The electronic structure of, and charge carrier transport in, organic semiconductors (small molecules or polymers) are greatly affected by the presence of electronic states that tail into the gap, above and below the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO), respectively. The origin of these states is generally not known, but ranges from molecular defects and impurities to static and dynamic disorder, grain boundaries an other film imperfections. Deep states with energy more than 100-200 meV away from the mobility edge act as traps, which considerably slow down the intermolecular hopping process and reduces the carrier mobility in these materials. This talk discusses the filling, or passivation, of these traps with very low densities of molecular n- and p-dopants, where the ratio of dopant molecules to matrix molecules is of the order of 10^-4-10^-3, instead of the usual 10^-2 and above generally used in doping studies. We demonstrate the very progressive filling of these trap states, with the concomitant super-linear motion of the Fermi level and the increase in mobility. A clear transition from trap filling to the addition of “free” carriers is seen, which allows a good estimation of the density of traps in the material. Modeling of Fermi level position and carrier mobility is done using a simple exponential distribution of trap states. Implications of ultra-low doping for device performance is discussed.

The energy level alignment at organic/organic and organic/electrode interfaces is of paramount importance for energy and charge transfer optimization in organic electronics, e.g. exciton dissociation in organic photovoltaic cells and charge injection in organic light emitting diodes. For these interfaces, vacuum level alignment is typically observed if the substrate Fermi-level is located within the energy gap of the overlayer. If the sample work function exceeds this limit an interface dipole forms to realign E_F within the energy gap and to establish electronic equilibrium across the entire heterostructure. Although Fermi-level pinning has been studied extensively, the microscopic origin of the interface dipoles is still controversially debated: The proposed mechanisms range from integer charge transfer to induced polarization. By employing photoelectron spectroscopy and scanning tunneling microscopy, we investigate the electronic properties of well-defined molecular heterostructures on a metal electrode, namely C₆₀ (sub)-monolayer films, which are prevented from direct electronic coupling with an Ag(111) substrate by sub-nanometer thick α-sexithiophene (6T) spacer films. This system is designed to induce Fermi-level pinning of C₆₀, because the work function of 6T/Ag(111) is lower than the electron affinity of C₆₀. Our results allow identifying integer long-range charge transfer from the metal to a fraction of the C₆₀ layer as cause for the observed interface dipoles, while the 6T interlayer and the other molecules in the first C₆₀ layer are neutral. The obtained structure is reminiscent of the active channel in an organic field effect transistor in the “on” state. Further studies on similar structures may thus contribute to obtaining a better understanding of charge accumulation and transport in organic-based transistors.

Organic semiconducting single crystals are currently investigated with respect to both their possible applications and their importance as model systems for understanding charge transport in organic semiconductors. Solution-grown single crystals based on the intrinsically polar 4-hydroxycyanobenzene (4HCB) molecule revealed recently interesting technological properties (such as three-dimensional anisotropic mobilities and ability to detect X-rays [1,2]), and the possibility to get unprecedented insights into molecular mechanisms of charge transport via infrared analysis [3]. In particular, accurate linearly polarized infrared (LP-IR) studies of 4HCB crystals unveiled optical properties like 2D spectral anisotropy (which allows to identify reproducibly and beyond any doubt the crystal axes with no need for X-ray or electrical measurements), 2D anisotropic hydrogen bonding degree, and a 3D anisotropic rotation of the electric dipole vector upon electrical polarization of the crystal. In addition, temperature-dependent measurements carried out using 4HCB crystals as channels in organic field-effect transistors allowed to detect visible spectral changes upon actual current flow, revealing the possible involvement of defined functional groups in the charge transport [4]. These topics will be discussed in relation with the crystallographic structure and with its variation upon temperature decrease (from room temperature to 140 K), as measured using synchrotron light-enabled X-rays crystal diffraction. The latter results will be compared with data from LP-IR measurements in the same temperature range, in order to achieve a detailed view of the relations between the crystallographic and LP-IR features at a given temperature, and of the general trend of these data with temperature decrease. Finally, the possible role of tautomeric forms of the 4HCB molecule within the crystal lattice with respect to charge transport will be discussed. References [1] B. Fraboni et al., Adv. Mater. 2009, 21, 1835 [2] B. Fraboni et al., Adv. Mater. 2012, 24, 2289 [3] A. Fraleoni-Morgera et al., J. Phys. Chem. C 2012, 116, 2563 [4] E. Capria et al., manuscript in preparation

The high degree of structural order and sufficient purity of single crystals of rubrene (c-rubrene) facilitate the studies of fundamental properties of organic semiconductors using this material as a model system. Such device architectures as field-effect transistors have proven to be a useful platform for probing charge carrier transport properties of c-rubrene. In this presentation, we report on demonstration of the photovoltaic effect at the Schottky interface of c-rubrene and aluminum electrode in the planar ITO/c-rubrene/Al and ITO/c-rubrene/LiF/Al device geometries. The mechanism of the effect formation is explained based on the reconstructed energy band diagrams of the ITO/c-rubrene/Al and ITO/c-rubrene/LiF/Al hetero-structures. The Fermi level in the bulk of c-rubrene lies at 0.55-0.6 eV above the onset of highest occupied molecular orbital (HOMO), which indicates that c-rubrene is a doped, p-type material. The photovoltage is equal to the built-in potential at the Schottky interface of c-rubrene with Al and defined by the work function difference between the ITO and Al electrodes. The magnitude of photovoltage can be significantly increased by the insertion of an ultra-thin LiF interlayer between c-rubrene and Al which induces surface traps for holes. The occupied electronic states that were found at 0.3 eV above the HOMO onset in the band gap of c-rubrene are discussed as probable dopants. This study was aimed at elucidating the details organic crystal/metal interface formation.

We will present our recent work on direct visualization of exciton diffusion in crystalline organic semiconductor materials. This is accomplished by mapping exciton location following excitation of organic semiconductor crystals with high temporal and spatial resolution using the ultrafast transient absorption microscopy. In these experiments, the pump laser excites the single crystals at a specific point, and the probe laser is spatially scanned to record an image. Images are collected at different times to monitor the motion and fate of the photo-generated excitons.

The key process in organic photovoltaics cells is the separation of an exciton, close to the donor/acceptor interface into a free hole (in the donor) and a free electron (in the acceptor). In an efficient solar cell, the majority of absorbed photons generate such hole-electron pairs but it is not clear why such a charge separation process is so efficient in some blends (for example in the blend formed by poly(3-hexylthiophene) (P3HT) and a C60 derivative (PCBM)) and how can one design better OPV materials. The electronic and geometric structure of the prototypical polymer:fullerene interface (P3HT:PCBM) is investigated theoretically using a combination of classical and quantum simulation methods. It is shown that the electronic structure of P3HT in contact with PCBM is significantly altered compared to bulk P3HT. Due to the additional free volume of the interface, P3HT chains close to PCBM are more disordered and, consequently, they are characterized by an increased band gap. Excitons and holes are therefore repelled by the interface. This provides a possible explanation of the low recombination efficiency and supports the direct formation of “quasi-free” charge separated species at the interface. This idea is further explored by using a more general system-independent model Hamiltonian. This talk will discuss how and when a combination of computational and theoretical models can truly contribute to organic electronics and will provide few examples of genuine material properties predictions based on computational chemistry methods.

In this presentation, we intend to cover two main topics: (i) The electronic structure and charge-transport properties of organic mixed-stack charge-transfer (CT) crystals. We have used density functional theory calculations and mixed quantum/classical dynamics simulations to study three representative CT crystals, DBTTFminus; TCNQ, DMQtTminus;F4TCNQ, and STBminus;F4TCNQ. The compounds are characterized by very small effective masses and modest electronminus;phonon couplings for both holes and electrons. In addition, the hole and electron transport characteristics are found to be very similar along the stacking directions. This similarity arises from the fact that the electronic couplings of both hole and electron are controlled by the same superexchange mechanism. Remarkable ambipolar charge-transport properties are therefore predicted for all three crystals.[1] (ii) Our recent progress in the understanding of the electron-phonon mechanism in oligoacene single crystals.[2] [1] L. Zhu et al., JACS 134, 2340 (2012) [2] Y. Li et al., Physical Review B 85, 245201 (2012)

The extrinsic energy levels in HOMO-LUMO energy gap (gap states) of organic materials are now recognized as a key to understand practical performance of organic devices. Gap states are known to work as trap, and are proposed to induce band bending also: however, the origin and nature of such states are not well understood. Gap states of organic semiconductors have been so far investigated by using indirect methods such electric measurements. For better understanding, direct observation to clarify the energy distribution and density-of-states is highly desired. Recently, ultraviolet photoemission spectroscopy (UPS) with pure light source was successfully applied to small molecule on graphite to observe gap states [T.Sueyoshi et al, APL 95, 183303 (2009)]. The study was limited to a couple of monolayer thickness region, but not for practically thick films as device scale partly due to charging problem. In this study, we have developed a new apparatus to investigate the gap states of various organic samples including small molecular films and polymers by using high-sensitivity UPS and photoelectron yield spectroscopy (PYS) without sample charging problem. Low energy light source with double monochromator enables us to investigate very weak levels such as gap state without ghost structure due to stray photon. Because of high sensitibity detection methods, even very thick films and crystals can be investigated. The results obtained by this apparatus will be reported as follows. For small molecule system, clear gap state with exponential distribution was observed for thick rubrene film. An additional gap state at rubrene/C60 interface was also observed. These states are important to discuss the property of p-n junction. Another type of gap state was observed for polyethylene terephthalate (PET) film, where weak gap state have a wide energy distribution over 4 eV. The results for organic single crystals (rubrene and pentacene) will be also reported.

We have performed an optical spectroscopy study of how the presence of oxygen and water affects the radical anion charge states in one of the most widely studied electron transporting conjugated polymers, poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bisthiophene)} (P(NDI2OD-T2)). By combining the results obtained from Charge Accumulation Spectroscopy (CAS)[1], a newly developed technique which allows optical quantification of the concentration of mobile and trapped charges in the polymer film, with electrical characterization of P(NDI2OD-T2) organic field-effect transistors (OFETs) we have been able to study the mechanism for storage and bias-stress degradation upon exposure to different atmospheres, including dry air/oxygen and humid nitrogen/water environments. We show that the stability upon oxygen exposure is limited by an interaction between the neutral polymer and molecular oxygen leading to a reduction in electron mobility in the bulk of the semiconductor. Using density functional theory (DFT) quantum chemical calculations we are able to ascribe the drop in mobility to the formation of a shallow, localized, oxygen-induced trap level, 0.34 eV below the delocalized lowest unoccupied molecular orbital (LUMO) of P(NDI2OD-T2), caused by a stabilizing interaction between the naphthalene unit of P(NDI2OD-T2) and molecular oxygen. [1] Di Pietro, R. and Sirringhaus, H. High Resolution Optical Spectroscopy of Air-Induced Electrical Instabilities in n-type Polymer Semiconductors. Adv. Mater. (2012).

