United States

With the ever-growing global demand for renewable energy sources, Cu&lt;sub&gt;2&lt;/sub&gt;ZnSnS&lt;sub&gt;4&lt;/sub&gt; (CZTS) thin films can complement the existing Silicon-based photovoltaic (PV) cell market. However, low carrier mobility and lifetime, non-uniformity, and defects in the film structure have reduced the widespread application of CZTS. During the manufacture of CZTS solar cells, the initial as-deposited films have a nanocrystalline structure. Therefore post-deposition annealing is critical to make the efficiency of the cell commercially viable. Conventional furnace annealing methods have failed to enable this technology&#39;s commercialisation, and the heating of the complete PV cell limits the selection of available substrates. This research explored alternative approaches, specifically diode and pulsed fibre laser annealing for CZTS films deposited on flexible Mo foils in different gas atmospheres. SEM, XRD, and Raman spectroscopy techniques were used to analyse the laser-annealed samples. Processing parameters were established by which grain enlargement was observable with the pure CZTS phase retained. The key findings build confidence in laser annealing as an alternative method to enhance the crystallinity, surface morphology, chemical composition, and electrical properties of CZTS thin films for next-generation PV cells. Laser annealing can potentially reduce the reliance on vacuum furnace heating of PV cells and enable annealing on more thermally sensitive substrates.

ICALEO 2023

Exploring Laser Annealing as an Alternative Approach to Improve CZTS Thin Film Quality for Photovoltaic Cells

solar

With the ever-growing global demand for renewable energy sources, Cu2ZnSnS4 (CZTS) thin films can complement the existing Silicon-based photovoltaic (PV) cell market. However, low carrier mobility and lifetime, non-uniformity, and defects in the film structure have reduced the widespread application of CZTS. During the manufacture of CZTS solar cells, the initial as-deposited films have a nanocrystalline structure. Therefore post-deposition annealing is critical to make the efficiency of the cell commercially viable. Conventional furnace annealing methods have failed to enable this technology's commercialisation, and the heating of the complete PV cell limits the selection of available substrates. This research explored alternative approaches, specifically diode and pulsed fibre laser annealing for CZTS films deposited on flexible Mo foils in different gas atmospheres. SEM, XRD, and Raman spectroscopy techniques were used to analyse the laser-annealed samples. Processing parameters were established by which grain enlargement was observable with the pure CZTS phase retained. The key findings build confidence in laser annealing as an alternative method to enhance the crystallinity, surface morphology, chemical composition, and electrical properties of CZTS thin films for next-generation PV cells. Laser annealing can potentially reduce the reliance on vacuum furnace heating of PV cells and enable annealing on more thermally sensitive substrates.

technical paper

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
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Managing Director, Precitec GmbH

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ICALEO® brings together the brightest minds, seasoned professionals, and pioneering researchers from around the world. With a laser-focused agenda, ICALEO offers an exceptional platform to delve into the cutting-edge advancements, diverse applications, and transformative research that define lasers and electro-optics.

Selective copper generation on polymers by reductive laser sintering represents an advantageous alternative to conventional mask-based approaches such as lithography or processes based on noble metals. A variety of applications in sensing, thermoelectronics and optoelectronics have already been demonstrated. For laser sintering, ultrashort-pulsed lasers enable precise control of the introduced energy and a reduced heat-affected zone. In turn, this type of laser can also be used to implement other processes such as ablation, modification, or the integration of embedded microfluidics and micro-optical systems. In previous studies, copper patterns with high spatial resolution have been achieved by using microscope objectives. However, design of microelectronic circuits also consists of broad conductive paths and contact areas, which result in time-consuming processing when hatching fine lines. The use of galvanometric scanners with long focal length lenses results in larger focal diameters that enable faster processing. However, this is also accompanied by higher laser powers, loading the heat-sensitive substrates and promoting reoxidation of the resulting copper structures, which can dramatically increase the achieved resistivities. 
Against this background, we report on the possibility to accelerate femtosecond reductive laser sintering of copper(II) oxide on the surface of transparent cyclic olefin copolymers by varying the focus size. By using different focusing conditions, different process speeds are achieved and the structural accuracies are controlled. The generated copper layers are investigated using scanning electron microscopy, energy-dispersive X-ray spectroscopy and 4-tip resistivity measurements to achieve high copper content at low resistivities in the resulting copper films at increased metallization rate.

