Presented at the 21st European Photovoltaic Solar Energy Conference and Exhibition, 4-8 September 2006, Dresden, Germany N-TYPE SOLAR GRADE SILICON FOR EFFICIENT P + N SOLAR CELLS: OVERVIEW AND MAIN RESULTS OF THE EC NESSI PROJECT. L.J. Geerligs, 1 C.J.J. Tool, 1 R. Kinderman, 1 I. Röver, 2 K. Wambach, 2 R. Kopecek, 3 T. Buck, 3 J. Libal, 3 R. Petres, 3 P. Fath, 3 P. Sánchez-Friera, 4 J. Alonso, 4 M. Acciarri, 5 S. Binetti, 5 S. Pizzini. 5 1 Energy research Centre of the Netherlands ECN, Petten, Netherlands 2 Deutsche Solar AG, Solar Materials, Freiberg, Germany 3 University of Konstanz, Germany 4 Isofotón S.A., Málaga, Spain 5 University of Milano-Bicocca, Milano, Italy ABSTRACT: An integrated R&D effort was performed, aimed at the industrial use of highly n-doped waste silicon, and at processing of this silicon to n-doped ingots, solar cells, and modules. An evaporation process was successfully applied to reduce the phosphorous concentration in the waste Si. Additional experimental n-type ingots were produced by blending (highly doped with undoped) silicon. Industrial p + nn + cell processes were developed with a front boron emitter and, alternatively, a rear aluminium emitter. A high and homogeneous carrier lifetime was observed in the n-type mc-Si which makes the material suitable even for a rear emitter process. Solar cell efficiencies of 14.7% (front emitter) and 14.4% (rear emitter) were obtained on mc-Si; while more than 16% efficiency was demonstrated on monocrystalline wafers. Cell interconnection by conductive adhesive was developed and performs well. Keywords: silicon, feedstock, bifacial 1 INTRODUCTION It is estimated that 2000-3000 ton/y of highly n- doped waste silicon is available, in principle, for the PV industry. At present this is not used, because there is no cost-effective technology for dopant removal. The aim of the NESSI project (supported by the EC 5 th Framework Programme) was twofold: 1) Develop purification and crystallisation techniques to make highly n-doped waste silicon available industrially for solar cell processing. The use of this silicon will reduce the shortage of silicon feedstock. 2) Develop industrial cell and module technology for n- type mc-Si wafers. Techno-economic assessment was part of both aims. Silicon solar cells based on n-type mc-Si wafers have certain advantages compared to p-type mc-Si. They are suited for very thin wafers because technology for open rear metallisation is relatively straightforward, and they are less sensitive to carrier lifetime degradation due to common defects [1]. The NESSI project has evaluated and tested the reduction of dopant concentration in the waste silicon, characterised and optimised high quality experimental n- doped ingots, developed several cell processes based on industrial processing techniques, optimised module production and characterised modules, and optimised wafer handling equipment for thin wafers. In this paper, a summary of the results of the project is given. More detailed results are available in references given in this paper. 2 EXPERIMENTAL 2.1 Dopant removal from waste Si The route for dopant removal is based on evaporation from the liquid silicon. Thermodynamic simulations were developed to model the removal and optimise dopant removal versus silicon loss. Experimental emphasis was put on phosphorous because this is more difficult to remove in this way than As or Sb. 2.2 Cell processes Two cell processes were developed; one with a rear Al-emitter, for which the process is very similar to the standard p-base cell process with Al-BSF; the other based on boron diffusion for the emitter, phosphorous diffusion for the BSF, and an open rear metallisation. The schematic cross sections of both cell types are illustrated in Fig. 1. Work on similar cell processes has also been performed by researchers elsewhere (ref. [2] and [3] respectively). Both cell processes were developed along two directions: one using laboratory processes where applicable, aimed at high efficiency and demonstrating potential of a process, and the other using industrial processing steps on (12.5cm) 2 wafers. Al doped emitter full rear Al contact n n + Front Ag contact ARC: SiN x p + Al doped emitter full rear Al contact n n + Front Ag contact ARC: SiN x p + SiN x or uncoated # Open rear Ag contact n p + n + Front Ag/Al contact ARC: SiN x or SiO 2 /SiN x stack SiN x or uncoated # Open rear Ag contact n p + n + Front Ag/Al contact ARC: SiN x or SiO 2 /SiN x stack Figure 1: Top: Cross-section of the monofacial Al rear emitter (FSF) solar cell. Bottom: Cross-section of the bifacial B front emitter (BSF) solar cell. rear-Al emitter ("FSF-process") The process was described in detail in ref [4]. After initial work on evaporated as well as screen printed Al layers for the emitter, it turned out that a screen printed Al layer which is co-fired with the front metallisation can result in cell results as good as or even better than the evaporated Al layer. The main deviations of the cell process from the current p-base industrial mc-Si process are in texture (the rear of the cell is preferably not