Session P22: Focus Session: Organic Electronics: FETs II

High Performance Solution Processable TFTs

Organic-based electronic devices offer the potential to significantly impact the functionality and pervasiveness of large-area electronics. We report on soluble acene-based organic thin film transistors (OTFTs) where the microstructure of as-cast films can be precisely controlled via interfacial chemistry. Chemically tailoring the source/drain contact interface is a novel route to self-patterning of soluble small molecule organic semiconductors and enables the growth of highly ordered regions along opposing contact edges which extend into the transistor channel. The unique film forming properties of soluble fluorinated anthradithiophenes allows us to fabricate high performance OTFTs, OTFT circuits, and to deterministically study the influence of the film microstructure on the electrical characteristics of devices. Most recently we have grown single crystals of soluble fluorinated anthradithiophenes by vapor transport method allowing us to probe deeper into their intrinsic properties and determine the potential and limitations of this promising family of oligomers for use in organic-based electronic devices. Co-Authors: O. D. Jurchescu$^{1,4}$, B. H. Hamadani$^{1}$, S. K. Park$^{4}$, D. A. Mourey$^{4}$, S. Subramanian$^{5}$, A. J. Moad$^{2}$, R. J. Kline$^{3}$, L. C. Teague$^{2}$, J. G. Kushmerick$^{2}$, L. J. Richter$^{2}$, T. N. Jackson$^{4}$, and J. E. Anthony$^{5}$ $^{1}$Semiconductor Electronics Division, $^{2}$Surface and Microanalysis Science Division, $^{3}$Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 $^{4}$Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802 $^{5}$Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055   

Solution-Processible Thin Film Transistors Using Surface-modified BaTiO3/Polymer Nanocomposites as Gate Insulators

Polymer/ceramic nanocomposites (NC) can exhibit high k and easily processible materials suitable for gate insulators in organic field-effect transistors (OFET). To obtain high k NCs, high volume fractions ($>$30 {\%}) of dielectric nanoparticles (NP) are needed. However, due to NP agglomeration at such high volume fractions, poor quality films with high leakage current are obtained. Recently, we have reported that phosphonic acids can strongly bind to BaTiO$_{3}$ (BT) NPs and provide enhanced dispersability of NPs in polymer hosts allowing increased volume loading. We report the use of phosphonic acid-modified BT NPs (30$\sim $50 nm) in poly(4-vinyl phenol) (PVP, k = 3.9) as gate insulators in OFET, which can be readily processed to high quality thin films by simple solution techniques. BT NPs modified with a phosphonic acid bearing a hydrophilic group afforded high quality NC thin films at high loading (up to 75 wt. {\%}) in PVP. Bottom-gate pentacene OFET devices were fabricated on the NC gate insulators. The improved film quality and increased capacitance density ($\sim $50 nF/cm$^{2}$, k $\sim $14) were reflected in a low threshold voltage ($\sim $1.1 V), a high on/off ratio ($\sim $2x10$^{5})$ and $\sim $10$^{5}$ fold decrease in leakage current as compared to that of unmodified BT.   

Evolution of the Unoccupied States in Alikali metal doped Copper-Phthalocyanine

The evolution of both the occupied and unoccupied states for Cs and Na-doped Copper-Phthalocyanine (CuPc) has been investigated with photoemission and inverse photoemission spectroscopy (IPES). From the IPES measurement, it is observed that, as the alkali metal doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts toward the Fermi level, and the shift becomes saturated when the LUMO edge is aligned with the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra. The evolution of the LUMO gives direct evidence for the origin of the doping-induced gap state in CuPc molecules. The intensity of the LUMO, as well as the gap state, for high Cs doping ratios clearly suggests that multiply charged CuPc species are formed in the doped film.   

