Although deuterated organic semiconductors have been widely employed in neutron reflectivity/scattering to study interfacial phenomena in organic electronic devices such as organic photovoltaics, there is still no clear understanding of the possible impacts of deuterium substitution on the optoelectronic properties of the organic semiconductors. Here, we first systematically study the isotope effect on structure, morphology and photovoltaic responses of regioregular poly(3-hexythiophenen)s (P3HT) by selectively substituting deuterium on its backbone or side chain. Deuteration on the backbone and side chain of P3HT leads to distinct optoelectronic responses in P3HT/PCBM bulk heterojunctions photovoltaics. Backbone deuteration of P3HT significantly changes the thin film crystallinity and morphology of P3HT/PCBM blends, and consequently the short circuit current. However, side chain deuterated P3HT exhibits a surprising reduced open circuit voltage as large as 0.23 eV, without a change in the short circuit current. This reduced open circuit voltage is not due to the reduced energy band offset between the HOMO of the deuterated P3HT donor and the LUMO of PCBM acceptor, but results from the increased recombination losses after side chain deuteration. Quantum chemistry calculations provide additional support that the electronic coupling between P3HT and fullerene is altered due to side chain deuteration, indicating that the increased VOC losses originate from the formation of interfacial charge transfer states.

Polymer field-effect transistors (FETs) have attracted intense interest in organic electronics, owing to their advantages of solution processability, low-cost large-area fabrications and low-temperature processing. Most high-mobility polymer field-effect transistors have been fabricated using chlorinated solvents. However, chlorinated solvents are not preferred for industrial manufacture as a result of their corrosive nature and environmental impact. Therefore, the development of a non-chlorinated solvent system for polymer device fabrication is a critical factor for practical applications of organic electronics.
In this study, non-chlorinated mixed solvents, tetrahydronaphthalene/p-xylene, were employed for device processing. The effects of mixed solvents on charge transport, morphology and the molecular packing of a donor-acceptor fused thiophene-diketopyrrolopyrrole copolymer were investigated. It was found that charge transport characteristics could be significantly improved in the field-effect transistors by controlling the mixed-solvent ratios. In addition, the low-voltage-driven devices were fabricated on damage-resistant ion-exchanged sodium aluminosilicate glass. The devices exhibited a remarkable mobility of 3.12 cm2V-1s-1, an on-off ratio over 10000 and a threshold voltage of -0.8 V in a low-voltage operation of -2 V in ambient conditions. Moreover, the devices exhibited stable on-off current ratios in 3000 continuous cycles and a negligible threshold-voltage shift of 0.2 V during bias stress testing. This study reveals that the combination of the non-chlorinated mixed solvents, coplanar donor-acceptor polymer and strong ion-exchanged substrates provide a promising path forward for practical organic electronics applications.

This lecture focuses on the challenging design and realization of new materials for creating unconventional electronic as well as excitonic circuitry. Fabrication methodologies to achieve these goals include high-throughput, large-area printing techniques. Materials design and synthesis topics to be discussed include: 1. Rationally designed high-mobility p- and n-type organic semiconductors for printed organic CMOS, 2. Self-assembled and printable high-k nanodielectrics enabling ultra-large capacitance, low leakage, high breakdown fields, minimal trapped interfacial charge, and radiation hardness, 3. Polycrystalline and amorphous oxide semiconductors for transparent and mechanically flexible electronics, 4) Merging these materials sets to produce a variety of high-performance thin-film transistor-based devices.

We demonstrate that supramolecular assembly and subsequent enhancement of field-effect mobility of organic conjugated polymers can, surprisingly, be achieved simply by adding small amounts of a poor solvent with lower boiling point, and which can hydrogen bond with the majority solvent. It was found that the addition of the lower boiling poor solvent, acetone, to a precursor solution of P3HT/chloroform can improve the field-effect mobility of P3HT films by a factor of up to approximately 4. The improvement is due to hydrogen bonding interactions between acetone and chloroform; the solvent mixture resides longer in the solution during film formation, compared to either of the individual components, due to its lower evaporation rate. P3HT is less soluble in the solvent mixture than in pure chloroform, which enables the supramolecular assembly of the P3HT chains, resulting in the enhancement of field-effect mobility. Hansen solubility parameters such as the dispersive (δD), the polar (δP), and the hydrogen bonding (δH) solubility parameters of the polymer and co-solvent were used to understand how the solvent mixture can enhance the supramolecular assembly of polymer chains when deposited from the solution state and subsequent thin film properties. In addition, hydrogen bonding between the main solvent and poor solvent was investigated by comparing the evaporation rate and spectroscopic properties between each pure solvent and the solvent mixture. Two-dimensional molecular ordering of the polymer film was controlled by varying the quantity of poor solvent added to the main precursor solution, and a correlation between the field-effect mobility and the molecular ordering of the P3HT films was demonstrated. This process oriented approach to defining solvent systems for π-conjugated semiconducting polymers could be attractive in the pursuit of low-cost, large-area electronic device fabrication methodologies.

The promise of low-cost processing and flexible circuitry has driven intense research in organic thin-film transistors (OTFTs). Since their discovery a remarkable enhancement in the performance of OTFTs has been accomplished by improvements in both materials and processing such that OTFTs now have charge carrier mobilities comparable to or higher than that of amorphous silicon. The utility of OTFTs has been demonstrated in flexible sensors, memories, and in the driving circuitry of organic displays. For complementary circuits (CMOS) ideally both p- and n-type OTFTs are patterned with reliable performance on a flexible substrate using solution-based methods such as stamping, ink-jet printing, Gravure printing, or solution shearing. CMOS circuits consume less power and are used for most logic circuits. However, to date relatively few high mobility n-channel materials have been developed, and many of the highest performing n-channel materials are not solution processable under ambient conditions.
TIPS-pentacene is a solution processable small molecule with a high field-effect hole mobility. Numerous studies have been devoted to controlling the morphology, crystal packing and alignment of TIPS-pentacene. The n-doping of TIPS-pentacene by strong electron donors has been confirmed by the detection of TIPS-pentacene radical anions with UV-Vis-NIR absorption spectroscopy. However, n-type transistors of TIPS-pentacene have not been reported. In this study, we investigated the n-doping of TIPS-pentacene films, as prepared by the solution-shearing technique, with 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) a reduced form of the recently reported vacuum processable dopant 2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I). Our findings indicate that high mobility, up to 6.81 cm^2/(Vs), and high on/off ratio, of 10^5-10^6, n-type OTFTs can be prepared by n-doping TIPS-pentacene. In contrast to most studies on doped OTFTs, our method does not lower the on/off ratio of the device even at high doping concentrations (> 50 mol%). This may be a consequence of the low miscibility of o-MeO-DMBI and TIPS-pentacene, or alternatively it may be that only trap states are sufficiently low in energy to activate the dopant for electron transfer. In either case, the doping efficiency is expected to be too low to observe an increase in the bulk conductivity, and the transistors maintain high on/off ratios. Interestingly, a common electron trap level and density has been found in many organic semiconductors; thus, a similar effect may be possible in other p-type materials.

Small-molecule organic semiconductors form the basis for the emerging field of organic optoelectronics. In order to better understand the intrinsic photo-physical and transport phenomena in this important class of materials, it is necessary to study samples of very high structural order and chemical purity. Such materials exist in the form of molecular single crystals that can be used for fabrication of high-performance prototype devices, such as field-effect transistors, photo-conductors and photo-voltaic cells, in which intrinsic properties of organic semiconductors can be investigated without parasitic effects of disorder (see, e.g., [1]). This talk will cover the recent progress in organic single-crystal device electronics. In particular, several new phenomena related to the previously discovered long-range triplet exciton diffusion and surface photocurrent generation (see, e.g., [2]) will be discussed. We will also present a novel method of Hall effect measurements in molecular crystals, complementary to the Hall measurements previously conducted in a traditional FET geometry [3].
**** REFERENCES:

(1). M. E. Gershenson, V. Podzorov, A. F. Morpurgo, "Colloquium: Electronic Transport in Single-Crystal Organic Transistors", invited review, Rev. Mod. Phys. 78, 973 (2006).
(2). H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, "Observation of long-range exciton diffusion in highly ordered organic semiconductors", Nature Mater. 9, 938 (2010).
(3). V. Podzorov et al., "Hall effect in the accumulation layers on the surface of organic semiconductors", Phys. Rev. Lett. 95, 226601 (2005).

Trap states in the band gap are of major concern also in organic semiconductors. In recent years several studies have been conducted to quantify the density of trap states (trap DOS). It was found that the trap DOS not only influences the charge transport itself, but is also important to understand energy level alignment at organic hetero-junctions as well as at the electrodes in OLEDs and OPVs. Therefore to improve organic electronic devices a proper understanding of trap DOS is essential.
In this study we quantify the spectral trap DOS for numerous organic semiconductors prepared under various conditions: single crystal FETs and thermally evaporated and solution processed TFTs of typical n- and p-type organic compounds are analyzed. This wide range of crystalline order leads to a variation of the trap DOS and conductivity by orders of magnitude.
In a first qualitative assessment, the impact of the trap DOS is visualized by proper rescaling of the measured transfer curves. In a second step, we quantify the trap DOS by numerically simulate the measured transfer curves using a FET model. Interestingly the trap DOS for organic n-type semiconductors is highly similar to that in p-type materials, provided a similar degree of crystallinity. We then compare the trap DOS of the organic semiconductors with measurements of typical inorganic semiconductors to highlight the intrinsic potential of organic semiconductor devices.

The steady-state characteristics of organic semiconductor devices are well understood and a wide range of tools and models exist to study the role of trap states under DC operation. For applications in electronic circuits, however, time-dependent phenomena associated with trapping- and detrapping play an important role in the device performance (e.g. switching speed of transistors).
We have developed a computational model to study both, the steady-state behavior of an organic device and its dynamic response to an AC voltage. When a DC voltage is applied, the occupation of traps and delocalized states is assumed to be in thermal equilibrium according to Fermi Dirac distribution. Under AC operation however, the device response is governed by the frequency-dependent trapping behavior: when the frequency of the driving field is sufficiently high, the equilibrium is disturbed, resulting in an altered conductive response compared to DC. Going beyond existing models, we include arbitrary trap density of states and extend the calculation to realistic device-structures, which allows us to compare simulations to experiments.
We have experimentally studied organic field effect transistors, SCLC-devices and MIS-structures by means of admittance-spectroscopy at different temperatures. In the SCLC-structure using rubrene single-crystals we found the conductance to increase at frequencies above ~10 kHz. This observation can be attributed to a specific trapping-frequency, beyond which charge carriers can no longer be captured by traps. Similarly, the deviations from DC response are also measured in OFETs.
Our method of characterizing the time-dependent charge transport provides additional insight into the effect of shallow and deep traps and their characteristic trapping/detrapping rates. It can be applied to a variety of device structures and provides a valuable instrument to study the frequency dependent conductivity of organic semiconductor materials. Once the material specific parameters are established, this model will also be a helpful tool to predict the AC performance of organic electronic circuits.

Development of functional materials and understanding of the microscopic mechanisms mutually benefit through their close interaction. To accelerate development of organic semiconductor films for industrial application to flexible electronics devices, it is essential to understand mechanisms of charge transport in conjunction with molecular-scale charge transfer. Here, we examine universality of the idea that practically attractive high-mobility charge transport in organic transistors is caused by band-like carrier dynamics using several different molecular systems as the active semiconductor layers. We employ Hall-effect measurement which examines the degree of intermolecular electronic charge coherence in various organic semiconductors.
We have measured Hall effects on various high-mobility organic transistors, for example solution-crystallized single-crystal transistor with 2,9-didecyl-dinaphtho[2,3-b:2&’,3&’-f]- thieno[3,2-b]thiophene (C10-DNTT). The results of measurements show Hall coefficient identical to inverse charge density, indicating well extended nature of the carriers accumulated at the surface of these semiconductors and band-like transport is realized in the systems such as rubrene single-crystal transistors. On the other hand, the devices of pentacene have exhibited obvious discrepancy between the Hall coefficient and inverse charge density at room temperature, meaning the free-electron like picture is partially violated. In pentacene single-crystal and thin-film transistors, under external pressure and lower temperatures, the deviation of the Hall coefficient and inverse charge density gradually diminishes and approaches to the free-electron like behavior. We discuss possible mechanism that large molecular fluctuation at room temperature partially destroys electronic coherence in pentacene devices. The result that the effect is more significant in simple rod-shaped pentacene as compared to the other high-mobility materials like C10-DNTT and rubrene is intriguing, which can give a prescription for designing molecules which cause high-mobility band transport.
Reference;
T. Uemura and J. Takeya et al., Phys. Rev. B 85, 035313-1-6 (2012).

We have fabricated 2-V operational organic thin-film transistors (OTFTs) with high thermal stability of more than 250 °C. We used two isomeric diphenyl derivatives of dinaphtho[2,3-b:2prime;,3prime;-f]thieno[3,2-b]thiophene (DPh-DNTT) as p-type organic channel layer, and a 2-nm-thick self-assembled monolayer (SAM) and a 4-nm-thick aluminum oxide layer as gate dielectrics. The transistors exhibited a field-effect mobility of 2.0 cm²/Vs under a 2-V operating voltage before heating, and the mobility changed from 2.0 cm²/Vs to 1.6 cm²/Vs after heating to 250 °C for 30 min in ambient air. Furthermore, we fabricated top-contact geometry organic TFTs by using a subfemtoliter inkjet^[1]. Silver nano particles were deposited on the semiconductor films as source and drain electrodes and sinterd at 120 °C for 3 h in nitrogen. The TFT with channel length 50 µm that operate 2 V and has a high field-effect mobility of 2.0 cm²/Vs.
These transistors were fabricated by vacuum evaporation and dipping processes. First, 30-nm-thick Al layer were thermally evaporated as gate electrode using shadow mask. Second, we formed gate dielectric layers. The gate dielectric layers were composed of 4-nm thick aluminum oxide layer which were formed by oxygen-plasma treatment and self-assembled monolayer SAMs (n-tetradecylphosphonic acid) which were prepared from a 2-propanol solution at room temperature^{[2, 3]}. Purified DPh-DNTT^[4] were deposited in vacuum through a shadow mask to form a 30-nm-thick as p-type organic semiconductors on the gate dielectric layer. Finally, a 100-nm-thick Au layer was evaporated through a shadow mask to form the source and drain electrodes.
One of the authors (T.Y.) is grateful to the research fellowships for young scientists of JSPS.
[1] T. Sekitani, Y. Noguchi, U. Zschieschang, H. Klauk, and T. Someya, Proc. Nat. Acad. Sci., 105, 4976 (2008).
[2] H. Klauk, U. Zschieschang, J. Pflaum, and M. Halik, Nature 445, 745 (2007).
[3] T. Yokota, T. Nakagawa, T. Sekitani, Y. Noguchi, K. Fukuda, U. Zschieschang, H. Klauk, K. Takeuchi, M. Takamiya, T. Sakurai, and Takao Someya, Appl. Phys. Lett. 98, 193302 (2011).
[4] K. Niimi, M. J. Kang, E. Miyazaki, I. Osaka, and K. Takimiya, Org. Lett., 13, 3430 (2011).

