Light-emitting diodes (LEDs) are devices that convert electrical energy into light. LEDs have many advantages over conventional light sources, such as high efficiency, long lifetime, low power consumption, environmental friendliness, and color tunability. LEDs have been widely used in various applications, such as display panels, lighting sources, sensors, and biomedical devices.
In this article, we will introduce some recent advances and perspectives on LEDs, focusing on their materials, architectures, and performance.
Materials for LEDs
The materials used for LEDs determine their emission properties, such as color, brightness, and stability. There are different types of materials for LEDs, such as inorganic semiconductors, organic molecules, quantum dots, and perovskites.
Inorganic semiconductors are the most mature and widely used materials for LEDs. They have high efficiency, stability, and reliability. However, they also have some drawbacks, such as high fabrication cost, limited color range, and difficulty in achieving flexible devices.
Organic molecules are carbon-based compounds that can emit light when excited by electric current. Organic LEDs (OLEDs) have some advantages over inorganic LEDs, such as low fabrication cost, large color gamut, and flexibility. However, OLEDs also suffer from low efficiency, short lifetime, and sensitivity to oxygen and moisture.
Quantum dots are nanoscale particles that can emit light of different colors depending on their size. Quantum dot LEDs (QLEDs) have high efficiency, color purity, and stability. However, QLEDs also face some challenges, such as toxicity of some quantum dot materials, difficulty in achieving uniform and stable films, and low outcoupling efficiency.
Perovskites are hybrid organic-inorganic materials that have emerged as promising candidates for LEDs. Perovskite LEDs (PeLEDs) have high efficiency, color tunability, and low fabrication cost. However, PeLEDs also have some issues, such as poor ambient and operational stability, hysteresis behavior, and light trapping in the device structure.
Architectures for LEDs
The architectures of LED devices affect their light extraction and emission mechanisms. There are different architectures for LED devices, such as planar structure, nanostructure, flexible structure, and transparent structure.
Planar structure is the simplest and most common architecture for LED devices. It consists of a thin film of emitting material sandwiched between two electrodes on a substrate. Planar structure has the advantage of easy fabrication and integration. However, it also has the disadvantage of low outcoupling efficiency due to the total internal reflection at the interface between the emitting layer and the substrate.
Nanostructure is an architecture that introduces nano-sized features into the LED device to enhance the light extraction and emission. Nanostructure can be achieved by using nanomaterials as the emitting layer or by patterning the substrate or the electrodes with nano-scale shapes. Nanostructure can improve the outcoupling efficiency by reducing the reflection and scattering losses at the interface. Nanostructure can also modify the emission mechanism by inducing different light-matter interactions, such as surface plasmon resonance, cavity resonance, or photon recycling.
Flexible structure is an architecture that enables the LED device to bend or stretch without affecting its performance. Flexible structure can be realized by using flexible materials as the substrate or the electrodes or by using thin film techniques to transfer the emitting layer onto a flexible substrate. Flexible structure can expand the application scope of LED devices to wearable electronics or curved surfaces.
Transparent structure is an architecture that allows the LED device to transmit light from both sides. Transparent structure can be achieved by using transparent materials as the substrate or the electrodes or by using micro-LED arrays to create transparent pixels. Transparent structure can enable novel applications of LED devices such as transparent displays or smart windows.
Performance of LEDs
The performance of LED devices is evaluated by several parameters, such as external quantum efficiency (EQE), luminance (L), color coordinates (x,y), color rendering index (CRI), lifetime (t), and power efficiency (PE).
EQE is defined as the ratio of photons emitted from the device to electrons injected into the device. EQE reflects the overall efficiency of LED devices. The highest EQE reported so far for different types of LED devices are: 72% for inorganic LEDs , 37% for OLEDs , 21% for QLEDs , and 20% for PeLEDs .
Luminance is defined as the amount of light emitted per unit area per unit solid angle from the device surface. Luminance reflects the brightness of LED devices. The highest luminance reported so far for different types of LED devices are: 10,000 cd/m2 for inorganic LEDs , 10,000 cd/m2 for OLEDs , 100,000 cd/m2 for QLEDs , and 10,000 cd/m2 for PeLEDs .
Color coordinates are defined as the position of the emitted light on the chromaticity diagram. Color coordinates reflect the color quality of LED devices. The ideal color coordinates for white light are (0.33, 0.33). The color coordinates reported so far for different types of LED devices are: (0.33, 0.33) for inorganic LEDs , (0.31, 0.32) for OLEDs , (0.32, 0.33) for QLEDs , and (0.31, 0.32) for PeLEDs .
CRI is defined as the ability of the emitted light to reveal the true colors of objects compared to a reference light source. CRI reflects the color fidelity of LED devices. The ideal CRI is 100. The CRI reported so far for different types of LED devices are: 95 for inorganic LEDs , 90 for OLEDs , 90 for QLEDs , and 85 for PeLEDs .
Lifetime is defined as the time required for the luminance or EQE of the device to decay to half of its initial value. Lifetime reflects the stability and durability of LED devices. The lifetime reported so far for different types of LED devices are: >100,000 hours for inorganic LEDs , >10,000 hours for OLEDs , >10,000 hours for QLEDs , and <100 hours for PeLEDs .
Power efficiency is defined as the ratio of luminous flux emitted from the device to electrical power consumed by the device. Power efficiency reflects the energy saving potential of LED devices. The highest power efficiency reported so far for different types of LED devices are: 300 lm/W for inorganic LEDs , 100 lm/W for OLEDs , 50 lm/W for QLEDs , and 40 lm/W for PeLEDs .
Conclusion
In this article, we have introduced some recent advances and perspectives on LEDs, focusing on their materials, architectures, and performance. LEDs have shown great potential in various applications, such as display panels, lighting sources, sensors, and biomedical devices. However, there are still some challenges and opportunities for further improvement and innovation of LED devices.