Peer review of: Charge and exciton dynamics of OLEDs under high voltage nanosecond pulse: towards injection la  (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
1. This is a very interesting paper on charge and exciton dynamics in 3 common polymer LEDs 
(Super Yellow (SY) PPV, F8BT, and PFO) at high pulsed current, whose key findings are claimed to 
“provide essential protocols for material and device design for injection lasing.” As expected, the 
authors did not observed electrically pumped lasing. And while their figures appear to show how 
close, or distant, their devices operate relative to the conditions required for electrically-pumped 
lasing, that gap is still not clear, and should made much clearer in the text and in the figures. 
2. If this work focused on highly efficient small molecule OLEDs, I would be more favorably 
disposed to accept it for publication in Nat. Comm. In its present form, it is difficult to see how 
relevant it is for highly efficient small molecule OLEDs. For example, in the PLEDs many excitons 
might be quenched because of their proximity to the metal cathode and losses to surface plasmons 
at the organic/metal cathode interface. However, it may still prove acceptable if the results could 
be compared to the earlier studies of the Adachi group on small molecule OLEDs at high current 
pulses. 
 
 
Reviewer #2: 
Remarks to the Author: 
Investigation of injecting organic lasers is important, but great challenging. In this article, the 
authors carried out research on the analysis of charge and excitons dynamic in OLETs through 
experiment and theoretical simulation. Based on the SY OLEDs, their results demonstrated that the 
complete excitons and charge dynamics starting from the charge injection to light emission, in a 
wide time range. They proposed theoretical model to simulate the experimental results and found 
good consistence, which further applied to F8BT and PFO polymers. This research is interesting, 
however, the following concerns should be fully addressed. 
For instance, in this article, the authors mentioned that their proposed theoretical model could 
accurately simulate the experimental results, not only for SY OLEDs but also for F8BT and PFO 
polymers. The question is that as for the charge injection, transport, exciton formation and 
annihilation in the device operation also mentioned by authors is very complex and may be 
affected by many factors, such as device structures, interface contact quality, layer thickness, and 
also the molecular solid-state structures, etc., how the theoretical model well match these three 
different polymers, without through elaborate control and modulation, especially the molecular 
structures in solid state. And how this can be concluded to be useful for other organic materials. 
The assumed conditions and approximate condition in the theoretical calculation should be detailed 
discussed in the text. 
Following above the question, the exciton decay pathways and rates are very sensitive to the 
experimental environment, thus it is not accurate to cite the STA, TTA and TPA values (Table 1) 
from literature directly, which may significantly affect the simulation results. 
As tabulated in Table S2, the measured singlet decay rate value of F8BT is 1.1 x 10-9 s-1, while 
the fitted value is totally different as 4.5 x 108 s-1. Same for the values of PFO in Table S3. What’s 
the reason? 
The statement in the last second sentence in “Discussion” section: “The electrical model predicts 
threshold of F8BT and PFO to be around 105 A cm-2 and 3 x 103 A cm-2 respectively, which is 
well below the experimentally achievable current densities (~ 200 A/cm2).” It is confused this 
sentence, below or above? 
As a typical OLED for further pulse investigation, except for the current density and brightness, 
other direct characterization data are also crucial, such as the stability of the devices under high 
current, if there is fast decay for the brightness? 
During the pulse measurement, how about the device performances and working conditions? The 
corresponding I-V curves should provided. 
The authors mentioned “Electrically pumped organic lasers…. the formation of excitons are all 
strongly non-linear and occur on timescales ranging from sub-ns to µs range”. References are 
needed here to prove this point. 
 
 
 
Reviewer #3: 
Remarks to the Author: 
In this manuscript, Viqar A. et al. tried to study the charge and exciton dynamics of organic light-
emitting diodes under high voltage nanosecond pulse: towards injection lasing, a very interesting 
topic in the field. The authors performed short pulse voltage to understand mechanism of exciton 
and charge carrier dynamics of OLEDs at high current density. In this work were well done, and 
the analysis of the results gives physical understanding of devices. Since this study is step for 
organic semiconductor material and device design for injection lasing, this reviewer would like to 
recommend for publication in Nature Communications, after considering the following comments. 
 
Page-2, Abstract do not suffice to understand their role of charge and exciton dynamics of OLED at 
high current density. “in a time scale spanning from the sub-ns to microsecond region” There is no 
any explanation or experimental data for microsecond region. “200 A cm-2 and 2 million cd m-2”, 
Base on author’s conclusion its necessary to applied several K A cm-2 to get lasing. Why didn’t 
Author try very short voltage pulse ~20; In that case they can apply high current to device. 
 
Page 3, “at high current density of 200 A cm-2 with a pulse duration of 300 ns” Prof. C. Adachi 
group reported good approaching for electrical pumping organic lasing base on BSBCz with very 
high current density ca. 3 K A cm-2 using short voltage pulse (400 ns) (Appl. Phys. Lett.106 
093301, 2015 and J. Appl. Phys. 118, 155501, 2015). It is better to compare their work, what is 
superior of this work comparing with Prof. C. Adachi group reported works? Could Author have try 
such high current to super yellow, F8BT and PFO based OLEDs? 
Page 4, “The experimental details of device fabrication and the test setup can be found in 
Supplementary Information” this reviewer couldn’t find any device fabrication information in SI 
Page 12, “The electrical model predicts threshold of F8BT and PFO to be around 105 A cm-2 and 3 
x 103 A cm-2 respectively, which is well below the experimentally achievable current densities (~ 
200 A/cm2)” I am not sure this is correct, Author should check carefully values 
 
Page 12, It is well known, for Laser diodes based on organic semiconductor materials have high 
threshold current densities that require the suppression of various inherent loss processes. One 
way to study such loss processes is to analyze the EQE roll-off in OLEDs. Author should discuss 
EQE roll-off behavior of their device.