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Spectroscopy and Photochemistry; General Theory 3

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Temperature-Dependent Photoluminescence of CHNHPbBr Perovskite Quantum Dots and Bulk Counterparts

Hee Chul Woo, Jin Woo Choi, Jisoo Shin, Sang-Hyun Chin, Myung Hyun Ann, and Chang-Lyoul Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01593 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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The Journal of Physical Chemistry Letters

Temperature-Dependent

Photoluminescence

of

CH3NH3PbBr3 Perovskite Quantum Dots and Bulk Counterparts Hee Chul Woo,† Jin Woo Choi, † Jisoo Shin,† Sang-Hyun Chin,†‡ Myung Hyun Ann,† and ChangLyoul Lee*,† †

Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology

(GIST), Gwangju 61005, Republic of Korea. ‡

Department of Physics, Research Institute of Physics and Chemistry (RINPAC), Chonbuk

National University, Jeonju, 54896, Republic of Korea. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Organic-inorganic lead halide perovskite is emerging as a potential emissive material for light emitting devices, such as, light emitting diodes (LEDs) and lasers, which has emphasized on the necessity of understanding its fundamental opto-physical properties. In this work, the temperature-dependent photoluminescence of CH3NH3PbBr3 perovskite quantum dots (QDs), polycrystalline thin film (TF) and single crystal (SC) have been studied. The opto-physical properties, such as exciton-phonon scattering, exciton binding energy and exciton decay dynamics, were investigated. The exciton-phonon scattering of perovskite is investigated which is responsible for both PL linewidth broadening and non-radiative decay of excitons. The exciton binding energy of QDs, TF and SC were estimated to be 388.2 meV, 124.3 meV, and 40.6 meV, respectively. The observed main exciton decay pathway for QDs is the phonon assisted thermal escape while that for TF and SC was the thermal dissociation due to low exciton binding energy.

TOC GRAPHIC


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After the significant improvement in photovoltaic device performances1,2, organic-inorganic lead halide perovskite (CH3NH3PbX3, X = Cl, Br, and I) has received an ever increased attention towards its use as emissive layer in light emitting diodes (LEDs) due to its outstanding optical and

electrical

properties,

such

as

narrow

full-width

half-maximum

(FWHM)

of

photoluminescence (PL), high PL quantum yield (PLQY), high charge mobility and easy tunable spectrum from visible to near infrared through halide composition change.3-6 Nevertheless, low exciton binding energy of bulk (3D) perovskite leads to easier exciton dissociation, which limits their radiative recombination.7 Therefore, the perovskite polycrystalline thin film (TF) with small grain size has been utilized to spatially confine excitons and improve the rate of radiative recombination.8,9 For conventional semiconductors, such efforts to further reduce grain size and confine excitons have led to the development of quantum dots (QDs), such as PbS, ZnS, CdS and CdSe, which have the lowest possible dimension (0D).10,11 Similarly, recent research interests have also got extended to perovskite QDs, because of their use as emissive layers in LEDs.12,13 The perovskite QDs show narrow FWHM of PL (~10 nm) and high PLQY (~80%) due to their quantum confinement effect, which enable high color purity and brightness, respectively.14,15 The passivation of perovskite QDs with long alkyl-chain surfactants can improve both the material stability and PLQY by reducing the surface defect density.16 Recently, Yan et al. have reported highly efficient CH3NH3PbBr3 perovskite QDs based green LED which shows the highest external quantum efficiency of 12.9 %.17 However, despite the recent remarkable progress of device performances in the perovskite QDs-LEDs, the necessity of understanding its fundamental emission characteristics is being constantly emphasized. In general, the emission characteristics of semiconductors are closely related to two important parameters, the exciton-phonon scattering and exciton binding energy, which play a key role for