Acceleration of Femtosecond Reductive Laser Sintering of Copper(II) Oxide for Conductive Copper Patterns on Cyclic Olefin Copolymers

Localized metallization of flexible and rigid dielectric materials is a powerful tool for surface engineering and modification. Metallic patterns are of great interest in various fields of science and technology, where they can find applications in the production of electronics, sensor devices, etc.&nbsp; Along with this, the vector of modern industrial microelectronics technologies is shifting annually towards the development and application of more environmentally friendly approaches. 
Deep eutectic solvents have been gaining popularity for the last 10 years as “green” solvents. Thus, the use of such systems seems extremely relevant, taking into account the replacement of traditional approaches with more environmentally friendly ones. 
We present a new approach based on deep eutectic solvents (DES)-assisted synthesis of conductive copper structures using regular technique of laser-induced liquid deposition of metals (LCLD). It was shown that it is possible to increase the deposition rate of copper by an order of magnitude by using deep eutectic solvents (DESs) as a medium for laser synthesis.&nbsp; As a results, the developed approach overcomes the previous limitations of the one-stage laser-assisted deposition technology, and opens the opportunities for new applications of this method, e.g. in micro-LEDs and displays technologies. We anticipate further development of this approach for fabrication of a wide range of conductive materials on various substrates with higher deposition rate.

Direct Laser Writing of Highly Conductive Copper Micropatterns from Deep Eutectic Solvents

Artificial intelligence is a trend that has been on innovation leader's radar for a long time. It is a technological enabler for a more sustainable future and will significantly increase business efficiency if used meaningfully. It affects our daily lives, and the way people and companies will work now and in the future. Insights into practical applications with various maturity degree will be presented from the perspective of a leading solution provider in the sheet metal industry. In times when there is a shortage of experienced operators, it is especially important to have a solution that enables machine operators to find the optimal machine settings and parameters to achieve the best performance and quality in shortest time. Artificial intelligence also supports the increase of the automatization degree of systems achieving a low-man production and ensures 24/7 production with assisting systems for monitoring the status of the laser cutting machines in operation and especially, during cutting. In addition to the monitoring, control procedures are presented to enhance the performance and quality demands of a laser cutting machine. A method for in-process quality control is presented prior to bending the sheet metals avoiding the processing of defect parts at an early stage.

Novel Multi-Material Lattice Structures Fabricated via Laser-Powder Bed Fusion Process

The capability to produce complexly and individually shaped metallic parts is one of the main advantages of the laser powder bed fusion (PBF‑LB/M) process. Development of material and machine specific process parameters is commonly based on results acquired from small cubic test coupons of about 10 mm edge length. Such cubes are usually used to conduct an optimization of process parameters to produce dense material. The parameters are then taken as the basis for the manufacturing of real part geometries. However, complex geometries go along with complex thermal histories during the manufacturing process which can significantly differ from thermal conditions prevalent during the production of simply shaped test coupons. This may lead to unexpected and unpredicted local inhomogeneities of the microstructure and defect distribution in the final part and it is a root cause of reservations against the use of additive manufacturing for the production of safety relevant parts. In this study, the influence of changing thermal conditions on the resulting melt pool depth of 316L stainless steel specimens is demonstrated. A variation of thermographically measured intrinsic preheating temperatures was triggered by an alteration of inter layer times and a variation of cross section areas of specimens for three distinct sets of process parameters. Correlations between the preheating temperature, the melt pool depth, and occurring defects were analyzed. The limited expressiveness of the results of small density cubes is revealed throughout the systematic investigation. Finally, a clear recommendation to consider thermal conditions in future process parameter optimizations is given.

