OPPORTUNITIES FOR SILICON EPITAXY IN BULK CRYSTALLINE SILICON PHOTOVOLTAICS M. Recamán Payo, Y. Li, I. Kuzma Filipek, A. Hajjiah 1 , A. Urueña De Castro, T. Borgers, E. Cornagliotti, L. Tous, R. Russell, S. Singh, M. Debucquoy, F. Duerinckx, J. Szlufcik and J. Poortmans 2 imec, Kapeldreef 75, B-3001 Leuven, BE 1 Kuwait University, P.O. Box 5969, Safat 13060, KW 2 imec, Kapeldreef 75, B-3001 Leuven, BE; KU Leuven, B-3001 Leuven, BE and UHasselt, B-3590 Diepenbeek, BE Corresponding author: phone: +32 1628 1478, fax: +32 1628 1501, e-mail: Maria.RecamanPayo@imec.be ABSTRACT: This work presents an overview of the opportunities in bulk crystalline silicon photovoltaics that have been explored using silicon epitaxy as doping technology. Epitaxy demonstrates to be an elegant and versatile technology which brings a lot of new opportunities to further simplify and improve the design and performance of bulk solar cells. Advantages are the doping profile flexibility, the reduced thermal budget, the absence of additional steps to remove glassy layers or activate dopants, the simplified integration of local doping by means of selective epitaxy, and the possibility of single-side deposition. The results presented herein demonstrate the potential of epitaxy by applying the process in three cell structures to grow a boron-doped layer. First, epitaxy is used to grow blanket doped layers as emitters on the full rear side of n-type PERT cells. Second, selective epitaxy is applied to locally grow the interdigitated emitter in n-type IBC cells. Third, selective epitaxy is applied to form the local BSF in p-type PERL cells. For each of these cell concepts, silicon epitaxy helped to simplify the reference BBr3 diffusion-based process, while keeping high efficiencies: 20.5 % for n-type PERT (226 cm 2 cell), 22.8 % for IBC (4 cm 2 cell) and at least +0.5 mA/cm 2 and +10 % escape reflectance for p-type PERL cells compared to the standard PERC. Keywords: boron, silicon epitaxy, selective epitaxial growth, emitter, back-surface-field. 1 INTRODUCTION The epitaxial growth of silicon on crystalline material consists of the regularly oriented growth of a crystalline silicon layer upon the substrate surface, which will work as a template for the growing layer. The doping of the epitaxial layer takes place by in-situ incorporation of the dopant source during the growth, which introduces a large degree of flexibility to design a doped profile in one step since both doping and thickness can be decoupled. The doping versatility by epitaxy introduces the technology as an attractive alternative to classical diffusion in bulk crystalline silicon solar cells, as there is no need after epitaxy for subsequent steps to drive-in the doped region, activate the dopants, heal any damage or remove glassy layers. Moreover, selective epitaxy (SEG) can be applied to locally create doped regions in a way that can further reduce the complexity of the cell fabrication. In selective epitaxy the net growth is the result of a balance between silicon deposition and etching upon substrates where part of the surface is “masked” with a dielectric(s) while the rest is “free” of dielectric(s). The overall result is the growth of a silicon layer solely on the areas “free” of dielectric, and the potential process simplification comes from the fact that the dielectric “mask” can be kept after epitaxy for passivation and optical purposes. This work explores the applications of boron-doped epitaxy in different bulk cell concepts such as p-type PERL, n-type PERT and IBC, and proves the potential and opportunities for this technology to simplify the process flow and, thus, reduce the cell cost-of-ownership while keeping a high efficiency performance. 2 EXPERIMENTS 2.1 P-type PERL: boron-doped local BSF Boron-doped SEG is presented as an alternative route for the fabrication of PERL cells by introducing the epitaxial step to create the local BSF, and replacing the contact firing by a short low temperature sintering in forming gas (FGA) (Fig. 1). Following this approach, PERL-type cells were fabricated where the rear dielectric stack, essential for both reflectance and passivation, was also working as a “mask” during the SEG of the local BSF. Fig. 1. Schematics of a p-type PERL solar cell (top); and solar cell flow for SEG PERL versus the reference PERC (bottom). 2.2 N-type IBC: boron-doped emitter The boron-doped epitaxial emitter of an IBC cell is grown using the phosphorous silicate glass (PSG) formed during the POCl3 diffusion of the BSF, as a “mask” during the SEG (Fig. 2). A first integration of this approach [1], which directly simplifies the baseline flow with BBr3 diffusion by skipping one dry oxidation and the associated cleaning, was already realized [2] on 10x10 cm 2 wafers p + -type local BSF oxide SiN x n + -type EMITTER p-type Si Al low T contact rear dielectric(s) 1. Emitter formation 2. Dry oxidation 3. PECVD SiOx/SiNx rear 4. PECVD SiNx front 5. Laser ablation rear 6. Al/AlSi 1% rear 7. Firing 8. Laser ablation front 9. Ni/Cu/Ag metallization front Reference PERC 1. Emitter formation 2. Dry oxidation 3. PECVD SiOx/SiNx rear 4. Laser ablation rear 5. SEG rear 7. Al/AlSi 1% rear 8. FGA 9. Laser ablation front 10. Ni/Cu/Ag metallization front SEG PERL 6. PECVD SiNx front 29th European Photovoltaic Solar Energy Conference and Exhibition 497