The ability to synthesize nanoparticles with well-defined functionalities in colloidal solution, which gradually evolved in the last three decades, transformed many research and industrial areas, from biomedicine to energy generation and storage.
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Reducing the size of metal or semiconductor crystals to the nanoscale results in unique electronic and optical properties, such as high quantum efficiencies, precisely controllable emission wavelengths, and improved light absorption. These findings inspired the development of novel nanomaterial-based optoelectronic devices, such as light-emitting diodes, lasers, photovoltaic cells, and photodetectors.
Semiconductor materials, such as silicon, germanium, gallium arsenide, and others, used in high-performance electronic components, light sources, solar cells, and light detectors accelerated the development of modern electronics throughout the 20th century.
Today, optoelectronic devices have become ubiquitous, are fabricated from a wide range of materials, and exist in diverse forms, from microscopic sensors and implants to mobile devices and TVs, where active components span meters in size. Solar farms use square miles of semiconductor panels to generate sustainable electricity, while miniature lasers and photodetectors revolutionized telecommunications.
Colloidal Nanomaterials as an Alternative to Traditional Semiconductor Technology
However, semiconductors at the heart of electronic and optoelectronic devices require high-purity materials and involve costly manufacturing processes. In contrast, emerging applications often require a combination of the electronic and optical properties of inorganic semiconductors (high charge carrier mobility, precise n- and p-type doping, and the ability to engineer the energy bandgap of the material) and cost-efficient solution-based fabrication techniques suitable for industrial-scale manufacturing.
In recent years, colloidal semiconductor quantum dots (QDs) have become a versatile platform for integrating proven inorganic semiconductor materials into high-performance optoelectronic devices by using solution-based fabrication methods at ambient conditions instead of costly, ultra-high-vacuum, high-temperature device manufacturing processes.
Colloidal QDs are a type of semiconductor nanocrystals with a typical size in the range of 2-20 nm. They comprise a crystalline inorganic semiconductor core coated with a shell of organic molecules. Due to the presence of inorganic and organic constituents, these colloidal nanocrystals combine the chemical processibility of molecular compounds with the semiconductor materials' well-understood electronic and optical properties.
Spectra Tuning and Solution Processing of QDs for Optoelectronics
The synthesis of colloidal QDs via wet chemical methods enables precise control over the size and composition of the QDs by tuning the precursor type and concentration, reaction time, and temperature during synthesis.
The spatial confinement of the electron-hole pairs within the nanoscale semiconductor core of the QD (often of the order of magnitude of a few atoms) leads to a shift in the semiconductor's bandgap to higher energies with the reduction of the size of the nanocrystal.
The semiconductor core can be enclosed within a shell of a different semiconductor, thus forming a heterojunction that further modifies the energy bandgap of the QD. As a result of the synthetic and size flexibility, the bandgap of the colloidal QDs, and therefore their absorption and emission spectra, can be tuned over a wide spectral range from ultraviolet to infrared wavelengths.
Ongoing research and development efforts aim to exploit the size-controlled tunability of the optical properties of the colloidal QDs together with their ultra-high emission quantum yields (nearing 100%) in advanced screen display technology and lighting applications.
Due to their narrowband, spectrally tunable emission, QDs enable improved color rendering and better coverage of the entire color space than the existing phosphor materials. Some of these applications, such as QD-based TV screens, have already become commercially available technology.
Colloidal QDs Enable Electrically-Pumped Lasers
The latest breakthrough in applying solution-processable colloidal nanomaterials comes from the Los Alamos National Laboratory in the USA. There, a team of researchers utilized a densely packed layer of QDs to develop commercially viable electrically-pumped lasers and light-emitting diodes based on colloidal QD technology.
To achieve this, the scientist incorporated a 50 nm-thick light emitting medium, consisting of custom-synthesized QDs, into an optical resonator without obstructing charge-carrier flows into the QD layer.
By carefully designing the multilayered structure, the Los Alamos team achieved good confinement and amplification of the emitted light within the device. This was a key to the efficient excitation of the quantum dots by the ultrashort (around 100 femtoseconds) electrical pulses and obtaining a stable light emission and amplification.
Solution-Processed QDs for Thin and Flexible Photovoltaics
Colloidal QDs also have great potential for applications in the next generation of light-sensing and solar energy harvesting technologies. Owing to their tunable bandgap, the absorbance of the QDs can be engineered over a wide range of wavelengths, which is especially attractive for creating inexpensive infrared photodetectors.
Recently, researchers from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia developed a novel thin-film architecture for QD-based photovoltaic (PV) devices that improved both the stability and power conversion efficiency of the device.
The KAUST team developed a two-step technique to sandwich a photosensitive layer of colloidal QDs between two ultrathin layers of indium oxide and zinc oxide. These oxide layers are designed to quickly extract negative or positive charges generated by photoexcited QDs to an external circuit. In addition, the layers provide encapsulation and protection against the environment.
Comparison with a traditional PV device demonstrated that the ultrathin QD-base solar cell performed as efficiently as the traditional counterparts. The newly-developed technology paves the way toward inexpensive, thin-film PV devices prepared by scalable solution-based processes such as roll-by-roll manufacturing.
Quantum-Confined Semiconductor Nanomaterials Can Boost Solar Energy Capture
In the future, QDs could enable conceptionally new photoconversion strategies arising from the unique physical properties of the quantum-confined colloidal nanomaterials. For example, the process of charge carrier multiplication in QDs can generate multiple electron-hole pairs from a single absorbed photon.
Another promising concept is the QD-based luminescent solar concentrator, which collects solar energy over a large area and converts it to luminescence that is concentrated and directed to relatively small PV cells. Such devices would generate electricity even in low light conditions and can be incorporated into architectural structures as transparent elements.
Continue reading: The Advances in Microsphere Nano-Imaging.
References and Further Reading:
Jung, H., Ahn, N. & Klimov, V.I. (2021) Prospects and challenges of colloidal quantum dot laser diodes. Nat. Photon. 15, 643–655. Available at: https://doi.org/10.1038/s41566-021-00827-6
De Arquer, F.P.G., et al. (2021) Semiconductor quantum dots: Technological progress and future challenges. Science 373, 6555. Available at: https://doi.org/10.1126/science.aaz8541
Roh, J., et al. (2020) Optically pumped colloidal-quantum-dot lasing in LED-like devices with an integrated optical cavity. Nat Commun 11, 271. Available at: https://doi.org/10.1038/s41467-019-14014-3
Liu, M., et al. (2021) Colloidal quantum dot electronics. Nat Electron 4, 548–558. Available at: https://doi.org/10.1038/s41928-021-00632-7
Los Alamos National Laboratory (2021). Paving the path to electrically-pumped lasers from colloidal-quantum-dot solutions. [Online]. Phys.org Available at https://phys.org/news/2021-09-paving-path-electrically-pumped-lasers-colloidal-quantum-dot.html
Los Alamos National Laboratory (2021). Decades of research bring quantum dots to brink of widespread use. [Online]. Phys.org Available at: https://phys.org/news/2021-08-decades-quantum-dots-brink-widespread.html
J. Riordon (2020). Colloidal quantum dot laser diodes are just around the corner. [Online]. Phys.org Available at: https://phys.org/news/2020-01-colloidal-quantum-dot-laser-diodes.html
Kirmani, A.R., et al. (2020) Colloidal Quantum Dot Photovoltaics: Current Progress and Path to Gigawatt Scale Enabled by Smart Manufacturing. ACS Energy Letters 5 (9), 3069-3100. Available at: https://doi.org/10.1021/acsenergylett.0c01453
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