Sijun Li

Bulk Interpenetration Network of Thermoelectric Polymer in Insulating Supporting Matrix

Thermoelectric properties of conjugated polymers are found to improve upon homogeneously distributing conjugated polymer into an insulating supporting matrix. The local one-dimensional charge transport along the interpenetration conductive network simultaneously leads to lower thermal conductivity, higher electrical conductivity without sacrifice of Seebeck coefficient, and thus a higher figure of merit ZT, as compared with neat conjugated polymer.

Epitaxy-Assisted Creation of PCBM Nanocrystals and Its Application in Constructing Optimized Morphology for Bulk-Heterojunction Polymer Solar Cells

PCBM (a C60 derivative) is so far the most successful electron acceptor for bulk-heterojunction polymer photovoltaic (PV) cells. Here we present a novel method epitaxy-assisted creation of PCBM nanocrystals and their homogeneous distribution in the matrix using freshly cleaved mica sheet as the substrate. The highly matched epitaxy relationship between the unit cell of PCBM crystal and crystallographic (001) surface of mica induces abundant PCBM nuclei, which subsequently develop into nanoscale crystals with homogeneous dispersion in the composite film. Both the shape and size of these nanocrystals could be tuned via choosing the type of matrix polymer, film thickness, ratio of PCBM in the composite film, and annealing temperature. Thus, the obtained thin composite film is removed from the original mica substrate via the flotation technique and transferred to a real substrate for device completion. The success of this method has been verified by the substantially improved device performance, in particular the increased short-circuit current, which is heavily dependent on the morphology of the photoactive layer. Therefore, we have actually demonstrated a novel approach to construct preferred morphology for high-performance optoelectronic devices via resorting to other specific substrates which could induce the formation of this type morphology.

Constructing thin polythiophene film composed of aligned lamellae via controlled solvent vapor treatment.

Thin poly(3-butylthiophene) (P3BT) film composed of aligned lamellae attached to the edge of the original film has been achieved via a controlled solvent vapor treatment (C-SVT) method. The polarized optical microscopy operated at both single-polarization and cross-polarization modes has been used to investigate the alignment of the fiber-like lamellae. A numerical simulation method is used to quantitatively calculate angle distributions of the lamellae deviated from the film growth direction. Prepatterned P3BT film edge acts as nuclei which densely initialize subsequent crystal growth by exhausting the materials transported from the partially dissolved film. The growth of new film upon crystallization is actually a self-healing process where the two-dimensional geometric confinement is mainly responsible for this parallel alignment of P3BT crystals. The solvent vapor pressure should be carefully chosen so as to induce crystal growth but avoid liquid instability which will destroy the continuity of the film. The combination of microfabrication technique and C-SVT method provides a novel method to fabricate hierarchical structure within thin polymer film with multiscale morphology via utilizing both up-bottom and bottom-up approaches.

A novel melting behavior of poly(3-alkylthiophene) cocrystals: premelting and recrystallization of component polymers

Poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT) cocrystals were prepared by solution casting and the cocrystals show two endothermic peaks in a differential scanning calorimetry (DSC) thermogram. The former endothermic peak is nearly fixed at the melting temperature of pure P3HT, while the latter one shifts gradually towards the P3BT melting point with increased P3BT content. This melting behavior of the polymer cocrystal is novel and different from the studies commonly reported in which the polymer cocrystal generally has one endothermic peak. After extensive analyses, including powder X-ray diffraction (XRD), synchrotron-based two dimensional grazing-incidence XRD, temperature-dependent UV-Visible spectroscopy and temperature modulated DSC, we propose the melting mechanism of P3HT/P3BT cocrystals involving three steps. In the first step, the P3HT component melts at the former endothermic peak around 241 °C. After that, the P3BT component recrystallizes when the coil-like P3HT chains escape from the cocrystal lattices. Finally, the P3BT component completely melts at the latter endothermic peak. These findings are beneficial to the understanding of the criteria for the formation and the melting behavior of polymer cocrystals.

Ternary Donor-Insulator-Acceptor Systems for Polymer Solar Cells

A series of block copolymers with fixed length of the semiconductor-block poly(3-butylthiophene) (P3BT) and varying length of the insulator-block polystyrene (PS) are synthesized. These copolymers are blended with phenyl-C61-butyric acid methyl ester (PCBM) for the bulk heterojunction photoactive layers. With appropriate insulator-block length and donor-acceptor ratio, the power conversion efficiency increases by one order of magnitude compared with reference devices with pure P3BT/PCBM. PS blocks improve the miscibility of the active layer blends remarkably. The P3BT-b-PS crystallizes as nanorods with the P3BT core covered with the PS-block, which creates a nanoscale tunneling barrier between donor and acceptor leading to more efficient transportation of charge carriers in the semiconductors.