title: KCN Chemical Etch for Interface Engineering in Cu2ZnSnSe4 Solar Cells.
abstract: The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.
authors.affiliation: †imec-partner in Solliance, Kapeldreef 75, 3001 Leuven, Belgium.|‡Department of Electrical Engineering (ESAT), KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium.|△Qatar Environment and Energy Research Institute (QEERI), Qatar Foundation, Doha, Qatar.
authors.forename: Marie
authors.initials: M
authors.lastname: Buffière
authors.affiliation: ∥imec division IMOMEC-partner in Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium.|⊥Institute for Material Research (IMO), Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium.
authors.forename: Guy
authors.initials: G
authors.lastname: Brammertz
authors.affiliation: ‡Department of Electrical Engineering (ESAT), KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium.|∥imec division IMOMEC-partner in Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium.|⊥Institute for Material Research (IMO), Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium.
authors.forename: Sylvester
authors.initials: S
authors.lastname: Sahayaraj
authors.affiliation: #Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium.
authors.forename: Maria
authors.initials: M
authors.lastname: Batuk
authors.affiliation: □Electronics and Information Systems department (ELIS), University of Gent, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium.
authors.forename: Samira
authors.initials: S
authors.lastname: Khelifi
authors.affiliation: ○Institut Jean Lamour, Université de Lorraine, Parc de Saurupt, CS 50840, 54011 Nancy, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Mangin
authors.affiliation: §Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes cedex 3, France.
authors.forename: Abdel-Aziz
authors.initials: AA
authors.lastname: El Mel
authors.affiliation: §Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes cedex 3, France.
authors.forename: Ludovic
authors.initials: L
authors.lastname: Arzel
authors.affiliation: #Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium.
authors.forename: Joke
authors.initials: J
authors.lastname: Hadermann
authors.affiliation: ∥imec division IMOMEC-partner in Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium.|⊥Institute for Material Research (IMO), Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium.
authors.forename: Marc
authors.initials: M
authors.lastname: Meuris
authors.affiliation: †imec-partner in Solliance, Kapeldreef 75, 3001 Leuven, Belgium.|‡Department of Electrical Engineering (ESAT), KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium.|∥imec division IMOMEC-partner in Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium.
authors.forename: Jef
authors.initials: J
authors.lastname: Poortmans
keywords: CZTSe; KCN; kesterite; photovoltaic; selective etching; surface