Electroluminescence in quantum dots arises from the radiative recombination of excitons generated through charge injection, offering unique advantages such as narrow emission spectra, tunable bandgaps, and high photoluminescence quantum yields. The process involves several key steps: charge carrier injection, exciton formation, and radiative recombination, each contributing to the overall efficiency of the device. Understanding the underlying physics is critical for optimizing performance and mitigating efficiency losses.

Charge injection into quantum dots is typically achieved through electron and hole transport layers in a device structure such as a quantum dot light-emitting diode (QLED). Electrons are injected from the cathode into the conduction band of the quantum dot, while holes are injected from the anode into the valence band. The energy level alignment between the transport layers and the quantum dots is crucial for efficient injection. Large energy barriers can lead to poor charge injection, increasing the driving voltage and reducing efficiency. For example, in CdSe-based quantum dots, the conduction band minimum is around 3.7 eV, requiring an electron transport layer with a low work function, such as ZnO (4.2 eV), to minimize the injection barrier. Similarly, hole transport materials like poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or poly-TPD are selected to match the valence band maximum of the quantum dots.

Once charge carriers are injected into the quantum dot, they form excitons—bound electron-hole pairs—through Coulombic attraction. The exciton binding energy in quantum dots is significantly higher than in bulk semiconductors due to quantum confinement, often exceeding 100 meV for CdSe quantum dots with diameters below 5 nm. This strong confinement enhances radiative recombination by localizing the electron and hole wavefunctions within a small volume. However, exciton formation competes with non-radiative processes such as Auger recombination and charge trapping. Auger recombination, where the energy from electron-hole recombination is transferred to a third carrier, becomes particularly significant at high carrier densities, leading to efficiency losses in high-brightness devices.

Radiative recombination of excitons produces electroluminescence with a wavelength determined by the quantum dot bandgap. The emission spectrum is narrow, typically with a full width at half maximum (FWHM) of 20–30 nm, due to the discrete energy levels imposed by quantum confinement. The color purity and tunability of quantum dot electroluminescence make them attractive for applications requiring precise color control. However, several factors influence the radiative recombination efficiency, including surface defects, charge imbalance, and electric field-induced quenching.

Surface defects on quantum dots act as trap states that capture charge carriers, preventing them from forming excitons or promoting non-radiative recombination. For instance, unsaturated bonds on the surface of CdSe quantum dots can create mid-gap states that trap electrons or holes. Passivation strategies, such as shell growth (e.g., ZnS shells on CdSe cores) or ligand exchange, are employed to reduce surface traps. A well-passivated quantum dot can achieve photoluminescence quantum yields exceeding 90%, but electroluminescence efficiency in devices is often lower due to additional losses.

Charge imbalance occurs when the injection rates of electrons and holes are unequal, leading to an excess of one carrier type. This imbalance increases the likelihood of non-radiative Auger recombination, where the excess carriers dissipate energy through interactions with other charges rather than emitting light. Optimizing the charge transport layers to balance injection is essential for maximizing efficiency. For example, in QLEDs, the electron mobility of ZnO is typically higher than the hole mobility of organic transport layers, leading to electron-dominated current. Introducing hole injection layers with higher mobility or incorporating electron-blocking layers can help restore balance.

Electric field-induced quenching is another loss mechanism where a strong internal electric field dissociates excitons before they can recombine radiatively. This effect is particularly pronounced in thick quantum dot layers or under high driving voltages. Reducing the quantum dot layer thickness and optimizing the device architecture to minimize the electric field across the emissive layer can mitigate this loss.

The external quantum efficiency (EQE) of quantum dot electroluminescent devices is the product of several factors: the charge balance factor, the fraction of excitons that recombine radiatively, the photoluminescence quantum yield of the quantum dots, and the outcoupling efficiency. State-of-the-art QLEDs have achieved EQEs exceeding 20%, with red-emitting devices often outperforming green and blue due to lower non-radiative losses. Blue-emitting quantum dots face additional challenges, such as higher Auger recombination rates and difficulty in maintaining stability under electrical excitation.

Material selection plays a critical role in determining device performance. For example, InP-based quantum dots are being explored as cadmium-free alternatives, though their lower exciton binding energy and broader emission spectra present challenges. Perovskite quantum dots offer high defect tolerance and high luminescence efficiency but suffer from instability under electrical bias. Each material system requires tailored device architectures to address its specific loss mechanisms.

In summary, electroluminescence in quantum dots is governed by charge injection dynamics, exciton formation, and recombination processes. Efficiency losses arise from surface defects, charge imbalance, Auger recombination, and electric field effects. Advances in quantum dot synthesis, surface passivation, and device engineering continue to push the boundaries of performance, making quantum dots a promising technology for high-efficiency light-emitting applications.