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high PLQY. The exciton-phonon scattering, related to the defect states, serves as a non-radiative decay pathway of excitons18 while higher exciton binding energy allows the formation of stable excitons, which contributes to radiative recombination.19 Therefore, both reducing the excitonphonon scattering and enhancing the exciton binding energy are of great importance for realizing high efficient perovskite LEDs.12 In addition, understanding of the radiative and non-radiative exciton decay dynamics provides the fundamental basis for the improvement of perovskite LEDs performances. In this regard, the systematic investigation on the excitons in perovskite in terms of exciton-phonon scattering, exciton binding energy and their decay dynamics would provide fundamental insights on the optical properties of perovskites. In this work, we have determined the exciton-phonon scattering and exciton binding energy of CH3NH3PbBr3 perovskite QDs through temperature-dependent PL experiment in the temperature range of 20-300 K and compared it with those of bulk perovskites, polycrystalline thin film (TF) and single crystal (SC). The factors mainly affect the non-radiative exciton decay dynamics of perovskite, have been investigated through the temperature-dependent experiments. The QDs are synthesized by the well-known precipitation method20 and deposited onto the glass substrate. The TF and free-standing SC are also prepared following the techniques mentioned elsewhere.21,22 The mean size of prepared QDs (< 10 nm) and the average grain size of TF (< 50 nm) were verified by electron microscopy (Figure S1). First of all, we have made a qualitative analysis of temperature-dependent PL which reveals several notable features, such as continuous blue shift of PL emission peaks with increasing temperature, relatively more stable PL emission of QDs compared to those of two bulk perovskites, high PL energy of QDs due to quantum confinement effect and phase transition of perovskite etc. Even though the qualitative analysis can provide an insight on opto-physical properties of CH3NH3PbBr3 perovskites, additional


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quantitative analysis is necessary for the better understanding of fundamental optical properties. Thus, the temperature-dependent PL results have been further interpreted via theoretical approach. The broadening of the PL linewidth has been quantitatively studied, which is strongly dependent on the exciton-phonon scattering. The exciton binding energies for QDs and other two perovskites are also estimated. The carrier recombination dynamics and non-radiative decay pathway of the excitons in CH3NH3PbBr3 perovskites have been further analyzed via theoretical investigations. Our experimental results with qualitative and quantitative interpretations provide comprehensive understanding of fundamental opto-physical properties of perovskite with different dimension, QDs, TF and SC.

Figure 1. Pseudo-color maps of temperature-dependent PL spectra of (a) CH3NH3PbBr3 perovskite QDs, (b) TF and (c) SC. The comparison of (d) PL emission peak energy and (e) integrated PL intensity as a function of temperature. The integrated PL intensity was normalized to the initial intensity. (f) The multi-peak PL emission of TF and SC at low temperature.


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The pseudo-color maps of temperature-dependent PL spectra of CH3NH3PbBr3 perovskites with different dimension, QDs, TF and SC, in the temperature range of 20-300 K are represented in Figure 1(a)-(c). The qualitative observation of experimental results provides several notable features, such as continuous blue shift of PL emission with increasing temperature for all three perovskites, which is counter-intuitive, and relatively stable PL intensity of QDs compared to two other perovskites in the whole temperature range. In addition, the higher PL emission energy in QDs than those of two other perovskites is a typical evidence of quantum confinement effect originated from their low dimension.23,24 The comparative normalized PL spectra of three perovskites for the whole temperature range are shown in Figure S2. Figure 1(d) represents the evolution of PL emission peak maximum of the three perovskites. PL spectra of all three perovskites were continuously blue shifted with the increase in temperature from 20 to 300 K. In general, the temperature dependence of semiconductor energy band gap, Eg, is a consequence of contributions from both electron-phonon interaction and the thermal expansion of the lattice.25,26 As the temperature increases, the Eg decreases due to enhanced electron-phonon interaction caused by increased phonon population and weak contribution of thermal expansion in most semiconductors.27,28 However, perovskites show the unusual blue shift of PL emission, i.e., widening of the Eg, upon increase in temperature. This cannot be explained by Varshini model28 which is often used for describing the typical red shift of the PL emission with increasing temperature in standard tetrahedral semiconductors. Recently, such unusual blue shift of the PL emission has also been reported in several semiconductors, such as lead salt semiconductors (PbS, PbSe and PbTe),26,29 copper(I) halides (CuX, X = Cl, Br, and I)30 and some ternary chalcopyrites containing Ag and Cu.31 Lee et al. have addressed this unusual blue shift of PL by valence band maxima (VBM) and conduction band minima (CBM).32 In


