Photographic Diagnosis of Crystalline Silicon Solar Cells by Electroluminescence

Abstract

Crystalline Si solar cells emit infrared light under the forward bias as so called “Electroluminescence, EL”. The photographic imaging of EL intensity gives spatial information of solar cell performance with high resolution in less than one second. The EL intensity is proportional to the total number of minority carriers in Si substrates determined by the minority carrier lifetime. The quantitative analysis under various injection conditions can reveal cell performances such as the open circuit voltage. Photographic imaging represents the spatial distribution of minority carrier number at a glance. Intrinsic recombination centers such as crystal defects and grain boundaries which reduce the minority carrier density can be clearly detected as dark parts (spots, lines and areas) due to decreased EL intensity. Extrinsic deficiencies such as substrate breakage and hidden mechanical cracks can also be found easily since they are strong drains of minority carriers. In addition, electrode breakdown and inferior contacts also become visible due to less injection carriers. The EL technique is a versatile diagnosis tool to get the improved performance and high reliability of solar cells.