Shaowei Shi

The role of conjugated side chains in high performance photovoltaic polymers

Four new D–A type copolymers, namely, PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT, were designed and synthesized to investigate the influence of conjugated side chain pattern on photovoltaic properties of conjugated polymers. All the four copolymers have an identical conjugated backbone comprising benzo[1,2-b:4,5-b′]dithiophene (BDT) donor unit and quinoxaline (Qx) acceptor unit, but with varying conjugated side chains, p-alkoxyphenyl or 2-alkylthienyl, attached to the donor and acceptor units, respectively. As evidenced by UV/Vis absorption spectra, electrochemical cyclic voltammetry, density functional theory (DFT), grazing incidence X-ray scattering (GIXS), transmission electron microscope (TEM) and photovoltaic measurements, the difference in conjugated side chain modulation led to totally different physicochemical properties. Among the four copolymers, PBDT-DFQX-TT exhibits the broadest absorption spectrum, the most close-packed structure as well as a finest fibril structure when blended with PC71BM. After systematic device optimization, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) photovoltaic devices based on the blends of PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT with PC71BM achieved 3.96%, 6.08%, 6.54% and 7.68%, respectively. By systematic varying the side chains of the copolymers from all phenyl groups to all thienyl ones, PCEs was increased by 250% from 3.96% to 7.68%. To date, PBDT-DFQX-TT is one of a few Qx-based PSCs that exhibits PCE exceeding 7.5%, and the results suggest that simultaneously modulating the conjugated side chains on both donor and acceptor units of copolymers could be an effective strategy for constructing high performance photovoltaic copolymers.

Naphtho[1,2b;5,6b′]difuran-based donor–acceptor polymers for high performance organic field-effect transistors

Two naphthodifuran-based donor–acceptor copolymers are presented. Via reasonable main-chain modification and side-chain engineering, remarkably dense π–π stacking spacings (<3.5 A) as well as high “edge-on” orientations are observed. When fabricated as organic field-effect transistors, high hole mobilities exceeding 5 cm2 V−1 s−1 are achieved at a moderate annealing temperature of 120 °C.

Reconfigurable Printed Liquids

Liquids lack the spatial order required for advanced functionality. Interfacial assemblies of colloids, however, can be used to shape liquids into complex, 3D objects, simultaneously forming 2D layers with novel magnetic, plasmonic, or structural properties. Fully exploiting all-liquid systems that are structured by their interfaces would create a new class of biomimetic, reconfigurable, and responsive materials. Here, printed constructs of water in oil are presented. Both form and function are given to the system by the assembly and jamming of nanoparticle surfactants, formed from the interfacial interaction of nanoparticles and amphiphilic polymers that bear complementary functional groups. These yield dissipative constructs that exhibit a compartmentalized response to chemical cues. Potential applications include biphasic reaction vessels, liquid electronics, novel media for the encapsulation of cells and active matter, and dynamic constructs that both alter, and are altered by, their external environment.

Enhancing the photovoltaic performance of quinoxalino[2,3-b′]porphyrinatozinc-based donor–acceptor copolymers by using 4,4′-bipyridine as a linear bidentate ligand additive

In this study, two donor–acceptor polymers, P(QP-TT) and P(QP-TT-Zn), were synthesized based on the 2,2′:5′,2′′-terthiophene (TT) donor unit and quinoxalino[2,3-b′]porphyrin (QP) or quinoxalino[2,3-b′]porphyrinatozinc (QP-Zn) acceptor unit. Bulk heterojunction polymer solar cells (PSCs) were fabricated. An alternative strategy for optimizing the photovoltaic devices by using a linear bidentate ligand additive, 4,4′-bipyridine (Bipy), is reported. The Bipy additive is totally different from the most widely used solvent additives in that it can form a coordination effect with metalloporphyrin-based polymers and leads to more ordered arrangements of polymers. Photovoltaic devices based on P(QP-TT-Zn) showed a notable improvement of power conversion efficiency (PCE) when a small amount of Bipy was added to the blend solution. Further optimization combined with thermal annealing and methanol treatment showed a significant improvement of PCE from 0.85 to 3.51%, which is the highest value among PSC devices based on porphyrin-based conjugated polymers. However, application of the same optimization methods to devices based on P(QP-TT) showed no improvements of photovoltaic performance. Morphology analysis revealed that blend films of P(QP-TT-Zn):PC71BM showed desired interpenetrating networks with some ordered packing in the thin film after adding the Bipy additive. The results suggest that the improvements of the photovoltaic performance are potentially due to the coordination effect between the nitrogen of Bipy and central zinc of P(QP-TT-Zn).

Nanomechanical Imaging of the Diffusion of Fullerene into Conjugated Polymer

The large Youngs modulus difference between chemically modified fullerene (PCBM) and a conjugated polymer was used to nanomechanically map the diffusion of PCBM into PTB7, a high-efficiency low-band-gap conjugated polymer. The sharp tip in nanomechanical atomic force microscopy ensures a high-resolution nanomechanical characterization of the diffusion front, with the intrinsic benefits of revealing the mechanical properties of the mixtures. Localized structure changes induced by diffusion were investigated by grazing incidence X-ray diffraction methods. We found a most unusual diffusion behavior that shows Case II characteristics, where a front of PTB7 saturated with PCBM moves into the pure PTB7 with a linear time dependence. This diffusion is due mainly to a majority fraction of the disordered PTB7 that has continuous paths for PCBM diffusion without obvious energetic barriers, and as diffusion proceeds, the paths for diffusion gets larger, leading to a step in the concentration profile. The donor/acceptor-dependent diffusion constants may also contribute to the observed Case-II-like diffusion front.

Atomic Force Microscopy Nanomechanical Mapping Visualizes Interfacial Broadening between Networks Due to Chemical Exchange Reactions

The interfacial broadening between two different epoxy networks having different moduli was nanomechanically mapped. The interfacial broadening of the two networks produced an interfacial zone having a gradient in the concentration and, hence, properties of the original two networks. This interfacial broadening of the networks leads to the generation of a new network with a segmental composition corresponding to a mixture of the original two network segments. The intermixing of the two, by nature of the exchange reactions, was on the segmental level. By mapping the time dependence of the variation in the modulus at different temperatures, the kinetics of the exchange reaction was measured and, by varying the temperature, the activation energy of the exchange reaction was determined.