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Extensive research in micro/nanoscale surface engineering has developed a variety of energy applications including improvement of light trapping performance in solar cells, and increased surfaced area on electrodes in batteries or fuel cells applications with extended life time. In this study, we are aiming to the evaluation of a cost effective surface texturing method based on rapid scanning of nanosecond laser pulses. In contrast to conventional laser-assisted methods, we have achieved highly uniform and controllable texturing means over arbitrary scanning area of semiconductor and metallic surfaces.

While utilization of renewable solar energy by converting to electricity via photovoltaic (PV) solar cells is one promising route to meet urgent energy needs without involving fossil fuel consumption or carbon dioxide emission, the challenge lies on reducing the cost per watt to compete with traditional fossil fuel technology. To this end, developing low cost PV manufacturing technologies at improved manufacturing and device efficiencies is primary challenge to ensure that solar energy is a viable and economic source for power needs. In this paper, recent efforts on short pulsed laser scribing processes of CIGS (Copper Indium Gallium Diselenide) thin film solar cells will be demonstrated. High repetition rate (~ 100 kHz) picosecond laser based results are compared with those by nanosecond laser. Advantages and limitations of picosecond laser scribing process will be discussed, and a tentative solution based on cost-effective nanosecond lasers will be proposed. A further improved scribing quality and accuracy will be also attempted by gas injection scheme.

Cost-effective laser patterning of indium tin oxide (ITO) thin film coated on flexible polyethylene terephthalate (PET) film substrate for touch panel was studied. The target scribing width was set to the order of 10 μm in order to examine issues involved with higher feature resolution. Picosecond-pulsed laser and Q-switched nanosecond-pulsed laser at the wavelength of 532nm were applied for the comparison of laser patterning in picosecond and nanosecond regimes. While relatively superior scribing quality was achieved by picosecond laser, 532 nm wavelength showed a limitation due to weaker absorption in ITO film. In order to seek for cost-effective solution for high resolution ITO scribing, nanosecond laser pulses were applied and performance of 532nm and 1064nm wavelengths were compared. 1064nm wavelength shows relatively better scribing quality due to the higher absorption ratio in ITO film, yet at noticeable substrate damage. Through single pulse based scribing experiments, we inspected that reduced pulse overlapping is preferred in order to minimize the substrate damage during line patterning.

There is increasing demand for functional polymeric optical coatings for plastic substrates. In the case of anti-reflective (AR) coatings, this is challenging because polymers exhibit a relatively narrow range of refractive indices. We synthesized a four-layer AR stack using hybrid polymer:nanoparticle materials deposited by resonant infrared matrixassisted pulsed laser evaporation (RIR-MAPLE). An Er:YAG laser ablated frozen solutions of a high-index composite containing TiO2 nanoparticles and PMMA, alternating with a low-index solution of PMMA. The optimized AR coatings, with thicknesses calculated using commercial software, yielded a coating for polycarbonate with relative transmission over 94%, scattering less than 5% and a reflection coefficient below 0.8% across the visible range.

We describe resonant infrared pulsed laser deposition (RIR-PLD) of cyclic olefin copolymer, a barrier and protective layer; for comparison, we describe RIR-PLD of polystyrene and poly(ethylene dioxythiophene) about which we already have significant knowledge. Film deposition based on resonant infrared laser ablation is a low-temperature process leading to evaporation and deposition of intact molecules. In this paper, we focus on deposition of this model barrier and protective material that is potentially useful in the fabrication of organic light emitting diodes. The films were characterized by scanning electron microscopy and Fourier-transform infrared spectroscopy. We also compared the properties of films deposited by a free electron laser and a picosecond optical parametric oscillator.

We report experimental studies on laser scribing of thin film solar cells using various types of short pulsed lasers (nanosecond, picosecond, and femtosecond temporal pulse widths), aiming to determine the optimum laser parameters for the scribing of multilayer structures of amorphous silicon (a-Si) and copper indium diselenide (CIS) based solar cells. Detailed laser scribing parameters such as repetition rate of the laser pulses, scanning speed of the sample and laser beam, individual pulse energy, laser wavelength, and direction of laser illumination (either from film side or from substrate side) are examined. Characteristics of each scribing conditions are evaluated in terms of morphology by atomic force microscopy (AFM) and scanning electron microscopy (SEM), chemical species analysis by Energy Dispersive X-ray Spectroscopy (EDS), and electrical conductance of interconnects by conductive AFM (c-AFM) and contact resistance measurement to determine the optimal laser scribing conditions. Further issues on defects in the films such as re-deposited debris, elevated molten rim and delamination, thermal damage to surrounding and/or underlying layers and inter-diffusion of materials at the interface are discussed on the basis of thermal/mass diffusion, thermal stress, and ablation-induced plasma formation, in order to demonstrate an efficient laser scribing of P1/P2/P3 of thin film solar cells.

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser ablation. The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (5% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic "co-matrices" that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 S/cm), but only when the ablation was carried out at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.

This paper promotes the general paradigm that a composite’s internal structure can be micro-tailored to achieve a multifunctional physical response through the use of the Field Aided Micro Tailoring (FAiMTa) technique. The FAiMTa technique relies on curing a polymer composite while in its liquid state in the presence of an electric field. The particles within the composite align themselves in the direction of the electric field and create an orthotropic composite structure. This technology can lead to composite materials having a micro-tailored structure mimicking biological systems. As an initial step towards this goal, uniformly orthotropic composites, which are prepared by the FAiMTa technique, are mechanically characterized. Two epoxy based systems are considered: a composite having micro-sized graphite particles whereas the other has micro-sized aluminum particles. Mechanical tests show the change of material properties according to direction of the particle alignment within the composite. Optical microscopy also confirms the created orthotropic microstructure. The next step in development of FAiMTa technique is the reduction of stress concentration near a geometric discontinuity by properly orienting particulate structures within the composite. Our on-going efforts toward optimization of the composites are briefly outlined.

