High Excitation Intensity Opens a New Trapping Channel in Organic

Nov 7, 2016 - We investigated the excited-state dynamics of CH3NH3PbBr3 perovskite nanoparticles (NPs) and bulk crystals under various excitation inte...
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High Excitation Intensity Opens a New Trapping Channel in Organic−Inorganic Hybrid Perovskite Nanoparticles Kaibo Zheng,*,†,‡ Karel Ž ídek,§ Mohamed Abdellah,†,# JunSheng Chen,†,⊥ Pavel Chábera,† Wei Zhang,† Mohammed J. Al-Marri,‡ and Tõnu Pullerits*,† †

Department of Chemical Physics and NanoLund, Lund University, Box 124, 22100 Lund, Sweden Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar § Regional Centre for Special Optics and Optoelectronic Systems (TOPTEC), Institute of Plasma Physics, Academy of Sciences of the Czech Republic, Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic # Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt ⊥ State Key Laboratory of Molecular Reaction Dynamics, Dalian, Institute of Chemical Physics, Chinese Academy of Sciences Dalian, 116023 Dalian, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: We investigated the excited-state dynamics of CH3NH3PbBr3 perovskite nanoparticles (NPs) and bulk crystals under various excitation intensity regimes using transient absorption spectroscopy. We confirmed the subband gap hole trap states with optical transition to the conduction band in both samples. In bulk crystals, the excited-state dynamics is independent of pump intensity. However, in NPs, pronounced intensity dependence appears. At low intensities, the hole trap states do not affect the excitedstate dynamics due to the potential barrier between the photogenerated holes and the surface trap states. When the excitation density is much higher than one per NP, charge accumulation makes hot holes overcome the barrier and get trapped with electrons long living in the conduction band (≫10 ns). This explains the high emissive properties of such NPs despite the existence of surface traps. However, in the application of emitting devices requiring high excitation intensity, the surface trapping becomes significant.

H

minimize nonradiative recombination. Due to the highly ionic nature and good stoichiometry of hybrid perovskite materials, point defects are not likely to reside within the volume of the perovskite NPs.9 Instead, the trap sites should be mainly from surface defects. Particularly in MAPbBr3 NPs, the ammonium functional group of the capping agent (octadecylamine, ODA) is likely to replace the organic cation in the perovskite, causing lattice disorder at the surface region.8 Pronounced surface defect states have also been verified by X-ray photoemission spectroscopy (XPS) in our previous study.13 Contrary to expectations, the trap densities of MAPbBr3 NPs evaluated by time-resolved PL spectroscopy and single-molecule spectroscopy are all less than one per NP.14,15 Apparently, most of the traps do not influence fluorescence. In order to reveal the

ybrid organic−inorganic lead halide perovskites APbX3 (A = organic ammonium cation; X = Cl−, Br−, I−) have shown promise for solar cell devices owing to their large absorption coefficient, broad absorption band, and excellent charge transport properties.1−3 Perovskitebased solar cell efficiencies have rapidly increased from below 10% to above 20% during the past 4 years, holding the record for solution-processed photovoltaic devices.4−7 Apart from their bulk form, perovskite nanoparticles (NPs) have been recently reported such as organic−inorganic CH3NH3PbBr3 (MAPbBr3) NPs and all-inorganic CsPbBr3 quantum dots (QDs).8,9 Owing to superior photoluminescence (PL) quantum yield (QY), such NPs are suitable materials for emitting devices such as displays, light-emitting diodes (LEDs), and lasers.10−12 We have previously shown that one reason for the enhanced PL QY of MAPbBr3 NPs is their relatively high exciton binding energy compared to that of bulk materials.13 Furthermore, in order to achieve high PL QY, the concentration of the defect/trap states should be low to © 2016 American Chemical Society

Received: August 12, 2016 Accepted: November 7, 2016 Published: November 7, 2016 1154

