Ribbon Growth on Substrate (RGS) silicon wafers are casted directly from the silicon melt onto reusable substrates. Material losses by wafer sawing are omitted and high production speeds can be achieved. However, multicrystalline RGS silicon as it is produced today incorporates high densities of crystal defects and impurities limiting the efficiency of the corresponding solar cells. The local impact of crystal defects on material quality is estimated via models developed by Donolato and Micard et al.. By theoretically negating the impact of grain boundaries and dislocations, charge carrier diffusion lengths are still limited to values o 100 mm. In addition to crystal defects which are common in other multicrystalline silicon materials, we found current collecting structures within grain boundaries. These structures can be associated with carbon and oxygen precipitation and are the cause for shunting phenomena. We conclude that high impurity concentrations are the dominant factor for limiting the performance of RGS silicon solar cells.

We report on measurements of crystal growth dynamics in semiconducting pure Ge and pure Si melts and in Ge Si ( = 25, 50, 75) alloy melts as a function of undercooling. Electromagnetic levitation techniques are applied to undercool the samples in a containerless way. The growth velocity is measured by the utilization of a high-speed camera technique over an extended range of undercooling. Solidified samples are examined with respect to their microstructure by scanning electron microscopic investigations. We analyse the experimental results of crystal growth kinetics as a function of undercooling within the sharp interface theory developed by Peter Galenko. Transitions of the atomic attachment kinetics are found at large undercoolings, from faceted growth to dendrite growth.This article is part of the theme issue 'From atomistic interfaces to dendritic patterns'.

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Kerf losses due to ingot and wafer sawing can be avoided by solidifying the silicon wafers directly from the melt by the Ribbon Growth on Substrate (RGS) process, thus significantly reducing the wafer cost. However, up to now solar cells made from standard RGS material suffered from shunting problems due to current collecting structures. This resulted in lower fill factor values and hence in lower efficiencies compared to solar cells made from block cast multicrystalline silicon (mc Si) materials. In this con tribution two novel RGS materials are presented and investigated. Solar cells processed from these new materials have fill factor values above 78%, comparable to those of mc Si. The increased fill factor values can be explained by the absence of current collecting structures as concluded from a comparative analysis of spatially resolved Light Beam Induced Current (LBIC) measurements and Electroluminescence (EL) images, and from infrared transmission microscopy investigations. Additionally the improved material quality resulted in open circuit voltage V OC values up to 608 mV. This enhanced material quality, in combination with increased fill factor values, resulted in record efficiencies above 16% (certified by Fraunhofer ISE CalLab). This represents a significant improvement compared to the former efficiency record of 14.4% for standard RGS material.