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contrast to conventional semiconductors, both VBM and CBM of ABX3 lead halide perovskites are formed by B ns - X mp anti-bonding orbitals and B np – X ms anti-bonding orbitals.33 Therefore, the thermal expansion of lattice results in slight potential energy changes in VBM and CBM leading to the widening of the Eg.32 Meanwhile, Yu et al. claimed that the contribution of electron-phonon interaction is negligible and the dominant contribution of lattice thermal expansion with positive temperature coefficient results in the widening of the Eg.34 The origin of blue shift of PL with increasing temperature is still under debate. Therefore, further investigations are strongly recommended for a better understanding of its fundamental origins. Figure 1(e) shows the temperature-dependent variation of the integrated PL intensity of CH3NH3PbBr3 perovskites. Each PL spectrum has been normalized at its initial PL intensity for easy comparison. The SC shows the exponential decrease of integrated PL intensity upon temperature increase. The TF also shows rapid decrease in integrated PL intensity but not as fast as that of the SC. It is generally suggested that rapid decrease of integrated PL intensity in both SC and TF resulted from low exciton binding energy, Eb, and/or the defects related to exciton quenching. Low Eb results in easier exciton dissociation, and thereby generates more of electrons and holes in the perovskites. In addition, CH3NH3PbBr3 perovskite SC has relatively long exciton diffusion length (> 17µm)35 due to their higher crystallinity. Therefore, for perovskites with higher crystallinity, once the charges are dissociated, they diffuse well through crystal domains and the rate of radiative recombination decreases. Since the SC has better crystallinity than that of TF, the PL intensity decreases more rapidly. On the other hands, the QDs show different temperature-dependent PL quenching characteristics compared to those of other two perovskites. The PL intensity is slightly increased in the low temperature region < 70 K, which is often observed in conventional semiconductor QDs, such as


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CdSe, CdTe, PbS etc.36-38 The integrated PL intensity of QDs reached the maximum at T = 70 K and thereafter it decreased. This enhancement of the integrated PL intensity indicates that there is a thermally activated redistribution of trapped carriers in the interfacial states between QDs and capping surfactants. As the temperature increases, the carriers trapped in the interfacial states overcome the shallow energy barriers and fall into the ground state through radiative recombination, which is added to the total PL intensity. When the temperature was further increased, the integrated PL intensity began to decrease, but the QDs still maintained about 40% of the initial integrated PL intensity even at RT. In addition, the rate of integrated PL intensity decrease upon temperature increase is much slower for QDs than the other two perovskites. The high PL stability of QDs even at RT is attributed to either good surface passivation and/or high exciton binding energy. Among three perovskites prepared in this study, only QDs have been synthesized with additional surfactants, which passivate their surfaces and prevent them from being aggregated. As a result, the surface defects of QDs, which act as non-radiative trap sites with deep trapping energy states, are well passivated by surfactants, and thus the non-radiative decay of the excitons is efficiently suppressed. In addition, the QDs must have higher exciton binding energy than TF and SC, which allows the radiative recombination of excitons at relatively high temperature. Multi-peak PL emission is another controversial issue which often appears in the low temperature PL experiments of organic-inorganic lead halide perovskite. Figure 1(f) shows the normalized PL of CH3NH3PbBr3 perovskites in the low temperature range of 20-80 K. The multi-peak PL emissions were observed for both TF and SC, but not for the QDs. The origin of such multi-peak PL emission with different energies is often explained by the phase coexistence18,39,40 and the emission from defect states.41 It is theoretically well known that three


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different crystal phases, orthorhombic (236.9 K),42 are available in CH3NH3PbBr3 perovskite at the temperature range of 20~300K. However, the explanation of multi-peak PL emission by the phase coexistence due to imperfection of the phase transition at the low temperature is excluded here, because the evolution of PL peak positions upon temperature increase is inconsistent with it. For example, the SC shows clear dual PL emission at low temperature (~20 K). The PL emission intensity at low energy decreases as temperature increases and disappears at ~80 K while the PL emission at high energy is continuously blue shifted. In addition, when the temperature further increases, a sudden red shift of PL emission at high energy is observed at ~140 K (Figure 1(d)) due to the phase transition from orthorhombic to tetragonal.43 Therefore, the PL emission at low energy and high energy is attributed to tetragonal and orthorhombic phases, respectively.43 Thus, if the multi-peak PL emission resulted from the phase coexistence, the PL intensity at low energy should have to be increased rather than becoming absent and PL emission at high energy was supposed to disappear when temperature increased from 20 to 80 K. From this result, it is assured that multi-peak PL emission at low temperature is not originated from the phase coexistence. It is assumed that the PL at low energy is attributed to emission from bound excitons which were trapped at shallow energy defects before recombination while the PL at high energy is from free excitons. Since, the density of defect states is limited and are filled as the temperature increases, relative PL intensity from bound exciton compared to that of free exciton decreases. The same explanation can be applied for the TF. As the temperature increases from 20 to 80 K, the PL from bound excitons with low energy disappears and that from free excitons survives. The power-dependent PL of TF and SC at 20 K were recorded to confirm the origin of multi-peak PL (Figure S3). The PL of both samples were normalized to the peak PL