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Figure 1. (a) GSA and TA spectra at a 2 ps delay time for BCs on a glass substrate and NPs in solution excited at 400 nm (fluence = 1 × 1013 photon/cm2, ⟨N⟩ np = 0.1) with the inset showing the optical transition. (b) TA spectra of BCs and NPs excited at 575 nm (fluence = 1 × 1013 photon/cm2, ⟨N⟩np = 0.1) with the inset showing the optical transition. (c) Early time TA kinetics at the band edge bleach (GSB1, 525 nm) and trap states (GSB2, 575 nm) of NPs with the inset showing the TA spectrum at 2 ps. (d) Schematics showing photoinduced charge generation in NPs under low excitation density.

evident that the thermalization of photogenerated hot charges is completed within this time scale. The TA spectra represent the optical response of the lowest excited states. The negative band of both spectra with minima at 530 nm for BCs and 524 nm for NPs can be assigned to the ground-state bleach (GSB), which is due to state filling of the band edge states. Accordingly, the positive signals at the blue side are due to excited-state absorption (ESA) of the photogenerated charges at the excited states. Those features are analogous to what has been reported before.17 In addition, we can also observe a pronounced negative tail below the band edge energy extending toward 700 nm in both BCs and NPs. Such a tail cannot be induced by stimulated emission sub-band gap transitions as no red side emission can be seen in time-resolved PL with similar excitation conditions in these samples (see the SI for details). However, there is a pronounced absorption tail below the band edge of the NPs and BCs, as shown in Figure 1a. It should be noted that light scattering can be excluded here as GSA is recorded by an integrating sphere. The tails in both GSA and TA therefore indicate the existence of sub-band gap states in NP samples with absorption transition strength. Such sub-band gap states have also been reported in MAPbI3 films and attributed to being excitonic trap states.18 In order to further confirm the existence of excitonic trap states in our MAPbBr3 samples, we measured TA spectra of both NPs and BCs with excitation directly below the band edge (575 nm). The TA spectra at 2 ps exhibit ESA (EA1) similar to the case with the excitation energy

nature of such trap states and understand why they do not reduce PL, we investigated here the excited-state dynamics by transient absorption (TA) spectroscopy of NPs compared with their bulk counterparts (bulk crystals, BCs). By varying the excitation intensity (0.5−13 excitations per NP), we found that efficient charge trapping starts in NPs when multiple excitations have filled the lowest-energy states. Spectral features in TA reveal that such trapping comes from hot charges accumulated at high energy levels. We propose a model that involves a potential barrier between excited charges and surface trap states that inhibits charge trapping under low excitation intensities. This explains the high PL QY of perovskite NPs despite numerous surface traps. In order to compare the charge carrier dynamics of NPs with bulk materials, we investigated both MAPbBr3 NPs and BCs with much larger size (>10 μm). Our previous studies have shown that such BCs have similar exciton binding energy and charge carrier dynamics as the bulk films.13 Detailed structural characterization reveals good crystallinity of both samples.16 The size of the NPs (8.2 nm; for details of sample characterization see the SI), however, is much larger than the exciton Bohr radius, leading to rather weak quantum confinement. Therefore, the edge of the ground-state absorption (GSA) spectrum of the NPs is only slightly blue-shifted compared to that of the BCs (see Figure 1). The TA spectra at 2 ps after photoexcitation with low-intensity 400 nm pulses are also plotted in Figure 1. From the kinetics (Figure 1c), it is 1155

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Figure 2. TA spectra at 2 ps delay time after thermalization of the photogenerated charges under excitation fluences from 5 × 1013 to 1.3 × 1012 photon/cm2 together with PL spectra with the same excitation condition for (a) BCs and (b) NPs. The inset in (a) illustrates the fwhm of the bleach vs 2/3 power of the excitation density fitted by the model Burstein−Moss band shift due to charge accumulation.

