Controllable Transformation between the Kinetically and Thermodynamically Stable Aggregates in a Solution of Conjugated Polymers

The Preparation of Dithieno[3,2-b:4,5-c']germole, and Its Application as a Donor Unit in Conjugated D-A Compounds

Group 14 metalloles have attracted much attention as core structures of conjugated functional materials. In this work, we prepared dithieno[3,2-b:4,5-c']germole as a new unsymmetrically condensed dithienogermole and benzo[4,5]thieno[2,3-c]germole as the benzene-condensed analog. The electronic states and properties of these unsymmetrically condensed germoles are discussed on the basis of the results of optical and electrochemical measurements with the help of quantum chemistry calculations on the simplified model compounds. The Stille cross-coupling reactions of bromodithieno[3,2-b:4,5-c']germole with di(stannylthienyl)- and di(stannylthiazolyl)benzothiadiazole provided conjugated donor-acceptor compounds that exhibited clear solvatochromic behavior in the photoluminescence spectra, indicating the potential application of the dithieno[3,2-b:4,5-c']germole unit as an electron donor in donor-acceptor systems.

Direct observation of conformations of a high-mobility n-type low-bandgap copolymer in solutions and solid films

The aggregation morphologies of conjugated polymers in solutions and solid films are important for their optoelectronic applications. Due to the amorphous state of the polymers, it remains a great challenge to determine their conformations in either liquids or solids. Herein, a ps/fs synchronized 2D IR technique is applied to investigate the molecular conformations of a high-mobility n-type low-bandgap copolymer, N2200, dissolved in CHCl₃ and CCl₄, and in solid films cast from both solutions by the vibrational cross-angle method. In CCl₄, the polymer forms more aggregates and folds more and the backbone dihedral angle of C-C(NDI)/C-S(Thiophene) of its average conformation is about 10° more distorted than that in CHCl₃ and the most stable conformation for a free molecule. Anti-intuitively, the solid films cast from both solutions have the same molecular conformation, and the conformation is similar to that of the polar CHCl₃ rather than the conformation of the less polar CCl₄. The results imply that the interaction between the polymer backbones is probably stronger than its interaction with CCl₄, which can naturally guide the rearrangement of polymer chains during the evaporation of solvent molecules. This work also implies that the balance and competition between the polymer/polymer interaction and the polymer/solvent interaction seem to be the dominant factors responsible for what morphology can form in a solid film cast from solution. It is not always true that different molecular conformations must exist in solid films grown from different solutions with different polarity or different extents of aggregates with different conformations.

Conjugated Polymers in Solution: A Physical Perspective

Excellent progress has been made in the optoelectronic properties of conjugated polymers by controlling solution-state aggregation. However, due to the wide variety and complex structures of conjugated polymers, it is still challenging to fully understand the complex aggregation process and microstructures both in solution and in the solid state. This Perspective focuses on the chain conformations and the aggregation of conjugated polymers in solution. We discuss the factors in detail which affect solution-state aggregation and microstructures from the perspective of polymer physics in solutions, including chemical structures and environmental conditions. Based on the understanding of multiple interactions of conjugated polymers in solution, strategies to regulate solid-state microstructures and obtain high-performance polymer-based devices from solution-state aggregation are summarized.

Crystallization of D-A Conjugated Polymers: A Review of Recent Research

D-A conjugated polymers are key materials for organic solar cells and organic thin-film transistors, and their film structure is one of the most important factors in determining device performance. The formation of film structure largely depends on the crystallization process, but the crystallization of D-A conjugated polymers is not well understood. In this review, we attempted to achieve a clearer understanding of the crystallization of D-A conjugated polymers. We first summarized the features of D-A conjugated polymers, which can affect their crystallization process. Then, the crystallization process of D-A conjugated polymers was discussed, including the possible chain conformations in the solution as well as the nucleation and growth processes. After that, the crystal structure of D-A conjugated polymers, including the molecular orientation and polymorphism, was reviewed. We proposed that the nucleation process and the orientation of the nuclei on the substrate are critical for the crystal structure. Finally, we summarized the possible crystal morphologies of D-A conjugated polymers and explained their formation process in terms of nucleation and growth processes. This review provides fundamental knowledge on how to manipulate the crystallization process of D-A conjugated polymers to regulate their film structure.

Recent Developments of n-Type Organic Thermoelectric Materials: Influence of Structure Modification on Molecule Arrangement and Solution Processing

Organic semiconductor (OSCs) thermoelectric materials have been studied widely due to their low thermal conductivity and solution processing characteristics. Currently, the high conductivity (up to 1000 s cm^-1 ) has boosted the performance of p-type organic thermoelectric materials substantially. In contrast, the development of n-type organic thermoelectric materials is still limited by their low mobility, inferior air stability, and poor doping efficiency, which is relevant to the molecule structure and dopant dispersion. Herein, the recent development of n-type organic thermoelectric materials was reviewed with an emphasis on molecule structure modification and solution processing. Methods for optimizing conjugate structure were summarized from the effects of conjugated backbone modification and side chains diversification on molecular stacking. The primary n-type dopants were also summarized briefly. Especially, the role of solution aggregation controlling on film preparation and properties was given special attention. Additionally, the emergence of organic diradicals with low lowest unoccupied molecular orbital energy level and no doping was introduced, which shows great potential in n-type organic thermoelectric materials. All these endeavors have led to the development of n-type OSCs materials. This Review is aimed at illustrating the state-of-the-art progress and providing some guideline for the design of organic thermoelectric materials in the future.