Soft X-Ray Emission and Absorption study of the Electronic Structure of the Organic Semiconductor Titanyl Phthalocyanine (TiO-Pc)

The electronic structure of thin films of the organic semiconductor titanyl phthalocyanine (TiO-Pc) has been investigated using synchrotron radiation-excited x-ray emission and absorption spectroscopies. The films were grown \textit{in-situ}, using organic molecular beam deposition. The C and N $K$-edge spectra display similarities with those from other metal-Pcs, while the O $K$-edge and Ti $L$-edge spectra support the premise that the titanyl species are spatially isolated. Good agreement is found between a calculation of the partial density of states and the measured spectra. The Ti $L$-edge spectra display marked differences with previous reports. Two energy-loss features are reported from resonant x-ray inelastic scattering of the Ti L-edge associated with Ti $3d$*-O $2p $and Ti $3d*$-N $2p$ charge transfer transitions. Our measurements will be discussed in the context of earlier soft x-ray studies of TiO-Pc, with particular attention paid to issues of contamination and beam damage.   

Charge transport mechanisms in phthalocyanine thin films

Devices consisting of phthalocyanine thin films sandwiched between gold electrodes were fabricated by organic molecular beam deposition. Samples with different organic layer thickness were deposited on sapphire substrates in-situ, using a shadow mask and a mobile sample holder controlled manually. The structural asymmetry of the devices determined by the different metal-organic interfaces is reflected in the I-V curves at positive and negative voltages. The logarithmic scale I-V plots can be fitted with linear functions of different slopes corresponding to different conduction regimes. At low temperatures a transition from the ohmic regime to a slope two space charge limited conduction mechanism is followed at higher voltages by a high slope linear dependence that tends to saturate when the voltage reaches maximum values. At higher temperatures the intermediary space charge limited regime disappears and the transition is from ohmic to high slope space charge limited. Traps with different energy and energy distribution determine the different conduction regimes. Shallow traps located at discrete energy levels control the transport at intermediate voltages while exponentially distributed traps determine the high voltage behavior. Work supported by AFOSR-MURI.   

Soft X-Ray Spectroscopic Studies of the Electronic Structure of Aluminum tris-8-hydroxyquinoline (Alq3)

The valence and core level electronic structure of the organic semiconductor aluminum tris-8-hydroxyquinoline (Alq$_{3})$ has been measured using synchrotron radiation-excited resonant x-ray emission spectroscopy (RXES), and x-ray photoelectron spectroscopy (XPS). Samples were in the form of thin films, grown \textit{in-situ} in an organic molecular beam deposition chamber attached to the spectrometer system. The films were found to be highly sensitive to photon induced beam damage, but this problem could be alleviated by continuous translation of the films during measurement. Our RXES measurements are compared to the results of density functional theory (DFT) calculations. The DFT calculated C, N and O partial densities of states are found to agree very well with the corresponding emission spectra. Our measurements will be discussed in the context of earlier soft x-ray studies of Alq$_{3}$, with particular attention paid to issues of beam damage.   

High Carrier Density and High Hole Mobilities of Ion Gel Gated Polymer Thin-Film Transistors

We report the comprehensive characterization of ion gel gated polymer thin-film transistors (IG-PTFTs), in which PQT-12 was used as the active layer and an ion gel comprising a polymer network swollen with an ionic liquid was used as the gate dielectric. The high capacitance of ion gels ($>$10 $\mu $F/cm2) can induce a very large hole density ($\sim $ 2 x 10$^{14}$ charges/cm$^{2})$ in the channel of polymer semiconductor layers in IG-PTFTs, leading to low operation voltages, high hole mobilities of $>$ 1 cm$^{2}$/V·s, and high ON currents. High ionic conductivities of ion gels ($>$ 1 mS/cm) enable fast response time ($\sim $ 1.5 ms at 80 {\%} ON/OFF) of IG-PTFTs. Temperature dependent measurements were carried out with IG-PTFTs. In the high temperature range (310 K $\sim $ 360 K), the device showed faster response time and little hysteresis due to increasing ionic conductivity with the operating temperature. At low temperature (20 K $\sim $ 185 K) where the ions are immobile, high ON currents between source and drain can be maintained with weak temperature dependence. Overall, the results demonstrate that the IG-PTFTs offer opportunities to probe transport of high 2-D charge carrier densities in semiconductors.   