been observed in CH3NH3PbI3 films. The full-width at halfmaximum (fwhm) of the bleach can be used as a measure of the 19 Burstein−Moss band shift ΔEBM g , given as

well above the band gap. The existence of EA1 confirmed state filling of the conduction band (CB) edge state, which comes from the dipole-allowed optical transition from sub-band gap trap states. There should also be bleach (GSB1) due to state filling in the band edge excited from the trap state (ST) at the red side; however, the detected signal there is overwhelmed by the scattering of the excitation pulse, which cannot be easily separated from the bleach. At the lowest excitation fluence (1 × 1013 photon/cm2), the average excitation number of an individual NP ⟨N⟩ is 0.1, taking into account the mean size and absorption cross section of the NPs (see the SI). From Figure 1c, it is clear that the rising of TA bleach at the band edge (GSB1, 300 fs) is faster than that of the trap state (GSB2, 700 fs). At the longer time scale, the TA kinetics of NPs at GSB1 and GSB2 follow the same trend (see the SI). Therefore, we can conclude that the charge carriers that are responsible for the band edge and the trap bleaching signals recombine together. The band edge bleach at the early time in Figure 1c represents state filling from thermally relaxed electrons as well as holes. The slower rising of the trap bleach compared to that of the band edge bleach therefore indicates that not all of the photogenerated charges at the band edge states contributed to the trap bleach. Our previous XPS study in such NPs revealed the sub-band gap defect states as a tail to the valence band (VB) edge, which is consistent with the absorption tail in GSA; therefore, we can conclude that the dominant type of traps are hole traps in our case.13 In this scenario, the GSB2 in Figure 1c should be mainly induced by filling of the CB edge levels by electrons, while GSB1 represents state filling of both the CB and VB. We conclude that the electron relaxation is slower and determines the growth of GSB2, whereas the fast hole relaxation determines the time scale of GSB1 growth. Photoinduced charge generation dynamics in NPs after excitation above the band edge and the corresponding bleach signals are summarized in schematics shown in Figure 1d. Charge carrier dynamics in NPs and BCs at high excitation intensities are very different. In BCs, initial accumulation of photogenerated charges near the edge of CBs and VBs occurs, leading to a shift of the band edge to higher energy due to the Pauli exclusion principle.19 This appears in TA as the carrierdensity-dependent Burstein−Moss blue shift and broadening of the band edge bleach, as shown in Figure 2a. The effect has also

ΔEgBM =

ℏ2 (3π 2n)2/3 * 2meh

(1)

where m*eh is the reduced effective mass and n is the carrier density. As shown in the inset of Figure 2a, the fwhm of the band edge bleach of BCs at 2 ps is proportional to n02/3 (n0 is the initial excitation density), in agreement with eq 1. In NPs, however, the broadening of the band edge bleach expands not only toward the blue side but also toward red, as shown in Figure 2b, and does not follow the Burstein−Moss shift theory. The enhanced negative signal at the low-energy side obviously cannot be attributed to the filling of states above the band gap. We can also exclude several other possible origins: Light scattering may occur in steady-state absorption due to aggregation and existence of large-sized nanoplatelet impurities reported in our previous studies.13 Such scattering would not significantly distort the TA signal because the diffused scattering would not affect pump-induced changes in absorption of the probe pulse. Sub-band gap defect emission was widely reported in semiconductor QDs.20 Such defect emission usually exhibits a broad emission band with a long lifetime depending on the trap states. No such sub-band edge emission band was observed in steady-state PL spectra of NPs excited with the same femtosecond laser pulse as that in TA measurements at both low and high intensity (5.5 × 1013 and 1.1 × 1015 photon/cm2), as shown in Figure 2b. The slight difference in the PL spectra of BCs in Figure 2a may be due to the different reabsorption by the crystals under various excitation intensities. In addition, absence of the defect emission can be further confirmed in time-resolved PL measurement in our previous work.13 Possible thermal effects on the excited-state dynamics at high excitation density have been discussed in the context of perovskite film samples for laser applications.21,22 However, in our case with the excitation levels far from the amplified spontaneous emission threshold, heating by laser excitation is limited ( 2 ps) and can be decomposed into a fast (τ1 = 79 ps) and a slow (τ2 = 80 ns) component with similar spectral features consisting of an ESA band at high energy and a GSB band at lower energy (Figure 3a). The fast component can be assigned to bimolecular recombination of multicarriers in BCs. In perovskite materials, if the recovery kinetics of the excited states are dominated by second-order nongeminate/free carrier recombination, the kinetics of carrier concentration n can be expressed as

