Organic Solar Cells: Energetic and Nanostructural Design

Preface
Contents
1 Basic Principles of Modern Organic Solar Cells
	1.1 Background
		1.1.1 Current Status of Solar Cells: c-Si
		1.1.2 Chasers of c-Si
		1.1.3 Motivation for Organic Solar Cell
		1.1.4 History
	1.2 Principles
		1.2.1 Photocurrent
		1.2.2 Photovoltage
		1.2.3 Cell Design
	1.3 Conclusions
	References
2 A Path to the Blended Junction
	2.1 Shock of Two-Layer Solar Cell
		2.1.1 Motivation
	2.2 Tracking During Interface Making
		2.2.1 Expectation
		2.2.2 Tracking
		2.2.3 Key: Direct PTC/Pc Molecular Contact
	2.3 Blended Junction
		2.3.1 Expectation
		2.3.2 Overcoming Experimental Hurdles
		2.3.3 Three-Layer Cell
		2.3.4 p-i-n Junction
		2.3.5 Inorganic/Organic Hybrid p-i-n Junction
		2.3.6 Inevitably Conceived Issue: Percolation
		2.3.7 Carrier Generation Model
	2.4 Conclusion
	References
3 Percolation Toward Lateral Junctions
	3.1 Motivation
	3.2 Percolation
		3.2.1 Substrate Temperature Control
		3.2.2 Crystalline–Amorphous Nanocomposite
		3.2.3 p-i-n Cells
	3.3 Vertical Junctions
		3.3.1 Concept
		3.3.2 Cell Fabrication
		3.3.3 Layer Width
		3.3.4 Exciton Diffusion Length
	3.4 Lateral Junctions
		3.4.1 Lateral Electron Transport in Organic Single Crystals
		3.4.2 Lateral Junctions
		3.4.3 Carrier Pathway Unit Cells
		3.4.4 Carrier Ranges
		3.4.5 Lateral Alternating Multilayered Junction
	3.5 Conclusion
	References
4 OPV with a Crystalline Organic Pigment Active Layer Up to 10 μm
	4.1 Introduction: Optical Design of Thickness of OPV and Its Photoelectric Conversion Layer
	4.2 Change in Relation Between Film Thickness of Active Layer and Solar Cell Characteristics of OPV by Crystallization
		4.2.1 Fabrication of Pigmented OPV Device with Crystallized Active Layer
		4.2.2 Relationship Between Active Layer Thickness and Solar Cell Characteristics
		4.2.3 Film Morphology and Crystallinity of 10-µm Crystallized OPV
		4.2.4 Relationship Between Active Layer Thickness and Absorption/Solar Cell Characteristics
	4.3 Effect of Buffer Layer and Antireflection Film
		4.3.1 Buffer Layer Selection for Stable Comparison
		4.3.2 Effect of Antireflection Film
	4.4 Summary and Outlook
	References
5 Polymer Solar Cells: Development of π-Conjugated Polymers with Controlled Energetics and Structural Orders
	5.1 Introduction
	5.2 Thiazolothiazole-Based Polymers
		5.2.1 Crystallinity of Thiophene–Thiazolothiazole Donor–Acceptor Polymers
		5.2.2 Control of Backbone Orientation in Thiophene–Thiazolothiazole Polymers
		5.2.3 Impact of Side Chain Topology in Solar Cell Performance
		5.2.4 Impact of Backbone Orientation in Solar Cell Performance
		5.2.5 Distribution of Backbone Orientation Through Film Thickness
		5.2.6 Correlation Between Thickness Dependence in FF and Backbone Orientation
		5.2.7 Summary
	5.3 Naphthobischalcogenadiazole-Based Polymers
		5.3.1 Naphthobischalcogenadiazoles
		5.3.2 Quaterthiophene–Naphthobisthiadiazole Polymer (PNTz4T): Comparison with Benzothiadiazole Analog
		5.3.3 Molecular Design of Polymers Using NTz and NOz Toward Reduction of Photon Energy Loss
		5.3.4 Summary
	5.4 Conclusion and Outlook
	References
6 Charge Carrier Dynamics in Polymer Solar Cells
	6.1 Introduction
	6.2 Photovoltaic Conversion in Polymer Solar Cells
	6.3 Optoelectronic Measurements
		6.3.1 Transient Absorption Spectroscopy
		6.3.2 Transient Photovoltage and Photocurrent Measurements
	6.4 Charge Generation Dynamics
		6.4.1 Amorphous Polymer Solar Cells
		6.4.2 Crystalline Polymer Solar Cells
		6.4.3 Polymer Solar Cells with Small Photon Energy Loss
		6.4.4 Ternary Blend Polymer Solar Cells
