Understanding Temporal Evolution of Electroluminescence Intensity in Lead Sulfide (PbS) Colloidal Quantum Dot Infrared Light-Emitting Diodes

Round 1

Reviewer 1 Report

In this paper Kim et al discussed transient effect in PbS CQD based LED

As is, i feel the paper very speculative and actually close to story telling. More data are required to support the claim.

 

In the method, more details need to be given to make your device reproducible, please add : a ref relative to PBS NC synthesis (Hines et al Adv Mat 2003), what is sheet resistance of ITO, where does PEDOT comes from, what is PEDOT thickness ? what is cleaning procedure for Zno CQD, which spectrometer is used, which parameter analyzer is used ?

 

Author claim that they use ITO to avoid charging during photoemission, do they also conduct ligand exchange to make the film more conductive ?

 

Figure 1 can be reshaped, Tem can be moved to next line to better balance top and bottom part. you use the word inset for something that is not an inset (same comment also hold for figure 6a). I also guess that in a paper dedicated to EL, a PL curve might be useful to add

Between figure 1 and 2, i expect a classic L-I-V curve at least up to 10 V. it is tough to understand transient effect without understanding of static properties

Main result of the paper is about the fact that there is a non steady state as bias is applied. Authors observe drop of current and rise of the light emission. From scratch authors want to attribute these effect to charge balance and electric field induced emission quenching, but as it is too briefly mentioned at the end of the paper, this might just be the result of heating. As current flow, resistance may drop and and resistance drop the injection resistance change. All these effect are combined together but the current set of data barely allow disentanglement. The only experience that is convincing to support charge trapping is discussed in fig 3, where authors show about strong hysteresis of the device with current and emission depending on the preconditioning of the charge. You should push this direction and support claim with band structure sketch

Why do you go up to 10V, ; are you sure there is no damage at such high voltage ? again an IV up to this bias is required

 

 

Page 5, line 176 i read « allow us to infer that the increase in the EL intensity during light emission is ascribed to the interfacial 1trapping of carriers at the surface of PbS CQDs close to the PEDOT:PSS and/or ZnO. » at the end, what do you infer, that interface are at the interface ? clarify your claim

 

Authors observe a blue shift under operation, there are new theory about that see for example Advanced Optical Materials 7 (14), 1801697

Also relative to this blueshift authors say joule heating is not leading to blue shift. Are you sure ? PbS NCs are among this weird material where temperature dependence (dEG/dT) can change of sign with particle size, see P hys. Rev. Lett. 81, 3539 – Published 19 October 1998,  to confirm your claim at least check the sign of the PL shift with T

Paragraph following fig 5 ispure speculation. Why dynamics in you LED should be connected to the oen of your FET based on another geometry and another material ?

In the caption of fig 6 theoretical is not suited as wording i will rather use fit

 

Minor comments :

english can be polished, see for example page 4 , line 137 « to understanding »

Author Response

Reviewer 1

[1] In the method, more details need to be given to make your device reproducible, please add : a ref relative to PBS NC synthesis (Hines et al Adv Mat 2003), what is sheet resistance of ITO, where does PEDOT comes from, what is PEDOT thickness ? what is cleaning procedure for Zno CQD, which spectrometer is used, which parameter analyzer is used ?

>>We added the experimental details as guided by the reviewer.

p.5: “For CQD washing, we centrifuged the synthesized solution, separating unreacted material. The supernatant was transferred to new tubes and centrifuged at 9000 rpm for 10 min. The precipitate is redispersed in hexane. We repeated the washing process several times and the sediments were re-dispersed in chlorobenzene (10 mL) to obtain a PbS CQD (~40 mg/mL).”

p.5: “To fabricate PbS CQD LEDs as structured in Figure 1(a), PEDOT:PSS (M124 HTL Solar from Ossila) was spin-coated onto the patterned indium tin oxide (ITO, 70 Ω□) substrate at 6000 rpm for 60 s.”

p.5: “After the injection process, the mixed solution was cooled in air. For washing process, we added acetone into the solution, followed by centrifugation at 9000 rpm for 10 minutes. After centrifugation, solvent was removed for another centrifugation in ethanol. The washing process was carried out more than twice.”

p.6: “The EL intensity of a PbS NIR QLED device was monitored at a driving voltage applied to the ITO side. With a photo-spectrometer (Hamamatsu TG-cooled series) and a semiconductor parameter analyzer (Keithley 2400), the EL intensity and the electrical current were plotted as a function of time, respectively, during light emission at a particular voltage over the turn-on voltage.”

