Nonfullerene acceptors (NFAs) have contributed significantly to the progress of organic solar cells (OSCs). However, most NFAs feature a large fused-ring backbone, which usually requires a tedious multiple-step synthesis, and are not applicable to commercial applications. An alternative strategy is to develop nonfused NFAs, which possess synthetic simplicity and facile tunability in optoelectronic properties and solid-state microstructures. In this work, we report two nonfused NFAs, BTCIC and BTCIC-4Cl, based on an A–D–A′–D–A architecture, which possess the same electron-deficient benzothiadiazole central core but different electron-withdrawing terminal groups. The optical properties, energy levels, and molecular crystallinities were finely tuned by changing the terminal groups. Moreover, a decent power conversion efficiency of 9.3 and 10.5% has been achieved by BTCIC and BTCIC-4Cl, respectively, by blending them with an appropriate polymer donor. These results demonstrate the potential of A–D–A′–D–A type nonfused NFAs for high-performance OSCs. Further development of nonfused NFAs will be very fruitful by employing appropriate building blocks and via side-chain optimizations.

There remains a lack of fundamental understanding in the role of backbone rigidity on the thermomechanical properties of conjugated polymers. Here, we provide the first holistic approach to understand the fundamental influence of backbone rigidity on an n-type naphthalene diimide-based conjugated polymer, denoted by PNDI-Cx, through insertion of a flexible conjugation break spacer (CBS). CBS lengths are varied from fully conjugated with zero alkyl spacer (PNDI-C0) to a seven-carbon alkyl spacer (PNDI-C7), with the CBS engineered into each repeat unit for systematic evaluation. Solution small-angle neutron scattering and oscillatory shear rheometry were employed to provide the first quantitative evidence of CBS influence over conjugated polymer chain rigidity and entanglement molecular weight (M e), demonstrating a reduction in the Kuhn length from 521 to 36 Å for fully conjugated PNDI-C0 and PNDI-C6, respectively, as well as a nearly consistent M e of ∼15 kDa upon the addition of CBS. Thermomechanical properties, such as elastic modulus and glass-transition temperature, were shown to decrease with an increasing length of CBS. An extraordinary ductility, upwards of 400% tensile strain before fracture, was observed for high-molecular-weight PNDI-C4, which we attribute to a high number of entanglements and disruption of crystallization. Furthermore, the deformation mechanism for PNDI-Cx was studied under strain through X-ray diffraction, polarized UV–vis spectroscopy, and atomic force microscopy. Overall, this work sheds light on the important role of backbone rigidity in designing flexible and stretchable conjugated polymers.

Donor-acceptor (D-A) type semiconducting polymers have shown great potential for the application of deformable and stretchable electronics in recent decades. However, due to their heterogeneous structure with rigid backbones and long solubilizing side chains, the fundamental understanding of their molecular picture upon mechanical deformation still lacks investigation. Here, the molecular orientation of diketopyrrolopyrrole (DPP)-based D-A polymer thin films is probed under tensile deformation via both experimental measurements and molecular modeling. The detailed morphological analysis demonstrates highly aligned polymer crystallites upon deformation, while the degree of backbone alignment is limited within the crystalline domain. Besides, the aromatic ring on polymer backbones rotates parallel to the strain direction despite the relatively low overall chain anisotropy. The effect of side-chain length on the DPP chain alignment is observed to be less noticeable. These observations are distinct from traditional linear-chain semicrystalline polymers like polyethylene due to distinct characteristics of backbone/side-chain combination and the crystallographic characteristics in DPP polymers. Furthermore, a stable and isotropic charge carrier mobility is obtained from fabricated organic field-effect transistors. This study deconvolutes the alignment of different components within the thin-film microstructure and highlights that crystallite rotation and chain slippage are the primary deformation mechanisms for semiconducting polymers.

Polymer gels are unique materials, which consist of a polymer network swollen in a solvent. The modulus and the overall state of the gel depend on thermodynamic parameters, such as strand length, structure, and chemical compatibility of the solvent. Scattering techniques have been used to study the gel structure and osmotic pressure and are discussed in this Tutorial pedagogically. These techniques are not always straightforward because of trapped inhomogeneities and complicated interactions. Nevertheless, there has been considerable progress in developing analytical tools to determine the internal structure and dynamics of polymer gels. In addition to these tools, we showcase modern spatially resolved techniques, such as Diffusing Wave Spectroscopy, and offer a perspective on similar methods such as Dynamic Small Angle Light Scattering and Differential Dynamic Microscopy, which can be used to study local and interface effects in gel systems as well as any anisotropy. These new tools can fundamentally advance our understanding of local gel dynamics, which have so far been too challenging to observe.

Flexible alkyl side chain in conjugate polymers (CPs) improves the solubility and promotes solution processability, in addition, it affects interchain packing and charge mobilities. Despite the well-known charge mobility and morphology correlation for these semi-crystalline polymers, there is a lack of fundamental understanding of the impact of side chain on their crystallization kinetics. In the present work, isothermal crystallization of five poly(3alkylthiophene-2,5-diyl) (P3ATs) with different side-chain structures were systematically investigated. To suppress the extremely fast crystallization and trap the sample into amorphous glass, an advanced fast scanning chip calorimetry technique, which is able to quench the sample with few to tens thousands of K/s, was applied. Results show that the crystallization of P3ATs was greatly inhibited after incorporation of branched side chains, as indicated by a dramatic up to six orders of magnitude decrease in the crystallization rate. The suppressed crystallization of P3ATs were correlated with an increased π-π stacking distance due to unfavorable side-chain steric interaction. This work provides a pathway to use side-chain engineering to control the crystallization behavior for CPs, thus to control device performance.Xiaodan Gu earned his Ph.D. from the Department of Polymer Science and Engineering at the University of Massachusetts Amherst, advised by Prof. Thomas P. Russell, focusing on the self-assembly of block copolymers and their lithographic applications. Subsequently, he started a post-doctoral appointment co-advised by Zhenan Bao and Michael F. Toney at Stanford University and SLAC National Accelerator Laboratory, where he studied the morphology of roll-to-roll printed electronics using real-time X-ray scattering techniques. He moved to the University of Southern Mississippi to start his independent career in 2016. His current research interests revolve around various fundamental polymer physics phenomena related to conjugated polymers and their derivative devices. His group studies structure, dynamic, and morphology of conjugated polymers and aim to link their molecular structures to their macroscopic properties through advanced metrology with an emphasis on scattering techniques. He was awarded ORAU Powe Junior Faculty Enhancement Award in 2019, ACS PMSE Young investigator, and a contributor to "Young Talents" special issue to Macromolecular Rapid Communications, "Emerging investigator" for 2020 Polymer Chemistry, and " Early Career Scholars" for 2021 Journal of Materials Research from MRS.