A Novel TLM Analysis for Solar Cells

Despite its widespread usage, the linear regression method used to obtain contact resistivity and sheet resistance from the transmission line model (TLM) can lead to huge errors. The method assumes that contact parameters (both electric and geometric) are identical across the probed structure, such as a solar cell. In contrast, the contact parameters display various process-associated inhomogeneities. This paper analyzes the limitations of the linear fit method by using mathematical analysis and measurements on the front-side Ni-Cu plated contacts of a passivated emitter and rear contact (PERC) solar cell. It presents a new evaluation method based on the compartmentalization of contact resistance and emitter sheet resistance of a TLM network. The relative increase in the uncertainty of the extracted contact parameters with decreasing contact resistance is verified and linked to strong error coupling. A large error (20%-180%) is observed in the extracted values of the contact resistance for an assumed (5%-40%) scatter in the sheet resistance values, when a linear fit is used on simulated data with absolutely no pre-estimated error in the contact resistance. The presented method enables the detection and elimination of the influence of bad contacts along with minimal error propagation, which is imperative for contacts with small opening width and lower values of contact resistivity.