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请同学们按照科技英文的翻译方法翻译附件文献，翻译过程中认真学习慕课《医学英文翻译》（西安交通大学、朱元主讲）相关词语转换、定语从句、长句的翻译等内容，注意截止时间（3月5日），按时上交。这次翻译的成绩将占本课程最终成绩的40%，希望大家能够重视！
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请同学们按照科技英文的翻译方法翻译下文，翻译过程中认真学习慕课《医学英文翻译》（西安交通大学、朱元主讲）相关词语转换、定语从句、长句的翻译等内容，注意截止时间，按时上交作业。这次翻译的成绩将占本课程最终成绩的40%，希望大家能够重视！ Highly efficient organic light-emitting diodes from delayed fluorescence The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal– organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescencebased OLEDs. The recombination of holes and electrons can produce light, in a process referred to as electroluminescence. Electroluminescence in organic materials was first discovered in 1953 using a cellulose film doped with acridine orange, and was developed in 1963 using an anthracenesinglecrystalconnectedtohigh-fieldcarrierinjectionelectrodes. Electrical charge carriers of both polarities were injected into the organic layers, and the subsequent carrier transport and recombination produced blue electroluminescence originating from singlet excitons; that is, fluorescence. According to spin statistics, carrier recombination is expected to produce singlet and triplet excitons in a 1:3 ratio, and this ratio has been examined for many molecular systems. The singlet excitons produced decay rapidly, yielding prompt electroluminescence (fluorescence). Two triplet excitons can combine to form a singlet exciton through triplet–triplet annihilation, which results in delayed electroluminescence (delayed fluorescence). Direct radiative decay of triplet excitons results in phosphorescence, but usually occurs only at very low temperatures in conventional organic aromatic compounds. The first demonstration of phosphorescent electroluminescence using ketocoumarin derivatives in 1990. However, the very faint electroluminescence was observed only at 77K, and with difficulty, and was assumed to be virtually useless even if included in rare-earth complexes, which should also involve both singlet and triplet excitons in electrical excitation. In 1999, efficient electrophosphorescence was first demonstrated using iridium phenylpyridine complexes that achieve an efficient radiative decay rate of ～106s-1 by taking advantage of the strong spin–orbit coupling of iridium. An internal electroluminescence efficiency of almost 100% was achieved, providing convincing evidence that OLED technology can be useful for display and lighting applications. In the work reported here, we used a novel pathway to attain the greatest possible electroluminescence efficiency from simple aromatic compounds that exhibit efficient thermally activated delayed fluorescence (TADF) with high photoluminescence efficiency. Figure 1a shows the energy diagram of a conventional organic molecule, depicting singlet(S1) and triplet(T1) excited states and a ground state(S0). It was previously assumed that the S1 level was considerably higher in energy than the T1 level, by 0.5–1.0eV, because of the electron exchange energy between these levels. However, we found that careful design of organic molecules can lead to a small energy gap (ΔEST) between S1 and T1 levels. Correspondingly, a molecule with efficient TADF requires a very small ΔEST between its S1 and T1 excited states, which enhances T1→S1 reverse intersystem crossing (ISC). Such excited states are attainable by intramolecular charge transfer within systems containing spatially separated donor and acceptor moieties. The critical point of this molecular design is the combination of a small ΔEST, of≤100meV, with a reasonable radiative decay rate, of >106s-1, to overcome competitive non-radiative decay path ways, leading to highly luminescent TADF materials. Because these two properties conflict with each other, the overlap of the highest occupied molecular orbital and the lowest unoccupied molecular orbital needs to be carefully balanced. Furthermore, to enhance the photoluminescence efficiency of a TADF material, the geometrical change in molecular conformation between its S0 and S1 states should be restrained to suppress non-radiative decay. Limited orbital overlap generally results in virtually no emission, as has been shown in benzophenone derivatives. Therefore, it was previously assumed that a high photoluminescence efficiency could never be obtained from molecules with a small ΔEST. Here we demonstrate that it is possible to realize a high photoluminescence efficiency and a small ΔEST simultaneously.
