Peer review of: Enhanced optical path and electron diffusion length enable high-efficiency perovskite tandems (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The primary novel concept in this manuscript is the process by which a thick metal halide perovskite 
film is formed by first spinning the precursor solution at a low spinning rate and then increasing the 
spinning rate just before the anti-solvent treatment. In this way is possible to both have a thick film 
and rapidly falling off the anti-solvent. There are also exciting advances in making a low bandgap 
quantum dot solar cell more efficient. I think these discoveries merit publication in Nature 
Communications, but would like to see the authors address a number of concerns expressed below. 

The authors report 28% power conversion efficiency for a four-terminal perovskite-Si tandem and 24 % 
efficiency for a perovskite-quantum dot solar cell tandem. At first these numbers seem to be 
extraordinary. However, the numbers are less impressive when one realizes how small the solar cells 
are. There is a graph in this paper showing record efficiencies. For many of those data points, the 
solar cell was a full square centimeter in size. The supplemental section says that the size of the 
quantum dot solar cell was 0.049 cm^2. The perovskite cell had an area of 0.053 cm^2. 
The “4T perovskite-Si” tandems is not really a tandem at all. The authors put a much larger perovskite 
solar cell stack on top of the Si cell and used the stack as a filter while measuring the Si cell efficiency. 
They then used the small perovskite cell to get an efficiency for the perovskite cell. Now that 
prototypes of 4T tandems have been demonstrated repeatedly by several research teams, I think the 
time has come for the community to stop obtaining numbers this way. It’s time to start building real 
4T tandems with matched areas. A recently reviewed a manuscript that recommends a proper protocol 
for measuring efficiencies of 4T tandems. 
I have no problem with the authors reporting the numbers for small cells, but I would also like to see 
them report the efficiency for 1 cm² device. They should let people know much the efficiency drops 
because of the additional series resistance in the electrode. Practical solar panels cannot be made with 
solar cells that are only 2 mm wide. 

It is not true that “The 4T tandem arrangement offers a higher theoretical PCE.” The theoretical limit 
for 2T and 4T tandems are the same. In the theoretical limit, the bandgaps for the 2T tandem would 
be the ideal ones. 

On page 3 the authors state “When we increased perovskite precursor solution concentration, 
crystallization is less controllable before anti-solvent dripping because of the high supersaturation of 
precursor solution and fast perovskite reaction rate during subsequent thermal annealing33,34. As a 
result, perovskite films form with a rough and wrinkled surface (Figure 1a-c), in line with previous 
reports23,24.” They have not accurately summarized the explanation provided in the papers they cite. 
The wrinkling occurs because compressive stresses arise during the film formation process. I do not 
agree that roughness arises because it is hard to control the crystallization. More precise wording is 
needed. 

The authors determined grain boundary size by simply looking at scanning electron microscopy images. 
Many people have asserted that there are actually multiple grains between the lines that are visible in 
the micrographs. The authors should make a comment on this subject. They might use words such as 
“apparent grain size.” A nice paper that was presented recently on this subject is 
10.1016/j.joule.2019.09.001. 

The authors should explain why zirconium-doped In2O3 is superior to ITO. 


Reviewer #2 (Remarks to the Author): 

The authors fabricate 1.63 eV perovskite solar cells with thick absorber layers that also have smooth 
morphology. This results in 19% power conversion efficiency (PCE) for a semi-transparent perovskite 
solar cell and in turn enabling perovskite-silicon tandem solar cells with a PCE of 28.2%. 

While the reported PCE is very impressive, the major conclusions of the work have been reported 
previously. 
Although the demonstrates boosted-solvent-extraction technique creates smooth and thick perovskite 
films, addition of the Lewis base is key to the electronic performance of the layer. However, the Lewis 
acid-base adduct approach to improve the properties of the perovskite has already been demonstrated 
before (ref 34). 
The authors report reduced trap densities in the perovskite thus enabling longer diffusion lengths (2.3 
um). However, there is very little data presented in this regard to support these claims. 
I am afraid this manuscript does not provide the sort of significant conceptual advance nor represent a 
sufficiently striking advance to justify publication in Nature Communications. 
For these reasons, I do not believe that the manuscript meets the requirements for publication in 
Nature Communications. 



Reviewer #3 (Remarks to the Author): 

In this manuscript, the authors reported a four terminals tandem perovskite-silicon solar cell with a 
record power conversion efficiency exciding 28%. The result was enabled by the combination of a 
thicker perovskite layer, which the authors achieved modifying the standard spinning and antisolvent 
procedures, and a defect passivation additive that result in a larger electron diffusion length. 
I found the work interesting but not sufficiently novel for publication. Besides a well-written story, this 
work is combining two optimisations of the perovskite film deposition and then applying them in a 
four-terminal tandem concept. Therefore I see it more as an incremental than a step improvement in 
the field. 
In conclusion, I cannot recommend publication since lack of novelty.