p+-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy

Abstract

Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few lm thick p þ-doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p þ-doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p þ-doped Si regions formed by the LFC process. The sample preparation is of great importance because the KPFM signal is very surface sensitive. Furthermore, the measurement is responsive to sample illumination and the height of the applied voltage between tip and sample. With other measurement techniques like micro-Raman spectroscopy, electrochemical capacitancevoltage, and energy dispersive X-ray analysis, a high local hole concentration in the range of 10 19 cm À3 is demonstrated in the laser fired region. This provides, in combination with the high spatial resolution of the doping distribution measured by KPFM, a promising approach for microscopic understanding and further optimization of the LFC process. V

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope

“…If the tip's work function remains constant, this allows for the determination of the sample's work function. [71,72] Furthermore, KPFM scanning along the cross-sectional surface of solar cells enables the characterization of the SP distribution, facilitating a comprehensive understanding of solar cell operation mechanisms. The combination of KPFM's plane potential scanning and C-AFM's photocurrent scanning permits the visualization of charge carrier photogeneration and transport pathways within the film samples, particularly highlighting the movements of carriers at the GBs and GIs.…”

Optimization of Laser Fired Contact Process for the Formation of an Al-BSF Layer

“…In older photovoltaic (PV) cells, lasers were mainly used for peeling and wafer cutting after the edge area process, but currently, they are used mainly as part of processes aimed at improving efficiency. Examples include the passivated emitter and rear contact (PERC) processes used to form local back contacts [3,4]. Compared with the cells of the full-area aluminum electrode, a solar cell with a PERC structure that forms a localized contact area can increase efficiency by lowering the recombination rate of electrons and holes.…”