Remember me
In this work, a ferroelectric dipole–methylamine lead iodide (MAPbI3) coupled x-ray detector is proposed in which barium titanate (BaTiO3) ferroelectric dipoles are randomly dispersed in the MAPbI3 absorption layer. Ferroelectric dipoles are demonstrated with strong coupling effects with the perovskites, which drive the mesoscopic assembly structure of perovskite grains to form conducting channels for carrier transport. Meantime, ferroelectric dipoles play an excellent role in passivating grain interfaces. Consequently, the introduction of ferroelectric dipoles brings higher carrier mobility lifetime product (μτ), higher sensitivity, better long-term stability, and lower detection limit to MAPbI3 direct-conversion x-ray detectors.
For the sensitivity measurement system shown in Fig. 1(a), a response current can be generated in the open circuit when the x ray is switched on. Figures 1(b) and S1 show the response signals of the open circuit system under different x-ray dose rates and different bias voltages. Some studies attribute the electrical signal to air ionization caused by x rays,25,2625. Y. He, W. Pan, C. Guo, H. Zhang, H. Wei, and B. Yang, Adv. Funct. Mater. 31, 2104880 (2021). https://doi.org/10.1002/adfm.20210488026. Y. Song, L. Li, W. Bi, M. Hao, Y. Kang, A. Wang, Z. Wang, H. Li, X. Li, Y. Fang, D. Yang, and Q. Dong, Research 2020, 5958243. https://doi.org/10.34133/2020/5958243 but ionization may not be the only source of this response current. The ionized gas should be electrically neutral in general when there is no applied bias voltage, and no macroscopic current will be generated due to the absence of the drifting field.2727. I. Langmuir, Phys. Rev. 26, 585 (1925). https://doi.org/10.1103/PhysRev.26.585 However, the response current at 0 V bias is still obvious under exposure by x ray with dose rates of 732 μGyair/s, which is 10−9 A compared to the dark current of 10−11 A.This current signal may originate from the compensation current after x-ray excited electrons emission, which is a common physical phenomenon used for materials characterization such as x-ray photoelectron spectroscopy. As shown in Fig. 1(b), in the measurement system, the x rays excite the electrons from the circuit, draining the electrons from ground to maintain an electrically neutral state, forming a compensation current signal. The characteristic of this compensation current is that as the dose rate increases, the slope of the current to the bias voltage gradually decreases,2828. J. Godlewski, R. Signerski, J. Kalinowski, S. Stizza, and M. Berretoni, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 252, 145 (1994). https://doi.org/10.1080/10587259408038220 indicating saturation of compensation current from the loop under high enough x-ray dose rates.The compensation current signal would increase the nominal sensitivity of the detector. This systematic error can be further amplified if divided by a small electrode area. In this work, to reduce the systematic error of x-ray sensitivity caused by the compensation current response, high dose rates of 100 kVp x-ray irradiation were chosen to reduce the additional error introduced by the compensation current as the slope gradually decreases with the increase in dose rates, and the compensation current of the blank sample at each bias and dose rate was measured and subtracted when calculating the x-ray net current of detector samples.
