This PDF file contains the front matter associated with SPIE Proceedings Volume 11476, including the Title Page, Copyright information, and Table of Contents.

The optical method to determine oxygen saturation in blood is limited to only tissues that can be transilluminated. With COVID-19 conventional finger pulse oximeters have surged in popularity; however, most studies of patient in-home oxygenation monitoring are conducted using only a few discrete measurement points per day. The status quo provides a single-point measurement and lacks 2D oxygenation mapping capability. We have demonstrated a flexible and printed sensor array composed of organic light-emitting diodes and organic photodiodes, which senses reflected light from tissue to determine the oxygen saturation. We use the reflectance oximeter array beyond the conventional sensing locations. The sensor is implemented to measure oxygen saturation on the forehead with 1.1% mean error and to create 2D oxygenation maps of adult forearms under pressure-cuff–induced ischemia. In addition, we present mathematical models to determine oxygenation in the presence and absence of a pulsatile arterial blood signal. The mechanical flexibility, 2D oxygenation mapping capability, and the ability to place the sensor in various locations make the reflectance oximeter array promising for medical sensing applications such as monitoring of real-time chronic medical conditions as well as postsurgery recovery management of tissues, organs, and wounds.

Both the semiconductor-dielectric interface and the presence of polymorphs in the semiconducting layer of an organic field-effect transistor (FET) dictate charge transport. Grazing incidence X-ray diffraction (GIXRD) from a side-chain-substituted thiophene DPP monomer (Ph-TDPP-Ph) reveal the presence of two triclinic phases. The FET carrier mobilities change by orders of magnitude depending on the ratio of the two triclinic phases. We further demonstrate the prospect of enhancing device performance of a DPP copolymer-based FET by texture poling of a ferroelectric dielectric layer. Such devices in conjunction with n-type sol-gel processed ZnO FET are promising in voltage inverter circuits.

Highly crystalline organic semiconductor layers patterned from a solution was inkjet printed for the fabrication of TFTs. A novel printing process enabled the micro-scale control of the film formation through the use of precision spatial placement of printed droplets over the device structure. The printing process used temporal and spatial control of the printed droplets to form a crystallization gradient. The gradient, first nucleated over an electrode that defines the TFT structure is used to recrystallize subsequently printed droplets; the initial printed film was then used to recrystallize subsequent printed regions as the channel region is deposited along the channel length. This printed gradient provides a larger grain structure to form within the active region of the TFT. Furthermore, by controlling the fluid dynamics through drying of the printed droplets, the film thickness profiles could be controlled within a specific region where the semiconductor crystallinity was enhanced

Despite decades of research, the dynamic performance of organic thin-film transistors (TFTs) remains well below that of the best inorganic TFTs. By analyzing the fundamental equations for the transit frequency of field-effect transistors, we examine the parameter space in which organic TFTs are expected to have a transit frequency of 1 GHz and derive the requirements for achieving this goal. We show that a transit frequency of 1 GHz at supply voltages of 3 V requires the channel length and the gate-to-contact overlaps to be smaller than 1 micron and the contact resistance to be well below 10 Ohm-cm.

Organic field effect transistors (OFETs) are the key players of the next generation flexible electronics. In this talk, I will focus on the meniscus guided coating (MGC) method for the OFET fabrications. If one can control the coating parameters well, the MGC method can ensure excellent crystallinity in the small molecules OFETS. This method is also highly compatible with large area deposition for mass production. Four elementary processing parameters (i) shearing speed (v), (ii) solute concentration, (iii) deposition temperature (T) and, (iv) solvent boiling point (Tb) are utilized to analyze crystal growth behavior in the meniscus-guided coating. I will also provide a generalized formula to estimate the effects of these fabrication parameters which can serve as the crystal growth guidelines for the MGC approach. I will also discuss how to use the ultra-low speed version of MGC to develop 2D monolayer organic crystal and long range in-plane ordering.