In this presentation, the overview of our recent progress for development of high performance printed unipolar and ambipolar CMOS circuits will be given. Firstly, we have developed high-performance inkjet-printed p- and n-channel or ambipolar OFETs by controlling crystallinity of printed semiconducting film. In addition, properties of OFET were optimized by careful selection of polymer gate dielectrics and insertion of interlayer to reduce contact resistance. The charge carrier mobility and operation voltage were highly improved by using high-k polymer dielectric blends, poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) and poly(methyl methacrylate) (PMMA). State-of-the-art p-type polymer semiconductors containing alkyl-substituted thienylenevinylene (TV) and dodecylthiophene (PC12TV12T) and n-type 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)) were used for various CMOS circuitry. Inkjet-printed PC12TV12T (p-channel) and P(NDI2OD-T2) (n-channel) OFETs typically showed a field-effect mobility of 0.5 ~ 0.8 cm2/Vs, and the operation voltage was effectively reduced below 5 V by using high-k P(VDF-TrFE):PMMA polymer blends. An interesting result was that the p-channel OFET characteristics could be tuned by controlling the mixing ratio of P(VDF-TrFE) in the blended dielectric. The mu;FET of the p-type OFETs was gradually improved by increasing the P(VDF-TrFE) content in the blended dielectric layer, while n-type OFET properties were slightly decreased or not significantly changed. These results can be attributed to interface dipole formation by -C-F bonds in P(VDF-TrFE) at the semiconductor/dielectric interface, which favors p-channel, rather than n-channel, transport. By optimizing the mixed ratio, balanced, high-electron hole mobilities of more than 0.1 cm2/Vs at low applied bias conditions below ±10V, and threshold voltages below ±3 V, were obtained at a 7:3 ratio of P(VDF-TrFE) to PMMA. Low-voltage (below 2 V ) operation of complementary inverters was successfully demonstrated, with high voltage gains of greater than 25, excellent noise margins of more than 75% of ideal values, and ideal inverting voltage at nearly half of the supplied bias (½VDD). Moreover, the complementary inverters were well incorporated to comprise NAND, NOR, OR, and XOR logic gates. In the end, high-speed complementary ring oscillator circuits at more than 78 KHz were achieved with 7:3 ratio dielectric blends, and low-voltage operation, even below VDD = 5 V, was achieved by optimizing various parameters such as gate dielectric capacitance, channel width and length, supplied bias, and overlap capacitance of the circuit configuration..

Printed, flexible electronics are gaining traction as low cost alternatives to conventional semiconductor devices in industries ranging from health care to national security. The performance of such devices is critically dependent upon alignment of the semiconducting unit at the nano- through macro-scales. Significant structure-process-property relationships that allow for enhancement of long-range order will be described. Of note, we have observed a lyotropic liquid crystalline (LC) phase in poly-(3-hexylthiophene) (P3HT) via solvent-evaporation induced self-assembly. In-situ polarized Raman spectroscopy was used to study the evolution of structure that eventually leads to a phase transition from an isotropic solution to an LC phase. Concurrent in-situ drain current measurements revealed variations in the current that correspond well with the time-lines for the phase transition, suggesting that P3HT undergoes a series of phase transitions ranging from isotropic, to liquid crystalline to finally, polycrystalline solid. These insights have further been applied to the design of an alternative, donor-acceptor (D-A) benzothiadiazole oligothiophene based copolymer material. For one family of copolymers, structural elements were found to provide for a span in hole mobility of approximately 3 orders of magnitude. DSC, XRD and AFM analysis were used to demonstrate a strong correlation between molecular ordering, thin film morphology and electrical performance, providing clear insights into critical structure-property relationships associated with charge carrier transport in D-A π-conjugated copolymers. Notably, poly(benzothiadiazole-sexithiophene) (PBT6) which possesses a highly crystalline, lamellar, π-π stacked edge-on orientation on FET substrates exhibits a hole mobility of ca. 0.2 cm2/Vs. While polarized optical microscopy (POM) of PBT6 provides evidence of liquid crystallinity, DSC allows for determination of 2 distinct melting points associated with this polymer. Using DSC as a guide, thermal annealing at a moderate temperature of only 70 °C effected mobilities as high as 0.75 cm2/Vs. The insights obtained through these studies may allow for simple, controllable, and cost-effective methodologies for achieving high performance in plastic electronic devices.

Although the performance of organic thin-film transistors (OTFTs) has increased significantly over the last decade, it still remains challenging to fabricate large area arrays of transistors with good device-to-device parameter uniformity. In terms of organic semiconductors, both soluble small molecules and polymers are viable materials classes with the former often being advantageous in terms of the higher charge carrier mobilities that can be obtained in OTFTs. Despite this significant advantage, however, the processing of highly crystalline small molecules into uniform thin films is often much more challenging than polymers. An attractive approach to these processing challenges is to combine the high mobility properties of small molecules with the advantageous processability of polymers and fabricate blend-based transistors. It has recently been shown that blending of small molecules with polymers can provide control of the crystallization of the small molecule component during film formation. Despite the great potential of the blend approach, however the methodology has not been applied to electron transporting (n-type) organic semiconductors, i.e. a key element for the development of next generation integrated circuits based on complementary logic. To this end, of particular importance is the development of n-type organic semiconductors that combine processing versatility with air stability. We hereby report on a new soluble and highly air stable n-type organic semiconductor containing an oxidised diketopyrrolopyrrole core (DPP-CN). By combining this novel and solution processable small molecule with a carefully chosen insulating polymer as the binder material, we are able to demonstrate, for the first time, blend OTFTs that exhibit both, high electron mobility (~0.7 cm2/Vs) and excellent operating stability when exposed to ambient air for prolonged period of time. Additionally, the small device to device parameter spread allows fabrication of integrated circuits such as all-organic unipolar as well as complementary inverters by combining the DPP-CN based blend OTFTs with high mobility p-type blend OTFTs. The work could have significant implications for the development of solution-processed organic microelectronics based on complementary logic architectures.

The high mobility measured in alkylated DNTT and BTBT molecules [1,2] raises the possibility that a band-like transport mechanism may be achievable in solution processed organic semiconductors. Therefore, an acoustic deformation potential model appropriate for transport in two dimensions was employed to estimate upper limits on the hole mobility of DNTT-C10 [1] and BTBT-C12 [2]. Density functional calculations are employed to determine the values of effective masses, deformation potentials, and elastic constants required to calculate the mobility along crystallographic axes.[3] According to the model, scattering by acoustic phonons places an upper limit on room temperature mobility within crystalline regions of these materials in the range 50-90 cm2/Vs. Comparison is made to results for a thin-film pentacene structure. The effective masses calculated for the holes in DNTT-C10 and BTBT-C12 are significantly less than those obtained for thin-film pentacene, and this difference is the origin of higher mobility of DNTT-C10 and BTBT-C12 within the model. The width (in the direction perpendicular to the plane of transport) of the π-bonded molecular core is identified as an important structural feature that affects mobility in 2D systems such as these. The effect of alkyl chains on the mobility will be discussed. This work is supported in part by AFOSR Grant FA9550-09-1-0436. [1] K. Nakayama et al, Adv. Mater. 23, 1626 (2011). [2] H. Minemawari et al, Nature 475, 364 (2011). [3] J. E. Northrup, Appl. Phys. Lett. 99, 062111 (2011).

Printed electronics for the next-generation smart devices require development of solution-processable high-performance organic semiconductors. However, despite the enormous versatility in designing organic molecules, practical industrialization has not been achieved using existing compounds such as a variety of acenes and heteroacenes because of the significant difficulty in realizing the high mobility, solubility, thermal stability, and large-scale productivity at the same time. Therefore, an essentially new molecular design that satisfies all of the above requirements is required. Here, we have developed a conceptually new V-shaped π-conjugated systems containing a thiophene ring at the center and naphthalene ring at both sides, so that the lowered symmetry induces solubility even with the five-ring fused oligoacene cores required to achieve efficient charge transport. We first developed a facile and versatile synthetic methodology for dinaphtho[2,3-b:2,3-d]thiophene (DNT-V). Specifically, starting from 2-methoxynaphthalenes, the compound is simply synthesized in four steps containing three key reactions. Interestingly, DNT-V itself exhibits a solubility of 0.10 wt% in toluene even at room temperature. These alkyl-substituted DNT-V derivatives (Cn-DNT-VW) exhibited improved solubility in common organic solvents in the range of 0.12-1.0 wt%. C6- and C10-DNT-VW having two hexyl (C6) or two decyl (C10) chains exhibited the obvious phase-transition temperatures of 197 and 150 °C, respectively, which is considerably higher than the values for other reported soluble organic semiconductors. Their overall packing structures are typical herringbone-type forms, which resemble those of high-performance semiconductors such as pentacene and dinaphthothienothiophene solids. Such conformation may be induced by two-dimensional intermolecular interactions. Notably, Solution-processed films based on such molecules have demonstrated high mobilities of up to 8 cm2 V-1 s-1. We will also present its device durability.

The evolution of organic electronics is now poised to enter the commercial phase, with the recent market introduction of the first prototypes based on organic transistors fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfill their potential. Organic solar cell efficiencies are currently increasing rapidly based on organic bulk heterojunction devices fabricated from solution. Central to these device efficiency improvements are the development of new photoactive semiconducting donor and acceptor materials, designed at the molecular level to optimise both absorption of the long wavelength region of the solar spectrum and generation of high cell voltages. This presentation will examine some of the key design strategies to control the molecular orbital energy levels and microstructure of donor polymer semiconductors and illustrate with examples and their characterisation. Specifically, the systematic reduction of the bandgap in a series of bridged ladder type indenofluorene copolymers, in combination with the progressive lowering of the HOMO energy level will be shown. Analogues of these polymers, along with DPP copolymers also exhibit high charge carrier mobilities, and transistor data will be presented.

Composite films of pentacene and azobenzene moieties were prepared and used as the active channel material in a top-contact, bottom-gate field-effect transistor. The transistors exhibited high field-effect mobility as well as large I-V hysteresis as a function of the gate bias history. The azobenzene moieties, incorporated either in the forms of self-assembled monolayers, discrete multilayered clusters, or on the surface of nanoparticles of gold at the dielectric surface, resulted in electric bistability of the pentacene-based transistor either by photo-excitation or gate biasing. The size of hysteresis, direction of threshold voltage shifts, response time, and retention time all depend on the substituent on the benzene ring. Detailed film structure analyses and correlation with the transistor/memory property are provided.

