Symposium S.EL13—Processing, Microstructure and Multifunctioning of Organic Semiconductors

The fabrication and manipulation of organic semiconductors as thin-film devices have met the challenge of combine structure-property relationships in practical devices even up to the nanoscale. This talk will focus on the use of pi-conjugated polymer precursors for electro-patterning nano-resolution patterns using conducting or current-sending AFM. The method involves the rational approach of preparing organic pi-conjugated polymers capable of crosslinking and also electrochemical activity. The interest is on light-emitting diode (LED) devices and photovoltaic (PV) devices. By using the precursor polymers, electrochemical methods capable of depositing as insoluble films or patterns enable patterning and multilayer formation. Application of a bias-voltage from an AFM tip is used for electronanopatterning but also junction-tunneling allows for signal voltage-current measurement – making them useful for memory devices or sensing. We will also describe the use of colloidal nanosphere lithography in producing arrays of conducting polymers with I-V behavior as probed by conducting SPM methods. These techniques demonstrate the ability to bring materials properties from bulk behavior up to the nanoscale.

High-performance organic electronic devices require organic semiconductor (OSC) thin films to possess desired microstructures. An effective approach is the use of graphene as a template for controlling growth behavior of OSC crystals. Unlike on ordinary substrates, the growth modes of OSCs on graphene are not only determined by the graphitic surface structure but also electronic characteristics of graphene. In this talk, I will discuss how OSC molecules assemble on graphene surface under the influence of graphene’s doping effect. Also, I will discuss a result that the OSC films grown on a graphene template under an optimized condition provide favorable vertical and lateral transport pathways for charge carriers and excitons in organic transistor and photovoltaic devices.

Polyaniline is a well known conducting polymer preferred owing to facile synthesis and various oxidation states. Polyaniline, on application of a potential bias attracts oppositely charged ions causing volume expansion. This reversible expansion is exploited to prepare bilayer actuator with cellulose acetate electrospun mat as the substrate. A stable deflection of over 1 mm was observed at 1 V. Effect of amount and properties of polyaniline on deflection is studied along with energy conversion efficiency. This electro-chemical actuator can have potential applications in biomimetic soft robotics.

Organic photovoltaics typically comprise two components in the photoactive layer – an electron donor and an electron acceptor – to overcome large excitonic binding energies in these Van der Waals systems. With typical exciton diffusion lengths in organic semiconductors on the order of 10 – 100 nm, the ideal structure of OPV photoactive layers comprises interdigitated crystalline domains of the electron donor and acceptor on the hundreds of nanometers lengths scale. Here, we present a novel strategy to form single-crystalline organic semiconductor nanoparticle arrays in situ during solution-phase deposition, with the size, orientation and spacing determined by nanostructured scaffolds at the substrate interface. Specifically, nanoporous scaffolds are introduced to the surface of device platforms that confine organic semiconductor nucleation at the air-solution-surface interface during a continuous dip coating process. These nuclei preferentially orient with their fast growth direction aligned parallel to the long axis of the pores. Subsequent crystallization proceeds beyond the scaffold to form uniform arrays of high-density, vertical nanorods with large exposed surface area. X-ray diffraction analysis has revealed that the vertical nanorods are oriented with the π-stack direction perpendicular to the substrate surface, the optimal orientation for light absorption and charge transport in organic solar cells and other devices with a sandwich electrode configuration. The height, diameter, and spacing of these nanorods are further tunable by varying the scaffold geometry and deposition conditions.¹ Critically, this generalizable method to form uniform nanoparticle arrays is compatible with continuous processing methods that will enable the large-scale manufacturing of such materials.²

1. Bai, X., Zong, K., Ly, J., Mehta, J. S., Hand, M., Molnar, K., Lee, S. S., Kahr, B., Mativetsky, J. M., Briseno, A., and Lee, S. S. “Orientation Control of Solution-Processed Organic Semiconductor Crystals To Improve Out-of-Plane Charge Mobility” Chemistry of Materials 29, vol. 17 (2017): 7571–7578.
2. Kong, X., Zong, K., and Lee, S. S. “Nanoconfining Optoelectronic Materials for Enhanced Performance and Stability” Chemistry of Materials 31, vol. 31 (2019):4953-4970.

A high-resolution microscopic gate-modulation imaging (μ-GMI) technique is recently developed to visualize spatial distribution of accumulated charges in operating organic field-effect transistors (OFETs) [1-4]. This technique utilizes high dynamic-range CMOS area-image sensor, which allows us to detect very slight change (up to 0.01%) of local optical absorption intensity in the channel semiconductor layers. The highly-sensitive and diffraction-limit μ-GMI becomes possible by taking advantage of the difference image sensing between at gate-on and gate-off states that are alternately biased. We successfully demonstrated that the technique is extremely useful for the microscope observations of channel charge and field distribution for studying carrier transport physics in OFETs [1,2], as well as for the rapid and collective inspection of thin-film transistor arrays [3,4].
Here we focus on the μ-GMI observations for model OFETs composed of high quality pentacene single-crystal channels that are fabricated by physical vapor transport technique and are attached on perfluorinated Cytop insulating layer coated on semi-transparent substrates. We show that anomalously large GM signals appear through the whole channel region, when the drain voltage, which is comparable to the applied gate voltage, is biased (i.e., saturation regime): The observed GM signal intensity reaches as high as ~1x10^-2, which is in contrast to the much weaker GM signals (~1x10^-4), as observed without the application of drain voltage. Peculiar feature is that the GM signal is considerably enhanced around the pinch-off point where the carriers should be depleted. We point out that the observation should be associated with the unexpectedly inhomogeneous distribution of positive and negative local GM signals in polycrystalline pentacene TFTs [1]. We discuss the origin of these “undefinable” operating channel states in terms of the electric-field effect on charged molecules in intrinsic organic semiconductors, on the bases of further experimental results on the polarization-dependent analyses of GM spectra as well as the effect of the use of trap-free perfluorinated insulator interfaces.
[1] S. Matsuoka, J. Tsutsumi, T. Kamata, and T. Hasegawa, J. Appl. Phys. 123, 135301 (2018).
[2] S. Matsuoka, J. Tsutsumi, H. Matsui, T. Kamata, and T. Hasegawa, Phys. Rev. Appl. 9, 024025 (2018).
[3] J. Tsutsumi, S. Matsuoka, T. Kamata, and T. Hasegawa, Org. Electron. 55, 187 (2018).
[4] J. Tsutsumi, S. Matsuoka, T. Yamada, and T. Hasegawa, Org. Electron. 25, 289 (2015).

In this research, we report the effect of crystallinity on the mixed conductivity of highly-ordered polypyrrole film. Polypyrrol-4,4’-biphenyldisulfonic acid (poly(Py:BPDSA:Py)) hybrid was synthesized by oxidative polymerization after connecting two pyrrole monomers with various stoichiometric ratios of BPDSA in the aqueous solution. The structural and electrical/electrochemical analysis revealed that the increase in BPDSA content enlarges the crystallite size from 18.9 to 52.1 nm, the ratio of crystalline to amorphous domains by 6 times, the electrical conductivity from 174 to 406 S/cm, and the volumetric capacitance from 85 to 672 C/cm³, while the doping concentration extracted from Hall measurement and electrochemical analysis remains unchanged. On the other hand, the resultant poly(Py:BPDSA:Py)-based organic electrochemical transistors (OECTs) showed the maximum value of transconductance (0.5 mS) at the intermediate composition of BPDSA, which can be attributed to the retardation of ion movement in highly crystalline polymeric film. The repeated OECT operation revealed the stable current on-off ratio over 2 x10² without the significant loss in transconductance. All these results suggests that the mixed conductivity of poly(Py:BPDSA:Py) can be strategically modulated by engineering the crystallite size.

Electrical doping of semiconducting polymers requires introduction of counter ions to maintain electroneutrality. These counter ions can modify electronic charge transport by changing the microstructure of the semiconductor and by electrostatic interactions between the charge carriers and their counter ion. We will discuss our recent work using single ion conducting polymers, known as polymeric ionic liquids, as the dielectric in transistors with semiconducting polymers. Polymeric ionic liquids (PILs) have tethered ionic functional groups on their backbone with an untethered counter ion leading to single ion conductivity. By appropriate molecular design of the PIL, the operation of polymer transistors can be switched from an electrical double layer mechanism to electrochemical doping of the bulk. The behavior of transistors with PIL gates reveals the differences in the electrical conductivity at the surface and bulk of semiconducting polymers at high charge carrier concentrations. The role of the infiltration of ions on the electronic density of states of semiconducting polymers can also be determined by measurements of the thermopower of electrochemical transistors. Our results show that electrochemical doping leads to a broadening of the DOS and helps to rationalize recent models for charge transport in semiconducting polymers at high carrier densities.

