Study On Design,Synthesis And Photovoltaic Performance Of Layered Ruddlesden-Popper Perovskite Materials

The power conversion efficiency of three-dimensional（3D）perovskite solar cells has acheieved 25.5%.Unfortunately,the instability of 3D perovskites against moisture,oxygen,and light has hindered their commercial applications.Recently,two-dimensional（2D）perovskites with the multiple-quantum well structure formed the alternating arrangement of inorganic sheets and organic interlayers have attracted much attention due to their high humidity stability.Compared to 3D perovskites,2D perovskites show poor photovoltaic performance due to a large exciton-binding energy,a wide bandgap and high trap density states.In this thesis,we focus on the design and synthesis of new 2D perovskites,controlling phases of layered Ruddlesden-Popper（RP）perovskites and the preparation of new 2D/3D perovskites,aiming to obtain high-efficieny and stable perovskite solar cells.（1）A novel organic cation is designed to reduce the dielectric confinement effect caused by the mismatch of dielectric constants between inorganic and organic layers,and thus decreases the exciton-binding energy.We synthesized 2-（4-fluorophenyl）ethylammonium cation（F-PEA⁺）by substitution the element on the 4-position of the phenyl components in the prototypical H-PEA⁺cation with the‘electron-pulling’fluoro group.Theoretical calculations show the dipole moment of F-PEA⁺increased by 1.33 D compared to that of H-PEA⁺.The increase in cation polarity can regulate the dielectric constant of organic layers.By the fitting of the temoerature dependent steady-state photoluminescence spectrum of the perovskite films,the exciton-binding energy of F-PEA₂MA₂Pb₃I₁₀ decreases by 30 me V compared to that of H-PEA₂MA₂Pb₃I₁₀.Moreover,the F group could increase the N-H···πhydrogen bond in organic cations,and reduce the interaction between organic cations and[Pb I₆].The decrease in the distortion of inorganic sheets contributes to red-shift absorption.In addition,the halogen group could enhance the interaction of organic cations in layer spaces,which benefits the crystallinity of the perovskite film.The planar structured perovskite solar cell based on F-PEA₂MA₂Pb₃I₁₀ shows over 90%enhancement comparing to the control reference of H-PEA₂MA₂Pb₃I₁₀.（2）In layered RP perovskite films,the low-n phases（n=1,2）could act as traps to capture charge carriers easily.The second work proposes an approach,i.e.,additive induced like Ostwald ripening,to reduce the low-n phases in the resulted RP perovskites.The additive,as ethylene glycol（EG）for example,has higher boiling points but less volatile than the host solvent DMF,and could selectively dissolve organic components.After spin-coating at room temperature,EG interactes with Cl-PEA⁺of low-n phase perovskites via the hydrogen bond.During a following annealing,EG induces a dissolution-recrystallization process in which the low-n phases recrystallize into the high-n phases（n≥3）and finally form a more uniform composition based on Ostwald ripening-like model.Thermal admittance spectroscopy indicates defect densities of the（Cl PEA）₂MA₃Pb₄I₁₃ device descrease by an order of magnitude after EG treatment.Finally,the PCE of the device improved by near 40%after EG treatment.（3）Polycrystalline films with multiphases and disordered crystal orientations naturally brings dense defects at grain boundaries.The third work shows that chiral 2D perovskite S-MBA₂Pb I₄（（S）-（-）-α-methylbenzylamine,S-MBA）as seed-like sites modulates the growth of MAPb I₃ and passivates the grain boundary of the 3D perovskite.The steric hindrance caused by the methyl group and chirality of S-MBA⁺is conducive to the formation of the phase pure 2D perovskite（n=1）.Moreover,the"symmetry breaking"of the chiral perovskite S-MBA₂Pb I₄ could induce the ordered crystal orientation of MAPb I₃.Space charge limited current measurement indictes the defect density of 2D/3D perovskite S-MBA-MAPb I₃ has reduced by nearly 50 percent compared to that of quasi 2D perovskite PA-MAPb I₃(1,3-diaminopropane,PA²⁺).The devices based on S-MBA-MAPb I₃ shows a near30%enhancement of PCE compared with the reference based on PA-MAPb I₃.