Suppressing Förster Resonance Energy Transfer in Close‐Packed Quantum‐Dot Thin Film: Toward Efficient Quantum‐Dot Light‐Emitting Diodes with External Quantum Efficiency over 21.6%

Abstract

the presence of Förster resonance energy transfer (FRET) among the close-packed QDs, [19][20][21][22][23][24] which thus limits the efficiency of QD LEDs (QLEDs). FRET is a nonradiative energy transfer process, in which the energy is down-transferred from a fluorescent donor to an acceptor via the dipoledipole coupling if the distance between donor and acceptor is smaller than the transfer radius (typically <10 nm). [24][25][26] In close-packed QD solids, interdot FRET can be very efficient due to the small distance between proximal dots. [25][26][27] Via interdot FRET, excitons therefore can migrate from small dots to nearby large dots, consequently leading to the redshift of the emission spectra. [24,[26][27][28] Along with the redshifted emission, the PL intensity is remarkably decreased, which is caused by the finite probability of nonradiative deactivation whenever an exciton transfers to a new dot with defects. [28] The defective dots function as effective quenching sites that can rapidly quench the excitons of surrounding QDs, leading to the reduction of QY of QD ensembles. Therefore, to improve the PL QY of QD solids, the interdot FRET should be suppressed. Since the FRET rate is very sensitive to the distance between donor and acceptor, [25][26][27]29] we can increase the interdot distance to reduce the FRET rate. This could be realized by tailoring the structure of QDs. For example, by designing giant QDs with thick shell, the devices exhibited an improved performance due to the suppression of interdot FRET. [22,[30][31][32] Alternatively, by embedding the QDs in a polymer matrix, the QDs can be separated spatially, which increases the interdot distance and thereby decreases the interdot FRET. [33][34][35][36][37] However, by physically blending the QDs with polymer, it is difficult to uniformly disperse the QDs within polymer matrix due to phase separation. [33] Recently, by chemically capping the surface of QDs with copolymer, a more homogeneous distribution of QDs is achieved. [34,36] Although increasing the shell thickness or embedding the QDs into a polymer matrix can lead to the suppression of FRET, [22,[30][31][32][33][34] the charge injection and transport are adversely affected by the thick shell or polymer matrix. Therefore, it remains challenging to obtain a device with nearunity internal quantum efficiency (IQE) though QDs with high PL QY of over 90% are routinely reported. Colloidal II-VI quantum dots (QDs) exhibit near-unity photoluminescence (PL) quantum yield (QY) when they are dissolved in dilute solution. However, when they are assembled into thin film, the PL QY is decreased significantly due to the presence of nonradiative Förster resonance energy transfer (FRET) among the close-packed QDs. In this work, the FRET is suppressed by developing a binary-QD light emission layer (EML), where blue-QDs are introduced as spacers to spatially separate the red-QDs. Due to the separation of red-QDs, the FRET among red-QDs is effectively suppressed and thus the emission properties of red-QDs are ...

Enhanced Direct White Light Emission Efficiency in Quantum Dot Light‐Emitting Diodes via Embedded Ferroelectric Islands Structure

“…[21][22][23][24][25] This phenomenon becomes more apparent by exhibiting the decrease in the PLQY in the QD film where the distance between QDs is reduced as shown in Figure S1, Supporting Information. [26] The PL characteristics of QD films showed an opposite trend compared to those of the QD solutions: the intensity of PL in the films is I red > I green ≈ I blue whereas the PL intensity in the solutions is I blue > I green ≈ I red . This resulted from the increased FRET process in the film state due to reduced distance between QDs.…”

“…This is due to the FRET from blue to green and red QDs, which is well consistent with the PLQY measurement, that is, when the mixed QDs formed a QD layer, PLQY of blue QDs significantly decreased as FRET increased due to the reduced dot-to-dot distance which was discussed in Figure 1. [21][22][23][24][25][26]28] However, the severe imbalance of the RGB ratio leads to degradation of white QLED performance because a turn-on voltage of a QLED would increase due to the high ratio of blue QDs with the larger bandgap as evidenced by the turn-on voltage of warm and daylight white QLEDs in Table 1. Therefore, smaller amount of blue QDs is desirable with the same CCT value.…”

Full-Color Quantum Dot Light-Emitting Diodes Based on Microcavities

“…Thus, FRET is probably the primary cause for the difference. A recent work [38] demonstrated that the FRET among red-QDs can be suppressed by blue-QDs as spacers. This implies that FTIR will be suppressed to different degrees for different color quantum dots depending on the ratio of RGB QDs.…”

37.1: Invited Paper: Efficient and Ultra‐Bright Quantum‐Dot Light‐Emitting Diodes

“…Then, by using the wide bandgap QDs as spacers, FRET among QDs can be effectively suppressed, leading to an improved QY of QD solids. As a result, QLEDs with binary-QD EML exhibit an EQE of over 22% [2]. We then improve the thermal stability of QDs by replacing the conventional OA ligand with DDT.…”