An Organic Laser Dye having a Small Singlet‐Triplet Energy Gap Makes the Selection of a Host Material Easier

Abstract

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))

Suppression of external quantum efficiency rolloff in organic light emitting diodes by scavenging triplet excitons

“…1b . Faster nonradiative relaxation of BSBCz triplet excitons was already reported, as the triplet exciton lifetime is in nano-second range 42 , 52 . Advantageously, due to suppressed STA in BSBCz-based devices (devices B, C), efficiency rolloff suppressed until the current density is 500 mA cm −2 , where after the device starts to breakdown due to joule heating under DC operation (Fig.…”

“…1a), since both of singlet and triplet energy levels of CBP is higher than those of DCNP. The radiative decay constant of DCNP is 1.0 × 10 8 s −1 [photoluminescence (PL) quantum yield = 0.83, fluorescence lifetime = 8.23 ns] 42 . On the other hand, nonradiative decay rate of DCNP triplets is 2.9 × 10 4 s −1 due to the long transient lifetime (35 µs) 42 .…”

“…The radiative decay constant of DCNP is 1.0 × 10 8 s −1 [photoluminescence (PL) quantum yield = 0.83, fluorescence lifetime = 8.23 ns] 42 . On the other hand, nonradiative decay rate of DCNP triplets is 2.9 × 10 4 s −1 due to the long transient lifetime (35 µs) 42 . Therefore, DCNP emitters accumulate triplets at higher current densities, thereby suppressing the rate of singlet exciton formation and hampering the singlet emission.…”

“…Major reason is that triplet scavenging mechanism realized using BSBCz as the host instead of CBP, led to the complete removal of triplets from the emitter molecules, thereby completely avoiding the emitter-emitter STA. The other reasons are negligible overlap of the DCNP emission spectrum with the BSBCz triplet absorption spectrum 42 , while faster triplet relaxation lifetime of BSBCz became an additional advantage 42,52 to complete suppression of emitter-host STA.…”

“…Thus, BSBCz fulfils all the aforementioned intrinsic requirements for the efficient host molecule. Previously, we reported true-CW laser operation using a guest-host matrix utilizing DCNP 41 as an active chromophore and BSBCz as the triplet scavenging host 42 . Therefore, with the intention to address the removal of triplets from the emitters under electrical excitation, thereby suppressing triplet induced issues, we introduced the same guest-host matrix into OLEDs in this work.…”