Peer review of: Bio-inspired vertebral design for scalable and flexible perovskite solar cells (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this paper, the authors reported the PEDOT:EVA as novel HTL materials for flexible perovskite solar 
modules. The PEDOT:EVA can improve the performance of flexible perovskite solar cells due to 
controlling the nucleation/crystallization rate of perovskite film and passivation effect. The topic of 
scalable and flexible perovskite solar modules is very important in perovskite solar cell society. This 
work is well worth to be accepted for publishing, but there are serious scientific issues that need to be 
revised and improved. 
1) All figure captions are confused to the reader. Please make more detail in all figure captions. 
2) IPCE should provide the integrated current in Figure 3e. 
3) Why IPCE exhibits the shoulder at the wavelength range from 700~750 nm. In addition, IPCE of 
the flexible cell is similar with rigid based cell. It is very strange because the transmittance of ITO/PET 
and ITO/glass is different. 
4) The bandgap and transmittance of PEDOT:EVE should be provided. 
5) The author claimed that the superior bending performance of the flexible cell is due to the excellent 
mechanical properties of PEDOT:EVA. However, origin of deterioration for flexible perovskite solar cell 
is low fatigue resistance of ITO substrate. [Energy. Environ. Sci. 8(3) 916, 2015; ACS appl. Mater. 
Interface 10 (5) 4697, 2018 etc.] So, I could not understand why PEDOT:EVA based flexible cell 
shows high mechanical stability when it bent at high bending curvature of ~3mm. In addition, Young’s 
modulus and Poisson’s ratio of PEDOT:EVA is lower than that of PEDOT:PSS. Generally, high Poisson’s 
ratio and Young’s modulus give bending properties. 
6) In Supplementary Tab. 7, Young’s modulus of PEDOT:PSS should be corrected. 
7) Not Young’s modules, it should be Young’s Modulus in the figure and manuscript. 



Reviewer #2 (Remarks to the Author): 

The authors demonstrated the PEDOT: EVA interfacial layer between the ITO and perovskite layers 
facilitates the oriented crystallization of perovskite on flexible substrate and sticks the device 
compactly to improve the flexibility. The resultant flexible PSCs achieved the efficiencies of 19.84% 
and 17.55% at 1.01 cm2 and 31.20 cm2, respectively. However, there are some unclear contents in 
the current manuscript. The work needs major revision before the reconsideration of publication in 
Nature Communications. The work shall also be considered to publish in more special journal. 
1. The manuscript published high impact journal of Nature Communication should demonstrate 
scientific principles rather than hype gimmicks. The work currently do not clearly show any new/shape 
scientific novelty. In addition, what are the differences between this work and other recent on on 
interface engineering related perovskite? (e.g. Nano Lett. 2019, 19, 2, 684, Adv. Mater. 2019, 31, 
1902902…etc.) 
2. The authors demonstrated that the PEDOT: EVA layer control the nucleation sites and crystal 
orientation growth. The statement does not carry important message. How does the bionic interface 
layer impact the nucleation and orientation growth? Why does the shorter nucleation time provide 
more time for grain growth to ensure the quality of perovskite film? 
3. In Fig. 2f, the authors should give a whole TRPL rather than just a part. It seems that there are 
some problems in Fig.2f. There is a concern to get such short TRPL (9.5 vs. 16.3ns) through fitting the 
data in Fig.2f. 
4. The interaction between Pb and carboxyl group has been reported in previous work, such as J. Am. 
Chem. Soc. 2019, 141, 14, 5781. 
5. The authors demonstrated the EQE in the manuscript, but Fig. 3e and Fig. S13 are IPCE. Moreover, 
most value in IPCE (EQE) (Fig. 3e) is lower than 85%, the integrated current from IPCE (EQE) is far 
lower than the Jsc. The authors must check all their data. 
6. Apart from reported high efficiency of the present work, there is no clear scientific novelty in the 
work. The high efficiency should be certified. The authors also should reveal something meaningful, 
not hype concepts. 



Reviewer #3 (Remarks to the Author): 

The authors of this manuscript prepared PEDOT:EVA suspension, and used it as an hole-transporting 
layer in flexible perovskite solar cells. They demonstrated that their PEDOT:EVA afford perpendicular 
crystal growth and enhanced flexibility of the whole device, and they also achieved a power conversion 
efficiency up to 19.84% for 1.01 cm2 flexible device. The fabrication of flexible device is an important 
issue in perovskite solar cells and the observation in this manuscript is interesting. Therefore, I think 
this manuscript can be accepted for publication after major revision as indicated in the following: 

1. Please provide the basic information of PEDOT:EVA films (morphology, transmittance, electrical 
conductivity, and etc.) These data would help readers to understand the characteristics of PEDOT:EVA. 

2. In this study, PEDOT:EVA showed the oriented crystal growth of perovskite layer. Please provide 
the reason why the different crystal orientation of perovskite were observed depending on different 
hole-transporting layers. 

3. PEDOT:EVA was found to have a higher work function than PEDOT:PSS. The detailed explanation 
about different energy levels between PEDOT:PSS and PEDOT:EVA seems to be required. 

