Operando recombination kinetics in perovskite nanocrystal films revealed by in situ time-resolved photoluminescence

Domain: physics
Published: 2025-12-12
Editorial outcome: accept

Abstract
Perovskite nanocrystals have shown great promise for optoelectronic applications. Understanding charge recombination under operational conditions, such as continuous-wave photoexcitation, is crucial for advancing device performance. Although time-resolved photoluminescence is widely used to study recombination kinetics, its reliance on ultrafast pulsed excitation fails to replicate the operational conditions. Here we develop an integrated spectroscopy platform that enables simultaneous acquisition of steady-state and time-resolved photoluminescence under continuous-wave illumination, with a wide range of photoexcitation intensity modulation. This approach resolves the long-standing mismatch between the typical time-resolved photoluminescence data and the actual continuous-wave operational behavior of perovskite nanocrystals. In contrast to the established Auger recombination model, which suggests accelerated recombination at high pump fluence, we demonstrate that the operando recombination kinetics is governed by the charge-carrier and trap-state interaction. This clarified recombination mechanism provides insight for designing perovskite nanocrystal films applied to efficient and stable light emission under high-power photoexcitation.

Introduction
Metal halide perovskite nanocrystals (PNCs), also known as perovskite quantum dots, represent a novel class of nanomaterials that exhibit a combination of efficient photoluminescence (PL)1, tunable bandgap2, and strong light absorption3. These prominent characteristics, in conjunction with the simple preparation procedures,4 render them highly promising for use in diverse optoelectronic devices5,6. In addition to the significant advances in material design and device fabrication, considerable efforts have been also made over the past decade to elucidate the unique physicochemical properties of PNCs, such as excellent defect tolerance7 and drastic polaronic features8, aiming to further extend the utilization of PNCs in specific practical applications.

Time-resolved PL (TRPL) techniques, including streak camera9,10, fluorescence upconversion11, and time-correlated single photon counting (TCSPC)12 are hitherto the most widely used approaches for investigating the energy conversion mechanism of PNCs. By analyzing the dynamics or kinetics of photogenerated excitons and charge carriers following the excitation of ultrashort optical pulses, one can extract detailed information about the radiative and nonradiative charge recombination processes13,14. However, despite the dramatic advantages, such as high temporal resolution, tunable detection window, and superior sensitivity15,16, it is recognized that typical TRPL techniques are perhaps incapable of revealing the landscape of charge recombination in PNC devices under real operational conditions. For example, color-conversion PNC devices are pumped by a stable continuous-wave (CW) light, while the TRPL measurements are performed under the excitation of ultrafast pulses. It is uncertain whether the observed phenomenon in TRPL experiments can accurately mirror the stable operational condition. At a comparable pump fluence, for instance, the peak power density of the ultrafast pulse is several orders of magnitude stronger than that of CW light. Therefore, a series of nonlinear optical processes can be detected by TRPL, while their occurrence is much less probable in real devices. This discrepancy poses a substantial challenge in certifying the strategy for improving material performance directly based on TRPL experimental results.

In the present study, we develop an integrated, in situ spectroscopy system that allows for the synchronous detection of steady-state PL and TRPL data. The sophisticated design of the photoexcitation method enables precise examination of the operando exciton/charge recombination kinetics of PNCs under the sustained excitation of a CW beam. As a proof of concept, the TRPL kinetics of photostable, close-packed PNC films were investigated by varying the CW excitation intensity over twenty fold. The standard steady-state PL spectra display notable discrepancies with the data of the typical TRPL measurement, whereas they are in excellent agreement with the in situ TRPL results. Specifically, with the enhancement of excitation intensity from 0.64 mW/cm2 to 12.74 mW/cm2, a superlinear increase in PL intensity accompanied by a prolongation of PL lifetime is recognized, which can be attributed to the charge-carrier and trap-state interactions, contradicting the widely established Auger recombination model. The operando charge recombination landscape, as elucidated by the proposed in situ TRPL technique, clarifies the efficacy of PNC films in the application of light-emitting devices working under high-power photoexcitation.

Results
To begin this study, we first synthesized colloidal cesium lead bromide (CsPbBr3) PNCs through the reported hot-injection approach with slight modifications4. The transmission electron microscopy (TEM) image reveals cubelike nanoparticles (Fig. 1a) with an average edge length of 10.4 nm (Supplementary Figs. S1 S2). An interplanar spacing of 0.42 nm is evidenced by high-resolution TEM (HR-TEM) (inset of Fig. 1a), in great consistency with the (110) plane of the cubic phase of CsPbBr3. Figure 1b illustrates the steady-state UV-vis absorption and PL emission spectra of the as-synthesized PNCs dispersed in n-octane. The absorption band edge (~2.429 eV) is inferred from the minimum of the second derivative of absorbance, as depicted in the inset of Fig. 1b. The PL spectrum exhibits a single peak centered at ~516 nm (corresponding to 2.402 eV) with a small full-width-at-half-maximum of 19 nm. The Stokes shift, defined as the energy difference between the absorption band edge and the PL peak, is thus determined to be ~27 meV, in line with the direct band gap structure of perovskite. The corresponding TRPL kinetics of the PNC solution measured by the typical TCSPC technique with varied pump fluence are presented in Fig. 1c. Prominent self-similarity is observed for TRPL decay profiles with a pump fluence below ~10 nJ/cm2, as attributed to the single exciton recombination; by contrast, an additional rapid decay component gradually emerges at elevated pump fluence (see the inset), which has been assigned to the Auger recombination of multiexciton17. Figure 1d shows the plot of PL intensity versus pump fluence, displaying an unambiguous derivation from the linear dependence at high pump fluence. This is in excellent agreement with the occurrence of non-radiative Auger recombination that reduces the photoluminescence quantum yield (PLQY) as mentioned above.

Now we move forward to close-packed PNC films that are applied for realistic optoelectronic devices. The films are fabricated by spin-coating the purified PNC colloidal solutions onto a quartz substrate, subsequently followed by thermal annealing and preillumination treatments, as schematically illustrated in Fig. 2a. Thermal annealing is beneficial for regulating the optical and electronic properties by intentionally sintering the nanoparticles, which has been well-established for both classical binary nanocrystals18,19 and lead halide PNCs20. Scanning electron microscopy (SEM) images reveal that the particle size dramatically increases from ~10 nm (Fig. 2b) to ~100 nm (Fig. 2c) following thermal annealing, while the cross-section thickness (Supplementary Fig. S3) and PL homogeneity (Supplementary Fig. S4) almost remain unaffected. The intensified and narrowed peaks identified in the X-ray diffraction (XRD) patterns, as shown in Fig. 2d, are indicative of an increase in grain size and an improvement in crystallinity. However, it also brings about a decrease in PLQY (Supplementary Fig. S5), which is attributed to the loss of surface ligands during annealing that increases the density of trap states21. To overcome this issue, the annealed PNC film is exposed to the irradiation of a CW beam (wavelength: 450 nm, power density: 14 mW/cm2) for 30 minutes prior to further optical measurement. It is anticipated that such a preillumination treatment can reconstruct the surface/boundaries of perovskite grains, thus improving the PL efficiency and stability of the film22–24. The comparison of PL emission spectra summarized in Fig. 2e shows a remarkable PL redshift as a result of size expansion upon thermal annealing; on the other hand, the preillumination treatment does not alter the PL profile, while it gives rise to an obvious enhancement in the PL emission intensity (Supplementary Fig. S6). Figure 2f displays the PL intensity evolution versus illumination time for the three types of PNC films, as derived from the long-term in situ PL spectroscopy measurement (Supplementary Fig. S7), which unambiguously manifests the efficacy of the sequential annealing and preillumination treatment on optimizing the photostability.

The integrated in situ PL spectroscopy technique proposed for interrogating the operando charge recombination kinetics of PNC films is presented in Fig. 3a. In brief, a CW beam and a high-frequency pulse beam, which are respectively generated from the continuous and picosecond laser diodes, are directed to the same position of the tested sample. A pair of neutral density filters (NDFs) is applied to accurately modulate the excitation intensity of the CW and pulse beams. By eliminating the excitation light with a long-pass filter (LPF), the pure PL signals are subsequently separated by a beamsplitter (BS) and collected by a spectrometer and a photomultiplier tube (PMT) spontaneously. The spectrometer records steady-state PL spectra as is, while the PMT connected to a TCSPC module is employed to output TRPL responses. It is worth noting that the femtosecond laser, as employed in typical TRPL measurements, has been replaced with a picosecond laser diode used as the pulse beam source for in situ TRPL testing. The picosecond laser diode possesses the capability to operate at a much higher frequency (5 MHz) than the femtosecond laser (6 kHz), a crucial factor in ensuring the acquisition of high-quality in situ TRPL data without significantly prolonging the accumulation time. Figure 3b displays the as-obtained in situ PL spectra, which are centered at 535 nm regardless of illumination period, aligning closely with the standard PL spectrum shown in Fig. 2e. The fluctuation of PL intensity is found to be less than 2% over the entire experimental period (Supplementary Fig. S8), validating the prominent stability of the integrated PL spectroscopy platform. Concomitant with the in situ PL spectra measurement, TRPL results are obtained by means of TCSPC. Three sets of TRPL kinetics with the same accumulation time of ~660 s, corresponding to the three regions denoted in in situ PL spectra as exemplified in Fig. 3c, share the identical time-resolved features. Firstly, the TRPL traces demonstrate a substantial baseline (ca. 2600 counts in our case) arising from the steady-state PL generated by the CW beam, which signifies the capability of examining charge recombination kinetics under continuous photoexcitation. Secondly, the three sets of TRPL kinetics are self-similar with a variation of average lifetime as small as ~4% (Supplementary Fig. S9 and Supplementary Table S1), indicative of excellent reproducibility for the in situ TRPL test. More importantly, the picosecond pulse intensity is sufficiently weak, so as not to pose interference to the steady-state PL spectra measurement (Supplementary Fig. S10) nor to introduce any pump fluence-dependent TRPL behavior (Supplementary Fig. S11). This provides a guarantee that the in situ TRPL functions in a perturbation manner, whereby the operando condition is solely determined by the CW light intensity and not interfered by the pulse beam.

Having experimentally verified the reliability of the integrated in situ PL spectroscopy platform, we conduct a systematic exploration into the operando charge recombination kinetics of PNC films. A series of representative steady-state PL spectra with varied CW pump fluences are presented in Fig. 4a (see Supplementary Fig. S12 for complete data). The PL intensity increases with the pump fluence while the spectral profile remains consistent. A closer inspection uncovers a superlinear relationship between the PL intensity and the CW pump fluence, as illustrated in Fig. 4b and its inset. For instance, with a multiplication of the PL signal measured at 0.64 mW/cm2 by twenty, it is still significantly lower than that measured at 12.74 mW/cm2. Such a superlinear dependence indicates that the PLQY is enhanced with the elevation of the excitation intensity. Interestingly, the typical TRPL data summarized in Fig. 4c elucidates accelerated decay kinetics at high pump fluence, which implies a more severe non radiative exciton/charge recombination process, as is the case in PNC colloidal solution (Fig. 1c). However, this is at odds with the increase in PLQY at high pump fluence as derived from steady-state PL spectra, which helps exclude the multi-exciton Auger recombination. Conversely, as visible in Fig. 4d, the in situ TRPL data demonstrate a more gradual decay profile when the intensity of the CW light is increased from 0.64 mW/cm2 to 12.74 mW/cm2. Notwithstanding the underlying mechanism that will be discussed later, in situ TRPL kinetics are phenomenologically in accordance with the superlinear pump fluence-dependent PL spectra. The stark difference between the typical TRPL and in situ TRPL results signifies the necessity of performing experiments in a perturbation manner for precisely elucidating the operando mechanism, where the condition of the CW excitation is strictly kept constant for both steady-state and time-resolved measurements. Besides, the PNC film demonstrates highly reproducible spectroscopic and kinetic features following the complete in situ TRPL testing period (Supplementary Figs. S13, S14), thus validating that the observed CW intensity-dependent behaviors arise from the intrinsic properties of PNCs rather than the irreversible alterations posed by light degradation or chemical decomposition.

In order to gain deeper insights into the photophysics of operando charge recombination in PNC films, we first compare their in situ TRPL results to those measured in colloidal PNC solutions. As illustrated in Fig. 5a, the in situ TRPL kinetics of the PNC solution present negligible alteration in response to a wide range of CW pump intensity variations. No additional rapid decay component is observed at high pump fluence, in contrast to the typical TRPL data shown in Fig. 1c, indicating that the multiexciton effect is minimized under CW photoexcitation. The distinctive in situ TRPL behaviors facilitate the ascription of the retarded TRPL decay to the specific microscopic structure of PNC films, whose formation is accompanied by the transition from highly dispersed-state nanoparticles in solution (Fig. 1a) to sintered close-packed large grains in solid-state configuration (Fig. 2c). In the meantime, the improved PLQY at high CW pump fluence in PNC films is analogous to the findings reported in hundred-nanometer-sized polycrystalline perovskites22.

In quantitative, provided that multi-body Auger recombination is negligible, the equilibrium-state classical continuity equation in polycrystalline perovskites is typically expressed as: dn/dt = G - k1n - k2n^2 = 0, where n is the carrier density at the equilibrium state, t is time, G is the rate of photocarrier generation, k1 is the rate constant for first-order nonradiative recombination, and k2 is the rate constant for bimolecular recombination. Once a perturbation is superposed on the equilibrium state, the expression should be rewritten as: d(n0 + \delta n)/dt = G - k1(n0 + \delta n) - k2(n0 + \delta n)^2, in which \delta n is the density of excess carrier with a value much smaller than the equilibrium-state carrier density in a perturbation case (\delta n << n0). A combination of the two aforementioned equations results in the classical continuity equation being reduced to the following form: d\delta n/dt = -(k1 + 2k2n0)\delta n, which is indicative of first-order reaction kinetics. In situ TRPL experiment fulfills the prerequisites of perturbation, therefore the time-dependent PL intensity is predicted to be described by the first-order kinetics model with a decay lifetime of \tau, IPL(t) = IPL(0) * exp(-t/\tau). Furthermore, in consideration of the inhomogeneity of the chemical environment for individual perovskite grains, it has been proposed that the TRPL data be fitted more accurately with a bi-exponential function25 or a stretched exponential function26. Here, we adopt the bi-exponential model for elucidation due to its higher fitting goodness than the stretched exponential model (Supplementary Fig. S15).

