Deep junction laser doping for contacting buried layers in silicon solar cells

A novel approach for contacting buried layers in silicon solar cells is presented in this work using laser doping to pattern, diffuse the surface layer and make electrical contact to the underlying silicon in a single step. Electron beam induced current images are used to determine the cross-sectional junction profiles for laser doping using both n- and p-type dopants on textured and planar surfaces. It is observed that using a processing speed of 0.5 m/s, junction depths of more than 11μm can be achieved independent of the dopant type and surface morphology. By laser doping using a boron spin on dopant source through a 100Ω/□ phosphorus emitter, direct contact can be made to the underlying p-type base silicon for a processing speed of 0.2 m/s. For processing speeds of 0.5–2 m/s, insufficient mixing of the dopants within the laser doped region can result in the formation of localised transistor structures forming in the peripheral areas of the laser doped region, reducing the ability to make direct contact to the underlying silicon without the risk of shunting to the surrounding n-type surface region. At higher processing speeds, no p-type region is formed. Initial work for using p-type laser doping to penetrate through an n-type emitter and contact the underlying p-type base silicon indicate that some shunting is present between the n-type emitter and heavily doped p-type silicon. However this shunting is reduced by increasing the depth of the laser doped regions.

► Demonstration of junction depths >10μm using laser doping.
► Ability to penetrate p-type contacts through an n-type emitter.
► Formation of localised transistor structures.