Symposium EP06—Organic Electronics—Materials and Devices

Despite all efforts to advance the performance of organic transistors, their transit frequencies remain far below 1 GHz. The reason is that previous efforts have focused mainly on parameters that have little or no impact on the high-frequency performance. By analyzing the fundamental equations for the transit frequency, the requirements for gigahertz performance are derived. Not surprisingly, the critical parameter in this quest is not the charge-carrier mobility, but the contact resistance.

A thorough understanding of tunnel currents in disordered organic semiconductors is essential to understand charge transport and injection into disordered organic semiconductors^1–3. Furthermore, tunnel currents provide an additional parameter space to design novel devices, e.g. organic Zener-Diodes^4,5, AC-driven OLEDs⁶, or inversion type OFETs^7,8.
In this presentation, a new analytic model to describe HOMO to LUMO tunneling in organic Zener diodes is presented. It is shown that the particular shape of the density of states in the organic layer determines the breakdown voltage in these diodes. Furthermore, the ideality factor of the diodes is shown to be limited by the width of the density of states and the roughness of the organic layer.
Based on this model, approaches to widen the design space of organic field-effect transistors are discussed. It is shown, how HOMO to LUMO tunneling can be used to facilitate ambipolar injection and generation of minority charge carriers in organic field-effect transistors⁷. Furthermore, the design and performance of Organic Tunnel Field-Effect Transistors is presented and ways to improve their performance are discussed.

1. Coropceanu, V., Cornil, J., da Silva Filho, D. A., Olivier, Y., Silbey, R. & Bredas, J.-L. Charge transport in organic semiconductors. Chem. Rev.107,926–952 (2007).
2. Schmechel, R. Gaussian disorder model for high carrier densities: Theoretical aspects and application to experiments.Phys. Rev. B66,235206 (2002).
3. Arkhipov, V. I., Emelianova, E. V, Tak, Y. H. & Bassler, H. Charge injection into light-emitting diodes: Theory and experiment. J. Appl. Phys.84,848–856 (1998).
4. Kleemann, H., Gutierrez, R., Lindner, F., Avdoshenko, S., Manrique, P. D., Lüssem, B., Cuniberti, G., Leo, K., Lüssem, B., Cuniberti, G. & Leo, K. Organic Zener Diodes: Tunneling across the Gap in Organic Semiconductor Materials. Nano Lett. 10,4929–4934 (2010).
5. Kleemann, H., Gutierrez, R., Avdoshenko, S., Cuniberti, G., Leo, K. & Lüssem, B. Reverse breakdown behavior in organic pin-diodes comprising C60 and pentacene: Experiment and theory. Org. Electron. 14,193–199 (2013).
6. Perumal, A., Fröbel, M., Gorantla, S., Gemming, T., Lüssem, B., Eckert, J. & Leo, K. Novel approach for alternating current (AC)-driven organic light-emitting devices. Adv. Funct. Mater. 22,(2012).
7. Al-Shadeedi, A., Liu, S., Keum, C.-M., Kasemann, D., Hoßbach, C., Bartha, J., Bunge, S. D. & Lüssem, B. Minority currents in n-doped organic transistors. ACS Appl. Mater. Interfaces 8,32432 (2016).
8. Lüssem, B., Tietze, M. L. M. L., Kleemann, H., Hoßbach, C., Bartha, J. W. J. W., Zakhidov, A. & Leo, K. Doped organic transistors operating in the inversion and depletion regime. Nat. Commun. 4,2775 (2013).

We have recently demonstrated approaches that tune the molecular structure to suppress energetic disorder at interfaces and suppress coherent lattice vibrations in small molecule semiconductors to enhance charge mobilities. We control chain conformations within high-k fluorinated polymers, minimize energetic disorder at the dielectric–semiconductor interface, and enhance charge mobilities in rubrene single-crystal devices. Copolymerization of vinylidene fluorene and bromotrifluoroetheylene (BTFE) monomers yields photopatternable, high-k fluoropolymer poly(vinylidene fluoride-bromotrifluoroethylene), P(VDF-BTFE). BTFE moieties allow crosslinking through thermal- or photocuring, and crosslinking of polymer chains perturbs the chain conformations within the polymer films. Devices incorporating hot-pressed, crosslinked P(VDF-BTFE) as the dielectric and rubrene single crystals as the active layer exhibit low interfacial trap densities compared to SiO2 and other polymer dieletrics, as well as charge mobilities exceeding 10 cm^2 /V s. These charge mobilities are more than an order of magnitude higher than mobilities of rubrene single-crystal OFETs comprising poly(vinylidene fluoride–tetrafluoroethylene) P(VDF-TeFE) copolymers as gate insulator layers, and are the highest charge mobilities reported to date for devices incorporating high-k (k > 3) polymer dielectrics. Furthermore, we show that alkyl side chains can disrupt key modes of lattice vibrations that limit transport. We explore a model series based on [1]benzothieno[3,2-b][1]-benzothiophene (BTBT) derivatives where we systematically add one or two octyl side chains. Single crystal field-effect transistors made from these materials show an enhancement in hole mobilities by nearly three orders of magnitude upon addition of alkyl side chains in active layer materials. Quantum calculations based on the static structure cannot predict this large variation in charge mobilities. Instead, inelastic neutron scattering spectra (of deuterated BTBT derivatives) show that the addition of alkyl side chains suppresses picosecond intermolecular lattice vibrations, thereby enhancing charge mobilities in devices. Thus, although a trade-off has been hypothesized between disrupting charge transport and enhancing solubility due to the addition of alkyl side chains, we instead show that the addition of alkyl side chains can enhance charge mobilities by multiple orders of magnitude.

With the advent of the Internet of Things (IoT), strong demand has grown for flexible and stretchable sensors. Particularly, sensors based on p-conjugated molecules covering small organic molecules and polymers have recently attracted great interest due to their high potential for use in flexible, low-cost, solution-processable, large-area electronics. Functional properties of organic active layers can be tailored by rational molecular design or surface functionalization to enhance their selectivity and sensitivity. Nanoscopically engineered organic semiconducting materials have emerged as promising building blocks for high-performance flexible sensors. In this talk, the development of high-performance organic and polymeric semiconductors will be presented with viable approaches to selectively tune the dominant polarity of charge carriers and achieve efficient charge transport, which embrace the rational design of conjugated backbones, side-chain engineering, microstructural and morphological control. Unconventional organic and polymeric nanomaterials covering single-crystalline nanowires, nanoporous films, core-shell nanomaterials, multiple-patterned plasmonic nanostructures, and chiral supramolecules will be described with their applications in flexible and wearable sensors including photodetectors, chemical and biological sensors. In addition, the fundamental charge transport and photophysical phenomena of molecule-based active layers will be discussed.

Organic electronics is a valuable technology for the development of novel applications in different fields, ranging from bioelectronics to smart packaging and displays. Transparency and compliance are often fundamental features for an effective device functionality and integration into complex systems. Several examples of organic devices, including diodes and Organic Field-Effect Transistors (OFETs) on transparent and ultra-flexible substrates, have been reported in literature. In the case of OFETs, however, the fulfillment of these characteristics is often related to the employment of small area, small throughput techniques which are not suited for the final development of complex applications and possibly up-scalable for production. Moreover, low voltage operation is complex to be obtained with cost-effective, large area techniques, in particular when organic materials are employed in order to improve transparency of devices. As a matter of fact, low voltage transparent OFETs fabricated using cost-effective, large area techniques with enhanced trasparency and conformability are absent in literature. Nevertheless, filling such a gap is fundamental in order to obtain fully functional systems that can be effectively employed in bioelectronics and electronics applications.
Here, an original approach for the fabrication of transparent, low voltage, all-organic FETs by means of large area techniques is presented. Two cost-effective, large-area techniques easily up-scalabe to an industrial size, namely inkjet printing and chemical vapor deposition, were employed for device fabrication. In particular, inkjet printing was used for patterning transistor electrodes using a PEDOT:PSS-based commerical ink, and for the deposition of the p-type, organic semiconductor, namely TIPS pentacene, from a custom-made solution. Chemical Vapor Deposition was used for the deposition of the organic insulator, namely Parylene C, in a uniform, 200-nm thick film allowing the low voltage operation. The overall transistor structure is perfectly transparent, as demonstrated by absorbance characterization in the range of visible light. More than 50 devices have been successfully fabricated over 175um-thick polyethylene terephtalate (PET) substrates: devices showed a quasi-zero threshold voltage (-0.10±0.05 V), a significant charge carrier mobility (0.25±0.07 cm²V^-1sec^-1), very low subthreshold slope (0.13±0.03 Vdec^-1). A good reproducibility of electrical parameters was obtained. Interestingly enough, these valuable performances were obtained for devices operating with voltages as high as 3 V. Finally, the fabrication process was successfully transferrend onto ultra-thin, Parylene C substrates for the development of compliant electronics that can easily conform on complex surfaces, such as the human skin in tattoo electronics applications. Several devices have been fabricated on 600nm-thick Parylene C substrates: electrical characterization carried out before and after transfering on the skin demonstrated that the device functionality is perfectly maintained. Moreover, the ultra-thin OFETs were employed for the fabrication of basic electronics circuits, such as pseudo-CMOS inverters, which were successfully tested on skin. These results pave the way for the development of transparent organic electronics for different kind of applications, including bioelectronics and tattoo-like electronics.

The optimization of organic thin-film transistors (TFTs) to address specific analog and digital circuit design requirements depends heavily on the choice of the materials employed, particularly the organic semiconductor and the gate dielectric. For a particular organic semiconductor, the performance must be reviewed with different combinations of substrate material, fabrication conditions and the choice of the gate dielectric material in order to achieve the optimum TFT characteristics for particular device and circuit needs. We have fabricated and characterized organic TFTs using the small-molecule organic semiconductor 2,7-diphenyl[1]benzothieno[3,2-b][1]-benzothiophene (DPh-BTBT) [1] in combination with a hybrid gate dielectric composed of aluminum oxide and a fluoroalkyl-phosphonic acid self-assembled monolayer. We find that the optimum substrate temperature during the vacuum deposition of the DPh-BTBT layer depends on the type of substrate on which the organic TFTs are fabricated. For vacuum-deposited DPh-BTBT to form a closed layer with the highest possible charge-carrier mobility, the semiconductor deposition should be carried out with the substrate held at a temperature of 100°C on silicon substrates and without substrate heating on flexible plastic substrates. XRD measurements indicate different orientations of the DPh-BTBT molecules under different fabrication conditions, which can be correlated to the TFT performance. Furthermore, fluoroalkyl-phosphonic acids with fluoroalkyl chain lengths ranging from 6 to 14 carbon atoms have been used to vary the thickness of the self-assembled monolayer in the gate dielectric, and its influence on the TFT characteristics has been studied. The fluoroalkyl terminal groups facilitate a shift of the TFTs’ turn-on voltage [2] in a deterministic manner, e.g., to exactly 0 V, if so desired from a circuit-design perspective, while taking advantage of the possibility to optimize the other TFT parameters by selecting the optimum fluoroalkyl chain length. We find that a medium fluoroalkyl chain length (10 carbon atoms in the fluoroalkyl chain) leads to the highest charge-carrier mobility (0.4 cm²/Vs on flexible plastic substrates, 1 cm²/Vs on silicon substrates) and the largest on/off current ratio (above 10⁶). [1] K. Takimiya et al., J. Am. Chem. Soc. 128, 12604, 2006. [2] U. Kraft et al., J. Mater. Chem. 20, 6416, 2010.

An ever increasing interest in the development and application of innovative optical and optoelectronic devices places greater emphasis for the advancement of new smart and functional materials that are readily processable. Significant progress has already been realised in the fields of organic light-emitting diodes (OLEDs) and photovoltaic cells (OPVs) through development of novel semiconducting materials. Here we discuss developments and advancements in materials design towards photonic structures that aid and improve light management in organic and inorganic/organic hybrid devices. Here, we cover systems targeted for use in light input-/output-coupling structures, anti-reflection coatings, waveguides, and beyond. Extension to architectures for heat management, important e.g. for a broad range of photovoltaic device platforms, will also be presented.

In field-effect transistors, charge is accumulated in only a-few-nanometer-thick surface of semiconductors interfacing gate-insulating layers, so structurally well-defined molecular-layer single-crystal films of organic semiconductors are highly beneficial in saving the material. We recently succeeded in growing highly homogeneous organic semiconductor films of ultrathin single crystals, so that high values of charge-carrier mobility exceeding 10 cm²/Vs is achieved everywhere in the dimensions of 10 cm x 10 cm substrates. Named as “organic single-crystal wafers” the multilayer films are ready to be fabricated to CMOS integrated circuits.
P- and n-type organic semiconductor materials are newly synthesized for high-mobility carrier transport based on strategies of minimizing effective masses and phonon scattering rates. Both layers are formed from solution by continuous crystallization at the edge of coating blades. With recent development of key processing technologies for printed LSIs, low-cost platforms for RFID tags, AD converters, data processors, and sensing circuitries will be displayed. In particular, developing simple integrated devices based on CMOS using the p-type and n-type printed organic FETs, successful rectification and identification are demonstrated at 13.56 MHz with printed organic CMOS circuits.

Solution-based productions of both metal and semiconductor patterned layers are the essential components for printed electronics technologies that have made considerable progress in recent years. Next important challenge is to combine and integrate the respective printing techniques, for realizing high-performance, highly-stable, and all-printed organic thin-film transistors (OTFTs). Here we demonstrate that stable operations become possible in solution-processed OTFTs, when they are composed of ultrafine silver electrodes that are produced by the “surface photo-reactive nanometal printing (SuPR-NaP)” technique [1]. The SuPR-NaP technique is based on a silver-nanoparticle chemisorption effect on photo-activated patterned perfluoropolymer surfaces, which allows easy, high-speed, and large-area manufacturing of ultrafine metal wiring.
In the fabrication of bottom-gate, bottom-contact-type OTFTs, we used spin-coated layer of perfluoropolymer, Cytop (Asahi Glass Co., Ltd.), that functions as a base layer for producing patterned photo-activated surface in the SuPR-NaP technique for producing source/drain electrodes but also as a gate dielectric layer (combined with base silica/Si substrates) [2]. As the surface of Cytop layer is extremely hydrophobic, we used unique “push-coating” technique to produce uniform polymer semiconductor channel layer on the Cytop layer with source/drain electrodes [3]. For reference, we also prepared OTFTs without Cytop layer on silica/Si substrates, where source/drain electrodes were fabricated by vacuum deposition of Au. We found that the OTFTs with hydrophobic Cytop layer exhibit much higher carrier mobility (more than 10 times larger) than that of the devices without Cytop layer on silica/Si substrates. Additionally, the performance of the former devices is featured by negligible hysteresis and small threshold voltage around 0 V, where threshold voltage shift due to gate bias stress is effectively suppressed, as compared to the large hysteresis of about 3 V in the latter devices. We discuss the origin of stable and highly-efficient operations of the printed OTFTs, in terms of the elimination of carrier traps in the high-quality gate dielectric/semiconductor interface realized with Cytop in the SuPR-NaP technique [4,5].

[1] T. Yamada et al., Nat. Commun. 7, 11402 (2016).
[2] G. Kitahara et al., Org. Electron. 50, 426-428 (2017).
[3] M. Ikawa et al., Nat. Commun. 3, 1176 (2012).
[4] C. Liu et al., J. Phys. Chem. C 117, 12337-12345 (2013).
[5] B. Blülle et al., Phys. Rev. Appl. 1, 034006 (2014).

Organic semiconductors can be integrated onto device structures in different physical forms such as single crystals, polycrystalline thin-films, or amorphous thin-films. There are many studies which have shown that by altering molecular orientation or packing, the extrinsic properties of an electronic device properties can be impacted. The structural order of the molecular solid profoundly influences the electronic properties, that in turn controls important properties, such as the transport gap and energetic level of the highest occupied molecular orbital (HOMO). While there are numerous photoemission spectroscopic measurements of organic semiconductors in thin-film structures, far fewer attempts have been made to determine the “fundamental” electronic properties for pristine organic single crystals.

We present results of photoemission measurements for single crystalline (SC) dinaphthothienothiophene (DNTT). DNTT is a small molecule-based thienoacene and has demonstrated carrier mobilities approaching 10 cm²/(V s) [1], is air-stable [2] and durable against accelerated temperatures and humidity conditions.[3] Electronic “band” structure measurements using a novel angle-resolved time-of-flight electron spectrometer is performed on SC-DNTT, and multiple highest occupied molecular orbitals are resolved of varying widths. Modest dispersion of the frontier HOMO is observed. Using polarized Raman spectroscopy, the orientation of the crystal unit cell can be identified. The electronic structure results will be discussed in context of the electronic structure calculations and charge carrier behavior of DNTT reported in the literature.

[1] W. Xie et al., Adv. Mater. 25, 3478 (2013)
[2] U. Zschieschang et al., Adv. Mater. 22, 982 (2010)
[3] N. K. Za’aba et al., Org. Electron. 45, 174 (2017)

Since the last decade, the synaptic electronics has attracted much more attention due to its computational parallelism, fault tolerance as well as energy efficiency. These excellent properties make them a prior candidate for overcoming the von Neumann bottleneck in modern computers and applicable for next-generation neuromorphic computing. While much attention has been paid on developing single artificial synapse to simulate basic synaptic functions, an integrated system including sensing and processing units for demonstrating synaptic circuit behavior like associative learning is highly desirable. Here we present a synaptic circuit for demonstrating the associative learning property in the human brain. Organic electrochemical transistors (OECTs) have been used as pivotal components in this circuit with vapor phase polymerized poly(3,4-ethylenedioxythiophene):Tosylate (PEDOT:Tos)/ Polytetrahydrofuran (PTHF) composite as an active layer. The PEDOT:Tos/PTHF-based OECT shows non-volatile characteristics with stable hysteresis window and long retention time (>200 minutes). The different mass ratio between PEDOT:Tos and PTHF have been investigated and they show critical roles on the short-term transient property as well as long-term memory behavior of the OECTs. Based on the non-volatile characteristics of OECT, short-term synaptic plasticity like paired-pulse facilitation (PPF), post-tetanic potentiation (PTP) and long-term synaptic plasticity like short-term memory (STM) to long-term memory (LTM) transition have been simulated. In addition, a pressure sensor and a photoresistor have been integrated on the gate terminal of non-volatile OECT to imitate pressure induced haptic memory and light induced iconic memory. Finally, by integrating pressure senor, photoresistor, volatile OECT as well as non-volatile OECT together, we successfully demonstrate the associative learning behavior and show that the adaptation of light can be reinforced by training with the presence of pressure. Our synaptic circuit can accelerate the development of next generation neuromorphic devices towards artificial intelligence.

