Direct Laser Interference Patterning (DLIP) is an established technology for producing textured and functional surfaces using beam-shaped laser radiation. It consists of producing high-intensity interference patterns by overlapping two or more laser beams at the material surface. In this work, new possibilities for producing textured surfaces on metals and polymers using high-throughput concepts for DLIP are presented. The first concept describes the development of a new DLIP optical head (called xDLIP) with an outstanding depth of focus of approximately 10 mm, which can be equipped with fs, ps or ns pulsed laser systems. This approach makes this device ideal to treat large areas as well as three-dimensional parts. In particular, a setup using an industrial robot system is shown. The second approach includes the combination of a new DLIP optical system with a polygon scanner, showing the possibility to treat metallic and polymer surfaces. This includes configurations for reaching 7.0 and 21.0 μm spatial periods at throughputs beyond 1 m2/min. Finally, DLIP is implemented into a roll-to-roll process using a high-power picosecond pulsed laser source, in which the main laser beam is shaped into two elongated beams which go through a scanner system. Using this setup, aluminum and copper foils with thicknesses of 20 μm and 9 μm, respectively, are processed.
Nature provides many examples of surface structures with multiple functionalities. Some of those, such as light management and self-cleaning, are of interest for increasing the efficiency of optoelectronic devices, such as OLEDs, and for adding new surface functions. However, mimicking and transferring these textures to polymers over large areas often requires complex processes at high costs. Here, we demonstrate a low-cost strategy to fabricate hierarchically textured polyethylene terephthalate (PET) films by plate-to-plate hot embossing. Laser-machined stainless-steel plates with doublescaled hole-like textures were used as master for hot embossing. The larger structure with a period between 30 µm and 70 µm and depths up to 8 µm was produced by direct laser writing (DLW), whereas the smaller structure featuring a period of 3 µm at a depth up to 2 µm was fabricated by direct laser interference patterning (DLIP). The textured surfaces of stainless steel were then molded onto PET films at a pressure of 42 MPa and a temperature of 85°C using a hydraulic press. Topographical characterization was performed by confocal microscopy and scanning electron microscopy. Experiments have shown an increased static water contact angle up to 105°. Furthermore, the hierarchically microtextured foils were studied as out-coupling layers in OLEDs, showing a potential increase in device efficiency of up to 57%. The results thus indicate a good suitability of the developed surfaces for use in highly efficient OLEDs with easy-to-clean properties.
Perovskite solar cells (PSC) are a promising low-cost energy source for niche markets, such as energy harvesting semitransparent windows, and colored or arbitrary shaped solar modules for portable power sources or building facades. Furthermore, the possibility to fabricate flexible solar modules allows the integration of the whole manufacturing process into a roll-to-roll facility with the potential of reducing dramatically the fabrication costs. In the quest for high efficiency flexible PSC, the absorbed sunlight can be maximized employing a light trapping technique, such as using a microstructured substrate capable to scatter or diffract the incoming light into multiple directions elongating the optical path in the absorber. This work presents a new strategy to pattern microstructures on polymers suitable as transparent substrates for flexible PSC with enhanced light trapping. This industrial compatible approach consists only on two processing steps. First, a cylindrical metallic stamp is structured using Direct Laser Interference Patterning (DLIP), and next, the stamp is used in a roll-to-roll hot embossing system to transfer the stamp pattern to polymeric foils. Optimizing the DLIP processing and hot embossing parameters, high-quality imprints were obtained with periodic features with a spatial period of 2.7 μm. PSC were deposited onto these structured substrates showing an increase in the light absorption and efficiency. Spectroscopic characterization using an integrating sphere suggests that the PSC efficiency increase is caused by an elongated optical path inside the perovskite due to scattering and diffraction in the visible spectrum.
In this contribution, Cr stamps were structured with periodic hole-like arrays using Direct Laser Interference Patterning. Using optimized laser processing parameters, homogeneous textures with different spatial periods and aspect ratios were produced, as observed with confocal microscopy. Then, these stamps were used as molds for patterning transparent polymers, namely polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA) and polycarbonate (PC) by plate-to-plate hot embossing. Adjusting the imprint time and temperature, replicas of the Cr stamps were fabricated reproducing faithfully the mold shape. Static water contact angle (WCA) measurements were done to study the wettability properties of these structured polymers. The results show that the produced topographies increase the WCA compared to the flat references up to an absolute maximum of 55° in the three polymers. For instance, a PET foil structured with a spatial period of 4.7 μm and a structured depth of 3 μm showed a WCA of 139° representing an increase of about 70% compared to a flat sample. The diffraction intensity of the patterned polymers was measured with an imaging optics and a spectrometer coupled to a goniometer in order to characterize their optical properties. It was found that for those samples processed with 80 or less applied laser pulses the diffraction peaks are clearly identified, whereas for large number of pulses there is more diffuse light travelling along random directions and the diffraction peaks become less defined. In conclusion, enhanced wettability and optical functionalities were achieved in transparent polymers imprinted with periodic microstructures.
Recently, product protection and tracking became increasingly important due to the spread of piracy and counterfeiting. A common anti-counterfeiting procedure is embedding holographic motives or logos onto the good. If the motive is engraved directly onto the material surface, these features are inseparable from the good adding a higher degree of security. Holographic coloring is achieved by fabricating periodic surface structures, where the dimensions of the spatial periods lie in the order of the wavelengths contained in the visible spectrum. However, the fabrication of such periodic features directly on the product surface at high resolution and manufacturing speed is still challenging. Direct Laser Interference Patterning (DLIP) is an industrial compatible method with high processing flexibility which allows the structuring of holographic motives with high resolution and throughput. In this work, DLIP is employed to produce diffraction gratings with variable spatial periods and feature heights on a transparent PET substrate, which is a polymer commonly used for mass consumer goods and packaging. A numerical model based on the finite element method was used to restrict the gratings’ geometrical parameters that maximize the diffraction efficiency in reflection mode before their fabrication. Then, using the design of experiment approach, the laser processing parameters (laser power, pulse-overlap, spatial period) were selected in order to maximize the experimental first-order diffraction intensity, measured with a photospectrometer. The results allow to find the optimum set of parameters to fabricate homogeneous gratings with a first-order reflected intensity up to 4 % of the light source intensity.
Here we investigate light trapping substrates and electrodes for enhancing the performance of organic photovoltaics (OPVs). Their power conversion efficiency (PCE) can be improved by a factor of 1.16 using laser patterned PET substrates and by a factor of 1.13 using commercial, structured display films. Furthermore, we prepare light trapping electrodes using as flexible conductive polymer with embedded TiO2 nanoparticles, improving the PCE by a factor of 1.08 as compared to a neat polymer electrode. However, nano-imprinted conductive polymer electrodes does not provide light trapping effect due to the small size (50 nm) of the structures. Moreover flexible OPV devices, integrating the above light trapping elements, show non-degraded performance after bending tests.
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