Review pubs.acs.org/CR
Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale Jonathan Rivnay,† Stefan C. B. Mannsfeld,‡ Chad E. Miller,‡ Alberto Salleo,*,† and Michael F. Toney*,‡ †
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
‡
5.1. Mobility Enhancement and Crystalline Strain Variation in Sheared TIPS−Pentacene Films 5.2. Film Morphology of P3HT/PCBM BHJ and Influence on OPV Performance 6. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References
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CONTENTS 1. Introduction 2. Relevant Length Scales in Organic Semiconductors 2.1. 1 Å−10 nm: Chemistry, Local Molecular Packing, and Defects 2.2. 1−100 nm: Interfaces, Crystallite Size, and Texture 2.3. 10 nm−100 μm: Phase Segregation, Alignment, and Domains 3. Characterizing a Vast Range of Size Scales 3.1. Why X-rays? 3.2. X-ray Scattering and Absorption 3.3. Common Non-X-ray-Based Techniques 4. Quantitative X-ray Techniques and Relevance for Structure−Property Investigations 4.1. Wide-Angle X-ray Scattering (WAXS) 4.1.1. Technique Basics and Experimental Configurations 4.1.2. Molecular Crystal Packing 4.1.3. Crystallite Size and Disorder 4.1.4. Crystalline Texture and Degree of Crystallinity 4.1.5. In-Plane Orientation and Alignment 4.2. X-ray Reflectivity (XRR) 4.3. Small-Angle X-ray Scattering (SAXS) 4.4. Near-Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS) 4.5. Resonant Soft X-ray Scattering and Reflectivity (r-SoXS/R) 4.6. X-ray Microscopy 4.6.1. Compositional Mapping 4.6.2. Orientational and Order Mapping 5. Importance of Quantitative Analysis for Modeling, Simulation, and Experiment
1. INTRODUCTION Organic semiconductors offer the potential for large-area deposition on substrates of flexible or unique form factors and can be deposited at ambient conditions at low cost. They can be chemically tuned or functionalized for specific applications and have therefore been targeted as active materials for displays, solid-state lighting, radio frequency tags, sensors, logic circuits, and solar cells.1,2 Conjugated organic molecules arrange in complex multiphase systems that are so sensitive to processing, chemistry, and local environment that detailed characterization has proven elusive, often requiring simplified and qualitative morphological descriptions. Furthermore, structural assemblies at a broad range of length scales (Ångstroms to centimeters) must be simultaneously controlled to impart optimal optoelectronic performance. Weak bonding interactions and highly asymmetric building blocks introduce structural variations in local molecular packing and larger scale order, which directly affects thermal and mechanical properties and all aspects of optoelectronic processes. Despite the intrinsic challenges in both controlling and describing organic semiconductor materials, the field has experienced explosive growth, with numerous applications near or at commercialization, challenging established technologies such as those based on amorphous silicon.1,3 Structural characterization with X-ray techniques has proven successful in uncovering a broad range of microstructural and morphological features from sub-Ångstrom molecular chemistry to device-scale alignment. With advances in instrumentation, light source brightness, and analytical techniques, the quality of data and determinable quantitative information has allowed for some of the most accurate and rich descriptions of organic semiconductors to date. It has become clear however
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Received: March 14, 2012
© XXXX American Chemical Society
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dx.doi.org/10.1021/cr3001109 | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
Review
Figure 1. Common organic semiconductors discussed in this review. Small molecules: pentacene, triisopropyl (TIPS) pentacene, diindenoperylene (DIP), and phenyl-C61-butyric acid methyl ester (PCBM). Polymers: poly(3-hexyl thiophene) (P3HT), poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (PBTTT), poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT), and poly-(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine (PFB).
that the use of quantitative characterization techniques is the key to obtaining such precise description of the molecular structure and microstructure of the materials of interest. This wealth of information combined with electrical characterization and modeling allows for the establishment of general design rules to guide future rational design of materials and devices. Thin films are commonly employed in a variety of device architectures, with the organic material sometimes used as an insulating dielectric layer, or playing the active role as a semiconducting or conducting layer (when doped). These optoelectronic properties derive from the conjugated bonding structure of the molecules with the most successful implementations taking the form of fused ringed polymers or small molecules (Figure 1). The performance of such systems depends on the desired application and is therefore measured in numerous ways. Light-emitting devices are often described by their efficiency, brightness, and color purityimportant to different degrees depending on whether they are used for display or for solid-state lighting purposes. Solar cells performance is reported based on power conversion efficiency and lifetime, organic thin film transistors (OTFTs) on charge carrier mobility, on/off ratio, operating voltage, and sensors on sensitivity and specificity. All of these properties depend on the intrinsic electrical properties of the materials, as well as on how they are modulated by extrinsic electrical or chemical stimulation. At the most basic level, regardless of the device geometry, the same general principles apply in understanding performance. These devices involve the introduction or
excitation of charge carriers into the system (whether from an electrode, electrochemical doping, or optical absorption), transport within the active materials, and/or charge collection or recombination. To study the basic electrical properties of organic semiconductors, devices such as organic field effect transistors or diodes are often the preferred choice. They allow for the study of charge transport in the two principal transport directions (in the plane of the substrate and along the substrate normal) and, with extensive device testing and modeling, can provide important information about transport efficiency or charge carrier trapping. Importantly, when combined with quantitative microstructural characterization methods, they offer a convenient means to relate these basic electrical properties with structural information. Despite this convenient platform to study structure−property relations, individual materials optimization is often addressed on a case-by-case basis in a trialand-error fashion. The past decade has seen a surge in detailed structure− property relation studies focusing on the influence of molecular arrangements on optical and charge transfer events, as well as on the role nanoscale, mesoscale, and macroscale structures play in governing charge transport and percolation. Moreover, with an increase in in situ or simultaneous structural/ optoelectronic measurements, time-resolved studies of charge carrier dynamics, and advanced computational and simulation techniques, organic electronics has progressed toward complete and general quantitative descriptions of relevant systems, which B
dx.doi.org/10.1021/cr3001109 | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
Review
Figure 2. Size scales and relevant morphological features in organic electronic devices. Within each schematic, the square denotes the enlarged region preceding it. Each type of physical feature shown is assigned a range of length scales on the scale at center, described in the text. Although many materials systems and device types/architectures are often investigated, two examples are chosen to display the various morphologies and microstructures possible. Top row: a two-component blend film typical of a bulk heterojunction used in organic photovoltaic devices. The morphology of phase separation is invariably more complex than the schematic shown here, with impure/mixed phases coexisting with pure crystalline phases. Bottom row: single-component small-molecule semiconductor film used in OTFTs. Domains refer to regions composed of similarly oriented grains.
Table 1. Important Basic Morphological Features Commonly Discussed in Organic Semiconductors, Their Relevant Size Scale, X-Ray-Based Technique Used to Probe That Feature, And the Most Important Optoelectronic Properties That Are at Play for Each Feature morphological or physical feature
size scale
appropriate experimental X-ray technique
molecular packing/chemistry