4. It has been reported that one of the main reason of limited flexibility in flexible perovskite solar cell 
is the limited flexibility of ITO electrode. Thus, the detailed study about the deformation of ITO 
electrode depending on different interlayer materials is suggested to be required in order to 
understand the mechanical properties of whole devices. 

5. In this study, benzene methyl sulfonic acid was used for synthesis of PEDOT, and then PEDOT:EVA 
suspension was prepared by miniemulsion method. What is the state of PEDOT (doped or non-doped)? 
Compared with the direct blending PEDOT:PSS+EVA polymer, which points are the advantage of your 
approach? And, if it is possible, could you provide the device performance or characteristics of 
PEDOT:EVA depending on different preparation conditions (ex: PEDOT/sulfonic acid ratio or PEDOT: 
EVA ratio or peroxide/EDOT ratio or molecular weight of EVA or etc.). 

Reviewer #4 (Remarks to the Author): 

This paper demonstrates a large-area flexible perovskite solar cell (PSC) with PEDOT:EVA (Poly(3,4-
ethylenedioxythiophene):poly(ethylene-co-vinyl acetate)) which acts as hole transporting and soft 
interfacial layers inspired by the biological crystallization and flexible structure of vertebrae. The 
PEDOT:EVA layer which is located between brittle ITO and perovskite films could improve the 
mechanical flexibility of PSCs through effectively absorbing and releasing the stress, like the cartilage 
between vertebrae. Furthermore, the high surface energy and hydrophobicity of PEDOT:EVA affords 
remarkable crystallization quality and enhanced stability of the device, respectively. In details, the 
binding energy of Pb atoms with carboxyl group is stronger than that of sulfonic radical and the 
number of the carboxyl group on PEDOT:EVA is less than that of sulfonic radical on PEDOT:PSS, 
resulting in fewer nucleating sites and larger grain size. The hydrophobicity of PEDOT:EVA can protect 
PSCs from the erosion by ion diffusion. Therefore, the PSCs based on PEDOT:EVA show the higher 
performance and better environmental stability then the typical PSCs based on PEDOT:PSS. Authors 
systematically investigated the effect of PEDOT:EVA in the interface of perovskite and ITO and the 
opto-electrical properties of PSCs with experimental and simulated studies. I suggest the acceptance 
of this manuscript in Nature Communications after addressing the following minor comments. 
1. Authors suggested a PEDOT:EVA interlayer between brittle ITO and perovskite films for the 
improved mechanical flexibility of PSCs. Although this approach can be a good solution for the flexible 
PSCs based on ITO electrodes, there are other recent approaches for the enhanced flexibility and 
performances based on flexible silver nanowire or carbon nanotube electrodes (Kang et. al, J. Mater. 
Chem. A 2019, 7, 1107; Jeon et al, Adv. Energy. Mater. 2019, 9, 1901204). Authors are 
recommended to compare merits of their approach with those other approaches. 
2. Authors synthesized the PEDOT:EVA and compared the properties with PEDOT:PSS in terms of 
modulus, pH values and so on. However, the electrical conductivity values of PEDOT:EVA film was not 
described. I wonder the electrical conductivity of PEDOT:EVA film compared to PEDOT:PSS. 
3. In flexible PSCs, the thickness of perovskite film critically affects the device performance and 
mechanical stability. I wonder the film thickness was optimized or not in this paper. If it is not, I 
recommend the authors show the studies about the performance and flexibility depending on the film 
thickness. 
4. Authors explained the binding energy of Pb atom to carboxyl group is much stronger than that of 
sulfonic radical. Authors should add proper references for the better understanding. 
5. Authors are recommended to add a comparison table in terms of the device performance and 
mechanical flexibility compared to other works to highlight the effect of PEDOT:EVA in the interface of 
perovskite and ITO. 

Reviewer #5 (Remarks to the Author): 

The authors have fabricated perovskite solar cells with PEDOT:EVA and achieved 19.84% PCEs on 
1.01 cm2 flexible PSCs with excellent mechanical stability. Furthermore, the authors measured several 
tool for comparison between PEDOT:PSS and PEDOT:EVA such as 2D-XRD, PL, TCSPC, XRD etc.,. 
However, there is no direct evidence of EVA as act like glue between ITO and perovskite. Generally, 
ITO is a brittle mineral that breaks when bent. But if the PEDOT:EVA acts as the glue in ITO, the 
property of ITO (conductivity or work function) can be changed. But there is no mentioned of thses 
results as well as perovskite. In addition, from the DFT calculation, the authors calculated only 
comparison between sulfonic radical and carboxylic group. This means that any carboxylic group with 
PEDOT can be acts as PEDOT:EVA materials? I believe that conclusion of this paper is not clear. Finally, 
the authors have emphasized that PEDOT:EVA acts as “vertebrae” but there is no “why”. The exact 
reason for how PEDOT:EVA does not damage the device and recovers again is missing. Therefore, I 
give rejection this paper to nature communication.