Influence of nickel(II) oxide nanoparticle addition on the performance of organic field effect transistors.

Here, the improved performance of organic field effect transistors (OFET) by doping inorganic nanoparticles into a semiconducting polymer as a channel layer is briefly reported. Nickel(II) oxide nanoparticle (NiOnp) was used as an inorganic dopant while regioregular poly(3-hexylthiophene) (P3HT) was used as a matrix polymer for the channel layer in the OFETs. The doping ratio of NiOnp was made 1 wt.% so that it would minimally influence the nanostructure of the P3HT channel layer. The results showed that the optical absorption spectrum of the P3HT film was slightly red-shifted by the NiOnp doping, which reflects the improved crystallinity of the P3HT domains in the P3HT:NiOnp films. The drain current of the OFETs with the P3HT:NiOnp films was significantly enhanced ca. three-to-seven fold by the NiOnp doping under appying gate voltages while the hole mobility of the OFETs P3HT:NiOnp films was improved as much as three fold by the NiOnp doping. The enhanced performance has been assigned to the role of NiOnp that has relatively higher hole mobility than the P3HT polymer.

Critical role of electrons in the short lifetime of blue OLEDs.

Designing robust blue organic light-emitting diodes is a long-standing challenge in the display industry. The highly energetic states of blue emitters cause various degradation paths, leading to collective luminance drops in a competitive manner. However, a key mechanism of the operational degradation of organic light-emitting diodes has yet to be elucidated. Here, we show that electron-induced degradation reactions play a critical role in the short lifetime of blue organic light-emitting diodes. Our control experiments demonstrate that the operational lifetime of a whole device can only be explained when excitons and electrons exist together. We examine the atomistic mechanisms of the electron-induced degradation reactions by analyzing their energetic profiles using computational methods. Mass spectrometric analysis of aged devices further confirm the key mechanisms. These results provide new insight into rational design of robust blue organic light-emitting diodes.

Direct measurement of extracellular electrical signals from mammalian olfactory sensory neurons in planar triode devices.

An artificial nose was developed to mimic aspects of sensory transduction of the peripheral mammalian olfactory system. We directly cultured and differentiated rat olfactory sensory neurons (OSNs) on indium-tin oxide electrodes of planar triode substrates without a coupling agent. Direct voltage (~50 µV) and current (~250 nA) signals were measured simultaneously when OSNs on the planar triode substrates were exposed to odorant mixtures. The response signals were sensitive to the concentration of the odorant mixture, with a typical lifetime, shape, and adaptation profile as seen in responses upon repeated stimulation in vivo. We found that the rising time to the peak current was ~161 ms, while the signal back to baseline was in 1.8 s, which are in agreement with the natural intracellular electrophysiological responses. These results provide the first evidence that mature OSNs grown in a planar triode device are able to detect direct electrophysiological responses to odorants.

All-polymer solar cells with bulk heterojunction nanolayers of chemically doped electron-donating and electron-accepting polymers.

We report the improved performance of all-polymer solar cells with bulk heterojunction nanolayers of an electron-donating polymer (poly(3-hexylthiophene) (P3HT)) and an electron-accepting polymer (poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)), which were both doped with 4-ethylbenzenesulfonic acid (EBSA). To choose the doping ratio of P3HT for all-polymer solar cells, various EBSA doping ratios (0, 1, 3, 5, 10, 20 wt%) were tested by employing optical absorption spectroscopy, photoluminescence spectroscopy, photoelectron yield spectroscopy, and space-charge-limited current (SCLC) mobility measurement. The doping reaction of P3HT with EBSA was followed by observing the colour change in solutions. The final doping ratio for P3HT was chosen as 1 wt% from the best hole mobility measured in the thickness direction, while that for F8BT was fixed as 10 wt% (F8BT-EBSA). The polymer:polymer solar cells with bulk heterojunction nanolayers of P3HT-EBSA (EBSA-doped P3HT) and F8BT-EBSA (EBSA-doped F8BT) showed greatly improved short circuit current density (J(SC)) and open circuit voltage (V(OC)), compared to the undoped solar cells. As a result, the power conversion efficiency (PCE) was enhanced by ca. 300% for the 6 : 4 (P3HT-EBSA : F8BT-EBSA) composition and ca. 400% for the 8 : 2 composition. The synchrotron-radiation grazing incidence angle X-ray diffraction (GIXD) measurement revealed that the crystallinity of the doped nanolayers significantly increased by EBSA doping owing to the formation of advanced phase segregation morphology, as supported by the surface morphology change measured by atomic force microscopy. Thus the improved PCE can be attributed to the enhanced charge transport by the formation of permanent charges and better charge percolation paths by EBSA doping.

