•A multi-level memory based on OFET is fabricated.•Li+@C60 is at the origin of the multi-level effect of memory OFETs.•Four logic states could be obtained with Li+@C60 as a charge trapping layer.We report on multi-level non-volatile organic transistor-based memory using pentacene semiconductor and a lithium-ion-encapsulated fullerene (Li+@C60) as a charge trapping layer. Memory organic field-effect transistors (OFETs) with a Si++/SiO2/Li+@C60/Cytop/Pentacene/Cu structure exhibited a performance of p-type transistor with a threshold voltage (Vth) of −5.98 V and a mobility (μ) of 0.84 cm2 V−1 s−1. The multi-level memory OFETs exhibited memory windows (ΔVth) of approximate 10 V, 16 V, and 32 V, with a programming gate voltage of 150 V for 0.5 s, 5 s, and 50 s, and an erasing gate voltage of −150 V for 0.17 s, 1.7 s, and 17 s, respectively. Four logic states were clearly distinguishable in our multi-level memory, and its data could be programmed or erased many times. The multi-level memory effect in our OFETs is ascribed to the electron-trapping ability of the Li+@C60 layer.Download high-res image (414KB)Download full-size image

We present controllable and reliable complementary organic transistor circuits on a PET substrate using a photoactive dielectric layer of 6-[4′-(N,N-diphenylamino)phenyl]-3-ethoxycarbonylcoumarin (DPA-CM) doped into poly(methyl methacrylate) (PMMA) and an electron-trapping layer of poly(perfluoroalkenyl vinyl ether) (Cytop). Cu was used for a source/drain electrode in both the p-channel and n-channel transistors. The threshold voltage of the transistors and the inverting voltage of the circuits were reversibly controlled over a wide range under a program voltage of less than 10 V and under UV light irradiation. At a program voltage of −2 V, the inverting voltage of the circuits was tuned to be at nearly half of the supply voltage of the circuit. Consequently, an excellent balance between the high and low noise margins (NM) was produced (64% of NMH and 68% of NML), resulting in maximum noise immunity. Furthermore, the programmed circuits showed high stability, such as a retention time of over 105 s for the inverter switching voltage. Our findings bring about a flexible, simple way to obtain robust, high-performance organic circuits using a controllable complementary transistor inverter.

•A new series of charged iridum(III) complexes is synthesized.•A deep-blue-emitting complex with an N,N-bidentate ligand is demonstrated.•A light-emitting device based on the charged complex was successfully fabricated.We herein report a series of charged iridium(III) complexes with a neutral bidentate ligand for blue phosphorescent emitter. A molecular design bearing a 2-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine ligand proved suitable for efficient blue emission according to the comparison of photoluminescent properties of the complexes. Its Commission Internationale de L’Eclairage x,y-coordinates (CIEx,y) in solution was estimated to be (0.17, 0.18), indicating that the complex with the ligand is a promising candidate for deep-blue emitter. We further demonstrated that this complex displays blue electroluminescence by successfully integrating it in a light-emitting device.This paper features the design, synthesis and properties of a new series of charged iridium(III) complexes for phosphorescent emitter.

•Influence of molecular orientation on organic solar cell performance is investigated.•A rubbing technique induces double horizontal orientations of α-6T and PTCBI.•The horizontal orientations enhance efficiency and operation stability of the cell.•The improved cell performance is discussed in terms of molecular orientation.The authors investigate the influence of molecular orientation of p-type molecules of alpha-sexithiophene (α-6T) and n-type molecules of 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI) on organic solar cell (OSC) performance. Deposition of α-6T and subsequently PTCBI on an α-6T buffer surface rubbed with a nylon cloth allows their horizontal orientations to be formed in separate layers. Power conversion efficiency and operation stability of a rubbed OSC are markedly improved when compared with an unrubbed OSC. The improved OSC performance is confirmed to be due to increases in light absorption, exciton diffusion length, energy difference between the p-type and n-type layers, and carrier collection by electrodes, which are caused by the rubbing-induced double horizontal orientations.

