1 Introduction The performance of commercial LEDs has improved tremendously over the past few years. Today commercial LEDs cover the entire spectral range from UV to IR. The brightness of InGaN-LEDs has been increased by more than an order of magnitude over the last 10 years. Internal Quantum Efficiencies (IQE) of 75% with corresponding wall plug efficiencies (WPE) above 50% have been demonstrated for blue LEDs [1]. The OSRAM Opto Semiconductors ThinGaN-technology has pushed the Light Extraction Efficiency (LEE) of LED chips beyond 80%. Thin GaN technology also provides scalability of LED chips: LED brightness and efficiencies can be scaled to larger chip areas without losses. However, the InGaN Internal Quantum Efficiency is neither independent of the energy gap (or emission wavelength) nor of the current density: while IQE of more than 75% can be achieved for blue InGaN LEDs (440 nm, 50 A/cm 2 ) the IQE drops to less than 40% for green InGaN LEDs (540 nm, 50 A/cm²). At high current densities this efficiency loss even worsens especially for long wavelengths ("droop"). Pushing InGaN-LEDs towards red emission, the IQE drops dramatically below 10%. For wavelength above 580 nm the InGaAlP material system provides very efficient yellow, amber and red LEDs. In the wavelength range between 500 nm and 580 nm, InGaAlP LEDs are not efficient anymore due to weak carrier confinement.
Real time in-situ microscopy imaging of surface structure and atom dynamics of heterogeneous catalysts is an important step for understanding reaction mechanisms. Here, using in-situ environmental transmission electron microscopy (ETEM), we directly visualize surface atom dynamics at manganite perovskite catalyst surfaces for oxygen evolution reaction (OER), which are ≥20 times faster in water than in other ambients. Comparing (001) surfaces of La0.6Sr0.4MnO3 and Pr0.67Ca0.33MnO3 with similar initial manganese valence state and OER activity, but very different OER stability, allows us to distinguish between reversible surface adatom dynamics and irreversible surface defect chemical reactions. We observe enhanced reversible manganese adatom dynamics due to partial solvation in adsorbed water for the highly active and stable La0.6Sr0.4MnO3 system, suggesting that aspects of homogeneous catalysis must be included for understanding the OER mechanism in heterogeneous catalysis.
The
stability of perovskite oxide catalysts for the oxygen
evolution
reaction (OER) plays a critical role in their applicability in water
splitting concepts. Decomposition of perovskite oxides under applied
potential is typically linked to cation leaching and amorphization
of the material. However, structural changes and phase transformations
at the catalyst surface were also shown to govern the activity of
several perovskite electrocatalysts under applied potential. Hence,
it is crucial for the rational design of durable perovskite catalysts
to understand the interplay between the formation of active surface
phases and stability limitations under OER conditions. In the present
study, we reveal a surface-dominated activation and deactivation mechanism
of the prominent electrocatalyst La0.6Sr0.4CoO3−δ under steady-state OER conditions. Using a
multiscale microscopy and spectroscopy approach, we identify the evolving
Co-oxyhydroxide as catalytically active surface species and La-hydroxide
as inactive species involved in the transient degradation behavior
of the catalyst. While the leaching of Sr results in the formation
of mixed surface phases, which can be considered as a part of the
active surface, the gradual depletion of Co from a self-assembled
active CoO(OH) phase and the relative enrichment of passivating La(OH)3 at the electrode surface result in the failure of the perovskite
catalyst under applied potential.
We investigated lateral charge carrier transport in indium gallium nitride InGaN/GaN multi-quantum wells for two different samples, one sample emitting green light at about 510 nm and the other emitting cyan light at about 470 nm. For the cyan light emitting sample we found a diffusion constant of 1.2 cm2/s and for the green light emitting sample 0.25 cm2/s. The large difference in diffusion constant is due to a higher point defect density in the green light emitting quantum wells (QWs) as high indium incorporation tends to reduce material quality
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