Ultrathin, oxygen-plasma-grown aluminum oxide layers form high-quality gate oxides in hybrid gate dielectrics for low-voltage organic thin-film transistors. In this work, we have investigated the materials properties of the AlOx layer, such as the thickness and the composition, using the techniques of high-resolution transmission electron microscopy and atom probe tomography to get atomic scale resolution information. We correlate these materials properties with the superior dielectric properties of the plasma-grown AlOx layer and the performance of AlOx-based hybrid gate dielectrics for organic TFTs.

Ultrathin hybrid gate dielectrics composed of an inorganic metal oxide and an organic self-assembled monolayer are useful for low-voltage organic thin-film transistors by providing a large gate dielectric capacitance and minimizing gate leakage. In this work, we focus on the role of the thin metal oxide in the hybrid gate dielectric, by investigating a plasma-grown aluminum oxide and the effects of the plasma power and duration on the growth of the aluminum oxide and the resulting properties of the gate dielectric in organic TFTs.

Low-cost, compact sensor systems for in-situ monitoring of marine conditions will provide critical information to understand how pollution and oxygen levels correlate to fish kills and aquaculture issues. The compatibility of organic electrochemical transistors (OECTs) with aqueous environments makes them promising sensor components in an ocean-sensing platform. This presentation will discuss the development of dissolved oxygen (DO) sensors, which has a detection limit to 3 ppm as required in marine studies. The challenges in balancing the oxygen reduction reaction with the doping/dedeoping process in OECTs will be shown and mitigated by using multiple voltage rails. The DO sensor will be used in conjunction with pH ion-selective transistors, to provide multi-modal measurements that assess how these parameters correlate to water quality. We will show the device sensitivity and stability under simulated marine environment.

My talk will deal with processing and characterization of conducting polymer films and devices for flexible, stretchable and healable electronics. I will particularly focus on micro-patterning of conducting polymer films for flexible and stretchable devices and on healing of conducting polymer films.

Dark-state triplet excitons lead to energy loss in organic light emitting diodes and photovoltaics; recently there has been considerable effort to convert this energy to improve device efficiency. While some observed improvement has been tied to control of the triplet state, alternate explanations cannot be ruled out as the influence of device architecture can mask underlying physics. In order to investigate triplet contribution to device output, we study single crystal 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF TES ADT) using orientation dependence of the magneto-photocurrent (MPC) in a transistor. MPC changes with magnetic field orientation in the crystal, showing triplet-controlled device output using the anisotropic spin character of the triplet.

We use organic field-effect transistor (OFET) measurements to perform a systematic study of the generation/annihilation of traps in organic semiconductors deliberately exposed to different stimuli. The first study involves the generation of a deep trap during repetitive transistor operation in ambient. The second study focuses on the dynamics of traps due to changes induced in the film microstructure in small molecule OFETs during exposure to solvents. Lastly, water-related traps in polymer films are investigated via real-time monitoring of the trap DOS spectrum during removal of water using a desiccant. A discrete peak responsible for threshold voltage instabilities was established upon removal of water from the SiO2 surface. The trap DOS spectrum provided a solid platform to understand the dynamics of trap formation/annihilation, which led to complete suppression of the discrete peak through a new surface passivation route that led to a remarkable enhancement in device performance.

Here we report a Vertical Hybrid Field Effect Transistor (VHFET) that shows an improved saturated output characteristics. Up till today the injection limited regime in vertical transistor was realized using an injection barrier as well as a buried semiconductor (SC) under the source contact. Using previous reported simulations and a new fabrication technique we successfully fabricated and characterized a functional device which operates at the injection limited regime without the need of a current limited source injection barrier. The new architecture shows better gate control with 5 ∙ 105 on/off ratio and 240 mV/dec subthreshold swing. Furthermore, we can set design rules for the vertical source contact to enable high performance Vertical Field Effect Transistors (VFET), some of which are applicable to any short-channel transistor.

