Study Of Optical Properties Of Two-dimensional Perovskite Crystals And Films

Using two-dimensional perovskite of（PEA）₂（MA）_n-1[Pb_nI_3n+1]（PEA=C₆H₅（CH₂）₂NH₃,MA=CH₃NH₃）,we study optical properties of crystals and films of（PEA）₂Pb I₄（shortened as PEPI）and（PEA）₂（MA）₂Pb₃I₁₀（shortened as PMPI₃）.Moreover,we also assemble the 2D/3D stacking structures in order to increase the stability of the three-dimensional perovskite film.The experimental results are summarized following:（1）Single crystals of PEPI and PMPI₃are prepared by cooling crystallization method,and they are characterized and optically tested respectively.Photoluminescence（PL）experiments at different temperatures and different excitation intensities have shown that crystal impurities and defects will affect the luminescence spectra,especially at low temperatures and at high excitation intensities.A micro-area PL spectrum measurement system is used to study the unstripped PEPI crystal.It is found that the full width at half maximum（FWHM）of the PL spectrum at the grain boundary is larger than that measured from other positions,with smaller emission peak energy.Furthermore,the PL of mechanically stripped PEPI and PMPI₃ crystals show that the defects and impurities are mainly on the crystal surface.（2）Films of PEPI and PMPI₃ are prepared by one-step solution method,and the effects of temperature and excitation light intensity on PL of perovskite crystals are studied.We found that the PMPI₃film is a mixed perovskite with a variety of different phases（n=1,2,3）.At low temperatures,the behavior of the PMPI₃film is consistent with that of PEPI;on the other hand,at room temperature,the behavior of PMPI₃ film is more similar to the three-dimensional perovskite.This 2D-3D transformation may be the reason why the two-dimensional perovskite of the heterogeneous mixture can be used as the active layer of solar cells.（3）Three-dimensional perovskite MAPb I₃ films are prepared using one-step anti-solvent method,and PEA or PEAI solutions are furtherly used to react with the MAPb I₃ film at a varity of times,in order to obtain a 2D/3D stacked structure.The results show that only the stability of MAPb I₃ film reacted with PEAI solution with 30 seconds is improved.The experiment proves that the formation of a two-dimensional perovskite cover layer on the surface of three-dimensional one can improve the stability of the film,however,this method may also destroy the surface integrity of the film resulting in the acceleration of degradation.