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intensity at high energy. Based on the fact that the density of defect states is limited, the relative intensity of PL at low energy is expected to decrease as the excitation laser intensity increases. As expected, the relative PL intensity at low energy for both SC and TF decreased and this result clearly reflects that the PL at low energy is from bound excitons. Figure S3 (c) and (d) shows the time-resolved photoluminescence (TR-PL) for both SC and TF which gives more direct evidence that strengthens our hypothesis explaining the origin of multi-peak PL. To determine the exciton lifetime of low and high energy PL, TR-PL curves were fitted by bi-exponential function and the average exciton lifetime was estimated. As a result, the average exciton lifetime of PL at low energy for SC was ~ 55.14 ns (~ 53.46 ns for TF) which is much longer than that of PL at high energy, ~ 28.19 ns (~ 17.15 ns for TF). The prolonged exciton lifetime of PL at low energy is attributed to the defect related recombination process. In contrast, the QDs show single peak PL with continuous blue shift throughout the whole temperature range. This might imply that the QDs do not undergo notable phase transition upon temperature changes and have high crystallinity without appreciable defect states which produce PL emission from bound excitons. Such a high phase stability could be explained by the sizedependence of phase transition. Recently, Li et al. has reported that thicker perovskite microplate shows easier phase transition compared to thinner one in the same temperature range.40 The zerodimensional QDs are spatially quantized to every direction, which means that it retains the thinnest form. The surface passivation not only increases the crystallinity of the QDs but also limits the freedom of ion redistribution to some extent.44 In addition, it also reduced the density of defect states in the surface of the QDs. As a result, the QDs are structurally very stable, which results in no phase transition throughout the whole temperature range and no PL emission from bound excitons. On the other hands, 3D SC is totally opposite compared to the QDs. It has high


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freedom of ions redistribution in every direction, which allows easier phase transition. Therefore, a sudden red shift of PL emission due to transition between orthorhombic and tetragonal phases45 has been observed at 140 K. In addition, no surface passivation induced much defect states in the surface of SC, which generate the PL from bound excitons. In this respect, it is expected that TF might be somewhere in the middle between SC and QDs. However, in our experiment, the abrupt red shift of PL peak emission in TF was not observed in the whole temperature range and this is attributed to limited dimension of TF compared to the SC. The PL linewidth broadening in temperature dependent experiments has long been used for elucidating the phonon scattering in the conventional semiconductors.18,45,46 Figure 2(a) shows the FWHM, Γ, of the PL spectra of three CH3NH3PbBr3 perovskites as a function of temperature. Since the intrinsic phonon broadening originated from the acoustic and optical phonons is required for analyzing the origin of FWHM boarding, only the single peak PL spectra from the temperature-dependent experiment have been used for extracting the FWHM values and multipeak PL spectra at low temperature have been excluded.46 Following equation describes the FWHM broadening with temperature change47 ߁ሺTሻ = ߁଴ + ߁௔௖ + ߁௅ை = ߁଴ + ߛ௔௖ ܶ + ߛ௅ை ܰ௅ை ሺܶሻ

(1)

where Γ0 is a temperature-independent inhomogeneous broadening term, which is determined by material size, shape and composition.48 The Γac and ΓLO are homogeneous PL emission broadening terms, which result from the acoustic and optical (LO) phonon scattering, respectively. The coefficient γac and γLO represent the weight of exciton-phonon coupling strengths. The NLO(T) is the Bose-Einstein distribution function which describes the occupation numbers of the respective LO phonons, given by, ܰ௅ை ሺܶሻ = 1/ൣ݁ ாಽೀ /௞ಳ ் − 1൧, where the ELO is


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phonon energy. The FWHM of each PL spectrum has been fitted to equation 3 and the resultant values of the Γ0, γLO and ELO for perovskites are listed in Table 1.

