A novel approach is presented for the controlled intramolecular collapse of linear polymer chains to give well-defined single-molecule nanoparticles whose structure is directly related to the original linear polymer. By employing a combination of living free radical polymerization and benzocyclobutene (BCB) chemistry, nanoparticles can be routinely prepared in multigram quantities with the size being accurately controlled by either the initial degree of polymerization of the linear chain or the level of incorporation of the BCB coupling groups. The latter also allows the cross-link density of the final nanoparticles to be manipulated. In analogy with dendritic macromolecules, a significant reduction of up to 75% in the hydrodynamic volume is observed on going from the starting random coil linear chains to the corresponding nanoparticles. The facile nature of the living free radical process also permits wide variation in monomer selection and functional group incorporation and allows novel macromolecular architectures to be prepared. Furthermore, the use of block copolymers functionalized with benzocyclobutene groups in only one of the blocks gives, after intramolecular collapse, a hybrid architecture in which a single linear polymer chain is attached to the globular nanoparticle.

We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction (BHJ) organic photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which has been shown to yield very high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC(71)BM). A surprisingly low degree of order was found in the polymer-far lower in the bulk heterojunction than in pure PTB7. X-ray diffraction data yielded a nearly full orientation distribution for the polymer π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction ("face-on"). By combining molecular orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, we propose a model describing the PTB7 molecular orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈20%) of the polymer in the PTB7/PC(71)BM blend is ordered. Energy-filtered transmission electron microscopy shows that the morphology of PTB7/PC(71)BM is composed of nanoscale fullerene-rich aggregates separated by polymer-rich regions. The addition of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO.

It has recently been shown that surface energy effects can cause selective segregation at the active layer interfaces of a bulk heterojunction (BHJ) organic photovoltaic device. The active layer interface composition has been suggested to impact device performance. In this study changes in the BHJ vertical composition profile of BHJ active layers cast on two hole transport layers (HTL) with significantly different surface energies (γ) are characterized using spectroscopic ellipsometry and near-edge X-ray absorption fine structure spectroscopy. Changes in the HTL γ are shown to significantly affect the BHJ interfacial segregation at the buried interface near the HTL while the composition near the free surface (air) of the BHJ is unaffected. Despite the significant differences in vertical segregation at the HTL interface, the performances of the resulting organic photovoltaic devices were relatively similar.

The buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast. The interface composition determines the type of charge transport measured with thin film transistors. These results have implications for organic photovoltaics device design and the use of transistors to evaluate bulk mobility in blends.

The large-scale manufacture of organic electronics devices becomes more feasible if the molecular orientation and morphology of the semiconductor can be controlled. Here, we report on a previously unidentified crystal shape of terraced nanoscale "ribbons" in thin films of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT). The ribbons form after a pBTTT film is heated above its highest temperature phase transition. In contrast to the wide terrace crystal shape previously reported, terraced ribbons have lateral widths of approximately 60 nm and lengths greater than 10 microm, with a common orientation between adjacent ribbons. Further, we report a simple and scalable flow coating process that can control the ribbon orientation without requiring special substrates or external fields. The degree of molecular orientation is small after coating but increases dramatically after the terraced ribbons are formed, indicating that an oriented minority templates the whole film structure. The large extent of orientation obtained in these polythiophene crystallites provides potential opportunities to exploit anisotropic electrical properties and to obtain detailed information about the structure of organic semiconductor thin films.