S. Lauzurica

Microprocessing of ITO and a-Si thin films using ns laser sources

Selective ablation of thin films for the development of new photovoltaic panels and sensoring devices based on amorphous silicon (a-Si) is an emerging field, in which laser micromachining systems appear as appropriate tools for process development and device fabrication. In particular, a promising application is the development of purely photovoltaic position sensors. Standard p–i–n or Schottky configurations using transparent conductive oxides (TCO), a-Si and metals are especially well suited for these applications, appearing selective laser ablation as an ideal process for controlled material patterning and isolation. In this work a detailed study of laser ablation of a widely used TCO, indium-tin-oxide (ITO), and a-Si thin films of different thicknesses is presented, with special emphasis on the morphological analysis of the generated grooves. Excimer (KrF, λ = 248 nm) and DPSS lasers (λ = 355 and λ = 1064 nm) with nanosecond pulse duration have been used for material patterning. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) techniques have been applied for the characterization of the ablated grooves. Additionally, process parametric windows have been determined in order to assess this technology as potentially competitive to standard photolithographic processes. The encouraging results obtained, with well-defined ablation grooves having thicknesses in the order of 10 µm both in ITO and in a-Si, open up the possibility of developing a high-performance double Schottky photovoltaic matrix position sensor.

Optical characterization of the heat-affected zone in laser patterning of thin film a-Si:H

In this paper we present an original approach to estimate the heat affected zone in laser scribing processes for photovoltaic applications. We used high resolution IR-VIS Fourier transform spectrometry at micro-scale level for measuring the refractive index variations at different distances from the scribed line, and discussing then the results obtained for a-Si:H layers irradiated in different conditions that reproduce standard interconnection parameters. In order to properly assess the induced damage by the laser process, these results are compared with measurements of the crystalline state of the material using micro-Raman techniques. Additionally, the authors give details about how this technique could be used to feedback the laser process parametrization in monolithic interconnection of thin film photovoltaic devices based on a-Si:H.

Chapter 13 – Laser-Induced Forward Transfer Techniques and Applications

Abstract Laser-induced forward transfer (LIFT) is a noncontact direct-write technique that enables the deposition of small volumes of material into user-defined, high-resolution patterns with a wide range of structural and functional materials. This chapter reviews the LIFT technique from its original development to the different approaches proposed to overcome some of its limitations. The physics and characterization methods associated with these processes are also presented. The last sections give a broad overview of the different applications and materials that have been addressed using these techniques; which cover simple metals and oxides to complex ceramics, polymers, biomolecules, and even living cells.

New strategies in laser processing of TCOs for light management improvement in thin-film silicon solar cells

Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells.

New laser-based approaches to improve the passivation and rear contact quality in high efficiency crystalline silicon solar cells

Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.

Laser induced forward transfer bioprinting of immune cells and chemoattractant proteins for immunological responses studies (Conference Presentation)

Laser bioprinting is a powerful tool in many biological fields due to its versatility in placing and construct different geometries of biological materials. The high accuracy and non-destructive nature of this method can be applied to the study of complex biological systems. In particular, single cell laser bioprinting helps to understand the relationships between cells and their local environment. Immunology is a transversal field that is governed by a complex network of genetic and signalling pathways subtending a network of interacting cells. In this context, mobility of the cells in a network along with their situation and the gene products they interact with, plays an important roll in the behaviour of the immune system. In this work we use a laser induced forward transfer blister assisted (BALIFT) approach to assess these cell-cell interaction and mobility in vitro. This method helps to understand properly the role of a cell in such networks to increase our knowledge of the immune system response. This work presents BALIFT bioprinting of single hematopoietic cells and chemoattractant proteins with high spatial resolution. In particular NK cells (natural killer), T-lymphocyte and chemokines and cytokine molecules are printed in specific patterns to study cell-cell and cell-environment interaction and cell migration. Whereby placing cellular components on a matrix previously designed on demand allow us to test the molecular interactions between lymphocytes and pathogens; as well as the generation of two-dimensional structures printed ad hoc in order to study the mechanisms of mobilization of immune system cells.

