Spatial Orientation and Order of Structure-Defining Subunits in Thin Films of a High Mobility n-Type Copolymer

Broadband polarized emission from P(NDI2OD-T2) polymer

We investigate the P(NDI2OD-T2) photophysical properties via absorbance and fluorescence spectroscopy, in association with the experimental approach baptized Stokes Spectroscopy, which provides valuable material information through the acquisition and analysis of the fluorescence polarization degree. By changing solvents and using different samples such as solutions, thick, and thin films, it is possible to control the polarization degree spectrum associated to the fluorescence emitted by the polymer's isolated chains and aggregates. We show that the polarization degree could become a powerful tool to obtain information related to the samples morphology, which is connected to their microscopic structure. Moreover, the polarization degree spectra suggest that depolarization effects linked to energy and charge transfer mechanisms are likely taking place. Our findings indicate that P(NDI2OD-T2) polymers are excellent candidates for the advancement of organic technologies that rely on the emission and detection of polarized lights.

In Situ Synthesis of Ternary Block Copolymer/Homopolymer Blends for Organic Photovoltaics

A detailed investigation of in situ-synthesized all-conjugated block copolymer (BCP) compatibilized ternary blends containing poly(3-hexylthiophene) (P3HT) and poly{[ N, N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dibcarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (PNDIT2) as donor and acceptor polymers, respectively, is presented. Both polymers are incompatible and show strong segregation in blends, which renders compatibilization with their corresponding BCPs promising to enable nanometer-phase-separated structures suitable for excitonic devices. Here, we synthesize a ternary block copolymer/homopolymer blend system and investigate the phase behavior as a function of block copolymer molecular weight and different annealing conditions. The device performance decreases on increasing annealing temperatures. To understand this effect, morphological investigations including atomic force microscopy, high-resolution transmission electron microscopy (HR-TEM), and grazing incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are carried out. On comparing domain sizes of pristine and compatibilized blends obtained from GISAXS, a weak compatibilization effect appears to take place for the in situ-synthesized ternary systems. The effect of thermal annealing is most prevalent for all samples, which, for the highest annealing temperature above the melting point of PNDIT2 (310 °C), ultimately leads to a change from the face-on to edge-on orientation of PNDIT2, as seen in GIWAXS. This effect dominates and decreases all photovoltaic parameters, irrespective of whether a pristine or compatibilized blend is used.

Temperature-dependent IR-transition moment orientational analysis applied to thin supported films of poly-ε-caprolactone

A novel experimental setup is described which enables one to carry out infrared transition moment orientational analysis (IR-TMOA) depending on temperature. By this, three dimensional molecular order parameter tensors of IR-active transition dipole moments with respect to the sample coordinate system can be determined in their thermal evolution (35 °C < T < 59 °C). As an example crystallinity and macroscopic order of poly-ε-caprolcatone are monitored. Both remain largely unaltered up to T ∼ 50 °C, above which they decrease. These reductions are explained as the melting of flat-on crystalline lamellae that make up about 34% of the crystalline material. The remaining crystallites are arranged into bulk-like, confined spherulitic structures and do not melt by more than (3 ± 3)%. Therefore, flat-on oriented lamellae are supposed to be kinetically favored by confinement during melt crystallization but are thermodynamically less stable than two-dimensionally confined bulk-like spherulites.