This conference plenary presentation was prepared for the Organic and Hybrid Field-Effect Transistors XXI conference at SPIE Optics + Photonics, 2022.

Organic mixed ionic/electronic conductors have gained considerable interest in bioelectronics, power electronics, circuits and neuromorphic computing. 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 revealing new classes of OMIEC active materials, as well as new opportunities enabled by such advances. We also employ varied biasing schemes to explore mechanisms of OECT degradation, expanding our understanding of performance degradation beyond aspects intrinsic to the OMIEC materials.

Semiconducting organic polymers are most often synthesized by linking an electron poor and an electron rich (hetero)aromatic building block via a transition metal catalyzed cross-coupling copolymerization. Researchers aiming at exploring applications and fundamental performance limits, for example for organic photovoltaics, organic photodetectors, and organic electrochemical transistors, often assume that the obtained material consists strictly of a perfect repetition of the depicted polymeric repeating unit, whereas this is likely not the case. In this contribution, we demonstrate a synthesis approach to obtain the depicted “perfect” structure of these types of polymers and the influence of material defects on the optoelectronic properties and device performance.

Solid-state order is a key parameter to tune charge carrier mobility in organic transistor materials. We recently introduced a universal crystal engineering core, to control the solid-state order of appended chromophores, and demonstrated simple trimeric systems with mobility < 10 cm2/ Vs. We are now exploring new pendant chromophores, to determine both the scope of compounds suitable to our crystal engineering modifier, and whether we can further tune charge transport based on heteroatom manipulation. Concurrently, we are developing a new crystal engineering strategy that eliminates the use of the carbon-carbon triple bond. The inherent flexibility of this type of bond leads to substantial freedom of motion along the long axis of the chromophore in the solid state, which we suspect increases transport disruptions due to the associated dynamic disorder. The new approach required the development of novel silapyrenes, and requires a new set of rules to determine how the size and shape of the silyl group influences the solid-state order of the chromophore. For this project, I will discuss how intermolecular electronic coupling is influenced by the inclusion of a pyrene core in the middle of an 'interrupted' acene.

The inorganic and hybrid (organic - inorganic) perovskites appear as an attractive, alternative solution for organic solar cells and light emitting diodes due to their large absorption coefficient, low exciton binding energy, and “long” carrier diffusion length in thin films. Thanks to their cost-effective and facile solution manufacturing perovskites have received enormous attention and showed great potential for industrialization and practical application. Despite the relatively high intrinsic - local charge carrier mobility determined by optical methods, the fabrication of reliable perovskites field-effect transistors with high performance at room temperature has remained as an enormous challenge. First of all, many strategies have been proposed to enhance the device performance, such as controlling the crystallization process, regulating the film microstructure, and adjusting the phase orientation. These approaches involve self-assembled monolayer, non-stoichiometry crystal engineering, mixed solvent, and additives. However, precise control of the grain size and crystallinity is still a challenge. First of all, the continuity and homogeneity of the films are the main limitations in an effective in-plane charge carrier transport. From one hand, strategy mainly used in photovoltaics, rapid nucleation supported by a slow crystal growth do not allow to obtain continuous and large area surface coverage without defects. Secondly, charge carrier transport in perovskite transistors, is obscured by serious ionic migration in both 3 dimensional (3D) and 2 dimensional (2D) structures. Ionic migration is induced by migration of both anions (I-) and cations (MA+, PB2+) in the presence of vacancies, supported by an external source – drain or source – gate field. This bring us to the fact, that all defects in perovskite structure created during not controlled nucleation or crystallization process are potential source of ions movement, which significantly reduce the in-plane charge carrier transport. From this reason, fundamental understanding of the perovskite formation process is the critical parameter which needs to be recognize and fully controlled, if high charge carrier transport, undisturbed by ions migration, is expected. In this report, the crystallization and growth kinetics of Sn(II)-based 2D perovskite, using 2-thiopheneethylammonium (TEA) as the organic cation spacer, was effectively regulated by the hot-casting method. Hot-casting is proposed as an effective approach to precisely control the crystallization and growth kinetics of (TEA)2SnI4 perovskite films enabling large grain sizes. With increasing crystalline grain size, the local charge carrier mobility, is found to increase moderately for ~ 25% from 13 cm2V–1s–1 to 16 cm2V–1s–1, as inferred by terahertz (THz) spectroscopy. In contrast, the FET operation parameters, including mobility, threshold voltage, hysteresis, and subthreshold swing improve substantially with increasing grain size, especially for large channel lengths. This behavior is mainly attributed to screening of the applied electric fields by ion migration that becomes severe for small grain sizes and high densities of grain boundaries. Electrical characterization at various temperatures (from 300K to 100K) presents an influence of the ion migration on the charge carrier transport in transistors architecture. Moreover, it is confirmed by grazing incident wide angle x-ray scattering and theoretical unit cell prediction that hot-casting method does not change the molecular organization in deposited thin films but only reduce the number of grain boundaries. These insights provide important guidance for the grain engineering for high-performance 2D perovskite FETs.

