Viscoelastic polymer semiconductors have the potential to be effective components in stretchable electronics. These malleable materials provide a simple and effective approach to realize stretchable field effect transistors and sensors.[1] For successful operation, the polymer film must be able to withstand large cyclic strains while maintaining electrical properties. Here, we show that in the stretching process, the elastomer substrate plays a critical role in the mechanical response of the semiconductor film. In particular, we explore the role of adhesion and near-surface modulus of a PDMS elastomer on the ability to achieve stretchable PDPP-4T films. We also show the use of PDMS tension on the stability of the film. We find that the increase in near-surface modulus of the PDMS and maintaining the PDMS in tension limits film wrinkling under large cyclic strain, and that an increase adhesion greatly reduces film delamination and propensity to tear. We show that through proper engineering of the elastomer substrate, the PDD-4T film has a surface roughness consistently below 3 nm for a strain range of 50% and for over 100 strain cycles. The local morphology and charge transport of the semicrystalline DPP-4T is also characterized in detail and shown to vary in a systematic and stable manner under this large strain range. These results demonstrate the ability to use low glass transition temperature polymers for intrinsically stretchable semiconductors given appropriate interactions with adjacent elastomer layers. [1] T. Sun, B. O’Connor, et al, Adv. Electron. Mater. 1600388, 2017.

Previously we have reported a general strategy to achieve solution-processed, doped films with a wide range of workfunctions (3.0–5.8 eV), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated (SC), heavily doped polymers. These SC doped polymers have been demonstrated to give good ohmic contacts processed ohmic contacts for high-performance light-emitting diodes, solar cells, photodiodes and field-effect transistors. For SC doped polymer, the mobile carriers on the polymer backbone are compensated by covalently-tethered counter-ions. The covalently-tethered counter-ion prevents dopant migration. The excess covalently-bonded counter-ions together with spectator anions provides solvent processability. We have now synthesized and characterized an extended series of SC p-doped polymer with different covalently-tethered counter-anions and spectator cations. We report the detail structural-properties of these materials on their workfunctions, film air and thermal stability, and hence device performance. We further found insignificant ion-layering in these doped polymer film by variable-angle X-ray photoemission spectroscopy. Therefore the WF of these SC p-doped films is strongly dominant by its chemical potential with minimal surface dipole contribution. The effect of ions in these doped polymers gained here is critical to the future development of materials for the emerging field of bioelectronics. [1]Tang, Choo, Ang, Keerthi, Tan, Nursyafiqah, Kugler, Burroughes, Png, Chua, Ho, Nature 539, 536-540 (2016) [2]Png, Ang, Teo, Choo, Tang, Belaineh, Chua, Ho, Nature Commun. 7:11948 (2016) [3]Seah, Tang, Png, Keerthi, Zhao, Guo, Yang, Ho, Chua, Adv. Funct. Mater. DOI: 10.1002/adfm.201606291 (2017) [4]Yang, Seah, Guo, Tan, Zhou, Matsubara, Nakamura, Png, Ho, Chua, Organic Electronics, 37, 491 (2016)

High performance transistors are indispensable building blocks for future flexible electronic circuits. In order to overcome performance limitations such as low switching speed and high driving voltage, it is necessary to shorten the channel length and use high mobility semiconductors. Vertical organic field-effect transistors (VOFET) may overcome these limitations since their channel length can be reduced to a few nanometer without being restricted by photolitographic resolution limits. Previously reported VOFETs consist of small molecules from vacuum deposition. However, these semiconductors do not show as high mobilities as achieved with new semiconductors deposited from solution and are less suited for low-cost applications because of cost considerations. Here, we present our investigations on the electrical performance of solution-processed VOFETs with the polycrystalline organic semiconductor 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacene) which shows mobilities up to 2 cm²/Vs. One major problem hindering the VOFET performance is contact resistance due to non-ohmic source/drain contacts as well as a non-linear channel resistance. Thus, we discuss the VOFET performance with regard to both kinds of resistances and reveal that the gate voltage induced modulation of contact resistance governs the IV curve for these VOFETs. The findings are supported by device simulations helping to create an improved understanding of the electric field and charge carrier distribution. We also demonstrate our results on channel resistance and compare experimental results with theoretical simulations proving that substantial improvements regarding contact resistance need to be made in order to obtain higher cutoff frequencies in organic transistors.

