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.
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.
An advantage of laser crystallization over conventional heating methods is its ability to limit rapid heating and cooling to
thin surface layers. Laser energy is used to heat the a-Si thin film to change the microstructure to poly-Si.
Thin film samples of a-Si were irradiated with a CW-green laser source. Laser irradiated spots were produced by using
different laser powers and irradiation times.
These parameters are identified as key variables in the crystallization process. The power threshold for crystallization is
reduced as the irradiation time is increased. When this threshold is reached the crystalline fraction increases lineally
with power for each irradiation time.
The experimental results are analysed with the aid of a numerical thermal model and the presence of two crystallization
mechanisms are observed: one due to melting and the other due to solid phase transformation.
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.
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.