We have successfully fabricated a fully printed organic thin-film transistor (TFT) device with high field effect mobility of more than 1.0 cm²/Vs, large on/off ratio of more than 10⁸, and low contact resistance of less than 10 kOmega;cm. Pseudo-CMOS integrated circuits were also created that exhibited good input-output characteristics with high-speed operation.
A 125-µm-thick PEN film was used as a substrate, upon which an amorphous fluoropolymer was spin-coated to form a base layer. A silver nanoparticle ink (DIC, JAGLT series) layer was applied using inkjet printing (Fujifilm Dimatix, DMP2830) and then sintered at 120 °C to form the gate electrode. An insulating material (Merck, lisicon^® D207) was then spin-coated to form a 1.3 µm-thick gate dielectric layer. Silver nanoparticle ink (Harima Chem., NPS-JL) was then inkjet printed and sintered at 120 °C to form the source-drain electrodes, which were modified using self-assembled monolayer (Merck, lisicon^® M001). Lastly, an organic semiconducting ink (Merck, lisicon^® S1200 series) was printed using dispenser equipment (Musashi Engineering, Image Master 350PC) and annealed at 100 °C for 1 min.
The on-current of the fabricated organic TFT device (W/L = 5000/25 µm) was 300 µA at an operation voltage of -40 V. The estimated field-effect mobility in the saturation region was 1.2 cm²/Vs, threshold voltage was 3.3 V, and on/off ratio was greater than 10⁸. We also fabricated a pseudo-CMOS inverter and 3-stage ring-oscillator [1,2]. The inverter circuit functioned well at low operating voltages of 5 V, with a signal gain of 15 at an operating voltage of 20 V, and 5 at 5 V. The oscillation frequency for the ring oscillator was 294 Hz at 20 V, which corresponds to a propagation delay of 570 µs per stage.
[1] T. C. Huang et al., IEEE Trans. Electron Devices, 58, 141 (2010).
[2] Fukuda, IEEE Electron. Device Lett., 32, 1448 (2011).

Development of soluble small-molecule organic semiconductors by trialkylsilylethyne-substitution allowed deposition from solution-based methods and yielded significant improvements in device performance and stability. In this presentation we report on organic thin-film transistors (OTFTs) fabricated with a soluble small-molecule organic semiconductor - 2,8-difluoro-5,11-bis(triethylgermylethynyl) anthradithiophene (diF-TEG ADT), obtained by a novel substitution route, trialkylgermyl functionalization, and we discuss the compatibility of this material with large-area electronics. Devices deposited by drop-casting the organic semiconductor at the surface of SiO2 dielectric exhibit charge carrier mobilities as high as 5.4 cm2V-1s-1 and on/off current ratios of 10^6, with 90% of the 104 measured transistors having mobilities greater than 1 cm2V-1s-1. By correlating the electrical properties with the structural data obtained from X-ray diffraction, we find that a good film crystallinity and optimal π-π overlap between adjacent molecules is responsible for this superior electronic behavior. Spray-coated devices from a very dilute solution in Chlorobenzene (0.15%) reach field-effect mobilities of 2.2 cm2V-1s-1 upon optimization of the film formation. This is achieved by accurately controlling processing parameters such as substrate temperature, flow rate and the distance between the spray nozzle and the substrate. This value represents the highest mobility reported to date in spray-coated OTFTs. Other characteristics for these devices include a very small threshold voltage (Vt = 0.5 V) and a good current on/off ratio (Ion/Ioff = 10^4). This result demonstrates the potential of diF-TEG ADT for incorporation in large-area, low-cost electronic applications.

Solution deposition of organic semiconductors (OSC) is a leading contender for producing large-area, inexpensive, and flexible organic electronics. We recently developed a solution deposition method for OSCs called the solution shearing method (SSM) which resulted in lattice strained OSCs that yielded higher performing organic field effect transistor (OFET) by tuning the electronic overlap. However, industrial applications of these lattice strained OFETs requires that the OFETS be patterned, and it is unclear whether the patterned OFETs will still maintain a lattice strained crystal packing. We have developed a surface functionalization procedure that utilizes hydrophobic/hydrophilic interactions on the interface to isolate lattice strained OSCs. We choose the surface functionalities so that charge traps are also passivated on the charge transport layer. We show the existence of strain though X-ray diffraction techniques, and study the charge transport properties of the patterned, lattice strained OFETs. We show that charge carrier mobilities up to 2 cm²V^-1s^-1 are possible in the patterned OFETs, and that strained OFETs can exist at patterning sizes of 100 mu;m and lower. We show that the pattern size can have an effect on the lattice strain possible in the OSCs, and investigate methods that enable sub 10 mu;m scale patterning using solution shearing.

“Printed electronics” is regarded as a realistic paradigm to manufacture light-weight, thin, and impact-resistant electronics devices by the patterned application of functional inks containing soluble or dispersed electronic materials. Organic semiconductors are most promising as the active semiconducting materials, because of the controlled solubility and processability at ambient conditions. However, conventional printing techniques are incompatible with the formation of organic semiconductor layers having high layer crystallinity, although this feature is indispensable in realizing high carrier mobility of organic thin-film transistors (OTFTs). Here we present a couple of novel printing techniques for manufacturing organic semiconductor films showing high layer crystallinity. One is double-shot inkjet printing (DS-IJP) for small-molecule semiconductors,[1] and the other is push-coating technique for semiconducting polymers [2]. We present and discuss that both of the processes are designed to form ideal fields for self-organization of the layered crystalline materials.
The DS-IJP technique introduces a technique of antisolvent crystallization into the microfluidic inkjet printing processes. In the process, the antisolvent ink is printed first and the semiconductor solution ink is then overprinted. We show that mixing fine droplets of an antisolvent and a solution of an active semiconducting component within a confined area on an amorphous substrate can trigger the controlled formation of exceptionally uniform single-crystal or polycrystalline thin films that grow at the liquid-air interfaces. Using this approach, we successfully fabricated OTFTs based on the printed single-crystal 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) showing high average mobility of 16.4 cm2/Vs.
We also present the simple push coating technique to manufacture plane semiconducting polymer films on highly hydrophobic surfaces without the material loss. The technique utilizes poly(dimethylsiloxane)-based trilayer stamp both as the pressing contact plate to form a thin solution layer and as the solvent extraction media from the solution. The use of the stamp is advantageous to enable the facile peeling from the semiconducting film after the film growth as well as the repeated use of the stamp. The planar film formation on hydrophobic surfaces also enables subsequent fine film patterning. Using this approach, we fabricated OTFTs based on the push-coated poly(3-hexylthiophene) (P3HT) showing high crystallinity and enhanced mobility.
[1] Nature 475: 364-367 (2011).
[2] Nat. Commun. 3:1176 doi: 10.1038/ncomms2190 (2012).

The performances of devices for organic electronics strongly rely upon the possibility of achieving high carrier mobility in the organic semiconducting active material. If small organic molecules are used in the active layer, crystal aggregation phenomena, as a result of vapor deposition processes, are usually exploited to produce materials with high charge mobility. Nevertheless the morphology of the organic active material can potentially alter the mobility of charge carriers and, consequently, the overall performances of devices. In this context, recent research efforts have been targeted to the definition of structure-property relationships, thus allowing the design of tailored materials and processes. However, a comprehensive understanding of the correlation between morphology at the nanoscale and charge transport properties is still elusive. In this respect, numerical simulations have the potential to shed light on phenomena occurring at the atomistic and nanometer scales and to provide quantitative information to be used in the design and optimization of devices.
In this work, we investigate these relationships with the use of realistic models mimicking the typical environment in a field-effect transistor (FET) configuration. The details of morphologies at the atomistic level of detail can be obtained by molecular dynamics (MD) simulations based on empirical potentials. To this end, suitable force-fields are implemented and validated. Aggregation phenomena and morphologies at the nanoscale are then compared with available experimental data. Charge carrier mobility is thus computed by performing electronic structure calculations making use of the morphology, with atomistic resolution, obtained by MD.
As a case study, we consider active organic layers constituted by substituted quaterthiophenes, which commonly exhibit p-type transport characteristics. Our simulations are based on the realistic description of the morphology at the interface with a polymer layer constituting the gate dielectric.
The interplay between the molecule/molecule and molecule/dielectric interactions induces a peculiar morphology of the organic phase at the nanoscale, which can be related to charge transport properties. The role of crystalline domains and grain boundaries with respect to bulk-like material is also addressed through the simulations of the concurrent growth of nanocrystals.
Our simulations account for the morphology-dependent behavior of the overall carrier mobility in realistic environments and provide a comprehensive picture of charge transport in organic materials on different length scales. Moreover, information about crystal aggregation can also be used to analyze phenomena of interest in the functioning of devices, including the interaction with metals constituting the contacts.

In this work, we show a study on the sensitivity to surface deformation in Pentacene-based Organic Thin Film Transistors (OTFTs) and its correlation with the intrinsic structural/morphological properties of the active layer thin film. All devices have been fabricated on flexible plastic foils (PET) using the same bottom gate/bottom contact structure. As previously demonstrated by several works we have observed that the surface deformation induced by an external mechanical stimulus gives rise to a marked, reproducible and reversible (within a certain rage of surface deformation) variation of the device output current. In this work we aim to demonstrate, also assisted by Atomic Force Microscopy (AFM) and X-Ray Diffraction (XRD) investigations, that this phenomenon is strongly related to a modification of charge carriers transport induced by morphological changes taking place within the organic semiconductor during the application of the mechanical stimulus. In order to shed more light onto this phenomenon, the morphology of the Pentacene films was intentionally modified by properly changing the deposition parameters. In particular, different sets of devices have been fabricated changing the deposition rate, but keeping constant all the other deposition parameters. As expected, we have observed that all the deposited thin films are characterized by very similar structural properties (observed in X-Ray investigations), however, a clear difference in the average domain dimensions was found. The electromechanical characterization, performed on different sets of samples, gave rise to very interesting results. In particular, a clear correlation between the average grain dimensions and the sensitivity to surface strain was found. Devices fabricated with a slower deposition rate, which are characterized by bigger domains, are characterized by a very high sensitivity to strain. On the contrary, the sensitivity can be strongly decreased by fabricating devices with smaller grain dimensions. For example, for a surface strain of 1%, the sensitivity of devices fabricated using the slowest deposition rate, 0.08 Å/s, is around 40%, whereas for the devices fabricated using the highest deposition rate, 11 Å/s, a much lower sensitivity was observed (3%). Finally we will show that this approach can be successfully extended also to other types of organic semiconductors, such as solution processable polymers and small molecules.
These results are particularly interesting as they demonstrate that the sensitivity to strain in pentacene based OTFTs can be predictably tuned by properly modulating the grain dimensions of the active layer. In particular, they indicate a possible way for optimizing the mechanical transduction for sensing applications, but also suggest a possible method for strongly limiting and eventually eliminating the OTFTs sensitivity to surface strain, which turns out to be a very important issue for the fabrication of flexible electronics.

In the field of molecular materials for organic electronics, fused acene-based organic semiconductors are key materials for the development of organic electronics, thanks to their outstanding properties and their good processability. Organic thin-film transistor (OTFTs) materials having an extended π-conjugate system such as polyacene and their heteroanalogs including pentacene often exhibit poor environmental stability. In this framework, derivatives of fused thienothiophene derivatives are a good promise between environmental stability and interesting solid state properties. Among them, DNTT(dinaphtho[2,3-]thieno-thiophene) showed exceptional high charge carrier mobilities in OTFTs and now represents the state of the art for molecular organic semiconductors.
In this work, we have designed and synthesized novel organic semiconductor based on thienothiophene core, Dithieno[2',3':4,5;3'',2'':6,7]benzothieno[3,2-b]dithieno[2,3-e:3',2']benzothiophene (4TBTT), which have an almost coplanar structure and crystallize into a herringbone arrangement, similar pentacene and DNTT. However, the most significant structure feature of the 4TBTT crystal packing is the presence of the 3-D network with strongly intermolecular multiple interactions. These interactions facilitate charge carrier transport. Additionally, the intermolecular transfer integral between adjacent molecules in crystal structure of 4TBTT are quite good, which means charge carrier transport can occur to any direction. The 4TBTT-based OTFTs on an octadecyltrichlorosilane-modified SiO2/Si substrate exhibit excellent field-effect performance.

We have been exploring, as organic semiconductors, molecules from the family of hydrogen-bonded pigments. These molecules are worldwide employed as pigments for inkjet toners, rugged outdoor paints, and cosmetics; they are already mass produced, cheap, and recognized for their stability and nontoxicity. These molecules have poor intramolecular conjugation, and thus fall outside of the accepted design rules for organic semiconducting compounds. Despite this, our results show that films of H-bonded pigments support air-stable ambipolar charge carrier transport with mobilities up to 2 cm2/Vs in organic field-effect transistors. We attribute the charge transport in these materials to the cooperative interactions between dye molecules mediated by H-bonding and π-stacking. Thus, molecules lacking intramolecular conjugation interact to produce an intermolecularly π-conjugated network. This communication will report our recent investigation of crystal structure and thin film formation of H-bonded pigments, and their effect on charge transport anisotropy in organic field effect transistors and diode devices. A particular emphasis will be put on the assessment of the structure-property relationships and solid-state packing of these materials.

Synthesis, characterization, and utility of a range of heterocycle-fused naphthalene building blocks recently developed will be presented. Isomeric naphthodithiophenes (NDTs) are useful electron donating building blocks both for small-molecular and polymer semiconductors [1], whereas naphtho[1,2-c:5,6-c&’]bis[1,2,5]thiadiazole (NTz) is a versatile electron deficient building block for the development of semiconducting polymers [2]. These new materials can be applicable to organic field-effect transistors and organic photovoltaics.
[1] Shinamura, S. et al. J. Org. Chem. 2010, 75, 1228; Shinamura, S. et al. J. Am. Chem. Soc. 2011, 133, 5024; Shinamura, S. et al. Org. Lett. 2012, 14, 4718; Osaka, I. et al. J. Am. Chem. Soc. 2010, 132, 5000; Osaka, I. et al. J. Am. Chem. Soc. 2011, 133, 6852.
[2] Osaka, I. et al. J. Am. Chem. Soc. 2012, 134, 3498; Osaka, I. et al. ACS Macro Letters 2012, 1, 437.

Indacenodithiophene (IDT) based low band gap polymers have been shown to be extremely versatile and were successfully implemented into organic photovoltaic cells (OPV) and organic field effect transistors (OFET). Depending on the solubilising alkyl chains attached to the IDT moiety, power conversion efficiencies of up to 6.5% were achieved in solar cells when blended with PC71BM. Similarly impressive results were obtained in OFET, in which hole mobilities higher than 3 cm2/Vs could be obtained when long alkyl side chains were used to solubilise the polymer. Those findings indicated that the solubilising alkyl side chains play an important role in the structure-property relationship and can be used to tune the opto-electronic properties of IDT containing polymers towards specific applications.
Four solubilising alkyl chains are attached onto the IDT moiety with always two alkyl chains positioned on opposite sides of the aromatic ring system. This arrangement of alkyl chains could potentially lead to steric hindrance and hinder a closer pi-pi stacking in IDT polymers. To minimize the steric hindrance, the central benzene ring of the IDT moiety was substituted for a thiophene ring, thus allowing all four solubilising side chains to be attached on the same side of the fused aromatic ring system. Herein we present the synthesis of this novel donor (DCPTT) from commercially available reagents and its incorporation into a series of donor-acceptor polymers. The discussion will be focused on the influence of the new side chain arrangement on molecular packing and its implications on charge carrier mobility.

