High-Speed Laser Beam Induced Current Imaging: A Complementary Quantitative Diagnostic Tool for Modules

Abstract

High-resolution imaging techniques based on thermography, luminescence imaging, and laser beam induced current (LBIC) probe and inspect solar cells and correlate the observed inhomogeneities to the cell performance. The imaging methods get increasingly qualitative as the size scales from cell level to module dimensions, where several solar cells are connected in series, as in silicon panels. Electroluminescence (EL) imaging is a standard measure of inspecting solar panels and extensively utilized in the manufacturing assembly line. The complementary method of obtaining images from the LBIC scanning technique, however, is rarely utilized at large panel dimensions. We report strategies to implement LBIC imaging at module levels and procedures to analyze in terms of microscopic and circuit parameters. LBIC is inherently quantitative and well suited to identify microscopic defects, since it involves measuring local photo-current. We demonstrate that the contrast features from LBIC in a specific cell region are directly associated with the shunt resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\text{sh}}$</tex-math></inline-formula> ) variation of that specific cell. Using a single diode equivalent circuit model, we show that the current contrast of the cell under study is purely a function of its <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\text{sh}}$</tex-math></inline-formula> , and an effective load resistance that includes shunt-resistance of the other series-connected cells. The resulting LBIC-map along with EL then becomes a complete diagnostic tool to quantitatively map and identify the defects prevailing in a solar panel. We overcome the key issue of the slow speed of LBIC based imaging by a modified scan routine for rapid screening of large areas (2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\bf m}$</tex-math></inline-formula> by 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\bf m}$</tex-math></inline-formula> ) panels under 5 min, at high resolution.