Poly(3-hexylthiophene-2,5-diyl) is among the most widely used conjugated polymers for opto-electronic applications. To enhance its properties, researchers have attempted to nanostructure this polymer using various processes including breath figure arrays, nanolithography and elaborated organic synthesis. We here demonstrate a simple process to nanostructure the conjugated polymer using self-assembly with polystyrene and selective removal of one of the phases. The influence of the molecular weight of each polymer on the thin film morphology was systematically studied by atomic force microscopy. Using this approach, we observe two types of nanostructure, namely, nanoporous and nanoisland structures, of which the dimensions can be tuned by modifying the molecular weight of each polymer in the blend. This simple process introduces a cost-effective alternative to produce thin films of conjugated polymer with average nano-features from 100 nm up to 500 nm which could be used in a wide range of applications.

•Various donor–acceptor concentration-graded devices were fabricated.•Improved donor–acceptor concentration gradient enhances the photovoltaic properties.•The increased open-circuit voltage results from lower reverse saturation currents.•Adjusting the dimensions of buffer and intermixed layers enhances fill factor.•Ideal active layer morphologies lead to an increase of 30% of the efficiencyIn the present work, we demonstrate that graded bilayer solar cells provide a very interesting alternative to the bulk heterojunction active layers commonly used in organic photovoltaic cells. One of the main advantages of this type of active layers is the possibility to optimize independently both donor and acceptor layers. Using various process methods, we obtain active layers that demonstrate a donor–acceptor vertical concentration gradient. These devices exhibit not only a high fill factor but also a remarkable increase in open-circuit voltage (Voc). In order to understand the influence of the film morphology over the device parameters, we provide a complete study using energy-dispersive x-ray spectroscopy elemental mapping of the device cross sections, showing evidence that ideal donor–acceptor concentration gradient are required to obtain high fill factors. Furthermore, we use a simple equivalent electrical model to extrapolate device parameters such as reverse saturation current for a clearer understanding of the origin of the Voc increase.

•A pentacene field-effect transistor based on a double-dielectric structure of CYTOP and SiO2 is fabricated.•Application of a positive switching voltage to a gate electrode induces a large threshold voltage shift from −4.4 to 4.6 V.•The threshold voltage shift is attributed to electron trapping at the CYTOP/SiO2 interface.•The change of threshold voltage is reversible and very stable.•A memory device is realized by utilizing the double-dielectric structure.The authors report controllable threshold voltage (Vth) in a pentacene field-effect transistor based on a double-dielectric structure of poly(perfluoroalkenyl vinyl ether) (CYTOP) and SiO2. When a positive switching voltage is applied to the gate electrode of the transistor, electrons traverse through the pentacene and CYTOP layers and subsequently trapped at the CYTOP/SiO2 interface. The trapped electrons induce accumulation of additional holes in the pentacene conducting channel, resulting in a large Vth shift from −4.4 to +4.6 V. By applying a negative switching voltage, the trapped electrons are removed from the CYTOP/SiO2 interface, resulting in Vth returning to an initial value. The Vth shift caused by this floating gate-like effect is reversible and very time-stable allowing the transistor to be applicable to a nonvolatile memory that has excellent retention stability of stored data.Graphical abstract

The influence of charge transfer from organic molecules to transition metal oxide on molecular orientation characteristics of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) was investigated. Absorption peaks originating from neutral and cationic states of α-NPD increased in absorbance when α-NPD was deposited on metal oxides (MoO2, MoO3, and WO3). Photoluminescence from this α-NPD was directional normal to the film plane. These results indicate that α-NPD is horizontally oriented near the metal oxide surfaces so that charge transfer from α-NPD to metal oxide occurs efficiently. Such horizontal orientation of α-NPD enhanced current density of hole-only α-NPD devices because of improvement of wave function overlap and charge transfer degree at the metal oxide/α-NPD interface.Graphical abstractHighlights► Charge transfer occurs from α-NPD molecules to transition metal oxide. ► Absorbance increased when α-NPD was deposited on the metal oxides. ► Photoluminescence from this α-NPD was directional normal to the film plane. ► The charge transfer induced a horizontal orientation of α-NPD near the metal oxides. ► Such horizontal orientation enhanced current density of hole-only devices.