Conductive fullerene (C60) units were designed to be arranged in one dimension close contact by locally organizing them with spatially constrained covalent bonds, in order to enhance the intramolecular charge transport. C60 side-chain polymers with high relative degrees of polymerization up to 1220 and fullerene compositions up to 53% were synthesized by ruthenium catalyzed, ring-opening metathesis polymerization of the corresponding norbornene-functionalized monomers. Combined molecular dynamics and quantum chemical calculations predicted that the intramolecular electronic interactions (i.e. charge transport) between pendant C60 units can be controlled by the length of the spacers linking the C60 units and the polymer main chain, corroborated by UV/vis absorption and photothermal deflection spectra. The electron mobility measured for the thin film field-effect transistor devices from the polymers was more than an order of magnitude higher than that of the monomers, as a result of the enhanced inter-fullerene interactions within the ultra-long polymer chains.

Understanding of charge injection in organic semiconductor devices is vital for understanding and optimizing device performance. The charge injection barriers in organic field-effect transistors (OFETs) seem to be far less critical as compared to organic light-emitting diodes (OLEDs). Counter intuitively, we show that the origin is image-force lowering of the barrier due to the gate bias at the source contact, although the corresponding gate field is perpendicular to the channel current. For organic diodes we derive an analytical equation to describe injection-limited currents. The derivation is based on a model for diffusion-limited currents. Using this formalism the diffusion current across an injection barrier can be calculated by incorporating an expression to account for the probability of creating a low energy injection pathway in the disordered semiconductor.

Organic field-effect transistors (OFETs) based on solution-processed conjugated semiconducting materials have undergone tremendous progress in recent years, primarily stimulated by the development of innovative and low-cost electronics. Advances both in terms of device performance and reliability have been achieved, which affords the practical application of these devices. However, there are still both physical and technological issues to be faced. Contact resistance represents a significant bottleneck to transistor performance and controlling charge injection from metal electrodes into the organic semiconductor is one of the major hurdles to achieve transistor scaling and dimensional reduction. Thus, understanding the microscopic processes that limit the charge injection in OFETs is important in order to improve the characteristics of the existing devices. To this end, OFETs based on electrolyte insulators have a considerable potential due to their extraordinary high capacitance value, which allows for device operation at extremely low voltages (typically less than 1 V). This can reveal new insight into phenomena that are no experimentally detectable with conventional inorganic or polymeric dielectrics.
Here we demonstrate a dependence of charge injection on the gate electrode work function by systematically investigating the contact resistance in electrolyte-gated OFETs. Our analysis reveals contact-limitations at the metal-semiconductor interface and shows that contact resistance increases as low work function metals are used as gate electrodes. Because the operating voltage in these devices is comparable to the respective variation of the work functions, it is possible to tune the contact resistance over more than one order of magnitude by properly choosing the gate metal.

The number of organic semiconductors that exhibit electrical performance in thin film transistors equivalent to or bettering that of amorphous silicon has increased rapidly in recent years. However in order to realise the potential of large-area organic electronic devices, factors such as device-to-device performance variation and electrical bias stability must be addressed. One of the most promising organic semiconducting systems known to date is based on mixtures of a small-molecule with an amorphous conjugated polymer that acts as the binder. In these blends, the small-molecule component ensures high charge carrier mobility, while the addition of polymer allows greater control over film processing and microstructure engineering. The resulting polycrystalline film is beneficial for charge transport but also has the potential for causing significant device-to-device performance variation. The size distribution of crystals and associated grain boundaries are often attributed with adversely affecting charge transport in polycrystalline organic thin films.
Here we report on the nature of grain boundaries formed between crystalline domains in high mobility hole transporting semiconducting blends based on the small-molecule 2,8-difluoro 5,11 triethylsilylethynyl anthradithiophene (diF-TES ADT) and the polymer poly(triarylamine) (PTAA). Transistors based on these blend films exhibit high hole transporting characteristics with field-effect mobilities exceeding 2 cm²/Vs. Surprisingly, this very high value is found to be independent of the degree of film crystallinity, as equivalent performance is seen between films comprising small crystalline domains (<10 mu;m diameter) and large crystalline domains (10-200 mu;m). This key finding suggests that the traditionally expected transport-limiting effects associated with grain boundaries may not be a limitation, in terms of both performance and device parameter uniformity, for these organic blend semiconductor based transistors. Lateral-current conductive atomic force microscopy (C-AFM) was employed to study the nature of these complex grain boundaries, which were found in two distinct forms: (i) open (50-200 nm gap between grains), and (ii) closed (no observable gap between grains). While open boundaries are seen to exhibit a modest resistance to hole transport, closed boundaries appear to provide no measurable barrier. Indeed instead of a decrease in hole conductivity across these boundaries, a distinct enhancement in hole current is measured; a result not reported in any other organic semiconducting system. We therefore propose that these closed grain boundaries formed in the binary system employed here are indeed highly conductive. This finding helps explain the relative independence of hole mobility on film microstructure as well as its exceptionally high value which in some cases approaches that measured for transistors based on single crystals of diF-TES ADT.

Organic field-effect transistors (OFETs) are now being used in active matrix backplanes, radiofrequency identification (RFID) tags, and chemical and biological sensing. To more effectively design circuits containing OFETs, the threshold voltage (VT) should be precisely tuned. One method that has been utilized for shifting VT is electrostatic charging of the dielectric. In this work, Kelvin probe microscopy (KPM) was used to visualize the charge in lateral organic field-effect transistors (OFETs). Instead of the conventional vertical architecture, the devices were fabricated laterally. This lateral geometry exposes a clean cross-sectional view of the OFET with access to all the interfaces while keeping the OFET operational. The KPM method allows the dielectric/OSC interface of the OFET to be visualized without any alteration of the OFET, which is not possible in conventional vertical device architectures. These devices consisted of a polymer gate dielectric, n- or p-channel organic semiconductors (OSCs) and gold source and drain electrodes. The dielectrics used in this study were polystyrene and poly(3-trifluromethyl) styrene, while the OSCs used were p-type pentacene and n-type copper-hexadecafluroro-phthalocyanine (F16CuPc).
The charge storage in the lateral OFET was visualized with the KPM and could be correlated to a threshold voltage shift during the transistor operation. This stored charge, which occurred inside the polymer dielectric layer, may be due to conventional bias stressing of the transistor or as a result of embedding charges. This technique allows for the observation of charge distribution between the two dielectric interfaces at the gate and OSC. By varying the embedded charges we can see definite shifts in the VT of the lateral OFETs, which can be used to specifically tune the VT. Higher charging levels resulted in greater changes in surface potential, which resulted in larger VT shifts. For the p-type OFET we saw that negative charging embedded negative charges into the dielectric, which shifted the VT to be more positive, while positive charging embedded positive charges which shifted VT more negative. Devices with an air gap dielectric were also used instead of the polymers; but the residual charges remained in the semiconductor after charging. The positive or negative charges embedded inside the dielectric can be related to the trap energy levels, which were determined by thermally stimulated discharge current (TSDC).

Tribocharging of materials, charge accumulation on materials upon touching or rubbing, has been known for millennia. However, the complete understanding of the molecular mechanism of polymer tribocharging, which is an important phenomenon in polymeric device manufacture, was missing majorly because of the lack of instrumentation to visualize the molecular/nano level events on the polymer surfaces upon their charging. Recently, we were able to see the nanodomains of charges on one-component polymer surfaces, ¹ which enabled us to understand the molecular mechanism of contact-charging of these surfaces. ² Here we aim to visualize the charge formation, migration and dissipation on the surfaces of polymer blends (which are often used in electronic devices) composed of two polymers with different electrical properties e.g. an insulator such as polydimethylsiloxane (PDMS) and a conductor such as polyaniline, (PANI) by using Kelvin Force Microscopy, KFM (to map the electrical potential), Magnetic Force Microscopy, MFM (to map the magnetic domains) and PeakForce Quantitative Nanomechanical Measurements, PF-QNM (to map mechanical properties) on polymer surfaces. PF-QNM maps serve for straightforward identification of the conducting (PANI) and insulator (PDMS) domains in polymer blends, making use of mechanical property differences. KFM mapping of the identified domains provides information about how the electrical potential changes on these surfaces after tribocharging and MFM shows the behavior of the accompanying magnetic domains.
This approach can as well be extended to understand the behavior of charge on surfaces of various two- or more-component polymer blends and can be used to tailor the electrical properties of devices made from these materials.
Literature:
[1] a) H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, B. A. Grzybowski, Science 2011, 333, 308; b) H. T. Baytekin, B. Baytekin, J. T. Incorvati, B. A. Grzybowski, Angew. Chem. Int. Ed. 2012, 51, 4843; c) H. T. Baytekin, B. Baytekin, S. Soh, B. A. Grzybowski, Angew. Chem. Int. Ed. 2011, 50, 6766; d) B. Baytekin, H. T. Baytekin, B. A. Grzybowski, J. Am. Chem. Soc. 2012, 134, 7223.
[2] a) R. G. Horn, D. T. Smith, Science 1992, 256, 362; b) R. G. Horn, D. T. Smith, A. Grabbe, Nature 1993, 366, 442; c) Jacobs, H. O., Whitesides, G. M. Science 2001, 291, 1763.
This work was supported by the US Department of Energy, Office of Basic Energy Sciences as part of the Non-Equilibrium Energy Research Center (NERC), an Energy Frontier Research Center.

Organic semiconductors not only offer potential cost advantages relative to inorganic materials, but they can also exhibit novel physical phenomena. Singlet fission is a spin-allowed process whereby an initially created singlet exciton spontaneously splits into a pair of triplet excitons. This process is of interest as a way to make higher efficiency solar cells, but the detailed mechanism has yet to be elucidated. The existence of a magnetic field effect on the excited state dynamics is often taken as evidence for the existence of singlet fission. We have developed a detailed theory that defines the conditions under which a magnetic field gives rise to observable effects on the singlet fission rate and ensuing triplet state dynamics. Picosecond time-resolved photoluminescence experiments on crystalline tetracene and amorphous rubrene are shown to be consistent with our theoretical predictions. For tetracene single crystals, the frequency of quantum beats in the delayed fluorescence signal reflect changes in both the triplet pair sublevel spacing and the number of accessible triplet states. For amorphous rubrene, the sign and magnitude of the magnetic field effect on the prompt and delayed fluorescence signal is sensitive to both the timescale of observation and the details of exciton diffusion and sample morphology. In both materials, the interplay between fission and fusion reactions and exciton diffusion leads to complex kinetics that determine the overall yield of free triplets. One interesting result is that the two triplets produced by singlet fission remain correlated with each other over relatively long timescales, ranging from 10-100 ns depending on the material. The ultimate utility of singlet fission for photovoltaic devices will be determined by the ability to harvest the triplet pairs, and this work provides insights into how these pairs evolve in different materials.

Charge trapping is generally avoided in the designing of organic electronic devices such as transistors and photodetectors. Here we report a device design which takes advantage of charge trapping for a high gain in photodetector [1]. In a hybrid photodetetor device with structure of ITO/PEDOT:PSS/4,4&’-Bis[(p-trichlorosilylpropylphenyl)phenylamino]-biphenyl (TPD-Si2): Polyvinylcarbazole(PVK)/PVK:ZnO or poly-3(hexylthiophene)(P3HT):ZnO nanoparticles/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Al, the photogenerated electrons were trapped by the ZnO nanoparticles which were close to the cathode side due to the vertical phase separation with the polymer. The trapped electrons at the interface of nanocomposite and cathode shift the lowest unoccupied molecular orbital (LUMO) of the polymer downwards and align the Fermi energy of the nanocomposite with that of the cathode, thus enables a high hole injection through cathode side under reverse bias. Since one single trapped electron in ZnO nanoparticle can cause the injection of more than one holes under reverse bias, there is an internal gain coming from this trap-electron-induced-hole injection effect. On the other hand, the hole injection barrier at the cathode side is so high that almost no holes can inject in the dark. The large hole injection barrier at cathode side as well as the insertion of the electron blocking layer of TPD-Si2:PVK at the anode side give a very low dark current density of 136 nA/cm^2 at -9 V. The combination of low dark current of the photodiode and high responsivity (721-1,001 A/W) of the photoconductor leads to a high specific detectivity of 3.4 × 10^15 Jones at 360 nm at room temperature, which is 2-3 orders of magnitude higher than that of existing semiconductor photodetectors. The reported low cost thin-film nanocomposite photodetectors have great potential for the existing applications of weak ultraviolet detection and can potentially open new application opportunities because of their flexibility, light weight and printability.
1.Fawen Guo, B.Y., Yongbo Yuan, Zhengguo Xiao, Qingfeng Dong ,Yu Bi and Jinsong Huang, A nanocomposite ultraviolet photodetector based on interfacial trap-controlled charge injection. Nature Nanotech, Accepted, 2012.

Organic semiconductors are suitable candidates for printable, flexible and large-area electronics. Alongside the endeavor addressed to the development of better performing organic semiconducting materials and fabrication procedures, a notable effort is being devoted to the investigation of new strategies to confer a multifunctional nature to the employed materials.
In this work, we report on the engineering of an electronic structure in a semiconducting film by blending two molecular components, a photochromic diarylethene (DAE) derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT&’s hole transport. In our study, the blend between DAE derivatives featuring different energy levels with an organic semiconducting polymer such as P3HT is used as a bi-component film forming the electroactive layer of organic thin-film transistors (OTFTs).Taking into account the hole transport levels of P3HT, we designed and synthesized a DAE molecule, DAE_1, featuring different ionization energy (IE) in its open and closed form with respect to the IE of P3HT. To explore the role of the energy levels in the two different DAE isomers with respect to the IE of P3HT we have extended our study to DAE_2 with higher IE of both isomers. In particular, we focus on the morphology dependence of the switching properties with varying the annealing temperature of the films. GIXD, AFM, KPFM, Differential Scanning Calorimetry, UPS and temperature dependent electrical characterization of organic thin-film transistors are employed to shed light onto the morphological as well as energetic interaction between the two different blend components.
The device illumination at defined wavelengths enabled reversible tuning of the diarylethene&’s electronic states in the blend, which resulted in modulation of the output current. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. [1] This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits.
[1] E. Orgiu et al., Nature Chem., 2012, 4, 675-679.

Organic and metal oxide semiconductors (OS and MOS) are widely used as charge-transporting materials (CTMs) in organic and hybrid photovoltaic devices such as bulk-heterojunction and solid-state dye-sensitized solar cells (ssDSSCs). In such applications, the mobility of charge-carriers in the CTM should be sufficiently high to ensure their efficient collection following photogeneration. Because the charge density in a photovoltaic device varies with the applied bias, knowledge of the charge density dependence of the carrier mobility is essential to improving our understanding of device performance under different operating conditions. Unfortunately, it remains difficult to reliably measure the charge density dependence of the charge-carrier mobility of CTMs by conventional techniques, so that it has become common to assume the mobility of many widely used CTMs to be constant with charge density.
We present a new technique, termed “Transient Optoelectronic Mobility extraction by Absorption Spectroscopy” (TOMAS), for the measurement of the charge-carrier mobility as a function of the charge density for a variety of commonly employed organic (such Spiro-OMeTAD and Poly[3-hexylthiophene]) and metal-oxide (such as Titanium Dioxide) semiconductors in architectures identical to that of ssDSSCs and bulk-heterojunction solar cells. We employ transient absorption spectroscopy (TAS) to monitor the decay of the charge density after light excitation simultaneously with time-resolved conductivity measurements. Having measured the relevant extinction coefficients, the TAS measurement allows us to directly estimate the photo-induced charge carrier density at the same time as the conductivity of the material. This allows us to calculate a mobility at any point along the charge density decay curve within a range of charge densities found from short-circuit to open-circuit conditions in typical solar cell operation.
Our results demonstrate that the charge-carrier mobility of the materials employed in this study show significant charge density dependence over the operating range of the devices, with absolute values depending on the device architecture. By comparing the conductivities and mobilities of the materials as a function of charge density, we gain new insights into how the charge-transport mechanism limits the performance of ssDSSCs, and indeed how the series resistance varies in the solar cell as it is scanned across the power quadrant. Corresponding solar cells were prepared, and the importance of the charge-carrier mobility and HTM p-doping on device performance is demonstrated. This technique represents an important characterization tool for new and existing charge-transporting materials in organic and hybrid solar cells.

