There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This Review summarizes recent progress in the development of polymer solar cells. It covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.

Polymer solar cells

Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Functionalized acenes and heteroacenes for organic electronics.

Charge-Transfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture

Polythiophenes are one of the most important classes of conjugated polymers, with a wide range of applications, such as conducting films, electrochromics, and field-effect transistors, which have been the subject of a number of older and more recent reviews. Much less attention has been paid to the light-emitting properties of this class of materials, although their unique properties present a number of opportunities unavailable from more popular polymeric light emitters such as polyfluorene or poly(p-phenylene vinylene). This article reviews achievements to date in applications of thiophene-based polymers and oligomers as electroluminescent materials. We demonstrate the basic principles of controlling the optical properties of polythiophenes through structural modifications and review the most important light-emitting materials created from thiophene derivatives. Special attention is paid to consequences of structural variations on the performance of light-emitting diodes fabricated with these materials.

Light-Emitting Polythiophenes†

Preface Editors Contributors Organic Light-Emitting Devices and Their Applications for Flat-Panel Displays Qing Wang, Gang Yu, and Jian Wang Light-Emitting Polymers Shidi Xun, Dmitrii F. Perepichka, Igor F. Perepichka, Hong Meng, and Fred Wudl Small-Molecule Materials for Organic Light-Emitting Diodes Shijian Su, Norman Herron, and Hong Meng Phosphorescent Polymer Light-Emitting Diodes Xu Han, Dmitrii F. Perepichka, Hong Meng, and Mang-Mang Ling Polarized Light Emission from Organic Light-Emitting Diodes Daniel Steiger and Christoph Weder Transparent Electrode for OLEDs Furong Zhu Vapor-Deposited Organic Light-Emitting Devices Michael S. Weaver Material Challenges for Flexible OLED Displays Peter F. Carcia Oxide Thin-Film Transistors for Active Matrix OLEDs Linfeng Lan, Weijing Wu, and Lei Wang Microstructural Characterization and Performance Measurements Zhigang Rick Li and Jeff Meth Index

Organic Light-Emitting Materials and Devices

[Liu, X. G.] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. Chinese Acad Sci, Int Ctr Mat Phys, Shenyang 110016, Peoples R China.;Liu, XG (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;liuxg@imr.ac.cn

Microwave-absorption properties of ZnO-coated iron nanocapsules

A new method of fabricating small metal-molecule-metal junctions is developed, approaching the single-molecule limit. The conductance of different conjugated molecules in a broad temperature, source-drain, and gate voltage regime is reported. At low temperature, all investigated molecules display sharp conductance steps periodic in source-drain voltage. The position of these steps can be controlled by a gate potential. The spacing corresponds to the energy of the lowest molecular vibrations. These results show that the low-bias conductance of molecules is dominated by resonant tunneling through coupled electronic and vibration levels.

Conductance of small molecular junctions.

Bright and stable blue emitters with narrow full-width at half-maxima are particularly desirable for applications in television displays and related technologies. Here, this study shows that doping aluminum (Al3+) ion into CsPbBr3 nanocrystals (NCs) using AlBr3 can afford lead-halide perovskites NCs with stable blue photoluminescence. First, theoretical and experimental analyses reveal that the extended band gap and quantum confinement effect of elongated shape give rise to the desirable blueshifted emission. Second, the aluminum ion incorporation path is rationalized qualitatively by invoking fundamental considerations about binding relations in AlBr3 and its dimer. Finally, the absence of anion-exchange effect is corroborated when green CsPbBr3 and blue Al:CsPbBr3 NCs are mixed. Combinations of the above two NCs with red-emitting CdSe@ZnS NCs result in UV-pumped white light-emitting diodes (LED) with an National Television System Committee (NTSC) value of 116% and ITU-R Recommendation B.T. 2020 (Rec. 2020) of 87%. The color coordinates of the white LED are optimized at (0.32, 0.34) in CIE 1931. The results suggest that low-cost, earth-abundant, solution-processable Al-doped perovskite NCs can be promising candidate materials for blue down-conversion layer in backlit displays.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700335

Hong Meng

Papers

Light-Emitting Polythiophenes†

Organic Light-Emitting Materials and Devices

Microwave-absorption properties of ZnO-coated iron nanocapsules

Conductance of small molecular junctions.

Aluminum-Doped Cesium Lead Bromide Perovskite Nanocrystals with Stable Blue Photoluminescence Used for Display Backlight.