As shown in Fig. 1(c), time-dependent current responses of the pure MAPbI3 detector and the 5% mole fraction ferroelectric dipole-MAPbI3 coupled detector at 210 V bias are tested, respectively. Current responses at other bias voltages show similar results, which is shown in Figs. S2 and S3. The net current densities vs series dose rates were recorded in Fig. 1(d), where the net current was obtained by subtracting dark current and compensation current from the x-ray photocurrent. The net current densities of both the two devices show strict linear relationships with dose rates. The x-ray sensitivities of the two devices at different applied bias voltages are shown in Fig. 1(e). It is found that the ferroelectric dipole-MAPbI3 coupled x-ray detector shows good repeatability according to 11 individual devices as the relative sensitivity differences (RSDs) of these individual devices are all among 5% (Fig. S4). Under 210 V bias voltage, the average x-ray sensitivity of the ferroelectric dipole-MAPbI3 coupled detector is 1.10×103 μC Gyair−1 cm−2, which is twice as high as that of the detector without dipole. It is worth noting that the theoretical limit sensitivity is just about 5.40×103 μC Gyair−1 cm−2 for ideal MAPbI3 single crystal without electrical injection, which is analyzed in supplementary material in detail. Figure 1(f) shows the current density-bias voltage (IV) curves of MAPbI3 detectors with and without ferroelectric dipoles. It is found that the ferroelectric dipole-MAPbI3 coupled detector not only shows higher x-ray photocurrent but also lower dark current.The ferroelectric dipole-MAPbI3 coupled device also shows reduced parasitic capacitance. As shown in Fig. 1(g), an obvious negative slope can be observed at the beginning of the IV curve from the detector without ferroelectric dipoles, implying a nominally negative resistance. This nominal negative resistance in the time domain corresponds to a negative capacitance, the detailed circuit model can be found in Refs. 2929. F. Ebadi, N. Taghavinia, R. Mohammadpour, A. Hagfeldt, and W. Tress, Nat. Commun. 10, 1574 (2019). https://doi.org/10.1038/s41467-019-09079-z and 3030. E. Ghahremanirad, A. Bou, S. Olyaee, and J. Bisquert, J. Phys. Chem. Lett. 8, 1402 (2017). https://doi.org/10.1021/acs.jpclett.7b00415. Each cavity in the MAPbI3 layer is equivalent to a capacitance. As the bias voltage is applied, these capacitors are first charged, and then discharged to the external circuit with the bias voltage decreasing, resulting in a negative slope at the beginning of the IV curve. The way these capacitors store charge may include the accumulation of x-ray-generated carriers and ions at the interfaces. In comparison, the ferroelectric dipole-MAPbI3 coupled detector hardly shows a negative slope, which is most likely ascribed to higher bulk density and interface passivation. Figure S8 gives the fitting analyses of the limit of detection (LoD), where the LoD of the coupled detector is about one order of magnitude lower than that of the pure MAPbI3 detector. In addition to the excellent performance, the scalable device is extremely crucial for flat-panel x-ray imaging detectors. As shown in Fig. 1(h), a 25 cm2 x-ray photoconductor thick layer based on MAPbI3 incorporated by ferroelectric dipoles, exhibiting uniform and homogenous surface morphology, is easily fabricated by blade coating. Finally, a pixel-by-pixel x-ray scanning image of a USB flash disk is carried by the ferroelectric dipole-MAPbI3 coupled x-ray detector, shown in Fig. 1(i), which exhibits the profile of the external shell and internal disk. Moreover, the golden fingers, small electronic components, and circuits of the internal disk are well recognized in the image.Figures 2(a) and 2(b) show the cross section and surface morphology of the photoconductor layers, respectively. The pure MAPbI3 shows a porous 3D stacking structure, which is consistent with previous works based on blade coating methods,9,25–279. Y. Li, E. Adeagbo, C. Koughia, B. Simonson, R. D. Pettipas, A. Mishchenko, S. M. Arnab, L. Laperrière, G. Belev, A. L. Stevens, S. O. Kasap, and T. L. Kelly, J. Mater. Chem. C 10, 1228 (2022). https://doi.org/10.1039/D1TC05338H25. Y. He, W. Pan, C. Guo, H. Zhang, H. Wei, and B. Yang, Adv. Funct. Mater. 