Moreover, due to the large surface area of the NPs, the heat transfer would be very efficient to the surrounding solvent. We conclude that the thermal effects cannot explain the long-lived sub-band gap feature in TA (for details of the discussion on heat dissipation, see the SI). Photochemical reaction is also not likely to happen in our case. First, the TA of each sample has been measured several times between high and low excitation conditions with reproducible dynamics; thus, irreversible photodegradation can be excluded. Second, the red bleach is formed within a few picoseconds. Such a time scale is much faster than reversible photoinduced structural changes such as ion diffusion reported in film samples (∼μs).23 Finally, as the NPs are well-dispersed in toluene solution, TA distortions due to refractive index changes as reported in conventional perovskite films24 are also excluded in our case. After excluding the above possibilities, the only remaining interpretation of the enhanced negative signal at long wavelength would be the change of the ratio between band edge bleach and trap bleach with different excitation density. As discussed above, the trap bleach in NPs can reflect state filling at both the conduction band minimum (CBM) and hole trap states (ST), while the band edge bleach represents state filling of an excited electron at the CBM and holes at the valence band maximum (VBM). Therefore, when the ratio between the band edge bleach GSB1 and the trap bleach GSB2 decreases, as illustrated in Figure 2b, it can be the consequence of two possible processes: (1) depopulation of ne,h at the CBM and VBM and (2) increased population of trap states ST. This means that more holes are trapped at higher excitation density for this kind of NPs. In order to clarify the excitation intensity

−

dn = Bn2 dt

(2)

n−1(t ) = Bt + C

(3) −1

In this scenario, ΔA should be a linear function of time. This is exactly what we observed in our case under low excitation conditions (for details, see the SI). The linearity of ΔA−1 vs time holds up to 100 ps, in good correspondence with the fast component that we extracted from SVD fitting. This confirms that the component does originate from the 1157

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Figure 4. Schematics of the excited-state dynamics of NPs under different excitation intensities.

band edge states. The fast component (74 ps) can be assigned to Auger recombination of the biexcitons in NPs. The assignment is confirmed by the linearity of ΔA−2 as a function of time at the early stage (up to 100 ps) in the TA kinetics of band edge bleach (for details, see the SI). A similar time scale of the biexciton lifetime can be found in the all-inorganic CsPbBr3 perovskite QDs system at relatively low excitation levels (⟨N⟩ ≈ 0.05).28 The lifetime of the slow component (1.4 ns) is still shorter than the intrinsic lifetime measured by time-resolve PL experiments, suggesting that the time constant is a combination of first-order and second-order (i.e., annihilation of charges) recombination. It should be noted that, statistically, the likelihood that the number of excitations is more than one in a NP can be significant due to the large size distribution (>40%) of the particles. We also note that both components consist of a non-negligible sub-band bleach tail, which means that state filling of the band edge also influences the trap states. It should be noted that, similarly to the BCs, the thermal relaxation of hot charges (400 nm excitation) at this excitation intensity is so fast that it is comparable to the response function of the laser pulse, as shown in Figure 1c. Therefore, the SVD procedure does not resolve the process. When ⟨N⟩ becomes larger than 1 (⟨N⟩ = 2.5), a component with a 1 ps lifetime appears. As in BCs, it is attributed to relaxation of the initial hot charge carriers to the band edge. The relaxation rate has slowed down, which indicates that the photogenerated charges start accumulating at the excited states at this excitation intensity. At the same time, the component related to Auger relaxation has become faster (35 ps) because of the involvement of higherorder (tri- and more-exciton) processes.29 Qualitatively different TA dynamics can be found when the excitation density is much more than 1 (⟨N⟩ = 13), as shown in Figure 3e. The fastest component reflecting electron and hole cooling is similar to the 1.2 ps component when ⟨N⟩ = 2.5 but with a longer lifetime of 4 ps, consistent with the earlier studies revealing a thermal phonon bottleneck effect.24 The bleach band in this component is slightly blue-shifted compared to the ⟨N⟩ = 2.5, indicating an enhanced Burstein−Moss energy shift due to the higher extent of charge accumulation. The second fast component (blue line, 14 ps) assigned to the Auger process has again become even faster, consistent with even larger involvement of higher-order Auger processes. However, the line shape of this component is significantly different for the two intensities, as shown in Figure 3f. Contribution of the band edge bleach (∼525 nm) at ⟨N⟩ = 13 is reduced compared to that at ⟨N⟩ = 2.5. This means that there should be charge depopulation of the band edge states (pink area in Figure 3f) and/or more charge population at trap states. In addition, we

