Martin Kaes

A New Approach to Prevent the Negative Impact of the Metastable Defect in Boron Doped CZ Silicon Solar Cells

A new reaction model concerning the boron-oxygen related degradation is presented, introducing a third recombination inactive state, that stabilizes the electrical parameters of Cz-Si solar cells, and the transition to this new inactive state is proven by experimental data. Furthermore, the stability under solar cell working conditions and the formation kinetics of this additional state are discussed

Correlating Multicrystalline Silicon Defect Types Using Photoluminescence, Defect-band Emission, and Lock-in Thermography Imaging Techniques

A set of neighboring multicrystalline silicon wafers has been processed through different steps of solar cell manufacturing and then images were collected for characterization. The imaging techniques include band-to-band photoluminescence (PL), defect-band or subbandgap PL (subPL), and dark lock-in thermography (DLIT). Defect regions can be tracked from as-cut wafers throughout processing to the finished cells. The finished cells defect regions detected by band-to-band PL imaging correlate well to diffusion length and quantum efficiency maps. The most detrimental defect regions, type A, also correlate well to reverse-bias breakdown areas as shown in DLIT images. These type A defect regions appear dark in band-to-band PL images, and have subPL emissions. The subPL of type A defects shows strong correlations to poor cell performance and high reverse breakdown at the starting wafer steps (as-cut and textured), but the subPL becomes relatively weak after antireflection coating (ARC) and on the finished cell. Type B defects are regions that have lower defect density but still show detrimental cell performance. After ARC, type B defects emit more intense subPL than type A regions; consequently, type B subPL also shows better correlation to cell performance at the starting wafer steps rather than at the ARC process step and in the finished cell.

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

Over 18% Efficient MC-SI Solar Cells from 100% Solar Grade Silicon Feedstock from a Metallurgical Process Route

Based on very promising results for solar cells manufactured in an industrial process reaching 16% efficiency we analyzed if solar grade (SoG) silicon feedstock is capable to match up with electronic grade silicon in high efficiency ranges. Thus we applied a reliable lab-type process on 5times5 cm2 wafers resulting in 2times2 cm2 untextured solar cells with an efficiency limit of 18.5% for floatzone (FZ) references. The 5times5 cm2 wafers were selected out of 12.5times12.5 cm2 phosphorous pregettered SoG-Si wafers characterized by lifetime measurements. The best solar cell out of 100% Elkem SoG-Si reached eta=18.1% stable efficiency certified by FhG-ISE CalLab. This is the highest value for this material reported so far

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.

Bulk passivation in silicon ribbons: a lifetime study for an enhanced high efficiency process

The effectiveness of hydrogenation either by deposition plus firing of a PECVD SiN layer in a conventional belt furnace or by remote H-plasma was compared quantitatively using spatially resolved lifetime measurements for EFG and string ribbon. Additionally the effect of a preceding phosphorous gettering on the hydrogenation and the presence of a screen printed rear side aluminum during firing was analyzed. Wafer areas with the presence of a rear side aluminum show additional lifetime improvements for both hydrogenation methods probably due to a gettering effect. With preceding P-gettering hydrogenation by SiN deposition plus firing is superior to remote H-plasma. A synergetic effect of a rear side aluminum as described elsewhere is not obtained. First high efficiency EFG solar cells using a PECVD SiN fired in a conventional belt furnace were processed with efficiencies in the 17-18% range.

Superior low-light-level performance of upgraded metallurgical-grade silicon modules

The industry is becoming critically sensitive to solar energy delivered in kilowatt-hours rather than in kilowatt at illumination peak intensity. This is because when comparing systems or modules, it is more relevant to compare the energy delivered during an entire day than the energy delivered during peak illumination, which happens for very few hours on the same day. For that reason, low-light-level performance is an important parameter that greatly influences the total energy yield of a PV system. This is especially important in low annual insolation regions such as northern Europe or the northeast United States. Low-light-level performance can vary significantly even within a particular PV technology. In this contribution, results of low-light performance of three modules are presented. The first module uses Calisolar upgraded metallurgical-grade (UMG) Si solar cells, the second module uses standard monocrystalline Si cells, and the third module uses standard electronic-grade (EG) Si cells. The modules were first tested at NRELs Outdoor Testing Facility. The low-light-level performance of the three modules indicated a markedly higher module output for the module with Calisolar UMG cells. Because angle of incidence, temperature, and spectral variations can significantly influence these data, these modules were also measured using a Spire indoor solar simulator. Measurements at 200, 400, 600, 800, and 1000 W/m2 corroborated our outdoor tests and the superior performance of the UMG-based modules. We observed that the shunt resistance of the module with Calisolar UMG cells is higher than that of the other two modules, which can explain the higher module output. Thus, as the light intensity decreases, the light IV curve moves toward the lower part of the diode characteristics where Rshunt and J02 (which describes recombination in the space charge region) dominate. In this low-light regime, a decrease of Rshunt drastically reduces the VOC and fill factor. Cells with higher Rshunt are less affected. Large-scale system outputs have also confirmed higher performance module output in low-light conditions for modules using Calisolar cells.