High-efficiency crystalline white organic light-emitting diodes.

Crystalline white organic light-emitting diodes (C-WOLEDs) are promising candidates for lighting and display applications. It is urgently necessary, however, to develop energy-saving and high-efficiency C-WOLEDs that have stable and powerful emission to meet commercial demands. Here, we report a crystalline host matrix (CHM) with embedded nanoaggregates (NA) structure for developing high-performance C-WOLEDs by employing a thermally activated delayed fluorescence (TADF) material and orange phosphorescent dopants (Phos.-D). The CHM-TADFNA-D WOLED exhibit a remarkable EQE of 12.8%, which is the highest performance WOLEDs based on crystalline materials. The device has a quick formation of excitons and a well-designed energy transfer process, and possesses a fast ramping of luminance and current density. Compared to recently reported high-performance WOLEDs based on amorphous material route, the C-WOLED achieves a low series-resistance Joule-heat loss ratio and an enhanced photon output, demonstrating its significant potential in developing the next-generation WOLEDs.

Crystalline organic thin films for crystalline OLEDs (II): weak epitaxy growth of phenanthroimidazole derivatives.

The ordered molecular arrangement of crystalline organic semiconductors facilitates high carrier mobility and light emission in organic light-emitting diode (OLED) devices. It has been demonstrated that the weak epitaxy growth (WEG) process is a valuable crystallization route for fabricating crystalline thin-film OLEDs (C-OLEDs). Recently, C-OLEDs based on crystalline thin films of phenanthroimidazole derivatives have exhibited excellent luminescent properties such as high photon output at low driving voltage and high power efficiency. Achieving effective control of organic crystalline thin film growth is crucial for the development of new C-OLEDs. Herein, we report the studies on morphology structure and growth behavior of the phenanthroimidazole derivative WEG thin films. The oriented growth of WEG crystalline thin films is determined by channeling and lattice matching between the inducing layer and active layer. Large-size and continuous WEG crystalline thin films can be obtained by controlling the growth conditions.

An Efficient Blue-Emission Crystalline Thin-Film OLED Sensitized by "Hot Exciton" Fluorescent Dopant.

Crystalline thin-film organic light-emitting diodes (C-OLEDs) can achieve a large light emission and a low Joule-heat loss under low driving voltage due to the high carrier mobility of the crystalline thin films. However, it is urgent for the C-OLEDs to improve their external quantum efficiency (EQE). Here, a novel strategy is proposed using a doped "hot exciton" material to sensitize a high PLQY blue emitter in C-OLEDs. Benefiting from the capability of the "hot exciton" material harnessing triplet/singlet excitons, the C-OLED exhibits an efficiency breakthrough with a maximum EQE of 6.2%, a much enhanced blue photon output with pure blue emission Commission International de L'Eclairage (CIE) (0.14, 0.15), a low turn-on/operation voltage of 2.6 V(@1 cd m-2 )/3.8 V (@1000 cd m-2 ), and a maximum power efficiency (PE) of 9.4 lm W-1 . This work unlocks the potential of C-OLEDs for achieving high photon output with high EQE.

High-efficiency blue-emission crystalline organic light-emitting diodes sensitized by "hot exciton" fluorescent nanoaggregates.

Sensitizing fluorescent materials is an effective way to maximally use excitons and obtain high-efficiency blue organic light-emitting diodes (OLEDs). However, it is a persistent challenge for present amorphous thin-film OLEDs to improve photon emission under low driving voltage, severely impeding the development of OLED technology. Here, we propose a novel OLED architecture consisting of a crystalline host matrix (CHM) and embedded "hot exciton" nanoaggregates (HENAs), which effectively sensitize blue dopant (D) emission. Owing to the advantages of the crystalline thin-film route, the device exhibits largely enhanced blue photon output [Commission International de L'Eclairage coordinates of (0.15, 0.17)], with a low turn-on/operation voltage of 2.5 V (at 1 cd/m2)/3.3 V (at 1000 cd/m2), an extremely low Joule heat loss ratio (7.8% at 1000 cd/m2), and a maximum external quantum efficiency (EQE) up to 9.14%. These areal photon output features have outperformed the present amorphous thin-film blue OLEDs with high EQE, demonstrating that the CHM-HENA-D OLED is promising for future OLEDs.

