Electro-Optical Performance of Organic Thin-Film Using HAT(CN)6 between Anode and Organic Materials

Round 1

Reviewer 1 Report

Review of Hong-Gyu Park and Sang-Geon Park for Coatings MDPI:

The authors reported a study of the electro-optical performance of a thin-film of HAT(CN)6, with different thickness layers, between a commercial anode (ITO substrate) and a commercial hole transport layer (α-NPD). The main goal of this study is to show that the driving voltage and efficiency could be improved by using the HAT(CN)6 thin-film as an interlayer between the anode and hole transport layer. Also, the authors show results that a variable nano-sized interlayer of HAT(CN)6 in the conventional OLED manufacturing could improve its current efficiency.

The content of the manuscript is of interest, in particular for the photonics applied field of displays. The authors show a broad and deep technical knowledge regarding the experimental procedures.

I consider the manuscript more technical than scientific. The authors must improve the scientific component of the manuscript, in particular, in the way they describe Figure 4, which is the scientific core of this subject. A small paragraph to describe the fundamental effect of the physical mechanism presented in this work, it is not acceptable, in particular in a scientific manuscript.

I also would like to see some minor, but essential, suggestions being addressed by the authors, to improve the quality of the manuscript, such as:

At line 94, the authors wrote: “Figure 1 shows transmittance-wavelength”. I suggest changing “transmittance-wavelength” to “spectral transmittance”, as well as in other sections of the manuscript. At line 98, the authors wrote: “the graph was shifted to a longer wavelength”. It is not the graph that is shifting, but the spectra. I recommend changing from “the graph was shifted” to “the transmittance spectrum from the thicker layers presented a bathochromic shift”. The title of the manuscript contains the words “Electro-Optical Performance”, which means that the authors also should comment and discuss what they wrote at the end of line 99, for example: a) Why there is a bathochromic shift as a function of the increasing thin-film layer? a) How this shift will affect the optical performance of a final product, i.e., the light-emitting device; b) How this shift will affect the optical performance of a final product, i.e., the light-emitting device. At line 107, the authors wrote “were evaluated as 0.6072, 0.7729875, 0.07840625 and 0.000080735 mA/cm2”. I recommend writing those quantities with the same significant digits, for example: “were evaluated as 0.60720, 0.77299, 0.07841 and 0.00008 mA/cm2”. a) There are no observational errors in these measurements? Please add them; b) Also, please, repeat if needed the same previous procedure in the entire manuscript. In the section “Introduction”, which was well written, the main focus of such section it is the electro-optical optimization of displays, in particular, OLED displays. In the lines from 165 to 169, the authors wrote about luminance, and they showed Figure 6. Besides the efficiency of a display, one of the most important things, regarding the optical performance, is that a display should have the optimal and proper CIE chromaticity diagram. a) There was any study made regarding the wavelength-dependent responsivity? If not, at least, the readers should understand why they did not. I recommend that the authors approach this vital subject. If yes, please, add the CIE chromaticity diagram with the proper discussion and analysis of it. Regarding External Quantum Efficiency: In the author's opinion, it is essential to do a wavelength-dependent study in this situation, as well as it is crucial in the Luminance analysis?

That being said, I would recommend the acceptance of the manuscript if the authors are willing to:

Drastically improve the discussion of Figure 4; Address the items 1 to 6, in particular, the optical performance of their device.

Author Response

Dear Reviewer,

 

Thank you for the thorough review and mindful comments.

We appreciate your constructive suggestions below and have made revisions accordingly with our utmost effort, which are explained below.

 

 

Reviewer 1

 

The authors reported a study of the electro-optical performance of a thin-film of HAT(CN)6, with different thickness layers, between a commercial anode (ITO substrate) and a commercial hole transport layer (α-NPD). The main goal of this study is to show that the driving voltage and efficiency could be improved by using the HAT(CN)6 thin-film as an interlayer between the anode and hole transport layer. Also, the authors show results that a variable nano-sized interlayer of HAT(CN)6 in the conventional OLED manufacturing could improve its current efficiency.

