| POLYMERS Vol.73 No.2 |
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COVER STORY
Optoelectronic Properties of Conjugated Polymers |
| COVER STORY: Highlight Reviews |
| Recent Progress in Polymer Solar Cells | Hideo OHKITA |
| <Abstract> The power conversion efficiency of polymer solar cells was less than 1% when they were first developed in the 1990s. Nonetheless, their efficiency has been steadily improved by subsequent developments of new materials, and finally more than 20% efficiency has been reported very recently. In this paper, I will review the history of efficiency improvement, summarize challenging issues necessary for further efficiency improvement in terms of photovoltaic parameters such as short-circuit current density, open-circuit voltage, and fill factor. Keywords: Polymer Solar Cell / Conjugated Polymer / Fullerene / Nonfullerene Acceptor / Ternary Blend / Short-Circuit Current Density / Open-Circuit Voltage / Fill Factor |
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| Room-Temperature Liquid Conjugated Polymer Electrets | Takashi NAKANISHI |
| <Abstract> A polymer design strategy that introduces flexible yet bulky and insulating branched alkyl side chains into the main chain backbone of rigid π-conjugated polymers can produce room-temperature liquid-conjugated polymers. Herein, we will provide an overview of the viscoelasticity, phase separation control, and optoelectronic functions of liquefied conjugated polymers, in particular, focusing on application development examples as electrets that stably retain electrostatically charged ion species injected through charging treatment. Keywords: Conjugated Polymers / Room-Temperature Liquids / Viscoelastic Polymers / Electrets / Mechanoelectric Generators / Mechanochromic Fluorescence / Compatibility Control |
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| Orientation Control of Semiconducting Polymers in Organic Solar Cells | Keisuke TAJIMA |
| <Abstract> In the quest to enhance the performance of electronic devices using organic thin films, the control of molecular orientation in organic semiconductors is an indispensable challenge. The problem becomes complex when considering not only the molecular structure, but also the thin film fabrication process and the interactions between molecules and interfaces. This article primarily provides an overview of the current state of molecular orientation control within thin films in organic solar cells. First, we discuss how orientation affects the primary processes in organic solar cells. Secondly, the principle of orientation control will be reviewed. Keywords: Organic Thin-Films / Molecular Orientation / Organic Electronics / Organic Solar Cells / Semiconducting Polymers |
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| COVER STORY: Topics and Products |
| Organic Photodiode Materials for Near-Infrared and Short-Wave Infrared Light Detection | Hidenori NAKAYAMA |
| <Abstract> We report on new materials for solution-processable organic photodiodes (OPDs) for near-infrared (NIR) and short-wave infrared (SWIR) detection compatible with CMOS image sensors. We selected a conventional structure (p-i-n) with a polymeric hole transport layer (HTL) that we originally made for organic light-emitting diodes. The HTL is free from acids and dopants, contributing to excellent device stability. The average roughness of the HTL on a 8-inch SiO2/Si wafer is less than 3 nm. For infrared sensing materials in the active layer, we developed novel non-fullerene acceptors (NFAs). An OPD targeting 940 nm achieved an external quantum yield (EQE) of 80% at the wavelength with a dark current in the order of 1×10－6 mA/cm－2 at －5 V. Another OPD targeting SWIR achieved EQE of 45％ at 1,100 nm with a dark current of 4×10－5 mA/cm－2 at －5 V. Novel NFAs targeting even longer wavelengths are under development. Keywords: Conjugated Polymer / Non-Fullerene Acceptor / Photodiode / Bulk Heterojunction / Organic Electronics / Image Sensor |
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| Stretchable Polymer Semiconductors Utilizing Hydrogen Bonding Interactions | Minoru ASHIZAWA |
| <Abstract> Conjugation breakers (CBs) with different H-bonding chemistries and linker flexibilities are designed and incorporated into a diketopyrrolopyrrole (DPP)-based conjugated polymer backbone. The impact of bonding interactions on polymer semiconductor morphology, mechanical properties, and electrical performance are systematically investigated. We observe that CBs with an H-bonding self-association constant ＞0.7 or a denser packing tendency are able to induce higher polymer chain aggregation and crystallinity in as-casted thin films, resulting in a higher modulus and crack on-set strain. In addition, we evaluate their electrical performances under mechanical strain in fully stretchable field-effect transistors. The polymer with the highest crack on-set strain has the least degradation in mobility as a function of strain. Overall, these observations suggest that we can finely tune the mechanical properties of a polymer semiconductor by modulating intermolecular interactions, such as H-bonding chemistry and linker flexibility. Such understanding provides molecular design guidelines for future stretchable semiconductors. Keywords: Organic Electronics / Stretchable Polymer Semiconductor / Hydrogen Bonding / Conjugation Breaker / Field Effect Transistor / Stretchable Transistor |
