Low Temperature Internal Gettering of Bulk Defects in Silicon Photovoltaic Materials

Abstract

Abstract. Multicrystalline silicon (mc-Si) substrates are widely used for photovoltaic cells. The minority carrier lifetime in mc-Si is affected by recombination associated with metallic impurities in many forms, such as point-like defects, precipitates and bound to or precipitated at structural defects such as dislocations. We have studied the effect of low temperature annealing on the lifetime and bulk iron concentration in as-received mc-Si wafers from different locations within a block. Lifetime measurements are made using a temporary iodine-ethanol surface passivation technique to minimize the occurrence of bulk hydrogenation which often occurs from dielectric films. In good wafers from the middle of the block the lifetime is reduced by annealing at 400 °C and 500 °C in a way which does not correlate with changes in bulk iron concentration. Lifetime improvements occur in relatively poor samples from the top and bottom of the block annealed at 300 °C, and also in samples from the bottom annealed at 400 °C. The improvement in bottom wafers correlates with iron loss from the bulk. Our work shows that under some conditions the lifetime in relatively poor as-grown wafers can be improved by low temperature internal gettering.

Hydrogénation effect on low temperature internal gettering in multicrystalline silicon

“…Set II samples were passivated with a temporary 0.1 M of iodine-ethanol (I-E) chemical solution at room temperature. More details of the I-E passivation process have been reported previously [14,15] The lifetime was measured using quasi-steady-state photoconductance (QSS-PC) with a Sinton WCT-120 tool. Lifetimes in this paper are the effective lifetime with bulk iron in the Fei state at an injection level of 1  10 15 cm -3 , and are denoted by Fei.…”

“…Lifetime and interstitial iron concentrations are measured on average and with spatial resolution at every annealing step. New results are compared with our previous work on low temperature internal gettering using a temporary iodine-ethanol passivation scheme in which samples were sourced from the same height positions and subjected to the same thermal treatment [14,15]. Possible differences between the cases are discussed.…”

“…Extensive research has been performed into external gettering using phosphorus, boron, or aluminium to remove metallic impurities from the bulk [7][8][9]. A number of studies have also been performed into low temperature internal gettering in which metallic impurities are redistributed within the material in configurations in which they are less detrimental to average lifetime [10][11][12][13][14][15].…”

“…Furthermore, it is possible that Krain et al's results were influenced by the hydrogenation effect due to silicon nitride passivation as reported in recent studies by Karzel et al and Liu et al [16,17]. In order to minimize a possible hydrogenation effect, we have recently performed a comprehensive study into low temperature internal gettering using a room temperature iodine-ethanol passivation scheme for samples from different height position of a commercially grown mc-Si ingot [14,15]. In bottom samples annealed at 400 °C, our results show lifetime can be improved by a factor of 7 and the interstitial iron concentration is decreased by 1.7 order of magnitude.…”