Recently, Associate Professor Xugang Guo’s group from the Department of Materials Science and Engineering published their latest research advance on unipolar n-type organic thin-film transistors in Advanced Materials, a top international journal specialized in materials science (impact factor = 19.791), under the title “Thiazole Imide-Based All-Acceptor Homopolymer: Achieving High-Performance Unipolar Electron Transport in Organic Thin-Film Transistors”.

High-performance n-type polymer semiconductors are critical for advancing the field of organic electronics, and most of these polymers adopt the donor–acceptor (D–A) type structure. The organic thin-film transistors (OTFTs) based on D–A polymers generally show high charge carrier mobility, but suffer from ambipolar transport with high off-currents (Ioffs) >10-8 A and small current on/off ratios (Ion/Ioffs) <105. The presence of electron-rich donor units in D–A polymers elevates their frontier molecular orbital levels and leads to narrow band gap, allowing both hole and electron injection thus the OTFTs could not be completely turned off. In order to suppress the hole injection and to achieve unipolar electron transport, all-acceptor (or acceptor-acceptor) combination should be an excellent approach. However, the highly electron-deficient acceptor units are challenging to synthesize and functionalize, therefore resulting in a limited library of materials. In addition, these electron-deficient units contain strong electron-withdrawing groups, generating significant steric hindrance and leading to twisted backbone and low electron mobility, typically <0.1 cm2 V-1 s-1.  The performance improvement of all-acceptor polymers relies on the development of new electron-deficient building blocks with good solubilizing capability, favored geometry, and optimized electrical property.

Figure 1. (a) The chemical structure of the all-acceptor homopolymer PDTzTI, the monomer structure and π-stacking distance measured by single-crystal XRD; (b) Organic thin-film transistor device structure and performance.

In this paper, Associate Professor Xugang Guo’s group designed and synthesized two novel imide-functionalized thiazoles, and subsequently developed the all-acceptor homopolymer PDTzTI (Figure 1a). Single crystal analysis of the monomer reveals its highly planar structure, close π-stacking distance, and the existence of intramolecular noncovalent N⋅⋅⋅S interaction, making the model compound very promising in constructing high-performance all-acceptor homopolymers. The strong electron-withdrawing capability of thiazole imide significantly lowers the frontier molecular orbitals, thus not only facilitates the electron injection and improves the stability of OTFT devices; but also simultaneously suppresses the hole injection and lowers the Ioffs. PDTzTI-based OTFTs exhibit unipolar n-type transport and a remarkable electron mobility of 1.61 cm2 V-1 s-1, low Ioffs of 10-10-10-11 A, and substantial Ion/Ioffs of 107-108 (Figure 1b). This is the highest reported mobility value for n-type all-acceptor homopolymers, comparable to that of electron transporting D–A polymers, but with much improved Ioffs and Ion/Ioffs in transistors. The results demonstrate that the all-acceptor approach is superior to the D–A one when appropriate electron-deficient building blocks are synthesized, which results in unipolar electron transport with more ideal transistor performance characteristics.

Research Assistant Yongqiang Shi and Research Assistant Professor Han Guo from Prof. Guo’s lab are the co-first authors of this paper. Undergraduate Yulun Wang (currently PhD candidate at Stony Brook University, US) performed the DFT calculation. This study was done in collaboration with Prof. Xinhui Lu from the Chinese University of Hong Kong (Synchrotron X-ray measurement). Financial support was provided by NSFC, Shenzhen Peacock Plan, Shenzhen Key Lab Funding and several other research grants.

Link to the paper：http://onlinelibrary.wiley.com/doi/10.1002/adma.201705745/full