Highly Efficient Three Primary Color Organic Single‐Crystal Light‐Emitting Devices with Balanced Carrier Injection and Transport

Organic single crystals have a great potential in the field of organic optoelectronics because of their advantages of high carrier mobility and high thermal stability. However, the application of the organic single crystals in light‐emitting devices (OLEDs) has been limited by single‐layered structure with unbalanced carrier injection and transport. Here, fabrication of a multilayered‐structure crystal‐based OLED constitutes a major step toward balanced carrier injection and transport by introducing an anodic buffer layer and electron transport layer into the device structure. Three primary color single‐crystal‐based OLEDs based on the multilayered structure and molecular doping exhibit a maximum luminance and current efficiency of 820 cd cm^?2 and 0.9 cd A^?1, respectively, which are the highest performance to date for organic single‐crystal‐based OLEDs. This work paves the way toward high‐performance organic optoelectronic devices based on the organic single crystals.     

Understanding the influence of doping in efficient phosphorescent organic light-emitting diodes with an organic p–i–n homojunction

We report on the development and detailed investigation of highly efficient p–i–n phosphorescent organic light-emitting diodes (PhOLEDs) using 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) as a single organic semiconductor matrix. Following optimization of doping concentration of both the phosphorescent emitter molecule and of the p- and n-type dopants, an external quantum efficiency (EQE) of 15% and a power efficiency (PE) of 28 lm/W are realized at a luminance of 1000 cd/m². These values are comparable to the state-of-the-art for conventional complex multilayered PhOLEDs. By analyzing the device characteristics (i.e. electroluminescence spectra, the current density–voltage behavior of single carrier devices, the transient electroluminescent decay, and the impedance spectroscopy response), we find that the device performance is closely linked to the charge carrier balance in the device, which in turn is governed by the interplay of the conductivities of the doped layers and the transport of each charge carrier species within the emitting layer.     

Fabrication and Characterization of Organic Single Crystal‐Based Light‐Emitting Devices with Improved Contact Between the Metallic Electrodes and Crystal

Organic single crystals have attracted great attention because of their advantages of high charge‐carrier mobility, high chemical purity, and potential for flexible optoelectronic devices. However, their intrinsic properties of sensitive to organic solvent and fragile result in a difficulty in the fabrication of the organic crystal‐based devices. In this work, a simple and non‐destructive technique of template stripping is employed to fabricate single‐crystal‐based organic light‐emitting devices (OLEDs). Efficient and uniform carrier injection induced by an improved contact between crystals and both top and bottom electrodes is realized, so that a homogeneous and bright electroluminescence (EL) are obtained. Highly polarized EL and even white emission is also observed. Moreover, the crystal‐based OLEDs exhibit good flexibility, and keep stable EL under a small bending radius and after repeated bending. It is expectable that this technique would support broad applications of the organic single crystals in the crystal‐based optoelectronic devices.     

High‐Color‐Rendering and High‐Efficiency White Organic Light‐Emitting Devices Based on Double‐Doped Organic Single Crystals

Organic single crystals with much higher carrier mobility and stability compared to the amorphous organic materials have shown great potential in electronic and optoelectronic devices. However, their applications in white organic light‐emitting devices (WOLEDs), especially the three‐color‐strategy WOLEDs, have been hindered by the difficulties in fabricating complicated device structures. Here, double‐doped white‐emission organic single crystals are used as the active layers for the first time in the three‐color‐strategy WOLEDs by co‐doping the red and green dopants into blue host crystals. Precise control of the dopant concentration in the double‐doped crystals results in moderately partial energy transfer from the blue donor to the green and red dopants, and thereafter, simultaneous RGB emissions with balanced emission intensity. The highest color‐rendering index (CRI) and efficiency, to the best of the authors' knowledge, are obtained for the crystal‐based WOLEDs. The CRI of the WOLEDs varies between 80 and 89 with the increase of the driving current, and the luminance and current efficiency reach up to 793 cd m^?2 and 0.89 cd A^?1, respectively. The demonstration of the present three‐color organic single‐crystal‐based WOLED promotes the development of the single crystals in optoelectronics.     