A representative in situ TRPL data in conjunction with the bi-exponential fitting results are depicted in Fig. 5b, where the fast (\tau1) and slow (\tau2) decay components have been suggested to arise from charge recombination at the grain interface/surface and within the volume of grains, respectively27,28. As the CW pump intensity is elevated from 0.64 to 12.74 mW/cm2 (Fig. 5c), the value of \tau1 increases by a factor of 37% (1.85 ns -> 2.53 ns) while the variation in \tau2 is less than 13% (see complete fitting results in Supplementary Fig. S16 and Supplementary Table S2). Concurrently, the weight of interface/surface recombination is reduced from 78% to 60%. It has been proposed that trap states are enriched at the grain interface/surface, where charge recombination occurs predominantly via a nonradiative path27,28. Therefore, the prolonged lifetime and decreased weight of the fast decay component of the in situ TRPL kinetics at elevated CW pump fluence collectively demonstrate that trap state-mediated nonradiative charge recombination is suppressed by amplifying the intensity of photoexcitation. This is in excellent agreement with the observed superlinear behavior of CW intensity-dependent in situ PL spectra. By visualizing the light emission patterns, wide-field PL microscopy images (Fig. 5d) further verify the improved PL efficiency of PNC films upon a strong CW illumination. Specifically, the PL imaging experiments are conducted by simultaneously reducing the excitation power (P) and prolonging the integrating time (\Gamma) with the same factor. While the product of P and \Gamma remains constant, the PL intensity exhibits a prominent decrease with the reduction in P (see detailed analysis in Supplementary Fig. S17), indicative of a higher PLQY by enhancing the CW illumination.

By taking all the aforementioned experimental results into account, the mechanism of charge recombination in PNC films under operational conditions can be excellently interpreted as below. As verified by TEM and SEM characterizations, PNCs undergo nanoparticles sintering to form large-size grains when the colloidal solution is subjected to spin-coating and thermally annealing to fabricate photostable, close-packed films (Fig. 5e). The sintered grains possess a size highly exceeding the exciton diameter of PNC (~7 nm)4, endowing PNC films with new photophysical properties analogous to those of three-dimensional polycrystalline perovskite films29. It should be also noted that a substantial proportion of surface ligands is inevitably eliminated during the process of thermal annealing21, which results in the formation of additional trap states for perovskite grains21,30. These trap states can be distributed either at the grain interface/surface or within the volume31, with the former being particularly significant in nonradiative charge recombination, due to the well-documented phenomenon of defect tolerance in perovskites32. The landscape of operando charge recombination revealed by in situ TRPL is illustrated in Fig. 5f, where n0 and \delta n denote the charge carriers generated by the CW beam and the pulse beam, satisfying the perturbation requirement, \delta n << n0. The decay of \delta n involves two pathways: trap state-mediated nonradiative charge recombination (dashed arrow) and radiative charge recombination (solid arrow), which correspond to the first and second terms, respectively, in the continuity equation d\delta n/dt = -(k1 + 2k2n0)\delta n. With the elevation of the CW pump intensity, the value of n0 is increased, which is anticipated to accelerate the TRPL decay; by contrast, the density of empty trap states (i.e., nonradiative recombination centers) is decreased owing to the trap filling by charge carriers, thus resulting in a reduced k1 and a retarded recombination kinetics. Under operational conditions, the density of charge carriers is several orders of magnitude smaller than the trap state density33–36. This indicates that the effect of photoexcitation on filling trap states is dramatically greater than that on increasing n0. As a consequence, the in situ spectroscopy results demonstrate a concomitant increase in the steady-state PL intensity and prolongation in the TRPL lifetime with the enhanced CW pump fluence, which mirrors the landscape of charge recombination in PNC films under realistic operational conditions that is not revealed by typical TRPL techniques.

In conclusion, the challenge of characterizing charge recombination in PNC films under continuous operational conditions is conquered through the proposal of an integrated in situ PL spectroscopy technique, which enables the acquisition of TRPL kinetics in a perturbation manner in conjunction with steady-state PL spectra measurements. The PL spectra manifest a superlinear dependence on the CW pump intensity, ranging from 0.64 mW/cm2 to 12.74 mW/cm2. Such a specific characteristic in close-packed films, distinct from that in colloidal solutions, contradicts typical TRPL results interpreted within the widely established framework of the Auger recombination model. Conversely, it aligns closely with the in situ TRPL data, which demonstrate a prolonged charge recombination by enhancing the CW pump fluence. With a combined analysis of the morphological and spectroscopic evolution in PNCs, we elucidate the fact that steady-state PL and TRPL behaviors can be collectively attributed to trap filling by charge carriers. The trap-filling effect suppresses nonradiative recombination with an increase in charge carrier density, according to the classical continuity equation, which retards the TRPL decay and improves the PL efficiency. The clarified operando charge recombination mechanism in PNC films highlights the significance of the interaction between photogenerated charge carriers and trap states on charge recombination kinetics, while the impact of Auger recombination under operation conditions is negligible, which justifies the efficacy of PNC films in light-emitting applications under high-power excitation levels.

Methods
Synthesis of CsPbBr3 PNCs
136 mg of PbBr2, 10 mL of ODE, 2 mL of OAm and 2 mL of OA were loaded into a 50 mL three-neck round-bottom flask and subjected to vacuum drying at 80 °C for 1 h to remove residual moisture and oxygen. The reaction mixture was then heated to 175 °C under a continuous flow of nitrogen. Upon reaching the desired temperature, 0.8 mL of pre-prepared Cs-oleate solution that had been preheated at 100 °C was swiftly injected to initiate nucleation and growth of CsPbBr3 nanocrystals. The reaction proceeded for 50 s before being rapidly quenched by the immersion in an ice-water bath to halt crystal growth. The as-synthesized PNC colloidal solution was then mixed with MeOAc at a volume ratio of 1:4 and centrifuged at 10,464 x g for 5 min. The resulting precipitate was redispersed in n-hexane and mixed again with MeOAc at a volume ratio of 2:1, which is followed by a second-round centrifugation at 10,464 x g for 5 min. The purified PNCs were redispersed in n-octane for optical characterizations and used as the precursor solution to fabricate PNC films.

Fabrication of PNCs films
Quartz substrates were cleaned in an ultrasonic bath for 30 min with detergent, deionized water, acetone, and isopropanol, respectively, and then treated with ultraviolet-ozone (UV-ozone) for 15 min. Subsequently, 25 \mu L of PNC solution were spin-coated onto the quartz substrate at 2000 rpm for 30 s. For thermal annealing treatment, the PNC films were heated at 150 °C for 30 min in air. For preillumination treatment, the annealed films were irradiated with continuous-wave light (wavelength: 450 nm, power density: 14 mW/cm2) for 30 min.

Structural characterization of PNCs
Transmission electron microscopy images were obtained on a FEI Talos F200X microscope with a 200 kV accelerating voltage. Dynamic light scattering measurement was conducted on a Malvern Zetasizer Nano ZS90 particle size analyzer. The surface morphology of the PNC films was characterized using a ZEISS Gemini SEM 300 field-emission scanning electron microscope, and the film thickness was determined by a Hitachi SU8010 field-emission SEM. X-ray diffraction patterns were obtained by using a Shimadzu XRD-7000 diffractometer with Cu K\alpha radiation performed in the 2\theta range of 10°–60° at a scan rate of 2° min−1.

Typical optical and spectroscopic characterizations
Ultraviolet-visible (UV–vis) absorption spectra were measured with an Agilent Cary 60 UV-Vis spectrophotometer operating at a scanning rate of 10 nm/s and covering a wavelength range of 300–800 nm. Steady-state photoluminescence (PL) spectra were measured by using an Edinburgh FLS980 spectrometer, and the PL quantum yield was determined with the same instrument equipped with an integrating sphere. For typical time-resolved PL measurement, a femtosecond pulsed laser (PHAROS, 190 fs, 6 kHz), in conjunction with an optical parametric amplifier (OPA, ORPHEUS) were utilized to generate the excitation beam at 450 nm. The beam was attenuated by a neutral density filter to control its intensity and then directed to the sample. The as-generated TRPL signals were collected by a photomultiplier tube (Hamamatsu H7421) and recorded by a time-correlated single-photon counting (TCSPC) module (PicoQuant, PicoHarp 300). To achieve the wide-field PL microscopy image, the 457 nm-CW light by a laser diode (Changchun New Industries, MBL-U-457 nm) was focused on the surface of PNC films by the objective lens (Nikon, CF Plan Apo, 150X), whereupon the PL signals were collected by a CMOS camera (Intelligent Scientific Systems, IsCMOS-TRC411) for visualization.

