Wire-sawing defects on multicrystalline silicon wafers grown by a directional solidification method

Residual stresses in multi-crystalline silicon photovoltaic wafers due to casting and wire sawing

In a multicrystalline silicon (mc-Si) wafer, trapping effects frequently occur in the carrier lifetime measurement based on the quasi-steady-state photoconductance (QSSPC) technique. This affects the accurate measurement of the carrier lifetime of an mc-Si solar cell by causing distortions at a low injection level close to the Pmax point. Therefore, it is necessary to understand this effect and effectively minimize the trapping-center density. In this study, the variations in the minority carrier-trapping effect of hydrogen at different annealing temperatures in an mc-Si were observed using QSSPC, time-of-flight secondary ion mass spectroscopy, and atom probe tomography. A trapping effect was confirmed and occurred in the grain boundary area, and the effect was reduced by hydrogen. Thus, in an mc-Si wafer, effective hydrogen passivation on the grain area and grain boundary is crucial and was experimentally proven to minimize the distortion of the carrier lifetime.

Etching methods for texturing industrial multi-crystalline silicon wafers: A comprehensive review

A practical challenge in slicing of low-cost multi-crystalline silicon (mc-Si) wafers by the fixed abrasive diamond wire sawing process is increased wire consumption due to greater wear of the diamond compared to slicing of the more expensive mono-crystalline silicon (mono-Si) wafers. In this paper, we present the results of scribing of mc-Si and mono-Si materials with two conical tip diamond indenters of the same geometry to understand the possible reasons for increased diamond wear in cutting of multi-crystalline silicon. Specifically, the scribing forces and the diamond indenter wear produced in scribing of the two silicon materials are analyzed. The results show that the forces generated in scribing of mc-Si are higher than in scribing of mono-Si. The higher forces in scribing of mc-Si are consistent with the corresponding higher tip radius of curvature (due to wear) of the diamond indenter compared to the tip radius produced in scribing of mono-Si. Scanning electron microscopy and confocal microscopy of the diamond indenters show that wear is primarily due to physical micro-fracture and blunting of the diamond. Raman spectroscopy shows evidence of stress-induced phase transformation of the diamond and the formation of compressive residual stress in the diamond. Plausible physical reasons, including the role of material inhomogeneity in mc-Si, for the wear of diamond during scribing are given.

Characterization of near-surface electrical properties of multi-crystalline silicon wafers

In this paper, we investigate the influence of different chemical treatments on the surface of multicrystalline Silicon (mc-Si) wafers, for both revealing grain boundaries and intra-grain defects. Electrical characterization by light beam-induced current (LBIC) was also carried out after the treatments. Several pieces of ~2 × 2 cm2, from mc-Si wafers, were mechanically polished and chemically etched and subsequently metallized with gold (on both surfaces) by sputtering, using optimized deposition times for doing transparent electrodes suitable for LBIC mapping. The surface treatments are discussed in terms of their capabilities to reveal the crystal defects and to provide the best conditions for efficient LBIC signals directly on silicon substrates, in the absence of a p–n junction. The best surface treatments allowing both revealing crystallographic defects and permitting the measurement of highly contrasted LBIC maps is the KOH etching. A large number of intra-grain defects are clearly revealed, being their electrical activity sensitively higher than that of the grain boundaries.

It was found that the addition of MnO2 particles into the HF/HNO3/H2O system could significantly improve the texturization etching performance on multi-crystalline silicon (mc-Si) wafer. For a wide component ratio range of HF/HNO3/H2O from HF-rich to HNO3-rich, by optimizing the MnO2 usage and the etching time, the addition of MnO2 particles always reduced the texture reflectance greatly. Low weighted average surface reflectance (Ra) for the AM1.5G sun spectrum in the wavelength range of 380–1100 nm was achieved on both the slurry wire sliced (SWS) mc-Si and the diamond wire sliced (DWS) mc-Si. Due to its excellent effect and simple processing, the MnO2/HF/HNO3/H2O etching system can be expected as a candidate for high-performance texturization on mc-Si wafer, especially on DWS mc-Si wafer.

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Defect diagnostics using scanning photoluminescence in multicrystalline silicon

We describe results of our theoretical and experimental studies performed to investigate the influence of defects and defect distributions in multicrystalline silicon (mc-Si) wafers on the solar cell's performance. Dislocation distributions were measured on wafers from various bricks of a mc-Si ingot. Solar cells were fabricated on sister wafers and characterized by a variety of methods. Cell performance can be accurately predicted from dislocation distribution of a mc-Si wafer using local N/P junction characteristics in a distributed network model. This analysis is applied to investigate changes in cell performance caused by dislocation propagation within a brick of mc-Si ingot. The theoretical results agree well with the measured performance of cells fabricated on wafers taken from different places in a brick.

A novel low cost process for texturing the diamond wire sawn (DWS) multi-crystalline silicon (mc-Si) wafers is demonstrated. The proposed scheme does not use any additives or metals and can be integrated into any standard industrial acid texturing tool. This process can be carried out at temperature ranges from 13–17 °C, which is normally higher than the temperature used for the conventional acid texturization process and thus making the process energy efficient. In this work, authors use HF rich acid solution for the generation of the porous silicon (Por-Si) layer, it is then followed by Por-Si dilution in an alkali solution. Process generates less porous, inverted rounded structures on the mc-Si wafer surfaces with excellent light trapping properties. Textured surfaces yield weighted average reflectance of 22.5% combined with a minority carrier lifetime up to 60 μs.

Potential cost reduction of buried-contact solar cells through the use of titanium dioxide thin films

Comprehensive cell to module optical loss analysis of metal assisted chemically etched inverted pyramid textured multi-crystalline silicon solar cells and modules by ray-tracing method

N-type silicon wafers present some definite advantages for photovoltaics, mainly due to the low capture cross sections of minority carriers for most metallic impurities. This peculiarity is beneficial for multicrystalline silicon (mc-Si) wafers. Most importantly, this peculiarity could be of a great interest when mc-Si ingots are produced directly from upgraded and purified metallurgical silicon feedstock. It is of a paramount importance to verify if the advantages of conventional n-type silicon also characterises n-type wafers provided by the direct metallurgical route. It is found, in raw wafers, that minority carrier diffusion lengths are 3 times higher in n-type than in p-type wafers, when the wafers are cut from the same ingot, which the bottom is p- type and the top is n-type. The results confirm the interest for n type silicon.

Relation between solar cell efficiency and crystal defect etching induced by acidic texturization on multicrystalline silicon material