Characteristics of protein-polymer nanobiocomposite films for protein devices.

Here we report the characteristics of protein-polymer nanobiocomposite films and their solid state devices. The protein-polymer nanobiocomposite films (thickness = approximately 125 nm) were prepared by spin-coating the solution of cytochrome c (cyt c) and poly(vinyl alcohol) (PVA) (cyt c:PVA = 3:1 by weight). To understand the characteristics of the cyt c-PVA films (nanolayers), we employed the optical absorption and surface morphology measurement and then fabricated planar diode-type solid state devices. The optical absorption measurement showed that the heme part of cyt c in the cyt c-PVA nanolayer was well kept after the coating process, while the crack-like surface was found from the atomic force microscopy measurement. The planar device showed an ohmic type dark current, but the current gradually increased as the incident light intensity increased. In particular, the (photo) current was strongly dependent upon the voltage, which was assigned to the insulating role of cyt c surrounding groups and PVA. This trend was supported by the slow rise and decay time via photo-switching experiment.

Effect of inorganic nanoparticle addition to the hole-collecting buffer layers in polymer solar cells.

We investigated the influence of nickel oxide (NiO) nanoparticles that are incorporated into the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] on the performance of polymer:fullerene solar cells. To understand the optimum composition of NiO nanoparticles, the composition of NiO nanoparticles was varied from 0 wt% to 23 wt%. Results showed that the optical transmittance was gradually decreased as the NiO content increased. However, the device performance (short circuit current density, fill factor, series resistance, power conversion efficiency) exhibited a two stage trend in a boundary of approximately 9 wt% NiO content. This trend was in good agreement with the trend of sheet resistance in the presence of slight discrepancy owing to the different charge transport geometry.

Dipole Moment- and Molecular Orbital-Engineered Phosphine Oxide-Free Host Materials for Efficient and Stable Blue Thermally Activated Delayed Fluorescence.

To utilize thermally activated delayed fluorescence (TADF) technology for future displays, it is necessary to develop host materials which harness the full potential of blue TADF emitters. However, no publication has reported such hosts yet. Although the most popular host for blue TADF, bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) guarantees high-maximum external quantum efficiency (EQEmax ) TADF devices, they exhibit very short operational lifetimes. In contrast, long-lifespan blue TADF devices employing stable hosts such as 3',5-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-3-carbonitrile (mCBP-CN) exhibit much lower EQEmax than the DPEPO-employed devices. Here, an elaborative approach for designing host molecules is suggested to achieve simultaneously stable and efficient blue TADF devices. The approach is based on engineering the molecular geometry, ground- and excited-state dipole moments of host molecules. The engineered hosts significantly enhance delayed fluorescence quantum yields of TADF emitters, as stabilizing the charge-transfer excited states of the TADF emitters and suppressing exciton quenching, and improve the charge balance. Moreover, they exhibit both photochemical and electrochemical stabilities. The best device employing one of the engineered hosts exhibits 79% increase in EQEmax compared to the mCBP-CN-employed device, together with 140% and 92-fold increases in operational lifetime compared to the respective mCBP-CN- and the DPEPO-based devices.

Two-dimensional photonic crystal arrays for polymer:fullerene solar cells.

