Colin Reese

Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers

The development of an electronic skin is critical to the realization of artificial intelligence that comes into direct contact with humans, and to biomedical applications such as prosthetic skin. To mimic the tactile sensing properties of natural skin, large arrays of pixel pressure sensors on a flexible and stretchable substrate are required. We demonstrate flexible, capacitive pressure sensors with unprecedented sensitivity and very short response times that can be inexpensively fabricated over large areas by microstructuring of thin films of the biocompatible elastomer polydimethylsiloxane. The pressure sensitivity of the microstructured films far surpassed that exhibited by unstructured elastomeric films of similar thickness, and is tunable by using different microstructures. The microstructured films were integrated into organic field-effect transistors as the dielectric layer, forming a new type of active sensor device with similarly excellent sensitivity and response times.

Patterning organic single-crystal transistor arrays

Field-effect transistors made of organic single crystals are ideal for studying the charge transport characteristics of organic semiconductor materials. Their outstanding device performance, relative to that of transistors made of organic thin films, makes them also attractive candidates for electronic applications such as active matrix displays and sensor arrays. These applications require minimal cross-talk between neighbouring devices. In the case of thin film systems, simple patterning of the active semiconductor layer minimizes cross-talk. But when using organic single crystals, the only approach currently available for creating arrays of separate devices is manual selection and placing of individual crystals—a process prohibitive for producing devices at high density and with reasonable throughput. In contrast, inorganic crystals have been grown in extended arrays, and efficient and large-area fabrication of silicon crystalline islands with high mobilities for electronic applications has been reported. Here we describe a method for effectively fabricating large arrays of single crystals of a wide range of organic semiconductor materials directly onto transistor source–drain electrodes. We find that film domains of octadecyltriethoxysilane microcontact-printed onto either clean Si/SiO2 surfaces or flexible plastic provide control over the nucleation of vapour-grown organic single crystals. This allows us to fabricate large arrays of high-performance organic single-crystal field-effect transistors with mobilities as high as 2.4 cm2 V-1 s-1 and on/off ratios greater than 107, and devices on flexible substrates that retain their performance after significant bending. These results suggest that our fabrication approach constitutes a promising step that might ultimately allow us to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications.

Organic single-crystal field-effect transistors

Organic molecular crystals hold great promise for the rational development of organic semiconductor materials. Their long-range order not only reveals the performance limits of organic materials, but also provides unique insight into their intrinsic transport properties. The field-effect transistor (FET) has served as a versatile tool for electrical characterization of many facets of their performance. In the last few years, breakthroughs in single-crystal FET fabrication techniques have enabled the realization of field-effect mobilities far surpassing amorphous Si, observation of the Hall effect in an organic material, and the study of transport as an explicit function of molecular packing and chemical structure.

Water-stable organic transistors and their application in chemical and biological sensors.

The development of low-cost, reliable sensors will rely on devices capable of converting an analyte binding event to an easily read electrical signal. Organic thin-film transistors (OTFTs) are ideal for inexpensive, single-use chemical or biological sensors because of their compatibility with flexible, large-area substrates, simple processing, and highly tunable active layer materials. We have fabricated low-operating voltage OTFTs with a cross-linked polymer gate dielectric, which display stable operation under aqueous conditions over >104 electrical cycles using the p-channel semiconductor 5,5′-bis-(7-dodecyl-9H-fluoren-2-yl)-2,2′-bithiophene (DDFTTF). OTFT sensors were demonstrated in aqueous solutions with concentrations as low as parts per billion for trinitrobenzene, methylphosphonic acid, cysteine, and glucose. This work demonstrates of reliable OTFT operation in aqueous media, hence opening new possibilities of chemical and biological sensing with OTFTs.

High-performance microscale single-crystal transistors by lithography on an elastomer dielectric

Organic single crystals have emerged as powerful tools for the exploration of the intrinsic charge transport properties of organic materials. To date, however, the limited number of fabrication techniques has forced a steep compromise between performance, reproducibility, range of feature sizes, gentle treatment of the single crystal, and facility of construction. Here the authors present a materials-general technique for the fabrication of single-crystal field-effect transistors with the use of a spin-coated elastomer gate dielectric and photolithographically defined source and drain electrodes. This allows the production of feature sizes and patterns previously impossible with reported elastomeric techniques yet yields devices with performance far superior to those fabricated on nonconformal dielectrics. The authors measure saturation-regime mobilities of 19.0 and 1.9cm2∕Vs for the semiconductors rubrene and pentacene, comparable to the best published values, and 2.4cm2∕Vs for tetracene, nearly double t...

Organic single crystals: tools for the exploration of charge transport phenomena in organic materials

The promise of organic semiconductors for use in new and existing electronic devices has initiated exploration of the underlying transport mechanisms in such materials. Single crystals offer a unique way to investigate charge transport as a function of molecular properties and arrangement. Recent developments in techniques show great promise for such exploration, and point to the convergence of theory and experiment for this important class of functional materials.

Band structure measurement of organic single crystal with angle-resolved photoemission

The electronic structure of bulk rubrene single crystal was studied with angle-resolved photoemission spectroscopy. Highly reproducible dispersive features were observed with nice symmetry about the Brillouin zone center and boundaries, representing the band structure measured for a bulk organic single crystal. The high quality of the surface was confirmed with scanning tunneling microscopy. The energy dispersion of the highest occupied molecular orbitals derived bands showed strong anisotropic behavior in the a-b plane of the unit cell. The measured band structure, however, differs unexpectedly from theoretical calculations in terms of the amount of the dispersion and the separation of the bands.

Overestimation of the field-effect mobility via transconductance measurements and the origin of the output/transfer characteristic discrepancy in organic field-effect transistors

Paramount to the rational design of electronic materials is the accurate characterization of their intrinsic properties. In particular, many applications of conducting and semiconducting soft materials have been driven by the development of materials with high, bias-stable field-effect mobility. Here, we demonstrate the effect of parasitic resistance and bias-dependent mobility on device electrical characteristics. Specifically, we analyze two of the most commonly employed test algorithms—the output and transfer curves—via a closed-form analysis. The analysis exhibits characteristics endemic to those published in literature, such as effective mobilities with maxima with respect to gate voltage that may lead to overstatements of mobility by manyfold. Furthermore, analysis reveals that common overestimation relative to intrinsic and output-estimated mobilities is caused solely by gate-bias-dependent mobility, and parasitic resistance can only lead to an underestimation of the effective mobility. We introduc...

Isotropic transport in an oligothiophene derivative for single-crystal field-effect transistor applications

Single-crystal organic semiconductors have proven invaluable tools in the exploration of charge transport in molecular materials. We employ the elastomeric, photolithographically patterned single-crystal field-effect transistor in the study of an alpha-substituted oligothiophene. The terminal units specify a symmetric layered motif, while allowing the oligothiophene cores to pack closely. Angle-resolved measurements of the field-effect mobility reflect the symmetric edge/face interactions and isotropic mobility. These measurements are supported by electronic structure calculations that show nearly equivalent intermolecular interactions along cell diagonals. These results reveal that the transport is diffusive and a minimum of fourfold symmetry is required for in-plane mobility isotropy.