Glossary

5.3 Light emission efficiency and quantum yield

3 min read•august 7, 2024

Light emission efficiency is crucial for optoelectronic devices. It's measured by quantum efficiency and yield, which tell us how well electrons turn into light. These concepts help us understand and improve the performance of LEDs and other light-emitting tech.

Factors like material quality and light extraction affect efficiency. By studying luminescence processes and minimizing losses, we can create brighter, more energy-efficient devices. This knowledge is key to advancing lighting, displays, and optical communication systems.

Quantum Efficiency and Yield

Internal and External Quantum Efficiencies

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  • (ηint\eta_{int}) represents the ratio of the number of photons emitted from the active region to the number of electrons injected into the active region
  • Influenced by factors such as material quality, defects, and processes
  • (ηext\eta_{ext}) is the ratio of the number of photons emitted from the device to the number of electrons injected into the device
  • Depends on both internal quantum efficiency and light extraction efficiency (ηextraction\eta_{extraction})
  • Relationship between internal and external quantum efficiencies: ηext=ηint×ηextraction\eta_{ext} = \eta_{int} \times \eta_{extraction}

Quantum Yield and Photoluminescence Quantum Yield

  • (Φ\Phi) is the ratio of the number of photons emitted to the number of photons absorbed by a material
  • Represents the efficiency of the luminescence process in a material
  • Photoluminescence quantum yield (PLQY) specifically refers to the quantum yield of photoluminescence processes (fluorescence and phosphorescence)
  • PLQY is often used to characterize the efficiency of light-emitting materials such as quantum dots and organic dyes
  • High quantum yield materials are desirable for applications such as and biological imaging

Luminescence Processes

Fluorescence and Phosphorescence

  • Fluorescence is a radiative process where an electron in an excited singlet state returns to the ground state by emitting a photon
  • Occurs rapidly, typically on the order of nanoseconds
  • Phosphorescence involves a transition from an excited triplet state to the ground state, accompanied by
  • Longer-lived process compared to fluorescence, often lasting microseconds to seconds
  • Triplet state is populated through intersystem crossing from the excited singlet state

Stokes Shift and Luminescence Efficiency

  • Stokes shift is the difference in wavelength between the absorption and emission peaks of a luminescent material
  • Arises from energy loss due to vibrational relaxation and other non-radiative processes
  • Larger Stokes shifts are advantageous for separating excitation and emission wavelengths in applications such as fluorescence microscopy
  • Luminescence efficiency is the ratio of the energy of emitted photons to the energy of absorbed photons
  • Influenced by factors such as quantum yield, Stokes shift, and spectral overlap between absorption and emission

Efficiency Factors

Light Extraction Efficiency

  • Light extraction efficiency (ηextraction\eta_{extraction}) is the ratio of the number of photons emitted from the device to the number of photons generated in the active region
  • Limited by total internal reflection at the device interfaces due to refractive index mismatch
  • Improved by techniques such as surface texturing, photonic crystals, and high-refractive-index encapsulants
  • Crucial factor in determining the overall efficiency of light-emitting devices (LEDs and organic LEDs)

Non-Radiative Losses

  • Non-radiative losses are processes that compete with and reduce the efficiency of light emission
  • Include mechanisms such as defect-related recombination, Auger recombination, and surface recombination
  • Defect-related recombination occurs at impurities or structural defects in the material, leading to non-radiative transitions
  • Auger recombination involves energy transfer from an electron-hole pair to a third carrier, which then relaxes through phonon emission
  • Surface recombination arises from dangling bonds and surface states at the material interfaces
  • Minimizing non-radiative losses is essential for achieving high luminescence efficiency and quantum yield in light-emitting materials and devices

Key Terms to Review (18)

