In this study, we investigated the dependence of electroluminescence (EL) efficiency on carrier distribution in the light-emitting layer (EML) of the device based on $\text{Eu}{\left(\text{TTA}\right)}_3\text{phen}$ ($\text{TTA}=\text{thenoyltrifluoroacetone}$ , $\text{phen}=1,10\text{-phenanthroline}$ ) doped $4,4^{\prime }\text{-}N,N^{\prime }$ -dicarbazole-biphenyl (CBP) system. We found that EL efficiency increases monotonously with increasing hole injection even when holes are the majority carriers. This phenomenon was attributed to the accumulation of holes in EML, which improves the balance of holes and electrons on $\text{Eu}{\left(\text{TTA}\right)}_3\text{phen}$ molecules, thus enhancing the EL efficiency. To further improve the balance of holes and electrons on $\text{Eu}{\left(\text{TTA}\right)}_3\text{phen}$ molecules, the injection of electron was gently decreased by modulating the thickness of Al and LiF layers. Interestingly, EL efficiency increases gradually to a maximum and then decreases rapidly with decreasing electron injection. As a result, the device with 80 nm Al and 1.2 nm LiF obtained the maximal current efficiency of 9.53 cd/A, power efficiency of 5.35 lm/W, and external quantum efficiency of 5.15%. Our experimental results demonstrated that the balance of holes and electrons on dye molecules is the precondition for the doped device to obtain high EL efficiency, when carrier trapping is the dominant EL mechanism.

The traditional risk evaluation of coal spontaneous combustion in goafs in China is unilateral and inaccurate, which can be attributed to the fact that only the change in coal properties is considered and the external environment of coal is ignored. According to latest research, the factors affecting coal spontaneous combustion in goafs is determined by the systems-theoretic accident model and processes, from which a risk index system is established. Then, a comprehensive risk evaluation model for coal spontaneous combustion in goafs is constructed with the analytic hierarchy process, unascertained measurement theory and combination weighting method, which divides the risk of coal spontaneous combustion into four levels. The model is tested in three goafs in different coal mines. Findings show that the model is objective and valid and can be applied to the risk evaluation of coal spontaneous combustion in goafs in coal mines.