Charge modulation spectroscopy (CMS) is an electro-optical method that allows direct probing of charge carriers in the active channel of working organic field-effect transistors (FETs). We have mapped the charge carrier distribution in a regio-regular poly(3-hexylthiophene) (rr-P3HT) FET by probing the Infra-Red Active Vibrational (IRAV) modes and the lowest polaron band induced by electrostatic doping in the medium-infrared spectral region (680-4000 cm-1). CMS measurements are implemented in reflectance mode using an FTIR spectrometer with confocal microscope, which enables characterization of actual device structures on opaque substrates. The CMS reflectance spectra display the typical spectroscopic signatures of the doped polymer and correlate well with photoinduced absorption and photoinduced reflectance spectra of P3HT films. Spatial mapping of the transistor channel enables the observation of charge carrier injection and carrier distribution along the active region of the device in different operating regimes. This demonstrates the potential of infrared CMS mapping to characterize charge carrier injection, transport and possibly dynamics in high mobility conjugated polymers, as well as the relationship between transport properties and polymer morphology.

Bias stress is an important phenomenon in organic field-effect transistors (OFETs). It refers to a continuous decrease of the source drain current (or equivalently, an increase of the threshold voltage) in an OFET under the accumulation conditions. There are several proposed mechanisms of the bias stress, such as a ground-state charge transfer across the interface between a semiconductor and an amorphous dielectric, a small electrical leakage or ionic conduction in gate insulators, effect of humidity on dielectric properties of gate insulators, modification of the contact resistance under the gate bias, and diffusion of protons in SiO2. Charge trapping in the semiconductor has also been suspected to be a source of the bias stress effect in OFETs. However, a detailed microscopic model based on charge trapping is still missing. In most of these mechanisms, the semiconductor-insulator interface and the presence of a material gate dielectric are essential for understanding the effect. An interesting question thus arises: does an OFET without any gate insulator also exhibit a bias stress effect? Here we report a study of the so-called “air-gap” single-crystal rubrene OFETs, showing that even in the absence of material gate dielectrics OFETs can still exhibit a bias stress effect, yet the mechanism of the effect in these devices is qualitatively different. By using air-gap OFET configuration and controlling the vapor content of the gap, we have successfully disentangled intrinsic (i.e., semiconductor related) and non-intrinsic (i.e., environment and device structure related) factors that contribute to the bias stress instability in OFETs. An intrinsic mechanism related to the relaxation of charge carriers in localized band tail states has been identified and formulated. This mechanism leads to a qualitatively different bias stress behavior compared to devices with material gate insulators. By varying the density of the localized states (traps) via intentional photo-oxidation of organic single crystals, we have further demonstrated a clear correlation between such density and the magnitude of the bias stress effect. The vapor content of the gap in these OFETs can also have a significant impact on the bias stress effect: polar gases introduced in the gap increase the bias stress rate. Such a behavior is explained in the framework of our model, based on formation of bound states between field-induced charge carriers and polar molecular species at the semiconductor&’s surface.

Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the hetero-interfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN2 single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transport is due to electrons, whose mobility exhibits band-like behaviour from room temperature to ~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The electron density decreases linearly with decreasing temperature, an observation that can be quantiatively explained based on the heterostructure band-diagram. Our results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces and show the potential of Schottky-gated organic heterostructures for the investigation of transport in molecular semiconductors.

The performance of electronic and opto-electronic devices depend critically on minimizing trap states both in the bulk and at interfaces. In addition, the understanding of the microscopic origin of trap states in organic semiconductors is of great conceptual and practical interest, and it is presently in the early stages. Therefore we study systematically and quantitatively the density-of-trap-states (DOS) in numerous organic semiconductors employing temperature-dependent space-charge-limited-current-spectroscopy to characterize the bulk, and full device simulation for field-effect transistors. Covering the broad range of crystalline order, starting from high quality single crystals, to evaporated or spin-coated thin films the trap DOS is found to be very low (10^16 / eV ccm) in crystals, and by several orders of magnitude higher in thin films. Chemical influences (exposure to various gases) and physical perturbations (ion and X-ray irradiation), as well as room-temperature annealing are found to modify the trap DOS in a distinct way. Interestingly, the over-all trap DOS in thin-film organic semiconductors is very similar to that in inorganic semiconductors, such as e.g. poly-Si and a-Si:H. Through detailed trap analysis at the dielectric-organic semiconductor interface we have introduced Cytop as the gate dielectric that yields high-quality field-effect devices with minimal hysteresis and strongly reduced trap states, both for n- and p-type materials. Using the same n-type material with various degrees of (dis)order we show the close correlation between charge trapping and charge transport. Thus, quantifying, understanding and minimizing charge trapping is of central importance in exploring the microscopic aspects and the application potential of organic semiconductors.

Conjugated organic semiconductors offer new opportunities for the controlled manufacturing of active electronic circuits by a combination of solution processing and direct printing. In recent years many classes of small molecule and conjugated polymer semiconductors with charge carrier mobilities above 1 cm2/Vs have been discovered. In this talk we will discuss recent insights into the charge and spin transport physics of these materials with a particular focus on the microscopic processes that limit transport in these systems and will discuss differences in the transport physics of high mobility conjugated polymers and molecular semiconductors.

This talk will review recent measurements and analysis of the anisotropic absorption and luminescence features of rubrene single crystals. The main vibronic progressions are described for single-photon absorption, two-photon absorption, and luminescence, and for all light polarizations. In rubrene, symmetry dictates that the purely electronic transition to the first molecular excited state, at an energy of 2.32 eV, only couples to one distinct light polarization, corresponding to the strongest absorption and luminescence features in the crystal. Transitions to higher vibronic states of appropriate symmetry, on the other hand, can couple to other light polarizations for which a weaker absorption and emission is observed. In addition to the spectroscopy of the first excited state using single-photon excitation, the next higher excited state that can be reached only by two-photon excitation and is situated at around ~3 eV above the ground state will also be discussed.

Singlet fission is a process where an initially created singlet exciton spontaneously splits into a pair of triplet excitons. This spin-allowed process is of interest as a way to make higher efficiency solar cells. We describe time-resolved experiments on solid-state tetracene and rubrene. The experiments are designed to determine the effects of coherence, sample morphology, temperature, and magnetic field on singlet fission. For both polycrystalline tetracene thin films and single crystals, quantum beats in the delayed fluorescence signal indicate the presence of coherent superpositions of triplet pair states created by direct transitions from the singlet state. In rubrene thin films, disorder effects lead to opposite magnetic field effects on the fission rate. In both materials, the interplay of singlet fission with singlet and triplet exciton diffusion leads to complex kinetics that determine the overall yield of free triplets.

Organic single-crystal semiconductors offer the systems of π-conjugated small molecules periodically arranged in a macroscopic length scale. Therefore, such solids are the most suited both to study fundamental physics of charge transport in the weakly interacting molecular assemblies and to fabricate the best-performing organic devices for printed and flexible electronics. This presentation focuses on the magnetotransport measurement to identify marginal intermolecular electronic coherence significantly dependent on compounds and on further technical challenge to apply microscopically band-transporting systems for practical applications to high-speed logic devices. The degree of fundamental intermolecular electronic charge coherence is discussed using the results of Hall effect on various single-crystal transistors under external pressure and varied temperatures. It turned out that the extent of the coherence depends on molecular systems, so that such an approach gives a prescription to design new molecular compounds because fully coherent materials exhibit high carrier mobility of approximately 10 cm²/Vs, indeed. Among many measured organic semiconductors, pentacene showed unique characteristics that the coupling strength between charge current and external magnetic, which gives the coherence parameter, continuously increases under pressure and upon cooling. The result indicates molecular fluctuation near room temperature plays a major role in establishing the coherent electronic states over at least several molecular distances to realize the band-like transport. On the technical side, the development of organic thin-film transistors (TFTs) itself has been intensively driven by the fact that they can be fabricated by easy and potentially low-cost processes. Furthermore, near-room-temperature processes enable semiconductor technologies to be realized on plastic substrates, raising expectation for post-silicon semiconductor industry. Solution-based processes are the most desired for producing high-performance organic TFTs in this regard. However, the transistor performances of such devices are not yet sufficiently high in terms of applications to practical logic circuitries. Since recent development of solution-crystallization processes successfully realized equally high carrier mobility even in the printed devices, the next challenge has become to minimize the channel length for high-speed charge accumulation. This presentation also covers our recent development of a new transistor configuration with two-layer gating function, so that charge is introduced through a “homo-junction” from charge-rich regions with the application on continuous gate electric field. This construction actually improved the high-frequency response of the solution-crystallized high-mobility transistors drastically with minimized contact resistances, so that the devices respond with the frequency in the order of 10 MHz.

Organic semiconductors are key materials for the advancement of electronic and optoelectronic devices, such as field-effect transistors, light emitting diodes and photovoltaic cells, because of their potential application in flexible electronics. Among organic devices, ambipolar transistors are very unique device, in which both electrons and holes are equally mobile, and we are able to emit light through the recombination of them [1,2]. One of the advantages of such ambipolar light-emitting transistors (LETs) based on several organic single crystals is the coexistance of high current density and high luminescent efficiency, which might be possible route to the electrical-driven organic laser. However, in these ambipolar transistors, many issues are still necessary to overcome. Here, I will present recent improvement in ambipolar transistors. One of the open issues in ambipolar transistors is the basic understanding of junction structure between p-type and n-type region. We fabricated single-crystal ambipolar transistors using a wide variety of molecules and tried to clarify it [3,4]. Although it was understood as the simple pn junction before, we finally found the existence of non-doped area between them, which size is decided by the density of trap states. Based on this knowledge, we reduced the effect of trap states by the optimization of fabrication process and dielectric layer. Particularly, using the high-capacitance electric-double layer of ion gel, we realized extremely low-voltage operation of ambipolar transistors [5]. We feedback these basic understandings to LETs. The reduction of trap states leads to the high current density, which is necessary for bright light-emitting devices, and the luminescent properties under high current density were investigated. Moreover, the micro-fabrication of optical resonators opens the new roots to functionalization of ambipolar transistors. These recent developments in ambipolar LETs will be presented. [1] J. Zaumseil et al., Nature Materials 2006, 5, 69-74. [2] T. Takahashi, T. Takenobu et al., Advanced Functional Materials 2007, 17, 1623. [3] S. Z. Bisri, T. Takenobu et al., Advanced Materials 2011, 23, 2753-2758. [4] A. Dadvand, T. Takenobu et al., Angew. Chem. Int. Ed. 2012, 51, 3837-3841. [5] Y. Yomogida, T. Takenobu et al., Advanced Materials 2012, DOI: adma.201200655.