Concentration Quenching: Concentration quenching refers to the reduction in photoluminescence intensity of a material as the concentration of luminescent centers increases. This phenomenon occurs because, at higher concentrations, energy transfer between nearby luminescent centers can lead to non-radiative decay pathways, resulting in decreased light emission efficiency and quantum yield. Understanding concentration quenching is essential for optimizing materials in optoelectronic applications, where effective light emission is crucial for device performance.
Dexter Energy Transfer: Dexter energy transfer refers to a non-radiative process in which energy is transferred from one excited molecule or atom to another through an exchange interaction. This mechanism is significant in the context of light emission efficiency and quantum yield, as it impacts how effectively a material can convert absorbed light into emitted light by facilitating energy transfer between nearby chromophores or luminescent centers.
Electroluminescence: Electroluminescence is the phenomenon where a material emits light in response to an electric current or strong electric field. This process is fundamental in various optoelectronic devices, as it plays a crucial role in how light-emitting diodes and other light sources operate, linking closely to the principles of semiconductor physics and materials science.
External Quantum Efficiency: External quantum efficiency (EQE) measures how effectively a light-emitting device converts injected electrical charge carriers into emitted photons. A higher EQE indicates that a larger proportion of the electrons or holes injected into the device results in light emission, which is critical for optimizing performance in various optoelectronic applications, including LEDs and laser diodes. Understanding EQE helps in evaluating the light emission efficiency and overall performance characteristics of these devices.
Förster Resonance Energy Transfer (FRET): Förster Resonance Energy Transfer (FRET) is a physical phenomenon where energy is transferred non-radiatively from an excited donor chromophore to an acceptor chromophore through dipole-dipole interactions. This process is critical in studying molecular interactions and dynamics, as it can reveal information about distances between molecules, making it essential for understanding light emission efficiency and quantum yield in optoelectronic devices.
Inorganic Semiconductors: Inorganic semiconductors are materials made primarily from elements or compounds that are not carbon-based, commonly exhibiting semiconductor properties. These materials, like silicon and gallium arsenide, play a crucial role in electronic devices, enabling efficient light emission and energy conversion processes, which are essential for applications such as LEDs and solar cells.
Internal Quantum Efficiency: Internal quantum efficiency (IQE) is the measure of how effectively absorbed photons are converted into electron-hole pairs in a semiconductor material. This concept is crucial in understanding the performance of optoelectronic devices, as it directly relates to the processes of radiative and non-radiative recombination, influencing light emission efficiency and overall device performance, particularly in light-emitting diodes (LEDs). High IQE indicates that a larger proportion of absorbed photons contribute to light generation, which is vital for optimizing device efficiency.
Lasers: Lasers are devices that emit coherent light through a process called stimulated emission. They have unique properties such as high intensity, directionality, and monochromaticity, making them essential in various fields like communication, medicine, and manufacturing.
Light-emitting diodes (LEDs): Light-emitting diodes (LEDs) are semiconductor devices that emit light when an electric current passes through them, leveraging the phenomenon of electroluminescence. The efficiency of LEDs is closely tied to the optical transitions in semiconductors, where electrons recombine with holes to release energy in the form of photons. The performance of LEDs can be influenced by their design, including factors like quantum well structures and light extraction techniques, making them versatile for various applications.
Non-radiative recombination: Non-radiative recombination refers to the process in semiconductors where charge carriers (electrons and holes) recombine without emitting photons. This phenomenon is significant because it impacts the efficiency of light-emitting devices, as energy lost through non-radiative processes does not contribute to light generation. Understanding this mechanism is crucial for enhancing the performance of optoelectronic devices, as it directly influences optical transitions and the overall light emission efficiency.
Organic Light-Emitting Diodes (OLEDs): Organic light-emitting diodes (OLEDs) are a type of light-emitting diode that uses organic compounds to emit light when an electric current is applied. These devices have revolutionized the display technology landscape, providing vibrant colors and high contrast in screens while being energy efficient. OLEDs are significant in the historical context of optoelectronic devices, their efficiency relates closely to light emission properties, and they demonstrate the importance of organic materials in modern optoelectronic applications.
Photoluminescence Spectroscopy: Photoluminescence spectroscopy is a technique used to study the light emission from a material after it has absorbed photons. It helps researchers understand various properties of materials, including energy levels, defect states, and electronic transitions, by analyzing the emitted light's wavelength and intensity. This technique is closely tied to understanding light emission efficiency and quantum yield, as these factors determine how effectively a material can convert absorbed light into emitted light.
Photon emission: Photon emission is the process by which a photon, a quantum of light, is released from an atom or molecule when it transitions from a higher energy state to a lower energy state. This process is fundamental to various optical phenomena and is closely tied to concepts like light emission efficiency and quantum yield, which measure how effectively a material can emit light when excited.
Quantum Yield: Quantum yield is a measure of the efficiency of a photophysical process, representing the ratio of the number of photons emitted or reacted to the number of photons absorbed. This concept is crucial in understanding how effectively materials convert absorbed light into luminescence, impacting both photoluminescence and electroluminescence processes. A high quantum yield indicates that a significant fraction of absorbed photons contribute to light emission, which is essential for the performance of light-emitting devices and understanding light emission efficiency.
Radiative Recombination: Radiative recombination is a process where an electron and a hole combine, releasing energy in the form of a photon. This process is crucial in optoelectronic devices, as it directly relates to light emission and efficiency, impacting how effectively devices like LEDs convert electrical energy into visible light.
Spontaneous Emission: Spontaneous emission is a process where an excited atom or molecule returns to its ground state and emits a photon without external stimulation. This natural process is fundamental in understanding how light interacts with matter, influencing various optical phenomena and the development of light-emitting devices.
Temperature Effects: Temperature effects refer to the influence of temperature on the performance and behavior of materials and devices, particularly in relation to light emission efficiency and quantum yield. These effects are crucial because they can significantly alter how well a material emits light, affecting both its efficiency and the overall performance of optoelectronic devices. Understanding temperature effects helps in optimizing the design and operation of these devices under varying thermal conditions.
Thermal Quenching: Thermal quenching refers to the phenomenon where the efficiency of light emission from a material decreases with increasing temperature. As the temperature rises, the energy states of electrons become increasingly populated by non-radiative processes, which leads to a reduction in the quantum yield of emitted light. This process is significant in understanding how temperature affects the performance and efficiency of optoelectronic devices.
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