Peer review of: Cathode engineering with perylene-diimide interlayer enabling over 17% efficiency single-junction (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
In this manuscript, the authors developed a new cathode interlayer material (PDINN) consisting of 
aliphatic amine side chains and a perylene diimide core, which can down-shift the work function of 
metal cathodes including Al, Ag, and Cu. Organic solar cells (OSCs) with PDINN as interlayer and Ag 
as cathode demonstrated a high power conversion efficiency (PCE) of 17.23% and high stability based 
on the hot PM6:Y6 system. The utilization of polar group functionalized conjugated materials, which 
was often nominated as water/alcohol soluble conjugated materials for improving the performance of 
organic optoelectronic devices including OSCs is not new. In the past ten years, numerous materials 
that featured polar groups (amine, phosphate, sulfate, and PEG etc.) and conjugated backbone (either 
electron-rich or electron-deficient) for use as electrode modifiers in solar cells have been reported in 
literatures. In this manuscript, the authors demonstrated considerable device performance 
enhancement based on their newly developed interface material PDINN, which is even superior to 
their previously reported cathode modification material PDINO. The prominent device results prove 
that PDINN is indeed a good modifier for a few high work function metals. Nevertheless, several 
critical scientific issues were not correctly explained in the current manuscript. 

1) An important starting point and critical claim of this paper is that their interface material can 
provide good adhesion via chemical interaction with the active layer (emphasized in the 
INTRODUNCTION part). However, there is no solid evidence to support this argument, which makes 
the discussion too speculative. The only supportive results are the variable-temperature 1H-NMR of 
the blended solutions consisting of PDINN and Y6. This is not convincing since one cannot use the 
results obtained in solutions to support the statement for solid states. Moreover, one cannot take it for 
granted that the improved device performance was offered by the improved adhesion since there are 
many other possibilities. I would suggest the authors conducting 1H-NMR and FTIR/Raman 
measurements using the samples in solid state to convincingly evidence the chemical interactions such 
as Hydrogen-bonding, which is indispensable for the new statement described in the manuscript. 
2) The authors ascribed the enhanced FF of the solar cells to the improved adhesion between interface 
layer and active layer, which lacks convincing evidences as I pointed out above. On the other hand, 
the function of enhanced electrical conductivity was underestimated, which would actually be the main 
reason for the improved device performance in my opinion. It is well known that high conductivity and 
the ability to form Ohmic contact is critical to an efficient interface material, which can promote charge 
extraction and improve FF. In this manuscript, much stronger electron spin resonance signal was 
observed for PDINN with respect to PDINO (Figure 1f). This is a clear implication for the more efficient 
self-doping and higher conductivity of PDINN than PDINO (please see the papers of Prof. Alex Jen and 
Prof. Yong Cao). Therefore, more experiments are needed to correlate the relationship between high 
device performance and high conductivity of interlayer materials. Meanwhile, the relevant discussion 
may need revisions. 
3) When studying the stability of devices based on PDINN interlayer material, the authors studied the 
air and thermal stability. It is also necessary to study the light stability of the devices, especially under 
ultraviolet light. The information about the device stability upon continuous illumination would be very 
helpful to prove the good adhesion of the interface layer. 
4) The authors did not show device stability data of Al cathode device in Figure 4(d). Is it because the 
stability of Al cathode device is poor? And from Figure 4(d), we can find that the stability of the 
PDINO/Cu device is similar to that of PDINN/Cu device. Therefore, one cannot state that PDINN can 
improve the device stability compared to PDINO. 
5) When PDINO or PDINN is used as the interlayer materials, the work function of Al is lower than that 
of Ag and Cu. And the performance of devices with Al as cathode is better than Ag and Cu for 
PM6:Y6/PDINO contact. Why did the Ag cathode afford the best performance instead of Al for the 
PM6:Y6/PDINN devices? 6) Please explain that why PDINN displays a blue-shifted absorption relative to that of PDINO (Figure 
1c). Does this correlate to the device performance? 
Generally, I would be happy to see the publication of this manuscript in Nature Communication if the 
above-mentioned issues could be reasonably addressed. 
 
