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Improving the efficiency of kesterite (Cu 2 ZnSn(S,Se) 4 ; CZTSSe) solar cells requires understanding the effects of Na doping. This paper investigates these effects by applying a NaF layer at various positions within precursors. The NaF position is important because Na produces Na-related defects in the absorber and suppresses the formation of intrinsic defects. By investigating precursors with various NaF positions, the sulfo-selenization mechanism and the characteristics of defect formation are confirmed. Applying a NaF layer onto a Zn layer in a CZTSSe precursor limits Zn diffusion and suppresses Cu-Zn alloy formation, thus changing the sulfo-selenization mechanism. In addition, the surface NaF layer provides reactive Se and S to the absorber layer by generating Na 2 Se x and Na 2 S x liquid phases during sulfo-selenization, thus limiting the incorporation of Na into the absorber and reducing the Na effects. Efficiency values of 11.16% and 11.19% are obtained for a flexible CZTSSe solar cell by applying NaF between the Zn layer and back contact and between the Cu and Sn layers, respectively. This study presents methods for doping with alkali metals and improving the efficiency of photovoltaics.

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Although Cu2ZnSnS4 (CZTS) has attracted attention as an alternative absorber material to replace CuInGaSe2 (CIGS) in solar cells, the current level of understanding of its characteristic loss mechanisms is not sufficient for achieving high power conversion efficiency. In this study, which aimed to minimize the characteristic losses across the devices, we examined the relations between the compositional ratio distribution in the absorber layer, subsequent defect formation, and surface electrical characteristics. A high‐temperature sulfurization process was used to improve the crystallinity of the absorber layer, which increased the uniformity of the compositional ratio distribution and consequently suppressed the formation of a ZnS secondary phase on the CZTS/MoS2 interface. Because defects and defect clusters generated in the absorber layer are shallower when the compositional ratio distribution is uniform, the electron‐hole recombination loss is reduced. These characteristics were confirmed by measuring the defect energy level using admittance spectroscopy and by analyzing the surface potential and current characteristics. These measurements revealed that improving the compositional ratio distribution suppresses the formation of deep‐level defects and reduces the rate of carrier recombination. In addition, improving the compositional ratio distribution substantially contributes to improving the series resistance and short circuit current density characteristics. Copyright © 2015 John Wiley & Sons, Ltd.