Integrated in situ PL spectroscopy measurement
The principle of the integrated in situ spectroscopy for the synchronous measurement of steady-state PL spectra and TRPL kinetics is illustrated in Fig. 3a. In detail, a continuous laser diode (Laserland, Wuhan RLTC Technology Co., Ltd., 1875A-450D-80mw-5v) and a picosecond laser diode (Edinburgh Instruments EPL-405, ~100 ps, 5 MHz) were used to generate the CW beam (wavelength: 450 nm) and pulse beam (wavelength: 405 nm), respectively, whose intensities were strictly modulated by the neutral density filters (Daheng Optics, GCO-074M) to meet the criteria of perturbation. By using a dichromatic mirror (Lbtek DM10-425SP), the CW and pulse beams were directed to the same position of the tested PNC film. A long-pass filter (Daheng Optics GCC-300112) was adopted to remove the excitation signals, and the PL signals were separated by a beamsplitter (Heng Yang Guang Xue HCBS1-025-70-VIS, splitting ratio: 7:3), whereby 70% of the signals were collected by a spectrometer (AvaSpec-ULS2048XL) and the remaining 30% of signals were recorded by a photomultiplier tube (Hamamatsu H7421) connected with the time-correlated single photo counting module (PicoQuant, PicoHarp 300).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I think the manuscript improved overall and the presentation is better. I can follow things now, and I find that the main point — that the trend is superlinearity under CW light and sublinearity under pulsed excitation — is clear. I think the authors convinced me on that point, and I agree this is probably a reflection of trapping, and that this approach can be used to study trapping. In a very stretched way, it can also be related to “operando” conditions, though that’s limited to optical illumination, not a real device bias... I personally would not call it operando and most device people as well would be confused.\n\nThe part about lifetime change is still shaky. The graph itself doesn’t convince me. The authors really try to sell it, but the experiments are so different — illumination power, laser system, repetition rate, everything. You can’t directly compare absolute lifetimes between these two regimes. It’s okay to show this data, but it shouldn’t be pushed as a key point.\n\nI was also a bit hesitant about data processing, but I think that even if there are some issues there, the main message is still sound.\n\nBased on this, I think the manuscript is generally publishable. It shows a certain level of robustness, especially if the authors are more honest about what is clearly demonstrated and what is more speculative.\n\nBut does it fit Nature Communications? I don’t think so. The authors show that under certain measurement conditions you can see effects of trapping. That’s good, but it’s not groundbreaking. From this point of view, it’s probably not for Nature Communications. The quantitative part also doesn’t feel robust enough, that’s my honest opinion.\n\nFinally, I know the other two reviewers were quite positive, but I don’t feel they looked deep enough into the data to really see these methodological nuances and what actually matters here. If the editor has doubts, I would suggest asking for the opinion of another spectroscopist, someone who really works on photochemical kinetics. I think that would help to evaluate the impact properly. In my view, the impact doesn’t fit Nature Communications. Scientific Reports or Communications Chemistry would be a more suitable.", "reviewer_2": "The authors have well revised the manuscript and answered the questions. This paper is suitable for publication with a minor addition.\n\nSince the authors are discsussing TRPL with a streak cameran on LHP QD< I encourage them to cite the most recent and precise work on the subject published bwlopw.\nhttps://pubs.rsc.org/en/content/articlelanding/2024/nr/d4nr01481b/unauth\nhttps://pubs.aip.org/aip/jcp/article-abstract/158/15/154702/2882245/Observing-strongly-confined-multiexcitons-in-bulk?redirectedFrom=fulltext\nhttps://pubs.acs.org/doi/abs/10.1021/acs.nanolett.3c03975\nhttps://pubs.acs.org/doi/abs/10.1021/acs.nanolett.3c03180", "reviewer_3": "I am satisfied that the authors have fully addressed the concerns raised in the initial review. I believe the authors have addressed the concerns of the reviewers generally very well. The revisions provide clear experimental details, improved data presentation, and quantitative analysis that support the conclusions. The rationale for the “operando” CW-bias TRPL approach is better articulated - I believe it is potentially useful for the authors to consider adding a one-sentence definition early in the Introduction clarifying the use of operandi to mean under steady-state CW optical conditions, emulating device illumination conditions, weather than any electrical bias.\n\nThe manuscript is consistent and much improved from the original manuscript - I believe this will be of benefit to the scientific field and nor warrants publication subject to perhaps some minor editorial corrections for clarity and consistency.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to the editor’s and reviewers’ comments:\n# Response to the editor’s comments\nIn particular, the referees collectively raise critical concerns influence of the CW light in\nmodifying the sample during the measurements, making the influence of photodegradation\neffects unclear, as well as whether the current approach provides operando data and questioning\nthe reproducibility of the experiments. In addition, reviewer #1 is concerned with data\npresentation, potentially exaggerating difference of in-situ TRPL to conventional TRPL.\nFurthermore, reviewer #2 is concerned with the choice of the system and comparison of pristine\nand annealed films.\nResponse:\nWe are grateful to the editor and reviewers for pointing out the issues of CW light-induced\nphotodegradation, experimental reproducibility, data presentation, and the study systems,\nwhich may pose potential problems in exploring the intrinsic operando behavior of charge\ncarriers.\n(1) Photodegradation and reproducibility issues. To avoid the occurrence of irreversible\nphotodegradation and ensure experimental reproducibility, we fabricate photostable PNC\nfilm via a successive thermal annealing and CW preillumination treatment. The presented\nresults (e.g., Figure 2f, Figure 3b, Figure 3c, Figure S7, and Figure S8) collectively verify\nthe suppression of photodegradation for the tested perovskite nanocrystal (PNC) films\nduring spectroscopic measurements. Furthermore, a series of steady-state and time-\nresolved confirmatory experiments have been added to verify the reproducibility (Figure\nS13 and Figure S14). More detailed discussion will be shown later to address the reviewers’\nconcerns.\n(2) Data presentation issue. The presentation of TRPL data has been improved according to\nReviewer-1’s suggestion. As shown in Figure 4c, Figure 4d, Figure 5a, and Figure 5b,\nall the typical and in-situ TRPL data share the identical vertical and horizontal scales. Upon\nthese revisions, the different features in TRPL kinetics are more clearly recognized now.\n(3) Research system issue. As stated above, we have replaced the previous research system\n(e.g., the unstable PNC films with or without an annealing treatment) with the improved,\nultrastable PNC film in the revised manuscript. Per the comment of Reviewer-2, the\nmanuscript has been modified to concentrate on the operando kinetics of charge\nrecombination in stable PNC film. The comparison between the unannealed and annealed\nfilms, in which irreversible photodegradation occurs during the optical experiments, has\nbeen removed from the manuscript. Such a revision engenders a more compact logic\nstructure and a more reliable conclusion.\n1\n# Response to the Reviewer-1’s comments\nThe paper by Cao et al. presents a spectroscopic study of quantum dot perovskite materials,\nwith a focus on their potential use in color conversion layers. The authors perform time-\ncorrelated single-photon counting (TCSPC) measurements under steady-state illumination bias\nand conclude that multiple processes are involved in controlling the emission dynamics,\nincluding Auger recombination and photodegradation-related photochemistry.\nThe study gives me mixed feelings. On one hand, the general approach is interesting.\nPerforming measurements under steady-state illumination bias is a promising way to\nunderstand the dynamics of materials under conditions relevant to actual device operation. On\nthe other hand, instead of carefully presenting and analyzing their results, the authors often push\nforward broad, principle-challenging claims and present their data in a way that can be\nmisleading. I find that the paper, in its current form, is not methodologically solid and would\nrequire substantial revision before it could be considered for publication. Furthermore, I feel\nthe work does not meet the high standards expected for Nature Communications, and would be\nmore suitable for a specialized journal such as Chemistry of Materials or Communications\nChemistry.\n1. The authors describe their measurements as \"operando dynamics.\" I believe this is\nmisleading. The term would be appropriate if the processes they observed were reversible,\nand if steady-state illumination accurately simulated device operating conditions. However,\nmany of the changes observed are irreversible, involving photodegradation and permanent\nchemical alterations. Therefore, the premise that the study represents true \"operando\"\nconditions is flawed. A major part of the introduction and context needs to be rethought\nand rewritten to reflect that the focus should be on photodegradation, not just dynamic\nprocesses during operation.\nResponse:\nWe thank the reviewer for highlighting the potential issues that may have arisen due to the\nirreversible alterations undergone during the CW measurements. Following a more exhaustive\nexamination, it has been determined that the irreversible PL enhancement/decline, as well as\nthe lifetime prolongation/shortening, should be ascribed to the chemical instability of the\nperovskite nanocrystal (PNC) films rather than the operando properties demonstrated in the\nprevious manuscript. In the revised manuscript, we have redesigned the tested sample and\nconducted a series of experiments to verify the experiment reproducibility and the capability of\nthe integrated in-situ PL platform in tracking operando TRPL kinetics, as detailed below:\n(1) Ultrastable PNC films are prepared through a two-step post-synthesis treatment method,\ncomprising annealing and preillumination. Thermal annealing is suggested to remove\nexcess organic ligand, thus preventing the occurrence of irreversible PL decline related to\nphoto-induced ligand detachment of the dispersed perovskite quantum dots (Figure 4a of\nthe previous manuscript). Concurrently, the preillumination treatment can improve both the\ncircumventing the irreversible PL enhancement of the annealed PNC film (Figure 4c of the\nprevious manuscript).\n) without annealing\nm.1.6\nannealing\no r preillumination\nn1.4\n(\ny\nsit1.2\nn\ne\nnt1.0\nL\ni\nP0.8\n0 2 4 6 8\nTime (min)\nFigure R1. PL intensity evolution of the PNC films with different treatments under continuous light\nillumination (corresponding to Figure 2f).\nAs verified by in-situ PL spectroscopy and summarized in Figure R1, the two-step treated\nfilm possesses excellent photostability for subsequent optical characterizations.\n(2) To guarantee that all the experimental findings are reversible and reproducible, we have\ncarefully controlled the CW light intensity to be lower than 15 mW/cm2.\nFigure R2. Steady-state PL spectra (left) and TRPL kinetics (right) of the PNC film before and after\na complete experimental period (corresponding to Figure S13 and Figure S14, respectively).\nFurthermore, both the steady-state PL spectra and TRPL kinetics for the same PNC film\nbefore and after a complete experimental period have been measured for quantitative\ncomparison (Figure R2). The variations in PL intensity and TRPL lifetime are less than\n5%, in great line with the robustness of the improved sample in spectroscopy measurements.\nAccordingly, the manuscript has been thoroughly revised. Firstly, additional context was\nincorporated to introduce the two-step treatment method and to highlight the resultant variation\nof photophysical properties. Secondly, more details of the operando PL measurements have\nbeen added in the manuscript, which demonstrates the stability of both the steady-state PL and\nTRPL units of the testing platform. Thirdly, as displayed above, the spectroscopic features are\ncarefully compared before and after the entire experimental period, excluding the impact of\nirreversible factors (e.g., undesired photodegradation) on the PL features observed in the\npresent work.\nPage 4: The films are fabricated by spin-coating the purified PNC colloidal solutions onto\na quartz substrate subsequently followed by thermal annealing and preillumination treatments,\nas schematically illustrated in Figure 2a.\nPage 4: Figure 2f displays the PL intensity evolution versus illumination time for the three\ntypes of PNC films, as derived from the long-term in-situ PL spectroscopy measurement (Figure\nS7), which unambiguously manifests the efficacy of the sequential annealing and\npreillumination treatment on optimizing the photostability.\nPage 5: Figure 3b displays the as-obtained in-situ PL spectra, which are centered at 535\nnm regardless of illumination period, aligning closely with the standard PL spectrum shown in\nFigure 2e. The fluctuation of PL intensity is found to be less than 2% over the entire\nexperimental period (Figure S8), validating the prominent stability of the integrated PL\nspectroscopy platform.\nPage 5: Concomitant with the in-situ PL spectra measurement, TRPL results are obtained\nby means of TCSPC. Three sets of TRPL kinetics with the same accumulation time of ~660 s,\ncorresponding to the three regions denoted in in-situ PL spectra as exemplified in Figure 3c,\nshare the identical time-resolved features……Secondly, the three sets of TRPL kinetics are self-\nsimilar with a variation of average lifetime as small as ~4% (Figure S9 and Table S1),\nindicative of excellent reproducibility for the in-situ TRPL test.\n4\n2. I am very disappointed by the presentation of Figures 2c, d, g, and h. It appears that the\nfigures were designed in a way that could mislead an uncareful reader. In comparing\nconventional TRPL and photoluminescence under illumination bias, the data should be\npresented on identical scales, and the excitation intensities should be normalized\nconsidering the excitation density in the sample. The authors fail to do this, instead\npresenting the datasets at very different scales and intensities to suggest that the results with\nand without bias are dramatically different — which is not true. After extracting and\nreplotting the data myself, I find that the kinetics are actually very similar; only the trends\nwith increasing illumination intensity differ somewhat. This is a serious flaw in data\npresentation.\nResponse:\nWe thank the reviewer for the important comments on data presentation. In the revised\nmanuscript, we have normalized scales and intensity scales for the typical TRPL and in-situ\nTRPL of the PNCs, as suggested by the reviewer. Figure 4c, Figure 4d, Figure 5a, and Figure\n5b now clearly show the difference between the typical and in-situ TRPL kinetics and that\nbetween the PNC solution and PNC film.\nRegarding the excitation density, it seems insignificant to normalize the values of the pulse\nbeam in typical TCSPC measurements and the CW beam in in-situ TCSPC measurements. For\nexample, as stated in the revised manuscript, the peak power density of the fs pulse is several\norders of magnitude stronger than that of the CW light, when their average pump fluences are\ncomparable. Specifically, the pump power of the pulse laser strongly depends on its repetition\nrate. Therefore, as Reviewer-2 pointed out, to circumvent this issue, the pump fluence of the\npulse beam should be shown in units of “J/cm2” instead of “mW/cm2”, which has been done\nin the present manuscript.\nRevisions:\nPage 6 & Page 7:\nPage 2: At a comparable pump fluence, for instance, the peak power density of the ultrafast\npulse is several orders of magnitude stronger than that of CW light. Therefore, a series of\nnonlinear optical processes can be detected by TRPL, while their occurrence is much less\nprobable in real devices.\n5\n3. Figure 4 clearly shows the effects of photodegradation and related permanent\nphotochemical changes. However, the authors do not attempt to quantify the contribution\nof these irreversible changes compared to other dynamic effects such as multiparticle\ninteractions and Auger processes. Without a quantitative analysis, it is very difficult to\njudge how significant the reported findings are. A thorough decomposition and estimation\nof different contributions are essential.