ABSTRACT Organic electrochemical transistors (OECTs) uniquely couple ionic and electronic transport, enabling high transconductance and low‐voltage operation for bioelectronic applications. While the Bernards–Malliaras model successfully describes lateral OECTs, it fails to capture the coupled space‐ and time‐dependent processes that govern vertical OECTs (vOECTs), particularly for disordered semiconductors and high ion concentrations. Here, we present a 2D numerical simulation that self‐consistently couples ion transport and electronic charge dynamics, validated against experimental data from n‐type poly(benzimidazobenzophenanthroline) (BBL) vOECTs. The simulations reproduce steady‐state and transient characteristics, revealing key physical mechanisms including diffusion‐dominated electronic transport, contact tunneling, energy loss at the semiconductor/electrolyte interface, and gate‐induced ion acceleration via band bending. The simulation also quantifies geometry‐dependent mobility discrepancies and anisotropic ionic transport between vertical and lateral architectures, consistent with recent reports on mixed ionic–electronic conductors. By bridging microscopic mechanisms with experimental observables, this work provides a predictive framework for vOECT operation and offers design guidelines for high‐performance, high‐density bioelectronic systems.

The development of stable and efficient perovskite solar cells (PSCs) hinges on the optimization of interfacial energetics and suppression of parasitic loss pathways. While poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl] phosphonic acid (MeO-2PACz) are among the most effective hole transport layers (HTLs) for inverted PSC architectures, each presents trade-offs between operational and reverse-bias stability. This study introduces a strategy to form a composite HTL comprising PTAA and MeO-2PACz that synergistically integrates both materials' advantages while overcoming their limitations. The composite HTL modulates the buried interface to the perovskite, effectively suppressing loss pathways and enhancing the uniformity of the HTL conductivity. Devices incorporating the composite HTL achieve a champion power conversion efficiency (PCE) of 22.83% without additional surface passivation, surpassing the ∼21% achieved by reference devices made using MeO-2PACz or PTAA alone. Moreover, they demonstrate exceptional operational durability and a markedly enhanced reverse bias tolerance. Accompanying drift-diffusion device simulations suggest a previously unexplored loss mechanism at molecular hole transport layers, related to losses induced by electron tunnelling from the perovskite to the hole-collecting contact. Such loss pathways are suppressed when the composite HTL is used, establishing it as a powerful and scalable route toward highly efficient, durable PSCs.

Electrochemical random‐access memory (ECRAM) devices are a promising candidate for neuromorphic computing, as they mimic synaptic functions by modulating conductance through ion migration. However, the use of a thick electrolyte layer (>40 nm) in conventional ECRAMs leads to an unavoidable tradeoff between synaptic weight updates and operating speed. To address this problem, a Cu‐based ultrathin ECRAM (UT‐ECRAM) that uses a single 5 nm HfOx active layer and a ≈1.2 nm AlOx liner is designed. The highly efficient gate‐tunable fast Cu‐ion transport in the AlOx/HfOx UT‐ECRAM enables 1) near‐ideal linearity in weight updates (0.45) even achieved with a pulse width (tw) of 50 μs, 2) dynamic multilevel retention of 104 s, and 3) reliable cycling endurance of 104 cycles. A numerical analysis based on device scaling quantitatively reveals that a relatively high concentration of field‐driven Cu ions (≈1020 cm−3) contributes to each synaptic weight update per gate voltage (VG) pulse in the UT‐ECRAM without becoming deactivated by traversing thicker layers. This improved gate sensitivity can ultimately overcome the linearity and the ratio/speed tradeoff relationships, paving the way for robust neuromorphic synaptic units.

Organic mixed ionic-electronic conductors (OMIECs), which can be used to build organic electrochemical transistors (OECTs), are of potential use in flexible, large-area and bioelectronic systems. Although hole-transporting p-type OMIECs are susceptible to oxidation, and oxygen leads to OECT instability, it is unclear whether oxygen also behaves as an uncontrolled p-dopant. We show that oxygen dissolved in a solvent can act as a p-dopant in OMIECs and OECTs by filling traps to enable effective electrochemical doping. To address the fact that the presence of oxygen simultaneously jeopardizes OECT stability, we develop a two-step strategy in which we first degas the solvent, and then dope the OMIEC in a controlled manner using a chemical dopant. Our approach improves the stability of both p-type and n-type OECTs, while increasing the on-off ratio, tuning the threshold voltage and enhancing the transconductance, charge carrier mobility, and the mu C* product-that is, the product of mobility and the volumetric capacitance.