The content of the manuscript is of interest, in particular for the photonics applied field of displays. The authors show a broad and deep technical knowledge regarding the experimental procedures.

 

Thank you for your thorough review.

 

I consider the manuscript more technical than scientific. The authors must improve the scientific component of the manuscript, in particular, in the way they describe Figure 4, which is the scientific core of this subject. A small paragraph to describe the fundamental effect of the physical mechanism presented in this work, it is not acceptable, in particular in a scientific manuscript.

 

Thank you for your comments.

According to your suggestions, a more detailed explanation of the energy level alignment, shown in Figure 4, has been added to the relevant text of the revised manuscript.

 

I also would like to see some minor, but essential, suggestions being addressed by the authors, to improve the quality of the manuscript, such as:

At line 94, the authors wrote: “Figure 1 shows transmittance-wavelength”. I suggest changing “transmittance-wavelength” to “spectral transmittance”, as well as in other sections of the manuscript.

Thank you for your review.

We have made the correction, accordingly.

Figure 1 shows spectral transmittance characteristics with HAT(CN)6 thickness variation.

 

At line 98, the authors wrote: “the graph was shifted to a longer wavelength”. It is not the graph that is shifting, but the spectra. I recommend changing from “the graph was shifted” to “the transmittance spectrum from the thicker layers presented a bathochromic shift”.

We have made the correction, accordingly.

As the thickness of the HAT(CN)6 increased, the transmittance spectrum from the thicker layers presented a bathochromic shift.

 

The title of the manuscript contains the words “Electro-Optical Performance”, which means that the authors also should comment and discuss what they wrote at the end of line 99, for example: a) Why there is a bathochromic shift as a function of the increasing thin-film layer?

First of all, given that the HATCN injection layer test primarily concerns the electrical properties of the material tested, several optical characteristics were separately taken into account, as follows.

Because a bottom-emission device also uses two metallic films with relatively high reflectivity as a cathode and anode, respectively, the resulting spectrum ends up being affected by the Fabry-Perot effect. In this regard, the distance between the anode and cathode must be kept constant for the optical characteristics of the spectrum to be appropriately examined. During the test, in an attempt not to adversely affect the electrical properties of other layers, only the thickness of the HTACN layer was increased, which led to an increase in the overall layer thickness. In light of this, a bathochromic shift is deemed to be unavoidable in this type of test. To reduce the resonant effect, the ITO anode was selected to be as thin as possible, i.e., 50nm. Even in the case of a top-emission device where the resonance effect is further enhanced, constructive interference was induced by adjusting the overall distance between the anode and cathode. Also, an optical-phase correction layer (also known as a capping layer) was additionally formed to adjust the spectra. a) How this shift will affect the optical performance of a final product, i.e., the light-emitting device;   b) How this shift will affect the optical performance of a final product, i.e., the light-emitting device.  

At line 107, the authors wrote “were evaluated as 0.6072, 0.7729875, 0.07840625 and 0.000080735 mA/cm2”. I recommend writing those quantities with the same significant digits, for example: “were evaluated as 0.60720, 0.77299, 0.07841 and 0.00008 mA/cm2”.

We have made the correction, accordingly.

The current density of the devices with ITO/ Interlayer X [X= HAT(CN)6 (1, 7, 20 nm)], or without Interlayer were evaluated as 0.60720, 0.77299, 0.07841 and 0.00008 mA/cm², respectively, at 2.6V.

a) There are no observational errors in these measurements?

No, there were no observational errors during the measurements.

Please add them; b) Also, please, repeat if needed the same previous procedure in the entire manuscript.

Thank you for your comments.

