Transmission electron microscopy analysis of secondary phases in Cu 2 ZnSnS 4 thin film solar cells
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Transmission electron microscopy analysis of secondary phases in Cu2ZnSnS4 thin film solar cells Wei Li, Ziheng Liu, Jian Chen, Fangyang Liu, Xiaojing Hao* School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia *Corresponding author: Xiaojing Hao; E-mail address: [email protected] ABSTRACT Secondary phases are likely to occur in the Cu2ZnSnS4 (CZTS) films since the CZTS is thermodynamically stable in only a narrow region of the phase diagram. The CZTS solar cell performance can be influenced by the existence and precipitated position of secondary phases. Therefore, locally investigate the distribution of secondary phases is important to further improve CZTS solar cell efficiency. In this study, two different kinds of transmission electron microscopy imaging techniques, bright field scanning TEM image (BF-STEM) and High-angle annular dark-field (HAADF) image, are applied to analyze the distribution of secondary phases. Due to the atomic number differences between CZTS and secondary phases, secondary phases are evident in the HAADF images. Therefore, HAADF image is a more powerful and convenient method to analyze the secondary phases than the BF-STEM image. INTRODUCTION Cu2ZnSnS4 (CZTS) attracts an increasing attention as a promising absorber material for thin film solar cells [1], because of the earth-abundant constituents, an optimum direct band gap [2] (1.4 to 1.6 eV [3]) and a high optical absorption coefficient of about 104 cm-1 in the visible range [4]. Cu2ZnSnS4 (CZTS) is thermodynamically stable in only a narrow region of the phase diagram and precise control of the composition is complicated by the loss of sulfur and tin at high temperature, particularly in the form of volatile SnS and sulfur vapor. Therefore secondary phases, such as ZnS, Cu2S, and Cu2SnS3 (CTS), could form depending on the processing conditions [5]. Secondary phases have several effects on photovoltaic device performance. First of all, when the valence (conduction) band of the secondary phases is higher (lower) than that of CZTS the lifetime of carriers will be decreased by carrier traps or recombination centers at the interface [6]. The formations of low band gap ternary phases (for example Cu2SnS3) are considered responsible in large part for the low Voc of devices. Otherwise, secondary phases can be regarded as an inactive high resistivity domain [6]. The valence band offsets and the conduction band offsets between ZnS and CZTS are reported over 0.9 eV, and 1.3 eV, respectively [6]. It indicates that ZnS phase in CZTS forms a high energy barrier for both electron and hole carriers and it may act neither as a recombination center nor a carrier trap but as a non-active high resistivity domain [6]. Moreover, secondary phases, such as Cu2S (monoclinic) and CuS (hexagonal), have crystal structures that differ from tetragonal CZTS are likely to form incoherent interfaces with high recombination velocities [5]. Secondary phase precipitated position can also influence the solar cell p
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