Shallow trap mediated temperature-dependent exciton and Mn2+ photoluminescence in CsPbCl3:Mn2+ nanocrystals

Abstract

As an effective means to endow lead halide perovskite nanocrystals (NCs) with new properties, ion doping has been widely applied. Transition metal Mn-doped CsPbCl3 is of particular interest, as the Mn doping introduces new emission bands, improves photoluminescence quantum yield (PLQY), and even enhances stability. However, it is still insufficient to catch insight of the energy transfer process and luminescence characteristics of these high defect tolerance NCs doped with Mn ion. Here, we systematically studied the energy transfer from exciton to Mn2+ ions in Mn-doped CsPbCl3 NCs by variable temperature steady-state fluorescence spectra and time-resolved spectra. The abnormal attenuation behavior of the exciton and Mn2+ with the change of temperature was observed, from which we predict the existence of shallow traps near the conduction band and these shallow traps will affect the luminescence properties by influencing the energy transfer process. We show a simple physical model invoking only shallow trap states that uniquely describe the temperature-dependent energy transfer between the exciton and the Mn2+ ion. The thermometric performance (30–290 K) was studied in terms of relative thermal sensitivity, in which the calculated relative sensitivity range is ∼0.12%–1% K−1.

Polar-Solvent-Free Sonochemical Synthesis of Mn(II)-Doped CsPbCl3 Perovskite Nanocrystals for Dual-Color Emission

“…During this process, exciton recombination decreased, resulting in gradually decreased band edge emission and improved radiative recombination from the 4 T 1 level to the 6 A 1 energy level, leading to higher PL intensity from Mn 2+ ions and the thermal activation of vibronic hot bands. 45,59 Such a trend is known as negative thermal quenching, which is usually observed in semiconducting materials. 45,60−62 When the temperature is further increased from 320 to 400 K, thermal quenching reduced the carriers located in the excitonic state, resulting in the decreased Mn 2+ PL intensity.…”

“…63 Meanwhile, the blue shift observed in Mn 2+ emission with increasing temperature was caused by the thermal expansion of the host (CsPbCl 3 ) lattice due to perturbation (k B T), resulting in a declined crystal field strength. 45,59 The d−d parity-forbidden transitions are greatly affected by the change in crystal field strength. Owing to phonon−electron coupling, the full-width half maxima (FWHM) value of the Mn 2+ emission peak broadened when the temperature is increased.…”

“…45,60−62 When the temperature is further increased from 320 to 400 K, thermal quenching reduced the carriers located in the excitonic state, resulting in the decreased Mn 2+ PL intensity. 59,63 This trend is due to the loss of ligands on the PNC surface, and it leads to the formation of defect states in PNCs. These defects act as nonradiative recombination centers, which can trap the photogenerated excitons or carriers.…”

“…These observed shifts in peak positions and changes in PL intensities as a function of temperature corroborated earlier findings. 41,45,59 From the temperature-dependent PL spectral profiles, the exciton binding energy (E b ) and exciton−photon scattering coefficients were determined. Figure 5b,c shows the variation in the integrated PL intensity of the exciton peak in pristine and 11.8% Mn-doped PNCs as a function of inverse temperature (1/T).…”

“…Based on temperature variation, three different PL trends were observed: (i) the PL intensity of the exciton exceeded that of dopant (Mn 2+ ) emission at low temperatures; (ii) it decreased gradually when the temperature ranged from 80 to 400 K, whereas the dopant (Mn 2+ ) exhibited two distinct temperature-dependent PL regimes, that is, PL intensity increased in the regime of 80 K ≤ T ≤ 320 K and decreased for temperatures above 320 K; (iii) the peak positions of exciton and dopant (Mn 2+ ) emission were blue-shifted as the temperature is increased from 80 to 400 K. At low temperatures (≤80 K), due to the competition between band-edge exciton recombination and ET from exciton to dopant (Mn 2+ ), excitonic recombination dominated the ET to Mn 2+ ions, causing the host to exhibit higher band-edge emission and rendering Mn 2+ sensitization inefficient. , When the temperature increased from 80 to 320 K (80 K ≤ T ≤ 320 K), thermal perturbations ( k B T ) increased and the carriers located at the excitonic states of the host (CsPbCl 3 ) or the carriers trapped in shallow states with low ionization energy received extra thermal energy ( k B T ) to transit directly to another energy level, which is likely to be a thermally excited 4 T 1 energy level of Mn ions. During this process, exciton recombination decreased, resulting in gradually decreased band edge emission and improved radiative recombination from the 4 T 1 level to the 6 A 1 energy level, leading to higher PL intensity from Mn 2+ ions and the thermal activation of vibronic hot bands. , Such a trend is known as negative thermal quenching, which is usually observed in semiconducting materials. ,− When the temperature is further increased from 320 to 400 K, thermal quenching reduced the carriers located in the excitonic state, resulting in the decreased Mn 2+ PL intensity. , This trend is due to the loss of ligands on the PNC surface, and it leads to the formation of defect states in PNCs. These defects act as nonradiative recombination centers, which can trap the photogenerated excitons or carriers.…”