Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by long-lived triplet excitons [singlet–triplet annihilation (STA)]. In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-λ5σ4-phosphinine (DCNP) as the emitter and 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP’s triplets are effectively scavenged by BSBCz while the DCNP’s singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1 wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff.

Quenching of singlets by long-lived triplets is a serious issue for lasing from organic laser dyes, especially under long pulse excitation duration. As a strategy to scavenge or manage unnecessary triplets, an organic laser dye is dispersed into a host material having high singlet and low triplet energy levels [a large singlet-triplet energy gap (ΔE ST )]. However, finding such a host material having a triplet scavenging capability is limited. In this study, we synthesized an organic laser dye, 2,6-dicyano-1,1-diphenyl-λ 5 σ 4 -phosphinine (DCNP), having a small ΔE ST of ~0.44 eV, and thus we were able to employ 4-4΄-bis[(N-carbazole)styryl] biphenyl (BSBCz) as a triplet scavenging host, i.e., the triplets formed on DCNP are easily transferred to BSBCz. A 1 wt.%-DCNP-doped BSBCz film was formed on a mixed-order distributed feedback grating, showed lasing with a low threshold value of ~0.86 µJ cm −2 and a FWHM of ~0.5 nm. Because of the suppressed singlet-triplet annihilation, we demonstrated DCNP-based laser devices operating under continuous-wave operation, with a low threshold of 72 W cm −2 and a long laser half-lifetime of ~3 min. Our results This article is protected by copyright. All rights reserved.3 demonstrated a possibility of the wider selection of host materials, easing a material design strategy of fabricating high-performance laser devices in future.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

Continuous-wave (CW) lasing is still difficult to realize in organic laser dyes, one reason being the thermal degradation caused by intense photoexcitation. A decrease in laser threshold suppresses the thermal degradation and, therefore, leads to long-lasting lasing from organic laser dyes. Here, we show that it is possible to decrease the laser thresholds by combining the organic laser dye 4,4-bis­[(N-carbazole)­styryl]­biphenyl, which has a small spectral overlap between the laser emission and the excited-state triplet absorption, with two-dimensional (2D) distributed-feedback (DFB) gratings. We used second-order 2D cross double and square lattice DFB gratings, which offer light feedback in two orthogonal directions, and second-order and mixed-order circular DFB gratings, which offer light feedback in radial directions. Among these grating structures, the mixed-order circular DFB grating structure led to the lowest lasing threshold of ∼0.015 μJ cm–2 under short-pulse photoexcitation because of the excellent optical feedback. Moreover, a low average-power threshold of 10 W cm–2 (a peak-power threshold of 1 kW cm–2) was obtained when laser devices with this grating structure were operated under 1 s of long-pulse photoexcitation with a repetition rate of 0.01 Hz. Additionally, using the mixed-order circular DFB grating structure improved the laser stability under CW photoexcitation since shorter exciton lifetimes in the optical resonator suppress the chemical decomposition. These results demonstrate the importance of choosing an optical resonator structure for improving organic laser performance.

Achieving continuous‐wave (CW) lasing in organic semiconductors is known to be a difficult task because long‐lived triplets quench radiative singlets via singlet–triplet annihilation (STA). To avoid STA and operate organic lasers in CW or long‐pulse photoexcitation, the triplets need to be removed from an organic laser gain medium. However, this triplet removal leads to a loss of excitons. In addition to removing the detrimental triplets, here it is reported a triplet recycling process, which includes triplet scavenging and successive triplet upconversion via triplet–triplet annihilation (TTA) to regenerate emissive singlet excitons in a laser medium. An anthracene derivative of 9‐(1‐naphthalenyl)‐10‐(4‐(2‐naphthalenyl)phenyl)anthracene (NaNaP‐A) and a laser dye of 4,4′‐bis[4‐(diphenylamino)styryl]biphenyl (BDAVBi) are used as the triplet recycling sensitizer and the emitting laser dye, respectively. In this laser system, NaNaP‐A can efficiently scavenge the triplets formed on BDAVBi because the triplet level is deeper for NaNaP‐A than for BDAVBi, and then NaNaP‐A successively recycles the triplets into the BDAVBi's singlet state via TTA. The TTA compensates and overcomes the STA in this laser system. Hence, these laser devices can be operated with long pulse widths of up to 10 ms. This unique triplet recycling behavior is confirmed by transient photoluminescence (PL) and electroluminescence (EL) studies.