\nResponse:\nWe appreciate the reviewer for reminding us of considering the influence of\nphotodegradation and related permanent photochemical changes on the observed spectroscopic\nvariations. As replied to the first comment, we have made substantial efforts to improve the\nstability of the tested sample and to validate the reliability of the in-situ spectroscopy platform,\nwhich are displayed in Figure R1 and Figure R2, respectively. The experimental results\ncollectively verify that photodegradation as well as any irreversible alterations are completely\nsuppressed in the revised manuscript.\nHaving excluded the interference of permanent photochemical changes, we further\nperform systematic quantitative analysis (Figure 5a-5c, Figure S15, Figure S16, Table S2),\naccompanied by optical imaging characterizations (Figure 5d), to rationalize the specific in-\nsitu TRPL features that substantially differ from the typical TRPL data. This ultimately allows\nus to develop an accurate physical model (Figure 5f) derived from the equilibrium-state\nclassical continuity equation, which excellently interprets the landscape of charge\nrecombination kinetics in PNC films under operational conditions.\nRevisions:\nPage 4: The films are fabricated by spin-coating the purified PNC colloidal solutions onto\na quartz substrate subsequently followed by thermal annealing and preillumination treatments,\nas schematically illustrated in Figure 2a.\nPage 4: Figure 2f displays the PL intensity evolution versus illumination time for the three\ntypes of PNC films, as derived from the long-term in-situ PL spectroscopy measurement (Figure\nS7), which unambiguously manifests the efficacy of the sequential annealing and\npreillumination treatment on optimizing the photostability.\nPage 7: As illustrated in Figure 5a, the in-situ TRPL kinetics of the PNC solution present\nnegligible alteration in response to a wide range of CW pump intensity variations. No\nadditional rapid decay component is observed at high pump fluence, in contrast to the typical\nTRPL data shown in Figure 1c, indicating that the multiexciton effect is minimized under CW\nphotoexcitation. The distinctive in-situ TRPL behaviors facilitate the ascription of the retarded\nTRPL decay to the specific microscopic structure of PNC films, whose formation is\naccompanied by the transition from highly dispersed-state nanoparticles in solution (Figure 1a)\nto sintered close-packed large grains in solid-state configuration (Figure 2c).\nequilibrium-state classical continuity equation in polycrystalline perovskites is typically\nexpressed as: 𝑑𝑛 /𝑑𝑡 = 𝐺−𝑘 𝑛 −𝑘 𝑛2 = 0, where 𝑛 is the carrier density at the\n0 1 0 2 0 0\nequilibrium state, 𝑡 is time, 𝐺 is the rate of photocarrier generation, 𝑘 is the rate constant\n1\nfor first-order nonradiative recombination, and 𝑘 is the rate constant for bimolecular\n2\nrecombination. Once a perturbation is superposed on the equilibrium state, the expression\nshould be rewritten as: 𝑑(𝑛 +𝛿𝑛)/𝑑𝑡 = 𝐺−𝑘 (𝑛 +𝛿𝑛)−𝑘 (𝑛 +𝛿𝑛)2, in which 𝛿𝑛 is\n0 1 0 2 0\nthe density of excess carrier with a value much smaller than equilibrium-state carrier density\nin a perturbation case (𝛿𝑛 ≪ 𝑛 ). A combination of the two aforementioned equations results\n0\nin the classical continuity equation being reduced to the following form: 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−\n1\n2𝑘 𝑛 𝛿𝑛, which is indicative of first-order reaction kinetics. In-situ TRPL experiment fulfills\n2 0\nthe prerequisites of perturbation, therefore the time-dependent PL intensity is predicted to be\ndescribed by the first-order kinetics model with a decay lifetime of 𝜏 , 𝐼 (𝑡) = 𝐼 (0)⋅\n𝑃𝐿 𝑃𝐿\n𝑒𝑥𝑝 (−𝑡/𝜏). Furthermore, in consideration of the inhomogeneity of the chemical environment\nfor individual perovskite grains, it has been proposed that the TRPL data be fitted more\naccurately with a bi-exponential function24 or a stretched exponential function25. Here we adopt\nthe bi-exponential model for elucidation due to its higher fitting goodness than the stretched\nexponential model (Figure S15).\nPage 7: A representative in-situ TRPL data in conjunction with the biexponential fitting\nresults are depicted in Figure 5b, where the fast (𝜏 ) and slow (𝜏 ) decay components have been\n1 2\nsuggested to arise from charge recombination at the grain interface/surface and within the\nvolume of grains, respectively26,27.\nPage 8: By visualizing the light emission patterns, wide-field PL microscopy images\n(Figure 5d) further verify the improved PL efficiency of PNC films upon a strong CW\nillumination. Specifically, the PL imaging experiments are conducted by simultaneously\nreducing the excitation power (P) and prolonging the integrating time () with the same factor.\nWhile the product of P and  remains constant, the PL intensity exhibits a prominent decrease\nwith the reduction in P (see detailed analysis in Figure S17), indicative of a higher PLQY by\nenhancing the CW illumination.\n7\n4. Figure 5 is highly qualitative and does not allow the reader to assess the confidence level of the conclusions or sufficiently support the interpretation of the spectroscopic data. Figure 5 is just a cartoon, not adding much to the study or supporting it. A more rigorous, controlled investigation correlating structural changes (upon annealing or in some other way) to optical properties, using systematical and quantitatively well-defined sample sets, would be needed to substantiate the proposed interpretations. As it stands, the data do not have the level of rigor expected for a Nature Communications publication.\nResponse:\nWe thank the reviewer for indicating the issue of qualitative analysis in the previous\nmanuscript, which looks insufficiently rigorous for supporting the physical model. In order to\naddress this concern, Figure 5 has been carefully revised from four aspects as detailed below:\n(1) Quantitative analysis. Within the framework of classical continuity equation in perovskite\nfilms, we have mathematically proved that charge recombination kinetics in a perturbation\nmanner follows the first-order reaction description (Page 7). This is premised on the fact,\n𝛿𝑛 ≪ 𝑛 , as has been manifested to be held for the in-situ TRPL measurement (Page 5,\nperovskite grains, the ensemble TRPL behavior generally turns out to be a multi-\nexponential or stretched exponential form. In line with the suggestion of Reviewer-2, we\nselect the bi-exponential function to fit the TRPL kinetics due to its higher fitting goodness.\n(Figure 5b, Figure 5c, Figure S16, Table S2). The quantitative analysis results help\nhighlight the significant role of the charge-carrier and trap-state interaction in determining\nthe TRPL features.\n(2) Comparison of PNC with different dispersion states. As discussed in the manuscript (Page\n7), the retarded TRPL decay with the increase of CW light intensity arises from the PNC\nsintering. Such phenomenon observed in close-packed PNC film is absent in PNC colloidal\nsolutions under the identical experimental conditions (Figure 5a), providing an additional\nsupport to the proposed physical model.\n(3) Additional visualization evidence. In order to more clearly showcase the CW light-induced\nbrightening of PNC films, wide-field PL imaging test was conducted for visualizing this\neffect (Figure 5d). The CW light intensity and the integrating time are simultaneously\nchanged to keep their product unchanged. If Auger recombination occurs at high CW\nintensities, the PL image will become dark with the elevation of light intensity and\nshortening of integrating time; otherwise, in the case of linear PL, the image brightness is\nanticipated to be unchanged. Interestingly, we have unambiguously observed an enhanced\nbrightness under the strong CW illumination, which differs from the predication of both\nAuger recombination and linear PL and is in excellent agreement with reversable charge-\ncarrier and trap-state interaction stated in our work.\n(4) Refined physical model. In the revised manuscript, the physical model has been improved\nby connecting the quantitative analysis with a more comprehensive consideration of the\nresults. Moreover, the physical symbols used in the mathematic equations are clearly\ndenoted in the schematic (Figure 5f), which explicitly uncover the effect of CW light on\nincreasing the PL efficiency. With these improvements, we believe that the physical model\nis more rigorous now and provides sufficient information to interpret the experimental\nfindings.\nRevisions:\nPage 7: As illustrated in Figure 5a, the in-situ TRPL kinetics of the PNC solution present\nnegligible alteration in response to a wide range of CW pump intensity variations. No\nadditional rapid decay component is observed at high pump fluence, in contrast to the typical\nTRPL data shown in Figure 1c, indicating that the multiexciton effect is minimized under CW\nphotoexcitation. The distinctive in-situ TRPL behaviors facilitate the ascription of the retarded\nTRPL decay to the specific microscopic structure of PNC films, whose formation is\naccompanied by the transition from highly dispersed-state nanoparticles in solution (Figure 1a)\nto sintered close-packed large grains in solid-state configuration (Figure 2c).\nPage 7: In quantitative, provided that multi-body Auger recombination is negligible, the\nequilibrium-state classical continuity equation in polycrystalline perovskites is typically\nexpressed as: 𝑑𝑛 /𝑑𝑡 = 𝐺−𝑘 𝑛 −𝑘 𝑛2 = 0, where 𝑛 is the carrier density at the\n0 1 0 2 0 0\nequilibrium state, 𝑡 is time, 𝐺 is the rate of photocarrier generation, 𝑘 is the rate constant\n1\nfor first-order nonradiative recombination, and 𝑘 is the rate constant for bimolecular\n2\nrecombination. Once a perturbation is superposed on the equilibrium state, the expression\nshould be rewritten as: 𝑑(𝑛 +𝛿𝑛)/𝑑𝑡 = 𝐺−𝑘 (𝑛 +𝛿𝑛)−𝑘 (𝑛 +𝛿𝑛)2, in which 𝛿𝑛 is\n0 1 0 2 0\nthe density of excess carrier with a value much smaller than equilibrium-state carrier density\nin a perturbation case (𝛿𝑛 ≪ 𝑛 ). A combination of the two aforementioned equations results\n0\nin the classical continuity equation being reduced to the following form: 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−\n1\n2𝑘 𝑛 𝛿𝑛, which is indicative of first-order reaction kinetics. In-situ TRPL experiment fulfills\n2 0\nthe prerequisites of perturbation, therefore the time-dependent PL intensity is predicted to be\ndescribed by the first-order kinetics model with a decay lifetime of 𝜏 , 𝐼 (𝑡) = 𝐼 (0)⋅\n𝑃𝐿 𝑃𝐿\n𝑒𝑥𝑝 (−𝑡/𝜏). Furthermore, in consideration of the inhomogeneity of the chemical environment\nfor individual perovskite grains, it has been proposed that the TRPL data be fitted more\naccurately with a bi-exponential function24 or a stretched exponential function25. Here we adopt\nthe bi-exponential model for elucidation due to its higher fitting goodness than the stretched\nexponential model (Figure S15).\nPage 7: A representative in-situ TRPL data in conjunction with the biexponential fitting\nresults are depicted in Figure 5b, where the fast (𝜏 ) and slow (𝜏 ) decay components have been\n1 2\nsuggested to arise from charge recombination at the grain interface/surface and within the\nvolume of grains, respectively26,27.\nPage 8: By visualizing the light emission patterns, wide-field PL microscopy images\n(Figure 5d) further verify the improved PL efficiency of PNC films upon a strong CW\nillumination. Specifically, the PL imaging experiments are conducted by simultaneously\n9\nreducing the excitation power (P) and prolonging the integrating time () with the same factor.\nWhile the product of P and  remains constant, the PL intensity exhibits a prominent decrease\nwith the reduction in P (see detailed analysis in Figure S17), indicative of a higher PLQY by\nenhancing the CW illumination.\nPage 8: The landscape of operando charge recombination revealed by in-situ TRPL is\nillustrated in Figure 5f, where 𝑛 and 𝛿𝑛 denote the charge carriers generated by the CW\n0,𝐼𝑖\nbeam and the pulse beam, satisfying the perturbation requirement, 𝛿𝑛 ≪ 𝑛 . The decay of\n0,𝐼𝑖\n𝛿𝑛 involves two pathways: trap state-mediated nonradiative charge recombination (dashed\narrow) and radiative charge recombination (solid arrow), which correspond to the first and\nsecond terms, respectively, in the continuity equation 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−2𝑘 𝑛 𝛿𝑛. With the\n1 2 0\nelevation of the CW pump intensity, the value of 𝑛 is increased, which is anticipated to\n0\naccelerate the TRPL decay; by contrast, the density of empty trap states (i.e., nonradiative\nrecombination centers) is decreased owing to the trap filling by charge carriers, thus resulting\nin a reduced 𝑘 and a retarded recombination kinetics. Under operational conditions, the\n1\ndensity of charge carriers is several orders of magnitude smaller than the trap state density32–\n35. This indicates that the effect of photoexcitation on filling trap states is dramatically greater\nthan that on increasing 𝑛 . As a consequence, the in-situ spectroscopy results demonstrate a\n0\nconcomitant increase in the steady-state PL intensity and prolongation in the TRPL lifetime\nwith the enhanced CW pump fluence, which mirrors the landscape of charge recombination in\nPNC films under realistic operational conditions that is not revealed by typical TRPL\ntechniques.\n10\n# Response to the Reviewer-2’s comments\nThis maniscript reports on the spectroscopy of lead-halide perovskite quantum dots (QD)\nunder operational conditions for LEDs. This is a very interesting and valuable manuscript and\nprobably should be published. But there are many questions to address.\n1. The authors use PL dynamics initially and then switch to kinetics. The initial point is that\nconsistency in terms should be used. Pick one or the other. But it turns out that that the\nsemantics matter. PL rarely measures dynamics. It mostly measures kinetics. There is a\nrecent discussion of the differences between kinetics and dynamics. And there are recent\nexamples of t-PL dynamics in LHP QD.\nResponse:\nWe thank the reviewer for highlighting the difference between dynamics and kinetics. As\nmentioned by the reviewer, the TCSPC data that display the PL decay profile as a result of\ncharge recombination should be labelled “kinetics” rather than “dynamics”. The term has\ntherefore been unified to “kinetics” in the revised manuscript.\nRevisions:\nPage 1, Title: Operando Recombination Kinetics in Perovskite Nanocrystal\nFilms\nRevealed by In-Situ Time-Resolved Photoluminescence.\nPage 1, Abstract: In contrast to the established Auger recombination model, which\nsuggests a more rapid TRPL kinetics at high pump fluence, it is demonstrated that the\ndependence of CW-driven PL intensity on pump fluence is governed by charge recombination\nkinetics in the context of charge-carrier and trap-state interactions.\nPage 2: The sophisticated design of the photoexcitation method enables precise\nexamination of the exciton/charge recombination kinetics of PNCs under the sustained\nexcitation of a CW beam. As a proof of concept, the TRPL kinetics of photostable, close-packed\nPNC films were investigated by varying the CW excitation intensity over twenty folds.\nPage 5: The integrated in-situ PL spectroscopy technique proposed for interrogating the\noperando charge recombination kinetics of PNC films is presented in Figure 3a.\nPage 6: Having experimentally verified the reliability of the integrated in-situ PL\nspectroscopy platform, we conduct a systematic exploration into the operando charge\nrecombination kinetics of PNC films.\nPage 8: With the elevation of the CW pump intensity, the value of 𝑛 is increased, which\n0\nis anticipated to accelerate the TRPL decay; by contrast, the density of empty trap states (i.e.,\nnonradiative recombination centers) is decreased owing to the trap filling by charge carriers,\nthus resulting in a reduced 𝑘 and a retarded recombination kinetics.\n1\n11\n2. The introduction begins with the good features of LHP QD for LED. But why not CdSe\nand other covalent QD that were investigated first? Why are LHP QD special. One answer\nis their unique lattice that is defect tolerant and does so by supporting liquid-like structural\ndynamics.\nResponse:\nWe thank the reviewer for the suggestion of emphasizing the motivation behind the\ninvestigation of the perovskite nanocrystals (PNCs) and the recommendation of important\nreferences.\nAs presented in the revised introduction of the manuscript, one of the most important\nreasons for selecting PNC, in agreement with the reviewer’s comment, is the unique\nphysicochemical properties of PNCs, such as the significant defect tolerance (A. L. Rogach,\net al. ACS Energy Lett., 2017, 2, 2071) and polaronic features (P. Kambhampati, et al. Nat.\nCommun., 2019, 10, 4962), differing from the traditional binary QDs. In the meantime, in\ncontrast to the rapid development of high-efficiency PNC optoelectronic devices, mechanistic\nstudies on the operando charge carrier kinetics still lag behind, which serves as another reason\nfor delving into the operando behaviors of PNCs.