Chapter 3: Origin of Strong Cathodoluminescence and Fast Photoresponse from Embedded
3.3. Results and Discussion
3.3.6. Low-Temperature Photoluminescence Study
To investigate the origin of high PL QY of Per NPs, we further performed the temperature- dependent PL measurements. Fig. 3.9(a,b) shows the PL spectra of N1P2 and bulk samples, respectively, in the temperature range 15-300 K. From the low-temperature PL analysis, we can estimate two important parameters: exciton-phonon scattering, and exciton binding energy, which play important roles in the high PL QY of the NPs.24 It is clear from the figure that the PL peak intensity is decreased and a gradual blue shift of the peak is observed as the temperature is increased from 15 to 300 K. In general, for semiconductors, with an increase in the temperature, the bandgap decreases, and a red shift is expected due to enhanced electron-phonon interaction caused by increased phonon population and weak contribution of thermal expansion.24 Note that perovskite materials show the unusual blue shift of PL emission, i.e., the bandgap increases with increasing the temperature.24, 25 Yu et al. argued that in perovskite semiconductor, the contribution of electron-phonon interaction is negligible and the dominant contribution of lattice thermal expansion with positive temperature coefficient results in the increase in the bandgap with the decreasing temperature.26 Fig. 3.9(c) shows the variation of the integrated PL intensity with temperature for Per NPs and the Per film. The integrated intensity of each PL spectrum has been normalized by PL intensity at room temperature (300 K) for easy comparison. At higher temperatures, the nonradiative recombination channels are more active, which results in the decrease in PL intensity. In the low-temperature region (<50 K), a more rapid change in PL intensity for the Per film is associated with the low exciton binding energy/defects related to PL quenching in the bulk film. For the Per film, the integrated PL intensity at 15 K is enhanced by 57 times as compared to that at 300 K, while for Per NPs, it is enhanced only by 33 times. Thus, the PL spectra of the Per film show high quenching of PL at room temperature due to the high density of nonradiative trap sites, which are inactivated at low temperatures, causing a higher intensity PL.
71 | S t r o n g C L , P L & P h o t o d e t e c t i o n o f E m b e d d e d P e r o v s k i t e N P s
Fig. 3.9: (a) Temperature-dependent PL spectra of Per NPs confined on NW (N1P2) (b) Comparison of temperature- dependent PL spectra of CH3NH3PbBr3 film. The inset shows the variation of PL intensity in temperature range 220- 300K (c) Variation of normalized integrated PL intensity with the temperature of sample N1P2 and Bulk. (d) Variation of FWHM of PL peak with temperature for sample N1P2 and Bulk film. (e, f) Arrhenius plot of samples N1P2 and Bulk film, respectively.
In contrast, Per NPs show much lower quenching of PL at room temperature due to the lower contribution of nonradiative recombination centers. Temperature-dependent PL line width was measured for explaining the exciton-phonon scattering. Fig. 3.9(d) shows the comparison of
temperature-dependent FWHM of PL peaks for Per NPs and Per film in the temperature region 120-300 K. Note that the low-temperature PL spectra show a single peak in the temperature range 300-120 K, while below 120 K, each spectrum splits into multiple peaks. For a better comparison of the evolution of FWHM with temperature for the two samples, the FWHM at each temperature is normalized to the FWHM at 300 K. For the Per film, the increase in FWHM with temperature is faster than that of Per NPs. The higher broadening in the Per film is due to the enhanced carrier- phonon scattering and nonradiative decay of excitons. We further investigated the exciton binding energy of Per NPs and Per film from the temperature-dependent integrated PL intensity using the Arrhenius equation
(3.1)
where I0 is the integrated PL intensity at very low temperature, A is a constant, Eb is the exciton binding energy, and kB is the Boltzmann constant. Fig. 3.9(e,f) shows the variation of integrated PL intensity with the inverse of temperature (1/T) for N1P2 and Per film samples, respectively.
The data points are well-fitted with the Arrhenius equation (3.1), and the obtained exciton binding energy for Per NPs is ∼113.1 meV, while that of Per film is ∼70.2 meV. The higher Eb value of Per NPs originates from the quantum confinement of carriers in the NPs. The lower Eb in Per films implies thermal escape of carriers and dissociation of excitons more easily than that in Per NPs.
Thus, at room temperature, the nonradiative decay process due to the thermal dissociation of excitons is less probable for Per NPs, which results in the high PL QY. In the low-temperature region (<80 K), the PL spectra of Per NPs and Per film contain multiple peaks. PL emission with multiple peaks that appear for organic-inorganic lead halide perovskite in the low-temperature PL measurements has remained a controversial issue. The origin of multiple-peak PL emission in low- temperature region is mostly explained by the coexistence of different phases of CH3NH3PbBr3, emissions from defect states, and emission from bound and free excitons. Depending on the temperature, CH3NH3PbBr3 crystals show three different phases, orthorhombic (<145 K), tetragonal (145-237 K), and cubic (>237 K), which result in PL peaks with different energies.
Chen et al. reported that the different sub-peaks of CH3NH3PbBr3 originate from bound and free excitons.25 In our case, three peaks are observed for the bulk film at low temperature, which may be related to the intrinsic emission and defect-related emission peaks. In the case of Per NPs,
73 | S t r o n g C L , P L & P h o t o d e t e c t i o n o f E m b e d d e d P e r o v s k i t e N P s
additional peaks are observed, which may be related to the coexistence of multiple phases of the Per nanocrystals. The defect contribution is expected to be lower in Per NPs. However, at low temperature, radiative and nonradiative peaks both contribute to the PL spectrum, and thus, the additional peaks appear in the spectrum.