Figure 2. (a) The FWHM of the steady-state PL spectra as a function of temperature for CH3NH3PbBr3 perovskites. (b) Estimated ∆FWHM for the comparison of exciton-LO phonon scattering. Table 1. Extracted PL linewidth broadening parameters. sample

γ0 / meV

γac / meV

γLO / meV

ELO / meV

QD

68.3

6.3

80.1

24.9

TF

39.6

0.23

161.9

31.2

SC

33.2

0.24

1375.1

52.7

Given the fitting results, the acoustic phonon scattering was almost negligible for all three perovskites with γacT ≪ γLONLO(T) (Figure S4). Therefore, the broadening of PL emission predominantly resulted from the LO phonon scattering rather than the acoustic phonon scattering. The best-fitted values of γLO for the QDs, TF and SC were 80.1 meV, 161.9 meV and 1.3751 eV, respectively. Smaller γLO of the QDs is attributed to quantum confinement effect, which is previously well reported for various QDs both theoretically49 and experimentally.36,50 The phonon energy, ELO, involved in LO phonon scattering also increased from 24.9 meV to


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31.2 meV and 52.7 meV for QDs, TF and SC, respectively. Smaller ELO value of the QDs implies that much phonons are produced, which would serve as a scattering center for nonradiative decay of excitons. To clarify the scattering strength of LO phonon for all three perovskites, the γLONLO values are plotted as a function of temperature in the Figure 2(b). This result demonstrated that the QDs show minimum FWHM broadening, which assures high color purity despite of high occupation numbers of LO phonons while both TF and SC show stronger FWHM broadening upon temperature increase due to their high γLO values. To have better insight on non-radiative relaxation dynamics of excitons in perovskites, the temperature-dependent PL intensity is quantitatively analyzed. Several exciton relaxation pathways, such as Auger non-radiative recombination,51 energy transfer between QDs,52,53 multiphonon relaxation,54 thermal escape of carriers54,55 and thermal dissociation of excitons54,56 are considered as non-radiative PL quenching processes in the perovskites. In this experiment, the incident laser (400 nm) had the photon fluence, jp of 2x108/cm2 per laser pulse with the intensity and repetition rate of 4 mW/cm2 and 40 MHz, respectively. The CH3NH3PbBr3 perovskite is known to have high absorption coefficient of ~ 105/cm at 400 nm.57 Then, the simple calculation gives absorbed carrier density, N0 of ~2x1013/cm3 which is small enough to rule out the bimolecular or Auger recombination in bulk perovksites.58 To find out whether the Auger nonradiative recombination matters or not in the QDs, the N0 is experimentally estimated by plotting the PL intensity as a function of laser intensity (Figure 3). The PL intensity curves were fitted with the functional form, I ∝ 1 − ݁ ିேబ ,59 and resultant N0 values were 0.22 and 0.12 at RT and 20 K, respectively, which are small enough to ignore the non-radiative recombination via Auger recombination60 in the QDs. The energy transfer between the QDs can also be ignored because the PL emission for both solution and thin film were identically same.48 This implies that the PL


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mainly come from the isolated QD. Multi-phonon relaxation denotes a non-radiative exciton decay via consecutive multiple phonon emissions, which is the one of the common PL quenching processes for small Eg semiconductor. If the Eg of semiconductors can be bridged by ~5 phonons, multi-phonon relaxation could compete with radiative recombination.54 However, our perovskite samples, having Eg within 2.2~2.45 eV with ELO values of several tens of millielectron volts, require at least 50 consecutive multiple phonons for such process. Consequently, multi-phonon relaxation is not favorable in the perovskites. Therefore, only thermal escape of carriers and exciton dissociation are two possible non-radiative relaxation pathways of excitons in the QDs. For the perovskite TF and SC, because of the continuum of energy bands, the thermal escape via stepwise absorption of several LO phonons was ignored. Thus the exciton dissociation is the only available exciton quenching process.

