🔬Quantum Dots and Applications Unit 9 – Quantum Dots: Displays and Lighting Tech

Fundamentals of Quantum Dots

• Quantum dots (QDs) are nanoscale semiconductor crystals typically ranging from 2-10 nanometers in diameter
• Exhibit unique optical and electronic properties due to quantum confinement effects resulting from their small size
• Composed of elements from groups II-VI (CdSe, CdTe, ZnS), III-V (InP, InAs), or IV-VI (PbS, PbSe) of the periodic table
• Size and composition of QDs determine their bandgap energy and, consequently, their emission wavelength
  • Smaller QDs emit shorter wavelengths (blue) while larger QDs emit longer wavelengths (red)
• Possess high quantum yield, meaning they efficiently convert absorbed light into emitted light
• Exhibit narrow emission spectra, enabling pure and saturated colors
• Offer tunable optical properties by controlling their size, shape, and composition during synthesis

Optical Properties and Light Emission

• QDs absorb photons with energy greater than their bandgap, promoting electrons from the valence band to the conduction band
• Excited electrons relax to the edge of the conduction band and recombine with holes in the valence band, releasing energy as photons
• Emission wavelength depends on the bandgap energy, which is determined by the size and composition of the QDs
  • Larger QDs have a smaller bandgap and emit longer wavelengths
  • Smaller QDs have a larger bandgap and emit shorter wavelengths
• Exhibit high color purity and narrow emission linewidths (full width at half maximum or FWHM) typically around 20-40 nm
• Possess large Stokes shift, meaning a significant difference between absorption and emission wavelengths, minimizing self-absorption
• Demonstrate high photostability and resistance to photobleaching compared to organic dyes
• Enable efficient energy transfer between QDs of different sizes through Förster resonance energy transfer (FRET)

Quantum Dot Synthesis and Fabrication

• Colloidal synthesis is the most common method for producing high-quality QDs
  • Involves the reaction of precursors in a coordinating solvent at elevated temperatures
  • Typical precursors include organometallic compounds (e.g., cadmium oleate) and chalcogen sources (e.g., trioctylphosphine selenide)
• Hot-injection method enables precise control over QD size and size distribution
  • Rapid injection of precursors into a hot solvent leads to burst nucleation and controlled growth
• Microwave-assisted synthesis offers faster reaction times and uniform heating compared to traditional hot-injection methods
• Surface passivation with a wider-bandgap semiconductor shell (e.g., ZnS) enhances quantum yield and stability
  • Core-shell structure confines charge carriers within the core, reducing surface defects and non-radiative recombination
• Ligand exchange processes can render QDs water-soluble or compatible with various matrices for device integration
• Purification techniques, such as precipitation and centrifugation, remove excess reagents and isolate monodisperse QDs

Display Technologies: LCD vs QLED

• Liquid Crystal Displays (LCDs) rely on a backlight, typically a white LED, and color filters to produce images
  • Color filters absorb a significant portion of the backlight, reducing efficiency
  • Limited color gamut due to the broad emission spectra of white LEDs and overlapping color filters
• Quantum Dot Light Emitting Diodes (QLEDs) utilize electroluminescence from QDs to generate light directly
  • QDs are sandwiched between electron and hole transport layers, forming a p-n junction
  • Applying a voltage injects electrons and holes into the QD layer, where they recombine and emit light
• QLEDs offer several advantages over LCDs:
  • Wider color gamut due to the narrow emission spectra of QDs
  • Higher efficiency as no color filters are required, and QDs have high quantum yields
  • Thinner displays due to the elimination of the backlight unit
  • Potential for flexible and transparent displays by using suitable substrates and electrodes
• Challenges in QLED development include improving charge injection efficiency, minimizing non-radiative recombination, and ensuring long-term stability

Quantum Dot Enhancement Films (QDEF)

• QDEF is a technology that integrates QDs into LCD backlights to improve color performance
• Consists of a thin film containing red and green QDs placed between the blue LED backlight and the LCD panel
• Blue light from the LED excites the QDs, which emit narrow-band red and green light
  • Resulting white light has a wider color gamut compared to conventional white LEDs
• QDEF allows for a more efficient use of the backlight, as the QDs absorb the blue light and re-emit it as red and green
• Enables LCD displays to achieve wider color gamuts, such as DCI-P3 or Rec. 2020, without sacrificing efficiency
• Offers a cost-effective solution for improving color performance in LCDs compared to developing full QLED displays
• Challenges include maintaining QD stability under prolonged exposure to high-intensity blue light and minimizing light scattering within the film

Lighting Applications and Efficiency

• QDs can be used to create high-efficiency, high-quality white light sources
• QD-based white LEDs generate white light by combining a blue LED with red and green QDs
  • Blue LED excites the QDs, which emit red and green light, resulting in a broad-spectrum white light
• QD-based white LEDs offer several advantages over conventional white LEDs:
  • Higher color rendering index (CRI) due to the broader emission spectrum
  • Tunable correlated color temperature (CCT) by adjusting the ratio of red and green QDs
  • Higher luminous efficacy (lumens per watt) due to the narrow emission spectra of QDs
• QDs can also be used in combination with phosphors to create hybrid white LEDs with improved color rendering and efficiency
• QD-based lighting can mimic natural daylight, offering potential benefits for human health and well-being
• Challenges include ensuring long-term stability of QDs under high-intensity light exposure and developing cost-effective manufacturing processes

Challenges and Future Developments

• Cadmium-based QDs face regulatory challenges due to the toxicity of cadmium
  • Development of cadmium-free alternatives, such as InP and ZnSe QDs, is crucial for widespread adoption
• Improving the stability and lifetime of QDs in various operating conditions (high temperature, humidity, and light exposure)
  • Encapsulation techniques and advanced shell materials can enhance QD stability
• Scaling up QD synthesis while maintaining high quality and consistency
  • Continuous flow reactors and microfluidic systems offer potential solutions for large-scale production
• Developing cost-effective and efficient patterning techniques for QD integration in displays
  • Inkjet printing, photolithography, and transfer printing are being explored
• Investigating the potential of QD-based lasers and single-photon sources for quantum computing and secure communication applications
• Exploring the use of QDs in other areas, such as solar cells, bio-imaging, and drug delivery
  • Surface functionalization and ligand engineering enable targeted delivery and enhanced biocompatibility

Real-World Applications and Case Studies

• Samsung's QLED TVs utilize QDEF technology to achieve wide color gamuts and high brightness
  • Flagship models offer 100% DCI-P3 coverage and peak brightness levels exceeding 1,500 nits
• TCL's 6-Series Roku TVs employ QDEF to deliver vivid colors and improved contrast
  • Combination of QDEF and local dimming enables deeper blacks and higher dynamic range
• Hisense's ULED TVs leverage QDEF to enhance color performance and overall picture quality
  • Proprietary algorithms optimize the backlight and QD emission for various viewing conditions
• Nanosys, a leading QD manufacturer, has partnered with various display makers to integrate their Quantum Dot technology
  • Their QDEF solutions are used in displays for TVs, monitors, and laptops
• Osram's Osconiq S 3030 QD LED offers high efficiency and color rendering for lighting applications
  • Combines a blue LED with red and green QDs to achieve a CRI of 90 and an efficacy of 173 lumens per watt
• Nanoco Group has developed cadmium-free QDs for use in displays and lighting
  • Their CFQD Quantum Dot technology is based on indium phosphide (InP) QDs, offering a more environmentally friendly alternative