Organic materials are essentially expected to meet emerging technological demands that realizing flexible and wearable organic field-effect transistor (OFET) devices. In this work, we first report the pentacene based OFET that fabricated on ultra-flexible 2 μm parylene C (PC) substrate. The ultra-flexible transistor was attached on silicon, glass and kapton flexible substrates and the characteristics were compared. Furthermore, the flexible OFET worked stable and obtained high mobility of 0.15 cm2V-1S-1 and current on/off ratio of 1.5×105 under bending condition. Then, the flexible nano-floating memory (NFGM) devices by using gold nanoparticles (Au NPs) and PC electrets as the charge store were realized, and the memory performances were demonstrated. In particular, the multi-level data storage were achieved from our flexible NFGM, which could be explained the deeper level trapping behavior of NPs and these energy levels of trapped sites in PC polymer electrets. The large memory window of 23 V were obtained by application of VGS pulse of ± 40 V. In further, the bending stability/mechanical stability test with stable retention property more than 105 s, and reliable cycling endurance over 500 cycles were obtained. The results highlight the utility of combing the charge store medium in flexible NFGM devices.

The main focus of development in the field of organic electronics is on light-emitting diodes, solar cells, and various types of field-effect transistors(FET). However, for sophisticated electronic circuits, not only transistors but also high-performance diodes are required, serving as key elements in rectifiers and voltage stabilizers for various circuit applications. Such devices are essential for high-frequency signal processing (e.g. RFID systems) or power conversion. Several diode parameters, such as maximum driving-current, switch-on voltage, transition frequency, on/off-ratio and non-ideality, are relevant, depending on the specific application. Most of these parameters are closely related to the mobility of the semiconductor materials in use. Due to the anisotropy of charge carrier transport in most high-mobility organic semiconductors, the mobility is far higher in lateral devices, such as FETs, than in vertical devices, such as diodes. Therefore, it is necessary to find material systems that offer high vertical mobilities. We present diodes targeted for rectification applications that are optimized for high current density, on/off-ratio, and switching speed. We employ a pin-diode design based on highly crystalline rubrene layers. Due to their crystallinity, these layers offer a high vertical mobility. We thoroughly studied the effect of variation in n- and p-doping on the layer properties and the resulting diodes. These devices show a very low turn-on voltage, stability of up to several 100 A cm−2 at constant power and on/off ratios of 1e6. These properties possibly allow for device operations beyond 1GHz which makes them ideal devices for signal rectification, needed for e.g. wireless communication.

Proper thin film transistor (TFT) operation requires that its contact resistance Rc remains only a fraction of its channel resistance Rch. The integration of thin films based on latest generation organic semiconductors into downscaled TFTs with short channel length and high capacitance dielectric results in devices with very low Rch. Matching this with a low enough Rc is very challenging, due to the notoriously poor charge injection into organic semiconductors. The viability of integrated circuit technologies based on organic TFTs hinges on solving this critical contact resistance issue. To properly address this, it is important to use a common metric based on simple, comparable contact resistance measurements. Rc is commonly measured using the Transfer Length Method (TLM) that involves the characterization of TFTs of different channel lengths in the linear regime. We find, however, that the precision and the absolute value of the extracted Rc is greatly influenced by the conditions used to characterize each TFT. This seriously complicates the comparison to other literature values. In this talk, we present an in-depth study of the TLM technique aimed at solving these particular problems. Our TLM structures are based on high mobility organic TFTs, fabricated with different technologies and topologies. We conduct a systematic comparison of voltage- and current-controlled measurements with constant lateral electric field and charge density. As a result, we delineate the conditions to conduct TLM characterization and data treatment for clean Rc extraction. We also identify the measurement parameters that count in establishing a good Rc benchmark.