recombination of multicarriers. The lifetime of the slow component is close to that of the radiative recombination established by the time-resolve PL measurement in our previous study.26 In addition, the slight red shift of the slow component spectrum can be observed compared to the fast component. This is identical to the so-called “biexciton shift”, which is widely reported in QD systems where the absorption transition is influenced by many-body effects if more than one electron−hole pair is generated in the system.27 We attribute the slow component of TA to first-order recombination of photogenerated electrons and holes. These two components with slightly different time constants can be found also at high excitation intensity, as shown in Figure 3b (1.3 × 1015 photon/ cm2). Besides that, an additional fast component with a lifetime of about 2 ps can be found at high intensity. Because the spectral line shape of the component mirrors well the state filling of accumulated charges, we assign the component to cooling of the initially excited hot carriers. This process has been shown to slow down at high excitation intensities,24 while at low intensity (Figure 3a), it is so fast that the SVD procedure cannot distinguish it. We also notice that although the sub-band edge tails that are related to the trap bleach can be observed in BCs, the contribution of them is small and they are independent of the excitation intensity. Such trap bleach is only due to state filling of the CBM, while the population of the trapped charges is negligible during TA measurement. In our previous study, we reported that for trap filling in MAPbBr3 BCs, due to the ultralong lifetime of the trap state, the trap filling would decrease the effective trap density, which depends on the repetition rate and power of excitation pulses.15 Here, the lowest excitation fluence in TA is 2 orders of magnitude higher than what was used in the time-resolved PL studies (∼1 × 1012 photon/cm2), leading to pronounced trap filling. Employing previous analyses, we can roughly estimate that more than 80% of the total traps in our BCs would be filled at an excitation fluence of 1.3 × 1015 photon/cm2 in TA measurements (for details, see the SI). Thus, the number of active traps that can accommodate excited holes is quite limited. In contrast to the BCs, intensity-dependent charge carrier dynamics in NPs are significantly influenced by the trap states. The lower panels of Figure 3 show the TA dynamics under three excitation fluences, which correspond to the average number of excitations per QD being less than 1 (⟨N⟩ = 0.5 in Figure 3c), slightly more than 1 (⟨N⟩ = 2.5 in Figure 3d) and much more than 1 (⟨N⟩ = 13 in Figure 3e). For the lowest excitation intensity, the SVD analyses provide two kinetic components, both representing the excited-state dynamics of 1158

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kinetic model are chosen in such a way that they lead to dynamics that corresponds well to the SVD fitting. It should be noted that the hole trapping leaves an electron to the CB. This leads to a negative trion with corresponding recombination apart from the usual Auger recombination. Such a process contributes to the nonradiative recombination pathway. In the simulation, we have set the trion recombination time constant to 250 ps, which has been reported for CsPbBr3 perovskite NPs.28 The TA kinetics is modeled as A(t ) = a(L(t ) + 0.5T(t ))

