Effects of alkali-metal block layer to enhance Na diffusion into Cu(in,Ga)Se 2 absorber on flexible solar cells Kenichi Moriwaki a,n , Maki Nomoto a , Shogo Ishizuka b , Atsushi Mukai a , Keigo Sato a , Hiroyuki Kobayashi a , Shigeru Niki b a FUJIFILM Corporation, Frontier Core-Technology Laboratory, 577 Ushijima, Kaisei, Ashigarakami-gun, Kanagawa 258-8577, Japan b National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan article info Article history: Received 25 September 2013 Received in revised form 1 October 2014 Accepted 27 October 2014 Keywords: Cu(In x Ga 1-x )Se 2 Sodium diffusion mechanism Alkali-metal block High efficiency abstract An alkali-metal containing (AC) layer, specifically sodium (Na), is necessary for improving Cu(In x Ga 1-x ) Se 2 (CIGS) solar cell performance on an alkali-free flexible substrate. However, we found that Na from the AC precursor layer diffused not only into the CIGS absorber layer but also into our flexible metal foil substrate, the surface of which was covered with an Al 2 O 3 layer, and thus creating Na depletion in the CIGS layer. In this work, newly developed alkali-metal block (AB) layers are introduced between the AC layers and the metal foil substrates in order to suppress Na diffusion into the substrates and enhance Na diffusion into the CIGS layers. Using different AB layer materials—Ti, TiO 2 , and TiN—and AC layers with systematically-varied thickness, Na concentrations in the CIGS layers and the solar cell efficiencies are investigated. Ti and TiN AB layers efficiently enhance the Na diffusion into the CIGS absorber layers. As a result, a high efficiency of 17.6% is obtained on the metal foil substrate using optimally prepared AC and AB layer of TiN. The AB layer controls the Na diffusion from the AC layer in one direction, and thus contributes to the precise control of Na concentrations in the CIGS layer. Hence, high efficiency solar cells on a variety of flexible substrates will be achieved comparable to those on soda-lime glass substrates. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Cu(In x Ga 1-x )Se 2 (CIGS) thin films are one of the most promising materials for photovoltaics [1], because of a high absorption coeffi- cient for solar radiation [2]. CIGS solar cells have achieved the highest efficiencies, up to 20.8% among all the thin film solar cells [3]. On the flexible polymer substrates, CIGS solar cells have achieved high efficiency 20.4% [4]. CIGS thin films have another advantage, i.e. their suitability in fabricating monolithically integrated structures that offer high-output voltage, high productivity and low production cost. Moreover, manufacturing CIGS solar cells on flexible substrates is highly attractive because it enables roll-to-roll deposition processing which has the potential of lowering costs and raising productivity compared with conventional in-line deposition processing used for rigid soda-lime glass (SLG) substrates. Furthermore, the low weight of flexible substrates contributes to easy installation [4–9]. Alkali-metals, specifically sodium (Na), have been widely acknowl- edged as beneficial dopants that contribute to improving p-type conductivity of the CIGS absorber layers and achieving high efficien- cies [5–13]. Generally, Na was diffused into the CIGS layers from the rigid SLG substrates. In contrast, alkali-free flexible substrates need additional methods to incorporate alkali-metals. These methods generally require introducing an alkali-metal containing (AC) layer such as sodium fluoride and SLG, before, during [10,11,13,14] or after CIGS growth [9,12]. In these methods, optimization of the AC layer thickness is crucial for cell performance. However, this thickness depends on the quality and uniformity of the substrate material, because on our metal foil substrates [5–8] this thickness was much larger than that on alkali-free glass substrates [11,14]. The purpose of this study was to investigate the precise control of Na concentration on our metal foil substrate to achieve reproducible high efficiency CIGS solar cells. Fig. 1 shows a schematic illustration of the cross-sectional structure. First, we analyzed the Na diffusion from the AC layer using the metal foil substrates. Furthermore, newly developed alkali-metal block (AB) layers were formed between the AC layers and anodic aluminum oxides (Al 2 O 3 ) layers of the metal foil substrates to suppress Na diffusion into the substrate. To investigate Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.10.039 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ81 465 86 1884; fax: þ81 465 86 1022. E-mail address: kenichi.moriwaki@fujifilm.com (K. Moriwaki). Solar Energy Materials & Solar Cells 133 (2015) 21–25