(Keynote) Physics, Materials and Applications of High-Mobility Organic Transistor Circuits

Abstract

Development of functional materials and understanding of the microscopic mechanisms mutually benefit through their close interaction. To accelerate development of organic semiconductor devices for industrial application to flexible and printed electronics, it has been essential to understand the mechanism of charge transport in conjunction with molecular-scale charge transfer. We examine the idea that high-mobility charge transport in newly developed solution-crystalized organic transistors is caused by band-like charge transport, using several different methods to probe coherence in the electronic charge. These materials, being essentially different from conventional organic semiconductors with basically incoherent hopping-like transport, often show high values of mobility exceeding 10 cm2/Vs so that high-speed organic transistor circuits are realized. We first employ Hall-effect measurement which differentiates the coherent band transport from site-to-site hopping. Rubrene single crystal transistors were the first to show the Hall voltage obviously indicating the band transport with the mobility exceeding 10 cm2/Vs, whereas the result of pentacene single crystal transistors with the mobility around 1 cm2/Vs suggested somewhat intermediate between coherent and incoherent charge transport. An interesting crossover from hopping-like to band-like transport is also found by gradually changing temperature and external pressure, clearly indicating involvement of intermolecular phonons. Stimulated by the experiments in the early stage, such materials as decyldinaphthobenzo-dithiophene derivatives (Cn-DNBDT) are newly synthesized to stabilize the molecular position in the crystal form by introducing steric hindrance in their p-conjugated cores. With the development of solution-crystallization method, arrays of the single crystal transistors are formed to the size of wafer scale on plastic substrates. The mobility is as high as 20 cm2/Vs for p-type compound and 4 cm2/Vs for an n-type compound. We measured spin relaxation time of the electric-field induced carriers down to 4.2 K using the large-area ultra-thin single-crystal transistors, so that electron-spin relaxiation is precisely investigated. We found that the spin-relaxation follows the Elliott-Yafet mechanism so that the spin life time is consistently elongated at low temperatures due to reduced phonon scattering via spin-orbit coupling. The result suggest that charge carrier mobility can as high as 650 cm2/Vs with minimized phonon scattering at the low temperature. This presentation also focuses on recent development of key technologies for printed LSIs which can provide future low-cost platforms for RFID tags, AD converters, data processors, and sensing circuitries. Such prospect bears increasing reality because of recent research innovations in the field of material chemistry, charge transport physics, and solution processes of printable organic semiconductors. With excellent chemical and thermal stability in recently developed new materials, we are developing simple integrated devices based on CMOS using p-type and n-type printed organic FETs. Particularly important are new processing technologies for continuous growth of the organic single-crystalline semiconductor “wafers” from solution and for lithographical patterning of semiconductors and metal electrodes. Successful rectification and identification are demonstrated at 13.56 MHz with printed organic CMOS circuits.