Progress in cleaning and wet processing for kesterite thin film solar cells Bart Vermang 1,2,a , Aniket Mule 1,3 , Nikhil Gampa 1,4 , Sylvester Sahayaraj 1,2 , Samaneh Ranjbar 1,5 , Guy Brammertz 1,6 , Marc Meuris 1,6 and Jef Poortmans 2,6 1 imec division IMOMEC – partner in solliance, Diepenbeek, Belgium 2 Department of Electrical Engineering, KU Leuven, Heverlee, Belgium 3 Department of Mechanical and Process Engineering, ETH Zurich, Zurich, Switzerland 4 Department of Engineering, Université Claude Bernard Lyon 1, Villeurbanne, France 5 Department of Physics, Universidade de Aveiro, Aveiro, Portugal 6 Inst. for Material Research (IMO) Hasselt University – partner in Solliance, Diepenbeek, Belgium a bart.vermang@imec.be Keywords: Photovoltaics, PV, thin film, kesterite, CZTS, wet processing, alkali, surface, cleaning, etching, passivation, chalcopyrite, CIGS. Kesterite solar cells, the new kid on the block The photovoltaic module manufacturing market is booming and presently dominated by Si modules, but also thin film (TF) photovoltaics (PV) remain very interesting due to their high potential for new applications and markets. TF PV can be completely processed on large rigid (e.g. glass) or flexible and thus lightweight substrates (e.g. steel) by use of monolithic integration (P1, P2 and P3 scribes). This is an interconnection technology that leads to a dark smooth appearance and is free of classic size limitations. Hence, such modules bring many exciting architectural choices with regards to size, shape, color and function; all important characteristics for building integrated PV (BIPV), a key future market. Additional advantages of TF PV are (i) the high energy yield and outstanding outdoor performance, (ii) the low energy consumption, short energy payback time and minimized material consumption, (iii) its proven reliability, (iv) the high productivity at the GW production level, and (v) the low production costs [1]. Cadmium telluride (CdTe) and copper indium gallium selenide/sulfide (CIGS) are the two main TF PV technologies, together they represent the largest TF PV market share and the highest energy conversion efficiencies, but unfortunately also toxicity and abundancy concerns. Indeed, two thirds of the TF PV market is provided by the two largest TF companies: First Solar (CdTe) and Solar Frontier (CIGS). Both companies are very successful in the development of their technologies, as demonstrated by their presence in the TF PV world record efficiency charts. These up-to-date record efficiencies are 21.0 % at cell level, as obtained by First Solar and Solibro (CIGS), and respectively 18.6 % (obtained by First solar) or 17.5 % (obtained by Solar Frontier) at module level. However, these technologies also embody clear toxicity (the use of poisonous Cd in CdTe PV) and abundancy (the use of rare elements In and Ga in CIGS PV) concerns [2]. Copper zinc tin selenide/sulfide (CZTS) is a non-toxic and earth-abundant alternative with great potential, but material and device improvements are essential. A novel (first solar cell developed in 1996) TF PV material is CZTS, which is derived from the related CIGS compound by substitution of In and Ga for Zn and Sn. This isoelectronic substitution produces a material with many similar properties of the parent compound. Indeed, the CZTS kesterite and CIGS chalcopyrite structures are known to be associated, where the advantage of CZTS lays in its composition of non-toxic and abundant materials. At present, top conversion efficiencies for small-area CIGS solar cells are typically above 20 % (e.g. NREL, ZSW, HZB, EMPA, Solibro, Solar Frontier, etc.), whereas CZTS solar cells are limited to efficiencies slightly above 10 % (e.g. IBM, Imec, IREC, Solar Frontier,