The Future Applications for Luminescent Material in Optoelectronics and Wearable Devices

Abstract

Optoelectronic devices, such as LEDs (light-emitting diodes) and OLEDs (organic light-emitting diodes), have a promising future with luminescent materials. These materials play a crucial role in enhancing device performance, efficiency, and functionality. Advances in luminescent materials, including quantum dots, perovskites, and organic dyes, are driving innovations in displays, lighting, sensing, and communication technologies. The future holds potential for even more efficient and versatile optoelectronic devices with the continued development of novel luminescent materials and fabrication techniques. Flexible and wearable devices are one of the future usages for luminescent materials compatible with flexible substrates. Future research may focus on enhancing the durability, stretchability, and comfort of these devices, opening up new opportunities in wearable technology, smart textiles, and flexible displays. This could involve optimizing the spectral characteristics, stability, and energy efficiency of luminescent materials to meet the demanding requirements of wearable devices.

1. Introduction

Optoelectronic devices that harness light energy have revolutionized various fields, from telecommunications to sensing and imaging [1]. The advent of luminescent materials, particularly two-dimensional (2D) materials, has opened new avenues for optoelectronics, LEDs, infrared photodetectors, and wearable photovoltaics [2]. Spectroscopy is closely associated with luminescence, which is the examination of matter’s general laws that govern radiation absorption and emission [3]. The three primary modes of luminescence are fluorescence, phosphorescence and chemiluminescence. Photoluminescence can be classified into two types: fluorescence and phosphorescence. In photoluminescence, the glow of a substance is caused by light, while in chemiluminescence, the glow is caused by a chemical reaction. Fluorescence and phosphorescence are dependent on the absorption and emission of light by substances, with longer wavelengths at lower energy levels and time is the most significant divergence [4, 5, 6]. Fluorescence results in an immediate emission, which is typically detectable only if the light source is continuously powered on. On the other hand, phosphorescent materials can store the absorbed light energy for a while and then re-emit the light, resulting in a persistent afterglow even after the light has disappeared [7]. The mechanism for relaxing the excited state is similar to that of fluorescence, but chemiluminescence is characterized by the unique generation of excited states. Unlike exothermic reactions, which release energy in the form of heat, certain chemical reactions produce electronically excited products. When the excited state is relaxed by luminescence due to the emission of photons, it is called chemiluminescence. When a biological enzyme catalyzes a reaction, it is called bioluminescence, even though the mechanism is the same [8].

2. The role of luminescent materials in optoelectronics

In the realm of optoelectronics, luminescent materials play a crucial role in the generation, manipulation, and detection of light. They are used to fabricate light-emitting diodes (LEDs), lasers, and photodetectors. Luminescent materials used in LEDs emit light of a specific wavelength when an electric current is passed through them. This property allows for the production of highly efficient and long-lasting lighting sources. In addition, luminescent materials are employed in lasers to achieve precise control over the emitted light, enabling applications in telecommunications, medical treatments, and scientific research [9]. Moreover, luminescent materials find application in photodetectors, which convert light into electrical signals. These devices are essential in various industries, including aerospace, automotive, and security systems. Luminescent materials enhance the sensitivity and response time of photodetectors, enabling them to detect light at low levels and rapid speeds. The ability to integrate luminescent materials into optoelectronic devices has revolutionized the field and opened up new avenues for technological advancements [10]. Figure 1 shows the mechanism of (a) LED with white color and (b) RGB-LED.

Figure 1.

Mechanism of providing light-emitting diode with white color. (a) Combination of three red-green-blue (RGB) LEDs and (b) mixing of different luminescent materials to generate the white color.

3. Applications of luminescent materials in wearable devices

The integration of luminescent materials into wearable devices has brought about a paradigm shift in the way we interact with technology. Luminescent materials offer unique properties that make them ideal for wearable applications. One such application is in flexible displays. This study demonstrates the great potential of photochromic fibers in wearable human-machine integration and advances the field of wearable human-computer interaction [11], where luminescent materials are used to create vibrant and energy-efficient screens. These displays can be seamlessly integrated into clothing or accessories, providing users with real-time information and enhancing their overall experience and medicine, as like as photomedicine for curing the cancers. The high potential of QLED to accelerate the adoption of photomedicine is expected to broadly cover multiple healthcare markets, including cancer treatment, periodontics, dermatology (especially cosmetic dermatology), and chronic wound and ulcer treatment [12]. The luminescent spectra of the 22 μm B-Si layered sample and the 31 μm B-Si layered sample are displayed in Figure 2a and b, respectively. Upon observing the figures, it becomes apparent that B-Si exhibits a strong visible emission at room temperature; however, the emitted spectrum shows slight variations. In Figure 3a, there are two distinct emission peaks. The first peak represents a sharp UV-blue emission ranging from 380 to 500 nm, while the second peak corresponds to a broader red emission centered on 650 nm. The inset of Figure 2a displays a camera image that captures a similar emission pattern. On the other hand, Figure 2b only exhibits a single emission peak in its photoluminescence spectra, which corresponds to the UV-blue emission. This observation is further confirmed by the camera image shown in the inset. The number of photoluminescence peaks in a spectrum provides valuable information regarding the diameter or crystal size of nanostructures [13].

