Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Eﬀect of various surface treatments on adhesion strength of magnetron sputtered bi-layer Molybdenum thin ﬁlms on soda lime glass substrate Brijesh Singh Yadav, Amol C. Badgujar, Sanjay R. Dhage ⁎ Centre for Solar Energy Materials, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500005, India ARTICLE INFO Keywords: Surface treatment Adhesion strength Mo thin ﬁlm Residual stress ABSTRACT CuInGaSe 2 (CIGS) solar technology is leading thin ﬁlm technology with champion photo-conversion eﬃciency exceeding 22%, stepping towards rapid commercialization. Sputtered, chemically inert, well adhered highly conducting Molybdenum (Mo) thin ﬁlm back contact on soda lime glass (SLG) substrate is essential for high photoconversion eﬃciencies in CIGS modules. The challenge in Mo deposition by sputtering lies in obtaining adherent ﬁlms while retaining high conductivity, essential for CIGS thin ﬁlm solar cells and various semi- conductor device applications as the metal contact. Apart from deposition conditions, the adhesion of the thin ﬁlm is also dependent on surface activation of a substrate. In the present work, inﬂuence and comparison of various surface treatment methods such as alkali treatment, ultrasonication and plasma treatment on large area SLG substrates subsequently sputtered with Mo had been investigated in detail. The surface of treated SLG substrates was characterized by Water contact angle, AFM, and XPS. Further, investigation of residual stress and adhesion strength of sputtered bilayer Mo thin ﬁlms on the surface treated SLG are being reported. 1. Introduction CuInGaSe 2 (CIGS) based thin ﬁlm solar cells are a promising can- didate for large-scale commercialization due to its potential for high eﬃciency and better thermal, chemical stability compared to its com- petitor thin ﬁlm technologies. Besides, it has a high optical absorption coeﬃcient (> 10 5 cm -1 ), requires less thickness ∼2 μm to absorb maximum part of the solar spectrum which enables the reduced raw material usage and decreasing the fabrication cost (Bär et al., 2009; Feurer et al., 2016). CIGS solar cell fabrication begins with sputter coated Molybdenum (Mo) on soda lime glass (SLG) substrate which acts as a back contact, likewise Mo thin ﬁlm back contact is also extensively being used in CZTS, CdTe and Sb 2 Se 3 thin ﬁlm solar cells (Garcia- Llamas et al., 2017; Krusin-Elbaum et al., 1987; Li et al., 2017; Lin et al., 1987). Moreover, sputtered Mo back contacts has wide range of applications, as an electrode in GaAs-based metal gate ﬁeld eﬀect transistors (MESFETs), silicon-based metal–oxide-semiconductor (MOS). In the case of CIGS thin ﬁlm solar cells, SLG is the most com- monly used substrate because it contains sodium. Incorporation of Na from the SLG to the CIGS absorber layer through Mo back contact is known to enhance the performance of cell (Bosio et al., 2014; Ishizuka et al., 2009). Mo is the best suitable material for forming back contact of CIGS and other thin ﬁlm solar cells, due to its low electrical re- sistivity, matching thermal expansion coeﬃcient with soda lime glass (SLG) and low contact resistance than other materials (Ma et al., 2013). Since multilayer stacks of thin ﬁlm solar cells (such as AZO/ZnO/CdS/ CIGS/Mo in the case of CIGS thin ﬁlm solar cells) are being developed on Mo which is popular substrate conﬁguration, the quality and func- tionality of the overall device are dependent upon electrical and me- chanical properties of Mo back contact. Mo layer as a back contact of complete stack thin ﬁlm solar cells undergoes various harsh processing such as thermal (selenization), electrochemical (electrodeposition) and laser scribing during the preparation of monolithically integrated solar cells. Suitable optimization of sputter process parameters such as sputtering power and deposition pressure can eﬀectively control elec- trical conductivity and adhesion which is well discussed and earlier reported by our group (Badgujar et al., 2015) and some of the other groups (Dai et al. 2014; Li et al., 2016) as well. Despite careful opti- mization of sputter process parameter, substrate cleaning and surface treatment play a crucial role inﬂuencing adhesion between SLG and Mo thin ﬁlm. Moreover, it is essential for scaling-up of a suitable process for large area back contact deposition. In past years, a variety of surface treatment methods such as ultrasonication using diﬀerent organic sol- vents (More et al., 2016), washing with various commercial detergents such as liqui-Nox, billco and RBS (Sundaramoorthy et al., 2010), plasma surface treatment technique (Bartella et al., 1987) and erosive acidic etching using Piranha, RCA-1, RCA-2 (Awadelkarim and Wang, 1999; Chen et al., 2001; Eske and Galipeau, 1999; Xianhua et al., 2006) http://dx.doi.org/10.1016/j.solener.2017.08.068 Received 26 May 2017; Received in revised form 18 August 2017; Accepted 23 August 2017 ⁎ Corresponding author. E-mail address: dhage@arci.res.in (S.R. Dhage). Solar Energy 157 (2017) 507–513 0038-092X/ © 2017 Published by Elsevier Ltd. MARK