Thermoreflectance imaging of defects in thin-film solar cells

We have identified and characterized various defects in thin-film a-Si and CIGS solar cells with sub-micron spatial resolution using thermoreflectance imaging. A megapixel silicon-based CCD was used to obtain noncontact thermal images simultaneously with visible electroluminescence (EL) images. EL can be indicative of pre-breakdown sites due to trap assisted tunneling and stress induced leakage currents. Physical defects appear at reverse bias voltages of 8 V in a-Si samples. Linear and nonlinear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as 4.5 V. Pre-breakdown sites with electroluminescence are investigated.