||Magnetoconductance responses in organic diodes
||Department of Photonics
magnetic field effect
charge transfer complex state
本篇論文研究有機二極體(organic diode)元件之磁電導響應(magnetoconductance)。有機二極體的磁電導響應一般與激發態的形成有關，由於激發態的一些光物理特性，包括激發子激發子交互作用(exciton-exciton interaction)、激發子載子交互作用(exciton-charge interaction)與解離(dissociation)等，皆與外加磁場有關，有機二極體的磁電導產生有相當複雜的成因。目前，我們可以說任何有關於激發態生成的原因皆有可能貢獻到磁電導響應的生成。本論文內容，使用兩種實驗上的手法來討論有機五環素(pentacene)二極體元件的磁電導響應，包括：摻混高電子親和力(high electron affinity)材料於五環素中用以誘導產生電荷轉移複合態(charge transfer complex state)，以及混入碳六十(fullerene)產生給體/受體(donor/acceptor)異質介面與介面電場(interfacial electric field)。其中，高電子親和力材料的使用是為了討論單載子(single carrier)傳輸所產生的負磁電導響應，而碳六十本身具備良好的電子傳輸能力，主要用來討論光電流的磁電導響應。
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.
List of Figures XI
1 Introduction 1
1.1 Brief Introduction to Organic Semiconductors 1
1.2 Applications on Spintronics 1
1.3 Development of Organic Spintronics 2
1.3.1 Spin Transport in Organic Semiconductors 3
1.3.2 Magnetic Field Effects (MFEs) in Organic Semiconductors
22.214.171.124 Magnetoconductance (MC) in Organic Diodes 9
126.96.36.199 Magentoelectroluminescence (MEL) in Organic Light-emitting Diodes 10
188.8.131.52 Magnetic Photocurrent (MPC) in Organic Photovoltaics
1.4 Motivation 13
1.5 Scope of the Researches in this Thesis 14
2 Theoretical Background 17
2.1 Excited States Related to Organic MFEs 17
2.2 Excitonic Models in Organic MFEs 18
2.2.1 Magnetic Field Modulated Intersystem Crossing (ISC(B))
2.2.2 Singlet Fission and Triplet Fusion in Organic Molecules 19
2.2.3 MC Responses Relative to the Singlet Polaron Pair Dissociation
2.2.4 MC Responses Relative to the Triplet Exciton Charge Reaction
2.2.5 MC Responses Relative to the Triplet Polaron Interaction (TPI)
2.2.6 Magnetic Field Modulated Carrier Mobility or Carrier
2.3 Bipolaron Model in Single Carrier Devices 27
2.4 Internal Magnetic Fields and Interactions Related to Organic MFEs 29
2.4.1 Spin-orbital Interaction (SOI) 29
2.4.2 Hyperfine Interaction (HFI) 31
2.4.3 Zeeman Effect 32
2.4.4 Exchange Interaction 34
3 Experimental Techniques 35
3.1 Device Fabrication 35
3.2 Measurement Setups 36
3.3 Line Shape Measurements 38
4 Identifying MC Components in Pentacene-based Diodes 43
4.1 Introduction to MC Components in Organic Diodes 43
4.2 Variation on Electrical Properties and MC Responses by Blending F4-TCNQ into Pentacene 45
4.3 Reduction of the Conductance in Pentacene:F4-TCNQ Diodes 47
4.4 Sign Inversion of the MC Response in Charge Injection Regime and MC Components in Photocurrent Regime 48
4.5 Function of F4-TCNQ on the Low-field Negative MC Response in Charge Injection Regime 49
4.6 Summary 53
5 Manipulating Positive and Negative MC Components in Pentacene:fullerene Heterojunction Devices by Introducing Interfacial and Built-in Electric Fields 55
5.1 Introduction to Low-field Negative MC Components in Organic Diodes
5.2 Triplet Polaron Pair Charge Reaction Induced Low-field Negative MC Response 56
5.3 Interfacial Electric Field Modulated Positive and Negative MC Responses 58
5.4 Suppression of the Low-field Negative MC Response Induced by Triplet Polaron Pair Charge Reaction under a Stronger Built-in Electric Field 60
5.5 Summary 63
6 Modulations in Line Shapes of Magnetoconductance Curves for Diodes of Pentacene:fullerene Charge Transfer Complexes 65
6.1 Introduction to the Correlation between Line Shapes of the MC and Internal Magnetic Fields (or Interactions) 65
6.2 Line shape narrowing of the MC response in pentacene:fullerene BHJ devices 66
6.3 Built-in Electric Field Modulated Electron-hole Separation Distance as well as Exchange Interaction 68
6.4 Reduction on the Saturation Field of the Positive MC Response by Modulating the Exchange Interaction 72
6.5 Summary 74
7 Conclusions about this Thesis and Outlook 75
Curriculum Vitae 87
List of Publication 87
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