Investigation of Lifetime-Limiting Defects After High-Temperature Phosphorus Diffusion in High-Iron-Content Multicrystalline Silicon

Abstract

Abstract-Phosphorus diffusion gettering of multicrystalline silicon solar cell materials generally fails to produce material with minority-carrier lifetimes that approach that of gettered monocrystalline wafers, due largely to higher levels of contamination with metal impurities and a higher density of structural defects. Higher gettering temperatures should speed the dissolution of precipitated metals by increasing their diffusivity and solubility in the bulk, potentially allowing for improved gettering. In this paper, we investigate the impact of gettering at higher temperatures on low-purity multicrystalline samples. To analyze the gettering response, we measure the spatially resolved lifetime and interstitial iron concentration by microwave photoconductance decay and photoluminescence imaging, and the structural defect density by Sopori etching and large-area automated quantification. Higher temperature phosphorus diffusion gettering is seen to improve metal-limited multicrystalline materials dramatically, especially in areas of low etch pit density. In areas of high as-grown dislocation density in the multicrystalline materials, it appears that higher temperature phosphorus diffusion gettering reduces the etch pit density, but leaves higher local concentrations of interstitial iron, which degrade lifetime.

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

“…The EXT process was chosen due to its enhanced iron gettering efficiency compared to the STD process. 9 In the as-grown state, chromium precipitates were detected by l-XRF along the random-angle grain boundary (Fig. 1), consistent with the behavior of other metals in silicon wherein metal precipitate nucleation is favored at bulk heterogeneous nucleation sites.…”

“…Iron, for example, has been well-studied, and kinetics process simulation tools exist to engineer its distribution in the material. [6][7][8][9]33 The impact of processing steps on chromium (both precipitated and interstitial) has not been studied as extensively, although the detrimental nature of the impurity is well-known. The maximum allowable chromium contamination in the silicon melt ranges from 1 Â 10 15 cm À3 to 2 Â 10 17 cm À3 depending on the growth process, device architecture, and target efficiency.…”

Firing and gettering dependence of effective defect density in material exhibiting LeTID

“…However, this behaviour would be surprising for a distributed (metal) impurity, since gettering should be more efficient in good grains like area I [10] due to the absence of competing internal gettering sites. It is also suspected that gettering can reduce the dislocation density [11].…”

Darwin at High Temperature: Advancing Solar Cell Material Design Using Defect Kinetics Simulations and Evolutionary Optimization

“…[ 77 ] Adding a low-temperature step drives this dissolved iron to segregate to the emitter, producing signifi cantly better material performance. [ 39,75 ] The average lifetime after the higher-temperature steps is now limited by high structural defect density, [ 83 ] where phosphorus diffusion is known to be relatively ineffective. [ 84,85 ] To quantify the contributions of the high-and low-temperature components of the diffusion profi le to the lifetime improvement seen in the GA process optimization results and the experimental samples, we simulate a sensitivity analysis of the time-temperature process variables.…”