Study On The Phase Stability Of Cesium-based Perovskite Solar Cells

Hybrid perovskite solar cells(PSCs)have developed rapidly with a certified power conversion efficiency(PCE)up to 25.7%.However,the easy decomposition of organic cation components(CH₃CH₂⁺(MA⁺);HC(NH₂)₂⁺(FA⁺))in heat leads to poor thermal stability of devices,futher limits the commercial application.Cs-based perovskites(Cs Pb I₃,Cs Pb I₂Br,etc.)have excellent thermal stability(above 300?),suitable band gap of?1.7-2.3 e V and low exciton binding energy,which make them ideal light absorption materials.However,the radius of Cs⁺of Cs-based perovskites is too small to support[Pb X₆]^4–octahedron,resulting in octahedral tilting and phase transformation.In addition,the preparation of Cs-based perovskites films often requires high temperature annealing,which leads to poor film quality and high defect density,and thus affects the performance of the PSC devices.Focusing on the phase stability and film defects of Cs-based perovskites,ionic liquids,peripheral substituents of hole transport materials,and long chain organic cation were adopted on the surface of perovskite layers or in the bulk of perovskite films to enhance the phase stability and passivate film defects.Besides,the mechanism of phase stability improvement was further revealled.The specific research contents and results are as follows:Firstly,ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate(BMIMBF₄)was adopted on the surface of Cs Pb I₂Br films to stabilize the cubic phase of Cs Pb I₂Br by perturbing Cs atoms.BMIMBF₄ modification colud passivate the interface defect and thus reduce the energy loss of Cs Pb I₂Br device.Moreover,the interaction between BMIMBF₄ and Cs Pb I₂Br changed the charge distribution on the surface of Cs Pb I₂Br films and thus improved the dipole moment,which was beneficial to the charge extraction and transfer.Therefore,the PCE and open circuit voltage(V_oc)of BMIMBF₄modified inverted devices were 13.2%and 1.07 V,respectively,which were 37.2%and18.9%higher than those of the control devices.Besides,the BMIMBF₄ modified devices remained 86.9%of the initial PCE after 1000 h storage in N₂,while the PCE of the control devices dropped to zero after 200 h.Secondly,low-cost hole transport layers(YT-MPF and YT-FF)were designed and synthesized by modified peripheral substituents to further reduce the energy loss and improve the performance of the device.The modification of peripheral groups regulated the electrostatic surface potentials(ESP).The YT-MPF with one fluorene group on each peripheral arm exhibited alternatively positive and negative ESP distribution,facilitating the efficient holes extraction and transport.Moreover,the prepared YT-MPF film was smooth and uniform with smaller roughness,which was conducive to reducing interface defects.Besides,YT-MPF showed larger hydrophobicity,which were beneficial to the improvement of phase stability.The Cs Pb I₂Br solar cell based on YT-MPF achieved a PCE of 16.0%and a V_oc of 1.29 V,which was superior to the traditional hole transport layers Poly[bis(4-phenyl)(2,4,6-tri Methylphenyl)a Mine](PTAA,14.7%,1.23 V)and 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene(Spiro-OMe TAD,14.7%,1.27 V).In addition,the device based on YT-MPF still maintained 77%of the initial PCE after 500 h in air with 25%relative humidity.Thirdly,the hydrophobic ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate(BMIMPF₆)was used to interact with Cs Pb I₂Br in the bulk of perovskite films.The addition of BMIMPF₆ decreased the energy of Cs Pb I₂Br and thus inhibited[Pb X₆]^4-octahedral tilting.Therefore,the phase stability of Cs Pb I₂Br was greatly improved and the black cubic phase of Cs Pb I₂Br films still remained after storage 7 months in air.Besides,the reduced structural distortion and the released lattice strain after BMIMPF₆ addition were conducive to improving phase stability.Therefore,the Cs Pb I₂Br solar cells with BMIMPF₆ achieved a PCE of 16.2%with excellent stability.The unencapsulated devices with BMIMPF₆ remained 98.9%,88.6%and 99.5%of the initial PCE after storage in N₂ for 1200 h,in air for 1000 h(30%relative humidity)and 200 thermal cycles(25-100?),respectively.Finally,two-dimensional Cs-based perovskites(BA₂Cs_n-1Pb_nI_3n+1(n=1,2,3,4,5))were prepared by introducing the long chain organic cation(Butylamine⁺,CH₃(CH₂)₃NH₃⁺,BA⁺)to modulate the intrinsic phase stability.The introduction of BA⁺released the lattice strain of n=?crystal plane,and in combination with the hydrophobicity,steric hindrance effect of BA⁺as well as the low formation energy of inorganic layer in two-dimensional layered perovskites,which were beneficial to suppressing[Pb X₆]^4-octahedral distortion,thus inhibiting phase transition and improving the intrinsic stability.The two-dimensional Cs-based perovskite films showed excellent phase stability in air with 45%relative humidity and in N₂ at 85?.Besides,the phase stability of two-dimensional Cs-based perovskite films increased with the decrease of n.Finally,two-dimensional Cs-based PSCs with a PCE of 9.5%were obtained(n=5).In this dissertation,the strategies including the introduction of ionic liquids BMIMBF₄,peripheral substituents modified hole transport materials on the surface of Cs Pb I₂Br layer;and BMIMPF₆,long chain organic cation BA⁺in the bulk of Cs Pb I₂Br films were adopted to improve the phase stability and passivate the defects of Cs-based perovskites,providing guidance for further improving the PCE and stability of Cs-based PSCs.