This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record . Please cite this article as doi: 10.1002/solr.201900260 This article is protected by copyright. All rights reserved Na-induced conversion of a notorious fine-grained residue layer into a working absorber in solution-processed CuInSe 2 devices Jihyun Moon † , Shanza Rehan † , Tanka Raj Rana, Byungsung O*, Seung Kyu Ahn*, SeJin Ahn* † These authors equally contributed to this work Dr. J. Moon, Dr. T. R. Rana, Dr. S. K. Ahn, Dr. S. J. Ahn Photovoltaics Laboratory, Korea Institute of Energy Research, Daejeon, Korea E-mail: notask@kier.re.kr , swisstel@kier.re.kr Prof. S. Rehan Faculty of Materials and Chemical Engineering, GIK Institute of Engineering Sciences and Technology, Topi, Pakistan Dr. J. Moon, Prof. B. O Department of Physics, Chungnam National University, Daejeon, Korea E-mail: byung@cnu.ac.kr Dr. S. J. Ahn Department of Renewable Energy Engineering, Korea University of Science and Technology (UST), Daejeon, Korea Keywords: (CISe solar cell, nanoparticle, Na, current density, carrier collection) Na has been reported to increase the efficiency of not only the vacuum-processed but also the solution-processed Cu(In,Ga)(Se,S) 2 (CIGSSe)-type solar cells. However, the physical mechanism underlying the improvement is significantly different depending on the nature of the processes, as exemplified by the experimental observation that the short circuit current density (J SC ) of the solution-processed devices reportedly increases upon Na addition while that of the vacuum-processed devices remains the same or even decreases. In this work, a systematic study was performed to elucidate the reason for this Na-induced J SC increase in solution-processed CuInSe 2 (CISe) devices. In our amorphous nanoparticle-based route, upon Na addition, the previously fine-grained bottom layer near the CISe/Mo interface was transformed into a large-grained layer, presumably due to the Na-Se liquid flux-assisted sintering. At the same time, Na also induced phase homogenization in the bottom layer in ed Article on, Dr. T. R. , Dr. T. aics Laborato aics Labor otask@kier.re otask@kie c ehan an f Materials an f Materials gy, Topi, Pak gy, Topi, P on, P , Pr rof. B. O of. B nt of Physics of Phys yung@cnu.ac yung@c Ahn nt of Renewa aejeon, Korea aejeon, Ko s: Accepte (CISe solar CISe so een reported t reporte processed Cu cessed m underlying underlying sses, as exem es, as ex J SC ) of the s ) of the o e vacuum vacuum- pr c study udy was wa A processed CuI processed