In conjugated-polymer:fullerene blend solar cells, free carriers are created when singlet excitons formed in the photoactive polymer donor diffuse to a nearby donor-acceptor interface and ultimately dissociate into free carriers. However, before dissociation can occur, the singlet exciton must pass through a sub-gap charge transfer exciton (CTX), or bound polaron pair state, where the electron and hole reside in different material phases. Specifically, in this work, donor-acceptor phase mixing and charge transfer excitons in bulk heterojunction solar cell films were imaged using confocal photoluminescence (PL) and Raman microscopy [1]. Spatially-resolved spectral analysis of PL was used to map fullerene diffusion and agglomeration due to thermal annealing as well as detect local changes in interfacial contact between donor, poly[2-methoxy-5-(3&’,7&’-dimethyloctyloxy-1,4-phenylene-vinylene] (MDMO-PPV), and acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), domains.
The results show that CTX emission was most intense at the periphery of micron-sized fullerene agglomerates, which correlates with fullerene depletion in the surrounding film during annealing. Additionally, spatial analysis of the CTX signal showed a red-shift in the onset of emission in PCBM rich areas of the film, consistent with bulk measurements, indicating that near-IR emission is due to the CTX rather than PCBM triplets. Raman scattering indicated that the polymer phase was essentially immobile during the annealing process and that fullerene agglomerates reside on top of a polymer-rich underlayer. Overall, we will show how spatially-resolved spectral analysis of PL emission, in conjunction with Raman spectroscopy, is an easy and informative way to assess phase mixing and changes in heterointerface density in conjugated polymer-fullerene blends due to processing.
[1] I. Riisness, C. Carach, and M.J. Gordon, Applied Physics Letters, 100, 073308 (2012).

It is well known that the efficiency of organic light-emitting diodes (OLEDs) decreases at higher current-densities. The three mechanisms that have been suggested in the literature are field-induced exciton dissociation, exciton-polaron quenching and exciton-exciton annihilation. For the case of phosphorescent OLEDs, triplet-triplet annihilation (TTA) has been argued to be the major cause or at least to be a significant contribution [1]. However, recently evidence has been presented suggesting that triplet-polaron quenching (TPQ) is the dominant mechanism [2]. The influence of field-induced dissociation is generally assumed to be small. The question arises what the interplay is between these mechanisms under realistic operating conditions and how this depends on the device parameters like layer thickness and dye concentration.
To answer this question, we have developed a new approach, by carrying out a kinetic Monte Carlo simulation of all processes in the OLED, including excitonics. The device is modeled as a three-dimensional simple cubic grid, in which each point represents a molecular site. The simulation includes the effects of the disordered energy level landscape and of possible dye aggregation on charge carrier transport, exciton generation under the explicit effect of the electron-hole Coulomb attraction, Foerster and Dexter exciton transfer, radiative and non-radiative exciton decay, exciton dissociation, TPQ and TTA.
Using this method, we have studied the roll-off as function of the layer thickness, dye concentration and exciton binding energy for a monochrome green OLED consisting of the green dye Ir(ppy)3 in a matrix of CBP, sandwiched in between alpha-NPD as hole transport layer and BCP and Alq3 as hole blocking and electron transport layers, respectively [3]. The simulation is based on experimentally accessible parameters obtained from the literature. The results support the point of view that TPQ, not TTA, is the main cause of the roll-off in the efficiency. Exciton dissociation is found to play a minor role; it slightly enhances the TPQ rate. Furthermore, we find that the calculated efficiency roll-off is excellently described in terms of an often-used empirical expression, containing the current density at which the efficiency has decreased by a factor of 2 and a steepness exponent as the only two parameters. The effect of a variation of the emissive layer thickness, dye concentration and charge carrier mobility is found to be described by a surprisingly simple relation, giving rise to design rules to optimize the efficiency.

We have long studied the relationship between crystal packing and device performance in acenes, demonstrating that two-dimensional pi-stacked arrays function best in transistor materials, while more one-dimensional arrays exhibit better performance in photovoltaic applications. We are now beginning to investigate the relationship of other structural parameters to device performance, requiring significant changes to our typical functionalization strategies to explore the broadest possible structure space and develop enhanced design rules. We have prepared soluble materials with significantly reduced symmetry, unusual variations in solubilizing substituents, dramatically increased conjugation length, and varieties of groups attaching the chromophore to the solubilizing moiety. We have studied the structure of these materials crystallographically, and determined their performance in simple devices. We will report how parameters such as crystallographic layering, unit cell anisotropy, and inter-layer steric interactions impact charge transport and device performance.

Systematic studies on the effect of dielectric in p-polymer (P3HT) and n-polymer (N2200) transport process were carried out. A range of dielectrics, which includes ferroelectric FE-PVDF, high-k paraelectric and low-k dielectrics were utilized for these studies. Temperature (T) dependent transport processes were studied in the range of 100 K < Tc < 400 K where Tc is the ferroelectric transition temperature. Switching and transient studies were performed for different combinations of dielectrics and semiconductors (p-type and n-type). Based on these results, we observe: (i) clear trend of weaker T dependence in FE-FETs due to polarization fluctuation dominated transport rather than the static dipolar disorder prevalent in high-k dielectrics [1] (ii) dielectric relaxation mechanism limited switching response below a characteristic channel length in case of FE dielectrics (iii) stronger effect of the dielectric relaxation for high-k dielectrics with random dipoles of block-co-polymer compared to homo-polymers. (iv) screening of the dielectric polarization effect to a large extent for polymers with conjugated core sheilded from the interface by long alkyl chains (as in the case of N2200).We suggest a mechanism to understand these results and discuss the implications in organic electronics.
Acknowledgement :SPS acknowledges CSIR for fellowship and KSN acknowledges DAE, Govt of India for funding.
Reference:
1. Satyaprasad P Senanyak, S Guha, K.S.Narayan ; Phys. Rev B, 115311,2012.

Current deep level transient spectroscopy (DLTS) with a bipolar rectangular weighting function in the unit of coulomb [1] has been applied to characterize traps in polymer-based organic light emitting diodes (OLEDs). OLEDs consist of the structure glass/ITO/fluorine-based polymer/LiF/Ca/Al. In order to distinguish between electron and hole traps observed in OLEDs, hole-only devices are fabricated consisting of the structure glass/ITO/fluorine-based polymer/Al. For OLEDs, the temperature-scan DLTS in the temperature range from 80 to 350 K reveals one broad peak around 160 K with the DLTS time constant of 91 ms and exhibits a sharp increase in intensity above 320 K suggesting deeper traps. A similar increase in intensity of temperature-scan DLTS above 320 K is also observed for hole-only devices with a broad peak around 180 K. Isothermal DLTS measurements, which are suitable for the decomposition of DLTS spectra with several traps, are performed for both devices at 250 and 340 K. Three hole traps are identified in hole-only devices, while additional three electron traps are identified in OLEDs. It is found that these traps are characterized by Gaussian distributions. Trap depths at the maximum of distributions are 0.25, 0.60 and 0.66 eV for hole traps and 0.19, 0.22 and 0.73 eV for electron traps. Other parameters of the Gaussian trap distributions including full width at half maximum and trap concentration will be presented. The authors acknowledge Sumitomo Chemical Company Ltd. for supplying us with the polymer material.
[1] Y. Tokuda, T. Shibata, H. Naitou, T. Katou and M. Katayama, Materials Research Society Fall meeting, U3.11, 2011.

Organic electronics has been heavily investigated in recent years as it shows tremendous potential for large-area device fabrication using low processing temperatures on flexible substrates. In the operation of an organic thin film transistor (OFET) the mobile charges are confined to within the very first few layers of the semiconducting material.^1,2 Hence, the deposition of the semiconducting material on the dielectric surface and the assembly of semiconductor at this interface is of great importance to the performance of devices, in particular the quality of the very first monolayer.
In this report we describe how the deposition conditions can influence thin film transistor device performance in the context of film morphology and growth from one monolayer up. Two alkyl substituted small molecules (C6-PTTP* and C6-NTCDI**) were deposited on high and low energy surfaces at varying deposition temperatures under vacuum. OFET devices with active layers that consist of just one molecular layer up to bulk, multilayer films, were fabricated. We discuss how the deposition conditions influence the thin film morphology and device performance and compare it to PTTP and NTCDI derived semiconductors that are directly attached to the surface via SAM deposition.
The vacuum deposited films were investigated via atomic force microscopy (AFM) and x-ray diffraction (XRD). The results can be understood in the context of the adhesive energy between the small molecule and the surface using simple surface energy calculations.³ The increased energy given to the molecules by vacuum deposition on higher temperature substrates leads to improved multilayer film structure and the highest reported mobility for the compound C6-PTTP has been achieved. We show that high fidelity self assembled monolayers on the surface of SiO2 consistently lead to higher mobilities, due in part to the quality of resulting thin film at the semiconductor/dielectric interface on this low energy surface.
* 5,5'-bis(4-hexylphenyl)-2,2'-bithiophene
**N,N&’-bis(hexyl)naphthalene diimide
[1] M.Mottaghi, G.Horowitz, Organic Electronics, 2006, 7, 528
[2] F.Dinelli, M.Murgia, P.Levy, M.Cavallini, F.Biscarin, Phys. Rev. Lett. 2004, 92, 116802
[3] C.Effertz, S.Lahme, P.Schulz, I.Segger, M.Wuttig, A.Classen, C.Bolm, Adv. Funct. Mater. 2011, 22, 415.

Implementation of complementary metal-oxide-semiconductor (CMOS) with organic semiconductors brings many benefits into integrated circuits such as feature-size scalability, high noise immunity, and low static power dissipation. However, CMOS requires both n- and p-channel field-effect transistors and thus two different organic semiconductors are used in many cases. In this study, we show doping an organic semiconductor is a useful method to achieve both n- and p-channel field-effect transistors using a single organic semiconductor. For doping, we co-evaporate fullerene (C₆₀), a well-studied organic semiconductor for n-channel field-effect transistors, and molybdenum trioxide as a p-type dopant. Although p-type doping C₆₀ is difficult due to its deep-lying highest occupied molecular orbital (HOMO) of 6.4 eV, we demonstrate p-channel C₆₀ field-effect transistors by doping with molybdenum trioxide with a large electron affinity of 6.7 eV. By changing doping concentration, the device performance of the p-channel C₆₀ field-effect transistors, such as mobility, threshold voltage, and on-off ratio is varied in a controlled manner. The hole field-effect mobility of the doped C₆₀ films grown at room temperature ranges from 1x10^-3 to 8.5x10^-3 cm²V^-1s^-1 at doping concentrations of 10~38 mol%. This work provides guidelines to use charge transfer doping to obtain both n- and p-channel field-effect transistors with a single organic semiconductor.

Characterizing organic semiconductors at the single molecule scale has greatly enhanced our understanding of intermolecular interactions, revealing new approaches to controlling film structure, while probing the electronic properties of organic interfaces. Pentacene has long been a model system for such studies. Here, though, we study monolayer and bilayer films of picene, a structural isomer of pentacene. We grow these films on Ag(111) by thermal evaporation in UHV and measure them in-situ using a low-temperature STM at ~77 K. Topographic STM measurements were used to establish the film structure. Much like pentacene, picene bonds with its molecular plane parallel to the surface, but unlike pentacene, picene forms dimers. Moreover, the work-function shift amounts to almost 1 eV (up to 2x the value for pentacene), suggesting that the molecule-surface distance is closer in this case. At the same time, the splitting of the LUMO, LUMO+1, and LUMO+2 molecular orbitals is somewhat larger than even semiempirically calculated values for the gas-phase. These measurements will be compared to first principles calculations made with the HSE functional to understand changes to the electronic structure with adsorption and the role of van-der-Waals interactions between flat-lying picene molecules.

Carrier transport in organic semiconductors (OSCs) has traditionally been modeled by thermally activated hopping between localized states.¹ In this model, three material properties dictate carrier mobility: the carrier localization length (α), the intrinsic attempt-to-jump rate (v₀), and the degree of energetic disorder, as quantified by the standard deviation (a_DOS) of the carrier density of states (DOS). Because each parameter directly influences mobility, it has proven difficult to derive independent values for them from fits to measured electrical conductivity (σ). This has led to general uncertainty regarding carrier transport that is both quantitative (e.g., reported values of v₀ vary by more than 7 orders of magnitude depending on what values are assumed for the other parameters^1,2) and qualitative (e.g., the effect of molecular structure on mobile carrier localization is often unclear). Furthermore, as OSCs become increasingly utilized for device applications in which engineering carrier concentration is critical, it is important to quantitatively assess the effects of dopants on carrier localization and the other hopping parameters.
The Seebeck coefficient (S) provides an additional transport parameter that is distinct from σ but likewise relatively simple to measure and which contains different dependences on the hopping parameters. Here we present a model for S and σ in the case of a Gaussian DOS that gives their separate explicit analytic dependences on a_DOS and α so that, when S and σ are both measured, uncertainties regarding the hopping parameters can be resolved.³ We then use this model in combination with experimental S and σ data to derive a_DOS, α, and v₀ for the prototypical high-mobility small-molecular OSC pentacene and the prototypical high-mobility polymer OSC poly(3,4-ethylenedioxythiophene) (PEDOT). This method indicates α to be larger than the molecular spacing for both materials, which the model further shows to be consistent with a very weak (band-like) dependence of mobility on temperature. We then show how bulk doping and a field-effect geometry impact the hopping parameters of pentacene differently, and analyze the impacts of additives and dopant type on the hopping parameters of PEDOT.
[1] R. Coehoorn, W. F. Pasveer, P. A. Bobbert, and M. A. J. Michels, Phys. Rev. B72, 155206 (2005).
[2] W. F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P. A. Bobbert, P.W. M. Blom, D. M. de Leeuw, and M. A. J. Michels, Phys. Rev. Lett.94, 206601 (2005).
[3] G.-H. Kim and K. P. Pipe. Phys. Rev. B86, 085208 (2012).