Blending a visible light-emitting organic semiconductor with an insulator alleviates the trap-limited nature of the electron current. Organic light emitting diodes (OLEDs) comprising such a blend as emissive layer exhibit a two-fold increase in luminous efficiency with only 10% semiconductor. Due to this low content of semiconductor, polymer-LEDs are more attractive than small molecule-based devices. However, polymers impose the difficulty of an inherently low miscibility. In a plain blend macro-phase separation can be avoided if the molecular weight is kept low, which, in case of the semiconductor, is a disadvantage as it suppresses charge carrier mobility. An alternative strategy is to impose a nano-confinement. We prepare aqueous nanodispersions of red (PPV) and blue (polyfluorene) emitting polymers, blended with polystyrene as insulator. We seem to fully suppress macro-phase separation in both cases. For the latter, the combination of nano-confinement and blending influences the phase morphology of the semiconductor in an unprecedented way. Fabricating OLEDs with an emitting layer consisting of nanoparticles poses a considerable challenge due to high operational current densities. We now succeed in fabricating such devices in a reproducible way at very decent efficiencies

There have been several reported on the use of “solution-technique” with a low-temperature process of 200-300 °C to achieve high-performance electronic devices[1]–[5]. In particularly, the Ink-jet printing (IJP) has attracted considerable attention in numerous electronic device fabrication areas because this technique requires low-power consumption and allows for direct-write patterning[6]-[10]. Direct printing of functional materials through IJP is more efficient technique for fabricating of new types of structures and devices with a low cost, low-temperature process, large-area fabrication and low environmental[11]-[12]. The fabrication of high-performance electronic devices through solution processing and direct printing requires accurately controlling the flow and spread functional liquid inks on solid surfaces, and this can be achieved by understanding the solid surface tension causing resolution morphologies on different surface energies of solid structures. In particular, when the conductive electron carrier transport in the semiconductor channel films of electronic devices is strongly related to crystalline morphologies, the evaporation behavior during the drying process plays a crucial role in controlling the thin-film morphology and the distribution of solute in ink-jet printed thin films. In addition, the relief-like phenomenon which is caused by the combination of capillary flow and pining of the contact line (CL) and is commonly observed during the evaporation of droplets, this is a deciding factor for the final thin film forming morphology of deposit patterns and their position on solid surface. The relief-like deposits are typically formed at the initial pinning of triple contact lines (TCL).
For utilizing printing technologies in the production of TFTs, understanding the relief-like morphologies of semiconductor films caused by capillary-flow-induced deposition on dielectric materials and consequently resulting in different electrical operating mechanisms is necessary. Here, we propose an approach of using IJP for directly-printing zinc oxide (ZnO), a highly transparent conducting oxide (TCO) material on the semiconductor layers of thin-film transistors (TFTs). The ZnO-based TFTs were fabricated on two dielectric materials of silicon dioxide (SiO₂) and silicon nitride (Si₃N₄), commonly used in the semiconductor industry and exhibited different surface energies. Our observation of the surface-energy patterns which caused two distinct channel widths under the same ink setting and the effects electrical properties indicated that understanding solution-based direct printing techniques for the deposition and direct-write patterning of functional materials is imperative. In conclusion, we demonstrated relief-like ZnO-based TFTs on two different dielectric materials of SiO₂ and Si₃N₄ exhibiting different solid surface tensions (contact angles) resulting in distinct ZnO thin-film deposition pattern morphologies with channel widths of 342.9 μm and 155.9 μm, respectively, the channel width for both designs was set to 1 μm in the ink-jet printer. The IJP technique facilitated fabricating excellent composition of continuous semiconductor thin-films and directly patterning semiconductor transistor channel widths, thus enabling the fabrication of transistor circuits without the involvement of complicated processing of photolithography system. Moreover, the IJP technique afforded an enhanced operation electrical characteristics of on-off current (I_ON-I_OFF) ratio value of approximately 10⁶ at a bottom-gate voltage of 30 V, and this is attributed to the clear capillary flow effect, which induced the thin-film channel to have distinct high/low resistance regions that resulted in enhanced conduction of electron carriers and electron flow path in the ZnO thin film.
Keywords: low-temperature process, ink-jet printing (IJP), transparent conducting oxide (TCO) materials, thin-film transistors (TFTs)

The ability to tailor the geometric arrangement and electronic structure of materials is appealing for a broad range of applications including energy storage, micro-electronics and electromagnetic shielding. This project aims to develop and study a series of new materials to gain insight into methods for controlling the electromagnetic properties of two-dimensional covalent organic frameworks (2D-COFs). 2D-COFs are porous, semi-planar and stackable macromolecular structures, which can serve as a scaffold for additive materials to yield novel performance. The experimental work reported herein focuses on the synthesis of a 2D-COF and the development of methods for incorporating ionic compounds and examining resultant changes in electronic structure.
In-situ and ex-situ methods of intercalation have been explored/optimized, and advanced structural and property characterization techniques have been employed on these new materials. Preliminary findings have already shown induced changes in the physical and electronic properties of perhaps the most common 2D-COF. The results of these studies show promising trends that will be expanded upon by future investigations.

Organic memory devices (OMEMs) are realized by integrating a layer of the crosslinkable dithienylethene XDTE in a solution-based multilayer OLED stack. The photochromic XDTE molecule can be switched reversibly by a photo- and/or electrically induced ring-opening/-closing reaction to two thermally stable states featuring different physical properties. The change in the energy level positions, which is responsible for controlling the hole injection by shifting the hole injection barrier, enables a high ON/OFF ratio approaching 10⁶. For future application as data storage element, electrically induced switching is of great importance, but the charge trapping landscape is affected by this switching mechanism. By incremental switching via current densities pulses, intermediate switching states are accessible. The current-response is analysed as a function of the fraction of closed isomer obtained via in-situ reflectance absorption spectra. In combination with impedance spectroscopy measurements, we study the role of trapping effects in this energetically anisotropic switching process.

The ultimate goal of energy-efficient fabrication of photovoltaic devices requires low energy consumption processes covering both synthesis of material and fabrication of device. Regarding to charge transport layer, it is worthwhile to synthesize nanomaterials and deposit films all at room temperature while still have good electrical properties. In our work, we propose and demonstrate a new nanocomposites of maghemite and iron hydroxide through a low energy consumption approach which is all room-temperature solution processes from the synthesis of the nanocomposites to the formation of high quality hole transport layer (HTL). Strategically adjustment of acidity for the conversion of prepared precipitation is demonstrated to achieve a component controllable maghemite and iron hydroxide nanocomposites which contributes to in-situ tunable work function of the nanocomposites HTL from 4.70 eV to 5.16 eV. Simultaneously, since the nanocomposites synthesized from this approach have the features of ultra-small size of 6–10nm and surfactant-free, high quality and efficient HTL films can be formed at room temperature. For organic solar cells using nanocomposite as HTL, the power conversion efficiency can be significantly improved by as much as 80% as compared with the optimized devices without HTL. Besides, both the efficiency and stability of the nanocomposite based organic solar cells are better than that of devices using poly (3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Consequently, the work contributes to the fabrication of simple, low-cost, and stable optoelectronics promoting green photovoltaics and flexible electronics.

A comparative study of thermally activated delayed fluorescence (TADF) properties of sulfone analogues are reported. Synthesis of multiple divinyl sulfones (DVSs) and their sulfone analogues have been explored via computational and experimental methods to determine the effects on the TADF properties. Utilization of DVS as the acceptor imparts several beneficial properties to the emitters: 1) increased separation between the donor and acceptor groups; 2) enhanced separation of the frontier orbitals; and 3) decrease in crystallinity. These properties lead to a smaller single-triplet energy gap thereby potentially improving TADF emission, and film imperfections caused by crystallization. Synthesis of selected compounds have been achieved via palladium catalyzed couplings followed by nucleophilic aromatic substitution using aromatic amine donors. Air versus argon solution photoluminescence, solution photoluminescent quantum yield, single-triplet state energy gap, density functional theory, thermal properties, and initial OLED results will be presented.

Organic electronic devices rely upon the work function of their electrodes for control over charge injection and built in electric fields. However, an explanation of the origin of work function modification remains unclear. With a representative polyelectrolyte set, we studied how the work function of electrodes can be modified by a combination of charge-based through-space interaction and surface interaction. The formation of a surface dipole due to a through-space interaction between the modifying layer and substrate results in a shifted work function, even when the modifying layer and substrate are separated by an insulating layer. Furthermore, we explored the effect of electrophoretic deposition of ionic polyelectrolytes inducing a work function shift. While the materials didn’t show any significant work function shift when they were deposited by spin-casting, the applied electric field in the electrophoretic deposition allowed polyelectrolytes to move to the oppositely charged electrode in the solution. Also, when electrospraying technique was used, a neutral polymer without any surface-interacting functional groups showed work function shift. This work helps us to better understand the origin of work function modification and to have a wide range of work function modifying materials for organic electronic devices.

Recent trends in the electronics industry have shown that organic semiconductors have the potential to replace silicon in some electronic devices due to their less stringent production environments and ability to offer new functionalities, such as large area flexible and self-healing architectures. Organic semiconductor films have traditionally lagged behind silicon devices due to the difficulty in predicting and controlling the molecular structure, which leads to unpredictable charge transport properties. In traditional organic film materials, the molecules lay flat in the first few molecular layers before transitioning to less favorable geometries for charge transport. Our collaboration has studied a molecular platform, tris(N-phenyltriazole) (TPT), that exhibits planar stacking through >20 molecular layers due to the π-π donor-acceptor intermolecular contacts between the electron-deficient tris(triazole) core and electron-rich peripheral phenyl units. Here, we present investigations of derivative molecules of TPT, which have different electron distributions and structures, to examine connections between packing structure and charge transport functionality. Molecular-resolution scanning tunneling microscopy is used to compare the molecular packing of these derivative molecules in the monolayer and to investigate the effects the structural and electronic modifications have on the stacking in the multilayer. Conductivity measurements are also made using a 4-point van der Pauw technique to evaluate the charge transport properties of these different molecules compared to an industry standard, pentacene. These results suggest that the TPT molecule and its derivatives are more conductive than that of pentacene. These studies may lead to new organic semiconductor material designs that have well-controlled structure and more predictable charge transport properties, making them more competitive with traditional silicon devices.

Ultra-flat patterning on polymer surfaces on the order of nanometers is extremely important for advances in organic electronics and energy technologies due to improvement of light transmittance at the interface, fine circuit patterning. Nanoimprinting is an effective patterning technique for polymer surface, which has productivity advantages such as simple, low cost, large area. We have previously demonstrated atomic step-and-terrace pattern formed both on soda-lime-silicate glass and polymer sheets such as poly(methyl methacrylate) (PMMA) and polyimide (PI) by the thermal nanoimprint method. In additional this atomic scale morphology contributed to transparent conductive oxide films deposited on nanopatterned PI showed good surface flatness and films deposited on glass resulted in improved crystallinity and crystal orientation ^[1,2,3].
Meanwhile, we have also reported fabrication of highly oriented and crystallized UV-sensitive oxide semiconductor thin films by an excimer laser annealing at room temperature ^[4,5], that the technique could improve the crystallinity and orientation of the UV-sensitive semiconductor films on polymer sheets at a temperature lower than the glass transition point of the polymer, when combined with the flat surface and sharp interface. In this work we report room-temperature fabrication of UV-sensitive semiconductor thin films such as ZnO or Ga₂O₃ on the originally developed ultra-flat polymer sheets with 0.3 nm-high atomic step-and-terrace surfaces. Atomic scale surface patterning with a vertical resolution of 0.3 nm was performed on PMMA sheets by thermal nanoimprinting utilizing a mold sapphire (α-Al₂O₃ single crystal) with atomic step-and-terrace structure on its surface. Thin films of UV-sensitive semiconductors such as ZnO or Ga₂O₃ were then formed on the patterned PMMA substrates by pulsed laser deposition (PLD) at room temperature. The oriented film revealed flat surfaces reflecting the step-and-terrace morphology on PMMA, having smaller RMS roughness in comparison with the bare substrate. In addition, XRD results indicated improvement of crystallinity and orientation. The influence of the film structure on the electrical and optical characteristics would also be reported.

[1] G. Tan et al., Nanotechnology 27 (2016) 295603
[2] Y. Akita, Y. Miyake et al., Appl. Phys. Ex-press 4 (2011) 035201.
[3] G. Tan et al., Polymer Journal Vol.48 (2016) 225-227.
[4] D. Shiojiri et al., J. Cryst. Growth 424 (2015) 38-41.
[5] D. Shiojiri et al., Applied Physics Express 9, (2016) 105502.

Conjugated polymers have a large range of applications, due to their characteristic electronic properties, and the variety of ways they can be processed. However, conjugated polymers with high mobility quickly degrade when processed in air. Therefore, a new roll-to-roll (R2R) processing technique is necessary in order to limit oxidation during polymer processing, while preserving the mechanical and electrical properties of the conjugated polymers.

In our proposed R2R design, polymer solution is slowly pumped onto the surface of a water-filled trough. When the polymer solution contacts the tub, the differing surface tension of the solvent and water causes the polymer solution to quickly spread and dry. The edge of the dry solution is pulled onto a substrate roller, freeing space on the water surface for more solution to spread and dry.

This R2R method provides an efficient method for applying small amounts of polymer to large area substrates with minimal waste. Furthermore, the fact that the polymer film is dry when depositing means that it is possible to layer two different polymer films onto the same substrate, including materials with non-orthogonal solvents. This is an advantage over other deposition methods, which cannot deposit “wet” solutions onto a dry film because the dry film will re-dissolve with the newly deposited polymer solution.

Meniscus-line-guided coating have attracted a lot of attentions in field of organic electronics due to the low fabrication cost and the compatibility with large area processing. Different solution processing approaches such as blade coating, bar coating, zone casting and etc, have been proposed and ameliorated to improve the molecular packing in the crystal. To control the crystallinity of the deposited organic semiconductors, the mechanisms behind crystal growth are actually very important. Here, we use blade coating method as a tool to systematically investigate effects of elementary factors on crystal growth rate or mass transfer rate with the semiconductor of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT). In the current study, we correlated four elementary parameters including shearing speed, concentration, solvent selection and deposition temperature. We have identified their roles in the meniscus-line-guiding coating process. Shearing speed showed an inverse proportion relationship with resulting thin film thickness, which could be used as thickness modulation parameter while the remained three factors are directly connected to growth rate. The experimental temperature range were set from 30^oC to 100^oC. By different combination of growth parameters, we achieve a crystal growth rate from 1x10^-11to 5x10^-10 kg/s. The equivalent thicknesses are well controlled below 40 nm by adjusting the shearing speed. These results could provide not only deep insight and guidance into crystal growth under meniscus-line-guided coating, but also reference for crystal growth analysis in other research fields.

Electrolyte-gated transistors (EGTs) or electric double layer transistors (EDLTs) have attracted significant research attention as potential switching devices for wide variety of thin-film devices because of their solution processability, low-voltage operations, and excellent device performance. Many different types of organic and inorganic semiconductors have been applied for high performance EGTs. In this work, Organic/inorganic hybrid complementary inverters operating at low voltages were successfully fabricated by transfer-stamping all the active components including organic p-type poly(3-hexylthiophene) (P3HT), inorganic n-type zinc oxide (ZnO), high capacitance solid electrolyte ion gel dielectric and conducting polymer. A semicrystalline homopolymer-based gel electrolyte was transfer-stamped on the semiconductors for low voltage EGT operation. For the ionogel stamping, the thermoreversible crystallization of network cross-links was utilized to improve the physical contact between the gel and the semiconductor layers. The p-type P3HT EGTs showed high hole mobility of 2.14 cm²/Vs, whereas the n-type ZnO EGTs exhibited high electron mobility of 1.82 cm²/Vs. By combining these p- and n-type EGTs in series, organic/inorganic hybrid complementary inverters were fabricated. The complementary inverter operated at low-voltages below 2 V with appropriate inversion characteristics including a high voltage gain of ~18.

Electrophoretic display (EPD) technology is a technology which have lots of advantages such as easy manufacturing process, being made of flexible material, requiring ultra-low power consumption, low-cost manufacture and, most of all, easy and convenient to read.
In this study, nano-size color particles were studied for application to electrophoretic display. Color particles were prepared by Emulsion polymerization and Seed polymerization. Polymerization method were proposed for the synthesis of monodispersed poly(vinyl acetate-co-divinylbenzene)[poly(VAc-co-DVB)] particles with different VAc/DVB feed ratios. Poly(Vinyl alcohol-co-divinylbenzene)[poly(VA-co-DVB)] particles were obtain-ed by the basic hydrolysis of poly(VAc-co-DVB) particles. The hydroxyl groups on poly(VA-co-DVB) particles have a suitably reactive functionality for surface covalent bond with reactive dye. So, Poly(VA-co-DVB) particles were reacted with reactive dye. Poly(VA-co-DVB) color nano particles were characterized by using SEM, IR, DSC, UV-vis.

