What Is The Difference Between Internal And External Quantum Efficiency

Quantum efficiency is a critical parameter in the field of photonics and optoelectronics, defining the effectiveness of devices such as solar cells, LEDs, and photodetectors. It measures how efficiently a device converts photons into electrons or vice versa. Understanding quantum efficiency is essential for optimizing the performance of these devices.

Internal quantum efficiency (IQE) refers to the efficiency of photon absorption and electron generation within a device. In contrast, external quantum efficiency (EQE) measures the efficiency of emitting or extracting photons from the device. Both metrics are vital for assessing the performance and potential improvements of optoelectronic devices.

Quantum efficiency impacts a wide range of applications, from enhancing solar cell efficiency to improving LED brightness and photodetector sensitivity. By optimizing both internal and external quantum efficiencies, researchers and engineers can significantly advance the performance and capabilities of various photonic devices.

Quantum Efficiency Basics

Photons and Electrons Interaction

Quantum efficiency measures how well a photonic device converts photons into electrons or vice versa. This conversion is fundamental to the operation of devices like solar cells, LEDs, and photodetectors. The interaction between photons and electrons is crucial because it determines the device’s efficiency and performance.

When photons strike a photonic device, they can be absorbed, generating electron-hole pairs. These pairs can then be separated and collected to produce an electric current. In light-emitting devices, the process is reversed: electrons and holes recombine to emit photons, producing light.

Conversion Efficiency

Conversion efficiency is a measure of how effectively a device transforms incident light into electrical energy or emits light from electrical energy. High conversion efficiency is essential for the optimal performance of photonic devices. Several factors influence conversion efficiency, including the material quality, device design, and operating conditions.

Role in Device Performance

Quantum efficiency directly impacts the performance of photonic devices. In solar cells, high quantum efficiency means more sunlight is converted into electricity, improving overall energy output. For LEDs, high quantum efficiency results in brighter and more efficient light emission. In photodetectors, higher efficiency enhances sensitivity and response time, making them more effective in various applications.

Internal Quantum Efficiency (IQE)

Definition of IQE

Internal quantum efficiency (IQE) is the ratio of the number of electrons generated to the number of photons absorbed by a device. It indicates how efficiently a device converts absorbed photons into electrons. IQE is a key parameter in evaluating the performance of photonic devices, as it directly affects their efficiency.

Mechanism of Photon Absorption

Photon absorption occurs when photons strike the active layer of a device and transfer their energy to electrons, creating electron-hole pairs. This process is influenced by the material properties and the quality of the device’s structure. Efficient photon absorption is crucial for achieving high IQE.

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Factors Affecting IQE

Material Quality

The quality of the material used in photonic devices significantly impacts IQE. High-quality materials with fewer defects allow for more efficient photon absorption and electron generation. Materials like high-purity silicon or compound semiconductors are often used to achieve high IQE.

Defects and Recombination

Defects in the material or device structure can lead to recombination, where electron-hole pairs recombine without generating an electric current. This reduces IQE. Minimizing defects through careful material selection and fabrication techniques is essential for improving IQE.

Measurement Techniques


Photoluminescence is a technique used to measure IQE by observing the light emitted from a device when it is excited by a light source. The intensity of the emitted light provides information about the efficiency of photon absorption and electron generation.


Electroluminescence involves measuring the light emitted by a device when an electric current passes through it. This technique helps assess IQE by evaluating how efficiently the device converts electrical energy into light.

External Quantum Efficiency (EQE)

Definition of EQE

External quantum efficiency (EQE) is the ratio of the number of photons emitted or extracted to the number of photons incident on the device. It measures how efficiently a device emits or extracts photons. EQE is a critical parameter for evaluating the overall performance of photonic devices.

Process of Photon Emission

Photon emission occurs when electron-hole pairs recombine in a light-emitting device. The efficiency of this process depends on several factors, including the material properties, device design, and surface conditions. Efficient photon emission is essential for achieving high EQE.

Factors Affecting EQE

Surface Recombination

Surface recombination occurs when electron-hole pairs recombine at the surface of the device, reducing the number of photons emitted. Surface passivation techniques, which involve coating the device surface with a material that reduces recombination, can improve EQE.

