The study of organic field-effect transistor based memory devices with polymeric electrets

  • 王 裕夫

Student thesis: Doctoral Thesis


In the past years organic field-effect transistor (OFET) based memory devices have attracted many attentions in organic electronics because of their unique advantages such as easily packed with integrated circuits low-temperature manufacturing and non-destructive reading out of digital signals In OFETs pentacene is usually used as a standard organic semiconductor for being an active layer in p-type device owing to its superior hole transport property between molecules However the researches of n-type OFET device was relatively backward comparing with p-type device This may attribute to the difficult synthesis and unstable structures under atmosphere environment for n-type semiconductors which restricts the electron transport ability of OFETs Therefore the researches for n-type semiconductor-based OFET memory device are still scarcity On the other hand the interfacial property between semiconductor layer and gate dielectric layer plays an important role for developing high performance OFET memories In this thesis we propose some simple routes to improving both electrical characteristics and electrical memory windows of n-type OFET memories A novel polyimide (PI) electret with a quasi-permanent electrostatic dipole on side chain structures was synthesized in this study We demonstrate that the carriers can be trapped by the polar groups near the semiconductor/PI interface and the electrical properties and memory effects of memory device were successfully controlled by the mixing ratios of PI electret Finally we also propose an easy method to improve the memory window and operation speed of n-type OFET-based photomemory devices by constructing a discontinuous pn-heterojnuction structure within the active layer that can effectively enhance charge carrier transportation at interfaces to broaden the memory window of memory devices In the first part a molecular design for the electret material of n-operating organic field-effect transistor-based (OFET) memories is introduced A large memory window and high operating speed were achieved while the polar groups are connected to the polymer chain of polyimide which plays the role of electret of a transistor memory device In fact the significant memory effect in the device with PI electret can be ascribed to the charge carriers that were rapidly trapped and released by the polar groups of PI electret The phase variation of electrical force microscopy (EFM) images showed that polarization field induces charge trapping states on the surface of electret layer and accumulates charged carriers within the conducting channel of OFET to achieve high-performance memory and transistor simultaneously In conclusion an extra-large memory window was also obtained by introducing photo-induced charge transfer effect In the second part a series of polyimides (PIs) containing different weight ratios of polar piperazinyl and cholesterol side chains denoted as PCPI was synthesized in this study These PIs were used as gate dielectrics of n-type organic field-effect transistors (OFETs) and as electrets of photo-assisted organic memories The thermal properties of the PI/PCPI composite films were improved by increasing the spatial distribution of the PCPI molecules to form a thermally stable dielectric film The performances of OFETs as PIs were used as gate dielectrics were gradually enhanced by increasing the mixture ratios of the PCPI molecules A dipole field which originated from the PCPI molecules into the OFETs was introduced to observe the special phenomenon of output current growth under a long operation time The application of these superior transistors with PCPI- and PI-mixed electrets to the field of organic memory resulted in a photo-assisted memory window of more than 38 V The mechanisms of the carriers trapped in and released from the PI electrets were elucidated Results showed that our devices possess excellent stability for OFETs and an extra-large memory window for organic memory devices In the third part a discontinuous pentacene layer was formed on the PI/PTCDI interface by controlling the parameters of thermal evaporation process The thicknesses of pentacene layers were varied from 0 nm (serve as standard device) to 10 nm The transmission electron microscopy (TEM) images show that the dispersion of the island type pentacene crystals on n-type PTCDI-C13H27 monolayer which confirms that the three dimension growth of pentacene molecules instead of the traditional layer-by-layer growth mechanism Interesting crystalline properties of pentacene films in in-plane and out of-plane directions were obtained by grazing angle x-ray diffraction (GIXRD) measurements The GIXRD results demonstrate that the pentacene crystals are trending to three dimension growth on PTCDI-C13H27 monolayer below 5 nm thickness and then transfer to layer-by-layer growth model when the thickness of pentacene layer is above 5 nm This discontinuous pn-heterojunction structure can reduce the chances of charge recombination of injected electrons in top contact geometry OFETs As this structure was applied in an organic memory device the electrical memory window of the device with 5 nm-thick pentacene layer was gradually extended to 47 6 V which is larger than the past reports for n-type memory device This result demonstrates that the inserted pn-heterojunction structure possesses more effective interface to lead the photo-induce charges dissociation into conducting channel and increase the density of minority carriers to extend the negative shift of VT Moreover as the operation time of programing and erasing process was reduces to 0 2 ms the electrical memory window is still maintain at 32 6 V which realizes the high performance and high-speed operation n-type photomemory device
Date of Award2016 Aug 30
Original languageEnglish
SupervisorWei-Yang Chou (Supervisor)

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