Ionizing radiation can be detected by directly converting it into an electrical signal. Only few and expensive inorganic semiconductors (e.g. CdTe, SiC) offer the possibility of realizing portable detectors that operate at room temperature. Organic semiconductors have been so far mainly proposed as detectors for ionizing radiation in the indirect conversion approach, i.e. as scintillators, which convert ionizing radiation into visible photons, or as photodiodes, which detect visible photons coming from a scintillator and convert them into an electrical signal. The direct conversion of ionizing radiation into an electrical signal within the same device is a more effective process than the indirect one, since it improves the signal-to-noise ratio and it reduces the device response time. We report here the use of organic semiconducting single crystals (OSSCs) as intrinsic direct ionizing radiation detectors, thanks to their stability, good transport properties and large interaction volume. X-ray detectors, based on low-cost solution-grown OSSCs are here shown to operate at room temperature, providing a stable linear response with increasing dose rate in atmosphere and in radiation-hard environments [1]. 2D X-ray detector matrixes, fabricated on plastic substrates with inkjet printed electrical contacts, can be operated at very low voltages (<30V) voltages. The intrinsic conversion of X-rays within the crystals allowed to fabricate all-organic optically transparent devices, indicating OSSCs as very promising candidates for a novel generation of low-cost, room temperature, portable X-ray detectors. [1] B.Fraboni et al. Adv.Mater. 24, 2289 (2012)

Despite the success of organic electronic devices, there are still significant hurdles to overcome towards understanding and controlling chemical doping of conjugated polymers. The processability of semiconducting polymers from solution becomes more difficult upon doping due to the formation of charge transfer complexes that form aggregates or insoluble “organic salts”. Here we report on the structural impact, changes in electrical conductivity, and thermal stability upon introducing two tetracyanoquinone derviatives into a series of thiophene polymers, including poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and poly(3-hexylthiophene) (P3HT). Ultraviolet-visible spectroscopy provided evidence of charge transfer, as a function of dopant concentration, with charge transfer peaks decreasing in intensity upon thermal treatment of the film. Synchrotron x-ray scattering demonstrated that all of the thiophene polymers formed ordered domains despite the presence of the molecular dopant. The molecular packing structure was substantially affected by doping with both dopant load and post thermal processing influencing each of the target polymers texture, for example the π-π d-spacing in PBTTT decreased by as much as 0.1 Å while the lamella layer spacing increased. As expected, increases in electrical conductivity scaled with dopant concentration, from 10E-6 S/cm to 10E-1 S/cm in pBTTT, but of special interest is how the conductivity was affected by thermal annealing due to its liquid crystalline phase behavior in neat films. The electrical conductivity was found to increase near the polymer liquid crystal transition temperature before decreasing below the as-cast conductivity at higher annealing temperatures. Overall the electrical conductivity and charge transfer was more stable in PBTTT than P3HT with annealing demonstrating the connection between molecular ordering and thermal stability in molecularly doped films.

Organic electronics has been the subject of intensive interest because of their potential in electronic devices. Major factors that determine the development of organic electronics in future are further increase in device performance, reduction of manufacturing costs, and extension of applications. Here, we present a fabrication technique for patterned organic crystals transistor arrays, which meets all the above-mentioned requirements. A mixture solution of organic semiconducting small molecule and insulating polymer was spin-coated onto a substrate with patterned surface wettability. Organic crystals, which are regarded as the ideal materials for high device performance, were formed within the patterned regions without any post treatment. The grain sizes were observed as large as hundreds of micrometers. High device performance and uniformity were obtained from the fabricated transistor arrays, exhibiting the field-effect mobility more than 1 cm^2/Vs and the on/off ratio of 10^8. Besides, patterning the active materials is desirable for turning organic crystals into a viable semiconductor technology. This solution-assisted process is simple and sufficient, and would highly reduce the manufacturing costs. Furthermore, it was found that the proposed method could create a very smooth semiconductor/dielectric interface. Thus, we were successful in presenting the first demonstration of patterned organic crystals transistor arrays on paper. Despite large surface roughness of paper substrates, the devices exhibited excellent electrical performance, possessing the highest values of both carrier mobility and on/off ratio for OFETs on paper so far. It promises that the proposed method also benefits for extending the applications of organic electronics. Reference [1] Y. Li, C. Liu, A. Kumatani, P. Darmawan, T. Minari, and K. Tsukagoshi, Org. Electron., 13, 264 (2012). [2] Y. Li, C. Liu, Y. Xu, T. Minari, P. Darmawan, and K. Tsukagoshi, Org. Electron., 13, 815 (2012).

Fabrication of highly controlled interfaces of organic materials is a crucial topic to solve the fundamental and practical issues of organic semiconductor devices. Fabrication of well-ordered organic p/n junction is required for high efficiency organic photovoltaic devices, which are expected to be next generation photon-to-current conversion devices. In this study, we demonstrated the epitaxial growth of C₆₀ (n-type) films on rubrene (p-type) single crystals with well-ordered organic p/n interface. C₆₀ thin films were deposited in vacuum on rubrene single crystals fabricated by a train-sublimation method. The morphology of the initial growth of C₆₀ with the film thickness of 1 nm was investigated by means of an atomic force microscopy. Small and rounded grains, which can be suggested to be polycrystal or amorphous, were grown along with the step edge of a rubrene surface. On the other hand, large grains with triangular and hexagonal shape were grown on the terrace, which can be suggested to be fcc (111) face of C₆₀. The crystal orientation of the C₆₀ film was investigated by a reflection high energy electron diffraction and an in-plane X-ray diffraction. The epitaxial growth of C₆₀(111) commensurate with rubrene (001) surface lattice was verified. The distribution of crystalline grains and random grains depended on the substrate temperature during growth and the deposition rate. High crystallinity and large grains were achieved at high substrate temperature and slow deposition rate. Pyramidal grains with the size of around 500 nm were obtained up to C₆₀ thickness of 30 nm with the substrate temperature of 150 °C and the deposition rate of 0.01 #8491;/s.

In this paper we present a study on the correlation between morphological/structural properties of organic semiconductor films and their sensitivity to surface strain. In order to do that, a detailed electromechanical characterization of Organic Field Effect Transistors, fabricated on flexible plastic substrates with different organic semiconductors, was performed. We will show 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. We will also demonstrate, assisted by Atomic Force Microscopy and X-Ray Diffraction investigations, that this phenomenon is strongly related to a modification of charge carriers transport induced by morphological and structural changes taking place within the organic semiconductor during the application of the mechanical stimulus. Several typologies of organic semiconductors have been investigated, such as thermally evaporated small molecules and solution processable small molecules and polymers. In particular, we have focussed our attention on Pentacene, TIPS-Pentacene, N1400 and P3HT and, interestingly enough, we have observed a clear correlation between the intrinsic structural and morphological properties of the active layer and the device sensitivity to surface strain. In particular, Pentacene film morphology was intentionally varied by modifying the deposition rate, and a linear dependence between the average domain dimensions and the device sensitivity to strain was observed. Moreover, when highly disordered polymeric films, as P3HT, have been used, a dramatic reduction of the device response to mechanical stimuli was noticed. Nevertheless, an unambiguous, fast and reproducible response has been obtained also for this material, which, being solution-processable, represents a valuable solution for the fabrication of low-cost pressure sensors for a variety of innovative applications. Starting from these results, matrices of OFET based mechanical sensors have been fabricated by means of inkjet printing. Thanks to the flexibility of the introduced structure, we will show that the presented system can be transferred on different surfaces (hard and soft) and employed for a wide range of applications. In particular, we have designed and fabricated a fully functional system based on a matrix of 64 elements that can be employed for detecting mechanical stimuli over larger areas, and we have demonstrated that such a system can be successfully employed for tactile transduction in the realization of artificial “robot skins”.

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 topics to be discussed include: 1. Rationally designed high-mobility p- and n-type organic semiconductors for printed organic CMOS, 2. Polycrystalline and amorphous oxide semiconductors for transparent and mechanically flexible electronics, 3) Self-assembled and printable high-k nanodielectrics enabling ultra-large capacitance, low leakage, high breakdown fields, minimal trapped interfacial charge, and device radiation hardness. 4) Combining these materials sets to fabricate a variety of high-performance thin-film transistor-based devices.

Electrolyte gated transistors are excellent testbeds for probing transport in organic semiconductors at tunable charge densities close to the metal-insulator transition. This talk will begin with a discussion of high capacitance electrolyte gates and new gel electrolyte materials based on ionic liquids that facilitate convenient fabrication of electrical double layer transistors and electrochemical transistors in which very large 2D or 3D charge densities are achieved. Discussion will progress to transport results for transistors based on poly(3-hexylthiophene) (P3HT) films and rubrene single crystals. For P3HT devices, full characterization of hopping transport parameters (e.g., carrier localization length, dielectric constant) as a function of temperature and magnetic field strength reveals increasing delocalization with carrier density and clear observation of the Hall effect for the first time. These observations are consistent with the approach to metallic conductivity. However, we will show that the onset of true metallic behavior is frustrated by doping induced disorder. Correspondingly, there is an interesting collapse of the carrier mobility in P3HT in the high carrier density regime, and this effect appears to be completely general for all types of polymer semiconductors. Interpretation of the polymer results is aided by comparison to results for electrolyte gated rubrene crystals. In rubrene crystal transistors, ions from the electrolyte do not penetrate the crystal so that charging is two-dimensional. We will show that the hole mobility in these systems also exhibits unusual non-monotonic variation with charge density that is related to the surface density of electronic states. AC conductivity measurements to probe the differential mobility as a function of temperature will also be described. The talk will conclude with a discussion of the overall prospects for employing electrolyte gates to enhance fundamental understanding of transport mechanisms in organic semiconductors at high charge densities.