[2] Author claim that they use ITO to avoid charging during photoemission, do they also conduct ligand exchange to make the film more conductive ?

We have not done ligand exchange because ligand exchange changes electronic and optical properties such as CQD band gap, trap density and molecular orbital energy levels. However, we believe it can provide insights into understanding the origin of temporal evolution of the EL intensity.

[3] Figure 1 can be reshaped, Tem can be moved to next line to better balance top and bottom part. you use the word inset for something that is not an inset (same comment also hold for figure 6a). I also guess that in a paper dedicated to EL, a PL curve might be useful to add

We reshaped Figure 1 as guided by the reviewer.

We admit that PL measurement would be useful in elaborating on the origin of the temporal evolution.  Unfortunately, we do not have PL curves for the QLED device because we don’t have the instrument for PL measurement in the NIR region. However, to elaborate on the exciton quenching mechanism investigation of the temporal evolution of PL will be helpful.  

 

[4] Between figure 1 and 2, i expect a classic L-I-V curve at least up to 10 V. it is tough to understand transient effect without understanding of static properties

Our system set up is designed to measure the emission intensity as a function of voltage. We manually scanned voltage and found the turn on voltage. The temporal EL intensity measurement was carried out over the turn-on voltage. To prevent device degradation due to high electric field, we did not do voltage scan for the sample for EL intensity. Instead, for other sample, we have the intensity vs voltage plot. (Unfortunately, we do not have I-V data at the extended region now.)

As shown below, over the voltage (10 V), the EL intensity increased with time.

To address the reviewer’s concern we showed the figures below and added sentences on p. 7.

p.7: “Prior to the charging/discharging experiments, we confirmed that, at a high voltage over 10 V, the EL intensity increased with voltage and time (Data not shown), indicating that the bias condition for the charging/discharging experiments has no significant electrical damage on the device.”

 

Figure S1

[5] Why do you go up to 10V, ; are you sure there is no damage at such high voltage ? again an IV up to this bias is required

 As shown in the below Figure S2, the current increased and decreased during charging and discharging, respectively, and gradually saturated. We believe that the gradual current change is due to charging/discharging process not by electric field induced degradation as concerned by the reviewer.

As mentioned above, over the voltage (10 V) in Figure S1, the EL intensity increased with time. We believe that voltage dependent EL intensity as well as time dependent EL intensity increase demonstrate that no damage was occurred at 10 V.

 

Figure S2

 

[6] Main result of the paper is about the fact that there is a non steady state as bias is applied. Authors observe drop of current and rise of the light emission. From scratch authors want to attribute these effect to charge balance and electric field induced emission quenching, but as it is too briefly mentioned at the end of the paper, this might just be the result of heating. As current flow, resistance may drop and and resistance drop the injection resistance change. All these effect are combined together but the current set of data barely allow disentanglement. The only experience that is convincing to support charge trapping is discussed in fig 3, where authors show about strong hysteresis of the device with current and emission depending on the preconditioning of the charge. You should push this direction and support claim with band structure sketch

We have to admit that it is pretty challenging to disentangle the origin of the temporal evolution of the EL intensity. First, the initial increase in the EL intensity can not be attributed to heating because, independent of the shift direction (red, or blue-shift) intensity increased which can not be explained by temperature dependent EL intensity from many previous reports. We understand the reviewer’s point but pinpointing the exact origin of the temporal evolution is almost impossible because of a lot of complicating factors including interface cross recombination and multi carrier emission. However, we would like to emphasize that, based on the interfacial charging data, EL intensity evolution is associated with interfacial trapping. We agree that we have to more focus on interfacial charging.

