Chapter 5: Origin and Tunability of Dual Color Emission in Highly Stable Solid-State
5.3. Results and Discussion
5.3.5. Temperature-Dependent Photoluminescence Study
123 | T u n a b i l i t y o f D u a l C o l o r E m i s s i o n i n M n D o p e d C s P b C l3 N C s
related emission, which almost vanishes after annealing. Thus, the annealing of the samples improves the structural properties of the samples and yields higher PL QY.
Based on the aforementioned results and literature report, the emission model of CsPbxMn1- xCl3 NCs is illustrated through the energy band diagram shown in Fig. 5.8.3, 14 Excited by 360 nm light, the CsPbCl3 host absorbs the photons and emits light at ~410 nm via the radiative recombination of excitons between the ground state and the excited state of CsPbCl3. Besides the radiative recombination, a nonradiative relaxation process also exists, which leads to energy loss through hole traps or electron traps resulting in low PL QY for the undoped sample. The Mn doping in CsPbCl3 NCs generates a new recombination pathway of excitons via the energy transfer from the excited state of host CsPbCl3 to Mn2+ and subsequent radiative recombination, which generates orange-red emission, as shown in Fig. 5.8.3, 4, 15 Only the excitons retaining enough thermal activation energy facilitate an intersystem crossing process and finally emit around 590 nm emission, showing a new excitons recombination pathway. This well-known recombination pathway of the d-d transition of Mn2+ ions is termed 4T1-6A1 transition.3, 16 In case of high doping concentration, defect-related emissions are observed, as denoted by the dashed lines in the band diagram.
Fig. 5.8: Schematic of the energy band diagram and PL emission mechanisms of CsPbxMn1-xCl3 NCs. The horizontal dashed lines represent the defect-related energy levels.
temperatures are scaled appropriately to enable a better comparison. Note that the temperature- dependent PL measurement was performed on a film of the NCs deposited on a Si substrate. Fig.
5.9(a) clearly shows that with the decrease in temperature, the intensity of the near band edge excitonic PL peak increases systematically. The increase in PL intensity with decreasing temperature is very common in semiconductors, which is mainly due to reduced carrier trapping and lower thermal quenching of PL at lower temperatures. At higher temperatures, the nonradiative recombination channels are more active, and excitons may be dissociated, which results in a decrease in PL intensity. Fig. 5.9(b) depicts the comparison of excitonic and Mn related PL peak intensity at different temperatures.
Fig. 5.9: (a) Temperature-dependent PL spectra of 3% Mn-doped CsPbCl3 NCs film. PL spectra at different temperatures are scaled up appropriately to enable better comparison. (b) The evolution of excitonic PL intensity (left Y-axis) and Mn related PL intensity (right Y-axis) with temperature for Mn3. (c) Deconvoluted PL spectra of Mn related peak at temperatures 80K and 350K. (d) Variation of integrated PL intensity of excitonic peak with the inverse of temperature and its fitting with the Arrhenius equation. The inset shows the variation of FWHM of the excitonic peak with temperatures and its Boson fit.
Interestingly, the intensity of Mn related emission peak decreases gradually with the decrease in temperature from 350K to 80K. The unusual temperature dependence of Mn2+ PL peak can be
125 | T u n a b i l i t y o f D u a l C o l o r E m i s s i o n i n M n D o p e d C s P b C l3 N C s
explained by the competitive recombination pathways between the near band edge recombination and 4T1 state to 6A1 state transition in Mn2+ after energy transfer from the host CsPbCl3.4 When we increase the temperature from 80 K to 350 K, the photoexcited carriers located at the excitonic state of CsPbCl3 get extra thermal energy (kBT) to transit directly to 4T1 state in Mn2+ ions. Thus the enhanced radiative recombination from 4T1 state to 6A1 state results in the higher intensity of Mn PL.4 Thus, the Mn related state shows negative thermal quenching behavior, which is beneficial for it practical applications. At low temperature, the more rapid excitonic recombination than the energy transfer to Mn2+ state is responsible for the low-intensity Mn-related PL as compared to the high band edge emission, as shown in Fig. 5.9(a). With the increase in temperature, the excitons dissociate and the excitonic recombination slows down and the energy transfer to the Mn2+ becomes more dominant, which results in the higher intensity of Mn PL at 350 K. From Fig. 5.9(a), it is clear that with the decrease in temperature, the Mn PL peak is partly quenched and red-shifted. The redshift of Mn-related PL peak with decreasing temperature is attributed to the decrease in crystal-field strength produced by thermal expansion of the host lattice and the thermal activation of vibronic hot bands.4 This type of redshift is also observed in Mn- doped ZnS and ZnSe NCs.17 Fig. 5.9(c) shows the deconvoluted PL spectra related to Mn doping at temperature 80K and 350 K. At high temperature, the decrease in weightage of the PL peak at higher wavelength (lower energy) reveals its nonradiative nature.
From the low-temperature PL spectra, we can also estimate two important parameters: (a) exciton binding energy (Eb) and (b) exciton-phonon scattering coefficient. We calculated the exciton binding energy of 3% Mn-doped CsPbCl3 NCs from the temperature-dependent integrated PL intensity of the excitonic peak, using the modified Arrhenius equation. Fig. 5.9(d) shows the variation of integrated PL intensity of excitonic peak with the inverse of temperature (1/T) fitted with the Arrhenius equation. The obtained Eb is 62.8 meV.18 Note that the low-temperature PL measurement was performed on a film of the NCs deposited on silicon substrates, where the spontaneous aggregation of the NCs may partly quench the PL intensity. Thus, the actual exciton binding energy in individual NCs may be higher than the above-calculated value. This relatively low exciton binding energy is consistent with the decrease in excitonic emission and an increase in Mn related emission at higher temperatures.
The inset of Fig. 5.9(d) depicts the variation of FWHM of excitonic PL peak with temperature.
It is clear from Fig. 5.9(d) that the FWHM of PL peaks is increased with increasing temperature.
The higher broadening with the increase in temperature is due to the enhanced exciton-phonon scattering and nonradiative decay of photogenerated excitons in perovskite NCs.19 The PL peak broadening is fitted using the Boson model with the following equation. 20, 21
/ (5.1)
where is the inhomogeneous broadening constant due to exciton-exciton scattering and σ is the exciton-acoustic phonon coupling coefficient. corresponds to the exciton-longitudinal optical phonon coupling coefficient or the Fröhlich coupling coefficient, while is the optical phonon energy. From the fitted data, the obtained parameters are as follows: = 91.9 meV, σ = 127.6 μeV/K, = 47.9 meV, and = 26.1 meV. These parameters indicate a strong exciton-phonon interaction in the doped CsPbCl3 NCs, and it leads to broadening of excitonic PL peak when the temperature is increased from 80 to 350 K.21 The obtained parameter is consistent with the reported value of optical phonon energy using Raman scattering experiment for CsPbCl3,which showed a
of 27.6 meV.22