Regulating Circularly Polarized Light Detection via Polar‐Phase Transition in Alternating Chiral‐Achiral Cations Intercalation‐Type Hybrid Perovskites

Abstract Circularly polarized light (CPL) detection has wide applications in many fields, where the anisotropy factor (g Iph) is an important indicator to characterize the CPL detection performance. So far, many materials with high g Iph have been reported, however, the exploration of the regulation of g Iph is still in its infancy. Herein, two novel alternating chiral‐achiral cations intercalation‐type chiral hybrid perovskites (CHPs), named (R/S‐1‐phenylpropylamine)(propylamine)PbBr4 (1‐R/S), exhibit above room‐temperature (RT) polar‐phase transition, which greatly regulates the g Iph value. The g Iph of 1‐R is 0.04 in high‐temperature phase chiral non‐polar (P212121) by applying 5 V bias, interestingly, with the temperature decrease, the g Iph value in low‐temperature phase chiral polar (P21) gradually increases (0.22@360K, 0.40@340K, 0.47@320K), and finally reaches a maximum of 0.5 at RT. Such value is not only the highest among 2D CHPs to date, but presents a 12.5‐fold amplification compared with 0.04. Further, this rare phenomenon should be attributed to the built‐in electric field induced by the polar photovoltaic effect, which sheds light on further obtaining CHPs with large g Iph.


Introduction
Circularly polarized light (CPL) detection has received widespread attention because of its wide applications in quantum optics, drug screening, magnetic recording, and security monitoring. [1]1h,2] The anisotropy factor of photocurrent (g Iph ) is generally used to assess their distinguishability in CPL photodetection devices, which is closely related to their outstanding semiconductor properties and strong chiral optical activity. [3]Therefore, it is critical to pursue direct CPL detectors with high g Iph .
4f] Although there have been many reports of CHPs with large g Iph , however, how to effectively regulate and amplify g Iph values also needs to be vigorously developed.
Changing polarity in CHPs may be an effective strategy for regulating the g Iph values.Structural phase transition is one of the effective ways to regulate polarity. [5]There have been many reported examples of changing polarity through phase transition regulation in non-chiral hybrid perovskites. [6]Considering that chirality is essential for achieving CPL detection, regulating polarity in CHPs is necessary.However, there are few studies of polarity regulation in CHPs, which are mainly in the regulation of ferroelectric properties. [7]7c] However, phase transitions modulating polar properties in chiral perovskites and the regulation of CPL detection g Iph value have not been explored.
Herein, novel chiral alternating cations intercalation (ACI)-type CHPs, named [(R/S)-PPA]PAPbBr 4 (1-R/S, (R/S)-PPA = R/S-1-phenylpropylamine; PA = propylamine), were obtained by using a cation alloying strategy.The differential scanning calorimetry (DSC), temperature-dependent dielectric constant (ʹ), and second harmonic generation (SHG), and in situ variable-temperature powder X-ray diffraction (PXRD) jointly demonstrate that 1-R has a reversible phase transition.Structure analysis shows that 1-R achieves a chiral-nonpolar (P2 1 2 1 2 1 ) to chiral-polar (P2 1 ) change from the high-temperature phase (HTP) to the low-temperature phase (LTP), thereby the generated chiral polar space group brings a obvious polar photovoltaic effect (PPE) and a 0.23 V photovoltage at room temperature (RT).The 1-R single-crystal (SC) device in HTP can only operate with an external bias for photodetection, and a small g Iph of 0.04 is obtained at a 5 V bias.Intriguingly, under the regulation of polar phase transition, the g Iph of 1-R gradually increases with the decrease of temperature (0.22@360K, 0.40@340K, 0.47@320K), and finally reaches 0.5 at self-powered mode in RT, which is the highest g Iph value reported in 2D CHPs to date.Thus, under the regulation of phase transition, 1-R achieved a rare phenomenon of regulation and amplifying g Iph (12.5-fold amplification) for the first time, which shed light on further obtaining CHPs with large g Iph.

