Annika Zuschlag

Improved iron gettering of contaminated multicrystalline silicon by high-temperature phosphorus diffusion

The efficacy of higher-temperature gettering processes in reducing precipitated iron concentrations is assessed by synchrotron-based micro-X-ray fluorescence. By measuring the same grain boundary before and after phosphorus diffusion in a set of wafers from adjacent ingot heights, the reduction in size of individual precipitates is measured as a function of gettering temperature in samples from the top of an ingot intentionally contaminated with iron in the melt. Compared to a baseline 820 °C phosphorus diffusion, 870 °C and 920 °C diffusions result in a larger reduction in iron-silicide precipitate size. Minority carrier lifetimes measured on wafers from the same ingot heights processed with the same treatments show that the greater reduction in precipitated metals is associated with a strong increase in lifetime. In a sample contaminated with both copper and iron in the melt, significant iron gettering and complete dissolution of detectable copper precipitates is observed despite the higher total metal concentration. Finally, a homogenization pre-anneal in N2 at 920 °C followed by an 820 °C phosphorus diffusion produces precipitate size reductions and lifetimes similar to an 870 °C phosphorus diffusion without lowering the emitter sheet resistance.

Recombination at Lomer Dislocations in Multicrystalline Silicon for Solar Cells

Lomer dislocations at small-angle grain boundaries in multicrystalline silicon solar cells have been identified as responsible for the dominating inherent dark current losses. Resulting efficiency losses have been quantified by dark lock-in thermography to be locally up to several percent absolute, reducing the maximum power of the cells. By electron beam induced current measurements and scanning transmission electron microscopy investigations, it is revealed that the strengths of the dark current losses depend on the density of Lomer dislocations at the small-angle grain boundaries.

Quantitative evaluation of grain boundary activity in multicrystalline semiconductors by light beam induced current : an advanced model

We present an advanced analytical model which applies to light beam induced current contrast profiles to determine reliably the effective surface recombination velocities (Seff) of grain boundaries (GBs) and diffusion lengths (Ldiff) in the grains, in cases where a GB is close to the studied one or when Ldiff of the neighboring grain differs. We introduce additionally a new method for a very accurate determination of the plateau value of the investigated linescan and make use of simultaneously fitting GB profiles measured at various laser wavelengths both aiming at increasing the accuracy of the Ldiff determination. Through several special case investigations, the various applications and limitations of the model are demonstrated. We discuss the influence of the electrical parameters of the semiconductor on the various zones of the profile as well as the influence of measurement technique parameters on the experimental profile and point out the need of an accurately determined small laser beam radius to e...

Nanomechanical control of an optical nanoantenna

We mechanically tune the feedgap of a single gold bowtie antenna by precise nanomanipulation with the tip of an atomic force microscope. At the same time, its optical response is determined via dark-field scattering spectroscopy.

Investigation of Lifetime-Limiting Defects After High-Temperature Phosphorus Diffusion in High-Iron-Content Multicrystalline Silicon

Phosphorus diffusion gettering of multicrystalline silicon solar cell materials generally fails to produce material with minority-carrier lifetimes that approach that of gettered monocrystalline wafers, due largely to higher levels of contamination with metal impurities and a higher density of structural defects. Higher gettering temperatures should speed the dissolution of precipitated metals by increasing their diffusivity and solubility in the bulk, potentially allowing for improved gettering. In this paper, we investigate the impact of gettering at higher temperatures on low-purity multicrystalline samples. To analyze the gettering response, we measure the spatially resolved lifetime and interstitial iron concentration by microwave photoconductance decay and photoluminescence imaging, and the structural defect density by Sopori etching and large-area automated quantification. Higher temperature phosphorus diffusion gettering is seen to improve metal-limited multicrystalline materials dramatically, especially in areas of low etch pit density. In areas of high as-grown dislocation density in the multicrystalline materials, it appears that higher temperature phosphorus diffusion gettering reduces the etch pit density, but leaves higher local concentrations of interstitial iron, which degrade lifetime.

Degradation and regeneration analysis in mc-Si

The performance of mc-Si PERC solar cells can be significantly affected by LeTID. The underlying mechanism causing LeTID is still unknown. This work compares the degradation and regeneration behavior under illumination and elevated temperature of an industrial mc-Si PERC solar cell to differently processed minority charge carrier lifetime samples. A strong degradation and also regeneration can be observed on lifetime level. Degradation and regeneration are strongly influenced by the applied process steps, like gettering, temperature load and surface passivation method. Therefore, lifetime studies offer a valuable possibility to identify further parameters influencing LeTID.

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.

Transition metal precipitates in mc Si: a new detection method using 3D-FIB

To investigate transition metal precipitates in Si, synchrotron based measurements, like micro x-ray fluorescence (μXRF) or detailed transmission electron microscopy (TEM) studies, are usually necessary. Transition metals are among the most detrimental defects in multi-crystalline (mc) silicon material for solar cell applications, due to their impact on minority charge carrier lifetime and possible shunt formation. We present another possibility to investigate transition metal precipitates by 3-dimensional focused ion beam (3D-FIB) cutting using a combined scanning electron microscope (SEM) SEM-FIB-system. This method is able to detect transition metal precipitates down to 5 nm in radius and provides additional information about the 3D shape, size and spatial distribution of the precipitates.

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.

Influence of dielectric layers and thermal load on LeTID

Light and elevated temperature induced degradation (LeTID) is observed for multicrystalline (mc) Si passivated emitter and rear cell (PERC) solar cells, strongly limiting solar cell parameters under operation conditions. In this contribution, we investigate the effect of surface passivation layer being present during the firing step based on lifetime samples. The LeTID effect is only observed if the surface passivation layer is present during the firing step. Samples without firing step show no LeTID. A re-passivation of the surface significantly changes the LeTID effect, showing that the whole sample treatment, temperature load and hydrogen content of a sample has to be taken into account investigating and evaluating LeTID.Light and elevated temperature induced degradation (LeTID) is observed for multicrystalline (mc) Si passivated emitter and rear cell (PERC) solar cells, strongly limiting solar cell parameters under operation conditions. In this contribution, we investigate the effect of surface passivation layer being present during the firing step based on lifetime samples. The LeTID effect is only observed if the surface passivation layer is present during the firing step. Samples without firing step show no LeTID. A re-passivation of the surface significantly changes the LeTID effect, showing that the whole sample treatment, temperature load and hydrogen content of a sample has to be taken into account investigating and evaluating LeTID.