Interpretation of lock-in thermography on thin film solar cells considering dissipative and Peltier contributions

We describe the measurement and modeling of lock-in thermograms of crystalline silicon on glass (CSG) thin film silicon solar modules for a range of bias voltages. For that purpose, a voltage series of lock-in thermograms for a single cell in the module is measured and compared to the area heat production distribution resulting from a Spice electronics simulation. The point spread function for the highly heat conductive layer on a thick glass substrate as well as Peltier effects at p and n contact holes are taken into account. This makes it possible to extract relevant cell performance parameters like the area diode saturation current and nonlinear edge shunting current densities. 1. Theory and simulation overview 1.1 Sample description and module layout The sample analyzed here was produced at the experimental production site of CSG Solar in Thalheim, Germany. It consists of a 3 mm glass substrate with a texture made from deposited glass beads for light-trapping, anti-reflection coating, the p-p-n silicon diode and back side contacting. CSG employs an unusual interdigitated contacting scheme [1], which consists of 0.5 mm wide Al pads spanning 2 cells each. Each contact pad has 30 contact holes, half of them contact the n layer of one cell and the other 15 contact the p layer of the next cell. The whole back surface of the cell is covered by these Al pads resulting in a low series resistance of the module but suppressing shunting effects due to its high resistance parallel to the cell axis [1,2]. 1.2 Influence of Peltier effects In contrast to standard wafer-based solar cells and many other thin film concepts, in the CSG contacting scheme p and n contacts are beside each other in the image plane and thermoelectric effects become visible. When a forward current is driven through the module, the metal–p contact (positive Peltier coefficient of p silicon, metal Peltier coefficient negligible) and the n–metal (negative Peltier coefficient of n silicon) contact both show pronounced cooling effects for typical biases of 300 to 500 mV/cell. Thermoelectric effects are of no consequence to the performance of the solar module and the energy taken from the environment at the contact holes needs to be dissipated at some place in the module. Roughly speaking, this happens during the recombination process, thus possibly several millimeters distant from the cooling effect at the contact hole. The corresponding terms are (q: heating/cooling power density, j: current density, Π: Peltier coefficient)