(4)

where a is the state-filling bleach signal of an electron−hole pair at the band edge of NPs and L(t) and T(t) are the timedependent populations of L and T levels (the electron alone gives half of the signal). For details of Monte Carlo simulations, see the SI. As shown in Figure 5, the simulated curves provide a very good fit to the experimental data. From the simulations, we have obtained four excitations for the trapping threshold, which means that the trapping will start for N larger or equal to 5. From the level of the long time scale signal, we have concluded that the average number of trapped holes is 6.8 for ⟨N⟩ = 30 and 3.2 for ⟨N⟩ = 13. X-ray photoelectron spectroscopy studies conclude that the trap states are mainly localized at the surface of the NPs.13 Furthermore, the synthesis strategy of NPs suggests that the terminal group of NH2 in a long-chain capping agent ODA would replace the MA and insert into the interspace among the PbBr64− octahedra,8 leading to a local lattice expansion or distortion. This would offset the energy band due to their impact on the stereochemical activity of the Pb2+ lone pair according to our previous studies.16 In this way, the potential barrier between the VB and the trap is formed. Such a potential barrier has a consequence that the surface traps in NPs would not affect the excite-state dynamics under light intensities within photovoltaic operating conditions with excitation density between 1013 and 1015 cm−3 (corresponding to a photon fluence of 109−1011 photon/cm2 per pulse).31 This also explains the high PL QY of MAPbBr3 NPs (20−80%) despite the expected high trap density on the surface (∼3.1 × 1011 cm−2).15 However, when utilized in emitting devices such as lasers, the excitation density is much higher to stimulate light amplification. The threshold for the bulk MAPbX3 film is about 10−20 μJ/cm2, corresponding to a fluence of 1014 photon/ cm2,32 which is close to the regime where trapping occurs. This means that charge-accumulation-induced trapping may be an additional factor to consider for device design. It should also be noted that simply exciting NPs with a high-energy photon (e.g., 400 nm, 3.1 eV in our case) does not induce such a trapping process to any significant extent. This can be proved by steadystate photoluminescence excitation (PLE) spectroscopy measurement (see the SI), where the PL QY of the NPs is independent of the excitation wavelength above the absorption edge. Clearly, no significant additional nonradiative recombination pathway is created when NPs are excited with highenergy photons. The main reason for this is the fast cooling rate of the hot charges compared to the slower trapping process, as illustrated in Figure 4. We also point out that in NPs the relatively large exciton binding energy shifts the equilibrium between the excitons and charges. In this case, the exciton levels reduce the filling effect to some extent. However, it will not change the physical picture regarding the trapping model that we propose.

Figure 5. TA kinetics at the band edge (525 nm) together with corresponding Monte Carlo simulations. The inset illustrates the corresponding multiexciton kinetic model with trapping and the barrier. Auger recombiniaton (AR), trion recombination (TR), radiative recombination (R), and trapping (T) are all included in the model. 1159

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In order to minimize the losses due to trapping, the material needs to be engineered to either diminish the surface traps or increase the energy of the potential barrier between the VB and the surface traps. In summary, we have investigated the pump-intensitydependent excited-state dynamics of CH3NH3PbBr3 NPs compared with that of large microcrystals by using TA spectroscopy. First, TA spectra with excitation above and below the band gap confirm the excitonic sub-band gap hole trap states from which the optical transition to the CB is allowed in both samples. In BCs, the excited-state dynamics only weakly depends on the pump intensity and the absorption from the trap states can be observed as a weak bleach tail in TA spectra. This is due to the limited density of trap states after the trap filling process. In NPs, the trap states do not affect the excited-state dynamics when the average number of excitations per NP is lower or comparable to 1 due to the potential barrier that isolates the photogenerated holes at low-energy VB levels from the surface trap states. When the excitation density is much larger than 1 per NP, charge accumulation occurs and the holes reside for a considerable time at the high-energy levels of the VB. From those levels, the potential barrier can be overcome and the trapping takes place. The trapped holes leave the corresponding excited electron in the CB with a lifetime much longer than 10 ns. The energy barrier for surface trapping may originate from structural modification induced by a surface capping agent. This also explains the high emission efficiency of such NPs despite the large surface trap density.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00352. Experimental details, detailed TA kinetics at various wavelengths, calculation of mean excitation numbers in NPs, and the trap filling process (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.P.). *E-mail: [email protected] (K.Zheng). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This Letter was made possible by NPRP Grant # NPRP7-2271-034 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. Collaboration within NanoLund is acknowledged.

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REFERENCES

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DOI: 10.1021/acsenergylett.6b00352 ACS Energy Lett. 2016, 1, 1154−1161