Understanding current paths and temperature distributions during ‘Laser Enhanced Contact Optimization’ (LECO)

We present results of diode network simulations modeling the current paths and current densities during the process ‘Laser Enhanced Contact Optimization’ (LECO). We model the contact interface assuming circular contact openings characterized by an area density, a specific contact resistivity and a contact radius. The simulated results indicate that current densities decrease with increasing contact radii but saturate against a maximum value for small contact radii. Current densities are found to be in the order of several MA/cm² depending on the applied terminal voltage. In a second part, a heat transfer model is introduced which uses the calculated current densities as input and returns the temperature distributions as a function of space and time. The results of the model indicate that high local temperatures (above 100°C reaching up to 8000°C) can be reached at the contact area while the surface temperature does not increase significantly (less than 20 K above room temperature). Furthermore, the results indicate that local temperatures are low for very small contact radii, then grow up to a maximum for contact radii in the order of ~ 50 nm and then decrease again due to decreasing current densities for large contact radii. Nevertheless, the results indicate that temperatures stay well below the silicon melting point if the contact area is in thermal contact with the silver or the silicon bulk. Only if the contact area is assumed to be surrounded by a material with low thermal conductivity (e.g., a glass layer) the model indicates temperatures above the silicon melting point. From these findings, we derive a descriptive hypothesis which can be seen as an extension of the ‘Current Fired Contacts’ hypothesis known from literature. We believe that contacts before LECO treatment need to meet certain activation criteria (contact size and heat insulating surrounding) in order to be ‘activated’ by the LECO process. If activated, the contacts grow (as described in literature) as semi spheres into the material and the growth stops due to decreasing current densities and hence decreasing temperature if contacts reach a certain size. Our calculations indicate that this size is in the order of typical junction depths (~ 300 nm). This agrees with the observation of decreasing shunt resistances of PERC cells with phosphorous emitters if these cells are overtreated with LECO.