To address the reviewer’s concerns we added a paragraph and sentences on p. 8-9. In the sentences we identified the interface where charge trapping occurs and elaborated on the effect of reductive surface passivation which is crucial in explaining the origin of the EL enhancement with time.

p.8-9: “During the charging process in Figure 3(b) at a high voltage (10 V), electrons are injected through Al and passivate the surface trap sites on the ZnO and PbS CQDs. The surface of ZnO CQDs are depleted of electrons due to adsorption of oxygen molecules as well as a high density of surface traps [29, 30]. A high injection of electrons fill and passivate the surface traps on the ZnO CQDs close to the PbS/ZnO interface. According to Weaver et. al, indeed, reductive passivation of surface traps through electron injection over the trap energy level of ZnSe CQDs led to large photoluminescence electro-brightening in the ZnSe CQDs, implicating interfacial carrier charging in determining luminescence intensity [31]. On the other hand, PbS CQDs are very well-known to form dynamic traps through which electrons are depleted, as will be discussed later. In contrast to the charging process, discharging process under application of -10 V depletes the charged electrons at the PbS/ZnO interface. Therefore, the subsequent bias at 1.5 V, after the charging process, causes a far higher current in comparison with that after the discharging process, as shown in Figure 3(b). It is important to note that the diode current increases with time at a higher voltage (10 V) while the current decreases at a lower voltage (5 V, Data not shown). At a high electric field, injected electrons fill the trap states fast and begin to passivate the interface while, at a low electric field, injected carriers are still in the process of being trapped. On the basis of the framework, the current decrease during the increase in the EL intensity at a low voltage (1. 5 V) implicates electron trapping at the PbS/ZnO interface, achieving charge balance.

In our previous studies, we investigated interfacial charging of PbS CQDs, finding that the time scale of carrier trapping associated with surface traps corresponds to tens of seconds which is consistent with that of the current and EL intensity change in Figure 2(b) [27, 28]. The long time scale over tens of seconds and the significant current change over an order of magnitude in Figure 3(a) in combination with the charging/discharging experiments in Figure 3(b) allow us to infer that the increase in the EL intensity during light emission is ascribed to the interfacial trapping of carriers at the PbS/ZnO interface.”

   

[7] Page 5, line 176 i read « allow us to infer that the increase in the EL intensity during light emission is ascribed to the interfacial 1trapping of carriers at the surface of PbS CQDs close to the PEDOT:PSS and/or ZnO. » at the end, what do you infer, that interface are at the interface ? clarify your claim

We addressed this issue in [6].

 

[8] Authors observe a blue shift under operation, there are new theory about that see for example Advanced Optical Materials 7 (14), 1801697

We carefully read it and it provides insights into unified mechanism integrating photoluminescence and electroluminescence coupled to resonators. The blue shift by hot electrons occur femto to picoseonds which corresponds to plasmon decay time constant which is distinctly different form our case. We appreciate to the reviewer’s comments.

[9] Also relative to this blueshift authors say joule heating is not leading to blue shift. Are you sure ? PbS NCs are among this weird material where temperature dependence (dEG/dT) can change of sign with particle size, see P hys. Rev. Lett. 81, 3539 – Published 19 October 1998,  to confirm your claim at least check the sign of the PL shift with T

As the temperature increases blue shift occurs. However, many previous reports consistently report that EL or PL intensity decreases with Joule heating. Particularly, according to Liu et al [44], as the temperature increases, photodarkening occurs due to photoionization of QDs followed by trapping of carriers. In the experiment, QDs are embedden in glass matrix. During irradiation, excited carriers passivate surface defects.

From the reference study including the one recommend by the reviewer, we found that Joule heating fails to explain increase the EL intensity. To address the reviewer’s point, we added sentences and references.

p.10-11: “We excluded the possibility of the Joule-heating effect through which the EL intensity decreases with a blue-shift as reported from many previous studies because, in our results, the EL intensity increased despite a blue-shift [44-46].”

p.12: “Importantly, the current decay during the increase in the EL intensity in Figure 2(b) is not well fitted to the stretched exponential function, clarifying that two different mechanisms compete to determine the EL intensity. It is noted that the contribution of the Joule-heating effect on the EL intensity is thought to be negligible because blue-shift was suppressed.”

