Air-stable solution processed Cu 2 ZnSn(S x ,Se (1-x) ) 4 thin film solar cells: influence of ink precursors and prepara
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Air-stable solution processed Cu2ZnSn(Sx,Se(1-x))4 thin film solar cells: influence of ink precursors and preparation process Xianzhong Lin1*, Jaison Kavalakkatt1, Martha Ch. Lux-Steiner1,2 and Ahmed Ennaoui1* Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany [email protected], [email protected] 2 Freie Universität Berlin, Germany 1
ABSTRACT Quaternary semiconductors, Cu2ZnSnS4 and Cu2ZnSnSe4 which contain only earth-abundant elements, have been considered as the alternative absorber layers to Cu(In,Ga)Se2 (CIGS) for thin film solar cells although CIGS-based solar cells have achieved efficiencies over 20 %. In this work we report an air-stable route for preparation of Cu2ZnSn(Sx,Se(1-x))4 (CZTSSe) thin film absorbers by a solution process based on the binary and ternary chalcogenide nanoparticle precursors dispersed in organic solvents. The CZTSSe absorber layers were achieved by spin coating of the ink precursors followed by annealing under Ar/Se atmosphere at temperature up to 580°C. We have investigated the influence of the annealing temperature on the reduction or elimination of detrimental secondary phases. X-ray diffraction combined with Raman spectroscopy was utilized to better identify the secondary phases existing in the absorber layers. Solar cells were completed by chemical bath deposited CdS buffer layer followed by sputtered i-ZnO/ZnO: Al bi-layers and evaporated Ni/Al grids. INTRODUCTION Although Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have achieved efficiencies over 20 % , the use of indium which is expensive somehow restrict the large scale development of CIGS-based thin film solar cells. It is crucial to develop new absorbing materials, which contain only earth-abundant constituents, for the further utilisation of the solar energy. Cu2ZnSn(SxSe(1-x))4 (CZTSSe), containing only earth-abundant constituents and exhibiting optimal optoelectronic properties for solar energy conversion such as tuneable direct optical band gap from 1.0-1.5 eV and high absorption coefficient in the visible range, has been considered as the alternative to CIGS as absorber layers for thin film solar cells. The solution-processed CZTSSe-based solar cells have achieved great success2-6. For example, hydrazine-based solution processed CZTSSe-based solar cells have reached 11 %5 which is the record efficiency for CZTSSe-based solar cells so far. It is well known that the optoelectronic properties of materials are closely related to qualities of the materials such as crystalline quality, chemical compositions and phase purity. However, according to the phase diagram reported by Olekseyuk et al. 7, the existence region of the phase pure CZTS is quite small, which implies that if the preparation conditions for materials is not well optimized, the occurrence of secondary phases like ZnS and Cu2SnS3 will be unavoidable. Moreover, these secondary phases may be detrimental to the device because they act as either recombination centre (when the band gap is
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