A novel intrinsically stretchable ABA triblock copolymer can be synthesized where A and B are poly­(3-hexylthiophene) (P3HT) and polyisobutylene (PIB) segments, respectively. The deformation of the self-assembled hierarchical nanostructure of the block copolymer thin film was clearly observed by grazing incidence small- and wide-angle X-ray scattering.

The rapid development of wearable electronic devices has prompted a strong demand to develop stretchable organic solar cells (OSCs) to serve as the advanced powering systems. However, to realize an intrinsically stretchable OSC is challenging because it requires all the constituent layers to possess certain elastic properties. It thus necessitates a combined engineering of charge-transporting layers and photoactive materials. Herein, we first describe a stretchable electron-extraction layer using a blend of poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN) and nitrile butadiene rubber (NBR, Nipol 1072). This hybrid PFN/NBR layer exhibits a much lower Derjaguin-Muller-Toporov modulus (0.45 GPa) than the value (1.25 GPa) of the pristine PFN and could withstand a high strain (60% strain) without showing any cracks. Moreover, besides enriching the stretchability of PFN, the terminal carboxyl groups of NBR can ionize PFN to promote its solution-processability in polar solvents and to ensure the interfacial dipole formation at the corresponding interface in the device, as evidenced by the Fourier transform infrared and ultraviolet photoelectron spectroscopy analyses. By further coupling the replacement of [6,6]-phenyl-C-butyric acid methyl ester (PCBM) with nonfullerene acceptors owing to better mechanical stretchability in the photoactive layer, OSCs with improved intrinsically stretchability and performance were demonstrated. An all-polymer OSC can exhibit a power conversion efficiency of 2.82% after 10% stretching, surpassing the PCBM-based device that can only withstand 5% strain.

A series of novel π-conjugated copolymers based on 2,2′-bis­(1,3,4-thiadiazole) (BTDz) have been developed. Among them, the BTDz-based donor–acceptor alternating copolymer with the (E)-1,2-di­(3-(2-ethyl­hexyl)­thiophene)­vinylene donor unit (PBTDzTV) exhibited a high solubility and high crystallinity. PBTDzTVs favorably self-assembled, forming face-on and edge-on multibilayer structures in thin nanoscale films. The relative volume fractions of these structures varied depending on the polymer’s molecular weight. The higher molecular weight polymer formed a higher volume fraction of the face-on structure; in particular, the polymer with a 26.6 kDa of number-average molecular weight made only the face-on structure. The device performance was improved as the polymer molecular weight and the volume fraction of the face-on structure increased. The bulk-heterojunction photovoltaic device based on PBTDzTV:PC71BM demonstrated the high power conversion efficiency (PCE) of 8.04% when the device was fabricated with the highest molecular weight polymer having the face-on structure.

ABC-type asymmetric star polymers containing a P3HT segment were successfully synthesized by polymer coupling/linking reactions by combining the Kumada catalyst-transfer polymerization and living anionic polymerization for the first time. The synthetic methodology involves the following three stage reactions: (a) the anionic linking reaction between ω-chain-end-functionalized P3HT with a bromobutyl moiety (P3HT-C4-Br) and living anionic PS end-capped with 1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-phenylethylene (1), (b) the transformation reaction of the 3-tert-butyldimethylsilyloxymethylphenyl (TBDMS) moiety at the junction between the P3HT and PS segments into the α-phenyl acrylate (PA) moiety through the hydroxymethylphenyl moiety, and (c) the anionic linking reaction between PA-in-chain-functionalized P3HT-b-PS and living anionic P2VP. Indeed, the well-defined ABC star polymers in which A, B and C are poly(3-hexylthiophene) (P3HT), polystyrene (PS), and poly(2-vinylpyridine) (P2VP), respectively, could be synthesized. The molecular weights and compositions of the star polymers were controllable by possessing extremely low Đ values ( Đ < 1.05). Two distinct transition temperatures (T g,PS+P2VP and T m,P3HT) were clearly observed in the DSC thermograms of the ABC star polymers, indicating the phase separation between the (PS+P2VP) and P3HT domains. The vibronic absorption of the ABC star polymer films based on the UV–vis spectroscopy indicated a high degree of ordering of the P3HT crystalline structures, supporting the isolation of the P3HT domains although the PS and P2VP segments are connected to P3HT at the core. In the AFM phase images of the ABC star polymer thin film surface, continuous fibril structures were clearly seen. GISAXS experiments confirmed the orientation of the fibril structures with the mean period distances depending on the P2VP arm lengths. The GIWAXS results showed that the P3HT crystalline domains in the microphase-separated P3HT domains align with an “edge-on” rich orientation and the π–π stacking distance in the range of 0.380–0.393 nm also depending on the length of P2VP segments.

The P3HT:PCBM (P3HT = poly(3-hexylthiophene, PCBM = phenyl-C61-butyric acid methyl ester) bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells using the AB diblock and ABA triblock copolymers (A = polystyrene derivative with donor-acceptor units (PTCNE) and B = P3HT) as compatibilizers were fabricated. Under the optimized blend ratio of the block copolymer, the power conversion efficiency (PCE) was enhanced. This PCE enhancement was clearly related to the increased short-circuit current (J(sc)) and fill factor (FF). The incident photon to current efficiency (IPCE) measurement suggested that the P3HT crystallinity was improved upon addition of the block copolymers. The increased P3HT crystallinity was consistent with the increased photovoltaic parameters, such as J(sc), FF, and consequently the PCE. The surface energies of these block copolymers suggested their thermodynamically stable location at the interface of P3HT:PCBM, showing the efficient compatibilizing performance, resulting in enlarging and fixing the interfacial area and suppressing the recombination of the generated carriers. Grazing incidence X-ray scattering (GIXS) results confirmed the superior compatibilizing performance of the ABA triblock copolymer when compared to the AB diblock copolymer by the fact that, after blending the ABA triblock copolymer in the P3HT:PCBM system, the enhanced crystallinity of matrix P3HT was observed in the excluded areas of the less-aggregated PCBM domains, changing the P3HT crystalline domain orientation from "edge-on" to "isotropic". This is, to the best of our knowledge, the first sequential effect (AB vs ABA) of the block copolymers on the compatibilizing performances based on BHJ OPV device systems.