English

• Polar semiconductors, materials possessing both spontaneous polarization and semiconducting properties, have gained substantial attention for optoelectronic devices due to their distinguishing polarization-generated photoelectric effects and cross-coupled functionalities [1-8]. Especially, polar semiconductors manifest a fascinating bulk photovoltaic effect (BPVE) [9], of which the photocurrent exhibits an angle dependence on light polarization, providing a distinctive path to underpin self-powered polarization-sensitive photodetection. Motivated by the captivating features of polar semiconductors, significant attention has been devoted to searching for appropriate candidates in a wide range of materials among inorganic compounds and organic-inorganic hybrids. The past decade has witnessed an explosion of research activities in polar inorganic Ⅲ–Ⅴ compounds, such as opening up the application space among laser diode [10], solar cells [11], optical windows [12], and photodetectors [13-15]. For instance, by combining an appropriate direct bandgap and high optical absorption coefficient (10⁴ cm⁻¹), GaAs show great potential in photodetection in the visible light range possess [16,17]. Despite those persistent and active efforts, those reported polar semiconductors suffer from the rigid structure and costly fabrication, which promotes the exploration of novel polar semiconductors.

  Recently, halide perovskites (HPs) have emerged as a promising category of semiconductors in photoelectronics fields due to their exceptional optoelectronic characteristics including high carrier mobility, long carrier diffusion length, and large light absorption coefficient [18-23]. As a prominent branch, the chemically driven executed dimensional reduction in two-dimensional (2D) HPs bestows them with exceptional structural flexibility and high stability, offering a broader platform for molecular engineering and structure design [24-27]. In particular, the promise of polar 2D HP semiconductors is invigorating [2,28,29], in which the combination of spontaneous polarization and remarkable semiconducting features endow polar 2D HPs significant potential in various photoelectric applications including self-powered photodetection [30], linearly [31-33] and circularly polarization-sensitive detection [34] and ionizing radiation detection [35]. Consequently, intensive efforts have been devoted to searching for rational strategies for obtaining 2D polar HPs. Particularly, the incorporation of aromatic cations with notable dipole moment can result in a considerable rise in dielectric constant and strong delocalization of the positive charge [36-38], introducing large polarization into 2D HPs, and facilitating the design of 2D polar HPs [39]. Unfortunately, those aromatic ammoniums with big steric hindrances exceed the tolerance range, not favoring the formation of a 2D inorganic framework consisting of corner-sharing metal-halide octahedra [40]. Within this portfolio, it is imperative to develop appropriate approaches to incorporate aromatic cations, maintaining an inorganic framework with excellent semiconducting performance.

  Herein, a mixed spacer cation ordering strategy is employed to assembly a polar 2D halide perovskite NMAMAPbBr₄ (NMPB, NMA is N-methylbenzene ammonium, MA is methylammonium) with alternating cation in the interlayer space. Driven by the incorporation of a small cation, MA, the perovskite layer revises the extreme distortion. The asymmetric hydrogen-bonding interactions between ordered mixed-spacer cations and 2D perovskite play a significant role in generating a strong internal electrostatic field, potentially favoring the production of charge carriers and the emergence of noteworthy photoelectric properties, such as the bulk photovoltaic effect (BPVE). Therefore, NMPB manifests excellent self-powered polarization-sensitive detection performance with a considerable polarization-related dichroism ratio up to 1.87. This work paves a new synthetic tool pathway for templating perovskite lattices, overcoming rigid limitations of conventional cations choice, which yields a versatile platform for designing novel polar semiconductors.

