There are many advantages of LEDs in energy and environmental conservation, but their short life in many outdoor applications prompt a necessity to have a detailed understanding of their degradations to prolong their lifetime, which can also conserve LED material and even expand their applications. Using ab initio density functional theory formulation, we identify the detail paths of the LED degradation in outdoor applications. We discovered that the main stressors are humidity and the light that is emitted from the LED chip itself. This is rather ironical. A mathematical model is developed based on the ab initio study, and excellent experimental agreements are found. With this model, we can predict the situations where no and slow degradations can be achieved, and these are verified experimentally here. We can also predict quantitatively the time to a specific degradation severity. Quality index of the housing material for LEDs can also be determined.

The introduction of iodobismuth ternary perovskites has solved the stability and toxicity issues raised by lead trihalide perovskites. However, their wide band gaps and low structural dimensionalities have limited the power conversion efficiency in solar cell applications. In this work, we have partially incorporated Rb+ into Cs3Bi2I9 perovskite and tuned its band gap for better optoelectronic results. The structural and optical characterizations reveal that the monovalent alkali cation Rb+ partially replaces the larger Cs+ cation in the Cs3Bi2I9 structure and lowers the band gap from 2.01 to 1.94 eV. The reduction in optical band gap is consistent with the electronic band structure calculated from density functional theory. Moreover, this compositional engineering method has reduced the nonradiative carrier recombination in the perovskite material. The solar cell device based on a Rb-incorporated perovskite photoabsorber has resulted in an enormous enhancement of photoconversion efficiency up to 1.17%. The champion cell also possesses better moisture and thermal stabilities than the pristine Cs3Bi2I9-based device. This work demonstrates the partial incorporation technique to tune the band gap of an iodobismuth ternary perovskite for better carrier extraction in solar cells.

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Quantum Dots (QDs) are being investigated in a hybrid white light LED structure which inculcates phosphor in the package with a blue LED chip as the light source recently. In this work, Zn doped CdS QD with ZnS shell together with green light emission phosphor is used. Upon prolonged operation, degradation of the LEDs due to the degradation of QDs is observed, which can limit its practical applications. The degradation includes intensity reduction as well as blue shift of the emitted wavelength from the white light. Three stages of degradation are observed, namely an enhancement state where light intensity is found to increase, followed by a rapid degradation stage where light intensity decreases rapidly, and finally a slower degradation stage where the degradation rate of light intensity slows down and continues till the end of the test. Through various detail material analysis, with confirmation from the density functional theory (DFT) calculations, we find that the degradation of the LEDs is due to the time evolving degradation of CdS core structure, beginning from the oxidation of sulfur vacancy of CdS QDs by the nearby oxygen atoms as a result of imperfection of the ZnS protective coating around the QDs in the presence of blue light. This oxidation renders a transformation of CdS into CdO at the initial stage. The final stage is the formation of CdSO4 via some intermediate processes.