References

48. Liu, C.; Kwon, Y. K.; Heo, J., Temperature-dependent brightening and darkening of photoluminescence from PbS quantum dots in glasses. Applied Physics Letters 2007, 90 (24), 241111.
49. Gaponenko, M. S.; Lutich, A. A.; Tolstik, N. A.; Onushchenko, A. A.; Malyarevich, A. M.; Petrov, E. P.; Yumashev, K. V., Temperature-dependent photoluminescence of PbS quantum dots in glass: Evidence of exciton state splitting and carrier trapping. Physical Review B 2010, 82 (12), 125320.
50. Turyanska, L.; Patane, A.; Henini, M.; Hennequin, B.; Thomas, N., Temperature dependence of the photoluminescence emission from thiol-capped PbS quantum dots. Applied Physics Letters 2007, 90 (10), 101913.

 

 

 

 

 

 

 

[10] Paragraph following fig 5 is pure speculation. Why dynamics in you LED should be connected to the oen of your FET based on another geometry and another material ?

Bias stress effect is coupled to surface traps which is closely related to disordered material systems such as QDs and organic semconductors. Importantly, PbS CQDs have been reported to form dynamic traps under high electric field, originating from field-induced ionization of capping molecules and nano morphology change of the CQD surface. Most of all, the unusually long decay time constant in the EL intensity allows us to correlate with dynamic trap formation typical of PbS CQDs.

To address the reviewer’s concern, we added sentences on p.11-12.

p.11: “Importantly, PbS CQDs have been reported to form dynamic traps under high electric field, originating from field-induced ionization of capping molecules and nano morphology change of the CQD surface [39]. In our previous study [27], indeed, we observed a time constant of tens of seconds in the time-domain measurements for pentacene/PbS CQD bilayer field-effect transistor devices, allowing us to connect the EL decay under high electric field to dynamic trap formation. From a far greater time constant than that with static electronic traps of fs to ms, we argued that dynamic traps originating from impurities associated with hydroxyl and hydrogen are present on the PbS CQD surface.”

p.12: “Importantly, the current decay during the increase in the EL intensity in Figure 2(b) is not well fitted to the stretched exponential function, clarifying that two different mechanisms compete to determine the EL intensity.”

 

[11] In the caption of fig 6 theoretical is not suited as wording i will rather use fit

 We corrected the word guided by the reviewer.

“The black and green curves represent fit and experimental data, respectively.”

 

 

 

 

 

 

Author Response File: Author Response.doc

Reviewer 2 Report

Current report by Kim and Park is and interesting study in a fast-growing field of QLEDs. Authors focused on a temporal evolution of electroluminescence (EL) by PbS QDs-based QLEDs in time scale up to 10 minutes. An interesting phenomenon of EL intensity increase in first two minutes and subsequent EL decrease in range of 2-10 min was observed. The EL increase at initially applied bias (up to 2 min) was assigned to the interfacial trapping of carriers at the surface of PbS CQDs close to the PEDOT:PSS and/or ZnO. After 2 minutes EL intensity decreases due to the charging of QDs that is a known quenching factor. Despite the quite well motivated conclusions and comprehensive experimental characterization of the observed phenomena I have few comments and questions that should be addressed before I can recommend paper for publication. I would like to take a look on the revised MS after authors improvements.

• Did authors consider the possible “cross”-recombination of electrons in ZnO and holes in adjacent QD? This may explain the observed temporal blue shift in EL spectra of QLED because of e-h recombination area moves from ZnO/PbS interface (CB-VB difference is 0.8 eV) to the PbS nanoparticle (bandgap 0.9 eV). Maybe the cross-recombination is even more efficient that own EL by PbS particles and it can explain the rise of EL intensity in starting 2 minutes and subsequent decrease of EL.
• Following previous comment, moving from PEDOT:PSS-PbS recombination area (CB-VB gap is 1 eV) the red shift in EL spectra should be observed (bandgap in PS is 0.9 eV), while authors observe a blue shift that might be an evidence that in initial 2 minutes a recombination at ZnO/PbS interface mainly works.
• If possible, authors should present in the manuscript a basic lighting characteristic of the fabricated device. I mean EQE, power and current efficiency at some test brightness (100 or 1000 Cd m-2).
• Some of important references on recent reports about QDs electroluminescence mechanism (1016/j.dyepig.2018.10.074) and deep red OLEDs performance (10.1016/j.dyepig.2019.108008, 10.1021/acsami.6b13689, 10.1016/j.dyepig.2019.108123) should be accounted.

