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Semiconductors are a widely used class of materials with electrical conductivities that fall between that of conductors and insulators. What has made doped versions of these materials the cornerstone of modern electronics is their ability to control the flow of electrons as part of a circuit, switching on and off as required, with a minimal physical device footprint.
Controlling the flow of electrons is not the only interesting application of semiconductor materials. Some semiconducting materials can be used to either convert electricity into light or, to do the reverse, converting absorbed light energy into electrical charge.
Optical devices typically make use of different materials for purely electronic applications. When choosing semiconductor materials for optical applications, there are a few key considerations to be made in terms of their optical properties.
The first of those is efficiency. A light-receiving semiconductor needs to have a high molar absorption coefficient in the wavelength region it will be exposed to. If the initial absorption process is of poor efficiency, this will set a low maximum limit of how much charge can be subsequently generated. For light-giving applications, the emission process needs to be favorable for an energy-efficient device.
The second consideration is the operating wavelength range and spectrum. Different semiconductors will emit different wavelengths of light depending on the underlying electronic structure of the material. Some compounds exhibit relatively narrowband emission whereas others may have broader, more complex spectra. For display applications, spectral purity is paramount for correct color reproduction and control but, there also needs to be sufficient coverage to generate all colors.
Light Emitting Diodes
Depending on the operating wavelength range, a huge number of semiconductor materials can be used to manufacture light-emitting diodes (LEDs). Common choices include indium gallium nitride (InGaN) for high-brightness green to UV LEDs and gallium phosphide (GaP) for yellow and green LEDs. Aluminum gallium indium phosphide (AlGaInP) and aluminum gallium arsenide (AlGaAs) are used for LEDs towards the redder end of the electromagnetic spectrum.
These days, LEDs are available in most wavelength ranges from infrared to UV. Broad-spectrum white LEDs were first based on Ce:YAG phosphor but inefficient and costly to produce. Now, materials such as gallium nitride (GaN) are preferred. In the last 20 years, there has been an explosion in cost-reduction and developments of semiconductors with emission properties suitable for lighting applications, to the point where LEDs are now commonly used in home lighting.
Laser diodes have many similarities to LEDs. They are optical semiconductor devices that convert electrical power to light, but rather than generating light through spontaneous emission, as happens in LEDs, they are used to generate stimulated emission via the lasing process.
There are a number of different types of laser diodes, including Fabry-Pérot diodes, quantum wells and vertical-cavity surface-emitting lasers (VCSEL) that differ in their construction and emission properties. Typical materials for laser diodes include GaN, GaAs and InP, the choice of which determines the emission wavelength.
For laser diodes, the optical band gap of the semiconductor plays an important role. The band gap describes the separation between the valence and conduction bands in the semiconducting material, which plays an important role in determining the absorption and emission properties of the material. Creating new materials often involves trying to engineer this band gap through chemical substitution and doping.
Photovoltaic Solar Cells
Semiconductors are at the heart of solar cells and their renewable energy generating abilities. Silicon-based devices are still the most widely used, with different dopants added to silicon to create the necessary p and n junctions. Many photovoltaic cells also contain other semiconducting compounds such as GaAs and geranium in various relative quantities.
The challenge for designing efficient solar cells is first to efficiently capture the full wavelength range of the solar spectrum. Successful absorption of a photon can lead to the formation of the electron-hole pairs that must reach the p-n junction to produce current. However, these charge carriers can undergo processes such as recombination, preventing the desired energy conversion process.
As well as traditional inorganic semiconducting materials like GaAs, researchers have been working on other types of semiconducting materials, including perovskites in an attempt to address the inherent limitations of silicon’s efficiency.6 Many of these materials have extremely promising performance characteristics though which of the materials will replace the dominance of silicon cells remains to be seen.
While many of the widely used semiconductors in optical devices are inorganic materials, organic semiconductors are a highly active area of research. Organic semiconductors offer many exciting physical properties, such as the ability to create flexible, and even wearable devices. Organic LEDs (OLEDs) are already a widely used optical application of organic semiconductors and there is great interest in making use of chiral semiconducting materials for polarization control of light.
The wealth of optical applications for semiconducting materials seems limitless and, with new semiconducting materials being created on a routine basis, more and more of these technologies will start to move from laboratory concepts into our daily lives.
References and Further Reading
Reineke, S. (2015). Complementary LED technologies. Nature Materials, 14(5), 459–460. https://doi.org/10.1038/nmat4277
Mills, A. (2005). Phosphors development for LED lighting. III-Vs Review, 18(3), 32–34. https://doi.org/10.1016/S0961-1290(05)01052-5
Wasisto, H. S., Prades, J. D., Gülink, J., & Waag, A. (2019). Beyond solid-state lighting: Miniaturization, hybrid integration, and applications of GaN nano-and micro-LEDs. Applied Physics Reviews, 6(4). https://doi.org/10.1063/1.5096322
Capasso, F. (1987). Band-Gap Engineering: From Physics and Materials To New Semiconductor Devices. Science, 235(4785), 172–176. https://doi.org/10.1126/science.235.4785.172
Dimroth, F. (2006). High-efficiency solar cells from III-V compound semiconductors. Physica Status Solidi C: Conferences, 3(3), 373–379. https://doi.org/10.1002/pssc.200564172
Deschler, F., Neher, D., & Schmidt-Mende, L. (2019). Perovskite semiconductors for next generation optoelectronic applications. APL Materials, 7(8), 7–10. https://doi.org/10.1063/1.5119744
Forrest, S. R. (2000). Active optoelectronics using thin-film organic semiconductors. IEEE Journal on Selected Topics in Quantum Electronics, 6(6), 1072–1083. https://doi.org/10.1109/2944.902156
Geffroy, B., le Roy, P., & Prat, C. (2006). Organic light-emitting diode (OLED) technology: Materials, devices and display technologies. Polymer International, 55(6), 572–582. https://doi.org/10.1002/pi.1974
Song, I., Ahn, J., Shang, X., & Oh, J. H. (2020). Optoelectronic Property Modulation in Chiral Organic Semiconductor/Polymer Blends. ACS Applied Materials and Interfaces. https://doi.org/10.1021/acsami.0c17211
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