High-quality epitaxial layers are directly related to internal quantum efficiency. The methods used to design such epitaxial layers are reviewed in this article. The ultraviolet C (UVC) light-emitting diode (LED) epitaxial layer structure exhibits electron leakage; therefore, many research groups have proposed the design of blocking layers and carrier transportation to generate high electron–hole recombination rates. This also aids in increasing the internal quantum efficiency. The cap layer, p-GaN, exhibits high absorption in deep UV radiation; thus, a small thickness is usually chosen. Flip chip design is more popular for such devices in the UV band, and the main factors for consideration are light extraction and heat transportation. However, the choice of encapsulation materials is important, because unsuitable encapsulation materials will be degraded by ultraviolet light irradiation. A suitable package design can account for light extraction and heat transportation. Finally, an atomic layer deposition Al2O3 film has been proposed as a mesa passivation layer. It can provide a low reverse current leakage. Moreover, it can help increase the quantum efficiency, enhance the moisture resistance, and improve reliability. UVC LED applications can be used in sterilization, water purification, air purification, and medical and military fields.

The external quantum efficiencies (EQEs) of perovskite quantum dot lightemitting diodes (QD-LEDs) are close to the out-coupling efficiency limitation. However, these high-performance QD-LEDs still suffer from a serious issue of efficiency roll-off at high current density. More injected carriers produce photons less efficiently, strongly suggesting the variation of ratio between radiative and non-radiative recombination. An approach is proposed to balance the carrier distribution and achieve high EQE at high current density. The average interdot distance between QDs is reduced and this facilitates carrier transport in QD films and thus electrons and holes have a balanced distribution in QD layers. Such encouraging results augment the proportion of radiative recombination, make devices with peak EQE of 12.7%, and present a great device performance at high current density with an EQE roll-off of 11% at 500 mA cm −2 (the lowest roll-off known so far) where the EQE is still over 11%.their PL QYs. Diverse approaches have been proposed to boost the performance of QD-LEDs via surface passivation, [19] modification of carrier transport layer, [20][21][22] and anion exchange. [12] QD-LEDs have therefore demonstrated an impressive quick increase of peak external quantum efficiencies (EQEs) from 0.01% to 21.3% within 4 years, suggesting perovskite QDs as promising new-generation optoelectronic semiconductor materials. [6,7,12,21] In spite of the rapid development of perovskite QD-LEDs, at present, the EQEs of these devices tend to be acquired at low current density and manifest significant loss at higher current density, which is known as efficiency roll-off. In best-performed perovskite QD-LEDs, [12] the maximum EQE was obtained at a current density lower than 1 mA cm −2 ; when the current density reached 100 mA cm −2 , the EQE became only about 1%, indicating the efficiency loss reached 95%. Such efficiency loss actually limits achievable brightness and leads to excessive power consumption.Apart from perovskite QD-LEDs, perovskite film-LEDs present similar issues and recently, a few studies have come up with several methods to minimize EQE droop. Zou et al. and Yang et al. concluded that it was luminescence quenching caused by Auger recombination that was responsible for the efficiency roll-off. [23,24] To reduce carrier density in quantum wells (QWs), increased QW width was acquired via different