On the Limitations of Small Cubes as Test Coupons for Process Parameter Optimization in Laser Powder Bed Fusion of Metals

Additive manufacturing of copper using laser powder bed fusion, enables the production of highly complex components, with excellent heat and electrical conductivity. However, the processing of copper by means of near-infrared laser radiation, which is commonly used, is challenging due to its high reflectivity. Nevertheless, it has been demonstrated that high densities and electrical conductivities can be achieved using high power laser systems. In order to process pure copper with reliable quality with different machines, it is essential to understand the conditions at which a continuous weld track is formed. For this purpose, weld tracks with varying laser power and scan speeds were welded on a copper substrate plate with an applied powder layer. The preheating temperature of the substrate plate and the beam size were varied to test different process conditions. The melt pool depths and widths were measured and a relationship was elaborated. Based on these results, cube samples with discrete weld tracks on top were manufactured. The melt pool depth was measured and compared with the predicted melt pool depth to investigate the transferability of the elaborated relationship from substrate to process conditions. It was found that with rising preheating temperature and for larger beam diameters at the same peak intensity, the weld width and weld depths increase. Furthermore, continuous weld tracks formed reliably in the keyhole welding regime. A good agreement between the weld depth of weld tracks on substrate and the elaborated relationship was revealed. However, the weld tracks were shallower than predicted.

Influence of Temperature and Beam Size on Weld Track Shape in Laser Powder Bed Fusion of Pure Copper Using Near-Infrared Laser System

The demands of the EV industry on the manufacturing processes are increasing rapidly in terms of productivity, strength, and electrical performance. Common laser tools are already reaching their performance limits in demanding fields of application, such as joining of copper and aluminum, or high-speed welding of thin metal foils. Innovative laser beam sources with new degrees of freedom in laser material processing are therefore needed to meet the increasing requirements. In this article, we examine selected laser applications in the EV industry, particularly in the areas of battery, e-motor and fuel cell production. In the case of electrical contacts, in addition to the requirements of high strength and low electrical resistance, there is also the difficulty that the welding depth may only be within a very narrow tolerance band, otherwise the parts would be destroyed if the component was shot through. In addition to the precise welding depth, the heat input is also a major factor in determining whether the component remains intact during the joining process. In this contribution results of fundamental process investigations when using two superimposed laser beams of high brilliance of the latest generation are presented. It is shown that both productivity and process stability can benefit from this new type of laser tool.

Latest Application Developments Across EV-Industries with Overlaying Highly Brilliant Laser Beams of the Next Generation

In order to achieve sustainable development goals, SDGs, decarbonization and low-carbonization are required. Electric and hybrid vehicles are indispensable for the conservation of natural environment, and the lightweight construction and the effective transfer of electricity become important. Thus, copper and aluminum have been becoming important materials because of their excellent electrical properties. However, in the welding of these materials, it is difficult to obtain strong joints, since there are problems in the brittle intermetallic compounds and the welding defects due to different melting points between copper and aluminum. Especially in joining of copper and aluminum by copper side irradiation, aluminum rich intermetallic compounds (IMC) shows brittle state, and mechanical strength decreases drastically. Therefore, mild heat input from copper to aluminum would be necessary to reduce the brittle IMC. Angled irradiation might result in the mild energy input to aluminum, because it can be expected that aluminum would be heated by the reflected light inside the keyhole generated in copper according to high light reflection of copper. In addition, stable welding can be expected by the superposed irradiation of blue and near-infrared lasers because of high light absorption rate of blue laser to copper. The angled and the superposed irradiation could achieve stable welding state, and the generation of aluminum rich IMC became smaller. Angled irradiation of near-infrared laser showed equivalent joining strength to the superposed irradiation of two wavelengths, and the combination of angled and superposed irradiation achieved the remarkable increase of joining strength in cross tensile test by 80%.