The chemical structure of donors (Ds) and acceptors (As) has been a limitation to the achievable power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) active layers of binary D-A mixtures in organic photovoltaics. While new syntheses can be used to generate Ds and As, a holistic strategy that simultaneously improves open circuit voltage, short circuit current, and fill factor is necessary and has been pursued using different approaches in organic photovoltaics (OPVs) to PCEs. This holistic approach, though, has been elusive, due to morphological constraints and the inherent electronic structures of the components, leading to performance trade-offs. Ideally, both the morphology and electronic structure would lead to a maximization of light absorption, enhancement of exciton splitting, and ease of carrier extraction. Adding a third component has been done, resulting enhancement of either the morphology or electronic structure leading to PCE improvement, but only incremental, at best. However, using quarternary D-A blends, double cascading energy level alignment in BHJ organic photovoltaic active layers are realized, enabling efficient carrier splitting and transport, without perturbing the desired BHJ morphology. This has led to record-breaking PCEs of 17.78% where, by electronic structure and morphology optimization, simultaneous improvements of the open-circuit voltage, short-circuit current and fill factor are realized. This strategy opens numerous avenues to optimize light absorption, carrier transport, and charge transfer state energy levels by the chemical constitution of the components. The chemical structures of the Ds and As offer control over electronic structure and charge transfer state energy levels, enabling manipulation of hole-transfer rates, carrier transport, and non-radiative recombination losses.

Organic solar cells (OSCs) are considered one of the most promising cost-effective options for utilizing solar energy in high energy/weight or semi-transparent applications. Recently, the OSC field has been revolutionized by the development of novel non-fullerene small molecular acceptors with efficiencies now reaching >16%. The device stability and mechanical durability of non-fullerene OPVs have received less attention and developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OPV commercialization. Here, we report our current understanding of the phase behavior of OPV mixtures and the relation of phase behavior to performance, processing needs (e.g., kinetic quenches), and morphological stability via meta-stability or vitrification. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to soft x-ray scattering and WAXS to determine critical morphology parameters and molecule packing. A large range of miscibility (from hyper-miscibility to strong hypo-miscibility) is observed, including complex temperature dependence that can be a mixture of upper- and lower critical solution temperature behavior for both the binodal and the liquidus. The measurements presented should help to create molecular structure-function relationships that would allow some predictive guidance on how desired phase behavior and vitrification properties can be targeted by specific chemical design. They also allow to predict how unstable binary systems can be stabilized with the correct ternary compound.

Organic mixed ionic/electronic conductors have gained considerable interest in bioelectronics, power electronics, circuits and neuromorphic computing. These organic, often polymer-based, semiconductors rely on a combination of ionic transport, electronic transport, and high volumetric charge storage capacity. Despite recent progress and a rapidly expanding library of new materials, the understanding of stability and transport/coupling of ionic and electronic carriers remain largely unexplored. We highlight recent synthetic and processing approaches used to tailor electrochemical device properties and stability, as well as new opportunities enabled by such advances. Our understanding of critical processes in electrochemical devices further requires us to study these materials in device-relevant conditions, fully considering the effects of ions and solvent on microstructure and transport. To this end, we report on recent efforts towards structure-property relations in high performance organic mixed conductors using ex-situ, in-situ, and operando scattering and spectroscopic techniques.

Perovskite solar cells (PSCs) have gained tremendous attention as potential materials for photovoltaics due to their high efficiencies approaching the best silicon solar cells and their compatibility with low-cost low-temperature fabrication methods (such as solution processing). Many solution-processing approaches have been adopted to manipulate perovskite formation including anti-solvent processing, inert-gas jet treatment, and additive-assisted processing. Some of these approaches allow room-temperature processing of perovskite thin films, while other approaches require post-deposition annealing. Perovskite film formation is complex, involving the formation of intermediates and/or metastable phases that strongly affect the final perovskite film microstructure. Therefore, understanding the mechanism of perovskite formation and the crystallization pathways is key for more facile control of perovskite formation. An effective way to understand the mechanisms of perovskite formation is using real-time X-ray scattering. Here, we use time-resolved x-ray scattering to investigate the perovskite formation of MAPbI₃-based perovskites and mixed cation (Cs, FA)PbI₃ perovskites in-situ during spin coating and the subsequent post-deposition treatments with different processing approaches such as additive-assisted processing, anti-solvent processing and N₂-gas jet treatment.

Time-resolved monitoring of the perovskite thin film processing reveals the formation of intermediate phases on the route of perovskite formation, whereas perovskite formation is dominated by a sol-gel process. For MAPbI₃-based perovskites processed with the MASCN additive, we show that the nature of the intermediate precursor phases and their formation/dissociation dynamics have an impact on the extent of nucleation and growth of perovskite phase affecting the microstructure of the perovskite film. Our findings reveal that the combination of two precursors (MASCN-precursor and DMSO-precursor) with fast and slow transformation rates contributes to the formation of micron-sized perovskite crystals, through seeding nuclei combined with the slow growth of the perovskite phase. For Cs-FA-containing perovskites, we show how the competition between the non-perovskite and perovskite phases formation are affected by the processing treatments. We find that dropping anti-solvent induces immediate crystallization from the bulk wet film, whereas applying N₂-gas works by depleting volatiles from the top-surface leading to interface-induced crystallization that occurs after reaching supersaturation, while both treatments result in dominant crystallization of non-perovskite hexagonal phase with the formation of seeds of perovskite phase. When neither treatment is applied, the as-cast film is mostly amorphous with little non-perovskite phase formation. We further show how the initial structure of the as-cast precursor film impacts the perovskite formation during the subsequent annealing treatment.

Our work highlights the importance of real-time investigation of perovskite film formation which can aid in establishing processing-microstructure-functionality relationships and help to provide a fundamental understanding of the mechanisms of perovskite formation.

This paper demonstrates that a thin polymeric film (10-80 nm) can be continuously drawn from the meniscus of a nonpolar polymer solution at an air-water-fluoropolymer interface using a roll-to-roll process: “interfacial drawing.” With this process, it is possible to control the thickness of the film by manipulating the concentration of the solution, along with the drawing velocity of the receiving substrate. We demonstrate the formation of thin films >1 m in length, and 1000 cm² in area, using our custom-designed apparatus. Interfacial drawing has three characteristics which compare favorably to other methods of forming and depositing polymeric thin films. First, the films are solidified prior to deposition, which means that they can be used to uniformly coat nonplanar, rough, or porous substrates. Second, these films can be stacked into multilayered architectures without risk of redissolving the layer beneath. Third, for some materials, the process yields films with superior mechanical compliance for applications such as wearable or flexible devices, compared to films produced by spin-coating. We demonstrate the utility of interfacial drawing by forming thin films of various semiconducting polymers, including the active layers of all-polymer bulk heterojunction solar cells as well as barrier coatings. As part of these demonstrations, we show how floating polymeric films can be transferred easily to diverse substrates, including those with rough and irregular surfaces, such as textiles and fabrics.

Meniscus-guided coating (MGC) represents a range of scalable methods for controlled solution-deposition of functional materials, such as organic semiconductors, polymers, perovskites and metal-organic frameworks. MGC is of special interest to the fabrication of thin film devices that rely on an active layer with a high unidirectional charge carrier mobility. During MGC a substrate is translated under a coating head, slot or blade which deposits the solution from a steady-state liquid meniscus. The thickness of the dry film may decrease or increase with the coating speed, depending on whether deposition takes place in a, respectively, evaporation-controlled or hydrodynamic (Landau-Levich) regime. The occurrence of these regimes has been reproduced and explained by models based on lubrication theory. In contrast to film thickness, so far little theoretical attention has been given to the effect of process parameters on the actual thin-film crystalline morphology, which is of critical influence to device performance. In this contribution we present our first effort to accounts for this. We develop a model that explains how substrate translation and solvent evaporation determine crystalline morphology in terms of the size, dispersity and shape anisotropy of spherulitic domains. The model reproduces the experimentally observed transition from macroscopically aligned structures, via stretched grains to isotropically impinged domains upon increasing the coating speed. Our calculations provide a framework that relates morphological characteristics to the deposition regime in MGC.