  As illustrated in Scheme 1, a mixed spacer cation ordering strategy is employed to assemble a polar 2D halide perovskite NMPB with alternating cation in the interlayer space. The second cation, MA⁺, with little steric effect is introduced to balance the excessive distortion of NPB inorganic layers, which results in the transformation of the perovskite layer from a 2D Pb₇Br₂₄ anionic network with corner- and face-sharing octahedra to a flat 2D PbBr₄ perovskite networks only with corner-sharing octahedra. NPB and NMPB crystallize from saturated HBr solution with stoichiometric amounts of Pb(CH₃COO)₂·3H₂O and corresponding organic amines. The phase purity can be proved by the matching powder X-ray diffraction (PXRD) patterns between the measured data and simulated results (Fig. S1 in Supporting information). The crystal structure analysis indicated that NPB crystallizes in the central space group Pbcn. As depicted in Fig. 1a, the corner-sharing and face-sharing [PbBr₆]⁴⁻ octahedra construct the inorganic frameworks and NMA cations stuff between layers. Rigid aromatics firmly connect the inorganic frameworks by N—H···Br hydrogen bonds (Fig. S3 in Supporting information). This inorganic plane is shown in Fig. 1b. To more accurately analyze the distortion within the inorganic stratum, we measure the equatorial angle (the Pb-Br-Pb angle) of NPB, which changes from 156.7° to 74.8°/80.4° along inorganic layers (Fig. 1c). The bond length distortion (Δd) is determined by calculating the evaluation of the octahedron using the fundamental building unit [PbBr₆]⁴⁻ [37]. Fig. S2 (Supporting information) shows the selected lengths for Δd. The value of Δd of NPB is calculated to be 1.63 × 10⁻³, which is an order of magnitude higher than those reported normal analogous, indicating the distorted organic framework. Different from NPB, NMPB consists of flat 2D PbBr₄ perovskite inorganic networks and organic layers including NMA⁺ and MA⁺, adopting a polar C2 space group at 273 K (Figs. 1d and e). From the equatorial angle of 168.1° (Fig. 1f) and Δd of 2.83 × 10⁻⁴, the inorganic skeleton of NMPB is much flatter, favoring the carrier transport and semiconducting properties. The coordinated rotations of [PbBr₆]⁴⁻ octahedra and the ordering of NMA and MA cations result in opposite shifts of positive and negative charge centers, leading to a parallel alignment of neighboring dipoles and the emergence of an overall non-zero spontaneous polarization in NMPB.

  Scheme 1

  

  Scheme 1.  Mixed spacer cation ordering strategy.

  DownLoad: Full-Size Img PowerPoint

  Figure 1

  

  Figure 1.  (a) The crystal structure of NPB. (b) The inorganic plane of NPB. (c) Bond angles in NPB. (d) Viewing from the a-axis of NMPB. (e) The structure of NMPB to clarity view. (f) Bond angles in NMPB.

  DownLoad: Full-Size Img PowerPoint

  Considering the non-centrosymmetric configurations, NMPB is expected to manifest a considerable SHG signal. The measurement of the SHG response of NMPB powder samples was conducted under 1064 nm laser irradiation, using the Kurtz and Perry method [41]. As exhibited in Fig. 2a, similar to KDP, the SHG response of NMPB gradually increases as the sizes of the power particles increase in the size range of 187−250 µm, which demonstrates that NMPB is type-Ⅰ phase-matchable. In Fig. 2b, a pair of endothermic/exothermic peaks in differential scanning calorimetry (DSC) verify the reversible structure phase transition with a temperature of 431 K, higher than the well-known ferroelectric BaTiO₃ (393 K) and demonstrate NMPB undergoing first-order type phase transition. Because of the high T_c, NMPB maintains stable polar performance in a wide temperature range and broadens the scope of practical applications. The DSC curve of NPB is shown in Fig. S4 (Supporting information). The polar nature of crystals can be demonstrated based on the pyroelectric effect. Pyroelectric measurements were performed along the polar b-axis direction of the single crystals of NMPB. As shown in Fig. 2c, the pyroelectric current shows an evident peak anomaly around the phase transition. As the temperature rises (dT/dt > 0, T < T_c), the initial steady state of electric dipoles has been disturbed, leading to the emergence of an external pyroelectric current to offset the internal built-in electrostatic field caused by electric polarization [42,43]. The P_s value is about 1.3 µC/cm². Upon nearing the phase transition temperature, the polarization diminishes until it reaches a state of complete disappearance beyond the T_c. The result is consistent with the DSC result. The presence of polarization plays a significant role in generating a strong internal electrostatic field, potentially favoring the production of charge carriers and the emergence of noteworthy photoelectric properties, such as the bulk photovoltaic effect (BPVE).

  Figure 2

  

  Figure 2.  (a) Powder SHG measurements at 1064 nm of KDP and NMPB. (b) DSC traces of NMPB in the heating and cooling runs. (c) Temperature-dependent polarization. Insert: Pyroelectric current is collected in the heating runs.

