Peer review of: Ultrathin-metal-film-based transparent electrodes with relative transmittance surpassing 100% (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The paper presents a design method and an experimental proof-of-concept for transparent 
dielectric/metal/dielectric DMD electrodes with relative optical transmittance over 100%. 
The results presented in the paper are certainly interesting for the community. Nevertheless, the 
paper has weak points that cannot be overseen. These are: 

1. The existing literature is not adequately cited. Characteristically, only for the metal 
meshes/nanowires the authors cite 10 papers (line 43 in the manuscript), which is more than the 
citations concerning the DMD electrodes. Many important papers are thus left uncited. A couple of 
important review papers on the DMD electrodes are: 
• Cao et al., Transparent electrodes for organic optoelectronic devices: a review, J. of Photonics for 
Energy, 4 (2014) 040990; 
• Zilberberg and Riedl, Metal-nanostructures - a modern and powerful platform to create transparent 
electrodes for thin-film photovoltaics, J. Mater. Chem. A Mater. Energy Sustain. 4, (2016) 14481 

2. The authors claim that to the best of their knowledge, finding design rules for the quantification and 
optimization of the transmittance over a broad spectrum in the visible has not been addressed before 
(line 89-94). In fact, many papers exist that tackle this issue. 

Apart from the optimization for single wavelength using admittance method, e.g. in the cited paper 
Kostlin and Frank, Optimization of transparent heat mirrors based on a thin silver film between 
antireflection films, Thin Solid Films 89 (1982) 287, or in Grosse et al., Design of low emissivity 
systems based on a three-layer coating, J. Non-Cryst. Solids 218 (1997) 38), there are different 
papers tackling electrode design for broadband transparency using transfer matrix methods. Some 
relevant papers here are: 

• Kim and Lee, Design of dielectric/metal/dielectric transparent electrodes for flexible electronics, J. of 
Photonics for Energy 2 (2012) 021215; 
• Ham et al., Design of broadband transparent electrodes for ﬂexible organic solar cells, 
J.Mater.Chem.A 1 (2013) 3076; 
• Bauch and Dimopoulos, Design of ultrathin metal-based transparent electrodes including the impact 
of interface roughness, Mater. Des. 104 (2016) 37. 

The design rules appearing in the present paper (regarding the choice of the metal, the bottom and 
top dielectric refractive index, thickness, etc) agree with the design rules that have been drawn in the 
literature. So, there is a question about the novelty of the present design approach. 

3. The effect of a higher DMD transmittance for the electrode-plus-substrate compared to the plain 
substrate has been reported in the literature, at least in: Kinner et al, Materials and Design 168 (2019) 
107663, for a broad window in the visible. 

4. The realized electrode of ZnO/Cu:Ag/Al2O3, despite its very high transparency, cannot be easily 
applied to devices like solar cells, OLED etc. The question is how would the carriers from the active 
device layers be efficiently extracted through the thick insulating Al2O3 layer. Also, the authors do not 
elaborate on how the device architecture would alter the electrode design, which is a critical issue in 
such electrodes. 

5. The authors do not discuss to the needed extent the influence of the layer roughness (metal and 
dielectric) on the optical and electrical properties of the electrode. They have included SEM images of 
the metal on a fused silica substrate that is much smoother than the PET substrate they finally use for 
experimental demonstration. Also, the metal is deposited on ZnO finally, which has its own roughness 
and influences the metal percolation threshold. Large roughness will affect the optical properties and 
should be considered in electrode design. 

In conclusion, although I think the results are interesting and worth reporting, I do not consider they 
consist a considerable advancement in the state of the art and, due to the mentioned weaknesses, I 
regret to advice that the paper is not suitable for publication in Nature Communications. After 
addressing the points above, the results certainly worth publishing in another journal. 



