Optimizations And Design Of Copper-based Chalcogenides For Photovoltaic Applications

The 3^rd generation thin-film solar cells based on Cu-based chalcogenides have attracted extensive attentions from industrial and academic communities in the past 3decades,due to its supoerior conversion efficiency,high absorptivity,high power density,excellent radiation-proof ability,and good match with solar spectrum.However,the cost of the 3^rd generation thin-film solar cells is still higher than that of silicon-based ones.Technical solutions with reduced cost are desired for the future development of the 3^rdd generation thin-film solar cells.It is generally believed that the non-vacuum process based on the nanoparticles of Cu-based chalcogenides,which can be used to fabricate the absorber layer of thin-film solar cells,is an effective approach to lower cost.The so called non-vacuum process consists 3 major steps:1)the synthesis of nanoparticles,2)the fabrication of thin films based on the suspensions or“inks”of nanoparticles,3)heat treatment for promoting grain growth.Benefiting from printable electronics and high-precision coating technologies,the technical solution for the fabrication of thin film based on the suspensions or“inks”of nanoparticles is ready.The major technical challenges of non-vacuum process are as follows.Firstly,the synthesis of single-phase chalcogenides is time-and money-consuming,and difficult.Secondly,the grain size of the absorber layer derived from nanoparticles is very small,even though after heat treatment.The numerous and random grain boundaries in the absorber layer lead to a high density of recombination center for photon-generated carriers,in turn result in failing to construct high performance photovoltaic device.To address these two challenges,we started from mechanism study,and finally developed a series of tecnnical strategies.In brief,we conducted exploratory studies on the synthetic methodologies for Cu-based chalcogenides,the mechanism of reaction and crystallization kinetics enhancement,and the rational design and preparation of nanoparticle precurosrs for non-vacuum process,to provide alternative technical solutions and to illuminate knowledge for the advanced thin-film solar cells.In terms of synthetic methodologies for Cu-based chalcogenides,we explored new approaches to improve the controllability of the phase and composition of the products,and to enhance the reaction kinetics,based on the understanding of the reaction pathway.First,according to the theory on the balance between complexes and ions in solution,we modified the precursors'reactivity through changing solvents composition.And then the phase-selective synthesis of zincblende-and wurtzite-strucutured CIGS was achieved,since the thermodynamic conditions to trigger the nucleation can be influenced.Second,inspired by the report on promoting crystallization kinetics of CIGSe by introducing trace antimony,we bring trace antimoby into the solvothermal synthesis of copper selenides.The phase-selective synthesis of Cu₂Se was realized becasuse the mass transfer in reaction is influenced by the mobile phase related to antimony.Third,the reaction kinetics of the synthesis of Cu-based chalcogenides was significantly improved by activating the Se precursor by NaBH₄,which is a strong reductant.The necessary reaction time to obtain single-phase CZTSSe was reduced to 10%of that of conventional solvothermal synthesis.Furthermore,the improved reactivity and solubility of Se in solvothermal system also enabled us to tune the S/Se ratio in CZTSSe more flexibly.In term of the mechanism of antimony-promoted reaction and crystallization kinetics,we chose CIGSe as the model materials.Through carefully analyzing the phase evolution of the products during solvothermal synthesis,we identified Cu₃SbSe₃ as the key species to form mobile phase,and revealed the the mechanism on the kinetics enhanced by the mobile phase through promoting mass transfer.Moreover,we optimized the Sb ratio in a fast manner via high-throughput experimentation.On the basic of scientific understanding of the mechanism on Sb-induced kinetics enhancement,we proposed CIGSe@Cu₃SbSe₃ core-shell nanoparticles as an ideal precursor for non-vacuum process.The CIGSe core is cladded by Cu₃SbSe₃,which can perform as a good absorber in photovoltaic devices,and can form mobile phase during heat treatment.The CIGSe@Cu₃SbSe₃ nanoparticles were synthesized through a carefully designed pathway.The heterogeneous interfaces between pre-formed CIGSe nanoparticles and solution induced the precursors of Cu,Sb,and Se to coprecipitate and form Cu₃SbSe₃.The thin films derived from CIGSe@Cu₃SbSe₃ nanoparticles performed a much better crystallization kinetics rather than that derived from pristine CIGSe nanoparticles.In summary,this dissertation put effort into rising to the major challenges in non-vacuum process for the fabrication of advanced thin-film solar cells based on Cu-based chalcogenides compound semiconductors.Specifically,we reinforced the synthetic methodologies for Cu-based chalcogenides,proposed a mechanism explanation on Sb-promoted formation and crystallization kinetics,and most importantly,designed,developed,and applied a strategy based on CIGSe@Cu₃SbSe₃ core shell nanoparticles to improve the crystallinity of absorber thin films deposited by non-vacuum process.This study provides several valuable techniques and enlightening knowledge for not only the3^rd generation thin-film solar cells,but also all-solid-sate lithium batteries,sensors,high-resolution display,next-generation light source,and other fileds involving the fabrication of functional devices by nanoscale materials.