We demonstrate for the first time that re-absorption and scatterings of photoluminescence (PL) significantly occur among electrospun light-emitting nanofibers. Electrospun nanofibers composed of a blend of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] and poly(ethylene oxide) with and without LiCF3SO3 were prepared in controlled numbers. Different numbers of fibers showed different PL spectra even though the PL spectra from individual fibers were almost the same. Results of simulated PL spectra using UV-vis absorption spectra measured with and without an integrating sphere revealed that the change of PL spectra is due to re-absorption and scatterings of PL among the fibers. Although most PL spectra of electrospun nanofibers have been performed with a sheet or mat of many electrospun fibers, our study clearly shows that measuring the PL spectrum of a single fiber is indispensable for precisely evaluating the aggregation states and the electronic states of conjugated polymers inside the fibers.

We report for the first time organic n-type nonvolatile memory transistors based on a fullerene (C60) semiconductor and an electron-trapping polymer, poly(perfluoroalkenyl vinyl ether) (CYTOP). The transistors with a Si++/SiO2/CYTOP/C60/Al structure show good n-type transistor performance with a threshold voltage (Vth) of 2.8 V and an electron mobility of 0.4 cm2 V−1 s−1. Applying gate voltages of 50 or −45 V for about 0.1 s to the devices induces the reversible shifts in their transfer characteristics, which results in a large memory window (ΔVth) of 10 V. A memory on/off ratio of 105 at a small reading voltage below 5 V and a retention time greater than 105 s are achieved. The memory effect in the transistor is ascribed to electrons trapped at the CYTOP/SiO2 interface. Because of the use of high-electron-mobility C60, the switching voltages of our memory transistors become significantly lower than those of conventional memory transistors based on pentacene.Graphical abstractHighlights► We report organic n-type nonvolatile memory transistors based on a fullerene semiconductor. ► The memory transistor can be programmed/erased by applying gate voltage pulses of 50/−45 V. ► The memory transistor shows a memory window 10 V and a retention time over 105 s. ► The memory effect originates from trapping and detrapping electrons at a CYTOP/SiO2 interface.

The authors investigate a relationship between substrate transfer speeds during vacuum vapor deposition and orientation characteristics of organic molecules. Results show that rod-shaped molecules of alpha-sexithiophene (α-6T) are oriented in a substrate transfer direction and an absorption dichroic ratio of 1.44 is obtained from the oriented α-6T molecule film when a high substrate transfer speed of 4 m s−1 is used. By combining the substrate transfer technique with homoepitaxial growth of α-6T molecules on a rubbed surface, the absorption dichroic ratio further increases to 4.29. Polarized electroluminescence (EL) characteristics are investigated using rod-shaped molecules of 4,4′-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi) as a light-emitting hole-transport layer. An EL dichroic ratio of 2.12 is obtained due to an orientation of DPAVBi molecules caused by combining two techniques.Graphical abstractHighlights► A long axis of organic molecules is oriented along a substrate transfer direction. ► Molecular orientation is enhanced as substrate transfer speeds are increased. ► Rod shapes of molecules are required to induce molecular orientation. ► Combination of substrate transfer with homoepitaxial growth enhances orientation. ► Polarized electroluminescence is observed from oriented molecule films.

Graded bilayer solar cells have proven to be at least as efficient as the bulk heterojunctions when it comes to the Poly(3-hexylthiophene) (P3HT) - [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) donor–acceptor system. However, control of the vertical concentration gradient using simple techniques has never been reported. We demonstrate that rubbing the P3HT layer prior to PCBM deposition induces major morphological changes in the active layer. Using the newly introduced energy-dispersive X-ray spectroscopy element mapping technique, we found that rubbing P3HT induces the formation of an ideal vertical donor–acceptor concentration gradient. Furthermore, the P3HT crystallites undergo a molecular reorientation from edge-on to face-on configuration inducing a better charge transport in the vertical direction. The combination of these two major morphological changes leads to the fabrication of high-performance solar cells that exhibit, to date, the record efficiencies for spin-coated graded bilayers solar cells.Keywords: conjugated polymer; molecular orientation; organic photovoltaics; P3HT; rubbing;

We have recently proposed that improvement of device performance using a buffer layer of molybdenum trioxide (MoO3) originates from interfacial charge generation at an interface of MoO3 and an organic hole-transport layer [17]. However, there is no clear experimental evidence enough to support the charge generation in our recent report. In this study, from comparison of current density–voltage characteristics of organic hole-only devices and ultraviolet/visible/near-infrared absorption spectra of composite films, we can conclude that the interfacial charge generation surly occurs to realize space-charge-limited currents of a wide variety of organic hole-transport layers. Moreover, a drastic increase in current density of a bilayer device of n-type C60 and p-type N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) by using a MoO3 layer can provide the evidence of the charge generation.Graphical abstractResearch highlights► Space-charge-limited currents of various organic layers are observed using thin MoO3. ► Electrical analysis enables estimation of hole mobilities of organic layers. ► Absorption spectroscopy reveals charge transfer occurring between organic and MoO3. ► Charge generation mechanism is proposed for enhanced device performance.

