Isoindigo‐Based Polymers with Small Effective Masses for High‐Mobility Ambipolar Field‐Effect Transistors

So far, most of the reported high‐mobility conjugated polymers are p‐type semiconductors. By contrast, the advances in high‐mobility ambipolar polymers fall greatly behind those of p‐type counterparts. Instead of unipolar p‐type and n‐type materials, ambipolar polymers, especially balanced ambipolar polymers, are potentially serviceable for easy‐fabrication and low‐cost complementary metal‐oxide‐semiconductor circuits. Therefore, it is a critical issue to develop high‐mobility ambipolar polymers. Here, three isoindigo‐based polymers, PIID‐2FBT , P1FIID‐2FBT , and P2FIID‐2FBT are developed for high‐performance ambipolar organic field‐effect transistors. After the incorporation of fluorine atoms, the polymers exhibit enhanced coplanarity, lower energy levels, higher crystallinity, and thus increased µ _e. P2FIID‐2FBT exhibits n‐type dominant performance with a µ _e of 9.70 cm² V^?1 s^?1. Moreover, P1FIID‐2FBT exhibits a highly balanced µ _h and µ _e of 6.41 and 6.76 cm² V^?1 s^?1, respectively, which are among the highest values for balanced ambipolar polymers. Moreover, a concept “effective mass” is introduced to further study the reasons for the high performance of the polymers. All the polymers have small effective masses, indicating good intramolecular charge transport. The results demonstrate that high‐mobility ambipolar semiconductors can be obtained by designing polymers with fine‐tuned energy levels, small effective masses, and high crystallinity.     

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.     

High Mobility 2D Palladium Diselenide Field‐Effect Transistors with Tunable Ambipolar Characteristics

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe₂), a noble‐transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field‐effect transistors (FETs) based on ultrathin PdSe₂ show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe₂ to exhibit electron‐dominated transport with high mobility (µ _{e (max)} = 216 cm² V^?1 s^?1) and on/off ratio up to 10³. Hole‐dominated‐transport PdSe₂ can be obtained by molecular doping using F₄‐TCNQ. This pioneer work on PdSe₂ will spark interests in the less explored regime of noble‐TMDCs.     

Bis‐Diketopyrrolopyrrole Moiety as a Promising Building Block to Enable Balanced Ambipolar Polymers for Flexible Transistors

A bis‐diketopyrrolopyrrole (DPP dimer, 2DPP) core is synthesized with much stronger electron deficiency than DPP by homocoupling of DPP. 2DPP‐based polymers, P2DPP‐BT, P2DPP‐TT, P2DPP‐TVT, and P2DPP‐BDT, are obtained. Top‐gated organic field‐effect transistors on plastic substrate are fabricated. Compared with their mono‐DPP‐based polymers, remarkable improvement of electron mobilities of P2DPPs is achieved. Meanwhile, their p‐channel performance becomes higher.     

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.     

2D Molecular Crystal Bilayer p–n Junctions: A General Route toward High‐Performance and Well‐Balanced Ambipolar Organic Field‐Effect Transistors

Ambipolar organic field‐effect transistors (OFETs) are vital for the construction of high‐performance all‐organic digital circuits. The bilayer p–n junction structure, which is composed of separate layers of p‐ and n‐type organic semiconductors, is considered a promising way to realize well‐balanced ambipolar charge transport. However, this approach suffers from severely reduced mobility due to the rough interface between the polycrystalline thin films of p‐ and n‐type organic semiconductors. Herein, 2D molecular crystal (2DMC) bilayer p–n junctions are proposed to construct high‐performance and well‐balanced ambipolar OFETs. The molecular‐scale thickness of the 2DMC ensures high injection efficiency and the atomically flat surface of the 2DMC leads to high‐quality p‐ and n‐layer interfaces. Moreover, by controlling the layer numbers of the p‐ and n‐type 2DMCs, the electron and hole mobilities are tuned and well‐balanced ambipolar transport is accomplished. The hole and electron mobilities reach up to 0.87 and 0.82 cm² V^?1 s^?1, respectively, which are the highest values among organic single‐crystalline double‐channel OFETs measured in ambient air. This work provides a general route to construct high‐performance and well‐balanced ambipolar OFETs based on available unipolar materials.