Organic electrochemical transistors (OECTs) operating in wet biological environments offer new possibilities for neuromorphic biosensors and bioelectronics. This work presents a device physics approach to develop an organic spiking neuron using a single OECT combined with passive RC components. The key condition is that charge carrier mobility decreases with ion concentration in the organic conductor. This leads to a Z-shaped current-voltage response that, when coupled with an external load, produces self-sustained oscillations. We model the system as a nonlinear oscillator described by a set of first-order differential equations, exhibiting a stable limit cycle. Through nonlinear dynamics and bifurcation theory, we construct a two-variable fast/slow model and identify the conditions for a Hopf bifurcation that triggers oscillatory behavior. The system's output can shift between sinusoidal spiking and relaxation oscillations by adjusting the external capacitor. Crucially, this neuron-like behavior is achieved using a single transistor without external amplifiers. This minimalistic design offers a promising pathway toward energy-efficient, low-cost, and biomimetic neuromorphic systems, with strong potential for integration in future bioelectronic devices.

Traditionally, the only ions allowed or welcomed by the microelectronic industry are those that act as fixed dopants. The issue of mobile ions became welcome primarily due to the memristor technology however, thin film transistor technologies benefit from them too. The electrochemical (transistor) random access memory (ECRAM) is emerging as a promising building block for multi-level neuromorphic computing. Using FAB-compatible materials we construct the ECRAM using CuO x as the gate and ions source (Cu+). The morphology of the HfO x gate insulator layer is tuned to render it ion transporting such that it can act as a uniform electrolyte layer. Lastly, the channel material is WO x with tungsten metal as the source/drain contact. While there are several reports of ECRAM devices, the operation mechanisms are not fully known/understood thus withholding progress of this field. Using the Sentaurus device simulator by Synopsis, including the hydrogen diffusion module, we simulate the mixed ionic electronic operation of the device. We have recently reported that by fitting the simulation to the device performance, we could identify the potentiation mechanism (i.e., insulator charging) and the occurrence of copper plating that takes place under high Cu+ ion flux (as in fast charging of Li batteries).[1] In the first part of the talk, we will expand on the chemical-physics details of the ECRAM device mentioned above. Next, we will present a new device architecture where we remove the WO x layer and study the ionic-electronic conduction of a modified HfOx layer. By fine-tuning the stoichiometry of the HfO x layer, it assumes both roles of ionic electrolyte and electronic conducting channel. Following detailed modelling of the measured properties, we introduce AlO 2 as an ionic barrier layer at the WOx/HfO x interface to enhance the ionic retention of the HfO x layer. Using the above three device architectures in conjunction with the mixed ionic-electronic device simulation we reveal the role of the two memory mechanisms: a) Electric field-activated ion transport and b) Structure-induced trapping by ion-barrier layers. Reference [1] Nir Tessler, Nayeon Kim, Heebum Kang, Jiyong Woo; Switching mechanisms of CMOS-compatible ECRAM transistors—Electrolyte charging and ion plating. J. Appl. Phys. 21 August 2023; 134 (7): 074501. https://doi.org/10.1063/5.0154153 Figure 1

AbstractThe effect of the electrolyte's counter‐ion in organic electrochemical transistors is often neglected. the influence of anions (i.e., counter ions) is investigated on organic electrochemical transistors (OECTs) with a PEDOT:PSS‐like semiconductor through device simulations. The study examined the effects of mobile anions on OECT performance under two scenarios: when anions are blocked by the semiconductor and when they can penetrate it. In each case, the OECT's ON and OFF states are analyzed. The findings show that when anions can penetrate the semiconductor, the ON/OFF ratio of the OECT remains unchanged while the transconductance significantly increases. In the ON state, the case of blocked anions is observed that the current is predominantly surface‐current, whereas it becomes volumetric only when anions can penetrate the semiconductor. Furthermore, the extreme case is explored in which anions remain stationary within the electrolyte. In this scenario, achieving a reasonable ON/OFF ratio necessitates an ion density within the electrolyte that is two orders of magnitude higher than the dopant density of the semiconductor. This work underscores the substantial influence of counter anions on OECT performance, highlighting their critical role in shaping device behavior.

