Cu?In,Ga?Se₂ Thin Film Solar Cellsfabricated By One-step Sputtering Route Based On Single Ternary Targetand Selenization Process

Tosolve the energy crisis and environmental problems,people are persistently searching the clean energies.Among them,solar cells attract more attentions due to its unique advantages.With the advantage of light weight,low price and transportation costs,and great application potential,thin film solar cells is considered to be one of the most effectiveroutesto reduce the photovoltaic?PV?costs.Due to the excellsence of large band gap with width adjustable range?1.02-1.67 e V?,high optical absorption,low manufacturing costs,strong radiation resistance,high photoelectric conversion efficiency?PCE?,stable performance of the battery,the copper indium gallium selenide?CIGS?thin film solar cells hasbecome one of the mainstream productsinthe new generation solar cellscurrently.The German ResearchCenter of Solar Energy and Hydrogen Energy?ZSW?is always the holder of the highest PCE record in CIGS thin film solar cells,and refreshesthe record to 22.6%in July2016 on the monolithic battery in laboratory.But it is difficult to achieve industrialization,as the evaporation sources need high temperatures and high vacuum?above 1000 degree?during film formation process,precisely control of film composition,polar altitude in facility requests,and difficult to actualize industrialization.Relatively speaking,the CIGS thin films prepared by magnetron sputtering is prone to accurately control of film quality and element ratio,low facility requests and manufacturing costs,which has become the first choice of industrialization.Generally,thestacked Cu-In-Ga film can be prepared by multi-step sputtering method with multi-target.Unfortunately,the Cu_xGa,Cu_xIn alloy phase and In single phase are easilyformed during the pos-selenization process.For the sake ofthe metal element distribution more uniform during thesputtering,one-step sputtering methodwithalloy targetwas firstlyproposedto fabricate CuInGa precursorsin 2009,which used by G.S.Chen.et al.However,thePCE of their fabricated CIGS thin film solar cellsisas low as 6.2%.Subsequently,the sandwich structures was designed and sputteredby addingCuGaalloy target and In target and the the PCE was improved to 9.5%.However,thecomplex fabrication process and the use of H₂Se as the selenization source increased the manufacturing costs due to the highly toxic nature of H₂Se.In the current research work,we choose one-step sputtering method with a ternary target to fabricateCuInGa precursors.In order to avoid the use of H₂Se,Se powder was usedas the selenization source during the elevated temperature rapid annealing treatment.Finally CIGS absorbers are prepared and high efficient CIGS thin film solar cells were also fabricated.The details are as follows:1.The influences of sputtering power on the metal element composition of Cu-In-Ga precursorsand properties of CIGS thin films were systematically investigated.Results showed that the deposition rate,composition rate,grain size,compactness and roughness of the precursorshave a close relation with the sputtering power.Also,the crystallinity?crystallization rate,grain size?,photoelectric properties,atomicstoichiometryof the absorbers can be influenced by the sputtering power.The CIGS absorbers prepared under the appropriate sputtering power showed the characteristics of copper-poor nature.The PCE of CIGS thin film solar cells?Mo/CIGS/CdS/ZnO/ITO?was about 10.3%.Aclear small granular layers in the interface layer between CIGS absorbers and substrate was observed,which canincrease the quantity of interface defects,as a recombination center of electron and hole,thus deteriorate the performance of the device.2.In order to optimize the interface,layered-selenization method was adopted for the purpose of thorough selenization.However,the result presented apparently stratification in absorbers.Undoubtedly this approach went against the formation of bulk crystalline blocks.So we studied selenization temperature in the effect of CIGS absorbersin crystallization rate,results showed that small particle layers significantly reduced underthe suitable selenization temperatureduring the phase formation process.Thereby theV_oc and PCE of CIGS solar cells devices were improved to 11%and 16.7%,respectively.And the highestPCE was up to 12%.3.We changed the microstructure of precursors by regulating argon flow in order to improve the Se atom activity and its effective spread in precursors.Found that prefabricated film with tooth shape and slight gap between the structureswas conductive to improve the crystallinity of the absorbers during the post-selenization process,and further affected the density of CIGS films.Finally,the small particles in the interface almost completely disappeared,further reduced the boundary defect andachieved the interface optimization.The PCEwas significantly improved andreached 12.3%.