Figure 3. PL intensity of the QDs as a function of laser intensity at (a) RT and (b) 20 K. Generally, the PL intensity of semiconductors per unit time can be expressed as ‫ܫ‬௉௅ ሺ‫ݐ‬ሻ =

௡ሺ௧ሻ ఛೝೌ೏

=

௡బ ఛೝೌ೏

݁ ି௧/ఛ

(2)

where n0 is the initial carrier population and τ is the temperature-dependent PL lifetime. Concerning previously discussed carrier dynamics, τ can be expressed as following equation,


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ଵ ఛ

=ఛ

ଵ ೝೌ೏

+ఛ

ଵ ೌ೎೟

+ ሺఛ

ଵ ೐ೞ೎

ሻ

(3)

where 1/τrad, 1/τact and 1/τesc represent the radiative recombination rate, thermal dissociation process rate and the thermal escape rate, respectively. Note that, the rate of thermal escape is only included for the QDs. The thermal dissociation and thermal escape rate are given by, ଵ ఛೌ೎೟

ଵ

= ఛ ݁ ିா್ /௞ಳ ் , ೌ

ଵ ఛ೐ೞ೎

ଵ

= ఛ ሺ݁ ாಽೀ /௞ಳ ் − 1ሻି௠ బ

(4)

where Eb is the activation energy for thermal dissociation process and kB is the Boltzman constant. 1/τ0 and 1/τa are the rate constants and m represent the number of LO phonons involved in the thermal escape of carriers. Then, the integration of equation 2 without thermal escape term gives ஶ

ே

బ ‫ܫ‬௉௅ ሺܶሻ = ‫׬‬଴ ‫ܫ‬௉௅ ሺ‫ݐ‬ሻ݀‫= ݐ‬ ଵା஻௘ షಶ್ /ೖಳ ೅

(5)

where B is a fitting parameter representing τrad /τ and Eb is the activation energy for exciton dissociation or exciton binding energy. For bulk organic-inorganic lead halide perovskite, it is already well known that various trap sites can exist in the material such as surface defects induced traps, ion vacancies due to the ion migration, etc.61 Therefore, another term considering the activation energy for bound excitons at such trap sites must be added to equation 5 that writes ஶ

‫ܫ‬௉௅ ሺܶሻ = ‫׬‬଴ ‫ܫ‬௉௅ ሺ‫ݐ‬ሻ݀‫= ݐ‬

ேబ

ଵା஺௘ షಶೌ /ೖಳ ೅ ା஻௘ షಶ್ /ೖಳ ೅

(6)

here, A is a fitting parameter and Ea is the activation energy for trap sites. The exciton binding energy and the activation energy for trap sites with other related parameters can be estimated by fitting integrated PL intensity of TF and SC with equation 6. For the QDs, however, the


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integrated PL intensity cannot be simply fitted by equation 8 due to the PL intensity enhancement below 70K. The contribution of thermally activated trapped carriers in the interfacial states between the QDs and surfactants is attributed to this PL intensity enhancement. Then, the thermal escape of carriers through quantized energy levels have to be considered, as a result, the equation 5 is modified as follows,37 ஶ

ே ାே ᇲ ௘ ష∆ಶ/ೖಳ ೅

బ బ ‫ܫ‬௉௅ ሺܶሻ = ‫׬‬଴ ‫ܫ‬௉௅ ሺ‫ݐ‬ሻ݀‫= ݐ‬ ଵା஻௘ షಶ್ /ೖಳ ೅ ା஼ሺ௘ ಶಽೀ /ೖಳ ೅ ିଵሻష೘

(7)

where N′0 and ∆E denotes the density of interface trap states and its activation energy, while C is the fitting parameter. Figure 4(a)–(c) represents the integrated PL intensity of three perovskites as a function of 1/T. The integrated PL intensity curves were fitted using equation 6 for TF and SC and equation 7 for QDs with corresponding ELO values which is extracted previously through fitting of the temperature dependent Γ. The best-fit values of Ea, Eb, ∆E and m for perovskites are listed in Table 2