Figure 2.

Luminescence spectra of transferred B-Si NSs on flexible substrate [13].

Figure 3.

Luminescence images of the sensor foil. Left: temperature gradient imaged through a Chroma 580 bandpass filter. Right: oxygen partial pressure for the same sensor foil imaged through an RG 650 nm long pass filter at an excitation wavelength of 366 nm [14].

Furthermore, luminescent materials are employed in the development of smart textiles [15], which can monitor vital signs, detect environmental conditions, and even generate power. By embedding luminescent materials into the fabric, wearable devices can become more responsive and adaptive to the wearer’s needs [16]. One of the most promising aspects of these luminescent materials is their sensitivity to temperature changes [17]. When exposed to different temperatures, the gels exhibit a distinct shift in their emission color. This makes them potentially useful as temperature sensors in various applications. For instance, luminescent materials can be used to create sensors that change color in response to temperature or chemical changes, alerting the wearer to potential dangers or providing valuable feedback [17]. Figure 3 represents the luminescence images of the sensor foil where the left picture shows temperature gradient imaged through a Chroma 580 bandpass filter and right picture shows oxygen partial pressure for the same sensor foil imaged through an RG 650 nm long pass filter at an excitation wavelength of 366 nm [14].

The unique properties of these luminescent materials make them ideal candidates for a variety of sensing applications. For instance, they could be used as coatings to monitor the structural integrity of pipes, cables, and other underwater structures critical to offshore energy operations. In addition, they could be used to detect chemical variations in liquids or to observe velocity gradients in fluid flow experiments [18, 19]. In the survey that Y. He1, S.C. Wang, and their colleagues have done before, it shows the luminescent material does not scatter any light in visible width waves, but it emits in UV light. For the coating in Figure 4a, the vanishing of luminescence under UV light is a sign of coating damage and may require recoating. By comparison, the attention of colorful light in Figure 4b suggests that the active coating is worn away [14].

Figure 4.

Schematic diagrams showing luminescent particles code posited as top layer in (a) or interlayer in (b), and the coating appearance in the damaged area.

4. Advancements in luminescent materials for optoelectronics and wearable devices

The field of luminescent materials is constantly evolving, with researchers and industry players striving to improve their properties and performance. Recent advancements have focused on enhancing the efficiency, stability, and tunability of luminescent materials. One such breakthrough is the development of perovskite-based luminescent materials as pointed in previous sections, which have shown exceptional performance in optoelectronic devices. These materials offer high quantum efficiency [20], narrow emission peaks [21], and excellent long-term stability [22], making them promising candidates for future applications.

Additionally, the integration of luminescent materials with nanotechnology has opened up new possibilities for optoelectronics and wearable devices [23]. Nanomaterials, such as quantum dots, exhibit unique size-dependent luminescent properties, allowing for precise control over the emitted light. These materials can be incorporated into flexible substrates [24], enabling the development of bendable and stretchable optoelectronic devices. Furthermore, the use of nanomaterials enhances the efficiency and color purity of luminescent materials, making them highly desirable for displays and lighting applications [12]. Luminescent materials can produce with various techniques; some of them are shown in Table 1.

Techniques	Extract from	Reference
Bioluminescence	Luciferase	[25]
Chemiluminescence	Enthalpy of the reactions	[26]
Crystalloluminescence	Formation of bonds between the atoms	[27]
Electroluminescence	Passing of electric current	[28]
Cathodoluminescence	Photons	[29]
Radioluminescence	Ionizing radiations like α, β, or γ rays	[30]
Sonoluminescence	Small bubbles in a solvent on excitation by ultrasonic waves	[28]
Thermoluminescence	Eating crystalline materials	[31]

Table 1.

The luminescence producing techniques.

