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Photon upconversion in the solid state has the potential to improve existing solar and infrared imaging technologies due to its achievable efficiency at low power thresholds. However, despite considerable advancements in solution-phase upconversion, expanding the library of potential solid-state annihilators and developing a fundamental understanding of their solid-state behaviors remains challenging due to intermolecular coupling affecting the underlying energy landscape. Naphtho[2,3-a]pyrene has shown promise as a suitable solid-state annihilator. However, the origin of its multiple underlying emissive features remains unknown. To this point, here, we investigate NaPy/poly(methyl methacrylate) thin films at varying concentrations to tune the intermolecular coupling strength to determine its photophysical properties at a range of temperatures between 300–50 K. The results suggest that the multiple emissive features present in the NaPy thin film emission at room temperature arise from a multidimensional I-aggregate (520 nm), an excimer (550 nm), and a strongly coupled J-dimer (620 nm). In addition, we find that at low temperatures, the emission spectrum is dominated by direct emission from the 1(TT) state. 

The widespread utilization of perovskite-based photovoltaics requires probing both the structural and optical properties under extreme operating conditions to gain a holistic understanding of the material behavior under stressors. Here, we investigate the temperature-dependent behavior of mixed A-site cation lead triiodide perovskite thin films (85% methylammonium and 15% formamidinium) in the range from 300 to 20 K. Through a combination of optical and structural techniques, we find that the tetragonal-to-orthorhombic phase transition occurs at ∼110 K for this perovskite composition, as indicated by the change in the diffraction pattern. With decreasing temperature, the quantum yield increases with a concurrent elongation of the carrier lifetimes, indicating suppression of nonradiative recombination pathways. Interestingly, in contrast to single A-site cation perovskites, an additional optical transition appears in the absorption spectrum when the phase transition is approached, which is also reflected in the emission spectrum. We propose that the splitting of the optical absorption and emission is due to local segregation of the mixed cation perovskite during the phase transition. 

Kagome materials are of topical interest for their diverse quantum properties linked with correlated magnetism and topology. Here, we report anomalous hydrostatic pressure (p) effect on ErMn₆Sn₆through isobaric and isothermal-isobaric magnetization measurements. Magnetic field (H) suppresses antiferromagneticT_Nwhile simultaneously enhancing the ferrimagneticT_Cby exhibiting dual metamagnetic transitions, arising from the triple-spiral-nature of Er and Mn spins. Counter-intuitively, pressure enhances bothT_CandT_Nwith a growth rate of 74.4 K GPa⁻¹and 14.4 K GPa⁻¹respectively. Pressure unifies the dual metamagnetic transitions as illustrated throughp-Hphase diagrams at 140 and 200 K. Temperature-field-pressure (T-H,T-p) phase diagrams illustrate distinct field- and pressure-induced critical points at (T_cr= 246 K,H_cr= 23.3 kOe) and (T_cr= 435.8 K,p_cr= 4.74 GPa) respectively. An unusual increase of magnetic entropy by pressure aroundT_crand a putative pressure-induced tricritical point pave a unique way of tuning the magnetic properties of kagome magnets through simultaneous application ofHandp.

 

The lack of viable solid‐state annihilators is one of the greatest hurdles in perovskite‐sensitized triplet–triplet annihilation upconversion (UC). Unfavorable singlet and triplet energy surfaces in the solid state have limited the successful implementation of many conventional solution‐based annihilators. To date, rubrene is still the best‐performing annihilator; however, this comes at the cost of a limited apparent anti‐Stokes shift. To this point, anthracene derivatives are promising candidates to increase the apparent anti‐Stokes shift. The well‐known green glowstick dye 9,10‐(bisphenylethynyl)anthracene (BPEA) and its chlorinated derivatives have already shown promise in solution‐based UC applications. Due to favorable band alignment of the perovskite and triplet energy levels of BPEA, it is conceivable that a wide variety of BPEA derivatives can be compatible with the perovskite‐based UC system. Here, the properties of the parent molecule BPEA and its derivatives 1‐chloro‐9,10‐(bisphenylethynyl)anthracene and 2‐chloro‐9,10‐(bisphenylethynyl)anthracene are investigated. Despite similar optical properties in solution, the different molecules exhibit vastly different properties in thin films. UC studies in lead halide perovskite/BPEA bilayer devices demonstrate the importance of intermolecular coupling on the resulting properties of the upconverted emission.

 

Temperatures below ambient room temperature (298 K) are ideal for perovskite-sensitized upconversion devices where maximum efficiency is reached at 170 K. Here, the underlying triplet diffusion rate governs the overall upconversion dynamics. 

At elevated temperatures SnSe is reported to undergo a structural transition from the low symmetry orthorhombic GeS-type to a higher symmetry orthorhombic TlI-type. Although increasing symmetry should likewise increase lattice thermal conductivity, many experiments on single crystals and polycrystalline materials indicate that this is not the case. Here we present temperature dependent analysis of time-of-flight (TOF) neutron total scattering data in combination with theoretical modeling to probe the local to long-range evolution of the structure. We report that while SnSe is well characterized on average within the high symmetry space group above the transition, over length scales of a few unit cells SnSe remains better characterized in the low symmetry GeS-type space group. Our finding from robust modeling provides further insight into the curious case of a dynamic order-disorder phase transition in SnSe, a model consistent with the soft-phonon picture of the high thermoelectric power above the phase transition.