Recent advances in designing and synthesizing novel organic semiconductors with superior electrical properties are primarily motivated by the fast growth of organic electronics for printable and flexible applications. Thienoacenes, which consist of ladder-type fused benzene/thiophene molecular structures, have drawn extensive research interests in the past few years due to their high field-effect mobility and easy solution processibility. To understand the charge transport and structure-property correlation in thienoacenes, we have carried out intensive electrical measurements in vapor-grown single crystals of dinaphtho[2,3-b:2&’,3&’ -f]thieno[3,2-b]thiophene (DNTT). Particularly, we employ both vacuum gap and electrolyte gates in DNTT-based single crystal field-effect transistors (FETs) for systematic transport studies in both low-charge-density (10¹⁰ cm^-2) and high-charge-density (10¹³ cm^-2) regimes. Vacuum-gap DNTT FETs on poly(dimethylsiloxane) (PDMS) substrates exhibit a large hole mobility up to 9.3 cm²V^-1s^-1 with negligible contact resistance with gold and hysteresis. Temperature-dependence of mobility between 260 K and 340 K indicates a diffusive bandlike transport (mobility increases with decreasing temperature), although an even broader temperature range is limited by the mechanical fragility of crystals under thermal cycle. These excellent transport behavior could be explained by a well-balanced 2D electronic structure of DNTT, which is further evident by a weak dependence of mobility on crystallographic directions from a separate mobility anisotropy measurement. When gated with high capacitance ionic liquids (ILs) so as to form an electrical double layer transistor (EDLT), the gate-induced charge density in DNTT reaches 2.0×10¹³ cm^-2, consistently determined by capacitance-voltage and displacement current measurements. The channel conductivity at such a high carrier concentration is not only a non-monotonic function of gate voltage, but exhibit a stable and reversible plateau, which is different from the conductivity peaks in IL-gated rubrene transistors. This conductivity behavior is very likely to be related to the filling of band states in DNTT. Remarkably, carrier mobility at charge density above 10¹³ cm^-2 reaches 6.4 cm²V^-1s^-1. Moreover, the threshold voltages (V_th) and turn-on voltages (V_on) in IL-gated DNTT EDLTs can be systematically tuned by using gate metals with different work functions, when high work function metals (such as Pt) generally give a more positive V_th (for p-type DNTT). The trend of V_th with gate work functions has also been confirmed by the capacitance-voltage measurement. These exciting results indicate that DNTT single crystal provides another ideal testbed besides rubrene for investigating charge transport behaviors in organic semiconductors.

Material design for solution processible semiconducting polymers most often targets molecules that readily develop long-range order to achieve high field-effect transistor (FET) mobilities. Examples of this strategy include pBTTT, a polythiophene derivative with a highly ordered molecular packing motif. In the past few years, however, there have emerged some polymers having FET mobilities comparable to or even higher than that of pBTTT, but appearing to have little long-range order. In this work, we examine one such material, an indacenodithiophene-benzothiadiazole (IDTBT) copolymer with FET mobility higher than 3 cm2 V-1 s-1, by using a combination of diffraction and spectroscopic techniques. Our results suggest that high FET mobility in IDTBT may solely arise from local molecular order, or conjugated ring coplanarity, which is consistent with recent reports showing that (1) charge transport is most efficient along the conjugated backbone and (2) intermolecular pi-pi stacking is highly defective in semiconducting polymers.

Solution-based crystallization plays a vital role in fabricating crystalline organic semiconductors for various organic electronics applications. In this talk, we discuss polymorphs and crystal structures resulted from novel solution-based crystallization /self-assembly and their effects on charge transport, in light of results from selected area electron diffraction and energy filtered TEM imaging. In the first part of the talk, the effect of solvent choice on the crystal structure and polymorphism of 5,11 bis (triethylsilylethynyl) anthradithiophene (TES ADT) is examined with selected area electron diffraction, grazing-incidence X-ray diffraction and UV Vis Spectroscopy. A new polymorph of TES ADT is identified and characterized with electron diffraction which gives a gamma angle of ~90 degree. This new unit cell is distinguishably different from the most common triclinic unit cell of TES ADT (gamma angle =106). The implication of polymorphs on charge transport will also be discussed. In the second part of the talk, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS Pentacene)/polymer blends will be compared as a function of polymer choice. Energy filtered TEM and selected area electron diffraction are used to study the solution-based crystallization and assembly. A conventional wisdom is that the crystallization of highly crystalline small molecule dominates the self-assembly process of the TIPS pentacene/polymer blends at room temperature. Here we show that the intermolecular interactions of TIPS pentacene and different polymers tune the vertical and lateral phase separation, leading to novel and potentially useful microstructures.

The semicrystalline structure of a series of well-defined poly(3-hexylthiophene)s (P3HT) of different molecular weights and high regio-regularity was characterized by x-ray scattering and AFM in as-cast and melt-crystallized films. Crystallization by cooling from the melt led to a substantial increase in crystallinity and a stronger alignment of the crystals within the films. The increase in crystallinity goes along with an increase in hole mobility in vertical direction by about one order of magnitude as measured by SCLC. Additionally the mobility depends on molecular weight and seemed to correlate with the long period of the semicrystalline structure in P3HT. Thus, the charge carrier mobility first increased with the molecular weight and finally saturated at lower values for higher molecular weights. The relation between crystallinity and mobility might explain some of the largely different mobility values reported in the literature for P3HT.

Film morphology plays a critical role in charge transport in semicrystalline conjugated polymers, where charge transport is a complex combination of transport through the amorphous and crystalline regions of the material. In organic thin film transistors (OTFTs), the molecular packing at the dielectric interface is of particular importance to device performance. Initial studies on poly(3-hexylthiophene) (P3HT) suggested that edge-on packing (pi-stacking direction parallel to the gate dielectric) is a prerequisite for high carrier mobility when in a OTFT configuration. However, recently a number of polymers have been developed including Polyera&’s N2200 that show high motilities with a highly face-on packing structure. In this work, we re-investigate the need for edge-on packing in P3HT and if this packing structure is required for efficient charge transport in P3HT based OTFTs. To achieve a sharp contrast between edge-on and face-on packing motifs, we employ a novel biaxial straining process. This biaxial alignment approach consists of consecutively straining the polymer film on a polydimethylsiloxane (PDMS) elastomeric substrate in two orthogonal directions and then printing the strained film onto a desired substrate. The oriented film is transferred to a variety of receiver substrates to investigate the resulting film morphology and charge transport characteristics. X-ray diffraction and UV-vis spectroscopy are employed to describe the morphology, and charge transport is analyzed in a bottom gate, bottom contact thin film transistor (TFT) configuration. We find that the biaxially strained films are largely isotropic in the plane of the film but move from an edge-on orientation found in the spin cast films to a highly face-on orientation. While biaxally strained films are highly face-on they are shown to have comparable charge mobility as their edge-on counterpart. The films are also shown to have similar on/off ratios, and sub-threshold slopes. Differences are observed in the channel length dependence between the unstrained and strained films attributed to an increased density of defects in the strained films. This work shows that efficient charge transport in face-on packing P3HT based OTFTs is achievable.

We report the performance of ambipolar organic field-effect transistors (FETs) based on low band gap copolymers, pDPPT2NAP-HD and pDPPT2NAP-OD. The polymer backbones are composed of electron-rich 2,6-di(thienyl)naphthalene (T2NAP) and electron-deficient diketopyrrolopyrrole (DPP) units with branched alkyl chains of 2-hexyldecyl (HD) or 2-octyldodecyl (OD). Both polymers were polymerized via Suzuki coupling, yielding optical band gaps of ~1.4 eV. In the transistor performance test, we observed decent ambipolar transport behavior in both polymer films while the pDPPT2NAP-OD showed one order higher hole and electron mobilities than the pDPPT2NAP-HD. Thermal annealing of polymer films increased carrier mobilities. 150 oC annealing was the best condition where film crystallinity was saturated and carrier mobilities were maximized. The highest hole and electron mobilities were 1.3 cm2/Vs and 0.1 cm2/Vs, respectively, which were obtained from pDPPT2NAP-OD. To investigate the origins of mobility dependence on the chemical structure and thermal annealing, polymer films are fully characterized by grazing incidence X-ray diffraction (GI-XD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) experiments. GI-XD data revealed that both polymers form highly crystalline films with edge-on orientation. The pDPPT2NAP-OD carrying the longer alkyl chains showed a higher tendency to form a long-range order of polymer chains. In addition, thermal annealing up to 150 oC improved polymer film crystallinity and developed longer range lamellar structure simultaneously. AFM and TEM images support the GI-XD data consistently. These features directly explained charge transport behavior. Moreover, density functional theory (DFT) rationalized the observed transistor result. In conclusion, this work demonstrates that the choice of chemical structure in the polymer backbone and the side alkyl chain makes a direct impact on film crystallinity and carrier mobility in polymer transistors.

Recent developments of p-type organic semiconductors, namely, derivatives of polyacenes, rubrene, fused thiophene, and thienoacenes have led to very high hole mobilities on the order of 1 cm2V-1s-1 in organic thin-film transistors (OTFTs). Deposition methods for thin films of these molecules under vacuum or inert atmosphere, however, suffer from severe limitations in the large-scale fabrication of the devices owing to the complexity of the process. Thus far, homogeneous polymers composed of donor and acceptor units in the repeating group have been reported to have an excellent mobility of 1.0-4.0 cm2V-1s-1. However, the high crystallinity and mobility of conjugated polymers always require high molecular weight, which sometimes hampers the solubility in solvents. Moreover, the reproduction of films with identical degrees of crystallinity and molecular orderness remains extremely challenging. Diketopyrrolopyrrole (DPP)-based chromophores are often employed as high-performance organic pigments and their solubility can be improved by modifying their chemical structure. The DPP moiety exhibits a planar conjugated bicyclic structure, which leads to strong π-π interactions. In this presentation, the facile synthesis of a DPP-based conjugated polymers, P(DPP-alt-DTT) P(DPP-alt-DTBSe), and P(DPP-alt-DTEBSe) is demonstrated. Interestingly, the field effect mobility of polymer TFTs reached 0.8 ~ 4.0 cm2 V-1 s-1 after annealing the film although the molecular weight (Mn) is relatively small. Using those semiconducting polymers, the nanowires were fabricated by anodized aluminum oxide (AAO) template method and self-assembly method in solution states. In 1-D structural nanowires, the polymer chain were highly confined to exhibit strong intermolecular interaction. The electrical properties were compared for high (HA-PNW) and low (LA-PNW) aspect ratio PNWs produced using two different preparation methods. Finally, a single PNW was used to fabricate a Vis-NIR photodetector device whose electrical performance was fully investigated.

A method for the preparation of uniaxially oriented thin films of molecular semiconductors is introduced. It relies on the crystal growth with a Bridgman-type process that decouples nucleation and growth phenomena. An effective thermal gradient of 5-15°C/mm has been used in which films are displaced from a hot zone to a cold zone at a constant rate of 2.5-5 µm/s. This allows to study the nucleation and growth phenomena for a variety of molecular structures. Films are composed of large monodomains that are uniaxially oriented along the gradient direction. If adequate gradient conditions are used, polymorphism can be suppressed. The orientation of the unit cell versus the gradient direction is discussed and tentatively explained based on nucleation theory.

