Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)

Visualizing a homogeneous blend in bulk heterojunction polymer solar cells by analytical electron microscopy

To increase efficiency of bulk heterojunctions for photovoltaic devices, the functional morphology of active layers has to be understood, requiring visualization and discrimination of materials with very similar characteristics. Here we combine high-resolution spectroscopic imaging using an analytical transmission electron microscope with nonlinear multivariate statistical analysis for classification of multispectral image data. We obtain a visual representation showing homogeneous phases of donor and acceptor, connected by a third composite phase, depending in its extent on the way the heterojunction is fabricated. For the first time we can correlate variations in nanoscale morphology determined by material contrast with measured solar cell efficiency. In particular we visualize a homogeneously blended phase, previously discussed to diminish charge separation in solar cell devices.

Tunable charge transport using supramolecular self-assembly of nanostructured crystalline block copolymers

Electronically functionalized block copolymers, combining covalently linked p-type and n-type blocks, show switching behavior of charge transport in organic field effect transistors (OFETs). The electronically active subunits self-assemble into continuous microdomains in a nanoscale regime, thereby forming percolation channels for holes or electrons or both depending on the composition and processing conditions. Here, we establish a charge transport-morphology relation for donor-acceptor block copolymers with two crystalline blocks. The n-type and p-type blocks self-assemble into two-dimensional lattices of π-π stacks and main chain polymer lamellae, respectively, over a broad composition range. Controlling the crystallization preferences of the two blocks by thermal annealing allows controlling the OFET polarity. Depending on the block ratio, the charge transport can be tuned from p-type to n-type or p-type to ambipolar, respectively. The impact of nanostructured phase separation is further delineated by X-ray diffraction, time-resolved spectroscopy, and scanning electron microscopy studies.

Molecular characterization of organic electronic films

Organic electronics have emerged as a viable competitor to amorphous silicon for the active layer in low-cost electronics. The critical performance of organic electronic materials is closely related to their morphology and molecular packing. Unlike their inorganic counterparts, polymers combine complex repeat unit structure and crystalline disorder. This combination prevents any single technique from being able to uniquely solve the packing arrangement of the molecules. Here, a general methodology for combining multiple, complementary techniques that provide accurate unit cell dimensions and molecular orientation is described. The combination of measurements results in a nearly complete picture of the organic film morphology.

Influence of molecular weight on the spectroscopic properties of a series of well-defined poly(3-hexylthiophene) polymers

A series of five poly(3-hexylthiophene) (P3HT) polymers were prepared, and their spectroscopic properties were studied. These polymers were well defined, with polydispersity indices no larger than 1.20 and regioregularities that were above 90% for each member of the series. The P3HT polymers were characterized using UV–vis and fluorescence spectroscopy, and their specific refractive index increments were measured in tetrahydrofuran. The influence of the molecular weight of these polymers on their maximum absorbance and emission wavelengths, molar extinction coefficients, and refractive index increments was studied.

Organic semiconductor growth and morphology considerations for organic thin-film transistors

Analogous to conventional inorganic semiconductors, the performance of organic semiconductors is directly related to their molecular packing, crystallinity, growth mode, and purity. In order to achieve the best possible performance, it is critical to understand how organic semiconductors nucleate and grow. Clever use of surface and dielectric modification chemistry can allow one to control the growth and morphology, which greatly influence the electrical properties of the organic transistor. In this Review, the nucleation and growth of organic semiconductors on dielectric surfaces is addressed. The first part of the Review concentrates on small-molecule organic semiconductors. The role of deposition conditions on film formation is described. The modification of the dielectric interface using polymers or self-assembled mono-layers and their effect on organic-semiconductor growth and performance is also discussed. The goal of this Review is primarily to discuss the thin-film formation of organic semiconducting species. The patterning of single crystals is discussed, while their nucleation and growth has been described elsewhere (see the Review by Liu et. al).([¹]) The second part of the Review focuses on polymeric semiconductors. The dependence of physico-chemical properties, such as chain length (i.e., molecular weight) of the constituting macromolecule, and the influence of small molecular species on, e.g., melting temperature, as well as routes to induce order in such macromolecules, are described.

Microstructural characterization and charge transport in thin films of conjugated polymers

The performance of semiconducting polymers has been steadily increasing in the last 20 years. Improved control over the microstructure of these materials and a deeper understanding of how the microstructure affects charge transport are partially responsible for such trend. The development and widespread use of techniques that allow to characterize the microstructure of semiconducting polymers is therefore instrumental for the advance of these materials. This article is a review of the characterization techniques that provide information used to enhance the understanding of structure/property relationships in semiconducting polymers. In particular, the applications of optical and X-ray spectroscopy, X-ray diffraction, and scanning probe techniques in this context are described.

Coarse-Grained Computer Simulations of Polymer/Fullerene Bulk Heterojunctions for Organic Photovoltaic Applications

We develop coarse-grained (CG) computer simulation models of poly(3-hexylthiophene) (P3HT) and P3HT/fullerene-C60 mixtures, in which collections of atoms from a physically accurate atomistic model are mapped onto a smaller number of "superatoms". These CG models allow much larger systems to be simulated for longer durations than is achievable atomistically, making it possible to study in molecular detail the morphology of polymer/fullerene bulk heterojunctions at length and time scales relevant to organic photovoltaic devices. We demonstrate that our CG models, parametrized at two state points, accurately capture the structure of atomistic systems at other points in the mixture phase diagram. Finally, we use our CG models to study the dynamic evolution of the microstructure of a P3HT/C60 bulk heterojunction in a system approaching the device scale.