The current density of the devices with ITO/Interlayer X [X= HAT(CN)6 (1, 7, 20 nm)], or without Interlayer were evaluated as 23.37750, 45.04625, 18.92525 and 0.00301 mA/cm², respectively, at 4.8V.

 

 In the section “Introduction”, which was well written, the main focus of such section it is the electro-optical optimization of displays, in particular, OLED displays. In the lines from 165 to 169, the authors wrote about luminance, and they showed Figure 6. Besides the efficiency of a display, one of the most important things, regarding the optical performance, is that a display should have the optimal and proper CIE chromaticity diagram. a) There was any study made regarding the wavelength-dependent responsivity?

Thank you for your constructive criticism. The present study primarily focused on the electrical properties of the proposed material, and thus the HATCN thin-film applied here is deemed to be too thin to be considered in examining the wavelength-dependent reactivity of the material.

A future study should focus on assessing the wavelength-dependent properties of the same material fabricated in the form of a thick film.

 

If not, at least, the readers should understand why they did not.

 

I recommend that the authors approach this vital subject.

If yes, please, add the CIE chromaticity diagram with the proper discussion and analysis of it. Regarding External Quantum Efficiency: In the author's opinion, it is essential to do a wavelength-dependent study in this situation, as well as it is crucial in the Luminance analysis?

External quantum efficiency is of great significance. Accurate measurement of the external quantum efficiency requires accurate measurement of the luminous intensity distribution, which is characterized by the front brightness and viewing angle.

Accordingly, it is worth noting that, in the present study, a bottom emission device with a wider viewing angle was used, and a thin ITO anode was applied so that the luminous intensity distribution could be kept as wide as possible. This means that the device structure applied in the present study can be reasonably assumed to result in the Lambertian luminous intensity distribution.

 

That being said, I would recommend the acceptance of the manuscript if the authors are willing to:

Drastically improve the discussion of Figure 4; Address the items 1 to 6, in particular, the optical performance of their device.

20nm HAT_CN, which was used as an interlayer, has bulk characteristics. As a result, it was found that even with the injection of holes, the initial current density failed to increase smoothly

However, as shown in the high driving voltage case in Figure 2, the device with 20nm HAT_CN exhibited a remarkably greater improvement in current density due to its carrier transport mechanism than the devices equipped with relatively thinner films of 1nm and 7nm, respectively.

The carrier transport mechanism between HAT(CN)6 and α-NPD is described as Figure 4 [11, 12]. The Fermi level is contiguous to the conduction band for HAT(CN)6. The Fermi level is contiguous to the HOMO level for α-NPD. To be more specific, the HOMO level for α-NPD is about 1.1eV away from the conduction band of HAT(CN)6. In this regard, the carrier-transport process from the HOMO level for α-NPD to the conduction band of HAT(CN)6 is deemed to be easy. Simply put, the device with 20nm HAT(CN)6, which has bulk characteristics, allows for easier carrier transport, and this is why the device exhibited remarkably better improvement in current density under the high driving voltage condition than the devices equipped with 1 nm and 7 nm thin-film HAT(CN)6. In contrast, the device without an interlayer showed the worst current density characteristics due to the hole injection barrier. HAT(CN)6 has a work function that varies with the thickness and thus can be used as an excellent interlayer matching a hole transport layer.

 

Thank you for the thorough review and insightful comments.

Author Response File: Author Response.docx

Reviewer 2 Report

See the attached file.

Comments for author File: Comments.docx

Author Response

Dear Reviewer,

Thank you for the thorough review and mindful comments.

We appreciate your constructive suggestions below and have made revisions accordingly with our utmost effort, which are explained below.

Referee report: Electro-Optical Performance of Organic Thin-Film 2 Using HAT(CN)6 between Anode and Organic 3 Materials

By Hong-Gyu Park and Sang-Geon Park

The authors of this paper, Hong-Gyu Park and Sang-Geon Park, present a large number of interesting results and a plethora of statements concerning the pertinent structures, i.e. organic thin-films.