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Figure 4. Integrated PL intensity as a function of 1000/T for (a) QDs, (b) TF and (c) SC. The non-radiative decay strength comparison for (d) QDs, (e) TF and (f) SC. Table 2. Exciton binding energy and other parameters. sample

Ea / meV

Eb / meV

m

∆E / meV

QD

-

388.2

1.3

15.1

TF

8.4

124.3

-

-

SC

9.9

40.6

-

-

Thus, from the results stated above, it can be clearly explained why the QDs show higher PL intensity (PLQY) than those of TF and SC. The QDs show much higher exciton binding energy (Eb) of 388.2 meV than TF and SC which have Eb of 124.3 meV and 40.6 meV, respectively. The resultant Eb are well in consistent with previously reported values.8,19,20,62 Higher Eb of the QDs guarantees the formation of highly stable excitons even at RT, which is originated from the


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quantum confinement effect due to their small sizes. In this respect, it can be expected that TF which has smaller crystal size with many grain boundaries will show higher Eb than that of SC because the generated excitons are more efficiently confined in small crystal grains. Given the Eb values for all three samples, exciton Bohr radius (ab) which determines the quantum confinement effect can be estimated. For semiconductor QDs, the quantum confinement effect23,24 on PL can be described as, ଷℏమ గమ

‫ܧ‬௉௅ = ‫ܧ‬௚ + ∗ మ − ‫ܧ‬௕ ଶ௠ ௗ

(7)

where EPL is the optical band gap energy of QDs determined from PL peak energy, Eg is the intrinsic band gap energy of the bulk perovskite and

, m* and d are the reduced Planck

constant, the effective mass of carriers and the size of QDs in nano-meter, respectively. In this equation, EPL and Eg can be replaced by the PL peak energy of QDs (2.31 eV) and SC (~2.27 eV) at 20 K. The effective mass of carriers is calculated to be ~0.081m0, where m0 is the mass of a free electron in vacuum. This result is well in accordance with previously reported effective mass of CH3NH3PbBr3 QDs.24,63 To further confirm the quantum confinement effect, the exciton Bohr radius, ab* has been estimated using the following equation, ܽ௕ ∗ = ߝ௥

௠బ ௠∗

ܽ௕

(9)

where εr is the relative permittivity of CH3NH3PbBr3 perovskite found to be ~16 from the literature64 while ab is Bohr radius of hydrogen in ground state (0.053 nm). Owing to the small effective mass of carriers, the resultant exciton Bohr radius is calculated to be ~10.5 nm. Thus, the excitons confined in 2ab* (~21 nm) should experience strong quantum confinement effect. In


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this regard, the PL from excitons in most of our QDs and some of small grains in TF show quantum confinement effect. Meanwhile, the average interfacial trap state energy between QDs and surfactants (∆E), which is responsible for the slight increase in PL intensity of the QDs below 70K, is estimated to be 15.1 meV. Since the surface-to-volume ratio is inversely proportional to the size of materials, it is expected that the QDs have higher surface defect density than other two perovskites. From the high PLQY and high stability of the QDs, however, it is assumed that most of surface defect states are well passivated by surfactants. Nevertheless, simultaneous production of interfacial traps between the QDs and surfactants during the synthetic and purification processes is inevitable and they contribute to slight increase in PL intensity. In contrast, in spite of their low surface-to-volume ratio compared to QDs, the TF and SC also have the surface defects and the calculated activation energy, Ea were 8.4 and 9.9 meV, respectively. The slight increase in PL intensity of QDs has been further investigated by temperaturedependent TR-PL measurement (Figure S5). For SC and TF, the average exciton lifetime continuously decreases as the temperature increases from 20 K to 150 K. However, in the same temperature range, the average exciton lifetime of QDs starts from 2.01 ns at 20 K, reaches its maximum, 2.12 ns at 70 K and thereafter decreases to 1.65 ns. This result is similar to what we have witnessed in the integrated PL intensity of QDs. The enhancement of PL intensity with prolonged exciton lifetime implies that there is an additional radiative recombination process at 70 K. The major difference between QDs and the others is the existence of surfactants that passivate the material surface. Thus, we attribute the slight increase in PL intensity of QDs at low temperature region to the carriers trapped at the interfacial states between the QDs and surfactants.