4.1 Circularly polarized luminescence (CPL)

One of the advancements in luminescent materials for optoelectronics is circularly polarized luminescence (CPL). Traditionally, circularly polarized light can be generated from unpolarized light through the physical method (Figure 5(a)). The emitted unpolarized light is first converted into linearly polarized light by the linear polarizer, and then further decomposed into left or right circularly polarized light through the quarter-wave plate. During this indirect physical process, at least 50% of energy will be lost [14]. Therefore, it is urgent to develop novel luminescent materials that can directly generate circularly polarized light. But in a new method referred to the geometric property of an object and Chiral luminophore, the direct polarized light can be produced with decreasing the lost energy as shown in Figure 5(b) [32].

Figure 5.

Two techniques for producing circularly polarized light. (a) Traditional method and (b) circularly polarized luminescence.

4.2 Perovskite quantum dots (PQDs)

Perovskite-based materials have emerged as promising candidates for optoelectronic applications due to their high photoluminescence quantum yield, tunable bandgap, and low-cost fabrication. PQDs offer excellent color purity and high brightness, making them suitable for displays and lighting [33]. Most studies have focused on colloidal quantum dots (QDs) due to their high photoluminescence (PLQY), tunable wavelength, and thin emission wavelength. Using the quantum confinement effect, the emission color of quantum dots can be controlled depending on the size and content of the quantum dots. These advantages make QDs useful in solar cells, lasers, light-emitting diodes (LEDs), and bioimaging [33].

4.3 Flexible and stretchable luminescent materials

The development of flexible and stretchable luminescent materials is crucial for wearable optoelectronic devices. Recent progress in materials engineering, including stretchable polymers, elastomers, and nanocomposites, has enabled the fabrication of wearable displays, health monitors, and smart textiles with conformal and stretchable light-emitting components [34]. Wearable displays that can adapt to the contours of the human body are of great interest for real-time visual expression and communication [35]. Figure 6a shows an electronic textile for wearable displays with DC-powered LEDs with luminous efficiency that you can compare with traditional OLEDs on glass [36]. On the other image, Figure 6b shows an electronic textile for wearable displays with phOLED, which has higher internal efficiency and lower driving voltage compared to OLED [37].

Figure 6.

(a) Fibrous OLEDs handwoven into clothing and (b) fibrous multicolored phOLEDs woven into everyday clothing [35].

4.4 Bioinspired luminescent materials

Inspired by natural photonic structures, researchers have developed biomimetic luminescent materials with enhanced light extraction efficiency and color-tuning capabilities. These materials, often based on photonic crystals, structural colors, and animals (insects) found in nature as shown in Figure 7, hold promise for applications in displays, lighting, and sensing [38]. The structures of some insects as Troïdes magellanus butterfly, Hoplia Correa beetle, fireflies, and moth eye are shown in Figure 8 with their SEM images [24].

Figure 7.

Image of optical structures and applications to LEDs, lasers, and sensors to improve the luminescence observed by living organisms [24].

Figure 8.

The hind wings of the Troïdes magellanus butterfly exhibit a uniform yellow color in sunlight and an increasing yellow-green color under ultraviolet illumination. Scanning electron microscopy (SEM) cross-sectional and side-view images show that the ridge structures with triangular cross sections are set as a lattice structure, with each ridge having a series of lamellae [39]. (B) Hoplia Correa beetle exhibits a purple-blue color. The scales covering the elytra are composed of alternating membranes of pure and mixed porous air cuticle layers, forming a periodic photonic structure [40]. (C) Fireflies can release bright light from their abdomens, which have complicated optical structures. Ventral scale mismatch improves emission extraction [41]. (D) The corneal surface of a moth eye is lined with nano pill structures—reflective construction increases light input [24, 42].

5. Challenges and future perspectives

The cost of luminescent materials can be a barrier to their implementation in large-scale applications. Some luminescent materials, such as rare-earth elements, are expensive and difficult to source. This restricts their use to niche markets and high-end devices. To drive widespread adoption, efforts are underway to develop cost-effective alternatives and scalable manufacturing processes (Figure 9) [43].

Figure 9.

The challenges and future perspectives of luminescence materials.

5.1 Challenges

5.1.1 Efficiency enhancement

Efficiency in luminescent materials refers to the ability to convert energy into light effectively. Advances in materials design and engineering can improve this by optimizing factors such as quantum yield, lifetime, and energy transfer processes. For instance, incorporating efficient energy transfer mechanisms or minimizing non-radiative decay pathways can enhance overall efficiency. This is crucial for applications like solid-state lighting and displays where energy consumption and performance are key considerations [44].

5.1.2 Stability

Stability is essential for the long-term performance and reliability of luminescent materials, especially in practical applications where exposure to various environmental factors is inevitable. Research focuses on developing materials with robust chemical and physical properties to withstand degradation over time. Strategies include designing materials with high chemical and thermal stability, encapsulating sensitive components, and exploring novel protection mechanisms to prevent degradation [45]. Some of the stable luminescence are PTM [45], K₂BaCa(PO₄)₂ [46], and so on as shown in Figure 10.