31, 2104880 (2021). https://doi.org/10.1002/adfm.20210488026. Y. Song, L. Li, W. Bi, M. Hao, Y. Kang, A. Wang, Z. Wang, H. Li, X. Li, Y. Fang, D. Yang, and Q. Dong, Research 2020, 5958243. https://doi.org/10.34133/2020/595824327. I. Langmuir, Phys. Rev. 26, 585 (1925). https://doi.org/10.1103/PhysRev.26.585 while the coupled layer promises a denser fiber-like structure oriented along the vertical direction. Meanwhile, it is found that the ferroelectric dipole tends to agglomerate to form small ferroelectric dipole clusters on the surface of MAPbI3 grains while uniformly distributed along the thickness of the film, as shown in Fig. S9.A possible mechanism is proposed here, as shown in Fig. 2(c). In the supersaturated pure MAPbI3 pigments, the environment of the MAPbI3 grains is isotropic, so there is no selectivity in the assembly of the grains during the drying process, thereby a loose three-dimensional porous structure is formed. When ferroelectric dipoles are introduced in the precursor, the polarized electric field of the dipoles will cause the dipoles to agglomerate into clusters under the action of electrostatic force.3131. R. N. Viswanath and S. Ramasamy, Nanostruct. Mater. 8, 155–162 (1997). https://doi.org/10.1016/S0965-9773(97)00004-4 These clusters are attached to the surface of some MAPbI3 grains so that there is a different electric field distribution on each surface of each MAPbI3 particle. For surfaces with the same polarization direction, they tend to repulse and for surfaces with opposite polarization directions, they tend to attract under the Coulomb force. This results in a selective assemble mode between the dipole-attached faces. In the vertical direction, the gravity compels grains to stick together as faces with the same polarization are mutually exclusive that make the stacking unstable. These make the coupling thick film produce different mesoscopic structure.Ferroelectric also improve the crystallinity of the MAPbI3 layer. Figure 2(d) shows the XRD diffraction patterns of the x-ray photoconductor layers based on the pure MAPbI3 and the MAPbI3 coupled with ferroelectric dipoles. After being coupled with ferroelectric dipoles, the sample shows reduced amorphous broad peaks at low angles and full width at half maximum (FWHM) compared to the pure MAPbI3, indicating an increase in crystallinity as well as sub-grain size. In addition, fine structures appeared in the diffraction pattern of the ferroelectric dipole-MAPbI3 layer, which also shows that the introduction of dipoles leads to better crystallinity of the perovskite layer. The fine diffraction peak spectrum calculated according to the stable relaxation perovskite lattice structures in the Materials Project database3232. A. Jain, S. P. Ong, G. Hautier, W. Chen, W. D. Richards, S. Dacek, S. Cholia, D. Gunter, D. Skinner, G. Ceder, and K. A. Persson, APL Mater. 1, 011002 (2013). https://doi.org/10.1063/1.4812323 does not fully correspond to the experimental results, as shown in Fig. S10. The corresponding crystal plane indexes of the main peaks are marked according to Refs. 33–3533. P. Fan, D. Gu, G. X. Liang, J. T. Luo, J. L. Chen, Z. H. Zheng, and D. P. Zhang, Sci. Rep. 6, 29910 (2016). https://doi.org/10.1038/srep2991034. X. Guo, C. McCleese, C. Kolodziej, A. C. S. Samia, Y. Zhao, and C. Burda, Dalton Trans. 