The choice of a staggered or coplanar geometry for organic thin-film transistors (TFT) has significant effects on the static and dynamic electronic properties of the transistors. Using two-port network analysis, we find that the parasitic capacitances and thus the unity current-gain (transit) frequencies are significantly more dependent on the gate-to-source overlap in the staggered TFTs than in coplanar TFTs, and that the transit frequency is higher overall when a coplanar geometry is implemented. We show that these differences are primarily attributed to the lower contact resistance in the coplanar TFTs (10 Ohm-cm) as well as smaller parasitic capacitances associated with the gate-to-contact overlaps.

The characteristics of low-voltage n-channel organic thin-film transistors (TFTs) based on the small-molecule semiconductors N,N’-bis(2,2,3,3,4,4,4-fluorobutyl)-(1,7 & 1,6)-dicyano-perylene-tetracarboxylic diimide (ActivInk N1100) and 2,9-bis(heptafluoropropyl)-4,7,11,14-tetrabromo-1,3,8,10-tetraazaperopyrene (TAPP) are compared. Staggered and coplanar TFTs were fabricated, and all measurements were performed in ambient air. In the coplanar TFTs, the source and drain contacts were treated with one of three different thiols. Overall, the two semiconductors provide similar performance, with electron mobilities up to 0.15 cm2/Vs, on/off ratios up to five orders of magnitude, subthreshold slopes as small as 130 mV/decade and contact resistances (measured using the transmission line method) as small as 21 kOhm-cm.

For non-ideal field-effect transistors (FETs) or thin-film transistors (TFTs), conventional current-voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, we explore the device physics of non-uniformly accumulated FETs and develop the generalized gated four-probe (G-GFP) technique, which detects dynamic change in carrier accumulation and transport. In experiments, FETs with controllable injection are fabricated and the G-GFP technique proves to avoid the errors brought about by conventional characterizations, to exclude the temperature-dependent injection, to detect in-situ the channel potential, and to clarify the gate-dependent mobility and origin of non-ideal characteristics.

Scaling down channel dimensions of organic and polymer semiconductor based thin-film transistors to submicron and nanoscale dimensions presents several challenges. Successful scaling will enable vastly improved device performance and hence the prospects are certainly very enticing. One of the biggest challenges is in making suitably good doped source and drain regions to facilitate relatively low-resistance contacts. A more thorough understanding of velocity saturation mechanisms and charge transport at high electric fields is also necessary. We describe charge transport in oxide and polymer TFTs with an emphasis on scaling. We show that for some TFTs, scaling of the channel width facilitates scaling down of channel length and a nanostripe or nanogroove array channel geometry has advantages. Finally, we describe hybrid TFTs – with multiple materials that are better suited to scaling down. The future prospects of scaled down TFTs are discussed.

Developing high-performance thin film transistors (TFTs) based on hybrid organic-inorganic perovskites has to date been surprisingly problematic. This is in-part attributed to ionic screening of the transistor channel under applied gate voltages, and subsequent difficulty in accumulating electronic charge. Not only does this make progress in TFT technology challenging, but it also excludes TFTs as effective probes of mobility, something for which TFTs are routinely employed by the organic semiconductor community. In this talk I discuss the use of contactless time-resolved microwave conductivity (TRMC) methods in evaluating mobility in semiconductors. I demonstrate how TRMC can be used to evaluate carrier mobility in powders of new semiconductors, eliminating the major bottleneck of device optimization. We can also resolve the mobility in-plane and out-of-plane of thin-films, as well as photo-induced changes in the relative permittivity of semiconductors.

In my presentation, I introduce the vertical organic field-effect concept and highlight the development of optimized transistor geometries leading to superior performance with an on/off ratio>10⁷, transition frequency approaching 100MHz and excellent operational stability. In the second part of the contribution, I focus on the potential of VOFETs for active-matrix OLED (AMOLED) display driving. Comparing typical TFT design guidelines for vertical and horizontal TFTs, it can be deduced that the VOFET design leads to a considerable reduction of area consumption within a pixel which ultimately enables a higher display resolution and a lower power consumption. The concept of vertical transistors is not restricted to organic semiconductors. Hence, I discuss in the third part of my presentation the potential of high mobility metal-oxide semiconductors for vertical thin-film transistors for GHz operation.

