Peer review of: Understanding coordination reaction for producing stable electrode with various low work functio (2020)

Reviewers' Comments: 
Reviewer #1: 
Remarks to the Author: 

The manuscript reported a series of Phen derivatives by utilising the coordination reaction to 
produce a stable electrode with various WFs, which can be tuned from 3.29–2.43 eV on the film of 
Phen derivative/ITO/ZnO. However, the molecular dipole moment, ESP and the amount of ET 
effects on ∆WF seem unconvincing for the paper's purpose. More work needs to be done to 
organize and sort out the relationship of gap state,∆WF, ∆e, driving voltage, and the electron 
injection/hole-blocking property. Therefore, I think there is still more work to do before publishing. 
The paper is inappropriate to be published in Nature Communications at present. 
 
1) It's not accurate to contribute the ∆WF to molecular dipole moment, ESP and the amount of ET 
estimated by DFT calculation with a single molecule. The corresponding calculation should be 
performed on Zn coordination compounds. 
2) Since the amount of ET showing the most potent effect on ∆WF, the amount of ET of the two-
molecule system should be calculated for further confirm. Supported references would better be 
supplied at the position of "it was found that the calculated amount of ET of a two-molecule 
system ....". Besides, there's no logical connection between the ESP and ET. 
3) Both 1H and 13C NMR spectra and Mass spectrometric analysis (high resolution)should be 
afforded for new compounds. 1H NMR data of p-Pyrrd-Phen was missed in SI. Also, the identity 
and purity of the compounds should be demonstrated. 
4) The wrong chemical structures of p-NMe2-Phen and p-Pyrrd-Phen were presented in the part of 
"Supplementary Section 1: Materials". 
5) ∆WF was shown in Figure 1, the detailed WF and specific evolution of work function and 
conductivity under photo-activation maybe introduced in the supporting information. 
6) The mechanism of coordination reaction between p-NMe2-Phen and Zn should be explained in 
detail. 
7) Calculated IP using DFT calculation should be given in the supporting information and illustrated 
in the manuscript. 
8) Compared with your previous work and other references, what is the advantages and 
disadvantages for this work? The list of OLED performance comparison may be listed. 
9) There are many spoken transitional words, for example, “In addition” was used so many times. 
The author should consider changing the words. 
10) Apparently, the idea of this work follows the reference “Nat. Commun. 10, 866 (2019)”, and 
you have used the electrode ZnO rather than Ag to and tried some new phenanthroline 
derivatives. But my question is that, compared with the former work, what is the superiorities for 
your research. 
11) In your MS, you have claimed the “coordination reaction” between ZnO and Phen derivatives, 
but there is not any experimental proof to support this conclusion, except for some calculations. 
But actually, the calculations should be as the supplementary or assistant proof to aid or further 
explain the experiments, rather than the crucial evidence. 
12) The most serious problem is that you should more professionally reconsider and analyse your 
UPS data, and more importantly, make it clear the energy levels for charge injection and 
transport. In fact, it is little sense to evaluate the “ΔWF” which actually infers the shift of vacuum 
level. The most important information from UPS is the HOMO or gap state. Others, like what 
means for “two gap states” and “the original HOMO” in Figure 1d? 
13) Please take care this, the thickness of 10 nm has beyond the range of solely electrode 
modification. 
14) The right two figures of Figure 1f cannot give the clear information. 
15) Apparently, the idea of this work follows the reference “Nat. Commun. 10, 866 (2019)”, and 
you have used the electrode ZnO rather than Ag to and tried some new phenanthroline 
derivatives. But my question is that, compared with the former work, what is the superiorities for 
your research. 
16) In your MS, you have claimed the “coordination reaction” between ZnO and Phen derivatives, 
but there is not any experimental proof to support this conclusion, except for some calculations. 
But actually, the calculations should be as the supplementary or assistant proof to aid or further 
explain the experiments, rather than the crucial evidence. 
17) The most serious problem is that you should more professionally reconsider and analyse your 
UPS data, and more importantly, make it clear the energy levels for charge injection and 
transport. In fact, it is little sense to evaluate the “ΔWF” which actually infers the shift of vacuum 
level. The most important information from UPS is the HOMO or gap state. Others, like what 
means for “two gap states” and “the original HOMO” in Figure 1d? 
18) Please take care this, the thickness of 10 nm has beyond the range of solely electrode 
modification. 
19) The right two figures of Figure 1f cannot give the clear information. 
20) The EQE–luminance characteristics of p-NMe2-Phen and p-Pyrrd-Phen were low than the 