Reviewer #2 (Remarks to the Author): 

Report on the manuscript: “Ultrathin-metal-film-based transparent electrodes with relative 
transmission surpassing 100%”. 
Today, due to the growing demand for transparent electrodes and flexible optoelectronic components, 
it is necessary to look for new solutions to make these electrodes. Indeed, ITO is the dominant 
transparent electrode nowadays but indium is rare and ITO is not flexible. It is therefore necessary to 
look for “Indium free transparent electrodes”. The introduction of the submitted article constitutes a 
particularly well-written state of the art concerning this field of investigation. 
-Just a comment about the bibliography. A great deal of works have already been published in this 
field, and also reviews summarizing these publications, some of which should be mentioned, such as: 
C. Guillén and J. Herrero, TCO/metal/TCO structures for energy and flexible electronics, Thin Solid 
Films 520 (2011) 1-17. 

Part 2.1 and 2.2 -The rigour of the authors' scientific approach must be stressed: first, a well-
reasoned and developed theoretical treatment, followed by a transition to an experimental level which 
makes it possible to corroborate the theoretical study. Figures 1 and 2 are well documented and 
commented, so that the theoretical development is well understood. However, some facts have 
already been known for a long time, such as the need to use high refractive index dielectrics... 
Nevertheless, the method used in this work makes it possible to grasp the coherence of the choices 
made, which is rarely the case in the earlier works published in this field. 

Part 2.3 –After estimating through theoretical study the optimum structure DMD’, the theoretical 
results are faced with the results of the experimental study. Nevertheless, between the theoretical 
ideal and the harsh reality of things there are often important differences. Thus, it is well known that 
the growth of Ag thin films follows the Volmer–Weber growth mode, which means that Ag films are 
neither homogeneous nor smooth, so that they exhibit a high surface roughness. As well, it is 
necessary to deposit a relatively large thickness of silver in order to obtain the coalescence, and hence 
the conductivity, of the silver clusters resulting from the three-dimensional growth of the silver layer. 
Such relatively large thickness of silver is unfavourable to good transmission of visible light This 
bottleneck has been over passed by authors of the present work. Indeed, they have previously shown 
that the introduction of a small percentage of Cu into the Ag layer (~2% Cu atomic concentration) 
allows two-dimensional growth, which permits achieving coalescence for very thin metal layer. 
Finally, it is shown that structures such as “PET substrate/24 nm ZnO/6.5nm Cu-doped Ag/56 nm 
Al2O3” allows achieving relative transmittance which surpasses 100% (Tr ~ 100.5%). 
Such result is quite spectacular and deserves real interest for its originality. Nevertheless, keeping in 
mind that this structure is being built to replace ITO transparent electrodes, one must question its 
functionality. Indeed, if the longitudinal sheet resistance of this electrode is 18.6 Ω/sq, one can 
question its transversal conductivity. The thickness of the top alumina layer, which is a perfect 
insulator, is 56 nm and the probability for an electron to pass through such an insulating layer 
thickness by tunnel effect is very low. Therefore using this electrode as bottom layer in optoelectronic 
device will probably result in a very poor performing device. In contrast, such structures should be 
very efficient as transparent heat mirror. Do these electrodes have been tested in optoelectronic 
components? 
-Have the authors probed the stability of their structures? Recent publications have shown that the 
stability of the DMD structures in the presence of Cu depends strongly on the nature of the dielectric 
used, if Cu does not diffuse in ZnO what is it with Al2O3? 
In conclusion, once the authors have responded to these comments, the manuscript will deserve 
publication. 

J C Bernède 


Reviewer #3 (Remarks to the Author): 

#### Summary #### 

This manuscript pertains to the development of new transparent conducting electrodes (TCEs), which 
have importance in optoelectronic devices. It advocates for using the dielectric-metal-dielectric (DMD) 
structure for next-generation TCEs, and proposes a quantitative design principle to expedite structure 
engineering of these DMD TCEs; in particular, it points out that there is no reason why the two 
dielectrics should be the same, and develops equations governing optimal material and film thickness 
choices. This development follows the principles of anti-reflective (AR) coatings (though without using 
that language) accounting for complex refractive indices. It then demonstrates the process 
experimentally using an ultra-thin Cu-doped Ag metal electrode that has been previously developed by 
the group. Using their principles, they improve film transmittance from ~88.1% to ~88.4% over the 
visible spectrum (400-700 nm). 