Acenes have been utilised as organic semiconductors for the last few decades. Attaching ethynyl groups to their core increases stability and solubility, allowing solution processing of these materials into opto-electronic devices. Side chain engineering can control the solid-state packing and affect charge transport in OFET devices. We report on the use of carbon side chains in place of the ubiquitous alkylsilyl groups, where subtle changes dramatically affect the crystal packing and therefore the applicability of these materials for use in organic electronics. Changes in sidechain can also affect the molecular properties and in particular the photostability of these materials.

At this moment, we are still far away from understanding many neurologic diseases. However, the emergence of different bioelectronic technologies opens the pathway to fathom them once and for all. Organic electrochemical transistors (OECTs) can assist here, thanks to their superior recording of neurological signals. Moreover, their flexible nature allows application in non-invasive scalp electrode devices. Nevertheless, the field of OECTs is still in its infancy, and clear, rational design criteria are lacking. Hence, dedicated backbone and side-chain engineering could enlighten the structure-performance relationships for next-generation conjugated polymers that are able to transport both ions and electrons in future OECTs.

Unique exciton states are being explored for use in quantum information systems. Excitons formed in organic semiconductors are long lived at room temperature owing to their high binding energies, and they can form quantum relevant multiexciton states like the spin-entangled triplet pair which is an intermediate during singlet fission. Electrical measurements of excitons provide complementary measurement to common optical based techniques by probing low density exciton dynamics and dark states. Here we tune singlet fission rates by using varied molecular packing to relate optical and device measurements to aid development of transistor-based exciton measurements.

Three n-type semiconducting polymers, P-0, P-50, and P-75 are investigated, in which the lactone group density were maximized by increasing the benzene content from 0% benzene (P-0), to 50% (P-50), and 75% (P-75), to enable co-planar polymers with deep-lying LUMO energy level. As a result, the polymer of P-75 exhibits a good polaron stability in air, allowing a high performing organic thermoelectrics with electrical conductivities (σ) of up to 12 S cm−1 and high power factor (PF) up to 13.2 μWm−1 K−2. These results suggest the significance of chemical design and provide new guidelines towards high performing n-type OTE devices.

Printed organic field-effect transistors 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. Here I will report on our recent efforts in making printed organic electronics (i) ultrathin, to enhance as much as possible conformability, (ii) fast switching, to enable more applications at Radio-Frequencies and (iii) edible, towards electronics systems made with non-toxic, ingestible materials, serving smart pharmaceuticals and food-tagging applications.

Recent advancements in the field of organic electronics and bioelectronics have focused on the processing of PEDOT:PSS with ionic additives to promote high electrical conductivity and device performance. The combination of excellent electrical properties and tunable structure are increasingly attractive, however, the mechanism responsible for these properties, specifically from a solution-phase self-assembly perspective, is unknown. Herein, we reveal how solution-phase self-assembly of PEDOT:PSS with ionic additives leads to multiscale ordering which influences thin film microstructure, electronic properties, enables new additive manufacturing modalities, and applications in aqueous environments.