We have shown that triazine compounds of 2-biphenyl-4-yl-4,6-bis-(4′-pyridin-2-yl-biphenyl-4-yl)-[1,3,5]triazine (DPT) and 2,4-bis-biphenyl-4-yl-6-(4′-pyridin-2-yl-biphenyl-4-yl)-[1,3,5]triazine (MPT) work as excellent electron-transport layers (ETLs) of organic light-emitting diodes (OLEDs). By replacing a typical ELT of tris(8-hydroxyquinoline) aluminum (Alq3) with the DPT ETL and the MPT ETL, driving voltages and power conversion efficiencies of OLEDs operated at a current density of 50 mA cm−2 are improved from 7.7 ± 0.2 V and 1.41 ± 0.10 lm W−1 (Alq3) to 6.0 ± 0.1 V and 1.62 ± 0.09 lm W−1 (DPT) and 5.2 ± 0.1 V and 1.88 ± 0.18 lm W−1 (MPT), respectively. Half lifetimes of the OLEDs operated at the same current are also enhanced from 3200 h (Alq3) to 4200 h (MPT) by using the MPT ETL. The reduction in driving voltage is found to arise from more efficient electron injection at interfaces of DPT/cathode (a barrier height of ≈0.61 eV) and MPT/cathode (a barrier height of ≈0.51 eV) than at an interface of Alq3/cathode (a barrier height of ≈0.73 eV) as well as better electron mobilities of DPT (8.7 × 10−5 cm2 V−1 s−1 at an electric field of 5.0 × 105 V cm−1) and MPT (1.4 × 10−4 cm2 V−1 s−1 at the same field) than that of Alq3 (4.1 × 10−6 cm2 V−1 s−1 at the same field [Naka et al., Synth. Met. 111–112 (2000) 331.]). A high-mobility electron-transport material of 4,7-diphenyl-1,10-phenanthroline (BPhen) is also used as an ETL for comparison of OLED characteristics.

We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron-sized structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by ∼100 mV), which leads to an overall enhancement of power conversion efficiency.

We demonstrate organic field-effect transistors (OFETs) with an ion-dispersed polymer for the gate dielectrics. By applying external electric field (Vex), the dispersed ions can migrate by electrophoresis and separated ion pairs form space charge polarization in the gate dielectrics. After Vex was applied, the drain current is increased over 7 times and threshold voltage is decreased from − 12.9 V to − 2.9 V. The shift direction of Vth is controllable by the polarity of the Vex. Results of ultraviolet/visible differential absorption study reveal that the active layer of OFETs is charged not only electrostatically but electrochemically with increasing the time after Vex was applied.

We demonstrated that the stability of organic solar cells (OSCs) under light irradiation is markedly enhanced by inserting a molybdenum trioxide (MoO3) buffer layer between an anode layer of indium tin oxide (ITO) and a p-type layer of 5,10,15,20-tetraphenylporphyrin (H2TPP) or N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD). The use of the MoO3 layer also enhanced open-circuit voltages and power conversion efficiencies of the OSCs due to an increase in built-in potential. From results of stability test of hole-only α-NPD devices, we concluded that the OSC degradation occurs near the ITO/p-type layer interface and that the use of the MoO3 layer can prevent the degradation at this interface.

We demonstrated that driving voltages, external quantum efficiencies, and power conversion efficiencies of organic light-emitting diodes (OLEDs) are improved by inserting a wide-energy-gap interlayer of (4,4′-N,N′-dicarbazole)biphenyl (CBP) between a hole-transport layer of N,N-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (α-NPD) and a light-emitting layer of tris(8-hydroxyquinoline)aluminum. By optimization of CBP thicknesses, the device with a 3-nm-thick CBP layer had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. We attributed these improvements to enhancement of a carrier recombination efficiency and suppression of exciton–polaron annihilation. Moreover, we found that the degradation of the OLEDs is caused by decomposition of CBP molecules and excited-state α-NPD molecules.

