The influence of in-diffusing atomic H into an annealed multicrystalline silicon (mc-Si) wafer on the concentration of interstitial iron [Fe i ] was investigated. Neighboring wafers with similar initial [Fe i ] were annealed with and without in-diffusing H. In-diffusion was realized by exposing the samples to a microwave induced remote hydrogen plasma at 400 C. [Fe i ] was detected based on lifetime measurements before and after dissociating the FeB complex. Surface passivation was achieved by a quinhydrone-methanol solution at room temperature to avoid further temperature steps or in-diffusion of H during surface passivation. From [Fe i ] measurements before and after the annealing steps with and without H, the influence of H alone on [Fe i ] could be accessed. The results were compared to previous experiments where the same SiN x :H layers were used as surface passivation for multiple [Fe i ] measurements of mc-Si samples before and after several anneals at 400 C. It could be shown that a H plasma atmosphere has a strong additional effect on the reduction of [Fe i ] compared to temperature effects alone. A formation of H-Fe i complexes associated with the passivation of the electrical activity of Fe i could be shown to be improbable because no depassivation of Fe i could be observed in subsequent annealing steps at 400 C.
Abstract-The emitter formation step (POCl 3 diffusion) in p-type crystalline silicon solar cell processing includes many variables, e.g., peak temperature, gas flows, temperature ramps, which can be optimized in order to improve material quality. Diffusion parameters of an 80-Ω/ emitter are varied, and the resulting change in electronic quality of multicrystalline silicon is analyzed. A detailed gettering analysis of multicrystalline material, surface passivated with hydrogen-rich amorphous silicon, after POCl 3 diffusion, and an additional gettering step combined with hydrogenation from SiN x :H is presented. The industrial-type diffusion leads to material of lower electronic quality than the extended reference diffusion. A major finding of this paper is the fact that results on different 5 × 5 cm 2 samples out of one 15.6 × 15.6 cm 2 wafer can vary significantly. Hence, conclusions about which diffusion is most efficient in gettering strongly depend on wafer position. An edge position close to crucible walls, for example, might improve less effectively than another position close to the crucible center. In fact, the opposite can also be shown, and samples originating from edge regions reach their highest lifetimes after gettering. This is explained by the different defect structure of the investigated samples. Structures exhibiting high gettering efficacy contain fewer recombination active grain boundaries and are predominantly free of extended defect clusters.
This investigation analyzes the dependency of minority charge carrier lifetime values at grain boundaries in multicrystalline silicon on the grain boundary type after P gettering and/or firing of SiNx:H layers deposited by plasma enhanced chemical vapor deposition. To get a broad statistics, a new method to determine the coincidence site lattice grain boundary types on large scale throughout entire 50 × 50 mm2 samples is combined with spatially resolved lifetime-calibrated photoluminescence measurements and mappings of the interstitial iron concentration. As an evaluation of the lifetime data at grain boundaries in comparison to the recombination activity of the bordering grains, lifetime contrast values are calculated. The correlation of this dependency on the grain boundary type with the impurity concentration is analyzed by the investigation of multicrystalline samples from two different ingots grown by directional solidification with different crucible material qual ities. A dependency of the efficacy of all applied processes on the grain boundary type is shown based on broad statistics-higher coincidence site lattice indexes correlate with a decrease of median lifetime values after all processes. Hydrogenation of both grains and grain boundaries is found to be more effective in cleaner samples. Extended getter sinks, as a P emitter, are also beneficial to the efficacy of hydrogenation. The lifetime contrast values are dependent on the degree of contamination of the multicrystalline silicon material. In cleaner samples, they rather decrease after the processes; in standard solar-grade material, they increase after POCl3 diffusion and decrease again after subsequent hydrogenation. No correlation with the interstitial iron concentration is found
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