(Invited) Organic Floating-Gate Transistors for Printable Nonvolatile Memory and Optoelectronic Device Applications

There has been considerable interest in the development of nonvolatile memories using organic materials owing to their potential use in flexible and printed electronic devices. In recent years, organic field-effect transistors (OFETs) having floating-gate structures as the charge storage layer have attracted increasing interest because they allow achieving large threshold voltage ( V th ) shifts and a relatively long retention time with a simple device configuration. However, the fabrication of floating-gate OFET memories by solution processes is generally difficult because it often suffers from the mixing of soluble organic materials at their interfaces. In previous studies, we have developed solution-processable floating-gate OFET memories by using top-gate/bottom-contact OFETs based on polymer semiconductors and the vertical phase separation behavior in the solution-processed composite film of a polymer insulator (PMMA or polystyrene) and a soluble small-molecule semiconductor (TIPS-pentacene) [1-3]. Here, I report the memory characteristics of solution-processed floating-gate OFET memories using a typical p-type polymer semiconductor of polythiophenes (P3HT and PBTTT) and an ambipolar polymer semiconductor based on diketopyrrolopyrrole (DPP) and their usefulness for the development of printable nonvolatile memories and image sensing devices with high functionality. Figure 1(a) shows the typical structure of our memory devices. Since a variety of orthogonal solvents are available for polymer semiconductors, the organic charge storage layer composed of PMMA and TIPS-pentacene molecules can be deposited onto the semiconductor layer by spin coating using butyl acetate. During thermal annealing at 100 ºC, TIPS-pentacene molecules segregate to the upper side of the composite film through vertical phase separation, and floating-gate electrodes consisting of TIPS-pentacene and organic tunneling layers are simultaneously formed by solution processes. Figure 1(b) shows the energy band diagram of the P3HT-based organic FET memory during the programming process. Because P3HT has a shallow LUMO level compared to the work function of the Au source-drain electrodes, the P3HT FET memory shows only a negligible V th shift when programmed in the dark. Under light illumination, the device exhibits a large V th shift of over 30 V by storing photogenerated electrons into TIPS-pentacene floating gates. To investigate the applicability of the OFET memories to large-area image sensors, the imaging of a black and white pattern using the recorded drain currents of the memory array under light illumination has been demonstrated [Fig. 1(c)]. We also found that P3HT FET memories with organic semiconductor floating gates exhibit light-intensity dependent synaptic characteristics when programmed under red light, enabling the enhancement of the functionality of organic image sensors. In contrast to OFET memories based on polythiophenes, DPP-DTT FET memories can be programmed in the dark because of the deep LUMO level and good electron transport property of DPP-DTT. It was also found that the electrical characteristics of DPP-DTT TFT memory devices depend on the difference of work function of source-drain and gate electrodes and tuning the energy level difference enables ambipolar carrier trapping and good retention characteristics [Fig. 1(d)]. The DPP-DTT FET memories having hole trapping characteristics also achieve NAND-like memory operations as memory devices are connected in series. References [1] F. Shiono, H. Abe, T. Nagase, T. Kobayashi, and H. Naito, Org. Electron. 67 , 109-115 (2019). [2] H. Abe, R. Hattori, T. Nagase, M. Higashinakaya, S. Tazuhara, F. Shiono, T. Kobayashi, and H. Naito, Appl. Phys. Express 14 , 041007 (2021). [3] M. Higashinakaya, T. Nagase, H. Abe, R. Hattori, S. Tazuhara, T. Kobayashi, and H. Naito, Appl. Phys. Lett. 118 , 103301 (2021). Acknowledgements This work was supported by the Mitsubishi foundation, the Mazda foundation, the Futaba research grant program, Kansai Research Foundation for Technology Promotion, and JSPS KAKENHI (Grant Nos. JP20K21007, JP21H04564, JP24K00931, and JP24K01330). Figure 1