Author Response

Reviewer 2

Current report by Kim and Park is and interesting study in a fast-growing field of QLEDs. Authors focused on a temporal evolution of electroluminescence (EL) by PbS QDs-based QLEDs in time scale up to 10 minutes. An interesting phenomenon of EL intensity increase in first two minutes and subsequent EL decrease in range of 2-10 min was observed. The EL increase at initially applied bias (up to 2 min) was assigned to the interfacial trapping of carriers at the surface of PbS CQDs close to the PEDOT:PSS and/or ZnO. After 2 minutes EL intensity decreases due to the charging of QDs that is a known quenching factor. Despite the quite well motivated conclusions and comprehensive experimental characterization of the observed phenomena I have few comments and questions that should be addressed before I can recommend paper for publication. I would like to take a look on the revised MS after authors improvements.

[1] Did authors consider the possible “cross”-recombination of electrons in ZnO and holes in adjacent QD? This may explain the observed temporal blue shift in EL spectra of QLED because of e-h recombination area moves from ZnO/PbS interface (CB-VB difference is 0.8 eV) to the PbS nanoparticle (bandgap 0.9 eV). Maybe the cross-recombination is even more efficient that own EL by PbS particles and it can explain the rise of EL intensity in starting 2 minutes and subsequent decrease of EL. Following previous comment, moving from PEDOT:PSS-PbS recombination area (CB-VB gap is 1 eV) the red shift in EL spectra should be observed (bandgap in PS is 0.9 eV), while authors observe a blue shift that might be an evidence that in initial 2 minutes a recombination at ZnO/PbS interface mainly works.

 

Considering a higher FET hole mobility of PbS in our experiments, it is plausible. But we don’t have concrete evidence to support it. Interfacial band alignment is pretty complicated and challenging to mention it. However, inspired by the reviewer’s insightful comment we have been encouraged to elaborate on the origin of the EL enhancement in the framework of energy band structure. We added a paragraph and sentences to reflect the reviewer’s excellent perspectives.

p.8-9: “During the charging process in Figure 3(b) at a high voltage (10 V), electrons are injected through Al and passivate the surface trap sites on the ZnO and PbS CQDs. The surface of ZnO CQDs are depleted of electrons due to adsorption of oxygen molecules as well as a high density of surface traps [29, 30]. A high injection of electrons fill and passivate the surface traps on the ZnO CQDs close to the PbS/ZnO interface. According to Weaver et. al, indeed, reductive passivation of surface traps through electron injection over the trap energy level of ZnSe CQDs led to large photoluminescence electro-brightening in the ZnSe CQDs, implicating interfacial carrier charging in determining luminescence intensity [31]. On the other hand, PbS CQDs are very well-known to form dynamic traps through which electrons are depleted, as will be discussed later. In contrast to the charging process, discharging process under application of -10 V depletes the charged electrons at the PbS/ZnO interface. Therefore, the subsequent bias at 1.5 V, after the charging process, causes a far higher current in comparison with that after the discharging process, as shown in Figure 3(b). It is important to note that the diode current increases with time at a higher voltage (10 V) while the current decreases at a lower voltage (5 V, Data not shown). At a high electric field, injected electrons fill the trap states fast and begin to passivate the interface while, at a low electric field, injected carriers are still in the process of being trapped. On the basis of the framework, the current decrease during the increase in the EL intensity at a low voltage (1. 5 V) implicates electron trapping at the PbS/ZnO interface, achieving charge balance.

