QLED vs. OLED Displays: A Physics-Based Comparison

Last Updated on September 8, 2024 by Narayan

Display technology has advanced significantly in recent years, with two of the most prominent types being QLED (Quantum Dot Light-Emitting Diode) and OLED (Organic Light-Emitting Diode) displays.

Understanding the physics behind these technologies helps to clarify their differences and potential applications. This article explores the fundamental principles of quantum mechanics, mechanisms of light emission and color generation, relevant mathematical equations, and a comparative analysis of QLED and OLED displays.

Fundamental Principles of QLED and OLED

QLED (Quantum Dot Light-Emitting Diode)

QLED displays use a layer of quantum dots, which are semiconductor nanocrystals, to produce light. The quantum dots are typically embedded in a film that is placed in front of an LED backlight.

The key to QLED technology is the ability of quantum dots to emit specific wavelengths of light when excited by the backlight, which enhances color and brightness. Quantum dots possess unique electronic and optical properties due to quantum confinement effects, which arise because of their nanoscale size (typically 2-10 nanometers).

OLED (Organic Light-Emitting Diode)

OLED displays, on the other hand, use organic molecules to produce light directly from the display pixels themselves. Each pixel in an OLED display is made up of organic compounds that emit light when an electric current passes through them. These organic materials include small molecules or polymers that are typically sandwiched between an anode and a cathode.

When a voltage is applied, electrons and holes are injected from the electrodes into the organic layer, where they recombine to produce excitons (electron-hole pairs) that subsequently decay, emitting light.

Mechanisms of Light Emission and Color Generation

QLED Displays

Light emission in QLED displays involves the following steps.

1. Excitation: The LED backlight emits blue light that excites the quantum dots.
2. Electron-Hole Recombination: Quantum dots absorb photons from the backlight, which promotes electrons from the valence band to the conduction band, creating excitons.
3. Energy Transfer and Emission: The excitons recombine, releasing energy in the form of visible light. The wavelength (color) of the emitted light is determined by the size of the quantum dots. Smaller dots emit shorter wavelengths (blue light), while larger dots emit longer wavelengths (red light).

The process of color generation in QLED displays is defined by the energy bandgap \((E_g)\) of the quantum dots, which dictates the energy of emitted photons as,
\[E = \frac{hc}{\lambda}\]
where \(E\) is the energy of the emitted photon, \(h\) is Planck’s constant, \(c\) is the speed of light, and \(\lambda\) is the wavelength of the emitted light.

OLED Displays

OLED displays emit light through the following process.

1. Charge Injection: Electrons and holes are injected into the organic layers from the cathode and anode, respectively.
2. Exciton Formation: These charges migrate through the organic layers and recombine to form excitons. The recombination of electrons and holes generates excitons in both singlet and triplet states.
3. Light Emission: The decay of these excitons to the ground state results in photon emission. The wavelength (color) of the emitted light depends on the energy gap of the organic molecules.

The color generation in OLEDs is dependent on the organic material’s molecular structure, where the electronic transitions between different molecular orbitals define the color of emitted light. The energy of the emitted photon can also be calculated using:
\[E = E_{\text{HOMO}} – E_{\text{LUMO}}\]
where \(E_{\text{HOMO}}\) represents the energy of the highest occupied molecular orbital, and \(E_{\text{LUMO}}\) denotes the energy of the lowest unoccupied molecular orbital.

Relevant Mathematical Equations

• Bandgap Energy:
  For quantum dots in QLED, the band gap is given by,
  \[E_g = \frac{h^2}{8R^2} \left( \frac{1}{m_e} + \frac{1}{m_h} \right) – 1.8 \frac{e^2}{\epsilon R}\]
  where (R) is the radius of the quantum dot, (m_e) and (m_h) are the effective masses of the electron and hole, respectively, (e) is the electron charge, and (\epsilon) is the dielectric constant.

• Quantum Yield:
  The efficiency of light emission, or quantum yield, is given by,
  \[\text{Quantum Yield} = \frac{\text{Number of photons emitted}}{\text{Number of photons absorbed}}\]

• Color Purity and Efficiency:
  The efficiency of an OLED display is determined by:
  \[\eta = \gamma \phi_{\text{ext}} \phi_{\text{int}}\]
  where \(\gamma\) is the charge balance factor, \(\phi_{\text{ext}}\) is the external quantum efficiency, and \(\phi_{\text{int}}\) is the internal quantum efficiency.

Comparative Analysis

Here’s the comparative analysis of QLED and OLED displays across different aspects.

Aspect	QLED	OLED
Optical Properties	Superior brightness and color volume due to quantum dot’s narrow emission spectra.	Better black levels and contrast ratios as each pixel emits light independently and can turn off completely.
Energy Efficiency	More energy-efficient at higher brightness levels due to the use of inorganic materials.	Less efficient at peak brightness but more efficient when displaying darker content, as it does not require a backlight.
Color Accuracy	Higher color accuracy and wider color gamut because quantum dots can be tuned to emit specific wavelengths.	Excellent color accuracy, but may suffer from color degradation over time due to organic material aging.
Brightness and Lifetime	Can achieve higher brightness levels and has a longer lifetime because quantum dots are more stable and less susceptible to burn-in.	Provides excellent viewing angles and faster response times but may suffer from shorter lifespans due to organic material degradation.
Impact of Nanomaterial Properties and Molecular Structure	Performance is affected by the size and composition of quantum dots, determining emission wavelengths and efficiency.	The efficiency of charge transport and light emission is dictated by the molecular structure and purity of organic materials, impacting overall performance.

Applications in Modern Displays

QLED and OLED technologies have found applications in a wide range of devices.

• QLED Displays: Commonly used in high-end TVs and monitors due to their brightness and color accuracy. Recent advancements include improvements in quantum dot synthesis, leading to even narrower emission spectra and better energy efficiency.
• OLED Displays: Predominantly used in smartphones, wearables, and TVs due to their excellent contrast ratios and flexibility. Innovations like tandem OLED structures and the use of new organic materials aim to extend the lifetime and improve the efficiency of OLED displays.

Future Prospects

• QLED: Future prospects include the development of electroluminescent quantum dots, potentially eliminating the need for a backlight and leading to self-emissive QLEDs.
• OLED: Ongoing research focuses on hybrid materials and phosphorescent emitters to enhance efficiency and extend lifespan.

Conclusion

Both QLED and OLED technologies offer unique advantages based on their underlying physics. QLED displays are better suited for environments requiring high brightness and color accuracy, while OLED displays provide unmatched contrast ratios and color richness in darker settings.

In the future, these technologies will likely complement each other, expanding the possibilities of modern display technology. Understanding the physics of each technology can help consumers make informed choices based on their needs.