\nThe relevant contexts and references have been added to the revised manuscript.\nRevisions:\nPage 2: In addition to the significant advances in material design and device fabrication,\nconsiderable efforts have been also made over the past decade to elucidate the unique\nphysicochemical properties of PNCs, such as excellent defect tolerance7 and drastic polaronic\nfeatures8, aiming to further extend the utilization of PNCs in specific practical applications.\n12\n3. The authors refer to t-PL measurements with streak cameras. They should cite the most\nrecent and most detailed t-PL measurements of LHP QD that reveal their dynamics and\nkinetics for particles in dispersion.\nResponse:\nWe thank the reviewer for this constructive suggestion. Accordingly, one of the recently\npublished references recommended by the reviewer (P. Kambhampati, et al. Nano Lett., 2024,\n24, 61) has been cited to replace the previous one in the revised manuscript.\nRevisions:\nPage 2: Time-resolved PL (TRPL) techniques, including streak camera9, fluorescence\nupconversion10, and time-correlated single photon counting (TCSPC)11 are hitherto the most\nwidely used approaches for investigating the energy conversion mechanism of PNCs.\n13\n4. My main concern is that the annealed QD film is so different from the pristine film that it\nmay not be meaningful to compare. The pristine film contains QD of 10 nm diameter. These\nare true QD. But the annealed film contains nanocrystals of 100 nm diameter which are not\nQD. Some NC are QD and some QD are NC but they are not the same as recently reviewed.\nThe main issue is the diameter relative to the exciton Bohr length which confers QD effects\nto excitonic structure and dynamics.\nResponse:\nWe thank the reviewer for pointing out the misuse of “QD” in this work and the potential\nconfusion it may have caused. As replied to the comments from Reviewer-1, the revised\nmanuscript now focuses on the photostable, close-packed PNC film.\nThe comparison between the PQD solution and the PNC film has been removed due to the\ninstability issue of the former during optical experiments, which causes irreversible\nphotodegradation. In addition, the annealing time has been prolonged to 30 minutes to yield a\nmuch more homogeneous and sintered PNC film with an apparent size far beyond the exciton\nBohr diameter. This minimizes the influence of the quantum confinement effect on PL kinetics\nand makes the landscape of charge recombination analogous to that of polycrystalline\nperovskite films. To eliminate any confusion, we have replaced the term “perovskite quantum\ndot (PQD)” with “perovskite nanocrystal (PNC)”.\nRevisions:\nThe title, abstract, and main text have been thoroughly revised in line with the\nreviewer’s suggestion.\n14\n5. Fig1g shows the Abs spectra of the two films which look totally different. Why is there a\nmore prominent excitonic peak for the annealed QD that increase in size to NC that are not\nQD.\nResponse:\nThe enhanced excitonic absorption peak accompanied by a redshift of the Abs spectra upon\nthermal annealing has been widely observed in PNC films (Kamat et al., ACS Mater. Lett., 2019,\n1, 8; J. Am. Chem. Soc., 2016, 138, 8603; Acc. Chem. Res., 2021, 54, 520).\nFigure R3. UV-vis absorption spectra of CsPbBr nanocrystal films with different thermal annealing\n3\nperiods (Reprinted with permission from et al., ACS Mater. Lett., 2019, 1, 8. Copyright 2019 American\nChemical Society.). With the prolongation of annealing time from 0 min to 60 min, the excitonic peak\ngradually becomes more prominent and shifts to the long-wavelength side.\nAs exemplified in Figure R3, this phenomenon has been suggested to be an indication of\nnanoparticle sintering (“Specifically, the appearance of a sharper excitonic peak in the bulk\nsamples indicates the growth of very homogeneously sized large particles compared to the\nensemble of small particles in the NC film samples.” This text is cited from Kamat et al., ACS\nMater. Lett., 2019, 1, 8). This interpretation seems consistent with the experimental data\nreported and presented.\n15\n6. Typo erros here: “Now we move forward to the PL mechanism of PQD films under the\noperational conditions of CCLs. Figure 2a displays 90 the steady-state PL emission spectra\nof the annealed film excited by a CW beam”. It should be unannealed film\nResponse:\nWe thank the reviewer for pointing out the typo errors. The manuscript has been thoroughly\nreorganized and carefully revised, and we that all the typos have been calibrated in the revised\nversion.\nRevisions:\nThe title, abstract, and main text have been thoroughly revised.\n16\n7. Fig2b. Why does the pristine filme show a sublinear saturation curve for intensity? It should\nslow slightly due to XX formation. It may also slow due to hot exciton trapping at the\nsurface which causes non-radiative recombination. The authors should do a PLE\nexperiment and measure the QY as a function of energy to verify if there are hot exciton\nsurface effects19, 20. Fig2c suggests that there is faster nonradiative decay by an excitation\ninduced process.\nResponse:\nWe thank the reviewer for the comments on the sublinear pump fluence-dependent PL\nbehavior in the unannealed PNC films. Following a thorough investigation, we find that the\ndecline in PL efficiency upon the elevation of CW pump fluence does not stem from the intrinsic\nphotophysics of PNCs, including multi-exciton Auger recombination and hot exciton effect.\nConversely, the process is irreversible; that is, when the CW light fluence is reduced to the\nprevious level following the entire in-situ PL experiment period, the PL intensity cannot fully\nrestore (Figure 4a of the previous manuscript), which is attributable to photodegradation (e.g.,\nligand detachment and QD aggregation/decomposition).\nThe issue of photodegradation brings about substantial difficulties in evaluating the impact\nof the operational conditions on charge recombination kinetics. In order to overcome this\nproblem, ultrastable PNC films have been fabricated, which do not undergo irreversible\nalterations upon long-term CW light irradiation. The present work focuses on the PL behavior\nof stabilized PNC films, where the sublinear pump fluence dependence is absent (Figure 4b).\nIn the revised manuscript, we have demonstrated that the multi-exciton effect is negligible\nunder CW excitation, and the enhanced superlinear PL features can be attributed to the trap-\nstate and charge-carrier interaction.\nRevisions:\nPage 6: A series of representative steady-state PL spectra with varied CW pump fluences\nare presented in Figure 4a (see Figure S12 for complete data). The PL intensity increases with\nthe pump fluence while the spectral profile remains consistent. A closer inspection uncovers a\nsuperlinear relationship between the PL intensity and the CW pump fluence, as illustrated in\nFigure 4b and its inset. For instance, by multiplying the PL signal measured at 0.64 mW/cm2\nwith twenty, it is still significantly lower than that measured at 12.74 mW/cm2. Such a\nsuperlinear dependence indicates that the PLQY is enhanced with the elevation of the excitation\nintensity.\nPage 8: The landscape of operando charge recombination revealed by in-situ TRPL is\nillustrated in Figure 5f, where 𝑛 and 𝛿𝑛 denote the charge carriers generated by the CW\n0,𝐼𝑖\nbeam and the pulse beam, satisfying the perturbation requirement, 𝛿𝑛 ≪ 𝑛 . The decay of\n0,𝐼𝑖\n𝛿𝑛 involves two pathways: trap state-mediated nonradiative charge recombination (dashed\narrow) and radiative charge recombination (solid arrow), which correspond to the first and\nsecond terms, respectively, in the continuity equation 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−2𝑘 𝑛 𝛿𝑛. With the\n1 2 0\nelevation of the CW pump intensity, the value of 𝑛 is increased, which is anticipated to\n0\naccelerate the TRPL decay; by contrast, the density of empty trap states (i.e., nonradiative\nrecombination centers) is decreased owing to the trap filling by charge carriers, thus resulting\nin a reduced 𝑘 and a retarded recombination kinetics. Under operational conditions, the\n1\ndensity of charge carriers is several orders of magnitude smaller than the trap state density32–\n35. This indicates that the effect of photoexcitation on filling trap states is dramatically greater\nthan that on increasing 𝑛 . As a consequence, the in-situ spectroscopy results demonstrate a\n0\nconcomitant increase in the steady-state PL intensity and prolongation in the TRPL lifetime\nwith the enhanced CW pump fluence, which mirrors the landscape of charge recombination in\nPNC films under realistic operational conditions that is not revealed by typical TRPL\ntechniques.\n17\n8. Fig2. The in situe t=PL is different from the dispersion. Why is this? Mainly the in situ is\nsingle exponential whereas the dispersion is bi-exponential as is commonly seen. Why the\nsingle exponential?\nResponse:\nWe thank the reviewer for this pertinent enquiry. As the time scales for the typical and in-\nsitu TRPL results are not aligned (0 – 170 ns vs. 0 – 36 ns, Figure 2c and Figure 2d in the\nprevious manuscript), the latter appears to be more consistent with single exponential functions.\nAs seen in Figure 5b of the revised manuscript, the in-situ TRPL kinetics demonstrate a high\ndegree of alignment with the bi-exponential function. In addition, quantitative fitting results\nbased on the bi-exponential functions are provided in Figure 5c and the relevant text.\nRevisions:\nPage 8: A representative in-situ TRPL data in conjunction with the biexponential fitting\nresults are depicted in Figure 5b, where the fast (𝜏 ) and slow (𝜏 ) decay components have been\n1 2\nsuggested to arise from charge recombination at the grain interface/surface and within the\nvolume of grains, respectively32. As the CW pump intensity is elevated from 0.64 to 12.74\nmW/cm2 (Figure 5c), the value of 𝜏 increases by a factor of 37% (1.85 ns  2.53 ns) while\n1\nthe variation in 𝜏 is less than 13% (see complete fitting results in Figure S16 and Table S2).\n18\n9. Fig2c and d should be plotted on the same time window for better comparison\nResponse:\nWe thank the reviewer for this great suggestion. In the revised manuscript, both typical\nand in-situ TRPL data now share an identical time scale (Figure 4c, Figure 4d, Figure 5a,\nFigure 5b).\nRevisions:\nPage 6 & Page 7:\n20\n10. Fig2e shows a shape change to the spectrum. It looks like there is an ASE peak. Is there a\npeak developing? Perhaps a subtraction procedure would be useful.\nResponse:\nWe thank the reviewer for the insightful comments regarding the potential incorporation\nof ASE. As shown in Figure R4, the normalized PL profiles exhibit excellent overlap,\nexcluding the occurrence of ASE. Meanwhile, under a CW pump intensity as low as ~102\nmW/cm2, it is fairly difficult to attain population inversion.\n30 mW/cm2\nm.) 700 mW/cm2\nor\nn\ny\n(\nsit\nn\ne\nnt\nL\ni\nP\n480 500 520 540 560 580\nWavelength (nm)\nFigure R4. Normalized PL spectra with different CW pump intensities (corresponding to Figure 2e of\nthe previous manuscript).\n21\n11. I am confused about the description of data in Fig2. The ex-situ data are in a film or a\ndispersion? Is it that in site and ex situ are for a film or for a device?\nResponse:\nWe thank the reviewer for highlighting the description issue. All the steady-state and time-\nresolved PL data in Figure 2 of the previous manuscript are collected on PNC films. In the\nrevised manuscript, the tested sample information has been added in the figure caption for all\nspectroscopic data.\nRevisions:\nPage 5: Figure 3 (a) Schematic illustration of the integrated in-situ PL spectroscopy\nplatform for the simultaneous acquisition of steady-state PL spectra and TRPL kinetics (see\ntext for details). BS: beam splitter, DM: dichromatic mirror, LPF: long-pass filter, M: mirror,\nNDF: neutral density filter, PMT: photomultiplier tube. Representative experimental results of\n(b) in-situ PL spectra and (c) TRPL kinetics of the PNC film measured in a synchronous manner.\nPage 6: Figure 4 (a) Steady-state PL spectra of the PNC film measured by in-situ PL\nspectroscopy at varied CW pump fluence. (b) Corresponding PL intensity versus CW pump\nfluence, which displays superlinear dependence. Inset: comparison of the PL spectra measured\nwith excitation intensity values of 0.64 and 12.74 mW/cm2, respectively, where the former\nspectrum has been multiplied by a factor of twenty. Pump fluence-dependent PL decay kinetics\nof the PNC film measured by (c) typical TRPL and (d) in-situ TRPL. The numbers denoted in\npanel (c) correspond to the fluence of the femtosecond pulse beam. In panel (d), the pump\nintensities of the CW beam for the presented TRPL data are 0.64, 1.27, 6.37, and 12.74 mW/cm2,\nrespectively. For the sake of comparison, the baselines of in-situ TRPL data have been\nsubtracted, and the peak intensities have been normalized.\nPage 7: Figure 5 (a) CW pump intensity-dependent in-situ TRPL kinetics for the colloidal\nPNC solution. (b) Representative in-situ TRPL kinetics (solid line) of the PNC film measured\nat a CW pump intensity of 0.64 mW/cm2 and the corresponding fitting results (dashed line),\nwhere 𝜏 and 𝜏 are the lifetimes of the fast and slow components derived from the\n1 2\nbiexponential function. (c) Lifetime values of the fast (green) and slow (orange) decay\ncomponents as a function of CW pump intensity. Also denoted in percentage are the weights of\nthe slow component for each set of in-situ TRPL data. (d) Wide-field PL images of the PNC film\nmeasured with desired excitation power and integrating time. Scale bar: 5 m. Schematics for\nillustration of (e) the morphological evolution during PNC film fabrication and (f) the operando\nrecombination mechanism with different CW excitation levels.\n22\n12. The t-PL kinetics should be fit to a biexponential and report of the two time constant sand\namplitudes.\nResponse:\nWe thank the reviewer for this constructive suggestion. The results of bi-exponential fitting,\nincluding both the time constants and amplitudes of the two exponential components, are\npresented in the revised manuscript (Figure 5b, Figure 5c, Figure S16, Table S2).\nRevisions:\nPage S17:\nSupplementary Fig. 16. In-situ TRPL decay kinettics of the PNC film under different CW pump\nintensities fitted to the bi-exponential mode.\nSupplementary Table 2. Fitting results of in-situ TRPL kinetics derived from the results shown in\nSupplementary Fig. 16.\nPump intensity 0.64 mW/cm2 1.27 mW/cm2 6.37 mW/cm2 12.74 mW/cm2\n𝑨 0.78 0.75 0.66 0.60\n𝟏\n𝝉 (𝒏𝒔) 1.85 1.92 2.31 2.53\n𝟏\n𝑨 0.22 0.25 0.34 0.40\n𝟐\n𝝉 (𝒏𝒔) 32.70 33.54 36.87 35.6.\n𝟐\n<𝝉> (𝒏𝒔) 8.70 9.80 13.91 15.65\n23\n13. They switch from a fs pulse to a ps pulse without much explanation. Why was this done?\nWhare are the pulse durations and pump wavelengths? Both should be reported. They\ndiscuss the non-perturbative regime. That is not relevant. The main thing is to be in the\nsingle exciton regime where <N> < 1. There are many ways to garuntee this situation for\nfs excitation14, 21-23. But why switch to ps excitation. Moreover for pulsed excitation the\nunit should be fluence in uJ / cm2 not mW/cm2. It is a pulse with an energy not a power.\nResponse:\nWe thank the reviewer for the comments on the excitation source of the TRPL\nmeasurements.\nFor the typical TCSPC experiment, a fs pulsed laser (PHAROS, 190 fs, 6000 Hz) is\nemployed in conjunction with an optical parametric amplifier (OPA, ORPHEUS) to generate\nthe excitation beam at 450 nm. The pump intensity of the femtosecond laser can be viably tuned\nover three orders of magnitude, thereby enabling the acquisition of pump fluence-dependent\nTRPL kinetics (see Figure 1c for PNC solutions and Figure 4c for PNC films).\nFor the in-situ, operando TCSPC experiment, the excitation source was substituted by a ps\nlaser diode (Edinburgh Instruments EPL-405, ~100 ps, 5 MHz). In this case, as long as the\nmulti-exciton Auger recombination caused by the pulse laser is suppressed, its intensity\ntunability is not important because the carrier population state is determined by the CW light.\nHowever, the sample is co-excited by a strong CW light and a weak pulse light, as shown in\nFigure 3c. The CW light generates TRPL baseline and noise, while the pulse light generates\nthe time-resolved signals; consequently, it requires a high repetition rate of the laser to shorten\nthe accumulation time and simultaneously guarantee the signal-to-noise ratio.\nThe rationale for changing the low-frequency fs laser to the high-frequency ps laser has\nbeen delineated in the revised manuscript. In addition, the pump intensity unit of the pulse\nbeams has been revised from mW/cm2 to nJ/cm2 or J/cm2, respectively, while the intensity of\nthe CW beam remains in the of mW/cm2 unit.