06/2015 ChemPhysChem, European in origin but international in scope, deals with all aspects of the overlapping areas between chemistry, physics, biology, and materials science. It is co-owned by Chemistry Publishing Society Europe (ChemPubSoc Europe) and published by Wiley-VCH. Contributions in ChemPhysChem cover a wide range of topics including atmospheric science, hard and soft matter, femtochemistry, nanoscience, complex biological systems, single-molecule research, http://www.chemphyschem.org clusters, colloids, catalysis, and surface science; experimental and theoretical studies can be published. ChemPhysChem publishes short Communications and long Articles, as well as Reviews, Minireviews, Highlights, Concepts, Essays, Book Reviews, and occasionally Conference Full text: Reports. Authors can submit manuscripts to ChemPhysChem online through our homepage (see left) by clicking on “Submit an Article” and following the simple instructions. wileyonlinelibrary.com
Most of the articles in this issue have already appeared online on wileyonlinelibrary.com. See www.chemphyschem.org under EarlyViewÒ.
DOI: 10.1002/cphc.201500229
Organic Electronics: Recent Developments John de Mello,*[a] John Anthony,*[b] and Soonil Lee*[c]
T
he increasing demand for large-scale displays, sensor arrays, lighting panels, and photovoltaic modules has generated a need for new high-performance electronic materials that can be readily deposited over large areas using low-cost ambient deposition techniques. While both inorganic and organic materials can potentially satisfy this need, organic materials have at[a] Prof. J. de Mello Centre for Plastic Electronics Department of Chemistry Imperial College London, Exhibition Road London SW7 2AZ (UK) E-mail: [email protected] [b] Prof. J. Anthony Department of Chemistry, University of Kentucky Lexington, Kentucky, 40506-0055 (USA) E-mail: [email protected] [c] Prof. S. Lee Department of Physics and Division of Energy Systems Research, Ajou University Suwon 443-749 (Korea) E-mail: [email protected]
ChemPhysChem 2015, 16, 1099 – 1100
tracted particular interest due to their favorable film-forming properties and the ability to control their physical characteristics through conventional wet chemistry.
T
he first commercial application of organic electronics has been displays based on organic light-emitting diodes (OLEDs). Since OLEDs are emissive devices, no backlight is required, which reduces energy consumption and decreases device complexity compared to conventional liquid crystal displays. OLED displays were first deployed in high-end mobile phones, but can now be found in a wide range of handsets, spanning the full price spectrum, and in televisions up to 77 inches in diagonal. The present generation of OLED displays are hybrid organic/inorganic devices, with the OLED pixels driven by a backplane of silicon transistors. In the longer term, the goal is to create ultra-thin, flexible displays made completely from organic components.
Organic electronics also holds considerable promise for solid-
state lighting and photovoltaics, where the possibility of using
1099
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Editorial conventional printing techniques to manufacture large-area organic devices could lead to major cost savings over conventional lighting and inorganic semiconductor technologies. A separate and growing area of interest is bioelectronics, where the soft nature of organic materials may enable the development of implantable or wearable sensors and devices that can augment or interact with biological systems.
C
ompared to OLED displays, these other application areas are still at an early stage of development, and significant advances are required on multiple fronts before commercial maturity is reached. Below we consider some of the key issues and challenges that are now facing the organic electronics community. Many creative solutions to these problems are proposed in the articles that follow.
T
he rational design of high-performance electronic materials has been a key goal of the organic electronics field since the earliest days, with researchers using a combination of chemical insight and predictive modeling techniques to guide the development of readily processed materials with good charge-transport properties and optimized optoelectronic characteristics. Beyond these basic physical requirements, however, there is a growing recognition that we must also start to address the commercial need for synthetic simplicity and environmentally sustainable manufacturing routes.
T
here is also a need for materials with improved tolerance for defects—whether introduced during synthesis, from processing, or as a consequence of environmental degradation during device operation. With the possible exception of microscale graphene and related materials, organic electronics rarely involves the use of ideal, defect-free materials. Typical thin-film organic materials are densely packed and highly disordered, with interactions between neighboring molecules greatly influencing the thin-film properties. Strategies for controlling molecular packing through the incorporation of conformationcontrolling side-groups or microstructure-influencing additives therefore play a critical role in ensuring the formation of films with the desired physical properties. The use of additives, in particular, has been widely reported in the literature, especially for bulk heterojunction solar cells, but it is only now that a proper understanding of their role is starting to emerge.
