Modulating hot carrier relaxation and trapping dynamics in lead halide perovskite nanoplatelets by surface passivation

Abstract

Two-dimensional (2D) lead halide perovskite (LHP) nanoplatelets (NPLs) have recently emerged as promising materials for solar cells and light-emitting devices. The reduction of LHP dimensions introduces an abundance of surface defects, which can strongly influence the photophysical properties of these materials. However, an insightful understanding of the effect of surface defects on hot carrier (HC) relaxation, one of the important properties of LHP NPLs, is still inadequate. Herein, the HC relaxation and trapping dynamics in pristine and surface passivated two-layer (2L) CsPbBr₃ NPLs have been investigated by using time-resolved spectroscopy. The results reveal that surface defects can trap HCs directly before they relax to the band edge, which accounts for the absence of the hot-phonon bottleneck (HPB) effect in LHP NPLs. After healing surface defects with a passivation agent, the relaxation time of HCs is extended from ∼73 to ∼130 fs in 2L CsPbBr₃ NPLs, indicating that the channel of HCs trapped by the surface defects can be effectively blocked. Accordingly, the HPB effect is activated in surface-passivated CsPbBr₃ NPLs. The finding of surface defect-related HC relaxation dynamics is important for guiding the development of high-performance LHP NPL devices related to HCs through surface defect engineering.

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Introduction

Two-dimensional (2D) lead halide perovskite (LHP) nanoplatelets (NPLs) have emerged as a new class of semiconductor materials for solar cells and light-emitting devices.^1–5 Compared to their bulk counterparts, atomically thin LHP NPLs exhibit many interesting characteristics, such as high exciton binding energy, large absorption cross sections, narrow emission peaks, adjustable band gap through thickness and composition control, easy surface modification,^6–10 and so on. However, as the dimensionality of LHPs decreases, the surface-to-volume ratio increases, inevitably producing an abundance of surface defects, which can strongly influence the photophysical properties of these atomically thin materials.^11,12 Although many studies on defect-related carrier dynamics in 2D LHP NPLs have been reported, most of them have focused on uncovering the impact of defects on the band edge carrier dynamics,^13–16 and understanding of how the hot carriers (HCs) interact with the surface defects in 2D LHP NPLs is still inadequate.

When a semiconductor is excited using photons with energies greater than its band gap, the photogenerated carriers in the semiconductor have significantly higher energies than the band edge before they dissipate the energy to the lattice and are often referred to as HCs. HCs carry a large amount of energy, which can drive chemical and physical processes.^17–19 For optoelectronic devices, longer-lived HCs are needed to further increase the power conversion efficiency to overcome the Shockley–Queisser limit.²⁰ In 0D and 3D-like LHPs,^21–23 the relaxation time of HCs can be significantly prolonged via the hot-phonon bottleneck (HPB) effect under high excitation fluences. However, many studies have reported that no HPB effect was observed in the HC relaxation of 2D LHP NPLs, and different views have been proposed for the HPB-free HC relaxation.^24–28 For example, Feldmann et al. and Ghosh et al. suggested that the lack of the HPB effect in 2D LHP NPLs is due to the reduced screening of the Coulomb-mediated carrier-phonon interaction in their low-dielectric surroundings.^24,25 Recently, Sun et al. attributed the absence of the HPB effect in 2D LHP NPLs to the efficient out-of-plane triplet-exciton-LO–phonon coupling because of the structural anisotropy.²⁷ In 2023, Bakulin's group observed that the highly excited HCs in monolayer WS₂ can be trapped during relaxation by defects.²⁹ Inspired by this work, the increased surface defects because of the increased ratio of surface-to-volume in 2D LHP NPLs may also account for the different HC relaxation dynamics compared with that in 0D and 3D-like LHPs. Therefore, it is necessary to gain a deeper understanding of the influence of surface defects on the HC relaxation process in 2D LHP NPLs.

