CaAl2O4:Eu is a persistent luminescence (PL) material in the blue light region with potentially wide commercial applications. With the doping of Nd, the decay time can be elongated to more than 19 h. These excellent properties are believed to be in close relation with the electronic structures of the dopants, the defects, and the host material. In this work, we attempt to achieve a better understanding of the PL mechanism of CaAl2O4:Eu based on first-principles calculations. The electronic structures of the host CaAl2O4, the luminescent center Eu, the O and Ca vacancies, and the dopant Nd are systematically studied. According to the calculations, the 4f and 5d levels of Eu are located within the band gap and slightly above the conduction band minimum (CBM), respectively. The electrons on the 4f levels can be excited into the 5d levels via ultraviolet radiation. The excited electrons on the 5d levels can move to the conduction bands and become free electron carriers. The electron carriers can be trapped for a short period by the empty defect levels below the CBM if they are very close to the defects and then return back into the conduction band. After the trap–release process, the electrons may re-enter the 5d levels of Eu and then move back to the 4f levels accompanied by light emission. The +2 charged-state O vacancies can serve as electron traps. The Ca vacancies cannot contribute to the PL property directly but can assist in stabilizing the +2 charged-state O vacancies. Nd dopants can serve as both electron donors and electron traps. These new insights into the electronic structures are useful for determining which materials may possess good PL properties, thereby motivating more experimental efforts in synthesizing improved PL materials.

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Mn4+ doped phosphors, as an alternative to rare-earth element doped phosphors, have attracted immense attention owing to their ultrahigh quantum efficiency of red emission for potential applications in high rendering white LEDs (light-emitting diodes). Their performance can be largely affected by quenching phenomena such as thermal quenching, concentration quenching and the quenching induced by some intrinsic/extrinsic defects. However, the quenching mechanisms due to the defect levels and host band are still incompletely understood. In this work, we carry out a comprehensive first-principles study on the underlying quenching mechanisms due to the defect levels of Mn4+ and other extrinsic/intrinsic defects, using the prototype oxide Y3Al5O12 (YAG), fluorides K2TiF6 (KTF) and ZnTiF6·6H2O (ZTF) as examples. From the comparison of the defect levels of Mn4+ with the host bands, we find that it is the very small energy difference between the defect levels of Mn4+ and the valence bands maximum (VBM) of YAG that causes the lower luminescence thermal stability of YAG:Mn4+, which we name as the hole-type thermal quenching mechanism. For the concentration quenching, it is nearly impossible for the Mn4+-Mn4+ pairs, previously considered as the main quenching centers, to appear in phosphors. A new quenching nature has been discussed. For the impurity ionic effects, the hole-type defects can largely stabilize the Mn ions in +4 states, thereby enhancing the emission intensity. These proposed mechanisms can offer deeper insights into the luminescence behavior of Mn4+ and a better practical understanding of the high photoluminescence quantum yield red phosphors by adjusting their chemical components.

In order to reduce the impact of renewable energy output and load fluctuations and improve the flexibility of the integrated energy system (IES), it is necessary to further promote user participation in integrated demand response (IDR). Therefore, this paper constructs a hierarchical framework that enables the transaction of IDR resources among users by combining the information interaction network established by blockchain and the energy management network established by energy management system. This paper also analyses the comfort of users, the cost of energy purchase by users and the cost of energy use by load aggregators, and then develops a two-layer optimization model. The results of the simulation show that the model and trading framework constructed in this paper can realize the trading of IDR resources among users, which effectively promotes the participation of users in IDR, reduces the cost of users and load aggregators, reduces the loss of IDR resources, enables more effective integration of dispersed IDR resources and improves the flexibility of IES.

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