As solution processing of crystalline organic semiconductor thin films is increasingly shown to achieve high performance organic thin film transistors (OTFTs), there is urgency to develop robust and reproducible manufacturing processes suitable for large area manufacturing. Traditional trial-and-error methods made possible by ease of processing must be complemented with a fundamental understanding of the underlying mechanisms of nucleation, crystallization, thin film formation and phase separation during solution processing of small-molecules and molecule-polymer blends. This talk will focus on our recent efforts to compare and contrast the mechanisms of nucleation and growth of small-molecule thin films via drop-, spin-, and blade-casting processes. We have used a variety of time-resolved methods to detect and locate phase transformation and crystallization during and post processing, including by quartz crystal microbalance with dissipation (QCM-D), grazing incidence wide angle X-ray scattering (GIWAXS) and fast polarized optical videomicroscopy. OTFTs were fabricated and tested to establish a correlation between device performance and the final microstructure and morphology of the thin films resulting from these processes.

We report on a unique growth mode of a widely used, high open circuit voltage electron donor material subphthalocyanine chloride (SubPc), wherein films of SubPc evolve from smooth into spherical yet crystalline features at thicknesses greater than 100 nm. This growth mode is accessed by Guard Flow- Enhanced Organic Vapor Jet Printing (GF-OVJP), which utilizes a carrier gas transport to sublimated organic vapor towards the substrate in the form of a focused jet, surrounded by an annular inert guard jet. GF-OVJP enables additive patterning of organic semi-conductors without the use of liquid solvents, and shows to enhance crystallization of the organic films, particularly when guard flow is employed. The resulting crystal structure, morphology and growth of SubPc, are studied using in-situ X-Ray Synchrotron diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The spectroscopic properties of films with varying thicknesses are examined as well. The unique aspects of the highly kinetic deposition method typical to GF-OVJP are discussed in terms of possible growth mechanisms and future utilization.

We are using a frequency-dependent electro-optic technique to measure the hole mobility in small molecule organic semiconductors, such as 6,13 bis(triisopropylsilylethynyl)-pentacene (TIPS-Pn). Measurements are made on semiconductor films in bottom gate field-effect transistors. Because of the buried metal layer effect the maximum response, due to absorption in the charge layer,will be for a dielectric film ~ 1/4 of a wavelength (in the dielectric), e.g. ~ 1 micron thick in the infrared. Results will be shown for FET's prepared with both spin-cast polymer and (higher k) inorganic dielectrics prepared by atomic layer deposition. At low frequencies the results are fit to solutions to a non-linear differential equation describing the spatial dependence of flowing charge in the FET channel, but FETs prepared on alumina dielectrics show interesting deviations from the mod at high frequencies, possibly due to increased contact impedance.

While several studies have spectroscopically identified impurities in pentacene and related films of π-conjugated molecules, it remains unclear which, if any, impurities lead to charge trapping in working devices. We describe experiments in which thin pentacene transistors were intentionally layered with three different trap-precursor candidates - pentacen-6(13H)-one (PHO), 6,13-dihydropentacene (DHP), and 6,13-pentacenequinone. Trapped charge was measured locally using electric force microscopy, the samples were illuminated, and the rate of trapped-charge clearing was studied versus the wavelength of the illuminating light. Only one defect, PHO, reacted with the pentacene cation during transistor operation to yield a high, uniform density of trapped charge. This reactivity finding disagrees with ab initio predictions of the reaction free energy of PHO and DHP, suggesting that kinetics and not thermodynamics may govern the reactivity of impurities with cations (and anions) in π-conjugated films. The agreement between the observed and calculated trap clearing action spectrum in the PHO experiment is excellent, proving that trap clearing involves an internal photoexcitation of the cationic trap species, and that electronic spectra of charged species in a film of π -conjugated molecules can be measured with a cantilever. Charge motion in organic semiconductors is not well understood. We describe microscopic measurements that yield new insight into carrier dynamics in organic semiconductors. In our measurements, charges undergoing random Brownian motion in a film cause fluctuations in electric field gradients above the sample surface; these electric field gradient fluctuations can be sensed as stochastic variations (e.g., noise) in the mechanical oscillation frequency of a charged atomic force microscope cantilever near the sample. The cantilever frequency noise is Fourier transformed to yield a power spectrum of frequency fluctuations that we study as a function of tip-sample separation and charge concentration in the sample. We have examined cantilever noise over triarylamine (TPD) and polythiophene (P3HT) transistors. The mobility of both films was inferred from transistor current-voltage data, and the expected frequency noise was predicted using a theory which assumes that charges move in the film (with no lateral electric field) via uncorrelated random hops. The observed frequency noise was orders of magnitude smaller than expected. To explain this finding, we developed a theory of frequency noise over a film that accounts for contributions from both dielectric fluctuations and moving charges and includes the effects of charge-charge interactions exactly. Suppression of fluctuations due to long-range charge-charge interactions in a film of π -conjugated molecules indicates that charge-charge repulsions may also limit the charge mobility.

The electronic properties of semiconductor-polymer thin films depend profoundly on their solid- state microstructure, which in turn depends on molecular weight and processing routes. Specifi- cally, low-molecular-weight macromolecules can adopt disordered crystalline domains of cofacially (π-stacked) oligomers when processed from solution, but high-molecular-weight polymers are typ- ically strongly entangled, leading to microstructures of molecularly ordered crystalline lamella embedded in an amorphous matrix. We examine the microstructure-depedent interplay between extended exciton states along planar polymer backbones, which can be considered as Wannier-Mott excitons in the limit of weak intersite coupling, and Frenkel excitons across π-stacked polymer chain segments. We combine optical spectroscopies, wide-angle X-ray diffraction, thermal probes, and theoretical modelling, focusing on neat poly(3-hexylthiophene) films with molecular weight vary- ing from 2-450 kDa. In microstructures adopted by high-molecular-weight materials, in which long molecular chromophores π-stack cofacially, a balance of intramolecular and intermolecular excitonic coupling results in high exciton coherence lengths within chains (sim; 20 thiophene units), with inter- chain coherence limited to sim; 3 chains. On the other hand, at low molecular weight, the interchain exciton coherence is more significant (sim; 5 chains) than in the high molecular weight regime, but spatial coherence is reduced to sim; 16 thiophene units within the chain. We thus develop a rigorous, fundamental description of primary photoexcitations in semicrystalline solid-state microstructures of π-conjugated polymers: two-dimensional excitons are defined by the solid-state-microstructure- dependent interplay between intramolecular and intermolecular spatial coherence.

Organic single crystals (SCs) have recently attracted considerable attention both for fundamental research and device applications because of some properties not found in inorganics and outstanding functionalities in optoelectronic devices. However, as most organic SCs are insulators, some surface science techniques like ultraviolet photoelectron spectroscopy (UPS) or low energy electron diffraction (LEED) have been hindered by serious charging of the samples during measurements. Such charging can be overcome by using a laser for photoconduction and we could experimentally determine the valence-band structure of rubrene SCs, which have a very light effective hole mass of 0.65 m0 [1]. More recently, we extended our work to picene SCs, where, in addition to angle-resolved UPS, we also measured LEED and demonstrated that no notable surface reconstruction occurs [2]. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs. Our results suggest that with the help of a laser a variety of surface science techniques is possible even on insulators. References: [1] S. Machida, Y. Nakayama, S. Duhm, Q. Xin, A. Funakoshi, N. Ogawa, S. Kera, N. Ueno and H. Ishii, Phys. Rev. Lett. 104, 156401 (2010). [2] Q. Xin, S. Duhm, F. Bussolotti, K. Akaike, Y. Kubozono, H. Aoki, T. Kosugi, S. Kera and N. Ueno, Phys. Rev. Lett. 108, 226401 (2012).

We have identified and characterized the structural features which appear in a thin film of regio-regular poly(3-hexylthiophene) (P3HT) drop-cast from dilute solution. Periodic structures observed at the edge of the film were interrogated by optical microscopy and atomic force microscopy. It is suggested that these structures originate from an instability at the leading edge of a liquid crystal-air interface. A liquid crystalline phase has been previously identified in P3HT as it undergoes film formation during solvent evaporation. This phase occurred concurrently with a transient enhancement in mobility of the semiconductor when deposited on a field effect transistor device substrate. In the present investigation, evidence that a solution of P3HT forms a liquid crystalline phase at higher concentration was obtained through the observation of distinct birefringence and characteristic textures under crossed polarizers using an optical microscope. The experimental results obtained here suggest that a solution of P3HT can form a lyotropic liquid crystal under defined conditions. This insight is expected to have profound implications for the design and development of polymer semiconducting materials and processes for large-area, solution processed, flexible electronics.

Bond anisotropy and orientation are critical parameters that strongly influence the electronic properties of many polymer-based devices, such as poly(2,5-Bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) thin film transistors. This is directly related to the high mobility along the one-dimensional back bone, some mobility along the pi-pi stacking direction, and very low mobility along the lamellar direction. To date, ordering in active layers of devices could only be characterized with X-ray or electron microscopy methods if the sample exhibited sufficient crystallinity. Here, we show that resonant scattering with polarized soft x-rays (P-SoXS) is not limited by the crystallinity in soft matter and can probe ordering of molecular orientation down to size scales below 10 nm [1]. We utilize its high sensitivity to probe the length scale of liquid crystalline ordering in PBTTT transistors. We find that charge mobility is exponentially dependent on the liquid crystalline correlation length. This correlation length relates more closely to device performance than other characteristics as probed by wide angle x-ray diffraction. We are in the process of measuring the correlation length with P-SoXS for other high mobility materials and will present initial results. - We thank I. McCulloch and M. Heeney (Imperial College, UK) for providing the PBTTT. [1] B. A. Collins et al., Nat Mater 11, 536 (2012).

Direct writing gives us the ability to deposit organic thin films from solution with controlled thickness, grain structure and orientation The alignment of molecules, which is in built in the deposition process plays a crucial role in the charge transport properties. We have investigated TIPS-Pentacene films deposited from toluene solution at various speeds via a combination of real time synchrotron x-ray scattering and polarized-light video mi-croscopy. Through video microscopy we observe a well-defined crystallization front that becomes less defined as the writing speed is increased. In synchrotron X-ray scattering we observe that the ordering process is an order of magnitude slower than what is seen under the optical microscope. The time resolved study gives an insight about the disordered state of the initial crystallization and the solvent annealing effect from the final ordering process. Also, Observations with substrate temperatures show compressive strain in the film, causing it to delaminate from the surface followed by cracks on cooling it back to room temperature. The effect of strain on the charge carrier mobility has also been verified by FET measurements.

