A Rigid Multiple Resonance Thermally Activated Delayed Fluorescence Core Toward Stable Electroluminescence and Lasing.

The investigation of organic light-emitting diodes (OLEDs) and organic laser devices with thermally activated delayed fluorescence (TADF) molecules is emerging due to the potential of harnessing triplets. In this work, a boron/nitrogen multiple-resonance TADF polycyclic framework fusing carbazole units (CzBNPh) was proposed. CzBNPh exhibited a narrowband emission (<30 nm), a unity photoluminescence quantum yield, and a fast radiative rate. Consequently, CzBNPh demonstrated a low distributed feedback (DFB) lasing threshold of 0.68 µJ cm-2 . Furthermore, the stimulated emission zone of CzBNPh was effectively separated from its singlet and triplet absorption, thereby minimizing the singlet-triplet annihilation under long-pulsed excitation ranging from 20 µs to 2.5 ms. Significantly, the enhanced rigid molecular conformation, thermal stability, and photo-stability resulted in improved lasing and electroluminescence stability compared to that of 5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene (DABNA)-core. These findings indicate the potential of CzBN-core as a promising framework for achieving long-pulsed wave and electrically-pumped lasing in the future.

Graphene-Like Conjugated Molecule as Hole-Selective Contact for Operationally Stable Inverted Perovskite Solar Cells and Modules.

Further enhancing the operational lifetime of inverted-structure perovskite solar cells (PSCs) is crucial for their commercialization, and the design of hole-selective contacts at the illumination side plays a key role in operational stability. In this work, the self-anchoring benzo[rst]pentaphene (SA-BPP) is developed as a new type of hole-selective contact toward long-term operationally stable inverted PSCs. The SA-BPP molecule with a graphene-like conjugated structure shows a higher photostability and mobility than that of the frequently-used triphenylamine and carbazole-based hole-selective molecules. Besides, the anchoring groups of SA-BPP promote the formation of a large-scale uniform hole contact on ITO substrate and efficiently passivate the perovskite absorbers. Benefiting from these merits, the champion efficiencies of 22.03% for the small-sized cells and 17.08% for 5 × 5 cm2 solar modules on an aperture area of 22.4 cm2 are achieved based on this SA-BPP contact. Also, the SA-BPP-based device exhibits promising operational stability, with an efficiency retention of 87.4% after 2000 h continuous operation at the maximum power point under simulated 1-sun illumination, which indicates an estimated T80 lifetime of 3175 h. This novel design concept of hole-selective contacts provides a promising strategy for further improving the PSC stability.

Compositional tuning in sputter-grown highly-oriented Bi-Te films and their optical and electronic structures.

Growth of Bi-Te films by helicon-wave magnetron sputtering is systematically explored using alloy targets. The film compositions obtained are found to strongly depend on both the sputtering and antenna-coil powers. The obtainable film compositions range from Bi55Te45 to Bi43Te57 when a Bi2Te3 alloy target is used, and from Bi42Te58 to Bi40Te60 (Bi2Te3) for a Te-rich Bi30Te70 target. All films show strong orientation of the van der Waals layers (00l planes) parallel to the substrate. The atomic level stacking of Bi2Te3 quintuple and Bi bi-layers has been directly observed by high resolution transmission electron microscopy. Band structure simulations reveal that Bi-rich Bi4Te3 bulk is a zero band gap semimetal with a Dirac cone at the Gamma point when spin-orbit coupling is included. Optical measurements also confirm that the material has a zero band gap. The tunability of the composition and the topological insulating properties of the layers will enable the use of these materials for future electronics applications on an industrial scale.

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials.

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived (>100 ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons.

Multicomponent chalcogenides, such as quasi-binary GeTe-Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 10(6) write-erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump-pump-probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.