Imaging the local forward current density of solar cells by dynamical precision contact thermography

In spite of many techniques of measuring the local lifetime or the local short circuit current, no nondestructive technique has been available for imaging the local forward current density, which determines the open circuit voltage and the fill factor of solar cells. We demonstrate the application of a new analyzing technique, enabling nondestructive “shunt hunting” in forward direction. A periodical forward current is applied to the Si solar cell, and only the dynamical temperature response is measured in contact mode with a resolution below 10 μK. Mechanical scanning of the T-sensor position yields a thermogram with a spatial resolution well below 1 mm and a current density resolution well below 1 mA/cm². First results show that both the edges of solar cells and their interior may have sites of a locally increased forward current. “Hot spots” measured in reverse bias direction only occasionally coincide with these “warm spots” measured in forward direction