Until recently the primary efforts of the organic electronics
community were directed towards form and function—developing materials and processes that are competitive with or superior to current technologies. This focus on performance has had the unintended effect of pushing aside the development of truly air-stable materials; in consequence, stringent environmental encapsulation against water and oxygen ingress remains a necessary part of any organic device. For today’s OLED displays, glass is the encapsulant of choice owing to its modest cost and unrivalled impermeability to oxygen and water. However, for the next generation of printed devices on plastic substrates, flexible thin-film encapsulants must be de-
ChemPhysChem 2015, 16, 1099 – 1100
www.chemphyschem.org
veloped that offer comparable levels of protection and transparency in a fully conformable form-factor.
Beyond the materials themselves, new methods for depositing and processing high-quality films are still needed, partly to enable new device configurations and partly to accommodate the unusual fabrication challenges posed by soft organic materials. Coating methods within the organic electronics field can be crudely divided into vacuum deposition and solution deposition, the former being preferred for many small molecules and the latter for polymers (plus certain suitably functionalized small molecules). Within these two categories, however, plenty of scope exists for developing new deposition procedures that offer improved materials quality and uniformity. Because many organic semiconductors have highly anisotropic physical properties, deposition methods that can selectively induce specific molecular orientations in thin films are likely to be essential for achieving reproducible performance from mass-manufactured devices.
C
ontrolled methods for multilayer thin-film deposition are of particular importance: the ability to fabricate precisely defined stacks of homogeneous or inhomogeneous films is critical for establishing the optical field and charge-carrier distributions required for optimal device performance. While vacuum deposition has always permitted the fabrication of complex multilayer devices, solution processing is more restrictive in its application (since materials must be deposited from ‘orthogonal’ solvents that do not dissolve or disturb previously deposited layers). However, solution processing also opens up the prospect of innovative manufacturing routes that could lead to massive reductions in production costs. Spin-coating, doctor blading, slot-die coating, inkjet printing and gravure printing remain the dominant solution-based deposition methods at present. However, new processing methods that are better suited to multilayer fabrication and are capable of yielding improved phase structure and crystallinity are still needed for achieving the highest possible performance in manufactured devices.
T
he issues described above are just some of those with which the organic electronics community is currently grappling. It is hoped that this special issue of ChemPhysChem will provide an illuminating snap-shot of the state of the art in organic electronics, revealing some of the latest approaches that are being used to address the grand challenges of the field. With diverse contributions from researchers in the areas of chemistry, physics, materials science, device engineering and biology, the articles that follow highlight the inherently interdisciplinary nature of the field. Innovative strategies are reported for lowering materials costs, probing fundamental charge transport and loss processes, improving device efficiencies and stabilities, enhancing device scalability, and targeting new applications areas. As these papers make abundantly clear, impressive progress is being made on many fronts but, as always, numerous challenges remain.
1100
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Most of the articles in this issue have already appeared online on wileyonlinelibrary.com. See www.chemphyschem.org under EarlyViewÒ.
DOI: 10.1002/cphc.201500229
Organic Electronics: Recent Developments John de Mello,*[a] John Anthony,*[b] and Soonil Lee*[c]
T
he increasing demand for large-scale displays, sensor arrays, lighting panels, and photovoltaic modules has generated a need for new high-performance electronic materials that can be readily deposited over large areas using low-cost ambient deposition techniques. While both inorganic and organic materials can potentially satisfy this need, organic materials have at[a] Prof. J. de Mello Centre for Plastic Electronics Department of Chemistry Imperial College London, Exhibition Road London SW7 2AZ (UK) E-mail: [email protected] [b] Prof. J. Anthony Department of Chemistry, University of Kentucky Lexington, Kentucky, 40506-0055 (USA) E-mail: [email protected] [c] Prof. S. Lee Department of Physics and Division of Energy Systems Research, Ajou University Suwon 443-749 (Korea) E-mail: [email protected]
ChemPhysChem 2015, 16, 1099 – 1100
tracted particular interest due to their favorable film-forming properties and the ability to control their physical characteristics through conventional wet chemistry.
T
he first commercial application of organic electronics has been displays based on organic light-emitting diodes (OLEDs). Since OLEDs are emissive devices, no backlight is required, which reduces energy consumption and decreases device complexity compared to conventional liquid crystal displays. OLED displays were first deployed in high-end mobile phones, but can now be found in a wide range of handsets, spanning the full price spectrum, and in televisions up to 77 inches in diagonal. The present generation of OLED displays are hybrid organic/inorganic devices, with the OLED pixels driven by a backplane of silicon transistors. In the longer term, the goal is to create ultra-thin, flexible displays made completely from organic components.
