| Compared to other light-harvesting materials,CZTS has attracted more attention in the application of solar cell,solar water splitting cell and photo-detector,due to more suitable band gap(1.5 eV),high absorption coefficient(104 cm-1),non-toxic and low-cost character.Micro-nanostructures,such as 1D nanowire,2D nanosheet,3D porous or hierar-chical structure,have been proved to play an important role in improving the efficiency of photoelectrodes.Currently,the impact of micro-nanostructure on the absorption ef-ficiency,charge separation efficiency and injection efficiency of CZTS photocathode hasn’t been systematically studied.The lack of effective micro-nanostructure control-ling method for CZTS thin film limits its performance improvement.On the other hand,the photovoltage and photocurrent of CZTS photocathode are limited by antisite or multivalency related deep defect levels and high grain boundary density/secondary phases,respectively,which lead to a much lower solar to hydrogen efficiency than its theoretical value.Therefore,to obtain high quality CZTS thin films with controllable micro-nanostructures is a key to improve solar water splitting performance.In this paper,we focus on improving the solar water splitting performance of CZTS photocathode by introducing micro-nanostructures,decreasing grain boundary density,eliminating defect and secondary phases.By introducing porous and hierar-chical structures,the charge separation efficiency of CZTS photocathode are improved.The solar water splitting performance is further improved by eliminating defects and secondary phases through Ge alloying and increasing sulfur pressure simultaneously.The main work in this paper is as followed:1.The photocurrent of nanocrystalline CZTS photocathode is improved by introducing porous structure with thiourea as pore-making agent.The photocur-rent density of a nanocrystalline CZTS photocathode with a dense structure is relative low due to the coupling of light absorption direction and charge separation direction.We found that porous structure can be introduced in a nanocrystalline CZTS thin film with a high thiourea vs metal ratio in sol-gel precursor solution.Further analysis show that thiourea is not only sulfur source and complexing agent in precursor solution,but also plays a role of pore-making agent.The decomposition of excess thiourea in pre-cursor film leads to the formation of porous structure.The saturate photocurrent of nanocrystalline CZTS photocathode with porous structure is 3 times higher than that of a CZTS photocathode with dense structure.A shorter collection distance of minority carrier distance is the main reason of photocunent improvement.2.The charge separation efficiency of nanocrystalline CZTS photocathode is further improved by introducing hierarchical structure through tuning anion ratios in precursor solution.Compared to dense and porous structures,hierarchical structure has the characters of 2D nanosheet and porous structure,which leads to a shorter charge collecting distance.We found that nanocrystalline CZTS thin films with dense,porous and hierarchical structures can be tuned by adjusting anion ratios in precursor solution.Further analysis show that the nanostructure of SnS2 intermediate phase is influenced by the anion ratios of Cl-vs acetate,which further determines the nanostructure of CZTS thin film.The nanocrystalline CZTS thin film with hierarchical structure has higher light absorption efficiency and charge separation efficiency than that of CZTS thin films with dense and porous structure.This structure-performance relationship can offer guidance to other photoelectrode materials.3.Grain boundary density,deep level defect and secondary phases were decreased by increasing sulfur pressure and Ge-alloying simultaneously,which yields a record photocurrent density and half-cell solar to hydrogen efficiency in CZTS-based photocathodes.The photovoltage deficit of CZTS based devices can be reduced by substituting Sn by Ge,which lowers the concentration of Sn2+ in CZTS.However,in current studies,the optimum Ge ratio in Ge-CZTS is obscure.What’s more,Ge element loss and secondary phases are observed in Ge-CZTS samples.By comparing Ge-CZTS with different Ge ratio sulfurized under different sulfur pressure,we found that a much higher sulfur pressure is needed for Ge-CZTS with higher Ge ratio.A high quality microcrystalline Ge-CZTS thin film with larger grain size,low-er Sn2+ concentration and less ZnS secondary phase is obtained by increiasing sulfur pressure and Ge alloying simultaneously.The improvement in crystalline and elimina-tion of ZnS secondary phase increases the photocurrent density and Ge alloying lowers the concentration of Sn2+,which improves the onset potential.After further surface modification,a photocurrent density of 11.1 mA/cm2@0 VRHE and HC-STH of 1.7u%is obtained by Ge-CZTS photocathode,which are the highest values in CZTS-based photocathodes. |
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