This PDF file contains the front matter associated with SPIE Proceedings Volume 11811, including the Title Page, Copyright information, and Table of Contents.

Even though record organic semiconductor mobilities are reported for organic semiconductor single crystals, making thin film crystals remains difficult. We will show our efforts to understand crystal formation, epitaxy, and transport. In particular, we will discuss our efforts to realize pinhole free films of numerous organic semiconductors with 100s microns scale grains, and how the materials able to undergo a transition from amorphous to crystalline correlate well with thermal properties. Homoepitaxial studies uncover evidence of point and line defect formation in these films, indicating that homoepitaxy is not always strain-free. Transistors made out of large-grained films of rubrene display charge carrier mobility of up to 3.5 cm2 V–1 s–1, very close to single crystal values, highlighting their potential for practical application. Finally, we will show efforts in achieving heteroepitaxial growth of a different molecular material on top of a crystalline organic template.

The charge carrier mobility in organic field-effect transistors (FETs) may be enhanced by a few orders of magnitude by an appropriate choice of the dielectric layer. Polymer ferroelectric dielectrics with their high dielectric constants are attractive for low-operating voltage FETs. However, due to the dynamic coupling of the charge carriers to the electronic polarization at the semiconductor-dielectric interface, polymer ferroelectric based organic FETs may result in low carrier mobilities. Selective electrical poling of the ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene) (PVDF-TrFE), is seen to greatly improve the performance of small molecule and donor-acceptor copolymer based FETs [1]. A combination of vertical and lateral poling of the PVDF-TrFE layer, which reduces the gate leakage current as well as mitigates polarization fluctuation driven transport, yields carrier mobilities upwards of 1 cm2/Vs in TIPS-pentacene and 0.5 cm2/Vs in diketopyrrolopyrrole based FETs under ambient conditions [2]. Other strategies for improving the performance of FETs involve dissolving the ferroelectric polymer in high dipole moment solvents and adding thin polymer buffer layers. The incorporation of magnetic nanoparticles in non-ferroelectric dielectrics is yet another approach for enhancing the dielectric constant. Ferrite nanoparticles with biomimetic peptide nanostructures as gate dielectrics have ramifications in low-operating voltage organic FETs [3]. This work was supported by National Science Foundation under Grant No. ECCS-1707588 [1] Laudari et al. Adv. Mater. Interfaces 6, 1801787 (2019). [2] Laudari et al. ACS Appl. Mater. Interfaces 12, 26757 (2020). [3] Khanra et al. ACS Appl. Nano Mater 1, 1175 (2018).

Sequential solution doping is a processing technique that allows a conjugated polymer film to be doped from a solvent that will not dissolve the polymer. We present here a method to predict the film doping level in cm-3 from the solution concentration used to dope the film. We show using four polymers and three different and newly synthesized dopants that the doping level can me modeled using a simple Langmuir isotherm. In addition, analysis of the UV/vis spectra shows filling of the density of states. Polymers with a sharper band edge demonstrate much high conductivity for the same hole density. We analyze a series of DPP polymers and show how the polymer order changes as a function of the doping level. A second recent discovery is that the anion in sequentially doped films can be exchanged with another anion after doping. This means that the reactive molecule used to doped the polymer can be removed and replaced with a different ion that is not reactive. We present a multi-ion Langmuir isotherm model and show that the film doping level in mixed ion solutions can also be predicted.

Crystals that twist as they grow are common but little known and introduce completely unexplored features to materials design. Here, we present growth-induced twists to molecular semiconductor crystals with the expectation that microstructure and continually precessing crystallographic orientations can modulate interactions with light, charge transport, and other optoelectronic processes. We have found that a variety of organic semiconductors and charge transfer complexes can be readily induced to grow from the melt as spherulites of tightly packed helicoidal fibrils. The twisting pitch can be controlled by the degree of undercooling after melting or through the incorporation of additives. Intriguingly, charge mobilities measured using field-effect transistor platforms have been found to increase with increasing extent of twisting. These results indicate crystal twisting to be a promising strategy for modulating the performance of optoelectronic devices.

Silylethyne substitution is a versatile approach to control solubility and crystalline order in high aspect ratio chromophores such as acenes. We have developed some simple rules to predict crystal packing in such substituted systems, and these rules were more recently refined by careful computational analysis. We also found that trialkylgermylethynyl-substituted acenes and heteroacenes followed nearly identical rules to the silyl derivatives. Due to their lower cost and more versatile synthetic variability, we have now begun to prepare trialkyl-carbon substituted alkynes to use in the crystal engineering of acenes and heteroacenes. We were rather surprised to discover that these carbon-based systems did not follow the same rules for packing as the silyl or germyl alkynes. We will present a systematic study of acene crystal packing in relation to several classes of carbon-based alkyne systems, the low-cost, scalable syntheses of these alkynes, computational assessment of the resulting crystal packings, and FET studies of select materials. We have also expanded the dimensionality of the backbones under study by incorporating pyrene units - the impact of pyrene insertion on the electronic structure of acenes will be covered in detail.