 
Reviewer #2 (Remarks to the Author): 
 
The manuscript by Yao et al. introduces a new cathode interlayer, aliphatic amine functionalized 
perylene diimide (PDINN). The PDINN interlayer complements the limits in cathode interlayer based on 
PDI, such as PDINO. The PDINN is much lower cost due to one step approach from cheap raw 
materials and has good solubility without the assistance of any acids. In OPVs devices, the PDINN has 
better adhesion with organic active layers due to hydrogen bonding from aliphatic amine group. This 
better adhesion enhances an ability in lowering work function of cathode, leading to higher built-in 
potential and higher VOC. This better ability in lowering work function makes it possible to replace Al 
cathode with Ag cathode which is more stable in the air. Secondary, the better adhesion also improves 
charge extraction and reduce recombination, leading to higher FF. As a result, using a PDINN cathode 
interlayer, the PCE of PM6/Y6 device exceeded 17% with better performance, stability and low cost 
than the traditional cathode interlayer, PDINO. 
 
The authors have applied the new cathode interlayer to improve quality of devices. Recently, there 
was a similar report which introduced new anode interlayer and reached to 17% of PCE [1]. Therefore, 
this report, introducing the cathode interlayer, does not seems to have noticeable novelty in terms of 
the efficiency of devices. However, with the better performance, the PDINN have an amazing 
attraction because of the low cost, simple step for synthesis even and good compatibility with stable 
cathode. These characteristics may provide a strong tool to many researchers and the PDINN might 
speed up development in solar cells researches. 
 
[1] Lin et al,. Adv. Mater. 2019, 31, 1902965 
 
A few questions and comments are as follows: 
 
2. Authors said “However, this approach is constrained by the intrinsic high surface tension of the 
interlayer bearing the polar group, which harms its deposition with a solution processed layer-by-layer 
technology and results in a poor interlayer contact with the active layer”. The authors are advised to 
provide the surface tension of the PDINN and PDINO. 
 
3. In figure 2c, the authors mention thickness insensitivity of the PDINN. However, there is no data to 
prove superiority of PDINN thickness insensitivity. PCE vs. PDINO thickness of OSC devices, would be 
needed. 
 
 
4. The (1 0 0) peak representing face on stacking is important for improving OSC devices. However, 
the (1 0 0) peak intensity of PDINN is lower than PDINO. Comments on the results would be needed. 
 
5. “With the good air and thermal stability of PDINN, we further investigated its device stability using 
Ag or Cu top electrode. After about 50 days, the PCEs of the PDINN / Ag-based and PDINN / Cu-based 
devices can retain 87% and 88% of their initial PCE value in a glove box filled with nitrogen (Figure 
4d)”. If the authors want to show the device air stability, the experimental data in air environment 
should be provided as well. 

 
 
Reviewer #3 (Remarks to the Author): 
 
Cathode interlayers are important to develop efficient and stable organic solar cells. For low-cost 
interlayers, they are generally insulate materials that are not suitable for practical application. The 
incorporation of aromatic compounds can enhance their conductivity, but this approach is associated 
with tedious purification steps, thus the interlayer can only accessible with high cost. This is an 
obstacle encountered for large-scale production and industrial application. In this manuscript, the 
authors reported a new low-cost interlayer (PDINN) based on perylene-diimide that can be facile 
synthesized by a typical one-step approach under a large-scale of 64 g in lab from cheap raw 
materials. PDINN offers stable electrode interface, high conductivity, effective work function tuning 
with air stable metals (Ag and Cu), and good contact with the nonfullerene active layer under a 
hydrogen-bond interaction, thus making the device working well with a record high efficiency of 17.23% 
with Ag (certified value of 16.77% by NREAL). Thus, PDINN is a low-cost (1.6 $ g−1) and effective 
interfacial engineering material that is prominent for thin film photovoltaic devices and cable of large-
scale production. Most specially, the approach of using noncovalent-bond interaction to improve of the 
adhesion of the interlayer on the active layer is interesting and a new concept to design new interlayer 
materials. This work can attract broad interest attract broad interest for both academia and industry. 
The followings are some suggested minors issues. 
 