Recently, development trends of electronic devices and optoelectronic devices are related to simple and economic next generation printing processes based on organic and organic/inorganic hybrid nano materials. The transfer printing process, which is one of the next generation printing processes, is attracting attention⁽¹⁾ because it is relatively dry, economical, and more stable by forming a reverse structure compared to the wet process. In this research, we investigated transfer printing process from enhanced hydrophobic interface of polyurethane acrylate (PUA) mold film via perfluoropolyether (PFPE)⁽²⁾. The energy release rate (G) of mold film has been successfully controlled for efficient transfer process, which was confirmed by contact angle measurement compared to the normal PUA. The transfer printed PEDOT:PSS layer exhibits comparable surface properties, and also induces favorable crystallinity of perovskite related to the spin-coated layer, which shows similar JSC and PCE (spin-coated vs. transfer printing: (12.85 vs. 12.33) %), and improved VOC. Furthermore, the stability of the device with transfer printed PEDOT:PSS achieved 90 % retention for about 40 days, which was affected by the preserved crystallinity of perovskite, and the inhibition of the degradation of ITO from XRD and XPS analysis, respectively. As a result, the transfer-printed hole extraction layer through interface control between mold film and PEDOT:PSS using the improved hydrophobicity contributes to maintaining the surface morphologies and device electrical properties, which correlates with the stable operation of perovskite photovoltaics. This work suggests a encouraging process of organic inter-layer fabricated by the controllable material-customized transfer films.

(1) Wang, Dong Hwan, et al. "Transferable graphene oxide by stamping nanotechnology: electron transport layer for efficient bulk heterojunction solar cells." Angewandte Chemie 125.10 (2013): 2946-2952.
(2) Yi, Minji, et al. "Enhanced Interface of Polyurethane Acrylate via Perfluoropolyether for Efficient Transfer Printing and Stable Operation of PEDOT: PSS in Perovskite Photovoltaic Cells." Applied Surface Science (2018).

Solution-processed active semiconductor doping would be a powerful strategy in order to improve CNTFETs performance in balanced charge transport. CNTs are promising material for high performance, large area printable and flexible thin film transistor. However, as synthesized single-walled carbon nanotubes (SWNTs) show non-uniform electrical properties due to the mixture of metallic and semiconducting SWNTs. Selective separation of semiconducting SWNTs (s-SWNTs) using conjugated polymers has been developed with the aim of utilizing their superior properties. We separated s-SWNTs with the high purity from as-grown mixed-type SWNTs using conjugated polymer with long alkyl side-chain length. Separated s-SWNTs were adapted for active layer of CNTFETs with or without dopant. Properties and doping effects of the CNTFETs active layer were revealed by UV-VIS spectroscopy, Raman Spectroscopy, low-temperature measurements and other experiments. We demonstrate that hole and electron charge carrier tuning can be easily achieved by simple spin coating in CNTFETs with s-SWNT/dopant bilayer structure. Furthermore, through this study, we implemented logic gates such as inverter and pass gate with simple solution processing.

The state-of-the-art commercial materials of organic light emitting diode (OLED), especially for a blue fluorescence, still need further improvement in terms of their efficiency. We have synthesized several diphenylsulfone-type thermally delayed activated fluorescence (TADF) host materials with diphenylsulfone (DPS) as acceptor unit, while carbazole (Cz), 3,6-di-tert-butyl-carbazole (tBCz), or 7,7-dimethyl-5,7-dihydroindeno [2,1-b] carbazole (DMICz) were utilized as donor units. The calculated S1/T1 energies of synthesized host materials, CztBCz-DPS and CzDMICz-DPS (with the standard B3LYP/6-31G*) are 3.35/3.00 eV and 3.13/2.86 eV, respectively. Deep blue emitting TADF devices with a color coordinate of (0.16,0.09) and (0.15, 0.09) were obtained for devices having CztBCz-DPS and CzDMICz-DPS emitter with bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) host. A molecular design of interlocking two phenyl units of DPS could help to reduce molecular motion, resulting in a shift of emission color to deep blue. Experimental values of electro-optical properties and time-resolved studies were investigated by the spectrometry on photoluminescence, UV-vis, and transient experiment to analyze a good overlap of the host emission to dopant absorption and delayed fluorescence. However, those TADF devices demonstrated still limited efficiency, about 3.7% (DPEPO: CztBCz-DPS 10%) and 5.7% (DPEPO: CzDMICz-DPS 10%). For a further improvement of TADF device efficiency, severe roll-off characteristics, and poor device stability, asymmetric DPS series compounds were applied for sensitizing host at TADF device architecture. The hyperfluorescence devices, with blue emitting layer composed of DPEPO: DPS-series-sensitivers (30%) :fluorescent dopant (5,9-Diphenyl-5H,9H-[1,4]benzazaborino [2,3,4-kl]phenazaborine; DABNA-1; 1%), showed 12 to 14% external quantum efficiencies at a color coordinate about (0.14, 0.07) depending on the type of asymmetric DPS-series-sensitizers. Although more detailed investigation is required, it is expected that sensitizers with tBCz-donor group were found to be more efficient compared with DMICz-donor materials.

Polymeric conjugated materials are convenient for developing future soft-material-based semiconductors, conductors, electronic and optoelectronic devices due to their inherent features. These features include being lightweight, flexible, cost efficient, malleable, and easily scaled. Similar to their inorganic counterparts, the addition of certain minority molecules, or dopants, to polymeric conjugated materials can significantly alter the electronic and optoelectronic properties of the host conjugated polymers or composites. This allows for tunability of a variety of electronic and optoelectronic applications. One way to improve device performance is through the process of thermal annealing. Annealing allows for a polymer matrix to self-assemble into a lower energy state, which leads to an increase in crystallinity and higher charge mobility. Previous research does not explicitly define how dopants can affect this process. This study involves an evaluation of the effects of annealing with doped P3HT films to demonstrate changes in optoelectronic and electronic properties.

Very high values of carrier mobility have been recently reported in newly developed materials for field-effect transistors (FETs) or thin-film transistors (TFTs). Concerning the precise characterizations of OTFTs or OFETs, we investigate the device physics of devices with non-ideal current-voltage relations:
(a) How to choose the analysis tool for contact resistance in OTFTs? Given the limitation of the transfer-line method in the difficulty in experiments and theoretical assumptions, we could perform analysis on a single device with transfer or output characteristics. These methods provide facile tools and whether their assumptions are valid are discussed. ^[1,2]
(b) How much contact resistance a FET or TFT can tolerate allowing the conventional current-voltage equations? Mobility in transistors with resistive contact can be underestimated with the presence of the injection barrier, whereas mobility in transistors with gated Schottky contact can be overestimated by more than ten times. This is because the band bending and injection barrier experience a complicated evolution on account of electrostatic doping in the semiconducting layer.^[3]
(c) How to assess the capacitance in OTFTs with high-k dielectrics? We used the capacitance of high-k alumina as an example to reveal the capacitance contributions from bulk, electric double layer, and pseudocapacitance. The series of three components could lead to very high apparent mobility, which is however not the properties of semiconductors. ^[4]
(d) How to compare disorders of charge transport in various OSCs can be directly compared in the same map? The generalized Einstein relation can unify various theoretical models and predict charge transport in OSCs with various crystallinity, by altering the variance of the density of states and delocalization degree in a Gaussian-distributed density of states. ^[5]

[1] Materials Today, 18, 79-96 (2015); Organic Electronics, 27, 253-258 (2015)
[2] Advanced Functional Materials, 25,758-767 (2015); Scientific Reports, 6, 298911(2016)
[3] Physical Review Applied, 8, 034020 (2017)
[4] Submitted
[5] Materials Horizons, 4, 608-618 (2017)

Organic photovoltaics (OPV) offer a competitive alternative to existing conventional solar cells and other emerging third generation solar cells due to their semi-transparency, light weight, mechanical flexibility, low cost, and ability to be printed with similar manufacturing techniques to newspapers and labels.¹ A bulk heterojunction (BHJ) photoactive layer is typically employed in OPVs that is processed almost exclusively from toxic organic solvents, such as chloroform, chlorobenzene, and 1,2-dichlorobenzene.² There is the need to move away from processing with chlorinated solvents and hence the development of eco-friendly processing technologies for solar cells is a recent target of the polymer solar cell community.^3.4 Unfortunately, a reduction in solar cell performance often results when chlorinated solvents are eliminated, which is attributed to the non-optimal microstructure of the light absorbing layer. This seminar presents recent work on unravelling the material-morphology-performance relationships in eco-friendly processed polymer solar cell photoactive layers with the use of advanced synchrotron X-ray characterization techniques, STXM and NEXAFS.⁵ Here we explore the microstructure of green solvent cast BHJ films and aqueous colloidal nanoparticle films for the poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) : phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) and poly(3-hexylthiophene) (P3HT) : PC₆₁BM donor-acceptor material systems. With additive engineering and customizing nanoparticulate colloidal inks we are able to reduce the material domain size to approach the typical exciton diffusion length in these organic semiconductor systems, demonstrating that eco-friendly processing of large area OPV is a realistic goal for the near future.

1. F.C. Krebs, et al., Adv. Mater. 2014, 26, 29–39.
2. S. Zhang, et al., Adv. Mater. 2018, 30, 1800868.
3. C. Xie, et al., ACS Appl. Mater. Interfaces. 2018, 10, 23225–23234.
4. N.P. Holmes, et al., Nano Energy. 2016, 19, 495–510.
5. N.P. Holmes, et al., Chem. Mater. 2018, 30, 6521−6531.

To ensure optimal performance, the development of organic semiconducting materials from their molecular and/or polymer constituents requires precise processing controls to optimize the solid-state molecular packing arrangements. Temperature, solution deposition rates, and secondary solvents are among the tools that can be used to fine tune solid-state packings. However, many of these tools are used in an ad hoc fashion, and process discovery remains largely in the realm of trial-and-error approaches.
We are developing all-atom molecular dynamics (MD) approaches to explore the solution processing of organic semiconductors. Here we will discuss recent advances in simulations that examine the free energies of mixing of solutions, and the influence of solution composition on molecular conformations and aggregation. For solvent and additive systems, diffusion parameters, solvation energies, and free energies of mixing are determined to provide insight into the solution environment. We then use these solvent systems to explore the solution behavior of complex donor-acceptor oligomers. The simulations reveal critical insight into the role of the secondary solvent additive during aggregation. These simulations are providing key atomic-scale insights as to how variations in processing solutions impact morphology development in organic semiconductors.

We have recently been exploring organic semiconductor-based thin films that feature crystalline grains of up to 1 mm in extent, termed microcrystalline films. We will show our efforts to understand their formation, epitaxy, and transistors. Homoepitaxial studies uncover evidence of point and line defect formation in rubrene films, indicating that homoepitaxy is not at equilibrium or strain-free. Point defects that are resolved as screw dislocations can be eliminated under closer-to-equilibrium conditions, whereas we are not able to eliminate the formation of line defects. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result of strain built into the thin film template, indicating that, perhaps in general, organic crystalline thin films may not adopt the exact lattice of a bulk crystal.

In terms of optoelectronic behavior, we have found that charge transfer (CT) states incorporating these long-range-ordered films can be highly delocalized, contributing to noticeably lower energy losses. Also, we have discovered that relative energies of CT states with respect to singlet and triplet energy levels are critical when considering devices that exploit multiple exciton processes such as singlet fission and its complement, triplet-triplet annihilation (or triplet fusion). We will discuss these aspects and their implications for more efficient organic solar cell function.

A new class of materials called mixed ionic/electronic conductors (MIECs) has the potential to be highly useful for many applications, ranging from biological sensors to battery electrode materials. The majority of current organic MIECs are created through the blending of two materials, where one component provides ionic conductivity, and the other electronic. This leads to a complex phase behavior that is still not fully understood. For rational design of MIECs, the connection between morphology and conduction must be better understood. To begin to answer these questions, guided by molecular dynamics simulations we synthesized two polythiophene derivatives bearing oligoethylene glycol side chains; one with an oxygen atom directly conjugated to the thiophene backbone, and one with a methylene spacer. Both polymers showed lithium uptake into crystalline domains, though lithium preferentially resided in the amorphous regions. By using a combination of electrochemical impedance spectroscopy and molecular dynamics simulations, we confirmed that ionic conduction occurs predominately in the amorphous regions. These polymers provide a valuable scaffold into investigating the interplay of morphology and ionic conduction.

Field-effect carrier mobility (m) is an important parameter to evaluate the performance of the organic field effect transistors (OFET). However, accurate measurement of this value is nontrivial. Apart from the recently discussed Schottky contacts induced mobility overestimation, the fringe effect resulting from the improper electrodes design is another disregarded source which would lead to mobility overestimation even the devices show good linearity in the transfer curves. We found that the OFET with unpatterned active layers or gate dielectrics can have mobility overestimation up to 210%. We systematically examine the fringe effect on mobility overestimation in the organic transistors with both vacuum-deposited and solution-processed semiconductor active layers. We introduce a correction-factor (C-factor) of the W/L defined by the source-drain electrodes to indicate the severity of the mobility overestimation caused by the fringe channel. By using electrodes with W/L ratio equals to one, the C-factor can reach 1.16 and 0.74 for vacuum-deposited and solution-processed OFETs, respectively. On the other hand, we also demonstrated the C-factor can be minimized to 0.1 when using properly designed electrodes (W/L≥ 40 for vacuum-deposited W/L≥ 20 for solution-processed transistors). In the talk, we will also provide recommendations on the design of device geometry and channel dimension so that common pitfalls of fringe effect in the device fabrication can be avoided. With a detailed comparison table of the reported mobilities, the current work will help to provide a blueprint to the researchers about how the fringe effect would affect the reliability of reported mobility values. We believe the improvement on accurate evaluation of carrier mobility in relation to the fringe effect will ensure the development of novel OFET materials towards a correct direction, rather than keep on reporting inflated mobility values which are actually wrong.

The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during solution deposition. As solvent vaporizes, molecules become electronically coupled as they aggregate to form a film, sometimes dramatically changing the exciton dynamics and thus the suitability for electronic devices. The dynamics of molecules in solution and in thin films of aggregates can be measured using transient absorption spectroscopy, but the exciton dynamics of intermediate aggregation states during thin film formation are typically unknown since measurements cannot be performed quickly enough to collect accurate transient absorption spectra of these species. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. A novel implementation of transient absorption spectroscopy is introduced that can measure transients with up to a 60 ps pump-probe time delay in one shot. The exciton dynamics measured during the solution deposition of a film is validated by comparing the initial exciton dynamics of the solution and the final exciton dynamics of the dry film to traditional transient absorption measurements. The exciton dynamics of intermediate aggregation states will be presented for the first time. The information gained using this technique can be used to modify environmental parameters during the film formation process to kinetically trap aggregates with exciton dynamics tailored for particular types of electronic devices.

Using high-surface tension solvents allowed us to grow 3 – 10 nm thin, highly-crystalline films of a N,N'-di((S)-1-methylpentyl)-1,7(6)-dicyano-perylene-3,4:9,10-bis(dicarboximide) (PDI1MPCN2) at the liquid/air interface of a drying droplet [1]. We find, that charge carrier mobilities in these electron conductive films is as high as 4 cm^2/Vs even for an only 3 nm thin PDI1MPCN2 film. Changing the solvent composition used for crystallization of our organic semiconductor also has allowed us to systematically tune the crystallinity and consequently the grain boundary density in thin films. From the temperature-dependent charge carrier mobility, we have extracted the density of states and compared it to Kinetic Monte Carlo simulations [2]. This combined theoretical and experimental approach has allowed us to identify, that it is rather the energetic barriers at grain boundaries than the usually identified energetic traps that limit charge carrier motion below room temperature. We also have revealed that the dipole moment of the PDI1MPCN2 is the cause for the energetic disorder at grain boundaries serving as clear guideline for future design of organic semiconductors with potentially no energetic barriers present at the grain boundaries. We furthermore have investigated in detail the dependence between charge carrier mobility and density at various temperatures and find surprisingly that above room temperature the charge carrier mobility decreases upon increasing the charge carrier density [3]. While the true cause for this suppression is currently unclear, we present evidence that the squeezing of charges closer to the semiconductor/dielectric by the gate dielectric field a subsequent scattering at this interface might cause the drop in mobility. We anticipate that our combined observations will help to understand the still debated nature of charge transport in high-quality organic semiconductors.

[1] I. Vladimirov, M. Kellermeier, T. Geßner, Z. Molla, S. Grigorian, U. Pietsch, L. S. Schaffroth, M. Kühn, F. May, and R. T. Weitz, Nano Letters 18, 9, (2018)
[2] I. Vladimirov, M. Kühn, T. Geßner, F. May, and R. T. Weitz, Scientific Reports 8, 14868, (2018)
[3] I. Vladimirov et al., in preparation

Polymer ferroelectrics are playing an increasingly active role in flexible memory application and wearable electronics. The relaxor ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene (PVDF-TrFE), although vastly used in organic field-effect transistors (FETs), has issues with the gate leakage current especially when the film thickness is below 500 nm, and low carrier mobilities that arise due to its inherent polarization fluctuation. The poling condition of PVDF-TrFE in organic FETs plays a large role in dictating the transport properties. The subthreshold swing and other transistor parameters in organic FETs can be controlled in a reversible fashion by switching the polarization direction in the PVDF-TrFE layer [1]. By using solution processed small molecule semiconductors such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and other donor-acceptor copolymers, we show that selective poling of the PVDF-TrFE layer dramatically improves FET properties compared to uniformly poled ferroelectric films. TIPS-pentacene FETs with selective poling the PVDF-TrFE layer show on/off ratios of 10⁵ and hole mobilities of 1 cm²/Vs under ambient conditions with operating voltages well below -5 V, without processing the organic semiconductor layer. Cross-sectional high-resolution transmission electron microscope images reveal some changes in the morphology upon selective poling the PVDF-TrFE layer. This study opens the prospect of achieving low-operating, high-performance, low-cost organic FETs by a combination of vertical and lateral polarization of the ferroelectric dielectric without any expensive patterning techniques.