For the successful implementation of organic electrochemical transistors in neuromorphic computing, bioelectronics, and real-time sensing applications it is essential to understand the factors that influence device switching times. This work describes a physical-electrochemical model of the transient response to a step of the gate voltage. The model incorporates 1) ion diffusion inside the channel that governs the electronic conductivity, 2) horizontal electron transport, and 3) the external elements (capacitance, ionic resistance) of the ion dynamics in the electrolyte. This work finds a general expression of two different time constants that determine the vertical insertion process in terms of the transport/polarization parameters, in addition to the electronic transit time. The work highlights the central role of the chemical capacitance in determining the modulation of the lateral conductivity. The different types of response of the drain current are classified, and the significance for synaptic operation in neuromorphic circuits is discussed. The model is confirmed by detailed simulations that enable to visualize the different ions distributions and dynamics.

The switching response in organic electrochemical transistors (OECT) is a basic effect in which a transient current occurs in response to a voltage perturbation. This phenomenon has an important impact on different aspects of the application of OECT, such as the equilibration times, the hysteresis dependence on scan rates, and the synaptic properties for neuromorphic applications. Here we establish a model that unites vertical ion diffusion and horizontal electronic transport for the analysis of the time-dependent current response of OECTs. We use a combination of tools consisting of a physical analytical model; advanced 2D drift-diffusion simulation; and the experimental measurement of a poly(3-hexylthiophene) (P3HT) OECT. We show the reduction of the general model to simple time-dependent equations for the average ionic/hole concentration inside the organic film, which produces a Bernards-Malliaras conservation equation coupled with a diffusion equation. We provide a basic classification of the transient response to a voltage pulse, and the correspondent hysteresis effects of the transfer curves. The shape of transients is basically related to the main control phenomenon, either the vertical diffusion of ions during doping and dedoping, or the equilibration of electronic current along the channel length.

Traditionally, new material-based technologies evolve first through chemistry and physics, with device engineering assuming a key role only towards commercialization. The device structure of an OLED pixel in any screen is a testament to the contribution of engineering to the challenge of achieving efficient and stable devices. In the talk, I will discuss two fields where this is yet to happen. The first is the organic electrochemical transistors, and the second is the solution-processed organic solar cells. Using both a detailed 2D chemical-physics semiconductor device model and a rate-equation level model, I will delve into the physics and electrochemistry of OECTs showing the role of electrode reactions and counter ions. In the context of OPVs, I will show that focusing on the performance at the maximum power point, instead of the short and open circuit, allows the design where the device contributes to charge generation and extraction.

Synaptic transistors (STs) with a gate/electrolyte/channel stack, where mobile ions are electrically driven across the solid electrolyte, have been considered as analog weight elements for neuromorphic computing. The current (ID) between the source and drain in the ST is analogously updated by gate voltage (VG) pulses, enabling high pattern recognition accuracy in neuromorphic systems; however, the governing physical mechanisms of the ST are not fully understood yet. Our previous physics-based simulation study showed that ion movement in the electrolyte, rather than the electrochemical reactions that occur in the channel, plays an important role in switching. In this study, we experimentally explore the properties of the HfOx electrolyte and show that by tuning the density of oxygen vacancies, it can assume the dual role of electrolyte and channel. We demonstrate analog synaptic behavior using a novel ST with a two-layer stack of CuOx/HfOx, where the CuOx is the gate and Cu ion reservoir, and the HfOx is the electrolyte and channel. To improve state retention and linearity, we introduce a Cu ion transport barrier in the form of a dense and stoichiometric Al2O3 layer. The CuOx/Al2O3/HfOx exhibits excellent state retention and improved potentiation and depression response. Energy dispersive spectroscopy mapping following potentiation confirms the role of the Al2O3 layer in confining the Cu ions in the HfOx layer. We also show that a two-step programming scheme can further enhance synaptic response and demonstrate high recognition accuracy on the Fashion-MNIST dataset in simulation.

Electrical doping of semiconductors is a revolutionary development that enabled many electronic and optoelectronic technologies. While doping of many inorganic and organic semiconductors is well-established, controlled electrical doping of metal halide perovskites (MHPs) is yet to be demonstrated. In this work, efficient n- and p-type electrical doping of MHPs by co-evaporating the perovskite precursors alongside organic dopant molecules is achieved. It is demonstrated that the Fermi level can be shifted by up to 500 meV toward the conduction band and by up to 400 meV toward the valence band by n- and p-doping, respectively, which increases the conductivity of the films. The doped layers are employed in PN and NP diodes, showing opposing trends in rectification. Demonstrating controlled electrical doping by a scalable, industrially relevant deposition method opens the route to developing perovskite devices beyond solar cells, such as thermoelectrics or complementary logic.

