Peer review of: Exciton diffusion in two-dimensional metal-halide perovskites (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Seitz et al. use transient photoluminescence microscopy to probe exciton diffusion in 2D metal halide 
perovskites. The authors find a correlation between exciton diffusion length and lattice rigidity, which 
is altered by employing organic spacer ligands. By varying the background excitation intensity, the 
authors show that trap state filling can fully eliminate subdiffusion of excitons. To my knowledge, this 
is the first study to use a time-resolved optical microscopy technique to image exciton diffusion in 2D 
perovskites. 
 
The manuscript is well written and thorough, especially considering the scope encompasses 2D 
perovskites with six different ligand spacers and also explores the dynamics of n = 2 perovskites. The 
technical execution and the analysis of the results are fine. I have a few questions regarding the 
relationship between the trap state density and the early time dynamics detailed below, but these are 
only for clarification rather than challenging the results and interpretations presented by the authors. 
 
While exciton diffusion has been previously probed using indirect methods (fluence-dependent 
annihilation or quenching measurements), using transient photoluminescence microscopy to probe 
exciton diffusion directly is a timely and natural addition to the literature. This work will further 
contribute to understanding the important of electron-phonon coupling on excitonic dynamics in 2D 
perovskites, and so after minor revisions, I can recommend it for publication in Nature 
Communications. 
 
Questions: 
 
- The authors demonstrate a clear correlation between average atomic displacement and the diffusion 
coefficient, implying that diffusivity is mostly controlled by the electron-phonon coupling and polaron 
formation. But ligands with different degrees of rigidity should also give rise to different trap state 
densities; for example, the quantum yield of phenethylammonium 2D perovskite thin films is higher 
than butylammonium. Could it now be possible that the quality of the crystals used could affect what 
the authors describe as a fundamental property of the material? In other words, can the authors fully 
rule out the influence of defects and traps on the 2D exciton diffusion, and why the early diffusion 
dynamics would be unaffected by the trap density? 
 
- Can the authors compare the intrinsic trap state density they measure (22 µm-2) with any literature 
values? 
 
- How does disorder, as opposed to trap state density, affect the diffusion dynamics? Is it enough to 
say that the longer time subdiffusion arises solely due to trap states, and not disorder, since the trap 
states are filled and the subdiffusion is eliminated under additional background excitation? The authors 
should provide more detail about the trap states giving rise to these subdiffusive dynamics. In 
quantum dot solids, this subdiffusion is thought to arise due to energetic disorder (e.g. from dot size 
polydispersity). Such energetic disorder can also arise in two-dimensional materials, where 
fluctuations on the order of 100 meV can arise (https://www.nature.com/articles/s41565-019-0520-0), 
resulting in levels which also could be filled under higher excitation densities. 
 
 
 
Reviewer #2 (Remarks to the Author): 
 Seitz et al. present a comprehensive and insightful study of exciton transport in layered 2D hybrid 
perovskites. Using transient photoluminescence microscopy, they measure the spatiotemporal 
dynamics of excitons in a variety of 2D perovskite structures and under varying experimental 
conditions (temperature, steady-state illumination). The most valuable new scientific insight is the role 
of the organic cation in electron-phonon scattering and its effect on exciton diffusivity, summarized in 
Fig. 3e and Fig. 4. Overall, the paper substantially extends our understanding of exciton photophysics 
in 2D perovskites and deserves to be published in Nature Communications. 
 
It is worth noting the topical similarity to another study recently published in Nature Communications 
(Deng et al., Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites, 
Nature Commun (2020), https://doi.org/10.1038/s41467-020-14403-z). However, I find that there is 
very little overlap in these two papers and that they complement each other nicely. The study by Deng 
et al. focused on layer thickness, whereas the manuscript by Seitz et al focuses on organic cation 
composition, trap states, and temperature. Of the two, I find the present manuscript by Seitz et al to 
be more insightful. Deng et al. provided a very nice benchmark data set for subsequent comparison, 
but the work by Seitz et al provides more explanation of the physical origin of the observed behavior. 
Both papers deserve to be published side-by-side in Nature Communications. 
 
A couple points of discussion: 
 
1) It may be worthwhile for the authors to discuss the possible role of free carriers vs excitons in their 
observed transport dynamics. Following the study by Gelvez-Rueda et al 
(https://doi.org/10.1021/acs.jpcc.7b10705), a surprisingly large fraction of excited states in 2D 
perovskites may exist as free charges – rather than excitons. This is more likely to impact the 
interpretation of measured diffusivity in thicker 2D perovskites (such as those studied by Deng et al), 
but it may be a point worth mentioning in this work. 
 
2) The discussion of polarons could be improved. While I personally agree with authors’ perspectives, 
the subject and claims in the paper are possibly more controversial. The section spanning pages 12-13 
could be improved if the authors first present the data in Figs. 3 & 4, then save their polaron 
interpretation for later in the Discussion section. As it’s written now, the paper reads as if the 
experimental data are confirming polaronic behavior that was previously expected; it undersells the 
novelty of the experimental results and unnecessarily raises doubt about the interpretation before the 
actual data (which are not in doubt) are presented. 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
In this manuscript entitled “Exciton diffusion in two-dimensional metal-halide perovskites”, Michael et 
al. presented a comprehensive study of exciton diffusion in two dimensional perovskites by transient 
photoluminescence microscopy. Exciton diffusion was shown to be strongly dependent on the organic 
ligands. Subdiffusive behavior of exciton at long-delay time was attributed to the trapping by defects. 
The manuscript is well written with high-quality data and the conclusions are for the most part 
supported by the experiments. 
 
I have the following suggestions for the authors to consider: 
 
1. It is remarkable that the exciton diffusion constant with PEA ligands is one order of magnitude 
higher than that with the BA ligand. However, it is unlikely that the polaron mass (which is determined 
by the electron-phonon coupling strength) is 10 times heavier in BA than PEA. Therefore, it is likely 
that other factors also play a role. For example, the optical phonon modes in BA probably are lower in 
frequency (softer) – will that play a role? Also, a softer lattice could also mean large structural 
disorder. Some further discussion on this point will be helpful. 
 
2. The temperature dependence data shown in Fig.4 is of a very narrow range and the error bar for 
the PEA sample is missing. So I am not sure if conclusions regarding coherent/ incoherent transport 
can be convincingly drawn. 
 
3. In fig.1d, it seems that the fittings of curves with large delay time give non-negligible positive value 
backgrounds (y0) in addition to the peaks. I suppose the curves should have 0 backgrounds otherwise 
the sigmas could be underestimated. Detail of fitting should be provided. 
 
4. The authors should compare their results with previous reported carrier diffusivities (mobilities) in 
two dimensional perovskites. 
 
5. Some typos: In figure 3b, MSD(t)^2 should be MSD(t); in line 185, it should be fig.3b; in line 187, 
it should be fig.3c.