CLEO ® /Europe-EQEC 2017 light absorption enhancement of crystalline silicon wafers by direct laser texturing for heterojunction solar cells

Light trapping strategies that lead to an enhancement of the light absorbed by a device is a direct way to improve solar cells behavior. Light trapping strategies that lead to an enhancement of the light absorbed by a device is a direct way to improve solar cells behavior. The texturization of one or more surfaces is an industry standard in solar cells manufacturing. In the particular case of crystalline- and multi-crystalline-silicon wafers, this is usually achieved by means of a chemical etching. The control of the final morphology is limited though, especially in multi-crystalline silicon wafers where the etching rate in basic solutions is dependent on the crystal orientation of the silicon grains. The use of laser sources has opened the door to new texturing techniques leading to different kind of morphologies. An example is the use of laser pulses together with chemical etching or the texturization by direct laser scribing [1, 2]. In this work, we study the changes in the incident light reflection of silicon samples when they are textured by direct laser beams and compare them with similar samples textured by standard chemical etching and optimized for its use in silicon heterojunction solar cells.

Assessment of geometry in 2D immune systems using high accuracy laser-based bioprinting techniques (Conference Presentation)

The immune system is a very complex system that comprises a network of genetic and signaling pathways subtending a network of interacting cells. The location of the cells in a network, along with the gene products they interact with, rules the behavior of the immune system. Therefore, there is a great interest in understanding properly the role of a cell in such networks to increase our knowledge of the immune system response. In order to acquire a better understanding of these processes, cell printing with high spatial resolution emerges as one of the promising approaches to organize cells in two and three-dimensional patterns to enable the study the geometry influence in these interactions. In particular, laser assisted bio-printing techniques using sub-nanosecond laser sources have better characteristics for application in this field, mainly due to its higher spatial resolution, cell viability percentage and process automation. This work presents laser assisted bio-printing of antigen-presenting cells (APCs) in two-dimensional geometries, placing cellular components on a matrix previously generated on demand, permitting to test the molecular interactions between APCs and lymphocytes; as well as the generation of two-dimensional structures designed ad hoc in order to study the mechanisms of mobilization of immune system cells. The use of laser assisted bio-printing, along with APCs and lymphocytes emulate the structure of different niches of the immune system so that we can analyse functional requirement of these interaction.

Silicon PV module customization using laser technology for new BIPV applications

It is well known that lasers have helped to increase efficiency and to reduce production costs in the photovoltaic (PV) sector in the last two decades, appearing in most cases as the ideal tool to solve some of the critical bottlenecks of production both in thin film (TF) and crystalline silicon (c-Si) technologies. The accumulated experience in these fields has brought as a consequence the possibility of using laser technology to produce new Building Integrated Photovoltaics (BIPV) products with a high degree of customization. However, to produce efficiently these personalized products it is necessary the development of optimized laser processes able to transform standard products in customized items oriented to the BIPV market. In particular, the production of semitransparencies and/or freeform geometries in TF a-Si modules and standard c-Si modules is an application of great interest in this market. In this work we present results of customization of both TF a-Si modules and standard monocrystalline (m-Si) and policrystalline silicon (pc-Si) modules using laser ablation and laser cutting processes. A discussion about the laser processes parameterization to guarantee the functionality of the device is included. Finally some examples of final devices are presented with a full discussion of the process approach used in their fabrication.

Characterization of UV laser ablation for microprocessing of a-Si:H thin films

Hydrogenated amorphous silicon has been widely studied last years, both from the basic research and industrial points of view, due to the important set of potential applications that this material offers, ranging from Thin Films Transistors (TFTs) to solar cells technologies. In different fabrication steps of a-Si:H based devices, laser sources have been used as appropriate tools for cutting, crystallising, contacting, patterning, etc., and more recent research lines are undertaking the problem of a-Si:H selective laser ablation for different applications. The controlled ablation of photovoltaic materials with minimum debris and small heat affected zone with low processing costs, is one of the main difficulties for the successful implementation of laser micromachining as competitive technology in this field. This work presents a detailed study of a-Si:H laser ablation in the ns regime. Ablation curves are measured and fluence thresholds are determined. Additionally, and due to the improved performance in optolectronic properties associated to the nanocrystalline silicon (nc-Si:H), some samples of this material have been also studied.