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Organic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C₆₀metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C₆₀OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 10⁷are realized. The devices have low subthreshold swing in the range of 80 mV dec⁻¹and a large transconductance of up to 8 mS mm⁻¹.

 

Faux‐hawk fullerenes are promising candidates for high‐performance organic field‐effect transistors (OFETs). They show dense molecular packing and high thermal stability. Furthermore, in contrast to most other C₆₀derivates, functionalization of the fullerene core by the fluorinated group C₆F₄CF₂does not increase their lowest unoccupied orbital position, which allows the use of air‐stable molecular n‐dopants to optimize their performance. The influence of n‐doping on the performance of OFETs based on the faux‐hawk fullerene 1,9‐C₆₀(cyclo‐CF₂(2‐C₆F₄)) (C₆₀FHF) is studied. An analytic model for n‐doped transistors is presented and used to clarify the origin of the increase in the subthreshold swing usually observed in doped OFETs. It is shown that the increase in subthreshold swing can be minimized by using a bulk dopant layer at the gate dielectric/C₆₀FHF layer instead of a mixed host:dopant layer. Following an optimization of the OFETs, an average electron mobility of 0.34 cm² V⁻¹ s⁻¹, a subthreshold swing below 400 mV dec⁻¹for doped transistors, and a contact resistance of 10 kΩ cm is obtained, which is among the best performance for fullerene based n‐type semiconductors.

 

Organic electrochemical transistors (OECTs) are highly versatile in terms of their form factor, fabrication approach that can be applied, and freedom in the choice of substrate material. Their ability to transduce ionic into electric signals and the use of bio-compatible organic materials makes them ideally suited for a wide range of applications, in particular in areas where electronic circuits are interfaced with biologic matter. OECT technology has attracted widespread interest in recent years, which has been accompanied by a steady increase in its performance. However, this progress was mainly driven by device optimization and less by targeting the design of new device geometries and OECT materials. To narrow this gap, this review provides an overview on the different device models that are used to explain the underlying physics governing the steady and transient behavior of OECTs. We show how the models can be used to identify synthetic targets to produce higher performing OECT materials and summarize recently reported materials classes. Overall, a road-map of future research in new device models and material design is presented summarizing the most pressing open questions in the understanding of OECTs. 

Organic Electrochemical Transistors are versatile sensors that became essential for the field of organic bioelectronics. However, despite their importance, an incomplete understanding of their working mechanism is currently precluding a targeted design of Organic Electrochemical Transistors and it is still challenging to formulate precise design rules guiding materials development in this field. Here, it is argued that current capacitive device models neglect lateral ion currents in the transistor channel and therefore fail to describe the equilibrium state of Organic Electrochemical Transistors. An improved model is presented, which shows that lateral ion currents lead to an accumulation of ions at the drain contact, which significantly alters the transistor behavior. Overall, these results show that a better understanding of the interface between the organic semiconductor and the drain electrode is needed to reach a full understanding of Organic Electrochemical Transistors. 

Doping organic semiconductors has become a key technology to increase the performance of organic light-emitting diodes, solar cells, or field-effect transistors (OFETs). However, doping can be used not only to optimize these devices but also to enable new design principles as well. Here, a novel type of OFET is reported—the vertical organic tunnel field-effect transistor. Based on heterogeneously doped drain and source contacts, charge carriers are injected from an n-doped source electrode into the channel by Zener tunneling and are transported toward a p-doped drain electrode. The working mechanism of these transistors is discussed with the help of a tunnel model that takes energetic broadening of transport states in organic semiconductors and roughness of organic layers into account. The proposed device principle opens new ways to optimize OFETs. It is shown that the Zener junction included between the source and drain of the vertical organic tunnel field-effect transistors suppresses short channel effects and improves the saturation of vertical OFETs.