08 Jul 18:56
by Sifan Li, Mubin He, Xin Jin, Weihang Geng, Chenglin Li, Xinsheng Li, Zhiyun Zhang, Jun Qian, and Jianli Hua
Chemistry of Materials
DOI: 10.1021/acs.chemmater.2c01025
08 Jul 18:45
by Ha Lim Lee,
Soon Ok Jeon,
Inkoo Kim,
Seung Chan Kim,
Junseop Lim,
Joonghyuk Kim,
Sangho Park,
Jun Chwae,
Won‐Joon Son,
Hyeonho Choi,
Jun Yeob Lee
An indolo[3,2,1-jk]carbazole-derived extended multiple-resonance structure, driven by the spin-vibronic model, is designed to reduce the gap between the singlet and triplet excited states and to increase absolute photoluminescence quantum yield. Pure blue organic-light-emitting diodes with a high external quantum efficiency over 30% and a narrow full-width-at-half-maximum of 23 nm are realized with extended multiple-resonance emitters.
Abstract
Achieving narrow-bandwidth emission and high external quantum efficiency (EQE) simultaneously is a challenge for next-generation blue-emitting organic light-emitting diodes (OLEDs). In this study, novel multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are developed by fusing an indolocarbazole unit with two carbazole skeletons using para-oriented nitrogen atoms. The resulting rigid and planar π-system without electron-accepting atoms exhibits pure blue photoluminescence at 470 nm, reaching a 100% quantum yield with a full-width-at-half-maximum (FWHM) of 25 nm. Higher-level quantum chemistry calculations confirm an MR effect within the extended π-conjugation and an enhanced triplet-to-singlet crossover (104 s−1) through a reduced energy gap (ΔE
ST) coupled with large spin-vibronic coupling mediated by low-lying triplet excited states. An OLED fabricated using the MR-TADF emitter with CIE color coordinates of (0.12, 0.16) exhibits a record high EQE of 30.9% and a small FWHM of 23 nm. With further optimization of the device structure, a high EQE of 33.8% is achieved without additional outcoupling enhancements owing to the near-perfect horizontal alignment of the emitting dipoles.
08 Jul 18:30
by Kleitos Stavrou,
Subeesh Madayanad Suresh,
David Hall,
Andrew Danos,
Nadzeya A. Kukhta,
Alexandra M. Z. Slawin,
Stuart Warriner,
David Beljonne,
Yoann Olivier,
Andrew Monkman,
Eli Zysman‐Colman
Pairing terminal emitters with suitable assistant dopants in hyperfluorescence organic light-emitting diodes can be restrictive. Here, exchanging boron substituents simultaneously tunes both the emission and absorption spectra of a multi-resonant TADF B,N-heptacene, allowing use with available blue TADF cohosts. This approach may circumvent the need for deeper-blue TADF cohosts by instead expanding hyperfluorescence compatibility to more stable cyan or green materials.
Abstract
Developing high-efficiency purely organic blue organic light-emitting diodes (OLEDs) that meet the stringent industry standards is a major current research challenge. Hyperfluorescent device approaches achieve in large measure the desired high performance by combining the advantages of a high-efficiency thermally activated delayed fluorescence (TADF) assistant dopant with a narrowband deep-blue multi-resonant TADF (MR-TADF) terminal emitter. However, this approach requires suitable spectral overlap to support Förster resonance energy transfer (FRET) between the two. Here, a color tuning of a recently reported MR-TADF B,N-heptacene core through control of the boron substituents is demonstrated. While there is little impact on the intrinsic TADF properties—as both singlet and triplet energies decrease in tandem—this approach improves the emission color coordinate as well as the spectral overlap for blue hyperfluorescence OLEDs (HF OLEDs). Crucially, the red-shifted and more intense absorption allows the new MR-TADF emitter to pair with a high-performance TADF assistant dopant and achieve maximum external quantum efficiency (EQEmax) of 15% at color coordinates of (0.15 and 0.10). The efficiency values recorded for the device at a practical luminance of 100 cd m–2 are among the highest reported for HF TADF OLEDs with CIEy ≤ 0.1.
08 Jul 18:30
by Hongyang Zhang,
Zhao Chen,
Longzhi Zhu,
Yongquan Wu,
Yuqing Xu,
Shuming Chen,
Wai‐Yeung Wong
By functionalizing the isoquinolyl and thienyl units of cyclometalated ligands, the newly designed Ir(III) emitters afford the champion efficiency in the Ir(III)-based OLEDs with the electroluminescent peak exceeding 760 nm.
