Session W49: Focus Session: Organic Electronics and Photonics - Thin Film Transistors

Pressure effect on organic field-effect transistors

Macroscopic transport in organic semiconductors is governed by intermolecular charge transfer, necessarily resulting in its sensitivity to molecular arrangement. The effect of external pressure in such soft materials is fundamentally important because of vulnerability in molecular displacement against relatively small force. Here, we introduce a method of measuring the effect of hydrostatic pressure on the conductivity in organic semiconductor crystals inducing high-mobility charge with the application of electric field. We performed four-terminal conductivity measurement to exclude extrinsic influence of the metal/semiconductor contact resistance. In addition, Hall coefficients are simultaneously measured to deduce the pressure coefficient properly. Using rubrene single-crystal transistors, variation of mobility under pressure turned out to be about 7 times larger than in the typical experiments reported for silicon and other inorganic semiconductors. Interestingly, the mobility starts to decrease with further increasing pressure above 600 MPa. The anomalous negative pressure effect indicate that the application of pressure not only diminishes distance between centers of adjacent molecules but relative positions of equivalent atoms in the two molecules.   

Determining the elastic constants of rubrene single-crystals

Organic single crystals have opened the doors to a new generation of high-performance organic electronic devices. Exceptional charge-transport properties combined with the advent of large-area patterning techniques make organic single crystals excellent candidates for flexible electronics applications. However, in order to effectively employ organic single crystals on mechanically flexible architectures, their mechanical properties need to be understood and characterized. In this presentation, the mechanical properties of rubrene single-crystals are investigated. Given the limited dimensions of as-grown crystals and associated handling difficulty, the elastic buckling instability is chosen as a metrology tool for determining the in-plane elastic constants. Our results show that ultrathin (200nm - 1000nm) rubrene crystals exhibit anisotropic wrinkling wavelengths as a function of crystallographic direction, which can be correlated to the anisotropic nature of its molecular packing. An adaptive intermolecular reactive bond order potential (AIREBO) is employed to calculate the nine elastic constants corresponding to orthorhombic rubrene.   

Tuning contorted hexabenzocoronene crystal structure and texture for organic field-effect transistors

Crystallography conducted on single crystals reveals contorted hexabenzocoronene (HBC) can adopt either herringbone (Pbcn) or slip-stack (P21/c) packing motifs. By adjusting the molecule-solvent interactions during solvent-vapor annealing (SVA), we can controllably crystallize thin films of HBC and access both packing motifs. In HBC films annealed with dichloromethane (DCM) vapor, molecule-solvent interactions are strong and yield highly oriented Pbcn crystals. However, in films annealed with hexanes vapor, molecule-solvent interactions are weaker and randomly oriented P21/c crystals form. In addition to tuning the molecule-solvent interactions via solvent choice, the interactions may also be modulated by selectively fluorinating the peripheral aromatic rings of HBC. With increased fluorination, we decrease molecule-solvent interactions during SVA. As such, we can coax these HBC derivatives to adopt the P21/c crystal structure even with DCM SVA. Further, more fluorinated HBCs form more oriented crystals when exposed to DCM vapors. Transistors fabricated with crystalline HBC active layers suggest that the mobilities of these devices are, to first order, determined by the extent of crystal orientation and less so by the crystal structure. The ability to independently access both crystal structures with varying degrees of orientation has allowed us to decouple their relative contributions to device performance.   

Quantitative assessments of the effect of microstructure on transport in organic semiconductors

From the fundamental standpoint, organic semiconductors are fascinating as they are neither crystalline nor amorphous and their microstructure plays a central role in governing charge transport. I will show that understanding disorder is the key to determining charge transport mechanism. We are particularly interested in cumulative disorder (paracrystallinity), where the lattice parameter takes on a Gaussian distribution about its mean value. The disorder parameter g allows us to rank materials quantitatively on a continuous scale, from a perfectly crystalline material (g $<$ 1\%) to an amorphous one (g $>$ 10\%). Surprisingly, even the polymers that are considered to have the highest crystallinity (PBTTT) have a g in the pi-stacking direction close to that of an amorphous material ($\sim$7\%). Furthermore, comparison of X-ray diffraction data and optical absorption data provides insight into the nature of the disordered phase as well. Using first principle calculations and tight binding methods, I will show that paracrystallinity in the pi-stacking direction provides a fundamental mechanism for the existence of an exponential distribution of localized tail states in the gap. The larger the degree of disorder the higher the trap density and the deeper their energy. Using disorder as a ranking parameter, I will discuss the differences in transport between small molecule and polymer films as well as their respective inherent limitations and bottlenecks.   