Organic semiconducting materials have garnered significant interest in recent years, largely due to their compatibility with large-area, flexible electronics. The most intensely-researched materials are single-component systems, such as pentacene, its derivatives, and its analogues. In contrast to these, materials composed of two compounds - one a donor and the other an acceptor - promise to yield interesting properties in terms of both fundamental physics and potential applications. Often, while the parent compounds of such materials are intrinsically unipolar semiconductors, the charge transfer complex resulting from their combination may exhibit novel properties; they can be ambipolar, metallic, or even superconducting. Broadening the field of organic electronics to include in-depth studies on such materials thus promises to increase its influence on modern materials research and applications.
In this presentation we describe the following binary compounds (in order of donor-acceptor): trans-stilbene - 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (STB-F4TCNQ), perylene - 7,7,8,8-tetracyanoquinodimethane in donor:acceptor ratios of 3:1 (P3T1), 2:1 (P2T1), and 1:1 (P1T1), and dibenzotetrathiafulvalene - 7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ). Materials were grown as single crystals by physical vapor transport and from solution, which permits access to their intrinsic electronic properties. The crystals were employed as the active layer in two types of devices - a diode-like “sandwich” structure and the organic field-effect transistor (OFET). This allowed us to extract two types of mobility - the bulk mobility evaluated from the space charge limited current measurements using the Mott-Gurney Law, and the surface mobility from OFET data. For the latter structure both top and bottom contacts were used in conjunction with various electrode materials. Electron, hole, and ambipolar conduction were detected depending on the contact and semiconductor material, as well as the donor-to-acceptor ratio. Properties such as mobility and resistivity were determined as a function of temperature. This yielded several interesting results, including observation of electronic transitions. Differential scanning calorimetry was employed to investigate the presence of solid-solid phase transitions. The nature of the transitions was examined in detail using low-temperature X-ray diffraction; this also allowed us to correlate molecular packing with electronic properties. All experimentally-determined values were compared to predicted electronic properties.

Semiconducting polymers have proven successful in applications ranging from thin film transistors to solar cells. Despite this progress, it is still difficult to dope polymers to control their electrical conductivity. Improvement in our ability to control the electrical conductivity is essential for emerging applications such as thermoelectrics. Understanding charge transport in polymeric materials doped by molecular charge transfer is complicated by their intrinsic disorder. Thermopower measurements allow direct access to the entropy per carrier in the material as an additional path elucidating charge-transport mechanisms. Here, we present a study of electrical transport and thermopower for three high hole-mobility alkylated polythiophene thin films: poly 3-hexylthiophene (P3HT), poly (2,5-bis(3-n-alkyl-2yl)thieno[3,2-b]thiophene) (PBTTT), and poly (2,5-bis(3-n-alkyl-2yl)thieno[3,2-b]thiophene), and poly(2,5-bis(thiphen-2-yl)-(3,7-ditridecantyltetrathienoacene)) (P2TDC13FT4), as a function of doping with the acceptor small molecule tetrafluorotetracyanoquinodimethane (F4TCNQ). By studying these materials as a function of processing conditions, we are able to correlate the changes in Seebeck coefficient with the electrical conductivity. Importantly we have also characterized the thermal stability of these blends at elevated temperatures and find enhanced stability for PBTTT and P2TDC13FT4 relative to P3HT. These results suggest a pathway for optimization of semiconducting polymers for applications in thermoelectric applications.

Contact charging of polymers 1 can have drastic negative effects on performance of the electrical devices bearing polymers (e.g. organic semiconductors) as active elements. Although contact charging is such a common phenomenon, prevention of charge on polymers still lacks a systematic approach. Such an approach can only be possible through clear understanding of the mechanism of charge formation on polymer surfaces. However, despite centuries of research, mechanism of contact-charging on polymers remained on debate until now. Some recent works from us 2 and others 3 gave insight into how charging on these surfaces is taking place. In these studies, it was shown that polymer charging results from breaking of bonds on polymer surfaces, and is associated with material transfer. Here we show that the overall charging of the surfaces can be controlled/manipulated at the molecular level. The results of these are monitored at the nano-level by edge modalities of Atomic Force Microscopy (AFM) such as Kelvin Force Microscopy (KFM), Peakforce QNM imaging and Magnetic Force Microscopy (MFM).
This new molecular control of the chemical species on polymer surfaces will help to avoid contact-charging on polymers that are extensively used in new generation devices, and prevent damage due to contact-charging on these devices.
Literature:
[1] a) R. G. Horn, D. T. Smith, Science 1992, 256, 362; b) R. G. Horn, D. T. Smith, A. Grabbe, Nature 1993, 366, 442; c) Jacobs, H. O., Whitesides, G. M. Science 2001, 291, 1763.
[2] a) H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, B. A. Grzybowski, Science 2011, 333, 308; b) H. T. Baytekin, B. Baytekin, J. T. Incorvati, B. A. Grzybowski, Angew. Chem. Int. Ed. 2012, 51, 4843; c) H. T. Baytekin, B. Baytekin, S. Soh, B. A. Grzybowski, Angew. Chem. Int. Ed. 2011, 50, 6766; d) B. Baytekin, H. T. Baytekin, B. A. Grzybowski, J. Am. Chem. Soc. 2012, 134, 7223.
[3] a) W. R. Salaneck, A. Paton, J. Appl. Phys. 1976, 47, 144; b) T. A. L. Burgo, T. R. D. Ducati, K. R. Francisco, K. J. Clinckspoor, F. Galembeck, S. E. Galembeck, Langmuir, 2012, 28, 7407; c) G. S. P. Castle, J. Electrostat. 1997, 40-41, 13; d) M. Williams, AIP Adv. 2012, 2, 010701; e) M. Sow, R. Widenor, A. Kumar, S. W. Lee, D. J. Lacks, R. M. Sankaran, Angew. Chem. Int. Ed. 2012, 51, 2695.
This work was supported by the US Department of Energy, Office of Basic Energy Sciences as part of the Non-Equilibrium Energy Research Center (NERC), an Energy Frontier Research Center.

Silver nanowires (AgNWs)/poly-(3,4-ethylenedioxythiophene/polystyrene sulphonate) (PEDOT: PSS) composite films as conductive electrode for OTFTs were prepared, and their optical and electrical properties were investigated. The conductive composite films used in this study afforded low sheet resistance of < 140 Omega;/sq and transmittance as high as 70 % in the visible region. For the composite film with 0.1 wt% of AgNWs, contact resistance as low as 2.7 × 10⁴ Omega; cm was obtained, as examined by Transfer length model (TLM) analysis, and work function of the corresponding film was 5.0 eV. Furthermore, the composite films were employed as source and drain electrodes for top-gate/bottom-contact organic thin-film transistors (OTFTs) based on solution-processed 5,11-bistriethylsilylethynyl anthradithiophene (TES-ADT) as organic semiconductor, and the resulting device showed high electrical performance with carrier mobility as high as 0.21 cm²/Vs.

Operational stability in organic thin-film transistors (TFTs) remains as a major issue to be resolved before organic TFT devices can be commercialized. In this study, we succeeded in suppressing threshold voltage shifts under continuous gate bias voltage stress by employing multi-gate dielectric layers consisting of parylene-C and amorphous fluoropolymers. Positive voltage shifts resulting from the parylene-C dielectric layer [1] were able to compensate for the negative voltage shifts caused by the fluoropolymer dielectric layer. We systematically varied the thicknesses of the two dielectric layers and evaluated their relationship with threshold voltage shifts under a continuous negative gate-source voltage.
Bottom-gate, top-contact pentacene TFT devices were fabricated using vacuum thermal evaporation processes. For the gate dielectric layers, a parylene-C (KISCO, dix-C) layer and amorphous fluoropolymer (DuPont^TM, Teflon^® AF 1600) were formed on the gate electrode. Several devices having two dielectric layers with different thicknesses were prepared. Next, a pentacene semiconducting material was deposited to form a 50-nm-thick channel layer on the multi-gate dielectric layers. Lastly, gold was deposited to form 50-nm-thick source/drain electrodes.
We considered the ratio of potential voltage drop in the parylene-C layer (V_P) to the gate and source voltage (X = V_P/V_GS), and investigated the relationship between X and change in threshold voltage (ΔV_TH) under constant bias stress voltages. ΔV_TH after continuous bias voltage stress conditions (V_GS = -40 V, V_DS = 0 V) for 3600 seconds increased from -1.0 to +4.2 V as X increased from values of 0 to 1. We found a proportional relation between X and ΔV_TH after 3600 seconds, which indicates that the bias stress effect can be controlled by systematically changing the thicknesses of the two dielectric layers.
TFT devices having multi-gate dielectric layers with optimized thicknesses (X = 0.34) exhibited remarkable stability under the long-term continuous gate-bias voltage stress. ΔV_TH after applying stress voltage for 48 hours was only -0.44 V without changes in other electrical parameters such as on/off ratio and field-effect mobility.
[1] Fukuda et al., Phys. Status Solidi A, 209, 2073 (2012).

The electrical performance of a thin film in an organic thin film transistor (OTFT) depends on the film morphology. A mixed solvent system was demonstrated to provide a useful method for controlling the morphologies of organic semiconductor layers in OTFTs. Control over the morphology of a 2-(6-((2-ethylhexyl)oxy)naphthalen-2-yl)anthracene (EH-NA) film using a THF/chloroform mixed system (with ratio of 1:5 w/w) improved the device electrical performance properties.

We report on how a copper atom in a copperphthalocyanine (CuPc) molecule can be decoupled from its environment. This is realized by trapping the molecule between two adjacent nanowires that are 1.6 nm apart on Au modified Ge(001) substrates. Using low-temperature scanning tunnelling microscopy and spectroscopy the structural and electronic properties of CuPc in this stable ‘molecular bridge&’ configuration have been studied. Charging of the core of the CuPc molecule, i.e. the double ionized Cu atom, occurs at sample biases larger than 4.7 V which is a sign of Coulomb blockade effect.

We have succeeded in improving electrostatic discharge (ESD) robustness of organic thin-film transistors (OTFTs) with polymer encapsulation layer on top of them. In particular, we obtained 2-V operation OTFTs withstanding over ESD stress of 100-V, which is 5 times as large as OTFTs without encapsulation. It has been reported that detaching of source/drain electrode is serious damage after applying 14-V ESD stress into the organic circuits, in previous our work[1]. Therefore we make encapsulation layer on top of the pentacene transistors with poly chloro para xylylene(Parylene) by chemical vapor deposition (CVD) to suppress that electrode comes off apart.
Organic transistors with SAM gate dielectrics were manufactured by vacuum evaporation and solution processes. First, we depo sited 25-nm-thick aluminum as a gate electrode on polyimide film substrate by thermal evaporation through the shadow mask. An aluminum oxide layer was prepared on the gate electrode by oxygen plasma treatment. Plasma power was 150 W and the exposur e time is 10 min. Next, we dipped the substrate into a solution of 5-mM n-octadecylphosphonic acid in isopropyl alcohol for
16 hours to form uniform 2-nm-thick SAMs [2]. The combination of SAMs on aluminum oxide functions as the gate dielectric layer.
30-nm thick pentacene was deposited by thermal evaporation to form the channel layer. Finally, we deposited 50-nm-thick Au by thermal evaporation onto the pentacene surface to make source/drain electrodes. The channel width and length are 500 mu; m and 40 mu;m, respectively. After that, we deposited 1-mu;m-thick parylene by CVD. To contact underneath electrode, we made via hole using green lazer.
[1] Wen Liu, Juin J. Liou, Kazunori Kuribara, Kenjiro Fukuda, Tsuyoshi Sekitani, Takao Someya, J. Chung, Yoon-Ha Jeong, Zhix in Wang, and Cheng-Li Lin, IEEE ELECTRON DEVICE LETTERS, 32, 967-969 (2011). [2] Hagen Klauk, Ute Zschieschang, Jens Pflaum and Marcus Halik, Nature, 445, 745-748, (2007).

Organic thin film transistors have currently been the subject of intensive research activity due to their promising attributes for the manufacturing of low-cost, large-area and flexible electronic devices. In particular, acenes and heteroacenes are efficient hole-transporting (p-type) semiconductors, whereas rylene diimides, especially perylene and naphthalene derivatives, are so far the most successful air-stable electron-transporting (n-type) materials.
In this study, we have synthesized new polymers for organic thin film transistors(OTFTs). The ability to extend the π system of polymers not only leads to broadened light absorption spectrum but also increases the electron affinities to facilitate electron injection and transport. After annealing, polymer films exhibit excellent OTFT characteristics under ambient stability.

Since superconductivity from potassium doped C₆₀ was discovered, alkali metal doping into organic molecular crystal has been gathering great interest both to realize potential replacement of inorganic superconducting oxides as well as to understand fundamentals involved with the generation of superconducting states in organic crystal. Recently, it has been reported that picene and phenanthrene exhibit superconductivity with relatively high critical temperature when they are doped with alkali metal. However, experimental reproducibility is not yet achieved due to the lack of stoichiometry control between alkali metal atoms and organic molecules, which restricts further study on their electronic states and transport property. Here, we present an electrical device technique to monitor and control the alkali metal doping level in organic thin films. The electrical current change across organic thin film is measured under the exposure of continuous and pulsed alkali metal vapor. The clear peak or plateau is observed from current - time curve, corresponding to the stoichiometric number of doped alkali metal atoms, which is confirmed by Raman spectroscopy. More details about the overall electrical transport property of stoichiometric alkali metal doped organic thin film device will be discussed.

We present a novel strategy for analyzing bias-stress effects in organic field-effect transistors (OFETs) based on a four-parameter double stretched-exponential formula. The formula is obtained by modifying a traditional single stretched-exponential expression comprising two parameters (a characteristic time and a stretched-exponential factor) that describe the bias-stress effects. The expression yields two characteristic times and two stretched-exponential factors, thereby separating out the contributions due to charge trapping events in the semiconductor layer-side of the interface and the gate-dielectric layer-side of the interface. The validity of our method was tested by designing two model systems in which the physical properties of the semiconductor layer and the gate-dielectric layer were varied systematically. We found that the gate-dielectric layer, in general, plays a more critical role than the semiconductor layer in the bias-stress effects, possibly due to the wider distribution of the activation energy for charge trapping. Furthermore, the presence of a self-assembled monolayer further widens the distribution of the activation energy for charge trapping in gate-dielectric layer-side of the interface and causes the channel current to decay rapidly in early stage. The novel analysis method presented here enhances our understanding of charge trapping and provides rational guidelines for developing competent OFETs with high performance.

We report a new polymeric semiconductor, PDPPDTSE, which is composed of DPP and selenophenylene vinylene selenophene, with a high field-effect mobility achieved through intermolecular donor-acceptor interactions. The field-effect mobility of OFET devices based on PDPPDTSE by spin-casting was found to be 4.97 cm2/Vs over predecessor polymeric semiconductors.

One of the important goals for organic molecule-based structures is to enhance electrical conductivity. Compared with inorganic semiconductors, such as Si and Ge, of which charge carriers can be extrinsically introduced by doping with elemental impurities, not many efficient ways are known to introduce charge carriers into organic structures. Direct metal or polyelectrolyte doping on organic structure or polarity induction by inserting polar molecule in organic structure are known to increase electrical conductivity. However, these methods are frequently struggled from the migration of metal and difficulty of dopant stoichiometric analysis as well as reproducibility. Here, we introduce a new strategic concept to increase charge carrier population by employing air stable radical in the skeletal backbone of 1D organic wire. We synthesized 2,2-diphenyl-1-picrylhydrazyl (DPPH#9679;) single crystalline 1D wire by solvent vapor annealing (SVA) process, followed by ex situ radical quenching using ascorbic acid to induce breakdown of delocalized pi-electron system. Electrical device measurements show that as-grown DPPH 1D wire exhibits very low electrical conductivity, while the quenched DPPH crystal by ascorbic acid shows about two orders of magnitude increased conductivity. More details about synthesis, characterization, device measurement, and proposed mechanism on the remarkably increased electrical conductivity will be discussed.