Rationally manipulating the functional substituents plays a crucial role in tuning the luminescence and lasing properties of organic gain media. Herein, a cyanophenyl-moiety, which exhibits relatively weaker electron affinity, is connected to 2,6-dicarbonitrile diphenyl-1λ 5 -phosphinine (DCNP) via para-linking. Resultantly, the appreciated locally-excited characteristics ensuring a large oscillator strength and high radiative rate can be reserved in DCNP-4-(4-cyanophenyl) (DCNP-pCN). Interestingly, the weak chargetransfer state from the relative donor (D)/acceptor (A) interplay enables small singlet-triplet splitting (ΔE ST ≈ 0.45 eV). Thus the triplets generated on DCNP-pCN can be efficiently scavenged by 4,4'-bis[(N-carbazole)styryl] biphenyl (BSBCz), which is used as the host with a lower-lying triplet energy level for DCNP-pCN. Moreover, benefitting from the mediation between the conjugated length extension and weak D/A interplay, the emission spectrum cannot be largely shifted, which can effectively suppress the overlap between the lasing emission of DCNP-pCN and the excited-state absorption of BSBCz, thereby avoiding detrimental singlettriplet annihilation. Thus, high-quality distributed feedback lasings with ≈2.0 μJ cm −2 thresholds are achieved, and the organic light-emitting diodes exhibit external quantum efficiency exceeding 2.0% without efficiency rolloff under high current injection, indicating the potential for electricalpumping organic lasings.

A continuous‐wave (CW) organic solid‐state laser is highly desirable for spectroscopy, sensing, and communications, but is a significant challenge in optoelectronics. The accumulation of long‐lived triplet excitons and relevant excited‐state absorptions, as well as singlet–triplet annihilation, are the main obstacles to CW lasing. Here, progress in singlet‐ and triplet‐state utilizations in organic gain media is reviewed to reveal the issues in working with triplets. Then, exciton behaviors that inhibit light oscillations during long excitation pulses are discussed. Further, recent advances in increasing organic lasing pulse widths from microseconds toward the indication of CW operation are summarized with respect to molecular designs, advanced resonator architectures, triplet scavenging, and potential triplet contribution strategies. Finally, future directions and perspectives are proposed for achieving stable CW organic lasers with significant triplet contribution.

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Films of the quasi‐2D perovskite based on 1‐naphthylmethylamine (NMA) are promising as the gain medium for optically pumped lasing and future electrically pumped lasing because of its low lasing threshold and small electroluminescence efficiency rolloff. However, reasons for the low threshold and small efficiency rolloff are still unclear. Therefore, exciton dynamics are investigated in NMA‐based quasi‐2D perovskite films. It is found that quenching of bright excitons by other excitons or charge carriers is unlikely in NMA‐based quasi‐2D perovskite films, which is one reason for the low lasing threshold and small efficiency rolloff. Moreover, thermally stimulated current measurements reveal that the defect levels inside the band gap of the NMA‐based quasi‐2D perovskite are shallow, with a depth of ≈0.3 eV, causing a decrease in nonradiative exciton recombination through the defects. Therefore, population inversion can be easily achieved, leading to the low lasing threshold as well. For fabrication of NMA‐based quasi‐2D perovskite laser devices with even lower lasing thresholds, a circular‐shaped optical resonator, and small‐molecule‐based defect passivation are used. Optically pumped lasing can be obtained from these devices, with a threshold of ≈1 µJ cm−2, which is one of the lowest values ever reported in any perovskite lasers.