In our previous studies, we investigated interfacial charging of PbS CQDs, finding that the time scale of carrier trapping associated with surface traps corresponds to tens of seconds which is consistent with that of the current and EL intensity change in Figure 2(b) [27, 28]. The long time scale over tens of seconds and the significant current change over an order of magnitude in Figure 3(a) in combination with the charging/discharging experiments in Figure 3(b) allow us to infer that the increase in the EL intensity during light emission is ascribed to the interfacial trapping of carriers at the PbS/ZnO interface.”

 

p.12: “In elaborating on the spectral shift during EL intensity variation, transition in the emission zone should be considered. Temporal blue-shift until 2 minutes can be interpreted that electron-hole recombination zone moved from the ZnO/PbS interface with an energy difference of 0.8 eV between the LUMO of the ZnO (-4.3 eV) and the HOMO of the PbS CQD (-5.1 eV) to the PbS CQD with a band gap of 0.9 eV even if the presence of interfacial dipole and surface defects complicate interfacial energy alignment.”

 

 

[2] If possible, authors should present in the manuscript a basic lighting characteristic of the fabricated device. I mean EQE, power and current efficiency at some test brightness (100 or 1000 Cd m-2).

 

Unfortunately, we do not have the facility to measure EQE and current efficiency for infrared wavelength region. From our experimental set up in which we measure the EL intensity with time over the threshold voltage, we could not do the typical EL characteristic measurements.    

 

[3]Some of important references on recent reports about QDs electroluminescence mechanism (1016/j.dyepig.2018.10.074) and deep red OLEDs performance (10.1016/j.dyepig.2019.108008, 10.1021/acsami.6b13689, 10.1016/j.dyepig.2019.108123) should be accounted.

 

We added the references.

6. Pidluzhna, А.; Ivaniuk, K.; Stakhira, P.; Hotra, Z.; Chapran, M.; Ulanski, J.; Tynkevych, O.; Khalavka, Y.; Baryshnikov, G. V.; Minaev, B., Multi-channel electroluminescence of CdTe/CdS core-shell quantum dots implemented into a QLED device. Dyes and pigments 2019, 162, 647-653.
7. Ledwon, P.; Motyka, R.; Ivaniuk, K.; Pidluzhna, A.; Martyniuk, N.; Stakhira, P.; Baryshnikov, G.; Minaev, B. F.; Ågren, H., The effect of molecular structure on the properties of quinoxaline-based molecules for OLED applications. Dyes and pigments 2020, 173, 108008.
8. Chapran, M.; Angioni, E.; Findlay, N. J.; Breig, B.; Cherpak, V.; Stakhira, P.; Tuttle, T.; Volyniuk, D.; Grazulevicius, J. V.; Nastishin, Y. A., An ambipolar BODIPY derivative for a white exciplex OLED and cholesteric liquid crystal laser toward multifunctional devices. ACS Applied Materials & Interfaces 2017, 9 (5), 4750-4757.
9. Ivaniuk, K.; Pidluzhna, А.; Stakhira, P.; Baryshnikov, G. V.; Kovtun, Y.; Hotra, Z.; Minaev, B.; Ågren, H., BODIPY-core 1, 7-diphenyl-substituted derivatives for photovoltaics and OLED applications. Dyes and pigments 2020, 175, 108123.

 

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

authors have only party adressed my comment so i wite again some of first round

 

What is PEDOT thickness

 

Regarding ZnO synthesis I read « «The washing process was carried out more than twice that does not look very accurate 

 

I ask for reshaping of figure 1 this is not what i see in the file v2

Part a is too small and TEM image should be bring at the bootom next by part c

 

I do not understand that you are able to male EL spectrum but not PL . only the spectrometer is a limitation ! make a film of QD, excite it with whatever source in the visible and put this in front spectrometer

 

Figure 3 is barely readable, increase size to suppress void and most important to make it readable

Same comment holds for figure 5, 6

On figure 6 an effort to better match the graph is required. In part b we barely see data below fit, can you change something to better show the two set of data even if they overlap

 

I ask for data L-I-V up to 10V where are they ?

If L is drifting at least provide I-V curve up to 10V

 

 

English from the authors looks as poor as mine

Sentence such as « We excluded the possibility of the Joule-heating effect through which the EL intensity decreases with a blue-shift as  reported from many previous studies because » seems to have a poor wording and this is not the only one

 

Additional details

Page 2 line 54 SiO2, the two need to be as index

Author Response

Reviewer 1

What is PEDOT thickness

>>We added the thickness information of PEDOT.