Future flexible consumer electronic devices require powerful organic field-effect transistors (OFETs) to fulfill the demanding performance targets for, e.g., flexible RFID tags or active-matrix display driving. To achieve the required switching speed and current density (e.g. for display driving), it is firstly important to improve the charge carrier mobility of the organic semiconductors but secondly, it is also essential to reduce the channel length. In this context, optimizing the geometry of OFETs led to the development of novel approachs for vertical organic field-effect transistors (VOFETs) [1] where the vertical channel can be scaled down to the range of <50 nm. Here we present VOFETs produced by a new, highly reliable integration process which allows to push the cutoff frequency, a main figure of merit, to new limits. In particular, geometrical aspects of the VOFET such as gate-source overlap and charge carrier injection/ extraction length are investigated to derive scaling laws allowing to predict dynamic device properties. Furthermore, we present an advanced patterning technique helping to maximize on/off ratio and leakage current. These improvements on device performance allow for alternating current operation above 10 MHz. Moreover, to evaluate the role of the organic semiconductor in these highly contacted limited devices, we investigate how semiconductor properties such as HOMO-LUMO gap, intrinsic doping, and mobility affect the VOFET performance and in particular device stability. Combining these finding on the device scaling and the influence of the semiconductor properties, we can provide a roadmap for future device improvement strategies.

One of the important challenges in the field of thin-film transistors is to improve designs that result in performance speeds to the GHz level. With polymer semiconductors, non-quasistatic circuits such as rectifiers in which the maximum frequency depends on the carrier transit time, have been demonstrated to work to a few 10’s of MHz. The challenge is to realize clocked sequential circuits that operate as speeds much larger than the few 10’s of kHz that have been demonstrated so far. One of the keys to this, which has received a lot of attention, is improved carrier mobility in new materials. It is also necessary to reduce the channel length considerably without serious degradation in performance. We present designs based on nanostriped polymer and other semiconductors that are very suitable for scaling down the channel length. We show that in such devices, we can achieve enhanced carrier densities and mobilities compared to regular planar devices at all channel lengths. Importantly, we can also potentially reduce the contact resistance on account of the high conductivities that result from this geometry. This will be helpful in reducing channel length. A nanostriped geometry also improves gate control and reduces short channel effects. With our optimized TFT model we demonstrate all these effects. We also compare our model predictions with experimental data from organic and polymer transistors.

The low-temperature processed organic FETs have attracted considerable attention owing to their potential application to low-cost, large-area, and flexible electronics. A use of double-layered gate insulators consisting of high-k material and polymer dielectrics is considered as effective techniques to reduce the operating voltage and interfacial trap densities. Especially, the self-assembled vertical phase separation of polymer dielectrics and organic semiconductors is easy but effective way to obtain the high-quality interface and higher FET performance. We have investigated the electrical properties and interfacial phenomena of the metal-insulator-semiconductor (MIS) diodes and the organic field effect transistors consisting of Ta/Ta2O5 (anodic oxidation)/ polystylene (PS)/ TIPS-pentacene/ MoO3/ Ag multilayers. A clear vertical phase separation of TIPS pentacene and PS layers were successfully obtained by controlling the ratio of TIPS-pentacene/PS blend and the sweep rate of meniscus coating of their solution dissolved in toluene, and it was confirmed by TOF-SIMS and cross-sectional TEM images. The operating voltage and sub-threshold swing values were successfully reduced by the high-k oxide (~50 nm-thick)/ PS (10-100 nm) double layered gate insulator. Optimized meniscus coating of TIPS-pentacene/ PS blends also contributed to the in-plane crystal-growth (pi-pi stacking) of TIPS-pentacene layers parallel to the sweep direction. The crystal size, thickness, and the field effect mobilities gradually increased with the decrement of sweep rate, and we obtained the field effect mobilities of higher than 0.3 cm2/Vs and low-operation voltages below 5 V. The interfacial electrical properties of MIS diodes were also investigated by the capacitance-voltage measurement and the displacement current measurement.