## 1 Introduction  
Colloidal quantum dots (CQDs) have been assembled into a variety of devices, including light-emitting diodes (LEDs), solar cells, and biomedical sensors [1; 2; 3; 4; 5]. Excellent spectral tunability induced by quantum confinement effect associated with adjustable optical band gaps, simple solution process, and highly efficient photoluminescence (PL) quantum yields have motivated extensive research towards quantum dot light-emitting diodes (QLEDs), emerging as a next-generation display technology which can compete with organic light emitting diodes [5; 6; 7; 8; 9]. Understanding of the underlying mechanism for efficient emission has been one of the leading research themes, revealing that suppression of electric-field induced electrostatic interactions, including quantum dot (QD) charging and electric-field assisted exciton dissociation is crucial in reducing the external quantum efficiency (EQE) roll-off due to non-radiative loss, enabling integration of highly efficient QLEDs [10; 11; 12]. The main issue in structuring highly efficient QLEDs is to confine electrons and holes to the QD emission layer, suppressing the non-radiative recombination process [13; 14; 15]. Charge injection in the QD emission layer can lead to an unbalanced charge state that causes the non-radiative Auger process. QDs with a high surface to volume ratio are favorable for the formation of trap sites which facilitates the non-radiative process, quenching luminescence. To reduce the surface states, core QDs were shelled by inorganic or organic layers, and a variety of surface ligand chemistries were applied for surface passivation [16; 17; 18]. To improve the EQE, researchers have carried out structural engineering of hetero-structured QDs with adjustment of the shell thickness and the composition of the core/shell structure [19; 20]. To prevent the non-radiative Auger recombination process due to QD charging, an interfacial alloy layer was introduced between the core and the shell QDs, mitigating the steepness of the confinement potential [19]. [PERSON] et al. demonstrated an enhanced EQE by the insertion of an intermediate layer (CdSe\({}_{\text{x}}\)S\({}_{1-\text{x}}\)) between CdSe QD layers, suppressing the Auger decay process [12]. The conduction band edge was engineered for a higher energy level to reduce electron injection, improving charge balance within the QDs. Various strategies including QD core/shell alloying, the formation of giant QD, and the adjustment of QD stoichiometries have successfully led to significant improvement in the EQE [21; 22; 23]. Electric field-induced luminescence quenching arising from QD charging and spatial separation of electrons and holes has also been considered one of the primary sources of the low EQE in QD LEDs [10; 11]. Therefore, understanding the mechanism of electric field-induced luminescence quenching is crucial in enhancing the EQE. Particularly, distinguishing between field-induced charging and field-induced luminescence quenching has remained elusive because the two effects occur simultaneously during the emission process, complicating the separation of the isolated effect on the EQE loss. [PERSON] et al. reported the fabrication of a test platform device in which multi-layers of QDs are sandwiched by SiO2 dielectric films [10]. This test structure enabled a fundamental study on the effect of the electric field on the EQE, excluding the effect of QD charging due to charge injection. They revealed that electric field-induced spatial separation of electrons and holes still occurs with the presence of the relaxed confinement potentials in the core/shell structures designed to extend either electrons or holes into the shell. Importantly, most of the charge carrier was localized over long time scales, revealing that the primary source of luminescence quenching is electric field-induced coupling to the surface states of QDs.