Polymer semiconductors, which are composed of a carbon-based π conjugated backbone, have been studied for several decades as active layers of multifarious organic electronic devices. They combine the advantages of the electrical conductivity of (semi-)conductors and the mechanical behavior of plastics.Therefore, they are poised to become one of the future's most promising modulable electronic materials. The properties of polymer semiconductors depend on their chemical structures and the multilevel microstructures in solid states. Despite great efforts, a clear picture of intrinsic chemical structures, microstructures, and device performancesis still far from being produced.
Recently, the team led by Jian Pei and Jie-Yu Wang at the College of Chemistry and Molecular Engineering, Peking University, published a comprehensive review in Chemical Reviews with the title “Polymer Semiconductors: Synthesis, Processing, and Applications”. In this review, they summarized the recent decades’ development of polymer semiconductors covering chemical structure design,synthetic strategies, multilevel microstructures, processing technologies, and functional applications. The multilevel microstructures of polymer semiconductors are especially emphasized, which play anessential role in determining the device performance of polymer semiconductors. The reviewprovided a panorama of research on polymer semiconductors and established a bridge between the chemical structures, microstructures, and device performances, paving the way for the further design and development of high-performance polymer semiconductors.
Fig.1Polymer Semiconductors: Synthesis, Processing, and Applications.
In this review,the rise and vigorous development of polymer semiconductors over the past few decades were first examined (Fig. 2). After the initial discovery of conductive polyacetylene with conductivity over 104 S cm-1, the research focus gradually shifted from conducting polymers to polymer semiconductors.This led to the emergence ofvarious functional organic electronic devices, and the field of organic electronics began to develop.With the development of synthesis strategies, a vastnumber of conjugated polymers were synthesized,and the structure-property relationships of polymer semiconductors were gradually resolved. Currently, there is growing interest in the complex solid-phase micromorphology, solution aggregation state, and processing methods of polymer semiconductors. Additionally, the further development of controlled doping has broughtnew vitality and vigor to conducting polymers. So far, polymer semiconductors have developed into a vast and feature-rich field.
Fig.2Timeline of key developments in the field of polymer semiconductors.
In the first part of the review, the authors summarized the principles of structural design and synthetic strategies of polymer semiconductors. (1) The energy level and the backbone planarity are key considerations for molecular design.The energy level is one of the fundamental factors that influence the photophysical, electronic, magnetic, and other properties of polymer semiconductors.On the other hand, the backbone planarity of polymer semiconductors affectselectron delocalization,intra- and inter-chainπ-orbital overlap, ultimately determining their optoelectronic properties. (2) The initialsynthesis of conjugated polymers was achieved through Ziegler–Natta polymerization or oxidation polymerization. In addition to Ziegler–Natta polymerization, the discovery of nickel- and palladium-catalyzed cross-coupling reactionsin the 1970s enabled efficient coupling between aromatic rings withfewer defects in the conjugated polymer. This made Suzuki and Stille coupling the most popular synthetic strategy for conjugated polymers to date. The review also highlighted the development of new polymerization methods, includingdirect arylation polymerization and chain-growth polymerization (Fig. 3).
Fig.3Schematic and chemical structure of transition-metal-catalyzed cross-coupling polymerization.
In the second part of the review, the authors discussed the multilevel microstructures of polymer semiconductors and their impacts on the charge transport process. Pei divided the multilevel microstructures of polymer semiconductors into quaternary structures and summarized the related characterization techniques.
Polymer semiconductors enable multiple solution processing techniques at atmospheric pressure with minimal equipment cost. The review discussed processing strategies to obtain “better microstructures” of polymer films for better device performance. Following the time sequence, the review focused on the key stages of polymer processing from solutions to solids and post-processing of the obtained films (Fig. 4). Each processing stage can critically affect the multilevel microstructures of the deposited polymer films and,therefore, the device performance. Inthe solution of polymer semiconductors, strong inter-chain interactions causepolymer chains to form aggregates. The polymer aggregates in solution before film fabrication are critical, as some thin-film morphologies of conjugated polymers are inherited from the features of the solution-state supramolecular aggregates. In the solidification stage, the crystallization kinetics of conjugated polymers from the solution significantly depend on various processing conditions such as solvent, temperature, concentration, etc. For deposited films of polymer semiconductors, the post-processing such as annealing and rubbing will further modulate the packing ofpolymer chain and their microstructures, allowing for further improvement of device performance.
Fig.4 Illustration of solution-based processing and the parameters to adjust microstructures and device performance of conjugated polymers.
In the third partof the review, the authors discussed the functional applications of polymer semiconductors (Fig. 5). To date, polymer semiconductors have been widely used in various optoelectronic devices, as well as in energy storage, conversion, and catalysis. The main applications of polymer semiconductors include: 1) organic field effect transistors, organic electrochemical transistors, organic thermoelectrics, and organic spin electronics thatutilize the charge transport properties of polymer semiconductors; 2) imaging, organic solar cells, and organic light-emitting diodes, which utilize the photophysical properties of polymer semiconductors; 3) applications such as batteries, supercapacitors, catalysis, and photoelectrodes, which utilize the redox properties of polymer semiconductors.
Fig.5Device schematic diagram of applications.
This tutorial review provides a comprehensive understanding of the chemical and physical properties of polymer semiconductors, guiding and inspiring rational material design, effective processing, and further development of functional applications.
Li Ding and Zi-Di Yu,Ph.D. students at Peking University, are the co-first authors of this paper.Prof. Jian Pei from Peking University is the corresponding author.This work was jointly supported by the National Natural Science Foundation, the Ministry of Science and Technology, and the Beijing National Laboratory for Molecular Sciences in China.
Original link for the paper: https://pubs.acs.org/doi/10.1021/acs.chemrev.2c00696