We report the application of two-dimensional (2D) photonic crystal (PC) array substrates for polymer:fullerene solar cells of which the active layer is made with blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The 2D PC array substrates were fabricated by employing a nanosphere lithography technique. Two different hole depths (200 and 300 nm) were introduced for the 2D PC arrays to examine the hole depth effect on the light harvesting (trapping). The optical effect by the 2D PC arrays was investigated by the measurement of optical transmittance either in the direction normal to the substrate (direct transmittance) or in all directions (integrated transmittance). The results showed that the integrated transmittance was higher for the 2D PC array substrates than the conventional planar substrate at the wavelengths of ca. 400 nm, even though the direct transmittance of 2D PC array substrates was much lower over the entire visible light range. The short circuit current density (J(SC)) was higher for the device with the 2D PC array (200 nm hole depth) than the reference device. However, the device with the 2D PC array (300 nm hole depth) showed a slightly lower J(SC) value at a high light intensity in spite of its light harvesting effect proven at a lower light intensity.

Effect of film thickness in hybrid polymer/polymer solar cells with zinc oxide nanoparticles.

We briefly report the effect of film thickness on the performance of hybrid polymer/polymer solar cells that were made using poly(3-hexylthiophene), poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and zinc oxide (ZnO) nanoparticles. The ZnO nanoparticles were introduced to improve the electron transport property of P3HT/F8BT blend films. Results showed that the open circuit voltage (V(OC)) was remarkably decreased by adding only approximately 0.5 wt% ZnO nanoparticles though the optical absorption spectra were not much changed due to the small amount of ZnO nanoparticles in the ternary blend films (approximately 1.9%). In contrast, the fill factor (FF) of devices was improved for the ternary blend devices with the ZnO nanoparticles due to the improved electron transport as evidenced by the reduced series resistance. The short circuit current density of devices was not much changed because of the enhanced charge transport. However, the addition of ZnO nanoparticles decreased the power conversion efficiency of devices owing to the larger influence of V(OC) compared to the FF improvement.

Pressing effect in polymer solar cells with bulk heterojunction nanolayers.

We report the effect of pressing light-absorbing layers on the performance of polymer solar cells. The light-absorbing active layer was prepared on the transparent conducting oxide coated substrates from solutions that contain a mixture of regioregular poly(3-hexylthiophene) and soluble fullerene molecules. The active layers were pressed using a home-built micro-press system by controlling temperature and pressure, followed by the top electrode deposition. The surface of the active layers pressed was examined using atomic force microscope, while the photovoltaic characteristics of devices were measured under simulated solar light illumination (air mass 1.5 G, 100 mW/cm2). Results showed that the dark current of devices was noticeably increased by pressing the active layer without respect to the pressing temperature. The highest power conversion efficiency was achieved for the device with the active layer pressed under 10 kgf at 70 degrees C. The result was explained in terms of surface morphology and thermophysical effect.

Significant Performance Improvement in n-Channel Organic Field-Effect Transistors with C60 :C70 Co-Crystals Induced by Poly(2-ethyl-2-oxazoline) Nanodots.

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C60 -Ih )[5,6]fullerene (C60 ) and (C70 -D5h(6) )[5,6]fullerene (C70 ) blend films, with an increase from 0.1 to 2.1 cm2 V-1 s-1 . The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C60 :C70 co-crystal structure within the blend films.

Improved Efficiency and Lifetime of Deep-Blue Hyperfluorescent Organic Light-Emitting Diode using Pt(II) Complex as Phosphorescent Sensitizer.