\nRevisions:\nPage 6: Figure 4\nPage 5: It is worth noting that the femtosecond laser has been replaced with a picosecond\n24\nlaser diode used as the pulse beam source for in-situ TRPL testing. The picosecond laser diode\npossesses the capability to operate at a much higher frequency (5 MHz) than the femtosecond\nlaser (6 kHz), a crucial factor in ensuring the acquisition of high-quality in-situ TRPL data\nwithout significantly prolonging the accumulation time.\nPage 10: For typical time-resolved PL measurement, a femtosecond pulsed laser (PHAROS,\n190 fs, 6 kHz), in conjunction with an optical parametric amplifier (OPA, ORPHEUS) were\nutilized to generate the excitation beam at 450 nm.\nPage 10: In detail, a continuous laser diode (Laserland, Wuhan RLTC Technology Co., Ltd.,\n1875A-450D-80mw-5v) and a picosecond laser diode (Edinburgh Instruments EPL-405, ~100\nps, 5 MHz) were used to generate the CW beam (wavelength: 450 nm) and pulse beam\n(wavelength: 405 nm), respectively, whose intensities were strictly modulated by the neutral\ndensity filters (Daheng Optics, GCO-074M) to meet the criteria of perturbation.\n25\n14. Fig3a shows the PLQY vs fluence for CW pumping. This is a fascinating result. It would\nbe useful to consider the QD in terms of competition kinetics between k_rad and k_nonrad.\nWhy not plot the k_rad and k_nonrad as a function of temperature since you have QD and\nk_decay. Similarly Fig3b shows the limetime vs temperature. It should be the rate constant\nnot the lifetime. Because the rate constant is linear in scale and the time constant is nolinear.\nSo the rate constant a better measure. Examining the rate constants vs temperature in an\nappropriate plot can be very revealing as shown here for unraveling excitonic spatial\ncoherence.\nResponse:\nWe thank the reviewer for the constructive suggestions concerning the pump fluence-\ndependent PLQY and the temperature-dependent TRPL experiments.\nIn the previous manuscript, the PLQY and temperature-dependent PL spectra were\ninvestigated with the aim of elucidating the disparate charge recombination mechanisms\nbetween the unannealed and annealed PNC films (e.g., the variation in exciton binding energy).\nHowever, as replied to Comment-7, the following investigations have revealed that the majority\nof the PL decline caused by CW excitation is irreversible, as attributed to undesired\nphotodegradation, rather than the anticipated multi-exciton Auger recombination. Therefore,\nthe revised manuscript focuses on the improved PNC film with excellent stability against the\nCW and pulsed illuminations. The relevant content to which the reviewer refers has been\nremoved from the current manuscript to maintain the content concise and the logic structure\ncompact. The approach to studying excitonic spatial coherence by temperature-dependent\nTRPL kinetics (P. Kambhampati, et al. Nano Lett., 2024, 24, 61), as mentioned by the reviewer,\nis both intriguing and useful, which will be showcased in our ongoing research work on the\nultrafast excitonic dynamics in PNCs.\n26\n15. Fig3c-d show the Intensity vs temperure 1/T. This is a useful plot but it could be done better.\nThe bright colors are distracting and not helpful to reading the data. I imagine the purpose\nof the temperature dependence is to see if there is some activation barrier to going from\nemitting states to non emitting states. Such a plot is best done as an Ahhrenius plot. The\ntemperature dependence should also report on the energy shifting and bandwidth narrowing\nvs temperature to unravel details relevant to this situation.\nResponse:\nWe thank the reviewer for the comments regarding the temperature-dependent PL behavior.\nBy fitting the temperature-dependent PL intensity to the Arrhenius equation, an activation\nenergy can be obtained as mentioned by the reviewer. Furthermore, we concur with the\nreviewer’s assertion that the variation in the emission peak and PL bandwidth against\ntemperature is meaningful and can be applied to quantitatively analyze the fine excited-state\nstructure and the phonon-electron interaction in PSCs. However, as outlined in the response to\nComment-14, the temperature-dependent PL results have been removed in the reorganized\nmanuscript.\n27\n16. The authors discuss the role of the surface and of surface ligands in controlling the PL\nkinetics. This point has been addressed in detail in CdSe QD. The idea is that there are\nsurface excitons that are shallow so are not populated at 300K. By 30K these shallow states\nbecome populated and emit. Their emission is broadened and redshifted from the exciton\nemission due to phonon progressions. By virtue of being at the interface of the QD core\nand the ligands, the interace is very sensitive to ligands for the PL behavior.\nResponse:\nWe thank the reviewer for the enlightening insights into the temperature-controlled surface\nexciton population and emission in QDs. It is unambiguous that the low-temperature TRPL\nkinetics will prove to be of significant utility, as detailed in the recommended references (P.\nKambhampati et al., Phys. Rev. B, 2013, 87, 081201; J. Chem. Phys., 2013, 138, 204705; Chem.\nPhys., 2015, 446, 92), for estimating the contribution of surface excitons on the assembled PL\nfeature. However, as stated in response to Comment-14 and Comment-15, the temperature-\ndependent PL data have been removed from the revised manuscript according to the\nreconstructed logic structure. For the sake of simplicity, we therefore would prefer not to delve\ninto the temperature-dependent behavior in the current stage.\n28\n# Response to the Reviewer-3’s comments\nThis manuscript presents a novel and potentially impactful methodology for studying\nphotoluminescence (PL) dynamics in perovskite quantum dot (PQD) films under operational\nconditions. The authors introduce an integrated “in-situ” spectroscopy setup that enables\nsynchronous acquisition of steady-state and time-resolved PL (TRPL) under continuous-wave\n(CW) illumination, offering a more realistic picture of device-relevant light emission than\nconventional TRPL. This approach addresses a well-known limitation in the field, where\nultrafast pulsed excitation used in TRPL experiments does not accurately reflect steady-state\noperational behavior. If validated and reproducible, this setup could become a valuable\ncharacterization tool for PQDs and other light-emitting materials.\n1. The methodology is promising and generally sound. However, several points require\nclarification or additional evidence. First, the effect of CW intensity on the TRPL decay\nshould be more rigorously assessed. The influence of laser pulse intensity is discussed, but\na more detailed kinetic analysis would strengthen the conclusions. For example, the use of\nbiexponential fitting alone may not fully capture the complexity of carrier dynamics,\nespecially in systems where trap-assisted recombination or Auger processes may play a\nrole. It would be beneficial to supplement the existing data with kinetic modeling to confirm\nthat the derived time constants are consistent with the proposed mechanisms.\nResponse:\nWe thank the reviewer for the constructive suggestions on improving the rigorousness by\nkinetic modelling. In the revised manuscript, it is first experimentally verified that the charge\nrecombination in annealed PNC films is distinct from that in highly dispersed PNC solutions\n(Figure 5a and Figure 5b). Subsequently, we present an in-depth quantitative analysis starting\nfrom the equilibrium-state classical continuity equation: 𝑑𝑛 /𝑑𝑡 = 𝐺−𝑘 𝑛 −𝑘 𝑛2 = 0 ,\n0 1 0 2 0\nwhich incorporates the terms of both nonradiative trap-assisted recombination and radiative\nrecombination. Multi-exciton Auger recombination, which may occur under the excitation of\nhigh-intensity pulse beams, is negligible for in-situ TRPL measurements, as derived from the\npump fluence-dependent data (Figure 4a-4d). In the case of perturbation (𝛿𝑛 ≪ 𝑛 , see\n0\nevidence in Figure S11), the continuity equation turns out to be: 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−\n1\n2𝑘 𝑛 𝛿𝑛 ,\n2 0\nin line with the first-order reaction model (see detailed derivation on Page 7),\nresulting in an exponential profile for 𝐼 (𝑡).\n𝑃𝐿\nFurthermore, by taking the inhomogeneity factor\ninto account, the TRPL kinetics are generally fitted with multi-exponential or stretched\nexponential functions. In our case, the experimental data can be adequately modelled by bi-\nexponential functions, thereby corroborating the recommendations proposed by Reviewer 2\n(Comment 12). The two exponential components obtained are attributed to charge\nrecombination at the grain interface/surface and within the volume of grains, respectively (J.\nHuang, et al. Science, 2015, 347, 519; N.-G. Park, et al. Nat. Energy, 2016, 1, 1). The relevant\nrevisions per the reviewer’s suggestion are summarized below.\nPage 7: In quantitative, provided that multi-body Auger recombination is negligible, the\nequilibrium-state classical continuity equation in polycrystalline perovskites is typically\nexpressed as: 𝑑𝑛 /𝑑𝑡 = 𝐺−𝑘 𝑛 −𝑘 𝑛2 = 0, where 𝑛 is the carrier density at the\n0 1 0 2 0 0\nequilibrium state, 𝑡 is time, 𝐺 is the rate of photocarrier generation, 𝑘 is the rate constant\n1\nfor first-order nonradiative recombination, and 𝑘 is the rate constant for bimolecular\n2\nrecombination. Once a perturbation is superposed on the equilibrium state, the expression\nshould be rewritten as: 𝑑(𝑛 +𝛿𝑛)/𝑑𝑡 = 𝐺−𝑘 (𝑛 +𝛿𝑛)−𝑘 (𝑛 +𝛿𝑛)2, in which 𝛿𝑛 is\n0 1 0 2 0\nthe density of excess carrier with a value much smaller than equilibrium-state carrier density\nin a perturbation case (𝛿𝑛 ≪ 𝑛 ). A combination of the two aforementioned equations results\n0\nin the classical continuity equation being reduced to the following form: 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−\n1\n2𝑘 𝑛 𝛿𝑛, which is indicative of first-order reaction kinetics. In-situ TRPL experiment fulfills\n2 0\nthe prerequisites of perturbation, therefore the time-dependent PL intensity is predicted to be\ndescribed by the first-order kinetics model with a decay lifetime of 𝜏 , 𝐼 (𝑡) = 𝐼 (0)⋅\n𝑃𝐿 𝑃𝐿\n𝑒𝑥𝑝 (−𝑡/𝜏). Furthermore, in consideration of the inhomogeneity of the chemical environment\nfor individual perovskite grains, it has been proposed that the TRPL data be fitted more\naccurately with a bi-exponential function24 or a stretched exponential function25. Here we adopt\nthe bi-exponential model for elucidation due to its higher fitting goodness than the stretched\nexponential model (Figure S15).\nPage 7: A representative in-situ TRPL data in conjunction with the biexponential fitting\nresults are depicted in Figure 5b, where the fast (𝜏 ) and slow (𝜏 ) decay components have been\n1 2\nsuggested to arise from charge recombination at the grain interface/surface and within the\nvolume of grains, respectively26,27.\n30\n2. A key point requiring clarification is the role of the CW light in modifying the sample\nduring measurement. The irreversible changes observed after pre-illumination (Figure 4)\nsuggest that CW exposure alters the material in a non-trivial way. This raises concerns\nabout whether the in-situ TRPL truly reflects steady-state behavior, or instead captures a\ntransient state of a material in flux. Can the authors demonstrate that measurements are\nreproducible on the same sample, and between different samples prepared under identical\nconditions? This would bolster the validity of the method and the conclusions drawn.\nResponse:\nWe thank the reviewer for the important comments on the stability and reproducibility of\nthe spectroscopic studies. As stressed by the reviewer, the in-situ TRPL kinetics cannot\naccurately reflect the operando features, if irreversible changes take place during the\nmeasurements. Based on in-depth exploration, it is verified that the CW light intensity-\ndependent irreversible PL and TRPL variations arises from the chemical instability of PNC\nfilms used in the previous manuscript. Therefore, the tested samples have been redesigned to\nguarantee their operational stability, and a series of experiments have been conducted to verify\nthe experiment reproducibility, as detailed below:\n(1) We propose a two-step post-synthesis treatment method, comprising annealing and\npreillumination, to prepare ultrastable PNC films. Thermal annealing is suggested to\nremove excess organic ligand, thus preventing the occurrence of irreversible PL decline\nrelated to photo-induced ligand detachment of the dispersed perovskite quantum dots\n(Figure 4a of the previous manuscript). Concurrently, the preillumination treatment can\nimprove both the PLQY and stability of the sintered PNC films in terms of defect self-\nhealing, thereby circumventing the irreversible PL enhancement of the annealed PNC film\n(Figure 4c of the previous manuscript).\n) without annealing\nm.1.6\nannealing\no r preillumination\nn1.4\n(\ny\nsit1.2\nn\ne\nnt1.0\nL\ni\nP0.8\n0 2 4 6 8\nTime (min)\nFigure R5. PL intensity evolution of the PNC films with different treatments under continuous light\nillumination (corresponding to Figure 2f).\nAs verified by in-situ PL spectroscopy and summarized in Figure R5, the two-step treated\nfilm possesses excellent photostability for subsequent optical characterizations.\n(2) To guarantee that all the experimental findings are reversible and reproducible, we have\ncarefully controlled the CW light intensity to be lower than 15 mW/cm2.\n31\nAfter test\n) m.\nor\nn10-1\n(\ny\nsit\nn\ne\nnt10-2\nL\ni\nP\n10-3\n0 20 40 60\nTime (ns)\nFigure R6. Steady-state PL spectra (left) and TRPL kinetics (right) of the PNC film before and after\na complete experimental period (corresponding to Figure S13 and Figure S14, respectively).\nBoth the steady-state PL spectra and TRPL kinetics for the same PNC film before and after\na complete experimental period have been measured for quantitative comparison (Figure\nR6). The variations in PL intensity and TRPL lifetime are less than 5%, in great line with\nthe robustness of the improved sample in spectroscopy measurements.\n(3) The reproducibility of the in-situ spectroscopy can be further verified by the stable real-\ntime PL spectra (Figure 3b) and TRPL kinetics (Figure 3c).\n(a) (b) 10000\n0 s\n10000 200 s\n400 s 9800\n600 s\no u\nnt s\n6\n8\n0\n0\n0\n0\n0\n0 8 1\n1 1\n0 0\n2 4\n0 0\n0 0\n0\n0 0\ns s\ns s o u\nnt s 9600\nP L\nc\nc\n9400\n2000 9200\n0 9000\n500 550 600 0 400 800 1200\nWavelength (nm) Time (s)\nFigure R7. Variation of the (a) PL spectra and (b) corresponding peak counts of the PNC film during\nin-situ PL and TRPL measurements (corresponding to Figure S8).\n(a) (b) (c)\n(I) (II) (III)\nFigure R8. In-situ TRPL kinetics recorded during an entire experimental period with different CW\nillumination durations (corresponding to Figure S9).\nIn quantitative, the variations in PL intensity and decay lifetime (Table S1) are also less\nthan 5%, which strongly manifest the reproducibility for both steady-state and time-\nresolved measurements.\nPage 4: Figure 2f displays the PL intensity evolution versus illumination time for the three\ntypes of PNC films, as derived from the long-term in-situ PL spectroscopy measurement (Figure\nS7), which unambiguously manifests the efficacy of the sequential annealing and\npreillumination treatment on optimizing the photostability.\nPage 5: Figure 3b displays the as-obtained in-situ PL spectra, which are centered at 535\nnm regardless of illumination period, aligning closely with the standard PL spectrum shown in\nFigure 2e. The fluctuation of PL intensity is found to be less than 2% over the entire\nexperimental period (Figure S8), validating the prominent stability of the integrated PL\nspectroscopy platform.\nPage 5: Concomitant with the in-situ PL spectra measurement, TRPL results are obtained\nby means of TCSPC. Three sets of TRPL kinetics with the same accumulation time of ~660 s,\ncorresponding to the three regions denoted in in-situ PL spectra as exemplified in Figure 3c,\nshare the identical time-resolved features……Secondly, the three sets of TRPL kinetics are self-\nsimilar with a variation of average lifetime as small as ~4% (Figure S9 and Table S1),\nindicative of excellent reproducibility for the in-situ TRPL test.\n33\n3. Further experimental details would also enhance reproducibility and interpretation. The\nauthors should provide the repetition rate of the pulsed laser and explain how they\nconfirmed that charge accumulation is avoided between pulses. Figure S8 is used to argue\nthat the pulsed beam does not affect the observed kinetics; however, time constant values\nare not provided, and should be included to support this claim.\nResponse:\nWe thank the reviewer for the constructive suggestions. The repetition rates of the fs and\nps lasers are 6 kHz and 5 MHz, respectively, which have been added to the Experimental\nSection of the revised manuscript. The interval between two pulses is therefore several orders\nof magnitude longer than the PL lifetime (~101 ns), which is imperative for avoiding charge\naccumulation.