Crystalline organic compounds designed for use in organic electronic devices frequently exhibit problems that hinder successful x-ray structure determination, as judged by conventional quality assurance criteria. Since electronic properties of organic materials are generally a function of the relative placement of electronically active groups within the crystal, it is often sufficient to know just the overall crystal packing. Although subtle variation in packing motifs, and hence electronic properties, is achieved by alteration of side chains, the structural vagaries of such appendages is rarely of much significance. The lack of cohesion between poorly ordered solubilizing groups, for example, regularly leads to crystal morphologies that are far from optimal for single-crystal x-ray structure determination. Problem crystals are often fragile, adopting flake, plate, lath, needle and fibrous shapes. To minimize crystal damage, non-standard mounting techniques and perseverance are often necessary. Stacking faults, twinning, partial and whole-molecule disorder are all too common. It is often possible to 'solve' a structure and obtain a chemically reasonable molecular model that adequately addresses all immediately relevant questions, only to find that it does not refine well enough to meet normal publication criteria. For example, alkyl chains (particularly the branched variety) used to impart solubility are often disordered in the solid state, and are sometimes completely unmodellable. This can happen even when packing of electronically important groups is established beyond any reasonable doubt. Thus, structural features intentionally built into organic materials to enhance electronic properties and processability regularly thwart production of high-quality crystal structures. To illustrate some common problems and their solutions (or partial solutions!), a series of difficult structures that ultimately gave useful structural results, albeit with not-so-good conventional quality assessment statistics, will be presented.

It is well established that processing conditions can strongly affect the crystallization of organic semiconductors, thereby controlling the charge mobility in these materials. Furthermore, the processing history can affect morphological factors other than the crystallinity - such as the crystal quality - that will also have an effect on charge mobility. We have utilized a variety of methods, such as melt-and-quench thermal histories and mixtures of orthogonal solvents, to create films with equivalent crystallinity that nevertheless differ in the charge transport properties. This enables us to systematically examine factors other than the crystallinity in semicrystalline polymeric semiconductors and semiconductor mixtures which affect the charge mobility. For example, we find that films cast from various solvents or solvent mixtures can exhibit different charge mobilities even when the crystallinity is invariant for various polythiophene materials.

Despite its long history of being a popular ‘lab rat&’ for over two decades, Poly(3-hexylthiophene) (P3HT) remains one of the most exciting/puzzling materials in the realm of organic semiconductors. Its optical and electronic properties are known to vary drastically under different processing conditions. The photoluminescence spectrum alone changes in different solvents as well as in different film morphologies due to interactions of the chromophores with their immediate environment. The formation of aggregate structures has been implied to have an important influence on the optical as well as electronical properties in that regard. Whereas conventional optical spectroscopy leaves the role of individual polymer chains hidden in the ensemble, single molecule spectroscopy can be a powerful tool in revealing the properties of single entities comprising the bulk material. P3HT, known for its relatively low photoluminescence quantum yield and a high charge formation yield makes optical detection of single chains an extraordinarily challenging matter. Single molecule spectroscopy carried out at low temperatures (4 K) provides stable conditions to accomplish this task. We employ the advantage of this technique to arrive at insight into individual building blocks of P3HT and unravel the disorder in the bulk. Single chromophoric units of P3HT exhibit blinking as well as surprisingly stable temporal behavior with spectral linewidths narrowed down to only a few meV. Whereas organic dyes or polymer materials like polyfluorene show inhomogeneous broadening of about 20 nm, we observe a constant linewidth and vibronic coupling energy of single chains over an unprecedented spectral range of over 150 nm. Our results lead to a better understanding of the morphology of single chains and may clarify the necessity of the aggregate picture. P3HT is unique in that the extraordinary high level of intrachain morphological variability promotes crystallization in folded chains and in the bulk. This process can be followed in-situ by single chain spectroscopy, leaving very little room for the assignment of true electronic aggregates.

Control of the crystallinity in thin layers of poly(3-hexylthiophene) (P3HT) is important for improving charge mobility and transport in next generation hybrid photovoltaics (PV) cells and flexible electronic devices[1]. Using X-ray diffraction and atomic force microscopy, we show that confinement of the thin film between a flexible mold (either smooth or nanopatterned) and a rigid substrate under applied pressure leads to a strong enhancement of chain alignment and overall crystallinity [2,3]. Surprisingly, this method leads to more crystalline films in thinner layers compared to thicker ones. Moreover, we found that the mold's material and the surface functionalization of the unlerlying substrate also strongly influence the formation of crystals in the P3HT films. We discuss the effect of sample preparation, applied pressure and temperature on crystallization of the thin film. 1) Kline R.J.; McGehee M.D.; Toney M.F. Nature Materials 2006, 5, 222-228. 2) Aryal M., Trivedi K., and Hu W., ACS Nano 2009, 3 (10) , 3085-3090. 3) Barbero, D. R.; Saifullah, M. S. M.; Hoffmann, P.; Mathieu, H. J.; Anderson, D.; Jones, G. A. C.; Welland, M. E.; Steiner, U. Advanced Functional Materials 2007, 17, 2419-2425.

The band structure of organic semiconductors are determined by the chemical formula of molecules, molecular composition of crystals and molecules arranged of different molecules into a single crystal. The intrinsic properties resulting from that molecular and crystalline composition and structure are not straightforwardly available from measurements and only roughly predicted from calculations. The measured electrical and optical properties of organic semiconductors are shaped by defects, dopants and contaminants. Therefore, the structure - property relations, explored for individual molecular semiconductor and molecular binary compound, are mostly extrinsic values and only partially useful for design of new, more efficient organic semiconductors. Thus, separation of external properties from intrinsic properties and generalization of those onto large families of organic compounds require extensive purification of used organic molecules and growth of crystals, as well as development of impurities and defects characterization methods suitable for organic semiconductors. The current results of purification, crystal growth and material characterization of mono-molecular crystal and molecular binary compounds will be discussed.

Molecular crystals such as organic semiconductors exhibit significant electron-phonon coupling, so that their transport properties are intimately linked to vibrations in the crystal. In these materials there is coupling to both intramolecular and intermolecular modes. The relatively large and anisotropic changes in intermolecular spacing with temperature in organic semiconductors mean that the low-frequency intermolecular vibrations also show strong temperature dependence, which in turn affects the temperature dependence of the mobility. In extreme cases the electron-phonon interaction can even lead to superconductivity at low temperatures. In binary crystals composed of two different molecules in a stoichiometric ratio the picture is complicated by the potential for charge transfer between the molecules. In mixed-stack structures (in which the two types of molecules alternate along the stack) in the ground state the molecules may be quasi-neutral (small charge transfer) or quasi-ionic (nearly complete charge transfer), and may change from one to the other as the temperature changes. The lowest-lying excited states in these compounds also involve charge transfer, and can be probed via optical absorption. In this talk I will present recent results from optical studies of a variety of binary organic semiconductor crystals and discuss the influence of the vibrational modes on the transport properties of the materials.

Electron-phonon coupling is important in understanding charge transport in organic semiconductor charge transfer (CT) compounds, and Raman spectroscopy is a noncontact method that can be used to investigate it. Crystalline CT compounds, which are composed of donor and acceptor molecules, can have unique electrical and optical properties that differ from those of their parent crystals. In this study we investigate CT compounds with perylene as the donor and tetracyanquinodimethane (TCNQ) as the acceptor. We present Raman analysis of three mixed-stack compounds with different ratios of donors and acceptors, i.e. PNT1 (N=1,2,3). The degree of charge transfer can be extracted by observing shifts in intra-molecular vibrational frequencies compared to those of the neutral parent molecules. Using Raman spectroscopy we observe that all three compounds have minimal charge transfer between the donors and acceptors in the ground state. We further explore electron-phonon coupling using resonant Raman spectroscopy by observing the enhancement of the intensity of certain vibrational modes under excitation at the first charge-transfer transition. With excitation wavelengths coincident with the charge-transfer band maxima, the observed enhancements of the vibrational mode intensities allow us to identify the vibrations that contribute strongly to the transfer integrals and thus have an impact on electronic transport. We compare these observations with theoretical calculations of the vibrational frequencies and transfer integrals. Intermolecular vibrational frequencies are highly dependent on structure, and owing to the large thermal expansion coefficients that most CT compounds exhibit these modes are strongly affected by changes in temperature. Using a combination of resonant and temperature-dependent Raman spectroscopy we are able to observe changes in structure and the resulting changes in intermolecular interactions that are relevant to charge transport.

In recent years π-conjugated polymers have emerged as an enabling class of semiconductor materials for printed organic thin-film transistors (OTFTs) which promise applications such as low cost RFID tags, e-paper and large area flexible displays. One of the main technological hurdles that has limited the commercialization of polymer based OTFTs is their rather low charge carrier mobility, which is typically below 0.1 cm²V^-1s^-1 . Over the last decade the Xerox Research Centre of Canada has developed a series of p-type semiconductor polymers including poly(3,3&’-dialkylquarterthiophene) (PQT) and poly(4,8-dialkyl-2,6-bis(3-alkylthiophen-2-yl)benzo[1,2-b:4,5-b&’]dithiophene (PBTBT) which have field-effect mobility of 0.1 and 0.3 cm²V^-1s^-1 respectively in solution processed devices. To further increase the performance a series of dye based (co)polymers have been investigated. This presentation will highlight a recent series of semiconductor polymers based on diketopyrrolopyrrole dyes which have improved performance and exhibit field-effect mobility up to 1.0 cm²V^-1s^-1 . The design, synthesis and device performance of these materials will be presented.

