Imaging of TCO lateral resistance effects in thin-film PV modules by lock-in thermography and electroluminescence techniques

The lateral sheet resistance of transparent conductive oxide (TCO) electrode in thin-film photovoltaic (PV) modules is a major component of series resistance losses that causes significant reduction in the fill-factor and output power. This paper presents the investigation of TCO lateral resistance effects in the encapsulated thin-film modules by lock-in thermography (LIT) technique, which is predominantly used for shunt investigation in the solar cells. The LIT technique has been employed under both dark and illuminated conditions to compare their spatial sensitivity for imaging TCO resistance effects in a module. The LIT images have also been compared with electroluminescence (EL) images to find a correlation between localized heating and voltage drop across distributed TCO layer resistances, and to determine their advantages and limitations. Experimental results show that both, DLIT and ILIT, exhibit a gradient in thermal signal along the cell width due to variation in power dissipation across the lateral resistance of TCO electrode. However, ILIT appears to be more sensitive for imaging TCO resistance losses due to less junction masking effect. The spatial sensitivity also depends on the width of cell in a module. For narrower cells, DLIT and EL techniques are observed to be more sensitive near the higher potential edge of a cell as compared to ILIT. The study concludes that the LIT technique is also a potential candidate for providing the spatially-resolved characterization of TCO resistive losses in thin-film modules.