# Optimization of Laser Fixed Contact Process for the Formation of an Al-BSF Layer
## 1 Introduction  
Research and development on new and renewable energy that can replace fossil fuels is underway in various countries. Among renewable energy sources, solar energy is a popular option, as it is the most readily available energy source, and is safe and reliable. Crystalline silicon (c-Si) solar cells currently account for more than 80% of the global solar cell market and have an efficiency of approximately 22% [1]. The goals of the solar cell industry include being able to reduce manufacturing costs (through the value chain of production systems) and to improve efficiency through technology research [2]. Recent research on high-efficiency c-Si solar cells has involved various aspects, such as texturing, doping, antireflection films, materials and structures of the front and rear electrodes, and surface passivation.  
The technical development of emitter formation and back field effect aspects are critical for manufacturing high-efficiency c-Si solar cells, and various studies have been actively conducted for this purpose. However, when using the thermal diffusion process in the doping process for emitter formation, it has been difficult to control the doping concentration and junction depth precisely. We have also seen that using the wet method for the texturing process results in there being no structural difference between the front and back surfaces, making it difficult to improve the rear field effect.  
Laser-fired contact (LFC) solar cells have several advantages compared to other local contact structures. For example, compared with the photolithography process, the process steps and procedures can be greatly reduced; when compared to the screen-printing process, warping of the wafer during the thin wafer process can be prevented. In older photovoltaic (PV) cells, lasers were mainly used for peeling and wafer cutting after the edge area process, but currently, they are used mainly as part of processes aimed at improving efficiency. Examples include the passivated emitter and rear contact (PERC) processes used to form local back contacts [3,4]. Compared with the cells of the full-area aluminum electrode, a solar cell with a PERC structure that forms a localized contact area can increase efficiency by lowering the recombination rate of electrons and holes. In another example, the laser-doped selective emitter (LDSE) process is a technique used to form a heavily doped region on an electrode formation area, which can improve efficiency by lowering the contact resistance.  
Most of these new laser-based concepts for developing c-Si photovoltaic efficiency improvement have been applied to either reduce the thermal load of the device, increase passivation on some interfaces, or improve solar cell Ohmic contacts. In the LFC process, a back surface field (BSF) is formed on the back surface, and a p+ layer is formed on the back surface of the p-type wafer, so that the electrons in the p region move to the back surface, to prevent losses caused by recombination. This can reduce the leakage current, and improve the open-circuit voltage (V\({}_{\mathrm{oc}}\)) and fill factor (FF) [5, 6, 7].  
The biggest problem in the LFC process has been minimizing the thermal damage between the silicon wafer and Al layer during the laser process. This indicates that while higher efficiency can be obtained by improving the rear electrode structure, the associated thermal damage has to be reduced. In the study reported here, a backside structure was formed using various lasers for the LFC process, and the thermal damage and characteristics applied to the silicon wafer were analyzed, in terms of laser selection and laser process conditions, so as to optimize the LFC associated with p-contact formation.