Fundamental Study on High-Quality Welding of Copper and Aluminum by Angled and Superposed Irradiation of Blue and Near-Infrared Lasers

The mobility sector is considered as a major contributor to global greenhouse gas emissions and air pollution. As a result, many countries have initiated the transition from fossil fuel-powered to electrified powertrains. This transformation of the powertrain concept will lead to a rapid increase in the production of electric vehicles and, therefore, to a high demand for so-called traction bat­ter­ies. As a production step of the traction batteries, a connection between the cell connector and the terminal of the battery cell is required. For this purpose, laser beam welding is a reliable and efficient joining technique. In order to ensure continuous quality of the welding process during production and in order to detect defects in real time, reliable process monitoring is required. In this study, spectral and acoustic emissions during laser beam welding were recorded using a laser welding monitor and an optical microphone. For determining possible correlations between the signals and weld defects, various failure cases were generated by the systematic placement of disturbance elements. These elements included a contaminated surface, a gap between the cell connector and the battery cell, and a misalignment of the cell connector. Based on the recorded signals, statistical metrics were calculated. Finally, weld seams with and without defects were compared to assess the capability of both sensor systems for detecting the weld defects.

Inline Failure Detection in Laser Beam Welding of Battery Cells: Acoustic and Spectral Emission Analysis for Quality Monitoring

The shape of the laser beam significantly influences the geometry of both the cutting front and the cutting kerf. The angle of the cutting front influences the absorptivity while the width of the kerf defines the amount of material, which has to be molten. The geometry of the kerf therefore determines the maximum possible cutting speed. These relations between absorptivity, the width of the cutting kerf, and the maximum cutting speed are described by a simple model considering the conservation of energy and rough geometrical approximations. In order to verify the prediction of the model, the geometry of the interaction zone was observed by means of online high-speed X-ray imaging. The geometry of the cutting front and kerf was recorded with a framerate of 1000 Hz during fusion cutting of 10-mm-thick samples of stainless steel. The results show an increase of absorptivity in case of an enlargement of the beam in the direction of the feed, while the cross-sectional area of the cutting kerf increases in case of an enlargement of the width of the beam in the transversal direction. Finally, these findings show a good agreement with the simpel geometric model and allows to identify optimum beam shapes for a maximum cutting speed.

Influence of Beam Shaping on the Geometries of the Cutting Front and Kerf to Maximize the Speed of Laser Cutting

Cutting thick plates is affected not only by the laser power but also by the cut kerf width and the melt flow dynamics that determine the ejection of the molten material. Employing the same laser beam intensity distribution for various thicknesses is the limiting factor when cutting thicker plates. This paper investigates fiber laser fusion cutting of 25 mm thick aluminium with dynamic beam shaping (DBS). While both static and longitudinal dynamic intensity distributions fail to cut this thickness with a 4 kW laser power, a cut through is achieved using annular and elliptical intensity distributions. However, an improvement of 45&nbsp;% in cutting speed can be achieved using an elliptical intensity distribution compared to an annular one. In order to understand the effect of the beam shape, an infrared thermal camera is used to study lateral heat propagation when using different process parameters. The more efficient heat propagation in the cutting direction employing elliptical intensity distribution decreases the power losses compared to the annular intensity distribution. Moreover, to analyze the melt flow when changing the DBS frequency, high-speed imaging is utilized to observe the molten material inside the cut kerf. Finally, the cut edge quality is investigated for different cutting conditions.

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Exploring Laser Annealing as an Alternative Approach to Improve CZTS Thin Film Quality for Photovoltaic Cells

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Acceleration of Femtosecond Reductive Laser Sintering of Copper(II) Oxide for Conductive Copper Patterns on Cyclic Olefin Copolymers

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