RESEARCH ARTICLE On the mechanical strength of monocrystalline, multicrystalline and quasi-monocrystalline silicon wafers: a four-line bending test study Josu Barredo 1 *, Vicente Parra 2 , Ismael Guerrero 2 , Alberto Fraile 3 and Lutz Hermanns 3 1 Centre for Modelling in Mechanical Engineering (CEMIM-F2I2), José Gutiérrez Abascal, 2, 28006, Madrid, Spain 2 DC Wafers Investments, S.L., Valdelafuente, León, Spain 3 Department of Structural Mechanics and Industrial Constructions, UPM, Madrid, Spain ABSTRACT Quasi-monocrystalline silicon wafers have appeared as a critical innovation in the PV industry, joining the most favorable characteristics of the conventional substrates: the higher solar cell efficiencies of monocrystalline Czochralski-Si (Cz-Si) wafers and the lower cost and the full square-shape of the multicrystalline ones. However, the quasi-monocrystalline ingot growth can lead to a different defect structure than the typical Cz-Si process. Thus, the properties of the brand new quasi- monocrystalline wafers, based on low and high crystal defect densities, have been for the first time studied from a mechan- ical point of view, comparing their strength with that of both Cz-Si monocrystalline and typical multicrystalline materials. The study has been carried out employing the four line bending test and simulating them by means of FE models. For the analysis, failure stresses were fitted to a three-parameter Weibull distribution. High mechanical strength was found in all the cases. However, the quasi-monocrystalline wafers characterized by large density of bulk defects, due to the noticeable den- sity of extended defects, showed lower fracture tensions. Copyright © 2013 John Wiley & Sons, Ltd. KEYWORDS quasi-monocrystalline silicon wafers; photoluminescence; four line bending test; nonlinear FE models; Weibull; size effect *Correspondence Josu Barredo, CEMIM-F2I2, José Gutiérrez Abascal, 2. 28006, Madrid, Spain. E-mail: jbarredo@etsii.upm.es Received 18 January 2012; Revised 7 November 2012; Accepted 22 January 2013 1. INTRODUCTION Solar market and technology are largely based on crystal- line silicon. Typically, two solar cell technologies can be developed according to the crystal features of the silicon substrates used: monocrystalline, by the so-called Czochralski method (Cz-Si), and multicrystalline (mc-Si), usually based on casting techniques (Bridgman-type furnaces). Cz-Si ingots imply higher solar cell efficiency because of the optimized monocrystalline surface texturing and lower amount of crystal defects, such as grain bound- aries and dislocations, than mc-Si materials. Higher oxygen concentrations are also expected, which can result in light-induced degradation patterns when considering Boron-doped materials [1]. In contrast, the manufacturing of mc-Si materials is relatively straightforward and more cost-effective. Also, Cz-Si wafers are usually presented in the market as pseudo-square substrates, whereas mc-Si is full square-shaped. Nowadays, highly innovative approaches are being de- veloped, in which notorious monocrystalline features from ingots manufactured according to ancient casting growth approaches can be reached [2,3]. As a result, the optimum wafer substrates share some of the advantages of the well- known Cz-Si monocrystalline crystals and the higher cost- effectiveness of multicrystalline. These wafers are usually labeled by the still scarce amount of manufacturers dealing with them as quasi-monocrystalline, pseudo-monocrystalline, monocrystalline cast or monocrystalline-like wafers, and they are supposed to offer up to 1.0–1.5% extra efficiency than the usual mc-Si substrates, depending on the wafer texturing process (alkaline-anisotropic or acid-isotropic). Quasi-monocrystalline materials still need to be thor- oughly studied and characterized, as a series of defects can be observed when comparing diverse zones in an in- dustrial ingot [4–6]. The obtention of very low breakage rates for both wafer, cell and module manufacturers is crit- ical for the PV industry [7,8]. The well-known thermal PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2014; 22:1204–1212 Published online 2 April 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2372 Copyright © 2013 John Wiley & Sons, Ltd. 1204