We acknowledge the support of this work through the National Science Foundation under Grant No. ECCS- 1707588

[1] A. Laudari, A. R. Mazza, A. Daykin, S. Khanra, K. Ghosh, F. Cummings, T. Muller, P. F. Miceli, and S. Guha, Phys. Rev. Appl. 10, 014011 (2018).

One of the key advantages of polymer semiconductors over traditions inorganic semiconductors is their mechanical behavior. The polymers typically have lower stiffness, greater ductility and toughness than their inorganic counterpart enabling effective integration in ultraflexible and stretchable devices. To exploit the full potential of these materials for mechanically demanding applications, the fundamental thermomechanical behavior of these materials must be accurately determined, including how their viscoelastic properties translate to device stability. However, capturing the thermomechanical properties of these materials can be challenging. A common approach is to use differential scanning calorimetry (DSC). But, DSC often doesn’t have the sensitivity to capture the thermal transitions of polymer semiconductors. Dynamic mechanical analysis (DMA) on the other hand offers a very sensitive and versatile tool to probe the thermomechanical behavior of these materials. In this talk, we describe specimen preparation on probing both bulk and then film thermomechanical properties of the polymers. We demonstrate DMA measurements that reveal thermal transitions including sub-glass transitions in a number of high performance polymer semiconductors, and how these transitions relate to the film mechanical behavior such as ductility, toughness, and creep. The implications of these properties on mechanical stability of devices will then be discussed. Furthermore, we show that the thermomechanical behavior of polymer-small molecule blends used in high performance solar cells is directly related to the long term device stability. We show that the small molecule diffusion measured by secondary ion mass spectroscopy can be directly related to thermal transitions, and how this dictates the blend film properties needed to ensure long-term morphological stability.

Although it is well known that intra- and intermolecular ordering greatly impact the electronic and optoelectronic properties of semiconducting polymers, the interrelationship between ordering of alkyl sidechains and conjugated backbones has not yet been fully detailed. A recent discovery clearly demonstrated that the tendency of the side-chains and the backbone to order is not synergistic [1], rather a competition was observed in six representative semiconducting polymers. The ordering of the respective polymer components is monitored by NEXAS, UV-vis spectroscopy, as well as diffraction and exhibits distinct signatures. Overall, a vertically layered multilayer nanostructure with ordered sidechain layers alternating with disordered backbone layers is observed as the equilibrium structure at room temperature, with sidechain ordering within the alkane layer exhibiting a coherence lengths >70 nm. In contrast, the side-chain are typically very amorphous, even in materials that show lamellar diffraction peaks. The long-range sidechain ordering has been exploited as a transient state to fabricate PBnDT-FTAZ films with an atypical edge-on texture and 2.5x improved OFET mobility. Overall, these observations imply that a new way towards molecular design needs to be pursued, which could possibly produce significantly improved ordering and thereby improve electronic and optoelectronic properties. We will review past and ongoing research towards perfect polymer crystals and monolayers [2] and propose the use of a design perspective that views the backbone and side-chain similar to classic semiconducting heterostructure bulk epitaxy. In other words, we need to design materials as completely ordered crystals and not partially ordered sublattices.

[1] J. H. Carpenter, M. Ghasemi, E. Gann, I. Angunawela, S. J. Stuard, J. J. Rech, E. Ritchie, B. T. O’Connor, J. Atkin, W. You, D. M. DeLongchamp, H. Ade, Advanced Functional Materials, https://doi.org/10.1002/adfm.201806977 (2019)
[2] M. Li, D. K. Mangalore, J. Zhao, J. H Carpenter, H. Yan, H. Ade, H, Yan, K. Müllen, P. WM Blom, Wojciech Pisula, Dago M Leeuw, Kamal Asadi, Nature communications 9, 451 (2018)

The design of molecules and polymers for solution-deposited organic semiconducting materials generally considers the chemical modulation of (i) the π-conjugated backbone to modify the electronic and optical characteristics and (ii) the alkyl side chains to govern solubility. As the solid-state material forms, physical interactions among these constituents play important, yet not well understood, roles in directing the molecular-scale packing arrangements that in part determine the final material properties. Further, the functions of the processing solutions are not clear, adding yet another layer of complexity in determining how materials self-assemble. In this presentation we will discuss the development of atomic-scale models that invoke thermodynamic, kinetic, and quantum-chemical methods to deliver insights into the processing and solid-state packings of organic semiconductors. The chemical knowledge developed through these investigations is beginning to refine and offer novel understanding essential to the development of next generation organic semiconducting active layers, and is opening new pathways for in silico materials development.

Though organic semiconductors have illustrated their potential as industry-relevant materials for electronics applications, there are few guidelines that can take one from designing molecules to constructing functional materials. Relatively weak, non-covalent intermolecular interactions determine the thermodynamic preferences for solid-state packing, while interface interactions and kinetic factors during processing also play important roles. For crystalline materials, crystal structure prediction remains a grand challenge, with an even greater challenge stemming from the need to predict and control the formation of the particular crystal polymorph that provides the best materials characteristics for a given application. Such predictive capabilities, which need to span and draw correlations from the atomic to the macro-scale, will require a united effort from researchers across multiple disciplines.

To address some of these challenges, we have initiated the construction of a database as a digital platform aiming to serve the community with a high-throughput computational workflow and a web-based user interface. Featuring combinatory inputs from synthetic chemists, theoreticians, experimentalists, and device physicists, the database employs a descriptor-based scheme to explore the correlations between molecules, their solid-state packings, and materials properties. Here, we illustrate database utility through the investigation of functionalized acenes as we refine the structure-packing model that has been proposed to offer predictive capabilities for this materials class.

Organic semiconducting polymers are promising candidates for stretchable electronics for their mechanical compliance. Donor-Acceptor type conjugated polymers have been the key drive for recent boost in device performance. Up to date, the effect of the conjugated backbone building block on the thermomechanical property of conjugated polymers has not been carefully studied, despite much work on their influence on the electronic property. In this talk, I will discuss our work on the structure and thin film thermomechanical property relationship for donor-acceptor polymers with systematically varied donor units on the conjugated polymer. To characterize the thermomechanical performance, the pseudo-free standing tensile test was used to obtain the full stress-strain curve. The glass transition temperature was measured for both thin and bulk films using AC-chip calorimetry and DMA, respectively. Thin film morphology was detected using AFM, UV-vis, and GIWAXS for further understanding. OFET devices were fabricated to test the electronic performance. The backbone structure and thermomechanical property relationship was established and applied to the design of new stretchable conjugated polymers.

Radical polymers are an emerging class of organic electronic materials that are composed of a non-conjugated macromolecular backbone and with stable open-shell moieties present on their pendant groups. These oxidation-reduction-active (redox-active) macromolecules have been oft-used in the realm of electrolyte-supported energy storage devices due to the rapid reaction kinetics associated with the oxidation and/or reduction of the high densities of radical sites present along the polymer chains. To date, their utilization in solid-state organic electronic devices has been limited due to the lack of conjugation along their macromolecular backbone and the idea that this lack of conjugation necessarily limits their ability to conduct charge effectively in the solid state. Here, we demonstrate that radical polymers, in fact, are able to achieve relatively high electrical conductivity values (i.e., > 20 S m^-1) if appropriate molecular design criteria are met.

Specifically, two key design rules exist for radical polymers, and these design rules are distinctly different from what is observed in common conjugated semiconducting and conducting polymers. First, in order to ensure that there is a high density of redox-active sites by which to pass charge in the solid state, radical polymers must be synthesized in such a manner that the fraction of pendant groups that have an open-shell character to them approaches unity. While a fairly stringent requirement, this can be accomplished in a straightforward manner by utilizing polymerization schemes that do not follow a radical-mediated pathway. Second, the glass transition temperature of the radical polymer must be lower than the degradation temperature of the radical polymer. This allows for thermal annealing of the radical polymer after it has been cast into a thin film. Through a combination of experiment and computation, we demonstrate that this annealing process is crucial to the formation of a thermodynamically-stable nanoscale structure that allows for electronic communication between the radical pendant groups. When these two design criteria are met, we have been able to demonstrate electrical conductivity values for tailored radical polymers in excess of 20 S m^-1. This places our pristine (i.e., not doped) radical polymer electrical conductivity on par with many common commercial grades of doped conjugated polymers [e.g., poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS)]. In addition to the device stability benefits associated with a lack of acidic dopants, the non-conjugated macromolecular backbones necessarily allow for high optical transparency values to be achieved across the visible spectrum for our radical polymers. For all of these reasons, radical polymers present themselves as useful materials in myriad optoelectronic and sensing device applications, and they offer the promise of replacing the current state-of-the-art of transparent conducting polymers and composite materials.

In particular, we will describe how specific nitroxide-based radical polymers are implemented as next-generation organic electronic materials in organic photovoltaic (OPV) devices and perovskite solar cells; organic field-effect transistors (OFETs); electrochromic devices; and bioelectronic sensors. The inclusion of the radical polymer layer improves the overall power conversion efficiency and stability of both OPV devices and perovskite solar cells. Additionally, using radical polymers as interfacial modifiers in pentacene-based OFETs both increases the observed mobility and decreases the contact resistance in the device. Finally, the high transparency in the visible spectrum allows for high-performance electrochromic devices that exhibit relatively rapid switching rates and are stable for a large number of cycles. Thus, we aim to show that radical polymers allow one to tie molecular design directly to device performance in a relatively linear manner.

Polymers capable of static electronic polarization, known as electrets, are of interest in applications such as electromechanical energy harvesting and as a means of pre-setting parameters and storing information in organo-electronic devices. Well-designed dielectrics with suitable flexibility and sufficiently low trap densities are desired for integration with semiconductors. Polymer thin films offer many advantages in terms of their flexibility and processability but also can introduce electronic trap states which have deleterious effects on mobilities and operating voltages of devices such as organic field effect transistors (OFETs). However, by intentionally biasing dielectrics through the application of a static electric field, trap states can be filled and static potential differences can be established to control organoelectronic device performance. In this work, we apply these principles using partially fluorinated styrenes to design polymer dielectrics for use in organoelectronic systems. Partial fluorination of styrene through the trifluoromethyl group results in polymers that exhibit multiple beneficial electronic effects both in the interface with pentacene based OFETs and in the bulk of the polymer layers themselves when compared with pure polystyrene. Pentacene-based thin film transistors fabricated with poly(3-trifluoromethyl styrene) dielectrics demonstrated a large reduction (ca. one order of magnitude) in interfacial traps as compared with plain polystyrene, in addition to larger and more stable static potentials following intentional biasing, providing the capability of shifting OFET threshold voltages to preferred values. These properties have potential to lead to electronic devices with lower power requirements and better controlled switching behavior. Further enhancements are possible by the additional incorporation of redox-active functional groups in the dielectrics. The dielectric tunability described here may be attractive for other systems in which charge density control is desired, such as chemical sensors and thermoelectrics.

This work funded by the U.S. Department of Energy, Office of Science Basic Energy Sciences under Award # DE-FG02-07ER46465.

Ferroelectricity, bistable ordering of electrical dipoles in a material, is widely used in sensors, actuators, non-linear optics and data-storage. Ferroelectric polymers are inexpensive lead-free materials that offer unique features such as the freedom of design offered by chemistry, the facile solution-based low-temperature processing, and mechanical flexibility. Among engineering polymers, odd-nylons are ferroelectric. Since the discovery of ferroelectricity in polymers, nearly half a century ago, a solution processed ferroelectric nylon thin-film has not been demonstrated due to the strong tendency of nylon chains to form hydrogen bonds. Therefore the initial enthusiasm about ferroelectric nylons quickly came to a halt due to the inability to solution process pinhole free ferroelectric thin-films, which is required for many of the envisioned microelectronic applications. In this contribution, we show solution processed transparent ferroelectric thin-film of nylons. Further we show that the nylon capacitor have superior performance compared to the capacitors made of the conventional ferroelectric polymer namely PVDF and its copolymers. Demonstration of ferroelectricity, and the way to obtain thin-films, makes odd-nylons attractive for applications in flexible devices, soft robotics, biomedical devices and even e-textiles.

Printed, flexible and even stretchable electronics have potential as low cost alternatives for devices in applications ranging from energy to health care to security. However, their successful commercialization relies on the design and development of sustainable, robust and reliable materials and processes. Molecular design coupled with solution behaviour play a significant role in determining a materials thin-film electronic performance. For instance, semiconducting polymer alignment at multiple length scales is a key consideration; and materials’ nano- through meso-structure can be manipulated in solution prior to device fabrication. Optimization of the polymer ink, prior to deposition is a key process step for the realization of robust and reproducible semiconducting solutions for flexible electronics applications.

Metal organic framework (MOFs) has emerged as an exciting class of porous materials, which can be structurally tuned by choosing organic-inorganic components based on the desirable application. For decades, metal organic framework with structural versality and tunable porosity have provided promising applications such as gas storage, drug delivery, sensing and catalysis. However, due to its low conductivity or insulating properties, the growth of metal organic frameworks in electronic applications is in its infancy. But by careful selection of organic-inorganic components and synthesis procedure, MOFs can exhibit conductivity and also porosity with diversified morphology. Here we report, a new methodology of synthesizing metal organic framework (IRMOF-8), using 2, 6-naphthalene dicarboxylic acid as organic linker with metal (M=Zn) via thermal annealing-based technique. Unlike conventional methods, which take about 72 hours, this technique can produce metal organic frameworks in 7 minutes at 250⁰C in organic solvents, exhibiting best performance characteristics. The synthesized naphthalene-based metal organic frameworks were confirmed by various characterizations techniques such as X-ray diffraction spectroscopy, thermogravimetric analysis, UV-visible spectroscopy, FT-IR spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The FT-IR spectra of these MOFs showed a stretching frequency at 1400 cm^-1confirming the co-ordination of 2,4 Naphthalene dicarboxylic acid to zinc (C-O-Zn). The crystalline properties of this MOF were analyzed and confirmed by X-ray diffraction spectroscopy. The chemical composition of these metal organic frameworks was analyzed by energy dispersive spectroscopy that confirmed the presence of carbon, oxygen, and respective metals (M=Zn). Thermal stability was analyzed by thermogravimetric that showed the decomposition of host framework at the temperature of 400⁰C-500⁰C. Scanning electron microscopy images of this MOF showed diversified morphologies for different metals (Zn) with the size ranging from 100 nm-300 nm. The yield of MOF synthesized via thermal annealing-based technique resulted about 80% that is twice of the conventional techniques. The electrical properties were determined using Kiteley source meter controlled by a Photo emission TEC.INC (PET) I-V test system. Perhaps, the proposed thermal annealing-based technique was successful in synthesizing conductive MOF suggesting that MOF based electronic devices can be constructed.

Conjugated polymers are showing ever increasing promise for both current and novel electronic applications. Much of the research for these materials has focused on optimizing electrical properties (such as OPV efficiency or electron mobility) at the detriment of understanding the mechanical properties which are necessary for device commercialization. One reason for this is that the active polymer layer is on the order of 100nm thick (or less) making direct mechanical characterization all but impossible. Our group has utilized the pseudo freestanding tensile test (film on water) to overcome this challenge enabling the calculation of parameters such as Young’s modulus and crack-onset-strain with minimal difficulty. This method has shown consistently lower modulus values than that obtained from buckling metrology. This may be due to a plasticization effect of the water, but the interaction of water with the thin film has not been sufficiently accounted for. Therefore, I will discuss a true free standing tensile test that provides direct mechanical characterization of conjugated polymeric thin films in air. This novel characterization technique was demonstrated on three polymer systems: polystyrene, poly(3-hexylthiophene-2,5-diyl), and polyfluorene at multiple film thicknesses to provide quantitative evidence of any measurable effect water induces in the pseudo freestanding tensile test. The film and bulk glass transition temperatures were measured via AC-chip calorimetry and DMA respectively. This enabled us to ascertain the effectiveness of this methodology across a broad range of dynamic systems. Furthermore, the free standing tensile test enabled in-situ characterization with darkfield microscopy under tensile strain thus providing further understanding in thin film deformation mechanics.

Phonons in organic semiconductors are crucial for determining their charge transport properties. Current theoretical models treat these phonons within the harmonic approximation. These models fail many times to predict important electronic properties such as the charge-carrier mobility. Since organic solids have weak intermolecular interactions, the anharmonic components of their low-frequency vibrations are expected to be significant. We hypothesize that the reason for the failure to predict the electronic properties is the neglection of these anharmonic components.

I performed temperature-dependent low-frequency Raman measurements of oligo-acenes to quantify their anharmonicity. Results show a much stronger change in the peak position of the low-frequency modes with temperature compared to inorganic semiconductors. Analysis of these peak shifts shows strong anharmonic behavior - contributed from both thermal expansion and phonon-phonon interactions - likely to have a significant impact on electron-phonon interactions. Comparison of a series of oligo-acenes shows an inverse correlation between anharmonicity and intermolecular electronic coupling. I will also show photoluminescence and reflectance measurements to corroborate the connection between vibrational anharmonicity and the electronic properties of organic semiconductors.

