Quantum Dot Color-Converting Solids Operating Efficiently in the kW/cm² Regime

Abstract

With rapid progress in the use of colloidal quantum dots (QDs) as light emitters, the next challenge for this field is to achieve high brightness. Unfortunately, Auger recombination militates against high emission efficiency at multiexciton excitation levels. Here, we suppress the Auger-recombinationinduced photoluminescence (PL) quantum yield (QY) loss in CdSe/CdS core− shell QDs by reducing the absorption cross section at excitation wavelengths via a thin-shell design. Studies of PL vs shell thickness reveal that thin-shell QDs better retain their QY at high excitation intensities, in stark contrast to thickershell QDs. Ultrafast transient absorption spectroscopy confirms increased Auger recombination in thicker-shell QDs under equivalent external excitation intensities. We then further grow a thin ZnS layer on thin-shell QDs to serve as a higher conduction band barrier; this allows for better passivation and exciton confinement, while providing transparency at the excitation wavelength. Finally, we develop an isolating silica matrix that acts as a spacer between dots, greatly reducing interdot energy transfer that is otherwise responsible for PL reduction in QD films. This results in the increase of film PL QY from 20% to 65% at low excitation intensity. The combination of Auger reduction and elimination of energy transfer leads to QD film PL QY in excess of 50% and absolute power conversion efficiency of 28% at excitation powers of 1 kW/cm 2 , the highest ever reported for QDs under intense illumination.

Aggregation-induced emission in lamellar solids of colloidal perovskite quantum wells

“…Specifically, because of an effective reduction of the dielectric constant in low-dimensional semiconductors ( 2 ), a near-unity photoluminescence (PL) quantum yield (η PL ), namely, that nearly 100% of the photoexcited electron-hole pairs (or excitons) recombine radiatively, has been observed in the individual core/shell CQDs in solution ( 3 ). However, upon aggregation and forming crystalline QD solids, the η PL values typically drop to ~10 to 20% or lower ( 4 – 7 ), with only a few studies reporting up to ~60% ( 8 , 9 ), which is undesirable for high-intensity photonic applications, such as light-emitting diodes (LEDs) ( 4 ), downconversion (DC) phosphors ( 10 ), and lasers ( 11 ). For example, in the state-of-the-art QD LEDs, it has been proven that the thin-film η PL represents the upper limit of the internal quantum efficiency in devices ( 12 – 14 ).…”

Recent Advances in Hybrid Optical Materials: Integrating Nanoparticles within a Glass Matrix

“…The mixed and separate QDs configurations achieved a luminous efficacy of 33 and 47 lm W −1 , respectively. By incorporating core/shell CdSe/CdS QDs (shell thickness ≈1.1 nm) in a sol–gel silica matrix, a PL QY in excess of 50% in kW cm −2 excitation regime was reported . This result suggests that QDs‐embedded hybrid materials are advantageous for high‐power applications, which is complementary to existing phosphor‐based w‐LEDs for bright monochromic lighting.…”

“…In this report, prior to the incorporation of particles into silica‐based sol, they modified QDs surface with 3‐mercaptopropyltrimethoxysilane (3‐MPS) in order to stabilize particles and control their size 47b. The following researches in this field have studied different combinations of NPs and glasses, ranging from metal‐, semiconductor‐, and dielectric‐based11a,51 NPs, and involving various matrices, such as borosilicate‐, silica/alumina‐,47b,50 polyniobate‐, and silica‐based sol–gel systems 11a,45,49c,51a. All these works suggest that surface modification of OANPs is critical to stabilize as‐synthesized NPs, but also facilitate the homogeneous dispersion of NPs in sol (Figure c) 47a,48.…”

“…It is also worthwhile to note that a NPs–glass interaction is not necessarily detrimental, but can be beneficial. For example, Wang et al observed the increase of PL quantum yield (QY) when QDs are incorporated into the silica matrix, which they attributed to improved surface passivation (due to the QDs‐silica matrix interaction) combined with good separation of QDs . Milliron and co‐workers demonstrated that the reconstructed NbO x glass network and NP–glass interactions (i.e., covalent bonding) led to prominent improvements of optical contrast and durability of the resultant nanocomposites—ITO NPs‐embedded NbO x film .…”

“…An alternative strategy for producing w‐LEDs is to incorporate various luminescent QDs,26i,31b which generally relies on incorporating core/shell QDs in a glass matrix via the low‐temperature sol–gel doping method . The use of core/shell nanostructure can passivate surface quenchers (thus confining excitons to the QD core), which not only enhances QY of QDs (up to >90% for CdSe/CdZnS QDs family), but also improves their photostability (preventing diffusion of oxygen into the QD core) 31b.…”