Nondestructive Contact Resistivity Measurements on Solar Cells Using the Circular Transmission Line Method

The contact resistivity ( $\rho _c$ ) of screen-printed contacts is a significant component of series resistance in industrial solar cells. Measuring  $\rho _c$ with standard techniques, such as the transmission line method (TLM), requires that the custom printed test structures be isolated from the device. In this article, we present a method for measuring  $\rho _c$ on solar cells using the circular TLM (cTLM), where the test structures are hidden within the busbars of the solar cell. This alleviates the need for the test structure to be isolated from the device, allowing for the fast and nondestructive measurement of  $\rho _c$ without causing additional shading on the cell. These measurements are performed with a semiautomatic tool, the ContactSpot-PRO, used for the accurate and systematic measurement of the  $\rho _c$ on the cTLM structures. Our results show that the cTLM technique compares well to the linear TLM technique on several samples fired through a  range of temperatures. However, numerical models generated using  Quokka3 and scanning electron microscopy images indicate that structural differences in screen-printed TLM and cTLM structures affect the actual contact resistivity of each pattern. The numerical models also show how nonuniform sheet resistance beneath and in-between the printed contacts can affect the measured  $\rho _c$ when using both the TLM and cTLM.