We realize a nonvolatile and rewritable memory effect in an organic field-effect transistor (OFET) structure using polymethylmethacryrate (PMMA) dispersed with 10-methyl-9-phenylacridinium perchlorate (MPA+ClO4−) as a gate dielectric. Applying a voltage between a top source-drain electrode and a bottom gate electrode induces electrophoresis of two ions of MPA+ and ClO4− towards the corresponding electrodes in the memory devices. The drain currents of the memory devices markedly increase from 10− 9 A to 10− 2 A under no gate voltage condition due to the strong space charge polarization effect. Our memory devices have excellent electrical bistability and retention characteristics, i.e. the memory on/off ratio reached 107 and the drain current maintained 40% of the initial value after 104 s.

We realize a uniform submicron-gap electrode by using an electrospun single fiber as a shadow-mask. By stretching an electrospun fiber, we can decrease the diameter of the fiber from 2 μm to 564 nm with its standard deviation of 57.7 nm. We place the fiber on the center of a Si/SiO2 substrate followed by the deposition of a molybdenum trioxide adhesion layer and Au electrode. After removing the fiber from the Si/SiO2 substrate, the submicron-gap gold electrode is formed. Characterization of the gap with scanning electron microscope revealed that the gap has a good uniformity; the average gap length is 865 nm throughout 2 mm gap width.

We demonstrated an acetalization reaction as a versatile method to immobilize aromatic aldehyde molecules on surfaces of metal oxides, silicon dioxide, and indium tin oxide. First, a trimethylsily (TMS) terminated surface was formed using a silylation reaction between a chloride group of trimethylsilychloride and a hydroxyl group of the substrate surfaces. Second, terephthalaldehyde (TPA) was immobilized on the surfaces using an acetalization reaction between the TMS-terminated surface and an aldehyde group of TPA. Results of contact angle, X-ray photoelectron, and ultraviolet absorption spectra revealed that the TPA molecules on the surfaces were well-packed with a high surface density.

A novel electrospinning process of uniaxially aligned submicron fibers was developed. The number of the fibers was precisely controlled by changing biased collector, and the diameter of the fiber was varied by post-deposition stretching process. This method realized the formation of number-controlled aligned poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/poly(ethylene oxide) (PEO) fibers with the systematic control of the diameter ranging from micrometer to submicrometer. Significant improvement of the uniformity of the fiber diameter was also observed by the stretching process.

Studies on the intrinsic degradation of organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinolinolato)aluminum (III) (Alq3) revealed that the operation stability of the OLEDs depends on the process pressure during device fabrication. Lowering of the pressure resulted in stable devices. In sharp contrast, differences in the initial device characteristics were marginal in all devices. Analyses with a quadrupole mass spectrometer indicated that the primary difference in the pressure during device fabrication was attributable to the amount of residual water. The results show that the degradation of OLEDs is associated with the electrochemical reaction of Alq3 with water.Enhanced stability of organic light-emitting devices (OLEDs) was observed in the device fabricated under ultra-high vacuum condition. Studies on the intrinsic degradation of the OLEDs based on tris (8-hydroxyquinoline) aluminium (Alq3) reveal that the degradation of the OLEDs is associated with the electrochemical reaction of Alq3 with water.

We demonstrate for the first time that re-absorption and scatterings of photoluminescence (PL) significantly occur among electrospun light-emitting nanofibers. Electrospun nanofibers composed of a blend of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] and poly(ethylene oxide) with and without LiCF3SO3 were prepared in controlled numbers. Different numbers of fibers showed different PL spectra even though the PL spectra from individual fibers were almost the same. Results of simulated PL spectra using UV-vis absorption spectra measured with and without an integrating sphere revealed that the change of PL spectra is due to re-absorption and scatterings of PL among the fibers. Although most PL spectra of electrospun nanofibers have been performed with a sheet or mat of many electrospun fibers, our study clearly shows that measuring the PL spectrum of a single fiber is indispensable for precisely evaluating the aggregation states and the electronic states of conjugated polymers inside the fibers.