Electrical Double‐Slope Nonideality in Organic Field‐Effect Transistors

Abstract

The field effect transistor (FET) is arguably one of the most important circuit elements in modern electronics. Recently, a need has developed for flexible electronics in a variety of emerging applications. Examples include form‐fitting healthcare‐monitoring devices, flexible displays, and flexible radio frequency identification tags. Organic FETs (OFETs) are viable candidates for producing such flexible devices because they incorporate semiconducting π‐conjugated materials, including small molecules and conjugated polymers, which are intrinsically soft and mechanically compatible with flexible substrates. For OFETs to be industrially viable, however, they must achieve not only high charge carrier mobility, but also ideal and comprehensible electrical characteristics. Most recently, nonideal double‐slope characteristics in the transfer curves of OFETs (i.e., high slope at low gate voltage and low slope at high gate voltage), have stirred debate, which has led to different mechanistic rationales in the literature. This review focuses on the general observations, mechanistic understanding, and possible solutions associated with phenomena that result in FETs with double‐slope characteristics. By surveying and systematically summarizing in a single source relevant literature that deals with the issue of double slope, the experimental framework and theoretical basis for interpreting and avoiding this electrical nonideality in OFETs is provided.

Recent Advances in the Bias Stress Stability of Organic Transistors

“…There are however side effects of the use of dopant additives, such as increases in the OFF‐state current and the undesirable diffusion of dopant molecules in the presence of an electric field or thermal energy, which has motivated the testing of nondopant additives. Phan et al blended fullerene (C 60 ) derivatives into donor‐acceptor p‐type polymer semiconductors, and observed that the presence of this additive suppresses bias stress instability: the blending of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC 61 BM) into poly[4‐(4,4‐dihexadecyl‐4H‐cyclopenta[1,2‐b:5,4‐b']dithiophen‐2‐yl)‐ alt ‐[1,2,5]thiadiazolo[3,4‐c]pyridine] (PCDTPT) had the effect that the first and 20th scan transfer curves are identical ( Figure b) . In other words, the bias stress–induced change in μ FET was significantly suppressed (Figure c).…”

“…c) Multiple scans of FET mobility with PC 61 BM blend and without with PC 61 BM blend, as a function of gate bias. Reproduced with permission . Copyright 2017, Wiley‐VCH.…”

Halogenated Tetraazapentacenes with Electron Mobility as High as 27.8 cm2 V−1 s−1 in Solution‐Processed n‐Channel Organic Thin‐Film Transistors

“…This phenomenon, also called double slope—high slope at low gate bias, but small slope at high gate bias in the transfer curves, has been commonly observed in high‐mobility OFETs . It has been attributed to different causes, such as contact resistance and charge trapping, based on studies on different molecular systems and device structures . However, there is still no general mechanism that is applicable to all molecular systems (e.g., p‐type and n‐type semiconductors) and device structures (e.g., thin films and single crystals).…”

“…It has been attributed to different causes, such as contact resistance and charge trapping, based on studies on different molecular systems and device structures . However, there is still no general mechanism that is applicable to all molecular systems (e.g., p‐type and n‐type semiconductors) and device structures (e.g., thin films and single crystals). To handle this problem, we used two ways to extract the field effect mobility from the transfer I – V curves.…”

Walking the Route to GHz Solution‐Processed Organic Electronics: A HEROIC Exploration

“…The first aspect to be considered refers to a circumstance occurred in the recent years, known as the “mobility hype.” In the past years, an increasing number of works have reported mobility values exceeding 10 cm 2 V −1 s −1 for organic semiconductors, measured through OFET structures and extracted using the gradual channel approximation (GCA) model, the validity of which has later become argument of discussion. In particular, a number of authors have debated the correctness of the use of such extraction method in actual instances where some of the highest mobility values were measured, identifying a certain number of recurring patterns in the data and in the extraction methods that substantially invalidate an important number of high mobility claims . In practice, the validity of the GCA can be compromised owing to poor charge injection into the semiconductor and to the dependence of charge injection on the voltage applied to the electrodes.…”