Understanding and reducing the boron-oxygen-related performance degradation in Czochralski silicon solar cells

The efficiency of silicon solar cells made on boron-doped Czochralski silicon is known to degrade under illumination. A boron- and oxygen-related metastable defect has been held responsible for this performance loss. This paper aims at clarifying the quantitative dependence of the metastable defect density on the boron and oxygen concentration. In order find knew strategies for the reduction of the degradation a fundamental understanding of the physical mechanism underlying the defect formation is essential. Therefore, we have investigated the time and temperature dependence of the degradation of the carrier lifetime and open-circuit voltage in detail. The defect formation process can be characterized by a two-step mechanism. The initial degradation is extremely fast taking place on a time scale of seconds and is not thermally activated, whereas the asymptotic degradation occurs on a time scale of hours and is thermally activated. The activation energy has been determined to be only 0.37 eV. The second part of the paper aims at optimizing the solar cell emitter diffusion process in order to reduce the performance loss. We show that it is possible to reduce the normalized defect density significantly by a factor of up to 3.5 with an optimized phosphorus diffusion at 850degreesC in a conventional quartz tube furnace using fast ramping conditions.