Nanostructured Top Contact as an Alternative to Transparent Conductive Oxides in Tandem Perovskite/c-Si Solar Cells

Round 1

Reviewer 1 Report

The authors carefully and clearly responsed the reveiwer's comments. I agree to publish it. 

Author Response

We thank the reviewer for his/her revision. We have made a full English grammar and style revision and we have tried to eliminate all the typos and errors in the manuscript.

Reviewer 2 Report

Please see the attachment.

Comments for author File: Comments.pdf

Author Response

The manuscript is well organized but it could be improved in terms of contents and quality of presentation. This reviewer suggests the publication after some minor revisions listed below:

1. In the introduction, the authors should give more information about perovskites with some relevant references (e.g. https://doi.org/10.1021/acs.chemrev.0c00107).

Although we have made an intensive search on tandem solar cells, we agree with the referee that a dedicated sentence and reference to perovskite is well deserved. Therefore, we have modified one of the paragraphs in the introduction section to include the reference suggested by the reviewer.

… kind of devices with a potential efficiency of more than 30 % [8]. In fact, perovskite is an appropriate choice for the upper cell. It present a large absorption at shorter wavelengths and its photovoltaic conversion efficiency is rapidly increasing in last few years [new reference #1, Ref 9 of the revised manuscript]. Mostly, the top contact…. 

[new reference #1] Jin Young Kim, Jin-Wook Lee, Hyun Suk Jung, Hyunjung Shin, and Nam-Gyu Park, High-Efficiency Perovskite Solar CellChemical Reviews 2020 120 (15), 7867-7918 DOI: 10.1021/acs.chemrev.0c00107

1. How is the thin AlN dielectric layer deposited as protective coating (sputtering, electrodeposition, etc.)? The general properties of AlN should be mentioned (https://doi.org/10.1109/TNANO.2020.3042234)

The AlN layer is used here as a protective coating. The deposition technique is not of importance as far as it produce a smooth finishing of the surface. On the other hand, we have proved that its thickness and optical properties does not degrade the overall performance of the cell. Our simulations have considered this issue that resulted in the selection of the given thickness of this layer. To complete the description of this layer we have added a sentence to the paragraph where the AlN layer is presented.

Finally, we have used a dielectric layer made of aluminum nitride (AlN) as a protective coating.  This material is being included in a wide variety of optoelectronic sensing devices because of its good biocompatibility and mechanic-electrical properties and typically fabricated by sputtering technique [new reference #2, Ref 40 of the revised manuscript]. The wavelength dependent refractive index data for AlN extracted from a recognized source [37]. Our design incorporates this material as a thin-film having a thickness of 50 nm. We have checked that the addition of this layer does not compromise the performance of the cell.

[new reference #2] M. Mariello et al., "Microstructure and Electrical Properties of Novel piezo-optrodes Based on Thin-Film Piezoelectric Aluminium Nitride for Sensing," in IEEE Transactions on Nanotechnology, vol. 20, pp. 10-19, 2021, doi: 10.1109/TNANO.2020.3042234.

 

1. Which type of perovskite has been used in the work? How the specific material can affect the results discussed here?

We thank the reviewer for his comment; this information was missed, and now included. The perovskite is (CH₃NH₃PbI₃):

………. with a 1D gold grating. The perovskite layer is based on the organic-inorganic hybrid Methylammonium lead iodide material (CH₃NH₃PbI₃). To control light absorption………………..

4. The conclusion should be enriched with the main quantitative results discussed in the work.

The conclusion is revised to include some quantitative results from the manuscript.

What are the real practical advantages of the configuration discussed in this work with respect to the state of the art? The authors should discuss this in the conclusions and report more relevant references in the introduction.

