Imaging and quantifying carrier collection in silicon solar cells: A submicron study using electron beam induced current

Abstract

In this work electron-beam-induced current (EBIC) is used to study the collection efficiency of emitters in industrial silicon solar cells. Laser-doped local emitters have been deployed industrially, yet in mas production they are designed wider than the screen-printed silver fingers to allow alignment tolerances. EBIC has allowed to image and quantify the laser-induced damage that occurs in these local emitter regions. A model is developed to account for such damage, so that losses in EQE could be quantified from the observed EBIC collection characteristics. The damaged regions present ~12% lower collection efficiency at short wavelength (300–500 nm) than the homogenous emitter. Sentaurus TCAD simulations reveal that eliminating such damage would improve cell efficiency by ~0.12%. Additional degradation is found in a region 1–2 µm wide adjacent to the silver fingers, which has not been detected before. It is also found that pulsed laser doping leads to ~15 µm long un-doped gaps, along the direction of laser movement. As laser doping becomes a key part of industrial cell fabrication, it is crucial to develop a better understanding of the potential pitfalls, and future improvements to the process. The versatility of EBIC imaging is also demonstrated using FIB milling to improve lateral resolution and study the depth profile of boron emitters in newly developed industrial i-TOPCon cells. EBIC imaging, in combination with advanced device simulations, have proven powerful tools to elucidate carrier collection characteristics and drawbacks, thus helping to understand and improve fabrication processes at industrial level.