Ralf Jonczyk

Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cells

Synchrotron-based microprobe techniques were used to obtain systematic information about the size distribution, spatial distribution, shape, electrical activity, chemical states, and origins of iron-rich impurity clusters in multicrystalline silicon (mc-Si) materials used for cost-effective solar cells. Two distinct groups of iron-rich cluster have been identified in both materials: (a) the occasional large (diameter ⩾1μm) particles, either oxidized and/or present with multiple other metal species reminiscent of stainless steels or ceramics, which are believed to originate from a foreign source such as the growth surfaces, production equipment, or feedstock, and (b) the more numerous, homogeneously distributed, and smaller iron silicide precipitates (diameter ⩽800nm, often ⩽100nm), originating from a variety of possible formation mechanisms involving atomically dissolved iron in the melt or in the crystal. It was found that iron silicide nanoprecipitates account for bulk Fe concentrations as high as 1014–...

Effects of grain boundary on impurity gettering and oxygen precipitation in polycrystalline sheet silicon

The effects of grain boundaries (GB) in polycrystalline sheet silicon on impurity gettering and oxygen precipitation were investigated by electron beam induced current (EBIC), deep level transient spectroscopy (DLTS), micro-Fourier-transform infrared spectroscopy (FTIR), and preferential etching/Normaski optical microscopy techniques. Both as-grown and thermally processed wafers were studied. A correlation between GB density and transition metal concentration was quantitatively established by combining DLTS and EBIC studies. It was found that four deep levels arising from Fe–B, Fe–Al, Cr–B, and Fei were present in the as-grown sample, and their concentrations decrease with increasing GB density. GB gettering was further verified by the presence of an EBIC image contrast halo around the GB. Preferential etching also revealed a precipitate density of 2×107 cm−2 on the GB. After processing, a clearly defined oxygen precipitate denuded zone formed around the GB with the interstitial oxygen concentration [Oi] ...

Ribbon Growth on Substrate and Molded Wafer-Two Low Cost Silicon Ribbon Materials for PV

This paper focuses on two very promising silicon ribbon materials currently produced for research: ribbon growth on substrate (RGS) by ECN solar energy and molded wafer (MW) by GE Energy. Both materials are investigated in terms of solar cell processing and characterisation. First cell results of large area 10times10 cm2 RGS cells are presented as well as results from 5times5 cm2 cells processed from 8times12 cm2 RGS and 12.5times12.5 cm2 MW wafers

Oxygen precipitate denuded zone in polycrystalline sheet silicon

The spatial variation of oxygen precipitation across the thickness of polycrystalline sheet silicon has been investigated by Fourier transform infrared (FTIR) microspectroscopy and preferential etching/optical microscopy. FTIR shows that interstitial oxygen is depleted near the top surface of the as-grown sample, thereby suppressing oxygen precipitation during subsequent annealing. Preferential etching and electron-beam-induced current imaging of polished cross sections revealed a 250-μm-wide precipitate denuded zone near the top surface. Evidently, growth-induced near-surface reduction of the oxygen profile keeps the oxygen supersaturation below a critical level for precipitate nucleation. Considering that the minority carrier diffusion length of current finished sheet silicon solar cells ranges from 50to100μm, it is anticipated that optimization of the 250-μm-wide precipitate denuded zone will improve solar cell performance.

Evaluation of Silicon Sheet Film Growth and Wafer Processing via Structural, Chemical and Electrical Diagnostics

This paper discusses data obtained via a complementary set of diagnostic procedures applied to continuous polycrystalline sheet silicon film us ed in the production of solar cell panels. The measurements include a three-pronged st ructural, chemical, and electrical diagnostic approach, which has enabled us to increase our understanding of defect/impurity/gettering device issues in this highly cost effe ctiv , but relatively defective and impure PV material. Electrically, the charge c arrier recombination behavior of grain boundaries (GBs) and intra-grain dislocations/stacking fault s/inclusions were studied by electron beam induced current (EBIC) and laser microwave photoconductance decay ( μ−PCD). Structurally, defects were delineated by preferenti al e ching/Nomarski optical microscopy. Chemically, FTIR, SIMS and SEM/EDS were a pplied. Of particularly interest was a comparative look at the degrading e ffects of either metallic impurities or oxide/nitride/carbide precipitates, recognizing tha t a competition exists between the grain boundaries and the precipitates for gettering of both light element and metallic impurities. This diagnostic approach has enabled us to confi dently proceed with systematic defect engineering changes in the sheet film growth process. These efforts have contributed to recent increases of approximately 10% in the avera ge output of AstroPower standard solar panels.

Single Wafer Casting

Converting silicon to wafers by a process of individually casting wafers has great cost advantages over conventional ingot casting methods. The Molded Wafer process demonstrates good silicon utilization, low processing cost, and offers efficiencies approaching that of conventional cast multicrystalline wafers. The ability to alter the casting mold offers freedom in wafer shape and presents the means to incorporate features such as metallization through holes at no added cost. These benefits make single wafer casting an economically viable option in times of silicon shortages and highly competitive product pricing

EBIC Study of Electrical Activity of Stacking Faults in Multicrystalline Sheet Silicon

The electrical activity of stacking faults (SFs) in multicrystalline sheet silicon has been examined by correlating EBIC(electron beam induced current), preferential defect etching, and microwave photo-conductance decay (PCD) lifetime measurements. Following a three hour 1060 0C annealing the interstitial oxygen concentration decreased from 14 to 4.5 x 1017 cm-3, during which time a high density of SFs were generated in the center of individual large grains. Subsequent EBIC contrast variation within individual large grains was correlated with the local SF density revealed by preferential etching. In addition, a more quantitative intra-grain lifetime was obtained from high spatial resolution PCD measurements. It was found that an SF density of 1 to 2 x 106 cm-2 produces a lifetime limitation in sheet silicon which corresponds to a recombination lifetime of ~2 µs.