Sieu Pheng Phang

Direct comparison of boron, phosphorus, and aluminum gettering of iron in crystalline silicon

This paper presents a direct quantitative comparison of the effectiveness of boron diffusion, phosphorus diffusion, and aluminum alloying in removing interstitial iron in crystalline silicon in the context of silicon solar cells. Phosphorus diffusion gettering was effective in removing more than 90% of the interstitial iron across a range of diffusion temperatures, sheet resistances, and iron doses. Even relatively light phosphorus diffusions (145 X/h) were found to give very effective gettering, especially when combined with extended low temperature annealing. Aluminum alloying was extremely effective and removed more than 99% of the implanted iron for a range of alloying temperatures and aluminum film thicknesses. In contrast, our experimental results showed that boron diffusion gettering is very sensitive to the deposition conditions and can change from less than 5% of the Fe being gettered to more than 99.9% gettered by changing only the gas flow ratios and the post-oxidation step. V C 2011 American Institute of Physics. [doi:10.1063/1.3569890]

Surface Passivation of Boron-Diffused p-Type Silicon Surfaces With (1 0 0) and (1 1 1) Orientations by ALD Al

Boron-diffused p+/n/p+ and undiffused silicon samples with (1 0 0) and (1 1 1) orientations passivated by aluminum oxide (Al₂O₃) that is synthesized by atomic layer deposition (ALD) have been investigated. Emitter saturation current densities of ~24, 29, and 33 fA/cm² were obtained for (1 0 0) samples with symmetrical 85Ω/□ B diffusions that were passivated by plasma-assisted, H₂O-based, and O₃-based ALD Al₂O₃, respectively. Compared with undiffused samples, it was found that the additional surface doping from the diffusion reduces recombination at the Al₂O₃/Si interface in the case of relatively low surface boron concentrations (<; 2×10¹⁹ cm^-3). The degree of surface passivation that is observed on (1 0 0) surfaces was generally better than on (1 1 1) surfaces, particularly for undiffused samples, but this difference effectively disappeared following the application of more negative charge by corona charging. From capacitance- voltage measurements, it was found that Al₂O₃ films on substrates with a (1 0 0) orientation display a higher negative fixed charge density Q_f than films on (1 1 1) samples. On the other hand, the interface state density D_it was not strongly influenced by surface orientation of the substrate. It appears that the difference in negative charge density is at least partly responsible for the differences in the observed passivation.

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Photoluminescence images of silicon wafers with non-uniform lifetime distribution are often smeared by lateral carrier diffusion. We propose a simple method to de-smear the photoluminescence images by applying the two-dimensional continuity equation. We demonstrate the method on simulated silicon wafers and measured photoluminescence-based lifetime image of multicrystalline silicon wafer. The de-smearing is very effective in recovering the actual lifetime for wafers with gradual changes in lifetime but is less effective around localised recombination centres with high contrast such as grain boundaries and dislocations. The method is sensitive to measurement noise; therefore, the implementation of suitable noise filtering is often critical.

O

The suitability of using a boron-rich layer (BRL) formed during boron diffusion as a gettering layer for n-type silicon solar cells is investigated. We have studied the gettering effectiveness, generation of dislocations and associated bulk lifetime degradation, and the impact of the BRL on the saturation current density, for different thickness of BRL and postoxidation conditions. Our results show that a BRL deposited using BBr3-based furnaces is very effective at gettering interstitial Fe, removing more than 99.9% of Fe, but that the gettered Fe is released back into the wafer when the BRL is oxidized thermally. While we have detected no significant bulk degradation due to dislocations for the diffusion conditions used, there remains a tradeoff between the gettering effect and the recombination in the boron-doped region. Although the BRL can be oxidized chemically at low temperature using boiling nitric acid without losing the gettering effect, the lowest saturation current density is obtained by means of thermal oxidation, thanks partly to a lower boron surface concentration in thermally oxidized samples.

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In this paper, we present measurements and modeling of the reduction in dissolved iron Fe; concentrations near grain boundaries in multicrystalline silicon (mc-Si) wafers. The measurements of the interstitial Fe concentrations are obtained via photoluminescence images taken before and after iron-boron pair dissociation. A simple diffusion-capture model was developed to characterize the removal of interstitial Fe by the gettering sites. The model is based on a numerical solution to the 1-D diffusion equation with two fitting parameters: the diffusion length of dissolved Fe atoms and the effective gettering velocity at the gettering site. By comparing the simulation with a controlled phosphorous gettering process, the model is shown to give good estimation of the diffusion length of Fe atoms. For as-cut multicrystalline silicon wafers from different parts of the ingot, that is, wafers with different average dissolved Fe concentrations [Fei], the diffusion lengths of Fe atoms are found to decrease with decreasing average [Fei] This suggests the presence of relaxation precipitation during the internal gettering of dissolved Fe by the grain boundaries in mc-Si during ingot cooling.