  DownLoad: Full-Size Img PowerPoint

  A comprehensive analysis of the optical and semiconducting features of NMPB single crystals is carried out to evaluate their potential for photodetection. At first, the ultraviolet-visible (UV–vis) optical absorption spectra were recorded via diffuse reflectance spectroscopy at room temperature. NMPB demonstrates an absorption cutoff around 490 nm, indicative of a bandgap measuring 2.40 eV shown in Fig. 3a. The first-principles density functional theory (DFT) was utilized to analyze the electronic structure of NMPB. Fig. 3b illustrates that the conduction band minimum and valence band maximum are at different positions in k-space, revealing the indirect bandgap nature of NMPB. The calculated bandgap value of 2.04 eV is observed to be slightly lower than the experimentally obtained value of 2.40 eV owing to the limitation of the DFT methods. Analysis of the partial density of states of NMPB revealed that the HOMO (highest occupied molecular orbital) is derived from the Pb-6s and Br-4p orbit, while the C-2p orbit plays a dominant role in the LUMO (lowest unoccupied molecular orbital) shown in Figs. 3c and d.

  Figure 3

  

  Figure 3.  (a) UV–vis absorption spectra for NMPB. Inset: calculated bandgap of NMPB. (b) The calculated energy band structure of the NMPB. The calculated (c) LOMO and (d) HUMO of NMPB by DFT.

  DownLoad: Full-Size Img PowerPoint

  Combining polar configuration and excellent semiconducting properties, NMPB is highly expected to manifest an intriguing BPVE and verify through experiment. Initially, we conducted the measurements of current-voltage (Ⅰ-Ⅴ) curves at various light power densities to evaluate the performance of the optoelectronic device. The photocurrent is directly proportional to the incident light intensity, due to the presence of a great number of photoinduced carriers. As illustrated in Fig. 4a, an impressive on/off ratio can be observed revealing the exceptional photoresponsive properties of NMPB single crystal. The intriguing BPVE shows under the power of 78.22 mW/cm² with an open circuit photovoltage of 0.05 V. The remarkable stability of our detectors is evidenced by the massive on/off photoswitching behaviors (Fig. 4b). NMPB possesses polar semiconductors’ BPVE, of which the photocurrent exhibits an angle dependence on light polarization providing a distinctive path to underpin self-powered polarization-sensitive photodetection. Following this, we fabricate polarized sensitive photodetectors based on NMPB single crystals, and the conductive silver adhesives are applied to both ends of the b-axis shown in Fig. 4c. Polarized light are successfully generated by employing a combination of polarizers and half-wave plates. As the half-wave plate rotates, a period is directly reflected on the current-time (I-t) diagram as peaks and troughs (Fig. S5 in Supporting information). A notable photocurrent change angle-dependent attribute is exhibited in the polar diagram (Fig. 4d), mapping out the measured photocurrent at different incident light (405 nm) polarization angles spanning from 0° to 360° under 0 V bias. In detail, the photocurrent experiences the minimum photocurrents (I_min) when polarized light is at 90° and 270°, aligned with the c-axis. Conversely, the maximum photocurrents (I_max) occur at 0° and 180°, parallel to the b-axis. According to the above results, a polarization-dependent dichroism ratio I_max/I_min is determined to be 1.87. The level of the polarization-dependent dichroism ratio is nearly on par with that of those reported 2D perovskites like (isoamylammonium)₂Cs₃Pb₄Br₁₃ (~1.2 at 405 nm) and (n-butylammonium)₂(MA)Pb₂Br₇ (~2.0 at 405 nm), which exhibits the potential of such polar hybrid perovskites polarization sensitive photodection.

  Figure 4

  

  Figure 4.  Photoelectric behaviors for NMPB: (a) Current-voltage chart along the b-axis of NMPB with different incident power (with 405 nm laser). Inset: open-circuit photovoltage of NMPB. (b) The current-time chart shows repetitive on/off switching at zero bias. (c) Schematic structure of the photodetector. (d) Polar plot of the normalized angle-resolved photocurrent as a function of polarization angle measured of NMPB at 405 nm.

  DownLoad: Full-Size Img PowerPoint

  In conclusion, a mixed spacer cation ordering strategy is employed to assemble a polar 2D halide perovskite NMPB with alternating cation in the interlayer space. In the crystal structure of NMPB, the asymmetric hydrogen-bonding interactions between ordered mixed-spacer cations and 2D perovskite layers give rise to a second harmonic generation response and a large polarization of 1.3 µC/cm². It is worth mentioning NMPB with excellent self-powered polarization-sensitive detection performance, showing a considerable polarization-related dichroism ratio up to 1.87. This work offers a new synthetic tool for templating perovskite lattices with controlled properties, overcoming limitations of conventional cations choice.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Qianxi Wang: Writing – original draft. Xiaoqi Li: Writing – review & editing. Fen Zhang: Methodology. Qingyin Wei: Supervision, Methodology, Conceptualization. Zengshan Yue: Supervision. Xiantan Lin: Supervision. Yicong Lv: Supervision. Xitao Liu: Writing – review & editing. Junhua Luo: Supervision.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22193042, 22125110, 22075285, 52473283, 21921001, U21A2069), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (No. ZDBS-LY-SLH024), and the Youth Innovation Promotion of Chinese Academy of Sciences (No. 2020307).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110405.

