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Manufacturing of CIGSe thin film solar modules involves typically one laser structuring step (P1) and two mechanical structuring steps (P2 and P3) for serial interconnection. In our approach, complete laser structuring is successfully demonstrated by application of short nanosecond laser pulses (<10 ns) with a single, visible wavelength of 532 nm. The P1 and the P3 trenches are scribed by induced and direct ablation, respectively. For the P2 scribe, the thermal input of the ns laser pulses is used to transform the CIGSe absorber layer locally into a highly conductive compound to provide proper electrical interconnection. These findings promise further simplification and flexibility to thin film solar cell production.
Laser-induced periodic surface structures (LIPSS) represent a unique route for functionalizing materials through the fabrication of surface nanostructures. Commercial AISI 316L stainless steel (SS316L) surfaces are laser treated by ultraviolet 300 ps laser pulses in a laser line scanning (LLS) approach. Processing parameters are optimized (pulse energy of 2.08 µJ, pulse repetition frequency of 300 kHz, and suitable laser scan and sample displacement rates) for the generation of low spatial frequency LIPSS over a large 25 × 25 mm2 area. Different angles of incidence of the laser radiation (0°, 30°, and 45°) and different linear laser beam polarizations (s and p) produce a plethora of rippled surface morphologies at distinct grains. Scanning electron microscopy and 2D Fourier transforms, together with calculations of the optical energy deposited at the treated surfaces using Sipe's first-principles electromagnetic scattering theory, are used to study and analyze in detail these surface morphologies. Combined with electron backscattering diffraction, analyses allow associating site-selectively various laser-induced-surface morphologies with the underlying crystalline grain orientation. Resulting grain orientation maps reveal a strong impact of the grain crystallographic orientation on LIPSS formation and point toward possible strategies, like multi-step processes, for improving the manufacturing of LIPSS and their areal coverage of polycrystalline technical materials.
In recent years, the improved understanding of the formation of laser-induced periodic surface structures (LIPSS) has led to an emerging variety of applications that modify the optical, mechanical, and chemical properties of many materials. Such structures strongly depend on the laser beam polarization and are formed usually after irradiation with ultrashort linearly polarized laser pulses. The most accepted explanation for the origin of the structures is based on the interference of the incident laser radiation with electromagnetic surface waves that propagate or scatter at the surface of the irradiated materials. This leads to an intensity modulation that is finally responsible for the selective ablation in the form of parallel structures with periods ranging from hundreds of nanometers up to some micrometers. The versatility when forming such structures is based on the high reproducibility with different wavelengths, pulse durations and repetition rate laser sources, customized micro- and nanometric spatial resolutions, and compatibility with industrially relevant processing speeds when combined with fast scanning devices. In this contribution, we review the latest applications in the rapidly emerging field of surface functionalization through LIPSS, including biomimetic functionalities on fluid transport, control of the wetting properties, specific optical responses in technical materials, improvement of tribological performance on metallic surfaces, and bacterial and cell growth for medical devices, among many others.
The polarization of the laser beam exhibits more substantial differences in laser micromachining as the angle of incidence deviates from zero. In the reported work, our focus was to explore the effects of circularly, p- and s-polarized laser on angled ultrashort pulse laser micromachining of micropillar arrays. The examination encompassed laser process factors, including angles of incidence, microstructure dimensions, and inter-pillar spacing. A comparison between the resulting structures demonstrated that p-polarized laser beam was the most efficient in material removal in angled laser micromachining, followed by circularly polarized laser. While the s-polarized beam exhibited the lowest ablation efficiency among the three. Such distinction is mainly attributed to the distinguishing reflectivity of the three states of polarization on tilted planes. The development of structural heights during ablation processes was examined, and potential defects in laser processing methodologies were interpreted. The dependency of structural heights on inter-pillar spacing was analyzed. This study bridges the gap between existing studies on angled ultrashort pulse laser machining and the influences of polarization on laser machining. The comparison between structures produced using laboratory-scale and industrial-scale laser systems also yielded pertinent recommendations for facilitating a smooth transition of angled laser micromachining from laboratory-scale research to industrial applications.