Integrated molecular diode as 10 MHz half-wave rectifier based on an organic nanostructure heterojunction.

Considerable efforts have been made to realize nanoscale diodes based on single molecules or molecular ensembles for implementing the concept of molecular electronics. However, so far, functional molecular diodes have only been demonstrated in the very low alternating current frequency regime, which is partially due to their extremely low conductance and the poor degree of device integration. Here, we report about fully integrated rectifiers with microtubular soft-contacts, which are based on a molecularly thin organic heterojunction and are able to convert alternating current with a frequency of up to 10 MHz. The unidirectional current behavior of our devices originates mainly from the intrinsically different surfaces of the bottom planar and top microtubular Au electrodes while the excellent high frequency response benefits from the charge accumulation in the phthalocyanine molecular heterojunction, which not only improves the charge injection but also increases the carrier density.

Asymmetric Conjugated Molecules Based on [1]Benzothieno[3,2-b][1]benzothiophene for High-Mobility Organic Thin-Film Transistors: Influence of Alkyl Chain Length.

Herein, we report the synthesis and characterization of a series of [1]benzothieno[3,2-b][1]benzothiophene (BTBT)-based asymmetric conjugated molecules, that is, 2-(5-alkylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-Tn, in which T and n represent thiophene and the number of carbons in the alkyl group, respectively). All of the molecules with n ≥ 4 show mesomorphism and display smectic A, smectic B (n = 4), or smectic E (n > 4) phases and then crystalline phases in succession upon cooling from the isotropic state. Alkyl chain length has a noticeable influence on the microstructures of vacuum-deposited films and therefore on the performance of the organic thin-film transistors (OTFTs). All molecules except for 2-(thiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene and 2-(5-ethylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene showed OTFT mobilities above 5 cm2 V-1 s-1. 2-(5-Hexylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene and 2-(5-heptylthiophen-2-yl)[1]benzothieno[3,2-b][1]benzothiophene showed the greatest OTFT performance with reliable hole mobilities (µ) up to 10.5 cm2 V-1 s-1 because they formed highly ordered and homogeneous films with diminished grain boundaries.

Organic High Electron Mobility Transistors Realized by 2D Electron Gas.

A key breakthrough in inorganic modern electronics is the energy-band engineering that plays important role to improve device performance or develop novel functional devices. A typical application is high electron mobility transistors (HEMTs), which utilizes 2D electron gas (2DEG) as transport channel and exhibits very high electron mobility over traditional field-effect transistors (FETs). Recently, organic electronics have made very rapid progress and the band transport model is demonstrated to be more suitable for explaining carrier behavior in high-mobility crystalline organic materials. Therefore, there emerges a chance for applying energy-band engineering in organic semiconductors to tailor their optoelectronic properties. Here, the idea of energy-band engineering is introduced and a novel device configuration is constructed, i.e., using quantum well structures as active layers in organic FETs, to realize organic 2DEG. Under the control of gate voltage, electron carriers are accumulated and confined at quantized energy levels, and show efficient 2D transport. The electron mobility is up to 10 cm2 V-1 s-1 , and the operation mechanisms of organic HEMTs are also argued. Our results demonstrate the validity of tailoring optoelectronic properties of organic semiconductors by energy-band engineering, offering a promising way for the step forward of organic electronics.

Multifluorination toward High-Mobility Ambipolar and Unipolar n-Type Donor-Acceptor Conjugated Polymers Based on Isoindigo.

Using a "multifluorination" strategy, ambipolar donor-acceptor conjugated polymer with hole and electron mobility (µh and µe ) up to 3.94 and 3.50 cm2 V-1 s-1 , respectively, and unipolar n-type donor-acceptor conjugated polymers with µe up to 4.97 cm2 V-1 s-1 is synthesized with isoindigo as acceptor units.

Electron-transporting polymers based on a double B←N bridged bipyridine (BNBP) unit.

In this communication, we report a series of polymer semiconductors based on a novel electron-deficient building block, double B←N bridged bipyridine (BNBP). These polymers show ambipolar or unipolar n-channel charge-transporting characteristics with electron mobilities in the range of 0.02-0.32 cm2 V-1 s-1 in organic thin film transistors.

A self-immolative and DT-diaphorase-activatable prodrug for drug-release tracking and therapy.

