The Chemical Conversion of a Thin Mo Sacrificial Layer at the Interface Between ITO and Widegap ACIGSe and Its Impact on GaO_x Formation

In (Ag,Cu)(In,Ga)Se₂ (ACIGSe)-based thin-film solar cells, molybdenum is commonly used as back contact. At elevated temperatures during absorber deposition, a MoSe₂ interlayer is formed [1], which is responsible for the ohmic contact leading to large fill factors [2]. However, for applications like bifacial devices or top cells of multijunction devices, transparent back contacts are required.<br/>For such purposes, various transparent conductive oxides (TCO) such as ZnO:Al (AZO), SnO:F (FTO), In₂O₃:Sn (ITO), In₂O₃:H (IOH) [3], [4], [5] have been suggested. One major obstacle found in these studies is the formation of GaO_x at the absorber/TCO interface at deposition temperatures above 500°C, which deteriorates the device performance. However, Keller et al. found that moderate amounts of GaO_x might not be detrimental, but thicker layers (several tens of nm) cause Ga depletion towards the back of the absorber, which negatively affects charge carrier separation [5].<br/>One opportunity to reduce the formation of GaO_x at the absorber/TCO interface is the addition of a thin, metallic Mo interlayer, which is thought to be converted into MoSe₂ during absorber preparation [6], [7].<br/>In our study, we examine the effect of a thin Mo interlayer on the device performance of solar cells based on an ACIGSe/ITO layer stack. The absorber material is optimized for a large band gap (~1.5 eV), achieved by a high GGI (~0.73), but low AAC (~0.09). While a cell without such an interlayer shows an efficiency of less than 1%, a similar cell with an about 10 nm thick Mo layer deposited on the ITO prior to absorber deposition exhibits an efficiency of 10.6%, with the fill factor being more than tripled. A combination of transmission electron microscopy (TEM) and hard X-ray photoemission spectroscopy (HAXPES) confirms that GaO_x forms at the back contact of both samples, however the layer thickness is significantly reduced upon Mo layer insertion. Analysis of the chemical composition of the interface suggests that the metallic Mo is nearly completely converted to MoSe₂ and MoO_x. Since the latter is a prominent material to realize charge carrier selective contacts for holes, we suggest its formation together with inhibiting the formation of GaO_x as an explanation for the drastically enhanced cell performance. These findings may provide crucial insights for the optimization of absorber/TCO interfaces in ACIGSe-based multijunction devices.<br/>[1] Bär, M.; Weinhardt, L.; Heske, C.; Nishiwaki, S.; Shafarman, W. N. (2008). Chemical structures of the Cu(In,Ga)Se₂/Mo and Cu(In,Ga)(S,Se)₂/Mo interfaces. <i>Physical Review B, 78</i>, 075404.<br/>[2] Kuo-Jui Hsiao, J.-D. L.-H.-S. (2013). Electrical impact of MoSe₂ on CIGS thin-film solar cells. <i>Phys.Chem. Chem. </i><i>Phys., 15</i>, 18174-18178.<br/>[3] Marc Daniel Heinemann, V. E.-W. (2015). Cu(In,Ga)Se₂ superstrate solar cells: prospects and. <i>Prog. Photovolt: Res. Appl., 23</i>, 1228-1235.<br/>[4] Tokio Nakada, Yutaka Hirabayashi, Takehito Tokado, Daiske Ohmori, Takahiro Mise. (2004). Novel device structure for Cu(In,Ga)Se₂thin film solar cells using transparent conducting oxide back and front contacts. <i>Solar</i><i> Energy, 77</i>(6), 739-747.<br/>[5] Jan Keller, Nina Shariati Nilsson, Asim Aijaz, Lars Riekehr, Tomas Kubart, Marika Edoff, Tobias Törndahl. (2018). Using hydrogen doped In₂O₃ films as a transparent back. <i>Progress in Photovoltaics, 26</i>(3), 159-170.<br/>[6] P.J. Rostan, J. Mattheis, G. Bilger, U. Rau, J.H. Werner. (2005). Formation of transparent and ohmic ZnO:Al/MoSe₂ contacts for bifacial Cu(In,Ga)Se₂ solar cells and tandem structures. <i>Thin Solid Films, 480-481</i>, 67-70.<br/>[7] Tokio Nakada. (2005). Microstructural and diffusion properties of CIGS thin film solar cells. <i>Thin Solid Film, 480-481</i>, 419-425.