Ionic-doping-induced nonvolatile switching in conductive polymer/inorganic complex for nonvolatile memory

Organic nonvolatile memories have received extensive attention in recent years due to their low cost and highly scalability. We have studied the nonvolatile switching property of a metal/conductive polymer (MEHPPV)/metal system which is induced by ionic doping under electric field. The switching phenomenon have been observed both in devices doped by electrochemical doping in liquid solution (TBAI) and in devices integrated with a solid electrolyte (RbAg4I5) in the device structure. The device can be switched from its high-resistance state (OFF) into its low resistance state (ON) by a threshold voltage with appropriate polarity and vice versa. The switching on/off ratio is more than 3 orders of magnitude with a switching time as short as 1us and the switching is reversible and repeatable. The resistance change is attributed to the reversible p-type doping of MEH-PPV by injecting/extracting iodide anions into/from the conductive polymer under the voltage bias above certain threshold amplitude. The results of Capacitance-Voltage (CV) measurements also indicated the ion migration in the polymer under the electric field.   

Chemical Vapor Sensing Using Dual Channel Hybrid Organic/Inorganic Field-Effect Transistors

We have developed a field-effect chemical sensing device architecture in which two semiconducting channels are employed, one of which is exposed to the analyte and is chemically sensitive. The second channel (usually silicon) is used for signal transduction/amplification. Such sensors work can work in many device modes including one that can be described as a ``chemical memory mode''. For the chemically sensitive channel, several classes of materials can be employed including small molecule organic semiconductors, conjugated polymers, and inorganic oxides such as SnO$_{x}$. With organic semiconductor channels, it is possible to demonstrate charge trapping of volatile organic molecules with significant dipole moments such as ketones and alcohols. We will describe the physics of operation of such sensors in various modes and also outline how the selectivity/sensitivity can be enhanced by incorporating organic receptors.   

Correlation of microstructure and magnetotransport in organic semiconductor spin valve structures

Magnetoelectronic devices based on organic semiconductors (OSC) hold promise due to the long spin relaxation time and the ability to tune relevant properties such as interface barriers. However, it is unclear to date whether magnetotransport effects observed in these systems is due to tunneling, or whether spin-coherent diffusive transport is also possible. We have studied magnetotransport in Co/OSC/Fe trilayer junctions, with 50 to 150 nm thick OSC layer, where tunneling would not be expected. Positive magnetoresistance (MR) is observed at T = 4.2 K for several OSCs and it persists up to T = 290 K for two systems: tris(8-hydroxyquinoline) Aluminum (III) (Alq3) and copper phthalocyanine (CuPc). In order to probe the origins of MR, we have done structural studies on Co/Alq3/Fe trilayer films by x-ray reflectivity and Auger depth profiling. The results indicate well-defined layers with modest interface roughness (3-5 nm) between the Alq3 and the surrounding FM layers. While these results rule out large-scale intermixing of Co or Fe into the OSC, they do not as yet rule out the existence of local defects, such as pinholes, in the OSC layers that could enable tunneling to occur.   

EPR Studies of Highly Interconnected Nanostructured Polyaniline Network

We present temperature-dependent X-band electron paramagnetic resonance susceptibility and linewidth studies of nanostructured polyaniline doped with perchloric acid (PANN/HClO$_{4})$. From analysis of the EPR data we determine that network has both Pauli- and Curie-like susceptibility with ${\rm X}^{P}$ of $\sim $2 x 10$^{-5}$ emu/mole-2-ring repeat unit and a localized spin density of $\sim $ 1 spin per 400 2-ring repeat units and exhibits a Lorentzian-like lineshape. The EPR linewidth from 100 K to room temperature exhibits two different linear regimes, where the linewidth increases linearly with increase in temperature. We will discuss the role of Korringa relaxation in determining the high temperature linewidth. We will consider the roles of disorder, localization and interfiber contact within the nanostructure network.   

Capacitance-voltage characterization of polythiophene-based field-effect transistors

We report on frequency-dependent capacitance-voltage characteristics of organic field-effect transistors based on (2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as the active polymer layer. The gate voltage and frequency behavior of the devices with the polymer spun on treated and untreated oxide gate dielectric are explored. The high quality of the devices (contact and channel properties) allows the use of traditional CV modeling to accurately describe the electrical characteristics of the intrinsic channel. The findings from this study provide new insight into charge trapping and transport in the field-accumulated channel of organic field-effect devices.