Leslie H. Jimison

Large modulation of carrier transport by grain-boundary molecular packing and microstructure in organic thin films

Solution-processable organic semiconductors are central to developing viable printed electronics, and performance comparable to that of amorphous silicon has been reported for films grown from soluble semiconductors. However, the seemingly desirable formation of large crystalline domains introduces grain boundaries, resulting in substantial device-to-device performance variations. Indeed, for films where the grain-boundary structure is random, a few unfavourable grain boundaries may dominate device performance. Here we isolate the effects of molecular-level structure at grain boundaries by engineering the microstructure of the high-performance n-type perylenediimide semiconductor PDI8-CN2 and analyse their consequences for charge transport. A combination of advanced X-ray scattering, first-principles computation and transistor characterization applied to PDI8-CN2 films reveals that grain-boundary orientation modulates carrier mobility by approximately two orders of magnitude. For PDI8-CN2 we show that the molecular packing motif (that is, herringbone versus slip-stacked) plays a decisive part in grain-boundary-induced transport anisotropy. The results of this study provide important guidelines for designing device-optimized molecular semiconductors.

High transconductance organic electrochemical transistors.

The development of transistors with high gain is essential for applications ranging from switching elements and drivers to transducers for chemical and biological sensing. Organic transistors have become well-established based on their distinct advantages, including ease of fabrication, synthetic freedom for chemical functionalization, and the ability to take on unique form factors. These devices, however, are largely viewed as belonging to the low-end of the performance spectrum. Here we present organic electrochemical transistors with a transconductance in the mS range, outperforming transistors from both traditional and emerging semiconductors. The transconductance of these devices remains fairly constant from DC up to a frequency of the order of 1 kHz, a value determined by the process of ion transport between the electrolyte and the channel. These devices, which continue to work even after being crumpled, are predicted to be highly relevant as transducers in biosensing applications.

Quantification of thin film crystallographic orientation using X-ray diffraction with an area detector.

As thin films become increasingly popular (for solar cells, LEDs, microelectronics, batteries), quantitative morphological and crystallographic information is needed to predict and optimize the films electrical, optical, and mechanical properties. This quantification can be obtained quickly and easily with X-ray diffraction using an area detector in two sample geometries. In this paper, we describe a methodology for constructing complete pole figures for thin films with fiber texture (isotropic in-plane orientation). We demonstrate this technique on semicrystalline polymer films, self-assembled nanoparticle semiconductor films, and randomly packed metallic nanoparticle films. This method can be immediately implemented to help understand the relationship between film processing and microstructure, enabling the development of better and less expensive electronic and optoelectronic devices.

Measurement of Barrier Tissue Integrity with an Organic Electrochemical Transistor

The integration of an organic electrochemical transistor with human barrier tissue cells provides a novel method for assessing toxicology of compounds in vitro. Minute variations in paracellular ionic flux induced by toxic compounds are measured in real time, with unprecedented temporal resolution and extreme sensitivity.

Microstructural Origin of High Mobility in High-Performance Poly(thieno-thiophene) Thin-Film Transistors