However, my main problem with the paper is that the great majority of these results are presented without any discussion or any “in depth” analysis.

A few examples, amongst many others, are given below:

On p.3 it is stated that:               This, somewhat, amazing result should be explained by the use of physical reasoning. The 95 devices showed an almost similar maximum transmittance of 91.5% in the wavelength range 96 including the visible light region.

Given that the ITO thin film is as thin as 50nm, and the HATCN film is thin as well, the transmittance at long wavelengths of 600nm or greater is almost at the same level as that of bare glass.

Also, the ITO substrate uses an optical-grade glass, and therefore at long wavelengths of 600nm or greater, the transmittance of only the ITO substrate is as high as 90 to 91%.

On p.3 it is also stated that: Here, we intended to examine how the current density changes with the thickness of HAT(CN6) by performing multiple measurements at random points under the high driving voltage condition, and further to determine where the highest current density occurred. This approach was considered highly effective in determining the optimal thickness of HAT(CN6).                     Why are these values chosen? were evaluated as 23.3775, 45.04625, 18.92525 and 0.00301225 mA/cm2, On p.3 it is also stated that:  The relevant explanations have been added to Figures 2 and 3, which can be summarized as follows.  At a high driving voltage of 4.8V, the device with the 20nm thick-film HAT(CN6) that has bulk characteristics exhibited a reduced difference in current density from that with 7nm thin-film HAT(CN6) due to the enhanced carrier transport; in the former device, the HOMO level for α-NPD is only about 1.1eV away from the conduction band of HAT(CN)6. Thank you for your comments.                   Without any explanation it is impossible to assess properly the practical value of this result. The current increase (2.38 times) of the device with 7 nm HAT(CN)6 relative to that with 20 nm HAT(CN)6 at a high driving voltage of 4.8V was significantly lower than the current increase (9.86 times) of the device with 7 nm HAT(CN)6 at a low driving voltage of 2.6V.           On p.5 it is stated that:the device with 20 nm HAT(CN)6 144 showed a remarkable current density increase to rapidly approach the values of the devices with each 145 of 1 nm and 7 nm thin-film HAT(CN)6, which are relatively thinner, due to the carrier transport 146 mechanism.                  Why is this current density approach remarkable? Again, an analysis of this phenomenon should be given!Similar to Answer c) above, the device with the 20nm thick-film HAT(CN6) exhibited a reduced difference in current density from that with 7nm thin-film HAT(CN6) due to the enhanced carrier transport; in the former device, the HOMO level for α-NPD is only about 1.1eV away from the conduction band of HAT(CN)6.    Thank you for your comments.     On p.5 it is stated that:It was possible to obtain an OLED with excellent driving voltage and current efficiency by optimizing the thickness of the thin film of HAT(CN)6 as the interlayer     The paper is in this way not suitable for publication, as the vast majority of the claims/results stated by the authors are not supported by a physical explanation.                   This is a very vague statement. What does "excellent" mean? How critical is the result for perturbations of this "optimal" thickness?    

Thank you for your comments.

In the present study, the characteristics of OLED devices were analyzed with variation of the thickness of HAT(CN6).

Due to the improved hole injection barrier and carrier transport effect, the device with 7nm thin-film HAT(CN6) exhibited the best characteristics in terms of current density–voltage (J–V)characteristics.

The device with 7nm thin-film HAT(CN6) also exhibited the best performance in terms of EL efficiency-voltage characteristics. This enhanced device performance is mainly attributed to the facilitated hole injection and carrier transport effect, thereby keeping the carrier balance from shifting.

As a result, 7nm thin-film HAT(CN6), which exhibited the best performance both in terms of driving voltage and current efficiency, was determined to have an optimal thickness for the device fabricated in the present study.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have considerably improved the readability of their manuscript and provided all the information needed for a good understanding of their many, interesting results.

I therefore recommend publication in your journal.