We used numerical methods (forward time iteration of rate equations) to study one dimensional ion transport in planar junctions between electrolyte and a conducting polymer. The transport of electronic and ionic carriers is defined by drift-diffusion equations.
The simulation study captures the essential physics of the following experiment: In a planar junction of electrolyte/Pedot:PSS film when a positive voltage is applied in the electrolyte cations are injected in the film and holes are extracted at the ohmic contact (Au electrode). Since cations compensate the sulphonate anions (on the PSS chain), the film is being dedoped and no reinjection of holes occurs to maintain charge neutrality. We have developed an analytical model to describe the behavior of this junction and to extract ion mobility. The model consists of two resistors in series; one ionic (dedoped area) and one electronic (rest of the film). Since the ionic resistor is much bigger than the electronic one, a fair approximation will be that all the potential drops there. Ion mobility can be extracted from the predicted dependence of the drift length with time.
In order to validate this model we treat the simulation data in the same way for different input values. We see that for sufficient fields the analytical model describes fully the behavior of the junction and ion mobility and density can be extracted from drift length and current versus time respectively.

Charge trapping and charge transport properties of triplet host doped with blue phosphorescent emitters were investigated using FIrpic, FNIrpic and FIr6. Hole only and electron only devices of blue emitting layer were fabricated to study the charge trapping & charge transport properties of blue triplet emitters. Hole trapping by dopant material was not observed in all blue phosphorescent emitters, but electron trapping was significant in all phosphorescent emitters. Electron trapping was the most significant in FCNIrpic dopant, while FIr6 and FIrpic showed similar electron trapping effect. However, electron transport property was better in FCNIrpic than in FIrpic & FIr6. Therefore, it can be concluded that FCNIrpic traps electrons, but transports electrons effectively compared with other dopant materials. The device performances of blue PHOLEDs were well correlated with the charge trapping and charge transport of dopant materials.

Remarkable charge mobilities (> 5 cm2/Vs) have been observed in organic field-effect transistors when active layers are comprised of single crystals. The high mobilities can be a consequence of eliminating grain boundaries which can be detrimental for charge transport in some systems. Nevertheless, processing can strongly affect measured mobilities; for example, it has been reported that charge mobilities in devices comprised of organic single crystals are higher when the active layer is deposited through thermal evaporated versus solution-processing. To explore the underlying phenomena which modulate charge mobilities as a function of processing, we measure the cumulative disorder in single crystals using X-ray diffraction. Our studies examine the relationship between solution processing (i.e. solvent choice and casting temperature), disorder, and charge mobilities within organic single crystals.

Organic Field-Effect Transistors (OFETs) have attracted much attention recently due to their advantages, such as low cost, easy solution processing, and availability for flexible devices. To achieve high performances OFETs, including the advantages, various efforts have been applied to organic semiconductors, dielectrics and electrodes. Among those efforts, especially, charge injection from electrodes is the one of key factors for the high performance OFETs. It could be controlled by interface engineering, inserted interface layer between organic semiconductor and electrodes. In this study, the high performances top-gated and bottom contact (TG/BC), [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) OFETs have been researched by introducing conjugated polyelectrolyte interfacial dipole layers, water-soluble polyfluorene (WPF), at electrodes/organic semiconductor interfaces as an electron injection layer. As inserting WPF layer to PCBM OFETs, significant enhancement was observed such as increased electron charge-carrier mobility (mu;e) and decreased threshold voltage. It was resulted from reducing the energy barrier between electrodes and semiconductors. These results were confirmed by measuring contact resistance and work-function of source and drain electrodes by transmission-line method (TLM), and UPS respectively. In addition, it was observed that in PCBM OFETs the device performance could be affected by the different ethylene-oxide side chain effect of WPF derivatives.

The performance of the organic devices is determined by both on intrinsic properties of organic semiconductor active layers and their interfaces facing on each layer. Especially in Organic Field-Effect Transistors (OFETs), it is obvious that efficient control over the semiconductor-dielectric interfaces have played a key role for enhancing charge transport in the active channel. In this study, we present remarkable enhancement of both hole injection and transport for OFETs based on the n-channel dominant poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) through the use of high-k P(VDF-TrFE) gate dielectric. Top-gated P(NDI2OD-T2) OFETs with P(VDF-TrFE) dielectrics showed mu;FET,h(~0.1 cm2/Vs) almost 100× greater than those with conventional poly(methyl methacrylate) (PMMA) dielectric (2×10-3 cm2/Vs). Next, we successfully developed ambipolar CMOS inverters (high voltage gain ~30) and Ring Oscillators (ROs) (oscillation frequency, fosc ~16.7 KHz) by novel soft etching methods for selectively patterning the gate dielectric using inkjet printer. Through this methodology, high quality gate dielectric layer can be patterned to ambipolar ICs as favorable charge carrier type in OFETs. ( P(VDF-TrFE) as dielectrics of P-channel OFET [Pull-up TR], and Polystyrene (PS) /P(VDF-TrFE) as dielectrics of N-channel OFET [Pull-down TR] )

Polymeric semiconductors have received extensive investigation over the few decades due to modulation of optical, electrochemical, physical and chemical properties by introduction of different chemical functional groups. Among the various functional groups, fluorinated molecules have attractive attention owing to the unique properties of fluorine atom. Fluorine atom can improve thermal stability because of the unique physical property of C-F bond, and also fluorinated alkyl chains with hydrophobicity prevent the penetration of water and oxygen. Intermolecular interactions are the most important factor which affects the molecular arrangement in organic crystal engineering. The fluorinated alkyl chains have weak intermolecular interactions owing to a symmetric distribution of charges, and lead to self-organization of fluorinated alkyl chains. The self-organization of fluorinated alkyl chains can lead to a more planar conformation of conjugated polymer backbone. Especially, the highly ordered molecular crystal of conjugated polymers is considered to induce efficient charge transport. In this presentation, we describe synthesis and characterization of a novel poly(semifluorododecyl-thiophene) composed of a semifluorinated alkyl chains and thiophene moieties to realize its full advantages. Furthermore, we demonstrated the bilayer ambipolar transistor and integrated circuit using chemical orthogonality of fluorinated polymer.

Organic Field Effect Transistors (OFETs) have shown a tremendous increase in performance during the last years (H. Klauk, Chemical Society reviews 39, 2643-66, 2010). However, despite this progress, the operational frequency and achievable saturation currents of OFETs are still limited by the low charge carrier mobility of organic semiconductors. These intrinsic limits render applications demanding high-frequency operation difficult for conventional OFETs.
Although it is possible to increase driving currents and transit frequency by aggressively scaling the channel length of the organic transistors (Ante et al., Small 8, 73, 2012), downscaling is only possible by advanced structuring techniques. This increases the production costs of OFETs and limits the advantages of organic electronics as low-cost technology. In this presentation, Vertical Organic Triodes (VOT) as a means to overcome this dilemma are discussed. In these vertical transistors, the channel length is defined by the thickness of the active layer, allowing for a control of the channel length with nanometer precision without the need for costly structuring techniques.
Our approach is based on a metal base organic transistor (Fischer et al. Journal of Applied Physics 111, 044507, 2012; Fischer et al., accepted by APL), consisting of a metallic emitter electrode, a thin layer of C60, a porous metal base electrode made of Al, a second C60 layer, and a collector electrode. Whereas the current injected at the emitter is controlled by the voltage applied between emitter and base, the majority of this current is transmitted through the porous base and collected by the voltage between base and collector. It is shown that by using doped injection contacts, these devices reach a comparable performance to inorganic bipolar transistors in terms of amplification and maximum driving currents. In particular, currents in excess of 1A/cm^2, a transconductance of 30mS, and operational frequencies close to the MHz regime are obtained while the driving voltage is less than 3 V.
Our C60-VOTs exhibit an increase in performance by an annealing step, leading to a tremendous increase in charge carrier transmission through the porous base electrode. We address this to morphological changes at the interface between base electrode and the top as well as the bottom C60 layer. Furthermore, the base electrode is electrically passivated by a native oxide, intentionally created by air exposure of the device after fabrication of this electrode. Our results prove charge accumulation in front of the thin oxide layer which facilitates a high transmission of charge carriers through the base electrode to the collector electrode.
Our progress in the field of metal base organic transistors reveals the possibility to switch high currents at high speed and opens new applications like DC-DC converter operating in the upper mA range or amplification for small loudspeakers at low voltages.

Organic semiconductor materials are interesting alternatives to inorganic semiconductors in applications where low cost, flexible or transparent substrates, and large area format is required. Currently they have been incorporated into organic thin-film transistors, integrated display driver circuits, photovoltaics artificial electronic skin, and radio frequency identification tags. One of our fundamental interests is to understand how we can ultimately perform rational design of organic semiconductors. In this talk, I will present recent results on understanding of molecular design rules for achieving efficient charge carrier transport and controlled growth of organic semiconductors.

Semiconducting polymers have a semicrystalline microstructure, where they are neither completely crystalline nor completely amorphous. Using electroluminescence, I will show that in such a microstructure transport occurs in a network of percolating crystallites, hence disorder in these crystallites limits transport. Using synchrotron radiation we measure disorder in a number of high-performance polymers and note that it depends on molecular weight. At high molecular weight, disorder saturates to approximately the same point for all high-performance polymers we investigated. Hence we hypothesize that in high molecular-weight semiconducting polymers the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for sustaining efficient long-range charge transport. This generalization explains the seemingly contradicting high performance of recently reported, strongly disordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials.

Device measurements in transistor based on semiconductor polymers are based on phenomenological models of transport that make a number of assumption on the energy distribution of states, their localization and their coupling. We show in this contribution how it is possible to describe from atomistic simulation the same parameters and to define a more rigorous connection between the polymer structure and electron transport properties. The difference between semicrystalline and amorphous polymers is discusses together with the effect of regioregularity with exemplifications from the most common polymers currently under investigation (P3HT, PPV derivatives, PBTTT). The results also provide insight on the chemical structure of low energy states.

Inducing Fermi-level pinning in an organic semiconductor at the interface to another material results in accumulation or depletion of electrons at the interface. Examples for the resulting band-bending at a pinned organic/electrode interfaces and organic/inorganic semiconductor heterojunctions will be introduced. The key roles of inorganic semiconductor surface termination and doping level on level pinning will be discussed. The energy level alignment at organic/organic semiconductor heterojunctions is essentially independent of deposition sequence, as long as supporting electrodes do not induce energy level pinning. When the energy levels become Fermi-level pinned due to an initial electronic non-equilibrium situation, long-range charge transfer can result. This creates an electric field right at and beyond the heterojunction. This effect is exemplified for prototypical donor and the acceptor interfaces, which are relevant for photovoltaic cells. Furthermore, it is demonstrated that light absorption induced charge separation at an organic heterojunction can be measured directly with photoemission spectroscopy.
Regarding charge transport, control of the organic semiconductor density of states is essential in (opto-) electronic devices. By engineering the electronic structure of two organic components, a photochromic diarylethene derivative and a semiconducting poly(3-hexylthiophene) (P3HT) matrix, it is possible to introduce photo-switchable and bistable energy levels in the P3HT hole transport level manifold. Organic field effect transistors with such two component blends exhibit reversible current variation upon light illumination with defined wavelength and thee photo-response is in the microsecond range. This approach is fully compatible with ink-jet and roll-to-roll printing on large area, enabling the design and fabrication of a new generation of multifunctional devices.

The bulk and interfacial electronic structures of organic materials are essential information to discuss and understand the properties of organic devices. So far, UV photoemission spectroscopy (UPS) has been widely applied to investigate the electronic structures in relation to organic light-emitting diodes, transistors, and solar cells. Conventional UPS is a powerful technique, but there remains some problems and limitations; sample charge-up, sensitivity, and probing depth. The first problem limits the thickness of sample typically below several tens nm. The second problem restricts the detection limit of observable density of states (DOS) of organic samples; energy states with extremely low DOS like gap states cannot be detected by conventional UPS. The third problem makes UPS is surface sensitive technique, which is not suited to explore buried interface. In this paper, we will report on our recent efforts to overcome these problems of UPS. The main topics are as follows.
Recently we succeeded to overcome the charge-up of organic sample by using photoelectron yield spectroscopy (PYS) and angle-resolved photoelectron spectroscopy (ARPES) with special tactics to relief sample charging. PYS was not disturbed by charge-up if the sample is negatively biased enough to emit photoelectron. The PYS results of organic single crystals (rubrene, anthracene) and very thick polymer films (PET, Nylon) will be presented. For UPS, the charge-up of rubrene single crystals was much reduced by visible laser illumination to enhance photoconductivity. High-resolution ARPES measurements along all the symmetry directions of the surface Brillouin zone of the rubrene single crystal will be also reported.
Concerning the sensitivity problem, recently Ueno and Kera et al demonstrated that monochromatized light source without satellite line enables to detect very weak DOS of a few monolayer thick samples with very low background. We extended the technique by using low energy photon (5-8eV) with extremely low stray light through double monochromator. This method enabled us to detect bulk gap states in thick rubrene and C60 films. In addition, the improvement of sensitivity can reduce the photon intensity to reduce the sample charge-up effect. PYS measurements measured by fA level current detection and pulse count detection by electron multiplier also enabled us to detect gap states in pentacene films and insulating polymer for tribo-charging. The above technique has another advantage to investigate buried interfaces: The reduction of charge-up, high sensitivity, and longer probing depth due to larger mean free path for slower electrons enable the detection of buried interfaces under thick organic layer. The results of rubrene/Au and rubrene/C60 interfacse will be reported.

Understanding the role of intermolecular interaction on through-space charge transfer characteristics in π-stacked molecular systems is central to the rational design of electronic materials. However, a quantitative study of charge transfer in such systems is often difficult because of poor control over molecular morphology.
In this study, we use the core-hole clock implementation of resonant photoemission spectroscopy to study the femtosecond charge-transfer dynamics in cyclophanes, which consist of two precisely stacked π-systems held together by aliphatic chains. We study two systems, [2,2]paracyclophane (22PCP) and [4,4]paracyclophane (44PCP), with inter-ring separations of 3.0 and 4.0 Å, respectively. We find that charge transfer across the π-coupled system of 44PCP is 20 times slower than in 22PCP. We attribute this difference to the decreased inter-ring electronic coupling in 44PCP.
We are able to quantitatively relate these charge transfer times to π-π coupling by controlling both inter-ring distance (and hence coupling) and the interaction of molecule to metal. These measurements illustrate the use of core-hole clock spectroscopy as a general tool for quantifying through-space coupling in π-stacked systems.
[1] Batra, A., G. Kladnik, et al. (2012). "Quantifying through-space charge transfer dynamics in π-coupled molecular systems." Nature Communications 3: 1086

Recent advances in developing donor-acceptor conjugated copolymers have led to great performance improvement in both organic photovoltaic cells and field-effect transistors (FETs). In contrast to the extensive spectroscopic studies on the photoinduced charge transfer and separation processes in these copolymers, little has been done to probe their charge transport properties on a microscopic scale. In this work, we combine charge modulation spectroscopy (CMS), photo-induced absorption (PIA) and chemical doping spectroscopy to interrogate separately the nature of hole and electron transport in a recently developed copolymer poly(N-alkyl diketopyrrolo-pyrrole (DPP)-dithienylthieno[3,2-b]thiophene (DTT)) (PDTP), which exhibits hole mobility up to 1.12 cm2V-1s-1 in a FET configuration. It is found that both hole and electron polarons exhibit two-dimensional delocalzation due to the strong intermolecular coupling. Yet the degree of delocalization is lower for electrons, indicating influence from deep electron trap states. Interestingly, a new charge induced transition with relatively long time response is observed, which has not been reported before in homopolymers. Temperature and gate-voltage dependent CMS results suggest that this new transition is unlikely originated from bipolarons. According to quantum chemical calculations, this new transition might be related to the existence of stagger-dimer stacking in copolymers due to the asymmetrical volume fractions of donor and acceptor moieties. The implication of these observations will be discussed in the context of the device performance, and the strategies to manipulate the charge transport properties of copolymers through molecular design will be addressed.

