Review Reports - Polydimethylsiloxane-Based Quantum Dot Color Conversion Layers for QD-OLED Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present a systematic study on the fabrication of quantum dot (QD) color conversion layers using a thermally curable polydimethylsiloxane (PDMS) matrix to address blue light leakage and low conversion efficiency in QD-OLED displays. By transitioning from a QD-only configuration to a PDMS-based composite, the study demonstrates a significant enhancement in green emission intensity and blue light absorption. The reported maximum color conversion efficiency (CCE) of 30.0% and output/input efficiency of 24.3% represent a notable improvement over the reference QD-only layers. However, while the technical results are promising, several critical physical mechanisms and characterization details require further substantiation to validate the authors' claims. I recommend Major Revision for this manuscript.

The manuscript attributes the improved performance to the PDMS matrix effectively enhancing the interaction between blue light and QDs, yet the underlying physical transition remains insufficiently explained. In Figure 3(d), the absorption coefficient of QD-only layers is shown to decrease with increasing concentration, a phenomenon the authors link to the scattering and red-shifting of the blue peak caused by the increased thickness of the QD layer. The authors must explicitly discuss how the PDMS matrix modifies the effective refractive index environment or light-trapping effects to suppress this decline in the absorption coefficient compared to pure QD films. Providing a comparative analysis of the absorption coefficients for the PDMS-based layers versus the QD-only layers is necessary to substantiate the "enhanced interaction" claim.
An observation is reported where the color conversion layer thickness decreases as the QD concentration increases, despite the higher total solid content. The authors qualitatively attribute this to the oleic acid ligands of the QDs reducing the viscosity and surface tension of the PDMS-QD solution, while also potentially delaying the curing kinetics. To scientifically validate this claim, the authors must provide quantitative rheological data, such as viscosity measurements of the PDMS-QD mixtures at varying concentrations and shear rates. Furthermore, since these ligands can act as plasticizers, a discussion or FTIR analysis regarding the cross-linking density of the PDMS matrix in the presence of high QD loading is required to ensure the mechanical integrity of the film.
The study mentions that at higher QD concentrations, the maximum conversion efficiency slightly decreases due to enhanced inter-dot energy transfer resulting from the reduced distance between QDs. While the efficiency trends in Figure 9 clearly show a peak followed by a decline, this interpretation of non-radiative loss is not sufficiently supported by dynamic spectroscopy. The authors should provide Time-Resolved Photoluminescence (TRPL) spectra to confirm the onset of concentration quenching or Förster Resonance Energy Transfer (FRET). A measurable decrease in the PL lifetime with increasing QD weight percentage is essential to confirm that the inter-dot spacing has reached a regime where non-radiative coupling dominates.
The authors demonstrate a shift in color coordinates toward the green region as a proof of improved color purity and reduced blue leakage. For high-performance display applications, color coordinates are extremely sensitive to even minor leakage in the blue spectrum, yet the calibration standards for these measurements are not fully detailed. The authors must explicitly state the calibration method for the spectroradiometer used in Figure 8 and provide the integrated current density values if applicable. Additionally, calculating the color gamut coverage relative to industry standards such as Rec. 2020 would provide a more rigorous context for the claimed "improved color purity".
The manuscript emphasizes the inherent thermal and environmental stability of the PDMS matrix as a key advantage over UV-curable systems. However, for a study focused on display applications, the provided stability evidence is largely qualitative and lacks data on the device's longevity under actual working conditions. The authors must include operational stability data, such as the retention of color conversion efficiency (CCE) under continuous blue OLED excitation at a fixed luminance over at least 100 to 500 hours. Since QD-OLEDs generate significant local heat, supplemental thermal stability data.

In conclusion , my decision of this manuscript is“major revision”.

Author Response

Response to Reviewer 1 Comments:

Thank you very much for taking the time to review this manuscript. We revised the manuscript according to reviewer’s comments and suggestions. Please find the detailed responses below and the corresponding revisions in the re-submitted files.

