Experimental Studies on the Anisotropic Thermoelectric Properties of Conducting Polymer Films

We reported general methods for studying the thermoelectric properties of a polymer film in both the in-plane and through-plane directions. The bench-mark PEDOT/PSS films have highly anisotropic carrier transport properties and thermal conductivity. The anisotropic carrier transport properties can be explained by the lamellar structure of the PEDOT/PSS films where the PEDOT nanocrystals could be isolated by the insulating PSS in the through-plane direction. The anisotropic thermal conductivity was mainly attributed to the lattice contribution from PSS because the polymer chain is oriented along the substrate

Morphology Control in TiO₂ Nanorod/Polythiophene Composites for Bulk Heterojunction Solar Cells Using Hydrogen Bonding

Hydrogen bond interactions between a dye adsorbed at the interface of TiO₂ nanorods and functionalized P3HT was used to control nanorod dispersion, increase interfacial area, and improve efficiency in solution-processable hybrid bulk heterojunction solar cells. A series of poly­(3-hexylthiophene-<i>b</i>-ethylene glycol) (P3HT-<i>b</i>-PEG) copolymers were prepared by a combination of Grignard metathesis polymerization and click chemistry. The short PEG segments in P3HT-<i>b</i>-PEG serve as a hydrogen bond acceptor. TiO₂ nanorods functionalized with N3-dye bearing multiple COOH groups function as both the electron acceptor and hydrogen bond donor. The strong preferential H-bonding interaction between TiO₂ nanorods and the PEG chain limits the aggregation of the TiO₂ nanorods and affords homogeneously dispersion of the nanorods within the polymer matrix to form an interpenetrating network. This structure provides large interfacial area between electron donor and acceptor and highly efficient transport pathways within the composite. Hybrid devices constructed from copolymers with 10 wt % PEG and N3-dye TiO₂ nanorods exhibit power conversion efficiency ∼50% higher than that of conventional P3HT homopolymer and N3-dye TiO₂ nanorods

Independent Tuning of the Band Gap and Redox Potential of Graphene Quantum Dots

The band gap and redox potential of semiconductor nanocrystals are two quantities of primary importance for their applications in energy conversion devices. Herein, we report on covalent functionalization of colloidal graphene quantum dots through a solution-chemistry approach and studies of their band gaps and redox potentials. We show that their band gaps and redox potentials can be independently controlled, the former by size and the latter by functionalization. The size and the functionalization dependence of the properties can be numerically reproduced with tight-binding calculations, which thus provides a simple theoretical tool to guide the design of graphene QDs with desired properties

Additive-Driven Assembly of Block Copolymer–Nanoparticle Hybrid Materials for Solution Processable Floating Gate Memory

Floating gate memory devices were fabricated using well-ordered gold nanoparticle/block copolymer hybrid films as the charge trapping layers, SiO₂ as the dielectric layer, and poly(3-hexylthiophene) as the semiconductor layer. The charge trapping layer was prepared <i>via</i> self-assembly. The addition of Au nanoparticles that selectively hydrogen bond with pyridine in a poly(styrene-<i>b</i>-2-vinyl pyridine) block copolymer yields well-ordered hybrid materials at Au nanoparticle loadings up to 40 wt %. The characteristics of the memory window were tuned by simple control of the Au nanoparticle concentration. This approach enables the fabrication of well-ordered charge storage layers by solution processing, which is extendable for the fabrications of large area and high density devices <i>via</i> roll-to-roll processing

Cooperative Assembly of Hydrogen-Bonded Diblock Copolythiophene/Fullerene Blends for Photovoltaic Devices with Well-Defined Morphologies and Enhanced Stability

We report the cooperative self-assembly of functionalized fullerenes and all conjugated block copolymers (BCPs) containing polythiophene derivatives in both segments to yield solar cells with well-defined nanostructures and enhanced morphological stability. Favorable hydrogen bonding interactions between the COOH-functionalized fullerene, bis-[6, 6]-phenyl C61-butyric acid (bis-PCBA), and the tetraethyleneglycol side chains of poly­(3-hexylthiophene)-<i>block</i>-poly­[3-(2,5,8,11-tetraoxadodecane)­thiophene] (P3HT-<i>b</i>-P3TODT) allows for high loading of bis-PCBA (up to 40 wt % to the blend) within the P3TODT domains, while preserving the lamellar morphology. Characterization by grazing incidence small-angle X-ray scattering, electron microscopy, and atomic force microscopy indicates that the periods of the structures range between 24 and 29 nm depending on the bis-PCBA loading. The hydrogen bond interactions between bis-PCBA and P3TODT segments further suppress crystallization and macrophase separation of the fullerenes, even under harsh annealing conditions (150 °C for 12 h). Bulk heterojunction solar cells prepared using P3HT-<i>b</i>-P3TODT/bis-PCBA exhibit a photoconversion efficiency of 2.04%, which is greater than that of a reference system, P3HT-<i>b</i>-P3TODT/bis-PCBM. Accelerated aging experiments reveal enhanced thermal stability as a result of the limited translational mobility of COOH-functionalized fullerene in P3HT-<i>b</i>-P3TODT relative to devices prepared using bis-PCBM in P3HT-<i>b</i>-P3TODT or P3HT. We believe that cooperative assembly using strong noncovalent interactions is a general approach that can be used to improve the processing, morphological stability, and aging of organic and hybrid photovoltaic devices