Search

Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light-emitting materials (OLEDS). Charge-transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi-equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4-[2-(4′-diphenylamino-biphenyl-4-yl)-phenanthro[9,10-d]imidazol-1-yl]-benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record-setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low-cost, high-efficiency fluorescent OLED materials. [ABSTRACT FROM AUTHOR]

In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron-hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor-acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA-NZP as the light-emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep-red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m−2 and a maximum external quantum efficiency (ηext) of ∼2.8%. The HLCT state is a new way to realize high-efficiency of EL devices. [ABSTRACT FROM AUTHOR]

Deep red (DR) and near-infrared (NIR) fluorescent emitters are often plagued by design difficulties due to the strong intramolecular interaction. Herein, two new DR and NIR molecules, 4-(7-(10-ethyl-10 H -phenoxazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)- N,N -diphenylaniline (TPAC2) and 4-(9-(10-ethyl-10 H -phenoxazin-3-yl)naphtho[2,3-c][1,2,5]thiadiazol-4-yl)- N,N -diphenylaniline (TPANZC2), characteristic with hybrid local and charge transfer were achieved via iteration with a theoretical combined experimental method. Both emitters realized the reverse intersystem crossing (RISC) via the hot exciton channel. As results, the non-doped organic light-emitting diodes (OLEDs) of TPAC2 and TPANZC2 achieved the maximum external quantum efficiency (EQE max) of 2.4% at 680 nm and 1.1% at 755 nm, respectively. Both conspicuous upward EQE curves demonstrated excellent resistance to efficiency roll-off. The ternary co-doping system further unleashed the potential in NIR emission, the co-doped device with 5 wt% TPANZC2 achieved an EQE of 4.6% with emission at 724 nm, which is among the best results of HLCT-NIR fluorescent OLEDs to date. [Display omitted] • Iterative strategy reasonably achieved deep red to NIR emission. • The co-doped NIR device achieved an EQE of 4.6%, is the best HLCT-NIR OLED performance to date. • EQE loss only 2.4% and 0.8% at 90% maximum luminescence. [ABSTRACT FROM AUTHOR]

The properties of excited states of emitters play an important role in OLEDs. How to obtain balanced hybrid localized charge transfer (HLCT) state by reasonably designing molecules is still a challenge. Herein, three HLCT states D-π-A type blue fluorophores with different LE/CT hybrid degree were designed and synthesized, namely 3,6-di-tert-butyl-9-(4-(6-(4-(4,5-diphenyl-4 H -1,2,4-triazol-3-yl)phenyl)pyren-1-yl)phenyl)-9 H carbazole (DTPCZPYTZ), 3,6-di-tert-butyl-9-(4-(4-(4-(4,5-diphenyl-4 H -1,2,4-triazol-3-yl)phenyl)naphthalen-1-yl)phenyl)-9 H -carbazole (DTPCZNATZ) and 3,6-di-tert-butyl-9-(4''-(4,5-diphenyl-4 H -1,2,4-triazol-3-yl)-[1,1':4′,1″-terphenyl]-4-yl)-9 H -carbazole (DTPCZPHTZ). DTPCZPHTZ achieved quasi-equivalent hybrid local and charge-transfer (HLCT) excited state and efficient "hot-exciton" channel, the non-doped deep-blue device based on DTPCZPHTZ has an electroluminescence emission peak (λ EL) of 427 nm with Commission International de L'Eclairage (CIE) of (0.157, 0.037), the maximum external quantum efficiency (EQE) is up to 5.7% and the efficiency roll-off is only 5% (under 1000 cd m−2), which is outstanding in the reported deep-blue emitters. This study proved that the quasi-equivalent hybrid HLCT state is beneficial to improving device performance. This work provides a flexible and convenient method for fine-tuning the excited states of emitters, by changing the number of benzene rings in the intermediate π bridge, the degree of LE/CT hybridization can be adjusted. Finally, quasi-equivalent hybrid deep-blue luminescent emitter and highly efficient non-doped deep-blue OLED is obtained. [Display omitted] • Construction of quasi-equivalent hybrid local and charge-transfer (HLCT) excited state and hot exciton channels. • Efficient non-doped deep-blue OLED with EQE up to 5.7% and CIE (0.157, 0.037). • Highly Suppressed efficiency roll-off of only 1%–5% at 1000 cd m−2. [ABSTRACT FROM AUTHOR]

This work provides a flexible and convenient method for fine-tuning the excited states of emitters, precise "surgery" was performed by targeting the molecular excited states through locally large twisted conformations, and finally obtained excellent hot exciton red emitters and high efficiency non-doped deep-red OLEDs. [Display omitted] • Deep-red emitters construct idea utilizing locally large twisted conformation. • Three emitters display hot exciton and aggregation-induced emission properties. • Breakthrough EQEs of hot exciton non-doped red devices up to 5.3% and 6%. The precise regulation of excited state is a challenge to emitters for achieving highly efficient non-doped deep-red electroluminescence (EL). Herein, we used triphenylamine and phenoxazine as donors and benzo[c][1,2,5]thiadiazole as an acceptor to construct a hot exciton skeleton. Then chemically modified the phenoxazine by locally large twisted conformation to obtain N , N -diphenyl-4-(7-(10-phenyl-10H-phenoxazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)aniline (TPAPH), 4-(7-(10-(4-ethoxyphenyl)-10 H -phenoxazin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)- N , N -diphenylaniline (TPAPHOC2), and 4-(3-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)-10 H -phenoxazin-10-yl)benzonitrile (TPAPHCN), thereby achieving excited state perturbation. As a result, three emitters exhibited aggregation-induced emission enhancement and hybridized local and charge transfers characteristics based on photophysical experimentals. In EL, the TPAPH-, TPAPHOC2-, and TPAPHCN-based non-doped devices achieved deep-red emission with Commission International de L'Eclairage coordinates of (0.68, 0.32), (0.69, 0.31), and (0.66, 0.34) and with emission peaks at 660, 669, and 647 nm, respectively. The maximum external quantum efficiencies were 5.3%, 3.7%, and 6.0%, respectively. The results of this study not only realize the non-doped deep-red OLEDs with high performance but also indicate that the presence of locally large twisted conformation presented an efficient manner for exploring precise excited state tuning for fluorescent emitter. [ABSTRACT FROM AUTHOR]