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| Size- and Surface-Controlled Synthesis of Conjugated Polymer Nanoparticles and its Applications for Biomedical Field | Seiichi OHTA |
| <Abstract> Conjugated polymer nanoparticles (called Pdots) have attracted attention as a new class of fluorescent material for bioimaging and biosensing. Thanks to the spread of the conjugated π electron system, they exhibit strong fluorescence with tunable color. In this article, we introduce the control method for the size and surface of Pdots and their applications for the biomedical field. Pdots were synthesized by the nanoprecipitation method, in which conjugated polymer and poly(styrene-co-maleic anhydride) dissolved in THF were dropped into water under sonication. By adjusting pH and salt concentration in the aqueous phase, we achieved size control of the synthesized Pdots in the range from 20 to 200 nm. Furthermore, since carboxylic groups are exposed on the surface of the synthesized Pdots, various functional molecules can be modified to the Pdot surface via simple carbodiimide chemistry. We demonstrated that by using DNA linker-mediated assembly of single strand DNA-modified Pdots, sensitive detection of cell surface markers can be achieved. Keywords: Conjugated Polymer / Nanoparticles / Bioimaging / Size Control / Surface Modification |
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| Polymer Transistors for Chemical Sensing | Yui SASAKI, Tsuyoshi MINAMI |
| <Abstract> The attention to the development of chemical sensors has increased owing to their beneficial abilities to visualize invisible chemical species related to diagnosis, environmental assessments, food analysis, etc. However, chemical sensors for real-sample analysis have not been fully established, in contrast to physical sensors. Hence, this topic summarizes methodologies using polymer materials for both receptor and transducer units in chemical sensor devices. Among sensor platforms, polymer field-effect transistors made of solution-processable π-conjugated semiconductive materials have been employed as transducers, because of their amplification abilities allowing highly sensitive detection in real-sample analysis. Moreover, molecularly imprinted polymers (MIPs) as molecular recognition materials provide three-dimensional recognition networks against specific analytes, which contributes to selective detection even in the presence of interferents. Indeed, a MIP-integrated polymer transistor has shown favorable sensor responses to salivary cortisol from the perspective of sensitivity, selectivity, and applicability to real-sample analysis. The demonstration revealed the potential of polymer materials as crucial components of chemical sensor devices. Keywords: Chemical Sensor / Polymer Transistor / Molecular Recognition / Real-Sample Analysis / Cortisol / Molecularly Imprinted Polymer |
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| Polymer Science and I: A Personal Account |
| Contribution to Society with Chemistry | Yu-I HSU |
| <Abstract> In this article, I describe why I chose chemistry, and the research what I did and what I am doing. Recently, I have been developing “salt-responsive” polymeric materials that are stable in freshwater but destabilize and degrade in seawater. These materials are expected to be next-generation marine biodegradable plastics. |
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| Front-Line Polymer Science |
| Structural Analysis of Polymer Crystals by Electron Microscopy | Shusuke KANOMI, Hiroshi JINNAI |
| <Abstract> The typical morphology of polymer crystals is plate-like crystals (lamellar crystals) with a thickness of about 10 nm. However, their original structure and structure formation mechanism still need to be fully understood. Since Keller revealed the structure of polyethylene single crystals, it was suggested that the folding polymer chains tilt with respect to the normal of the lamellar crystal surface （chain tilt）. The chain tilt inside the lamellar crystals has remained ambiguous due to the lack of proper analytical techniques. Nanodiffraction imaging （NDI）, a state-of-the-art electron microscopy technique, makes it possible to image the polymer chains tilting inside the lamellae. Such knowledge advances a deeper understanding of the thermodynamics of polymer chains in lamellar crystals. Keywords: Semicrystalline Polymer / Lamellar Crystal / Crystal Structure / Transmission Electron Microscopy / Electron Diffraction / Nanodiffraction Imaging |
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