以昇陳

A sky‐blue‐exciplex‐forming host constructed by π–donor and π–acceptor with bipolar π‐spacers is used to fabricate single‐emissive‐layer all‐fluorescent white organic light‐emitting diodes, leading to stable warm white emission with a record‐high power efficiency of 69.6 lm W⁻¹ and a long T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m⁻².

Abstract

Exciplex‐forming hosts with thermally activated delayed fluorescence (TADF) provide a viable opportunity to unlock the full potential of the yet‐to‐be improved power efficiencies (PEs) and stabilities of all‐fluorescent white organic light‐emitting diodes (WOLEDs), but this, however, is hindered by the lack of stable blue exciplexes. Here, an advanced exciplex system is proposed by incorporating bipolar charge‐transport π‐spacers into both the electron‐donor (D) and the electron‐accepter (A) to increase their distance for hypsochromic‐shifted emission while maintaining the superior transporting ability. By using spirofluorene as the π‐spacer, 3,3′‐bicarbazole as the D‐unit, and 2,4,6‐triphenyl‐1,3,5‐triazine as the A‐unit, a π–D and π–A exciplex with sky‐blue emission and fast reverse intersystem crossing process is thereof constructed. Combining this exciplex‐forming host, a blue TADF‐sensitizer, and a yellow conventional fluorescent dopant in a single‐emissive‐layer, the fabricated warm‐white‐emissive device simultaneously exhibits a low driving voltage of 3.08 V, an external quantum efficiency of 21.4%, and a remarkable T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m⁻², accompanied by a new benchmark PE of 69.6 lm W⁻¹ among all‐fluorescent WOLEDs.

The specific detectivity of near‐infrared organic photodetectors is found to be limited by non‐radiative recombination, which is also known to be the main contributor to open‐circuit voltage losses. Investigation of the relation between open‐circuit voltage, dark current, and noise current allows the calculation of an intrinsic upper limit of D* as a function of the optical gap.

Abstract

Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime. However, near‐infrared photodetection has proven to be challenging and, to date, the true potential of organic semiconductors in this spectral range (800–2500 nm) remains largely unexplored. In this work, it is shown that the main factor limiting the specific detectivity (D*) is non‐radiative recombination, which is also known to be the main contributor to open‐circuit voltage losses. The relation between open‐circuit voltage, dark current, and noise current is demonstrated using four bulk‐heterojunction devices based on narrow‐gap donor polymers. Their maximum achievable D* is calculated alongside a large set of devices to demonstrate an intrinsic upper limit of D* as a function of the optical gap. It is concluded that OPDs have the potential to be a useful technology up to 2000 nm, given that high external quantum efficiencies can be maintained at these low photon energies.

New blue‐emitting thermally activated delayed fluorescence (TADF) emitters are designed and synthesized for blue‐emitting organic light‐emitting diodes (OLEDs) by incorporating rigid donor units with an oxygen‐bridged boron acceptor unit. As a result, a deep‐blue OLED with a maximum external quantum efficiency of 29.3% and CIE coordinates of (0.142, 0.090) is realized.

Abstract

New blue (DBA‐SAB) and deep‐blue (TDBA‐SAF) thermally activated delayed fluorescence (TADF) emitters are synthesized for blue‐emitting organic‐light emitting diodes (OLEDs) by incorporating spiro‐biacridine and spiro‐acridine fluorene donor units with an oxygen‐bridged boron acceptor unit, respectively. The molecules show blue and deep‐blue emission because of the deep highest occupied molecular energy levels of the donor units. Besides, both emitters exhibit narrow emission spectra with the full‐width at half maximum (FWHM) of less than 65 nm due to the rigid donor and acceptor units. In addition, the long molecular structure along the transition dipole moment direction results in a high horizontal emitting dipole ratio over 80%. By combining the effects, the OLED utilizing DBA‐SAB as the emitter exhibits a maximum external quantum efficiency (EQE) of 25.7% and 1931 Commission Internationale de l'éclairage (CIE) coordinates of (0.144, 0.212). Even a higher efficiency deep blue TADF OLED with a maximum EQE of 28.2% and CIE coordinates of (0.142, 0.090) is realized using TDBA‐SAF as the emitter.

Nature Photonics, Published online: 19 October 2020; doi:10.1038/s41566-020-00702-w

Organic long‐persistent luminescence from an organic thermally activated delayed fluorescence (TADF) compound is demonstrated. It is also shown that this phenomenon can occur in a number of chemically distinct host matrices. Furthermore, this work reveals the important role that photoinduced charges play in TADF‐emitter‐doped films, which has a significant impact on their photophysical properties.

Abstract

Organic long‐persistent luminescence (OLPL) is one of the most promising methods for long‐lived‐emission applications. However, present room‐temperature OLPL emitters are mainly based on a bimolecular exciplex system which usually needs an expensive small molecule such as 2,8‐bis(diphenyl‐phosphoryl)dibenzo[b,d]thiophene (PPT) as the acceptor. In this study, a new thermally activated delayed fluorescence (TADF) compound, 3‐(4‐(9H‐carbazol‐9‐yl)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (CzPhAP), is designed, which also shows OLPL in many well‐known hosts such as PPT, 2,2′,2″‐(1,3,5‐benzinetriyl)‐tris(1‐phenyl‐1‐H‐benzimidazole) (TPBi), and poly(methyl methacrylate) (PMMA), without any exciplex formation, and its OLPL duration reaches more than 1 h at room temperature. Combining the low cost of PMMA manufacture and flexible designs of TADF molecules, pure organic, large‐scale, color tunable, and low‐cost room‐temperature OLPL applications become possible. Moreover, it is found that the onset of the 77 K afterglow spectra from a TADF‐emitter‐doped film is not necessarily reliable for determining the lowest triplet state energy level. This is because in some TADF‐emitter‐doped films, optical excitation can generate charges (electron and holes) that can later recombine to form singlet excitons during the phosphorescence spectrum measurement. The spectrum taken in the phosphorescence time window at low temperature may consequently consist of both singlet and triplet emission.