560†nm InGaN micro-LEDs on low-defect-density and scalable (20-21) semipolar GaN on patterned sapphire substrates.

We demonstrate InGaN-based semipolar 560 nm micro-light-emitting diodes with 2.5% EQE on high-quality and low-defect-density (20-21) GaN templates grown on scalable and low-cost sapphire substrates. Through transmission electron microscopy observations, we discuss how the management of misfit dislocations and their confinement in areas away from the active light-emitting region is necessary for improving device performance. We also discuss how the patterning of semipolar GaN on sapphire influences material properties in terms of surface roughness and undesired faceting in addition to indium segregation at the proximity of defected areas.

Variation of Interfacial Interactions in PC61BM-like Electron-Transporting Compounds for Perovskite Solar Cells.

The synthesis, characterization, and incorporation of phenyl-C61-butyric acid methyl ester (PC61BM)-like derivatives as electron transporting materials (ETMs) in inverted perovskite solar cells (PSCs) are reported. These compounds have the same structure except for the ester substituent, which was varied from methyl to phenyl to thienyl and to pyridyl. The three latter derivatives performed better than PC61BM in PSCs, mainly attributed to the specific interactions of the fullerenes with the perovskite layer, as evidenced by X-ray photoelectron spectroscopy (XPS) and steady-state and time-resolved photoluminescence (SS- and TRPL) measurements. The experimental results were fully supported by density functional theory (DFT) calculations, which showed that the strongest interactions were exhibited by the compound possessing the pyridyl substituent.

Dynamics of Additive Migration to Form Cathodic Interlayers in Organic Solar Cells.

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC71BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating process, i.e., the majority of PEG accumulates at the bottom substrate, while the minority resides in the film. During electrode evaporation, PEG molecules kinetically "trapped" near the film surface migrate to the organic/Al interface to reduce the interfacial energy. This diffusion-limited process is enhanced with the initial concentration of PEG in the solution and with thermal annealing after metal deposition. In contrast, annealing the film before metal deposition stalls PEG migration. This mechanism is supported by corresponding OSC devices showing that Voc increases with PEG content at the interface, up to a saturation value associated with the formation of a continuous PEG interlayer. Presence of a continuous interlayer excludes the driving force for further migration of PEG to the interface. Revealing this mechanism provides practical insight for judicious selection of additives and processing conditions for interfacial engineering of spontaneously generated interlayers.

Bioinspired gradient materials via blending of polymer electrolytes and applying electric forces.

Free-standing and supported films with a lateral gradient in composition were prepared using blends of poly(acrylic acid) (PAA)/sodium salt and its copolymers with acrylamide (AAm) in an applied electric field. The gradients were stabilized by complexation of carboxylate groups with metal species. To find the favorable conditions and components for successful blending and interaction with Fe and Ce species, we studied blending of the two PAA samples with molecular weights of 2000 and 15 000 Da with two copolymers of AA and AAm (with 10 and 70 wt % of AA units) and interaction of these blends with Fe(III) and Ce(IV) ions. The structure of the hybrid (blend) films was studied using differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy, X-ray diffraction, and optical microscopy. To ensure blend miscibility and efficient interaction with metal ions, the copolymer containing 70 wt % AA units has been used. The surface enrichment with metal species was observed at all experimental conditions studied in this work. For lateral gradient film formation, 15 000 Da PAA has been used to avoid uneven distribution of the homopolymer in the film, observed for 2000 Da PAA. The gradient films were characterized by XPS. The lateral gradient of functionality such as COONa group or Fe content has been obtained at different strengths of electric field applied during film formation. The use of lower voltage allows one to prevent NaOH formation and creates more favorable conditions for development of a gradient polymer film. The Ce content gradient was not observed due to formation of large Ce oxide particles (> or = 750 nm), masking the gradient of functionality. For the first time, free-standing films with a lateral gradient in composition were prepared using an applied electric field.