In situ characterization of stresses, deformation and fracture of thin films using transmission X-ray nanodiffraction microscopy.

The use of hard X-ray transmission nano- and microdiffraction to perform in situ stress and strain measurements during deformation has recently been demonstrated and used to investigate many thin film systems. Here a newly commissioned sample environment based on a commercially available nanoindenter is presented, which is available at the NanoMAX beamline at the MAX IV synchrotron. Using X-ray nanoprobes of around 60-70 nm at 14-16 keV and a scanning step size of 100 nm, we map the strains, stresses, plastic deformation and fracture during nanoindentation of industrial coatings with thicknesses in the range of several micrometres, relatively strong texture and large grains. The successful measurements of such challenging samples illustrate broad applicability. The sample environment is openly accessible for NanoMAX beamline users through the MAX IV sample environment pool, and its capability can be further extended for specific purposes through additional available modules.

A new tetracyclic lactam building block for thick, broad-bandgap photovoltaics.

A new tetracyclic lactam building block for polymer semiconductors is reported that was designed to combine the many favorable properties that larger fused and/or amide-containing building blocks can induce, including improved solid-state packing, high charge carrier mobility, and improved charge separation. Copolymerization with thiophene resulted in a semicrystalline conjugated polymer, PTNT, with a broad bandgap of 2.2 eV. Grazing incidence wide-angle X-ray scattering of PTNT thin films revealed a strong tendency for face-on π-stacking of the polymer backbone, which was retained in PTNT:fullerene blends. Corresponding solar cells featured a high open-circuit voltage of 0.9 V, a fill factor around 0.6, and a power conversion efficiency as high as 5% for >200 nm thick active layers, regardless of variations in blend stoichiometry and nanostructure. Moreover, efficiencies of >4% could be retained when thick active layers of ∼400 nm were employed. Overall, these values are the highest reported for a conjugated polymer with such a broad bandgap and are unprecedented in materials for tandem and particularly ternary blend photovoltaics. Hence, the newly developed tetracyclic lactam unit has significant potential as a conjugated building block in future organic electronic materials.

Plasmon-enhanced four-wave mixing by nanoholes in thin gold films.

Nonlinear plasmonics opens up for wavelength conversion, reduced interaction/emission volumes, and nonlinear enhancement effects at the nanoscale with many compelling nanophotonic applications foreseen. We investigate nonlinear plasmonic responses of nanoholes in thin gold films by exciting the holes individually with tightly focused laser beams, employing a degenerated pump/probe and Stokes excitation scheme. Excitation of the holes results in efficient generation of both narrowband four-wave mixing (FWM) and broadband multiphoton excited luminescence, blueshifted relative to the excitation beams. Clear enhancements were observed when matching the pump/probe wavelength with the hole plasmon resonance. These observations show that the FWM generation is locally excited by nanoholes and has a resonant behavior primarily governed by the dimensions of the individual holes.

Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope.

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.

Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope.

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.

High-Entropy Mixtures of Pristine Fullerenes for Solution-Processed Transistors and Solar Cells.

The solubility of pristine fullerenes can be enhanced by mixing C60 and C70 due to the associated increase in configurational entropy. This "entropic dissolution" allows the preparation of field-effect transistors with an electron mobility of 1 cm(2) V(-1) s(-1) and polymer solar cells with a highly reproducible power-conversion efficiency of 6%, as well as a thermally stable active layer.

Mapping fullerene crystallization in a photovoltaic blend: an electron tomography study.

The formation of fullerene crystals represents a major degradation pathway of polymer/fullerene bulk-heterojunction thin films that inexorably deteriorates their photovoltaic performance. Currently no tools exist that reveal the origin of fullerene crystal formation vertically through the film. Here, we show that electron tomography can be used to study nucleation and growth of fullerene crystals. A model bulk-heterojunction blend based on a thiophene-quinoxaline copolymer and a fullerene derivative is examined after controlled annealing above the glass transition temperature. We image a number of fullerene nanocrystals, ranging in size from 70 to 400 nanometers, and observe that their center is located close to the free-surface of spin-coated films. The results show that the nucleation of fullerene crystals predominately occurs in the upper part of the films. Moreover, electron tomography reveals that the nucleation is preceded by more pronounced phase separation of the blend components.