Corrugated 2D antimony‐halide perovskites such as Cs3Sb2Cl9 (CSC) are promising candidates for single‐source white‐light emission due to their ultra‐broadband spectra. However, CSC has a serious luminescence quenching phenomenon due to inadequate confinement of excitons. By the homovalent substitution of trivalent antimony cation Sb3+ by a small amount of trivalent rare earth (RE) cations RE3+, the photoluminescence intensities from high‐quality Cs3(Sb1−xREx)2Cl9 (CSRC) (RE = Ce, Sm, Nd, Y, Er, etc.) films at room temperature (RT) are over two orders of magnitude higher than that of CSC film. Especially, the photoluminescence quantum yield (PLQY) for the Cs3(Sb0.995Er0.005)2Cl9 film is 9.5% at RT, which is much higher than A3B2X9 perovskites previously reported for single‐source white‐light lighting. Furthermore, the Cs3(Sb0.995Er0.005)2Cl9 film exhibits an ultra‐broadband emission with the full width at half maximum reaching 554 meV at RT, resulting in a “warm” white‐light with the CIE coordinate (0.33, 0.46) and the correlated color temperature of 5450 K. The PLQY enhancement can be considered as the fact that a high activation energy by bandgap widening effect and Type‐I‐like “straddling” band alignment between Cs3Sb2Cl9 and Cs3Er2Cl9 lead to reducing nonradiative losses and increasing radiative recombination channels. Meanwhile, the spectral broadening can be considered to be attributed to strong effect of electron–phonon interaction.
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<sup>–</sup>, Br<sup>–</sup>) or mix cations (e.g. Cs<sup>+</sup>) into I-based perovskite matrix. Moreover, incorporating Br<sup>–</sup> 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, <i>λ</i> = 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<sup>+2</sup> and I<sup>–</sup> 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<sup>+2</sup> and I<sup>–</sup> are close to stoichiometric proportion. For optical absorptionspectra, the optical bandgaps of the perovskite films increase with doping concentration of Br<sup>–</sup> increasing. For TPL, the perovskite films with <i>x</i> = 0 and <i>x</i> = 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<sup>–</sup> 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<sup>–</sup> ions is observed at the same temperatures. This indicates that the Br<sup>–</sup> 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 <i>x</i> = 0 are 186 meV and 37.5 meV, respectively. The HT excitonic binding energy first increases and then decreases with the Br<sup>–</sup> concentration in I-based perovskite film increasing. The minimal variation of TPL peak position and FWHM (full width at half maximum) at <i>x</i> = 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.
The chlorine-based organometallic halide perovskite (Cl-OHP) film with a (001)-preferred orientation and good crystallization has been synthesized by a hybrid sequential deposition process. The photoluminescence and absorption spectra of the Cl-OHP film in the blue light region have been investigated at operating temperatures ranging from 10 to 350 K. The Cl-OHP film shows a strong exciton-related emission of which the exciton binding energies at low temperature and high temperature are 136 meV and 41 meV, respectively. It is found that the blueshift from excitonic luminescence is initially observed at temperature below 175 K, and then, the redshift occurs from 175 to 350 K. Meanwhile, the bandgap of the Cl-OHP film widens with the increase in operating temperature. The nonmonotonous shifts on the emission peak energy are attributed to the competition between the Stokes effect and bandgap widening. This should contribute to the understanding of photophysical processes in Cl-OHP materials and devices.
Broadband emission of 1400–2100 nm is achieved in Er–Tm codoped ZnO (ETZO) films at an excitation of photon energies higher than the band gap of ZnO. Room temperature (RT) and temperature-dependent photoluminescence (PL) spectra of the films annealed at different temperatures, together with their corresponding structural characterization results, reveal that the defect states of ZnO play an important role in the energy transfer (ET) processes for the broadband emission. At low temperatures, an intensive defect-related peak at 1938 nm concomitantly with a much-enhanced broadband emission was observed in the PL spectra. A dominant ET channel from ZnO to Tm3+ ions by the recombination of the defect-related state is suggested. The important role of the defect states for the broadband emission is further confirmed. Moreover, RT electroluminescence is also realized in the ETZO films with an Al/Ni/ETZO/p-Si/Al device structure, and a similar broadband emission spectrum was observed, illustrating the same luminescence mechanism as that in the PL under above-band-gap excitation. These results pave the way for the practical application of ETZO films as infrared broadband optical amplifiers and light emitters.
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