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Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average quantum well composition measured by atom probe tomography and quantitative scanning transmission electron microscopy quantitatively agrees with measurements by X-ray diffraction. Atom probe tomography is further applied to study the distribution of indium atoms in non-polar a-plane (11-20) InGaN quantum wells. An inhomogeneous indium distribution is observed by frequency distribution analysis of the atom probe tomography measurements. The optical properties of non-polar (11-20) InGaN quantum wells with indium compositions varying from 7.9% to 20.6% are studied. In contrast to non-polar m-plane (1-100) InGaN quantum wells, the non-polar a-plane (11-20) InGaN quantum wells emit at longer emission wavelengths at the equivalent indium composition. The non-polar a-plane (11-20) quantum wells also show broader spectral linewidths. The longer emission wavelengths and broader spectral linewidths may be related to the observed inhomogeneous indium distribution. [ABSTRACT FROM AUTHOR]

Organic-inorganic hybrid perovskite solar cells form a new type of thin film photovoltaic technology, which has achieved extraordinary improvements in power conversion efficiency in a relatively short time. To further improve the efficiency and stability of the perovskite solar cells, it is critical to understand and control the microstructure of both the functional materials and their interfaces. Much effort has already been made to understand the microstructure of perovskite solar cells and its influence on their performance. This has proved particularly challenging due to the fragile nature of the organic-inorganic perovskites and the consequent potential for generating artefacts through the application of the characterization methods themselves. In this progress report, an overview of some of the more commonly used characterization methods is given, their possible impact on the materials analyses is evaluated, and the latest developments in the understanding of the microstructure of perovskite solar cells are summarized. The heterogenic nature of the individual perovskite grains and the polycrystalline film as a whole is illustrated, the features and properties of the grain boundaries and the effect they can have on solar cell performance are described, and the interface characterization between the layers in the solar cell devices is discussed. [ABSTRACT FROM AUTHOR]

Organic-inorganic hybrid perovskite solar cells with mixed cations and mixed halides have achieved impressive power conversion efficiency of up to 22.1%. Phase segregation due to the mixed compositions has attracted wide concerns, and their nature and origin are still unclear. Some very useful analytical techniques are controversial in microstructural and chemical analyses due to electron beam-induced damage to the 'soft' hybrid perovskite materials. In this study photoluminescence, cathodoluminescence, and transmission electron microscopy are used to study charge carrier recombination and retrieve crystallographic and compositional information for all-inorganic CsPbIBr2 films on the nanoscale. It is found that under light and electron beam illumination, 'iodide-rich' CsPbI(1+ x)Br(2− x) phases form at grain boundaries as well as segregate as clusters inside the film. Phase segregation generates a high density of mobile ions moving along grain boundaries as ion migration 'highways.' Finally, these mobile ions can pile up at the perovskite/TiO2 interface resulting in formation of larger injection barriers, hampering electron extraction and leading to strong current density-voltage hysteresis in the polycrystalline perovskite solar cells. This explains why the planar CsPbIBr2 solar cells exhibit significant hysteresis in efficiency measurements, showing an efficiency of up to 8.02% in the reverse scan and a reduced efficiency of 4.02% in the forward scan, and giving a stabilized efficiency of 6.07%. [ABSTRACT FROM AUTHOR]