Reviewer #1: The authors present a systematic study on the fabrication of quantum dot (QD) color conversion layers using a thermally curable polydimethylsiloxane (PDMS) matrix to address blue light leakage and low conversion efficiency in QD-OLED displays. By transitioning from a QD-only configuration to a PDMS-based composite, the study demonstrates a significant enhancement in green emission intensity and blue light absorption. The reported maximum color conversion efficiency (CCE) of 30.0% and output/input efficiency of 24.3% represent a notable improvement over the reference QD-only layers. However, while the technical results are promising, several critical physical mechanisms and characterization details require further substantiation to validate the authors' claims. I recommend Major Revision for this manuscript.

Comments 1: The manuscript attributes the improved performance to the PDMS matrix effectively enhancing the interaction between blue light and QDs, yet the underlying physical transition remains insufficiently explained. In Figure 3(d), the absorption coefficient of QD-only layers is shown to decrease with increasing concentration, a phenomenon the authors link to the scattering and red-shifting of the blue peak caused by the increased thickness of the QD layer. The authors must explicitly discuss how the PDMS matrix modifies the effective refractive index environment or light-trapping effects to suppress this decline in the absorption coefficient compared to pure QD films. Providing a comparative analysis of the absorption coefficients for the PDMS-based layers versus the QD-only layers is necessary to substantiate the "enhanced interaction" claim.

Response 1: Thank you for the insightful comments.

Reviewer 2 noted that the calculation of the absorption coefficient based on the exponential dependence of the transmitted light is not appropriate for our system because our system is strongly influenced by light scattering. We agree with this comment. For this reason, we removed Figure 3(d) from the revised manuscript to avoid potential misinterpretation. Instead, we added converted images in Figure 8(b) of the revised manuscript to more clearly describe the underlying physical mechanism. We observed significant blurring and light diffusion in the converted images of the PDMS-QD layers, which can be attributed to multiple scattering. This scattering increases the effective optical path length of blue photons within the layer. This light-trapping effect enhances the probability of photon absorption by QDs, leading to improved color conversion efficiency and suppressed blue leakage compared to QD-only films. We added these sentences in the Results and discussion section of the revised manuscript.

Comments 2: An observation is reported where the color conversion layer thickness decreases as the QD concentration increases, despite the higher total solid content. The authors qualitatively attribute this to the oleic acid ligands of the QDs reducing the viscosity and surface tension of the PDMS-QD solution, while also potentially delaying the curing kinetics. To scientifically validate this claim, the authors must provide quantitative rheological data, such as viscosity measurements of the PDMS-QD mixtures at varying concentrations and shear rates. Furthermore, since these ligands can act as plasticizers, a discussion or FTIR analysis regarding the cross-linking density of the PDMS matrix in the presence of high QD loading is required to ensure the mechanical integrity of the film.

Response 2: Thank you for the insightful and constructive comments.

We fully agree that quantitative analysis would provide a more rigorous validation of our claim regarding the effect of the oleic acid ligands on the viscosity of the PDMS-QD solution. We also considered performing viscosity measurements. However, more than 10 g of QDs are required for viscosity measurements. Since the cost of purchasing 1 g of QDs is approximately $2,000, about $20,000 would be required to measure the viscosity. Due to our financial constraints, we are unable to allocate such a substantial amount solely for viscosity measurements.

Instead, we observed that as the QD concentration increases, the PDMS-QD solution becomes less viscous due to the presence of QDs dispersed within the PDMS matrix. From this observation, we indirectly infer that the viscosity decreases with increasing QD concentration. Furthermore, when the QD loading exceeds 42.1 wt%, the PDMS does not cure properly, to the extent that thickness measurements using a surface profiler could not be performed. This indicates that oleic acid ligands interfere with the network formation of PDMS. However, at QD concentrations below 42.1 wt%, we confirmed the formation of sufficiently rigid films that are not scratched during surface profiler measurements. We included these sentences in the Results and discussion section of the revised manuscript.