Charge and energy transfer across interfaces are fundamental processes in organic semiconductor based electronics and optoelectronics. In organic photovoltaics, photocurrent generation often requires the dissociation of Frenkel excitons into free electrons and holes at donor/acceptor interfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between an electron and a hole, resulting in an energetic trap at the interface [Phys. Rev. Lett. 2008, 101, 196403]. Using model organic semiconductor interfaces and femtosecond nonlinear laser spectroscopies, we provide real time views on how the initial Frenekl exciton dissociation leads to hot charge transfer excitons across the interface [Nature Mater. 2012, 11, in press]. We show how hot charge transfer exciton cooling and collapse of charge separation sets the fundamental time limit for competitive charge separation. We also discuss competitions between charge and energy transfer channels [Science 2011, 334, 1541-1545; J. Am. Chem. Soc. 2012, 134, in press].

Practical interest in PEDOT:PSS is mainly driven by its widespread application as solution processable electrode layer in organic electronic devices. Scientifically, the transport properties of this material are as intriguing as puzzling despite substantial investigations. Here, we unravel the mechanism and magnitude of the in-plane conductivity of PEDOT:PSS films of thin films of different PEDOT:PSS ratios, with and without high boiling solvent (HBS). Combining transport measurements and TEM imaging we find that charge transport takes place in a 3D network built up from quasi-1D filaments.
For native, i.e. non-HBS-treated PEDOT:PSS of all studied ratios, the temperature dependence of the conductivity is well described by a stretched exponential. The stretching exponent 0.5 indicates quasi-1D variable range hopping (VRH). Analysis of the field dependent conductivity shows a behavior that is consistent with quasi 1-D VRH. For native PEDOT:PSS the conductivity prefactor vs. PEDOT loading follows a power law with power 3.5 over three orders of magnitude. This is characteristic of a percolating network in the critical regime. Combined, these observations suggest that conductance takes place via a percolating network of quasi-1D filaments. Using transmission electron microscopy (TEM) filamentary structures are indeed observed both in solution and in dried films. For HBS-treated PEDOT:PSS, the temperature dependence of the (enhanced) Ohmic conductivity still indicates quasi-1D VRH, but the low characteristic temperature T0 indicates that the system is close the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with PEDOT loading, indicating the conduction is no longer limited by a percolation of filaments. The lack of observable changes in TEM upon addition of the HBS suggests that the changes in conductivity are not due to large morphological modifications but rather due to a smaller spread in the conductivities of individual filaments, or a higher probability for neighboring filaments to be connected.

Along with recent remarkable progress in the development of high-mobility organic semiconductors, high-speed and low-voltage organic transistors is desired to realize many attractive applications, such as flexible logic circuits for active-matrix displays, and logic processers for wearable smart sensors with microcomputers. It is necessary for high-speed operations to realize high carrier mobility even in short-channel devices, therefore the contact resistance between an organic semiconductor and electrodes should be minimized. We have reported that organic single-crystal field-effect transistors (SC-OFETs) having ‘micro&’ air-gap structures yield high effective mobility even with the channel length L of as short as several mu;m. High hole mobility of 10 cm2/Vs in rubrene field-effect tramsistors (FETs) and electron mobility of ~ 4 cm2/Vs in PDIF-CN2 FETs are demonstrated for L=5 mu;m using SiO2/Si substrates. In the air-gap structures, hole (or electron) can be injected through molecularly-flat organic crystal surfaces contacted electrostatically with electrodes. The contact resistances in these devices are estimated to be as low as ~ 0.5 kOhm cm at VG of -4 V for rubrene devices. In a new device structure with L=4.5 mu;m in which the gate parasitic capacitance was reduced using insulator structures, the cutoff frequency of the rubrene air-gap device was estimated to be as high as 25 MHz for VD of -15 V, which is the fastest speed reported for p-type OFETs operating in ambient conditions.
To produce large output current and reduce the operating voltage, it is desired to increase the density of carriers to be injected in organic semiconductors. In this study, we report air-gap SC-OFETs gated with ionic liquids, in which the carrier density in a semiconductor channel is highly increased to ~1.2 mu;F/cm2. Short-channel rubrene air-gap SC-OFETs were fabricated with L=4.5 mu;m and then ionic liquid was injected into the gap space between an organic single-crystal semiconductor and a gate electrode using capillary force. Ionic liquid was held in the gap space due to capillarity. Using N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13-TFSI) as an ionic liquid, effective mobility in the rubrene devices was measured to be ~ 1 cm2/Vs with the applied drain and gate voltages of below 1 V. The cut-off frequency of the transistors was measured to be 0.5 MHz with the drain voltage of as low as -1 V and the gate voltage of -0.2 V. The present results demonstrate that even higher-speed organic transistors can be realized with shorter devices operating at extremely low voltages below 1 V for future applications to organic complementary circuits.

The performance of organic electronics devices is governed by the microscopic details of charge transfer processes between localized states. The transition rate for a single hop of a charge carrier in a semiconducting polymer is assumed to be thermally activated. As the temperature approaches absolute zero, the predicted conductivity becomes infinitesimal small in contrast to the measured finite conductivity. The semi-classical description breaks down, because even at absolute zero ground-state oscillations exist that allow nuclear tunnelling through classical barriers. Here we present a uniform description of charge transport in semiconducting polymers based on a full quantum mechanical treatment of the system. To suppress the omnipresent disorder we analyse electrical transport in semiconducting polymers at high carrier density. The energy barriers to be overcome then arise only form polarization of the surrounding phonon bath. The resulting expression for the macroscopic current shows a power-law dependence on both temperature and voltage. The predictions are experimentally verified using chemically doped in-plane diodes and ferroelectric field-effect transistors.

The realization of organic injection laser is of interest for those who are studying in the field of organic optoelectronics. Although a number of important findings have been reported on materials and device structures, organic injection laser has yet to be demonstrated. One of the reasons of the difficulties is the low charge density in the bulk undoped organic materials, and thus it remains difficult to achieve high conductivity and high current density.
In this study, we adopted polymer-based light-emitting electrochemical cells (LECs) as a platform device for constructing polymer injection lasers. The LECs are unique devices which a high-charge density p-i-n junction can be formed just by the application of the voltage. However, although significantly high conductivities have been reported in LECs, they haven&’t been considered for the injection laser thus far. This may be because of the lack of uniformity in the active layers due to the strong phase separation between the light-emitting polymers and the polyelectrolyte, which causes unfavorable scattering of the emitted light for laser action.
Here, we prepared a LEC in which the active layer is enough uniform to observe optically pumped amplified spontaneous emission (ASE) by utilizing a polymer-compatible ionic liquid as an electrolyte. The active layer of our LEC is a blend film of a light-emitting polymer of poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) and an ionic liquid of tetradecyltrihexylphosphonium(trifluoromethylsulfonyl)amide [P66614-TFSA] (10:1 ratio). The blend film was uniformly transparent without distinct phase separation, thus it behaves as a good optical slab waveguide. We achieved reasonably low ASE threshold of 4.1 mu;J cm-2 in the blend film which is similar to the value in the neat F8T2 film. For the electrical pumping, we fabricated a planar device with gold contacts of which the interelectrode distance of 10 mu;m. At room temperature, the linear shaped emission started to observe from 2.6 V, which is close to the bandgap of F8T2 of 2.4 eV. This implies charge carrier injection and transport are very efficient due to the high charge density in the active layer. The achieved current density at 6 V was 36 A cm-2. Furthermore, a combination of frozen-junction and pulse-driving techniques realized significantly high current density over 1,000 A cm-2, which is exceeding the estimated current density for lasing. Although the obtained spectra have not yet showed ASE characteristics even at the high current density, we think the reasonably low ASE threshold and the significantly high current density in the ionic liquid-based LECs strongly accelerates the development of polymer injection lasers.

Mixed conductivity (electronic / ionic) in conducting polymers plays a key role for a variety of organic electronic devices including light-emitting electrochemical cells, dye-sensitized solar cells, batteries, electrochromic devices and biosensors. Although electronic transport in organic electronic materials is relatively well understood (Gaussian disorder, correlated disorder, etc), a fundamental understanding of ion transport is missing, mainly due to the lack of a direct method to assess ion mobility.
We have developed a new experimental technique that relies on the electrochromic phenomenon and allows us to estimate ion mobilities in conducting polymers. Using one dimensional planar junctions between a conducting polymer and various electrolytes we were able to inject common ions and directly observe their transit through the film.
We have measured the mobilities of proton, metal ions and organic ions in PEDOT:PSS. In all cases the mobilities were high and close to mobilities in water, which was consistent with the large swelling due to hydration of the film. Crosslinking the film decreased its swelling and the ion mobility. In contrast to PEDOT:PSS, PEDOT:TOS expresses poor ionic conductivity. Hydroscopic additives such as mixture of peptides and proteins enhanced ionic conductivity. The comparison of ion mobility in PEDOT:TOS with various ratios of these additives will be discussed.

Solution-processable polymeric thin-film transistors (PTFTs) are being investigated for various applications such as large-area, low-cost, and flexible electronics based on electrical and material properties. Hitherto, there have been very few polymer semiconductors reported to date with mobilities in excess of 1 cm2/V-s. One of promising approaches to realize high mobility polymers is to design copolymers with reinforcing intramolecular interactions via packing, which results in enhanced molecular pi-orbital overlap. One such family of copolymers is based on a donor-acceptor architecture and utilizing the diketopyrrolopyrrole (DPP) acceptor block. Recently, we have obtained high device performance in solution-processable PTFTs consisting of the furan version of DPP with C20 alkyl chain and thiophene block with mobilities of 2-8 cm2/V-s, which is compatible with those reported thiophene version of DPP with C20 alkyl chain. We will also evaluate charge transport at high carrier densities employing electrolytic gating. Studies of charge transport can provide important information in understanding the origins of high mobility in some of DPP-based systems. Whereas quasi-DC experiments provide only a partial picture charge transport and device physics in high mobility DPP based FETs, our non-quasi-static measurements allow time-resolved charge transport studies including the computation of carrier velocity distributions. We employed a 4-point-probe configuration to measure field-effect mobility in the linear region to enable more accurate activation energy measurements, especially at low temperatures. We also compare the activation energy vs. field-effect mobility in a few important polymer semiconductors to gain a better understanding of transport of DPP systems and make appropriate comparisons.

Light emitting organic field-effect transistors (OFETs) are being investigated as a potential light source. For light emission both holes and electrons have to be present in the semiconductor. In the ambipolar regime these charges are accumulated simultaneously by biasing the gate in between the source and drain voltage. Surprisingly, reported light emissions are often strongest when the ambipolar OFETs are operated in the unipolar regime. This is unexpected, in the unipolar regime the light emission should be completely suppressed because then only one type of charge carrier is accumulated.
Here we investigate an electroluminescent diketopyrrolopyrrole copolymer. The potential in the transistor channel is probed by scanning Kelvin probe microscopy as a function of gate bias. The measurements reveal that the recombination zone is instable in the ambipolar regime. We show that this instability is a result of the presence of injection barriers and demonstrate the relation to the anomalous unipolar light emission from the OFET.
The photocurrent and electrical transport have been measured and numerically analyzed. The highest electroluminescence intensity was obtained when the transistor was biased in the two unipolar regimes. We show that the unipolar light emission is quantitatively explained by injection of minority carriers into deep tail states of the semiconducting polymer. This injection starts to dominate when the electrode Fermi level sits about midgap, i.e. when substantial injection barriers are present. The density of the injected minority carriers is small. Hence they are relatively immobile and they recombine close the contact with accumulated majority carriers. The unipolar light output is characterized by a rather constant efficiency that is independent of gate bias. We argue that light emission from OFETs predominantly originates from the unipolar regime when the charge transport is injection limited.

Efficient charge injection is an important issue for organic field-effect transistors (FET). Various approaches can be used to optimize injection of either holes or electrons. However, for ambipolar devices it is more difficult to improve injection of both charge carriers at the same time. Here we demonstrate a simple process to significantly improve ambipolar charge injection in bottom contact/top gate polymer field-effect transistors by adding single-walled carbon nanotubes (SWNT) to the semiconducting polymer at concentrations well below the percolation limit. Such polymer/carbon nanotube hybrid systems are easily produced by selective dispersion of SWNT in the conjugated polymer solution by ultrasonication. Even at very low nanotube concentrations the charge injection of both holes and electrons, for example, into poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene) (PFO) is significantly enhanced leading to reduced threshold voltages and lower contact resistances than in FETs with pristine polymer films. This effect can be extended to other polymer systems such as polythiophenes and P(NDI2OD-T2). It can be maximized by patterning layers of pure carbon nanotubes onto the injecting electrodes before spincoating the pristine polymer semiconductor leading to almost ohmic contacts for polymers, which usually show only highly Schottky barrier limited charge injection. This improved injection of holes and electrons allows for a much wider range of accessible polymers for ambipolar and light-emitting transistors.

Poly(9,9 '-dioctylfluorene-co-benzothiadiazole) (F8BT) is currently one of the most promising material for use as active layers in polymeric electronic devices, such as polymer light-emitting diodes (PLEDs) and the polymer field effect transistor (P-FET). It is well known that both polymer structure and dynamics affects either the luminescence or transport properties in thin films, making worthwhile to investigate them. Molecular relaxation and Dynamical aspects of F8BT were first investigated by Dynamical Mechanical Thermal Analysis (DMTA) measurements, Diferencial Scanning Calorimetry (DSC) and 1H Nuclear Magnetic Resonance (NMR) experiment. The results revealed the presence of two main relaxation process, which occurs at about 225 K (β-relaxation) and 370 K (α -relaxation). The crystallization were also measured by DSC and Wide Angle X-Ray Diffraction (WAXD), ocurring around 470 K. The nature of the molecular processes of such relaxations were investigated by specific NMR experiment, such as DIPSHIT and CONTRA. The results showed that, in the temperature range of 220 to 373 K, the lateral chain execute molecular rotations with average correlation times ranging from 10-4 - 10-7 seconds. On the other hand, from 300 to 350K the backbone carbons execute slow libration motions with reorientation angles that increase as a function of temperature. Those results could be correlated with the Current-Voltage characteristcs of both thick (3 µm) and thin (200 nm) ITO/F8BT/Al devices carried out a several temperature (70 until 490 K) and modeled assuming a Gaussian density of states. Also, AC measurements were performed, revealing strong influence of the molecular relaxation on the real impedance, specially for the α -relaxation and crystalization. The behavior of the drift mobility was also studied by Time of Flight tecnhque (TOF) and Photo-CELIV measurements. The result showed relatively abrubt change on the activation energy near both relaxations (β and α) and also near the crystalization. Our results suggest that the electronic states are strogly influenciate by the molecular processes observed for F8BT.

