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

REVIEWER COMMENTS 

Reviewer #1 (Remarks to the Author): 

This paper analyses current-voltage curves in terms of space charge limited currents. This method is 
popular for deducing charge transport parameters in organic semiconductors but I am not aware of its 
being used before for perovskite solar cells. With the aid of a drift diffusion model that allows for 
mobile ions that greatly modify perovskite cell current-voltage characteristics, and by fabricating 
electron and hole only devices, the authors have been able to find electron and hole mobilities I 
believe for the first time. Many useful experimental results are presented in addition to the mobilities 
such as the temperature dependence of the permittivity. 

I recommend the paper is published **provided** more explanation is provided on the materials 
science underlying the results obtained from experiment and model. I refer to 
a) the reason why the electron mobility exceeds the hole mobility and how sample dependent this 
result is. 
b) the origins of the observed frequency dependence of the permittivity 
c) identification of the dominant mobile ionic species 



Reviewer #2 (Remarks to the Author): 

Review for the manuscript "Space-charge-limited electron and hole currents in hybrid organic-
inorganic perovskites" by Alvar et al. 

The paper reports on the measurement and simulation of unipolar halide perovskite devices with the 
purpose of accurately determine the electron and hole mobility by correctly accounting for ionic 
motion. This is done by implementing ionic motion in a drift-diffusion solver and then fitting or 
reproducing the experimental data with that simulation tool. 
The general idea is very nice and timely. But I do have some technical questions and issues that I 
don't understand that could be quite important for the complete story. 
The authors highlight the necessity to include the frequency dependent permittivity, especially the low 
frequency part influenced by ions. However, if I understand correctly the extremely high permittivity is 
a result of the ion movement and therefore already part of the simulation. That means if we run the 
simulation with the permittivity shown in Fig. 1a, we would be counting the effect of ions twice. This 
confuses me a lot. Could you clarify? If you were to simulate the plate capacitance (not sure this is 
possible with the simulation tool) of a sample with ion movement, would it show the trend shown in 
Fig. 1a for a (nearly) constant permittivity used in the simulation? This is what I understood should 
happen. 
The second big issue that I have are the low mobilities. First, the unit of mobility is cm²/Vs and the 
authors have forgotten the V. I briefly thought they may be reporting diffusion constants rather than 
mobilities but I assume it is just a missing V in the unit. Assuming this is correct, the mobilities are 
very low, the hole mobility being lower than that of typical organic semiconductors. If that is correct, 
something is really odd either with the samples or with the analysis method. Let us assume the hole 
mobility is on the order of 1e-6 cm²/Vs, then the diffusion length even for a high bulk lifetime of 1µs 
would be on the order of 3nm. Solar cells are typically 300-800nm thick and usually show little signs 
of charge collection problems and also little signs of imbalanced collection. How can this be compatible? 
Did the authors ever try to make a solar cell from their MAPI and see how efficiency and charge 
collection are like and then simulate it and see whether it is all self-consistent? at the moment, it 
seems not self-consistent. 

There is also a statement saying "We note that our extracted mobilities are comparable to values 
obtained by electrode-based mobility measurement 
techniques31, 40, 43, 63, 64 and lower than the ones obtained by electrode-free techniques27, 28, 
31." 
I didn't check all the references, but only the first one (31). Here, I note that the range of values 
reported is huge going from 10^-5 to 10^3 cm²/Vs. It is not credible to me that the whole range is 
correct and it is only a matter of what subset of the carrier population is probed. I also don't believe 
that this is sample to sample variation. I think the only credible explanation for this is that in some 
cases, the mobility measurement doesn't measure mobility but something else or that the analysis 
method that translates experimental results into mobilities is flawed. I also note that SCLC (an 
electrode based technique) is listed rather in the higher range of values. Same holds for Hall, also a 
technique with electrodes. This contradicts the author's statement that suggests that such low 
mobilities are the "normal" scenario for electrode based techniques. I think they are the "normal" 
scenario for wrong data analysis. The latter sounds harsh but is just based on me believing in the fact 
that mobilities have to be consistent with e.g. photovoltaic behaviour. If a solar cell works, its mobility 
and lifetime cannot be infinitely low. 
The mobility we would need to learn something for perovskite based devices is a mobility that helps 
explain functionality within a sensible model. These low mobilities in my opinion do not help to explain 
functionality but I am open to hear the opinion of the authors. But currently, I have the feeling that 
whatever this number really means it wont help without a lot of further explanation. 



