Efficiency limiting factors of quantum dots light-emitting diodes (QLEDs) are studied by a machine learning approach using features taken from published data. Prototypical structure of Cd-based QLEDs is transparent conductive oxide (TCO)/hole transport layer (HTL)/quantum dots (QDs)/electron transport layer (ETL)/Al, fabricated by printing processes. The most important factor is the hole injection barrier from HTL to QD layer (about 1 eV in CdSe QLEDs). A mechanism of the hole injection in such QLEDs is discussed using device simulation, and the experimental results that support the mechanism are shown. In addition, other efficiency limiting factors - the electron mobility of HTL and carrier balance in QD layer - are experimentally shown.
A four-level model consisting of a higher triplet excited state (T2), the lowest singlet and triplet excited states (S1 and T1), and the ground state was previously used to understand emission properties of a thermally activated delayed fluorescence (TADF) emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). In this report, we discuss the four-level model in more detail and apply to the other two TADF emitters, i.e., 1,2,4,5-tetrakis(carbazol-9-yl)-3,6-dicyanobenzene (4CzTPN) and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), in order to determine their excited-state structures. It is suggested that in all the emitters T2 lies between S1 and T1 and play an essential role in the emitting process. In 4CzTPN, phosphorescence from T2 is clearly observed around 100 K as in 4CzIPN. Compared to the other two emitters, 2CzPN has a wider energy gap between S1 and T1 so that delayed fluorescence at room temperature is thought to be mixed with phosphorescence. Because of this mixing, the spectrum characteristic of phosphorescence from T2 in 2CzPN cannot be identified.
In stoichiometric GeTe-Sb2Te3 films, reversible phase change optical recording materials, Sb addition is used widely in order to improve the crystallization speed, the data retention time, and the stability of cyclic operation, but its mechanism is not clear. V-VI compounds (GeTe, SnTe) are well known as compounds in which the ferroelectric structural phase transition occurs. The dielectric constant (epsilon) 0 changes largely due to a change in the short-range order of weakly bound nearest neighbors in the crystal to more strongly bound nearest neighbors in the amorphous phase. Therefore, it is necessary to discuss the effect of Sb addition in the stoichiometric GeTe-Sb2Te3 composition upon the dynamic properties of crystallization and the dielectric constant change (Delta) (epsilon) 0 based on the ferroelectric phase transition.
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