Evaluation of localized vertical current formation in carrier selective passivation layers of silicon solar cells by conductive AFM

Abstract

Carrier selective contacts are of growing interest in the development and optimization of high efficiency silicon solar cell concepts. In particular, the passivation mechanism of ultra-thin oxide layers in interaction with poly-Si layers came into focus and the origin of tunnel currents and the so called pinhole conductivity is discussed. Many process parameters and their influence on the passivation effect are not clear, yet. The present study investigates the electrical properties in Si/SiOx/poly-Si layer system from different processes. For this purpose, high-resolution electrical evaluation of the current path density through the interfacial oxide is investigated by conductive AFM using a newly developed image calculation software tool to determine the vertical current path density. We compared two thicknesses of poly-Si (n+ PECVD) layers each at optimum annealing temperature (corresponding to highest i-VOC). The influence of three annealing temperatures (at optimum passivation, below and above) is investigated for an ozone oxide and pinhole densities are analyzed by the TMAH method. Finally, the optimum properties of the layer stack for three interfacial oxides (each at optimum passivation) are studied.