Heterojunction a-Si/poly-Si solar cells on mullite substrates A. Focsa a, ⁎ , I. Gordon b , G. Beaucarne b , O. Tuzun a , A. Slaoui a , J. Poortmans b a InESS-CNRS-ULP, 23 rue du Loess, F-67037 Strasbourg, France b IMEC, Kapeldreef 75, B-3001 Leuven, Belgium Available online 17 December 2007 Abstract Crystalline silicon thin films (thickness b 10 μm) on ceramic substrates provide an alternative approach to bulk silicon based cells, provided the electronic quality of the silicon film is high enough and the junction structure is optimized. In this work, we present the structural and electrical properties of a-Si/poly-Si heterojunction solar cells fabricated on mullite ceramic substrates. The polycrystalline silicon films were formed by high temperature chemical vapour deposition on mullite ceramics coated with spin-on flowable oxides (FOx) serving as intermediate layers. The average size of the grains and their distribution were investigated by optical microscopy and EBSD technique. It is found that more than 60% of the surface of polysilicon films grown on FOx is covered by large grains of 4–12 μm, and that grains are (110) preferentially oriented. Finally, we present the photovoltaic data on test solar cells made on such heterojunction polysilicon based films. An open-circuit voltage of 500 mV was obtained for the polysilicon layers with a heterojunction emitter. This is the highest reported open-circuit voltage for polycrystalline solar cells with a p–n heterojunction on mullite ceramics. It is also a new record for thin-film silicon solar cells deposited by CVD onto ceramic substrates. The limiting factors as well as possible improvements are discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon; Polycrystalline; Thin-film; Ceramics 1. Introduction Among the alternative approaches to bulk silicon based cells, there is the use of polycrystalline silicon thin films (pc-Si TF) on foreign substrates. Solar cells utilizing thin-film polycrystalline silicon can achieve photovoltaic power conversion efficiencies higher than 19% [1]. Important issues in the development of pc- Si TF solar cells are the choice of the substrate and the growth method since reasonably high growth rates and an overall reduction of the number of steps are essential in order to obtain a cost-effective process. Different ceramic materials were tested for Si deposition by different groups [2–6]. However, mullite ceramic (3Al 2 O 3 –2SiO 2 ) seems to have many advantages over the others. From reflectance measurements, the mullite is found to reflect 80–90% of the incident light and can therefore act as a good back reflector. Another important factor is the matching of the thermal expansion coefficients of mullite (5 × 10 - 6 °C - 1 ) and Si (3.9 × 10 - 6 °C - 1 ) and mullite ceramics can sustain high temperature processes like CVD or diffusion. Thus, the silicon thin film becomes sufficiently elastic to accommodate the induced stress. More importantly, such ceramic materials can be easily made by the tape casting method [7], which is current- ly used for the production of cheap alumina substrates for electronic packaging. With this technique, the substrate's dimensions can be tailored from 20 μm to 1 mm in thickness, up to 1 meter wide and in a continuous way. The main draw- back of such ceramic materials is possible contamination of deposited silicon during the growth process [8]. In order to avoid potential contamination, intermediate layers – also called diffusion barriers – would be necessary to confine impurities in the substrate during high temperature steps. The barrier materials that have been mostly reported include PECVD oxide [9] and oxide–nitride–oxide (ONO) [6,10]. Flowable oxide (FOx) films made from silicate are an interesting alternative to conventional barrier films. Indeed, the technology of sol–gel thin film forming and coating is simple. The solution containing the silicon oxide precursor is applied to a substrate by spinning, while the resulting films are uniform over a large area and strongly adherent. Furthermore, the equipment needed Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 6896 – 6901 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +33 388106329; fax: +33 388106335. E-mail address: Alexandru.Focsa@iness.c-strasbourg.fr (A. Focsa). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.12.097