A Study On Current Hysteresis And Its Origin In OTFTs

Organic thin-film transistor (OTFT), a field effect device comprising organic semiconductors, electrodes and insulators, is the core component of organic integrated circuits. In recent years, OTFTs have attracted widespread attention for its low preparation temperature, easy processing, potential in large-area and flexible manufacturing. Over the past 30 years of both theoretical and technological research of OTFTs, significant progress has been m-ade in its device performances. But the large-scale application of OTFTs is still restricted by two obstacles, one of which is the low mobility, resulting in large operation voltages, and the other of which is the poor reliability, especially the instability of threshold voltages. The bias-dependence of the threshold voltage causes the hysteretic behavior of the current. Origin of the current hysteresis was interpreted in different ways during the years. For example, charging and recharging of the electron traps of=SiOH groups on SiO2 surface, the electrochemical reaction of the H2O/O2 redox couples, polaron/bipolaron transformation in conjugated polymers, et al. However, none of these theories can give a comprehensive interpretation of the origin of current hysteresis in OTFTs.In this work, single walled carbon nanotubes (SWCNTs) are embedded into a conjugated polymer, poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). By varying the channel length of the composite OTFTs from 10μm to 1,000μm, a systematic investigation of current hysteresis is conducted and a new model is proposed. The interaction of bipolaron with the H2O/O2 redox couple causes current hysteresis in OTFTs. In short channel length devices, in which SWCNTs "percolate" through the source and drain, hysteresis similar to that of SWCNT transistors is observed and it is caused mainly by the H2O/O2 electrochemical reaction. In long channel length devices, in which SWCNTs form "sub-percolation" pathways in the composite, conduction of the channel is dominated by electron hopping in F8T2, and the current hysteresis in these devices originates from the bipolaron formation facilitated by the H2O/O2 electrochemical reaction. The hysteresis gap in sub-percolation devices is sensitive to channel length, illumination and temperature. Contrary to the bipolaron mechanism discussed in literatures, the bipolaron is formed at the off-state with positive gate bias instead of the on-state with negative gate bias. The bipolaron manifest itself in the BP2+-OH- complex by combination with OH-, the electrochemical reaction products. The bipolaron-induced hysteresis is also a thermally activated process, with an apparent activation energy of 0.29 eV extracted for the dissociation of the BP2+-OH-complex in temperature-varying characterization.In sum, the model of "electrochemical reaction facilited bipolaron formation and dissociation" is a genetic model, which not only applies to low-mobility OTFTs, but also to devices with high mobilities semicoductors like carbon nanotubes and graphene. The proposion of the model will benifit the large-scale application of OTFTs.