DT-diaphorase, which catalyzes the reduction of various biological substances like quinones, is overexpressed in some malignant tumors. However, exploiting this attractive property for the controlled release of an active drug from a prodrug is yet to be fully taken advantage of. Herein we report a DT-diaphorase-based prodrug for concomitant drug-release imaging and cancer chemotherapy. This prodrug system is composed of two camptothecin (CPT) moieties as the active anticancer drug, a DT-diaphorase-responsive quinone propionic acid moiety and a set of self-immolative linkers. The presence of DT-diaphorase leads to the release of two CPT molecules and restores the fluorescence of the latter, thereby realizing the fluorescence monitoring of the DT-diaphorase level as well as the tracking of CPT release. Upon internalization by DT-diaphorase overexpressing cells, the prodrug can release fluorescent CPT and exhibit high cytotoxicity (half-maximal inhibitory concentration 0.71 µM) towards the cancer cells. This prodrug features on-demand enzyme-biomarker-triggered drug release as well as self-monitoring of drug release, therapeutic effect and biomarker level. This new strategy may provide an effective approach for constructing prodrugs with enhanced drug loading as well as controllability for drug release and tracking.

Growth of Highly Oriented Ultrathin Crystalline Organic Microstripes: Effect of Alkyl Chain Length.

The growth of organic semiconductor with controllable morphology is a crucial issue for achieving high-performance devices. Here we present the systematic study of the effect of the alkyl chain attached to the functional entity on controlling the growth of oriented microcrystals by dip-coating. Alkylated DTBDT-based molecules with variable chain lengths from n-butyl to n-dodecyl formed into one-dimensional micro- or nanostripe crystals at different pulling speeds. The alignment and ordering are significantly varied with alkyl chain length, as is the transistor performance. Highly uniform oriented and higher-molecular-order crystalline stripes with improved field-effect mobility can be achieved with an alkyl-chain length of around 6. We attribute this effect to the alkyl-chain-length-dependent packing, solubility, and self-assembly behavior.

Highly Crystalline Films of Organic Small Molecules with Alkyl Chains Fabricated by Weak Epitaxy Growth.

Because side-chain engineering of organic conjugated molecules has been widely utilized to tune organic solid-state optoelectronic properties, the achievement of their high-quality films is important for realizing high-performance devices. Here, highly crystalline films of an organic molecule with short alkyl chains, 5,8,15,18-tetrabutyl-5,8,15,18-tetrahydroindolo[3,2-a]indole[30,20:5,6]quinacridone (C4-IDQA), are fabricated by weak epitaxy growth, and highly oriented, large-area, and continuous films are obtained. Because of the soft matter properties, the C4-IDQA molecules can adjust themselves to realize commensurate epitaxy growth on the inducing layers and exhibited good lattice matching in the thin film phase. The crystalline phase is also observed in thicker C4-IDQA films. The growth behavior of C4-IDQA on the inducing layer is further investigated, including the strong dependence of film morphologies on substrate temperatures and deposition rates due to the poor diffusion ability of C4-IDQA molecules. Moreover, highly crystalline films and high electron field-effect mobility are also obtained for the small molecule N,N'-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (C8-PTCDI), which demonstrate that the weak epitaxy growth method could be an effective way to fabricate highly crystalline films of organic small molecules with flexible side chains.

Fully Integrated Organic Nanocrystal Diode as High Performance Room Temperature NO2 Sensor.

Organic diodes consisting of molecular nano-pyramid structures sandwiched between metal and strained nano-membrane electrodes are created. The robust and smooth contacts provided by self-curled metal layers render the molecular nano-pyramids efficent channels for detecting nitrogen dioxide airflow.

Large-area perovskite nanowire arrays fabricated by large-scale roll-to-roll micro-gravure printing and doctor blading.

Organic-inorganic hybrid halide perovskite nanowires (PNWs) show great potential applications in electronic and optoelectronic devices such as solar cells, field-effect transistors and photodetectors. It is very meaningful to fabricate ordered, large-area PNW arrays and greatly accelerate their applications and commercialization in electronic and optoelectronic devices. Herein, highly oriented and ultra-long methylammonium lead iodide (CH3NH3PbI3) PNW array thin films were fabricated by large-scale roll-to-roll (R2R) micro-gravure printing and doctor blading in ambient environments (humility ∼45%, temperature ∼28 °C), which produced PNW lengths as long as 15 mm. Furthermore, photodetectors based on these PNWs were successfully fabricated on both silicon oxide (SiO2) and flexible polyethylene terephthalate (PET) substrates and showed moderate performance. This study provides low-cost, large-scale techniques to fabricate large-area PNW arrays with great potential applications in flexible electronic and optoelectronic devices.