Low-voltage organic phototransistors based on naphthyl end-capped oligothiophene nanofibers

Organic crystalline semiconductors with highly ordered molecular packing could be vital components in novel low power consumption light sensors due to their unique light absorption and charge carrier transport properties. In this work, we show that nanofibers made from naphthyl end-capped bithiophenes can be used for low-voltage, high responsivity organic phototransistors (OPTs). Density functional theory (DFT) calculations have been carried out to estimate the device properties related to charge transport and photon absorption. In terms of the calculation of the reorganization energy upon charge transfer, we used an extended method to include inter-molecule interactions. Experimentally, the device performance of the 5,5-bis(naphthyl)-2,2′-bithiophene (NaT2) nanofiber OPTs has been compared with that of a thin film OPT with inferior molecular ordering. The better photoresponsivity of the nanofiber OPT compared with the thin film OPT under monochromatic illumination at various wavelengths suggests that the NaT2 nanofiber-based OPTs have great potential to be used as high performance nano-scale light detectors. Moreover, the absorption of the nanofiber-based OPTs is also polarization sensitive, which provides another advantage of nanofiber-based devices in terms of potential novel sensor design.     

The doping effect of cesium-based compounds on carrier transport and operational stability in organic light-emitting diodes

The doping effect of cesium compounds (i.e., Cs₂CO₃, CsN₃ and CsF) doped electron injection layer (EIL) on charge transport properties and operational stability of organic light-emitting diodes (OLEDs) was systematically investigated in this work. It has been found that device characteristics and lifetime are highly dependent on the doping constituent materials. The doping of cesium compounds in EIL can improve the charge injection and transport of OLEDs, due to the increase in conductivity and reduction in electron injection barrier. Apart from the difference in electrical characteristics, the operational stability of OLEDs is strongly influenced by the doping mechanism of different cesium compounds in the EILs. The OLED device using Cs₂CO₃ as the n-type dopant for the EIL shows a superiority in both electrical property and operational lifetime.     

Novel Approach for Alternating Current (AC)‐Driven Organic Light‐Emitting Devices

A novel approach for alternating current (AC)‐driven organic light‐emitting devices is reported, which uses the concept of molecular doping in organic semiconductors. Doped organic charge‐transport layers are used to generate charge carriers within the device, hence eliminating the need for injecting charge carriers from external electrodes. Bright luminance of up to 1000 cd m^?2 is observed when the device is driven with an AC bias. The luminance observed is attributed to charge‐carrier generation and recombination, leading to the formation of excitons within the device, without injection of charge carriers through external electrodes. A mechanism for internal charge‐carrier generation and the device operation is proposed.     

Interface engineering to enhance charge injection and transport in solution-deposited organic transistors

Soluble small molecule organic semiconductors combine the high-performance of small molecule organic semiconductors with the versatile processability of polymeric materials, but the control of device performance and uniformity is challenged by the complex film microstructure formed in these materials, and its strong dependence on processing conditions. These films crystallize via a nucleation and growth mechanism that can be difficult to control. In this study we used highly fluorinated self-assembled monolayers (SAMs) to modify the surface of the source and drain contacts and improve the performance of organic thin-film transistors (OTFTs) through controlling film microstructure and lowering the contact resistance. We reached charge carrier mobilities as high as 5.7 cm²/V in 2,8-Difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), one order of magnitude greater than what we obtained in devices on untreated substrates, and on par with the value reported for single crystal devices. Kelvin probe measurements distinguished an increase in the work function between 0.28 eV and 0.5 eV, depending on the molecular structure of the SAM. Selected area electron diffraction (SAED) confirmed the preferential “edge-on” molecular orientation of the semiconductor. We discuss the device performance in relation to the film morphology and contact resistance.     