Electrochromic (EC) materials and their correspondent EC devices (ECD) that change colors on application of voltages have attracted great research interests in the past several decades since they have commercial applications as smart windows, auto dimming rearview mirrors and static displays.
Viologen has been widely investigated as an organic EC material since it has excellent redox properties accompanied with distinct color changes. However, most viologens only show color change from transparent or translucent yellow to blue. In order to make viologens more colorful, we designed and synthesized novel materials by inserting thiophene moiety into bipyridine. These chromophores are expected to have abundant color changes. Further, these new viologen-conjugates possess phosphoric acid groups, which could help them anchor on the titanium dioxide (TiO₂) more firmly.
A layer of nanostructured TiO₂ film (4.0 μm thick, TiO₂ particles are about 20 nm large) was coated on conducting F-doped tin oxide (FTO) glass, and the new viologen-conjugate was adsorbed onto TiO₂ particles by chemisorption. The films of new viologen-conjugate/TiO₂/FTO were characterized by using cyclic voltammetry. Color changes from transparent at 0 V to purple at -0.8 V and rust red at -1.1 V were observed. Next, Prussian blue (PB) was electrodeposited onto FTO glass and used as counter electrode. And then ECD was assembled by combining the working electrode of new viologen-conjugate/TiO_2/FTO, counter electrode of PB/FTO, and gel electrolyte. The ECD was characterized by using UV-vis spectrophotometer and electrochemical analysis system. The ECD showed a broad transmittance contrast over 60% at +2.5 V and -2.5 V.

Reference:
1. Rong, Y.; Kim, S.; Su, F.; Myers, D.; Taya, M., Electrochimica Acta, 2011, 56, 6230-6236.
2. Weng, D.; Shi, Y.; Zheng, J.; Xu, C., Organic Electronics, 2016, 34, 139-145.
3. Pan, M.; Ke, Y.; Ma, L.; Zhao, S.; Wu, N.; Xiao, D., Electrochimica Acta, 2018, 266, 395-403.
4. Itaya, K.; Ataka, T.; Toshima, S., Journal of American Chemical Society, 1982, 104, 4767–4772.

Programmable control of electron transport through organic molecules is a crucial step for designing integrated electronic devices for energy storage. Recent advances in molecular electronics have brought us closer towards achieving the ultimate limits in miniaturization and spatial and functional control over electronic performance. Despite recent progress, however, we still lack a full understanding of molecular-scale electron transport and how these properties are affected by chemical identity and sequence. In this work, we directly measure single molecule conductance using a scanning tunneling microscope-break junction (STM-BJ) technique. Oxazole-terminated molecules are found to exhibit interesting quantum interference phenomena through central phenyl rings and terminal oxazole rings, which can be used for controlling charge transport. In particular, the quantum interference of the central phenyl group follows a quantum circuit rule such that G_para/G_meta = 6, whereas the c-type terminal oxazole ring shows constructive quantum interference. We can further precisely tune the conductance of oligophenyls via aromatic interaction with different background molecules. In this way, our work provides the fundamental electron and charge transport information to inform future programmable molecular electronics design.

Understanding the molecular origin for better control of polymorphism is critical for the development of high performing, large area production of organic electronics. We explore a series of organic semiconductor systems with the same conjugated core and various bulky side chains to understand the polymorphic phase space. We demonstrate that substituting a bulkier silicon atom for a carbon atom in the side-chains compromises the packing environment such that they inhibit side chain rotation. We prove that either allowing, or preventing, the rotation of bulky sidechains triggers cooperative transition or nucleation and growth, respectively. We investigate the impact of both types of polymorphic transitions on electronic performance. By inducing nucleation and growth, we can access two kinetically stable polymorphs and study their electronic performance. From triggering cooperative transition, we have in situaccess to different polymorphs with rapid reversible polymorphic transition for applications in next-generation smart multifunctional materials. This work offers a simple molecular design tool to access both polymorphic transition pathways and incorporate their advantages to organic semiconductors.

Design rules and application spaces for closed-shell conjugated polymers have been investigated to a significant extent in the field of organic electronics, and this has allowed for significant breakthroughs to occur in myriad device platforms [e.g., organic field-effect transistors (OFETs) and organic light-emitting devices (OLEDs)] such that significant laboratory and commercial efforts have come to fruition. Conversely, organic electronic materials that are based on the emerging design motif that includes open-shell stable radicals have not been evaluated in such detail, despite the promise these materials show for charge transfer, light-emission, and spin manipulation platforms. Moreover, recent results have demonstrated that the materials performance of hybrid conjugated closed-shell and open-shell systems will allow for future applications to harness both of these platform design archetypes in order to generate composites that combine the performance of current state-of-the-art conjugated polymer systems with the novel functions provided by open-shell species. Thus, establishing the underlying physical phenomena associated with the interactions between both classes of materials is imperative for the effective utilization of these soft materials.

Here, we demonstrate that Förster resonance energy transfer (FRET) is the dominant mechanism by which energy transfer occurs from a common conjugated polymer to various radical species using a combination of experimental and computational approaches. Specifically, we determined this by monitoring the fluorescence quenching of poly(3-hexylthiophene) (P3HT) in the presence of three radical species: (1) the galvinoxyl radical; (2) the 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO) radical; and (3) the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical. Both in solution and in the solid-state, the galvinoxyl and PTIO radicals showed fluorescence quenching that was on par with that of a common fullerene electron-accepting derivative, phenyl-C₆₁-butyric acid methyl ester (PCBM). This was due to the considerable overlap of their absorbance spectrum with the fluorescence spectrum of the P3HT species, which indicated that isoenergetic electronic transitions existed for both species. Conversely, TEMPO showed minimal quenching at similar concentrations in solution and at similar loadings in the solid state due to the lack of such an overlap. To support the determination of the mechanism, and to rule out photoinduced charge transfer, a potential competing process, pump-probe transient absorption measurements were employed. An increased rate of exciton decay in P3HT was observed when blended with the galvinoxyl radical and PTIO radical, consistent with an excited state transfer mechanism. Moreover, signals corresponding to the anion species of the quenchers were not observed, suggesting that charge transfer was not occurring at an appreciable rate for the compositions evaluated here. Additionally, computational studies suggested that FRET would occur at a significantly faster rate than photoinduced charge transfer. These computationally-predicted FRET rates were calculated from the spectral overlap of the P3HT fluorescence and the radical quencher absorbance spectra, while charge transfer rates were calculated by extracting Marcus theory parameters for the composite P3HT-quencher systems from density functional theory (DFT) calculations. Therefore, these computational results support the steady-state and time-resolved fluorescence experiments, and the results highlight the precise interactions between open-shell small molecules and closed-shell conjugated polymers in optoelectronic applications. Additionally, these findings suggest that long-range energy transfer can be accomplished in applications when radicals that can act as FRET acceptors are utilized, forming a new design paradigm for future optoelectronic applications.

Semiconducting conjugated polymer thin film transistors (TFT) with mobilities in the range of 0.1~20 cm²/Vs have been reported by man groups. There is a demand for higher mobility polymer TFTs; therefore, an accurate understanding of the physics of charge transport is necessary. However, a complete theoretical understanding and quantitative description of charge transport in such semiconductors has been difficult to attain. The mobilities and mean free paths are simply too low (on the order of intermolecular distance ~3 Å) for the application of conventional semiconductor transport theories. In this presentation, we will describe a very general solution based on the statistical nature of charge transport and introduction of a factor (mean free path factor) related to the probability of mean free path exceeding the minimum transport length. We are then able to apply the Boltzmann transport equation (BTE) with appropriate scattering mechanisms and obtain very good results that agree well with experiment[1]. This approach is very well suited to thin-film transistors based on polymer and organic semiconductors, and also many amorphous oxide semiconductors with room temperature mobilities in the range 5~50 cm²/Vs[2]. We combine the mean free path factor, BTE solutions with an extended multiple trap and release (MTR) model that takes into account traps in the device to get an overall response.
Several scattering mechanisms are considered based on the material properties. It is found that at low and intermediate temperatures, free carrier scattering by the trapped charge carriers is dominant. Carriers trapped in the semiconductor traps or at the semiconductor-dielectric interface can be considered immobile Coulomb scattering centers, and can scatter mobile carriers efficiently. At high temperatures, basically the room temperature (300K) or above, carrier-phonon scattering starts to dominate. An important phonon scattering mechanism in the polymer thin film is the optical deformation potential scattering. Due to the structure of the conjugated polymers, the backbone chain generates low frequency optical phonons, and the optical deformation potential is fairly large in these soft materials. This leads to very effective carrier scattering and decrease the mobility at high temperatures. Other scattering mechanisms including polar optical phonon scattering and acoustic phonon scattering are also considered into the evaluation of the band mobility for different temperatures and carrier concentrations.
The resulting band mobility of conjugated polymers is in the range of 20~40 cm²/Vs, corresponding to 0.1~20 cm²/Vs for the effective MTR mobility in a TFT device. In the case of low carrier concentration, the carrier free path is assumed to approximately obey the Poisson distribution with occurrence number of zero (exponential distribution) and the mean free path is the average of the Poisson distribution. Although the mean free path of the carrier can might be smaller than the intermolecular distance, there is still a fraction of free carriers that can survive travelling past that distance. This fraction of carriers is considered to participate the actual band transport. Thus, the apparent MTR mobility is further reduced by the factor of the mean free path survivor fraction. We compare results from our calculations with experimental data[1] for donor-acceptor polymers. The agreement is very good and attests the validity of our approach. Our work also points to clear directions that TFT device structures must possess for high performance.

Refs:
[1] T. J. Ha, et al. "Charge transport study of high mobility polymer thin-film transistors based on thiophene substituted diketopyrrolopyrrole copolymers." Physical Chemistry Chemical Physics 15.24 (2013): 9735-9741.
[2] X. Wang, A. Dodabalapur. " Trapped carrier scattering and charge transport in high-mobility amorphous metal oxide thin film transistors." Annalen Der Physik (2018) (accepted).

Recently transparent-composite-electrodes (TCEs) were introduced as a substitute for indium-tin-oxide (ITO) in bulk heterojunction organic solar cells (OSCs). In this study, an anode structure consisting of TiO₂/Ag/TiO₂ TCE with optimized layer thicknesses was implemented as an OSC to demonstrate its potential as a replacement for ITO. In addition, the compatibility of hole-transfer-layer (HTLs) of either MoO₃ or poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were investigated in terms of their compatibility with the TCE. Corresponding control samples of ITO based OSCs were fabricated using PEDOT:PSS and MoO₃ as the HTL. It is found that MoO₃ HTLs benefited the OSC that utilizes a TCE as the anode. When using the MoO₃ the as HTL, TCE based OSC has better performance than ITO-based OSC; and, it even rivalled the performance of conventional ITO-based OSC that utilizes PEDOT:PSS as the HTL. Outcomes of this study confirmed that even with the slightly lower transmittance in the visible portion of the light spectrum, TCEs did not degrade the light absorption in OSCs, and the efficiency of transporting photo-induced carriers in TCEs anodes was comparable or even better than ITO.

With the increasing concerns for the environment and the progress in non-invasive diagnosis, monitoring volatile organic compounds (VOCs) have attracted considerable attention. Ammonia and acetone are of particular interest because they are related to chronic kidney disease and diabetes disorder, respectively. Multi-functional donor-acceptor conjugated polymers are potential candidates for building reliable ammonia and acetone sensors due to the recent advance in the field of thin film transistor. In this report, we demonstrate that the transistor mobility, the device stability and the sensitivity of sensors can be greatly enhanced by incorporating functional groups directly on the polymer main chain. The fluorinated isoindigo polymers exhibit hole mobility with over an order improvement than their un-fluorinated counterparts. The novel polymer transistors sensor can discriminate various VOCs and detect 100 ppb ammonia gas and 50 ppm acetone gas within 2 min in air. This is the most sensitive polymer transistor-based sensor towards acetone ever reported. The influence of fluorination is systematic studied to understand the intermolecular interaction between the analytes and fluorine atoms. Exceptional sensitivity to ammonia is attributed to the hydrogen bond forming between fluorine and ammonia and the sensitivity to acetone is due to the changing in the energy levels of frontier orbitals of the polymers. Fluorination of conjugated polymers enhance the transistor and gas sensing performance, which is essential for building fast response and high sensitivity sensor platform.

Dual gate transistors based on rubrene/rubrene(p+p channels) and rubrene/DFH-4T(α,ω-Diperfluorohexyl-quarterthiophene)) (p+n channels) were fabricated. Traditional dual gate transistors using only one semiconductor active layer will inevitably result in bad injection in one channel. To maximize the charge injection area for both top and bottom channels, we adopted a staggered architecture with dual semiconductor layers. Gold source and electrodes are sandwiched between the bottom channel and the top channel. In the rubrene/rubrene dual layer dual gate OFETs, the average hole mobilities in the saturation regime for both bottom and top channel are around 5 cm² V^-1 s^-1. The output characteristics for both channels exhibit good linearity in small V_ds region, suggesting the charge injection is effective. Depending on the bias configuration, the threshold voltage of the bottom or top channel can be continuously modulated from 35.2 V to -45.6 V. In the rubrene/DFH-4T devices, the operating mode of the dual gate devices can be controlled as a p-channel, n-channel or ambipolar OFET. The AFM (Atomic force microscope) and XRD (X-ray diffraction) characterizations show that the growth of the top DFH-4T semiconductor is regulated by the bottom rubrene single crystals with distinct orientation. The dual gate transistors have the potential to be further constructed into inverters and complex logic circuits.

Recently, stamping transfer process using by thin films or soft mold has been considered by advanced technology to overcome limits of wet coating such as spin or dip coating, which are composed of deposition of large area and specific region, the material loss, and penetration of solvents. Although the adhesion energy between stamp and targets was minimized for a successful transfer process, many researches have not discussed this factor. In this research, we introduced wetting coefficient related with the adhesion energy. We investigated the adhesion energy depending on the surface energy of the stamp. We applied polyurethane acylate (PUA) as the stamp, of which the surface energy was modified to increase the transfer reproducibility. As a results, high-surface-energy PUA was used to form organic bulk hetero junction (BHJ) layer onto PEDOT:PSS/ITO substrates. The transferred device revealed a comparable efficiency, 95% relative to spin coating device. In order to find a decrease of fill factor of transferred device, we observed charge recombination and resistance through impedance spectroscopy.[1] Second, we applied stamping transfer to formation of inter-layer in planer-type perovskite photovoltaics. We have successfully fabricated the device with transferred inter-layer, 6,6-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM), onto perovskite layer by dry stamping transfer condition. The device exhibited enhanced J_sc and efficiency, which were caused by improved coverage of inter-layer on perovskite layer, correlated with increased electron mobility and exciton dissociation.[2] Finally, we fabricated organic photodetectors (OPDs) by stamping transfer process for photo sensitive BHJ layer. We confirmed a comparable performance of the transferred OPD, compared to the device from spin coating. Especially, the dry transferred device exhibited a superior durability (over 90%) for 350 hours to the spin coated device, because of morphology stabilization of photo sensitive layer, which led to suppression of degradation of the layer and burn-in loss.[3] This work provides a promising alternative process which can improve the device operation durability without burn-in loss by a simple and controllable transfer films.
K. M. Kim, W. Jang, S. C. Mun, S. Ahn, J. J. Park, Y. Y. Kim, E. Kim, O O. Park and D. H. Wang, Org. Electron., 2016, 31, 295.
S. Ahn, W. Jang, S. Park, D. H. Wang, ACS Appl. Mater. Interfaces, 2017, 9, 15623
W. Jang, D. H. Wang, ACS Appl. Mater. Interfaces, 10.1021/acsami.8b13375

Doping conjugated polymers is an effective way to tune their electronic properties for thin-film electronics applications. Chemical doping of semiconducting polymers involves the introduction of a strong electron acceptor or donor molecule that can undergo charge transfer (CT) with the polymer. The CT reaction creates electrical carriers on the polymer chain while the dopant molecules remain in the film as counterions. To dope polymer films, we employ a sequential process (SqP) in which a pure polymer layer is deposited first, followed by infiltration of the dopant in a second step using a semi-orthogonal solvent. SqP overcomes the problems typically incurred by blend-doping, where the polymer and dopant are mixed in solution, which results in aggregation at high concentration. The exceptional film quality achievable with our SqP doping method allows us to employ electrical measurements over macroscopic length scales, such as Van der Pauw conductivity measurements as well as AC Hall effect and impedance measurements of carrier mobility.

This work focuses on the use of substituted icosahedral dodecaborane (DDB) clusters of the form B₁₂(OR)₁₂ as a new class of dopant molecules, where R is a substituted benzyl group. The redox potentials of DDBs can be rationally tuned via modification of the R-group substituents without a significant change to the size or shape of the dopant molecule. These tunable dopants provide a unique handle on the energetic offset that governs the driving force for doping via integer CT. Here, we disentangle the effects of energetic offset on the production of free and trapped carriers in DDB-doped poly-3-hexylthiophene (P3HT) films.

In DDB-doped P3HT films, in general, we find that conductivity and polaron absorption amplitude increase with increasing reduction potential, yielding conductivities on the order of 12 S/cm. Since DDBs tend to localize electron density on their core, we have shown that the bulky corona of substituents on the clusters provide spatial isolation of the counterion. The polaron is therefore Coulombically shielded from the counterion, thereby reducing electrostatic interactions, resulting in highly delocalized and mobile carriers with mobilities upt 0.1 cm²/Vs and effective conductivities up to 32 S/cm even in very non-crystalline polymer films. In these films, nearly all carriers are free, whereas it has been shown that small molecule dopants like F4TCNQ trap 95% of carriers, due to their smaller size and inevitable proximity of the counterion. Given the high conductivity but distinct loss off crystallinity observed, we explore the properties of these DDB-doped films as potential thermoelectric materials. The low crystallinities should lead to low thermal conductivities, even while the electrical conductivity is maintained, allowing optimization of the thermoelectric figure of merit.