19


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It is important to clarify which factors primarily affect the non-radiative PL quenching of perovskites for opto-electronic applications. It can be revealed by comparing the non-radiative strength of exciton dissociation (‫ି ݁ܤ‬ா್/௞ಳ ் ), thermal escape of charge carriers (‫ܥ‬ሺ݁ ாಽೀ /௞ಳ ் − 1ሻି௠ ) and exciton quenching through trap states (‫ି ݁ܣ‬ாೌ/௞ಳ ் ) of perovskites. Figure 4 (d)-(f) describe the non-radiative decay strength of perovskites. For both TF and SC, bound excitons thermally overcome the activation energy (Ea) predominantly decayed (Figure 4 (e)-(f)) at lower temperature under 170 K (100 K for SC). However, as temperature increases, excitons overcome the exciton binding energy (Eb) and start to dissociate. Therefore, the primary non-radiative exciton decay process for both TF and SC at RT is the thermal dissociation due to their low Eb. On the other hands, the QDs show unique non-radiative exciton decay dynamics compared to other two perovskites. The main non-radiative exciton decay pathway is the phonon assisted thermal escape of carriers rather than the thermal dissociation. In the whole temperature range, the strength of thermal escape dominates the reduction of PL intensity and the strength of thermal dissociation is almost negligible (Figure 4 (d)). The high Eb of QDs prevents excitons from the thermal dissociation, it will conserve high PL intensity (PLQY) through the radiative recombination. In addition, the probability of exciton-phonon coupling, non-radiative PL quenching process, also increases as the temperature increases because the amount of stable exciton increases due to high Eb of the QDs. This result is well in consistent with our previous finding that the population of LO phonon scattering with excitons increases upon temperature increase due to low ELO of the QDs. From these results, it is concluded that the non-radiative exciton decay dynamics of perovskites QDs differs from bulk perovskite and it has to be carefully considered for various opto-electronic application of perovskites.


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In summary, we have investigated the temperature-dependent (20-300 K) PL of CH3NH3PbBr3 perovskite QDs in comparison with TF and SC. The continuous blue shift upon temperature increase have been observed for all three perovskites. For the whole temperature range, the QDs show more stable PL compared to TF and SC. The phase transition has only been observed in the SC as a sudden red shift of PL emission peak at 140 K and no obvious phase change was observed for QDs. Such high PL and phase stability of QDs must be due to their structural stability coming from high crystallinity and effective surface passivation by ligands. In addition, the QDs have shown clear quantum confinement effect with their enlarged band gap, resulting in blue shifted PL. The optical phonon scattering influences more to the PL linewidth broadening than acoustic phonon and the estimated optical phonon energy, ELO, are 24.9 meV, 31.2 meV and 52.7 meV for QDs, TF and SC, respectively. Smaller ELO of QDs implies increase in NLO, which means more of phonons serve as scattering center for non-radiative decay of excitons. The exciton binding energies, Eb of QDs, TF, and SC were calculated to be 388.2 meV, 124.3 meV, and 40.6 meV, respectively. The primary non-radiative exciton decay pathway for QDs is the phonon assisted thermal escape while that for TF and SC was the thermal dissociation due to low exciton binding energy. Higher Eb of QDs is due to the quantum confinement effect that enables the formation of stable exciton even at RT. Such high Eb of QDs is desirable for their use as an emissive material in LEDs. However, despite high PLQY by high Eb, the PL intensity of QDs also decreases upon temperature increase due to increased non-radiative exciton decay through the thermal escape. Therefore, for perovskite QDs, further study on the optimization of both Eb and LO phonon interaction is necessary for LEDs applications. Our in-depth investigation provides a fundamental insight into emission properties of organic-inorganic lead halide perovskites with different dimension.


21


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AUTHOR INFORMATION Author Contributions Hee Chul Woo and Jin Woo Choi contributed equally to this work. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning and Ministry of Education, Korea (MSIP; NRF2016R1A2B4013003) and a grant funded by GIST 2018 (Research on Advanced Optical Science and Technology). ASSOCIATED CONTENT Supporting Information. Further details on experimental methods, synthesis recipes for QDs, TF and SC, whole spectra of temperature-dependent PL for three perovskites, PL of SC, TF and QDs at RT, electron microscope images of SC, TF and QDs, comparison of exciton-phonon scattering strength are available.


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