Valence band structure of rubrene single crystals in contact with an organic gate dielectric

Organic field effect transistors (OFETs) using crystalline organic semiconductors are of great interest because of their well-defined structural and electronic properties to study the intrinsic charge carrier transport mechanisms in π-conjugated molecular solids, as well as to unravel their potential to be applied as a novel type of electronic device. In the present study, the valence band structure of the channel region of an OFET is proposed based on photoemission results of a well-defined interface between a dielectric molecular monolayer and single crystals of 5,6,11,12-tetraphenyltetracene (rubrene) which is known to exhibit the highest field effect mobility of all organic semiconductors at room temperature. Commensurate growth of clusters of tetratetracontane (TTC; n-C₄₄H₉₀) on the rubrene single crystal surface and their morphological transformation into a uniform overlayer were observed by atomic force microscopy. Photoelectron spectroscopy measurements at various electron take-off angles were then conducted to derive the valance band width of the rubrene single crystal covered by the TTC overlayers. The valence band width at this hetero-interface was found to be equivalent to that of the pristine rubrene, which suggests an unchanged ‘band effective mass ?²(d²E/dk_||²)’ of accumulated holes even at the vicinity of hydrocarbon-based gate dielectrics.     

Efficient single layer RGB phosphorescent organic light-emitting diodes

We report efficient single layer red, green, and blue (RGB) phosphorescent organic light-emitting diodes (OLEDs) using a “direct hole injection into and transport on triplet dopant” strategy. In particular, red dopant tris(1-phenylisoquinoline)iridium [Ir(piq)₃], green dopant tris(2-phenylpyridine)iridium [Ir(ppy)₃], and blue dopant bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium [FIrpic] were doped into an electron transporting 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) host, respectively, to fabricate RGB single layer devices with indium tin oxide (ITO) anode and LiF/Al cathode. It is found that the maximum current efficiencies of the devices are 3.7, 34.5, and 6.8 cd/A, respectively. Moreover, by inserting a pure dopant buffer layer between the ITO anode and the emission layer, the efficiencies are improved to 4.9, 43.3, and 9.8 cd/A, respectively. It is worth noting that the current efficiency of the green simplified device was as high as 34.6 cd/A, even when the luminance was increased to 1000 cd/m² at an extremely low applied voltage of only 4.3 V. A simple accelerated aging test on the green device also shows the lifetime decay of the simplified device is better than that of a traditional multilayered one.     

Great improvement of operation-lifetime for all-solution OLEDs with mixed hosts by blade coating

We investigated some effective device designs and fabrication methods for long operation-lifetime all-solution-processed Phosphorescent OLEDs (PhOLEDs) and fluorescent OLEDs with mixed-hosts system and thin Poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(4-sec-butylphenyl) diphenylamine)] (TFB). The all-solution-processed green PhOLEDs had high current efficiency (30.3 cd/A) and long operation-lifetime. The best half-lifetime of green PhOLEDs with thin HTL, MH-hosts EML and optimized deposition was 310 h at an initial luminance 1000 cd/m², 250 h at an initial luminance 500 cd/m² for green PhOLEDs with thin HTL, and MH-hosts EML, and the lifetime of triple layer PhOLEDs device was only 0.5 h for the same materials. The red PhOLEDs exhibited a high current efficiency (10.93 cd/A) and half-lifetime with 157.9 h at an initial luminance 500 cd/m². For the blue fluorescent OLEDs, the thin polymer TFB, mixed-hosts EML, double EMLs and optimization deposition yield a high current efficiency (5.68 cd/A) and long operation-lifetime with 117.7 h at an initial luminance 500 cd/m². Single host fluorescent device had half-lifetime of 73.5 h only at an initial luminance 100 cd/m². Finally, by doping red emitter Rubrene into stable blue device, we achieved soft yellow OLEDs with high efficiency (10.87 cd/A) and 8 fold improvement operation-lifetime (1200 h). We believe that such all-solution-processed OLEDs which showed greatly improved operational lifetimes would be suitable for the indoor supportive lighting with natural colors.     

