Highly Efficient and Low Turn-On Voltage Quantum Dot Light-Emitting Diodes by Using a Stepwise Hole-Transport Layer

Highly efficient red quantum dot light-emitting diodes (QD-LEDs) with a very high current efficiency of 16 cd/A were demonstrated by adopting stepwise hole-transport layers (HTLs) consisting of 4,4′-<i>N</i>,<i>N</i>′-dicarbazole-biphenyl (CBP) combined with <i>N</i>,<i>N</i>′-dicarbazolyl-3,5-benzene (mCP). The mCP layer plays two important roles in this kind of QD-LEDs. One is that it can block the electron to leak into the HTL due to its higher LUMO (LUMO = the lowest unoccupied molecular orbital) energy level than that of CBP; and the other is it can separate the carrier accumulation zone from the exciton formation interface, which is attributed to the stepwise hole-transport layer structure. Moreover, the lower HOMO (HOMO = the highest occupied molecular orbital) energy level of mCP decreases the hole-injection barrier from the HTL to the QD emitting layer, which improves the charge carrier balance injected into the QD layer, reducing the turn-on voltage of QD-LEDs fabricated with the stepwise HTL structure

Exploring the Effect of Band Alignment and Surface States on Photoinduced Electron Transfer from CuInS₂/CdS Core/Shell Quantum Dots to TiO₂ Electrodes

Photoinduced electron transfer (ET) processes from CuInS₂/CdS core/shell quantum dots (QDs) with different core sizes and shell thicknesses to TiO₂ electrodes were investigated by time-resolved photoluminescence (PL) spectroscopy. The ET rates and efficiencies from CuInS₂/CdS QDs to TiO₂ were superior to those of CuInS₂/ZnS QDs. An enhanced ET efficiency was surprisingly observed for 2.0 nm CuInS₂ core QDs after growth of the CdS shell. On the basis of the experimental and theoretical analysis, the improved performances of CuInS₂/CdS QDs were attributed to the passivation of nonradiative traps by overcoating shell and enhanced delocalization of electron wave function from core to CdS shell due to lower conduction band offset. These results indicated that the electron distribution regulated by the band alignment between core and shell of QDs and the passivation of surface defect states could improve ET performance between donor and acceptor

Photoinduced Charge Separation and Recombination Processes in CdSe Quantum Dot and Graphene Oxide Composites with Methylene Blue as Linker

The charge separation and recombination processes between CdSe quantum dot (QD) and graphene oxide (GO) composites with linking molecule methylene blue (MB⁺) were studied by femtosecond transient absorption spectroscopy. Anchoring MB⁺ molecules on GO results in significant changes in steady-state and transient absorption spectra, where the exciton dissociation time in the CdSe QD-MB⁺-GO composite was determined to be 1.8 ps. Surprisingly, the ground state bleaching signal increased for MB⁺-GO complex was found to be 5.2 ps, in relation with electron transfer from QD to GO. On the other hand, the strong electronic coupling between MB<b>^•</b>-GO radical and GO prolonged charge recombination process (≥5 ns) in QD-MB⁺-GO composites. Charge separation and recombination processes at the interface between semiconductor QDs and graphene can thus be modulated by the functionalized dye molecules

Size- and Composition-Dependent Energy Transfer from Charge Transporting Materials to ZnCuInS Quantum Dots

We studied the energy transfer processes from organic charge transporting materials (CTMs) to ZnCuInS (ZCIS) quantum dots (QDs) with different emission wavelength by steady-state and time-resolved photoluminescence (PL) spectroscopy. The change in the PL excitation intensity of the ZCIS QDs and the PL decay time of the CTMs clearly demonstrated an efficient energy transfer process in the ZCIS/CTM blend films. It was found that the efficiency of Förster resonance energy transfer significantly increases with increasing the particle size and decreasing the Zn content in the QDs, which is well consistent with the estimated Förster radii (<i>R</i>₀) varying from 3 to 5 nm. In addition, the PL quenching of the QDs related to the charge separation process was also observed in some of the samples. The energy transfer and charge separation processes in the films were well explained based on the band alignment between the ZCIS QDs and CTMs

Photoinduced Charge Separation and Recombination Processes in CdSe Quantum Dot and Graphene Oxide Composites with Methylene Blue as Linker

The charge separation and recombination processes between CdSe quantum dot (QD) and graphene oxide (GO) composites with linking molecule methylene blue (MB⁺) were studied by femtosecond transient absorption spectroscopy. Anchoring MB⁺ molecules on GO results in significant changes in steady-state and transient absorption spectra, where the exciton dissociation time in the CdSe QD-MB⁺-GO composite was determined to be 1.8 ps. Surprisingly, the ground state bleaching signal increased for MB⁺-GO complex was found to be 5.2 ps, in relation with electron transfer from QD to GO. On the other hand, the strong electronic coupling between MB<b>^•</b>-GO radical and GO prolonged charge recombination process (≥5 ns) in QD-MB⁺-GO composites. Charge separation and recombination processes at the interface between semiconductor QDs and graphene can thus be modulated by the functionalized dye molecules

Ultrafast Carrier Dynamics and Hot Electron Extraction in Tetrapod-Shaped CdSe Nanocrystals

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB₂, CB₃₍₂₎, and CB₄) to the lowest electronic state (CB₁) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states

Ultrastrong Absorption Meets Ultraweak Absorption: Unraveling the Energy-Dissipative Routes for Dye-Sensitized Upconversion Luminescence

Dye sensitization is becoming a new dimension to highly improve the upconversion luminescence (UCL) of lanthanide-doped upconversion nanoparticles (UCNPs). However, there is still a lack of general understanding of the dye–UCNPs interactions, especially the confused large mismatch between the inputs and outputs. By taking dye-sensitized NaYF₄:Yb/Er@NaYF₄:Nd UCNPs as a model system, we not only revealed the in-depth energy-dissipative process for dye-sensitized UCL but also confirmed the first ever experimental observation of the energy back transfer (EBT) in the dye-sensitized UCL. Furthermore, this energy-dissipative EBT restricted the optimal ratio of dyes to UCNP. By unearthing all of the energy loss behind the EBT, energy transfer, and energy migration processes, this paper sheds light on the further design of effective dye-sensitized nanosystems for UCL or even downconversion luminescence

Doping Lanthanide into Perovskite Nanocrystals: Highly Improved and Expanded Optical Properties

Cesium lead halide (CsPbX₃) perovskite nanocrystals (NCs) have demonstrated extremely excellent optical properties and great application potentials in various optoelectronic devices. However, because of the anion exchange, it is difficult to achieve white-light and multicolor emission for practical applications. Herein, we present the successful doping of various lanthanide ions (Ce³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Er³⁺, and Yb³⁺) into the lattices of CsPbCl₃ perovskite NCs through a modified hot-injection method. For the lanthanide ions doped perovskite NCs, high photoluminescence quantum yield (QY) and stable and widely tunable multicolor emissions spanning from visible to near-infrared (NIR) regions are successfully obtained. This work indicates that the doped perovskite NCs will inherit most of the unique optical properties of lanthanide ions and deliver them to the perovskite NC host, thus endowing the family of perovskite materials with excellent optical, electric, or magnetic properties