INCREASING SHORT MINORITY-CARRIER DIFFUSION LENGTHS IN SOLAR-GRADE POLYCRYSTALLINE SILICON BY ULTRASOUND TREATMENT

We have found that ultrasound treatment (UST) has a profound effect on the recombination rate in as-grown, B-doped cast polycrystalline sili con wafers for photovoltaic applications. As determined by surface pho tovoltage measurements of the minority carrier diffusion length L, the UST increases the corresponding lifetime by almost an order of magnit ude. The maximum enhancement takes place in the wafer regions with the shortest L values. For L > 20 mum, both positive and negative changes of L after UST are revealed at different wafer regions. The UST effec t is temperature dependent and exhibits maximum influence at about 60- degrees-C. Enhanced dissociation of Fe-B pairs by UST is identified as a mechanism which leads to a negative change of large L values, and a complex post-treatment relaxation. A positive change of L is attribut ed to the influence of ultrasound vibrations on crystallographic defec ts.