RESEARCH ARTICLE Avalanche breakdown in multicrystalline solar cells due to preferred phosphorous diffusion at extended defects Jan Bauer 1 * , Dominik Lausch 2 , Horst Blumtritt 1 , Nikolai Zakharov 1 and Otwin Breitenstein 1 1 Max Planck Institute of Microstructure Physics, Halle, Germany 2 Fraunhofer Center for Silicon Photovoltaics CSP, Halle, Germany ABSTRACT Multicrystalline solar cells break down strongly at reverse voltages well below the theoretical limit. Previous explanations were based on assuming a constant depth of the junction below the surface. In this work, preferred phosphorous diffusion at special line defects in grain boundaries is shown to lead to spikes in the p–n junctions even below flat surfaces. The curvature radii of the spherical p–n junction bending are measured by electron beam-induced current to be in the range of 300–500 nm, leading to the observed type III avalanche breakdown voltages. Copyright © 2012 John Wiley & Sons, Ltd. Supporting Information may be found in the online version of this article. KEYWORDS avalanche breakdown; breakdown voltage; multicrystalline solar cells; p–n junction; alkaline texture *Correspondence Jan Bauer, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany. E-mail: jbauer@mpi-halle.mpg.de Received 4 November 2011; Revised 7 December 2011; Accepted 28 March 2012 1. INTRODUCTION Electrical breakdown in solar cells gained much interest in solar cell industry as well as in research because of its importance for reliability and long-time stability of multicrystalline (mc) silicon solar cells and modules. However, the physical mechanisms of breakdown in mc-Si solar cells are multiple, and some of them are not fully understood yet. Therefore, they are in the focus of recent research to support and enable technological solutions for reducing breakdown issues in solar cells. Assuming a plane p–n junction, silicon solar cells with a base doping concentration of 1  16 cm 3 should break down at a voltage of 60 V [1]. The main breakdown mechanism in this voltage range is electron multiplication in the space charge region, which is commonly named avalanche effect or avalanche breakdown (AB) [2]. However, the observed global breakdown voltages in mc-Si solar cells, that is, the breakdown voltage taken from its current–voltage (I–V) characteristic, are in the range of 13 V for typical acidic texturized Si solar cells [3] and 15 up to 20 V for alkaline texturized solar cells [4,5], which is far below the theoretical limit. Furthermore, solar cells do not break down on the entire area of their p–n junction. In fact, these breakdown sites are strongly localized, and if the reverse current density is high enough, hot spots may appear at breakdown sites [6,7]. The global breakdown voltage of ideal, that is, defect- free p–n devices, is naturally influenced by the doping concentration of the base because the electric field under reverse bias depends strongly on the base doping concentra- tion [1]. For non-ideal p–n junctions, as solar cells, it was shown in [8] that a higher doping concentration lowers the global breakdown voltage in solar cells, influencing in particular the breakdown behavior of solar cells made from upgraded metallurgical grade (umg) Si feedstock [9]. If no other breakdown mechanism would occur in solar cells, only the base doping would determine the breakdown voltage. However, the breakdown behavior of solar cells is much more complex, and a short overview about the different parameters influencing breakdown in solar cells is given in the following. One noticeable influence on the global breakdown voltage of solar cells is the general surface roughness, which is mainly determined by the texturization process. It was shown by Nievendick et al. that surface trenches caused by preferred grain boundary (GB) etching during the texturization step lead to different global breakdown voltages [10]. Strongly texturized solar cells, which show deeper texturization trenches, break down at lower reverse voltages than weakly texturized solar cells, which have PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2013; 21:1444–1453 Published online 2 June 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2220 Copyright © 2012 John Wiley & Sons, Ltd. 1444