45, 3806 (2016). https://doi.org/10.1039/C5DT04420K35. H. Zhang, M. Tao, B. Gao, W. Chen, Q. Li, Q. Xu, and S. Dong, Sci. Rep. 7, 8458 (2017). https://doi.org/10.1038/s41598-017-09109-0.Figures 3(a) and 3(b) give the steady-state photoluminescence (PL) and time-resolved PL spectra of the thick layers with and without ferroelectric dipoles, respectively. It is found that both the two samples show radiative recombination PL peaks with the same position at 1.50 eV, indicating that ferroelectric dipoles do not introduce doping impurities into MAPbI3. The average carrier lifetimes of samples with and without ferroelectric dipoles are 132.5 and 41.2 ns, respectively. Surprisingly, the heterogeneous ferroelectric dipoles do not introduce extra defects but played a significant role in passivation, thereby improving the carrier lifetime. The carrier mobility lifetime products (μτ) derived by Hecht's drifting model of samples with and without ferroelectric dipoles are 9.92 × 10−5 and 2.83 × 10−5 cm2·V−1, respectively, as shown in Fig. 3(c), which shows that the introduction of ferroelectric dipoles increases the drift distance of carriers. Ferroelectric dipoles also contribute to lower trap-state density in MAPbI3 layers. As shown in Fig. S11, assuming that the relative permittivity of MAPbI3 falls between 25.7 and 70,36,3736. J. M. Frost, K. T. Butler, F. Brivio, C. H. Hendon, M. V. Schilfgaarde, and A. Walsh, Nano Lett. 14, 2584 (2014). https://doi.org/10.1021/nl500390f37. Q. Lin, A. Armin, R. C. R. Nagiri, P. L. Burn, and P. Meredith, Nat. Photonics 9, 106 (2015). https://doi.org/10.1038/nphoton.2014.284 the trap-state density ranges of samples with and without ferroelectric dipoles, derived from Mott–Gurney law,3838. N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials ( Oxford University Press, 2012). are 7.07 × 109–2.15 × 1010 and 7.63 × 1011–2.32 × 1012 cm−3, respectively.As shown in Fig. 3(d), the MAPbI3 layer without dipoles has no hysteresis loop, and the slight shift is caused by accumulated charges due to the high resistivity. For the MAPbI3 layer coupled with ferroelectric dipoles, significant hysteresis loops can be observed both in the dark state and under the light. The slight enhancement of the remnant polarization may come from the bulk photovoltaic effect of BaTiO3 ferroelectric dipoles that provides an extra polarization field.39,4039. W. T. H. Koch, R. Munser, W. Ruppel, and P. Würfel, Solid State Commun. 17, 847 (1975). https://doi.org/10.1016/0038-1098(75)90735-840. S. Pal, S. Muthukrishnan, B. Sadhukhan, N. V. Sarath, D. Murali, and P. Murugavel, J. Appl. Phys. 129, 084106 (2021). https://doi.org/10.1063/5.0036488 The polarization field may reduce the electron binding energy of MAPbI3, thereby reducing the energy required for x-ray excitation of electron-hole pairs, so that the current density increases at the same dose rate. As shown in Fig. 3(e), the core binding energy of Pb2+ and I- both move to the lower direction. The binding energy shift of Pb2+ is smaller than that of I- because the 4f orbital of lead ions has a stronger shielding effect. The surface work function of the MAPbI3 layer coupled with ferroelectric dipoles showed an around 0.1 eV shift toward low energy direction and the Fermi edge showed the same shift [Fig. 3(f)]. It seems that the polarization field of dipoles only adds a constant potential to the surface energy band, which helps to reduce the work function of the MAPbI3 layer.The current response of thick polycrystalline perovskite films under constant bias usually exhibits a logarithmic variation in the time domain due to the equivalent capacitance generated by ion accumulation.29,3029. F. Ebadi, N. Taghavinia, R. Mohammadpour, A. Hagfeldt, and W. Tress, Nat. Commun. 10, 1574 (2019). https://doi.org/10.1038/s41467-019-09079-z30. E. Ghahremanirad, A. Bou, S. Olyaee, and J. Bisquert, J. Phys. Chem. Lett. 8, 1402 (2017). https://doi.org/10.1021/acs.jpclett.7b00415 Such a material property is equivalent to a resistance–capacitance parallel circuit, and such a current response is called RC characteristic current. As shown in Figs. 4(a) and 4(b), the detector coupled with ferroelectric dipole shows a relatively insignificant RC characteristic response while the detector without dipoles exhibits obvious RC characteristic current under the same test condition (210 V bias, 100 kVp, 2.22 mGyair/s). The RC characteristic current may be caused by the ion drift current or the change of the injection due to the accumulation of ions at the interface. After 2 h of continuous x-ray irradiation, the response current of the ferroelectric dipole-MAPbI3 coupled detector dropped about 6%. However, the