This article represents the editorial preface for the proceedings of the Symposium CC “Laser and plasma processing for advanced applications in material science” held from May 11th to 15th 2015 in the Lille Grand Palais, France, during the annual Spring Meeting of the European Materials Research Society (E-MRS).
Structuring of thin-film photovoltaic modules requires basic knowledge of the laser – thin-film interaction in order to adapt the accessible laser parameters, like wavelength, power, repetition rate and scribing speed whilst taking into account the specific material properties of the layer. We have studied the nanosecond laserablation behavior of corresponding layers (i) of silicon based thin-film solar cells with a-Si/µc-Si tandem absorber type and (ii) of back contact and absorber layer of CIGSe solar cells. The respective ablation threshold fluences were determined as integrative parameters describing the specific laser – material interaction. For the threshold determination we used two different methods and developed a new analytical approach taking into account scribing through the glass substrate as it is preferred for most structuring processes. This was done by analyzing the thin film ablation results by means of optical microscopy, profilometry, scanning electron microscopy (SEM). Moreover, we determined the incubation coefficient of the regarded material layers which allows us to predict quantitatively the influence of the spot overlap on the scribing threshold.
Micromachining experiments were performed with Ti:sapphire laser pulses (130 fs - 150 fs, 800 nm, approximately 10 Hz) in air. Employing the direct focusing technique, highly absorbing titanium nitride (TiN) and weakly absorbing polyimide (PI) and polymethylmethacrylate (PMMA) served as target materials. The lateral and vertical precision of the laser ablation and morphological features were characterized by scanning force (SFM), scanning electron (SEM) and optical microscopy. For TiN, incubation can be observed, i.e. the single-pulse surface damage threshold (0.26 J/cm2) is by a factor of two greater than the threshold for 100 pulses. Ablation rates below 10 nm per pulse can be achieved. The evolution of sub-wavelength ripples is presented in dependence on pulse number and laser fluence, respectively. The incubation behavior of the polymers can be described by an accumulation model as for TiN. Experiments on PI with varying focal lengths result in the same modification thresholds. Different polarization states of light (linear, circular) lead to a variation of the ablation rate and to various morphological patterns in the ablation craters (wavelength ripples, cones). Swelling of PMMA occurred at fluences below the ablation threshold.
The selective emitter is a well-known technology for producing highly doped areas under the metallization grid to improve the solar cell performance. In this work, the influence of laser irradiation on phosphoric acid coated multicrystalline silicon PV-wafers on the wafer surface structure, the phosphorous depth distribution and the electrical contact resistance within the laser treated area as well as the electrical series resistance of laserprocessed solar cells was evaluated. Different laser processing settings were tested including pulsed and continuous wave (cw) laser sources (515 nm, 532 nm, 1064 nm wavelength). Complementary numerical simulations using the finite element method (FEM) were conducted to explain the impact of the laser parameters on the melting behavior (melt duration and geometry). It was found that the melt duration is a key parameter for a successful laser Doping process. Our simulations at a laser wavelengths of 515 nm reveal that low-repetition rate (<500 kHz) laser pulses of 300 ns duration generate a melt duration of ~0.35 µs, whereas upon scanning cw-laser radiation at 532 nm prolongates the melt duration by at least one order of magnitude. Experimentally, the widely used ns-laser pulses did not lead to satisfying laser irradiation results. In contrast, cw-laser radiation and scan velocities of less than 2 m/s led to suitable laser doping featuring low electrical resistances in the laser treated areas.
The removal of a 75- to 90-nm-thick passivating silicon nitride antireflection coating from standard textured multicrystalline silicon photovoltaic wafers with a typical diffused 90-Ω/sq-emitter upon irradiation with near-infrared femtosecond laser pulses (790 nm central wavelength, 30 fs pulse duration) is studied experimentally. The laser irradiation areas are subsequently characterized by complementary optical microscopy, scanning electron microscopy and depth profiling chemical analyses using secondary ion mass spectrometry. The results clarify the thin-film femtosecond laser ablation scenario and outline the process windows for selective antireflection coating removal.