Abstract
Advances in achieving high external quantum efficiency (EQE) of near-infrared (NIR) organic light-emitting diodes (OLEDs) are lagging behind that of the visible-light OLEDs, according to the energy gap law. Herein, two structurally simple NIR-phosphorescent Ir(III) complexes, DTCNIr and PTCNIr, with the cyclometalated ligands functionalized by the 1-phenylisoquinoline-4-carbonitrile moiety and thieno/benzo[b]thiophene moiety are handily accessed within three synthetic steps. The introduction of the cyano unit can significantly lower the lowest unoccupied molecular orbitals whereas incorporating the conjugated group can elevate the highest occupied molecular orbitals of the newly designed Ir(III) complexes. The intramolecular charge transfer (ICT) transitions are enhanced due to the increased donor–acceptor interaction inside the metallophosphor. As a result, the emissions are red-shifted to the NIR region with fast radiative decay. A maximum external quantum efficiency (EQE) of 8.11% with the emission peak at 726 nm for DTCNIr and a maximum EQE of 6.39% with the emission peak at 763 nm for PTCNIr are achieved in the NIR OLEDs by using these Ir(III) materials as the dopant emitters, a champion efficiency in the Ir(III)-based OLEDs with the emission peak exceeding 760 nm.
08 Jul 18:19
by Thanh Ba Nguyen,
Hajime Nakanotani,
Chihaya Adachi
An overlooked charge-transfer (CT) interaction in interfacial triplet–triplet upconversion (TTU) process in blue organic light-emitting diodes (OLEDs) is now revealed. Such interaction is conventionally avoided due to color impurity and external electroluminescence quantum efficency quenching. Here, a well-designed CT interface between hole-transporting/electron-blocking material and blue TTU material resulted in a significant improvement of the blue TTU-OLED.
Abstract
Two low-energy triplets can generate one singlet via triplet–triplet upconversion (TTU), and result in an exciton production yield exceeding 25% in conventional fluorescence-based organic light-emitting diodes (OLEDs). In most cases, since such low-energy triplets induce no serious OLED degradation, TTU-OLEDs are the only commercialized blue OLEDs so far. Herein, it is clarified that the charge-transfer (CT) interaction at a hole-transport/emitter-layer interface is an overlooked pathway to enhance TTU yield significantly. First, a small energy offset at the interface enables the formation of a high-energy CT exciton. Second, the lower energy triplet state originated from an anthracene moiety in the emitter layer collects the interfacial triplet CT. Third, due to the high-density interfacial triplets formation, TTU at the interface contributes to the electroluminescence from the emitter layer or blue dopants even at low current density. This finding underlines the important role of the CT interface to exploit the full potential of TTU in pure-blue OLEDs.
28 Jun 19:25
by Makoto Tadokoro,
Masaki Itoh,
Ryota Nishimura,
Kensuke Sekiguchi,
Norihisa Hoshino,
Hajime Kamebuchi,
Jun Miyazaki,
Fumiya Kobayashi,
Motohiro Mizuno,
Tomoyuki Akutagawa
A new H-bonded crystal of [RuIII(Him)3(Im)3](1) with six HIm (imidazole) was prepared so as to obtain a high-temperature proton conductor. The crystal is constructed through N−H⋅⋅⋅N−H-bonds between the inter-RuIII complexes and has a rare Icy-c* cubic net without crystal anisotropy. The proton conductivity is due not only to rotations and hopping motions of HIm, but also induced by isotropic whole-molecule rotation of 1.
Abstract
A new H-bonded crystal [RuIII(Him)3(Im)3] with three imidazole (Him) and three imidazolate (Im−) groups was prepared to obtain a higher-temperature proton conductor than a Nafion membrane with water driving. The crystal is constructed by complementary N−H⋅⋅⋅N H-bonds between the RuIII complexes and has a rare Icy-c* cubic network topology with a twofold interpenetration without crystal anisotropy. The crystals show a proton conductivity of 3.08×10−5 S cm−1 at 450 K and a faster conductivity than those formed by only HIms. The high proton conductivity is attributed to not only molecular rotations and hopping motions of HIm frameworks that are activated at ∼113 K, but also isotropic whole-molecule rotation of [RuIII(Him)3(Im)3] at temperatures greater than 420 K. The latter rotation was confirmed by solid-state 2H NMR spectroscopy; probable proton conduction routes were predicted and theoretically considered.
