PROCESS INDUCED VARIATION IN THE MEASUREMENT OF MINORITY CARRIER LIFETIME OF SILICON IN SOLAR CELLS FABRICATION

摘要

The minority carrier lifetime in solar cells fabricated with boron doped Czochralski silicon (Cz-Si) is reduced significantly by a Cz-specific metastable defect. The solar cells fabricated from such material show a reduction of absolute conversion efficiency by 1～2%. This paper reports the study of the variation in the effective minority carrier lifetime in silicon with the advancement of wafers into different processing steps during solar cell fabrication in a conventional commercial production line. The results clearly showed a trend of increasing lifetime as the fabrication process advances from the bare wafers. In order to minimize the error, measurements were made on five different positions of each test wafer and the results were averaged out. Carrier lifetime was found to be improved in almost all the sample wafers after every step under consideration with plasma edge isolation process as an exception. A comparative study of effectiveness of different dielectric films for silicon surface passivation was also carried out through the effective carrier lifetime measurement. By applying the optimized dielectric film of silicon nitride as antireflection coating as well as passivation layer, solar cell of conversion efficiency as high as 17.12 % was fabricated starting with solar grade Cz-Si wafers of active device area ～ 147 cm~2 in a conventional solar cell fabrication line.