Magnetoconductance responses in organic diodes

  • 黃 偉順

Student thesis: Doctoral Thesis

Abstract

This thesis investigated magnetoconductance (MC) responses in organic diodes MC responses in organic diodes commonly relate to the formation of excited states Since some of the photo-physical properties of excited states have strong dependences with applied magnetic field including exciton-exciton interaction exciton-charge interaction and dissociation MC responses in organic diodes could be very complicated Everything proposed possibly contribute to the observed MC response In this thesis two ways to identify the MC response in pentacene-based organic diodes are proposed Those are inducing charge transfer complex states (CT complex states) in pentacene diodes by blending 2 3 5 6-tetrafluoro-7 7 8 8-tetracyanoquinodimethane (F4-TCNQ) into pentacene and creating a donor/ acceptor interface as well as an interfacial electric field in pentacene-based diodes by introducing fullerene into pentacene F4-TCNQ is used to study the MC response of the pentacene diode under charge injection regime Fullerene is especially used to investigate the MC under photocurrent owing to fullerene provides good electrical properties on charge transport Brief descriptions about the researches in this thesis (Chapter 4 5 and 6) are showed in the following In chapter 4 we investigate the MC responses in photocurrent unipolar injection and bipolar injection regimes in pentacene-based diodes Both photocurrent and bipolar injection contributed MC responses show large difference in MC line shape which are attributed to triplet-polaron interaction (TPI) modulated by the magnetic field dependent singlet fission and the intersystem crossing (ISC) of the polaron pair respectively By blending F4-TCNQ into pentacene all the MC responses are suppressed but the MC response at unipolar injection regime is enhanced which is attributed to the induced CT complex states This work successively identify the MC responses between single carrier contributed MC and exciton related MC by the induced CT complex states The work in chapter 5 investigates a distinct low-field negative MC response under illumination in pentacene:fullerene diodes The low-field negative MC response is attributed to a triplet polaron pair ((PP)3) charge reaction associated with the hyperfine interaction Applying a magnetic field can reduce the intensity of the reaction causing the negative MC response Additionally the low-field negative MC response changes with the blending ratios of fullerene in pentacene active layer the applied bias voltages and applying device structure with bulk heterojucntion (BHJ) or planar heterojunction (PHJ) configuration because the modulations of the internal electric field Consequently the results indicate that the (PP)3 charge reaction induces the low-field negative MC which is correlated with the local hyperfine field external magnetic field and electric field In chapter 6 the work investigates that the applied electric field modulates the line shapes of MC responses in pentacene:fullerene diodes under illumination We attribute the line shapes of MC curves herein are correlated with the strength of Columbic interaction of opposite charges in CT complexes Applying the reversed bias increases the built-in electric field (Ebuilt-in) of the diodes to offset the exchange interaction of CT complexes and narrows the line shapes of MC responses The saturation field of MC curves in pentacene:fullerene diodes is 752 Oe when the device is biased at 0 4 V and decreases to 212 Oe at a reversed bias of -1 0 V The line shapes of MC curves for the diode made of pristine pentacene or fullerene as the active layer show a marginal effect by the applied bias Our results indicate the correlations of MC responses with the exchange interaction of CT complexes as modulated by the applied electric field
Date of Award2014 Aug 1
Original languageEnglish
SupervisorTzung-Fang Guo (Supervisor)

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