The efficiencies of green and red perovskite light-emitting diodes (PeLEDs) have been increased close to their theoretical upper limit, while the efficiency of blue PeLEDs is lagging far behind. Here we report enhancing the efficiency of sky-blue PeLEDs by overcoming a major hurdle of low photoluminescence quantum efficiency in wide-bandgap perovskites. Blending phenylethylammonium chloride into cesium lead halide perovskites yields a mixture of two-dimensional and three-dimensional perovskites, which enhances photoluminescence quantum efficiency from 1.1% to 19.8%. Adding yttrium (III) chloride into the mixture further enhances photoluminescence quantum efficiency to 49.7%. Yttrium is found to incorporate into the three-dimensional perovskite grain, while it is still rich at grain boundaries and surfaces. The yttrium on grain surface increases the bandgap of grain shell, which confines the charge carriers inside grains for efficient radiative recombination. Record efficiencies of 11.0% and 4.8% were obtained in sky-blue and blue PeLEDs, respectively.

Multiple-cation lead mixed-halide perovskites (MLMPs) have been recognized as ideal candidates in perovskite solar cells in terms of high efficiency and stability due to decreased open-circuit voltage loss and suppressed yellow phase formation. However, they still suffer from an unsatisfactory long-term moisture stability. In this study, phosphorus-containing Lewis acid and base molecules are employed to improve device efficiency and stability based on their multifunction including recombination reduction, phase segregation suppression, and moisture resistance. The strong fluorine-containing Lewis acid treatment can achieve a champion PCE of 22.02%. Unencapsulated and encapsulated devices retain 63% and 80% of the initial efficiency after 14 days of aging under 75% and 85% relative humidity, respectively. The better passivation of Lewis acid implies more halide defects than Pb defects at the MLMP surface. This unbalanced defect type results from phase segregation that is the synergistic effect of Cs and halide ion migrations. Identifying defect type based on different passivation effects is beneficial to not only choose suitable passivators to boost the efficiency and slow down the moisture degradation of MLMP solar cells, but also to understand the mechanism of defect-assisted moisture degradation.

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.