Electrochemically-stable ligands bridge the photoluminescence-electroluminescence gap of quantum dots

Colloidal quantum dots are promising emitters for quantum-dot-based light-emitting-diodes. Though quantum dots have been synthesized with efficient, stable, and high colour-purity photoluminescence, inheriting their superior luminescent properties in light-emitting-diodes remains challenging. This is commonly attributed to unbalanced charge injection and/or interfacial exciton quenching in the devices. Here, a general but previously overlooked degradation channel in light-emitting-diodes, i.e., operando electrochemical reactions of surface ligands with injected charge carriers, is identified. We develop a strategy of applying electrochemically-inert ligands to quantum dots with excellent luminescent properties to bridge their photoluminescence-electroluminescence gap. This material-design principle is general for boosting electroluminescence efficiency and lifetime of the light-emitting-diodes, resulting in record-long operational lifetimes for both red-emitting light-emitting-diodes (T95 > 3800 h at 1000 cd m−2) and blue-emitting light-emitting-diodes (T50 > 10,000 h at 100 cd m−2). Our study provides a critical guideline for the quantum dots to be used in optoelectronic and electronic devices.

The author demonstrated record-long operational lifetimes for both red-emitting QLEDs (T95 > 3800 hours at 1000 cd m-2 ) and blue-emitting QLEDs (T50 >10,000 hours at 100 cd m-2 ). Through a general but previously overlooked degradation channel in QLEDs, i.e., operando electrochemical reactions of surface ligands with injected charge carriers, they developed a strategy of applying electrochemically-inert ligands to QDs with excellent luminescent properties to bridge theirphot luminescence electroluminescence gap. This is an interesting working and it is meaningful for the QLED application especially for the commercial application. However; some questions should be addressed before accepted.
1 In Page 7, why take the 10mA cm-2 for testing the CdSe/CdS, and for the CdSe/CdS/ZnSand CdSeS/ZnSeS/ZnS with 100mA cm-2？Why use the 30mA cm-2 but not the 100mA cm-2 for the testing? 2 In Page 8, the author demonstrated that completely replace all carboxylate and cadmiumcarboxylate ligand, however the -COO peak appeared in the Fig. 3a. How to explain this?
Reviewer #3 (Remarks to the Author): The paper "Electrochemically-stable ligands bridge the photoluminescence..." is very interesting and well written paper. It emphasizes that the issue that material design requirements for EL applications does not always coincide with what is good enough for PL applications. The device performance is impressive and as far as the device performance and its relation to the presence of different ligands, I find the paper to be rigorous and providing several different indications that converge to the same conclusion. If you choose the ligand wrong -your film PL will have very little in common with its PL. There is also significant part of the paper that mentions electrochemical activity. Unfortunately, I am not an expert in this. I can't tell if the improved results are due to better coverage and/or passivation of the surface or that there is indeed an electrochemistry that involves one ligand and not the other. I hope that this important issue can be answered by one of the other referees. Assuming this part is correct than I believe the paper is suitable for publication.

Our revision and responses:
We thank the reviewer for his/her positive comments on the scientific quality of our manuscript. Following the reviewer's suggestions, the first sentence on page 6 (highlighted) has been modified to "In literature, chemical reduction of cadmium-salt ligands on the cadmium chalcogenide QDs was reported 26,27 ". In addition, we have gone through the entire manuscript to correct the spelling, grammar and word use.

Responses to Reviewer #2
The author demonstrated record-long operational lifetimes for both red-emitting QLEDs (T95 > 3800 hours at 1000 cd m -2 ) and blue-emitting QLEDs (T50 >10,000 hours at 100 cd m -2 ). Through a general but previously overlooked degradation channel in QLEDs, i.e., operando electrochemical reactions of surface ligands with injected charge carriers, they developed a strategy of applying electrochemically-inert ligands to QDs with excellent luminescent properties to bridge their photluminescence electroluminescence gap. This is an interesting working and it is meaningful for the QLED application especially for the commercial application. However; some questions should be addressed before accepted.
1 In Page 7, why take the 10 mA cm -2 for testing the CdSe/CdS, and for the CdSe/CdS/ZnS and CdSeS/ZnSeS/ZnS with 100 mA cm -2 ？Why use the 30 mA cm -2 but not the 100mA cm -2 for the testing?
Our revision and responses: We thank the reviewer for the valuable comment. The purpose of measuring the time-dependent PL QY of the QDs in single-carrier devices is to investigate the redox activity of the QDs. The CdSe/CdS QDs with Cd(RCOO) 2 ligands are more reactive than the CdSe/CdS/ZnS QDs and CdSeS/ZnSeS/ZnS QDs with Zn(RCOO) 2 ligands, because the standard reduction potential of Zn 2+ is ~0.3 V more negative than that of Cd 2+ (Supplementary Fig. 2). Therefore, a current density of 10 mA cm -2 applied to the electron-only devices with the CdSe/CdS QDs is sufficient to demonstrate that the CdSe/CdS-RNH 2 QDs QDs are stable while the CdSe/CdS-Cd(RCOO) 2 QDs are not. In contrast, a higher current density of 100 mA cm -2 is applied to the electron-only devices with the CdSe/CdS/ZnS QDs (or the CdSeS/ZnSeS/ZnS QDs) to distinguish behavior of the QDs with different surface ligands. Regarding the hole-only devices (ITO/PEDOT:PSS/poly-TPD/PVK/QDs/CBP/MoO x /Au), the injection of electrons becomes evident when the current density is larger than ~50 mA cm -2 (corresponding to a high driving voltage of ~10 V, see figure below). Therefore, a current density of 30 mA cm -2 is applied to ensure that the charge carriers transported in the hole-only devices are exclusively holes, but not electrons.
In the revised manuscript, we have added a few sentences in the captions of Fig.2  Our revision and responses: Sorry for the confusion caused by the peak labels in Fig. 3a of the original manuscript. For the sake of clarity, we have marked the peak at ~1460 cm -1, which is originated from the bending vibration of -CH 2 -groups in either types of ligands. The revised Fig. 3a (also see below) demonstrates that the cadmium-carboxylate ligands have been completely replaced by the amine ligands.

Responses to Reviewer #3
The paper "Electrochemically-stable ligands bridge the photoluminescence..." is very interesting and well written paper. It emphasizes that the issue that material design requirements for EL applications does not always coincide with what is good enough for PL applications. The device performance is impressive and as far as the device performance and its relation to the presence of different ligands, I find the paper to be rigorous and providing several different indications that converge to the same conclusion. If you choose the ligand wrong -your film PL will have very little in common with its PL.
There is also significant part of the paper that mentions electrochemical activity. Unfortunately, I am not an expert in this. I can't tell if the improved results are due to better coverage and/or passivation of the surface or that there is indeed an electrochemistry that involves one ligand and not the other. I hope that this important issue can be answered by one of the other referees.
Assuming this part is correct than I believe the paper is suitable for publication.

Our revision and responses:
We thank the reviewer for his/her encouraging comments. No action needed.