Effects of Divalent Cations on Phase Behavior and Structure of a Zwitterionic Phospholipid (DMPC) Monolayer at the Air−Water Interface

Effects of divalent cations (Ca²⁺, Mg²⁺, Ni²⁺, and Zn²⁺) on a zwitterionic phospholipid monolayer at the air−water interface are investigated by surface pressure−area isotherms and in situ X-ray scattering. Divalent cations lower the surface pressure for the fluid (LE) to condensed (L₂) phase transition in a strongly ion-specific manner. Surprisingly, the two-dimensional lattice dimensions and the tilt of the lipids’ alkyl tails in the L₂ phase show a nearly ion-nonspecific dependence on the excess surface pressure above the transition pressure. An empirical “universal” relationship was found between the tail tilt and the excess pressure, with the tails in the L₂ phase always displaying a tilt of 29° at the transition. A practical implication of these results is that, regardless of the divalent cation present, the microscopic details of the lipid tail packing in the L₂ phase can be deduced at any surface pressure once the transition pressure is obtained from isotherms

Molecular Orientation and Performance of Nanoimprinted Polymer-Based Blend Thin Film Solar Cells

In this work, we have used synchrotron-based grazing incidence X-ray scattering to measure the molecular orientation and morphology of nanostructured thin films of blended poly­(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester blends patterned with nanoimprint lithography. Imprinting the blend films at 150 °C results in significant polymer chain orientational anisotropy, in contrast to patterning the film at only 100 °C. The temperature-dependent evolution of the X-ray scattering data reveals that the imprint-induced polymer reorientation remains at high temperatures even after the patterned topographic features vanish upon melting. Photovoltaic devices fabricated from the blend films imprinted at 150 °C exhibit a ∼21% improvement in power conversion efficiency compared to those imprinted at 100 °C, consistent with a polymer chain configuration better suited to charge carrier collection

Nanostructured Surfaces Frustrate Polymer Semiconductor Molecular Orientation

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C₆₁-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction

Photo-Cross-Linkable Azide-Functionalized Polythiophene for Thermally Stable Bulk Heterojunction Solar Cells

We have synthesized photo-cross-linkable azide-functionalized poly­(3-hexylthiophene) to explore improvements in the thermal stability of bulk heterojunction solar cells. Exposing blends of photo-cross-linkable polythiophene and [6,6]-phenyl-C₆₁-butyric acid methyl ester to ultraviolet light preferentially cross-linked the polythiophene without degrading its optical or electrical properties. X-ray scattering measurements showed that cross-linking slightly compacted the polythiophene chain lamellar stacking while increasing the polymer crystal coherence length by 20%. Optimized solar cells having cross-linked active blend layers retained 65% of their initial photovoltaic power conversion efficiency after 40 h of thermal annealing at 110 °C, while devices using un-cross-linked commercial polythiophene underwent significant phase separation and retained less than 30% of their initial efficiency after annealing

Multiphonon Relaxation Slows Singlet Fission in Crystalline Hexacene

Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10–100 ps) but significantly slower than pentacene (80–110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family