Advances in Colloidal Quantum Dots for Applications in the Visible Spectrum

Electronic states and wavefunctions of band-edge electrons and holes dictate optical, electronic, optoelectronic, and chemical properties of materials. Due to this customizability, QDs have been actively pursued as a new generation of functional materials in recent years. In comparison to organic optical and optoelectronic materials, semiconductor nanoparticles are thermodynamically stable inorganic single crystals rendering outstanding optical, chemical, and thermal stability. Contrasting with inorganic phosphors and bulk semiconductors, QDs have a monolayer coating of organic ligands that allows them to be compatible with inexpensive and energy-saving solution techniques.

The controlled production of high quality QDs is essential to fully exploiting their great potential. Fortunately, the synthetic chemistry of semiconductor materials has advanced rapidly in the past 20 years. Using their patented technology of “greener approaches” as a foundation, our vender (NNCrystal US Corporation) further developed scale-up schemes and established a world-class industrial production facility for fabricating high quality quantum dots. This progressive resource offers excellent accessibility to this unique class of optical and optoelectronic materials for scientists and engineers worldwide.

Specifically, the application-relevant products of quantum dots are made using core/shell structures, i.e., well-controlled epitaxial growth of wide-bandgap shell semiconductor onto core QDs. The organic ligands of these core/shell QDs are optimized for high-efficiency as well as stable emission and have recently been released to the market by our vendor with industrial-standard quality control (see Figure 3).

On the market, these quantum-dot enhanced LCD TVs are known as QLED TVs, or more precisely Quantum-Dot TVs, which offer otherwise-not-available color quality and color resolution (Figure 4). In Figure 4, three sets of characters are embedded in three backgrounds with similar color tone. The regular LCD can only display the blue characters (Figure 4, left). With the same blue-emitting GaN LEDs, the Quantum-Dot LCD can offer a clear display of all three sets of characters (Figure 4, right) when using green and red-emitting QDs as the back-lighting units.

In 2014, a paper published in Nature demonstrated that it is possible to fabricate high-performance quantum-dot light-emitting-diodes (QLEDs) with outstanding device lifetimes.1 The QLEDs with different electroluminescence colors not only have outstanding color quality and information-display properties as shown in Figure 4 but are also fabricated through inexpensive solution processes resulting in their rapid worldwide progress as the next generation display technology. In addition to their use in the display industry, QLEDs are also explored as possibilities for solid-state lighting.

Other types of optoelectronic devices centered around QDs, such as solar cells and photodetectors, are also under intense investigation in both industry and academic laboratories.2-4 Contrasted with competitive technologies, devices based on QDs are fabricated with low-cost solution techniques, critical for large-scale applications such as solar cells. After 20 years of development effort, the performance of both quantum-dot solar cells and photodetectors has continued to advance rapidly, with power-conversion efficiencies approaching that of industrial products.