The emerging research field of organic bioelectronics has developed rapidly over the last few years and elegant examples of biomedically important applications including for example in-vivo drug delivery and neural interfacing have been demonstrated. The organic electrochemical transistor (OECT), capable of transducing small ionic fluxes into electronic signals in an aqueous environment, is an ideal device to utilise in bioelectronic applications. To date, nearly all OECTs have been fabricated with commercially available PEDOT:PSS, heavily limiting the variability in performance. We have previously shown that tailor-made semiconducting polymers are fully capable of matching the performance of PEDOT:PSS. To capitalise on this discovery and the versatility of the organic chemistry toolbox, further materials development is needed. In my talk I will discuss our recent work in this area covering examples of both molecular and polymeric semiconducting materials and their performance in bioelectronic devices.

Chemists have the capability to tune the electronic, redox, and optical properties of π-conjugated molecules and polymers, which in turn are used as the building blocks to develop organic semiconductors. In these materials, the nature of the molecular-scale solid-state packing arrangement dictates performance, rendering knowledge as to how materials processing impacts these arrangements critical. Currently, there exist few hard-and-fast rules that combine molecular design and process conditions in the discovery of new organic semiconductors, as the understanding that links these domains is quite limited. Here, we will discuss the development of data-enabled tools, including an open-access database dedicated to crystalline organic semiconductors, that aim to seek out physicochemical understanding across the multiple physical and chemical scales that dictate performance and offer machine-based discovery and design of new generations of organic semiconductors.

Organic Field Effect Transistors (OFETs), while showing a lot of promise, currently suffer from a number of limitations. Organic doping can help to overcome these limitations. It opens up a number of new possibilities by offering a way to define majority charge carriers, control the charge carrier density, threshold voltage etc. precisely and produce devices with better performance, stability, and reproducibility. The doping techniques explored in OFETs thus far have been in the range of a few wt.%, which has limited the use of doping to contact doping or a thin doped layer at the gate dielectric interface. Furthermore, the high doping concentrations used place serious limitations on the doping efficiency that can be achieved. Here we demonstrate the successful use of low doping in the 100ppm range throughout the bulk of the organic semiconductor layer of an OFET with the use of a rotating shutter.

In this presentation, we will describe our results with field-emission type source and drain contacts in which the metal electrodes are tapered to a sharp point or an array of sharp points. The enhanced electric field as a consequence of this geometry results in more facile injection of charges from the metal to the band states of the semiconductor. The on-resistance of short channel TFTs is lowered and the channel lengths of TFTs can be reduced below 100 nm without adversely affecting the shape of the current-voltage characteristics.

Organic field-effect transistors (OFETs) have found a wide range of uses due to their attractive properties. A great deal of effort has been expended on boosting their mobilities, which tend to be low. Given this, accurate estimation of the mobility is crucial. We have developed a web application that automates or simplifies several of the steps required to estimate the mobility from experimental data. The app can be accessed at ofetanalysisapp.shinyapps.io/ofetanalysisapp. The app takes as inputs a file with the data and pieces of information like the number of OFETs and their channel lengths. The app has features that enable the user to mark OFETs as outliers, which are excluded from subsequent calculations. It fits nonlinear regression models to compute estimates of the mobility as well as the threshold voltage. The app provides several visualizations that give the user insight into the nature of the data. The estimates computed by the app can be downloaded in an Excel file so the user can perform further analysis. The use of the app is illustrated with a dataset from one of our OFET experiments.

Vertical organic field effect transistors (VOFET) and vertical static induction transistor (VSIT) have shown the advances of large on-current, high compatibility with sensors, ease of solution-processing and good mechanical flexibility. However, there has been no concise and explicit theories for these transistors. Here, we draw the physical images of the mechanisms, derive the electrostatic potentials, and propose the simple current-voltage relations for these vertical organic transistors. The theory has been verified by numerical simulation and are consistent with experimental results. The theories also provide guidances for device designing toward sharp turn-on properties, a large on-off ratio and good saturation degree.

