Well-Balanced Carrier Mobilities in Ambipolar Transistors Based on Solution-Processable Low Band Gap Small Molecules

Abstract

We synthesized a solution-processable low band gap small molecule, Si1TDPP-EE-COC6, for use as a semiconducting channel material in organic thin film transistors (OTFTs). The Si1TDPP-EE-COC6 is composed of electron-rich thiophene−dithienosilole−thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) and carbonyl units. SiTDPP-EE-COC6-based OTFTs with Au source/drain electrodes were fabricated, and their electrical properties were systematically investigated with increasing thermal annealing temperature. The hole and electron mobilities of as-spun Si1TDPP-EE-COC6 were 3.3 × 10 −4 and 1.7 × 10 −4 cm 2 V −1 s −1 , respectively. The carrier mobilities increased significantly upon thermal annealing at 150°C, yielding a hole mobility of 0.003 cm 2 V −1 s −1 and an electron mobility of 0.002 cm 2 V −1 s −1 . The performance enhancement upon thermal annealing was strongly associated with the formation of a layered edge-on structure and a reduction in the π−π intermolecular spacing. Importantly, the use of atomically thin single-layer graphene (SLG) source/drain electrodes that were grown by the chemical vapor deposition (CVD) method further increased the carrier mobilities. The 150°C annealed Si1TDPP-EE-COC6-based OTFTs with SLG source/drain electrodes exhibited a hole mobility of 0.011 cm 2 V −1 s −1 and an electron mobility of 0.015 cm 2 V −1 s −1 . The improved electrical performances of the SLG OTFTs were attributed to the stepless flat surface of the SLG electrodes and the better interfacial contact between the Si1TDPP-EE-COC6 molecules and the SLG electrodes compared to the Au electrodes. This work suggests that careful chemical design is essential to enhance balanced ambipolar transistor performance based on small conjugated molecules, and the SLG is a good electrode material to promote the carrier mobilities.

Structure–Property Relationships of Semiconducting Polymers for Flexible and Durable Polymer Field-Effect Transistors

“…The electron mobilities of the DPP-based FETs were approximately 5−100 times lower than the hole mobilities. This strong asymmetry between hole and electron mobilities was attributed mainly to the lower hole injection barrier from the Au electrode (with a high work function of ∼5.1 eV) to the semiconductor; 24,25 this will be further explained below. After thermal annealing at 100 and 150 °C, the electrical properties, including the carrier mobility and on/off current ratio (I on /I off ), were improved significantly.…”

High Performance Solution Processed Organic Field Effect Transistors with Novel Diketopyrrolopyrrole-Containing Small Molecules

“…Kim et al . 33 , 34 also used DPP as an acceptor and dithiensilole as the core donor, achieving ambipolar transport characteristics with hole mobility of 0.011 cm 2 V −1 s −1 and electron mobility of 0.015 cm 2 V −1 s −1 . However, OFETs utilizing DPP-based small molecules show a much lower performance than those utilizing DPP-based polymers.…”

Photoresponsive Transistors Based on a Dual Acceptor-Containing Low-Bandgap Polymer

“…Organic π-conjugated polymers have drawn a great deal of attention in the last few decades due to the merits of economical cost in fabrication, large-area printing compatibility, mechanical robustness, and manageable optoelectronic properties for application in polymer field-effect transistors (PFETs). − Electrical performance of PFETs has been enhanced significantly through the logical design of polymer structures (i.e., both polymer backbones and substituted alkyl side chains) as well as painstaking development of the device fabrication process. − Recently, a variety of donor–acceptor (D–A) low-bandgap polymers, typically consisting of electron-donating and electron-accepting groups alternating along the polymer backbone, have displayed impressive carrier mobilities. ,− In particular, carrier mobilities over 1 cm 2 /(V·s) have been achieved from pyrrolo­[3,4- c ]­pyrrole-1,4­(2 H ,5 H )-dione (DPP)-based conjugated copolymers mainly because of the large amount of interchain interactions via planar geometries of DPP moieties. − It is worth noting that these D–A polymers can harvest a wide range of wavelength photons because their bandgaps are low. , …”