4.2 Auger recombination and multi-exciton dynamics

Auger recombination in quantum dots is a non-radiative process where energy from electron-hole recombination excites a third charge carrier. It competes with radiative recombination, reducing quantum yield and efficiency, especially in applications with high carrier densities like lasers and LEDs.

Multi-exciton states occur when multiple electron-hole pairs are excited in a single quantum dot. Their dynamics involve radiative and non-radiative recombination, including Auger processes. Understanding these phenomena is crucial for developing high-performance quantum dot-based devices and gaining insights into nanoscale physics.

Auger Recombination in Quantum Dots

Definition and Significance

• Auger recombination is a non-radiative recombination process where the energy released from an electron-hole pair recombination excites a third charge carrier (electron or hole) to a higher energy state
• Auger recombination competes with radiative recombination in quantum dots and can reduce the quantum yield and overall efficiency
• The rate of Auger recombination in quantum dots is higher than in bulk semiconductors due to the strong spatial confinement of charge carriers, which increases the probability of carrier-carrier interactions
• Auger recombination is particularly important in quantum dot-based applications involving high carrier densities (lasing, light-emitting diodes), where it can limit the performance and efficiency of the devices

Factors Affecting Auger Recombination Rates

• The size of the quantum dot plays a crucial role in determining the Auger recombination rate
  • As the size of the quantum dot decreases, the spatial confinement of charge carriers increases, leading to enhanced Auger recombination rates
• The composition and structure of the quantum dot can influence the Auger recombination rate
  • Core-shell quantum dots with a type-I band alignment can suppress Auger recombination by spatially separating the electrons and holes, reducing the probability of carrier-carrier interactions
• The shape of the quantum dot can affect the Auger recombination rate by modifying the electronic structure and the overlap of the electron and hole wavefunctions
  • Elongated or asymmetric quantum dots may exhibit reduced Auger recombination rates compared to spherical quantum dots
• The surface chemistry and passivation of the quantum dot can impact the Auger recombination rate by influencing the density of surface trap states and the efficiency of surface passivation
  • Proper surface passivation can minimize non-radiative recombination channels and reduce the Auger recombination rate
• Temperature affects the Auger recombination rate in quantum dots
  • At higher temperatures, the increased thermal energy can promote carrier-carrier interactions and enhance the Auger recombination rate
• The carrier density in quantum dots directly influences the Auger recombination rate
  • Higher carrier densities lead to an increased probability of carrier-carrier interactions and, consequently, higher Auger recombination rates

Dynamics of Multi-Exciton States

Formation and Processes

• Multi-exciton states in quantum dots occur when multiple electron-hole pairs (excitons) are simultaneously excited within a single quantum dot
• The formation of multi-exciton states depends on factors such as the excitation power, the size and composition of the quantum dot, and the temperature
• The dynamics of multi-exciton states in quantum dots are governed by various processes, including radiative recombination, Auger recombination, and exciton-exciton interactions

Radiative and Non-Radiative Recombination

• Radiative recombination in multi-exciton states can lead to the emission of multiple photons, resulting in phenomena such as biexciton and triexciton emission
• Auger recombination in multi-exciton states is typically faster than in single-exciton states due to the increased probability of carrier-carrier interactions, leading to a rapid depletion of the multi-exciton population
• Exciton-exciton interactions in multi-exciton states can result in phenomena such as exciton-exciton annihilation, where one exciton recombines non-radiatively and transfers its energy to another exciton, promoting it to a higher energy state

Factors Affecting Auger Rates

Quantum Dot Size and Composition

• Quantum dot size plays a crucial role in determining the Auger recombination rate
  • As the size of the quantum dot decreases, the spatial confinement of charge carriers increases, leading to enhanced Auger recombination rates
• Quantum dot composition and structure can influence the Auger recombination rate
  • Core-shell quantum dots with a type-I band alignment (CdSe/ZnS) can suppress Auger recombination by spatially separating the electrons and holes, reducing the probability of carrier-carrier interactions

Surface Chemistry and Carrier Density

• Surface chemistry and passivation of the quantum dot can impact the Auger recombination rate
  • Proper surface passivation (using ligands like oleic acid) can minimize non-radiative recombination channels and reduce the Auger recombination rate
• Carrier density in quantum dots directly influences the Auger recombination rate
  • Higher carrier densities (achieved through high excitation power) lead to an increased probability of carrier-carrier interactions and, consequently, higher Auger recombination rates

Implications of Auger Recombination vs Multi-Exciton Dynamics

Impact on Quantum Dot Applications

• Auger recombination and multi-exciton dynamics can limit the efficiency and performance of quantum dot-based optoelectronic devices (LEDs, lasers, solar cells)
  • In LEDs, Auger recombination can reduce the quantum yield and the overall efficiency of the device, particularly at high current densities
  • In lasers, Auger recombination can increase the lasing threshold and limit the maximum achievable output power
  • In solar cells, Auger recombination can reduce the power conversion efficiency by competing with the extraction of photogenerated carriers

Mitigation Strategies and Fundamental Insights

• Strategies to mitigate the effects of Auger recombination and multi-exciton dynamics in quantum dot applications include:
  • Engineering the size, composition, and structure of the quantum dots (using core-shell structures or alloyed compositions) to suppress Auger recombination and enhance radiative recombination
  • Optimizing the surface passivation and ligand chemistry (using organic or inorganic ligands) to minimize surface-related non-radiative recombination channels
  • Employing charge-separating architectures, such as type-II quantum dot heterostructures (CdSe/CdTe), to reduce the spatial overlap of electrons and holes and minimize Auger recombination
• Understanding and controlling Auger recombination and multi-exciton dynamics in quantum dots is crucial for developing high-performance quantum dot-based devices (single-photon sources, quantum light-emitting diodes, quantum dot lasers)
• The study of Auger recombination and multi-exciton dynamics in quantum dots provides valuable insights into fundamental physical processes (carrier-carrier interactions, exciton dynamics, non-radiative recombination mechanisms) in nanoscale systems