Stable, High-Efficiency Voltage-Dependent Color-Tunable Organic Light-Emitting Diodes with a Single Tetradentate Platinum(II) Emitter Having Long Operational Lifetime.

Voltage-dependent, color-tunable organic light-emitting diodes (OLEDs) are appealing tools that can be used for the visualization of electronic output signal of sensors. Nonetheless, the literature-reported color-tunable OLEDs that have a simple single-cell device structure suffer from relatively low efficiency, pronounced efficiency roll-off, color-aging, and short operation lifetime, all of which limit their practical applications. Here, a novel co-host-in-double-emissive-layer (CHIDEL) device, designed to enhance the performance of color-tunable OLEDs with the use of a single tetradentate Pt[O^ N^ C^ N] emitter, is described. When Pt-X-2 is used as a single emitter in an optimized CHIDEL device, a white OLED with tunable Commission International de I'Eclairage (CIE) coordinates from (0.47, 0.44) at 3 V to (0.36, 0.48) at 11 V, a high color rendering index of 82, and high external quantum efficiency (EQE) of up to 20.75% can be achieved. By using Pt-X-4 as a single emitter, the voltage-dependent color-tunable CHIDEL device, with CIE coordinates shifted from (0.56, 0.43) at 3 V to (0.42, 0.55) at 11 V, demonstrates a high luminance of beyond 90 000 cd m-2 and a high EQE of 23.23% at a luminance of 1300 cd m-2 . A long-lifetime time to 90% of the initial luminance (LT90 ) of almost 20 000 h is demonstrated for the color-tunable OLED with Pt-X-4 emitting dopant.

Impact of excitation energy on the excitonic luminescence of cesium lead bromide perovskite nanosheets.

An excitation-energy-dependent luminescence phenomenon is reported in cesium lead bromide (CsPbBr3) perovskite nanosheets. At 10 K, the relative integrated luminescence intensity between the trapped exciton (TX) emission and the free exciton (FX) emission shows an interesting tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such phenomenon, we develop a quantitative model on the basis of the biological population growth theory. A good agreement between experiment and theory is obtained. It is thus revealed that the lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level shall be the major cause of the observed phenomenon. These findings may help to deepen the current understanding of the complex luminescence mechanisms of these emerging light-emitting materials.

Photoluminescence signatures of thermal expansion, electron-phonon coupling and phase transitions in cesium lead bromide perovskite nanosheets.

In this article, photoluminescence (PL) behaviors of inorganic cesium lead bromide (CsPbBr3) nanosheets are investigated in a broad temperature range from 5 to 500 K. As can be seen from the temperature evolution of the PL peak position, the bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron-phonon coupling for the temperature variation from 5 K to 360 K. As the temperature is further increased, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift, again at ∼360 K. Such turning behavior is consistent with an orthorhombic-tetragonal (γ-ß) phase transition at this critical temperature. For the PL linewidth, it shows a continuous broadening at temperatures beyond 40 K, suggesting the dominant role of phonon scattering, especially at high temperatures. These findings of temperature dependent photophysical properties of CsPbBr3 nanosheets may provide useful information of their further applications in the emerging nano photo-electronic devices.

Luminescence and thermal behaviors of free and trapped excitons in cesium lead halide perovskite nanosheets.

Very recently, all-inorganic perovskite CsPbX3 (X = Cl, Br, I) nanostructures such as nanoparticles, nanoplates, and nanorods have been extensively explored. These CsPbX3 nanostructures exhibit excellent optical properties; however, the photophysics involved is not yet clear. Herein, the emission properties and luminescence mechanism of CsPbBr3 nanosheets (NSs) were investigated using steady-state and time-resolved photoluminescence (PL) spectroscopic techniques. Moreover, two kinds of excitonic emissions (Peak 1 and Peak 2) are observed at low temperatures (<80 K) under the conditions of low excitation level. They are revealed to stem from the radiative recombination of trapped and free excitons by examining their spectral features and emission intensity dependences on excitation power. Thermally induced exchange between the two kinds of excitons is found and modeled quantitatively; this has led to the determination of an activation energy of 13 meV. Thermal redistribution of trapped excitons and thermal expansion-induced blueshift of the bandgap are jointly responsible for the abnormal temperature dependence of the position of Peak 1, whereas the latter is predominant for the monotonic blueshift of the position of Peak 2 with an increase in temperature. These results and findings shed some light on the complicated luminescence mechanism of CsPbBr3 NSs.