Recent Progress in High‐Mobility Organic Transistors: A Reality Check

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin‐film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current–voltage characteristics, known as the “kink” or “double slope,” has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field‐effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier‐mobility‐related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm² V^?1 s^?1, respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high‐performance p‐type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n‐type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next‐generation organic semiconductors.     

Well‐Balanced Ambipolar Conjugated Polymers Featuring Mild Glass Transition Temperatures Toward High‐Performance Flexible Field‐Effect Transistors

Conjugated polymers, which can be fabricated by simple processing techniques and possess excellent electrical performance, are key to the fabrication of flexible polymer field‐effect transistors (PFETs) and integrated circuits. Herein, two ambipolar conjugated polymers based on (3E,7E)‐3,7‐bis(2‐oxo‐1H‐pyrrolo[2,3‐b]pyridin‐3(2H)‐ylidene)benzo[1,2‐b:4,5‐b′]difuran‐2,6(3H,7H)‐dione and dithienylbenzothiadiazole units, namely PNBDOPV‐DTBT and PNBDOPV‐DTF2BT , are developed. Both copolymers possess almost planar conjugated backbone conformations and suitable highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels (?5.64/?4.38 eV for PNBDOPV‐DTBT and ?5.79/?4.48 eV for PNBDOPV‐DTF2BT ). Note that PNBDOPV‐DTBT has a glass transition temperature (140 °C) lower than the deformation temperature of polyethylene terephthalate (PET), meaning well‐ordered molecular packing can be obtained on PET substrate before its deformation in mild thermal annealing process. Flexible PFETs based on PNBDOPV‐DTBT fabricated on PET substrates exhibit high and well‐balanced hole/electron mobilities of 4.68/4.72 cm² V^?1 s^?1 under ambient conditions. After the further modification of Au source/drain electrodes with 1‐octanethiol self‐assembled monolayers, impressively high and well‐balanced hole/electron mobilities up to 5.97/7.07 cm² V^?1 s^?1 are achieved in the flexible PFETs. Meanwhile, flexible complementary‐like inverters based on PNBDOPV‐DTBT on PET substrate also afford a much high gain of 148. The device performances of both the PFETs and inverters are among the highest values for ambipolar conjugated polymers reported to date.     

Uniaxial Alignment of Conjugated Polymer Films for High‐Performance Organic Field‐Effect Transistors

Polymer semiconductors have been experiencing a remarkable improvement in electronic and optoelectronic properties, which are largely related to the recent development of a vast library of high‐performance, donor–acceptor copolymers showing alternation of chemical moieties with different electronic affinities along their backbones. Such steady improvement is making conjugated polymers even more appealing for large‐area and flexible electronic applications, from distributed and portable electronics to healthcare devices, where cost‐effective manufacturing, light weight, and ease of integration represent key benefits. Recently, a strong boost to charge carrier mobility in polymer‐based field‐effect transistors, consistently achieving the range from 1.0 to 10 cm² V^?1 s^?1 for both holes and electrons, has been given by uniaxial backbone alignment of polymers in thin films, inducing strong transport anisotropy and favoring enhanced transport properties along the alignment direction. Herein, an overview on this topic is provided with a focus on the processing–structure–property relationships that enable the controlled and uniform alignment of polymer films over large areas with scalable processes. The key aspects are specific molecular structures, such as planarized backbones with a reduced degree of conformational disorder, solution formulation with controlled aggregation, and deposition techniques inducing suitable directional flow.     

Design of High‐Mobility Diketopyrrolopyrrole‐Based π‐Conjugated Copolymers for Organic Thin‐Film Transistors

Since the report of the first diketopyrrolopyrrole (DPP)‐based polymer semiconductor, such polymers have received considerable attention as a promising candidate for high‐performance polymer semiconductors in organic thin‐film transistors (OTFTs). This Progress Report summarizes the advances in the molecular design of high‐mobility DPP‐based polymers reported in the last few years, especially focusing on the molecular design of these polymers in respect of tuning the backbone and side chains, and discussing the influences of structural modification of the backbone and side chains on the properties and device performance of corresponding DPP‐based polymers. This provides insights for the development of new and high‐mobility polymer semiconductors.     

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

Molecular engineering of tetraazapentacene with different numbers of fluorine and chlorine substituents fine‐tunes the frontier molecular orbitals, molecular vibrations, and π–π stacking for n‐type organic semiconductors. Among the six halogenated tetraazapentacenes studied herein, the tetrachloro derivative (4Cl‐TAP) in solution‐processed thin‐film transistors exhibits electron mobility of 14.9 ± 4.9 cm² V^?1 s^?1 with a maximum value of 27.8 cm² V^?1 s^?1, which sets a new record for n‐channel organic field‐effect transistors. Computational studies on the basis of crystal structures shed light on the structure–property relationships for organic semiconductors. First, chlorine substituents slightly decrease the reorganization energy of the tetraazapentacene whereas fluorine substituents increase the reorganization energy as a result of fine‐tuning molecular vibrations. Second, the electron transfer integral is very sensitive to subtle changes in the 2D π‐stacking with brickwork arrangement. The unprecedentedly high electron mobility of 4Cl‐TAP is attributed to the reduced reorganization energy and enhanced electron transfer integral as a result of modification of tetraazapentacene with four chlorine substituents.