Bifacial heterojunction silicon solar cells by hot-wire CVD with open-circuit voltages exceeding 600 mV C. Voz a, * , D. Mun ˜oz a , M. Fonrodona a , I. Martin a , J. Puigdollers a , R. Alcubilla a , J. Escarre b , J. Bertomeu b , J. Andreu b a MNT-Departament d’Enginyeria Electronica, Universitat Politecnica de Catalunya, c/ Jordi Girona 1-3, Campus Nord C4, 08034 Barcelona, Spain b CerMAE-Departament de Fisica Aplicada i Optica, Universitat de Barcelona, Avda. Diagonal 647, 08028 Barcelona, Spain Available online 4 January 2006 Abstract Double-sided (bifacial) heterojunction silicon solar cells have been fabricated by Hot-Wire CVD on both p- and n-type crystalline silicon substrates. In these devices, doped microcrystalline silicon layers are combined with thin intrinsic amorphous silicon buffers. Such heterojunction with intrinsic thin layer concept is applied to obtain both the low temperature deposited emitter and back surface field contact. Especially remarkable is the performance of the solar cell fabricated on p-type c-Si. This device yields a total area (1.4 cm 2 ) conversion efficiency of 13.3%, with an open-circuit voltage of 619 mV, short-circuit current density of 29.0 mA cm  2 and fill factor of 74.1%. The substrate temperature is kept below 200 -C during the whole fabrication process. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon; Heterojunction; Solar cell; Hot-Wire CVD 1. Introduction The cost of high efficiency crystalline silicon (c-Si) modules is hindering the progress of the PV industry as a viable alternative for clean energy production. Therefore, cell manufacturers are searching different approaches that could allow the desired cost reduction. For instance, considering that c-Si wafers represent about 30–50% of the module price, the final cost would be significantly reduced by increasing the number of wafers processed from a given ingot. Nowadays, very thin c-Si wafers (<200 Am) can be produced with moderate kerf losses. However, in c-Si cells with minority carrier diffusion lengths similar to the wafer thickness, rear surface recombination becomes important [1]. Although thermal oxidation very effectively passivates the c-Si surface [2], thin wafers tend to warp at the high temperatures (¨1000 -C) involved in the process. On the other hand, low cost multicrystalline silicon wafers are not compatible either with high temperature steps due to strong lifetime degradation [3]. Therefore, low temperature surface passivation schemes using thin films of hydrogenated amorphous silicon (a-Si:H) [4,5], silicon carbide (a-SiC x :H) [6] or silicon nitride (a-SiN x :H) [7,8] have gained special interest due to their compatibility with both thin and low quality c-Si substrates. The heterostructure formed between a-Si:H and c-Si is particularly interesting, since intrinsic films can be used for passivation and doped ones to form either the emitter junction or the back surface field (BSF) contact. Certainly, the most successful result is the Heterojunction with Intrinsic Thin-layer (HIT) cell developed by Sanyo Electric Co. with record 19.5% conversion efficiency in mass-produced modules [9]. Sanyo uses Plasma-Enhanced Chemical Vapour Deposition (PECVD) to grow the a-Si:H thin films, but very little is known about the specific technological process. The HIT concept is used on both sides of n-type Czochralski (CZ) wafers, i.e., p-doped/ intrinsic and n-doped/intrinsic a-Si:H stacks are used to form the emitter and BSF contact, respectively. These double-sided structures have been called bifacial HIT cells in the literature [10]. Such excellent results by Sanyo moved many groups worldwide to investigate heterojunction silicon solar cells. Most groups use p-type c-Si wafers, though the best doping type of the base is still subject of discussion [11]. Several groups have already reported encouraging results with a-Si:H emitters deposited by PECVD. For instance, Tucci and de Cesare have reported a conversion efficiency (g ) of 17% (total 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.11.099 * Corresponding author. Tel.: +34 934017488; fax: +34 934016756. E-mail address: cvoz@eel.upc.edu (C. Voz). Thin Solid Films 511 – 512 (2006) 415 – 419 www.elsevier.com/locate/tsf