Although the organic light-emitting diode (OLED) has been successfully commercialized, the development of deep-blue OLEDs with high efficiency and long lifetime remains a challenge. Here, a novel hyperfluorescent OLED that incorporates the Pt(II) complex (PtON7-dtb) as a phosphorescent sensitizer and a hydrocarbon-based and multiple resonance-based fluorophore as an emitter (TBPDP and ν-DABNA) in the device emissive layer (EML), is proposed. Such an EML system can promote efficient energy transfer from the triplet excited states of the sensitizer to the singlet excited states of the fluorophore, thus significantly improving the efficiency and lifetime of the device. As a result, a deep-blue hyperfluorescent OLED using a multiple resonance-based fluorophore (ν-DABNA) with Commission Internationale de L'Eclairage chromaticity coordinate y below 0.1 is demonstrated, which attains a narrow full width at half maximum of ≈17 nm, fourfold increased maximum current efficiency of 48.9 cd A-1 , and 19-fold improved half-lifetime of 253.8 h at 1000 cd m-2 compared to a conventional phosphorescent OLED. The findings can lead to better understanding of the hyperfluorescent OLEDs with high performance.

Nano-crater morphology in hybrid electron-collecting buffer layers for high efficiency polymer:nonfullerene solar cells with enhanced stability.

Organic solar cells based on solution processes have strong advantages over conventional silicon solar cells due to the possible low-cost manufacturing of flexible large-area solar modules at low temperatures. However, the benefit of the low temperature process is diminished by a thermal annealing step at high temperatures (≥200 °C), which cannot be practically applied for typical plastic film substrates with a glass transition temperature lower than 200 °C, for inorganic charge-collecting buffer layers such as zinc oxide (ZnO) in high efficiency inverted-type organic solar cells. Here we demonstrate that novel hybrid electron-collecting buffer layers with a particular nano-crater morphology, which are prepared by a low-temperature (150 °C) thermal annealing process of ZnO precursor films containing poly(2-ethyl-2-oxazoline) (PEOz), can deliver a high efficiency (12.35%) similar to the pristine ZnO layers prepared by the conventional high-temperature process (200 °C) for inverted-type polymer:nonfullerene solar cells. The nano-crater morphology was found to greatly enhance the stability of solar cells due to improved adhesion between the active layers and ZnO:PEOz hybrid buffer layers.

Pronounced Side Chain Effects in Triple Bond-Conjugated Polymers Containing Naphthalene Diimides for n-Channel Organic Field-Effect Transistors.

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R1)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R2)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR1-R2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm2 V-1 s-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.

Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors.

We report the development of low operating voltages in inorganic-organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrO x gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer "Super Yellow" acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10-2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered.

Efficient Deep Red Light-Sensing All-Polymer Phototransistors with p-type/n-type Conjugated Polymer Bulk Heterojunction Layers.

Here we demonstrate deep red light-sensing all-polymer phototransistors with bulk heterojunction layers of poly[4,8-bis[(2-ethylhexyl)-oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)). The device performances were investigated by varying the incident light intensity of the deep red light (675 nm), while the signal amplification capability was examined by changing the gate and drain voltages. The result showed that the present all-polymer phototransistors exhibited higher photoresponsivity (∼14 A/W) and better on/off photoswitching characteristics than the devices with the pristine polymers under illumination with the deep red light. The enhanced phototransistor performances were attributed to the well-aligned nanofiber-like morphology and nanocrystalline P(NDI2OD-T2) domains in the blend films, which are beneficial for charge separation and charge transport in the in-plane direction.

Strong Composition Effects in All-Polymer Phototransistors with Bulk Heterojunction Layers of p-type and n-type Conjugated Polymers.

We report the composition effect of polymeric sensing channel layers on the performance of all-polymer phototransistors featuring bulk heterojunction (BHJ) structure of electron-donating (p-type) and electron-accepting (n-type) polymers. As an n-type component, poly(3-hexylthiopehe-co-benzothiadiazole) end-capped with 4-hexylthiophene (THBT-4ht) was synthesized via two-step reactions. A well-studied conjugated polymer, poly(3-hexylthiophene) (P3HT), was employed as a p-type polymer. The composition of BHJ (P3HT:THBT-4ht) films was studied in detail by varying the THBT-4ht contents (0, 1, 3, 5, 10, 20, 30, 40, and 100 wt %). The best charge separation in the P3HT:THBT-4ht films was measured at 30 wt % by the photoluminescence (PL) study, while the charge transport characteristics of devices were improved at the low THBT-4ht contents (<10 wt %). The photosensing experiments revealed that the photosensivity of all-polymer phototransistors was higher than that of the phototransistors with the pristine P3HT layers and strongly dependent on the BHJ composition. The highest (corrected) responsivity (RC) was achieved at 20 wt %, which can be attributable to the balance between the best charge separation and transport states, as investigated for crystal nanostructures and surface morphology by employing synchrotron-radiation grazing-incidence wide-angle X-ray scattering, high-resolution/scanning transmission electron microscopy, and atomic force microscopy.

