Photographic diagnosis of crystalline silicon solar cells utilizing electroluminescence

The photographic surveying of electroluminescence (EL) under forward bias was proved to be a powerful diagnostic tool for investigating not only the material properties but also process induced deficiencies visually in silicon (Si) solar cells. Under forward bias condition, solar cells emit infrared light (wavelength around 1000 to 1200 nm) whose intensity reflects the number of minority carriers in base layers. Thus, all the causes that affect the carrier density can be detected, i.e., the minority carrier diffusion length (or in other words, lifetime), recombination velocity at surfaces and interfaces, etc. (intrinsic material properties), and wafer breakage and electrode breakdown, etc. (extrinsic defects). The EL intensity distribution can be captured by Si CCD camera in less than 1 s, and the detection area simply depends upon the optical lens system suitable to the wide range of 1 cm–1.5 m. This fast and precise technique is superior to the conventional scanning method such as the laser beam induced current (LBIC) method.