Photogenerated Intrinsic Free Carriers in Small-molecule Organic Semiconductors Visualized by Ultrafast Spectroscopy.

Confirmation of direct photogeneration of intrinsic delocalized free carriers in small-molecule organic semiconductors has been a long-sought but unsolved issue, which is of fundamental significance to its application in photo-electric devices. Although the excitonic description of photoexcitation in these materials has been widely accepted, this concept is challenged by recently reported phenomena. Here we report observation of direct delocalized free carrier generation upon interband photoexcitation in highly crystalline zinc phthalocyanine films prepared by the weak epitaxy growth method using ultrafast spectroscopy. Transient absorption spectra spanning the visible to mid-infrared region revealed the existence of short-lived free electrons and holes with a diffusion length estimated to cross at least 11 molecules along the π-π stacking direction that subsequently localize to form charge transfer excitons. The interband transition was evidenced by ultraviolet-visible absorption, photoluminescence and electroluminescence spectroscopy. Our results suggest that delocalized free carriers photogeneration can also be achieved in organic semiconductors when the molecules are packed properly.

High Mobility Ambipolar Diketopyrrolopyrrole-Based Conjugated Polymer Synthesized Via Direct Arylation Polycondensation.

A diketopyrrolopyrrole-based conjugated polymer, PDPP-4FTVT, which exhibits ambipolar transport behavior in air with hole and electron mobilities up to 3.40 and 5.86 cm(2) V(-1) s(-1), respectively, is synthesized via direct arylation polycondensation. Incorporation of F-atoms in ß-positions of thiophene rings dramatically improves the efficiency of direct arylation polycondensation.

A DT-diaphorase responsive theranostic prodrug for diagnosis, drug release monitoring and therapy.

A DT-diaphorase-activatable theranostic prodrug, which contains camptothecin, a self-immolative linker and a trigger group, has been developed for the detection of DT-diaphorase, tracking of drug release and selectively killing cancer cells over-expressed with DT-diaphorase. This strategy may offer a new approach for the development of enzyme-catalyzed theranostic anticancer therapeutics.

An organic quantum well based on high-quality crystalline heteroepitaxy films.

A meaningful organic quantum well based on crystalline heteroepitaxy films is constructed. The quantum confinement effect is demonstrated by its reflections on optics and electrics: the blueshift of the optical characteristic peaks and the negative differential resistance at room temperature. The realization of an organic quantum well indicates the highly delocalized transport mechanism in well-defined organic crystalline systems and promises novel organic "quantum" optoelectronic devices.

Benzothienobenzothiophene-based conjugated oligomers as semiconductors for stable organic thin-film transistors.

Two benzothienobenzothiophene (BTBT)-based conjugated oligomers, i.e., 2,2'-bi[1]benzothieno[3,2-b][1]benzothiophene (1) and 5,5'-bis([1]benzothieno[3,2-b][1]benzothiophen-2-yl)-2,2'-bithiophene (2), were prepared and characterized. Both oligomers exhibit excellent thermal stability, with 5% weight-loss temperatures (T(L)) above 370 °C; no phase transition was observed before decomposition. The highest occupied molecular orbital (HOMO) levels of 1 and 2 are -5.3 and -4.9 eV, respectively, as measured by ultraviolet photoelectron spectroscopy. Thin-film X-ray diffraction and atomic force microscopy characterizations indicate that both oligomers form highly crystalline films with large domain sizes on octadecyltrimethoxysilane-modified substrates. Organic thin-film transistors with top-contact and bottom-gate geometry based on 1 and 2 exhibited mobilities up to 2.12 cm(2)/V·s for 1 and 1.39 cm(2)/V·s for 2 in an ambient atmosphere. 1-based devices exhibited great air and thermal stabilities, as evidenced by the slight performance degradation after 2 months of storage under ambient conditions and after thermal annealing at temperatures below 250 °C.