1. To decrease the work function of cathode, generally, a highly polar groups is introduced to increase 
its dipole moment. As the authors point out, this approach is usually constrained by the poor adhesion 
of the interlayer on the active layer brought by the polar group. That is why fewer reported interlayer 
worked efficiently with air stable metals that can be widely used in different active layers. In Page 18, 
the authors tried different active layers to confirm that PDINN is a universal interlayers that can be 
widely used. When make discussions, can the authors correlated this advantage with this special 
amine groups? 
2. Thermal stability is an important issue for real application, and the authors have given some 
evidence that PDINN shows better stability. Also, thermogravimetric analysis should be carried out to 
further evaluate the thermal stabilities. 
3. In Figure S5, from morphology, the authors give a strong evidence that PDINN does show a better 
adhesion on the active layers than that of PDINO. This observation together with the variable-
temperature and titration 1H NMR experiments (Figure S6) shows that intermolecular interaction can 
help PDINN layer form a good contact with the actively layers during its layer-by-layer deposition. To 
help the readers had a better understand on the conclusion, more details explanation on the 
experimental results of PDINN (Fig 5Sd) and PDINO (Fig 5Sb) should be given. 
 
 
 
Reviewer #4 (Remarks to the Author): 
 
The authors present a novel PDI derivative bearing aliphatic amine sidechains (denoted as PDINN) as 
a cathode interlayer material for highly efficient organic solar cells. Compared to the commonly 
employed PDIDO, the new material stands out by its ability to form hydrogen-bonds with the active 
organic layer, thereby improving the adhesion between the layers. Also, stable metals such as Au or 
Ag coated with PDINN exhibit work functions of ca. 4 eV, which renders these modified metals suited 
for electron-injecting/extracting contacts. Notably, self-doping results in a fairly high conductivity of 
the PDINN layers, making the device performance less sensitive to the thickness of the PDINN. As a 
consequence, highly efficient OSCs are fabricated from different polymer:NFA blends with stable Ag 
and Cu electrodes. 
 
The performance data presented in the work are indeed impressive as is the tolerance of the newly 
developed cathode interlayer material versus the choice of the cathode metal and the processing 
conditions. The material is clearly superior to PDINO, which is now widely applied for the production of 
state of the art NFA blends (e.g. J. Yuan, Y. Zhang, L. Zhou, G. Zhang, H.-L. Yip, T.-K. Lau, X. Lu, C. 
Zhu, H. Peng, P.A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. Li, and Y. Zou, Joule 3, 1140 (2019)). I, 
therefore, believe that the results presented in this work will have a large impact on the future 
research the OPV community. I also appreciate that the authors do a great job to justify most of their 
claims by providing detailed data on the polymer and device characteristics (e.g. the ability of 
hydrogen bonding to the photoactive material by performing variable temperature and titration NMR 
spectroscopy). Therefore, I recommend publication of this work. 
 
Nevertheless, revision is mandatory, in particular regarding the analysis of the physical and device 
properties. 
 
The authors analyze the dark JV-characteristics of their devices to deduce values of the series and 
shunt resistances and the ideality factor. This approach is based on the modified Shockley-diode 
equation and it is very common for the study of inorganic semiconductors, which because of doping 
and high carrier mobilities exhibit high electrical conductivities. This presumption is not fulfilled in 
organic semiconducting layers as has been conclusively demonstrated by several groups in the past 
(see e.g. Figure 3 in K. Tvingstedt and C. Deibel, Adv. Energy Mater. 6, 1502230 (2016) or the work 
by U. Würfel, D. Neher, A. Spies, and S. Albrecht, Nat. Commun. 6, 6951 (2015)). Therefore, this 
analysis and the corresponding interpretation (e.g. the calculated Voc) needs to be removed from the 
paper. 
 
