Temperature-dependent photoluminescence in hybrid iodine-based perovskites film

Lead halide perovskite has attracted much attention due to its high absorption coefficient, long carrier diffusion length, low binding energy, and low cost. The stability of intrinsic crystal structure in I-based perovskite can be theoretically estimated by calculating cubic structures factor and octahedral factor. Experimental methods to solve the stability of structure in I-based perovskite could be mainly to either incorporate anions (e.g. Cl–, Br–) or mix cations (e.g. Cs+) into I-based perovskite matrix. Moreover, incorporating Br– into I-based perovskite leads its band gap to widen, which might be used as a top-cell material to tandem solar cell. However, in order to understand photo-physics process of anion-mixed and/or cation-mixed perovskites, it is essential to further investigate the optical properties such as absorption spectrum, photoluminescence (PL), temperature-dependent PL (TPL) behavior, etc. In this work, anion-mixed and/or cation-mixed perovskite thin films with high quality crystallization and (110) prereferral orientation are synthesized by one-step solution method. All mixed perovskite films are characterized by using X-ray diffraction (Rigaku D MAX-3C, Cu-Kα, λ = 1.54050 Å) and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific Escalab 250Xi). A set of strong peaks of the mixed perovskite films at 14.12° and 28.48°, is assigned to (110) and (220) lattice plane of orthorhombic crystal structure of I-based perovskite, due to preferred orientation. The Pb 4f and I 3d doublet peaks, corresponding to Pb+2 and I– states, are observed in XPS spectra. It should be noted that in the absence of other valence states of Pb and I component at lower/upper binding energy, the chemical element composition ratio of Pb+2 and I– are close to stoichiometric proportion. For optical absorptionspectra, the optical bandgaps of the perovskite films increase with doping concentration of Br– increasing. For TPL, the perovskite films with x = 0 and x = 0.05 show abnormal red-shifts in a temperature range from 10 to 100 K. The following blue shifts in a temperature range from 125 to 350 K emerge, which is mainly attributed to band gap widening. However, incorporating more Br– into I-based perovskite leads the TPL spectra to monotonically blue-shift. A linear relationship between the TPL peak position and the doping concentration of Br– ions is observed at the same temperatures. This indicates that the Br– anion in I-based perovskite plays a crucial role in determining the optical properties. The low-temperature and high-temperature (HT) excitonic binding energy at x = 0 are 186 meV and 37.5 meV, respectively. The HT excitonic binding energy first increases and then decreases with the Br– concentration in I-based perovskite film increasing. The minimal variation of TPL peak position and FWHM (full width at half maximum) at x = 0.0333 are 13 nm and (25.8 ± 0.5) meV, respectively, suggesting higher temperature stability in optical property. This should contribute to understanding the relationship between temperature-dependent electrical and optoelectronic performance for hybrid mixed perovskite materials and devices.