To investigate the effect of surface defects on the HC relaxation process, in this work, pristine and surface passivated two-layer (2L) CsPbBr₃ NPLs were synthesized, and their ultrafast HC dynamics were comprehensively investigated by using both transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopy. The direct trapping of HCs by surface defects during relaxation was observed. The process of HC relaxation was found to slow down in passivated 2L CsPbBr₃ NPLs, which arises from the reduced surface defects by the addition of a passivation agent. The regulation of HC relaxation time by surface defect engineering in 2L LHP NPLs can facilitate their application in photovoltaic and optoelectronic devices related to HCs.

Results and discussion

In the present study, our main aim is to examine the effect of surface defects on the process of HC relaxation in 2D LHP NPLs. Considering that the surface defect density and volume ratio of NPLs increase with the decrease of NPL thickness, 2L ultrathin CsPbBr₃ perovskite NPLs were selectively prepared following a reported method with slight modifications (see the Experimental section in the ESI† for details).^30,31 As presented in Fig. 1a, to heal the surface defects of the as-prepared CsPbBr₃ NPLs, a hexane solution of PbBr₂, oleic acid, and oleylamine was used according to a previously reported literature method.³² Herein, the as-synthesized and surface-passivated NPLs are named A-NPLs and P-NPLs for easy distinction. Fig. 1b–e show transmission electron microscope (TEM) images of the synthesized CsPbBr₃ NPLs. The TEM images reveal that the morphology of the NPLs is maintained after the surface passivation process in our experiments. Both the A-NPLs and P-NPLs prepared in our experiments display a rectangular shape with an average length of ∼12 nm and width of ∼7 nm (Fig. 1b, d, and S1†), respectively. The average thickness of the A-NPLs and P-NPLs with a value of ∼1.5 nm is confirmed by the TEM images of NPLs standing on their side (Fig. 1c, e, and S1†). Therefore, the measured thickness (∼1.5 nm) in our experiments corresponds to the NPL comprising two monolayers, according to the ∼0.6 nm lattice parameter of the CsPbBr₃ cubic crystal structure.^31,33 The as-prepared NPLs adopt a cubic perovskite phase, which will be demonstrated below via X-ray diffraction (XRD) analysis.

Fig. 1 (a) Schematic of the passivation method that converts A-NPL into P-NPL. The gray octahedron in the lattice of A-NPL denotes the Pb²⁺ vacancy. TEM images of tiled (b) and vertical (c) A-NPLs. TEM images of tiled (d) and vertical (e) P-NPLs.