The transport properties and device performance of organic semiconductors is determined by the microstructure and morphology of polycrystalline thin films, including grain size, domain orientation and grain boundaries. For device applications, such as field-effect transistors, controlling the domain orientation and grain boundaries is particularly important as this mediates charge transport in the channel of the device. More generally, the opportunity to fabricate single crystal domains to match the device architecture would be a very promising pathway to unlock the intrinsic properties of organic and hybrid semiconductors. This presentation will discuss recent efforts to reduce and even eliminate grain boundaries in solution-processed organic and hybrid semiconductor films, with such applications as high performance field-effect transistors, photodetectors and light emitting devices envisioned. Approaches based on patterning the microstructure of thin films post-deposition or in situ will be discussed, as well as additive manufacturing-based coating and printing methods of delivering materials with single crystal-like properties at designated locations.

The brain can perform massively parallel information processing while consuming only ~1- 100 fJ per synaptic event. I will describe a novel electrochemical neuromorphic device (ENODe) that switches at record-low energy (<0.1 fJ projected, <10 pJ measured) as well as voltage (< 1mV, measured), and displays a large number of distinct, non-volatile conductance states within a ~1 V operating range. The tunable resistance behaves very linearly, allowing blind updates in a neural network when operated with the proper access device. ENODes also display outstanding endurance achieving over 10⁹ switching events with very little degradation. I will describe our recent efforts at scaling and materials selection, allowing us to reach 20 ns write pulses and operation at high temperature (up to 120°C). These properties are very promising in terms of the ability to integrate with Si electronics to demonstrate online learning and inference. ENODes are electrochemical devices where gated proton drift induces changes in the electronic states of a semiconductor channel. The peculiarities of the physics of these devices will be discussed along with their consequences on device design and performance.

The recent emphasis of point-of-care diagnostics and preventative health monitoring has led to increasing interest in electronic devices aimed at noninvasive continuous health monitoring. Organic electronic materials are well suited for bioelectronics applications because of their biocompatibility and low Young’s modulus which is comparable to that of human tissue. In particular, organic electrochemical transistors (OECTs) show promise due to their ability to convert ionic signals, which are typically found in biological systems, to electronic outputs with high gain. Furthermore, the low operating voltages (<1 V) are ideal for biological samples due to the sensitivity of cells/proteins to an applied bias. As a result, OECTs have been implemented for biosensing, ion delivery, and neural recording.

The majority of OECTs are based on the commercially available conductive polymer blend PEDOT:PSS because of its high electronic and ionic mobilities. However, PEDOT chains in PEDOT:PSS are heavily doped due to the negative charge of sulfonate anions on the PSS units, and thus OECTs must operate in depletion mode. Thus, OECTs require significant voltages (+0.8 V vs Ag/AgCl) to hold the OECT in its off state, leading to increased power consumption and device instability caused by parasitic reactions with ambient oxygen. In this work, we mitigate these inherent problems with PEDOT:PSS by introducing small molecule amine de-dopants in solution which can reduce PEDOT chains. With these additives, we effectively de-dope PEDOT:PSS to achieve electronic characteristics similar to an intrinsic semiconductor. The resulting de-doped PEDOT:PSS is used to fabricate highly stable OECTs that operate in enhancement mode while preserving the exceptional electronic and ionic transport (µ_h+ ~ 2 cm² V^-1 s^-1, C* ~ 40 F cm^-3).

This presentation describes the chemical interactions between the PEDOT:PSS and amine de-dopant molecules to understand the relationship between the film microstructure and resulting electronic structure and transport properties. We demonstrate the relationship between de-dopant concentration and OECT performance, showing a tradeoff in mobilities and transconductance with increasing de-dopant concentration. Then, we use cyclic voltammetry and ultraviolet photoemission spectroscopy to probe the electronic structure of PEDOT, showing a deeper highest-occupied molecular-orbital (HOMO) level is responsible for the shift in OECT threshold voltage. Furthermore, we investigate microstructure of the de-doped PEDOT:PSS films using grazing incidence x-ray scattering measurements (GISAXS/GIWAXS), Raman spectroscopy, and atomic force microscopy to rationalize the difference in performance across the studied amines. Finally, we develop design criteria for selecting de-doping additives to further improve the stability and performance of enhancement-mode PEDOT:PSS OECTs for low-power wearable and implantable electronic devices.

Biological nerves have evolved toward functionalities to efficiently process complex information in real-time. Unlike biological nerve system, von Neumann based computing system performs in centralized and sequential form which has difficulty in event driven and parallel operation. In this regard, there has been many efforts to adopt the superior characteristic of biological nervous system and the concept of neuromorphic electronics have emerged. As the synaptic behavior can be implemented through single artificial synaptic device, it can emulate the functions of biological sensory and motor neurons when combined with sensors and actuators. Making the artificial nervous system desirable for soft robotics and neuroprosthetics have been widely investigated.
Here, artificial nervous systems were developed by using organic electronics to emulating peripheral nervous system. Pressure sensors (artificial mechanoreceptors), organic ring oscillators (artificial nerve fibers), and synaptic transistors were integrated to emulate the biological sensory nerve system. By forming hybrid reflex arc which connects artificial sensory nerve and biological motor nerves the applicability for neural prostheses was verified and the biological motor nerves were actuated depending on external pressure information. In addition, by integrating a stretchable artificial synapse, and a polymer actuator (an artificial muscle), stretchable artificial sensorimotor nervous system was developed. The optogenetics was emulated by contracting the artificial muscle with light stimulation. In parallel, the system distinguished the alphabet Morse code which showed the potential as an optical wireless communication method for human-machine interface. Furthermore, by integrating a triboelectric sensor for an artificial auditory system, artificial auditory system was emulated. We modulated morphology of organic semiconductors of the artificial synapses to implement the specific recover time of biological synapse facilitation. Our organic artificial nerve systems suggest promising strategy for bioinspired electronics, soft robotics, and neuroprosthetics.

Over recent years several new classes of conjugated polymers have shown promise as materials for polymer field-effect transistors with high field-effect mobilities. Many of the recently discovered high mobility polymers, in particular donor-acceptor copolymers, owe their excellent charge transport properties to a low degree of energetic disorder associated with a well-defined backbone conformation with small variations in torsion angles. In this presentation we will present our current understanding of the transport physics of these materials and focus in particular on the relationship between molecular structure, thin film processing and charge transport and thermoelectric properties of these materials. We will discuss new approaches for the doping of these polymers and for understanding their thermoelectric physics.

The performance drop of organic field-effect transistors (OFETs) at high temperature is usually attributed to unstable and disrupted charge transport in the channel. However, the impact of charge injection process is overlooked and rarely discussed. Here, we proved that the contact resistance has a significant influence on the behavior of the OFETs at high temperature. Two different device structures with different contact resistance, bottom-contact-bottom-gate (BCBG) and top-contact-bottom-gate (TCBG), of an Isoindigo-based polymer, were characterized from room temperature to 220°C in air. The BCBG device witnessed a substantial threshold (V_T) shifting, dropping on-current and unstable output curve at temperatures above 180°C. Conversely, the TCBG devices can maintain close-to-ideal V_T, high mobility, stable output curves and high on/off ratio of 10⁶ at 220°C. The contact resistance of the BCBG devices, estimated using the gated-transmission line method, is much higher than that of the TCBG devices in the whole range of measured temperature. More importantly, at fixed gate voltage, the ratio of contact to channel resistance (R_C/R_Ch) of BCBG devices increases with increasing temperature that makes them heavily contact-limited at elevated temperature. The TCBG devices, on the other hand, show no sign of being contact-limited even at 220°C. This study indicated the importance of charge injection and device architecture on designing thermal resistant OFETs as well as fundamental understanding of the charge transport in conjugated polymers in extreme conditions.

Electronic materials require flexibility in order to accomplish conformal integration on nonplanar dynamic surfaces such as skins, internal organs, and textiles for wearable and implantable devices. Mechanical flexibility of electronic materials has been achieved by geometrical engineering of hard materials or by molecular design of intrinsically deformable π-conjugated polymers. The latter approach is further amenable to large-scale, low-cost solution processing. However, high intrinsic deformability of π-conjugated polymers is attained at the expense of reduced crystallinity and/or conjugation length, either by the synthetic approach to introduce dynamic bonds or through nanoconfinement to enhance chain dynamics. On the other hand, exceptional molecular ordering is the primary requirement for disorder-free charge transport. Towards this end, single crystals or long-range ordered crystalline films of organic semiconductors have been pursued, achieving charge carrier mobility above 10 cm²V^–1s^–1for both small molecules and polymers. Despite their promising performances, single crystals or crystalline films are not tolerant to mechanical deformation. It has been revealed that strain allows a shift toward non-equilibrium packing within the elastic deformation regime (the strain window limited up to about ±2%); strain above this limit leads to brittle fractures.

