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.

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. Here we describe 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. We find a general expression of two different time constants that determine the vertical insertion process in terms of the kinetic parameters, in addition to the electronic transit time. We thus obtain a generalization of Bernards-Malliaras model for the exponential transient, as well as a more general set of nonlinear equations that explain the large perturbation effects. The basic 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.

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.

Electrical doping of semiconductors is a revolutionary development that enabled many electronic and optoelectronic technologies. While doping of many inorganic and organic semiconductors has been well-established, controlled electrical doping of metal halide perovskites is yet to be demonstrated. In this work, we achieve efficient n- and p-type electrical doping of metal halide perovskites by co-evaporating the perovskite precursors alongside organic dopant molecules. We demonstrate that the Fermi level can be shifted by up to 500 meV towards the conduction band and by up to 400 meV towards the valence band by n- and p-doping, respectively, which increases the conductivity of the films. The doped layers were 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.

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 (I_D) between the source and drain in the ST is analogously updated by gate voltage (V_G) 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 HfO_x 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 CuO_x/HfO_x, where the CuO_x is the gate and Cu ion reservoir, and the HfO_x 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 CuO_x/Al₂O₃/HfO_x 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 HfO_x 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.

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 MAPbI_3−xCl_x TFTs (cations) and provide a good estimate of their density (∼10¹⁷cm⁻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.

The 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.

While it is known that too low built-in potential is detrimental to cells' performance, there is no consensus regarding the importance of maximizing the internal electric field or the built-in potential for achieving the highest power conversion efficiency of non-fullerene acceptor (NFA) organic solar cells. We use one of the prototypical NFA bulk heterojunction solar cells to show a direct correlation between the built-in potential, the open circuit voltage, the fill factor, and the device's efficiency. This is achieved using statistical analysis of devices made of two different cathodes (Ag, Mg) and a simple, intuitive model for the solar cell's current–voltage characteristics. Designing device structures with enhanced built-in potential (internal electric field) is crucial for surpassing the 20% efficiency limit.

The roll-to-roll printing production process for hybrid organic–inorganic perovskite solar cells (PSCs) demands thick and high-performance solution-based diffusion blocking layers. Inverted (p-i-n) PSCs usually incorporate solution-processed PC₇₀BM as the electron-transporting layer (ETL), which offers good electron charge extraction and passivation of the perovskite active layer grain boundaries. Thick fullerene diffusion blocking layers could benefit the long-term lifetime performance of inverted PSCs. However, the low conductivity of PC₇₀BM significantly limits the thickness of the PC₇₀BM buffer layer for optimized PSC performance. In this work, we show that by applying just enough N-DMBI doping principle, we can maintain the power conversion efficiency (PCE) of inverted PSCs with a thick (200 nm) PC₇₀BM diffusion blocking layer. To better understand the origin of an optimal doping level, we combined the experimental results with simulations adapted to the PSCs reported here. Importantly, just enough 0.3% wt N-DMBI-doped 200 nm PC₇₀BM diffusion blocking layer-based inverted PCSs retain a high thermal stability at 60 °C of up to 1000 h without sacrificing their PCE photovoltaic parameters.