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.

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) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of T; maximum density was attained at T = 270-300 K. α-NPD molecules were randomly oriented at T = 342 K, and their horizontal orientation on the substrate became dominant as T 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.

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))