Organic Photocurrent Multiplication

Preface
Contents
1 Photocurrent Multiplication in Amorphous Silicon Carbide Films
1.1 Photocurrent Multiplication in Amorphous Silicon Carbide Films
1.1.1 Background
1.1.2 Motivation
1.1.3 Discovery of Photocurrent Multiplication
1.1.4 Spectral Dependence
1.1.5 Light Intensity Dependence
1.1.6 Transient Response
1.1.7 Interfacial Structure
1.1.8 Multiplication Mechanism
1.1.9 Temperature Dependence
1.2 Photo-Modulation of Photocurrent Multiplication
1.2.1 Basic Idea
1.2.2 Multiplication Suppression
1.2.3 Suppression Wavelength Region
1.2.4 Suppression Mechanism
1.3 Conclusion
References
2 Photocurrent Multiplication in Organic Semiconductor Films
2.1 Discovery
2.1.1 An Original Idea: Taking Out the Interface
2.2 Photocurrent Multiplication in n-Type Organic Semiconductor Films
2.2.1 Multiplication Rate Reaching Ten Thousand-Fold
2.2.2 Identifying the Interface
2.2.3 Transient Response
2.2.4 Multiplication Mechanism—Tunneling Injection of Electrons
2.2.5 Hole Traps—Origin of Multiplication
2.3 Photocurrent Multiplication in p-Type Organic Semiconductor Films
2.3.1 Multiplication Behavior
2.3.2 Influence of the Kinds of Electrode Metals
2.3.3 Specifying the Interface
2.3.4 Transient Response
2.3.5 Multiplication Mechanism—Tunneling Injection of Holes
2.4 Generality
2.5 Room-Temperature Multiplication
2.5.1 Multiplication Rate
2.5.2 Spectral Sensitivity
2.5.3 Influence of the Kinds of Metals
2.5.4 Multiplication Mechanism
2.6 Conclusion
References
3 Analyses of Multiplication Behaviors—Structural Trap
3.1 Multiplication Mechanism
3.2 Transient Response
3.2.1 Two Components of Photocurrent
3.2.2 First Component: Primary Photocurrent
3.2.3 Second Component: Tunneling Injection
3.2.4 Direct Tracing of Multiplication Process
3.2.5 Required Charges for Multiplication
3.3 Fowler-Nordheim (FN) Analyses
3.3.1 FN Plots of Multiplied Photocurrent [6, 7]
3.3.2 Estimation of Field Concentration Width (d)
3.3.3 Estimation of Trap Density
3.4 TSC Measurements
3.4.1 Energetic Depth of Traps
3.4.2 Trap Depth Versus Electric Field
3.5 Structural Trap Model
3.5.1 Experimental Findings
3.5.2 Model Proposal
3.6 Clues for the Origin of Structural Traps
3.6.1 Deposition Rate
3.6.2 Material Dependence
3.6.3 Relationship Between Trap Density and Onset Charge
3.7 Conclusions
References
4 Morphology of Organic/Metal Interface and Photocurrent Multiplication Behaviors
4.1 Photocurrent Multiplication and Structural Trap Model
4.2 Morphology of Organic Films and Multiplication Behaviors
4.2.1 Motivation
4.2.2 Three Perylene Pigments with Different Side Chains
4.2.3 Multiplication Rates
4.2.4 Morphology of Organic Films
4.2.5 Crystallinity of Organic Films
4.2.6 Multiplication Induction by Crystallization
4.2.7 Energetic Barrier Height at Organic/Metal Junctions
4.2.8 Explanation Based on Structural Trap Model
4.2.9 Lateral Spatial Separation of Trap Sites and Injection Sites
4.2.10 Identification of Interfacial Crystallization Effect
4.3 Metal Morphology Observed by SEM and Multiplication Behaviors
4.3.1 Motivation
4.3.2 Effects of Au Deposition Conditions on Perylene Pigment Film [15]
4.3.3 Resistivity-Heated Deposition Versus Sputtering Deposition
4.3.4 Deposition Rate
4.3.5 Morphology of Au Films Observed by SEM
