many different heteroatom organic blocks, such as carbazole, [ 10 ] pyridine, [ 11 ] oxadiazole, [ 12 ] acridine, [ 13 ] phosphine oxide, [ 14 ] etc., are introduced to meet these requirements, because the frontier orbitals of host materials can be delicately tuned with different groups. [ 15 ] These differences in heteroatom compounds give us more options in material's design and, however, bring some unknown risks in chemistry. It is primarily resulted from the fact that organic functional groups consist of heteroatoms always serve as the activated sites by themselves or activate adjacent positions. For example, Qiao and co-workers studied the molecular stability under device operation and found that C-C bond is signifi cantly stronger than C-S, C-P, or C-N bonds. [ 16 ] Thus, there is still a quest for host materials constituted by pure hydrocarbon (PHC) backbone in academia. But in practical, these heteroatom-free hosts still lag far behind the development of heteroatom hosts. Take the sky-blue emitter FIrpic as guest for example, the best results based on PHC hosts were just around 10% in EQE, [ 17 ] while the heteroatom hosts can achieve >20% EQE, approaching the ≈100% IQE in theory. [10][11][12][13][14] There could be big room for improvement of PHC hosts; we would therefore go forward to explore which is the effective way to arrange the hydrocarbon blocks for high efficient hosts.In this communication, two new spirofl uorene-based PHC materials, SF33 and SF34, are reported by combining two 9,9′-spirofl uorene blocks in different linking ways. SF33 adopts symmetrical confi guration with meta-meta linkage, while SF34 has unsymmetrical confi guration with meta-ortho linkage. Most of previously researches about PHC hosts adopt symmetrical or repeated units as SF33, as it can increase the molecular weight for the balance between suffi cient glass-transition temperature and suitable volatility. [ 17d ] But by breaking the symmetry of SF33 in this case, SF34 presented similar thermal property but quite different electrical and optical properties. In thermogravimetric analysis (TGA) and differential scanning calorimetry measurements ( Figure S1 and S2, Supporting Information), SF33 and SF34 exhibited good thermal stability. The decomposition temperatures ( T d ), which correspond to 5% weight loss upon heating during TGA, were measured to be 433 °C and 407 °C for SF33 and SF34, respectively. In addition, SF33 and SF34 also have a high glass-transition temperature ( T g ) of 193 °C and 177 °C, respectively.The signifi cant difference between SF33 and SF34 is in the spectra measurements. The UV-vis absorption and photoluminescence (PL) spectra of SF33 and SF34 in dilute hexane solution (1 × 10 −6 mol L −1 ) were measured to investigate their photophysical properties ( Figure 1 ). SF33 and SF34 present Organic light-emitting diodes (OLEDs) have been widely touted as next-generation displays and solid-state lighting technologies because of their superior characteristics. [ 1 ] However, effi ciency and lifetime...

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The electrical doping nature of a strong electron acceptor, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN), is investigated by doping it in a typical hole-transport material, N,N'-bis(naphthalen-1-yl)-N,N'-diphenylbenzidine (NPB). A better device performance of organic light-emitting diodes (OLEDs) was achieved by doping NPB with HATCN. The improved performance could, in principle, arise from a p-type doping effect in the codeposited thin films. However, physical characteristics evaluations including UV-vis absorption, Fourier transform infrared absorption, and X-ray photoelectron spectroscopy demonstrated that there was no obvious evidence of charge transfer in the NPB:HATCN composite. The performance improvement in NPB:HATCN-based OLEDs is mainly attributed to an interfacial modification effect owing to the diffusion of HATCN small molecules. The interfacial diffusion effect of the HATCN molecules was verified by the in situ ultraviolet photoelectron spectroscopy evaluations.

Optogenetics, photostimulation of neural tissues rendered sensitive to light, is widely used in neuroscience to modulate the electrical excitability of neurons. For effective optical excitation of neurons, light wavelength and power density must fit with the expression levels and biophysical properties of the genetically encoded light‐sensitive ion channels used to confer light sensitivity on cells—most commonly, channelrhodopsins (ChRs). As light sources, organic light‐emitting diodes (OLEDs) offer attractive properties for miniaturized implantable devices for in vivo optical stimulation, but they do not yet operate routinely at the optical powers required for optogenetics. Here, OLEDs with doped charge transport layers are demonstrated that deliver blue light with good stability over millions of pulses, at powers sufficient to activate the ChR, CheRiff when expressed in cultured primary neurons, allowing live cell imaging of neural activity with the red genetically encoded calcium indicator, jRCaMP1a. Intracellular calcium responses scale with the radiant flux of OLED emission, when varied through changes in the current density, number of pulses, frequency, and pulse width delivered to the devices. The reported optimization and characterization of high‐power OLEDs are foundational for the development of miniaturized OLEDs with thin‐layer encapsulation on bioimplantable devices to allow single‐cell activation in vivo.