How to render molecular crystals highly deformable to reconcile the conflict of mechanical flexibility and exceptional molecular ordering? An intriguing strategy offered by biological systems is the contractile tail of bacteriophage T4 virus—its tail is comprised of a protein crystal capable of undergoing strain-induced cooperative structural transition to reach a staggering 60% contraction, thereby injecting DNA into the bacteria host. A similar strategy has been applied to synthetic materials, whose atomic/molecular cooperativity enables strain accommodation mechanisms such as shape memory effect (recovery of mechanically deformed shape by thermally induced phase transition), superelasticity (mechanically induced reversible shape change through stable-to-metastable phase transition) and ferroelasticity (mechanical shape reconfiguration through deformation twinning and detwinning). On the contrary, mechanically deformable molecular crystals realized through cooperative structural transition are rare and has not been reported for electronic materials and applications.

We discover that mechanically-induced cooperative structural transitions can serve as a stress-releasing mechanism when mechanical deformation surpasses the intrinsic elastic limit. Upon mechanical shearing or bending, synchronized molecular rotation and/or displacement give rise to cooperative structural transitions, i.e. superelastic polymorph transition and ferroelastic deformation twinning in organic semiconductor single crystals. Through this mechanism, organic semiconductor single crystals can tolerate an unprecedented 13% strain while maintaining >70% of the unstrained charge carrier mobility. Moreover, thermomechanical cooperative structural transitions further enable shape memory effect, paving ways to designing ultraflexible single crystal electronic devices that can memorize shape and function at once.

Polymer semiconductors are promising materials for stretchable electronics owing to the opportunity to achieve intrinsically stretchable films. In stretchable devices, the films are expected to repeatedly deform under cyclic strain while being operationally stable. While there have been a number of successful demonstrations, the physical origin of the deformation behavior remains elusive. This is in part due to the difficulty in probing the stress-strain behavior of the semiconductor film owing to its thin nature and in-plane cyclic loading. Here, we employ a novel method to probe the stress-strain behavior of polymer semiconductor films under large cyclic strains. This is achieved by laminating the films onto a thin elastomer substrate and testing the composite behavior in a dynamic mechanical analyzer (DMA). The elastomer provides support to handle the film but is thin enough that the semiconductor of interest can be probed. The DMA provides a highly sensitive tool to extract detailed mechanical behavior of the composite with fine control of the sample temperature and strain rate. We use this film laminated on elastomer (FLOTE) method to study a range of conjugated polymers and compare the results to the thermomechanical relaxation of the polymers. We find that the viscoelastic characteristics of the polymer not only impact film deformation in tension but also contraction upon strain removal. We find that many DA polymers have stress-strain behavior characteristic of cold-drawing at room temperature and are unstable under cyclic strain loading. However, we show that stretchability can be improved by varying the molecular structure of the polymer to modify viscoelastic behavior, demonstrated by exploring structural variants of PBnDT-FTAZ. Furthermore, we show that the polymer semiconductor elastomer composites have features consistent with Mullins’ effect that is commonly observed in elastomer composites. These features include strain-softening after the first strain cycle, stress-strain behavior that tracks with a virgin sample upon greater extensions, and a permanent set that changes with applied strain. This behavior is exploited to demonstrate high-performance stretchable field-effect transistors that are stable over a 40% cyclic strain.

Semiconducting polymers are attractive materials for flexible and stretchable electronics owing to favorable mechanical properties. These properties are afforded by the viscoelastic nature of the polymers, which directly impact the mechanical and thermodynamic stability of the film. Recently, dynamic mechanical analysis (DMA) have been shown to be a sensitive probe to capture thermal relaxations in conjugated polymers. This is particularly useful to study recently developed donor-acceptor (DA) type copolymers that have been shown to have complex thermomechanical characteristics without a clear glass transition. In addition, DMA probes thermal transitions under the effect of direct mechanical loads, thus, provides a complete picture of thermomechanical properties of the polymer including thermal relaxations, storage and loss moduli, stress relaxation, etc. In this talk, we report on the origin of the thermal relaxations in the representative DA polymer PBnDT-FTAZ. This is done through an analysis of the DA polymer with systematic changes to the molecular structure. We then extend this analysis to a broad set of high performance DA polymers showing the common origin of the relaxation behavior associated with side-chains, localized backbone motion, and chain slippage. We then show that these transitions have a significant impact on film toughness demonstrating that localized relaxation largely associated with polymer side-chains dictate the ductile to brittle transition of the film. Finally, we take the information gained by the thermomechanical behavior of the polymers to demonstrate high performance stretchable organic field effect transistors, and morphologically stable high efficiency organic solar cells.

The mechanical robustness of polymer solar cells (PSCs) is of great importance to ensure the long-term stability and enable their use as power-generators in flexible and stretchable electronics. Here, we present a comparative study of the mechanical properties of small-molecule acceptor (SMA)-based, polymer acceptors (PA)-based, and fullerene-based PSCs. We chose ITIC, P(NDI2OD-T2), and PCBM as three representative acceptor materials and blended them with the same polymer donor. To understand the difference between the mechanical properties of SMA-based and PA-based PSCs, we control the number-average molecular weight (M_n) of P(NDI2OD-T2) from 15 to 163 kg mol^-1 in all-PSCs. The high M_n PA-based-PSCs exhibited a high strain at fracture of 31.1%, which is 9- and 28-fold higher than those of SMA-PSCs and PCBM-PSCs, respectively. The superior mechanical robustness of all-PSCs is attributed to using a PA above the critical molecular weight (M_c), which produces tie molecules and polymer entanglements that dissipate substantial mechanical strain energy with large plastic deformation. The connectivity between the crystalline domains generated by PA tie chains leads to high charge mobilities and photovoltaics performances of all-PSCs. Also, this feature explains very high donor:acceptor composition tolerance of all-PSCs in the photovoltaic and mechanical performances. Therefore, our work highlights the importance of incorporating high M_n PAs above the M_c for producing the PSCs with excellent mechanical robustness and device performance.

Electronic devices in the future sustainable societies require maximum function with minimum amount of constituent materials and energy cost for production. Electronic functions are often originated from two-dimensional material properties so that device components of large area and small thickness saves material consumption. The fundamental electronic functions such as switching “zero” and “one” in the digit of information are governed only by a nanometer-scale thin layer at the very surface of the semiconductor material due to the very short penetration length of electric field. Future ten-meter scale large-area display also needs to be more “two-dimensional” to save materials and save energies to carry and equip on billboards.
This presentation focuses on recently developed ultrathin organic semiconductor single crystalline films that is suitable for large-area production with low energy consumption; the films is easily formed to large area from solution at relatively low temperature at 80 degrees centigrade. Extremely thin crystal films are controllably grown to a-few molecular layers with the thickness of only 10 nm, so that material cost can be only 0.01 Euro per cm2. The talk begins with material chemistry of designing the semiconductor molecules that causes very high carrier mobility more than 10 cm2/Vs for p-type and 3-5 cm2/Vs for n-type organic single-crystal transistors. Furthermore, Recent development of key technologies for printed integrated circuits which can provide future low-cost platforms for RFID tags and sensing circuitries. Finally, a technology for large-area light-weight display sheets will be demonstrated.
Such prospect bears increasing reality because of recent research innovations in the field of material chemistry, charge transport physics, and solution processes of printable organic semiconductors. With excellent chemical and thermal stability in recently developed new materials, we are developing simple integrated devices based on CMOS using p-type and n-type printed organic FETs. Particularly important are new processing technologies for continuous growth of the organic single-crystalline semiconductor “wafers” from solution and for lithographical patterning of semiconductors and metal electrodes. Successful rectification and identification are demonstrated at 13.56 MHz with printed organic CMOS circuits.

With printable single-junction organic solar cell efficiencies reaching 16%, the economic viability of a >20% efficient, printable, and non-toxic panel is more achievable than ever before. To realize such a device near its limiting performance, however, a much more quantitative and granular model of how molecular ordering affects charge generation is required. The development of resonant soft X-ray scattering (RSoXS) has considerably aided nanostructure characterization yet is still rather undeveloped. I will discuss using new analytical models of RSoXS going beyond the simplistic 2-phase assumptions and that quantify the device donor-acceptor interfaces toward characterizing interfacial molecular orientation. We have, furthermore, developed an in-situ photophysical measurement suite, including time-delayed collection field. This suite and new device analysis quantifies the full excited state population dynamics on the exact same devices within which we measure the nanostructure. This has resulted in unprecedentedly quantitative structure-property relationships in model systems that reveal the effect of the mixed amorphous interface. With this new capability, functional relationships that dictate device performance from interfacial aggregation, mixing, and molecular orientation can be used to tailor molecular architecture and device processing that achieves the promise of truly competitive large scale solar harvesting.

