Towards more accurate imaging of the local saturation current density in solar cells by using alternative PL evaluation methods

We have performed 2-dimensional inhomogeneous solar cell simulations using the circuit-simulation program Spice and obtained photoluminescence (PL), electroluminescence (EL), and dark lock-in thermography (DLIT) images for various illumination and biasing conditions. These images were evaluated by established methods for retrieving images of the saturation current density J₀₁. We found that DLIT delivers blurred but quantitatively correct J₀₁ images, but PL generally delivers too weak local maxima of J₀₁. This is due to the used model of independent diodes, which is too simple to describe the local diode voltage in inhomogeneous cells precisely. In reality lateral balancing currents exist, which are not regarded in established PL evaluation methods. For solving this problem we have checked two alternative PL evaluation methods, which are already published but not established yet. Especially the Laplacian-based method has the potential to overcome this problem. For our simulated images the method yields correct results. However, in spite of some improvements for the determination of the calibration image, our first experimental results obtained by using this method do not agree yet with lock-in thermography based J₀₁ results.