Quantum Dots: Advantages, Disadvantages, and Working Principles

This article explores the advantages and disadvantages of quantum dots, including their working principles.

What are Quantum Dots? | Working Principle

Quantum dots are incredibly small semiconductor particles, measured in nanometers. Their tiny size gives them unique optical and electronic properties compared to larger particles. Different types of quantum dots emit specific frequencies when exposed to electricity or light. The emitted frequency can be fine-tuned by adjusting the size, shape, and material composition of the dots.

Figure 1: Structure of Quantum Dots

The figure above shows a typical quantum dot structure. Often, Cadmium Selenide (CdSe) forms the inner core, surrounded by an outer shell like Cd 1-x Zn x S. This is then often coated in silica to prevent oxidation. The outer shell acts as an absorber.

When a photon interacts with a quantum dot, an electron moves from its valence band to the conduction band, leaving behind a “hole” in the valence band.

Figure 2: Working of a Quantum Dot Solar Cell

The figure above depicts how a quantum dot solar cell functions. Quantum dots can capture excess photon energy, which is normally lost to heat, through a process called multiple exciton generation. Incident light passes through the transparent electrode of the solar cell and onto the light-absorbing layer of quantum dots, creating electron-hole pairs (e-/h+). These charged particles then separate and travel to their respective electrodes, generating an electric current.

Benefits or Advantages of Quantum Dots

Here are some key advantages of using quantum dots:

• Enhanced Displays: They are extensively used in the television industry for their ultra-high-definition colors and wider viewing angles.
• Improved Optoelectronics: They boost the output of photovoltaics, light sensors, photocatalysts, and other optoelectronic devices by efficiently absorbing light. They’re used in solar cells to produce energy around the clock.
• Solar Energy Harvesting: Quantum dot solar cells can absorb a broad spectrum of light, from ultraviolet and visible to infrared, allowing energy production during both day and night, making solar cells more efficient.
• Superior to Dyes: CdSe quantum dots are significantly brighter than fluorophore dyes, up to twenty times brighter in some cases.
• Tunable Emission: The wavelength of emitted light can be easily adjusted within the 400 to 4000 nm range, allowing for the creation of different colors.
• Cost-Effective Production: Quantum dot cells can be manufactured using low-cost solution-phase chemistry methods and are compatible with high-speed printing techniques.
• Long Lifespan: A thin layer of quantum dots on normal glass can have a lifespan of up to 14 years.
• Chip Compatibility: They integrate well with existing chip technology.

Drawbacks or Disadvantages of Quantum Dots

Despite their advantages, quantum dots also have some drawbacks:

• Toxicity: CdSe-based quantum dots are highly toxic and require a stable polymer shell for safe handling.
• Shell Effects: These protective shells can alter the optical properties of the dots, and controlling the particle size can be challenging.
• Biodegradation Concerns: The degradation of quantum dots within living organisms has been a topic of study and concern.
• Lower Efficiency: The overall energy conversion efficiency of quantum dot devices can be lower compared to other technologies.
• Temperature Sensitivity: Quantum dots may require lower operating temperatures.
• Production Yield: Achieving high device production yields can be difficult.
• De-coherence: They can suffer from de-coherence issues.