Back-contact back-junction silicon solar cells under UV illumination F. Granek n , C. Reichel Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, D-79110 Freiburg, Germany article info Article history: Received 8 December 2009 Received in revised form 4 May 2010 Accepted 14 May 2010 Keywords: c-Si Back-contact UV-stability Front surface ﬁeld abstract The performance of n-type Si back-contact back-junction (BC-BJ) solar cells under illumination with high energy ultraviolet (UV) photons was investigated. The impact of the phosphorus doped front surface ﬁeld (FSF) layer on the stability of the front surface passivation under UV illumination was investigated. Lifetime samples and solar cells without the front surface ﬁeld showed a signiﬁcant performance reduction when exposed to ultraviolet light. The surface saturation current density (J 0e ) increased from 48 to 446 fA/cm 2 after the UV exposure. At the same time the efﬁciency of the BC-BJ solar cells without the FSF diffusion reduced from 19.8% to 14.3%. In contrast to the lifetime samples and solar cells without the FSF diffusion, the tested n + nn + structures and the BC-BJ solar cells with applied FSF diffusion proﬁles were signiﬁcantly more stable under UV exposure, i.e. J 0e increased only by a factor of 25% and the efﬁciency of these cells decreased only 0.3% abs by the UV illumination. Finally it was shown that the performance of the UV-degraded solar cells without FSF could be improved during a forming gas anneal (FGA). Due to application of FGA the efﬁciency almost fully recovered from 14.3% to 19.6%. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Back-contact back-junction (BC-BJ) silicon solar cells represent an attractive high-efﬁciency cell structure. In mass production, BC-BJ solar cells achieve average efﬁcien- cies of 22.4% as presented by De Ceuster et al. [1]. For comparison, standard silicon solar cells currently have efﬁciency in the range of 16–18%. Recently a new record efﬁciency of 23.4% for a large area (149 cm 2 ) BC-BJ solar cell was announced by Swanson [2]. Due to the fact that the collecting p–n junction is placed on the rear surface of BC-BJ solar cells, and that most of the photo- generation takes place close to the front side, the requirements on the front surface passivation quality are very high. Thus, a low front surface recombination rate is one of the critical factors inﬂuencing the efﬁciency of the back-junction solar cells. The front surface passivation scheme needs not only to be of very high quality. It is also essential that the applied front surface passivation scheme is stable under solar cell operating conditions. Especially the stability of the surface passivation under the high energy ultraviolet (UV) part of the solar spectrum is of main importance in order to maintain high device performance during the long-term ﬁeld operation of the photovoltaic modules. In our previous work [3] the front surface passivation scheme using a phosphorus doped front surface ﬁeld (FSF) and a stack system of a thermal oxide and a PECVD silicon nitride for the BC-BJ solar cells was developed and studied. Also other positive effects of the FSF on the performance of the BC-BJ solar cells were investigated, such as improvement of the low-illumination performance [4] and improvement of the lateral transport of the majority carriers [5]. In the present work the inﬂuence of the exposure of the front surfaces of the BC-BJ solar cells to UV light is investigated. Additionally the inﬂuence of the FSF diffusion on the UV stability of lifetime samples and on the performance of the BC-BJ solar cells is presented. 2. Inﬂuence of the UV light on the front surface passivation of BC-BJ solar cells The impact of the UV illumination of the oxide passivated silicon surfaces was investigated by many authors in the microelectronic ﬁeld [6,7]. In the ﬁeld of the silicon solar cells the work of Gruenbaum was pioneering. Already in 1988 Gruenbaum et al. [8,9] reported that the efﬁciency of some of the point-contact concentrator solar cells, developed at the Stanford University by Sinton et al. [10], decreased after exposure to concentrated sunlight. The decrease in solar cell performance was caused by an increase in the front surface recombination velocity. The studies of Gruenbaum et al. showed that the ultraviolet component of the incident light spectrum caused damage on the front surface. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.05.038 n Corresponding author. Tel.: + 49 761 4588 5355; fax: + 49 761 4588 9250. E-mail address: ﬁlip.granek@ise.fraunhofer.de (F. Granek). Solar Energy Materials & Solar Cells 94 (2010) 1734–1740