Synthesis And Spectroscopic Studies Of Hybrid CH₃NH₃PbBr₃ Perovskites

Organic-inorganic hybrid perovskite semiconductors with three-dimensional crystal structure ABX₃(A=CH₃NH₃,CH(NH₂)₂,B=Pb,Sn,X=I,Br,Cl)have been widely used in the frontiers of diverse optoelectronics.Meanwhile,aided by a set of advanced steady and transient spectroscopic techniques,investigation deep into the underlying optoelectronic superiorities of these semiconductors has been largely advanced.Despite this fact,consensus with respect to the most basic static spectroscopic features of any certain prototypical hybrid metal halide perovskite remains hardly achieved.This is mainly due to the apparent diversities in the observed spectroscopic results that rely largely on the material quality in terms of crystallinity and defect levels.In this thesis,we took a closer look at the static and time-resolved spectroscopies of one of the most studied prototypical hybrid metal halide perovskites,CH₃NH₃PbBr₃,both in its polycrystalline thin-film and sizable single crystal(serval millimeter scale)forms.In brief,measurements including the surface morphology characterization,UV-Vis absorption,static and time-resolved photoluminescence were conducted and the results were analyzed according to the fully matured theoretical essentials of semiconductor optics as listed below.By the aid of X-ray diffraction,both the polycrystalline CH₃NH₃PbBr₃ thin film and the CH₃NH₃PbBr₃ single crystal were confirmed to be of cubic crystal phase.The surface roughness analysis by AFM revealed that the single crystal shows atomically flat pristine surfaces.In contrast,the polycrystalline thin film,composed of different size nanocrystals with much enlarged surface-volume rations,shows apparently discontinuities and abundant grain boundaries.In general,semiconductors characterized by direct bandgap like CH₃NH₃PbBr₃ demonstrate light absorption spectrum of two components as a function of the absorbed photon energies according to the Elliott theory,i.e.,a sum of discrete excitonic contributions below the band gap and continuous inter-band absorption above the band gap.By fitting the absorbance with the Elliott function,the two-component contributions were decoupled,in the meantime,the band gap(E_G)and exciton binding energy(E_B)were extracted.In particular,E_G=2.20 eV and E_B=10 meV were determined for CH₃NH₃PbBr₃ single crystal while E_G=2.40 eV and E_B=40 meV for CH₃NH₃PbBr₃polycrystalline thin film.Apart from the discrete excitonic effect that lead to augmented absorption below the band gap,the intensity of the inter-band absorption above the band gap was also largely enhanced beyond that expected for free-carrier absorption by the Coulombic enhancement factor?.This explains why the inter-band absorption of CH₃NH₃PbBr₃ is enhanced in a much stronger way in its polycrystalline thin-film form over its large single-crystal form.In addition,the absorption of the single crystal has a very sharp band edge,yielding an Urbach energy of E_U?18.7 meV.The static photoluminescence of the CH₃NH₃PbBr₃ single crystal exhibits two components as function of photon energies.In this scenario,one PL-component peaking at 581 nm with relatively high intensity is attributable to the luminescence from the crystal bulk,while the other one peaking at 545 nm with relatively lower intensity is attributable to the luminescence from the surface state.Interestingly,the short-wavelength PL-component demonstrated by the high-quality single crystal overlaps well with the mono-component PL demonstrated by the polycrystalline thin film characterized by extraordinarily large surface-volume ratios.The asymmetric line-shape of the long-wavelength PL-component was also elaborated with respect to the steepness feature of the band gap absorption,which further corroborates that this component originates from the crystal bulk.The transient fluorescence decay of CH₃NH₃PbBr₃ single crystal phase has the characteristic of two-phase decay,which is related to its small excitonic binding energy.Driven by the thermodynamic energy K_BT(?26 meV)at room temperature,the excitons generated by photoexcitation first dissociate rapidly,forming the multi-particle electron-hole plasma,and then exhibits the single exponential decay kinetic characteristic.The fluorescence decay of the polycrystalline film does not exhibit biphasic characteristics,which indicates that the excitons produced in the polycrystalline film do not dissociate into new excited phase when the binding energy of the exciton is greater than the thermodynamic energy at room temperature,the fluorescence decay of polycrystalline films can be well fitted by triple exponential function under the influence of exciton pairing,Coulomb binding and electrostatic shielding effect.