Solution-processable semiconducting materials in organic solar cells (OSCs) enable the roll-to-roll printing of functional devices. However, the dissolution of these materials commonly requires toxic solvents, thus hindering the translation to industry. To enable large scale printing in industry, they should be formulated as inks using a less toxic medium such as alcohol or even water.
Researchers have fabricated OSCs by dispersing semiconducting materials in water/alcohol with the help of surfactants. However, significant surfactants remained in the active layer even after extensive dialysis, reducing the device performance and solar cell life-time. Recently, devices were fabricated using a dispersion of poly-3-hexylthiophene (P3HT) with indene-C₆₀ bisadduct (ICBA) in methanol and showed a 3.8% device efficiency without the need of a surfactant. However, in this case, the performance was highly dependent on the batch of polymer used for dispersion. Hence, there is a requirement of an appropriate surfactant which not only controls the size of nanoparticles but also increases nanoparticle stability whilst maintaining device efficiency.
To address this problem, we synthesized P3HT end-capped with pyridine. We expected this pyridine-P3HT to be protonated on addition of acid and act as a surfactant by making an electrical double layered around the nanoparticles, inducing a repulsive interaction and preventing aggregation, however under thermal annealing the pyridinium salt would decompose removing all trace of the acid. By using a combination of 1.5 wt% pyridine-P3HT and trifluoroacetic acid as additives, stable P3HT:ICBA nanoparticle dispersions of up to 30 mg/mL in methanol were achieved. The nanoparticles in these dispersions were 120 ± 5 nm, and the dispersions were stable up to 60 days. Inverted architecture OSC built using these nanoparticle dispersions exhibited 3.4% efficiency, a result that is comparable to the state-of-the-art P3HT:ICBA solar cells fabricated under optimized conditions using chlorinated solvents.

Since the discovery of the first conductive polymer, organic electronics helped disclosing a whole new world of possibilities in the realization of, among several other applications, flexible displays, mechanically compliant (but also implantable and bioresorbable) sensors and biosensors, and tactile sensors for next generation artificial skin applications. Despite these remarkable achievements and more than 5 decades of technical advancements, the field of application of organic electronic devices is still relatively narrow, due to the intrinsic properties of the semiconductive materials usually employed. In particular, common limitations are the low switching speed caused by the intrinsic low mobility of the charge carriers within the most common polymers and small molecules used (this limitation preventing most of the organic transistor to be used for logic applications), and the limited devices resolution, the latter issue being mainly due to the difficulty of fabricating short channel transistors while at the same time using low-cost and large area fabrication techniques.

In order to overcome these apparently unavoidable issues, during the last 15 years a new type of organic transistor has been introduced, namely the vertical organic transistor (vOFET). This peculiar “vertical approach” allows to obtain short channels without the need of expensive fabrication techniques, thus representing a very interesting technological advancement with respect to pre-existing OFETs. In this work we report about a novel and easy approach to the realization of vertical organic transistors that includes the use of Parylene C as both the “core” of the vertical structure and the gate insulator. The fabrication process has been optimized in order to have the possibility to obtain highly flexible and ultra-conformable transistors, being such devices fabricated on micrometer-thick films (either Parylene C or polyamide-imide) that, after the device fabrication, can be conveniently detached from the carrier substrate allowing it to be transferred onto whatever kind of surface. Moreover, we will also demonstrate that such approach is particularly suitable for the fabrication of sensing devices. In particular, we employed a double-gated organic transistor called organic charge modulated FET (OCMFET), which has established itself during the past 10 years as a versatile sensor with an ultra-high sensitivity.

The combination of the proposed novel and convenient approach to the fabrication of vertical organic transistors (with the possibility of realizing high-resolution and faster switching devices) and the remarkable features of the OCMFET represents an interesting advancement within the organic electronics scenario, in that it will allow to obtain reference-less, ultra-sensitive sensors arranged in conformable high-density arrays, while keeping the process low-cost, thus offering considerable advantages to the field of organic sensing and biosensing.

Research on near-infrared- (NIR-) emitting materials and devices has been propelled by fundamental and practical application demands surrounding information-secured devices and night-vision displays to phototherapy and civilian medical diagnostics. However, the development of stable, highly efficient, low-cost NIR-emitting luminophores is still a formidable challenge owing to the vulnerability of the small emissive bandgap toward several nonradiative decay pathways, including the overlapping of ground- and excited-state vibrational energies and high-frequency oscillators. Suitable structural designs are mandatory for producing an intense NIR emission. Herein, we developed a series of deep-red to NIR emissive iridium(III) complexes (Ir1–Ir4) to explore the effects of electron-donating and electronwithdrawing substituents anchored on the quinoline moiety of (benzo[b]thiophen-2-yl)quinoline cyclometalating ligands. These substituents help engineer the emission bandgap systematically from the deep-red to the NIR region while altering the emission efficiencies drastically. Single-crystal X-ray structures authenticated the exact coordination geometry and intermolecular interactions in these new compounds. We also performed an in-depth and comparative photophysical study in the solution, neat powder, doped polymer film, and freeze matrix at 77 K states to investigate the effects of substitution on the excited-state properties. These studies were conducted in conjunction with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Most importantly, the –CH₃ substituted Ir1, unsubstituted Ir2, and –CF₃ substituted complex (Ir4) were promising novel compounds with bright phosphorescence quantum efficiency in doped polymer films. Using these novel molecules, deep-red to NIR emissive organic light-emitting diodes (OLEDs) were fabricated using a solution-processable method. The unoptimized device exhibited maximum external quantum efficiency (EQE) values of 2.05% and 2.11% for Ir1 and Ir2, respectively.

Conjugated polymers can be used in mechanically flexible and low cost thermoelectric (TE) devices, but their thermoelectric performance must be improved to make them commercially viable. The performance of thermoelectric materials depends on the electrical conductivity, Seebeck coefficient and thermal conductivity. In polymer based TE materials the polymer needs to be doped to become electrically conductive. The higher the doping concentration, the more electrically conductive the material becomes, but generally at the cost of a decrease in the Seebeck coefficient. Blending of π-conjugated polymers has been proposed as a method to minimize the tradeoff between electrical conductivity and the Seebeck coefficient, thus potentially allowing higher power factors to be reached. By blending two polymers, the total density of states (D.O.S.) will be manipulated, which may be used to alter the energy dependence of charge transport in the TE material. The major parameters that we expect to impact the power factor in polymer blends are the mobility ratios between the two pure polymers and the shape of the D.O.S. (i.e., the disorder and the energy offsets between the D.O.S. distributions of each polymer). Here, we modified a model introduced by Bässler and Arkhipov to theoretically probe how these two parameters impact thermoelectric performance. These calculations are then used to fit experimental data of various polymer blends with varying mobility ratios and D.O.S. distributions. We find that adding a polymer with a narrower D.O.S. and higher mobility with respect to host polymer can lead to an enhancement in the power factor.

Hybrid organic-inorganic semiconducting interfaces have attracted attention in photodiodes and field-effect transistors (FETs) due to the realization of intrinsic p-n junctions and their mechanical flexibility. Organic copolymers based on diketopyrrolopyrrole (DPP) cores have also gained significant interest for application in FETs because of their high p-type carrier mobilities as well as high power conversion efficiencies in solar cell structures. In addition, grazing-incidence x-ray diffraction (GIXRD) reveals crystalline stability of phenyl-capped DPP-based monomers [1]. With the difficulty of developing high-mobility n-type organic semiconductors due to the necessity of low LUMO levels and ambient environment stability, solution processable inorganic materials are an excellent alternative. ZnO is an intrinsic n-type semiconductor which is non-toxic and sol-gel processable, creating avenues for film patterning [2] and fully solution processed devices. These ZnO films are also able to be easily treated, reducing lattice defects through UV-Ozone processing. This decreases the dark current and increases responsivity in hybrid photodetectors and yields improved electrical performance in FETs. Additionally, cross-sectional electron microscopy techniques reveal hidden characteristics within the morphologies of the films and provide insight as to how processing conditions impact FET and photodiode operation.

We acknowledge the support of this work through the National Science Foundation under Grant No. ECCS- 1707588

[1] A. Pickett, M. Torkkeli, T. Mukhopadhyay, B. Puttaraju, A. Laudari, A.E. Lauritzen, O. Bikondoa, J. Kjelstrup-Hansen, M. Knaapila, S. Patil, S. Guha, Correlating Charge Transport with Structure in Deconstructed Diketopyrrolopyrrole Oligomers: A Case Study of a Monomer in Field-Effect Transistors, ACS Appl. Mater. Interfaces, 10 (2018) 19844-19852.
[2] A. Pickett, A. Mohapatra, A. Laudari, S. Khanra, T. Ram, S. Patil, S. Guha, Hybrid ZnO-organic semiconductor interfaces in photodetectors: A comparison of two near-infrared donor-acceptor copolymers, Org. Electron., 45 (2017) 115-123.

Organic photovoltaic (OPV) cells have drawn increasing attention for decades due to the advantages of low cost, flexibility, lightweight, solution processability, and potential applications in large area devices. The development of new materials, nanomorphological control and device design has led to significantly improved solar cell efficiency up to 15% up to now. Over the long journey of three decades, many photoactive dyes have been systematically explored in metal(electrode)/dye/transparent oxide electrode (typical PV device stack geometries). One family of these promising dyes are squaraine (SQ) dyes which have a typical squaraic bone with four carbon atoms centered in their respective molecules. They are notable for their exceptionally high absorption coefficients extending from the green to the near-infrared. However, the theoretical understanding from molecular level remains challenging in terms of functional group modulation such as diphenylamnino and dissobutylamine groups. Based on experimental results, it shows that SQ donor molecules with diphenylamnino groups has Fill Factor (FF) has high as 0.73 compared with only 0.63 for SQ molecules with dissobutylamine groups. In this work, we use the first-principles simulation to gain a molecular understanding of the close correlation between structure and property. Furthermore, multiple physical parameters have been systemically calculated such as frontier orbitals, light-absorbing capacities, exciton binding energies, intramolecular charge transfer (ICT) properties of SQ, and the exciton dissociation rates at the interface of SQ and C₆₀. The results show that the two functional groups affect the performance of the cells by changing the charge distribution inside the SQ molecules and the separation process of excitons at the interface. The results indicate that the functional group modulation is an effective strategy to enhance the performance of small molecular weight organic thin film solar cells.

Organic field-effect transistors (OFETs) based on organic semiconducting materials have intensive interest both in the academic and industrial fields because of their application in low-cost and flexible electronics. The structure-property relationship of novel organic semiconducting materials has been explored with the development of device engineering technique leading to understanding charge transport mechanism and device physics. In this work, we have focused on the investigation of structure-property relationship of conjugated molecules based on quinoidal structure. The quinoidal structure has been considered as a promising building block to achieve efficient charge transport due to their high structural planarity arising from a double bond linkage between aromatic rings. Despite the merits as conjugated moiety, the incorporation of the quinoidal platform into the conjugated polymer backbones has suffered synthetic difficulty. Here, some derivatives of quinoidal small molecules and polymers with various quinoidal platforms were synthesized, and their structure-property relationship was investigated. Moreover, field-effect transistors based on these compounds were fabricated and characterized about electrical properties. Among them, OFETs based on poly(quinoidal thiophene-bithiophene) (PQuT-BT) prepared by off-center spinning showed the unprecedent highest hole mobility of 8.09 cm² V^-1 s^-1 among reported quinoidal polymers.^[1] Furthermore, we fabricated OFET devices based on this polymer via printing methods applying unidirectional shear forces leading to high molecular alignment and device uniformity in large area.

[1] Y. Kim, H. Hwang, N. K. Kim, K. Hwang, J. J. Park, G. I. Shin, D. Y. Kim, Adv. Mater. 2018, 30, 1706557.

Flexible transparent electrodes have drawn increasing interest due to the rapid development of flexible electronics and sensors. Metal networks is an emerging novel transparent electrode, which has shown potential applications in organic electronics and wearable sensors. However, the techniques which can realize facile fabrication of non-transfer, large-area and low-roughness metal-network are desperately desired. Our work demonstrates a print-compatible method for fully interconnected metal networks directly on the flexible substrate. We use blade-coat process to create polymer-sphere networks on the flexible substrate and then convert them into polymer networks. We combine evaporation process to transfer the networks into interconnected metal structures on the substrate. We demonstrate such transparent electrodes as the top electrodes both in the spin-coat and printed polymer photodiode and light emitting diode. This technique is low-cost, print compatible and can achieve the low-roughness and large-area transparent electrodes directly on the flexible substrate, which shows potential applications in large-area organic electronics.

Organic mixed ionic electronic conductors (OMIECs) are semiconducting polymers able to efficiently transport both ionic and electronic charge, and efficiently transduce ionic and electronic currents. This has led to their success as the channel materials in organic electrochemical transistors (OECTs) for biological sensing applications, amongst others. While the density of electronic charge and net ionic charge is readily addressed through a variety of electrical and spectroscopic techniques, OMIEC systems present a complicated and dynamic composite system which undergoes solvent swelling and intercalates significant concentrations of charge balanced anions and cations. To date the total of composition OMIEC systems in device relevant conditions remains a vexing and unresolved question. Employing ex situ and in situ x-ray fluorescence and adsorption spectroscopy, in concert with electrochemical impedance and gravimetric techniques, we report the quantification of the dynamic composition of mixed conducting thin films of the benchmark conjugated polymer/polyelectrolyte blend, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and a series of oligo-ethylene glycol side chain conjugated alternating copolymers. With this suite of characterization tools, the potential dependent mass fluctuations have been deconvoluted into their constituent anion, cation, and solvent components. This work reveals the complex interplay between conjugated polymer and electrolyte in OMIEC systems. Large degrees of swelling occur, as does a significant accumulation of anions and cations with these films, which to some degree is irreversible. The implications of these compositional effects on microstructure, charge transport (electronic and ionic), and device performance will be addressed.

In recent years, researchers have explored various methods of shaping the active semiconductor layer to improve thin film transistor (TFT) performance. The shaping has varied in methods and architectures from micron scale etched stripes for amorphous metal oxides¹, to hundreds of nanometers wide printed polymer stripes², and also to textured or grooved substrates for various materials including organics and polymers^3,4. It has been reported by these and other works that patterning can improve morphology of polymers and it has been observed that patterning improves key transistor characteristic across many TFT material systems. We present a detailed analysis of the electronic origins of improvements seen by nanopatterned devices and a means to optimize TFT device design to enhance these improvements. Our work specifically focuses on the unique advantages of nanostripe patterned TFTs, which are defined as narrow semiconductor stripes atop a gate dielectric with air or a low permittivity dielectric between each stripe. A nanostripe TFT device features multiple strips with a common gate and common source and drain contacts.
There are three main origins of nanostripe enhancement—first is improved morphological order of the disordered materials especially polymers, as previously reported by several groups; second is greatly enhanced induced carrier densities which is especially advantageous for disordered semiconductors which display carrier mobilities that are approximately linearly dependent on carrier concentration; third is improved gate control, similar to FinFET schemes seen in silicon and other semiconductor transistors. With the combined effects of all three, nanostripe devices can expect substantially improved drive currents and gate control over their unpatterned thin-film counterparts. This presentation will focus on the latter two sources of nanostripe enhancement since they are more broadly applied to all disordered semiconductor systems.
By using an experimentally verified TCAD model developed in Silvaco Atlas for the high mobility polymer semiconductor, PDPP-TNT, we show how optimizing the patterning of TFTs into nanostripes maximizes enhancement of foundational transistor performance metrics, such as increased carrier mobility, lower of off-current, reduced threshold voltage, and improved subthreshold swing. Just by adjusting the geometry of the nanostripe—namely by finding the optimum stripe width and stripe pitch, and making no adjustments to any other material parameters, we show that the drive current can be improved by a factor of 13. The nanostripe geometry focuses electric fields along the edges of stripe, maximizing trap state filling in those areas, to greatly improve carrier transport. While similar effects are seen in nanogroove and FinFET structures, nanostripes (which sit atop the dielectric) generate the most local enhancement. Further experimentation is underway to improve the accuracy of the TCAD model.

1. Lee, S., Shin, J. & Jang, J. Top Interface Engineering of Flexible Oxide Thin-Film Transistors by Splitting Active Layer. Adv. Funct. Mater. 27, (2017).
2. Gentili, D. et al. Logic-Gate Devices Based on Printed Polymer Semiconducting Nanostripes. Nano Lett. 13, 3643–3647 (2013).
3. Chen, H. J. H., Chen, J., Chen, S. & Huang, J. Study of Organic Thin Film Transistors on UV-Curable Dielectrics with Periodic Patterns Fabricated by Nano Imprint Technology. 96, 5–6 (2010).
4. Luo, C. et al. General Strategy for Self-Assembly of Highly Oriented Nanocrystalline Semiconducting Polymers with High Mobility. Nano Lett. 14, 2764–2771 (2014).

Understanding charge transport in organic semiconductors is an important step in the development of high-performance electronics employing these materials. Intermolecular motions are postulated to limit carrier mobility in organic electronics due to the formation of transient charge carrier localization, which prevents band-like transport. Raman spectroscopy is a powerful technique to nondestructively investigate such vibrational modes of organic crystals. These phonons are spatially localized within the lattice and correlate to the underlying crystallographic structure of the material. This provides a straightforward means to investigate the crystallinity, orientation, composition, and phase with high spatial resolution and no sample preparation. Here we present a polarization-orientation Raman spectroscopy analysis of single-crystalline organic semiconductor tetracene. We find five distinct vibrational modes in the low-energy regime (<25 meV) that are ascribed to intermolecular lattice phonons. Polar plots indicate that these motions are have A_g character, in agreement with group theory, directed within the ab plane of single-crystalline tetracene (001), as confirmed by electron diffraction. We then correlate the geometry of these phonon modes with the anisotropic field-effect mobility found within this plane and utilize density functional theory modeling of the corresponding molecular motions to examine the potential impact of orbital overlap between molecules on carrier transport. Collectively, these data suggest that transient localization due to Raman-active intermolecular motion has directionality within the single-crystalline structure of tetracene molecules and, more generally, the lattice motion strongly influences the inherent charge transport behavior and anisotropic character of organic semiconductors.