Fig. 2a shows the XRD patterns of the prepared A-NPLs and P-NPLs in our experiments, with the standard pattern of the CsPbBr₃ cubic phase (PDF# 54-0752) added for comparison.³⁴ From the XRD data, we clearly identified that the phase of CsPbBr₃ NPLs is retained after passivation and they have a cubic phase consistent with the standard pattern. Three intense diffraction peaks appearing at ∼15.6, ∼21.9, and ∼30.9° in the XRD patterns are assigned to the (100), (110), and (200) crystallographic facets, respectively.³⁵ It is well known that the contribution to the electronic structure near the band edges of CsPbBr₃ primarily arises from Br 4p and Pb 6p orbitals, while the Cs ion plays a minor role.^36,37 This is because cubic CsPbBr₃ consists of periodically arranged Pb–Br octahedra that show strong interactions, while the isolated Cs ions at the corners of the cube exhibit weak interactions. Therefore, the optical properties of CsPbBr₃ NPLs can be easily modulated through variation of the surface Pb and Br states since internal defects are negligible compared to surface defects due to their massive exposed surfaces. In general, two types of lead and bromine vacancy defects (V_Pb and V_Br) exist on the surface of CsPbBr₃ NPLs. To confirm the difference in the surface chemical states of A-NPLs and P-NPLs, X-ray photoelectron spectroscopy (XPS) measurements were performed on the two samples. Fig. 2b–d show the XPS spectra of Cs 3d, Pb 4f, and Br 3d for pristine A-NPLs and surface passivated P-NPLs. The Cs 3d XPS spectra display no change after passivation treatment, confirming the weak interaction between Cs⁺ and the PbBr₆⁴⁻ octahedron as discussed above. In the Pb 4f XPS spectra, two main peaks appear in the curves of both samples and two minor peaks can be seen at low binding energy sites after fitting. The main and minor peaks are assigned to the inner and surface Pb ions,^38–40 respectively. After passivation, the intensity ratio between inner and surface ions increases and the peaks shift to a higher binding energy side, indicating that the level of Pb–Br species increases at the surface. In other words, the V_Br defect density on the CsPbBr₃ surface of NPLs decreases after passivation due to Pb ions only bonding with Br ions in the crystal lattice. In addition, as shown in Fig. 2d, the increase in the level of Pb–Br species in P-NPLs can also be confirmed by the shift of the peaks in the Br 3d spectra. By calculating the peak areas in XPS data and atomic sensitivity factors, the relative ratios of Cs, Pb, and Br in A-NPLs and P-NPLs are found to be 1 : 1 : 2.8 and 1 : 1.1 : 3.5, respectively. Therefore, the XPS data above prove that the surface defects of V_Br and V_Pb are lower for the P-NPLs than the A-NPLs.

Fig. 2 (a) XRD patterns of pristine and surface passivated 2L CsPbBr₃ NPLs. (b) XPS spectra of Cs 3d (b), Pb 4f (c), and Br 3d (d) of pristine and surface passivated 2L CsPbBr₃ NPLs. (e) Steady-state absorption and PL spectra of pristine and surface passivated 2L CsPbBr₃ NPLs in hexane solution. (f) Normalized PL decay dynamics of pristine and surface passivated 2L CsPbBr₃ NPLs. The open circles and curves in (f) are the experimental data and biexponential fitting results, respectively.

To explore the optical properties of the A-NPLs and P-NPLs, steady-state UV-vis absorption and PL spectra were first recorded. As shown in Fig. 2e, the absorption spectrum of A-NPLs shows a sharp exciton absorption peak at ∼430 nm with a half-maximum full width of ∼11 nm. The narrow and single absorption peak suggests that the NPLs have a good size distribution and shape uniformity. The PL peak of A-NPLs appears at ∼436 nm in the emission spectrum (Fig. 2e, pink shadow). After passivation, the spectral shapes of the excitonic absorption and PL emission remain almost unchanged, implying that the thickness of the NPLs does not change, which is consistent with the results of TEM. Both the excitonic absorption and PL emission peaks of NPLs display a slight blue shift after passivation, which can be attributed to the enhancement of the dielectric effect. Specifically, the intensity of the PL emission of P-NPLs is significantly enhanced compared with that of the A-NPLs (Fig. 2e), with the PL quantum yield (QY) increasing from 15.2% to 48.7% (see Note 1 in the ESI† for the method). This increase of PL QY in passivated NPLs indicates the suppression of defect-related non-radiative carrier recombination after introducing the passivation agent. Next, time-resolved PL (TRPL) measurements were conducted on A-NPLs and P-NPLs under excitation at 375 nm to study the effect of passivation on the exciton recombination process of NPLs. As illustrated in Fig. 2f, both the PL decay curves of A-NPLs and P-NPLs detected at 435 nm can be well fitted with a biexponential function (see Note 2 in the ESI†), and the corresponding decay parameters are listed in Table S1.† However, compared with the pristine A-NPLs, the average PL lifetime of P-NPLs is prolonged from 3.68 ± 0.57 ns to 7.24 ± 0.90 ns due to the highly suppressed non-radiative carrier recombination channel in P-NPLs (Table S1†), further confirming the decrease in surface defect density after passivation in NPLs.^41,42

