An Investigation of the Structure-Property-Performance Relationship for Organic Semiconductors in Polymer and Electrolyte-Gated Organic Field-Effect Transistors

  • Raja Khan

Student thesis: Phd

Abstract

Research in organic semiconductors (OSCs) has demonstrated promising application of organic field-effect transistors (OFETs) as biosensors in medical diagnostics, food and environment monitoring. Iterative improvement in OSC material design and deposition strategy has contributed to enhanced charge transport properties of OFETs, critical for realising commercially viable devices. To date, little research has been conducted on the use of unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene (BTBT) small-molecules that can potentially yield superior intrinsic electronic properties as well as highly ordered thin-films, all the while possessing favourable properties for facile solution processing. A library of previously littler or never studied before unsymmetrical BTBT molecules consisting of a decane chain at the C2 position, and various functional groups, namely, phenyl, methoxy, diamino, and trifluromethyl groups (–Ph, –OMe, –NMe2, and –CF3) at the C7 position of the BTBT moiety were investigated. The results clearly highlighted the influence the four different functional groups had on thermal and electrochemical properties of these materials, thus influencing the thin-film microstructure and performance in OFET devices. The introduction of the –Ph, –OMe, and –NMe2 functional groups contributed to the presence of several liquid crystalline phases in these material. Stark differences were observed in the HOMO energy levels of the small-molecules; the BTBT small-molecules with –OMe and –NMe2 had significantly higher lying energy levels compared to –Ph and –CF3 containing BTBT molecules. Transistors fabricated using Ph-BTBT (1.89 cm2 V-1 s-1), OMe-BTBT (1.87 cm2 V-1 s-1), and CF3-BTBT (1.54 cm2 V-1 s-1) blends with poly[4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-benzothiadiazole (IDTBT), yielded higher average charge carrier mobility values compared to pristine IDTBT (1.02 cm2 V-1 s-1), blends of the prototypical 2,7-dioctyl BTBT (C8-BTBT) (1.19 cm2 V-1 s-1) and NMe2-BTBT (0.08 cm2 V-1 s-1) devices, processed under the same conditions, demonstrating the potential for realising improved performance through design of unsymmetrical BTBT molecules. Thin-film morphology analysis revealed that the BTBT blends retained their crystalline properties when processed in the blend explaining the mobility values, with the exception of NMe2-BTBT blends (hence the poor performance). This work clearly shows that the design and synthesis of unsymmetrical BTBT molecules can be used to optimise electrochemical, thermal, and self-assembling properties of the materials for specific large-area organic transistor fabrication, all the while maintaining high mobility values. Electrolyte-gated field-effect transistors (EGOFETs) are considered more adept for biosensing application than OFETs, and similarly, to enable optimum performances of EGOFET, significant understanding of OSC structure-property relationship and its interplay with EGOFET device performance is required. The novelty of this work was the first in-depth investigation of the OSC structure-property interplay with EGOFET device performance for a large of assortment of all-donor polymers, donor-acceptor copolymers, and OSC polymer/small-molecule blends. The work attempted to establish if an alternative material design strategy is required for EGOFETs or if current state-of-the-art OFET materials are equally robust in EGOFET devices. Surprisingly, the study showed an almost complete reversal in trend between performances in OFET and EGOFETs. In OFETs, the OSC polymer/small-molecule blends yielded the highest charge carrier mobility values with poly(3-hexylthiophene-2,5-diyl) (P3HT) exhibiting one of the lowest average mobility value. This was in stark constant to the blend thin-films yielding the poorest charge carrier mobility values in EGOFETs with P3HT yielding one of the highest. A clear trend could be seen between the smoothness of the thin-film and µ values in
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorSacha Howell (Supervisor), Michael Turner (Supervisor) & Michelle Webb (Supervisor)

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