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 a conducting polymer have been grown by resonant infrared matrix-assisted pulsed-laser evaporation
(RIR-MAPLE). Properties of the thin films such as surface morphology and electrical conductivity have
been investigated as a function of laser wavelength, fluence, and pulse structure. Using a free-electron laser
whose wavelength is continuously tunable throughout the mid-infrared region (2-10 μm), we are able to deposit
polymer films from various liquid matrices by resonantly exciting selective vibrational modes of the solvent. An
Er:YAG laser operating at 2.94 μm is used to study the effects of different laser pulse durations. In the case of
poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), it is found that only specific excitation
wavelengths and pulse durations lead to the deposition of smooth and functional polymer films.
Multi-layered polymer light-emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant
infrared pulsed-laser deposition of the polymer layers. The light emitter used was poly[2-methoxy-5-(2-
ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and in some cases a layer of the hole-transport polymer
poly(3,4 etylenedioxythiophene:polystyrenesulfonate) (PEDOT:PSS) was also laser deposited, resulting in a device
structure of ITO/PEDOT:PSS/MEH-PPV/Al. Fourier transform infrared (FTIR) spectroscopy confirmed
that neither of the laser-deposited polymers was significantly altered by the deposition process. Laser-fabricated
devices displayed electroluminescent spectra similar to those of conventional spin-coated devices, but the differences
in electrical characteristics and device efficiency were substantial. These discrepancies can probably
be attributed to surface roughness of the deposited polymer layers. With the appropriate refinement of the
deposition protocols, however, we believe that this process can be improved to a level that is suitable for routine
fabrication of organic electronic components.
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.
Polymer light emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant infrared laser
ablation of the light emitting layer. The light emitting polymer used was poly[2-methoxy-5-(2-ethylhexyloxy)-
1,4-phenylenevinylene] (MEH-PPV) and was deposited into the device structure ITO/MEH-PPV/Al. Fourier
transform infrared (FTIR) spectroscopy confirmed that the laser-deposited polymer was not drastically altered by
the deposition process. Laser-fabricated devices displayed similar properties such as electroluminescence spectra
and IV characteristics as conventional spin-coated devices. The dependence of these device properties on laser
fluence was investigated, and showed no strong dependency. Peak emission wavelengths of electroluminescence
spectra were all within 10 nm of electroluminescence spectra of spin coated devices and showed only slight peak
broadening. These results are technologically important in that shadow mask technology can be incorporated
into this method to arbitrarily pattern substrates with light emitting polymers.
Ultrashort laser pulses interacting with brittle dielectrics in the mid-infrared region of the spectrum produce a number of novel effects which are potentially useful in materials processing and analysis. These include the texturing of the surface, the generation of hydrodynamic instabilities, and a surprisingly efficient and gentle ablation behavior. Nevertheless, the mechanism of infrared laser ablation remains somewhat mysterious. Here we present evidence for a mechanism of explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser operating in the wavelength region from 2 - 10 micrometer. The unusual pulse structure of the free-electron laser -- which produces 1-ps micropulses at intervals of 350 ps in a macropulse lasting up to 4 microseconds -- makes it possible to test separately the effects of intensity and fluence. We show in particular that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light.
Ultrashort-pulse lasers are increasingly being used for laser-induced surface modification, texturing and marking of insulators. Ultrashort pulses interacting with insulators in the vibrational IR produce a number of novel effects of potential utility in materials processing and analysis applications, including the creation of microbumps, microdimples, generation of hydrodynamic instabilities, and creation of smooth ablation craters. This paper describes recent results in the study of ultrashort-pulse laser interactions with surfaces when the irradiation is in the 2- 10 micrometers range. The laser source was a tunable, free- electron laser with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 4 microsecond(s) , with an average power of up to 3W. This unusual pulse structure makes possible novel test of the effects of resonant vibrational excitation, controlling the ratio of absorption depth to thermal diffusion length, and desorption and ionization by resonant excitation. The mechanisms underlying these effects, including vibrational excitation and relaxation dynamics, as well as their implications for materials-modification strategies, are discussed with reference to recent experimental examples.
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.
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.
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.
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.
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.
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.
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