Characterization of defects in photovoltaics using thermoreflectance and electroluminescence imaging

Thermal and electroluminescence (EL) imaging techniques are widely accepted as powerful tools for analyzing solar cells. We have identified and characterized various defects in photovoltaic devices with sub-micron spatial resolution using a novel thermoreflectance imaging technique that can simultaneously obtain thermal and EL images with a mega-pixel silicon-based CCD. Linear and non-linear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as -5V. Pre-breakdown sites with electroluminescence are observed. The wavelength flexibility of thermoreflectance imaging is explored and thermal images of sub-micrometer defects are obtained through glass that would typically be opaque for infrared light. Image sequences show a 10μs thermal transient response of a 15μm defect in a polysilicon solar cell. Nanosecond reverse bias voltage pulses are used to detect breakdown regions in thin-film a-Si solar cells with EL.