response current of the detector shows a full recovery after the following on-and-off x ray. This indicates that the continuous irradiation of x rays increases the parasitic capacitance of the detector, possibly due to the destruction of chemical bonds and the ionization of atoms by irradiation of x rays, increasing the concentration of ions, which results in the increase in capacitance as the accumulation of extra ions at interfaces driven by the applied bias voltage. For pure MAPbI3 detectors, the RC characteristic current is quite significant both before and after continuous irradiation. After continuous x-ray irradiation of 2 h, the current signal intensity reduced by about 14%, and gradually recovered to about 90% of the steady state before continuous irradiation as the x-rays were turned on and off. These results suggest that ferroelectric dipoles at the interface may block the migration or accumulation of ions due to the additional potential barrier introduced by the polarization field. Figures 4(c) and 4(d) present schematic diagrams of surface/interface potential with and without the dipole polarization field, respectively. For pure MAPbI3, the potential distribution on the interface and the surface increases monotonically with the direction of the electric field and the intrinsic ions drift along the corresponding direction driven by the electric field. After ferroelectric dipoles are introduced, the equipotential surfaces near the dipole on the surface and the interface of MAPbI3 form an approximate saddle surface distribution in space (Figs. S14–S16). This causes the potential distribution to fluctuate, producing several local extrema. For cations, the local minimum of the potential is the potential well, and for anions, the local maximum of the potential is the potential well. These potential wells provide thermodynamic metastable states for ions trapped within, thus reducing the ion migration and accumulation in MAPbI3.Figures 4(e) and 4(f) show the current responses of the MAPbI3 x-ray detector coupled with and without ferroelectric dipoles to the x-ray signal (100 kVp, 2.22 mGyair/s) at the bias of 210 V, respectively. After being stored in a dry nitrogen atmosphere for 4 weeks, the net current decreased by about 5% for the detector coupled with ferroelectric dipoles, while that of the other one dropped by about 15% from its initial value. These results suggest that ferroelectric dipoles can also help improve the dark state stability of MAPbI3 x-ray detectors, which may be attributed to the protection of the perovskite particle surface by chemically inert ferroelectric dipoles.In conclusion, the ferroelectric dipole coupled MAPbI3 x-ray detector shows improved sensitivity and stability compared with the MAPbI3 x-ray detector. X-ray imaging confirms that the coupling detector has good spatial resolution and density resolution. In addition, a neglected system error and a more accurate sensitivity measurement method are proposed.
The National Natural Science Foundation of China (No. 52203342), the Guangdong Basic and Applied Basic Research Foundation (No. 2022A1515011613), the Guangdong Youth Talent Program (No. 2021QN02Y300), and the Young Elite Scientist Sponsorship Program by Cast of China Association for Science and Technology (No. YESS20210285) are acknowledged for financial support.
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Ziyao Zhu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (lead); Methodology (lead); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Huiwen Chen: Investigation (equal); Methodology (equal). Bo Zhao: Investigation (supporting); Methodology (supporting). Weixiong Huang: Investigation (supporting); Methodology (supporting). Qianqian Lin: Conceptualization (supporting); Writing – review & editing (supporting). Xuefeng Yu: Conceptualization (supporting); Funding acquisition (supporting); Writing – review & editing (supporting). Yunlong Li: Conceptualization (lead); Data curation (equal); Formal analysis (lead); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (lead); Supervision (lead); Validation (lead); Visualization (lead); Writing – original draft (equal); Writing – review & editing (lead).