Air-Stable Solution-Processed Thin-Thin Film Transistors with Hole Mobilities of 3.5 cm$^{2}$/Vs

We report on organic thin-film transistors fabricated on a novel soluble small molecule organic semiconductor difluoro bis(triethylgermyl) anthradithiophene. Fabrication techniques are all applicable at room temperature and ambient pressure, and include drop-casting, spin-coating, and spray deposition. Devices exhibit remarkable electronic properties, including charge carrier mobilities as high as 3.5 cm$^{2}$/Vs, on/off current ratios of 10$^{5}$, and good environmental and operational stability. Chemical treatment of the contact surface with self-assembled monolayers allows us great control of the crystalline order within the organic semiconductor layer. Because thin-film microstructure defects such as grain boundaries reduce the charge transport capabilities of the active layer, high quality single crystals are grown by physical vapor transport for comparison. By correlating the electrical properties with the structural data obtained from X-ray diffraction, we find that a good $\pi -\pi $ overlap is responsible for this superior electronic behavior.   

Variable-range hopping transport and Hall effect measurements in electrolyte-gated P3HT

Extensive charge transport measurements (1.5 -- 250 K) at gate-tuned hole concentrations between 1*1$^{20}$ and 9*10$^{20}$ cm$^{-3}$ have been made on a single ion-gel gated poly-(3-hexylthiophene) (P3HT) thin film transistor. We report observation of a robust Hall effect, having rational trends with magnetic field, gate voltage, and temperature, and yielding hole concentrations close to those measured via the charging current. At high doping we observe transitions from apparent band transport, to 3D Mott variable range hopping (VRH), to Efros-Shklovskii (ES) VRH on cooling. At lower doping ES VRH is observed at all temperatures. A detailed analysis of the temperature and field-dependence of the VRH resistivity provides information on the localization length and dielectric constant as a function of doping, providing significant insights into the approach to the insulator-metal transition in this system and the nature of the Coulomb-gapped density of states. Work at UMN supported by NSF MRSEC   

Charge Density Dependent Hole Mobility and Density of States Throughout the Entire Finite Potential Window of Conductivity in Ionic Liquid Gated Poly(3-hexylthiophene)

Ionic liquids, used in place of traditional gate dielectric materials, allow for the accumulation of very high 2D and 3D charge densities ($>$10$^{14}$ {\#}/cm$^{2}$ and $>$10$^{21}$ {\#}/cm$^{3}$ respectively) at low voltage ($<$5 V). Here we study the electrochemical gating of the benchmark semiconducting polymer poly(3-hexylthiophene) (P3HT) with the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]). The electrochemical stability of [EMI][FAP] allowed the reproducible accumulation of 2 x 10$^{21}$ hole/cm$^{3}$, or one hole (and stabilizing anion dopant) per every two thiophene rings. A finite potential/charge density window of high electrical conductivity was observed with hole mobility reaching a maximum of 0.86 cm$^{2}$/V s at 0.12 holes per thiophene ring. Displacement current measurements, collected versus a calibrated reference electrode, allowed the mapping of the highly structured and extremely broad density of states of the P3HT/[EMI][FAP] doped composite. Variable temperature and charge density hole transport measurements revealed hole transport to be thermally activated and non-monotonic, displaying a activation energy minimum of $\sim $20 meV in the region of maximum conductivity and hole mobility. To show the generality of this result, the study was extended to an additional four ionic liquids and three semiconducting polymers.   

Conductivity Maxima on the Surface of Organic Semiconductor Crystals at High Charge Densities

We have previously achieved effective carrier mobility up to 3.2 cm$^{2}$V$^{-1}$s$^{-1}$ at charge densities larger than 10$^{13}$ cm$^{-2}$ in rubrene electrical double layer transistors (EDLTs) gated with ionic liquids (ILs). At lower temperatures when a larger gate bias can be applied, the maximum attainable charge density reaches 6.5*10$^{13}$ cm$^{-2}$ (0.34 holes per rubrene molecule), and more remarkably, two pronounced maxima in channel conductivity have been reproducibly and stably observed. This feature, which has not been reported for any EDLTs gated with electrolytes, is independent of ionic liquid composition, current-voltage sweep rate, and crystallographic directions of rubrene crystals. We have identified that the first and second conductivity peaks occur at charge densities of 2.0*10$^{13}$ cm$^{-2}$ and 5.2*10$^{13}$ cm$^{-2}$, respectively. Capacitance-voltage (C-V) measurements at different frequencies have also revealed two maxima at the same gate voltages as in current-voltage measurements. Collectively, these observations imply that the conductivity maxima at high charge densities are very likely related to the electronic band structure on the surface of rubrene crystals.   