The limitations in large-scale fabrication of all the materials used in a given device is of importance in solar cells. Usually, the top contact for thin film cells [the top cell of the tandem device] is made of transparent conductor oxides (TCO), that are based on indium. As indium has a limited supply in the earth’s crust, it should be replaced by other available materials and structures without compromising the operation of the device. Here, the most important parameters to validate the alternative are low reflectivity and absorption losses, besides low electrical resistance. Large finger contacts made of metals have low resistivity but also have a large reflectivity and induce shadowing effect.   One of the effective solutions is the nanostructures based on metals that can be customized to have low reflectivity and high scattering directivity. Applying these structures to tandem devices is an optical challenge to distribute the optical energy adequately between the two subcells forming the device.  Here, we present a design for an alternative to TCO-based devices that uses a metallic 1D grating. Using the optical model, we calculated the geometrical parameters and arrangement to have this design work efficiently when compared with TCO-based devices, taking advantage of removing indium-based layers from the device. These points are presented and discussed at various location through the revised manuscript  (abstract, introduction, and conclusions).

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors propose technological procedures for photovoltaic cells, in order to improve the penetration of solar radiation, and consequently their electrical performance (short circuit, efficiency, etc.). The idea is clear but the results presented are very insufficient to qualify the proposed technology. I propose to value the proposed structures:

• by adding electrical results of simulations and experiments in terms of photovoltaic conversions, current-voltage characteristics, optimal performance and efficiency in particular. 
• discussing further and comparing these results with standard silicon cells (monocrystalline, ...). 

Author Response

Comments and Suggestions for Authors

The authors propose technological procedures for photovoltaic cells, in order to improve the penetration of solar radiation, and consequently their electrical performance (short circuit, efficiency, etc.). The idea is clear but the results presented are very insufficient to qualify the proposed technology. I propose to value the proposed structures:

• by adding electrical results of simulations and experiments in terms of photovoltaic conversions, current-voltage characteristics, optimal performance and efficiency in particular. 

This paper is mainly focused on the use of the optical model (radiative energy budget) that also contains the conversion from photon absorption to charge carriers generation and short-circuit current. The detailed analysis of the electrical model is not considered for any of the compared structures.  Indeed, the validation of the technology should require a dedicated experimental realization of the proposed design. Unfortunately, currently we have not the resources to make it happen.

• discussing further and comparing these results with standard silicon cells (monocrystalline, ...). 

Perovskite-cSi tandem cell have been chosen because of their high efficiency figures and by the thin-film technology of the perovskite cell. When checking the reported efficiency of tandem cells we find it very competitive with monochrystalline Si cells (see for example, https://www.nrel.gov/pv/cell-efficiency.html )

The theoretical limit of single junction solar cells calculated by Shockley–Queisser is 29 %, this limit is higher for multi-junction devices. The simplest form of multi-junction devices that combine cost effective fabrication and efficiency limit higher than 30 % is the tandem devices. The selection of the top and bottom materials is wide and includes both organic and inorganic materials. One of the promising selection a device having perovskite on top and crystalline silicon on bottom. Therefore, we select this system to apply our design.

Reviewer 4 Report

Authors presented  an innovative arrangement for tandem solar cells based on perovskites on C-Si. In this proposal, a multilayer grating is arranged in a periodic 1D structure on a c-Si surface. There are also some  regions left where the perovskite cell has been removed on the exposed  area of the device. Cost-effective and comparable performance are the benefits of this arrangement over the usual  devices.  The  long-circuit current of the whole device has been optimized by optimizing the short-circuit current of its structure. Two parameters were optimized: the width and thickness of the active layer in the perovskite cell, which includes the metal contact. This device offers comparable performance at a lower price than normal devices. 

Author Response

We really appreciate the comments of the review as they reflect the main findings of the paper. In any case, the revision process has been directed to improve the clarity and the relevance of this contribution.

Reviewer 5 Report

In this manuscript “Nanostructured top contact as an alternative to transparent conductive oxides in tandem perovskite/c-Sci solar cell”, the authors have analyzed a tandem cell combining crystalline Silicon (c-Si) and perovskites cells. A comparison has been made between two solar cell designs. The comparison shows a “slight increment” in the performance. There are some questions to be addressed and are as follows:

1. Using nanostructure top contact in the conventional design has just slightly increased the results as addressed by the authors in the abstract. Using this design, the cost of the device will be increased for this slight increment. Do you think this approach should be applied to other tandem cells for good results? What is the novelty of work?
2. The methodology part should include more computational details for the design.
3. In the results section, the comparison should be made with fill factor, QE, and V_oc of each design for better understanding.