We present a device simulation of lead-halide perovskite-based thin film transistors (TFTs) containing mobile charged species to provide physical reasoning for the various experimental reports. We study the output characteristics for a range of scan duration (1/speed), average mobile ion densities, and N- and P-channel TFTs. We then directly compare our results to published data by Zeidell et al. [Adv. Electron. Mater. 4(12), 1800316 (2018)] and show that if the transistor’s measurement procedure is such that the ions’ effects are apparent, and then, our model can resolve the sign of the mobile ions in their MAPbI3−xClx TFTs (cations) and provide a good estimate of their density (∼1017 cm−3 at 200 k). Interestingly, we find that effects previously associated with channel screening are due to the ion-blocking of the charge extraction and that the incomplete saturation often reported is due to ion-induced channel shortening. Utilizing the same perovskite materials as in solar cells would allow researchers to improve their understanding of the mechanisms governing solar photovoltaics and improve their performance.

AbstractThe study of the net magneto‐capacitance, C(B), in thin films of the conducting polymer Poly(3 4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is presented. In the films there are charged electrically‐conducting PEDOT‐rich regions surrounded by PSS insulating charged material. The high‐conductivity grade PEDOT:PSS thin films are studied at low temperature, where the hopping conduction is dominant. It is experimentally observed that there is a finite net magneto‐capacitance, C(B), in the temperature range T = 1.7–3 K. Thus, the observed C(B) is a direct evidence for an intra‐site Coulomb interaction, U ≈ 2 meV, among the mobile charge carriers in the PEDOT‐rich regions. The lifting of spin degeneracy motivates PEDOT for use in spintronic applications.

Hysteresis in organic electrochemical transistors (OECT) is a basic effect in which the measured current depends on the voltage sweep direction and velocity. This phenomenon has an important impact on different aspects of the application of OECT, such as the switching time and the synaptic properties for neuromorphic applications. Here we address the combined ionic and electronic kinetic effects that cause the dominant hysteresis effects. We use a combination of tools consisting on basic analytical models, advanced 2D drift-diffusion simulation, and the experimental measurement of a Poly(3-hexylthiophene) (P3HT) OECT, working in an accumulation mode. We develop a general transmission line model considering drift electronic transport and ionic injection and diffusion from the electrolyte. We provide a basic classification of the transient response to a voltage pulse, according to the dominant ionic or electronic relaxation time, and the correspondent hysteresis effects of the transfer curves according to the general categories of inductive and capacitive hysteresis. These are basically related to the main control phenomenon, either the vertical diffusion of ions during doping and dedoping, or the equilibration of electronic current along the channel length.

The Bernards-Malliaras model, published in 2007, is the primary reference for the operation of organic electrochemical transistors (OECTs). It assumes that, as in most transistors, the electronic transport is drift only. However, in other electrochemical devices, such as batteries, the charge neutrality is accompanied by diffusion-only transport. Using detailed 2D device simulations of the entire structure while accounting for ionic and electronic conduction, we show that high ion density (>1019 cm-3) results in Debye screening of the drain-source bias at the electrodes' interface. Hence, unlike the drift-only current in standard FETs or low ion density OECTs, the current in high ion density OECTs is diffusion only. Also, we show that since in OECTs, the volumetric capacitor and the semiconductor are one, the threshold voltage has a different meaning than that in FETs, where the semiconductor and the gate-oxide capacitor are distinct entities. We use the above insights to derive a new model useful to experimentalists. Lastly, we fabricated PEDOT:PSS fiber-OECTs and used the results to verify the model.

Although p-type organic mixed ionic electronic conductors (OMIECs) are susceptible to oxidation, it has not yet been considered as to whether oxygen could behave as an uncontrolled p-dopant. Here, oxygen dissolved in solvents is shown to be behave as a p-dopant, that fills traps to enable more effective electrochemical doping in OMIECs and organic electrochemical transistors (OECTs). Yet the presence of oxygen also jeopardizes OECT stability. A two-step strategy is introduced to solve this contradictory problem, where first the solvent is degassed, and second the OMIEC is doped in a controlled manner using a chemical p-dopant. This strategy has a remarkable impact on OECT stability in air and water, while simultaneously increasing on-off ratio, tuning the threshold voltage, and enhancing the transconductance, mobility and the µC* product. This simple solution-processing technique is easily implemented, low-cost, highly effective in air and water, and effective in materials systems with different polymers, solvent and dopants. Overall, the data herein suggests that combining chemical doping with solvent degassing could be a broadly applicable technique to improve essential criteria needed to realize organic bioelectronics and more complex OMIEC circuitry.

Accounting for the perovskite ionic transport and reactions reveals the importance of the electron blocking (hole transporting) layer in determining device stability.