The performance of semiconductor devices is fundamentally governed by charge-carrier dynamics within the active materials1–6. Although advances have been made towards understanding these dynamics ...under steady-state conditions, the importance of non-equilibrium phenomena and their effect on device performances remains elusive7,8. In fact, the ballistic propagation of carriers is generally considered to not contribute to the mechanism of photovoltaics (PVs) and light-emitting diodes, as scattering rapidly disrupts such processes after carrier generation via photon absorption or electric injection9. Here we characterize the spatiotemporal dynamics of carriers immediately after photon absorption in methylammonium lead iodide perovskite films using femtosecond transient absorption microscopy (fs-TAM) with a 10 fs temporal resolution and 10 nm spatial precision. We found that non-equilibrium carriers propagate ballistically over 150 nm within 20 fs of photon absorption. Our results suggest that in a typical perovskite PV device operating under standard conditions, a large fraction of carriers can reach the charge collection layers ballistically. The ballistic transport distance appears to be limited by energetic disorder within the materials, probably due to disorder-induced scattering. This provides a direct route towards optimization of the ballistic transport distance via improvements in materials and by minimizing the energetic disorder. Our observations reveal an unexplored regime of carrier transport in perovskites, which could have important consequences for device performance.Charge-carrier dynamics are fundamental to the operation and performance of semiconductor devices. In methylammonium lead iodide perovskites, carriers in the non-equilibrium regime after excitation propagate ballistically over 150 nm within 20 fs.
Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in ...these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH3(CH2)3NH3)2PbI4 and hexylammonium lead iodide (CH3(CH2)5NH3)2PbI4, both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton–phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm–1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 and 137 cm–1. Using the determined optical phonon energies, we analyzed photoluminescence broadening mechanisms. At low temperatures (<100 K), the broadening is due to acoustic phonon scattering, whereas at high temperatures, LO phonon–exciton coupling is the dominant mechanism. Our results help explain the broad photoluminescence line shape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton–phonon coupling in these materials.
The generation, control and transfer of triplet excitons in molecular and hybrid systems is of great interest owing to their long lifetime and diffusion length in both solid-state and solution phase ...systems, and to their applications in light emission
, optoelectronics
, photon frequency conversion
and photocatalysis
. Molecular triplet excitons (bound electron-hole pairs) are 'dark states' because of the forbidden nature of the direct optical transition between the spin-zero ground state and the spin-one triplet levels
. Hence, triplet dynamics are conventionally controlled through heavy-metal-based spin-orbit coupling
or tuning of the singlet-triplet energy splitting
via molecular design. Both these methods place constraints on the range of properties that can be modified and the molecular structures that can be used. Here we demonstrate that it is possible to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic insulating nanoparticles. This allows the classically forbidden transitions from the ground-state singlet to excited-state triplets to gain oscillator strength, enabling triplets to be directly generated on molecules via photon absorption. Photogenerated singlet excitons can be converted to triplet excitons on sub-10-picosecond timescales with unity efficiency by intersystem crossing. Triplet exciton states of the molecules can undergo energy transfer to the lanthanide ions with unity efficiency, which allows us to achieve luminescent harvesting of the dark triplet excitons. Furthermore, we demonstrate that the triplet excitons generated in the lanthanide nanoparticle-molecule hybrid systems by near-infrared photoexcitation can undergo efficient upconversion via a lanthanide-triplet excitation fusion process: this process enables endothermic upconversion and allows efficient upconversion from near-infrared to visible frequencies in the solid state. These results provide a new way to control triplet excitons, which is essential for many fields of optoelectronic and biomedical research.
Abstract
We propose a computational design framework to design the architecture of a white lighting system having multiple pixelated patterns of electric-field-driven quantum dot light-emitting ...diodes. The quantum dot of the white lighting system has been optimised by a system-level combinatorial colour optimisation process with the Nelder-Mead algorithm used for machine learning. The layout of quantum dot patterns is designed precisely using rigorous device-level charge transport simulation with an electric-field dependent charge injection model. A theoretical maximum of 97% colour rendering index has been achieved with red, green, cyan, and blue quantum dot light-emitting diodes as primary colours. The white lighting system has been fabricated using the transfer printing technique to validate the computational design framework. It exhibits excellent lighting performance of 92% colour rendering index and wide colour temperature variation from 1612 K to 8903 K with only the four pixelated quantum dots as primary.
Lead halide perovskites are promising materials for various optoelectronic device applications such as solar cells, light-emitting diodes, and lasers. Three-dimensional perovskites, for example, ...CH3NH3PbI3 and CsPbBr3, have been demonstrated to be high-gain active media for low-threshold lasing. In contrast, layered perovskites, for example, (CH3(CH2)3NH3)2PbI4, are known to be difficult to show lasing oscillation especially at room temperature, despite their robustness for the environment. Here we reveal the bottleneck for the lasing oscillation in layered perovskites through systematic experiments on time-resolved photoluminescence and transient absorption. It is found that the energy transfer to a long-lived exciton state with triplet nature is enhanced by increasing pumping fluence or by introducing a high-Q microcavity, hindering the formation of population inversion. These results are consistent with a coupled rate-equation model as well as previous works and paves the way for designing low threshold layered perovskite lasers.