In order to investigate the HC relaxation dynamics in CsPbBr₃ NPLs, we carried out systematic TA measurements on the pristine and surface passivated samples under the above band gap excitation (360 nm). Fig. 3a–d display the typical TA mapping and spectra at selected time delays of A-NPLs and P-NPLs. The TA spectra of both samples are dominated by a strong and narrow photobleaching (PB) feature centered at ∼430 nm, matching the peak of the excitonic transition in the steady-state UV-vis absorption spectra (Fig. 2e), which can be attributed to the state-filling effects.⁴³ In addition, the TA spectra also present two photoinduced absorption peaks located beside the PB signal, which may be caused by the absorption of the photoinduced carriers, band gap renormalization, and biexciton effect.^25,44,45 However, the exciton PB signals of pristine CsPbBr₃ NPLs show a faster recovery than that of surface passivated CsPbBr₃ NPLs, which is consistent with the result obtained in TRPL measurements (Fig. 2f) and further suggests the reduction of surface defects in passivated CsPbBr₃ NPLs. We have further monitored and compared the exciton PB dynamics of pristine and surface passivated CsPbBr₃ NPLs extracted from Fig. 3a and b. As shown in Fig. 3e, the exciton PB dynamics detected at ∼430 nm confirms the slow recovery in passivated CsPbBr₃ NPLs. The PB recovery dynamics can be fitted with a multi-exponential decay function (solid curves in Fig. 3e). The decay time constants are 0.32 ± 0.05 ps (61%), 130 ± 13 ps (24%), and 4170 ± 400 ps (15%) for pristine CsPbBr₃, and 0.36 ± 0.07 ps (60%), 210 ± 15 ps (24%), and 9205 ± 900 ps (16%) for passivated CsPbBr₃ NPLs. The two fast decay components can be assigned to the trapping of carriers by shallow and deep defect states,¹⁶ and the slow component could be attributed to carrier radiative recombination.⁴⁶ Meanwhile, as illustrated in Fig. 3f, the buildup of PB dynamics with a sub-picosecond time constant was observed, which reflects the process of HC generation by photon excitation with energy larger than the band gap relaxing from a high energy level to form the subsequent exciton.⁴⁷ The HC relaxation time of CsPbBr₃ NPLs can be extracted by exponential fitting and increases from ∼73 fs to ∼130 fs after surface passivation. We speculate that the slow relaxation rate of HCs in P-NPLs observed under the same experimental conditions can be attributed to the suppression of HCs trapped by surface defects. The surface defect density controls and determines the competition between HC relaxation and carrier trapping in NPLs. Since the probability of HCs being trapped by surface defects in A-NPLs is higher, the observed HC lifetime is shorter, which is reflected in the rapid rise of the exciton PB dynamics. Similarly, the trapping of highly excited HCs in monolayer WS₂ during cooling by defects has been demonstrated in recently reported work.²⁹

Fig. 3 TA mapping as a function of probe wavelength and time delay under 360 nm excitation with a pump power of 0.3 μW for pristine (a) and surface passivated (b) 2L CsPbBr₃ NPLs. (c) and (d) TA spectra probed at the selected delay times extracted from (a) and (b), respectively. (e) Normalized exciton dynamics of pristine and surface passivated 2L CsPbBr₃ NPLs. (f) The zoomed-in profiles of (e) in the earlier time delay. The open circles in (e) and (f) represent the experimental data, and the curves show the multi-exponential fitting results after the consideration of the instrument response function (IRF, ∼120 fs).