20 Jun 16:57
by Hartmut Yersin,
Rafał Czerwieniec,
Larisa Mataranga‐Popa,
Jan‐Michael Mewes,
Gang Cheng,
Chi‐Ming Che,
Masaki Saigo,
Shuji Kimura,
Kiyoshi Miyata,
Ken Onda
Experimental and theoretical studies of the recently designed organic donor–acceptor molecule show an ultra-small gap of ∆E (1CT(S1)-3CT(T1)) ≈ 10 cm–1 (≈1 meV) and ultra-fast reverse intersystem crossing (RISC) of >109 s–1, realized by polarity tuning of the 1,3CT and 3
ππ* states to near resonance and evidenced by fs to µs time-resolved spectroscopy. OLED devices showing external quantum efficiency of 19% confirm efficient singlet and triplet exciton harvesting.
Abstract
The electronic structure and photophysics of the recently designed organic direct singlet harvesting (DSH) molecule are explored, in which donor (D) and acceptor (A) are held at distance by two bridges. One of the bridges is functionalized with fluorene. This structure leads to an ultrasmall singlet–triplet energy gap of ∆E (S
1−T
1) ≈ 10 cm−1 (≈1 meV) between the charge transfer states 1,3CT and shows an energetically close-lying 3
ππ* state localized on fluorene. Dielectric constant variation of the environment leads to state crossing of 3
ππ* and 1,3CT near ε = 2.38 (toluene), as confirmed through time-dependent density functional theory (DFT) and state-specific DFT/polarizable continuum model excited-state calculations. Transient absorption (TA) and time-resolved luminescence in the femtosecond to microsecond regimes show rates of intersystem crossing (ISC) and reverse ISC (rISC) of >109 s–1. Thus, a strictly mono-exponential short-lived photo-luminescence decay (431 ns) is observed, revealing that rISC is no longer the bottleneck responsible for long thermally activated delayed fluorescence. Ultrafast TA displays a time constant of ≈700 fs, representing the relaxation time of DSH and its solvent environment to the relaxed 1CT state with a molecular dipole moment of ≈40 D. Importantly, OLED devices, emitting sky-blue light and showing high external quantum efficiency of 19%, confirm that singlet and triplet excitons are harvested efficiently.
20 Jun 16:55
by Alessio Cesaretti, Tommaso Bianconi, Mariafrancesca Coccimiglio, Nicolò Montegiove, Yogajivan Rout, Pier Luigi Gentili, Rajneesh Misra, and Benedetta Carlotti
The Journal of Physical Chemistry C
DOI: 10.1021/acs.jpcc.2c01423
14 Jun 15:41
by Huanyu Zhou,
Shin Jung Han,
Amit Kumar Harit,
Dong Hyun Kim,
Dae Yoon Kim,
Yong Seok Choi,
Hyeokjun Kwon,
Kwan‐Nyeong Kim,
Gyeong‐Tak Go,
Hyung Joong Yun,
Byung Hee Hong,
Min Chul Suh,
Seung Yoon Ryu,
Han Young Woo,
Tae‐Woo Lee
A protocol to fabricate highly efficient organic light-emitting diodes that use an intrinsically stretchable 2D-contact electrode topped with graphene is reported. As a benefit of the fast carrier mobility with complete 2D contact with the organic material and the tunable work function of the 2D-contact stretchable electro (TCSE), the limited charge injection of the widely used silver-nanowire-based stretchable electrode is solved.
Abstract
Intrinsically stretchable organic light-emitting diodes (ISOLEDs) are becoming essential components of wearable electronics. However, the efficiencies of ISOLEDs have been highly inferior compared with their rigid counterparts, which is due to the lack of ideal stretchable electrode materials that can overcome the poor charge injection at 1D metallic nanowire/organic interfaces. Herein, highly efficient ISOLEDs that use graphene-based 2D-contact stretchable electrodes (TCSEs) that incorporate a graphene layer on top of embedded metallic nanowires are demonstrated. The graphene layer modifies the work function, promotes charge spreading, and impedes inward diffusion of oxygen and moisture. The work function (WF) of 3.57 eV is achieved by forming a strong interfacial dipole after deposition of a newly designed conjugated polyelectrolyte with crown ether and anionic sulfonate groups on TCSE; this is the lowest value ever reported among ISOLEDs, which overcomes the existing problem of very poor electron injection in ISOLEDs. Subsequent pressure-controlled lamination yields a highly efficient fluorescent ISOLED with an unprecedently high current efficiency of 20.3 cd A−1, which even exceeds that of an otherwise-identical rigid counterpart. Lastly, a 3 inch five-by-five passive matrix ISOLED is demonstrated using convex stretching. This work can provide a rational protocol for designing intrinsically stretchable high-efficiency optoelectronic devices with favorable interfacial electronic structures.
Open Access