Fluorescence imaging is an indispensable tool in biology, with applications ranging from single‐cell to whole‐animal studies and with live mapping of neuronal activity currently receiving particular attention. To enable fluorescence imaging at cellular scale in freely moving animals, miniaturized microscopes and lensless imagers are developed that can be implanted in a minimally invasive fashion; but the rigidity, size, and potential toxicity of the involved light sources remain a challenge. Here, narrowband organic light‐emitting diodes (OLEDs) are developed and used for fluorescence imaging of live cells and for mapping of neuronal activity in Drosophila melanogaster via genetically encoded Ca2+ indicators. In order to avoid spectral overlap with fluorescence from the sample, distributed Bragg reflectors are integrated onto the OLEDs to block their long‐wavelength emission tail, which enables an image contrast comparable to conventional, much bulkier mercury light sources. As OLEDs can be fabricated on mechanically flexible substrates and structured into arrays of cell‐sized pixels, this work opens a new pathway for the development of implantable light sources that enable functional imaging and sensing in freely moving animals.

Top‐emitting organic light‐emitting diodes (OLEDs) are of interest for numerous applications, in particular for displays with high fill factors. To maximize efficiency and luminance, molecular p‐doping of the hole transport layer (p‐HTL) and a highly reflective anode contact, for example, made from silver, are used. Atomic layer deposition (ALD) is attractive for thin film encapsulation of OLEDs but generally requires a minimum process temperature of 80 °C. Here it is reported that the interface between the p‐HTL and the silver anode of top‐emitting OLEDs degrades during an 80 °C ALD encapsulation process, causing an over fourfold reduction in OLED current and luminance. To understand the underlying mechanism of device degradation, single charge carrier devices are investigated before and after annealing. A spectroscopic study of p‐HTLs indicates that degradation is due to the interaction between diffusing silver ions and the p‐type molecular dopant. To improve the stability of the interface, either an ultrathin MoO3 buffer layer or a bilayer HTL is inserted at the anode/organic interface. Both approaches effectively suppress degradation. This work shows a route to successful encapsulation of top‐emitting OLEDs using ALD without sacrificing device performance.

Organic light‐emitting diodes (OLEDs) can emit light over much larger areas than their inorganic counterparts, offer mechanical flexibility, and can be readily integrated on various substrates and backplanes. However, the amount of light they emit per unit area is typically lower and the required operating voltage is higher, which can be a limitation for emerging applications of OLEDs, e.g., in outdoor and high‐dynamic‐range displays, biomedical devices, or visible‐light communication. Here, high‐luminance, blue‐emitting (λpeak = 464 nm), fluorescent p–i–n OLEDs are developed by combining three strategies: First, the thickness of the intrinsic layers in the device is decreased to reduce internal voltage loss. Second, different electron‐blocking layer materials are tested to recover efficiency losses resulting from this thickness reduction. Third, the geometry of the anode contact is optimized, which leads to a substantial reduction in the in‐plane resistive voltage losses. The OLEDs retain a maximum external quantum efficiency of 4.4% as expected for an optimized fluorescent device and reach a luminance of 132 000 cd m−2 and an optical power density of 2.4 mW mm−2 at 5 V, a nearly eightfold improvement compared to the original reference device.

tremendous improvement in device lifetime and efficiency. However, ensuring robust stability in applications that require high luminance compared with typical displays (typical display luminance <500 cd m −2 ) remains challenging. Since the thermal conductivity of organic semiconductors is relatively low (typically, <1 W m −1 K −1 ), [7] and no efficient channel to dissipate heat is present in typical OLED stacks, resistive heating can be significant, particularly during high-brightness operation. [8] In addition to intrinsic resistive heating, heating can also occur due to environmental effects during OLED operation (e.g., when OLEDs are used in automotive applications) or due to the need for a certain process temperature during fabrication or encapsulation of devices. For instance, atomic layer deposition (ALD) is a promising technique to form thin, highly protective encapsulation layers that are intrinsically conformal and unlikely to be disrupted by local defects; [9] however, state-of-the-art ALD processes require a relatively high temperature during the deposition (typically, >80 °C). [9][10][11] The influence of elevated temperature (due to either resistive or external heating) on device performance has been extensively investigated and is seen as an important factor in OLED degradation. [12][13][14] In general, an increase in temperature can induce morphological changes and may promote crystallization of one or several OLED layers, which in turn can reduce device performance and may lead to catastrophic failure via electric breakdown. [15] Because the thermal durability of organic materials is usually closely correlated to the glass transition temperature (T g ), a straightforward method to enhance device stability is to adapt existing materials for higher T g or to develop new high-T g materials. [16,17] However, great care has to be taken to retain appropriate functionality of the material in the final OLED, e.g., in terms of charge transport or charge blocking performance. Another strategy is to blend an existing low-T g material with a higher T g material. This can result in an intermediate T g and thus in improved morphological stability of the film, even though this effect has been found to be relatively modest. [18] Given that state-of-the-art OLEDs generally comprise 4,7-Diphenyl-1,10-phenanthroline (BPhen) is widely used to create the electron transport layer (ETL) in organic light-emitting diodes (OLEDs) because of its high electron mobility and good compatibility with alkali metal n-dopants. However, the morphology of these ETLs is easily altered by heating due to the relatively low glass transition temperature (T g ) of BPhen and this change often reduces the performance of OLEDs. Here, an enhancement in the thermal stability of OLEDs when doping their BPhenbased ETLs with cesium (Cs) is reported. To investigate the role of the Cs dopant in the BPhen matrix, the crystallization features of Cs-doped BPhen films with different doping concentrations are examined. Next, the electrical and optical ...