To verify the speculation proposed above and deeply understand the impact of surface defects on HC relaxation in CsPbBr₃ NPLs, TA measurements on pristine and surface passivated CsPbBr₃ NPLs under near band edge excitation (405 nm) were further performed in our experiments. As shown in Fig. S2 and S3,† similar to that of 360 nm excitation, the exciton PB dynamics of CsPbBr₃ NPLs recover with a much slower rate after surface passivation due to the suppression of defect-related non-radiative carrier recombination. However, compared with the 360 nm excitation, the difference is that the exciton PB dynamics of pristine and surface passivated CsPbBr₃ NPLs show a more rapid rising process and reach their maximum simultaneously, as illustrated in Fig. 4a. The rapid rise of the PB dynamics under 405 nm excitation is because the carriers generated in CsPbBr₃ NPLs excited with the wavelength close to the exciton absorption are mainly near the band edge, which can be detected immediately by the probe pulse after photoexcitation, resulting in a fast PB dynamics buildup rate. This result further confirms that the accelerated HC relaxation in pristine CsPbBr₃ NPLs under 360 nm excitation (Fig. 3f) results from the direct trapping of HCs by surface defects. Next, the pump intensity-dependent HC relaxation dynamics were measured under 360 nm excitation to examine whether the HPB effect was present in CsPbBr₃ NPLs. As displayed in Fig. 4b, the buildup of PB dynamics in pristine CsPbBr₃ NPLs exhibits no dependence on the pump intensity, implying a lack of the HPB effect in this material, which is in accord with earlier reports.^26,27 As discussed above, the high density of surface defects in the pristine CsPbBr₃ NPLs can trap the HCs directly, and then the excess energy carried by HCs can rapidly dissipate into the surroundings. As a consequence, the HCs in pristine CsPbBr₃ NPLs can maintain a fast relaxation rate even under high pump intensity and produce a suppressed HPB effect. Conversely, as shown in Fig. 4c, with increasing pump intensity, the relaxation rate of HCs in surface passivated CsPbBr₃ NPLs slows down gradually, which can be attributed to the HPB effect. This is because the decrease in the surface defect density of the passivated CsPbBr₃ NPLs leads to the reduced probability of HC trapping. As a result, the increase in pump intensity could lead to the slowing down of the HC relaxation rate in passivated CsPbBr₃ NPLs.

Fig. 4 (a) Normalized exciton dynamics of pristine and surface passivated CsPbBr₃ NPLs following 405 nm excitation with a pump power of 0.6 μW. Pump intensity dependence of the normalized exciton dynamics of pristine (b) and surface passivated (c) CsPbBr₃ NPLs excited at 360 nm. The pump powers used from the top line to the bottom are 0.43, 2.1, 4.3, and 11.5 μW. The open circles are the experimental data, and the curves show the exponential fitting results after the consideration of the IRF (∼120 fs). (d) Normalized UV-vis spectra (solid line) and PL spectra (dotted line) of pristine 2L, 3L, and 4L CsPbBr₃ NPLs. The wavelengths corresponding to the arrows represent the pump wavelengths used in the collected TA spectra in (e). (e) Normalized exciton dynamics of pristine 2L, 3L, and 4L NPLs excited at 360 nm, 375 nm, and 384 nm, respectively. (f) A schematic illustration of two relaxation pathways for photoinduced hot electrons in 2L CsPbBr₃ NPLs. The purple vertical arrow indicates the initial excitation with a 360 nm pump pulse. The red wavy arrow and blue curved arrow denote hot electron relaxation to the band edge and are directly trapped by the surface defects, respectively. For simplicity, we only depict the relaxation of hot electrons here, and we expect that the relaxation process of hot holes is the same as that of hot electrons.