Organic electronics also holds considerable promise for solid-
state lighting and photovoltaics, where the possibility of using
1099
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Editorial conventional printing techniques to manufacture large-area organic devices could lead to major cost savings over conventional lighting and inorganic semiconductor technologies. A separate and growing area of interest is bioelectronics, where the soft nature of organic materials may enable the development of implantable or wearable sensors and devices that can augment or interact with biological systems.
C
ompared to OLED displays, these other application areas are still at an early stage of development, and significant advances are required on multiple fronts before commercial maturity is reached. Below we consider some of the key issues and challenges that are now facing the organic electronics community. Many creative solutions to these problems are proposed in the articles that follow.
T
he rational design of high-performance electronic materials has been a key goal of the organic electronics field since the earliest days, with researchers using a combination of chemical insight and predictive modeling techniques to guide the development of readily processed materials with good charge-transport properties and optimized optoelectronic characteristics. Beyond these basic physical requirements, however, there is a growing recognition that we must also start to address the commercial need for synthetic simplicity and environmentally sustainable manufacturing routes.
T
here is also a need for materials with improved tolerance for defects—whether introduced during synthesis, from processing, or as a consequence of environmental degradation during device operation. With the possible exception of microscale graphene and related materials, organic electronics rarely involves the use of ideal, defect-free materials. Typical thin-film organic materials are densely packed and highly disordered, with interactions between neighboring molecules greatly influencing the thin-film properties. Strategies for controlling molecular packing through the incorporation of conformationcontrolling side-groups or microstructure-influencing additives therefore play a critical role in ensuring the formation of films with the desired physical properties. The use of additives, in particular, has been widely reported in the literature, especially for bulk heterojunction solar cells, but it is only now that a proper understanding of their role is starting to emerge.
Until recently the primary efforts of the organic electronics
community were directed towards form and function—developing materials and processes that are competitive with or superior to current technologies. This focus on performance has had the unintended effect of pushing aside the development of truly air-stable materials; in consequence, stringent environmental encapsulation against water and oxygen ingress remains a necessary part of any organic device. For today’s OLED displays, glass is the encapsulant of choice owing to its modest cost and unrivalled impermeability to oxygen and water. However, for the next generation of printed devices on plastic substrates, flexible thin-film encapsulants must be de-
ChemPhysChem 2015, 16, 1099 – 1100
www.chemphyschem.org
veloped that offer comparable levels of protection and transparency in a fully conformable form-factor.
Beyond the materials themselves, new methods for depositing and processing high-quality films are still needed, partly to enable new device configurations and partly to accommodate the unusual fabrication challenges posed by soft organic materials. Coating methods within the organic electronics field can be crudely divided into vacuum deposition and solution deposition, the former being preferred for many small molecules and the latter for polymers (plus certain suitably functionalized small molecules). Within these two categories, however, plenty of scope exists for developing new deposition procedures that offer improved materials quality and uniformity. Because many organic semiconductors have highly anisotropic physical properties, deposition methods that can selectively induce specific molecular orientations in thin films are likely to be essential for achieving reproducible performance from mass-manufactured devices.
C
ontrolled methods for multilayer thin-film deposition are of particular importance: the ability to fabricate precisely defined stacks of homogeneous or inhomogeneous films is critical for establishing the optical field and charge-carrier distributions required for optimal device performance. While vacuum deposition has always permitted the fabrication of complex multilayer devices, solution processing is more restrictive in its application (since materials must be deposited from ‘orthogonal’ solvents that do not dissolve or disturb previously deposited layers). However, solution processing also opens up the prospect of innovative manufacturing routes that could lead to massive reductions in production costs. Spin-coating, doctor blading, slot-die coating, inkjet printing and gravure printing remain the dominant solution-based deposition methods at present. However, new processing methods that are better suited to multilayer fabrication and are capable of yielding improved phase structure and crystallinity are still needed for achieving the highest possible performance in manufactured devices.
T
he issues described above are just some of those with which the organic electronics community is currently grappling. It is hoped that this special issue of ChemPhysChem will provide an illuminating snap-shot of the state of the art in organic electronics, revealing some of the latest approaches that are being used to address the grand challenges of the field. With diverse contributions from researchers in the areas of chemistry, physics, materials science, device engineering and biology, the articles that follow highlight the inherently interdisciplinary nature of the field. Innovative strategies are reported for lowering materials costs, probing fundamental charge transport and loss processes, improving device efficiencies and stabilities, enhancing device scalability, and targeting new applications areas. As these papers make abundantly clear, impressive progress is being made on many fronts but, as always, numerous challenges remain.
1100
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