Recently, it was reported an interesting effect observed on a dicyanomethylene-substituted thienoquinoid oligomer: a three-orders of magnitude increase in the electron mobility upon annealing of the spin-coated film. This increase in charge-carrier mobility was also followed by the appearance of an absorption peak, around 1000nm, not seeing in the solution UV-vis spectrum. These experimental results suggest that the annealing procedure is changing the way these molecules pack and this new order is responsible for the observed change in mobility and absorption spectrum. Here we plan to investigate this structure-property relationship by means of quantum-chemical calculations. We will simulate aggregates of two or more molecules and compute relevant parameters to describe the electron mobility (such as the electronic coupling) and the absorption spectrum (such as the excited states) in order to rationalize the structure-property relationships observed experimentally. Exploratory TD-DFT calculations indicate that there is an excited state around 1000nm when a dimer is used in the calculation, indicating that intermolecular interactions play an important role in the observed effect.

Information and communication technology (ICT) is calling for solutions enabling lower power consumption, further miniaturization and multifunctionality requiring the development of new device concepts and new materials. A fertile approach to meet such demands is the introduction of the spin degree of freedom into electronics devices, an approach commonly known as spintronics. This already lead to a revolution in the information storage (GMR readheads) in the last decades. Nowadays, the challenge is to bring spintronics also into devices dedicated to logics, communications and storage within the same material technology [1].
In this context the electric control of the magnetoresistance represents one of the most promising issues enabling both further miniaturization and multifunctional operation of spintronic devices. Likewise, also the electronics community is committed to follow the Moore&’s law, and one of the promising approaches is the use of arrays of crossbar memristors capable of information processing and storing (‘stateful&’ logic) [2].
We show that an electrically controlled magnetoresistance can be achieved in organic devices [3] combining magnetic bistability (spin-valve) and resistance switching effects. In such devices the GMR effect can be turned ON and OFF by a programming bias that sets the device in low or high resistance state respectively. The magnitude of the GMR depends on the bias history and can be recovered up to the pristine value [4].
We show [4] that such devices operate like Magnetically Enhanced Memristors (MEM). MEMs can be operated in both memory and logic gate applications merging together spintronic and electronic approaches towards new future device concepts [5].
[1] Awschalom, D. D. & Flatte, M. E. “Challenges for semiconductor spintronics”, Nat. Phys. 3, 153 (2007)
[2] Borghetti, J. et al. “Memristive' switches”, Nature 464, 873 (2010)
[3] Dediu, V. A. et al. “Spin routes in organic semiconductors”, Nat. Mater. 8, 707 (2009)
[4] Prezioso, M. et al. “A Single-Device Universal Logic Gate Based on a
Magnetically Enhanced Memristor”, Adv. Mater. 2012, DOI: 10.1002/adma.201202031
[5] Sanvito S. & Dediu V. A. "News from the Organic Arena", Nature Nanotechnology 7, 696 (2012)

We report here on the experimental observation of electron spin transport in tris-(8-hydroxyquinoline) aluminum (Alq3) over a distance of 150 nm in contrast to previous reports where holes were the spin carrier, even though electron transport is preferred in Alq3. In order to facilitate the injection of spin polarized electrons into and extraction from Alq3, a thin layer of Ca was inserted between the ferromagnetic electrodes and Alq3 in spin-valve devices. In addition to the Co/Ca/Alq3/Ca/NiFe spin-valve structures, Co/Alq3/NiFe devices were fabricated in parallel where hole spin injection is expected. A mechanical shadow mask and thermal evaporation in a vacuum deposition system were used for the device fabrication and the mask was changed in a glove box under Ar gas without exposure to ambient air between deposition steps in order to avoid any oxidation and contamination. The key difference between the two spin valve test structures is the introduction of a 1 nm thick Ca layer between the 150 nm thick Alq3 film and ferromagnetic transition metals (Co and NiFe). We demonstrate that the insertion of this low work-function alkali metal enables electron spin injection into Alq3 because the Fermi level of pristine Ca (2.9 eV) is closely aligned with the lowest unoccupied molecular orbital (LUMO) of Alq3 (asymp; 3.0 eV). In contrast, hole spin injection is dominant in the Co/Alq3/NiFe structure where the Fermi levels of Co (asymp; 5.0 eV) and NiFe (4.5 - 5.0 eV) are more closely aligned to the highest occupied molecular orbital (HOMO) of Alq3 (asymp; 5.8 eV). The devices with Ca as an intermediary electron injection layer show symmetric current-voltage (I-V) characteristics, which indicates that the charge injection properties are identical on either side of the Alq3. By using this band-engineered contact scheme, we observe positive magnetoresistance as large as 4% at 4.5K attributed to electron spin transport. In contrast, we observe asymmetric I-V characteristics in the Co/Alq3/NiFe devices and no spin-valve effect. We conclude that the improved band alignment and Alq3&’s higher electron mobility relative to hole transport are the main reasons for the clear magnetoresistance signal in the devices with Ca which is lacking in the devices without Ca layers. The observation of positive magnetoresistance in the Co/Ca/Alq3/Ca/NiFe test structure further supports our conclusions. In this presentation, we will discuss in detail our experimental procedure and observations in our Alq3 spin-valve devices, as well as the experimental challenges that arise due to device degradation.

ABSTRACT: Very recently it was discovered that the spatially varying fringe fields emanating from an unsaturated ferromagnet layer can have a large effect on the magneto-conductive properties of an organic semiconductor [1]. In contrast to the normal magnetic field effect observed in organic semiconductors [2], which may be caused by the hyperfine fields varying roughly at the hopping length scale [3], the fringe fields vary on a length scale almost two orders of magnitude larger than the hopping length.
We have incorporated fringe fields into our recent theory [4] of organic magneto-resistance to explain the puzzling experimental phenomena. Analysis of the fringe field distributions and their gradients shows that the spatial variation of the fringe fields across a typical hopping distance can be comparable to that of the hyperfine fields. Calculating the magneto-resistance within the framework of [4] yields several results in general agreement with the experiment such as how the fringe fields change the magneto-resistance line shape, and the dependence of the magneto-resistance on the distance between the organic semiconductor and the ferromagnetic layer.
We acknowledge support from an ARO MURI and stimulating discussions with P. A. Bobbert.
[1] F. Wang et al. Phys. Rev. X 2, 021013 (2012)
[2] O. Mermer et al. Phys. Rev. B 72, 205202 (2005)
[3] P. A. Bobbert et al. Phys. Rev. Lett. 99, 216801 (2007)
[4] N. J. Harmon and M. E. Flatté Phys. Rev. Lett. 108, 186602 (2012); Phys. Rev. B 85, 075204 (2012); Phys. Rev. B 85, 245213 (2012)

In the last years, various magnetic field effects in the current and light emission of devices of organic semiconductors, such as OLEDs, have been found. Consensus is growing that these effects are caused by 1) the existence of pairs of spin-carrying species (electrons, holes, triplet excitons) with very weakly coupled spins, 2) magnetic field-dependent interconversion between the spin states of these pairs, 3) the occurrence of reactions of these pairs towards reaction products with rates that depend on the spin states. The consequence of 1)-3) is that the amounts of produced reaction products depend on the applied magnetic field B, provided that the reaction is sufficiently slow as compared to the electronic spin precession time. This then leads via various routes to the observed effects. The magnetic field-dependent interconversion can be caused by hyperfine coupling of the electron spin to nuclear spins, exchange effects, electronic dipole-dipole interactions, differences in g-factors of the different spin-carrying species, and magnetic-field gradients. The most interesting aspects are that the effects can occur at room temperature and at very small magnetic fields. There are striking analogies between the magnetic field effects observed in organic devices and equivalent effects studied in the field of spin chemistry. In this talk I discuss various theoretical aspects of organic magnetic field effects and analogous effects in spin chemistry. In addition, I address the question how the size of the effects can be increased, so that they can be technologically exploited. One way to go is the realization of one-dimensional organic systems, which are theoretically predicted to have the potential of showing extremely high magnetoresistance. Such systems have recently been fabricated by inserting organic molecules into the channels of a zeolite crystal, forming isolated molecules wires. The current through such wires can be practically switched off at room temperature by a magnetic field of only a few mT. The effect can be explained by a spin-blockade in the creation of doubly negatively charged singlet bipolarons on the molecules. It should be possible to realize similarly large effects in specific donor-acceptor copolymers. This would lead to a convenient exploitation of organic magnetic field effects with many possible technological applications.

Magneto-dielectric functions refer to the phenomena where electric and magnetic parameters are internally coupled. Magneto-dielectric functions provide an effective mechanism to develop internally coupled electric and magnetic orders towards the development of multiferroic properties. Primarily, magneto-dielectric functions can be obtained by combining magnetic and electric structures through materials design, namely an approach of using ground states. Recently, we observed that optically-generated intermolecular excited states can exhibit a magneto-dielectric phenomenon in an organic semiconducting donor:acceptor system containing poly(N-vinylcarbazole) (PVK) as an electron donor and 1,2,4,5-Tetrachloro-3-nitrobenzene (TCNB) as an electron acceptor. This experimental observation presents a new approach of using intermolecular excited states to generate magneto-dielectric functions. Using optically-generated intermolecular excited states can lead to three major unique properties. First, it allows an optical tuning on magneto-dielectric functions by using photoexcitation. Second, the long-range Coulomb interaction between intermolecular excited states can be used as a convenient tool to influence short-range spin-spin interaction within individual intermolecular excited states. Third, it can enable convenient engineering method of using materials mixing technique to control intermolecular excited states in generating magneto-dielectric functions. This presentation will discuss the fundamental mechanisms of electric-magnetic coupling in intermolecular excited states based on π-π and π-d interactions.

In this talk several important advances in the field of Organic Spintronics and magnetic field effect (MFE) of organic films and optoelectronic devices that have occurred during the past two years from the Utah group will be surveyed and discussed. (i) Organic Spintronics: We demonstrated spin organic light emitting diode (spin-OLED) using two FM injecting electrodes, where the electroluminescence depends on the mutual orientation of the electrode magnetization directions [1]. This development has opened up research studies into organic spin-valves (OSV) in the space-charge limited current regime. (ii) Magnetic field effect: We demonstrated that the photoinduced absorption spectrum in organic films (where current is not involved) show pronounced MFE [2]. This unravels the underlying mechanism of the MFE in organic devices, to be more in agreement with the field of MFE in Biochemistry. (iii) Spin effects in organic optoelectronic devices: We demonstrated that certain spin ½ radical additives to donor-acceptor blends substantially enhance the power conversion efficiency of organic photovoltaic (OPV) solar cells [3]. This effect shows that studies of spin response and MFE in OPV devices are promising.
References: [1] Nguyen et al., Science 337, 204 (2012); [2] Gautam et al. PRB 85, 205207 (2012); [3] Zhang et al. Nature Commun. 3, 1043 (2012).
In collaboration with T. D. Nguyen, E. Ehrenfreund, B. Gautam, and T. Basel
* Supported by the DOE grant 04ER46109 [1]; NSF Grant # DMR-1104495 and MSF-MRSEC program DMR-1121252 [2,3].

Spintronic applications based on organic semiconductors (OSCs) have been widely investigated because of not only their ease, large-area, and solution processability but also their exceptionally long spin lifetimes due to their light element compositions. Traditional studies have focused on discovering magnetoresistance using a multilayer structure, like an organic spin valve, where a spin polarized current is injected and transport through OSCs. Unlike the spin polarized current, a pure spin current is a flow of electrons&’ angular momentum without a simultaneous flow of one&’s charges, where carrier spins mainly play a key role in transmitting, processing and storing information. However, to exploit this potential, a method for direct conversion of spin information into an electric signal is indispensable. Here we realize the spin-to-charge conversion in a solution-processed conducting polymer, PEDOT:PSS. We show that the pure spin current can be induced by pumping of spins through exciting ferromagnetic resonance in an adjacent magnetic insulator, Yttrium Iron Garnet, and that this pure spin current generates an electric voltage across the polymer film. All experimental characteristics of the generated voltage are fully consistent with it being generated by the inverse spin Hall effect in the conducting polymer. In contrast to inorganic materials, the conducting polymer displays coexistence of high spin-current to charge-current conversion efficiency and long spin lifetime. Our discovery not only provides a basis for studying the process of spin-to-charge conversion in organic materials, but also opens a route for a new generation of molecular-structure-engineered spintronic devices, that could lead to important advances in plastic spintronics.

Recently organic semiconductors become a promising candidate for spintronics due to their weak spin-orbit coupling and hyperfine interaction. The reasonably large magnetoresistance (MR) at low temperature has been observed in the organic spin-valves (OSV) from both molecular and polymeric materials [1, 2]. However, the MR effect at room temperature (RT) was scarcely achieved on the OSV. The mechanism on spin transport is still far-from well understood[1], for example there is a clear debate on whether the observed MR is due to carrier transport within the organic layer or tunneling through the thin regions.
In this work, we have prepared the vertical spin-valves using fullerene (C60) as the spacer [2]. The device structure is La0.67Sr0.33MnO3 (LSMO)/C60 (L nm)/AlOx (1 nm)/Co, (L = 5-40 nm). In contrast to negative MR displayed on the LSMO/C60/Co OSV&’s reported[3], all of our devices exhibit a positive MR (ΔR/R) and especially an appreciable value at RT (3.65% and 0.75% for the C60 thickness of 8 nm and 28 nm, respectively). A possible mechanism on the MR polarity is proposed. Furthermore, we observe a largely temperature independent MR on the OSV&’s with a thin C60 spacer (e.g. 8 nm) however a strong temperature dependence of MR curve in the case of a thick C60 layer (ge; 20 nm). Based on systematic temperature- and thickness- dependent MR and I-V characterization, we have obtained clear evidences that the MR of C60-based spin-valves originates from the tunneling of spin-polarized electrons through the organic layer at the low thickness of C60, however at the thickness of ge;20 nm the spin-polarized electrons are injected into and subsequently hopping transport within the C60 spacer. Film micro-structures (crystallinity, surface morphology, etc.) of C60 are also examined and it is found to have a remarkable impact on MR value.
REFERENCES:
[1] V. Dediu, L. E. Hueso, I. Bergenti and C. taliani, Spin routes in organic semiconductors, Nat Mater., 2009, 8(9), 707
[2] M. Gobbi, F. Golmar, R. Llopis, F. Casanova and L. E. Hueso, Room -temperature spin transport in C60- based spin valves, Adv. Mater., 2011, 23, 1609
[3] R. Lin, F. Wang, M. Wohlgenannt, C. He, X. Zhai and Y. Suzuki, Organic spin-valves based fullerene C60, Synth. Met. 2011, 161, 553

Accessing the still unexplored dynamical properties of electrons residing at hybrid interfaces between ferromagnetic metals and organic molecules will provide fundamental information about the microscopic mechanisms behind the extremely promising spin filtering properties of such hybrid systems [1,2, 3]. We use two-photon photoemission (2PPE) to optically pump and probe a hybrid electronic state forming at the prototypical interface between cobalt and the metallorganic complex Tris(8-hydroxyquinolinato)aluminium (Alq3). We generate a transient spin polarization in the hybrid interface state, and follow its behavior in four dimensions: energy, time, spin and momentum. We find that due to the strong hybridization between Co and Alq3, electrons are confined at the Co-Alq3 interface for times in the range between 0.5-1 ps, and that the confining potential is strongly spin dependent. Such spin trapping behavior explains the microscopic origin of the spin-filtering properties of hybrid interfaces and opens new avenues for engineering efficient next-generation spintronics devices based on tunable organic spin filters.
[1] Barraud et al., Nature Physics 6, 615-620 (2010)
[2] Atodiresei et al., Phys. Rev. Lett. 105, 066601 (2010)
[3] Cinchetti et al.,Phys. Rev. Lett. 104, 217602 (2010)