Novel RGS materials with high fill factors and no material-induced shunts with record solar cell efficiencies exceeding 16%

Abstract

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 contribution 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 VOC 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.