Many advanced materials have been developed for organic field-effect transistors (OFETs) or thin-film transistors (TFTs) based on organic and organic hybrid materials. However, although many new OFETs exhibit superior characteristic parameters (such as high mobility), most of them show nonideal performances that have strongly limited progress in the design of molecules, the understanding of transport mechanisms, and the circuit applications of OFETs. In this review, the device physics of ideal and nonideal OFETs is discussed first to understand the factors that limit effective mobility in semiconducting channels, distort the potential distribution, or reduce the drift electric field. Then, recent advances in optimizing the material combinations, device structures, and fabrications of OFETs toward ideal transistors are discussed. Based on the good control of materials and interfaces, some new and novel concepts to utilize the nonideal properties of OFETs to build low-power circuits and integrated sensors are also discussed.
We developed a facile
method for fabricating large-area, two-dimensional (2D), organic,
highly crystalline films and extended it to organic thin-film transistor
arrays. Tilted spinning provided oriented flow at the three-phase
contact line, and a 2D crystalline film that consisted of layer-by-layer
stacked 2,7-diocty[1]benzothieno[3,2-b]benzothiophene
(C8-BTBT) was obtained facilely for organic thin-film transistors
(OTFTs). The extracted field-effect mobility is 4.6 cm2 V–1 s–1, but with nonideal features.
By applying this method to microdroplet arrays, an oriented crystal
was fabricated, and the channel region for OTFTs was covered by adjusting
the spinning speed. By tuning the tilt angle (θ) of the revolving
substrate, we fabricated high-performance OTFT arrays with average
and maximum mobilities of 7.5 and 10.1 cm2 V–1 s–1, respectively, which exhibited high reliability
factors of over 90% and were close to that of ideal transistors. These
results suggest that high-quality crystalline films can be obtained
via a facile tilted-spinning method.
Revealing the intrinsic electrical properties is the basis of understanding new functional materials and developing their applications. However, in nonideal field-effect transistors (FETs), conventional current-voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, a generalized gated four-probe (G-GFP) technique is developed, which detects dynamic changes in carrier accumulation and transport. The technique is suitable for exploring the intrinsic properties of semiconductors in FETs with arbitrary contacts and in any operational regimes above the threshold. Application to simulated transistors confirms its accuracy in probing the evolution of channel potential, drift field, and gate-dependent carrier mobility for devices with a contact-limited operation and disordered semiconductors. Comparative experiments are performed based on FETs with various materials, device structures, and operational temperatures. The G-GFP technique proves to exclude the various injection properties, to detect in situ how carriers are accumulated, and to clarify carrier mobility of the semiconductors. In particular, the well-known "double-slope" features in the current-voltage relations are controllably generated and their origins are identified. The approach could be used to explore electronic properties of newly developed materials such as organic, oxide, or 2D semiconductors.
In article number 1903889, Chuan Liu and co‐workers review the origins and critical factors that lead to organic field‐effect transistors (OFETs) with deviations from the ideal device models in terms of device physics. The recent progress in the optimization strategies and new perspectives for utilizing nonideal OFETs are also presented.
The
crystallization of organic or perovskite semiconductors reflects
the intermolecular interactions and crucially determines the charge
transport in opto-electronic devices. In this report, we demonstrate
and investigate the use of an ultrasonicated dispenser to guide the
formation of crystals of organic and perovskite semiconductors. The
moving speed of the dispenser affects the match between the concentration
gradient and evaporation rate near the three-phase contact lines and
thus the generation of various crystallization morphologies. The mechanism
of crystallization is given by a relationship between the calculated
concentration gradient profile and the degree of crystal alignment.
Highly ordered, aligned crystals are achieved for both organic bis(triisopropylsilylethynyl)-pentacene
and perovskite MAPbI3 semiconductors. Absorption spectra,
Raman scattering spectroscopy analysis, and grazing incidence wide-angle
X-ray scattering measurement reveal the strong anisotropy of the crystalline
structures. The aligned crystals lead to remarkably enhanced electrical
performances in an organic thin-film transistor (OTFT) and perovskite
photodetector. As a demonstration, we combine the OTFT with photodetectors
to achieve an active matrix of normally off, gate-tunable photodetectors
that operate under ambient conditions.
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