Blade coating of Tris(8-hydroxyquinolinato)aluminum as the electron-transport layer for all-solution blue fluorescent organic light-emitting diodes

Organic light-emitting diodes (OLEDs) with a low driving voltage and efficient blue fluorescence were fabricated through blade coating. Tris(8-hydroxyquinolinato)aluminum (Alq₃) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq₃ through a solution process is difficult because of its extremely low solubility organic solvents, a result of its symmetrical molecular structure. In this study, Alq₃ was successfully deposited through blade coating at a very low concentration below 0.1wt%. The OLEDs contained co-dopants BUBD-1 and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), and a high-band-gap host 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as the emission layer with the following structure: ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:2% BUBD-1:1% DSA-Ph (50 nm)/TPBI (30 nm)/LiF (0.8 nm)/Al (100 nm)or ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:3% BUBD-1 (50 nm)tris(8-hydroxyquinolinato)aluminum (Alq₃; 10 nm)/LiF (0.8 nm)/Al (100 nm). 2,7-disubstituted fluorene-based triaryldiamine(VB-FNPD)is the cross-linking transporting material. The device exhibited a peak current efficiency of 5.67 cd/A for Alq₃ and 5.76 cd/A for TPBI. The device with Alq₃ has operated lifetime seven times higher than the device with TPBI.     

Analysis of charge transfer complex at the interface between organic and inorganic semiconductors

Improving the electrical performance of organic semiconductors is critical to use them for optoelectronic applications. In this study, we analyze the mechanism of charge transfer complex (CTC) formation at the interface between organic and inorganic semiconductors through extensive optical and electrical measurements. N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and molybdenum oxide (MoO₃) were sequentially deposited to form a donor/accepter heterojunction structure. The CTC formation and conductivity of the films were determined using UV–visible spectroscopy and transmission line method, respectively. Compared with the single layer devices, the donor/accepter heterojunction exhibits significantly enhanced conductivity. In addition, the conductivity and CTC generation efficiency of the heterojunction display strong dependence on NPB layer thickness, which originates from the variation of dipole interactions at the heterojunction interface. These results provide useful insights on interfacial doping properties, which is potentially beneficial for enhancing the understanding of organic/inorganic interfaces.     

Optoelectrical characteristics of organic light-emitting devices fabricated with different cathodes

Organic light-emitting devices (OLEDs) with various cathode structures were prepared on indium tin oxide (ITO) substrates by vacuum sublimation technique, and the effects of the device cathodes on the electroluminescence (EL) characteristics of OLEDs were studied in terms of the luminance, efficiency, driving voltage and threshold voltage. The results demonstrate that the optical and electrical performance of OLEDs depend on the properties of the devices' cathodes and the characteristics of the cathode–organic interface and the organic–organic interface. The optoelectrical performance of a device with composite cathodes is better than that of the devices with metal alloy and pure metal cathodes. The improvement in the device performance can be attributed to a more efficient electron injection at the cathode–organic interface, a better balanced hole and electron recombination in the light-emitting layer and fewer accumulated charges near the organic–organic interface.     

Improving mobility and electrochemical stability of a water-gated polymer field-effect transistor

Water-gated organic transistors have attracted considerable attention in the field of biosensors, thanks to their capability of operating in the aqueous environment typical of biological systems at very low voltages (∼1 V). Some examples have been recently reported in the literature, employing different organic materials as the active semiconducting layer, ranging from small molecules to single crystals. Here we report on water-gated polymer-based organic-field effect devices using poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) as the active layer. Very promising electronic performances, in terms of mobility and operating voltages are obtained; notably, the charge carrier mobility is in the order of 0.08 cm²/V s, which is of the same order of magnitude of values reported for single-crystal based water-gated devices, and consistent with values reported for solid-state polymer dielectric transistors. Moreover, the pBTTT-based device shows improved electrochemical stability, as compared to previously reported polymer based water-gated devices. Importantly, good functioning of the device is demonstrated also when water is replaced by physiological-like solutions. Critical to the transistors operation, besides the good transport properties of the active material, is the key-role played by alkyl side chains and ordered morphology of the polymer at the interface with the liquid environment, which we highlight here for the first time. Our contribution overall provides a useful step towards the development of bio-organic sensors, with enhanced properties in terms of sensitivity and stability, and for a successful exploitation of organic based field effect transistors in biotic/abiotic interfaces.     