Ultrashort-pulse lasers are at an increasing rate being used for laser-induced surface modification of insulators, including ablation. Ti:sapphire chirped-pulse amplifier systems, with fundamental wavelengths in the near infrared, can produce efficient ablation and other desirable surface modifications with little collateral damage because the laser energy is deposited on a time scale much shorter than thermal diffusion times. Little is known, however, about how ultrashort pulses interact with insulators at wavelengths in the vibrational infrared. This paper describes surface modifications achieved by picosecond laser irradiation in the 2 - 10 micrometer range. The laser source was a tunable, free- electron laser (FEL) with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 4 microseconds, with an average power of up to 3 W. This unusual pulse structure makes possible novel tests of the dependence on fluence and intensity, as well as the effects of resonant vibrational excitation. As model materials systems, we studied calcium carbonate, its isoelectronic cousin sodium nitrate, and fused silica. Particularly intriguing are surface modifications achieved by tuning the laser into vibrational resonances of the target materials, or by tailoring the energy content of the pulse. The mechanisms underlying these effects, and their implications for materials-modification strategies, are discussed.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Calcium carbonate (CaCO3) and its structural relatives, the phosphates and hydroxyapatites, are natural crystals which are similar to the minerals found in such hard tissues as teeth and bone. We have recently demonstrated that laser- induced material removal in calcium carbonate occurs with high efficiency when irradiating with a free-electron laser at the fundamental asymmetric stretch mode of the carbonate group near 7 micrometers ; related studies show that the same thing is true in the isoelectronic sodium nitrate, and we expect it to operate in the phosphates as well when irradiated near the resonant 9 micrometers band. The mechanism of material removal appears to be the ejection of CO followed by a calcination reaction which produces CaO. Among the features which make CaCO3 such an interesting model material is that it also has a characteristic, temperature-dependent thermoluminescence - thus making it possible, by the study of the light emitted by the crystal prior to and after ablation, to estimate the temperature reached by the crystal in the early stages of laser ablation. Wavelength-dependent photoluminescence, photoacoustic and plume-spectroscopic studies show that efficient evaporative 'hole drilling' occurs at the infrared wavelengths corresponding to carbonate or nitrate vibration modes. However, where electronic or vibrational defects are excited by visible or infrared lasers, respectively, the mechanisms of material removal are photomechanical fracture in the former case and exfoliation or subsurface explosive vaporization in the latter.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Pulsed-laser irradiation on a solid surface induces a highly efficient surface cleaning of submicron-sized particulates and undesirable organic overcoat films. The nanosecond-pulsed UV laser irradiation, shortly after the deposition of a thin liquid film on the surface, induces rapid vaporization of the liquid film and removal of particulates (`steam cleaning'). The laser beam also causes ablative photodecomposition of organic film contaminants on the surface (`dry cleaning'). A laser cleaning tool is constructed with an aim toward practical use based on an industrial grade KrF excimer laser. The tool includes a computer-controlled laser beam scanning system and a stable liquid film deposition unit, providing a cleaning rate of over 200 cm2/min. The cleaning strategy and the physical mechanisms of the laser cleaning techniques are also studied.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The transient temperature and pressure field development in the excimer laser-induced vaporization of liquids in contact with a solid surface is studied. A thin silicon film, which has temperature-dependent optical properties, is embedded between an absorbing chromium film and a transparent fused quartz substrate. Static reflectivity measurement is performed to determine the thin film optical properties at elevated temperatures. The transient backward reflectance responses from the silicon layer are compared with heat transfer modeling results. The backward reflectance probe is not affected by the creation of bubbles and is successfully employed for the first time to measure non-intrusively the temperature development during the rapid vaporization process. The optical reflectance probes are applied from the front-side and back-side of the sample simultaneously to monitor the dynamic bubble nucleation behavior and transient temperature development, respectively, at various ambient pressures using a high- pressure cell. The investigation on the effect of ambient pressure on the bubble nucleation threshold combined with the surface temperature measurement determines the thermodynamic state of the superheated metastable liquid at the interface and subsequently the explosion pressure.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The formation of bubbles at a liquid-solid interface due to acoustic cavitation depends particularly on the preconditions of the interface. Here, it is shown that following laser- induced bubble formation at the interface the acoustic cavitation efficiency is strongly enhanced. Optical reflectance measurements reveal that this observed enhancement of acoustic cavitation due to preceding laser-induced bubble formation, which could be termed as a memory effect, decays in a few hundred microseconds. By performing a double-pulse experiment using two excimer lasers the influence of process parameters, such as liquid temperature and salt concentration, on the temporal decay of the memory effect has been studied. An analysis of the experimental results by a diffusion model is presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Melting and solidification of a silicon film by continuous wave laser beam irradiation has been studied. The silicon film melting and recrystallization is controlled by the temperature distribution in the semiconductor. Calculations have been carried out for a range of laser beam parameters and material translational speeds. The temperature field development also has been monitored with localized transient reflectivity measurements. During transient heating of semitransparent materials at the nanosecond scale, the thermal gradients across the heat affected zone are accompanied by changes in the material complex refractive index. These changes, coupled with wave interference, modify the energy absorption and thus the temperature field in the target material. These affects are taken into account in a rigorous manner using thin film optics theory.