	6.5 Charge Recombination Dynamics
		6.5.1 Bimolecular Recombination Dynamics
		6.5.2 Charge Carrier Lifetime
	6.6 Challenging Issues and Concluding Remarks
	References
7 First-Principles Investigations of Electronically Excited States in Organic Semiconductors
	7.1 Introduction
	7.2 Effects of Molecular Aggregation on Electronic States: Polarization and Delocalization
		7.2.1 Ionization Potential and Electron Affinity from the Gas to Solid Phase
		7.2.2 Polarization Energies of Excited States
		7.2.3 Excited States in Molecular Aggregates
	7.3 Electronic Structure Calculations for Optoelectronic Properties
		7.3.1 Many-Body Green’s Function Method Within GW Approximation
		7.3.2 Benchmark Results for Exciton Binding Energy
		7.3.3 Fragment Molecular Orbital Method
	7.4 Pentacene Clusters
		7.4.1 Polarization Energies of Charged and Excited States
		7.4.2 Delocalized Electronic States
	7.5 Pentacene/C60 Planar Heterojunction
	7.6 Concluding Remarks
	References
8 Open-Circuit Voltage in Organic Solar Cells
	8.1 Introduction
	8.2 Theoretical Background of Open-Circuit Voltage in Organic Solar Cells
		8.2.1 Photoconversion Mechanisms in Organic Solar Cells
		8.2.2 Empirical Understanding of Open-Circuit Voltage in Organic Solar Cells
		8.2.3 Equivalent Circuit Model
		8.2.4 Detailed Balance Theory
	8.3 Increasing Open-Circuit Voltage in Organic Solar Cells by Modifying Donor/Acceptor Interface
		8.3.1 Effect of Energy-Level Alignment at Donor/Acceptor Interface on Open-Circuit Voltage in Organic Solar Cells
		8.3.2 Doping for Controlling Open-Circuit Voltage in Organic Solar Cells
		8.3.3 Organic pn Homojunction Solar Cells
		8.3.4 Enhancement of Open-Circuit Voltage in Organic Solar Cells by Monolayer Cascade Energy Structure at Donor/Acceptor Interface
		8.3.5 Importance of Donor/Acceptor Interfacial Crystallinity to Reduce Open-Circuit Voltage Loss in Organic Solar Cells
		8.3.6 Controlling Energy Level of Acceptor Dye Molecule to Reduce Open-Circuit Voltage Loss in Organic Solar Cells
	8.4 Conclusion and Future Prospects
	References
9 Parts-Per-Million-Level Doping Effects and Organic Solar Cells Having Doping-Based Junctions
	9.1 Introduction
		9.1.1 Motivation
		9.1.2 History
	9.2 Principles
		9.2.1 Charge Transfer
		9.2.2 Ionization
	9.3 ppm-Level Doping Method
		9.3.1 Purification
		9.3.2 Ultra-Slow Deposition at 10–9 nm s−1
	9.4 pn-Control by Doping
		9.4.1 Single Films
		9.4.2 Blended Films
	9.5 Doping Efficiency
		9.5.1 Band-Mapping
		9.5.2 Low Doping Efficiency in Single Films
	9.6 Junction Formation
		9.6.1 Ohmic Junctions
		9.6.2 Homojunctions
		9.6.3 Band-Mapping of Organic pn-Homojunctions
	9.7 Doping Sensitization in Blended Films
	9.8 ppm-Level Doping Effects
		9.8.1 Organic Semiconductor Films
	9.9 Bulk-Doped Organic Single Crystals
		9.9.1 Doped Homoepitaxy
		9.9.2 Hall Effects
		9.9.3 High Ionization Rate
		9.9.4 Scattering
		9.9.5 Doping-Induced Trap Formation
		9.9.6 Future Prospects
		9.9.7 Conclusion
	References
10 Proposal for Future Organic Solar Cells
	10.1 Is Blended Junction Necessary?
		10.1.1 Bipolar Band-Conductive Organic Semiconductor-Exciton Dissociation Using Single Organic Semiconductor
		10.1.2 Long Exciton Diffusion Length—Doped Organic Single-Crystal Solar Cell
		10.1.3 Large Dielectric Constant—Organic/Inorganic Hybrid Cell
	10.2 Advanced Lateral Junctions—Beyond Shockley–Queisser Limit
	10.3 Recombination Suppression
		10.3.1 Non-radiative Recombination Dissipated to Molecular Vibration
		10.3.2 Non-radiative Recombination Via Carrier Traps
	10.4 Conclusion
	References