Using direct-write electron-beam lithography, low-voltage organic thin-film transistors (TFTs) with channel lengths and parasitic gate-to-source and gate-to-drain overlaps as small as 100 nm have been fabricated on flexible polymeric substrates. Despite the small channel lengths and gate-to-contact overlaps, these TFTs display good static current-voltage characteristics, including on/off current ratios of nine orders of magnitude, subthreshold swings of about 100 mV/decade, turn-on voltages of 0 V, negligibly small threshold-voltage roll-off, and contact resistances below 1 kOhm-cm. TFTs with such small critical dimensions are of interest for high-frequency applications.

In this talk, I will focus on the meniscus guided coating (MGC) method for the OFET fabrications. We will demonstrate and elucidate why the organic monolayer OFETs developed by MGC would show outstanding contact resistance values under staggered structure especially during low source-drain bias (VDS) operations. The device under study is 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) monolayer with the van der Waals integration of metal electrodes. I will deviate the access resistance component and interface resistance component of the contact resistance. We noticed access resistance of organic semiconductor is extracted under -1 mV drain-source bias, while the Schottky diode at the metal-organic interface is negligible. On the other hand, the diode effect at the metal-organic interface can be amplified by increasing the VDS level and eventually dominates the device performance.

With our marine ecosystems under threat from climate change, there is an urgent need to continuously monitor marine conditions. One key indicator is the dissolved oxygen level, but existing sensors are limited by size and costs that preclude widespread non-intrusive monitoring. This work reports a new dual-gate design based on organic electrochemical transistors (OECTs) to track dissolved oxygen concentration in seawater, a highly challenging matrix owing to its high ionic strength and multitude of chemical interferents. We present the novel operating principle, by deriving the channel conductance with respect to potentials on the two gates. The sensor achieved a detection limit of 0.5 ppm dissolved oxygen concentration in seawater. The device demonstrated reliable operation over five days and was capable of monitoring oxygenation changes arising from the photosynthesis cycles of saltwater macro-algae.

While there have been many impressive demonstrations of neuromorphic computation in recent years, input stimuli provided to this hardware generally still take a form designed for von Neumann processors. For example, in a CCD detector an array of pixels is sampled at fixed intervals in time. Here we have taken inspiration from the human retina and demonstrated an event-driven sensor which pre-processes optical signals by design. Using a thin film semiconductor as one dielectric layer of a bilayer capacitor, we demonstrate a device which changes its capacitance under illumination. When in series with a resistor, and a constant bias is applied across this device, the voltage dropped across the resistor will spike temporarily as the capacitor (dis)changes, before returning to its equilibrium value. The result is a sensor which spikes in response to changes in illumination, but otherwise outputs zero voltage. This design hence inherently filters out non-pertinent information such as static images, providing a voltage only in response to movement. Using a simple model based on Kirchhoff’s Laws, we are able to parameterize this device and accurately reproduce its behavior in simulations. It is hoped that this work represents the first step towards a paradigm shift for the design of sensing systems for neuromorphic computation, and artificial intelligence in general.

Organic electrochemical transistors (OECTs) have been demonstrated in a wide range of applications such as analyte detection, neural interfacing, impedance sensing and neuromorphic computing. Majority of OECTs use PEDOT:PSS and liquid electrolytes. In this talk, I will discuss the development of biomaterials as solid state electrolytes and conjugated polyelectrolytes (CPEs) as semiconductors for OECTs. The biogels that consist of gelatin and glycerol with high ionic conductivity are used as solid electrolytes. We establish a relation between morphology and protonic-conductivity of the gels, allowing for the fabrication of gel-based OECTs with desirable functionalities, good ON/OFF ratio and transconductance, fast-switching speed, and good stability in ambient air. Anionic CPEs are used as mixed conductor materials for OECTs to replace PEDOT:PSS. CPE-based OECTs operate in the accumulation mode, which allows for much lower energy consumption in comparison to commonly used depletion mode PEDOT:PSS devices. The physical and electrical properties of CPE-K have been fully characterized to allow a direct comparison to other top performing OECT materials. CPE-K demonstrates an electrical performance that is among the best that have been reported in the literature for OECT materials.

My talk will deal with processing and characterization of conducting polymer films and devices for flexible, stretchable and healable transistors. Self-healing electronic materials are highly relevant for application in biology and sustainable electronics. We observed mechanical and electrical healability of PEDOT:PSS thin films. Upon reaching a certain thickness (about 1 µm), PEDOT:PSS thin films damaged with a sharp blade can be healed by simply wetting the damaged area with water. The process is rapid, with a response time on the order of 150 ms. Significantly, by blending with other polymers, the films are transformed into autonomic self-healing materials without the need of external stimulation. This reveals a new property of PEDOT:PSS and enables its immediate use in flexible and biocompatible electronics, such as electronic skin and bio-implanted electronics, placing conducting polymers on the front line for healing applications in bioelectronics.