Tailoring of a visible-light-absorbing biaxial ferroelectric towards broadband self-driven photodetection

In terms of strong light-polarization coupling, ferroelectric materials with bulk photovoltaic effects afford a promising avenue for optoelectronic devices. However, due to severe polarization deterioration caused by leakage current of photoexcited carriers, most of ferroelectrics are merely capable of absorbing 8–20% of visible-light spectra. Ferroelectrics with the narrow bandgap (<2.0 eV) are still scarce, hindering their practical applications. Here, we present a lead-iodide hybrid biaxial ferroelectric, (isopentylammonium)2(ethylammonium)2Pb3I10, which shows large spontaneous polarization (~5.2 μC/cm2) and a narrow direct bandgap (~1.80 eV). Particularly, the symmetry breaking of 4/mmmFmm2 species results in its biaxial attributes, which has four equivalent polar directions. Accordingly, exceptional in-plane photovoltaic effects are exploited along the crystallographic [001] and [010] axes directions inside the crystallographic bc-plane. The coupling between ferroelectricity and photovoltaic effects endows great possibility toward self-driven photodetection. This study sheds light on future optoelectronic device applications.

Organic-inorganic hybrid perovskites have emerged as a revolutionary class of electroactive materials in the photoelectric and photovoltaic fields. However, to date, the visible-light-absorbing ferroelectric materials are extremely scarce. In this work, the authors report a new visible-lightabsorbing molecular ferroelectric of (isopentylammonium)2(ethylammonium)2Pb3I10, which possesses the narrowest optical bandgap of molecular ferroelectric systems. It is remarkable that excellent self-powered photoactivities have been demonstrated in a broadband range of 365-670 nm, which are superior than conventional perovskite oxides. In my opinion, this work emphasizes that the importance of hybrid perovskite in photo-ferroelectric and sheds light on their potentials for future photonic device application. Such interesting findings will attract broad interests and wide readerships in the field of material science, chemistry, physics and electronics. Therefore, I strongly recommend the publication of this article in Nature Communications after minor revisions. 1. How the author evaluates thermal stability of this compound, which is quite essential to the subsequent practical applications. It is recommended that thermogravimetric and differential thermal analysis would be a clear indicator in this respect. 2. Compound 1 undergoes obvious symmetry breaking phase transitions from paraelectric phase to ferroelectric phase. The symmetry (elements) variation between its ferroelectric and paraelectric states should be elucidated clearly. 3. Dielectric, ferroelectric and photodetection measurements were performed on crystal samples. If the samples are single crystal samples, which direction is tested? How to grow large crystals in the stable solutions? The detailed process should be provided in supporting information for others to reproduce the crystal growth.
Reviewer #3 (Remarks to the Author): Shiguo et al., reports photon detection with a ferroelectric material with a bandgap smaller than previously reported material. In principle, it is claimed self-driven can be achieved. It should be of interest for Nature communications. However, to be published in Nature communications, the following comments need to be addressed: The manuscript is on ferroelectricity and in-plane bulk photovoltaic effects (BPVEs) in 2D lead-iodide hybrid perovskite [(CH 3 ) 2 CH(CH 2 ) 2 NH 3 ] 2 (CH 3 CH 2 NH 3 ) 2 Pb 3 I 10 . The coupling between ferroelectricity and visible-light-driving in-plane BPVEs is rare. Therefore, these preliminary results are interesting for the applied physics community.
However, the clarity of the manuscript needs to be improved as follows: Q1. The author confirms that the drift mobility is about 0.5 cm 2 V −1 s −1 in the molecular crystal. Is the semiconductor P-type or N-type? What is the mobility of its main carrier? √Answer: Great thanks for the reviewer's positive comments on our manuscript.
Ultraviolet photoelectron spectroscopy (UPS) measurements have been performed to explore the semiconductor properties of our compound (abbreviated as PEPI, based on the 3 rd reviewer's suggestion). As shown in the following Figure 1, UPS result shows that the Fermi level of PEPI deviates from the middle of the bandgap and is close to the valence band, indicating that PEPI should be the P-type semiconductor. [1][2][3][4] That is, hole transport is the main carrier in this semiconductor material. Such results resemble some other inorganic-organic hybrid perovskite, such as CH 3 NH 3 PbI 3 single crystal. 5

