Peer review of: Space-charge-limited electron and hole currents in hybrid organic-inorganic perovskites (2020)


RR2
 REVIEWERS' COMMENTS:
Reviewer #1 (Remarks to the Author):
  In this resubmission, the authors have answered the concerns raised by the reviewers. I am happy to
 recommend this version for publication in Nature Communications.
Reviewer #2 (Remarks to the Author):
  Review for the revised version of the paper "Space-charge-limited electron and hole currents in hybrid
 organic-inorganic perovskites" by Alvar et al.
 The key issue I had in the previous round was with the extremely low values of the mobility that do
 not seem to be consistent with any photovoltaic behaviour. In order to rationalize their findings, the
 authors have provided a simulated and experimental JV curve using Langevin-type recombination.
 This implies that they have set the SRH lifetime to infinity, i.e. assume that they are in the radiative
 limit. Unlike in organics, the bimolecular recombination coefficient found in literature is determined by
 radiative recombination and can therefore be calculated from the absorption coefficient and intrinsic
 carrier concentration of the material (see e.g. https://www.nature.com/articles/s41467-017-02670-2).
 Therefore, k_rad is known to a certain degree and doesnt depend on mobility. The papers cited by the
 authors (Sherkar et al. and Wehrenpfennig et al.) also find similar values.
 If the typically cited range of mobilities of 1 - 100 cm2/Vs is correct, Langevin type recombination
 would be much faster than that and it makes no sense to assume that recombination was diffusion
 limited. Even if due to the presence of grain boundaries the external mobility was much slower (orders
 of magnitude than that), the Langevin theory should really use the local mobility that corresponds to
 what is determined by non-contact measurements. So I dont find it credible to use Langevin theory
 here.
 The JV curve of the solar cell shows a Voc of ~1.05V for a MAPI cell. Thus, the cell has a Voc that is
 ~270mV away from the radiative limit (1.32 V for a band gap of 1.6eV). Thus, the non-radiative
 recombination current exceeds the radiative recombination current by a factor exp(270/25.8)= 3.5e4.
 Under these circumstances it is most likely not appropriate to calculate the JV curve using what is
 essentially radiative recombination only. Certainly, part of the voltage loss will be due to interface
 recombination, part due to bulk recombination. How exactly the loss is distributed, we dont know. But
 my original argument still holds. The mobilities are too low to explain half way decent photovoltaic
 functionality in a realistic scenario (of finite SRH lifetimes that everybody reports who looks at
 transient optical measurements). The electron mobilities are on the edge of being just about credible if
 the lifetime of the bulk was very long. But the hole mobilities dont seem to be credible. What is
 causing them to be or appear so low, I dont know. Generally, to understand what the authors did in
 Fig. S7, I would need to see a full list of variables including all contact and interface layers. At the
 moment, I dont even understand how the curve can reproduce Voc while only having a Langevin
 recombination rate in the bulk. That should really give the radiative limit if correct values for band gap
 and effective DOS are used.
 The general problem I have here is that to a certain degree I can only trust the authors that this is
 what their simulations are giving and there is no other way to simulate the data. As a reviewer
 (without access to the software and without the time to reproduce everything from scratch) I can only
 judge whether the data makes sense to me and it still doesnt. What will the publication of the article
 cause when it is read?
 People who do SCLC on perovskites will see that they have to include ions in drift diffusion, which they
 probably already figured out themselves. Only problem is that 99% cannot do that because they dont
 have the code for it and no time to program one of their own. So those 99% only have the data as a

 take home message and that will say what most of you thought the mobility of holes in MAPI is, is off
 by 6 to 8 orders of magnitude. What will the reader say? He/she will probably just be confused. As I
 am.
 So, the take home message of the paper is not helpful to all those readers, who are not in the author's
 group and can use the code and I dont see that the paper develops in a direction where it would be
 helpful. Now one can of course still publish it because it contains interesting insights for the minority
 of people who can work with it. It can inspire people to write their own code with ions or get access to
 one. The question is whether Nature Communcations is then the right venue, because here I would
 have the expectation that there is a useful take-home message for a broad readership.
 An interesting additional fact is that in a rather similar paper about ions and SCLC, published this year
 (ACS Energy Lett. 2020, 5, 376−384), the authors have concluded that they cannot determine the
 mobility from the SCLC data (and therefore only study upper limits to trap densities).
 Reviewer #3 (Remarks to the Author):
 The authors addressed precise discussion to the all questions regarding the concerns on the
 ambiguities in the previous discussion. I do now believe this work with current status is giving good
 knowledge to the readers about the importance of SCLC measurement on perovskite. I therefore think
 this manuscript is ready to be published.