Light Extraction Efficiency

Light extraction efficiency refers to how effectively photons generated within a device are emitted from its surface. Device design, including the shape and structure of the surface, plays a crucial role in light extraction. Techniques like texturing the surface or using reflective coatings can enhance light extraction efficiency, thereby improving EQE.

Measurement Techniques

Photovoltaic Cells

In photovoltaic cells, EQE is measured by illuminating the cell with light of different wavelengths and measuring the generated current. This technique provides information about the cell’s efficiency across the solar spectrum, helping identify areas for improvement.

Light-Emitting Devices

For light-emitting devices, EQE is measured by driving the device with an electric current and measuring the emitted light. This technique helps evaluate the efficiency of light emission and identify factors that may limit performance.

Key Differences

Photon Absorption vs. Emission

The primary difference between internal and external quantum efficiency lies in their focus on photon interactions. Internal quantum efficiency (IQE) deals with the absorption of photons and the generation of electron-hole pairs within the device. It measures how effectively photons are converted into electrons.

On the other hand, external quantum efficiency (EQE) focuses on the emission or extraction of photons from the device. EQE measures how efficiently a device emits photons or converts them into usable light or electricity. This distinction is crucial for understanding and improving the performance of photonic devices.

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Influence of Device Design

Device design plays a significant role in determining both IQE and EQE. For IQE, the quality of the active layer, thickness, and material purity are critical factors. A well-designed device with minimal defects and optimal layer thickness will have a higher IQE.

For EQE, the design must ensure efficient light extraction. This includes optimizing the surface structure, using anti-reflective coatings, and incorporating light-trapping mechanisms. Effective device design can significantly enhance both IQE and EQE, leading to better overall performance.

Impact of Environmental Conditions

Environmental conditions can also affect IQE and EQE. Factors such as temperature, humidity, and exposure to light can influence the efficiency of photon interactions. High temperatures can increase recombination rates, reducing IQE. Similarly, surface degradation due to environmental exposure can lower EQE by increasing surface recombination.

Protective coatings and robust device structures can help mitigate these effects, ensuring consistent performance even under varying environmental conditions.

IQE vs. EQE in Various Applications

Solar Cells

Efficiency Metrics

In solar cells, quantum efficiency is a critical metric. Internal quantum efficiency measures how effectively the cell absorbs sunlight and generates electron-hole pairs. External quantum efficiency evaluates how well these electron-hole pairs are collected and converted into electricity.

Influence on Overall Performance

High IQE and EQE are essential for maximizing solar cell efficiency. A cell with high IQE absorbs more sunlight, while high EQE ensures that the generated electrons are effectively extracted and used. Optimizing both efficiencies can lead to significant improvements in solar cell performance, making them more efficient and cost-effective.


Brightness and Efficiency

For LEDs, brightness and efficiency depend heavily on both IQE and EQE. High IQE ensures that more electrons recombine to produce photons. High EQE ensures that these photons are efficiently emitted as light.

Role in Light Output

The light output of an LED is directly proportional to its EQE. By improving IQE, more photons are generated within the LED. By enhancing EQE, more of these photons are emitted as visible light, resulting in a brighter and more efficient LED.


Sensitivity and Response Time

In photodetectors, sensitivity and response time are key performance indicators. High IQE means more photons are absorbed and converted into electrons, increasing sensitivity. High EQE ensures that these electrons are efficiently collected, improving response time.

Importance in Sensing Applications

High quantum efficiency is crucial for sensing applications. Photodetectors with high IQE and EQE can detect lower levels of light with greater accuracy and speed. This makes them ideal for applications in fields like medical imaging, environmental monitoring, and communications.

Improving Quantum Efficiency

Material Innovations

High-Quality Semiconductors

Using high-quality semiconductors can significantly improve quantum efficiency. Materials with fewer defects and higher purity can absorb and emit photons more efficiently. Innovations in semiconductor fabrication are continuously enhancing material quality, leading to better device performance.

