The effect of additional sulfur on solution-processed pure sulfide Cu 2 ZnSnS 4 solar cell absorber layers
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The effect of additional sulfur on solution-processed pure sulfide Cu2ZnSnS4 solar cell absorber layers Zhengfei Wei1, Miao Zhu1, James D. McGettrick1, Gabriela P. Kissling2, Laurence M. Peter2 and Trystan M. Watson1 1 SPECIFIC, College of Engineering, Swansea University, Engineering East, Bay Campus, Swansea, SA1 8EN 2 Department of Chemistry, University of Bath, Bath, BA2 7AY, UK ABSTRACT To reduce the amount of chalcogen needed in the post-annealing process, we demonstrate significantly increased sulfur incorporation into pure sulfide CZTS films achieved by increasing the thiourea content of DMSO-based precursor solution. The increase of sulfur content was confirmed by thermogravimetric analyses (TGA). To understand how the elemental distribution across the CZTS layer is affected by extra thiourea, a systematic compositional study was carried out using X-ray photoelectron spectroscopy (XPS). XPS depth profiling reveals increased sulfur incorporation in the final CZTS films when more thiourea is added to the solution. The grain size was reduced slightly with increased sulfur content and the surface morphology was changed significantly. The effect on the surface of the CZTS film has been investigated using scanning electron microscopy (SEM), Raman spectroscopy, and XPS. External-quantum-efficiency (EQE) measurements with an electrolyte contact were used to investigate the optoelectronic properties of the deposited CZTS films. INTRODUCTION Solution-processed Cu2ZnSn(S,Se)4 (CZT(S,Se)) quaternary thin film solar cells have attracted enormous interest as an economic alternative to other industrially-proven thin film technologies such as CIGS and CdTe [1, 2]. Recently, many laboratories have fabricated CZT(S,Se)-based solar cells with close to or above 10% power conversion efficiency using nonvacuum processes [3-6]. The highest efficiency of 12.6% has been obtained with a solution process using hydrazine as the solvent and selenium added into the high temperature annealing process [3]. Generally, toxic materials such as S, Se, H2S and H2Se are present in excess during high-temperature annealing processes in order to form high quality CZT(S,Se) films [3, 7-9]. The drawback of using these toxic materials is an increase in process complexity, cost and safety hazards. To avoid the use of hydrazine, the less toxic and environmentally benign solvent, dimethyl sulfoxide (DMSO) has been widely used to fabricate comparably high efficiency devices [4, 6, 10]. The thiourea is added as a sulfur source and stabilizer in the solution. In this paper, we report that increasing the concentration of thiourea in the DMSO-based CZTS precursor solution can enhance the final sulfur content after rapid high temperature (>550 ºC) annealing process. Thermogravimetric analyses (TGA) revealed noticeable changes on the thermal profile and weight losses of precursor solutions when the amount of sulfur was varied. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that the amount of thiourea, annealing temperature and time signif
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