Cryocooler based HTS SQUID photoscanning system [photovoltaic device evaluation]

A high temperature superconductor (HTS) SQUID photoscanning system for nondestructive evaluation of photovoltaic devices has been developed. The SQUID is cooled to its operational temperature of 77 K by a commercial Stirling-type cryocooler with a maximum cooling power of 3W at 80 K. The system can be operated completely unshielded. For this type of system, an HTS washer-type dc SQUID with a small sensing area of about 500 /spl mu/m/spl times/500 /spl mu/m has been fabricated. To stabilize the working point of the SQUID in the cryocooler based system a new direct-coupled (dc) read-out electronics with automatic bias voltage tuning has been used. The samples analyzed are multicrystalline silicon wafers with lateral p-n junctions and complete solar cells fabricated on such substrates. The HTS SQUID photoscanning system has a field noise level of 7 pT/H/sup 1/2/ at 1 kHz, the modulation frequency of the exciting laser beam. A typical signal-to-noise ratio (SNR) of 50 has been achieved for solar cell analysis. It has been be demonstrated that typical defects and structural features of solar cells and multicrystalline substrates can be characterized.