Solvent-Resistant Organic Transistors and Thermally-Stable Organic Photovoltaics Based on Crosslinkable Conjugated Polymers

Conjugated polymers in general are unstable when exposed to air, solvent, or thermal treatment, and these challenges limit their practical applications. Herein we have developed a simple, but powerful approach to achieve solvent-resistant and thermally stable organic electronic devices with improved air-stability, by introducing a crosslinkable group into a conjugated polymer. To demonstrate this concept, we have synthesized polythiophene with crosslinkable groups attached to the end of alkyl chain. Photo-crosslinking of crosslinkable P3HT dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport. This is the first demonstration of solvent-resistant organic transistors. Furthermore, the bulk-heterojunction organic photovoltaics (BHJ OPVs) containing crosslinkable P3HT show an average efficiency higher than 3.3{\%} after 40 h annealing at an elevated temperature of 150$^{\circ}$C, which represents one of the most thermally-stable OPV devices reported to date. This enhanced stability is due to an in-situ compatibilizer that forms at the P3HT/PCBM interface and suppresses macrophase separation.   

Surface enhanced Raman spectroscopic studies of the metal-semiconductor interface in organic field effect transistors

The performance of organic field-effect transistors (FETs) largely depends on the nature of interfaces of dissimilar materials. Metal-semiconductor interfaces, in particular, play a critical role in the charge injection process. Here, Raman spectroscopy is used to investigate the nature of the Au-semiconductor interface in pentacene based FETs. A large enhancement in the Raman intensity (SERS) is observed from the pentacene film under the Au layer. The enhancement is evidence of a nano-scale roughness in the morphology of the interface, which is further confirmed by electron microscopy images. The morphology of the interface is investigated by SERS as a function of the pentacene layer thickness and the Au layer thickness. The Raman spectra are found to be extremely sensitive in detecting small changes in the morphology of the interface in the sub-nanometer range. Changes in the Raman spectra are further tracked after biasing and ageing the devices. Evolution of these Raman spectra is correlated with degradation in device performance. Finally, FETs based on other donor-acceptor semiconductors are probed by Raman scattering and contrasted with those of the pentacene-based devices.   

Ultra-thin body poly(3-hexylthiophene) transistors with improved short-channel performance

The microstructure and charge transport properties of binary blend of regioregiolar (rr) and regiorandom(RRa) poly(3-hexylthiophene) (P3HT) are investigated. X-ray diffraction study shows vertical phase separation in the blend films, with rr-P3HT crystallized at the semiconductor/dielectric interface. These thin film transistors with layered structure preserve high field effect mobility when rr-P3HTcontents are reduced to as low as 5.6{\%} where the channel thickness is only a few nanometers. As a result of this ultra-thin active layer at interface, short channel effects due to bulk currents are eliminated, suggesting a new route to fabricate high performance, small size and reliable electronic devices.   

Physical Characterization of Functionalized Silk Material for Electronic Application and Devices

Naturally harvested spider silk fibers are investigated for their physical properties under ambient, humidified, iodine-doped, pyrolized, sputtered gold and carbon nanotube coated conditions. The functional properties include: humidity activated conductivity; enhanced flexibility and carbon yield of pyrolized iodized silk fibers; full metallic conductivity and flexibility of micron-sized gold-sputtered silk fibers; and high strain sensitivity of carbon nanotube coated silk fibers. Magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared spectroscopy (FTIR) are used to explore the nature of ambient and functionalized spider silk fiber, and significant changes in amino acid-protein backbone signature are correlated with gold sputtering, and iodine-doped conditions. The application of gold-sputtered neat spider silk fibers for making four terminal flexible, clean, ohmic contacts to organic superconductor samples and carbon nanotube coated silk fibers for heart pulse monitoring sensor are demonstrated. The role of silk thin film in organic thin film transistor will be briefly discussed.   

Spectroscopic signatures of ambipolar injection in narrow gap donor-acceptor polymer transistors

Donor-Acceptor (D-A) copolymers have recently emerged as versatile materials for use in a large variety of device applications. Specifically, these systems possess extremely narrow bandgaps, enabling ambipolar charge transport when integrated in solution-processed field-effect transistors (OFETs). However, the fundamentals of electronic transport in this class of materials remain unexplored. We present a systematic investigation of ambipolar charge injection in D-A conjugated polymers polybenzobisthiadiazole-dithienopyrrole (PBBTPD) and polybenzobisthiadiazole-dithienocyclopentane (PBBTCD) using infrared spectroscopy. We observed a significant modification of the absorption edge in both PBBTPD- and PBBTCD-based OFETs under the applied electric field. The absorption edge reveals hardening under electron injection and softening under hole injection. The most straightforward interpretation of the observed band edge modification is in terms of the linear Stark effect, implying the existence of a built-in electrical dipole moment in these polymers. Additionally, we carried out microscopic IR measurements to characterize the ambipolar injection profile between electrodes.