Influence of Chemical Structure on the Refractive Index of Imide‐Type Polymers

The influence of different substituents on the optical properties of poly(amide imide)s and polyimides is systematically investigated. All of the polymers exhibit good solubility and film‐forming ability, high thermal stability, and glass transition temperatures of 162–209 °C for poly(amide imide)s and of 209–274 °C for polyimides. The optical transmittance of the films at 450 nm is above 80% for all the studied polyimides and poly(amide imide)s containing benzonitrile units linked in the 2,6‐position. The values of refractive index for poly(amide imide)s are in the range of 1.623–1.748 and for polyimides in the domain of 1.672–1.768.

     

Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Polymers for Optoelectronic Applications

Over the last decade, great progress has been made in the field of organic electronics. Advancements in organic syntheses as well as in device engineering enabled preparation of polymer solar cells with power conversion efficiency (PCE) exceeding 8%–9%. In search for new polymers suitable for photovoltaic applications, push–pull polymers containing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) motif as an electron deficient (pull) unit emerged as very promising candidates. This Trend Article summarizes research on TPD‐based polymers with a special emphasis on the structure–property relationships.     

Liquid‐Crystalline Nano‐optomechanical Actuator

The synthesis and characterization of organic nanoparticles composed of a polymer network of azobenzene moieties and capable of reproducible, photoinduced mechanical actuation are reported. The molecules within the nanoparticles undergo co‐ordinated, reversible isomerization between cis‐ and trans‐conformations in response to ultraviolet and visible electromagnetic radiation, resulting in a reversible 20% height contraction of nanoparticles adsorbed on a substrate. The kinetics of the actuation response as a function of light intensity and duration are reported and closely match the molecular kinetics of azobenzene photoisomerization. The results support the proposed mechanism of co‐ordinated molecular conformational changes resulting in observable nanoscale actuation.     

D‐A Conjugated Polymers Based on Tetracyclic Acceptor Units: Synthesis and Application in Organic Solar Cells

Two electron‐deficient tetracyclic monomers, 6,9‐dibromo‐7,8‐bis(octyloxy)‐11H‐indeno[2,1‐b] quinoxalin‐11‐one ( M1 ) and 1,3‐bis(5′‐bromo‐4′‐hexylthiophen‐2‐yl)‐9H‐indeno[1,2‐b]thieno[3,4‐e]pyrazin‐9‐one ( M2 ), are synthesized through condensation of ninhydrin with aromatic diamines in good yields. Copolymerization of M1 with thiophene (Th) and dithieno[3,2‐b:2′,3′‐d]silole (DTS), and M2 with DTS affords three new D–A conjugated copolymers, P1 – P3, respectively . The optical, electrochemical, and photovoltaic properties of P1–P3 are investigated. P3 exhibits a broad absorption (300–1100 nm) and a low bandgap of 1.24 eV. Solar cells based on P3 /[6,6]‐phenyl C71 butyric acid methyl ester (PC₇₁BM) blends afford a power conversion efficiency of 1.93% and a very broad spectral response (300–1020 nm).

     

Reaction Kinetics and Thermodynamic Aspects of Thermoreversibly Cross‐Linked Polymer Networks

The Diels–Alder reaction of furan and maleimide is used to prepare reversibly cross‐linking polymer gels consisting of furan‐bearing polymer chains and bismaleimides. The roles of chain length, stoichiometry, and concentration on gelation kinetics and other properties are evaluated. Additionally, kinetics of the Diels–Alder reactions in the gelling system are compared with the kinetics of small molecule systems. By combining spectroscopic and rheological techniques, a mechanochemical analysis is performed.

     

Synthesis and Photovoltaic Performance of a [1,2,3]Triazolo[4,5‐g]quinoxaline‐Based Low‐Bandgap Polymer

A narrow‐bandgap conjugated polymer, PFDTBTzQ‐2OC1, is prepared by alternating [1,2,3]triazolo[4,5‐g]quinoxaline and 9,9‐didodecyl‐fluorene. With a bandgap of 1.63 eV, this polymer has wide absorption ranging from 300–760 nm in film. Bulk heterojunction solar cells fabricated by blending PFDTBTzQ‐2OC1 with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester exhibit a maximum power conversion efficiency of 1.31%, with a short‐circuit current density of 1.98 mA cm^–2, an open‐circuit voltage of 0.74 V, and a fill factor of 0.47.

     

Influence of Smectic Liquid Crystallinity on Lamellar Microdomain Structure in a Main‐Chain Liquid Crystal Block Copolymer Fiber

The microlamellar and smectic liquid crystal (LC) structures of a block copolymer of a main‐chain LC polyester connected at both ends with poly(ethyl methacrylate) are investigated by fiber X‐ray scattering. In the as‐spun fiber, the lamellae are parallel to the fiber axis, while the smectic layers are perpendicular to it. Annealing the as‐spun fiber at a temperature higher than the isotropization temperature (T_i) of the LC segment preserves the lamellae, but the LC structure disappears. Further annealing the fiber at T < T_i improves the lamellar stacking coherence and aligns the smectic layers parallel to the lamellae. In contrast, annealing the as‐spun fiber at T < T_i conserves the smectic layers and arranges the lamellae in parallel to the smectic layers. Thus, the liquid crystallinity affects the lamellar ordering and orientation.

     

Carbazole‐Substituted Triphenylamine and Diketopyrrolopyrrole Alternating Copolymer for Photovoltaic Cells

A new alternating conjugated polymer (PTCDPP) of carbazole‐substituted triarylamine and diketopyrrolopyrrole is prepared and characterized in detail. The polymer exhibits two strong absorption bands at 345 and 600 nm. With highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of ?5.13 eV and ?3.67 eV, PTCDPP displays an energy gap of 1.66 eV. PTCDPP‐based bulk heterojunction solar cells with a structure of fluorinated tin oxide (FTO)/TiO₂/PTCDPP:[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/MoO₃/Ag are fabricated. The devices are optimized by adjusting the composition of the PTCDPP:PCBM active layer, thermal treatment, and addition of processing additives. The device based on PTCDPP:PCBM (1:4, w/w) shows a power conversion efficiency (PCE) of 2.31%, with a short‐circuit current of 4.17 mA cm^?2, an open‐circuit voltage of 0.79 V, and a fill factor of 0.35. The best cell performance (2.65% PCE) is achieved by using 1,8‐diiodooctane (3%, v/v) as a processing additive and annealing the active layer at 80 °C.

     

Advanced Functional Polymers for Increasing the Stability of Organic Photovoltaics

The development of new advanced polymers for improving the stability of OPV is reviewed. Two main degradation pathways for the OPV active layer are identified: photochemically initiated reactions primarily starting in the side chains and morphological changes that degrade the important nanostructure. Chemical units can be introduced that impart an increased stability. Similarly, the morphological degradation of the optimal nanostructure can be reduced. Active polymers and blends with acceptor material are used to create nanoparticle links with controlled size. Most of these advanced polymers and processing methods have only been utilized in small‐scale devices prepared by standard techniques such as spin coating, but a few cases of roll‐to‐roll processed solar cells with heat‐cleaved side chains are discussed.