Organic-inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution ...processability
. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers
. However, CW lasing has not yet been realized at room temperature because of the 'lasing death' phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet-triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers.
Quasi‐2D metal halide perovskite films are promising for efficient light‐emitting diodes (LEDs), because of their efficient radiative recombination and suppressed trap‐assisted quenching compared ...with pure 3D perovskites. However, because of the multidomain polycrystalline nature of solution‐processed quasi‐2D perovskite films, the composition engineering always impacts the emitting properties with complicated mechanisms. Here, defect passivation and domain distribution of quasi‐2D perovskite films prepared with various precursor compositions are systematically studied. As a result, in perovskite films prepared from stoichiometric quasi‐2D precursor compositions, large organic ammonium cations function well as passivators. In comparison, precursor compositions of simply adding large organic halide salt into a 3D perovskite precursor ensure not only the defect passivation but also the effective formation of quasi‐2D perovskite domains, avoiding unfavorable appearance of low‐order domains. Quasi‐2D perovskite films fabricated with a well‐designed precursor composition achieve a high photoluminescence quantum yield of 95.3% and an external quantum efficiency of 14.7% in LEDs.
Quasi‐2D perovskites are engineered in terms of defect passivation and exciton confinement to reach high emitting efficiency. Compared with preparing the precursor solution with stoichiometric quasi‐2D compositions, simply adding a large organic halide salt into the 3D perovskite precursor ensures not only defect passivation but also effective formation of quasi‐2D perovskite domains, avoiding unfavorable appearance of low‐order domains.
Quasi-2D perovskites have attracted wide attention as the emitter of light-emitting diodes (LEDs) in recent years because of the ease of obtaining high external quantum efficiencies (EQEs). However, ...the quick degradation under continuous operation and significant EQE roll-off at high current densities are issues that need to be overcome for future practical applications using quasi-2D perovskite LEDs (PeLEDs). In this context, we discuss the mechanism of the degradation and EQE roll-off on the basis of ion migration. The migration of ligand cations though domain boundaries of quasi-2D perovskite films induces the gradual loss of defect passivation at the boundaries, which results in the reversible PeLED degradation and severe EQE roll-off. When the device operation time is long, the mobile cations enter and interact with the electron transport layer, leading to the stage of irreversible PeLED degradation. The device degradation mechanisms we discovered here are constructive for developing quasi-2D PeLEDs with better operational durability.
Excess/unreacted lead iodide (PbI2) has been commonly used in perovskite films for the state‐of‐the‐art solar cell applications. However, an understanding of intrinsic degradation mechanisms of ...perovskite solar cells (PSCs) containing unreacted PbI2 has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI2 crystals under continuous light illumination. Unreacted PbI2 undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI2 is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI2 crystals in perovskite films is very important to improve device operational stability for diverse opto‐electronic applications in the future.
Degradation of perovskite solar cells with excess PbI2 is investigated. Excess PbI2 in perovskite films undergoes photodecomposition (photolysis) under illumination, which produces lead and iodine and accelerates the degradation of PSCs.
Organic light-emitting diode (OLED) technology is promising for applications in next-generation displays and lighting. However, it is difficult-especially in large-area mass production-to cover a ...large substrate uniformly with organic layers, and variations in thickness cause the formation of shunting paths between electrodes
, thereby lowering device production yield. To overcome this issue, thicker organic transport layers are desirable because they can cover particles and residue on substrates, but increasing their thickness increases the driving voltage because of the intrinsically low charge-carrier mobilities of organics. Chemical doping of organic layers increases their electrical conductivity and enables fabrication of thicker OLEDs
, but additional absorption bands originating from charge transfer appear
, reducing electroluminescence efficiency because of light absorption. Thick OLEDs made with organic single crystals have been demonstrated
, but are not practical for mass production. Therefore, an alternative method of fabricating thicker OLEDs is needed. Here we show that extraordinarily thick OLEDs can be fabricated by using the organic-inorganic perovskite methylammonium lead chloride, CH
NH
PbCl
(MAPbCl
), instead of organics as the transport layers. Because MAPbCl
films have high carrier mobilities and are transparent to visible light, we were able to increase the total thickness of MAPbCl
transport layers to 2,000 nanometres-more than ten times the thickness of standard OLEDs-without requiring high voltage or reducing either internal electroluminescence quantum efficiency or operational durability. These findings will contribute towards a higher production yield of high-quality OLEDs, which may be used for other organic devices, such as lasers, solar cells, memory devices and sensors.
Degradation of organic light-emitting diodes (OLEDs) operated continuously at a constant current density is investigated using photoluminescence techniques. The OLEDs contained the thermally ...activated delayed fluorescence emitting dopant (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). OLED degradation proceeds mainly on the basis of excited-state instability of host molecules rather than processes related to 4CzIPN. Additionally, the electrochemical instability of radical cations and anions influences long-term OLED degradation. The formation of exciton quenchers and nonradiative carrier recombination centers acts to reduce OLED luminance. These findings highlight the need for new host material development to fabricate more stable TADF-OLEDs.
The influences of film density and molecular orientation on the carrier conduction and air stability of vacuum-deposited amorphous organic films of ...N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) were investigated. The substrate temperature (T sub) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of T sub; maximum density was attained at T sub = 270–300 K. α-NPD molecules were randomly oriented at T sub = 342 K, and their horizontal orientation on the substrate became dominant as T sub decreased. Hole current and air stability were clearly raised by increasing the film density by 1 to 2%; these effects were, respectively, attributed to enhanced carrier hopping between neighboring α-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin-film devices with α-NPD films than control of molecular orientation.
In this study, we investigate the lasing properties of 4,4′-bis(N-carbazole)styrylbiphenyl thin films under electrical pumping. The electroluminescent devices incorporate a mixed-order distributed ...feedback SiO2 grating into an organic light-emitting diode structure and emit blue lasing. The results provide an indication of lasing by direct injection of current into an organic thin film through selection of a high-gain organic semiconductor showing clear separation of the lasing wavelength from significant triplet and polaron absorption and design of a proper feedback structure with low losses at high current densities. This study represents an important advance toward a future organic laser diode technology.
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