Electric potential mapping by thickness variation: A new method for model-free mobility determination in organic semiconductor thin films

Charge transport, with charge carrier mobility as main parameter, is one of the fundamental properties of semiconductors. In disordered systems like most organic semiconductors, the effective mobility is a function of the electric field, the charge carrier density, and temperature. Transport is often investigated in a space-charge limited current (SCLC) regime in thin film single carrier devices, where an electric current is driven in the direction perpendicular to the surface. Direct evaluation of the current–voltage characteristics, however, is problematic, because parasitic contributions from injection or extraction barriers can falsify results.     

Determination of diffusion lengths in organic semiconductors: Correlation with solar cell performances

Organic semiconductors are promising candidates for future applications in solar energy conversion. Recent investigations of bulk heterojunction (BHJ) semiconductors have suggested a density of states and transport mechanisms by multiple trapping close to those observed in disordered inorganic thin films. That is why we have applied to BHJ thin films experiments that are currently used for disordered semiconductors. In addition to the steady state photoconductivity we have tested the ability of the steady state photocarrier grating (SSPG) technique to provide information on the minority carrier diffusion length. We found that SSPG can be applied to P3HT:PCBM thin films leading, for the best sample, to a diffusion length of the order of 125 nm. From the comparison of the transport parameters obtained on thin films with the performances of the devices integrating the latter, we conclude that SSPG is a very powerful tool for optimizing the BHJ thin film properties before their incorporation in solar devices.     

Simulation of OLEDs with a polar electron transport layer

Organic light-emitting diodes (OLEDs) rely on the use of functional materials with suitable energy levels and mobilities for selective charge carrier injection and transport of one species only at the respective electrode. Until recently, however, the dipolar nature of many organic semiconductors has been largely ignored in this context. In particular, electron transports layers (ETLs) often exhibit spontaneous orientation polarization leading to interfacial charges that modify the electrical potential landscape inside a hetero-layer device.Here we demonstrate that the effect of polar ETLs can be simulated using the well-established Poisson and drift-diffusion formalism, if these interfacial charges are taken into account. Impedance spectroscopy is used in order to validate our approach and to characterize the polarity of the material. Finally, simulations allow to quantify the impact of polar ETLs on device performance.     

Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.     

掺杂发光体对红色有机电致发光的影响(英文)

为了研究掺杂发光体对红色有机电致发光二极管的增强效果,将DCJTB和C545T分别掺入Alq3,制备了双发光层的OLED器件,器件结构为玻璃/ITO/4 ,4′,4″-tris[2-naphthylphenyl-1-phenylamino]triphenyla-mine (2T-NATA)/N,N′-di (naphthalene-1-yl)-N,N′-diphenyl benzidine ( NPB)/tris-(8-hydroxyquinoline)aluminum ( Alq3):4-(dicyanomethylene)-2-tert-butyl-6 (1 ,7 ,7 ,7-tetramethyljulolidin-4-yl-vinyl)-4 H-pyran(DCJTB)/Alq3:10-(2-benzothiazolyl)-2 ,3 ,6 ,7-tetrahydro-1 ,1 ,7 ,7 ,-tetramethyl-1 H,5 H,11 H-(1)-benzopy-ropyrano-(6 ,7-8-i ,j)quinolizin-11-one (C545T)/Alq3/LiF/Al ,并且将其与单发光层的红、绿光器件相比较。实验结果表明,与单发光层的红光器件相比,加入绿光发光层的红光器件的发光特性被增强了,这种双发光层器件的最优掺杂比例为[Alq3:(2 .5 %)C545T]/[ Alq3:(1 .5 %)DCJTB](质量分数) ,在电压为11 .5 V时得到最大发光亮度为6 830 cd/ m2,在11 V电压时能得到4 .59 cd/A的最大电流效率。但是,这种方法的缺点是削弱了红光的色纯度。