Author Response

Comments and Suggestions for Authors

In this manuscript “Nanostructured top contact as an alternative to transparent conductive oxides in tandem perovskite/c-Sci solar cell”, the authors have analyzed a tandem cell combining crystalline Silicon (c-Si) and perovskites cells. A comparison has been made between two solar cell designs. The comparison shows a “slight increment” in the performance. There are some questions to be addressed and are as follows:

1. Using nanostructure top contact in the conventional design has just slightly increased the results as addressed by the authors in the abstract. Using this design, the cost of the device will be increased for this slight increment. Do you think this approach should be applied to other tandem cells for good results? What is the novelty of work?

The actual application of this strategies depends on the technology used for each subscell. Our design takes advantage of the thin multilayer structure of a working perovskite cell. Light can interact quite efficiently with the proposed grating structure. Other multijunction or tandem solar cells fabricated with an upper thin film technology can use this strategy.

The novelty of the work can be expressed in two directions: one of them is the demonstration of how the ITO layer can be replaced without decreasing the performance of the cell. The calculated short-circuit current of our design is 3% larger than the one obtained for the planar cell arrangement. The other direction is the use of the grating structure to increase the absorption at the active upper layer without diminishing the efficiency of the c-Si subcell.

We have tried to stress the importance of these points in the writing of the revised version of the manuscript.

1. The methodology part should include more computational details for the design.

Following the reviewer suggestion, we have added some additional information about the computational details of the simulation.

We calculated the optical performance of both the reference and proposed designs with the finite element method implemented in COMSOL Multiphysics. The nanostructured array is infinite along the grating direction and can be computationally reduced to a unit cell as shown in figure 1b,c.  Then, periodic boundary conditions applied on the sides of this unit cell to repeat it infinitely. The optical source is placed on top of the whole structure with a customized orientation that resembles the incidence conditions.  The optical model evaluates absorption at each active layer, as well as the total reflection of the tandem cell. A detailed description of the optical model and the underline equation are discussed and reported elsewhere [41].

1. In the results section, the comparison should be made with fill factor, QE, and V_oc of each design for better understanding.

We agree with the reviewer that a full analysis including electronic and optical calculation are important. However, the layer stack of both junctions for the planar and the proposed devices are the same, i.e. the band diagram of the device is preserved. Besides, the work function of ITO is almost the same of Au. Therefore, we did not expect a significant change in the electronic behavior between both designs. For this, we focus in our study on optical performance. This appears clearly when doing optical and electrical modeling where the parameter of interest is the short-circuit current, while other electronic parameters the open circuit voltage is almost the same [x1]. If the layer arrangement changes then an electrical study is necessary to fully evaluate the design. This is why most of the available electrical solvers are one-dimensional.

[x1] Mirsafaei, Mina, et al. "The influence of electrical effects on device performance of organic solar cells with nano-structured electrodes." Scientific reports 7.1 (2017): 1-8.

Reviewer 6 Report

The article is interesting but a nice English polishing is required as well as a proofreading. For example, line 70, a "be" is missing within the sentence.

The Table 1 could be updated since it is quite confusing. The authors do not present their own result work but the previous structure considered. This table could be of more importance if the structure defined by the authors was presented (replace the top layer and add the dielectric layer).

 

The authors should provide a comparison between the perovskite thickness they have chosen and the litterature.

 

 

 

 

Author Response

The article is interesting but a nice English polishing is required as well as a proofreading. For example, line 70, a "be" is missing within the sentence.

We have made a full English grammar and style revision and we have tried to eliminate all the typos and errors in the manuscript.

Authors thank to Dr. Irene Alda for her critical reading and English grammar and style revision of the manuscript.

