 1.38 × 10⁻⁶. 4 cd·m -2 Regarding luminous efficiency, device A4 achieved a maximum external quantum efficiency (EQE) of up to 9.52%, which is approximately 55% higher than that of device A1 (6.14%). This indicates that the interface layer constructed in this invention effectively reduces the hole injection barrier and significantly improves the carrier injection balance, thereby enhancing device efficiency.

[0095] (5) Device operational stability test: Device brightness decay curve ( Figure 8 The display shows that at 100 cd·m -2 At initial brightness, the half-life (T) of device A4 50 The duration of action reached 10.87 h, approximately 3.7 times that of Example 1 (2.92 h). The improved long-term stability is mainly attributed to the neutral and stable chemical properties of the molecules in Example 4, which avoids the electrode corrosion and interface quenching caused by the acidic materials in Example 1; at the same time, its dense monolayer structure isolates the direct contact between the light-emitting layer and the ITO electrode, effectively suppressing non-radiative recombination quenching.

[0096] (7) Performance summary of devices A1 to A4:

[0097]

[0098] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for improving the performance of copper-iodine cluster electroluminescent diodes by utilizing methoxy-substituted carbazole phosphonate group self-assembled monolayers, characterized in that: A self-assembled monomolecular interface layer constructed from methoxy-substituted carbazole phosphonic acid molecules is introduced between the anode and the copper-iodine cluster light-emitting layer. The interface dipole effect of the interface layer is used to regulate the anode work function, thereby reducing the hole injection barrier and passivating anode surface defects, thus synergistically improving the efficiency and stability of the device.

2. The method according to claim 1, characterized in that, The methoxy-substituted carbazole phosphonate molecule is used as a hole injection interface material in the device. Compared with the unsubstituted carbazole phosphonate molecule (2PACz), the molecule significantly enhances the vertical dipole moment of the molecule by introducing a methoxy electron-donating group and π-conjugated extended framework, thereby achieving deep-level energy level modulation of the interface.

3. The method according to claim 2, characterized in that, The methoxy-substituted carbazole phosphonic acid molecule is selected from the following structures: [2-(3,6-dimethoxy-9H-carbazole-9-yl)ethyl]phosphonic acid (MeO-2PACz) or [4-(3,6-bis(2,4-dimethoxyphenyl)-9H-carbazole-9-yl)butyl]phosphonic acid (XS4).

4. The method according to claim 1, characterized in that, The self-assembled monomolecular interface layer is chemically neutral and is used to replace the traditional acidic poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) (PEDOT:PSS) hole transport layer, avoiding the corrosive effect of the acidic environment on the indium tin oxide (ITO) anode. At the same time, the dense interface layer plays a physical isolation role, inhibiting the diffusion of electrode metal ions to the light-emitting layer and chemical quenching, significantly improving the operational stability of the device.

5. The method according to claim 1, characterized in that, The self-assembled monomolecular interface layer forms a dense monomolecular covering layer by undergoing a dehydration condensation reaction between phosphonic acid groups and hydroxyl groups on the anode surface, anchored by covalent bonds.

6. The application according to claim 1, characterized in that, The phosphonic carbazole-based self-assembled monolayer material is at least one of [4-(3,6-bis(2,4-dimethoxyphenyl)-9H-carbazole-9-yl)butyl]phosphonic acid, [2-(9H-carbazole-9-yl)ethyl]phosphonic acid, and [2-(3,6-dimethoxy-9H-carbazole-9-yl)ethyl]phosphonic acid.

7. The method according to claim 1 or 3, characterized in that, By adjusting the number of methoxy groups or the length of aryl conjugation, the modified anodic work function was further increased to below -5.3 eV, matching it with the HOMO energy level of the copper-iodine cluster luminescent layer, and the hole injection barrier was reduced to below 0.4 eV.

8. A copper-iodine cluster electroluminescent diode prepared by the method described in any one of claims 1 to 7, characterized in that, The device exhibits a reduced turn-on voltage and an improved external quantum efficiency (EQE); in particular, when using XS4 with a π-conjugated system as the interface layer, the device achieves an EQE of over 9.5% and a maximum luminance exceeding 1.3 × 10⁻⁶. 4 cd·m -2 At 100 cd·m -2 Operating life T at initial brightness 50 It can last for more than 10 hours.

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