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.

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.

To elucidate the internal chemical physics of measured CMOS-compatible electrochemical random-access memory (ECRAM) devices, we constructed a 2D semiconductor device simulation, including ions and electrochemical reactions, and used it to fit measured devices. We present the results of a device simulation model that includes Cu⁺ ions’ diffusion and the charge transfer reaction between the WO_x conduction band electron and Cu⁺ (i.e., “Cu plating”). Reproducing the linear response of ECRAM devices, the effect of charging HfO_x by the Cu⁺ ions is sufficient, and WO_x is not being doped by the Cu⁺ ions. While potentiation is supported by the formation of an electron channel, an efficient depression requires the formation of high positive charge density at the channel material. At higher Cu⁺ flux, Cu⁺ reaches and penetrates the WO_x layer. While this effect enhances the potentiation response, it also initiates the “plating” reactions. Including this reaction is essential to reproducing the data of devices exhibiting sub-linear responses. We suggest that electron trapping by ions (i.e., plating) would constitute a long-term degradation process even for H⁺ based devices.

A solar cell device is presented. The solar cell device comprises a layered structure comprising an electron transport layer and a hole transport layer and a heterojunction interface region between the electron transport and hole transport layers configured to define at least one charge generation region forming at least one junction between them, wherein at least one of the electron transport layer and the hole transport layer comprises at least one modulated doping layer at a predetermined distance from said at least one junction, said at least one modulated doping layer thereby inducing variation of an energy band structure at a vicinity of said at least one junction generating electric field applied to charge carriers increasing efficiency of generation and/or collection of the charge carriers.