  
  
• 1. [1]

     M.M. Yang, Z.D. Luo, Z. Mi, et al., Nature 584 (2020) 377–381. doi: 10.1038/s41586-020-2602-4

  2. [2]

     Y. Peng, Y. Zhang, X. Wang, et al., Angew. Chem. 136 (2024) e202319882. doi: 10.1002/ange.202319882

  3. [3]

     J.W. Bennett, I. Grinberg, A.M. Rappe, J. Am. Chem. Soc. 130 (2008) 17409–17412. doi: 10.1021/ja8052249

  4. [4]

     M. Huang, M. Yu, R. Si, et al., New J. Chem. 47 (2023) 783–797. doi: 10.1039/d2nj04889b

  5. [5]

     S. Hiroto, M. Wakita, M. Chujo, Chem. Asian J. 17 (2022) e202200808. doi: 10.1002/asia.202200808

  6. [6]

     K. Fedoruk, D. Drozdowski, M. Maczka, et al., Inorg. Chem. 61 (2022) 15520–15531. doi: 10.1021/acs.inorgchem.2c02206

  7. [7]

     I.H. Choi, M.S. Kim, C. Kang, et al., Appl. Surf. Sci. 571 (2022) 151279. doi: 10.1016/j.apsusc.2021.151279

  8. [8]

     W. Guo, X. Liu, Z. Sun, Innov. Mater. 2 (2024) 100075. doi: 10.59717/j.xinn-mater.2024.100075

  9. [9]

     X. Chen, K. Xu, X. Zhang, et al., Acta Phys. Sin. 72 (2023) 237201. doi: 10.7498/aps.72.20231786

  10. [10]

      P. Li, J. Wang, H. Chen, et al., Chin. Chem. Lett. 33 (2022) 1017–1020. doi: 10.1016/j.cclet.2021.06.084

  11. [11]

      S.P. Philipps, F. Dimroth, A.W. Bett, Chapter I-4-B - High-Efficiency Ⅲ–Ⅴ Multijunction Solar Cells, Academic Press, 2018, pp. 439–472.

  12. [12]

      F. Zafar, A. Iqbal, Proceed. Math. Phys. Engin. Sci. 472 (2016) 20150804. doi: 10.1098/rspa.2015.0804

  13. [13]

      S.L. Tsai, J.S. Wu, H.J. Lin, et al., Phys. Status Solidi. 5 (2008) 2167–2169. doi: 10.1002/pssc.200778510

  14. [14]

      M.D. Thompson, A. Alhodaib, A.P. Craig, et al., Nano Lett. 16 (2016) 182–187. doi: 10.1021/acs.nanolett.5b03449

  15. [15]

      Z. Li, Z. He, C. Xi, et al., Adv. Mater. Technol. 8 (2023) 2202126. doi: 10.1002/admt.202202126

  16. [16]

      Y. Lu, S. Feng, Z. Wu, et al., Nano Energy 47 (2018) 140–149. doi: 10.1109/dsaa.2018.00024

  17. [17]

      S. Debnath, M. Meyyappan, P.K. Giri, ACS Appl. Mater. 16 (2024) 9039–9050. doi: 10.1021/acsami.3c17477

  18. [18]

      J. Zhou, P. Xie, C. Wang, et al., Angew. Chem. Int. Ed. 62 (2023) e202307646. doi: 10.1002/anie.202307646

  19. [19]

      R. Zhao, T. Wu, Y. Hua, et al., Chin. Chem. Lett. 36 (2025) 109587. doi: 10.1016/j.cclet.2024.109587

  20. [20]

      W. Yu, F. Li, T. Huang, et al., Innovation 4 (2023) 100363.

  21. [21]

      J. Wu, R.L. Tang, S.P. Guo, Chin. J. Struct. Chem. 43 (2024) 100291.

  22. [22]

      J. Lim, M. Kober-Czerny, Y.H. Lin, et al., Nat. Commun. 13 (2022) 4201. doi: 10.1038/s41467-022-31569-w

  23. [23]

      L. Chouhan, S. Ghimire, C. Subrahmanyam, et al., Chem. Soc. Rev. 49 (2020) 2869–2885. doi: 10.1039/c9cs00848a

  24. [24]

