þ 0.4% Efficiency gain by novel texture for String Ribbon solar cells J. Cichoszewski a,n , M. Reuter a , J.H. Werner a,b a Steinbeis Center Photovoltaics, Roteb¨ uhlstr. 145, 70197 Stuttgart, Germany b Institute for Photovoltaics (ipv), University of Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany article info Article history: Received 1 August 2011 Received in revised form 15 January 2012 Accepted 24 January 2012 Available online 3 March 2012 Keywords: Ribbon silicon Texturisation Metallic catalyst Solar cell abstract This work introduces metal assisted etching for surface texturing of saw-damage free crystalline silicon independent from the crystallographic orientation. A noble metal serves as a catalyst for the chemical etching in an aqueous solution containing hydrofluoric acid and nitric acid. Metal assisted textured String Ribbon solar cells show a reduction DR eff ¼16% rel for the effective reflection R eff compared to untextured String Ribbon solar cells. This improvement results in an increase DJ sc ¼þ1.4 mA/cm 2 of the short circuit current density J sc . Therefore, the median solar cell efficiency Z increases by DZ ¼þ0.4% from Z untex ¼14.1% to Z tex ¼14.5%. & 2012 Elsevier B.V. All rights reserved. 1. Introduction State-of-the-art industrial solar cells utilize a surface texture to reduce front surface reflection and thus increase their power conversion efficiency. For standard mono- and multi-crystalline silicon wafers wet etching processes result in cost effective and reliable surface texturing. For mono-crystalline silicon, an alkaline etchant attacks the (100) surfaces and results in random pyra- mids with (111) oriented side walls [1]. For multi-crystalline silicon wafers from casted silicon blocks an acidic solution etches the silicon surface at the saw-damage introduced by the slicing process during wafering; thus a rough texture is formed [2]. Non- standard wafering methods like String Ribbon [3,4], SiGen [5] or SLIVER [6,7] result in wafers without saw-damage and with crystallographic orientations close to (111) or (110). Therefore, the standard wet etch processing does not result in a surface texture on these materials [7,8]. The String Ribbon method uses two high temperature-resistant strings which stabilize the edges of the ribbon growing from the Si- melt during crystallization. This unique growth method yields Si ribbons with thickness 4100 mm without any sawing process and thus results in flat wafers without any sawing damage [9]. Due to near-equilibrium solidification directly from the molten silicon, the surface solidifies in the preferential (111) or (110) surface orientation. Therefore, neither standard acidic nor alkaline etch solutions have so far resulted in a satisfying texture of String Ribbon material [8]. In contrast, our metal assisted surface texture results in a drastically reduced surface reflection and thus in increased solar cell efficiency. This work presents metal assisted etching (MAE) [10–12] as a surface texturing method for non-standard crystalline silicon material and shows an efficiency gain for MAE textured solar cells. Metal nanoparticles catalyze the etch reaction by supplying additional holes and thereby enforcing the etching process in acidic solution containing HF and HNO 3 [12]. While most pub- lications treat with application of either silver or gold as catalyst, in this paper we want to emphasize the excellent catalytic properties of palladium [13] for silicon etching. Due to its great performance the amount of catalytic metal (Pd) is reduced significantly thus influencing positively the process costs. Since the MAE etching process is fully controlled by metal catalyst neither crystal orientation nor absence of saw damage influences the texturization. The MAE texturization increases the efficiency for String Ribbon solar cells of industrial size (80  150 mm 2 ) from Z untex ¼ 14.1% to Z tex ¼ 14.5%. 2. Results and discussion 2.1. Texture The metal assisted surface texturing method consists of three processing steps. Firstly, we deposit palladium nano-particles on the silicon surface as a catalyst for the metal assisted etching. Secondly, an acidic solution locally etches into the wafer and yields a porous silicon layer. Thirdly, a potassium hydroxide solution polishes off the porous silicon and structures the surface. Finally, wet chemical cleaning removes any metal residue [14]. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2012.01.031 n Corresponding author. Tel.: þ4971131589434; fax: þ4971131589435. E-mail addresses: jakub.cichoszewski@stw.de, jakub.cichoszewski@ipe.uni-stuttgart.de (J. Cichoszewski). Solar Energy Materials & Solar Cells 101 (2012) 1–4