Reviewer #3 (Remarks to the Author): 

The authors present an interesting results on demonstration of space charge limited current with 
respect to temperature dependence, scan rate dependence, and frequency dependence. The 
correlation between experimental data and simulation well supports the hypothesis and conclusion of 
this work. Also the manuscript is well written. All the current voltage measurements were performed 
as staircase measurement and measurement history is not provided. As concerned in the research 
society, in the literature and even by the author, the hysteresis originated by ion movement is a main 
issue to understand the space charge limited current. I do believe this work will give a direction of 
unraveling the device physics of perovskite based optoelectronic devices, after the authors provide 
more details about reproducibility/measurement history and also address my following concerns. 

1. Line 75: how do you know this high dielectric constant is a result of slow moving ions? What is the 
timescale at which ions move? 

2. Line 87: Is it still an ohmic contact with having organic semiconductor layer even though it is thin 
layers? Can author confirm that this asymmetric current voltage is only attributed to the electron 
injection barrier at the Ag bottom electrode? Can author explain or confirm that is not due to the C60 
layers on top of perovskite? 

3. Line 111: on the same note to above question 2, Can author explain what is the reason for this 
assumption? It was also not clearly described in ref 17 (doi.org/10.1002/aelm.201900935). 

4. Line 125: Which factor causes the ‘4 x Vm’? In this frequency rate indeed the ions are moving, 
which causes a change in the dielectric constant, but that also means the JV curves of the sclc 
measurement are strongly affected, which could make it arguable to be able to fit these JV curves. 

5. Line 132: With organic semiconductor layer on the bottom metal electrode (PTAA) in comparison to 
direct spin coat on metal electrode, the morphology and crystallinity, grain size of perovskite thin films 
should be confirm if they are still the same perovskite, like via XRD or SEM. What is the thickness of 
perovskite thin-films? 

6. Fig. S2: JV curves are staircase measurement, which means the measurement is conducted under a 
constantly applied electric field. It doesn’t provide an information about the history of the 
measurement. Therefore the ions might have already moved or are moving during the scan, and 
thereby affecting the extracted current (both the shape of JV curves and the magnitude). 

7. Line 135-140: The SCLC part has slope equal 2. However, since 1) the ions are affecting the JV 
curve (and this influencing the slope) and 2) The SCLC regime has not yet been reached, its hard to 
validate if the author can actually extract an effective mobility from there. For that to be possible to 
slope has to be fitted, for which the effect of ion density (and their movement in this case) has to be 
known. 

8. Fig S3: It might be a good plan to perform a pulsed measurement and perform the fitting with low 
dielectric constant. If that would fit it would be the same as the fast measurement with high dielectric 
constant. However, it seems impossible to set 1 single value (the dielectric constant) that has to take 
into account all the possible effects from ions. (proven by the observed hysteresis). 

9. Line 177: What is the mechanism behind an increase ion mobility causing enhancement of the 
screening of the electric field, but thereby reducing the hysteresis. 

10. Line 207: Fig 4 needs to be Fig. 5 

11. Line 213: Figs 5c and 5d, need to be 5a and 5b. 

12. In Fig. 2 for electron only device, there is a rapid increase in current near at -2V but not in hole 
only devices. However it’s found in the Fig 3a for electron only device that the rapid increase is getting 
effective as scan rate decrease. Moreover these rapid changes are shown in both electron only and 
hole only device but in opposite temperature. It is very hard to understand this characteristic curve 
with respect to neither temperature dependent, electron only/hole only, nor scan rate. I believe that 
the author should note this point in the main text.