Electrotuneable Radical Polymers for Thin-Film Electronic Device Applications
Authors: Singh Deepa , Magnan François , Gilroy Joe B. , Fanchini Giovanni 
Published: July 2022

Polyradicals – organic radical polymers in which each repeating unit contains a singly occupied molecular orbital (i.e. an unpaired electron spin) – are unique alternatives to their π-conjugated and semiconducting counterparts for several applications.[1] Unique of polyradicals are tunable charge states at their repeating units (see figure) which enable multi-stable charge transport regimes at the nanoscale. In this talk, we will present the use of thin films of polyradicals in transparent and flexible thin-film nanoelectronics. Although field-effect thin-film transistors (FETs) and flash memory devices ("memristors") based on radical polymers have been often proposed, memristor stability was frequently limited to a few writing cycles, in spite of the excellent quality of the active layer, and no FETs have been demonstrated, even though evidence of polyradical doping has been offered.[2]

Here, the design criteria for flash memory devices are reviewed.It will be shown, using a combination of Kelvin-probe force microscopy (KPFM), electrical transport and optical measurements, that single-layer flash memory devices can be demonstrated from 6-oxoverdazyls, a class of radical polymers from which ultra-thin and ultra-smooth organic thin flims are advantageously processable.[3] As a case study, ultrathin devices in which the active layer is formed by a 15-nm homogeneous film of a poly-norbornene-6-oxoverdazyl (PN-6OV) polyradical synthesized by a dry vacuum polymer deposition technique are presented and compared with the corresponding devices of poly-6-oxoverdazyls (P6OV) synthesized by wet chemistry. [4] We will show that high performance is associated to the presence three tunable charge states in each monomer: positive, neutral, and negative, and also depends on the length of the pendant groups to which the radical repeating units are attached. We will demonstrate that careful engineering of the anode and cathode work functions, specifically aligning them with the negative and positive energy levels of the polyradical, is vital to maximize the on/off current ratio and ensure flash operation. The possibility to achieve electro-tunable poly-6-oxoverdazyl radical polymers by different techniques offer uniques opportunities for their use in a variety of different contexts, for example in transparent and/or flexible electronics, and where compatibility with different substrates is required.

In the last part of our talk, we will present how a vertical device architecture, with drain-source contacts sandwiching the active layer of a strongly correlated 6-oxoverdazyl polyradical, leads to on/off ratios >103 in p-type PR-FETs. [4] Hole injection thus occurs by contact doping via tunable charge states at the polyradical-electrode interface. PRFETs are superior to existing organic FETs as they combine memristor and transistor functions in one mem-transistor device, offering unique potential for synaptic and spintronic applications.

[1] Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W. A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science
2018, 359, 1391-1395

[2] Nguyen, T. P.; Easley, A. D.; Kang, N.; Khan, S.; Lim,S-M.; Rezenom, Y. H.; Wang, S.; Tran, D. K.; Fan, J.; Letteri, R. A.; He, X.; Su, L.; Yu, C-H.; Lutkenhaus, J. L.; Wooley K, L. Polypeptide organic radical batteries. Nature, 2021, 593, 61

[3] Ezugwu, S.; Paquette, J. A.; Yadav, V.; Gilroy, J. B.; Fanchini, G. Design Criteria for Ultrathin Single-Layer Flash Memristors from an Organic Polyradical. Adv. Electron.
Mater.
2016, 2, 1600253

[4] Singh, D.; Magnan, F.; Gilroy, J. B.; Fanchini, G. Transparent and flexible field-effect transistors and mem-transistors with electroactive layers of solution-processed organic polyradicals, https://arxiv.org/abs/1910.10212

Figure 1