While most high-efficiency polymer solar cells (PSCs) are made of bulk heterojunction (BHJ) blends of conjugated polymers and fullerene derivatives, they have a significant morphological instability issue against mechanical and thermal stress. Herein, we developed an architecturally engineered compatibilizer, poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP), that effectively modifies the sharp interface of a BHJ layer composed of a P3HT donor and various fullerene acceptors, resulting in a dramatic enhancement of mechanical and thermal stabilities. We directly measured the mechanical properties of active layer thin films without a supporting substrate by floating a thin film on water, and the enhancement of mechanical stability without loss of the electronic functions of PSCs was successfully demonstrated. Supramolecular interactions between the P2VP of the P3HT-g-P2VP polymers and the fullerenes generated their universal use as compatibilizers regardless of the type of fullerene acceptors, including mono- and bis-adduct fullerenes, while maintaining their high device efficiency. Most importantly, the P3HT-g-P2VP copolymer had better compatibilizing efficiency than linear type P3HT-b-P2VP with much enhanced mechanical and thermal stabilities. The graft architecture promotes preferential segregation at the interface, resulting in broader interfacial width and lower interfacial tension as supported by molecular dynamics simulations.

We investigate the photovoltaic properties and charge dynamics of all polymer solar cells (all-PSCs) based on poly­[(N,N′-bis­(2-octyl­dodecyl)­naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)] (P­(NDI2OD-T2)) and its fluorinated analogue, poly­[(N,N′-bis­(2-octyl­dodecyl)­naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl)-alt-5,5′-(3,3′-difluoro-2,2′-bithiophene)] (P­(NDI2OD-T2F)), as the acceptor polymer and poly­[4,8-bis­(5-(2-ethyl­hexyl)­thiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophene-alt-5-octyl-4H-thieno­[3,4-c]­pyrrole-4,6­(5H)-dione] (PBDTTTPD) as the donor polymer. The PBDTTTPD:P­(NDI2OD-T2)-based device has a high open-circuit voltage (V OC) of 1.03 V but suffers from low power conversion efficiency (PCE) of 2.02% with a short-circuit current density (J SC) and fill factor (FF) of 4.45 mA cm–2 and 0.44, respectively. In a stark contrast, the PCE of PBDTTTPD:P­(NDI2OD-T2F)-based PSC dramatically increases to 6.09% (V OC = 1.00 V, J SC = 11.68 mA cm–2, and FF = 0.52). These results are attributed to the fluorination, which removes the energetic barrier for hole transfer and promotes the formation of the donor/acceptor blend morphology with suppressed phase separation and enhanced intermixed phases. The detailed charge dynamics examined by femtosecond transient absorption spectroscopy suggests the significantly increased hole transfer efficiency and larger populations of long-lived polarons for PBDTTTPD:P­(NDI2OD-T2F).

In this work, we develop mechanically robust and high-performance organic thin-film transistors (OTFTs) based on poly­(3-hexylthiophene) (P3HT) regioblock copolymers (block-P3HTs). These block-P3HTs consist of regioregular (rre) and regiorandom (rra) P3HTs, where the highly crystalline rre block allows efficient charge transport while the amorphous rra block provides mechanical robustness and interdomain connection. To examine the effects of the molecular architecture on the OTFT performance and stretchability, we prepare a series of block-P3HTs having different number-average molecular weight (M n) values of rra blocks (from 0 to 32 kg mol–1) and a fixed M n of rre blocks (11 kg mol–1). Thin films of all of the block-P3HTs exhibit a high charge-carrier mobility due to the formation of well-developed edge-on crystallites from the rre blocks confined within the rra domains, leading to a hole mobility of 1.5 × 10–1 cm2 V–1 s–1, which is superior to that of the rre P3HT homopolymer. In addition, the mechanical toughness of block-P3HT thin films is remarkably enhanced by the rra block. While the rre P3HT homopolymer thin film shows a brittle behavior with an elongation at break of only 0.3%, the elongation at break of the block-P3HT thin films increases by a factor of 100, yielding 30.2% with increasing M n of the rra block, without sacrificing the electrical properties. In particular, a noticeable enhancement of both elongation at break and toughness is observed between M n values of the rra block of 8 and 20 kg mol–1, indicating that the critical molecular weight of rra P3HT plays an important role in determining the mechanical response of the block-P3HT thin films. This study provides guidelines and strategies to improve the mechanical properties of organic electroactive materials without the disruption of optoelectrical properties, which is critical to fabricate high-performance soft electronics.