Characterization Techniques and Crystal Structures
Microcrystals of 1-R/S were prepared by mixing Pb(OAc) 2 , R/S-1-phenylpropylamine (R/S-PPA), and propylamine (PA) in a hot concentrated HBr solution.After a slow cooling process, bulk crystals (1-R, 7 × 4 × 3 mm 3 ) were obtained (see inset in Figure 1a).The PXRDs show that 1-R/S matches well with the simulated diffraction peaks, indicating its phase purity (Figure S1, Supporting Information).Furthermore, 1 exhibits a high thermal stability up to 520 K, which is evidenced by the thermogravimetric analysis (TGA) curve (Figure 1a).6c] Moreover, the phase transition of 1-R is also confirmed by the significant anomaly around T c in its ʹ measurement (Figure 1c).For convenience, we refer to the phase below T c as the LTP and the phase above T c as the HTP.Besides, a distinct SHG intensity peak in RT is 0.92 times that of KDP (see inset in Figure 1d), emphasizing that 1 belongs to a non-centrosymmetric space group in LTP.6a,b] Furthermore, in situ variable-temperature PXRD measurements also demonstrate a significant reversible phase transition of 1-R near T c (detail see Figure S2, Supporting Information).
To further clarify the phase transition mechanism, the structures of 1-R/S in different phases are determined by single-crystal X-ray diffraction.Since 1-R and 1-S are antisymmetrical isostructural (Figure S3, Supporting Information), 1-R is chosen for depicting the LTP and HTP crystal structure in detail.Structure analysis reveals that 1-R is a 2D monolayered ACI-type CHP, which crystallizes in a monoclinic system with the chiral polar space group of P2 1 in LTP.1-R is composed of the inorganic layers connected by corner-sharing octahedral PbBr 6 and two organic cations (including chiral aromatic PPA + and achiral chain PA + cation) arranged between interlayer space (Figure 2a).Specifically, four adjacent Pb 2+ ions are bridged by Br − ions to form a distorted square on the ab plane, where the bond length of Pb-Br is 2.920-3.103Å and the bond angle of Pb-Br-Pb is 142.074-155.95°,respectively (Figure S4, Supporting Information).Besides the inorganic skeletons, both PPA + and PA + cations are alternately arranged in the interlayer space and opposite in a 1:1 manner along the c-axis (Figure 2a).Further, these cations are firmly anchored between two inorganic perovskite skeletons through N-H⋅⋅⋅Br bonds (Figure S5, Supporting Information), resulting in the formation of 1-R (Figure 2a).
As the temperature increases above T c , 1-R crystallizes in the orthorhombic system of the chiral non-polar space group P2 1 2 1 2 1 (Figure 2b).Although no obvious changes are found in the inorganic skeleton, the thermal ellipsoids of organic cations tended to increase relative to LTP, suggesting the possibility of an ordered-disordered transition (Figure S6, Supporting Information).To further compare the changes of the polar axis in LTP and HTP, the 2 1 helical axes of LTP and HTP are displayed in de-tail (Figure 2b and Figure S7, Supporting Information).It can be seen that the short-chain PA + in HTP (Figure S8a,c, Supporting Information) is closer to a straight chain than the twisted state under LTP (Figure S7a,c, Supporting Information), which may account for its loss of the 2 1 helical axis along a-and c-axis in LTP (Figure 2a,b and Figure S7, Supporting Information).The PA and PA cations are arranged approximately in parallel, which also satisfies the symmetry requirement of non-polar space groups through cation reorientation [6c] (Figure 2a,b).Thus, a symmetry breaking with 222F2 as the Aizu symbol is found, which fits one of the 88 types of ferroelectric symmetry-breaking [6a,b] (Figure 2c).Therefore, the DSC, variable-temperature dielectric and SHG, and structure analysis indicate that 1-R is a potential ferroelectric material, and the crystal symmetry achieves the transition from HTP chiral non-polarity to LTP chiral polarity.