Next, we explore the dependence of the HC relaxation dynamics on the layer of CsPbBr₃ NPLs to further validate the trapping of HCs by surface defects. The CsPbBr₃ NPLs with 3Ls and 4Ls were further prepared in our experiments. As displayed in Fig. 4d, the steady-state absorption spectra of 3L and 4L CsPbBr₃ NPLs show exciton absorption peaks at ∼454 nm and ∼465 nm, and the corresponding PL peaks appear at ∼463 nm and ∼474 nm in the emission spectra, respectively. To facilitate the comparison, the steady-state absorption and PL spectra of the 2L CsPbBr₃ NPLs are also included in Fig. 4d. It should be noted that in order to ensure that the excess energy carried by photogenerated HCs in different layers of NPLs is equal, the TA spectra of 2L, 3L, and 4L CsPbBr₃ NPLs were collected using excitation wavelengths of 360 nm, 375 nm, and 384 nm (as denoted by the arrows in Fig. 4d), respectively. In addition, as shown in Fig. S4,† the amplitudes of the exciton PB dynamics for CsPbBr₃ NPLs with different layers were similar, indicating that the carrier densities generated in different layers of CsPbBr₃ NPLs are very close, allowing a more accurate comparison of HC relaxation dynamics. As shown in Fig. 4e, the exciton PB dynamics exhibit a slow recovery rate as the number of layers of CsPbBr₃ NPLs increases. The HC relaxation times of 3L and 4L CsPbBr₃ NPLs are ∼82 fs and ∼119 fs (Fig. S5†), respectively. This can be explained by the fact that as the number of layers of CsPbBr₃ NPLs increases, the surface defect density and volume ratio decrease, leading to a reduction in surface defect-related non-radiative carrier recombination processes. Meanwhile, with the increase of the number of CsPbBr₃ NPL layers, the rising process of the exciton PB dynamics becomes slower, indicating a relatively lower energy loss rate for the photogenerated HCs. As discussed above, one possible difference as the number of layers of CsPbBr₃ NPLs decreases is that more defects acting as HC capture centers are present at the surfaces of NPLs, leading to a fast relaxation of HCs. Therefore, layer-dependent HC relaxation dynamics observed in Fig. 4e further confirm that HC trapping by surface defects in CsPbBr₃ NPLs could occur before the relaxation of the HCs with excess energy.

Based on the experimental observations obtained above, the scenario of HC relaxation dynamics in 2L CsPbBr₃ NPLs under excitation with the photon energy above the band gap can be clearly described. As shown in Fig. 4f, two relaxation pathways for photoinduced HCs exist in CsPbBr₃ NPLs. One pathway is that HCs relax to the band edge by emitting phonons (red wavy arrow). The second pathway is that HCs are directly trapped by surface defects before they relax to the band edge (blue curved arrow). The second channel can be effectively blocked by surface passivation, and the HPB effect can be activated in passivated CsPbBr₃ NPLs.

Conclusions

In summary, we have studied the influence of surface defects on photoinduced HC relaxation dynamics in 2L CsPbBr₃ NPLs with femtosecond TA spectroscopy. Two relaxation pathways for photoinduced HCs exist in CsPbBr₃ NPLs including HC relaxation through interaction with phonons and direct trapping by surface defects. The pristine CsPbBr₃ NPLs display no HPB effect due to the direct trapping of the HCs by surface defects before they relax to the band edge. The trapping channel of HCs by defects can be effectively blocked after surface passivation, and the relaxation time of HCs is correspondingly prolonged from ∼73 fs to ∼130 fs. As a result, the HPB effect can be activated in the passivated CsPbBr₃ NPLs. This provides a viable means to modulate the HPB effect in CsPbBr₃ NPLs. These results herein are of importance for developing high-performance LHP NPL devices related to HCs through surface defect engineering.

Author contributions

This article was written by Yanshen Zhu and Zhen Chi after discussions with all authors. Zhen Chi and Lijun Guo supervised the research. Yanshen Zhu synthesized the samples and conducted all the characterization studies. Yanshen Zhu performed TA experiments with the help of Shida Luo and Yuting Zhang. Yanping Liu, Yulu He, and Tianfeng Li participated in the data analysis and manuscript revision.

Data availability

Experimental data are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (22103024 and 22373031), the Science and Technology Development Project of Henan Province (242102310445), and the Key Scientific Research Project of Colleges and Universities in Henan Province (23A510013).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr02560a

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