Nanostructures and Quantum Dots

Nanostructures and quantum dots are advanced materials that can enhance quantum efficiency. These materials have unique optical properties that allow for more efficient photon absorption and emission. Incorporating nanostructures and quantum dots into device designs can significantly boost IQE and EQE.

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Device Engineering

Optimizing Layer Structures

Optimizing the layer structures within a device is crucial for improving quantum efficiency. This involves carefully designing the thickness and composition of each layer to maximize photon interactions. Properly engineered layers can reduce recombination losses and enhance both IQE and EQE.

Surface Passivation Techniques

Surface passivation involves coating the device surface with a material that reduces recombination at the surface. This technique can significantly improve EQE by ensuring that more photons are emitted rather than recombined at the surface. Effective surface passivation is essential for high-performance photonic devices.

Advanced Measurement Methods

Time-Resolved Spectroscopy

Time-resolved spectroscopy is an advanced technique used to measure quantum efficiency. It involves observing the behavior of photons and electrons over time, providing detailed insights into the efficiency of photon interactions. This method helps identify areas for improvement and optimize device performance.

Quantum Yield Analysis

Quantum yield analysis measures the efficiency of photon emission in light-emitting devices. It provides a direct assessment of EQE, helping to identify factors that affect light output. This analysis is crucial for optimizing LEDs and other light-emitting devices.

Challenges and Future Directions

Overcoming Recombination Losses

Recombination losses are a major challenge in achieving high quantum efficiency. These losses occur when electron-hole pairs recombine without generating useful current or light. Overcoming these losses involves improving material quality, optimizing device design, and employing advanced passivation techniques.

Enhancing Light Extraction

Light extraction is critical for achieving high EQE. Techniques like surface texturing, anti-reflective coatings, and photonic crystals can enhance light extraction by reducing reflection and scattering. Improving light extraction ensures that more photons are emitted from the device, boosting overall efficiency.

Potential Breakthroughs

New Materials

New materials with superior optical properties can revolutionize quantum efficiency. Innovations in materials science, such as perovskites and two-dimensional materials, offer promising potential for enhancing photon interactions. These materials could lead to significant breakthroughs in photonic device performance.

Novel Device Architectures

Novel device architectures can also play a crucial role in improving quantum efficiency. Designs that incorporate advanced materials, optimized layer structures, and innovative engineering techniques can achieve higher IQE and EQE. Future research will continue to explore these possibilities, driving progress in the field of photonics and optoelectronics.

Frequently Asked Questions

What is quantum efficiency?

Quantum efficiency is a measure of how effectively a photonic device converts incident photons into electrons or emits photons from electrons. It is crucial for determining the performance of devices like solar cells, LEDs, and photodetectors.

How is internal quantum efficiency different from external quantum efficiency?

Internal quantum efficiency (IQE) refers to the efficiency with which photons are absorbed and converted into electrons within a device. External quantum efficiency (EQE) measures the efficiency of emitting or extracting photons from the device, considering external factors like surface recombination and light extraction.

Why is quantum efficiency important in solar cells?

Quantum efficiency in solar cells determines how effectively they convert sunlight into electrical energy. High quantum efficiency leads to better performance and higher energy output, making solar cells more efficient and cost-effective.

How can quantum efficiency be improved?

Improving quantum efficiency involves enhancing material quality, optimizing device structures, and employing advanced techniques like surface passivation and nanostructuring. These improvements help reduce recombination losses and enhance light extraction.

What factors affect internal quantum efficiency?

Factors affecting internal quantum efficiency include material quality, presence of defects, and recombination processes within the device. High-quality materials and minimizing defects can significantly improve IQE.


Quantum efficiency is a fundamental concept in photonics and optoelectronics, playing a crucial role in the performance of devices like solar cells, LEDs, and photodetectors. By understanding and optimizing internal and external quantum efficiencies, significant advancements can be achieved in these technologies.

Future research and innovations will continue to focus on enhancing quantum efficiency through better materials, advanced device structures, and novel engineering techniques. These efforts will pave the way for more efficient, reliable, and powerful photonic devices, driving progress in various fields and applications.

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