D. Munoz-Martin

Study of the Surface Recombination Velocity for Ultraviolet and Visible Laser-Fired Contacts Applied to Silicon Heterojunction Solar Cells

In this study, we investigate the effect of the laser-firing process on the back surface passivation of p-type silicon heterojunction solar cells. For that purpose, two different nanosecond laser sources radiating at ultraviolet (UV) (355 nm) and visible (532 nm) wavelengths are employed. First, we optimize the laser-firing process in terms of the electrical resistance of locally diffused point contacts. Specific contact resistance values as low as 0.91 and 0.57 mΩ·cm2 are achieved for the visible and ultraviolet laser sources, respectively. In addition, the impact of the laser-firing process on the rear surface passivation is studied by analyzing the internal-quantum-efficiency curves of complete devices. Low surface recombination velocities in the range of 300 cm/s are obtained for the ultraviolet laser with a 1% fraction of contacted area. This value increases to about 700 cm/s for the visible laser, which indicates a significantly higher recombination at the contacted area. The best heterojunction solar cells with rear laser-fired contacts are obtained for the ultraviolet laser and reached a 17.5% conversion efficiency.

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.

Parameterization of a-Si crystallization by continuous-wave green laser irradiation: from single spot to large area

Abstract. An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. In the present work, thin-film amorphous-silicon samples were irradiated with a continuous-wave green laser source. Laser irradiated spots were produced by using different laser powers and irradiation times. Micro-Raman spectroscopy was used to study the crystallization induced on the irradiated surface. Both laser peak power density and irradiation time are identified as key variables in the crystallization process, but within the parametric window considered, the enhancement of the crystalline factor, is more sensitive to the power density than to the irradiation time. The optimum parameters are then used for crystallizing a large sample area by means of overlapped laser scanned lines. Ellipsometric data experimentally show that the whole volume of a micron-thick sample is crystallized.

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.

Multiple-pulse effects during printing of silver paste using laser induced forward transfer (Conference Presentation)

Laser induced forward transfer (LIFT) technique has been used for printing of various materials ranging from flexible metallic contacts to conductive silver lines. In this study, we are focusing on the printing of an industrial-grade silver paste formulated for the metalization of the front side of solar cells. Printing of industrial silver pastes using the LIFT technique is challenging because the high viscosity of the silver paste allows only a small window of process parameters for reproducible and well-defined material transfer. In this work, we are examining the multiple-pulse effects during the printing of silver paste. Time-resolved imaging and characterization of the ejected silver paste voxels are performed to examine the influence of process parameters on the morphology of transferred paste dots and lines. We have observed that by firing repeating laser pulses below the transfer energy threshold it is possible to print smaller volumes of paste, which yields an opportunity to print lines with higher resolution. We also show that it is possible to print well-defined dots (voxels) of the paste using pulse energies near transfer threshold values. However, regarding the printing of lines, there is a strong interaction effect between adjacent voxels. This influence is so important that a distance between adjacent laser pulses threshold has been evaluated to print lines. The printing of single voxels has been achieved above the evaluated threshold value, while no printing could be achieved below the threshold. This distance threshold represents a limitation to the LIFT process of high viscosity pastes, which indicates that a compromise must be done between voxel size and laser frequency.