This talk focuses on improving the switching speed of electrolyte-gated transistors (EGTs). EGTs have organic or inorganic semiconductor channels; the key feature of all EGTs is the use of an electrolyte as the gate dielectric. The high capacitance electrolyte provides EGTs with attractive characteristics, but it also leads to slow switching speeds. We discuss the main causes for signal propagation delay in EGT inverters including parasitic resistances and capacitances and then show that by mitigating these factors delay times can be decreased below 100 ns. This allows switching at frequencies up to 10 MHz. Analysis suggests further improvements are possible.

Carrier mobility in conjugated polymers continues to increase with recent reports of field-effect mobilities exceeding 10 cm2/V.s. Charge transport is intrinsically dependent on processes occurring across multiple lengthscales. In order to access order parameters at the molecular scale we use charge modulation spectroscopy combined with theory.1 This technique allows us to further differentiate field-induced and doping-induced charges and study how their delocalization depends on local structure and disorder. Furthermore, we study the mesoscale organization of polymers using new techniques in the transmission electron microscope. By combining 4D STEM2 and HRTEM we are able to study the microstructure across a range of length-scales in real space and reciprocal space. These techniques are used on homopolymers and donor-acceptor copolymers and allow to extract information about the microstructure that is typically not visible by inspection. Such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport in conjugated polymers.

The combination of vacuum electrospray deposition (ESD) and scanning tunnelling microscopy (STM) is used to investigate the conformation and assembly of traditional p-type OFET materials (pBTTT and IDT-BT) and of novel fused electron-deficient rigid-rod polymers. High-resolution STM images clearly demonstrate that one of the main drivers for the polymer self-assembly is the maximization of alkyl side-chain interdigitation. Furthermore, a careful analysis of the STM data is used to establish the presence of synthetic defects in the polymer backbone, to ascertain their conformation, to infer torsional potential energies and to define the geometry of intermolecular couplings with sub-monomer resolution.

In this talk, I will we present new approaches based on highly ordered organic multilayer structures which have the potential to increase device performance by orders of magnitude. Triclinic rubrene crystals are created by heating of amorphous layers and can be electrically doped during the epitaxial growth process to achieve hole or electron conduction. Analysis of the space charge limited current in these films reveals record vertical mobilities of 10cm2/Vs. To demonstrate the performance of this materials system, pin-diodes with record threshold frequency of 3.7 GHz are built, paving the way for future high-performance organic electronic devices based on carefully designed heterostructures. Furthermore, these structures allow comparatively long minority carrier diffusion lengths, allowing to realize the first organic bipolar transistors.

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

Field-effect transistors based on organic and hybrid metal halide perovskite semiconductors provide a controlled means of studying the charge transport physics of these materials and are also of interest for a broad range of applications in electronics, optoelectronics or bioelectronics. In this talk we will provide a general overview of the structure-property relationships that determine the charge transport physics and performance of hybrid perovskite FETs and we will also discuss our current understanding of the factors that govern and limit the operational stability of conjugated polymer FETs for applications in large area electronics.

Organic and metal-halide perovskites semiconductors share many attractive attributes for application in an expanding range of emerging opto-electronics. However, extensive efforts to translate their advantageous characteristics to viable commercial technologies, have highlighted the numerous technological challenges that still remain. In this presentation I will discuss our recent efforts to address some of these challenges by combining perovskites with organic semiconductors. I will show how physical blending of the two components in combination with other building blocks borrowed from printed electronics, can lead to prototypical devices, such as transistors, memories, and memristors, with intriguing functionalities and performance.

Photogeneration and transport of mobile photocarriers in high-mobility crystalline organic semiconductors remain underexplored. The photo-Hall effect was used to address the fundamental charge transport properties of these functional molecular materials, without the need for fabricating complex transistor devices or chemical doping. Here we demonstrate a photo-Hall effect in organic semiconductors, using a benchmark molecular system rubrene as an experimental platform. We show that this technique can be used to directly measure the charge carrier mobility and photocarrier density, disentangle the surface and bulk transport phenomena, as well as deepen our understanding of the mechanism of photoconductivity in these high-performance materials.