Interface- and Temperature-Sensitive Linear Electric Field Effects on Exciton Absorption of CH₃NH₃PbI₃ Perovskite Films

The influence of applied electric field (FA) on the absorption spectra of a methylammonium lead tri-iodide (MAPbI3) crystalline film sandwiched between a fluorine-doped tin oxide (FTO) layer and a polymer film of poly(methyl methacrylate) (PMMA) (Sample I) and sandwiched between a compact layer of titanium oxide (cp-TiO2) and a PMMA film (Sample II) has been investigated by first harmonic modulation spectroscopy on various temperatures in the range of 290–60 K. The linear electroabsorption spectra, EA(1f), of the MAPbI3 film in Sample I showed very different behaviors in the temperature range above and below 200 K. EA(1f) spectra show a shape similar to the second derivative of the exciton absorption band having a Gaussian profile, and switching between positive and negative signs is generated at 290 K on alternating polarity of FA. With decreasing temperature, the same tendency of FA polarity-dependent EA(1f) switching was maintained until ∼200 K. At temperatures below 200 K, the inversion of the EA(1f) signal was found with the same field direction of FA, and the signal intensity increased with decreasing temperature. In sample II, the polarity-dependent EA(1f) signal was negligibly small at room temperature but became prominent with decreasing temperature and followed the same trend as that in Sample I observed at temperatures below 200 K, indicating that the substrate on which the MAPbI3 film was coated played a conclusive role as the origin of the polarity-dependent EA(1f) spectra. The second-derivative-like shape of the interface- and temperature-sensitive EA(1f) spectra is interpreted in terms of the polarity-dependent linear Stark shift of the exciton absorption band whose peak position depends on the temperature and substrate on which the MAPbI3 film is coated

Illumination Power-Dependent Electroabsorption of Excitons in a CH₃NH₃PbI₃ Perovskite Film

Electroabsorption (E-A) spectra of methylammonium lead triiodide (MAPbI3) are found to depend on the power density of the illumination light (IL-D) in both tetragonal and orthorhombic phases when the modulation frequency (M-F) of the applied electric field (F) is low. As IL-D increased, the E-A intensity decreased and the shape of the E-A spectra altered from that similar to the second derivative of the exciton absorption band having a Gaussian profile to the one similar to the first derivative, when the M-F of F is low. When the M-F of F is high, E-A spectra were independent of the power density of illumination. Orientational polarization of excitons is considered to be induced by F having a low M-F with strong photoirradiation, following ion migration of MA+ and I– along the applied field direction enhanced by photoirradiation

Temperature-Dependent Electroabsorption and Electrophotoluminescence and Exciton Binding Energy in MAPbBr₃ Perovskite Quantum Dots

Organic–inorganic lead halide perovskite nanocrystals have attracted much attention as promising materials for the development of solid-state light-emitting devices, but the existence of free or bound excitons or the formation of trap states remains under debate. We recorded the temperature-dependent electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra, that is, electric-field-induced change in absorption and photoluminescence spectra, for methylammonium lead tribromide (MAPbBr3) colloidal perovskite nanocrystals, that is, quantum dots (QD), doped in a poly­(methyl methacrylate) film in the temperature range of 40–290 K. Based on the results, the binding energy of the exciton (electron–hole pair) was estimated. The exciton binding energy of QD of MAPbBr3 estimated from the absorption and E-A spectra (∼17 meV) is nearly the same as that of a MAPbBr3 polycrystalline thin solid film, while the exciton binding energy estimated from the temperature-dependent PL spectra (∼70 meV) is much greater than that estimated from the absorption profile. The frequency dependence of the E-A intensity observed at 40 and 290 K for the modulated applied electric field indicates a slow ion migration in nanocrystals, which follows the modulation of the applied electric field at a frequency less than 500 Hz. The observed E-A spectra were analyzed with an integral method on assuming the Stark effect; the magnitudes of the changes in electric dipole moment and polarizability following photoexcitation were determined at each temperature from 40 to 290 K. E-PL spectra show that the PL of QD of MAPbBr3 is quenched on the application of an external electric field; the extent of quenching is much greater for trap emission than for exciton emission. Exciton–phonon scattering, which is responsible for the line broadening of the PL spectra, is also discussed based on the temperature-dependent PL spectra

Temperature-Dependent Electric Field-Induced Optical Transitions of 2D Molybdenum Disulfide (MoS₂) Thin Films: Temperature-Dependent Electroabsorption and Absorption

Two-dimensional (2D) layered MoS2 nanosheets (NSs) possess many unique properties and hold great potential for various applications. Herein, MoS2 NSs were synthesized by a hydrothermal method. The as-synthesized MoS2 NSs are crystalline and layered. Absorption and electroabsorption (E-A) spectra of MoS2 doped in a poly­(methyl methacrylate) (PMMA) thin film were measured at different temperatures (290–40 K). The E-A spectra detected at the second harmonic of the modulation frequency of the applied electric field were analyzed using an integral method by considering the Stark effect as a dominant feature. The absorption spectra consist of seven transitions, among which five transitions are contributed to the E-A spectra. It is found that the changes in the electric dipole moment and polarizability of each transition determined at different temperatures increase substantially with decreasing temperature. Electronic resonance states identified for low-energy excitonic bands of MoS2 NSs showed prominence E-A signals. The study is essential to understand the electronic structure in the photoexcited state, which is important for applications of MoS2 NSs to optoelectronic devices

Inhomogeneous Photoluminescence Characteristic in Carbon Nanodots and Electrophotoluminescence Measurements

Photoluminescence (PL) spectra, time-resolved PL spectra, and PL decay profiles have been observed for carbon nanodots (CDs) with different excitation wavelengths in an embedded solid film and in solution. PL excitation spectra have been also observed with different monitoring wavelengths. Then, it is found in both solid film and solution that not only the location of the PL spectra but also the peak of the excitation spectra show a significant red shift, as the excitation and monitoring wavelengths become longer, respectively, indicating that the emitting states of the excitation-dependent PL are the real state to which direct absorption occurs from the ground state, not the transient trapped states produced by photoexcitation. It is shown that not only the excitation-dependent PL but also the excitation-independent PL with a peak at ∼375 nm exist. The lifetimes of both PL emissions are very sensitive to the surroundings. Multiple emitting states that give excitation-dependent PL are ascribed to the inhomogeneous properties in prepared carbon nanodots, which is supported by the fluorescence lifetime image measurements. Electrophotoluminescence spectrum, that is, the electric-field-induced change in PL spectrum, has also been observed for the excitation-independent PL of CDs embedded in a poly­(methyl methacrylate) film, and the magnitude of the change in electric dipole moment and molecular polarizability following emission has been determined