We are increasingly living in a hyper-connected environment where humans, smart devices and robots live in synergy together. The continued development of soft organic devices¹ for bio-integrable and even neuro-integrable sensory systems will augment human abilities and drive new applications as health diagnostics, surgery and predictive analytics. In my talk, I will discuss materials design and strain engineering techniques to develop electronic intelligent materials with stretchability, sensitivity and robust mechanical properties, such as self-healing^2,3. In addition, I will also discuss our recent progress in developing new scalable electronic skin platform technologies⁴ for systems capable of far greater perception and intelligence. It is envisioned that such electronic skins can be useful in future distributed conformable electronic skins⁵, neuro-prosthetic devices and wearable exo-suits in the increasingly digital and augmented human era.

1. Tee, B. C. K. & Ouyang, J. Soft Electronically Functional Polymeric Composite Materials for a Flexible and Stretchable Digital Future. Adv. Mater. 0, 1802560 (2018).
2. Tan, Y. J., Wu, J., Li, H. & Tee, B. C. K. Self-Healing Electronic Materials for a Smart and Sustainable Future. ACS Appl. Mater. Interfaces 10, 15331–15345 (2018).
3. Cao, Y. et al. Self-healing electronic skins for aquatic environments. Nat. Electron. 2, 75–82 (2019).
4. W. W. Lee et al., “A neuro-inspired artificial peripheral nervous system for scalable electronic skins,” Sci. Robot., vol. 4, no. 32, p. eaax2198, Jul. 2019.
5. Tian, X. et al. Wireless body sensor networks based on metamaterial textiles. Nat. Electron. 2, (2019).

Semiconductor-based catalysts can convert solar energy into chemical fuels such as hydrogen, hydrogen peroxide, or hydrocarbons produced via carbon dioxide reduction. Long overlooked due to stability concerns, some organic semiconductors have recently emerged as promising electrocatalysts and photocatalysts for operation in aqueous environments. Our attention has focused on nanocrystalline pigment-type organic semiconductors, which stand out due to stability and performance. Generally, we have found a high selectivity of organic semiconductors for oxygen reduction reactions, by both one-electron and two-electron pathways. The products of these reactions are superoxide or hydrogen peroxide. We find this occurs on numerous organic semiconductors and conducting polymers in a pH range from 1 to 12. The large pH stability window is remarkable when compared with inorganic counterparts. We have found that while photogenerated electrons reduce oxygen, the fate of the holes represents a complex picture. When the nanocrystalline semiconductor is used as a photocathode and efficient p-type transport is available, photogenerated holes can easily be extracted to an external circuit. If this is not possible or inefficient, the hole will either oxidize electron-donors in solution or precipitate autooxidation reactions of the semiconductor itself. The factors affecting this photocorrosion effect will be discussed, as well as principles leading to stabilization of the semiconductor catalyst. The possibilities of solar energy conversion into the high-energy molecule H₂O₂ enabling carbon-neutral energy storage in liquid form, in contrast to gaseous H₂, will be covered. Organic semiconductors have potential to become a powerful class of intrinsic catalysts, tunable by molecular design.

In this talk, I will present an electronic platform based on an n-type conjugated polymer for detecting glucose as well as generating power from bodily fluids. We employ the n-type semiconducting material in an accumulation mode organic electrochemical transistor (OECT). The polymer is engineered to have specific interactions with the enzyme. This micron-scale device detects glucose without the need of a mediator and shows excellent sensitivity and selectivity over five orders of magnitude wide detection range. When integrated into an enzymatic biofuel cell, the same polymer serves as the anode, converting the glucose metabolism in an aqueous medium into power. The mediator-free glucose-oxygen fuel cell outperforms other reported systems at physiological glucose concentrations. This is the first-time use of n-type polymer in an energy generation device with the capability to operate in human fluids. Such polymers open up an avenue for the development of self-powered microscale sensors that run on metabolites produced by the body. These devices can be fabricated on flexible substrates, paving the way for implantable self-powered metabolite sensing.

Solution processable meniscus-line-guided (MGC) coating is an excellent candidate to realize highly crystallized organic semiconductor thin films in the fabrication of organic field effect transistors (OFETs). The basic processing parameters, including shearing speed (v), concentration (c), solvent boiling point (T_b) and deposition temperature (T), are pivotal to achieve fine control over the crystallinity of the deposited organic thin films and therefore, understanding of the interconnections between these parameters are highly desired. Here, we examine the effects of different fabrication factors on the crystal deposition rate systematically under MGC approach. The active layer material in current study is the 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) organic semiconductor. It is verified that v and c are connected to crystal deposition rate by means of mass conservation. The crystal deposition rate is further related to T and latent heat (L, proportional to T_b) in the Arrhenius form. Furthermore, the optimum organic thin film (roughness R_q < 0.25 nm and average mobility μ_ave = 5.88 cm² V^-1 s^-1) is achieved under a mass transfer rate of 6 × 10^-11 kg s^-1to 15 × 10^-11 kg s^-1. Our findings provide valuable information in understanding the crystal growth under the MGC method, which is believed to be one of the highest potential approaches for the mass production of OFETs.

Sample surface potential measurement is one of the important topics for material research. The state-of-art surface measure technologies are Kelvin Probe Force Microscope (KPFM) and Electrostatic Force Microscope (EFM). Although those technologies can measure the surface of the sample in nanoscale, the equipment, in general, is sensitive to the environment due to the measurement noise and require extra amplifiers to improve the signal-to-noise ratio. This research aims at functionalizing the AFM probes by integrating an OFET onto the tip for localized electrical signal amplification. The OFET-AFM tip structure includes the source, drain, organic semiconductor and dielectric coating. The patterning of the source and drain electrodes are developed by the Focused Ion Beam (FIB). The meniscus-guided solution-processing method is employed to grow ultrathin (~30 nm) C₁₀-DNTT crystal which will be transferred to the electrodes by the FIB milling method. We believe the ultrathin organic semiconductor crystal allows the contact resistance between electrodes and semiconductors to be low enough (< 100 Ωcm) for decent transistor operation. Furthermore, the FIB milling can minimize the thermal damage, which is caused by the thermal evaporation of metal to the organic semiconductor during the fabrication process. As a result, building up a small scale (channel length less than 2 micrometer) OFET device at the AFM tip becomes possible. The Parylene coating (~500 nm) serves as the dielectric layer of the OFET and allows the OFET-AFM probe to measure the voltage signal from the sample surface. The surface potential of the probe will act as the gate bias of the OFET, thus to control the channel current of the device. We believe the OFET-AFM probe can open up a new direction in small scale biomedical applications.

Organic/Polymeric Semiconductor (OSC) based devices have been under extensive study for the past three decades due to their intrinsic potential advantages such as lightweight, flexible, biocompatible, low toxicity, abundant material availability, low cost of processing, etc. A phototransistor incorporates the properties and functions of a transistor and photodetector. In this study, a phototransistor based on a donor/acceptor (D/A) pair (photo-doping) was studied and demonstrated. Unlike in organic photovoltaics (OPV) where 1:1 proportion by mass of the donor:acceptor is typically utilized to make up the active layer, that ratio appears to be too high for phototransistor applications. It has been reported that the D/A 1:1 concentration leads to a low overall phototransistor device performance, lack of I-V curve saturation (kink effect), and other undesired bi-polar transistor behaviors. By systematically adjusting the materials doping concentrations and ratios, as well as device fabrication techniques, we were able to demonstrate a much-improved performance of a p-type phototransistor. Specifically, a high-performance OFET phototransistor was achieved via a very small amount of Phenyl-C71-butyric acid methyl ester (PCBM) doped into a Poly(3-hexylthiophene) (P3HT) host. With this work, a greater understanding behind the optimization of D/A based phototransistors is advanced.