The Table 1 could be updated since it is quite confusing. The authors do not present their own result work but the previous structure considered. This table could be of more importance if the structure defined by the authors was presented (replace the top layer and add the dielectric layer).

We have included the material and geometrical parameters of the planar and nanostructured cells in Table I. This surely will help the reader to know where the design has been modified.

Layer	Material (planar cell -> nanostructured)	Thickness (planar cell -> nanostructured)
Protective	None -> AlN [37]	0 -> 50nm
Top contact	ITO [29] -> Au [35]	100 nm -> 20 nm
Hole Transport Layer	SPIRO-OMeTA [30]	160 nm
Active Layer	Perovskite [31]	260 nm -> 750 nm
…	…	…
…

The table caption has been also modified:

Table 1. Tandem perovskite/c-Si solar cell layer structure. Both the materials and thicknesses of the layers are included for the planar cell and the one presented in this paper (nanostructured). We have also included the references where the optical constant are taken from.

The authors should provide a comparison between the perovskite thickness they have chosen and the litterature.

The diffusion length of the photo-generated charge carriers in perovskites is large compared with other materials arranged as thin-films. This length is over 1 μm [new reference 3], hence the thickness of the active layer should be limited to this length to effectively extracted these charges. The practical thickness of the active layer investigated in tens of articles and found to range from 250nm to 800 nm. For example, an optimized thickness of 750nm was reported [x2]. The optimum thickness of the active layer depends on the layer structure of the device and the incorporated charge transport layer, fabrication procedures, etc. Based on this, we see that the optimized thickness lies within the common range of the active layer for the perovskite cell.

Following the reviewer suggestion, we make this point clear in the text by adding the following:

The diffusion length of the photo-generated charge carriers in perovskites is about 1 μm, so we limit the maximum height of the perovskite layer in our calculations to this value [new reference 3, Ref 42 of the revised manuscript].

 

[new reference 3] Loi, Maria Antonietta, and Jan C. Hummelen. "Perovskites under the Sun." Nature materials 12.12 (2013): 1087-1089.

[x2] Du, Tian, et al. "Light-intensity and thickness dependent efficiency of planar perovskite solar cells: Charge recombination versus extraction." Journal of Materials Chemistry C 8.36 (2020): 12648-12655.

 

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

I really wanted the authors to put more value on technological performance to improve the cell efficiency . 

Author Response

Reviewer #3. 2nd round

 

I really wanted the authors to put more value on technological performance to improve the cell efficiency . 

 

In this contribution we have focused on the replacement of a technical solution – the use of transparent conductive oxides – that have some issue related with the availability of materials, cost, and processing. In our understanding, the cell efficiency is more related with the photovoltaic conversion. This has more to do with the choice of the photovoltaic materials. In this sense, we have considered the case of a quite promising technology (tandem cells based on perovskites and c-Si cells). As declared in the manuscript, our design has demonstrated a slight improvement in efficiency over the planar design. With this result, we have fulfilled our objective to propose an alternative for the top electrode. Besides, our solution does not degrade the photovoltaic performance of the cell. If we consider the efficiency of the cell in terms of material availability, this is strongly improved when using noble metals, that are easily obtained and used in the optoelectronic industry.

 

We agree with the reviewer that a detailed analysis of the electrical response of the device could complete the description of our design. However, as far as the materials of the perovskite cell are kept the same, the band diagram of this cell is preserved. On the other hand, ITO and Au have a similar value of their work function. This is why we do not expect a significant change of the electrical behavior. Actually, some references (as for example [x1]) analyze the electric parameters involved in different situations of layer structures.  While, the optical characteristics are changed strongly in the proposed design. This means that our design is mainly a pure optical problem, which needs an optimization to achieve the goal discussed above. This is where the optical model helps to understand better how our design compares with the reference one.

 

[x1] Mirsafaei, Mina, et al. "The influence of electrical effects on device performance of organic solar cells with nano-structured electrodes." Scientific reports 7.1 (2017): 1-8.

