Instantaneous Photocarrier Transport at the Interface in Perovskite Solar Cells to Generate Photovoltage

Round 1

Reviewer 1 Report

The submitted paper “Instantaneous photocarrier transport at the interface in perovskite solar cells to generate photovoltage” by T. Mochizuki, I. Kawayama, M. Tonouchi, Y. Nishihara, M. Chikamatsu , Y. Yoshida and H. Takato is devoted to terahertz emission spectroscopy study of perovskite solar cell structure with built-in electric fileds. The authors found the external bias value at which the terahertz transient changes polarity and attributed this phenomena to the change of electron motion direction. The results look original and can be of interest. I would recommend the authors to write more clearly what is the practical impact of this work: maybe the authors expect to determine the band bending from terahertz emission waveforms or something else? In my opinion modeling of the emission processes is required (like in M.B. Johnston et al. PRB, 2002, v. 65, 165301 ; S.A. Bereznaya et al. Infrared Physics & Technology, 2016, v. 77, 2016, pp. 100-103 etc.) in the future work to extract the parameters of the structure and to better explain the observed features, on the other hand the results of similar works show that the models include many parameters and basically the qualitative picture can be obtained.

The are several misprints in the text: “tansport” in the Title, “TiO2lectron” in line 3.

I think this communication can be accepted for publication in Photonics after the above mentioned minority changes are made.  

Author Response

Thank you for reviewing our manuscript. We are grateful for your important comments. We have revised the manuscript in accordance with the comment as underlined. Specifically, we revised the abstract, introduction and summary to emphasize the practical impact of the work, mentioned the future work to extract the physical parameters of the perovskite solar cells with a proper modelling of the emission processes, and collected misspelling. Here, we would like to resubmit the revised paper for publication in Photonics.

Reviewer 2 Report

The authors are reporting the experimental observations of photo carrier transport of perovskite solar cells by using THz emission spectroscopy technique. The authors explain the demonstrated phase inversion of the THz emission with the Photo-Dember effect. The experimental demonstrations are interesting and would be useful for scholars.

On the other hand, the work is not well organized, creates confusion at first read, leading to a wrong interpretation of the conducted work. A further emphasis on the novelty of the observation must be given. Moreover, even if the introduction provides sufficient background, references are missing in major parts of the discussion. Overall a novel and well conducted work, yet, some points must be further shaped to help readers better understand the outcome and the novel conclusions

I would like to reconsider for publication after a major revision.

I would like to thank all the Authors for their efforts and I kindly ask them to address my comments and suggestions below:

1)            The English language and style are understandable, yet an overall spell check is required. Some phrases and paragraphs are not clear at a first read.

(Not only)Just few to address:

Line 64:” A portion of the dipole radiation originating from this current exits the sample as THz radiation and is observed. ”

Line 197: “will be possible to obtain more detailed knowledge on the carrier dynamics in the perovskite solar cell by measuring with high precision how the terahertz radiation changes to parameters including excitation wavelength, intensity and sample recipe.”

2)            I would suggest the authors to avoid numeric values at the abstract.

Moreover the phrase is misleading and not converging with the findings explained “The waveform of the THz radiation inverted at -12 V of reverse bias”

Since the experimental observation states: in line 119 “The peak intensity decreases with the reverse bias up to -6 V then increases to -10 V.”

3)            Most importantly the, the work is not well organized, creates confusion at first read, leading to a wrong interpretation of the conducted work. As stated in the above comment; the experimental findings are very interesting and novel yet not very clearly addressed throughout the text: I would suggest the authors to give more emphasize on the E-Field emission response with the applied bias changes.

4)            The introduction provides sufficient background yet references are missing in the literature examples given, furthermore, some phrase regarding the proof of concept must be strengthen with citations showing similar observations.

Line 18: “various techniques including ink-based spin-coating and blade coating, which”

Line 38:” hole transport layer should be higher than that of the active layer.”

Line 41: “work functions referred to as band bending, can also block the carriers and lower the voltage.

Line43:” to avoid those blocking and most simply band bending is calculated by solving Poisson’s equation with Anderson’s rule referring to those gaps and work function.”

Line 46:” Therefore the interface can have a dense impurity level or fixed charge.”

Line 62:” these electron-hole pairs behave as electron-hole plasma.”

