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Phone: þ61 2 612 555 38, Fax: þ61 2 6125 0506The recombination parameters of aluminium-oxygen complexes in silicon have been reassessed by applying lifetime spectroscopy on several n-and p-type intentionally Al-contaminated and control samples, using a single-level defect model. The presence of the control samples has allowed greater accuracy for the extraction of the recombination lifetime. The uncertainty ranges of the parameters have been tightened significantly by simultaneously fitting the lifetime on several samples. The electron/hole capture cross section ratio k was reassessed to be 380, in the uncertainty range of 330-460. A direct comparison of the n-and p-type samples has shown that those complexes are much more recombination-active in p-type silicon than in n-type silicon at low and intermediate injection levels.

We present solar cells fabricated with n-type Czochralski-silicon wafers grown with strongly compensated 100% upgraded metallurgical-grade feedstock, with efficiencies above 20%. The cells have a passivated boron-diffused front surface, and a rear locally phosphorus-diffused structure fabricated using an etch-back process. The local heavy phosphorus diffusion on the rear helps to maintain a high bulk lifetime in the substrates via phosphorus gettering, whilst also reducing recombination under the rear-side metal contacts. The independently measured results yield a peak efficiency of 20.9% for the best upgraded metallurgical-grade silicon cell and 21.9% for a control device made with electronic-grade float-zone silicon. The presence of boron-oxygen related defects in the cells is also investigated, and we confirm that these defects can be partially deactivated permanently by annealing under illumination. V

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Abstract:This paper deals with the impact of dopant compensation on the degradation of carrier lifetime and solar cells performance due to the boron-oxygen defect. The boron-oxygen defect density evaluated by lifetime measurements before and after degradation is systematically found proportional to the total boron concentration, showing that compensation cannot reduce light-induced degradation. This result is confirmed by a comparison of upgraded-metallurgical grade silicon solar cells having identical boron, oxygen and carbon but different compensation levels and in which the degradation is found more severe when the compensation is stronger.

Czochralski (Cz)‐grown upgraded metallurgical‐grade (UMG) silicon wafers degrade significantly during high‐temperature processes, eroding their appeal as a low‐cost alternative to conventional electronic‐grade silicon wafers. However, the thermal degradation in UMG wafers can be delayed by utilizing a prefabrication annealing step. Based on this, a high‐efficiency solar‐cell process is modified by selecting a single‐boron diffusion step and applying phosphorus‐doped polycrystalline films as electron‐selective contacts with excellent impurity‐gettering properties to minimize the thermal budget. The application of this modified high‐efficiency solar‐cell process to n‐type UMG‐Cz wafers results in a solar cell with a conversion efficiency of 22.6% on a cell area of 2 × 2 cm2.