Adv. Mater. 2010, 22, 697–701 2010 WILEY-VCH Verlag Gm In recent years, semiconducting polymers have been widely studied for their potential applications in low-cost, printed, and flexible electronic devices. Carrier mobility in these materials has been steadily increasing, approaching that of hydrogenated amorphous silicon. The best-performing polymeric semiconductors exhibit a high degree of order and are typically semicrystalline. Charge transport in semicrystalline polymers is controlled at several length scales. In the ordered regions of the material, conjugated polymer chains stack in lamellar sheets with p–p interactions between neighboring chains. In addition to ordered crystallites, the microstructure of semicrystalline polymers comprises disordered regions. The spatial arrangement of the crystallites and the disordered regions affect transport via trapping at defects and the percolation properties of the crystalline and disordered networks. Therefore, in order to develop accurate models of charge transport, it is important to understand the relationship between the morphology of the film, its microstructure and its electronic properties. Identifying transport mechanisms and bottlenecks is of particular relevance to the design of materials with improved performance. In an effort to improve mobility by controlling the microstructure of the polymer, a family of poly(2,5bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes, PBTTT) was recently synthesized. When processed on self-assembled monolayers such as octyltrichlorosilane (OTS) and heated into the liquid crystalline regime, thin films of PBTTT can achieve high room-temperature mobilities up to 0.7–1 cm V 1 s . X-ray diffraction (XRD) strongly suggests that crystallites in PBTTT films are significantly more ordered than those found in thin films of other regio-regular poly(thiophenes) such as poly(3hexyl-thiophene) (P3HT) or poly(3-30 0 0-didodecylquarterthiophene) (PQT-12) The superior electronic performance of annealed PBTTT films is attributed to a highly organized mesoscale morphology that arises from annealing the material through its liquid-crystalline phase. Indeed, atomic force microscopy (AFM) reveals the existence of large (a few hundred nm) terraces that are interpreted as crystalline grains. As a result, transport across the film is thought to be greatly enhanced due to the lower areal density of grain-boundaries, which are known to impede charge transport. Furthermore, because of the liquid-crystalline nature of PBTTT, the regions between the crystalline domains are likely to have a more ordered morphology compared to that of other polymers. To first order, transport in semicrystalline polymers can be understood as a combination of transport through a network of crystallites separated by defects that trap charge. If the regions between the crystallites are more ordered, mobility should be increased due to the correspondingly improved intergranular transport. Thus, according to the previous discussion, it is expected that PBTTT films exhibit a much lower trap density than films of P3HT or PQT-12, and such reduced trap density is the reason for the higher mobility in PBTTT. In this work, we investigate explicitly factors that limit charge transport in PBTTT transistors. We combine a study of charge transport in PBTTT thin films by thin-film transistor (TFT) measurements with structural andmorphological characterization performed by AFM and transmission electronmicroscopy (TEM). Charge transport is analyzed using the mobility edge (ME) model, which has been successfully applied to other semicrystalline polymers before. The great advantage of the ME model is that it allows us to deconvolute the effect of traps and estimate the mobility of the mobile charge in the film, thereby providing means to compare structure–property relationships between different polymers. TFTs were prepared in the bottom-gate staggered configuration. Highly doped silicon wafers with 200 nm of thermal oxide were used as substrates and were cleaned prior to undergoing UV irradiation in an ozone furnace for 20min. Substrates were submerged in octadecyltrichlorosilane (OTS) to form amonolayer on the dielectric surface. Semiconducting polymers solutions, 0.5wt% for both PBTTTwith a C14 alkyl chain (Mw1⁄4 70 kDa) and P3HT (Mw1⁄4 64 and 158 kDa) in 1,2-dichlorobenzene (DCB), were deposited on the substrates via spin-coating. Films of PBTTT were annealed at 180 8C for 10min and, then, cooled down slowly through the mesophase region. To fabricate transistors, 80-nm-thick gold contacts were thermally evaporated

PEDOT:TOS with PEG: a biofunctional surface with improved electronic characteristics

Devices based on conducting polymers offer great promise for interfacing with cells. Here, we use vapour phase polymerisation to create a biofunctional composite material of the conducting polymer poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:TOS) and the biologically relevant poly(ethylene glycol) (PEG). On the addition of PEG, electroactivity of the PEDOT is maintained, conductivity is increased, and its performance as the active material in a transistor is unaffected. Both direct and indirect biocompatibility tests prove that PEDOT:TOS and PEDOT:TOS:PEG are biocompatible and nontoxic to mammalian cells. A functionalised PEG (PEG(COOH)) was additionally introduced into PEDOT:TOS to showcase the potential of this material for use in applications requiring biofunctionalisation.