Comments 3: The study mentions that at higher QD concentrations, the maximum conversion efficiency slightly decreases due to enhanced inter-dot energy transfer resulting from the reduced distance between QDs. While the efficiency trends in Figure 9 clearly show a peak followed by a decline, this interpretation of non-radiative loss is not sufficiently supported by dynamic spectroscopy. The authors should provide Time-Resolved Photoluminescence (TRPL) spectra to confirm the onset of concentration quenching or Förster Resonance Energy Transfer (FRET). A measurable decrease in the PL lifetime with increasing QD weight percentage is essential to confirm that the inter-dot spacing has reached a regime where non-radiative coupling dominates.

Response 3: Thank you for the valuable comments.

According to the reviewer’s suggestion, we performed time-resolved photoluminescence (TRPL) measurements of the PDMS-QD solutions and added the corresponding decay curves in Figure 9(c) of the revised manuscript. We observed that the PL lifetime of the 42.1 wt% sample is longer than that of 8.3 wt%. This prolonged decay is attributed to enhanced self-absorption and photon recycling in the high-density QD environment, where the emitted photons are more likely to be reabsorbed by neighboring QDs before escaping the layer, effectively extending the measured PL lifetime. Furthermore, in the PDMS-QD layer, multiple scattering significantly increases the effective optical path length of photons, enhancing both blue photon absorption and reabsorption of emitted photons. However, at higher concentrations, excessive self-absorption leads to repeated photon recycling processes, during which photons are increasingly lost via non-radiative recombination pathways. In addition, QD agglomeration at 42.1 wt% may further exacerbate self-absorption and contribute to the reduction in maximum efficiency. We added these sentences in the Results and discussion of the revised manuscript.

Comments 4: The authors demonstrate a shift in color coordinates toward the green region as a proof of improved color purity and reduced blue leakage. For high-performance display applications, color coordinates are extremely sensitive to even minor leakage in the blue spectrum, yet the calibration standards for these measurements are not fully detailed. The authors must explicitly state the calibration method for the spectroradiometer used in Figure 8 and provide the integrated current density values if applicable. Additionally, calculating the color gamut coverage relative to industry standards such as Rec. 2020 would provide a more rigorous context for the claimed "improved color purity".

Response 4: Thank you for these valuable comments regarding the reliability of chromaticity measurements and the evaluation of color purity.

To ensure measurement accuracy, the spectroradiometer was periodically calibrated using a standard luminance source provided by the supplier. The spectroradiometer was warmed up for at least 30 min prior to data acquisition, and dark calibration was performed before each measurement to eliminate the influence of dark current and electronic noise. All measurements were conducted in a light-tight dark box under fixed optical geometry to minimize ambient light and systematic errors. We added these sentences in the Materials and Methods section.

We agree that the color gamut coverage relative to industry standards such as Rec. 2020 is important for clarifying “improved color purity”. However, in this work, the emission is based on green color conversion from a blue OLED and therefore does not represent a full RGB system required for a complete Rec. 2020 gamut evaluation. Instead, the color-converted images of the QD-only layer and the PDMS-based QD layer are presented in Figure 3 and Figure 8, respectively, and directly compared. In addition, by comparing the corresponding color coordinates with the standard green coordinates of Rec. 2020, it is demonstrated that the PDMS-based QD color conversion layer exhibits reduced blue leakage and enhanced green emission compared to the QD-only layer. We added these sentences in the Results and discussion section of the revised manuscript.

Comments 5: The manuscript emphasizes the inherent thermal and environmental stability of the PDMS matrix as a key advantage over UV-curable systems. However, for a study focused on display applications, the provided stability evidence is largely qualitative and lacks data on the device's longevity under actual working conditions. The authors must include operational stability data, such as the retention of color conversion efficiency (CCE) under continuous blue OLED excitation at a fixed luminance over at least 100 to 500 hours. Since QD-OLEDs generate significant local heat, supplemental thermal stability data.

Response 5: Thank you for suggesting stability measurements of the color conversion layers. We fully agree that the operational and thermal stabilities under continuous blue OLED excitation are critical factors for display applications. However, such stability tests require a dedicated experimental setup, including stable device operation under a fixed luminance and long-term monitoring over extended periods. We note that this type of investigation constitutes a comprehensive reliability study that goes beyond the primary scope of the present work, which focuses on the optical performance and color conversion characteristics of the PDMS-based QD system. Nevertheless, to the best of our knowledge, systematic studies on the operational stability of QD-based color conversion systems for blue OLEDs remain limited, despite their importance for practical applications. Therefore, a systematic evaluation of long-term operational stability under realistic device conditions is currently under investigation and will be reported in future work.