\nFigure R9. In-situ TRPL kinetics as a function of repetition rate for the ps laser.\nFigure R9 illustrates the TRPL kinetics with different repetition rates. It is evident that the\nTRPL kinetics are not altered with the repetition rate, indicating the absence of charge\naccumulation.\nIn addition, per the reviewer’s suggestion, the fitting results as well as the decay time\nconstants obtained for the in-situ TRPL kinetics with different pulse intensities have been\nprovided in the revised manuscript. The variation in the time constant is less than 3%, thereby\nverifying that the weak ps pulses do not introduce any pump fluence-dependent TRPL behavior\n(see below).\nRevisions:\nPage 5: More importantly, the picosecond pulse intensity is sufficiently weak, so as not to\npose interference to the steady-state PL spectra measurement (Figure S10) nor to introduce any\npump fluence-dependent TRPL behavior (Figure S11).\n190 fs, 6 kHz), in conjunction with an optical parametric amplifier (OPA, ORPHEUS) were\nutilized to generate the excitation beam at 450 nm.\nPage 10: In detail, a continuous laser diode (Laserland, Wuhan RLTC Technology Co., Ltd.,\n1875A-450D-80mw-5v) and a picosecond laser diode (Edinburgh Instruments EPL-405, ~100\nps, 5 MHz) were used to generate the CW beam (wavelength: 450 nm) and pulse beam\n(wavelength: 405 nm), respectively, whose intensities were strictly modulated by the neutral\ndensity filters (Daheng Optics, GCO-074M) to meet the criteria of perturbation.\nPage S12:\nSupplementary Fig. 11. In-situ TRPL of PNC films as a function of pulsed excitation intensity, where\nthe intensity of the CW beam remains unchanged (~7 mW/cm2). The circles are experimental data,\nand the solid curves are bi-exponential fitting results. The pulse intensities and as-obtained decay\ntime constants have been displayed in the corresponding panels.\n35\n4. A few specific points are noted below:\nSmall changes: • Figure 2a and line 90: Figure says unannealed, while line 90 says annealed.\n• Figure 4. b) and d) is practically impossible to distinguish between the solid line and the\ndotted line. Change colour scheme. • Line 130, accompanied..\nResponse:\nWe thank the reviewer for pointing out these errors and inaccuracies. Following a thorough\nreorganization and revision of the manuscript, all typographical and formatting errors have been\nrectified.\nRevisions:\nThe main text have been thoroughly revised according to the reviewer’s suggestions.\n36\n5. For Figure 2, the conditions (pump intensities) for CW and TRPL measurement are quite\ndifferent, which implies different charge carrier generation and recombination rates (DOI:\n10.1063/1.5143121) and can potentially explain differences in the trends presented in the\npaper. On the other hand, “in-situ TRPL” measurements employ CW illumination as a base,\nwhich they show can affect the PL response of the system over time (Figure 4). If the\nsample changes over time due to the CW, how can we rely on the “in-situ” TRPL\ninformation obtained? Can the authors show that the “in-situ” measurements are\nreproducible? Can the measurements be repeated in the same system already measured and\nobtain the same results? If not, can the experiments be repeated and obtain the same\nbehaviour?\nResponse:\nWe thank the reviewer for these valuable inquiries, which have been addressed through the\nimplementation of additional experiments in the revised manuscript, as detailed below.\n(1) As the reviewer has noted, the TRPL features are found to be strongly dependent on the\npump intensity. The experimental condition of typical time-resolved studies (i.e., under the\nexcitation of ultrashort pulses) significantly differs from the operational conditions of PNC\ndevices (i.e., under the excitation of CW light). Therefore, the former may not precisely\nreflect the case in realistic PNC materials. In the revised manuscript, we performed two\ncomparative studies: (i) the different behaviors between the typical and in-situ TRPL\nkinetics are attributed to the different photoexcitation conditions, in line with the reviewer’s\nconsideration; (ii) the different behaviors between the dispersed PNC solution and the\nclose-packed PNC film are attributed to the variation in the microscopic structure.\n(2) By improving the sample fabrication procedures, the issue of PL instability has been settled,\nas validated in Figure 2f, Figure S13, and Figure S14. In addition, the reproducibility of\nthe in-situ spectroscopy platform has been solidly verified upon a series of confirmatory\nexperiments in the revised manuscript (Figure S8, Figure S9, and Table S1). Detailed\ninformation can be found in the response to the second comment from Reviewer-3.\nRevisions:\nPage 2: … it is recognized that typical TRPL techniques are perhaps incapable of revealing\nthe landscape of charge recombination in PNC devices under real operational conditions. The\nworking devices are pumped by stable continuous-wave (CW) light or constant electric field,\nwhile the TRPL measurements are performed under the excitation of ultrafast pulses.\nPage 7: As illustrated in Figure 5a, the in-situ TRPL kinetics of the PNC solution present\nnegligible alteration in response to a wide range of CW pump intensity variations. No\nadditional rapid decay component is observed at high pump fluence, in contrast to the typical\nTRPL data shown in Figure 1c, indicating that the multiexciton effect is minimized under CW\nphotoexcitation. The distinctive in-situ TRPL behaviors facilitate the ascription of the retarded\nTRPL decay to the specific microscopic structure of PNC films, whose formation is\n37\naccompanied by the transition from highly dispersed-state nanoparticles in solution (Figure 1a)\nto sintered close-packed large grains in solid-state configuration (Figure 2c). In the meantime,\nthe improved PLQY at high CW pump fluence in PNC films is analogous to the findings reported\nin hundred-nanometer-sized polycrystalline perovskites21.\nPage 5: Figure 3b displays the as-obtained in-situ PL spectra, which are centered at 535\nnm regardless of illumination period, aligning closely with the standard PL spectrum shown in\nFigure 2e. The fluctuation of PL intensity is found to be less than 2% over the entire\nexperimental period (Figure S8), validating the prominent stability of the integrated PL\nspectroscopy platform.\nPage 5: Concomitant with the in-situ PL spectra measurement, TRPL results are obtained\nby means of TCSPC. Three sets of TRPL kinetics with the same accumulation time of ~660 s,\ncorresponding to the three regions denoted in in-situ PL spectra as exemplified in Figure 3c,\nshare the identical time-resolved features……Secondly, the three sets of TRPL kinetics are self-\nsimilar with a variation of average lifetime as small as ~4% (Figure S9 and Table S1),\nindicative of excellent reproducibility for the in-situ TRPL test.\n38\n6. The authors claim the following (line 105): “According to what has been extensively\ndocumented in previous studies, the emergence of a rapid PL decay component at high\npump fluence is attributed to the Auger recombination of multi-excitons, which, however,\nis ruled out due to the ultralow intensity of the pulsed laser in our case.” And use Figure S8\nto show that the kinetics do not change by changing the intensity of the pulsed laser, but 2\nquestions emerge from this statement:\n(a) The authors employ high CW pump intensities. The power of illumination to excite a\nsample in a TRPL experiment has a strong influence on the concentration of\nphotogenerated carriers, which in turn determines the type of recombination process\nthat dominates in the system, including Auger recombination for high fluences ((DOI:\n10.1039/d0c904950f), (DOI: 10.1063/1.5143121), (DOI: 10.1002/adfm.201910004).\nHave authors consider Auger recombination as a result of the CW illumination in the\n“in-situ” TRPL, or what was the criteria to discard this phenomenon as a result of the\nCW illumination?\n(b) For Figure S8, the authors make the following statement: “The intensity of the pulsed\nbeam has nothing to do with the observed in-situ TRPL kinetics in our case…”.\nHowever, it seems they do not provide time constant values for figure S8 that can\ncorroborate this statement. I would recommend including this information.\nResponse:\nWe thank the reviewer for the important comments on the pump intensity-dependent TRPL\nbehaviors.\n(1) The occurrence of Auger recombination under CW illumination is suggested to be\nminimized in the present work. Since Auger recombination is a multi-body process, if it\nplays a noneligible role under CW illumination in the present experiment, the TRPL\nkinetics should be accelerated by increasing the pump intensity, which is at odds with the\nexperimental results (Figure 4d).\n(2) Per the reviewer’s suggestion, the fitting results of in-situ TRPL data with varied pulse\nintensities have been shown in Figure S11 of the revised manuscript, displaying a\nfluctuation of decay time constant as small as ~3%.\nRevisions:\nPage 6: Interestingly, the typical TRPL data summarized in Figure 4c elucidates\naccelerated decay kinetics at high pump fluence, which implies a more severe nonradiative\nexciton/charge recombination process, as is the case in PNC colloidal solution (Figure 1c).\nHowever, this is at odds with the increase in PLQY at high pump fluence as derived from steady-\nstate PL spectra, which helps exclude the multi-exciton Auger recombination. Conversely, as\nvisible in Figure 4d, the in-situ TRPL data demonstrate a more gradual decay profile when the\nintensity of the CW light is increased from 0.64 mW/cm2 to 12.74 mW/cm2. Notwithstanding the\n39\nunderlying mechanism that will be discussed later, in-situ TRPL kinetics are\nphenomenologically in accordance with the superlinear pump fluence-dependent PL spectra.\nPage S12:\nSupplementary Fig. 11. In-situ TRPL of PNC films as a function of pulsed excitation intensity, where\nthe intensity of the CW beam remains unchanged (~7 mW/cm2). The circles are experimental data,\nand the solid curves are bi-exponential fitting results. The pulse intensities and as-obtained decay\ntime constants have been displayed in the corresponding panels.\n40\n7. The authors are using a sum of exponentials to obtain 2 different time constants and then\nobtain an average value of the lifetime. What does this lifetime represent?\nResponse:\nWe thank the reviewer for the questions regarding the significance of the fitting results.\nAs suggested by Reviewer 2 and in accordance with the reported studies (J. Huang, et al.\nScience, 2015, 347, 519; N.-G. Park, et al. Nat. Energy, 2016, 1, 1), the in-situ TRPL kinetics\nare fitted to bi-exponential functions, which yields two decay lifetimes related to charge\nrecombination at the grain interface/surface and within the volume of grains, respectively.\nRevisions:\nPage 8: A representative in-situ TRPL data in conjunction with the biexponential fitting\nresults are depicted in Figure 5b, where the fast (𝜏 ) and slow (𝜏 ) decay components have been\n1 2\nsuggested to arise from charge recombination at the grain interface/surface and within the\nvolume of grains, respectively32.\n41\n8. It has been reported that the laser intensity influences the TRPL decay and the dominant\nrecombination mechanisms, but this information is often hard to evaluate from a\nbiexponential fitting (DOI: 10.1039/d0c904950f). Have the authors consider using a kinetic\nmodel to analyse the data and corroborate that the same conclusions can be reached?( DOI:\n10.1002/adfm.201910004) (DOI: 10.1039/D0CP04950F)\nResponse:\nWe thank the reviewer for the constructive comments on the kinetic model for TRPL fitting.\nIn the revised manuscript, having verified the negligible contribution of the multi-exciton Auger\nrecombination, the quantitative analysis was initiated with the equilibrium-state classical\ncontinuity equation: 𝑑𝑛 /𝑑𝑡 = 𝐺−𝑘 𝑛 −𝑘 𝑛2 = 0 , by taking the nonradiative trap-\n0 1 0 2 0\nassisted recombination and radiative recombination into account (Figure 4a-4d). The in-situ\nTRPL operates in a perturbation case (𝛿𝑛 ≪ 𝑛 , see evidence in Figure S11), leading to the\n0\ntransformation of the continuity equation to the first-order kinetic form, 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−\n1\n2𝑘 𝑛 𝛿𝑛 (see detailed derivation on Page 7). It can thus be concluded that the integration\n2 0\nequation is, in fact, an exponential decay function. In addition, the complexity arising from the\nfilm inhomogeneity may result in a derivation from the rigorous single exponential prediction\nof the TRPL decay profile. According to the proposal of Reviewer 2, which is also consistent\nwith the fitting results, we select the bi-exponential function to describe the TRPL kinetics.\nFrom a statistical perspective, the as-obtained two exponential components are pertinent to\ncharge recombination at the grain interface/surface and within the volume of grains,\nrespectively (J. Huang, et al. Science, 2015, 347, 519; N.-G. Park, et al. Nat. Energy, 2016, 1,\n1).\nRevisions:\nPage 7: In quantitative, provided that multi-body Auger recombination is negligible, the\nequilibrium-state classical continuity equation in polycrystalline perovskites is typically\nexpressed as: 𝑑𝑛 /𝑑𝑡 = 𝐺−𝑘 𝑛 −𝑘 𝑛2 = 0, where 𝑛 is the carrier density at the\n0 1 0 2 0 0\nequilibrium state, 𝑡 is time, 𝐺 is the rate of photocarrier generation, 𝑘 is the rate constant\n1\nfor first-order nonradiative recombination, and 𝑘 is the rate constant for bimolecular\n2\nrecombination. Once a perturbation is superposed on the equilibrium state, the expression\nshould be rewritten as: 𝑑(𝑛 +𝛿𝑛)/𝑑𝑡 = 𝐺−𝑘 (𝑛 +𝛿𝑛)−𝑘 (𝑛 +𝛿𝑛)2, in which 𝛿𝑛 is\n0 1 0 2 0\nthe density of excess carrier with a value much smaller than equilibrium-state carrier density\nin a perturbation case (𝛿𝑛 ≪ 𝑛 ). A combination of the two aforementioned equations results\n0\nin the classical continuity equation being reduced to the following form: 𝑑𝛿𝑛/𝑑𝑡 = −𝑘 𝛿𝑛−\n1\n2𝑘 𝑛 𝛿𝑛, which is indicative of first-order reaction kinetics. In-situ TRPL experiment fulfills\n2 0\nthe prerequisites of perturbation, therefore the time-dependent PL intensity is predicted to be\ndescribed by the first-order kinetics model with a decay lifetime of 𝜏 , 𝐼 (𝑡) = 𝐼 (0)⋅\n𝑃𝐿 𝑃𝐿\n𝑒𝑥𝑝 (−𝑡/𝜏). Furthermore, in consideration of the inhomogeneity of the chemical environment\nfor individual perovskite grains, it has been proposed that the TRPL data be fitted more\naccurately with a bi-exponential function24 or a stretched exponential function25. Here we adopt\nthe bi-exponential model for elucidation due to its higher fitting goodness than the stretched\nexponential model (Figure S15).\nresults are depicted in Figure 5b, where the fast (𝜏 ) and slow (𝜏 ) decay components have been\n1 2\nsuggested to arise from charge recombination at the grain interface/surface and within the\nvolume of grains, respectively26,27.\n43\n9. Could the authors provide the repetition rate employed for the laser? How did the authors\ncorroborate that the repetition r ate was adequate for the system, particularly at preventing\ncharge accumulation in the material? (DOI: 10.1039/d0c904950f)\nResponse:\nWe thank the reviewer for drawing attention to the potential issues that may be caused by\nthe high repetition rate in TRPL experiments. The repetition rates are 5 MHz and 6 kHz,\nrespectively, for the ps and fs pulses, as provided in the revised manuscript.\nFigure R10. In-situ TRPL kinetics as a function of repetition rate for the ps laser.\nFurthermore, in order to ensure the suppression of charge accumulation, the TRPL kinetics are\nmeasured as a function of repetition rate for comparison. As shown in Figure R10, the TRPL\nkinetics are not influenced by the repetition rate, excluding the occurrence of charge\naccumulation in our experiments.\nRevisions:\nPage 10: For typical time-resolved PL measurement, a femtosecond pulsed laser (PHAROS,\n190 fs, 6 kHz), in conjunction with an optical parametric amplifier (OPA, ORPHEUS) were\nutilized to generate the excitation beam at 450 nm.\nPage 10: In detail, a continuous laser diode (Laserland, Wuhan RLTC Technology Co., Ltd.,\n1875A-450D-80mw-5v) and a picosecond laser diode (Edinburgh Instruments EPL-405, ~100\nps, 5 MHz) were used to generate the CW beam (wavelength: 450 nm) and pulse beam\n(wavelength: 405 nm), respectively, whose intensities were strictly modulated by the neutral\ndensity filters (Daheng Optics, GCO-074M) to meet the criteria of perturbation.\n44\n10. As mentioned in Figure 4, the samples seem to change under CW conditions over time,\nwhich could affect the results of “in-situ” TRPL. Could the authors comment on the\ninfluence of the CW on “in-situ” TRPL regarding sample relaxation/charge accumulation\nbetween laser pulses?\nResponse:\nWe thank the reviewer for the constructive comments.