| Chalcopyrite structure Cu(In,Al)Se2(CIAS)thin film has been considered as one of the most promising materials for thin film solar cell application due to its high optical absorption coefficients(>105 cm-1),high stability under sun-light irradiation and continuous adjustable optical band-gap from 1.04 eV(CuInSe2,CIS)to 2.67 eV(CuAISe2,CAS).Up to now,the highest photovoltaic conversion efficiency(PCE)of 16.9%for the CIAS thin film solar cell has been obtained through the co-evaporation deposition.However,the co-evaporation method generally suffers from relative low throughput,complex and precise operation,which show a detrimental effect on large-scale production.Moreover,the PCE of CIAS thin film solar cell still exhibits an obvious gap compared with the PCE of 22.6%for Cu(In,Ga)Se2(CIGS)thin film solar cell,which can attribute to the insufficient and superficial analysis on the preparation of physical property of CIAS thin films.Hence,in order to further improve the PCE of CIAS thin film solar cell and link to the industrialized production,CIAS thin films have been fabricated by the low-cost selenization of magnetron sputtered stacked precursors.In this work,a series of fundamental key issues such as synthesis mechanism,composition modulation and microstructure change of the CIAS thin films have been investigated sufficiently and deeply,and the performance of CIAS thin film solar cells has also been explored.Besides,we have systematically and detailedly researched the influence of V element doping on the growth mechanism and intrinsic defects of CIAS thin films.All these studies are aim at laying a good foundation for improving the PCE of CIAS thin film solar cell.The major innovative research results obtained are as follows:1.The influence of the major selenization process parameters such as temperature,pressure and time on the physical properties of CIAS thin films has been studied systematically.The synthesis mechanism of CIAS thin film through this approach has been explored.The results clearly reveal that the elevated selenization temperature can effectively facilitate the formation of single phase CIAS thin film and result in the better crystal quality.Meanwhile,the content of Al element in CIAS thin films gradually decreases with the increasing of selenization temperature and lead to a red-shift of the optical band-gap.This result can be attributed to In ions partially replaced by Al ions can affect the hybridization degree of Al-Se and In-Se and then result in the move down of the conduction band bottom.Besides,Relative low selenization pressure can indeed suppress the formation of the unwanted phases such as CuSe and decrease the defects in thin film.Thus,the CIAS thin films with the smooth and compact morpho logy and excellent optical property can be prepared.In addition,the change of selenization time can briefly reveal the synthesis mechanism of CIAS thin film,that is,the selenization time drives structural phase transition from coexisting phases of CIS and CIAS to single CIAS phase.2.Effects of composition modulation such as A1 element content,Cu element content and Se/S ratio on the physical properties of CIAS films were investigated.Here,we emphatically analyze the reason that why high quality CIAS thin film can be obtained within a wide range of Cu content.With the increase of Al content,the(112)diffraction peak of CIAS thin film gradually shifts to higher angle,the grain size decreases and surface roughness increases obviously,which eventually induce a detrimental effect on the device performance.Besides,XRD and Raman measurements reveal that the phase purity of thin films strongly relies on the Cu content,that is,the secondary phases such as Cu(In,Al)3Se5 and cars could be formed under the over Cu-poor and Cu-rich state.However,the microstructural parameters of thin films have little relevance with the Cu content.With the increase of Cu content,the conductivity of CIAS thin films gradually improves and the optical band-gap exhibits a decrease trend.The(112)diffraction peak of CIAS thin film gradually shifts to higher angle with a decrease of Se/S ratio,which due to the Se2-gradually replaced by S2-and the ionic radius of S2-cation is smaller than that of Se2-.Furthermore,the corresponding surface roughness and grain size decrease with the increase of Se/S ratio.3.We have systematically researched the influence of pretreatment process parameters(e.g.preheating temperature and time)on and the physical properties of CIAS thin films and performance of the resulting solar cell devices.The phase transition mechanism of Cu-In-Al precursors in the pretreatment process has been investigated.The XRD patterns of Cu-In-Al precursors show that the phase structure of Cu-In-Al precursor is strongly relied on the preheating temperature,that is,the Cu-In alloy phase gradually transforms to the Cu-Al alloy phase with the increase of the preheating temperature.Meanwhile,the crystallinity of these CIAS thin films with a pretreatment process is better than that of as-deposited CIAS thin films,and these films exhibit the more smooth and compact surface morphologies and larger grain size.Photoluminescence spectra show that the pretreatment of precursors can effectively reduce the lattice defects of CIAS thin films.When the preheating temperature is optimized to 300 ?,the largest grain size of CIAS thin film and its best photovoltaic performance is achieved.In addition,within a certain range of preheating time,the crystal quality of CIAS thin film improves obviously with an increase of preheating time,the corresponding microstructure becomes more smooth and dense and the surface roughness degree decreases sharply.However,long preheating time could induce the decomposition of CIAS phase and increase the surface roughness degree.Thus,the best crystallinity and largest grain size of CIAS thin film can be obtained under the preheating time is 10 min,which exhibits the minimum grain boundary and improve dramatically the collection of photo-generated carriers.4.The influence of V group element doping(Sb or Bi)on the physical properties of CIAS thin films has been investigated.We mainly explore the effects of Sb and Bi element doping on the grain growth mechanism of CIAS thin films.Our results indicate that Sb doping has a negligible impact on the crystal structure of CIAS thin films.Interestingly,it can dramatically improve the crystallinity of CIAS thin films,reduce the lattice defects,promote the grain growth and decrease the surface roughness degree.In this work,the volatile Sb2Se3 compound plays a role as fluxing agents which are beneficial to facilitate the crystal growth of CIAS thin films during the selenization process can be confirmed.Besides,the electrical property of CIAS thin film has been optimized through Sb doping,which could reduce the grain boundary by increasing the grain size and then enhance dramatically the collection of photo-generated carriers.Besides,the crystallinity of CIAS thin film can also improve obviously by Bi element doping.It can suppress the lattice defects and increase the grain size of CIAS thin films.The crystal growth promotion mechanism in the CIAS thin films with Bi doping can be presumably attributed to the low melting point Bi element act as the fluxing agents during the selenization process.On the other hand,the optical property of thin films has little relevance with the V element doping. |
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