Reviewer #2:
Organic-inorganic hybrid perovskites have emerged as a revolutionary class of electroactive materials in the photoelectric and photovoltaic fields. However, to date, the visible-light-absorbing ferroelectric materials are extremely scarce. In this work, the authors report a new visible-light-absorbing molecular ferroelectric of (isopentylammonium) 2 (ethylammonium) 2 Pb 3 I 10 , which possesses the narrowest optical bandgap of molecular ferroelectric systems. It is remarkable that excellent self-powered photoactivities have been demonstrated in a broadband range of 365-670 nm, which are superior than conventional perovskite oxides. In my opinion, this work emphasizes that the importance of hybrid perovskite in photo-ferroelectric and sheds light on their potentials for future photonic device application. Such interesting findings will attract broad interests and wide readerships in the field of material science, chemistry, physics and electronics. Therefore, I strongly recommend the publication of this article in Nature Communications after minor revisions.

Q1.
How the author evaluates thermal stability of this compound, which is quite essential to the subsequent practical applications. It is recommended that thermogravimetric and differential thermal analysis would be a clear indicator in this respect.
√Answer: Great thanks for the reviewer's positive comments on our manuscript.
According to the suggestion, thermogravimetric-differential scanning calorimetry (TG-DSC) analysis was carried out on a Netzsch STA 449C unit within the temperature of 300-1100 K with a heating rate of 10 K min -1 . As shown in the following Figure 3, the TG-DSC curve indicates that PEPI can thermally stable up to 560 K without any thermal decomposition (Please see Supporting Information in revision).

Reviewer #3:
Shiguo et al., reports photon detection with a ferroelectric material with a bandgap smaller than previously reported material. In principle, it is claimed self-driven can be achieved. It should be of interest for Nature communications. However, to be published in Nature communications, the following comments need to be addressed:

Q1.
There are many other materials including perovskite used as photon detectors. The motivation using ferroelectric material needs to be clarified. For self-driven, with photovoltaic effect, photons will generate voltage/current. In this case, any such material can be said with self-powering function? It is not well described in the main text. The material used has phase change with different temperature. Probably such device can be used for temperature sensing? √Answer: Thanks a lot for the reviewer's positive comment on our manuscript.
According to the suggestion, we have added more details about the self-driven photodetection (Please see manuscript in revision). Self-powered photodetection, refers to sensing light signals without an external driving force to separate the photoexcited electron-hole pairs. Traditional self-powered photodetection systems mainly consist of heterojunction and Schottky barriers. Ferroelectrics, capable of instinctive spontaneous polarization, are holding a promise for next-generation self-powered photodetectors. First of all, the large photovoltage achieved in ferroelectrics could afford a controllable power supply in single-phase homogeneous materials, breaking through the bandgap limitation for typical built-in asymmetry systems (Schottky barriers or p-n junctions). 11 Secondly, in contrast to conventional asymmetric systems of Schottky barrier and p-n junction, ferroelectrics don't need complicated interface engineering and fabrication process. 12 Thirdly, spontaneous polarization in ferroelectrics can be modified by electrical field, offering multiple electrically tunable functionalities. 13 In addition, ferroelectric creates an internal electric field at least one order of magnitude higher than that in the p-n junction. 14 By the way, symmetry-breaking ferroelectric phase transition simultaneously accompanies with the change of pyroelectric currents. In principle, almost all the polar materials might be expected to manifest pyroelectric effects and enable response to thermal change. Under external variable radiation of thermal energy, an electric current will be produced in such material, being proportional to the rate of temperature change. 15 As depicted in Figure 3b in manuscript, our crystal exhibits obvious pyroelectric response during the phase transition. That is, such device has potential applications in temperature sensing.  Q3. The use of "1" as the presentation of the material reads very strange, this should be replaced with other words. √Answer: Thanks a lot for the reviewer's useful suggestion. According to the suggestion, we have replaced "1" with "PEPI" to name our compound (isopentylammonium) 2 (ethylammonium) 2 Pb 3 I 10 (Please see the revised manuscript).

Q2. Application for
Q4. The detection speed seems very fast, but seems beyond author's instrument detection limit. It is good to see the exact response time for such a fast detector.