4.3.6 Explanation by Structural Trap Model
4.4 Metal Morphology Observed by AFM and Multiplication Behaviors
4.4.1 Motivation
4.4.2 Morphology of Au Film on 1, 4, 5, 8-Naphthalenetetracarboxylic Dianhydride (NTCDA) [16]
4.4.3 Revised Structural Trap Model
4.4.4 Three-Dimensional Energy Structure
4.4.5 Effects of Metal Nanoparticle Size
4.4.6 Rear Surface of Au and In Films
4.5 Essential Factors in the Structural Trap Model
4.5.1 Spatial Gap
4.5.2 Molecular-Sized Roughness
4.5.3 True Nature of Molecular-Sized Roughness
4.6 Conclusion
References
5 Photocurrent Multiplication in Organic Single Crystals—Molecular Blind Alleys
5.1 Background
5.2 Motivation
5.3 Multiplication Behaviors of Organic Single Crystals
5.3.1 Naphthalene Tetracarboxylic Anhydride (NTCDA)
5.3.2 Single-Crystal Growth
5.3.3 Cell Fabrication
5.3.4 Multiplication Rate
5.3.5 Spectral Sensitivity
5.3.6 Response of Multiplied Photocurrent
5.3.7 Multiplication Mechanism
5.4 Real Nature of Structural Trap
5.4.1 Anomalous Characters of Traps at Organic/Metal Interface
5.4.2 Structural Trap Model Hypothesis
5.4.3 The Idea Behind the Hypothesis
5.4.4 Physically Existing Nanostructure Acting as Structural Trap
5.4.5 Spatial Gap
5.4.6 Molecular-Size Roughness
5.5 Molecular Blind Alley Model
5.5.1 Why Single Crystal?
5.5.2 Molecular Steps
5.5.3 Molecular Steps Acting as Molecular Blind Alleys
5.5.4 Molecular Step Density Versus Multiplication Rate
5.5.5 Molecular Kinks Acting as Molecular Blind Alley Sites
5.5.6 Deliberate Formation of Molecular Blind Alleys
5.5.7 Molecular Blind Alleys in Polycrystalline NTCDA Films
5.6 Future Perspective
5.6.1 High Resolved AFM Observation of Steps and Kinks
5.6.2 Multiplication Control via the Design of Molecular Blind Alleys
5.6.3 Design of Molecular Blind Alleys by Nanoimprint Lithography
5.7 Conclusion
References
6 Photocurrent Multiplication at Organic Heterojunctions
6.1 Background
6.2 Motivation
6.3 Photocurrent Multiplication at CuPc/Me-PTC Heterojunction
6.3.1 CuPc/Me-PTC Heterojunction
6.3.2 Multiplication Rate
6.3.3 Identification of Interface
6.3.4 Multiplication Suppression by Superimposed Light
6.3.5 Multiplication Mechanism
6.3.6 Suppression Mechanism
6.3.7 Structural Trap at Organic Heterojunction
6.4 Organic Heterojunctions Incorporating Hole and Electron Transporting Layers
6.4.1 Multiplication Rate
6.4.2 Action Spectra
6.4.3 Multiplication Mechanism
6.4.4 Structural Traps at HTL/n-OSC and ETL/p-OSC HTL (ETL)/n-OSC (p-OSC) Heterojunctions
6.5 Conclusion
References
7 High-Speed Response Devices
7.1 Background
7.1.1 Transient Response of Photocurrent Multiplication
7.1.2 Structural Trap Model
7.2 Numerical Calculation
7.2.1 Motivation
7.2.2 Simplified Model for Calculation
7.2.3 Time Development of Hole Accumulation
7.2.4 Surface Mobility
7.2.5 Keys of High-Speed Response
7.3 High-Speed Response Devices Having C60:CuPc Co-deposited Films
7.3.1 Motivation
7.3.2 Cell Structure
7.3.3 Photoresponse of Multiplied Photocurrent
7.3.4 Factors of High-Speed Response
7.4 High-Speed Devices Having Double-Layered Structure
7.4.1 Motivation
7.4.2 Double-Layered Structure
7.4.3 Transient Response
7.4.4 Film Morphology
7.4.5 Dark Current Suppression
7.4.6 Multiplication Rate Versus Applied Voltage
