Functional polymer dielectrics applied in PTCDI-C13H27-based sensors memory and transistors

Abstract

This thesis focuses on the applications of N N’-ditridecyl-3 4 9 10-perylene tetracarboxylic diimide (PTCDI-C13H27)-based optoelectronic devices and the evaluation of anisotropic charge transport capability in a pentacene cluster via electronic coupling energy mapping using quantum chemical calculation PTCDI-C13H27 is a potential n-type organic semiconductor with high electron mobility In this thesis the applications of PTCDI-C13H27-based optoelectronic devices are grouped into three subjects with particular focus on functional dielectric layers for organic thin-film transistors (OTFTs) The three subjects are (a) the time-dependent current growth phenomenon of PTCDI-C13H27-based OTFTs using a dielectric buffer layer consisting of strong and weak dipole units (b) the optoelectronic properties of PTCDI-C13H27-based OTFTs which include a switch OTFT and a sensor OTFT with potential for use in in-cell devices and (c) a photoinduced memory effect in PTCDI-C13H27-based OTFTs through phenolic dielectric buffer layers Besides charge transport capability of pentacene in various crystal polymorphisms was also discussed in the thesis by quantum chemical calculation In the first part of this thesis we describe an unusual phenomenon of time-dependent current growth that was first observed in OTFTs particularly n-type transistors We obtained the time-dependent growth of the drain current and nearly hysteresis-free electricity under DC bias stress for PTCDI-C13H27-based OTFTs with a polyimide (PI) dielectric layer These phenomena are attributed to (a) a reduction in trap state density located at the interface between the PI layer and semiconductor (b) the effective gate field enhanced by electric dipoles within the PI and (c) a low interface trap lifetime A polymer dielectric with moderate polar groups is suitable for application in stable organic devices In the second part we demonstrated that the control of the operating voltage for OTFT-based photosensors (photo-OTFT) is an essential factor that can enhance the photocurrent/dark current ratio (P) This phenomenon is due to the reduction of the contribution of field-effect current to output current under dark state In this study we analyzed a highly sensitive flexible organic photosensor made from cross-linked poly(4-vinylphenol) (C-PVP) as a polymer dielectric layer and PTCDI-C13H27 as an n-type active layer on a transparent polyethersulfone (PES) substrate by tuning both source-drain and source-gate voltages to the neighbor of threshold voltage (Vt = 3 0 V) Interestingly a maximum P was obtained when the operating voltage was reduced to around Vt The time-response characteristics and sensitivity of the PTCDI-C13H27-based photosensor were clearly investigated Considerable interest has been given to the flexible in-cell remote touch screen that comprises both photosensitive and switch OTFTs In this work both OTFT-based switch (switch-OTFT) and photo-OTFT were shown to form on the flexible PES substrate using the same fabrication process The electrical characteristics of switch-OTFT under bending states were discussed in terms of photoluminescence and time-resolved photoluminescence measurements as well as quantum theory calculations In the third part we discussed our discovery on the photoinduced phenomenon of current enhancement and memory effect in which was discovered in a PTCDI-C13H27-based OTFT structure of non-volatile memory (NVM) devices This phenomenon was observed after illumination with 532 nm laser for 2 min The C-PVP electret layer which consists of hydroxyphenyl group was assembled in NVM The retention of photoinduced memory effect in C-PVP-based NVM was more than 1 day Two kinds of dipoles namely instantaneously strong and quasi-permanent caused the photoinduced memory effect in C-PVP-based NVM An instantaneously strong dipole (27 9 D) was introduced by the excitation process of PTCDI-C8H17 dimer (a derivative of PTCDI family) The quasi-permanent dipole in C-PVP electret layer was generated and aligned by the instantaneously strong dipole The accumulated charge varied with electric field time-dependent capacitance measurement and quantum chemical simulation which were used to analyze the photoinduced memory effect To further verify whether the hydroxyphenyl group is the key factor of photoinduced memory effect in NVM we employed four types of electret layers [poly(4-vinylphenol) (PVP) poly(styrene) (PS) poly(vinyl alcohol) (PVA) and PS doped with 1 wt% 1 2-dihydroxybenzene (catechol)] to fabricate NVMs Results indicated that only the electret of NVMs had hydroxyphenyl group or similar group and the photoinduced memory effect could be observed The results in this study could be used as reference in designing electret materials for photoinduced n-type NVMs In the fourth part We proposed an approach for evaluating the charge transport capability of pentacene molecules in various crystal polymorphisms in the framework of Marcus theory of electron-transfer reactions Crystal-induced reorganization energy (?reg) and electronic coupling energy (J) which are the key factors that determine charge hopping mobility in various molecular configurations of crystal clusters are calculated by using quantum chemical calculation In particular we examined the effects of the edge-to-face angle (?ef) and tilt angles (?tilt) of pentacene molecules in crystal clusters on charge mobility because the two parameters are difficult to determine experimentally In addition to the effects of lattice constants we found that ?tilt and ?ef exert a large impact on the charge mobility capability of pentacene in various crystal polymorphisms Our mapping results provide a complete understanding of crystal-induced ambipolar transport capability of pentacene molecules We highlight that the electron mobility of pentacene molecules are possibly as fast as hole mobility i e balanced ambipolar transport unlike the previous idea of hole-dominated transport

Date of Award	2015 Aug 17
Original language	English
Supervisor	Wei-Yang Chou (Supervisor)