Quantifying the uniformity of solar cells using thermal imaging diagnosis

Local hot spots and localized heating in solar cell can be detected by Infrared (IR) thermography via thermal images of the surface temperature distribution. This paper presents the method to quantify the quality of solar cells using thermal imaging analysis. By the infrared thermal imaging method, the local parallel resistances of solar cell have been analyzed, and the indices for qualifying localized heating in solar cell are proposed. The indicators, H-index, A-index and L-index, are derived using a cone shape model based on statistical analysis of thermal images. The experiment was carried out with 20 unused and no-encapsulated single crystalline solar cells, 10 cm × 10 cm in size. Among the test cells, eight of those cells were selected and compared with four different values of parallel resistances. From a comparison, one cell has a uniform temperature profile while the others exhibit local hot spots. The electrical performance measurements, such as performance at Standard Test Condition, dark IV characteristics, were also done under reverse bias conditions for comparison. The IR thermal images were taken while the cells were being tested under reverse bias in the dark and the air temperature was controlled at 25 °C. The analysis results can be summarized as follows (1) L-index relates to Tmax closely, (2) the lower the L-index, the better the temperature uniformity (3) the proposed indicators and statistical values of IR thermal analysis are in line with the resulting parallel resistances, and (4) IR thermal analysis in conjunction with the dark IV characteristic can be used to examine local structural defects in a whole cell. Moreover, the electrical parallel resistance can be calculated and verified using the proposed indicators and statistical values obtained by IR thermography.