      T. Zhang, K. Xu, J. Li, et al., Natl. Sci. Rev. 10 (2023) nwac240. doi: 10.1093/nsr/nwac240

  25. [25]

      L. Ye, W.X. Zhang, Chin. J. Struct. Chem. 43 (2024) 100257.

  26. [26]

      L. Mao, W. Ke, L. Pedesseau, et al., J. Am. Chem. Soc. 140 (2018) 3775–3783. doi: 10.1021/jacs.8b00542

  27. [27]

      W. Li, S. Sidhik, B. Traore, et al., Nat. Nanotechnol. 17 (2021) 45–52.

  28. [28]

      S.I. Ohkoshi, K. Nakagawa, K. Imoto, et al., Nat. Chem. 12 (2020) 338–344. doi: 10.1038/s41557-020-0427-2

  29. [29]

      C. Han, J.A. McNulty, A.J. Bradford, et al., Inorg. Chem. 61 (2022) 3230–3239. doi: 10.1021/acs.inorgchem.1c03726

  30. [30]

      Z. Han, W. Fu, Y. Zou, et al., Adv. Mater. 33 (2021) 2003852. doi: 10.1002/adma.202003852

  31. [31]

      C.D. Liu, C.C. Fan, B.D. Liang, et al., ACS Mater. Lett. 5 (2023) 1974–1981. doi: 10.1021/acsmaterialslett.3c00539

  32. [32]

      M. Li, S. Han, B. Teng, et al., Adv. Opt. Mater. 8 (2020) 2000149. doi: 10.1002/adom.202000149

  33. [33]

      L. Li, L. Jin, Y. Zhou, et al., Adv. Opt. Mater. 7 (2019) 1900988. doi: 10.1002/adom.201900988

  34. [34]

      C. Wang, G. Li, Z. Dai, et al., Adv. Funct. Mater. 34 (2024) 2316265. doi: 10.1002/adfm.202316265

  35. [35]

      W. Guo, H. Xu, Q. Fan, et al., Adv. Opt. Mater. 12 (2024) 2303291. doi: 10.1002/adom.202303291

  36. [36]

      P.P. Shi, S.Q. Lu, X.J. Song, et al., J. Am. Chem. Soc. 141 (2019) 18334–18340. doi: 10.1021/jacs.9b10048

  37. [37]

      X. Li, W. Ke, B. Traoré, et al., J. Am. Chem. Soc. 141 (2019) 12880–12890. doi: 10.1021/jacs.9b06398

  38. [38]

      X. Hu, H. Xu, Y. Liu, et al., J. Phys. Chem. Lett. 13 (2022) 6017–6023. doi: 10.1021/acs.jpclett.2c01435

  39. [39]

      H.Y. Zhang, Z.X. Zhang, X.J. Song, et al., J. Am. Chem. Soc. 142 (2020) 20208–20215. doi: 10.1021/jacs.0c10686

  40. [40]

      Y. Liu, J. Guo, H. Zhou, et al., J. Am. Chem. Soc. 146 (2024) 8198–8205. doi: 10.1021/jacs.3c12756

  41. [41]

      S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798–3813. doi: 10.1063/1.1656857

  42. [42]

      C. Wang, N. Tian, T. Ma, et al., Nano Energy 78 (2020) 105371. doi: 10.1016/j.nanoen.2020.105371

  43. [43]

      H. Ryu, S.W. Kim, Small 17 (2019) 1903469.

• 

  Scheme 1  Mixed spacer cation ordering strategy.

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) The crystal structure of NPB. (b) The inorganic plane of NPB. (c) Bond angles in NPB. (d) Viewing from the a-axis of NMPB. (e) The structure of NMPB to clarity view. (f) Bond angles in NMPB.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Powder SHG measurements at 1064 nm of KDP and NMPB. (b) DSC traces of NMPB in the heating and cooling runs. (c) Temperature-dependent polarization. Insert: Pyroelectric current is collected in the heating runs.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) UV–vis absorption spectra for NMPB. Inset: calculated bandgap of NMPB. (b) The calculated energy band structure of the NMPB. The calculated (c) LOMO and (d) HUMO of NMPB by DFT.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Photoelectric behaviors for NMPB: (a) Current-voltage chart along the b-axis of NMPB with different incident power (with 405 nm laser). Inset: open-circuit photovoltage of NMPB. (b) The current-time chart shows repetitive on/off switching at zero bias. (c) Schematic structure of the photodetector. (d) Polar plot of the normalized angle-resolved photocurrent as a function of polarization angle measured of NMPB at 405 nm.

  下载: 全尺寸图片 幻灯片
•