Figure 10.

Optical properties of PTM-3NCz. (a) Chemical structure. (b) Boundary molecular orbital. (c) Comparison of photostability between PTM and PTM-3NCz. (d) Photograph of PTM and PTM-3NCz in dilute cyclohexane solution under UV light [45].

5.1.3 Mechanism understanding

Understanding the fundamental mechanisms behind luminescence is crucial for optimizing material design and performance. This involves studying the processes involved in light emission, such as electron transitions, energy transfer mechanisms, and material interactions. Advanced spectroscopic techniques and theoretical modeling are used to elucidate these mechanisms, providing insights into how to control and manipulate luminescent properties at the molecular and nanoscale levels [47].

5.2 Future perspectives

5.2.1 Display technology

Luminescent materials play a key role in display technology, offering advantages such as high brightness, wide color gamut, and energy efficiency. Organic light-emitting diodes (OLEDs) and quantum dot-based displays are prominent examples. Ongoing research focuses on improving device efficiency, lifespan, and manufacturability, as well as exploring new display concepts such as flexible and transparent displays for emerging applications in wearables, automotive, and augmented reality [48]. An example of this application is given in Figure 11.

Figure 11.

(a) Schematic illustration of the fabrication process of PLTs and the as-prepared fibrous membrane under UV light irradiation. (b) Application of CsPbBr3@HPβCD@PFOS composites in patterned display, white light-emitting diodes (WLEDs) and wearable optoelectronics. SEM (YAG back-scattered electron detector) images of the (c) control fiber and (d) CsPbBr3@HPβCD fiber. (e) TEM image showing the CsPbBr3@HPβCD fiber without or with PFOS coating as indicated. Note: The top half of the CsPbBr3@HPβCD@PFOS fiber was selectively etched by n-hexane to expose the inner CsPbBr3@HPβCD. (f) Close-up TEM image of the CsPbBr3@HPβCD@PFOS fiber. Inset: HRTEM image of the CsPbBr3 crystal.

5.2.2 Lighting

Luminescent materials are revolutionizing the lighting industry by enabling energy-efficient and environmentally friendly lighting solutions. Solid-state lighting technologies, such as LEDs and phosphor-converted LEDs, utilize luminescent materials to generate light with high efficiency and color quality. Research aims to further enhance efficiency, color rendering, and spectral control while reducing costs and environmental impact, driving the adoption of these technologies in general lighting applications [49].

5.2.3 Biomedical imaging

In biomedical imaging, luminescent probes offer advantages such as high sensitivity, multiplexing capability, and noninvasiveness, making them valuable tools for diagnostics and research. Fluorescent dyes, quantum dots, and upconversion nanoparticles are commonly used for applications such as fluorescence microscopy (as shown in Figure 12(a)), molecular imaging, and drug delivery tracking. Ongoing research focuses on improving probe specificity, biocompatibility, and imaging depth for applications in disease diagnosis, drug development, and personalized medicine [51]. Figure 12(b) shows a cell that has been pictured by wide-field fluorescence microscope.

Figure 12.

(a) Anatomical representation of the wide-field fluorescence microscope setup. (b) Fluorescence microscope image of a cell, where the mesh of keratin average filaments is shown in green, actin filaments in red, and nuclear DNA in blue [50].

5.2.4 Materials science and nanotechnology

Materials science and nanotechnology play pivotal roles in advancing luminescent materials and their applications. Researchers explore novel synthesis methods, nanostructuring techniques, and hybrid material systems to tailor luminescent properties for specific applications. Key areas of focus include improving material stability, enhancing quantum efficiency, and achieving precise control over optical properties. These advancements drive innovation across various fields, from electronics and photonics to healthcare and energy, opening up new opportunities for luminescence-based technologies [46].

6. Conclusion

The applications of luminescent materials in optoelectronics and wearable devices are vast and promising. From leveraging the unique optical properties of 2D materials in LEDs and photodetectors to harnessing wavelength conversion and emission properties for sensing and imaging, these materials offer exciting opportunities. However, challenges remain, including improving quantum efficiency prediction, developing environmentally benign materials, and enhancing material lifetime and scalability. As the demand for energy-efficient lighting, innovative sensing functions, and advanced optoelectronic devices continues to grow, the luminescent materials market is poised for substantial expansion. Overcoming the hurdles through data-driven approaches, composition and nanoarchitecture engineering, and photonic engineering will be crucial in unlocking the full potential of these materials in next-generation technologies.

Acknowledgments

The authors have used Grammarly software to edit the language.