The manuscript has been substantially revised by taking care of all issues raised by the reviewer.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript entitled” Polydimethylsiloxane Based Quantum Dot Color Conversion Layers for QD-OLED Applications”, submitted to micromachines, describes the assembly and characterization of QD-OLEDS with conversion layers based on QD dispersed in PDMS.

The manuscript must be fully reformulated by answering the following the points:

1 must include the absorption or the excitation spectra of OLEDS and QDs in panels c) and d).
The calculation of the absorption coefficient is based on the exponential dependence of the transmitted light with the absorption coefficient times the thickness of the layer. The authors should give a reasonable explanation for this dependence as light scattering is important and so invalidates the Beer-Lambert
The authors must specify the blue light wavelength used for the calculation of the absorption coefficient.
The decrease of the absorption coefficients with the QD concentration was attributed to the shift of the blue intensity peak toward longer wavelengths. This is very unreasonable and much probably results from the increase of the scattering with the concentration of QDs.
The authors claim that the red shift of the blue light with increasing QD concentration is considered to be due to the scattering of blue light by QDs. This is unreasonable and is probably due to different absorption coefficients of the OLEDs blue light by the QDs that varies with the wavelength. The authors must correlated using a plot that include the luminescence spectrum of the OLEDS and the absorption spectrum of the QDs.
The authors must explain why the PDMS-based QD color conversion layers exhibit significantly lower blue leakage compared with the QD-only layers. The authors claim that PDMS matrix effectively enhances the interaction between blue light and QDs. How this occurs? This is probably the result from lower light scattering due to a better dispersion of the QDs in PDMS.

The authors must explain the meaning of the sentence “As the QD concentration increases, the maximum color conversion efficiency and the maximum output/input efficiency decrease due to enhanced energy transfer among QDs resulting from the reduced distance between QDs” . How does the homo- energy transfer changes the distance between the QDs? The authors must instead discuss how the self-absorption and light scattering in the QDs layer can influence the above-mentioned efficiencies.

In conclusion the manuscript must be shortened and fully reformulated before it can be considered for publication.

Author Response

Response to Reviewer 1 Comments:

Reviewer #1: The manuscript entitled “Polydimethylsiloxane Based Quantum Dot Color Conversion Layers for QD-OLED Applications”, submitted to micromachines, describes the assembly and characterization of QD-OLEDS with conversion layers based on QD dispersed in PDMS.

The manuscript must be fully reformulated by answering the following the points:

Comments 1: 1 must include the absorption or the excitation spectra of OLEDS and QDs in panels c) and d).

Response 1: Thank you for pointing this out.

According to reviewer’s suggestion, we revised Figure 1 by combining the previously separated OLED EL and QD PL spectra into a single panel and additionally included the absorption spectrum of green-emitting QDs. This revision allows for a clearer visualization of the spectral overlap between the blue OLED emission and the QD absorption. We incorporated this updated figure into the revised manuscript.

Comments 2: The calculation of the absorption coefficient is based on the exponential dependence of the transmitted light with the absorption coefficient times the thickness of the layer. The authors should give a reasonable explanation for this dependence as light scattering is important and so invalidates the Beer-Lambert.

The authors must specify the blue light wavelength used for the calculation of the absorption coefficient.

The decrease of the absorption coefficients with the QD concentration was attributed to the shift of the blue intensity peak toward longer wavelengths. This is very unreasonable and much probably results from the increase of the scattering with the concentration of QDs.

Response 2: Thank you for the insightful comments.

We fully agree that light scattering plays an important role in our system, which makes the simple application of the exponential Beer-Lambert relationship inappropriate. In accordance with the reviewer’s comments, we have removed Figure 3(d) and the corresponding discussion on the absorption coefficient from the revised manuscript to avoid potential misinterpretation.