Line 129:” the polarization inside the sample P and the current J are as follows:”

Line 151:” contribution of trapped charge, fixed charge and mesoporous layer at the interface is not known at present”

Line 193 “Large reverse voltages required to invert the waveform indicate that the photo-Dember effect is the dominant source of the transient current of the samples without external biases”

5)            As the authors mainly conclude the experimental observations hypothesis on the photo demper effect: The photo demper effect must be better explained to help readers from diverse backgrounds. More over the correlation with the electron transport phenomena must be further explained with citations and examples from the literature showing proof similar observations.

6)            Some strong statements need to be better identified. The correlation with their findings must be better explained.

“These electron-hole pairs behave as electron-hole plasma.” This statement must be strengthening with references.”

 “They are separated by an electric field inside the device or a difference of mobility between electrons and holes on a picosecond time scale to produce a transient local current inside.”

7)            In line 76 “Reverse bias stronger than -12 V caused an inversion of waveforms of the THz radiation.”

Inversion of the THz time domain signal implies a phase change under various tensions. I ask the authors to show the phase and amplitude response of the THz emission under different bias potentials.

Moreover, the LTEM method is basically using the material system under investigation as a photoconductive switch, such that the terahertz emission becomes directly proportional with the system geometries and their intrinsic photo carrier and/or electron transport properties. To this concern observing a phase change upon reverse potential is already a trivial observation. Such that one may see the phenomena directly by using a standard PCA. I kindly ask the authors to better explain these phenomena. And also the correlation related to the perovskite layers and the photo demper effect is not very clearly stated. Further comment and explanation needed.

8)            In line 80 “In addition, the contribution of transient currents due to the internal electric field is not negligible and is estimated to be about one order of magnitude weaker than the Photo-Dember effect. Since the sum of the photo-Dember effect and the effect of the interfacial electric field determines the intensity and waveform of terahertz radiation, a more sophisticated analysis will give us information on the internal electric field of lead halide perovskite from terahertz radiation.”

I would suggest the authors to show the dynamics under diverse excitation potentials under a fixed bias.

9)            In correlation with the previous comment:

The authors hypothesize the finding simply addressing to the possible photo demper effect. This is possible yet, as expressed in the text: For semiconductors, the relationship between the electric field A of the terahertz radiation, the polarization inside the sample P, and the current J is already well studied. This way the authors may also reveal the electron transport phenomena related physical quantities (electron number density, plasma frequency, complex conductivity and similar) upon modelling with the given equation to the observed time domain signals. This is one of the major points of using terahertz emission spectroscopy. This is missing in the work.

Further explanation and comment needed.

10)        In Figure 3: I ask the authors to use the common THz nomenclature... such that the time domain signals are plot in terms of the E-field intensity, amplitude is generally used to identify the FFT amplitude and not the E-field intensity. I kindly ask the authors to correct the plot accordingly.

11)          Following the comment 7):  same behavior should have been observed also under positive potentials, comment and further explanation needed. “The peak intensity decreases 119 with the reverse bias up to -6 V then increases to -10 V.” 

And also: In figure 2. I kindly ask the authors to show the I-V characteristics covering the bias voltages used (-12 to 12 V). More over a further comment needed, as the given plot does not coincide directly (correlated yet) with the observations. Further comment is needed.

12)          In line 103:” 103 measurements in a solar simulator.” I kindly ask the authors to give more details on the measurement technique and better identify what is a solar simulator.

Author Response

Thank you for reviewing our manuscript. We are grateful for your important comments. We have revised the manuscript in accordance with the comment as underlined. The details of the collection made are as follows:

1)  Thank you for the comment. We collected the misspelling and collected phrases and paragraphs based on an English proofreading.

2) Thank you for the comment. We revised the abstract to avoid using numeric values and being misleading.

3) Thank you for the comment. We revised the abstract, introduction, discussion, and summary to emphasize on the contribution of the field emission.

4) Thank you for the comment. We added the relevant citations (Number 2,3,8,9,10,25, and 41) to the manuscript.

5) Thank you for the comment. We revised the introduction and discussion to better explain the photo-Dember effect and explain the observed phenomena citing comparable observations.

6) We changed the statements there referring the work of Baranowski et al. (#25) and changed the explanation in the discussion and added more statements to be more understandable.

7) Thank you for the important comment. While the phase response is unavailable due to our technical problems, we added more details of our measurements and discussion on the results comparing with our previous reports to avoid confusion.

8) Thank you for the important comment. Changing the excitation potentials under a fixed bias will be of a great help to get more information about the carrier dynamics. We will do it in the future work because further experiment is currently unavailable, and we added the reason of small number of experimental parameters to the experimental setup and results sections.