Regarding ZnO synthesis I read « «The washing process was carried out more than twice that does not look very accurate 

>>We modified the sentence to clarify the washing process.

p.5-6: “After the injection process, the mixed solution was cooled in air. For washing process, we added acetone (35 mL) into the solution (10 mL), followed by centrifugation at 9000 rpm for 10 minutes. After centrifugation, the solvent was removed and ZnO CQDs were dried for 1 minute. We added ethanol (10 mL) into the conical tube containing ZnO CQDs for another centrifugation.”

I ask for reshaping of figure 1 this is not what i see in the file v2

Part a is too small and TEM image should be bring at the bootom next by part c

We reformatted Figure 1(b) and 1(c) as guided by the reviewer. 

I do not understand that you are able to male EL spectrum but not PL . only the spectrometer is a limitation ! make a film of QD, excite it with whatever source in the visible and put this in front spectrometer

I commented about this issue on the R1 version as below.

“We admit that PL measurement would be useful in elaborating on the origin of the temporal evolution.  Unfortunately, we do not have PL curves for the QLED device because we don’t have the instrument for PL measurement in the NIR region. However, to elaborate on the exciton quenching mechanism investigation of the temporal evolution of PL will be helpful.”

I commented on the facility issue. The process is not as simple as the reviewer mentioned. The spectral noise by the pumping laser as well as ambient light (we use back box) made the data pretty noisy. Spectral sensitivity provided by the spectrometer and the area of the optical fiber made it difficult to PL measurements. We plan to do it next in external resource center which is beneficial in elaborating on the exciton quenching mechanism. (I am not sure if it is appropriate or not to mention that the student who did experiment will be back in about 6 months for some reasons. We could not do the experiments.)

 

 

Figure 3 is barely readable, increase size to suppress void and most important to make it readable

Same comment holds for figure 5, 6

On figure 6 an effort to better match the graph is required. In part b we barely see data below fit, can you change something to better show the two set of data even if they overlap.

>>We reformatted Figures as guided by the reviewer.

 

It is not part b It is part (a)

>>We replaced the red curve to red dash to distinguish the experiment from the fit.

 

I ask for data L-I-V up to 10V where are they ?

If L is drifting at least provide I-V curve up to 10V

 

>>In the first round of revision we answered that we don’t have the I-V data because, as mentioned before, the student is not working now. In the first round of revision, we added two data figures to address the reviewer’s concerns about device stability from electrical damage. We admit that we should have measured I-V. From the data attached, we believe that the device is working normally at such a high voltage.

 

To address the reviewer’s concern we showed the figures below and added sentences on p. 7.

p.7: “Prior to the charging/discharging experiments, we confirmed that, at a high voltage over 10 V, the EL intensity increased with voltage and time (Data not shown), indicating that the bias condition for the charging/discharging experiments has no significant electrical damage on the device.”

 

 

 

 

 

 

Figure S1

 As shown in the below Figure S2, the current increased and decreased during charging and discharging, respectively, and gradually saturated. We believe that the gradual current change is due to charging/discharging process not by electric field induced degradation as concerned by the reviewer.

As mentioned above, over the voltage (10 V) in Figure S1, the EL intensity increased with time. We believe that voltage dependent EL intensity as well as time dependent EL intensity increase demonstrate that no damage was occurred at 10 V.

 

 

 

 

 

 

 

 

 

Figure S2

 

 

English from the authors looks as poor as mine

Sentence such as « We excluded the possibility of the Joule-heating effect through which the EL intensity decreases with a blue-shift as  reported from many previous studies because » seems to have a poor wording and this is not the only one

>>We significantly modified and rephrased sentences as recommended by the reviewer and believe that it has been improved. (The modified sentences are highlighted in the manuscript.)

 

 

 

 

 

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

I'm satisfied with answers by authors to my questions and thus I can recommend to accept this paper for publication In Applied Sciences journal. 

Author Response

We appreciate to the reviewer's approval to our revised manuscript.