Optical Properties
Considering the crystallization in the chiral space group, the chiral performance of 1 was studied further.3b,4e,8a-c] The PXRDs of the thin films show that the films have a high degree of orientation along the (0 0 l) direction (Figure S9, Supporting Information).4g] These results demonstrate the inherent chiral optical activity of 1.The difference in PL intensity between 1-R and 1-S crystals at 77 K has been measured under 405 nm left-handed ( − ) and right-handed ( + ) CPL excitation with a power of 125 μW (Figure S10a,b, Supporting Information).The obvious intensity difference can be observed between the  − and  + CPL light in 490 nm for the chiral 1-R and 1-S crystals (Figure S10a,b, Supporting Information).To further quantify the degree of the circularly polarized PL, a parameter P was introduced, which is defined as P = (I left − I right )/(I left + I right ), wherein I left and I right represent the intensity of the leftand right-handed circularly polarized PL, respectively.The estimated /P/ of 1-R and 1-S crystals are 16.6% and 18.3%, respectively.8e] Moreover, solid-state diffuse reflectance UV-vis spectrum (DRS) exhibits that the extinction band edge of 1-R is around 420 nm, and the optical band gap is determined to be 3.05 eV.Further, the theoretically calculated electron band structure shows that 1-R features a direct bandgap (Figure S11a, Supporting Information).The calculated bandgap of 1-R is 2.76 eV, which is comparable to the experimental value.The partial density of states shows that the electronic structure and optical bandgap of 1-R are mainly dominated by the inorganic skeleton derived from the Pb-6p and Br-4p orbitals, which is further supported by the charge density distribution of VBM and CBM [9] (Figure S11b,c, Supporting Information).These characteristics, such as chiral, polar, and di-rect bandgap semiconductors, suggest that 1-R is suitable for CPL detection. [10]

CPL Detection and Regulation of g Iph
The polar axis of 1-R in LTP is on the b-axis, which is verified by the electron polarization value of 2.291 nC cm −2 calculated by the point charge model along the b-axis [11] (Figure S12, Supporting Information).So the Ag/ 1-R SC /Ag electrode devices formed along the b-axis were used to probe the possible influence of polarity on their optoelectronic properties (Figure 4a).Firstly, the photoconductivity of 1-R in the HTP phase was investigated, and the device temperature was raised to 380 K (> T c ) by a variable temperature heating stage.Figure S13, Supporting Information exhibits the current-voltage (I-V) curves of the 1-R device in the dark and under 405 nm light illumination.1-R presents a relatively high dark current of 0.38 nA at 10 V bias.As the optical power density increases, the photocurrent increases to 4.68 nA at 682.16 mW cm −2 with a switching ratio of 12.The inability to measure the current-time (I--t) curve at 0 V bias indicates that 1-R cannot achieve self-powered mode in HTP.Therefore, 5 V bias is added, and the results exhibit that the current increases as the optical power density increases (Figure S14, Supporting Information).Further, the CPL detection of 1-R was tested under 5 V bias.As shown in Figure 4e, the photocurrent of RCP is larger than that of LCP under the same light intensity (288 mW cm −2 ), revealing the ability to distinguish between RCP and LCP.Accordingly, the ability to distinguish CPL is evaluated by the g Iph , which is derived from the formula g Iph = 2(I R − I L )/(I R + I L ), where I R and I L represent the photocurrent under 405 nm RCP and LCP irradiation.The g Iph value is calculated to be a relatively small 0.04 at 5 V bias (Figure 4e).For comparison, the 1-R device was subjected to CPL detection at room temperature at 5 V bias with a g Iph of 0.1 (Figure S19c, Supporting Information).These values are smaller than that of many reported CHPs [3b,4] (Table S4, Supporting Information).
To further investigate the effect of phase transition on material photoconductivity and CPL detection, the same 1-R device is progressively cooled to RT.3b,4d] Typically, the PPE is closely associated with the built-in electric fields in CHPs that can spontaneously separate photogenerated electron and hole pairs, giving the material great potential for self-powered CPL detection. [12]Moreover, the 1-R device further tested its photoconductivity and CPL detection at RT.The I-V curves of 1-R are shown in Figure S16, Supporting Information.Interestingly, 1-R exhibits an extremely low dark current (≈3.2 pA) at 10 V bias, which is 118 times smaller than that of HTP.Such a low dark current indicates that 1-R SC has a high quality and a low intrinsic carrier concentration, which facilitates high-performance photodetection.Further, the photocurrent increases sharply under 405 nm light irradiation and eventually rises to 3.68 nA at a light intensity of 576.08 mW cm −2 (Figure S16, Supporting Information).This considerable photocurrent and ultra-low dark current thus generate a large on/off ratio of 1.1 × 10 3 , a value about 100 times larger than that in HTP.Noteworthy, 1-R has significant photocurrent generation at a very weak photocurrent density of 99.5 nW cm −2 (see inset in Figure S16, Supporting Information), which is comparable to those of some excellent 2D perovskite materials, for example, (BA) 2 PbBr 4 (80 nW cm −2 ) and (FPEA) 2 (MA)Pb 2 I 7 (100 nW cm −2 ). [13]Moreover, the 0.23 V photovoltage at RT has also been verified by I-V and the timedependent voltage (V--t) curves (Figure 4b,c), and the photovoltaic can remain stable after multiple on/off cycles (Figure 4c), which prompted us to investigate its self-powered photoelectric detection.
Figure S17, Supporting Information shows the I-t curve of 1-R at different light intensities under 0 V bias.The results present that the current increases gradually with the increase of light intensity, indicating that the material has excellent photoresponse stability under self-powered mode (0 V bias).Furthermore, the self-powered CPL detection performance of 1-R has been studied.As shown in Figure 4d and Figure S18, there is a significant increase in g Iph value as the temperature decreases below T c (0.22@360K, 0.40@340K, 0.47@320K), and finally reaches a maximum g Iph value of 0.5 at RT.3b,4e,f] This phenomenon indicates that the chiral polarity change of 1-R in the structure is regulated by the polar phase transition process, and thus the magnitude of the g Iph value is successfully adjusted.In addition, compared to a g Iph value of 0.04 for HTP, this value has a 12.5-fold amplification to 0.5 for the RT phase, which is also the highest value among 2D CHPs reported so far [3b,4] (Figure 4f, and Table S4, Supporting Information).4d] Therefore, the driving force provided by the PPE is distinct under the excitation of different helicities.4d] This interesting phenomenon shows that under the influence of phase transition (HTP to LTP), not only does the reversible change of the material from chiral-non-polar to chiral polarity be achieved, but regulate and amplify the g Iph value from 0.04 to 0.5 (12.5-fold amplification), which is also an important reference for constructing CHPs with high g Iph .Furthermore, the stability of the 1-R device in a working state can be proved by cycle-repeating experiments at RT and long-term photocurrent stability at 380K @5 V bias (Figure S20, Supporting Information).

