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.

The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Organic electronics are broadly anticipated to impact the development of flexible thin-film device technologies. Among these, solution-processable π-conjugated polymers and small molecules are proving particularly promising in field-effect transistors and bulk heterojunction solar cells. This Perspective analyzes some of the most exciting strategies recently suggested in the design and structural organization of π-functional materials for transistor and solar cell applications. Emphasis is placed on the interplay between molecular structure, self-assembling properties, nanoscale and mesoscale ordering, and device efficiency parameters. A critical look at the various approaches used to optimize both materials and device performance is provided to assist in the identification of new directions and further advances.

Molecular design and ordering effects in π-functional materials for transistor and solar cell applications.

We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene−diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm2 V−1 s−1, which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC71BM gave a power conversion efficiency of 5.4%.

/pdf/thieno-3-2-b-thiophene-diketopyrrolopyrrole-containing-2m3u5pjo25.pdf

Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices

Microelectronic circuits/arrays produced via high-speed printing instead of traditional photolithographic processes offer an appealing approach to creating the long-sought after, low-cost, large-area flexible electronics. Foremost among critical enablers to propel this paradigm shift in manufacturing is a stable, solution-processable, high-performance semiconductor for printing functionally capable thin-film transistors — fundamental building blocks of microelectronics. We report herein the processing and optimisation of solution-processable polymer semiconductors for thin-film transistors, demonstrating very high field-effect mobility, high on/off ratio, and excellent shelf-life and operating stabilities under ambient conditions. Exceptionally high-gain inverters and functional ring oscillator devices on flexible substrates have been demonstrated. This optimised polymer semiconductor represents a significant progress in semiconductor development, dispelling prevalent skepticism surrounding practical usability of organic semiconductors for high-performance microelectronic devices, opening up application opportunities hitherto functionally or economically inaccessible with silicon technologies, and providing an excellent structural framework for fundamental studies of charge transport in organic systems.

https://dr.ntu.edu.sg/bitstream/10220/10007/1/14.%20A%20stable%20solution-processed%20polymer%20semiconductor%20with%20record%20high-mobility.pdf

A stable solution-processed polymer semiconductor with record high-mobility for printed transistors

Plastic electronics based on organic semiconductors attract considerable attention due to the promise of low cost, lightweight, and fl exible large area electronic devices such as organic fi eld effect transistors (OFETs), [ 1 ] organic photovoltaics (OPVs), [ 2 ] electrochromic devices, [ 3 ] and organic light emitting diodes (OLEDs). [ 4 ] Of the semiconductors, regioregular poly(3alkylthiophenes) rr-P3ATs are the most studied and researched material. [ 5 ] This is predominately due to the development of Grignard Metathesis (GRIM) that provides the facile synthesis of structurally pure rr-P3AT with narrow polydisperities. [ 6 ] The pristine rr-P3ATs are known to form into a nanofi brillar morphology promoted by the pi-stacked polymer backbone and lamellar packing of the alkyl side chain. It is postulated that this morphology is the reason for its high charge mobility ( ∼ 0.1 cm 2 /Vs). [ 7 ] The nanofi brillar structures act as a charge transport pathway where holes can hop from one polymer chain to another down the nanofi bril. Unfortunately, these nanofi brillar structures have some major vulnerabilities that can be detrimental to charge transport such as large interfi brillar grain boundaries that trap charges and variable morphologies that are diffi cult to reproduce, both resulting in reduced charge mobilities. [ 8 ]

Transistor paint: high mobilities in small bandgap polymer semiconductor based on the strong acceptor, diketopyrrolopyrrole and strong donor, dithienopyrrole.

New solution processable 4-(2-hexyldecan)-4H-bisthieno[2,3-d:3',2'-b]pyrrole and 4,4'-dialkyl-2,2'-bithiazole-based copolymers (PBTzDTPs) are synthesized with excellent FET performance. These novel copolymers have considerable potential in printable electronics as they have high charge carrier mobilities, excellent air stability, good solution processibility, and no requirement for post-deposition thermal annealing, all requirements for this field of application. The thin film transistors fabricated from PBTzDTPs achieve field effect mobilities as high as 0.14 cm 2 V -1 s -1 with current on/off ratios up to 10 6 without thermal annealing. In addition, the devices exhibit stable performance in air, showing no significant degradation over 60 days. Moreover, the polymers described here provide an excellent example of the systems in which higher mobility performance does not require higher crystalline, long-range ordered structures. Such a system appears to be particularly promising for rapid fabrication techniques, where kinetic conditions usually prevent the development of long-range order.

Transistor Paint: Environmentally Stable N-alkyldithienopyrrole and Bithiazole-Based Copolymer Thin-Film Transistors Show Reproducible High Mobilities without Annealing

Eight novel polymer materials for transistor applications were prepared based on the fused ring building block selenolo[3,2-b]thiophene. The molecular structure of selenolo[3,2-b]thiophene was determined by single-crystal X-ray diffraction and was further modified for use as a monomer during the Stille synthesis of various copolymers containing substituted and unsubstituted thiophenes, thiazoles, dithienopyrroles (DTP) and benzothiadiazole subunits. The resultant polymers were analyzed for UV-Vis absorption and show a significant red shift when compared to regioregular poly(3-hexylthiophenes) (P3HT), attributed to the increased planarity and conjugation of the polymer backbone. Cyclic voltammetry experiments on the new polymers show that almost all of the new structures have a higher oxidation potential compared to P3HT, making them more stable toward oxidative doping under ambient atmospheres. The polymers were also evaluated for their electrical properties through their performance as the active layer in field effect transistors. The polymers showed relatively high mobility values, of which the highest observed value is 0.11 cm2 V−1s−1, with on/off ratios of about 105.

Sarada P. Mishra

Papers

Transistor paint: high mobilities in small bandgap polymer semiconductor based on the strong acceptor, diketopyrrolopyrrole and strong donor, dithienopyrrole.

Transistor Paint: Environmentally Stable N-alkyldithienopyrrole and Bithiazole-Based Copolymer Thin-Film Transistors Show Reproducible High Mobilities without Annealing

Mixed selenium-sulfur fused ring systems as building blocks for novel polymers used in field effect transistors