Comments 3: The authors claim that the red shift of the blue light with increasing QD concentration is considered to be due to the scattering of blue light by QDs. This is unreasonable and is probably due to different absorption coefficients of the OLEDs blue light by the QDs that varies with the wavelength. The authors must correlated using a plot that includes the luminescence spectrum of the OLEDS and the absorption spectrum of the QDs.

Response 3: Thank you for pointing this out.

We fully agree that the observed red shift originates from the wavelength-dependent absorption of the QDs rather than from light scattering. Specifically, the QDs exhibit stronger absorption at shorter wavelengths within the blue emission range of the OLED, as shown in Figure 1(c). Therefore, the relatively less-absorbed longer wavelength components become more dominant at higher QD concentrations, resulting in an apparent red shift of the blue peak. We included these sentences in the Results and discussion section of the revised manuscript.

Comments 4: The authors must explain why the PDMS-based QD color conversion layers exhibit significantly lower blue leakage compared with the QD-only layers. The authors claim that PDMS matrix effectively enhances the interaction between blue light and QDs. How this occurs? This is probably the result from lower light scattering due to a better dispersion of the QDs in PDMS.

Response 4: Thank you for the valuable comment.

To clarify the effect of the PDMS matrix on blue leakage, we added the color conversion images of the QD-only and PDMS-based QD layers in Figure 3(c) and Figure 8(b), respectively. The color-converted images become increasingly blurred and diffused with increasing QD concentration and layer thickness, indicating enhanced light scattering within the PDMS-QD layers. This scattering increases the effective optical path length of blue photons within the layer, thereby enhancing the probability of absorption by the QDs. Consequently, blue leakage is significantly suppressed compared to that in QD-only layers.

We added these sentences in the Results and discussion section of the revised manuscript.

Comments 5: The authors must explain the meaning of the sentence “As the QD concentration increases, the maximum color conversion efficiency and the maximum output/input efficiency decrease due to enhanced energy transfer among QDs resulting from the reduced distance between QDs” . How does the homo- energy transfer changes the distance between the QDs? The authors must instead discuss how the self-absorption and light scattering in the QDs layer can influence the above-mentioned efficiencies.

Response 5: Thank you for the insightful comments.

Following the reviewer’s suggestion, we added the TRPL results of PDMS-QD solutions in Figure 9(c) and revised the discussion to focus on the roles of self-absorption and light scattering. The PL decay becomes significantly prolonged as the QD concentration increases, indicating enhanced self-absorption in the high-density QD environment. In such systems, photons emitted by the QDs are more likely to be reabsorbed by neighboring QDs before escaping the sample, resulting in an extended decay time due to photon recycling processes. Furthermore, in the PDMS-QD layer, multiple scattering increases the effective optical path length of photons, which enhances the probability of blue photon absorption and contributes to improved color conversion efficiency and reduced blue leakage compared to QD-only films. However, at higher QD concentrations, excessive self-absorption leads to repeated reabsorption, during which photons are increasingly lost via non-radiative pathways. In addition, QD agglomeration can be observed in Figure 8(c) at higher concentrations, which may further intensify self-absorption and reduce the maximum color conversion efficiency.

We added these sentences in the Results and discussion section of the revised manuscript.

In accordance with the reviewer’s comment, we have thoroughly revised the entire manuscript to eliminate redundant descriptions, streamline the presentation, and improve the overall logical flow. The manuscript has been shortened and fully reformulated to address all issues raised by the reviewer.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for the careful revision and the detailed point-by-point responses. The authors have made a sincere and substantial effort to address the concerns raised in the previous round of review. The revised manuscript is clearer, better supported, and improved in overall presentation.

In particular, the additional explanations, revised figures, and supplementary experimental clarification have helped strengthen the manuscript. Although a few issues could still be explored further in future work, I find that the authors have provided reasonable responses within the current scope of the study, and the remaining points do not preclude publication.

Overall, I believe the manuscript is now suitable for publication in its present form.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors consider most of my comments and the final version of the manuscript was largely improved. I consider the present version adequate to be published.