\nFirst, in the previous manuscript, the observed sample variation during the long-term CW\nexperiment has been verified to be photodegradation in our subsequent experiments. This\ncomplicates the evaluation of the impact of operational conditions on the intrinsic photophysics\nof PNCs. Therefore, as stated in the response to the second comment of Reviewer-3, we have\nimproved the film fabrication and ensure that all the operando behaviors reported in the revised\nmanuscript are irreversible and reproducible. This has been collectively supported by the\nphotostability and spectral stability experiments (Figure 2f, Figure 3c, Figure S8, Figure S9,\nFigure S13, Figure S14).\nSecond, we concur with the reviewer’s assertion that the CW intensity-dependent TRPL\nbehavior is attributable to the varied charge population. According to the derived kinetic model\n(see schematic illustration in Figure 5f), the enhanced charge population in trap states\nsimultaneously gives rise to an increase in PL efficiency and a prolongation in TRPL decay\ntime constant. However, we would like to stress that this is substantially different from the\n“charge accumulation issue” discussed in Comment 9. The charge accumulation caused by a\nhigh laser repetition rate has been excluded by Figure R10. In addition, if the CW light poses\nthe issue of charge accumulation, the baseline of the in-situ TRPL kinetics (i.e., TRPL signals\nbefore 𝑡 = 0) should not be a constant, which is not reflected in the presented data (Figure 3c).\n# Response to the Reviewer-1’s comments\n1. The revised version of the manuscript addresses many of the issues raised in the initial\nround of review, both by myself and by the other reviewers. The data are now presented\nmore consistently, and the discussion is better structured. However, I must say that the\nrevised paper feels like an entirely different manuscript. A major part of the original work\n— the time evolution of the device — has now been removed. As a result, the focus and\nscope have shifted significantly.\nResponse:\nWe are grateful to the reviewer for concluding that the manuscript has been improved by\nour revisions in terms of data presentation and discussion.\n1\n2. I still find the distinction between what the authors call \"operando\" and \"non-operando\"\ndata rather unclear. In Figures 4 and 5, the two datasets are now presented on identical\nscales, which is good. However, they are background-corrected differently, and the\noperando data clearly contain dark counts not present in the non-operando measurements.\nThis background difference — particularly the dark counts around negative times —\nappears to be the main source of the observed changes in decay behavior.\nResponse:\nWe thank the reviewer for reminding us of paying attention to the potential mistakes\noutcome in data analysis due to the different backgrounds in typical (i.e., “non-openrando”)\nand in-situ (i.e., “operando”) TRPL data. As outlined in the Experimental Section of the\nmanuscript, the essential distinction between the typical and in-situ TRPL measurements lies\nin the implementation of the CW actinic light superimposed on the pulsed measuring light.\nFigure R1. Representative typical and in-situ TRPL traces displayed in a linear scale. (The data were\ntaken from those illustrated in Figure 4b and Figure 4c of the manuscript.) (a) Original TRPL traces. (b)\nThe pair of traces shown in panel (a) after shifting the baseline to zero. (C) Comparison of the background\nsignals under the same ordinate.\nIn an ideal case devoid of any noise, the background signal at negative time (t < 0) is\nanticipated to be zero in the case of typical TRPL, or to be a positive constant owing to the\nbiased CW light in the case of in-situ TRPL. Here, we note that the presence of a background\nsignal in an in-situ TRPL trace conforms to the principle of in-situ measurement, reflecting the\noperational condition.\nIn reality, both typical and in-situ TRPL traces bear noise and dark counts as shown in\nFigure R1 (a), (b). It is seen unambiguously that the peak count of the typical TRPL trace is\nsubstantially higher than that of the in-situ TRPL trace, since the intensity of the pulsed laser\nhas been strictly controlled in the latter to satisfy the “perturbation” criterion.\nFigure R1 (c) compares the ‘noise and dark counts’ of the different kinds of TRPL traces\nunder the same absolute ordinate. Clearly, the levels of ‘noise and dark counts’ are comparable\nbetween the pair of TRPL traces. Therefore, either the background difference or the dark\ncounts cannot inflict any aberration on the observed changes in decay behavior.\nNote the different ordinate scales of the pair of traces.\nIn the manuscript, we have normalized the TRPL traces and presented them on identical\nscales, so as to better illustrate the pump-intensity dependence of the TRPL kinetics.\nConsequently, with reference to the typical TRPL traces, both the background and the signal of\nin-situ TRPL traces are seemingly amplified. Nevertheless, based on the above analysis and\nfor the sake of presentation clarity, we would like to maintain the previous data\npresentation as employed in the manuscript.\n3\n3. Overall, the differences between operational and non-operational conditions have become\nquite small. Yet, due to inconsistent background levels across datasets and a lack of\nrigorous analysis of this background, it remains extremely difficult to quantify these\ndifferences in any meaningful way.\nResponse:\nThe distinction between the types of TRPL data is embodied by the significantly\ndifferent dependences of the PL kinetics on excitation intensities.\nFigure R2. Excitation intensity-dependent PL decay kinetics measured by (a) typical TRPL and (b) in-\nsitu TRPL techniques, respectively (corresponding to Figure 4c and Figure 4d in the main text).\nAs shown in Figure R2, with the enhancement of excitation intensity, the decay kinetics\nare accelerated for the typical TRPL while retarded for the in-situ TRPL.\nThe background issue has been comprehensively discussed in the above response, which\nmanifests that the visual difference in the background level of the two types of TRPL\nmeasurements stems from the data processing method adopted, while the absolute background\nlevels are actually comparable. Furthermore, given that the maximum TRPL counts are at least\ntwo orders of magnitude higher than the background counts, the impact of the background on\nthe dependence of TRPL kinetics on excitation intensity is negligible.\n4\n4. Fundamentally, I struggle to see what new insight or capability the technique\nactually provides. The results are not convincingly quantified, and in the current\nform I cannot recommend the manuscript for publication. Even if the authors were\nto conduct a more robust background analysis and present more convincing data, I\nam not convinced the changes would be significant enough to yield meaningful\nconclusions about the material's behavior — certainly not at a level suitable for\nNature Communications.\nResponse:\nWe thank the reviewer for pointing out the potential issue that the scientific insights seem\nunclear in the current manuscript. The significance of the present study can be demonstrated\nfrom the following two aspects:\n(1) Operando charge recombination mechanism in PNC films. A correct understanding of\nthe charge recombination mechanism is crucial for the rational design and optimization of\nhigh-efficiency PNC materials. According to the widely reported studies, where\nrecombination kinetics are characterized by traditional TRPL techniques, PL efficiency is\nsuggested to decline upon the elevation of excitation intensity due to multi-exciton Auger\nrecombination. This is generally accompanied by a shortened PL lifetime, consistent with\nthe typical TRPL results presented in this manuscript. However, as demonstrated by the\nexcitation intensity-dependent PL spectra, the PLQY increases with illumination.\nSuch a\ncontradiction stems from the inconsistent testing conditions between the PL spectra\nand typical TRPL measurement: the sample is excited by a CW beam in the former and\nby a high-energy pulsed laser in the latter.\nTo overcome this technical limitation, we propose the in-situ TRPL method, which\nallows for measuring the recombination kinetics under CW illumination in a perturbation\ncase. Using this new technique, the TRPL lifetime is observed to increase with CW\nexcitation intensity. This finding not only rationalizes the CW light-induced PL\nefficiency enhancement (see analysis and discussion in the manuscript) but also\nessentially resolves the longstanding mismatch between typical pulsed TRPL results\nand the CW operational behavior.\n(2) Methodology of operando dynamics/kinetics characterization.\nDevolping operando characterization techniques, aiming to elucidating the mechanism\nof photochemical and photophysical reactions under real operational conditions, has\nbecome a cutting-edge challenge in the fields of photochemistry (Nat. Commun., 2016, 7,\n11918), electrochemistry (Nat. Energy, 2018, 3, 46), biochemistry (Nat. Commun., 2022,\n13, 547), and photoelectronics (Nat. Commun., 2023, 14, 8000). For instance, regarding\nmaterials employed for light emission, by moving research from characterizing\nmaterials/molecules in a passive state to diagnosing active systems, it allows the accurate\nmechanisms affecting quantum efficiency and working stability in real-world applications\nto be precisely identified.\nFurthermore, the proposed in-situ TRPL technique possesses superior sensitivity and\n5\nsimplicity in comparison to other classical time-resolved techniques (e.g., femtosecond\ntransient absorption spectroscopy). These advantages, combined with the excellent\nstability and reproducibility (see verification in Figure 3b, Figure 3c, Figure S8, and\nFigure S9), are thus anticipated to provide the technique a variety of applications in\nrelevant research areas.\n6\n# Response to the Reviewer-2’s comments\nThe authors have well revised the manuscript and answered the questions. This paper is suitable\nfor publication with a minor addition.\nSince the authors are discsussing TRPL with a streak cameran on LHP QD< I encourage them\nto cite the most recent and precise work on the subject published bwlopw\nResponse and revision:\nWe are grateful to the reviewer for the positive view of our work and for concluding that\nour work is ready for publication.\nWe also thank the reviewer for recommending a series of noteworthy references\nconcerning the TRPL studies in PNC by means of the streak camera. In the revised manuscript,\ntwo of the recommended papers have been cited in the appropriate sections as listed below:\nPage 2: Time-resolved PL (TRPL) techniques, including streak camera9,10,…such as high\ntemporal resolution, tunable detection window, and superior sensitivity15,16,…\nReferences:\n[10] Strandell, D., Mora Perez, C., Wu, Y., Prezhdo, O. V. & Kambhampati, P. Excitonic\nquantum coherence in light emission from CsPbBr metal-halide perovskite nanocrystals. Nano\n[16] Strandell, D. P. & Kambhampati, P. Light emission from CsPbBr metal halide perovskite\n3\nnanocrystals arises from dual emitting states with distinct lattice couplings. Nano Lett. 23,\n11330–11336 (2023).\n7\n# Response to the Reviewer-3’s comments\nI am satisfied that the authors have fully addressed the concerns raised in the initial review. I\nbelieve the authors have addressed the concerns of the reviewers generally very well. The\nrevisions provide clear experimental details, improved data presentation, and quantitative\nanalysis that support the conclusions. The rationale for the “operando” CW-bias TRPL\napproach is better articulated - I believe it is potentially useful for the authors to consider adding\na one-sentence definition early in the Introduction clarifying the use of operandi to mean under\nsteady-state CW optical conditions, emulating device illumination conditions, weather than any\nelectrical bias.\nThe manuscript is consistent and much improved from the original manuscript - I believe this\nwill be of benefit to the scientific field and nor warrants publication subject to perhaps some\nminor editorial corrections for clarity and consistency.\nResponse and revision:\nWe are grateful to the reviewer for the positive view of the revisions we made and for\nconcluding that our work is ready for publication.\nWe also thank the reviewer for the suggestion of improving the clarity of operando\nmeasurements. Accordingly, the manuscript has been revised as below:\nPage 2: For example, color-conversion PNC devices are pumped by a stable continuous-wave\n(CW) light, while the TRPL measurements are performed under the excitation of ultrafast pulses.\nPage 2: The sophisticated design of the photoexcitation method enables precise examination of\nthe operando exciton/charge recombination kinetics of PNCs under the sustained excitation of\na CW beam.\n# Response to the Reviewer-1’s comments\n1. I think the manuscript improved overall and the presentation is better. I can follow things\nnow, and I find that the main point — that the trend is superlinearity under CW light and\nsublinearity under pulsed excitation — is clear. I think the authors convinced me on that\npoint, and I agree this is probably a reflection of trapping, and that this approach can be\nused to study trapping. In a very stretched way, it can also be related to “operando”\nconditions, though that’s limited to optical illumination, not a real device bias... I\npersonally would not call it operando and most device people as well would be confused.\nResponse:\nWe sincerely thank the reviewer for acknowledging that the manuscript has been improved\nin terms of data presentation, particularly in highlighting the distinct TRPL behaviors observed\nunder CW and pulsed excitation.\nRegarding the definition of operando conditions, it varies depending on the type of device.\nFor instance, in perovskite photovoltaics, the operando condition typically involves\nillumination under simulated sunlight with a specific load, whereas for perovskite LEDs, it\nrefers to operation under a sustained DC voltage. These examples likely align with what the\nreviewer had in mind for the definition of “operando” condition, which typically incorporates\nan electric field (e.g., the sunlight induced internal field in photovoltaics and the bias field\napplied by DC voltage in LEDs), which cannot be replicated in a purely optical experiment.\nBesides, an important application of PNCs lies in color conversion device, where a PNC\nfilm is excited by CW short-wavelength light to generate PL at desired wavelengths. The\nproposed in-situ TRPL platform is specifically designed to characterize recombination kinetics\nunder conditions that closely mimic the operando environment of such PNC-based color\nconversion devices.\nThis point was also raised by Reviewer 3 in the previous review round, who suggested:\n“The rationale for the “operando” CW-bias TRPL approach is better articulated - I believe\nit is potentially useful for the authors to consider adding a one-sentence definition early in the\nIntroduction clarifying the use of operandi to mean under steady-state CW optical conditions,\nemulating device illumination conditions, weather than any electrical bias.” In response, we\nhave revised the manuscript with a focus on color conversion applications. The relevant text\nnow appears on Page 2 and reads:\n“For example, color-conversion PNC devices are pumped by a stable continuous-wave\n(CW) light, while the TRPL measurements are performed under the excitation of ultrafast pulses.\nIt is uncertain whether the observed phenomenon in TRPL experiments can accurately mirror\nthe stable operational condition.”\n1\n2. The part about lifetime change is still shaky. The graph itself doesn’t convince me.\nThe authors really try to sell it, but the experiments are so different — illumination\npower, laser system, repetition rate, everything. You can’t directly compare\nabsolute lifetimes between these two regimes. It’s okay to show this data, but it\nshouldn’t be pushed as a key point.\nResponse:\nWe thank the reviewer for the comments on the different experimental parameters between\nthe typical TRPL and the proposed in-situ TRPL measurements.\nIndeed, the discrepancy in photoexcitation conditions between pulsed laser-based TRPL\nand CW-driven color conversion applications serves as the key motivation for developing the\noperando kinetics characterization method in this study. As the reviewer rightly pointed out, the\ncarrier behavior under CW excitation cannot be adequately captured or interpreted using\nconventional TRPL; conversely, it aligns well with the results obtained through our in-situ\nTRPL approach.\nA detailed discussion on this point can be found on Page 6 of the manuscript, which further\nsupports the effectiveness of the in-situ TRPL technique in revealing recombination kinetics\nunder operando-like conditions.\n2"}]

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