Side chains in conjugated polymers have been primarily utilized as solubilizing groups. However, these side chains have roles that are far beyond. We advocate using side chain engineering to tune a polymer’s physical properties, including absorption, emission, energy level, molecular packing, and charge transport. To date, numerous flexible substituents suitable for constructing side chains have been reported. In this Perspective article, we advocate that the side chain engineering approach can advance better designs for next-generation conjugated polymers.

Side Chain Engineering in Solution-Processable Conjugated Polymers

The purpose of this review is to provide a basic physical description of the exciton diffusion in organic semiconductors. Furthermore, experimental methods that are used to measure the key parameters of this process as well as strategies to manipulate the exciton diffusion length are summarized. Special attention is devoted to the temperature dependence of exciton diffusion and its relationship to Forster energy transfer rates. An extensive table of more than a hundred measurements of the exciton diffusion length in various organic semiconductors is presented. Finally, an outlook of remaining challenges for future research is provided.

Exciton diffusion in organic semiconductors

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize la...

Light Harvesting for Organic Photovoltaics

Low-power upconversion via sensitized triplet–triplet annihilation (TTA-UC) is a useful and versatile process that allows for the conversion of optical radiation into photons of higher energy. While this effect had been known to occur in solution for some 50 years, it was only very recently realized for the first time in polymers. The possibility to realize the TTA-UC process in solid materials is important for a variety of applications that range from the optimization of photovoltaic devices to bioimaging. This article provides a brief introduction to the field, summarizes the underlying photophysical aspects, reviews the design concepts for TTA-UC in polymers, and summarizes the recent advances in the development of such materials.

Low-power photon upconversion through triplet–triplet annihilation in polymers

The promise of wearable and implantable devices has made stretchable organic semiconductors highly desirable. Though there are increasing attempts to design intrinsically stretchable conjugated polymers, their performance in terms of charge carrier mobility and maximum fracture strain is still lacking behind extrinsic approaches (i.e., buckling, Kirigami interconnects). Here, polymer crosslinking with flexible oligomers is applied as a strategy to reduce the tensile modulus and improve fracture strain, as well as fatigue resistance for a high mobility diketopyrrolopyrrole polymer. These polymers are crosslinked with siloxane oligomers to give stretchable films stable up to a strain e = 150% and 500 strain-and-release cycles of 100% strain without the formation of nanocracks. Organic field-effect transistors are prepared to assess the electrical properties of the crosslinked film under cyclic strain loading. An initial average mobility (μavg) of 0.66 cm2 V−1 s−1 is measured at 0% strain. A steady μavg above 0.40 cm2 V−1 s−1 is obtained in the direction perpendicular to the strain direction after 500 strain-and-release cycles of 20% strain. The μavg in the direction parallel to strain, however, is compromised due to the formation of wrinkles.

/pdf/inducing-elasticity-through-oligo-siloxane-crosslinks-for-tjt8ue3sst.pdf

Inducing Elasticity through Oligo‐Siloxane Crosslinks for Intrinsically Stretchable Semiconducting Polymers

We present a simple and efficient strategy for the synthesis of acrylate-functionalized oligofluorenes. Within one single synthetic step photopatternable oligofluorenes with variable acrylate contents and molecular weights are formed. The Yamamoto condensation yields well defined oligomers by the direct polymerization of fluorene acrylates. The acrylate contents of the oligomers was varied from 10% to 100% by a statistical co-oligomerization. Photolithography experiments showed that for the species with highest acrylate content a crosslinking time of only 30 seconds leads to well resolved polyfluorene patterns. A minimum lateral resolution of 1 µm was achieved.

Tailoring fluorene-based oligomers for fast photopatterning

In this contribution we show that random fluorene cooligomers with photo reactive acrylate units can be prepared in a simple 1-step Yamamoto synthesis. The acrylate functionalities are preserved quantitatively under Yamamoto conditions. NMR and Maldi-ToF measurements point to an almost statistical incorporation of the comonomers into the oligomer chain. Maldi-ToF analyses give a further insight into the chain compositions, and we found fluorene-only oligomers to be present in low quantities. Thin films of the aromatic amine containing cooligomer show a blue fluorescence, the benzothiadiazole oligomer shows yellow photoluminescence, and the bithiophene oligomer emits orange-red light upon excitation. Compared to pure fluorene oligomers with a HOMO of 5.7 eV the HOMO levels of the TPD and bithiophene derivatives are increased to 5.25 and 5.31 eV, respectively, whereas the HOMO level of the benzothiadiazole oligomer is decrased to 5.85 eV. Photolithography experiments reveal that a careful optimization of th...

Three Color Random Fluorene-Based Oligomers for Fast Micrometer-Scale Photopatterning

In this paper we report the synthesis, characterisation and photopatterning of polydisperse fluorene oligomers. The oligomers were synthesised using the Yamamoto coupling. The molecular weights of the oligomers and as a consequence the nematic-isotropic transition temperatures were tailored by an endcapping reaction. With the endcapping species we introduced reactive acrylate functionalities. Subsequent photocrosslinking through a test mask led to the formation of fluorescent microstructures with a resolution of 1 μm.

Synthesis and Photopatterning of Fluorene Based Reactive Mesogens

An efficient one‐step synthesis of 9,9‐di(2‐ethylhexyl)‐2,7‐fluorene oligomers via an endcapping reaction is reported. Controlled endcapping demands a full conversion of functional groups and thus ...

Synthesis of oligofluorenes by endcapping

The synthesis of alternating photo-crosslinkable fluorene co-oligomers via Suzuki cross coupling is reported. The co-oligomers were characterized using NMR and GPC, the results showing effective control of molecular weight. The oligomers have low Tgs of 53-63 °C and are thermally stable up to ≈300 °C. Using TPD as co-monomer, a blue-emitting oligomer was obtained, a yellow-fluorescent material for benzothiadiazole, and a red (film) and yellow-green (solution) emitting material for bithiophene. The energy levels are very similar to those of randomly linked oligomers. The photo-crosslinking behavior was studied and under optimized conditions a lateral resolution of 600 nm was obtained.

Synthesis and Properties of Alternating Fluorene‐Based Oligomers for Sub‐µm Photopatterning

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