transistors (OFETs), organic light emitting diodes and polymer solar cells (PSCs), developing ecofriendly processing methods suitable for industrial fabrication has become an important topic of recent investigations. [1,2] To this end, a great deal of research efforts have been devoted to phasing out halogenated solvents such as chloroform and chlorobenzene, which have been prevalent for optimizing the fabrication of lab-scale OFET and PSC devices, because they are detrimental to both human health and the environment. In this context, a wide range of halogenfree solvents including toluene, xylenes, and trimethylbenzenes have been recently proposed as greener solvent alternatives and some notable progress has been made. [3][4][5][6][7][8][9][10][11] However, it remains in question whether those halogen-free solvents, which are often claimed to be "green solvents," are indeed practically applicable at an industrial-scale for sustainable manufacturing of organic electronics due to the serious health hazards and harmful environmental impacts that they can pose (e.g., median lethal dose LD 50 for chlorobenzene: 1110 mg kg −1 , toluene: 5580 mg kg −1 , water: >90 000 mg kg −1 , and De minimis % limit allowed to release for chlorobenzeneThe authors report the development of a desirable aqueous process for ecofriendly fabrication of efficient and stable organic field-effect transistors (eco-OFETs) and polymer solar cells (eco-PSCs). Intriguingly, the addition of a typical antisolvent, water, to ethanol is found to remarkably enhance the solubility of oligoethylene glycol (OEG) side chain-based electroactive materials (e.g., the highly crystalline conjugated polymer PPDT2FBT-A and the fullerene monoadduct PC 61 BO 12 ). A water-ethanol cosolvent with a 1:1 molar ratio provides an increased solubility of PPDT2FBT-A from 2.3 to 42.9 mg mL −1 and that of PC 61 BO 12 from 0.3 to 40.5 mg mL −1 . Owing to the improved processability, efficient eco-OFETs with a hole mobility of 2.0 × 10 −2 cm 2 V −1 s −1 and eco-PSCs with a power conversion efficiency of 2.05% are successfully fabricated. In addition, the eco-PSCs fabricated with water-ethanol processing are highly stable under ambient conditions, showing the great potential of this new process for industrial scale application. To better understand the underlying role of water addition, the influence of water addition on the thinfilm morphologies and the performance of the eco-OFETs and eco-PSCs are studied. Additionally, it is demonstrated that the application of the aqueous process can be extended to a variety of other OEG-based material systems.

In the last decade, extensive academic and industrial efforts have been devoted to developing efficient conjugated polymers (CPs) for organic electronics. Specifically, the relationship between the molecular structures, properties, and...

Discrete oligomers (i.e., highly monodisperse) can provide a deep understanding of chain-length-dependent properties of polymers and their self-assembly behaviors. Herein, discrete oligo­(3-hexylthiophene)­s (D-o3HTs) with a dispersity (Đ) of 1.0 and degree of polymerization (DP) between 6 and 18 were obtained through a simple synthetic procedure of 3-hexlythiophene trimer-based polymerizations and automated column chromatography purification. As the DP of D-o3HTs increases, longer conjugation lengths cause red shifts in their optical properties and yield tunable crystalline properties. Interestingly, D-o3HTs with DP ≤ 9 have a dominant face-on Form II structure in thin films, while a fiber morphology is also not observed in thin films. In contrast, D-o3HTs with DP ≥ 12 assemble into a dominant edge-on Form I structure in thin films and show highly ordered fiber morphologies. In addition, Bragg rod patterns are observed in thin films by transmission electron microscopy and grazing incidence X-ray scattering with these patterns being distinctive when compared to those for conventional regioregular poly­(3-hexylthiophene) with Đ = 1.1. Finally, the formation of 2-dimensional flowerlike nanostructures with overall micrometer dimensions is obtained from D-o3HTs via solvent-mediated self-assembly. These results offer an understanding of self-assembly behaviors of discrete conjugated polymers, leading to exquisite control over their crystallinity and nanoscale morphology.

Interface engineering of evaporative emulsion droplets containing block copolymers (BCPs) provides an effective route to generate nonspherical particles. Here, we demonstrate the impact of length-controlled nanorods (NRs) on the interfacial properties of BCP emulsions to produce anisotropic BCP particles. A series of lamellae- and cylinder-forming polystyrene-b-poly­(4-vinylpyridine) (PS-b-P4VP) and a series of NRs with different lengths (l) are coassembled, and selective arrangement of the NRs on the P4VP domain at the particle surface enables the production of striped football (prolate) and convex lens-shaped (oblate) particles. In particular, the ratio of the NR length to the size of the NR-hosting domain (l/L), which is varied from 0.07 to 3.60, is the key parameter in determining the location of the NRs in the BCP particles as well as the final particle shape. The oblate particles are generated only in the range of 0.36 ≤ l/L ≤ 0.96, whereas the prolate particles are produced for much wider range of l/L ≥ 0.83 without upper limit. This difference is attributed to larger entropic penalty for the NRs confined within the P4VP cylinders than the entropic penalty for those within the lamellae. To better understand and support our experimental observations, we performed dissipative particle dynamics simulation and calculated the free energy for the NR/BCP assembly within the emulsion droplets.

We present a new series of fullerene derivatives that exhibit solubility in ethanol/water solvent mixtures and implement these materials to fabricate polymer solar cells (PSCs) using environmentally benign solvents. In order to simultaneously optimize the processability of the fullerenes in ethanol/water solvent mixtures and device performance, different fullerene mono-adducts were designed by introducing oligoethylene glycol side chains with different lengths and number of branches. As a result, we achieved power conversion efficiencies up to 1.4% for PSCs processed from benign ethanol/water mixtures in air. Significantly, the new alcohol/water-soluble fullerene derivatives displayed electron mobilities up to 1.30 × 10 −4 cm 2 V −1 s −1 , 150 times higher than those of a previously reported alcohol-soluble fullerene bis-adduct, owing to efficient packing of the fullerenes. Femtosecond transient absorption spectroscopy revealed the acceptor side chain to markedly impact geminate and/or nongeminate charge recombination in the PSCs. In addition, side chain optimization of these fullerenes produced well-intermixed morphologies with high domain purity when blended with p-type polymer to provide hole and electron transport pathways. Our results provide important guidelines for the design of electroactive materials for safe and environmentally benign fabrication of PSCs and other organic electronic devices.