A star-shaped 1,3,5-triazine/cyano hybrid molecule CN-T2T was designed and synthesized as a new electron acceptor for efficient exciplex-based OLED emitter by mixing with a suitable electron donor (Tris-PCz). The CN-T2T/Tris-PCz exciplex emission shows a high ΦPL of 0.53 and a small ΔET-S = -0.59 kcal/mol, affording intrinsically efficient fluorescence and highly efficient exciton up-conversion. The large energy level offsets between Tris-PCz and CN-T2T and the balanced hole and electron mobility of Tris-PCz and CN-T2T, respectively, ensuring sufficient carrier density accumulated in the interface for efficient generation of exciplex excitons. Employing a facile device structure composed as ITO/4% ReO3:Tris-PCz (60 nm)/Tris-PCz (15 nm)/Tris-PCz:CN-T2T(1:1) (25 nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm), in which the electron-hole capture is efficient without additional carrier injection barrier from donor (or acceptor) molecule and carriers mobilities are balanced in the emitting layer, leads to a highly efficient green exciplex OLED with external quantum efficiency (EQE) of 11.9%. The obtained EQE is 18% higher than that of a comparison device using an exciplex exhibiting a comparable ΦPL (0.50), in which TCTA shows similar energy levels but higher hole mobility as compared with Tris-PCz. Our results clearly indicate the significance of mobility balance in governing the efficiency of exciplex-based OLED. Exploiting the Tris-PCz:CN-T2T exciplex as the host, we further demonstrated highly efficient yellow and red fluorescent OLEDs by doping 1 wt % Rubrene and DCJTB as emitter, achieving high EQE of 6.9 and 9.7%, respectively.

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm−1 was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs.

We report a new efficient exciplex-forming system consisting of a biscarbazole donor and a triazine-based acceptor. The new exciplex was characterized with a high photoluminescence quantum yield up to 68% and effective thermally activated delayed fluorescence behavior. The BCzPh:3P-T2T (2:1, 30 nm) blend was examined not only as an emitting layer (device D1) but also a reliable co-host of fluorescent and phosphorescent emitters for giving highly efficient exciplex-based organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 15.5 and 29.7% for devices doped with 1 wt % C545T (device D2) and 8 wt % Ir(ppy)(acac) (device D4), respectively. More strikingly, a strongly enhanced lifetime ( T = 16 927 min.) of the C545T-doped device was obtained. The transient electroluminescence measurement as well as capacitance-voltage and impedance-voltage correlations were utilized to explore the factors governing the high efficiency and stability. The obtained results clearly show that the energy transfer and charge transport is highly efficient; they also show the photoelectric semiconducting characteristics of exciplex-based OLEDs, which are significantly different from those of unipolar host-based reference devices D3 (Alq: 1 wt % C545T) and D5 (CBP: 8 wt % Ir(ppy)(acac)). Our works have established a systematic protocol to shed light on the mechanisms behind exciplex-based devices. The combined results also confirm the bright prospect of the exciplex-forming system as the co-host for highly efficient and stable OLEDs.

Four bipolar molecules oDOXA, mDOXA, oOXA, and mOXA composed of a hole‐transport carbazole (donor, D) and an electron‐transport 1,3,4‐oxadiazole (acceptor, A) bridged with different π‐spacers (biphenyl or o‐terphenyl) are synthesized, characterized, and used as host materials for various colored phosphorescent OLEDs (PhOLEDs). The highly twisted geometry established via multiple ortho/meta‐connections effectively inhibits direct electronic D–A coupling and gives these bipolar molecules similar high triplet energies (≈2.70 eV). In addition, distinctive bipolar transport capabilities are observed by time‐of‐flight technique (μh ≈ μe ≈ 10−5–10−6 cm2 V−1 s−1). The D/A connection topology is found to subtly govern the physical properties, rendering these new molecules suitable for serving as bipolar host materials. Among the four host materials, oOXA using the tandem ortho‐linkage terphenyl as a linker outperforms the other three hosts in terms of the device efficiency, in which the maximum external quantum efficiencies (ηext) of the corresponding PhOLEDs are as high as 19.4%, 21.3%, 20.9%, 20.1%, and 19.0% for blue, green, yellow, orange, and red PhOLEDs, respectively. Moreover, a single‐host multilayered warm‐white OLED based on oOXA also shows remarkable efficiency (19.8%, 42.4 cd A−1, and 38.6 lm W−1) with high color‐rendering index of 86.8 and stable chromaticity.

Abstract. This paper presents a high performance piezoelectric micro energy harvester (PMEH) fabricated on stainless-steel substrate with metal MEMS process. The PMEHs fabricated in this study are with simple unimorph or bimorph cantilever structure with one or two layers of high quality lead zirconate titanate (PZT) piezoelectric films deposited by aerosol deposition method (ADM) and a glued tungsten proof mass (4 mm* 6 mm* 1 mm). The thickness of the PZT active layer are 18μm for unimorph PMEH and 10μm thick on both sides for bimorph PMEH. The length and width of the cantilever structure is 9mm and 6mm. With all the experimental process optimized, the results show that unimorph PMEH has a maximum output power of 122μW tested with optimal load under 0.5 g acceleration vibration level in resonant frequency around 120Hz. The corresponding value of bimorph PMEH is 304μW. The normalized power density (NPD) for unimorph PMEH and bimorph PMEH are 18.9mW·cm -3 ·g -2 and 46.81mW·m -3 ·g -2 respectively, which outperformed all previous published PMEHs based on either silicon or stainless steel substrates. IntroductionThe output performance of piezoelectric micro energy harvesters (PMEH) with area smaller than 1 cm 2 has great improvement over the past decade. The performance of the piezoelectric materials, the device structure design, the fabrication process, and the substrate materials are all crucial factors to optimized the output perfomance of PMEHs. Early studies of PMEHs are mostly based on silicon substrate with conventional MEMS processes [1][2][3][4], the chip area of these PMEHs are usually small and with higher resonant frequencies in around kHz range. Although the normalized power density (NPD) of silicon based PMEHs can be high, the devices are difficult to find real field applications because the mechanical vibration frequencies are usually in low frequencies. Furthermore, silicon and ceramic piezoelectric material are both brittle material which tends to break easily under strong vibration levels. There are growing studies of PMEHS based on metal substrates, especially stainless steel [5][6][7][8]. Metal substrate materials like stainless steel are ductile materials show not only much better mechanical strength over silicon but can be pre-stressed to avoid tensile cracks of ceramic piezoelectric materials. The metal MEMS processes of cantilever stucture are also simpler and cheaper because the metal substrates are usually chosen as the final beam thickness and buck micromachining backside etching for silicon is not required. The output power of metal substrate based PMEHs therefore showed better performance in lower frequency but the NPD are in general smaller than silicon based devices with high performance thin piezoelectric layers. In this study, high performance unimorph and bimorph PMEHs with around 10 μm and 20 μm high quality lead zirconate titanate (PZT) deposited by aero deposition method (ADM) on 60 μm stainless steel substrates are fabricated and tested. The device is design to have natural fre...