Layers

Crystalline silicon (c-Si) solar cells featuring a high-temperature processed homojunction have dominated the photovoltaic industry for decades, with a global market share of around 93%. Integrating commercially available crystalline silicon solar cells with high-efficiency perovskite solar cells is a viable pathway to increase the power conversion efficiency, and hence achieve low levelized electricity costs for the photovoltaic systems. However, the fabrication process for this type of cell is challenging due to the many, and often conflicting, material processing requirements and limitations. Here, we present an innovative design for a monolithic perovskite/silicon tandem solar cell, featuring a mesoscopic perovskite top subcell and a high-temperature tolerant homojunction c-Si bottom subcell. The improved temperature tolerance of the c-Si bottom cell permits significantly increased flexibility in the design and fabrication of the perovskite cell. We demonstrate an efficiency of 22.5% (steady-state) and a Voc of 1.75 V on a 1 cm2 cell. The method developed in this work opens up new possibilities in designing, fabricating and commercialising low-cost high-efficiency perovskite/c-Si tandem solar cells.

Carrier de-smearing of photoluminescence images on silicon wafers using the continuity equation

In this work, we present two techniques for spatially resolved determination of the dopant density in silicon wafers. The first technique is based on measuring the formation rate of iron-acceptor pairs, which is monitored by band-to-band photoluminescence in low injection. This method provides absolute boron concentration images on p-type wafers, even if compensating dopants such as phosphorus are present, without reference to other techniques. The second technique is based on photoluminescence images of unpassivated wafers, where the excess carrier concentration is pinned by a high surface recombination rate. This rapid technique is applicable to either p- or n-type wafers, when the bulk carrier lifetime is much longer than the transit time to the surface. The relative sensitivities and advantages of the two techniques are discussed.

Tradeoffs Between Impurity Gettering, Bulk Degradation, and Surface Passivation of Boron-Rich Layers on Silicon Solar Cells

We compare the recombination properties of a large number of grain boundaries in multicrystalline silicon wafers with different contamination levels and investigate their response to phosphorous gettering and hydrogenation. The recombination activity of a grain boundary is quantified in terms of the effective surface recombination velocity SGB based on photoluminescence imaging and 2-D modeling of the emitted photoluminescence signal. Our results show that varying impurity levels along the ingot significantly impact the grain boundary behavior. Grain boundaries from the middle of the ingot become more recombination active after either gettering or hydrogenation alone, whereas grain boundaries from the top and bottom of the ingot have a more varied response. Hydrogenation, in general, is much more effective on gettered grain boundaries compared with as-grown grain boundaries. A close inspection of their injection dependence reveals that while some grain boundaries exhibit little injection dependence before gettering, others show a relatively large injection dependence, with their SGB increasing as the injection level decreases. The former type tend not to be recombination active after both gettering and hydrogenation and are less likely to impact the final cell performance, in comparison with grain boundaries of the latter type.

Investigating Internal Gettering of Iron at Grain Boundaries in Multicrystalline Silicon via Photoluminescence Imaging

Compared with phosphorus diffusions, conventional boron diffusions for n-type solar cells are not effective at impurity gettering without the presence of a boron-rich layer. In this paper, we investigate the gettering effectiveness of light phosphorus diffusions for removing Fe impurities, applied on an underlying boron diffusion, similar to the buried emitter concept, as an option for achieving effective gettering on boron diffused substrates. Our experimental results on monocrystalline silicon samples demonstrate that the underlying boron diffusion does not affect the gettering effectiveness of the phosphorus diffusion, even though much of the phosphorus diffused region is overdoped by the boron diffusion. Furthermore, we investigate the gettering effectiveness of low surface concentration phosphorus diffusions that can result in reduced recombination in the n+ region. Our results show that the gettering effectiveness decreases when the surface phosphorus concentration is reduced, either through manipulating the deposition gas flows or through subsequent driving in. Driving in the surface phosphorus concentration from 2 × 1020 to 3.5 × 1019 cm-3 decreased the gettering effectiveness by about one order of magnitude.

Monolithic perovskite/silicon-homojunction tandem solar cell with over 22% efficiency

The removal of dissolved iron from the wafer bulk is important for the performance of p-type multicrystalline silicon solar cells. In this paper we review some recent progress in understanding both external and internal gettering of iron. Internal gettering at grain boundaries and dislocations occurs naturally during ingot cooling, and can also be driven further during cell processing, especially by moderate temperature anneals (usually below 700 °C). Internal gettering at intra-grain defects plays key a role during such precipitation annealing. External gettering to phosphorus diffused regions is crucial in reducing the dissolved iron concentration during cell processing, although its effectiveness depends strongly on the diffusion temperature and profile. Gettering of Fe by boron and aluminum diffusions is also found to be very effective under certain conditions.