Mechanical flexibility is one of the key advantages of organic semiconducting films in applications such as wearable-electronics or flexible displays. Besides, the flexibility of organic materials facilitates the fabrication of organic field-effect transistors (OFET) using a high-throughput technique, such as roll-to-roll processing on flexible substrates. The present study is aimed to gain a deeper insight into the effect of strain on charge transport properties of the organic semiconductor films. We report here a fabrication process for the preparation of high-performance flexible C₆₀-based top-gate OFET devices using flexible PET substrate and Paralyene-C as gate dielectric. The C₆₀ films with pronounced grainy morphology were deposited by Hot Wall Epitaxy technique. The fabricated OFET devices exhibited low leakage current, high performance transistor characteristics with no hysteresis, typical electron mobility of about 0.5 cm²/Vs, a low threshold voltage V_T= -0.1V, and “on-off ratio” of 10⁵. The OFET devices have been characterized by curling the substrates concavely and convexly, to apply varying values of compressive and tensile strain, respectively. The electron mobility was found to increase upon applied compressive strain, while it decreases for the tensile strain. The observed change in electron mobility is almost one order of magnitude higher compared to the change in the charge mobility reported earlier for pentacene-based OFETs. Moreover, the strain influence of electron mobility in C₆₀ films was found to be strongly anisotropic with respect to the direction of applied strain. When the applied strain was parallel to the direction of transport, recorded change was over 100% but for the perpendicular case, change was observed to be lower and non monotonous. The observed anisotropy implies that the change in the charge mobility in C₆₀ films is mostly dominated by the intermolecular coupling factor rather than the energy distribution of the traps, and thus it has the ability to discriminate between these two factors. The observed strain effect on the electron mobility in C₆₀-based OFET devices has been quantitatively described within an Extended Gaussian Disorder model (EGDM) for the hopping charge transport. Due to polycrystalline morphology of C₆₀ films the charge mobility is controlled by grain boundaries. We suggest that the grain boundaries absorb the majority of the applied strain as they form the weakest link in organic thin films in term of mechanical deformation. Analysis within the EGDM for the OFET mobility suggests that the observed strain dependence of the electron transport is dominated by a change in the effective charge hopping distance over the grain boundaries in polycrystalline C₆₀ films. Finally, based on our findings, we predict that in organic roll-to-roll processing technology, for a better management of strain induced variability, the direction of current should be kept perpendicular to the major rolling axis.

The level of charge carriers mobility, exceeding 1 cm²/Vs, achieved in both p- and n-channel organic field-effect transistors (OFETs), envisage the possibility to demonstrate high-speed, flexible complementary circuits, if a suitable level of downscaling can be implemented. In this contribution we show effective and high-yield approaches for the controlled downscaling of two critical features determining OFETs functionality: the channel length and the dielectric thickness. First we show that by a combination of digital direct-writing techniques, such as inject-printing and fs-laser ablation, it is possible to achieve sub-micron channel lengths and that these metallic electrodes can be used to fabricate high-performance OFETs. Moreover, reduction of operating voltages of OFETs is mandatory to enable their adoption in real applications. Many methods are under investigation, with the goal of preserving the overall fabrication process compatibility with plastic substrates (e.g. low temperature). In this work we investigate for the first time the use of Pulsed Laser Deposition (PLD) to deposit thin high-k dielectric layers on top of organic semiconductors to achieve low-voltage operation in high-mobility top-gate OFETs. PLD enables high deposition rates, while limiting the substrate temperature and kinetic energy of the oxides impinging on the underneath polymer layers. We demonstrate the feasibility of realizing low voltage OFETs, with low leakage and high gate break-down field, thanks to the combination of very thin high-k oxides and polymer buffer layers.

Monodisperse conjugated oligomers are receiving significant attention in fundamental and applied science due to their interesting optical, optoelectronic, and charge transport properties. These “low molecular weight” polymers serve as model structures for the corresponding polymer analogues, which are inherently polydisperse. Here we report the synthesis, electronic structure, morphology, and charge transport properties of monodisperse didodecylquaterthiophene (DDQT) oligomers up to 24 thiophene units in chain length. After the effective conjugation length is reached (22-mer), the electronic structure showed convergence behavior to the corresponding polymer, poly(3,3&’&’-didodecyl-quaterthiophene) (PQT-12) and a decrease of the degree of crystallinity with increasing molecular weight. X-ray crystal structure analysis of the dimer (DDQT-2) showed that terminal thiophenes exhibit syn-conformations, which is similar to the terminal syn-conformations found in the trimer (DDQT-3). DDQT-2 also exhibits a rare bending of the terminal alkyl side chains in order to prevent steric hindrance with neighboring hydrogens attached to core thiophenes. Grazing incidence X-ray scattering measurements reveal a morphology evolution from small molecule-like packing to polymer-like packing in thin films, with this morphology transition occurring near the effective conjugation length. The phase transition from random to orientation in the film from is also observed. In structure-property study, we correlated the electronic/molecular structure and morphology to charge transport and determined that carrier mobilities are most sensitive to chain orientation and crystallinity. The pure oligomers showed an increase in mobility with decreasing chain length. This clearly indicates that molecular weight is not a decisive factor for improved carrier mobility in low molecular weight region, but rather the degree in crystallinity and crystal orientation.

Crystalline polymer semiconductors can exhibit high charge carrier mobilities exceeding 1 cm2/Vs. However, the degree of crystallinity is not easily reproducible in solution-processed devices and can vary significantly with deposition conditions, leading to a wide mobility spread. Reduction of the degree of crystallinity in organic semiconductor is usually considered to be very undesirable, although this can offer more reproducible device performance due to isotropic nature of charge transport. In such materials mobility is expected to be significantly lower due to reduced long range molecular order. In this work we demonstrate indeofluorene copolymer field effect transistors with hole mobility of up to 0.3 cm2/Vs. Device parameters are very reproducible, and charge transport is essentially isotropic in the plain of the film. Additionally, devices show excellent ambient stability, with only 15% mobility decrease over few months, attributed to high copolymer Ionization Potential (IP) of 5.8 eV. Surprisingly, Differential Scanning Calorimetry (DSC) and microstructure characterisation methods (XRD, AFM) show no evidence of large crystalline domains in the polymer film. Moreover, the domain size seems to be below the measurement limit of the equipment. Nevertheless, measured charge transport is exceptionally high for such low-ordered microstructure, and points at efficient molecular orbital overlap in the copolymer films. We attempt to establish structure-performance relationship and uncover the reasons for high charge carrier mobility by analysing electrical transport in different channel length devices at various electric fields to extract energetic and positional disorder for this copolymer.

Organic electronics have the potential for the development of electronic products that are non-toxic and environmentally friendly. Nature represents an immense reservoir of inspiration for scientists searching for materials that are biodegradable and biocompatible. In nature, many pigments are hydrogen-bonded small molecules as opposed to van der Waals bonded synthetic molecules like pentacene, phthalocyanines or oligothiophenes. These natural molecules show H-bonding as well as pi-stacking interactions, producing highly-ordered thin films with crystalline textures unlike the typical Herringbone pattern found in many molecular semiconductors [1 ]. H-bonded, natural or nature-inspired semiconductor materials have been recently implemented in organic field effect transistors and inverter circuits, and afforded performances on par with state-of-the-art synthetic organic materials [2 , 3 , 4 ]. Among the investigated materials are naturally-occurring indigo and tyrian purple together with a large family of synthetically produced indigoids, anthraquinones and acridones. Most of these pigments are ambipolar semiconductors with balanced electron and hole mobilities in the range of 0.01 to 0.5 cm2/Vs, coupled with air stability of both electron and hole channels. Field effect mobilities as high as 2 cm2/Vs were recorded in air stable epindolidione and quinacridone (the typical yellow and magenta pigments used in inkjet printers, with a retail price below 1 USD/kg). No degradation in OFET performance was observed for samples stored and measured in air over a period exceeding 3 months. 1 E. D. Glowacki, G. Voss, L. Leonat. M. Irimia-Vladu, S. Bauer, N. S. Sariciftci, Indigo and Tyrian Purple, from Ancient Natural Dyes to Modern Organic Semiconductors, Isr. J. Chem. (2012), 52 1-12. 2 M. Irimia-Vladu, E. D. Glowacki, P. A. Troshin, G. Schwabegger, L. Leonat, D.K. Susarova, O. Krystal, M. Ullah, Y. Kanbur, M. A. Bodea, V. F. Razumov, H. Sitter, S. Bauer, N. S. Sariciftci, ”Indigo - A Natural Pigment for High Performance Ambipolar Organic Field Effect Transistors and Circuits”, Adv. Mater. (2012), 24(3) 375-380. 3 E. D. Glowacki, L. N. Leonat, G. Voss, M. Badea, Z. Bozkurt, M. Irimia-Vladu, S. Bauer, N. S. Sariciftci, “Ambipolar Organic Field Effect Transistors and Inverters with the Natural Material Tyrian Purple”, AIP Advances (2011), 4, 042132. 4 Y. Kanbur, M. Irimia-Vladu, E. D. Glowacki, G. Voss, M. Baumgartner, G. Schwabegger, L. Leonat, M. Ullah, H. Sarica, S. Erten-Ela, R. Schwödiauer, H. Sitter, Z. Küccedil;ükyavuz, S. Bauer, N. S. Sariciftci, “Vaccum Processed Polyethylene as a Dielectric for Low-operating Voltage Organic Field Effect Transistors”, Organ. Electron. (2012), 13, 919-924.

Mobility is one of the key factors that measure the performance of organic field-effect transistors (OFETs). Since the first demonstration of OFETs, the field-effect mobility (mu;_FET) has increased much over the past three decades. The mu;_FET of typical pentacene OFETs is currently comparable to that of amorphous Si transistors. Yet, the mobility value still matters as we can expand the range of OFET applications with higher mu;_FET. Here we present a method to increase the mu;_FET using a small-molecule-based gate dielectric material. Our results show that the new dielectric material enhances the mu;_FET of pentacene OFETs more than three times compared to the pentacene OFETs made on OTS-treated SiO2. This increase is achieved by 1) improvement of the crystallinity of the semiconducting layer; 2) reduction of detrimental effects from the grain boundaries; 3) reduction of undesirable scattering from the dielectric-semiconductor interface. Correlating these effects and the OFET performance, we introduce a new way to improve the mu;_FET.

Pyrene is well-known as one of the smallest polycyclic aromatic hydrocarbon (PAH) molecules that exhibit strong π-π stacking in solution or in the solid state. The electron and hole mobility of single crystalline pyrenes were found to be 3.0 cm ² V^-1s^-1 and 1.2 cm ² V^-1s^-1 at room temperature, respectively. Therefore, molecules with the pyrene moiety as their constituents are prime target matarials as high performance of organic field-effect semiconductor materials. Pyrene-based organic field effect transistors (OFETs) have not been actively investigated and only a few pyrene derivatives have been studied as an active channel layer in OFETs. We produced a new series of pyrene end-capped thiophene oligomers via Suzuki coupling in one-step process. The three kinds of thiophene-pyrene oligomers were generated. Using those molecules which have not been known so far, we fabricated the OFETs and investigated their electrical properties as an organic semiconductor field-effect transistor. Pyrene end-capped thiophene oligomers show a good p-type OFET performance through the optimization of the thin film deposition conditions.