The data that support the findings of this study are available from the corresponding author upon reasonable request.
REFERENCES
1. J. P. Grunz, H. Huflage, J. F. Heidenreich, S. Ergün, M. Petersilka, T. Allmendinger, T. A. Bley, and B. Petritsch, Invest. Radiol. 56, 785 (2021). https://doi.org/10.1097/RLI.0000000000000789, Google ScholarCrossref2. B. Kreisler, Eur. J. Radiol. 149, 110229 (2022). https://doi.org/10.1016/j.ejrad.2022.110229, Google ScholarCrossref3. M. J. Willemink, M. Persson, A. Pourmorteza, N. J. Pelc, and D. Fleischmann, Radiology 289, 293 (2018). https://doi.org/10.1148/radiol.2018172656, Google ScholarCrossref4. T. Flohr, M. Petersilka, A. Henning, S. Ulzheimer, J. Ferda, and B. Schmidt, Phys. Med. 79, 126 (2020). https://doi.org/10.1016/j.ejmp.2020.10.030, Google ScholarCrossref5. T. G. Flohr, K. Stierstorfer, C. Süss, B. Schmidt, A. N. Primak, and C. H. McCollough, Med. Phys. 34, 1712 (2007). https://doi.org/10.1118/1.2722872, Google ScholarCrossref6. H. Wei, Y. Fang, P. Mulligan, W. Chuirazzi, H. H. Fang, C. Wang, B. R. Ecker, Y. Gao, M. A. Loi, L. Cao, and J. Huang, Nat. Photonics 10, 333 (2016). https://doi.org/10.1038/nphoton.2016.41, Google ScholarCrossref, ISI7. R. Zhuang, X. Wang, W. Ma, Y. Wu, X. Chen, L. Tang, H. Zhu, J. Liu, L. Wu, W. Zhou, X. Liu, and Y. Yang, Nat. Photonics 13, 602 (2019). https://doi.org/10.1038/s41566-019-0466-7, Google ScholarCrossref8. J. Zhao, L. Zhao, Y. Deng, X. Xiao, Z. Ni, S. Xu, and J. Huang, Nat. Photonics 14, 612 (2020). https://doi.org/10.1038/s41566-020-0678-x, Google ScholarCrossref9. Y. Li, E. Adeagbo, C. Koughia, B. Simonson, R. D. Pettipas, A. Mishchenko, S. M. Arnab, L. Laperrière, G. Belev, A. L. Stevens, S. O. Kasap, and T. L. Kelly, J. Mater. Chem. C 10, 1228 (2022). https://doi.org/10.1039/D1TC05338H, Google ScholarCrossref10. J. Peng, Y. Xu, F. Yao, H. Huang, R. Li, and Q. Lin, Matter 5, 2251 (2022). https://doi.org/10.1016/j.matt.2022.04.030, Google ScholarCrossref11. Y. Song, L. Li, M. Hao, W. Bi, A. Wang, Y. Kang, H. Li, X. Li, Y. Fang, D. Yang, and Q. Dong, Adv. Mater. 33, 2103078 (2021). https://doi.org/10.1002/adma.202103078, Google ScholarCrossref12. M. Xia, Z. Song, H. Wu, X. Du, X. He, J. Pang, H. Luo, L. Jin, G. Li, G. Niu, and J. Tang, Adv. Funct. Mater. 32, 2110729 (2022). https://doi.org/10.1002/adfm.202110729, Google ScholarCrossref13. W. Zhu, W. Ma, Y. Su, Z. Chen, X. Chen, Y. Ma, L. Bai, W. Xiao, T. Liu, H. Zhu, X. Liu, H. Liu, X. Liu, and Y. Yang, Light: Sci. Appl. 9, 112 (2020). https://doi.org/10.1038/s41377-020-00353-0, Google ScholarCrossref14. M. Overdick, C. Baumer, K. J. Engel, J. Fink, C. Herrmann, H. Kruger, M. Simon, R. Steadman, and G. Zeitler, IEEE Trans. Nucl. Sci. 56, 1800 (2009). https://doi.org/10.1109/TNS.2009.2025041, Google ScholarCrossref15. M. Danielsson, M. Persson, and M. Sjölin, Phys. Med. Biol. 66, 03TR01 (2021). https://doi.org/10.1088/1361-6560/abc5a5, Google ScholarCrossref16. Y. Huang, L. Qiao, Y. Jiang, T. He, R. Long, F. Yang, L. Wang, X. Lei, M. Yuan, and J. Chen, Angew. Chem. Int. Ed. 58, 17834 (2019). https://doi.org/10.1002/anie.201911281, Google ScholarCrossref17. X. Geng, H. Zhang, J. Ren, P. He, P. Zhang, Q. Feng, K. Pan, G. Dun, F. Wang, X. Zheng, H. Tian, D. Xie, Y. Yang, and T. L. Ren, Appl. Phys. Lett. 118, 063506 (2021). https://doi.org/10.1063/5.0040653, Google ScholarScitation, ISI18. J. Peng, C. Q. Xia, Y. Xu, R. Li, L. Cui, J. K. Clegg, L. M. Herz, M. B. Johnston, and Q. Lin, Nat. Commun. 12, 1531 (2021). https://doi.org/10.1038/s41467-021-21805-0, Google ScholarCrossref19. T. Udayabhaskararao, M. Kazes, L. Houben, H. Lin, and D. Oron, Chem. Mater. 29, 1302 (2017). https://doi.org/10.1021/acs.chemmater.6b04841, Google ScholarCrossref20. C. Liu, Y. B. Cheng, and Z. Ge, Chem. Soc. Rev. 49, 1653 (2020). https://doi.org/10.1039/C9CS00711C, Google ScholarCrossref21. H. Hu, M. Singh, X. Wan, J. Tang, C. W. Chu, and G. Li, J. Mater. Chem. A 8, 1578 (2020). https://doi.org/10.1039/C9TA11245F,
Comments (0)