Joris Libal

Effect of compensation and of metallic impurities on the electrical properties of Cz-grown solar grade silicon

In this work we present a study of a p-type Czochralski-grown Si ingot which was grown using 10% solar grade silicon (SoG-Si). As the SoG-Si contains a relatively high concentration of impurities including phosphorus, the electrical properties of the as-grown wafers from this ingot are affected by both the compensating dopants and other impurities. Measurements of the minority charge carrier lifetime in the as-grown material reveal very low values (4–8μs). The Hall mobilities at room temperature correspond to normal values for Czochralski silicon in the upper part of the ingot (which solidifies first) and decrease significantly toward the bottom of the ingot. Segregation leads to an accumulation of impurities toward the lower parts of the ingot as well as to a stronger increase in phosphorus than of boron, the latter of which results in a high compensation level (i.e., an increasing resistivity). A priori, both effects could be responsible for the degradation of the electrical properties in the lower part...

Large-Area Back-Contact Back-Junction Solar Cell With Efficiency Exceeding 21%

In this study, high-efficiency solar cells are presented with the use of low-cost industrially available technologies. This results in the so-called ZEBRA concept: a litho-free process in which standard 156 × 156 mm2 monocrystalline n-type Cz-Si wafers are processed into high-efficiency interdigitated back-contact solar cells. In our first attempt, we obtained energy conversion efficiencies of more than 20% on 239 cm2 area. With the help of a 3-D simulation of the device, a further improvement to more than 21.5% was determined. We indentify the open-circuit voltage and the large serial resistance as the main losses impeding from reaching the simulated efficiency. The cell results after an extensive optimization are presented in this study, focusing on the improvement of the fill factor of the cells. The optimized solar cells show an improvement in the energy conversion efficiency up to 21%. Furthermore, the simple metallization and the module interconnection design of the ZEBRA cells allow for a bifacial application. Indoor and outdoor I - V measurements on a bifacial one-cell-module show an enhancement in the total generated power of more than 12% as compared with a monofacial module.

EBIC, EBSD and TEM study of grain boundaries in multicrystalline silicon cast from metallurgical feedstock

Grain boundaries in multicrystalline silicon material grown from metallurgical feedstock, were investigated in detail using Electron Beam Induced Current (EBIC), Electron Back-Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) techniques. The EBSD analysis showed that small angle grain boundaries, with misorientation angles lower than 2°, gave high EBIC contrast, i.e., high recombination activity. EBIC combined with TEM showed that at low temperatures, silicon oxide was found to be recombination centers both at grain boundaries and on decorated dislocations in the bulk. The grain boundaries containing multi-metallic silicides were found to have random misorientations and showed strong contrast in the EBIC image. Clean twins showed less or no contrast in the EBIC image. The metallic precipitates observed in the sample contain mainly nickel silicide with an iron rich core.

Impact of Extended Defects on the Electrical Properties of Solar Grade Multicrystalline Silicon for Solar Cell Application

Aim of this work is to study the electrical properties and the minority charge carrier recombination behaviour of extended defects in multicrystalline silicon (mc-Si) ingots grown from solar grade silicon (SoG-Si) feedstock. The pure metallurgical SoG-Si feedstock has been produced directly by carbothermic reduction of very pure quartz and carbon without subsequent purification processes.This mc SoG-Si is studied by temperature-dependent Electron Beam Induced Current measurements and PhotoLuminescence spectroscopy and the potentiality of the combination of these two techniques in the identification of the defects which limit the quality of the base material is shown. The EBIC mapping technique shows the presence of electrically active grain boundaries at room temperature while dislocations result inactive. Dislocations become active only at temperatures lower than 250K, indicating a moderate level of metal decoration. The most detrimental defects in this material seem to be the grain boundaries and impurities dissolved in the matrix. Furthermore, the PL spectra reveal the presence of oxygen and carbon related complexes. In this work we show that the knowledge about the defect related recombination processes acquired by a combined application of EBIC measurements and PL-spectroscopy is of particular importance to tune the proper solar cell process step to be applied on such material.

Evaluation of cell to module losses for n-type IBC solar cells assembled with state of the art consumables and production equipment

In this paper we present an investigation on different aspects of the module assembly for back contacted n-type solar cells; in our case implemented on ISCs “Zebra” IBC cell. With all contacts on the rear, new possibilities and challenges arise. The study is conducted on one cell mini-modules, “rear half”- and “front half”- mini-modules. It is aiming to minimize the overall cell to module (CTM) loss and to maximize module robustness against climate chamber tests. Electrical losses are evaluated using different interconnection schemes whereby optical losses are studied by utilizing novel encapsulant layers; long term stability is probed with climate chamber testing (TC200/400), PID and surface polarization tests.