Low-cost sensors for in situ monitoring of ocean conditions will provide critical information to understand how nutrients and oxygen levels correlate to fish kills and aquaculture issues. The compatibility of organic electrochemical transistors (OECTs) with aqueous environments makes them promising sensor components in an ocean sensing platform. Our OECTs is based on PEDOT as channel and carbon as electrodes. Here we design the sensor for detecting dissolved oxygen (DO) and explain the challenges in balancing the oxygen reduction reaction with the doping/dedeoping process in OECTs. We will show the device sensitivity and stability with respect to DO concentration under simulated marine environment.

Vertical organic transistors are an attractive alternative for short channel transistors, in particular, vertical organic permeable base transistors can achieve record-high transition frequencies of up to 40 MHz and drive large current densities of kA cm-2. Here, we design a novel architecture for vertical organic field-effect transistors (VOFETs) with superior electrical performance and simplified low-cost processing. By using electrochemically oxidized aluminum oxide as the charge blocking layer in VOFETs, direct leakage current paths between source and drain can be effectively suppressed, enabling VOFETs with a high on-off ratio and exceptional transconductance. Our anodization technique is easy to be realized and can be readily used on both n-type and p-type organic semiconductor thin films to achieve high-performance VOFETs, opening a new pathway to build, e.g. complementary circuit composed of vertical organic transistors.

Filtering capacitors are essential components in electronic circuits for converting high frequency alternating current (AC) into direct current (DC) with minimal energy dissipation. Yet the bulkiness and rigid configuration of traditional filtering capacitors (e.g. Aluminum electrolyte capacitors) impose size restrictions. Supercapacitor offers the potential of high capacitance density to function as miniaturized filtering capacitors. However, the charging/discharging speed of supercapacitors tend to be slow below 100 Hz and limit their use in ac filtering. To understand the mechanisms of the charging/discharging process in electrode materials, this study measures electrochromic properties of the conducting polymer PEDOT, one of the most widely used electrode materials in organic supercapacitors. The contributions of redox reactions and electrical double layer to the total capacitance were quantified. We found that redox reactions were active and dominant when the switching frequency was higher than 100 Hz. The rate of redox reactions for PEDOT changes with different applied potential due to the changing conductivity of PEDOT . The main bottleneck to fast kinetics were due to the interfacial impedance between the Faradaic materials and the current collector electrode. These results will guide future optimization of the materials and morphology design for filtering capacitor electrode.

Using Kelvin Probe Force Microscopy (KPFM), we performed surface-potential measurements on operating organic thin-film transistors (TFTs). Several parameters inaccessible through current-voltage measurements were determined, namely the source and drain resistances separately, the threshold voltage and the electric field along the channel. We show that the source resistance is always higher than the drain resistance, and non-linear intrinsic behavior is demonstrated in some cases. The threshold voltage extracted by KPFM is different from that extracted from current-voltage measurements. By analyzing the tip response using calibration samples, the electric field at the source/channel interface can be estimated within a small error margin.

Achieving gigahertz transit frequencies in low-voltage organic thin-film transistors (TFTs) will require a contact resistance below about 1 Ohm-cm [1,2]. A general approach to reduce the contact resistance in organic devices is to modify the surface of the metal contacts with a chemisorbed interface layer, ostensibly by reducing the nominal injection barrier. Combined with a thin gate dielectric, this approach can enable contact resistances below 30 Ohm-cm and transit frequencies above 10 MHz at low voltages in coplanar organic TFTs [3,4]. However, further reduction of the contact resistance depends strongly on non-idealities of the interface other than the nominal barrier height according to the Schottky-Mott limit. We show a detailed study on the efficacy of interface layers based on various thiols to improve the contact resistance in coplanar dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) TFTs. We compare the contact resistance of multiple sets of TFTs to results from ultraviolet photoelectron spectroscopy measurements and find strong evidence that Fermi-level pinning prevents a significant reduction of the contact resistance below about 100 Ohm-cm in DNTT TFTs. Therefore, we conclude that this approach may not be a generally sufficient method by itself to eliminate the contact resistance in organic TFTs.