A. Exner

Nanostructured interfaces in polymer solar cells

The morphology in organic photovoltaics plays a key role in determining the device efficiency. We propose a method to fabricate bilayer devices with controlled nanostructured interfaces by combining nanoimprinting and lamination techniques. This technique allows us to achieve a network structure of donor-acceptor material with a ∼80 nm periodicity and ∼40 nm width. These structures have an abrupt interface between the donor and acceptor materials and show an increased effective interfacial area and photovoltaic performance compared to bilayer solar cells. In contrast to blend films, they will allow an in depth analysis of the influence of morphology on interfacial physical processes.

Low-Cost Thermo-Optic Imaging Sensors: A Detection Principle Based on Tunable One-Dimensional Photonic Crystals

Infrared (IR) sensors employing optical readout represent a promising class of devices for the development of thermographic imagers. We demonstrate an infrared radiation detection principle based on thermally tunable one-dimensional (1D) photonic crystals acting as optical filters, integrated with organic and inorganic light emitting diodes (OLEDs and LEDs, respectively). The optical filters are composed of periodically assembled mesoporous TiO2 and SiO2 layers. Due to the thermal tunability of the transmission spectrum of the optical filter, the intensity of light passing through the filter is modulated by temperature. The tuned spectrum lies in the visible region and, therefore, can be directly detected by a visible-light photodetector. The thermal response of the luminance of the OLED-photonic crystal ensemble is 3.8 cd m(-2) K(-1). Furthermore, we demonstrate that the local temperature profile can be time and spatially resolved with a resolution of 530 by 530 pixel, thus enabling a potential application as an infrared imaging sensor featuring low power consumption and low fabrication costs.

Ultra-thin titanium oxide

We demonstrate the fabrication of ultra-thin titanium oxide films by plasma-induced surface oxidation. Ellipsometry measurements indicate an oxide thickness of about 2 nm. Electrical characterization was performed on microscale and nanoscale metal-insulator-metal tunneling diodes. Electrical fields up to 22 MV/cm were applied without destroying the titanium oxide films. The current-voltage-characteristic of the diodes are found to be asymmetric with respect to zero bias when employing electrodes with different work functions. The permittivity of the ultra-thin titanium oxide was determined to be less than 6, which is the smallest permittivity that has been reported for titanium oxide.

Patterning Poly(3-Hexylthiophene) in the Sub-50-nm Region by Nanoimprint Lithography

We use thermal and room temperature nanoimprint lithography (NIL) for directly patterning the photoactive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) in the sub-50-nm region. Different types of molds were used to directly imprint the desired structures into P3HT thin films. Good pattern transfer is achieved independent of the presence of other underlying polymer layers or the type of substrate incorporated. Further, we discuss the future application of this technology to the fabrication of ordered heterojuction organic photovoltaic devices and demonstrate that the NIL process involved does not damage the polymer or alter its chemical or electrical properties.

Tunable thermoresponsive TiO2/SiO2 Bragg stacks based on sol–gel fabrication methods

Thermoresponsive TiO2/SiO2 one-dimensional photonic crystals (Bragg stacks) fabricated via sol–gel processing methods represent a promising class of environmentally responsive nanostructures featuring optically encoded temperature and humidity detection. The thermo-optic response of the layer materials is amplified by their inherent porosity owing to adsorption/desorption of ambient humidity into the mesoporous multilayer structure. Based on a comprehensive analysis of the impact of layer thickness, refractive index and thermo-optic coefficient on the stop band position, and width of various Bragg stack architectures, design criteria for thermoresponsive Bragg stacks operating in the visible range of the optical spectrum are put forward. A large and well-defined thermo-optic signature is expected for material combinations featuring individually high thermo-optic coefficients with the same sign or allowing for large changes in the effective refractive indices due to water adsorption in the porous layers reinforcing the thermo-optic response, as observed in the TiO2/SiO2 couple. Important practical aspects of the performance of thermoresponsive Bragg stacks are addressed, including the hysteresis properties of TiO2/SiO2 Bragg stacks during multiple heating/cooling cycles, as well as response and recovery times (~2–4 s) of the multilayer system during external changes in ambient humidity.