Infrared (IR)-to-visible up-conversion organic light-emitting diodes (OLEDs) using various novel low-bandgap organic donors and/or acceptors as the organic IR sensitizing layer were fabricated with an IR sensitivity beyond 1 μm. The novel donors and acceptors have narrow bandgaps of 0.9 ~ 1.2 eV and showed strong absorption in the visible as well as IR region till 1200nm. IR photodetectors are first fabricated to evaluate the new donors and acceptors as the IR sensitizer in the IR-to-visible up-conversion OLEDs. The multilayered photodiode structures with an electron blocking layer (EBL) and a hole blocking layer (HBL) are used to reduce its dark current, thus resulting in high detectivity. Most of photodetectors with novel IR sensitizers showed strong IR sensitivity in the near-IR wavelengths from 700 nm to 1200 nm. The photodetectors with novel IR sensitizers showed detectivities higher than 10¹¹ Jones in the multi-spectral region (300-1100nm) and the maximum detectivity of 3.0× 10¹² Jones at the wavelength of 800 nm due to significantly reducing dark current (~1 × 10^-6 mA/cm² at -0.1V). Using this novel low-bandgap organic materials as the IR sensitizer, IR-to-visible up-conversion OLEDs were fabricated with an IR sensitivity up to 1,200 nm. The IR up-conversion OLED successfully converted invisible near-IR light of 700nm-1200nm directly to visible green light with a peak emission wavelength of 520 nm. This is the very first report of all-organic IR-to-visible up-conversion OLED with near-IR sensitivity beyond 1100 nm which Si-based photodetectors cannot offer.

Solution processable semiconducting polymers are of great interest because of their potential for use in a myriad of different device applications. Most recently, their promise for use as thermoelectric materials has come into focus as an area where they might hold significant advantages over their inorganic counterparts. The thermoelectric performance in these materials can be tuned by adjusting the doping level and as such, the molecular doping of conjugated polymer films has become an area of particular importance for these materials. To carry out the doping process, our group employs two different two-step sequential processing techniques, one based on solution processing and the other based on thermal evaporation. The purpose of this talk is to provide a detailed comparison between these two doping methods.
In the first step of the solution sequential processing method, a pristine polymer layer is formed via spin-coating a polymer solution. In the subsequent doping step, a semi-orthogonal solvent is chosen for the dopant that swells but does not dissolve the underlying polymer layer. The dopant is then driven homogeneously inside the swollen polymer film by spin-coating the dopant solution on top of the pre-formed polymer underlayer. Because this solution-based method relies on the polymer layer swelling in the presence of a semi-orthogonal solvent, dopant intercalation is largely independent of the polymer layer thickness, provided the solvent fully penetrates throughout the extent of the polymer layer.
For the evaporation sequential doping method, thermal evaporation is used to place the dopant on top of the pre-existing polymer layer, where heating of the underlying polymer film is often used to help intercalate the dopant inside the polymer film from the gas phase. To date, most studies investigating doping by this process have been carried out on ultra-thin polymer films on the order of ~20 nm in thickness. For practical use, much thicker device architectures must be implemented and it has until now remained a mystery how this evaporative doping process would scale to multi-micron thick films for thermoelectric applications.
Here, we perform a systematic comparison between these two techniques by examining infiltration of the dopant 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) inside increasingly thick polymer films of poly(3-hexylthiophene-2,5-diyl) (P3HT). Structural changes induced by the incorporation of the dopant species are evaluated using grazing-incidence wide-angle x-ray scattering (GIWAXS). In addition, changes in intensity and location of the low-energy P1 polaron absorption band are measured as a way of characterizing the degree of free charge carrier delocalization. Finally, conductivity measurements, coupled with measurements of the Seebeck coefficient, illuminate how the thermoelectric power factor changes as increasingly thick polymer films are used. We find, quite surprisingly, that the power factor obtained for these materials with the two different doping methods are largely indistinguishable, and even approach the same level within the error of the measurement at increasing film thicknesses. Thus, when thermal evaporation is undesirable due to constraints in cost or time, solution processing provides a suitable method of dopant intercalation for fabricating polymer-based thermoelectric devices.

Electrical doping of semiconducting polymers remains challenging despite the successes of molecular design of materials for transistors, solar cells, and light emitting diodes. We will discuss our recent work in understanding the mechanisms of electrical doping of semiconducting polymers and the impact of processing on electrical properties. We will present results showing how the introduction of dopants from the vapor phase can help to unravel the interplay between morphology and electrical conductivity in polymers. Using model systems, such as poly(3-hexylthiophene), we find that structural order can increase upon the introduction of dopants that both stiffen the backbone and occupy free volume in the material. Measurements of thermopower of doped polymers provide a means to understand changes in the electronic density of states (DOS) upon doping. Unlike field-effect doping, our results show that chemical doping leads to a broadening of the DOS making it more difficult to predict electronic properties as a function of carrier concentration. Design rules for molecular structures that lead to efficient electrical doping will also be discussed.

Neuromorphic computing could address the inherent limitations of conventional silicon technology in dedicated machine learning applications. Recent work on silicon-based spiking neural networks and large crossbar-arrays of two-terminal memristive devices has led to the development of promising neuromorphic systems but implementing an efficient artificial neural networks in hardware remains a significant challenge. Organic electronic materials offer an attractive alternative to such systems and could provide neuromorphic devices with low-energy switching and excellent tunability, while being biocompatible and relatively inexpensive.

This talk describes state-of-the-art organic neuromorphic devices and provides an overview of the current challenges in the field and attempts to address them¹. We demonstrate a novel concept based on an organic electrochemical transistor²and show how some challenges in the field such as state-retention and linear conduction tuning can be overcome³. We investigate chemical doping of the active organic semiconductor material with additive amines for improved oxidation state stability over a large range and enhanced device-to-device variability, an essential characteristic for efficient large-scale arrays. Finally, we demonstrate that this device can be entirely fabricated on flexible substrates, introducing neuromorphic computing to large-area flexible electronics and opening up possibilities in brain-machine interfacing and adaptive learning of artificial organs.

¹ van de Burgt et al. Nature Electronics, 2018
² van de Burgt et al. Nature Materials, 2017
³ Keene et al. J Phys D, 2018

Improving the performance and reliability of organic electronic devices is necessary for their successful commercialization.New materials combined with improved synthetic routes have so far lead the way to the development of state-of-the-art organic electronic devices and their application in a range of emerging forms of electronics. An alternative and less studied approach towards high performing organic semiconductors (OSCs) is the use of molecular dopants as a mean for tuning their physical properties and often the resulting device characteristics. In organic thin film transistors (OTFTs) doping results to improved charge carrier mobility, by eliminating the threshold voltage while reducing parasitic contact resistances. Although highly useful, detailed understanding of the doping mechanism still remains challenging, with the most understood process being that of the integer charge transfer. The key requirement for doping to occur is the minimum energy offset between the dopant and the host material. In the case of n-type materials the dopant has to donate an electron from its highest unoccupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the host. Although numerous studies have been reported on p-type doping of organic semiconductors, work on molecular n-doping remains surprisingly limited. The main reason for this is the difficulty of identifying suitable dopants that are stable and efficient.
Here, we report the use of a new molecular dopant that is able to n-dope a number of organic semiconductors, which are then used as the channel materials in OTFTs. We show that key transistor parameters such as charge carrier mobility, contact resistance and threshold voltage improve dramatically upon addition of the dopant. The doping effect was studied using different methods including Electron Paramagnetic Resonance (EPR) and temperature dependent field-effect electron transport measurements. The impact of the dopant on the morphology of the OSCs has also being studied using Atomic Force Microscopy (AFM) and X-Ray diffraction (XRD) measurements, yielding interesting insights on the impact of doping on the layer microstructure for the first time. Overall, this work highlights that controlled doping of organic semiconductor materials is the key for improving the electronic characteristics of n-channel OTFTs.

In organic field-effect transistors (OFETs), the charge accumulation region is within several nm away from dielectric interface. For the monolayer crystals of small-molecule organic semiconductor materials, all the molecules contribute in forming the conductive channel and receive benefits from long-range in-plane ordering. A meniscus-guided solution-processing method is employed to grow monolayer crystals of C₁₀-DNTT with single-crystalline domain size as large as millimeters. Intrinsic field-effect mobility of >12 cm²V^-1s^-1is achieved. More importantly, the ultrathin thickness minimizes the charge injection barrier for the top-contact electrodes, allowing an ohmic hole injection and a low contact resistance (R_c W) of 67±15 ohm cm. The low contact resistance ensures low voltage drops at the metal/semiconductor interface, allowing the device to operate even at V_DS = -1 mV. It opens a new direction of low-power OFET that operates at pico-watts, which can only be achieved by high-mobility short-channel OFETs with very low contact resistance. The physics origin of forming such low contact resistance of OFETs will also be discussed.

Organic optoelectronic devices (OLED, OFETs, etc.) contain at least one, if not multiple instances of overlayers deposited onto organic semiconductors. The generated interface is inherently flawed with issues such as non-ohmic contact, overlayer delamination, or deposition induced damage arising. Traditionally, this is addressed by physical vapor deposition of yet another layer or by reengineering the materials in the device stack. In contrast, a reaction based approach allows for a wider range of function to be installed via molecular components in an organized and oriented manner, all while take advantage the inherent reactivity of the organic molecules which comprise the semiconducting layer. We have developed this approach via a “click-like” Diels-Alder chemistry whereby prototypical acene films (tetracene or pentacene) can be appended with a variety of small molecules to form an interfacial layer only ~5 Å thick. This chemistry is then applied towards improving the metal on semiconductor contact. As a demonstration of principle, Diels-Alder chemistry is utilized to form covalent bonds linking the organic semiconductor with a deposited metal contact thereby eliminating the poor adhesion present in this system. Application of the chemistry towards contact potential shifts is presented, while work towards sensing applications concludes the talk.

An important requirement for organic thin-film transistors (TFTs) is a reproducibly small and well-controlled contact resistance. A popular method for determining the contact resistance is the transmission line method (TLM), which has the drawback that it is valid only under the assumption that the contact resistance is linear, which is generally not the case. Another drawback is that TLM cannot differentiate between the source resistance (R_S) and the drain resistance (R_D). Using surface potentiometry on operating TFTs [1], resistances at the source and drain contacts can be measured independently from the potential drops measured in the transistor channel. We have carried out KPFM measurements on TFTs based on dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) fabricated in the bottom-gate, bottom-contact (inverted coplanar) configuration. A 10-nm-thick layer of aluminum oxide deposited by atomic layer deposition was used as the gate dielectric, and gold was used for the source and drain contacts. In some of these TFTs, the bias sequences applied during the KPFM measurements induced changes of the contact behavior, and in some cases the current-voltage characteristics of the contacts became non-linear. On these TFTs, potential drops (U_DROP) were observed on the KPFM profiles both at the source and at the drain contacts. From the simultaneous measurement of the drain current I_D, the contact characteristics I_D(U_DROP) were obtained. When the I_D(U_DROP) relationship is linear, the contact resistance can be obtained from this relationship. In some of the TFTs examined here, the current-voltage characteristics of the source and drain contacts are nonlinear, following a power law with an exponent between 1.5 and 3. In this case, we were able to estimate the contact resistance only to within an order of magnitude. Also, the potential drop at the source contact appears to be significantly larger than the potential drop at the drain contact, as one would expect from a contact exhibiting a large misalignment of the energy levels. To understand how each contact contributes to the total contact resistance, similar measurements were performed at various gate-source voltages. In addition, the measurements were repeated to observe the evolution of the contact characteristics with time.

References
[1] G. De Tournadre et al., J. Appl. Phys. 119, 125501 (2016)

Recent advances in bioelectronics are making the gap between electronic systems and human body ever closer. Despite these recent successes, the majority of bioelectronic devices still rely on electrode materials which are physically and mechanically dissimilar to biological tissues. Biological tissues are typically soft and contain large amounts of water with dissolved ionic species. In contrast, most inorganic materials and dry polymers in bioelectronic devices exhibit much higher elastic moduli with virtually no water content. Among many engineering materials, hydrogels show a great promise as ideal interfacing materials to biological tissues, owing to their unique tissue-like mechanical property, water-rich nature, superior biocompatibility, and ease in engineering. However, conventional hydrogels typically lack electronic conductivity, and the ionic conductivity of hydrogels in physiological conditions is very low. Unlike conventional hydrogels, conducting polymer hydrogels uniquely offer both electronic and ionic conductivity, and have been extensively explored for bioelectronic applications, among which hydrogels based on PEDOT:PSS are particularly promising owing to their favorable electrical, mechanical properties, and biocompatibility. Despite recent developments of various PEDOT:PSS hydrogels, several challenges still remain as unresolved questions in the field. For example, electrical conductivity of PEDOT:PSS-based hydrogels are typically low (less than 1 S cm^-1) and their poor adhesion with other engineering solids significantly limits their utility in bioelectronic device applications. Moreover, the limited set of accessible advanced fabrication strategies for PEDOT:PSS hydrogels further restrain their impact in applications. In this talk, we will discuss our series of recent developments on preparation, robust adhesion, and 3D printing of highly conductive PEDOT:PSS hydrogels. We first start with a simple yet effective method to prepare highly conductive pure PEDOT:PSS hydrogels. Then, we will discuss a novel strategy to facilitate robust bonding of PEDOT:PSS hydrogel on various commonly-used device substrates. Lastly, we will discuss the development of a novel direct ink writing 3D printable PEDOT:PSS ink and the resultant printed structures, uniquely enabled by the unprecedented 3D printing capability.

Organic electrochemical transistors (OECTs) are captivating significant attention owing to their applications in various fields such as molecular sensing, neuromorphic computing, digital logic circuits, cell culture analysis and printed electronics. However, building complementary circuits requires high-performance complementary p-type (hole-transporting) and n-type (electron-transporting) materials. Up to date, most reported OECTs are based on p-type polymers, with n-type lagging far behind mainly due to the lack of efficient and stable n-type conducting polymers. Herein, we report that polythiophene based polymers specifically poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-hexylthiophene-2,5-diyl) (P3HT), conventionally known as p-type polymers, can be electrochemically doped to be n-type. The electronic conductivity of both PEDOT and P3HT was estimated to be 744 and 640 mS cm^-1, respectively. Both, PEDOT and P3HT can work successfully as efficient n-type channel material for OECTs.

Contact effects represent a significant hurdle in the pursuit of the promised potential of organic thin-film transistors (OTFTs). For effective use in radio-frequency identification (RFID) and the other myriad technologies commonly ascribed to OTFTs, devices must operate in the high kHz to low MHz range, which requires a combination of high-mobility semiconductor and short channel length. However, transistors with short channel lengths must be accompanied by a drastic reduction in contact resistance. The channel conductance scales with channel length, whereas contact resistance is independent of this reduction in scale and, without a proportional decrease in contact resistance, this results in a reduced effective device mobility. For these reasons, the development of functional devices based on organic semiconductors requires significantly diminished contact resistance.
In this study, we focus on a simple, yet effective method to engineer OTFT electrodes with exceptionally low contact resistance and high device performance. We fabricate staggered-structure OFETs in the bottom-contact, top-gate configuration using Ti/Au source and drain contacts treated with pentafluorobenzenethiol (PFBT), and Cytop as the top-gate dielectric, while as semiconductors we tested both small molecules and polymers. We evaluated the performance of OFETs using the same architecture with contact deposition rates varied between 0.5 Å s^-1 and 2.5 Å s^-1. In 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene (diF-TES ADT) devices we found an increase in mobility from µ = 3.7 cm² V^-1 s^-1 obtained at a rate of at 2.5 Å s^-1 to µ = 19.2 cm² V^-1 s^-1 at 0.5 Å s^-1. Similarly, in the copolymer indacenodithiophene-co-benzothiadiazole (C₁₆IDT-BT), the mobility increases from µ = 3.3 cm² V^-1 s^-1 at 2.5 Å s^-1 to µ = 12.0 cm² V^-1 s^-1. To determine if these results were due to a difference in semiconductor film microstructure, we performed microbeam grazing incidence wide-angle X-ray scattering (µGIWAXS) and NEXAFS but find no significant differences. We assessed the contact resistance of these devices using the gated transmission line method (gated TLM) and found that the largely improved mobility at 0.5 Å s^-1 coincides with a significant reduction in contact resistance, with the lowest value of 200 Ωcm being obtained in the polymer devices. From scanning Kelvin probe microscopy, we discovered that the origin of the reduced contact resistance is the presence of high work function surface domains resulting from local SAM order, that facilitate injection into the semiconductor. The massively improved performance in both small-molecule and polymer semiconductors indicates that the deposition rate of source and drain contacts is an important parameter to modify in order to reach the low contact resistances necessary for the integration of OFETs into existing technologies.¹
Lamport et al., Nature Commun, accepted

Carrier injection from metal electrodes to semiconductors and electrical transport in semiconducting active layers are the most two important key actions in electronic devices. Generally, two types of semiconductors can be considered: One being organic semiconductors (OSCs) based on carbon based molecules and the other being inorganic semiconductors (ISCs). The former has a clean surface without dangling bonds in π-bonded system, and the latter is in a σ-bonded one and frequently has dangling bonds on its surface. By reflecting these differences between the two types of semiconductors from their bonding nature, the Schottky limit of metal-semiconductor (MS) interfaces is commonly observed for OSCs, while the Bardeen limit is more frequently seen for ISCs. Consequently in OSCs, the difference in energy level between the Fermi level (E_F) of electrode and the valence band maximum (VBM) or the conduction band minimum (CBM) becomes a Schottky barrier height against hole and electron injection, respectively. E_F of Au is generally located near VBM while that of Ca is near CBM of OSCs, and hitherto hole injection has been more popularly observed when Au electrode is employed, but efficient electron injection needs air-unstable metals with high E_F, such as Ca. This has been one of the major problems for long years that carrier injection of electrons has been greatly difficult compared to those of holes in OSCs since stable metals with low E_F, such as Au, Cu, and Ag, are typically employed. Contrarily, a barrier height for carrier injection is nearly independent of E_F for ISCs owing to the disorder induced gap states (DIGS) and metal induced gap states (MIGS) inside a band gap made by disorder/dangling bonds on the surface and metal-semiconductor interface structure. Such a phenomenon is known as Fermi level pinning leading to a vacuum level shift at the MS interface known as the Bardeen limit.
Simultaneous injection of holes and electrons (ambipolar carrier injection) is possible in field-effect transistors (FETs) of single-crystalline OSCs (sc-OSC). Recombination of holes and electrons generate excitons, and light can be emitted with energy release in a radiative process. Such ambipolar injection and subsequent light emission have attracted much attention from the viewpoint of optoelectronics since no chemically doped pn junction is required for light-emitting FETs (LE-FETs). Instead, pn junction, a fundamental component of electronic devices, can automatically be made at the recombination zone of holes and electrons in a single crystal active layer. However, serious problems are still left for optoelectronic applications that hetero-structure electrodes (combination of two different metals with high and low E_F) are required for ambipolar injection to achieve the highly efficient ambipolar carrier injection.
Here, we report an intriguing new concept of electrode for sc-OSCs, which can provide high efficiency beyond those of highly conductive metals like Au for holes and Ca for electrons [1]. The new electrode consists of a bilayer made of a polycrystalline thin film of OSC (pc-OSC), which is made of the same material as a sc-OSC used as an active layer, and linear-chain alkane tetratetracontane (TTC) coated by a metal (M: Au, Ca) thin film: M/pc-OSC/TTC. According to the unique carrier injection mechanism, highly efficient (higher than those of pure Au and Ca) and equivalently balanced hole and electron injection is realized. Air stable Au/pc-OSC/TTC electrodes showed high ambipolar injection efficiency, and bright light emission with very high current density of 25 kAcm^–2 is obtained.
[1] T. Kanagasekaran, H. Shimotani, R. Shimizu, T. Hitosugi, K. Tanigaki, Nature Communications, 8, 999 (2017).

