Sven Seren

Concentration and penetration depth of H introduced into crystalline Si by hydrogenation methods used to fabricate solar cells

The hydrogenation of crystalline Si by methods used to passivate defects in Si solar cells has been studied by infrared spectroscopy. For these experiments, floating-zone Si that contained Pt impurities that act as traps for H was used as a model system in which H could be directly detected. In this model system, the concentration and indiffusion depth of H were determined for different hydrogenation treatments so that their effectiveness could be compared. The postdeposition annealing of a hydrogen-rich SiNx surface layer was found to introduce H into the Si bulk with a concentration of ∼1015cm−3 under the best conditions investigated here.

Review on Ribbon Silicon Techniques for Cost Reduction in PV

The shortage of Si feedstock and the goal of reducing Wp costs in photovoltaics (PV) is the driving force to look for alternatives to ingot grown multicrystalline (me) Si wafers which have the highest share in the PV market. Ribbon Si seems to be a very promising candidate as no kerf losses occur, resulting in reduced Si costs per Wp. In addition, there is no need for the energy consuming crystallization of the ingot and therefore energy payback times can be significantly reduced. The higher defect density in ribbon Si materials has to be taken into account during cell processing, but ribbon materials already commercially available show excellent efficiencies, while for the most promising techniques efficiencies are significantly lower, but very promising. In this presentation an overview of ribbon Si technologies currently under research will be given, based on available data on crystal growth as well as solar cell processing and cell parameters

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...

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

Screen-printed ribbon growth on substrate solar cells approaching 12% efficiency

Ribbon growth on substrate (RGS) solar cells have been processed at the University of Konstanz using an adapted industrial-type fire-through SiN process. An efficiency of 12.3% has been reached on a 5/spl times/5 cm/sup 2/ cell. This is the highest efficiency obtained on this very promising and cost-effective material using an industrial-type cell process. An important factor for the increase in efficiency was the reduced oxygen concentration of almost an order of magnitude in the current RGS wafer material compared to former RGS material. Enhanced J/sub sc/, V/sub oc/ and L/sub eff/ values in the range of 100 /spl mu/m as well as lifetimes above 4 /spl mu/s demonstrate the potential of the new low oxygen RGS material. Efficiencies well above 13% should be possible, provided a surface texture is applied and shunting mechanism can be avoided.

Near 13% Efficiency Shunt Free Solar Cells on RGS Wafers

Direct casting of silicon into wafers allows to produce wafers much more cheaply than in traditional block casting methods. RGS (ribbon growth on substrate) is such a method. In order for RGS to be cost effective sufficient cell efficiencies must be realized. In this paper we present a 12.9% efficient screen printed RGS cell. This is an increase of 0.6% absolute compared to the best previous result reported in January 2005. The increase was achieved by reducing shunting in cells made on RGS material

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.

Dopant mapping in highly p-doped silicon by micro-Raman spectroscopy at various injection levels

Micro-Raman spectroscopy has been used to investigate the acceptor distribution in highly p-doped silicon. As an example, the dopant distribution in crystalline thin-film layers, as developed for solar cells, was mapped. The method is based on the analysis of the Fano-type Raman peak shape which is caused by free charge carriers. For calibration of the Raman acceptor measurements (excitation at a wavelength of 532 nm), we used mono-crystalline reference samples whose acceptor concentration was determined by electrochemical capacitance voltage. We find a significant influence of light induced free charge carriers on the peak shape which results from typical Raman excitation. Thus, the selection of a suitable intensity is important to avoid a too low signal-to-noise ratio on the one hand and systematic errors due to light induced carriers on the other hand. Different evaluation methods, i.e., peak asymmetry versus peak width analysis, are compared in respect to interference caused by random noise of the spe...

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.