Charging and Charged Species in Quantum Dot Light-Emitting Diodes

Despite recent rapid advances in improving quantum dot light-emitting diodes, many fundamental aspects of the device operating mechanism remain unresolved. Through transient electroluminescence and time-resolved photoluminescence measurements, the effects of offset voltage on charging and charge transport are examined. First, capacitive charging occurs with a time constant of ∼500 ns, followed by electron transport through quantum dots with a mobility of ∼10–5 cm2 V–1 s–1. Hole injection then initiates an electroluminescence rise that is independent of offset voltage. The photoluminescence lifetime is also unaffected by the offset voltage, indicating no injection of charges into the quantum dots or on their surfaces prior to the voltage pulse. A slower equilibration to steady-state electroluminescence is dependent on the offset voltage, indicative of another charging process. Elemental mapping shows that ZnO deposition from solution can lead to the diffusion of charged species into the quantum dot layer, which may cause the slower process

Capturing Phase Evolution during Solvothermal Synthesis of Metastable Cu₄O₃

The metastability of Cu₄O₃ has long hindered the synthetic preparation of bulk samples with substantial crystallinity. The lack of suitable samples has thwarted the detailed understanding of the magnetic properties of Cu₄O₃ and the ability to tune its properties. While Cu₄O₃ was recently shown to form in solvothermal reactions, the results are unpredictable, and the crystals are small. We developed a new, more uniform synthesis technique using sealed fused silica tubes. Interrogation of the solid and liquid phases resulting from this reaction has shed more light on the kinetic evolution of copper-containing phases and the microstructural correlation between different precipitation products. We find that direct conversion of the intermediate phase Cu₂(NO₃)­(OH)₃ to Cu₄O₃ is a likely consequence of dimethylformamide (DMF) triggered <i>in situ</i> reduction. The optimal reduction environment should be more straightforward to attain given the improved reliability of our method, and it remains under investigation. We verify the formation of Cu₄O₃ via X-ray diffraction, Raman microscopy, and SQUID magnetometry