Unraveling the effect of shell thickness on charge injection in blue quantum-dot light-emitting diodes

Although huge progress has been made for quantum-dot light-emitting diodes (QLEDs), blue devices remain a short slab limiting the development of QLED based applications. For blue devices, a fundamental and key question-how the shell thickness affects the charge injection-remains unclear by far. Herein, we show a comprehensive investigation on the effect of shell thickness on the charge injection by employing blue CdxZn1-xSeyS1-y/ZnS core/shell QDs with various shell thicknesses as the emission layers. We demonstrate that, during the electroluminescence onset processes, the fast and slow components of the rising edges characterized by transient electroluminescence spectra are strongly associated with the electron and hole injection processes, respectively. We find that the shell thickness influences the injection of electrons more dramatically than that of holes. A thick shell suppresses the electron injection and consequently reaches a more balanced charge injection into QDs, hence leading to an enhanced efficiency.