The non-invasive, in operando characterization of opto-electronic properties of organic semiconductors is of special interest as it enables information on microscopic processes governing the performance of a device and sheds light on its real lifecycle.
In this contribution we present a new approach to cope with this challenge by utilizing fluorescent Tetraphenyldibenzoperiflanthene (DBP) molecules as optically addressable sensors, deterministically positioned at very low concentration within archetypical N,N′-di-(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine/Tris(8-hydroxyquinolato)aluminium (NPB/Alq₃) organic light-emitting diodes (OLEDs) as model system.
Observing variations in fluorescence intensity of the optically excited DBP sensor molecules during device operation allows for a correlation with the charge carrier distribution within a dynamic range of four orders of magnitude in current density (10^-4 – 10⁰ mA/cm²) under forward bias [1]. Due to the deliberate incorporation of the molecular probes at a well-defined depth within the OLED stack as well as utilization of a µ-photoluminescence technique this information is gathered with nanometer length scale resolution. The resolution is determined by opto-electronic processes like Langevin capture of injected charge carriers and Förster type resonant energy transfer and can be estimated to around 15 nm. Under reverse bias condition the molecular sensing method proves sensitive to uncompensated dipoles at the NPB/Alq₃ heterointerface, which results from the preferential orientation of the molecular Alq₃ dipoles during layer deposition [2]. The optical data is substantiated by complementary macroscopic impedance measurements performed on identical devices. In conjunction with the impedance measurement, drift-diffusion simulations [3] of the OLED stack characteristics are carried out to further corroborate the correlation between the observed photoluminescence changes and the opto-electronic device properties.
The broad variety of molecular host-guest systems, renders this approach a universal tool for analyzing not only opto-electronic phenomena in many different organic electronic device structures but, for example, also in biological soft matter systems.

[1] Nothaft et al., ChemPhysChem, 2011, 12, 2590-2595.
[2] Noguchi Y., Brütting W., Ishii H., Jpn. J. Appl. Phys., 2019, 58, SF0801.
[3] Altazin et al., Org. Electron., 2016, 39, 244-249.

Model heterojunctions have the potential to elucidate fundamental mechanisms for charge transfer at these interfaces. We demonstrate the epitaxial growth of single-crystalline p-n junctions on a graphene substrate using two organic small molecules: Zinc phthalocyanine (ZnPc) as a donor-type material (p-type), and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) as an acceptor-type material (n-type). The morphology of a p-n junction was demonstrated to vary upon deposition sequence, as an epitaxial PTCDA layer grown on top of ZnPc exhibits a nanopillar structure, while a ZnPc layer grown on PTCDA shows a bulk, island-like structure. The epitaxy mechanism follows a “point-on-line” rule where organic crystalline layers grow on top of each other by sensing and registering the spacings and periodicities of the lattice plane it grows on, which leads to different molecular packing motifs and resultant different morphologies. Preliminary grazing incidence wide-angle X-ray scattering (GIWAX) and high-resolution TEM studies show that both types of p-n junctions are highly orientated and molecules have a preferred face-on packing motif. This work together with conducting AFM results that show diode characteristics of p-n junctions in the dark suggest that graphene-templated p-n junctions can favor vertical charge carrier transport, and thus be suitable for use in organic photovoltaics (OPV). From this study, we can better understand the packing/orientation of molecules in vertically aligned nanostructures and how the geometrical arrangement, and resultant morphology, will affect the electronic properties and photovoltaic device design.

Organic field-effect transistors (OFETs) based flexible electronic devices are the key players of the next generation wearable electronics. The conventional fabrication processes of flexible OFETs usually require layer by layer deposition which requires thermal and chemical compatibility among different layers. Fabrication errors induced in each layer will accumulate and eventually affect the overall the performance of device. Here, we introduce a lamination method to fabricate ultra-thin flexible OFETs from separately prepared layers. Compared with the vacuum sublimated method where thin film Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) is directly deposited onto the PEN substrate, the proposed method allows us to modify the growth mode of the DNTT by regulating the surface energy of the substrate, thus high crystallinity DNTT can be potentially obtained. Furthermore, the transferred gold electrodes onto the PMMA which can not only avoid the thermal damage of the organic semiconductors, but also serve as the encapsulation layer for the OFET operation. We will show how to align the DNTT layer with the electrodes and top gate vertically under the microscope by lamination. Another major advantage of our lamination method is one can repeatedly using them onto different devices by delamination. The ultimate effective mobility of the flexible OFETs is targeting at 1.5 cm²V^-1s^-1. Last but not least, the proposed method allows us to fabricate individual layers at the same time which in principle would also boost up the production efficiency. The ultra-flexible OFETs can achieve a bending radius of 2 mm or less without significant degradation in electrical performance.

Transistors have many uses for electrical systems including acting as a gate switch and enhancing signals. However most transistors are not compatible with biological systems due to the metallic materials they are made up of. For these instances, organic transistors made with carbon based materials are used. For my research I have been working on a similar carbon based FET (field effect transistor) using Glassy Carbon (GC) and Graphene (GR). With these two materials I hope to fabricate a transistor that will be compatible in biological systems as well as have high thermal and electrical properties as compared to conventional FET transistors. This type of transistor could be used to enhance electrochemical signals in vivo to act as a highly sensitive biosensor.
To test the properties of these combined materials, the electron mobility and resistivity was experimentally determined using the Hall Effect method along with Hall Bar type devices. Three devices were tested for the electron mobility; one fabricated with Graphene on top of Glassy Carbon and the other two were made with just a single layer of Glassy Carbon or Graphene respectively. A device made of gold was also tested as an experimental control.
The transistors are characterized by determining the Transconductance, this is an electrical characteristic of transistors relating an output current with a given source voltage through the device. In order to determine the Transconductance of the transistor devices, numerous designs will be fabricated to test different sizes and design types. This will also be done with different material types (GR/GC, GC, and GR) similar to the mobility testing to have comparable results.
Preliminary results from the Hall Experiments showed that the Graphene had the highest mobility and Glassy Carbon the lowest. The combination device (GR/GC) had a measured mobility between the two controls. These results show that the stacked material (GR/GC) has properties that lie between the two, which is typical of compound materials.

Poly(9,9-di-n-octylfluorenyl-2,7-diyl), also known as PFO, is one of the most well-studied light emission polymers which can exhibit different microstructures depending on processing methods and conditions. One microstructure that attracts much attention is β-phase, in which the intermonomer torsion angle is ~180°, resulting in a planar-zigzag chain conformation. Researchers have shown that with a very small amount of β-phase in PFO (~1.3%), the luminance and external quantum efficiency (EQE) of OLEDs can triple^[1]. In this work, we report on the generation of β-phase in a 10% benzothiadiazole-containing PFO-based copolymer, poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (ADS233YE), and the improvements in OLED performance that ensue.

The β-phase was induced into ADS233YE thin films by either solvent vapour annealing or dipping the sample in a solvent/non-solvent mixture. The existence of β-phase was confirmed by absorption spectra, and both photoluminescence (PL) spectra and photoluminescence quantum efficiencies (PLQE) of pristine and β-phase-containing samples were measured and compared. ADS233YE OLEDs with β-phase were found to perform better than without, including luminance (at 10 V, 5000 cd m⁻² to 5700 cd m⁻², i.e. 14% increase), EQE (peak value, 1.09% to 1.93%, i.e. 77% increase), luminance efficiency (peak value, 3.96 cd A⁻¹ to 6.26 cd A⁻¹, i.e. 58% increase), and luminous power efficiency (peak value, 2.20 lm W⁻¹ to 3.69 lm W⁻¹, i.e. 68% increase). Possible reasons for these enhancements include a lower ionisation potential in β-phase-containing samples due to an extension of the conjugation length, more balanced hole and electron mobility caused by the self-doping effect of β-phase chain segments, and more efficient energy transfer from fluorene to benzothiadiazole moieties, mediated by the β-phase, all of which are currently being investigated.

Our study shows that controlling chain conformation by inducing β-phase into a PFO-based copolymer can be a simple but effective way to boost OLED performance. It will be interesting to see if this approach can be generalised to other PFO-derivatives and even other polymer families, providing us with new options to optimise OLED devices.

Reference:
[1] H.-H. Lu, C.-Y. Liu, C.-H. Chang, S.-A. Chen. Adv. Mater. 2007, 19, 2574 – 2579.

Understanding the structure-property relationship of organic semiconducting polymers is critical for their future development for technological applications. To this end, studying suitable model systems is needed to expand our knowledge of how to modify the chemical structure of materials to get the desirable performance necessary for commercialization. The triplet states play a vital role in the optoelectronic properties of organic semiconductors with direct consequences for their use in devices. It is, thus, vital to have a deep understanding of the formation, transport and harvesting of triplet excitons in such materials. We report a photophysical study of a series of copolymers containing the light emitting
polymer 9,9-dioctylfluorene (PFO). These comprise varying amounts (0.5, 1, 2.5 and 5%) of cyclometalated PyTPAPt(acac) complex or the corresponding organic PyTPA ligand embedded into the backbone of the PFO. Interest in such compounds stems from the large enhancement in spin-orbit coupling induced by the heavy platinum atoms making triplet emission partially allowed. Additionally, the morphological diversity of PFO allows to study the effect of film morphology on the photophysics. We investigate the temperature dependence of the photoluminescence in the range between 5K and 300K as well as the room temperature electroluminescence. Strong emission quenching with increasing platinum content is observed in both the glassy and β phases of the fluorene moiety with phosphorescence only observed at low temperature. The phosphorescence is ascribed to fluorene triplet emission from its energy and linewidth. We explore how trapping at defects and bimolecular recombination affect the emission efficiency. Analysis of photoluminescence measurements at different excitation intensities indicates triplet-triplet annihilation leading to emission losses while phosphorescence temperature dependent intensity measurements demonstrate a
distribution of trap sites for the triplet excited state. Finally, at higher platinum contents triplet emission from both the planar β phase and disordered glassy phase of PFO is observed despite fluorescence arising only from the β phase. We explore this phenomenon in the context of energy transfer between the different excited state energy levels.

