Localised Back Surface Field Formation via Different Dielectric Patterning Approaches Jie Cui, Jack Colwell, Zhongtian Li and Alison Lennon The School of Photovoltaic and Renewable Energy Engineering, The University of New South Wales, Sydney, NSW 2052 Australia Keywords: Localised contact, back surface field, alloying ABSTRACT Full-area screen-printed aluminium has been used for decades in industry for p-type metallisation of silicon solar cells. The effectiveness of this rear surface metallisation method largely lies in the formation of an aluminium-doped p+ back surface field (BSF) during the firing process which serves to shield the minority carriers from the high surface recombination velocity of the metal-silicon interface. However, the effective rear surface recombination velocity can be even further reduced by limiting the metal-silicon contact area and forming localised BSF regions. In this paper we report on the effect of different dielectric patterning methods on the formation of local BSF regions. Line openings were patterned using boron laser doping, laser ablation and aerosol jet chemical etching in a silicon dioxide/silicon nitride dielectric layer. Local BSF regions were then formed by firing aluminium that was screen-printed over the entire rear surface. The local BSF regions were imaged using scanning electron microscopy with the depth of the p+ BSF regions being visualised using selective etching. It is shown that thicker BSF regions and less void formation can result with closer spacing of openings for all patterning methods. Although the thickest BSF regions were obtained using chemical etching, it was difficult to eliminate the effects of the wider 60-70 µm line openings that resulted with the aerosol jet etching method compared to the 25 µm wide laser lines. It is also shown that the use of boron laser doping to form the openings in the dielectric layer did not result in thicker BSF regions than observed when laser ablation was used to form the openings. Contact author: Jie Cui – j.cui@unsw.edu.au 1. INTRODUCTION Rear localised aluminium-alloyed contact structures, which reduce the effective rear surface recombination velocity (SRV) by forming localised back surface field (BSF) regions through patterned dielectrics, can enable higher cell energy conversion efficiencies in industrial production than cells employing a full area aluminium-alloyed BSF [1]. Prior research has shown that the local BSF formation is sensitive to: (i) the dielectric opening size; (ii) spacing or pitch; and (iii) the aluminium firing process [2-7]. Discontinuous BSF regions and Kirkendall voids have been reported due to poor selection of pitch size and firing condition [8]. It is also reported that intentionally adding silicon into the aluminium paste can significantly improve the contact geometry by reducing contact depth and increasing BSF thickness [4]. In most of the previous studies, localised openings in rear passivation dielectrics were formed by either laser ablation [9] or screen printing etching paste [5] [10]. However, boron laser doping [11] and chemical etching using inkjet or aerosol printing [12] can also be used to pattern the dielectric layer. Boron laser doping is performed by laser scribing the rear dielectric layer which is covered with a boron spin-on dopant source. Localised heat generated by the laser beam effectively ablates the dielectric and melts the underlying silicon, while the boron dopant simultaneously diffuses into silicon and forms a boron doped p+ layer in the localised openings. Although laser ablation and doping can be potentially performed at high throughput [13], the laser process can result in laser-induced damage, which can increase recombination and limit device voltages [14]. Chemical patterning methods which can be achieved with no high temperature processing (e.g., [12]) can eliminate damage to the silicon wafer and minimise effects on temperature-sensitive hydrogen passivation processes [15],[16]. The direct etching method, which uses either an inkjet or aerosol printer, can etch silicon dioxide and silicon nitride dielectrics with lower chemical usage and generation of less hazardous chemical waste than existing immersion etching [12]. However, the properties of the local BSF regions