Validation of the organic electrochemical transistor for in vitro toxicology

BACKGROUND The gastrointestinal epithelium provides a physical and biochemical barrier to the passage of ions and small molecules; however this barrier may be breached by pathogens and toxins. The effect of individual pathogens/toxins on the intestinal epithelium has been well characterized: they disrupt barrier tissue in a variety of ways, such as by targeting tight junction proteins, or other elements of the junctions between adjacent cells. A variety of methods have been used to characterize disruption in barrier tissue, such as immunofluorescence, permeability assays and electrical measurements of epithelia resistance, but these methods remain time consuming, costly and ill-suited to diagnostics or high throughput toxicology. METHODS The advent of organic electronics has created a unique opportunity to interface the worlds of electronics and biology, using devices such as the organic electrochemical transistor (OECT), whose low cost materials and potential for easy fabrication in high throughput formats represent a novel solution for assessing epithelial tissue integrity. RESULTS In this study, OECTs were integrated with gastro-intestinal cell monolayers to study the integrity of the gastrointestinal epithelium, providing a very sensitive way to detect minute changes in ion flow across the cell layer due to inherent amplification by the transistor. MAJOR CONCLUSIONS We validate the OECT against traditional methods by monitoring the effect of toxic compounds on epithelial tissue. We show a systematic characterization of this novel method, alongside existing methods used to assess barrier tissue function. GENERAL SIGNIFICANCE The toxic compounds induce a dramatic disruption of barrier tissue, and the OECT measures this disruption with increased temporal resolution and greater or equal sensitivity when compared with existing methods. This article is part of a Special Issue entitled Organic Bioelectronics - Novel Applications in Biomedicine.

A Boltzmann-weighted hopping model of charge transport in organic semicrystalline films

We present a model of charge transport in polycrystalline electronic films, which considers details of the microscopic scale while simultaneously allowing realistically sized films to be simulated. We discuss the approximations and assumptions made by the model, and rationalize its application to thin films of directionally crystallized poly(3-hexylthiophene). In conjunction with experimental data, we use the model to characterize the effects of defects in these films. Our findings support the hypothesis that it is the directional crystallization of these films, rather than their defects, which causes anisotropic mobilities.

Interfacial effects in thin films of polymeric semiconductors

The surface onto which polymeric semiconductors are cast from solution plays an important role in determining the electrical transport properties of the polymeric thin film. The authors use synchrotron-based x-ray diffraction to show that even moderate roughness (rms∼5 A) can affect the texture of semicrystalline poly(thiophene) thin films. Moreover, the authors use a novel optical characterization technique (surface plasmon resonance spectroscopy) to characterize the appearance of electronic states in the bandgap of thin films (∼20 nm) of regioregular poly(thiophene). Such states may be due to the heterointerface between the thin Au substrate and the polymer.

Microstructural effects on the performance of poly(thiophene) field-effect transistors

The performance of polymer field-effect transistors is highly dependent on their processing history. For instance, thermal processing plays a role in micro-structure development and consequently in device performance. A transport model was developed based on the semiconductor micro-structure where highly mobile states are located in the crystalline areas and defects and disordered regions correspond to areas where carriers are trapped. By applying this model to electrical characterization data of PQT-12 (a regio-regular polythiophene), it is found that annealing tightens the energetic distribution of the traps. Films quenched from the melt performed worse than annealed films due to an increased trap density and broader energy distribution of the traps. X-ray diffraction in grazing and specular geometry was carried out at the Stanford Synchrotron Radiation Laboratory on PQT-12 thin films to reconcile the predictions of the transport model with the micro-structure of the PQT-12 thin films. In all cases the polymer crystallites are textured with the π-stacking direction in the plane of charge transport and the rocking curves indicate the existence of a population of highly oriented crystallites. Annealing the as-spun films improves the crystallinity and texture, in agreement with the transport model. Quenching produces defects in the films, which are likely to produce traps, thereby lowering the carrier mobility.