This work is supported by Grant-in-Aid for Scientific Research of JSPS (18H03883, 17H05326, 18H04304), JST-CREST, and the bilateral country research program.

The realization of organic thin-film transistors (TFTs) showing transit frequencies of 10 MHz or more at usefully low voltages of about 3 V or less requires that the contact resistance be reduced well below 100 Ohm-cm [1]. Thus, several approaches have been developed over the past decade to improve the contact resistance in organic TFTs, including the use of interface layers to improve charge injection [2]. In a bottom-gate, bottom-contact (BGBC) organic TFT architecture, low contact resistance can be achieved by using metal source and drain contacts functionalized with a thiol monolayer. This allows tuning of the work function of the contacts to more closely match the energy levels of the organic semiconductor layer, as well as enabling in-plane pi-stacking morphology of the organic semiconductor layer across the channel-contact edge [3]. For example, by using a fluorinated thiol, such as pentafluorobenzenethiol (PFBT) on gold source and drain contacts, the contact resistance in p-channel BGBC TFTs can be significantly reduced below the target 100 Ohm-cm. Here, we employ this approach in flexible BGBC TFTs based on the small-molecule semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DPh-DNTT) [4], utilizing a 5.3-nm-thick hybrid gate dielectric composed of plasma-grown aluminum oxide and an alkylphosphonic acid self-assembled monolayer which allows TFT operation with voltages of about 2 to 3 V. The TFTs have small contact resistance (<50 Ohm-cm), steep subthreshold swings (<70 mV/dec) and large on/off current ratios (>10⁸), even for channel lengths as small as 1 µm. In addition, S-parameter and direct small-signal current-gain measurements on flexible sub-micron channel length TFTs show transit frequencies as high as 20 MHz. [1] A. Yamamura et al., Sci. Adv.,4, eaao5758, 2018; [2] S. Choi et al., ACS Appl. Mater. Interfaces, 8, 24744, 2016; [3] J. W. Ward, Adv. Funct. Mater., 24, 5052, 2014; [4] K. Niimi et al., Org. Letters, 13, 3430, 2011.

Printed polymer field-effect transistors (FETs) have been considered for many novel applications towards large area and flexible electronics, since they can enable pervasive integration of electronic functionalities in all sorts of appliances, their portability and wearability. However, printed polymer FETs fabricated with scalable tools fail to achieve the minimum speed required for example to drive high-resolution displays or to read the signal from a real-time imager, where a transition frequency (f_T), i.e. the highest device operative frequency, above 10 MHz is required. Such goal is even more critical to achieve with low operating voltages and on cheap plastic foils. Here, we demonstrate that high-frequency, low-voltage, polymer field-effect transistors can be fabricated on plastic with the sole use of a combination of scalable printing and digital laser-based techniques. These devices achieve f_T in the MHz range already at 2 V, and reaches a record 14 MHz f_T at 7 V. These devices can be successfully integrated into a rectifying circuit on plastic operating at 13.56 MHz, allowing to supply a DC voltage to RF devices and tags fabricated with cost-effective production processes.

Recently, flexible electronics based on paper or paper-like substrates have attracted considerable attention as a possible next-generation technology capable of replacing plastic-based electronics. Indeed, compared with conventional flexible substrates employed in electronic devices, paper features ubiquity, extremely low costs, eco-sustainability, as well as flexibility and bendability. Thank to these merits, in the last years the use of paper or paper-like substrates has been explored in different applications, ranging from basic electronic components to complicated devices and circuits. Basic electronic component are essential in every paper electronics application. In particular, the transistor is the fundamental component in most electronics systems. In order to develop flexible paper electronics, inexpensive and highly efficient fabrication methods are necessary. Printing techniques, such as inkjet printing, has gained a growing interest for depositing functional materials. Thank to scalability and manufacture cost, inkjet printing on paper is more competitive than other approaches. However, so far only few works report about thin film transistors (TFTs) printed on paper substrates. In this work, a low voltage organic field effect transistor (OFET) fabricated by inkjet printing on paper is presented. In particular, Bottom Gate- Bottom Contact devices were fabricated. A PEDOT:PSS-based commercial ink was employed for patterning gate, source and drain contacts by means of inkjet printing. A thin film of Parylene C has been deposited as organic insulator by means of chemical Vapour Deposition. Finally, a p-type organic semiconductor, namely TIPS Pentacene, was inkjet-printed through an home-made ink. Alternatively, an n-type organic semiconductor, namely ActivInk^TM N1400, was deposited through spin coating. All devices were fabricated on a p_e:smart paper type 2 from Felix Schoeller. This is a 185 ± 10 µm thick flexible paper-based substrate covered with a nonporous surface coating and a hydrophilic primer layer. Reproducible electrical performances were obtained on a set of more than 50 devices. In particular, low operating voltages (as high as 5 V), significant charge carrier mobility (in the range 0,2 cm²V^-1sec^-1 for p-type OFETs and 0,01 cm²V^-1sec^-1 for n-type OFETs), and a quasi-zero threshold voltage were recorded. Taking advantage on the fact that both p-type and n-type OFETs have been fabricated, complementary electronic circuits have been successfully fabricated and tested. In particular, complementary inverters with a valuable gain (10 V/V) and noise margins (of about 2 V) were initially fabricated. Starting from such basic building blocks, other logic ports, namely AND, OR, NAND and NOR, were fabricated and tested. These results pave the way for a future development of complex electronic systems that can be fabricated on paper by means of cost-effective techniques and with enhanced properties of portability (enabled by low voltage operation) and functionality (enabled by complementary circuit logic).

Organic vapor jet deposition encompasses several variations that have emerged for the direct deposition and additive patterning of organic semiconductor materials, as well as auxiliary materials used in organic electronic applications. In this talk, we will review the key principles involved and show examples of additive patterning of OLEDs, OTFTs, and OPV cells, as well as chemical vapor deposition of parylene as a barrier film. We discuss key parameters controlling deposition rate, doping ratios, patterning resolution, and the potential for lab-scale as well as high throughput fabrication of devices.

Conductive Polymers (CPs) are attractive materials for use in organic electronic devices such as photovoltaics, light emitting diodes, and field effect transistors, due to their electronic conductivity, optical transparency, and mechanical flexibility compatible with lightweight substrates. Here we present oxidative Chemical Vapor Deposition (oCVD) synthesis of polymeric conductor 3,4-polyethylene dioxythiophene (PEDOT) using a volatile liquid oxidant, vanadium oxytrichloride (VOCl₃). The oCVD process is a versatile deposition technique for fabricating CPs due to its unique combination of characteristics, including: formation of conformal coatings, processing at low temperatures, solvent-free synthesis, uniformity of growth, mechanical flexible films, industrial scale-up, and substrate-independence. The influences of deposition temperature and a fraction of oxidant saturation pressure (P/P_sat) on the electrical conductivity, optical transparency, and texture characteristics of oCVD PEDOT films are systematically studied. Grazing Incidence X-ray Diffraction (GIXRD) analysis revealed that the orientation of the oCVD PEDOT film is dependent on the deposition temperature and (P/P_sat) of the oxidant vapor. The orientation of oCVD PEDOT changed from predominantly edge-on orientation (h00) to predominantly face-on orientation (0K0) with an increase in deposition temperatures and a decrease in P/P_sat of the oxidant vapor. The optical properties of oCVD PEDOT are of essential importance for application as transparent conductors while there is a trade-off between transparency and sheet resistance. It was found that the figure of merit (FOM) which is defined as the ratio of direct current conductivity (σ_dc) to optical conductivity (σ_op) increases by a decrease in P/P_sat of the oxidant vapor.

Naphthalene diimide (NDI)-based copolymers with bithiophene (BT) or dithienylethene (TVT) are widely used in organic electronic devices as n-type materials and, in general, these can form large crystal domains through NDI-driven self-assembly. However, they still have limitations on improving electron transport mainly due to the lack of electrical connection between crystal domains. An interconnected network of small crystals with mixed orientation could provide an effective approach to increase the electron mobility (μ_e) of a low crystalline polymer. We demonstrate this approach using NDI-based copolymers (PNDI-FTVT), with dithienylethene having fluorine in the 3-position instead of hydrogen. The two fluorine substituents on the FTVT unit impart a dipole moment in the polymer backbone, leading to a fast and robust aggregation process (mainly a kinetic process), which suppresses NDI-driven self-assembly process (a thermodynamic process). This kinetic process limits the growing crystals; thus, the resulting smaller crystals improve electrical connection between crystal domains. In addition, robust aggregates driven by FTVT units in amorphous domains afforded a thermally stable morphology in the solid state. This microstructure resulted in improved μ_e of transistor devices by lowering energetic disorder as well as the reinforced film morphology at elevated temperature.

The electronic properties of solution-processable small molecule organic semiconductors have rapidly improved in recent years, rendering them highly promising for various low-cost large-area flexible electronic applications. Here we present solution shearing as a promising technique to generate high quality organic thin-films for organic transistors. In solution shearing, organic solution is sandwiched between a heated substrate and a shearing blade. As the shearing blade moves, meniscus is formed, through which solute deposits on the substrate to form a thin film. When the evaporation rate of the solvent at the meniscus matches that of the shearing rate, the crystals can be grown along the direction of shearing, resulting in aligned crystalline films. To further enhance crystallinity, the substrate can be patterned with alternation solvent wetting and dewetting regions, to limit nucleation rate and impede lateral crystal growth. This resulted in highly aligned TIPS-pentacene crystals with mobility enhancement by 1 order of magnitude. Furthermore, we demonstrate that crystal size of small molecule organic semiconductor can be controlled during solution shearing by tuning the shape and dimensions of the micropillars on the blade. Increasing the size and spacing of rectangular pillars increases the crystal size, resulting in higher thin-film mobility. We attribute this phenomenon as the microstructure changing the degree and density of meniscus line curvature, thereby controlling the nucleation rate. We also show that solution shearing can be performed on a curved surface using a curved blade. Lastly, we demonstrate that by specifically designing the pattern and tuning surface wettability of the electrodes on the substrate, aligned organic crystals can be patterned specifically between source and drain electrodes without the use of complex equipment. Simple organic-based logic circuits were demonstrated using our technique. In summary, these demonstrations legitimizes solution shearing as a highly viable technique for the formation of high quality organic semiconductor thin-film that would expand its applicability towards printed flexible electronics.

Additive printing provides a facile and useful approach for fabrication of electronic devices as it allows for the deposition and patterning of a wide range of solution-processable materials including organic small molecules and polymers, nano-/micro-particles, sol-gels, biomaterials, composites and others, thus enabling large-area, custom, and on-demand fabrication of complex functional components and systems. Some of the common challenges associated with printing electronics are limitations on materials set imposed by the choice of print technique and a usual need to carry out a number of manual interventions during processing, for example, changing material, carrying out post-deposition processes, and assembly of separate components into a complete device. These factors restrict the complexity of electronics that can be directly digitally fabricated and reduces the connection between the design and the final printed part. In order to address these issues, approaches that allow side-by-side and layer-by-layer deposition of a broad set of materials are needed, and have been developed by integrating multiple print techniques into a single, unified platform.

In this presentation, an example of an additive manufacturing approach for electronics that integrates processes such as fused-deposition modelling, paste extrusion, ink-jet printing and pick-and-place, as well as in-line metrology methods will be described. Using these tools and techniques a number of printed and hybrid devices have been made including: 1) biodegradable chemical sensors based on organic composite conductors, 2) structural input devices that provide tactile feedback using embedded magnets, and 3) thermally reversible sorbent systems for concentration of gasses such as CO₂. In addition, the combination of digital fabrication and in-line metrology allows for on-the-fly update of the design during printing based on characterization of the device as it is being made, thereby enabling some level of continuous autonomous optimization during the print.

Polymer semiconductors nowadays have been investigated for use in future electronics that have good electrical performance, mechanical compliance and large scale processability. The versatility of polymers has been exploited to design semiconductors with remarkable performance in various electronic devices. Although comparable electrical performance to traditional inorganic semiconductors can be achieved by polymeric semiconductors, it is still challenge to simultaneously introduce stretchability at the molecular level, in order to create mechanically robust semiconducting polymers. Typically, mechanical resilience and ductility of polymer active thin layer can be tailored through chemistry design, such as inserting functional units in polymer backbone or manipulating alkyl side chains. However, such methodologies involve complicated chemistry process, and triggered batch-to-batch variations. We therefore develop a new approach by physically adding organic small molecules, which possess high boiling point and low vapor pressure, into semiconducting polymers. By delivering such small molecules between polymer chains, the lamellar packing distance of semiconducting polymer can be modulate from 24.6 to 27.9 Å. This 3 Å volume opened using our method indeed greatly improve the thin film deformability due to the enhanced polymer chain dynamics in nanoscale. The semiconducting film can be stretched up to 300% without cracks. Finally, a charge carrier mobility of 1.8 cm²V^-1s^-1 is achieved as the active layer stretched under 100%, which is one of the best performance among stretchable polymer semiconductors in the research community.

Screen printing has been widely applied in electronic industry, since it has several distinct advantages such as large scale, low temperature processability, low cost and efficient productivity. However, due to the requirement of high viscosity of ink, and the relatively large printed film thickness, screen printing is usually used to fabricate metal electrode, while it remains a significant challenge to fabricate high quality organic small molecule semiconductors (OSCs) crystalline film. Herein, a simple method for the fabrication of various small-molecular OSCs crystalline film using ultra-low viscosity OSCs/insulating polymer blends by screen printing is reported. The film thickness is reduced to tens of nanometers by tuning the squeezing velocity and concentration of ink. Meanwhile, the quality and crystal domain size is largely improved by using screen printed banks prior to printing to confine OSCs crystallization and minimize nucleation sites, resulting in higher devices performance. As a result, the as-prepared C₈-BTBT organic field-effect transistors (OFETs) and device array achieved an excellent performance with the highest hole mobility up to 12.10 cm² V^-1s^-1, and the on/off ratio is up to 8×10⁹, which is on par with its single crystal counterparts. In addition, the uniform distribution of mobility in 8×8 array indicated the scalability of this method. Meanwhile, this method can also be applied for other OSC materials such as C₆-DPA. The high-performance OFET device demonstrated that screen printing is suitable for the scalable manufacture of organic electronic circuit. The successful fabrication of organic semiconductor layer by printing method is the key step toward full printed flexible electronics.

As the demand for high performance displays increases, the required performance for solution processed transistors for backplanes has increased. Target mobilities to drive AMOLED displays are now in the 10 cm²/Vs range, competitive with amorphous oxides. While advances in materials continue, over-all ink design and film processing present additional opportunities for device improvement. Two general approaches to improved devices have emerged: production of aligned channels via directional coating and production of vertically structured channels from blend-based inks. Using insitu X-ray and optical probes, we will explore the development of structure in these platforms. Due to the pi-stacking motif of most semiconducting polymers, the local mobility is intrinsically anisotropic. Therefore fabrication of aligned films enables further optimization of device performance. Blade coating is an excellent prototyping tool for production deposition techniques such as slot die coating and, due to it’s direction character, can result in aligned films. We report results from blade coating a number of high performing polymer materials. Depending on polymer and deposition regime, highly aligned films can be produced. The aligned films typically exhibit significant (c.a. 5 times) greater mobility than isotropic films. Detailed morphological studies suggest that the improvement in transport is related to alignment of the inter-crystallite regions (tie-chains). In-situ studies indicate that the coating induced alignment nucleates near the air interface and is facilitated by the presence of a lyotropic liquid crystal phase. In blend-based inks, vertical segregation between the active material and the matrix can result in advantageous ordering at either the buried or air interface. This can lead to improved device performance in either bottom or top gate architectures. In-situ probes again provide detailed insights into the development of vertical stratification and active layer crystallization.