Johannes Junge

Detailed Investigation of Surface Passivation Methods for Lifetime Measurements on P-Type Silicon Wafers

The effect of five different common surface passivation techniques on the measured bulk lifetime values of multi- and monocrystalline p-type silicon wafers was investigated. Mono-[Czochralski (Cz) and floatzone (FZ)] and multicrystalline [mc and edge-defined film-fed growth (EFG)] silicon wafers were either deposited with a dielectric passivating layer of SiNx, Al2O3, or amorphous silicon (a-Si) or were passivated chemically with 0.08 M iodine-ethanol (IE) or 0.07 M quinhydrone-methanol (QM) solutions. The temporal stability of annealed and nonannealed Cz wafers that were passivated with QM and IE was tested. The lifetime values of EFG, mc, and FZ wafers that were subjected to repeated QM passivation and mc wafers that were subjected to IE passivation without a surface etching between passivations were found to decrease with each passivation. Lifetime values of a set of 11 mc wafers that were passivated with Al2O3 were found to decrease about 30% after a period of four weeks in darkness. The decrease was reversible by annealing the samples. The lifetime values of annealed Cz samples that were passivated with Al2O3 and a-Si were found to decrease by >;20% within 5 h of annealing. Subsequent tests on 200-Ω·cm FZ material did (for a-Si) and did not (for Al2O3) show surface passivation degradation over this time period. Neighboring mc wafers were passivated dielectrically or wet chemically with IE or QM and characterized with photoluminescence imaging. All mc wafers that were subjected to dielectric passivation methods that include annealing at 400 °C displayed a greater area of high lifetime values but fewer areas of very high lifetime values, providing visible evidence of internal gettering and/or defect redistribution.

μXRF investigations on the influence of solar cell processing steps on iron and copper precipitates in multicrystalline silicon

The material quality of multicrystalline silicon is influenced by crystal defects and contaminations like transition metal precipitates. During solar processing these defects can be restructured and change their electrical activity. The purpose of this work is to study the impact of different solar cell processing steps on the distribution and electric activity of transition metal precipitates like iron and copper. Therefore, neighbouring wafers of a multicrystalline silicon ingot, intentionally contaminated with iron and copper were investigated by μXRF (X-Ray Fluorescence Microscopy) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, to determine the distribution of transition metal precipitates. Afterwards, several solar cell processing steps were applied to these samples. The same sample areas were measured by μXRF again to determine the influence of the applied processing steps on the observed transition metal precipitates. Therefore, a different behaviour of iron and copper precipitates could be observed as expected, due to their different dissolution and diffusion coefficients in silicon. Additionally, the same processing steps were applied to a second set of samples to evaluate the effect of processing steps on the minority charge carrier lifetime and the recombination activity of grain boundaries.

Advanced processing steps for high efficiency solar cells based on EFG material

In the past few years the quality of Edge-defined Film-fed Growth (EFG) material has strongly improved and can now compete with most standard multicrystalline materials. The maximum conversion efficiency of solar cells based on high quality EFG material is at the moment mostly limited by the applied solar cell processing steps. The state-of-the-art high efficiency process at the University of Konstanz (UKN) in combination with some additional processing steps is presented. The latter include hydrogen passivation of bulk defects, texturisation of the front surface by remote SF6 plasma (most samples shown here were textured at IMEC), surface passivation using a silicon oxide / silicon nitride stack and the application of Laser Fired Contacts (LFC). Single additional processing steps are investigated as well as various combinations of additional processing steps.

Investigations on the recombination activity of grain boundaries in MC silicon

This paper focuses on the influence of the effective intra-grain minority charge carrier diffusion length and surface recombination velocity at grain boundaries on solar cell parameters. Both can be extracted in principle from Light- and Electron Beam Induced Current measurements (LBIC and EBIC). Multicrystalline floatzone (mc FZ) silicon with different grain sizes was processed to solar cells with and without hydrogenation step, followed by LBIC and EBIC characterization. A theoretical model is used which can be applied to measured LBIC or EBIC profiles in order to obtain values for the effective intra-grain diffusion length and the recombination velocity at grain boundaries. Efficiencies reached on the processed solar cells (up to 16.0%) are the highest reported so far for material with such a small grain size, and the positive effect of hydrogenation can clearly be seen. The obtained results are very useful for other cost effective small grained mc silicon materials.