Interface Stoichiometry Control of InP Heterostructure Quantum Dots for Efficient Light-Emitting Diodes

Colloidal quantum dots (QDs) have garnered extensive attention in optoelectronic applications due to excellent optical and electrical properties. A significant characteristic of colloidal QDs is the size-dependent optical properties. The band gap can be tuned by adjusting the particle size, enabling QDs to cover the entire visible light spectrum and near-infrared region which allows the QDs as a light-emitting diodes (LEDs), bio-imaging, photodetector, and solid-state lighting. The unique size-dependent characteristic of QDs is originate from quantum confinement effect which is observed when the QDs have smaller diameter than exciton Bohr radius. For example, the exciton Bohr radius of InP is 15 nm and 5.3 nm for CdSe. The size limitation of QDs compels the sophisticated synthesis of the emissive core materials on a nanometer scale. However, there is a rise in the surface-to-volume ratio when the size of the particles diminishes, leading to a significant increase in surface imperfections.<br/><br/>Indeed, the InP core exhibits photoluminescence quantum yields (PLQYs) under 1 % due to the surface defects. Forming a core/shell heterostructure in nanocrystals helps the core materials emit photons effectively. Epitaxially grown shells passivate the surface defects, such as dangling bonds, and provide the better confinement of electron/hole wavefunctions with type I band alignment. This strategy allows the InP-based QDs, which have heavy metal-free composition, to achieve near-unity PLQY. For a shell material, group II-VI materials such as ZnSe and ZnS are widely employed because they possess the wide energy band gap and low lattice mismatch with InP core. However, there is a fundamental interface misfit between the III-V composition of the InP core and the II-V composition of the shell materials. The heterogeneity between the III-V core and II-VI shell could impede radiative recombination or charge carrier transport.<br/><br/>In this study, we investigate the effects of interface stoichiometry in InP-based core/shell structure QDs on optical and electrical properties. We control the chemical composition at interface of InP core/shell QDs to improve the charge carrier injection efficiency in QD-based LEDs (QD-LEDs) by in-situ selenium treatment. This strategy allows the excess indium and zinc ions to be anchored on the surface during the InP core synthesis, resulting in the formation of In/Zn thin alloy layer. The surface stoichiometry of the in-situ Se-treated InP core can be tuned by controlling the feed ratio of indium and zinc precursor. The change of binding energy of selenium depending on the In/Zn feed ratio in X-ray photoelectron spectroscopy (XPS) analysis confirmed the formation of In/Zn alloy layer. Photoluminescence (PL) analysis showed that the InP core with In/Zn alloy layer exhibit higher PLQYs than InP core with Zn-rich interface even after thick shell growth. Ultraviolet photoelectron spectroscopy (UPS) analysis revealed that the in-situ Se treatment modifies the energy band alignment, leading to an enhanced charge carrier balance in QD-LEDs. As a result, the QD-LEDs based on the Se-treated InP QDs show peak current efficiency as 31.5 cd/A and a maximum luminance of 26370 cd/m² for green emission.