existing Bphen under high current (Figure 2e). Why? 
21) 22) “In addition, we see from Fig. 2e that the external quantum efficiency (EQE) of the 
inverted OLED using p-MeO-Phen is lower than that of the inverted OLED using BPhen, although p-
MeO-Phen causes a larger ∆WF than BPhen. Thus, it is reasonable to assume that the 
characteristics of inverted OLEDs are dominated by not only ∆WF, but also the hole-blocking 
property around a cathode.” How about the relationship between ∆WF and the hole-blocking 
property for the EQE of five Phen derivatives? Who plays the leading role? 
22) In the evaluation of electron injection efficiency and air stability of cathode-modification layers 
in inverted OLEDs. The Luminance–voltage and current density–voltage characteristics of cOLED 
employing p-Pyrrd-Phen were almost the same as the PEI (Figure 3d-e). How about the 
advantages of p-Pyrrd-Phen than PEI? 
23) Provide the details about the decay test in the light-emitting area of inverted OLEDs. It was 
clear that the initial brightness of inverted OLED-3 is significantly higher than others, but the 
Luminance–voltage and current density–voltage characteristics of OLED-2 and OLED-3 are almost 
the same (Figure 3b-d). The detailed measurement could directly affect the decay test. 
24) “On the other hand, the relatively low operational stability of the cOLED employing p-MeO-
Phen, where the electron injection efficiency is insufficient, may be caused by the accumulation of 
the carrier recombination region rather than the stability of p-MeO-Phen45–47.” The author 
explained the relatively low operational stability of the cOLED employing p-MeO-Phen was caused 
by the accumulation of the carrier recombination region rather than the stability of p-MeO-Phen. 
Please confirm it with detailed measurements and recheck the data. 
25) Why the EQE–current density curves characteristic of cOLEDs employing p-MeO-Phen was 
higher than p-Pyrrd-Phen, but there was a significant difference in the Luminance–voltage and 
(inset) current density–voltage characteristics (Figure 4b-c)? How to explain? Please recheck the 
data. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
The authors demonstrates that low work function contacts can be achieved using phenanthroline-
based interlayers that coordinate to the metal and ZnO cathodes. They further show for inverted 
OLEDs that the electron injection efficiency and air stability of a phenantorline-modified electrode 
are higher than those of a LiF-modified electrode, the "benchmark" of OLEDs. 
The authors carry out UPS experiments on in situ prepared cathodes and phenanthroline-based 
interlayers (5 nm) and show the formation of gap states that are attributed to through-bond 
charge transfer (nitrogen atoms bonding/coordinating with the Zn at the ZnO surface). The charge 
transfer at the interface is given as the main factor for the resulting low work function. 
I find the paper to be well written and the results useful. I think the paper could be improved 
however with a few additions. 
1. If I understood the paper correctly, the UPS experiments were carried on in situ prepared ZnO 
cathodes and phenanthroline-derivative films, whereas for the OLED 2, the ZnO were exposed to 
air before spin-coating the phenanthroline derivative (also in air). Here, the surface of the ZnO 
may not be as reactive as for the in situ prepared ZnO/phenanthroline-derivative system and the 
WF-lowering mechanism may then be different (see e.g. J. Mater. Chem. C, 2020, 8, 173). 
Carrying out UPS of ZnO films exposed to air and then inserted into the UHV system for 
phenanthroline-derivative deposition may give a more precise comparison between the UPS and 
OLED 2. 
2. The gap states evident in the UPS are convincing evidence for charge transfer given the WF 
shift. However, additional evidence od charge transfer from XPS or FTIR would be helpful, but this 
is not a "game breaker". 
 
 
 
Reviewer #3: 
Remarks to the Author: 
The work by Fukagawa et al, presents a novel way to modify the workfunction of zinc oxide. The 
approach is based on the interaction between phenanthroline variaties and ZnO. Quite impressive 
changes in the WF are observed. 
The authors present fundamental analysis of the modified electrodes and apply these electrodes in 
efficient OLEDs. The latter confirmes that indeed the WF can be tuned as witnessed by the effect 
on the driving voltage. 
I think this is a very nice work, and have only a few questions that would require some 
explanation or comment from the authors. 
The authors use a very thin ZnO layer, what is the reason for such a thin layer. Is this required to 
obtain good electron injection? Are the authors certain the ITO is fully covered by such a thin ZnO 
layer? How is this verified? 
Did the authors verify if these modified ZnO electrodes can also be effective charge extraction 
layers for photovoltaic devices? 
What is the role of HAT-CN between the HTL and the metal electrode. Judging from the energy 
levels shown in Fig. 2. B, it would not be expected to lead to efficient doping of the HTL.