Synthesis,Photophysical Properties,and Stability of Organic-Inorganic CH3NH3PbI3 Perovskite Semiconductor

Author:Rubn Segovia

Supervisor:gao bo


Degree Year:2019





Methylammonium lead iodide CH3NH3PbI3(MAPbI3),a hybrid organic-inorganic semiconductor from the perovskite family,is destined to be one of the most important semiconductors in the optoelectronic industry of the near future,in particular for next generation photovoltaic solar cells.Its easy and low-cost synthesis through solution at low temperature without any need of sophisticated equipment make this material ideal for manufacturing at industrial-scale.Due to its hybrid composition,MAPbI3 exhibits properties of inorganic semiconductors such as high absorption coefficient and large electron–hole diffusion lengths.On the other hand,it presents low exciton binding energy and large Bohr radius common in Wannier-Mott exciton type and typical of organic semiconductors.In addition,MAPbI3 shows the potential like polymer-organic semiconductors to combine with flexible substrates,extending its application to flexible electronics,in contrast with rigid conventional semiconductors such as Si,Ge,or CdSe.Nevertheless,as a relatively new material,MAPbI3 still has many characteristics and behaviours that are not well understood.In particular,degradation and stability due to ambient factors such as humidity and temperature fluctuation are still important issues of this semiconductor affecting its optical and electronic properties,thereby further investigations are necessary.The work presented in this thesis mainly studies the structural and photophysical properties of different MAPbI3 morphologies.In addition,the impact of long term humidity exposure and thermal cycling on the structural and photophysical properties of MAPbI3 microwires is also investigated.Moreover,a novel crystallization process for obtained arrays of microwires is presented.First,3D MAPbI3 crystals with diverse shapes and sizes such as films,wires,rods,plates,flower-like,leaf-like,sheaf-like,and low-dimensional nanoparticles were prepared through different crystallization processes.Besides,a dynamic crystallization method for obtaining oriented and well-ordered MAPbI3 microwires is developed.This technique is a novel crystallization process based on the evaporation of the solvent while the substrate is moving forwards and backwards in one direction,promoting the assembling and growth of well-ordered microwires parallel to the motion direction.The obtained crystals were structural characterized by AFM,SEM,and XRD,showing different surface topography along with different crystalline unit cell dimensions.The molecular vibration properties were studied using Raman spectroscopy,presenting similar spectra all the morphologies studied.The spontaneous emission was studied by means of photoluminescence(PL).The PL peak position varies for the different crystal shapes,which illustrates the morphology-dependent emission of MAPbI3.Moreover,the influence of microstructure on the vibrational and emission properties was investigated by analyzing various Raman and PL spectra measured at various positions along the same crystal.All the crystal morphologies studied presented a homogeneous chemical composition,in contrast with the PL emission that varies along the same crystal from the same sample,which indicates spatial-dependent recombination mechanisms in MAPbI3.In addition,we investigated the degradation of MAPbI3 microwires caused by laser light and humid air.We show that high intensity laser light induces MAPbI3decomposition into the precursor materials methylammonium iodide(CH3NH3I)and lead iodide(PbI2).While long exposure to humid air induces a redshift in wavelength of the PL emission.Raman and XRD measurements suggest that the redshift of the PL was originated from structural disorder caused by the incorporation of H2O molecules in the crystal lattice,and by radiative recombination through moisture-induced subgap energy trap states.Finally,with the intention to mimic a thermic fluctuation condition under which MAPbI3-based optoelectronic devices could operate,we studied the PL emission behavior of MAPbI3 microwires under thermal cycling.The microwires were compelled to follow thermal cycles from 40oC(tetragonal crystal phase)to 80oC(cubic crystal phase),which induced a recursive tetragonal-to-cubic and cubic-to-tetragonal crystal structure transition.The results indicate a thermal cycling-dependent PL and a gradual crystalline structure deformation due to a reiterated change on the microwires crystal lattice,resulting on a variation of the electronic bandgap along the heating-cooling process.Thermal cycling-reflectance measurements corroborated the alteration of the electronic band structure.