Scanning electron microscopical examination of the impact of laser patterning on microscopic inhomogeneities of Cu(In,Ga)(Se,S)2 absorbers produced by rapid thermal processing

Laser scribing of the Mo back electrode is commonly applied to define the cell structure of Cu(In,Ga)(Se,S)2 (CIGSSe) thin film solar cells. The patterning process was performed on laboratory samples using ns and ps pulse length laser processes. After structuring, CIGSSe absorbers were processed by rapid thermal processing (RTP) of stacked elemental layer precursors. Microscopic inhomogeneities were investigated on different sample positions. For samples structured with ns pulse, the absorber morphology in the laser line vicinity is different as compared to the morphology in the unstructured cell area. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show significant changes in the absorber grain size and chemical composition. Close to the laser line, the typically observed Ga accumulation on the back contact is less pronounced and more Ga is incorporated closer to the surface leading to a smaller grain size. The observed changes are attributed to partial damaging of a diffusion barrier between glass and Mo induced by the ns laser process, which allows diffusion of sodium from the glass substrate into the absorber during RTP. The enhanced Ga incorporation closer to the surface is an indication for the influence of sodium on the local phase development during RTP. The damages of the diffusion barrier can be effectively prevented by the application of a ps laser scribing process. CIGSSe absorbers processed on samples structured with ps pulse length do not show the described microscopic inhomogeneities around the laser line.

► Scanning electron microscopy on Cu(In,Ga)(Se,S)2 solar cell absorbers
► Laser patterning with ns laser pulse damages the sodium diffusion barrier.
► Improved laser patterning with ps laser pulse leaves diffusion barrier intact.
► Additional sodium changes phase development during absorber formation.
► Gallium content is increased at surface and decreased at backside of absorber.