OVERVIEW ON RECENT IMPROVEMENT FOR INDUSTRIALLY APPLICABLE PERL-TYPE SI SOLAR CELL PROCESSING J. Horzel 1) , P. Choulat 1) , E. Cornagliotti 1) , T. Janssens 1) , J. John 1) , I. Kuzma-Filipek 1) , V. Prajapati 1) , A. Rothschild 1) , R. Russell 1) , S. Singh 1) , E. Sleeckx 1) , L. Tous 1) , A. Uruena De Castro 1) , B. Vermang 1) , M. Ngamo 2) , P. Jaffrennou 2) , J. Penaud 2) , F. Duerinckx 3) , A. Cacciato 3) , J. Poortmans 1) 1) Imec v.z.w., Kapeldreef 75, B-3001 Leuven, Belgium, tel: +32 16 28 8097, fax: +32 16 28 15 01, jorg.horzel@imec.be 2) TOTAL Gas & Power, 2 place Jean Millier - la défense 6, F-92078 Paris la défense Cedex, France 3) Photovoltech, Grijpenlaan 18, B-3300 Tienen, Belgium ABSTRACT: An industrially applicable process flow for the formation of rear side passivated solar cells has been suggested by IMEC in 2005. Since then considerable improvements in solar cell efficiencies have been obtained with that type of process. In parallel simplifications and cost reductions were needed to bring it to production. IMEC’s Industrial Affiliate Programme (IIAP) offers industrial partners the possibility to actively join development efforts with IMEC and other partners sending residents to work at IMEC as part of a research team. Selected development results of this IIAP are presented here. On large area homogeneous emitter CZ-Si solar cells FhG-SE Cal-Lab confirmed solar cell efficiencies of 20.0 and 20.3% for screen printed Ag and reliable plated Cu front contacts, respectively. Fill factors of 80% have been achieved for fully screen printed (front and rear) solar cells with low cost rear side dielectric stack layers. Cu plated contacts show reliable module data in IEC test conditions. Latest results with average solar cell efficiencies of 20.4% over the length of a CZ-Si ingot with top efficiencies of 20.6% are just an intermediate step. IMEC sees a road to >21% efficient i-PERL solar cells in production. Keywords: rear side passivation, process optimisation, Cu plating 1 INTRODUCTION Solar cells with well passivated front emitters and rear surfaces (having local rear contacts) achieved so far the highest solar cell efficiencies amongst all single junction c-Si solar cells operated at 1 sun, AM1.5 conditions [1,2]. Simplified process flows to manufacture such devices had been proposed by G. Agostinelli et al. [3] and E. Schneiderloechhner et al. [4] accompanied by patent applications. These types of devices have entered in the meanwhile industrial solar cell production and record solar module efficiencies [5- 8] have been reported (screen printed solar cells with front and rear contacts). In times of destructive cut-throat competition in the photovoltaic sector it seems essential to focus on next evolutionary steps for c-Si solar cell production and to prepare the roll-out of that technology node. IMEC offers industrial partners the possibility to join an affiliate program (IIAP) in which they can actively participate in research by sending residents to IMEC taking part in common development. In that way partners can optimise their R&D efforts and cost and benefit from common interests with other partners without losing their own IP. It is the goal to create in this partnership new IP as a strong team in shorter time than possible by individual entities. IMEC intends to give its partners technological advantage over competition aiming for lower CoO. This contribution gives examples for improvement achieved in the recent months with i-PERL processing at IMEC as part of the IIAP. 2 PROCESS IMPROVEMENT Rear side passivated solar cells with local rear contacts to Si have a higher efficiency potential than earlier aluminium back surface field (Al-BSF) solar cells. To exploit that potential it is essential to come to simple and cost-effective process flows combining improved rear sides, improved light trapping and front surface improvements. Furthermore, it is essential to address cost reduction potential and wafer material aspects determining the final device and module performance in the field. In the following we give some examples for such improvement. 2.1 Rear side surface conditioning Unlike for Al-BSF solar cells where surface morphology did not play a major role as Al paste consumed several micrometer of Si in an alloying process, the rear surface topography plays an important role in i-PERL solar cells. It affects to a large extent passivation, rear contact formation and light trapping. With a tendency to use thinner starting wafer thickness in future all those factors become more pronounced. Figure 1: Efficiency dependence on rear side polishing Si removal for two-side textured surfaces. IMEC demonstrated [9] that rear surface polishing is beneficial to fabricate advantageous i-PERL Si solar cells with local Al contacts formed by laser opening rear side dielectrics, applying an Al layer on top of the complete rear surface and subsequent firing. The reason why this polishing step is required has to do with remaining non- perfections in the rear side reflector properties for such a process flow. As this flow is at this moment a good 27th European Photovoltaic Solar Energy Conference and Exhibition 1602