Study On Solution Processed High-Efficiency Copper Indium Selenium Thin Film Solar Cells:High Pressure Selenization,Copper-Rich Absorber And Alkali Doping

Chalcopyrite structured copper indium gallium selenide(Cu(In,Ga)Se₂,CIGS)thin film solar cells are very promising for large-scale commercial applications due to their high efficiency,easy integration and flexibility.However,the absorbers of current highly efficient CIGS solar cells are all deposited by vacuum methods,which require complicated fabrication process and expensive equipment and have low throughput.Fabrication of CIGS absorbers from solution can greatly reduce production cost,more importantly,solution process is adaptable to roll-to-roll industrial production.In this dissertation,Cu In(S,Se)₂(CIS)and CIGS thin-film absorber materials were fabricated from dimethyl formide(DMF)based precursor solutions.The effects of selenization pressure,the stoichiometric ratio of the precursor compounds in the solution,and the alkali metal ion doping on the device performance were systematically studied.This dissertation includes three parts.(1)The effect of selenization pressure on the quality of the CIS absorber film and device performance.The pressure of the argon inert gas was adjusted and the property of the absorber films obtained under different pressures and the solar cell performance were studied.The results show that the absorbers selenized under normal pressure(one atmosphere)are composed of loosely connected grains,leading to deposition of CdS into the absorber inside and formation of conductive channels inside the absorbers and thus low device performance.The absorbers selenized under increased argon pressure(1.4 to 1.6 atmosphere)are composed of densely packed grains with smooth and flat surface,resulting in uniform growth of CdS buffer layer and thus significantly improved device performance,A power conversion efficiency(PCE)of 10.3%is achieved from CIS absorber selenized under increased pressure.This is the first time that high pressure selenization was used to fabricate chalcopyrite thin film solar cell,which is a new and effective strategy that can be applied to improve the electronic property of multiple-element composed thin-film semiconductors.(2)The effect of the stoichiometric ratios of copper to indium on the performance of CIS solar cells.The ratio of copper to indium,from Cu-poor(Cu/In<1)to Cu-rich(Cu/In?1),were controlled by the feed-in composition in the precursor solution and property of the absorber films and their device performance were systematically studied.Characterizations by XRD,Raman,SEM,SIMS,GDOES,J-V,EQE,etc.show that CIS films prepared under Cu-rich condition have better morphology,better electrical properties and higher photovoltaic performance.The Cu-rich absorbers have stoichiometric compositions,smoother surfaces,and epitaxial deposited CdS buffer layers;they have higher conductivities,smaller potential fluctuations(?_g)and band gap fluctuations(?_opt),and lower Urbach energy(E_U);the Cu-rich CIS/CIGS devices show lower ideality factors(n)and reverse saturation current densities(J₀).The results show that the bulk and surface defects of the absorber films obtained under the Cu-rich condition are much lower than those under Cu-poor condition,which can effectively reduce the bulk and heterojunction interface recombination.The Cu-rich absorber films achieved 14.5%efficient CIS(the highest efficiency among all solution-based CIS solar cells)and 15.2%efficient CIGS solar cells.The n,J₀,E_U,?_g and?_opt of the 15.2%device are respectively1.43,2.0×10^-12 A/cm²,13.7 meV,16.94 meV,and 32.61 meV,comparable to that of vacuum-based CIGS devices with efficiency beyond 20%,showing great potential of solution processed Cu-rich absorber as low cost and high efficiency thin film absorbers.(3)The effect of alkali metal ion doping on the property and photovoltaic performance of Cu-rich CIS absorber films.By directly adding Li Cl,Na Cl,KCl or Rb F to the precursor solution,the effect of alkali metal ion doping on the electronic property and device performance of DMF solution processed Cu-rich absorbers were studied.The results have shown that:(1)all alkali metal ions promote the grain growth of CIS absorber as evidenced by the increased grain size and well densely compacted grains.Among all the alkali metal ions investigated,Na has the most significant influence on the morphology of the film;(2)Li doping has no effect on device performance whereas Na,K and Rb improved device efficiency by 23%,7%and 9%.The improvement in device performance mainly comes from open circuit voltage and fill factor.Compared to the reference device without doping,devices with Na,K or Rb doping have lower E_U and n values,indicating reduced band tailing,bulk recombination,and interface recombination.In addition,Rb doping has achieved 14.4%efficient CIS device with the highest open circuit voltage(537 m V)of all CIS solar cells reported so far.In summary,this dissertation has effectively improved the electronic property of DMF solution processed CIS/CIGS absorbers through new absorber fabrication strategies including increased pressure selenization,fabrication of the absorber under Cu-rich condition,and alkali metal ion doping via solution approach.This dissertation not only significantly enhanced device performance but only revealed the mechanism behind the effectiveness of these new strategies.The results of this dissertation demonstrate the great potential of solution based multi-element thin film semiconductors for low-cost and large-area photovoltaics.