We introduce femtosecond wide-field transient absorption microscopy combining sub-10 fs pump and probe pulses covering the complete visible (500–650 nm) and near-infrared (650–950 nm) spectrum with ...diffraction-limited optical resolution. We demonstrate the capabilities of our system by reporting the spatially- and spectrally-resolved transient electronic response of MAPbI3–x Cl x perovskite films and reveal significant quenching of the transient bleach signal at grain boundaries. The unprecedented temporal resolution enables us to directly observe the formation of band-gap renormalization, completed in 25 fs after photoexcitation. In addition, we acquire hyperspectral Raman maps of TIPS pentacene films with sub-400 nm spatial and sub-15 cm–1 spectral resolution covering the 100–2000 cm–1 window. Our approach opens up the possibility of studying ultrafast dynamics on nanometer length and femtosecond time scales in a variety of two-dimensional and nanoscopic systems.
The existence of step-like contrast structure for a class of singularly perturbed optimal control problem is presented by contrast structure theory. By means of direct scheme of boundary function ...method, we construct the uniformly valid asymptotic solution for the singularly perturbed optimal control problem. As an application, an example is given to illustrate the main result in this paper.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, ODKLJ, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Printed quantum dot (QD) light emitting diodes (QLEDs) over a large scale have received much interest in the last decade owing to the demand for the next generation of self-emissive large-area ...displays. To realize printing technology for QLEDs, a pioneering study suggested using a co-solvent system or a polymer additive in CdSe-based QLEDs. However, limitations on printed QLEDs with eco-friendly QDs, including the selection of ink solvent, environmental degradation of QDs in the air by temperature and O
2
/H
2
O level, and viscosity, make it difficult to achieve higher luminescence and external quantum efficiency (EQE) than that of ionic-bonded Cd-based QDs. Herein, we design an air-processable and stable ink with a photoinitiator (PI) mediating cross-linkage between eco-friendly QDs for inkjet-printed QLEDs. Once QD inks with a PI are deposited on the desired surface, their film polymerizes
in situ
through radical formation induced by ultraviolet (UV) exposure. Cross-linking reactions between ligands in the QDs reduce the distances between them, leading to flattening of the surface and enhancement of environmental stability in air. Printed InP-based green QLEDs demonstrated maximum luminescence values of 3600 cd m
−2
at 10 V on ITO/glass for the very first time. Finally, large-scale InP red/green/blue QLEDs directly printed with a bird image were fabricated on the flexible substrate.
Large-scale printed InP RGB quantum dot (QD) light emitting diodes (QLEDs) are realised by an air-processable and stable ink with a photoinitiator (PI) mediating cross-linkage between eco-friendly QDs for next generation self-emissive display.
Cadmium‐free quantum dot light‐emitting diodes (QLEDs) have held the potential to revolutionize the next‐generation displays with their advantages in color gamut, luminance intensity, and solution ...processibility. As a promising way of realizing large‐area QLED display production, inkjet printing has been intensively studied on Cd‐based QLEDs but lacks exploration in fabricating Cd‐free devices. Here, we developed Cd‐free RGB inkjet‐printed QLEDs with tailored hole transport layers (t‐HTLs) using Cd‐free QDs including InP/ZnSeS red and green QDs and ZnTeSe/ZnSe/ZnS blue QDs. With the t‐HTLs, QD ink erosion on the bottom charge transport layer was remarkably suppressed, while the efficient hole transport was maintained, which kept high device performance, especially in the QLED lifetime. With bank structures, Cd‐free QLED pixels were well defined within the size of 60 μm × 160 μm. Based on the t‐HTL structure and the bank structures, inkjet‐printed Cd‐free RGB QLED pixel arrays were demonstrated. This study bridges the gap between existing Cd‐free QLED technologies and the future commercialization of Cd‐free self‐emissive QD displays.
A tailored hole transport layer is employed for QD‐LEDs through a comprehensive study on the jetting behaviour of QD inks and the interface between QDs and the transport layer. Inkjet‐printed RGB multi‐colour microscale arrays based on Cd‐free QD‐LEDs is realised.
A comprehensive study of the various device architectures of patterned-, stacked-, and mixed-type quantum-dot light-emitting diodes (QD-LEDs) for smart white lighting has been performed by way of ...computational simulation and experimental device fabrication. The layout of the patterned-type QD-LED has been optimized by a rigorous charge transport simulation and a numerical grid searching color optimization method. The patterned-type QD-LED devices are fabricated using a unique transfer printing technique to validate design concepts. The architectural dependency on the color of the stacked-type QD-LED has been computationally and experimentally explored by the charge transport simulation with the electric-field-dependent carrier hopping model and by fabricating the devices with a multi-step spin-coating process. The mixed-type QD-LED has also been experimentally analyzed by the QD volume mixing ratios in the mixture solution. These three types of patterned, stacked, and mixed QD-LED device architectures show potential for various applications of functionalized next-generation smart white lighting systems.
Device architectures of patterned-, stacked-, and mixed-type quantum-dot light-emitting diodes (QD-LEDs) for the next-generation smart white lighting have been analyzed and optimized by computational charge transport simulation and experiments.
PDF