The information provided on the TAS measurements is insufficient and it is, therefore, totally unclear 
which conclusion can be drawn from these data: what was the pump fluence: 
• how large is the carrier density at this excitation condition? 
• why was the probe wavelength 633 nm, which excitation is dominating the signal at this wavelength 
and why did you use an IR-sensitive InGaAs to measure at this wavelength? 
• what was the architecture of the measured sample (was the layer on PEDOT or on glass, was there a 
top electrode or not)? 
I am also concerned with the claim that “the PDINN CIM heals surface traps”. Which surface traps? On 
the PM6:Y6 surface? Or at the interface to the CIM? 
Clearly, results from additional measurements on a neat PM6:Y6 need to be provided, measurements 
have to be performed at different fluence in order to identify the order of recombination (see e.g. 1 I.A. 
Howard, R. Mauer, M. Meister, and F. Laquai, J. Am. Chem. Soc. 132, 14866 (2010)) and the transient 
spectra have to be displayed. 
 
 
I am concerned with several of the claims on page 16 (and the corresponding graphs in Figure 3. 
• Why should an p-n junction from between the active PM6:Y6 blend and the CIM. The CIM is 
(probably n-doped) but the active layer is not p-doped (at least, I did not find any evidence for p-
doping in the manuscript)? 
• What crucial information can be drawn from the PiFM image in Figure (e)? How relevant is this image 
for the understanding of the function of the interlayer? 
• The authors conclude from the results in Figure (f) that “that the interlayer can also server as the 
light harvest layer when the interlayer is contact with the donor domain due to the charge transfer 
effect from donor to interlayers interlayer”. First, the photocurrents in Figure (f) are very small but 
also, the authors do not provide spectral information which would be needed to draw conclusions 
about the origin of the photocurrent 
• The author claim that the rectification behavior in Figure (g) is induced “by an interfacial dipole layer 
due to the interaction between the amine group of interlayers with electron-deficient part of the n-OS 
acceptors”. But given the fact that for these devices, the Y6 acceptor layer is sandwiched between a 
high workfunction PEDOT:PSS anode an a low workfunction CIM/Ag cathode, rectification is expected 
anyhow. This is because PEDOT:PSS does not inject electrons and CIM/Ag does not inject holes. I my 
opinion, the shown rectification behavior has nothing do to with an interfacial dipole layer but it is 
simply related to the difference in work function between the top and bottom contact. 
 
 
Other more minor issues: 
 
The abstract is poorly written, with grammar mistakes or missing words (e.g. “requires a highly polar 
groups” or “offering effective down-shifting work function of air stable cathodes”) 
 
The authors demonstrate in Supplementary Figure 4 that PDINN is more stable under prolonged 
heating (compared to PDINO) but they don’t explain why this is the case. 
 
In Supplementary Figure 5, the authors employ PiFM to show the superior wetting ability of PDINN on 
PM6:Y6. However, as the technique is not properly introduced (PiFM is not a common technique in the 
OPV field), it is not clear what is actually shown in the graphs. For example, the image in Figure (b) 
differs from that in Figure (c) by the larger feature size and the larger signal range. Which information 
is used to conclude that film formation of PDINO on PM6:Y6 is poor? 
 
The authors employ lateral JV experiments to determine the conductivity of the interlayer. However, 
the JVs from these measurements in Supplementary Figure 7 are plotted on a log-lin scale. It’s 
therefore, not obvious that the current is indeed linear in voltage as would be expected for purely 
ohmic behavior. 
 
Figure 2 shows the JVs of the best devices while table 2 provides the average values with standard 
deviations. Nevertheless, a graph with the full device statistics must be provided as well (e.g. as a 
Supplementary Figure) 
 
On page 15, the text reads: “The OSCs with methanol threated (without CIM) and with PDINN or 
PDINO CIM exhibit the slopes of 1.42, 1.24 and 1.19 kT/q, respectively” which is not consistent with 
the numbers of Figure 3(a). 
 
More information needs to be provided on the device stability measurement. Have the samples been 
kept in the dark or under illumination between the JV measurements? Unfortunate, neither the caption 
nor the text provides these details. Also, what exactly is shown in the inset of Figure 4(d)?