Multijunction solar cell mesa isolation: Correlation between process, morphology and cell performance

Multijunction solar cells must be electrically isolated from one to another at the end of the fabrication process; a step known as mesa isolation. In this study, three different techniques are assessed to perform this step: saw-dicing, wet etching and plasma etching. Triple junction solar cells were fabricated with each process and the open-circuit voltages were measured in order to compare the impact of each technique on the device performance. An optional wet treatment is also proposed to clean the sidewalls after the mesa isolation process. The mesa sidewalls were characterized by scanning electron microscopy and atomic force microscopy to assess the profile length and roughness respectively. The cell performance was then correlated to the sidewall length and roughness for all three isolation techniques. This study indicates that a plasma etching process followed by a wet clean is the process that maximizes the solar cell performance, thanks to a short profile length and a low sidewall roughness.