7.4.7 Transient Response Versus Applied Voltage
7.4.8 Factors of High-Speed Response
7.5 Requirements of Organic Multiplication-Type Photosensors
7.6 Conclusion
References
8 Effect of Oxygen and Water on Photocurrent Multiplication Rates
8.1 Background
8.2 Motivation
8.3 Effect of O2 Under Ex-Situ Conditions
8.3.1 Ex-Situ Conditions
8.3.2 p- and n-Type OSC Cells
8.3.3 p-DQ Cells
8.3.4 n-NTCDA Cells
8.3.5 O2 Enhancement Mechanism at p-DQ/Au Junction
8.3.6 O2 Suppression Mechanism at n-NTCDA/Au Junction
8.3.7 A Concept of Multiplied Gas Detection
8.4 Effects of H2O on Photocurrent Multiplication Observed Under In-Situ Conditions
8.4.1 Irreversible Change Due to H2O Adsorption
8.4.2 Motivation
8.4.3 In-Situ Conditions
8.4.4 Cells
8.4.5 Effect of O2 Under In-Situ Conditions
8.4.6 Effect of H2O
8.4.7 Enhancement/Suppression Mechanisms of O2 Under In-Situ Conditions
8.4.8 Multiplication Rate Enhancement Mechanism of H2O
8.4.9 Future Work
8.5 Conclusion
References
9 Multiplied Photocurrent Oscillation with Negative Resistance
9.1 Motivation
9.2 Cell Structure and Measurement Conditions
9.3 Photocurrent Oscillation and Voltage-Controlled Measurements
9.4 S-shaped Negative Resistance: Current-Controlled Measurements
9.5 Negative Resistance and Heterogeneity
9.6 Thickness Dependence
9.6.1 H2Pc Thickness
9.6.2 NTCDA Thickness
9.7 Heterogeneous Accumulation Model
9.8 Feedback Effect
9.9 Generality
9.10 Removal of H2Pc Roughness
9.10.1 Mechanical Pressing
9.10.2 H2Pc Roughness
9.10.3 Oscillation Behavior and H2Pc Roughness
9.10.4 Oscillation Disappearance
9.10.5 Conclusion
References
10 Avalanche Multiplication in Perylene Molecular Crystals
10.1 Background
10.2 Motivation
10.3 Photocurrent Multiplication Rate in Perylene Single Crystals
10.3.1 Perylene Single Crystals
10.3.2 Surface-Type Cells
10.3.3 Dark Current
10.3.4 Multiplication Characteristics
10.3.5 Photoresponse
10.3.6 Carrier-Traveling Distance
10.4 Impact Ionization
10.5 Ionization Rate
10.6 Mean Free Path
10.7 Conclusion
References
11 Progress in Organic Photocurrent Multiplication
11.1 Aim of This Chapter
11.2 Historical Outline
11.3 Blended Junction
11.4 Blocking Layer
11.4.1 Blocking Layer at Active Interface
11.4.2 Blocking Layer at Counter Interface
11.5 Carrier Trap
11.5.1 Uniform Trap
11.5.2 Interfacial Traps
11.5.3 Incomplete Percolation Traps
11.6 Universality of Multiplication Concept—Hybrid System
11.7 Multiplication in Charge Transfer Region
11.8 Essence of Progress
11.9 Organic Photocurrent Multiplication (OPM) Devices
11.10 Conclusion
References
12 Perspective on Organic Photocurrent Multiplication
12.1 Aim of This Chapter
12.2 Artificial Fabrication
12.2.1 Electrode Metal Having the Tip of the Needle Shape
12.2.2 Molecular Blind Alley Traps Made by Nanoimprint Lithography
12.2.3 Micrometer-Sized Thickness Heterogeneity Made by Nanoimprint Lithography
12.3 Direct Design of Molecular Blind Alley
12.3.1 Detailed Observation of Steps and Kinks
12.3.2 Control of Multiplication Behaviors by Designing Steps and Kinks
12.4 Insulating Layer
12.4.1 Metal/Insulator/Semiconductor (MIS) Junction
12.4.2 Spatial Separation
12.4.3 How to Break the Tunneling Balance Between Electrons and Holes?
12.5 Organic Avalanche Multiplication
12.6 Conclusion
References