 

Taking these previous reasoning into account, we preferred not to focus on the 3% increase in efficiency of our design with respect to the planar arrangement. Instead, we emphasize the advantages of the nanostructured design, and the methodology to find the optimum geometrical and material arrangement. At the same time, we have included some sentences both in section 3 and 4:

…Therefore, the optimum geometry is in region II, where the design delivers a short circuit current higher than the reference planar tandem cell. When choosing the geometrical arrangement of the grating lying in region II of both Fig. 2.a and 2.b, we also need to fulfill the matching current condition. This situation …

… that uses an ITO top contact. Reaching this value assures that the replacement of the ITO top electrode does not degrade the efficiency of the cell.  The computational evaluation considers …

Reviewer 5 Report

In this manuscript “Nanostructured top contact as an alternative to transparent conductive oxides in tandem perovskite/c-Sci solar cell”, the authors have analyzed a tandem cell combining crystalline Silicon (c-Si) and perovskites cells. A comparison has been made between two solar cell design. The comparison shows “slight increment” in the performance. There are revisions that must be addressed. 

1. Authors have proposed that multilayer nanostructured design, do add in text which type and specifications of nanostructures are being used for grating.

Author Response

Reviewer #5. 2nd round

In this manuscript “Nanostructured top contact as an alternative to transparent conductive oxides in tandem perovskite/c-Sci solar cell”, the authors have analyzed a tandem cell combining crystalline Silicon (c-Si) and perovskites cells. A comparison has been made between two solar cell design. The comparison shows “slight increment” in the performance. There are revisions that must be addressed. 

1. Authors have proposed that multilayer nanostructured design, do add in text which type and specifications of nanostructures are being used for grating.

Our design is described in Section 2, where we have directed the reader to well established contributions where the perovskite-cSi tandem cell is described (references 8, 27 and 28). A detailed description of the planar structure and the 1D multilayer structure is also presented in table I and Fig. 1. To clarify this point, we have added some sentences where the arrangement of the perovskite cell, that is the one that forms the grating, is specifically describe

 

… they are ordered from top to bottom. When moving from the planar tandem cell to our design, we consider that the grating constitutes the upper cell. The materials of the core of the perovskite cell are the same for both designs. Table 1 also contains…

 

The structure is detailed in the following paragraph of the paper:

.....The perovskite multilayer cell structure is distributed as a 1D rectangular grating on top of the cSi cell, which includes a gold thin-film 20 nm thick on top of it(refractive index from [32]). Figures 1.b, and 1.c show the top perovskite cell layers arranged as a multilayer grating with a period P = 800 nm. As we see in Fig. 1.c, the width of the perovskite cell layers and the gold contact is the same, W,........

We think that this information, along with the data given in Table I and the drawings of Fig. 1, completes the description of the original and modified device.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

From my understanding, the authors have been trying to theoretically demonstrated an efficient tandem perovskite/c-Si cell with a nanostructured top contact. Unfortunately, the whole article was not well organized and the clue is not clearly presented. The title is kind of misleading and the abstract also does not manifest the points of the paper. Besides, I highly suggested the authors to carefully read all the references cited and make sure they are relevant to the statements. For example, ref 13, 14, 15, and 16 are needed to be directly related to the main text. Further, quite some of references are reviews, which are not sufficient for a detailed study of the solar cells since they may not provide some significant information in details. Also, the optically modelling is not a novel method for optimization solar cells by theoretically understanding photon absorption and integrated short-circuit current. The authors need to scientifically organize the paper again and at the current stage, I don’t think the paper can be accepted.

Reviewer 2 Report

The authors designed the perovskite/Si tandem solar cells. This idea is interesting. However, this reviewer do feel some concern about the structure. I agree that the nano perovskite structure on top of Si can reduce the light reflection from the cell surface. But the cell leave some openings (non-perovskite covered area) that short wavelength light can pass through to the bottom Si cell. Due to this, the efficiency of the cell cannot be better than the cells with short wavelength light all absorbed in the top cell. Besides, the metal contact on top of the perovskite can also cause some light loss. By considering these two factors, I cannot be persuaded that the advantages of this design can lead to better overall performance. 

Sorry!