#### Major Comments #### 

The proposed design principle simplifies the thin film engineering for DMD TCEs by providing 
constraints on the search space, particularly in light of them pointing out that the two dielectrics need 
not be the same material. The majority of work in DMD TCEs uses the same dielectric on both sides of 
the metal, so highlighting this is valuable to the community; it would be helpful to place this insight in 
a more prominent place in the paper. While the principles of their theoretical development are all 
accessible at an undergraduate level, they have nicely laid out the constraints and reduced the effort 
to engineer a film based on the DMD structure. 

The quantitative portion of the paper would be clearer if the authors used the language of AR coatings; 
the proposed DMD structure for a TCE seems built around this idea, and the enhanced transmission of 
an optical film with an AR coating--even with a reflective metal interface--is well established. While it 
may not be trivial to optimize, any optical engineer is familiar with the principle to improve 
transmittance. The opening of section 2.1 could be much more concise by referencing AR principles 
then moving to the particular structure for the design constraints. If there is a clear argument as to 
why their structure is not an AR coating (or an application of the principle) it should be explicit. If the 
structure is, in fact, an application of AR coating principles, the title and article should be revised to 
reflect this. 

In the experimental portion, while the low RMS roughness of the ultra-thin Cu-doped Ag film is indeed 
impressive, the authors should make clear that this is not the first report of this film. The authors 
have previously reported similar results on a film in ACS Appl. Mater. Interfaces 2019, 11, 
27216−27225, which was not mentioned in the context of the low RMS roughness, instead making the 
comparison to a pure Ag film (line 248). The exposition in section 2.3 could be considerably simplified 
and shortened by referencing this prior work. 

The claim of all previous work on metallic-material-based TCEs showing relative transmittance of 
<100% is dubious (e.g. lines 19-21). A quick search reveals, for instance, NPG Asia Materials, 10, 
309–317 (2018), which reports improved transmittance (using the AR language) with a sheet 
resistance of <20 ohm / sq. The authors should revise their claims to account for this and other 
developments. 

For many optoelectronic devices, the transmittance vs. angle is important. Calculations and 
experimental data for transmittance vs. angle would be very useful to evaluate the potential 
performance of this structure in device applications. 

#### Detailed Notes #### 

Figure 3a: Scale of grains in SEM and AFM are different. Explain. What new information does the SEM 
provide that the AFM does not? 
Figure 3b: Enhance contrast to show the structure more clearly. 
Figure 3c: What is the shaded region? I assume this is meant to show the visible range over which the 
device is being optimized, but it is not clear. 
Figure 4c: What are error bars on experimental data? Zoom in on transmittance axis to show 
fluctuations in experiment and theory 

Figure S3: It would be helpful to see the modeled transmittance of bare PET in this figure as well. 
Figure S4: This schematic has been published multiple times (e.g. ACS Appl. Mater. Interfaces, 11, 
27216−27225 (2019)). Explicit attribution may be necessary. 
Figure S5: As the thin, low RMS film deposition is already published, is this figure necessary? 

As this electrode is ultimately for use in a device, the quantitative derivation would be more useful if 
n1 is allowed to be arbitrary (i.e. the index can be set to the device rather than air). This would permit 
the DMD electrode to be adjusted to a particular application. 

It would be useful to have Figure 4c and Figure S2 on the same plot to evaluate how the optimized 
structure performs at different wavelengths relative to the bare PET substrate. For instance, at 400nm, 
the structure performs worse than the PET, and overall there appears to be more curvature. While this 
might be expected, the noted broadband transmittance (lines 150-154) should be evident with this 
data. 

Line 34: replace "by" with "because of" for the sentence meaning 
Lines 279-280: Can the "consistency" be quantified? Do the modeled and experimental transmittance 
agree within error?