Modulation-Doped In2 O3 /ZnO Heterojunction Transistors Processed from Solution.

This paper reports the controlled growth of atomically sharp In2 O3 /ZnO and In2 O3 /Li-doped ZnO (In2 O3 /Li-ZnO) heterojunctions via spin-coating at 200 °C and assesses their application in n-channel thin-film transistors (TFTs). It is shown that addition of Li in ZnO leads to n-type doping and allows for the accurate tuning of its Fermi energy. In the case of In2 O3 /ZnO heterojunctions, presence of the n-doped ZnO layer results in an increased amount of electrons being transferred from its conduction band minimum to that of In2 O3 over the interface, in a process similar to modulation doping. Electrical characterization reveals the profound impact of the presence of the n-doped ZnO layer on the charge transport properties of the isotype In2 O3 /Li-ZnO heterojunctions as well as on the operating characteristics of the resulting TFTs. By judicious optimization of the In2 O3 /Li-ZnO interface microstructure, and Li concentration, significant enhancement in both the electron mobility and TFT bias stability is demonstrated.

Acidity-Controlled Conducting Polymer Films for Organic Thermoelectric Devices with Horizontal and Vertical Architectures.

Organic thermoelectric devices (OTEDs) are recognized one of the next generation energy conversion platforms because of their huge potentials for securing electricity continuously from even tiny heat sources in our daily life. The advantage of OTEDs can be attributable to the design freedom in device shapes and the low-cost fabrication by employing solution coating processes at low temperatures. As one of the major OTE materials to date, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been used, but no study has been yet carried out on its acidity control even though the acidic components in OTEDs can seriously affect the device performance upon operation. Here we demonstrate that the addition of aniline (a weak base) can control the acidity of PEDOT:PSS and enhance the performance of OTEDs. In particular, the vertical OTEDs with aniline-doped PEDOT:PSS films (active area = 1.0 cm2) could continuously generate electricity (0.06 nW) even at low temperatures (<38 °C) when they were mounted on a desk lamp (power = 24 W).

Broadband pH-Sensing Organic Transistors with Polymeric Sensing Layers Featuring Liquid Crystal Microdomains Encapsulated by Di-Block Copolymer Chains.

We report broadband pH-sensing organic field-effect transistors (OFETs) with the polymer-dispersed liquid crystal (PDLC) sensing layers. The PDLC layers are prepared by spin-coating using ethanol solutions containing 4-cyano-4'-pentyl-biphenyl (5CB) and a diblock copolymer (PAA-b-PCBOA) that consists of LC-philic block [poly(4-cyano-biphenyl-4-oxyundecyl acrylate) (PCBOA)] and acrylic acid block [poly(acrylic acid) (PAA)]. The spin-coated sensing layers feature of 5CB microdomains (<5 µm) encapsulated by the PAA-b-PCBOA polymer chains. The resulting LC-integrated-OFETs (PDLC-i-OFETs) can detect precisely and reproducibly a wide range of pH with only small amounts (10-40 µL) of analyte solutions in both static and dynamic perfusion modes. The positive drain current change is measured for acidic solutions (pH < 7), whereas basic solutions (pH > 7) result in the negative change of drain current. The drain current trend in the present PDLC-i-OFET devices is explained by the shrinking-expanding mechanism of the PAA chains in the diblock copolymer layers.