Substitution of C atoms in a polymer backbone by N atoms allows for the facile tuning of the energy levels as well as the backbone conformation and packing structures of conjugated polymers. Herein, we report a series of three polymer acceptors (P_As) with N atoms introduced at different positions of the backbone, and investigate how these N atoms affect the device performances of all-polymer solar cells (all-PSCs). The three P_As, namely, P(NDI2DT-BTT), P(NDI2DT-PTT), and P(NDI2DT-BTTz), are composed of naphthalenediimide (NDI) and benzothiadiazole (BT)-based derivative (dithiophene-BT (BTT), dithiophene-thiadiazolepyridine (PTT), and dithiazole-BT (BTTz)) units. The PTT and BTTz units are synthesized by replacing the C atoms in BT and thiophene, respectively, with N atoms, which effectively tune the optical, electrochemical, and charge-transporting properties of the corresponding P_As. The all-PSCs using PBDB-T as a polymer donor and P(NDI2DT-PTT) as the P_A exhibit a significantly enhanced power conversion efficiency (PCE) of 6.95%, whereas the all-PSCs based on the other P_As show relatively lower PCEs (6.02% for PBDB-T:P(NDI2DT-BTT) and 1.43% for PBDB-T:P(NDI2DT-BTTz)). The high PCE of the PBDB-T:P(NDI2DT-PTT) device is due to superior charge transfer and charge dissociation, resulting from the closely-matched energy levels between PBDB-T and P(NDI2DT-PTT), as well as the more favorable BHJ morphology with improved miscibility. Importantly, the P(NDI2DT-PTT)-based all-PSC device shows improved air stability compared to the P(NDI2DT-BTT)-based device, which is most likely due to a decreased lowest unoccupied molecular orbital level of the P_A. Our findings suggest that incorporation of N atoms into the P_As is an effective strategy for improving the efficiency and stability of all-PSCs.

One of the biggest issues within BHJ light-harvesting films is macro-phase separation in domains of the donor and acceptor materials leading to severe PCE drop. To alleviate this problem, incorporating compatibilizer has been one solution by increasing adhesion between the phases, reducing the interfacial tension, and stabilizing morphology. In this study, a novel acceptor–donor–acceptor (A–D–A) triad type small molecule (SM), 5TRh-PCBM, which has an oligothiophene segment as the central core and fullerene-derivatives as the end groups, was synthesized. 5TRh-PCBM was added and evaluated into traditional fullerene-based blend (P3HT:PC₆₁BM), and high-efficiency blend system (PTB7-Th:PC₇₁BM) and (PBDB-T:PC₇₁BM), respectively. Interestingly, with incorporating of 5TRh-PCBM, PCEs of PTB7-Th:PC₇₁BM:5TRh-PCBM blends were 9.37%(1:1.5:0), 10.09%(1:1.5:0.125), and 9.72%(1:1.5:0.25). Furthermore, thermal stability of PTB7-Th:PC₇₁BM:5TRh-PCBM (1:1.5:0.25) blend was improved retaining approximately 85% of its initial PCE at the annealing condition of 80°C for 120 hrs. Blend morphology was investigated through OM, GIWAXS, and RSoXS varying compatibilizer content ratio and annealing time. When 5TRh-PCBM was incorporated into each blend system, there were no apparent morphological transitions and macro-phase separation after thermal annealing. This thermally stable tolerance is likely due to the fullerene derivative end groups of 5TRh-PCBM as a binder which can prevent thermal diffusion of PC₆₁BM (or PC₇₁BM) and suppress its aggregation within each of the BHJ films. This compatibilization strategy demonstrated the useful guidelines of molecular engineering to achieve highly efficient and stable OSCs through controlling and stabilizing blend morphology.

Perylene diimide (PDI) derivatives are organic semiconductor that, among others, have been identified as one of the best classes of non-fullerene acceptors (NFAs) for organic photovoltaic (OPV) devices, with the best power conversion efficiency surpassing 10%.^[1] Recently, our group has developed a series of N-annulated PDI dimers, that have demonstrated excellent processability from all types of solvents, including environmentally friendly 2-Me-THF and 2-Me-anisole, and can achieve respectable performances in OPV devices.^[2-4] The synthetic methods required to produce some of these have proven to be scalable and efficient, making them excellent candidates for the up-scaling of green printed electronic devices. We had previously identified one of said PDI dimers, with ethyl-hexyl side chains on the pyrrolic position, to be a suitable NFA for green solvent and air processed OPV.^[3]

This presentation will first discuss the rather unique self-assembly properties of this specific PDI NFA under the influence of processing solvent additives such as 1,8-diiodooctane (DIO), which can cause the crystallization of the molecular material in a polymeric matrix. This phenomenon was studied, and the aggregation effect was controlled under specific conditions using the benchmark polymer P3HT.^[4] To further investigate these findings and probe the phenomenon impact on a higher performing, and potentially scalable system, we identified a suitable medium band-gap benzodithiophene-quinoxaline polymer. The presentation will then focus on our effort to understand this system and translate it to scalable processing. First, diphenyl ether (DPE) was selected as a non-halogenated counterpart to DIO to make our system completely halogen-free, and effect of DPE on the PDI molecular material was studied. Then, a comparison of spin-coated and slot-die coated active layer films was made looking at film properties, morphology evolution and OPV device performance. We identified dependencies between these parameters and the selected coating method and worked with slot-die coating to obtain superior OPV results. The results obtained demonstrated that solvent additives might play an additional beneficial role in the device fabrication of printed organic active layers, highlighting the need of further exploration of additives with dual or even multiple functions to optimize the processing involved in green printed electronics.^[5]

[1] J. Zhang, Y. Li, J. Huang, H. Hu, G. Zhang, T. Ma, P.C.Y. Chow, H. Ade, D. Pan, and H. Yan, J. Am. Chem. Soc. 139 (45), 16092–16095 (2017).
[2] S.M. McAfee, S.V Dayneko, P. Josse, P. Blanchard, C. Cabanetos, and G.C. Welch, Chem. Mater. 29, 3, 1309-1314 (2017).
[3] S.V. Dayneko, A.D Hendsbee, and G.C. Welch, Small Methods 2, 1800081 (2018).
[4] F. Tintori, A. Laventure and G.C. Welch, Soft Matter, 15, 5138-5146 (2019)
[5] F. Tintori, A. Laventure and G.C. Welch, ACS Appl. Mater. Interfaces, 11, 42, 39010-39017 (2019).

Traditionally, the fabrication of ultrathin organic field-effect transistors (OFETs) is limited by both poor electrical performance in devices with fewer than 2-5 molecular layers and poor scalability of these methods. Here, we employ a floating film transfer method which uses the spontaneous spreading of polymer solutions over the surface of a green solvent to fabricate floating large area, highly uniform monolayer films which can be transferred to an arbitrary substrate for use in organic electronic devices. This method enables achieving high charge carrier mobility in material-efficient ultrathin films, which is comparable with values achieved in spin-cast thick film.

Introduction: The photovoltaic market is growing and although Silicon solar cells is considered to be the dominant technology, there is significant interest in research and development of other low-cost novel technologies. Organic Solar Cells (OSCs) has a niche market that require lightweight and flexible structures however their application is still limited because of low efficiencies and stability issues when compared to their inorganic counterparts. OSCs can clearly be a low cost technology, but to be able to take it to commercialisation, there is a need to develop new and efficient materials that are both stable and scalable. One of the great advantages of organic solar cells is the myriad of combinations of polymer materials that can be easily synthesised to form donor-acceptor blends, but the key is the performance in a device architecture.