Conclusion
In summary, two novel chiral ACI-type perovskite materials 1-R/S were obtained.Many characterization techniques and crystal structure analyses show that 1-R undergoes a chiral non-polar (P2 1 2 1 2 1 ) to chiral polarity (P2 1 ) structural transition process from HTP to LTP, and the generated chiral polar space group brings the PPE and 0.23 V photovoltage in RT phase.Further, the optoelectronic performance measurements show that 1-R can only operate at a certain applied voltage at HTP, and a small g Iph of 0.04 is obtained under a 5 V bias.Surprisingly, with the effect of PPE and photovoltage at self-powered mode, the g Iph value of 1-R gradually increases with the decrease of temperature, and finally reaches 0.5 at RT, which is not only by far the maximum value among 2D CHPs, but also presents a 12.5-fold amplification compared with that of 0.04 in HTP.Thus, a rare g Iph regulation and amplification phenomenon (12.5-fold amplification) was achieved for the first time by 1-R under the regulation of polar phase transition, which also provides a reference for further obtaining CHPs with large g Iph .

Figure 1 .
Figure 1.Phase transition properties of 1-R.a) TGA curve, Inset: bulk crystal of 1-R; b) DSC trace in heating and cooling processes; c) Temperature dependence of the dielectric constant; d) Temperature-dependent variation of SHG optical signals.Inset: Comparison of SHG signals of 1-R and KDP at RT.

Figure 2 .
Figure 2. a,b) Packing structure of 1-R in LTP and HTP, respectively, the yellow lines present the 2 1 helix axes along the c-axis in HTP; c) Equatorial plane projection of point groups and spatial symmetry operations of space groups in LTP and HTP.PbBr 6 octahedra: Medium green.

Figure 3 .
Figure 3. a) Circular dichroism spectrum of 1-R/S films; b) Solid-state diffuse reflectance UV-vis spectrum of 1-R/S.The optical bandgap of 1-R revealed in the inner window.

Figure 4 .
Figure 4. a) The schematic of single crystal device; b) I-V curve in the dark and light illumination with varied intensities at RT; c) V--t curve at RT; d,e) CPL measurement at 0 V-and 5 V-biases under 288 mW cm −2 in LTP and HTP, respectively; f) The anisotropy factors of some reported 2D chiral hybrid perovskites.