Internal quantum efficiency (IQE) measures how effectively a material or device converts absorbed energy into a desired output. This metric is particularly relevant in optoelectronic devices, where conversion often involves light and electricity. IQE quantifies the intrinsic performance within the material, focusing on internal processes.
Defining Internal Quantum Efficiency
In a photodetector or solar cell, internal quantum efficiency (IQE) describes how many charge carriers (electrons or holes) are generated for each photon absorbed by the material. Conversely, in a light-emitting diode (LED), IQE represents the number of photons emitted for each electron-hole pair that recombines inside the device.
The term “internal” emphasizes that IQE considers only photons successfully absorbed by the active material, bypassing losses from reflection or transmission. For instance, if 10 photons strike a solar cell and 2 are reflected, IQE is calculated based on the 8 photons that entered the material. This distinguishes IQE from external quantum efficiency (EQE), which accounts for all incident photons, including those lost before absorption. EQE incorporates external factors like light extraction or collection losses, while IQE isolates the material’s conversion capability.
Factors Influencing Efficiency
The internal quantum efficiency of a material or device is governed by several underlying physical mechanisms. Material quality plays a significant role, as the purity and crystal structure of the semiconductor directly impact its performance. Imperfections such as impurities or dislocations within the crystal lattice can create pathways for energy loss. These defects can lead to non-radiative recombination, where absorbed energy is dissipated as heat rather than being converted into the desired output, such as light or electrical current.
Recombination mechanisms within the material are also a major determinant of IQE. Radiative recombination is the desired process, where an electron and a hole combine to emit a photon, producing light in an LED or generating an electron-hole pair that contributes to current in a solar cell. However, non-radiative recombination pathways, where energy is lost as heat, compete with radiative processes. A high IQE signifies that a greater proportion of recombination events result in radiative emission or carrier generation, rather than heat loss.
Temperature can also significantly influence IQE. As temperature increases, the likelihood of non-radiative recombination pathways often rises, which can reduce the overall internal quantum efficiency. This is because higher thermal energy can promote carrier trapping at defects or activate alternative non-radiative processes. Furthermore, the concentration of charge carriers, known as carrier density, affects the rates of both radiative and non-radiative recombination, thereby impacting IQE.
Importance in Modern Technology
Internal quantum efficiency is a significant metric across various modern technologies, directly influencing device performance and enabling advancements. In Light Emitting Diodes (LEDs), a high IQE directly translates to brighter and more energy-efficient light sources. This improved efficiency means less electrical energy is wasted as heat, contributing to reduced energy consumption and promoting greater sustainability in lighting applications. For example, modern high-brightness LEDs can achieve IQE values exceeding 90% in some cases, leading to substantial energy savings compared to traditional lighting.
For solar cells, also known as photovoltaics, a high internal quantum efficiency means that a larger fraction of the absorbed sunlight is effectively converted into electrical energy. This directly increases the power output and overall efficiency of solar panels. Solar cells with higher IQE can generate more electricity from the same amount of sunlight, making solar energy more cost-effective and viable for widespread adoption. Advances in material science and device architecture have pushed IQE in silicon solar cells to typically range from 70% to over 90% across the visible spectrum.
In photodetectors and cameras, a robust IQE leads to enhanced sensitivity. This allows these devices to accurately detect fainter light signals, which is particularly beneficial for low-light imaging, scientific instrumentation, and various sensing applications. For instance, in scientific cameras used for astronomy or medical imaging, high IQE ensures that even a small number of incoming photons can be reliably converted into a detectable electrical signal, improving image clarity and data acquisition.