Background: Currently, the organic blends need to be tested through trial and error and by fabricating the full devices which needs time and effort. In this work, we use the contactlless technique of Time resolved microwave conductivity (TRMC) that can be used as a powerful diagnostic characterisation tool to screen and select the most promising materials and organic blend ratio before it goes into the device fabrication stage. It uses a pump-probe technique to measure the transient change in the microwave reflectivity of a sample inside a microwave cavity in the dark and when pumped with light, usually a laser pulse. The relative change in the absorbed microwave power from the dark signal to the excited signal is directly proportional to the photoconductance. Apart from being a contactless technique, it is also a non-intrusive photo-conductivity measurement technique that can measure dynamics of charge carrier generation and recombination and also study post-bleaching behaviour.

Method : To validate the TRMC as a screening tool, we identify a few blend materials from literature with known efficiencies and compare the TRMC figure of merit with the device efficiency results. The donor-acceptor blend samples for the TRMC are prepared by spin coating or blade coating on quartz substrates. To investigate the material properties better, photoconductivity decay analysis is also performed and photocarrier lifetimes are extracted. The ‘figure of merit‘ for TRMC measurements used to evaluate the quality of the absorber material will be discussed and some preliminary results of screening a few polymer blends will be presented.

Conclusion : Our results so far show that there is a clear correlation between material quality and TRMC figure of merit and its extracted parameters. Hence TRMC characterisation can be used as a fast and high throughput technique to screen new polymer blends and predict device performance.

In this presentation we report the development of novel semiconductors for mechanically flexible and more ductile transistors and circuits. Particularly, we show that “ultra-soft” polymers comprising naphthalenediimides (NDI) units co-polymerized with “rigid” and “flexible” organic units can change how charge transport is affected by mechanical stress, demonstrating that polymer backbone composition is more important that film degree of texturing. This strategy enables to reduce the elastic modulus of the semiconducting film by >2-4x while retaining charge transport characteristics. In addition, , new polycrystalline molecular semiconductor films based on perylenediimide molecules are plasticized by using an innovative polymer additive design strategy enabling polymer localization at the fragile grain boundaries. This approach preserves charge transport characteristics in a TFT architecture as well as greatly enhances film bendability and charge transport stability upon multiple bending (> 1000 x vs < 10 for the pristine molecular film).

Biological nerves have evolved toward functionalities to efficiently process complex information in real-time. Unlike biological nerve system, von Neumann based computing system performs in centralized and sequential form which has difficulty in event driven and parallel operation. In this regard, there has been many efforts to adopt the superior characteristic of biological nervous system and the concept of neuromorphic electronics have emerged. As the synaptic behavior can be implemented through single artificial synaptic device, it can emulate the functions of biological sensory and motor neurons when combined with sensors and actuators. Making the artificial nervous system desirable for soft robotics and neuroprosthetics have been widely investigated.
Here, artificial nervous systems were developed by using organic electronics to emulating peripheral nervous system. Pressure sensors (artificial mechanoreceptors), organic ring oscillators (artificial nerve fibers), and synaptic transistors were integrated to emulate the biological sensory nerve system. By forming hybrid reflex arc which connects artificial sensory nerve and biological motor nerves the applicability for neural prostheses was verified and the biological motor nerves were actuated depending on external pressure information. In addition, by integrating a stretchable artificial synapse, and a polymer actuator (an artificial muscle), stretchable artificial sensorimotor nervous system was developed. The optogenetics was emulated by contracting the artificial muscle with light stimulation. In parallel, the system distinguished the alphabet Morse code which showed the potential as an optical wireless communication method for human-machine interface. Furthermore, by integrating a triboelectric sensor for an artificial auditory system, artificial auditory system was emulated. We modulated morphology of organic semiconductors of the artificial synapses to implement the specific recover time of biological synapse facilitation. Our organic artificial nerve systems suggest promising strategy for bioinspired electronics, soft robotics, and neuroprosthetics.

In this research, we report the effect of crystallinity on the mixed conductivity of highly-ordered polypyrrole film. Polypyrrol-4,4’-biphenyldisulfonic acid (poly(Py:BPDSA:Py)) hybrid was synthesized by oxidative polymerization after connecting two pyrrole monomers with various stoichiometric ratios of BPDSA in the aqueous solution. The structural and electrical/electrochemical analysis revealed that the increase in BPDSA content enlarges the crystallite size from 18.9 to 52.1 nm, the ratio of crystalline to amorphous domains by 6 times, the electrical conductivity from 174 to 406 S/cm, and the volumetric capacitance from 85 to 672 C/cm³, while the doping concentration extracted from Hall measurement and electrochemical analysis remains unchanged. On the other hand, the resultant poly(Py:BPDSA:Py)-based organic electrochemical transistors (OECTs) showed the maximum value of transconductance (0.5 mS) at the intermediate composition of BPDSA, which can be attributed to the retardation of ion movement in highly crystalline polymeric film. The repeated OECT operation revealed the stable current on-off ratio over 2 x10² without the significant loss in transconductance. All these results suggests that the mixed conductivity of poly(Py:BPDSA:Py) can be strategically modulated by engineering the crystallite size.

Semiconductor-based catalysts can convert solar energy into chemical fuels such as hydrogen, hydrogen peroxide, or hydrocarbons produced via carbon dioxide reduction. Long overlooked due to stability concerns, some organic semiconductors have recently emerged as promising electrocatalysts and photocatalysts for operation in aqueous environments. Our attention has focused on nanocrystalline pigment-type organic semiconductors, which stand out due to stability and performance. Generally, we have found a high selectivity of organic semiconductors for oxygen reduction reactions, by both one-electron and two-electron pathways. The products of these reactions are superoxide or hydrogen peroxide. We find this occurs on numerous organic semiconductors and conducting polymers in a pH range from 1 to 12. The large pH stability window is remarkable when compared with inorganic counterparts. We have found that while photogenerated electrons reduce oxygen, the fate of the holes represents a complex picture. When the nanocrystalline semiconductor is used as a photocathode and efficient p-type transport is available, photogenerated holes can easily be extracted to an external circuit. If this is not possible or inefficient, the hole will either oxidize electron-donors in solution or precipitate autooxidation reactions of the semiconductor itself. The factors affecting this photocorrosion effect will be discussed, as well as principles leading to stabilization of the semiconductor catalyst. The possibilities of solar energy conversion into the high-energy molecule H₂O₂ enabling carbon-neutral energy storage in liquid form, in contrast to gaseous H₂, will be covered. Organic semiconductors have potential to become a powerful class of intrinsic catalysts, tunable by molecular design.

The transport properties and device performance of organic semiconductors is determined by the microstructure and morphology of polycrystalline thin films, including grain size, domain orientation and grain boundaries. For device applications, such as field-effect transistors, controlling the domain orientation and grain boundaries is particularly important as this mediates charge transport in the channel of the device. More generally, the opportunity to fabricate single crystal domains to match the device architecture would be a very promising pathway to unlock the intrinsic properties of organic and hybrid semiconductors. This presentation will discuss recent efforts to reduce and even eliminate grain boundaries in solution-processed organic and hybrid semiconductor films, with such applications as high performance field-effect transistors, photodetectors and light emitting devices envisioned. Approaches based on patterning the microstructure of thin films post-deposition or in situ will be discussed, as well as additive manufacturing-based coating and printing methods of delivering materials with single crystal-like properties at designated locations.

Electrical doping of semiconducting polymers requires introduction of counter ions to maintain electroneutrality. These counter ions can modify electronic charge transport by changing the microstructure of the semiconductor and by electrostatic interactions between the charge carriers and their counter ion. We will discuss our recent work using single ion conducting polymers, known as polymeric ionic liquids, as the dielectric in transistors with semiconducting polymers. Polymeric ionic liquids (PILs) have tethered ionic functional groups on their backbone with an untethered counter ion leading to single ion conductivity. By appropriate molecular design of the PIL, the operation of polymer transistors can be switched from an electrical double layer mechanism to electrochemical doping of the bulk. The behavior of transistors with PIL gates reveals the differences in the electrical conductivity at the surface and bulk of semiconducting polymers at high charge carrier concentrations. The role of the infiltration of ions on the electronic density of states of semiconducting polymers can also be determined by measurements of the thermopower of electrochemical transistors. Our results show that electrochemical doping leads to a broadening of the DOS and helps to rationalize recent models for charge transport in semiconducting polymers at high carrier densities.

Over recent years several new classes of conjugated polymers have shown promise as materials for polymer field-effect transistors with high field-effect mobilities. Many of the recently discovered high mobility polymers, in particular donor-acceptor copolymers, owe their excellent charge transport properties to a low degree of energetic disorder associated with a well-defined backbone conformation with small variations in torsion angles. In this presentation we will present our current understanding of the transport physics of these materials and focus in particular on the relationship between molecular structure, thin film processing and charge transport and thermoelectric properties of these materials. We will discuss new approaches for the doping of these polymers and for understanding their thermoelectric physics.