Studies On Lead-less Or Lead-free Perovskite Solar Cells

Organic-inorganic lead halide perovskite with the formula of APbX₃?A:CH₃NH₃⁺?MA⁺?or NH?CH₃?₂⁺?FA⁺?;X:Cl^-,Br^-or I^-?possess several appealing features such as adjustable band gap,controllable components,and superior optoelectronic properties,and which have recently attracted a tremendous amount of attention in the optoelectronic community.The power conversion efficiency of inorganic organic halide lead perovskite solar cells have currently certified 22.7%.However,stability and lead toxicity issues of them have significantly limited their commercial applications.In this thesis,we focus on to explore the suitable Pb substitution elements via the appropriate Pb substitution method,achieving the improvement of its intrinsic stability while preparing the lead-less or lead-free perovskite materials,which without sacrificing the performance of materials and the corresponding solar cells simultaneously.In the first work,we use the strontium?Sr?which has the ionic radius almost identical to Pb²⁺(Sr²⁺=132 pm,Pb²⁺=133 pm)to explore the potential replacement of Pb.The Sr-joined binary metal perovskites have been synthesized and characterized.Here we uncovered the dramatic influence of Sr element substitution on the optoelectronic properties of perovskite films,including up-shifting conduction band edge energy level,increasing exciton binding energy and trap density states due to the isoelectronic impurity of electric neutral substitution.The Pb⁰ has been successfully suppressed for the Sr-joined materials and thus the stability of the perovskite can be markedly improved.The corresponding perovskite solar cell using a planar n-i-p structured perovskite device(ITO/ETL/MASr_aPb_1-a-a I_3-xCl_x/HTL/Au)exhibits an excellent conversion efficiency of16.3%?a=0.05?under AM 1.5 simulated sunlight.The characterization with impedance and transient decay measurements reveal an increased trap states induced by substitution Pb with Sr in the hybrid halide perovskites,and thus provide a deep understanding of the role of external elements on device physics.Strain effects on vacancies,electronic states and stability have been disclosed for perovskite structured materials?ABX₃?including metal oxides and organic-inorganic hybrids critical for energy storage and generation.Ion substitution has been pursued as an attractive solution to alter the crystallization or induced electronic of organic-inorganic hybrid halide lead perovskite.In our work,we introduce crystallization of restriction ABX₃ composition with moderate zinc?Zn?substitution to relax strain and achieving ordered CH₃NH₃?Zn:Pb?I_3-xCl_x crystal via an appropriate lattice constriction within BX₆octahedra.This structure is beneficial to restrain the crystal defects and enhance the intrinsic stability.The highly crystalline CH₃NH₃?Zn:Pb?I_3-xCl_x perovskite with ordered large grains can contribute to the outstanding photovoltaic performance of corresponding devices.The Zn-Pb binary metal halide perovskite has been identified to have an efficiency of 20.06%using spiro-OMeTAD HTM and 18.2%with good stability using poly-TPD HTM.This is expected to have important practical consequences as the need for further improving perovskite thin film quality and device performance by understanding substitution effect and crystal features.Work is in progress to further improve the performance of this promising system by optimizing the crystal structure and composition forms of this novel perovskite compounds and charge selective layers.