Peer review of: Simultaneous enhanced efficiency and thermal stability in organic solar cells from a polymer acce (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this work, a non-fullerene acceptor named BTTT-2Cl was synthesized, and a donor/acceptor blend 
with PZ1 polymer additive was reported with good thermal stability. Considering the novelty and 
technical issues, I cannot recommend publishing this work in this journal. Please see the detailed 
comments below: 

(1) The novelty of the new acceptor BTTT-2Cl is limited, the core of the molecule is same as Y6 (ref. 
46), but the PCE of PM6/BTTT-2Cl based device is lower than that of PM6/Y6 based device (13.8% vs. 
15.5%). What's the advantage of this new molecule compare with Y6? 
(2) What's the original reason of the higher VOC of device with PZ1? The addition amount of PZ1 is 
only 1%, the enhanced VOC should be discussed. 
(3) The stability of PZ1 based device is very good. The authors claimed the reason for this good 
stability is "suppressing thermal-induced BTTT-2Cl aggregation in the PM6 polymer domains". In Fig. 
3B, this reviewer found PZ1 can reduce the thermal-induced aggregation (crystallization), but even in 
amorphous OPVs, the thermal stability can not be that good. In my opinion, this is maybe one of the 
reasons for the good stability, but not the major one, some additional studies are needed. 
(4) The original chemical/physical interactions between PZ1 with PM6/BTTT-2Cl which induced the 
excellent stability should be the most important part of the work, but missing now. 
(5) The whole part of "space environments, including Moon, Low Earth Orbital Satellite and Mars" 
should be removed. Since the authors only considered about the temperature change, but missed 
irradiations in space. The irradiation stability (like X-ray, Gamma-ray) of solar cells is very critical to 
evaluate the stability in space, that's why some other type cells (like CIGS, GaAs, perovskite) show 
good potential in space. The authors cannot claim that OPV can be used in space (Moon, Low Earth 
Orbital Satellite and Mars….) without figuring out the irradiation instability. This part will seriously 
mislead readers. 
(6) In some previous works, some polymer additives were introduced and employed in OPV for better 
PCE or stability. What about the unique advantages or new mechanisms of this PZ1 polymer additive? 



Reviewer #2 (Remarks to the Author): 

This manuscript describes the role of PZ1 as a third component in ternary organic solar cells. The 
PM6:BTTT-2Cl:PZ1 device achieved a power conversion efficiency of 15.1% with enhanced all 
photovoltaic parameters compared to control binary device. The authors argued that addition of PZ1 
improves the morphology and thermal stability. Overall, the manuscript is well organized and can be 
accepted for publication in this journal after considering the following issues. 

1. Photovoltaic performances of PM6:PZ1 devices are should be given. 
2. How about the thermal stability of PM6:PZ1 devices? Based on the contents of manuscript, 
PM6:PZ1 devices could demonstrate superb thermal stability. 
3. Did all thermal stability test be carried out in N2-filled glovebox or ambient air? It should be notified 
in the manuscript. 
4. In Figure 2C, the authors argued that the lifetime from 0.36us to 0.34us by adding PZ1 indicates 
less trap-assisted recombination of the PZ1-doped device. However, in Figure 3F, the authors 
mentioned that small change of lifetime in the PZ1-doped device after 24h thermal stress from 0.34 to 
0.36 indicates good thermal stability of the PZ1-doped device. In my opinion, 0.36 and 0.34 within 
~20 ns changes in Figure 2C are almost identical values in the error range of measurements like 
Figure 3F. It is hard to say less recombination in PZ1-doped device by Figure 2C. 


Reviewer #3 (Remarks to the Author): 

In this manuscript, the authors have reported that a small amount of PZ1 additive can enhance both 
the efficiency and stability of the PM6:BTTT-2Cl-based OSCs. The PCE of the device is enhanced from 
13.8% to 15.1%. The device exhibit excellent thermal stability after annealing at 150 oC for 800 h. 
The effect of PZ1 additive on the film morphology, device performance and device thermal stability is 
thoroughly studied. The excellent thermal stability of the OSC device is very interesting and important. 
Recently, the PCE of OSCs has exceeded 17%, so the stability of OSCs become one of the most 
important issue but this issue is rarely solved. This work is a great example to improve the stability of 
OSCs and the results are amazing. Overall, I think this manuscript is very important and that it should 
be accepted by Nat. Commun. after revision. The following is some suggestions for revision of the 
manuscript. 
(1) Why a tiny amount (1%) of PZ1 can greatly improve the stability at high temperature. The authors 
need to give the reason. The thermal stability is the most striking point of this work but the thermal 
stability is not thoroughly studied and discussed. More study on thermal stability, including the reason, 
the evolution of AFM image under thermal annealing, the evolution of GIWAXS data, etc.. 
(2) The thermal behaviors (e.g. DSC curves, phase transition temperature) of the materials 
themselves need to be provided and discussed. This information will be very important to understand 
the thermal morphology stability of the OSC devices. 
(3) Figure 3b, the AFM images of the active layers under thermal annealing. For the device without 
PZ1, the morphology does not change obviously from 0 h to 5 h but change dramatically from 5 h to 
24 h. More data need to be provided to show the evolution of the morphology. Does the large 
crystalline domains suddenly appear or grow gradually? 
(4) The Figure 3c is not in accordance with Figure S16. In Figure S16, The 001 dot indicated with red 
arrow in fig. b and fig. c can also be clearly observed in fig. f. In Fig 3c, the diffraction bands cannot 
be observed in the line cut curves (the purple line at the bottom). The authors need to double check 
the data. 
(5) In the GIWAXS measurements section, the authors claimed that addition of PZ1 results in a 
favorable blend film with more obvious π-π stacking of BTTT-2Cl molecules. However, the PZ1 doped 
devices showed the improved hole-mobility and depressed electron-mobility. These results seem 
contradictory. Please discuss it in the main text.