9) Thank you for pointing out. One of the major goals of our work is to extract meaningful physical quantities upon modelling and analyzing the THz emission. We added further explanations and current limitations of our situation on the discussion section and the summary section.

10) We added FFT signals to Fig. 3. However, detailed analysis will be done in the future work because of strong influence from response function of the LT-GaAs module and we added explanations on the introduction and discussion to clarify this point.

11) Thank you for the comment. The measurement under positive potentials and I-V measurement covering large bias voltages were not done because of the limitations of the durability of the perovskite solar cell. Usually applying large bias voltage instantly damage the perovskite solar cell and LTEM measurements under reverse bias was ended by the destruction of the sample. We added the comment on this and we will do further experiments after improving the durability of the samples.

12)  Details of the solar simulator was added to the experimental setup section. Thank you.

Reviewer 3 Report

The paper shows photocarrier transport on perovskite solar cells interface using THz emission. They showed that the transport mechanism changed with applied vias and explained the transport mechanism based on the photo-dember effect. Overall, the paper is well described, and the conclusions are consistent with the results obtained. It shows a good method to study charge transport in solar cells and interesting mechanistic results that can optimize solar cell efficiency by changing the applied bias.  

Comments: 

1. Figure 1b looks messy when including the laser pulse representation. Please, consider removing the pulse, only leaving the energy diagram. 
2. The paper includes a clear explanation for the first emission peak in figure 3a; however, there is no explanation for the second peak. Do the authors know the origin of the second peak?. As it´s the opposite signal to the first one, it might be related to charge recombination. Please, add a comment on that in the draft.  
3. The decay shown in the emission curve in figure 3a from 6 to 9 ps has a slope that changes with applied bias. Can this slope relate to electron transfer frequency or charge carrier density?. Please, consider giving a physical interpretation of this slope.
4. Changes in the amplitude area observed at -6V in figure 3 can be related to hole trapping?. Please, consider this in the model and comment. 
5. The role of gold in enhancing the terahertz emission is interesting. What is the role of gold in enhancing the terahertz emission? Please, give a bit more explanation on this role. Besides, the degradation study is not adding much to the history, and I think it is unnecessary to include it here. 
6. It would be nice to pump the sample at different energies to confirm the proposed mechanism. Please, consider including this study in the draft. 

The paper could increase the impact by including some calculations on TiO2, perovskite coupling, and bandgap. So please, consider including this calculation in the draft. 

Author Response

Thank you for reviewing our manuscript. We are grateful for your important comments. We have revised the manuscript in accordance with the comment as underlined. The details of the collection made are as follows:

1) The laser pulse representation was removed from Figure 1(b).

2) The observed waveform is expected to be strongly affected by the response function of the LTEM setup. The second peak and change in slope at 6-9 ps is likely to be derived from the response function. To clarify this point, we added a detailed explanation on LTEM to the experimental setup section.

3) Thank you for pointing out the possibility of contribution of hole trapping. We added some discussions about this.

4) We added a bit more explanations of the possibility of gold to enhance the emission with a reference.

5) I agree with your comment that it would be nice to pump the sample at different energies to confirm the proposed mechanism. We will do it in the future work and added the explanation of the limitation of the current experiment to the experimental section.

6) I agree with your comment that the paper could increase the impact by including some calculations on TiO2, perovskite coupling, and bandgap. Due to our limitation of capacity, calculations with a result that would increase the impact of our work could not be done and we will do it in the future work and briefly mentioned this point in the summary.

Round 2

Reviewer 2 Report

I would like to thank all the Authors for their efforts.

The authors have answered my comments satisfactorily for the parts of major importance.

On the other hand it would have been fruitful if the authors would have been able to add further experimental results on the phase response and results related to the excitation potentials under a fixed bias. The authors prefer to keep some of the further parameters of interest for a possible future publication/investigation. Since this is a matter of preference, I respect the authors decision and I would not insist on the subject. I would recommend publication in its present form.

With my best regards,

P.S. Just a side note that i think may help the authors: regarding the phase response analysis: I would suggest the authors to evaluate the phase data acquired from the FFT analysis(after doing the phase interpolation first) with out out unwrapping the phase!This way the authors may reveal the phase shifts more easily. I hope this would help to solve out some of the technical difficulties they have encountered. 

P.P.S please discard the "P.S," notes, if does not make sense.