P-P orbital interaction: via magnesium isovalent doping enhances optoelectronic properties of halide perovskites

Article abstract of DOI:10.1039/d0cc02150d

p-p orbital interaction through Mg(ii) isovalent doping in methylammonium lead chloride perovskite significantly enhances the electronic properties while not affecting the optical bandgap. This chemical behaviour shows promising applications to optoelectr

Full text of DOI:10.1039/d0cc02150d

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p–p orbital interaction via magnesium isovalent
doping enhances optoelectronic properties of
halide perovskites†
Cite this:DOI: 10.1039/d0cc02150d
Received 24th March 2020,
Accepted 19th November 2020
ab
a
Feng Qian, ‡a Jue Gong, ‡b Mingyu Hu,
Chunyu Ge,
and Jing Feng*a
bc
Nitin P. Padture, *b Yuanyuan Zhou
*
DOI: 10.1039/d0cc02150d
rsc.li/chemcomm
p–p orbital interaction through Mg(II) isovalent doping in methylammo- cause the bonding electrons in Pb–Cl network to be less mobile.
nium lead chloride perovskite significantly enhances the electronic In addition, in contrast to that for the 6s2 electrons of Pb2+, the
properties while not affecting the optical bandgap. This chemical density distribution for the 2p6 electrons of Mg2+ are in much
behaviour shows promising applications to optoelectronic devices.
closer proximity to the nucleus, contributing to tighter chemical
bonding between Mg2+ and Clꢀ, and thus, higher chemical
Owing to the suitable optical bandgap of 2.9 eV, methylammo- stability of the perovskite.15 As such, herein we demonstrate
nium lead trichloride (CH3NH3PbCl3 or MAPbCl3) perovskite is Mg-doping (Fig. 1c) as a promising strategy for improving the
potentially useful in various applications such as deep-color electronic properties of MAPbCl3 while maintaining its bandgap.
light-emitting diodes, lasers, transparent photovoltaics, and Based on this approach, UV photodetectors with enhanced
ultraviolet (UV) photodetectors.1–6 However, the high electro- performance and stability are successfully fabricated.
negativity of Clꢀ can have an adverse effect on carrier mobilities
In order to confirm the Mg-doping mechanism in MAPbCl3, we
of this perovskite material,7 resulting in poor electronic proper- first synthesized Mg-doped MAPbCl3 (denoted as Mg:MAPbCl3)
ties and limiting the device performance. For mitigating this single-crystals using the antisolvent-vapor-assisted crystallization
issue, composition engineering approaches may be used, which method.16 Here, PbCl2, MACl, and MgCl2 are co-dissolved in
entail the replacement of either MA+ or Clꢀ with alternative dimethyl sulfoxide in molar ratios of 1 : 1 : 0.15 to form the
ions.8–13 But such practices inevitably bring about changes in precursor solution. Note that the atomic ratios in the precursor
the key optical bandgap of MAPbCl3. For improving the electro- solution do not necessarily represent the composition of the final
nic properties, while maintaining the wide-bandgap nature, product. Subsequently, the solution was placed in an atmosphere
one can only resort to substitution of the B-site cations (Pb2+) in of dichloromethane antisolvent vapor at room temperature, where
MAPbCl3. As a group IIA element, Mg2+ has a different valence
electron configuration (2s22p6) from Pb2+ (6s2). As shown in
Fig. 1a, when forming chemical bonds with Clꢀ (3s23p6), the p–p
orbital interaction between Mg2+ and Clꢀ is expected to give rise
to more polarizable electrons and thus higher carrier mobilities
in materials because of greater spatial orbital overlap, as com-
pared with the s–p interaction between Pb2+ and Clꢀ shown in
Fig. 1b. Furthermore, due to relativistic effects, Pb 6s orbitals are
stabilized through spatial contraction,14 which will additionally
a Faculty of Materials Science and Engineering, Kunming University of Science and
Technology, Kunming, Yunnan 650093, China. E-mail: jingfeng@kmust.edu.cn
b School of Engineering, Brown University, Providence, Rhode Island, 02912, USA.
E-mail: nitin_padture@brown.edu
c Department of Physics, Hong Kong Baptist University (HKBU), Hong Kong SAR,
China. E-mail: yyzhou@hkbu.edu.hk
Fig. 1 (b) Schematic illustrations of orbital interactions in (a) Mg–Cl and
(b) Pb–Cl bonds. (c) Schematic illustration of Mg-doping in MAPbCl3 via
cation substitution.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc02150d
‡ These authors contributed equally.
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This is because there are negligible effects of s or p orbitals of Mg
on the Pb or Cl orbital energy levels near the bandgap (see Fig. S3,
ESI,† for more details).
X-ray photoelectron spectroscopy (XPS) studies were con-
ducted to elucidate the effects of Mg-doping on the chemical
bonding in the perovskite structure. As shown in Fig. 3a, XPS
survey scans confirm the existence of Pb, Cl, and C elements in
both Mg:MAPbCl3 and pure MAPbCl3 single-crystal samples. By
scrutinizing the binding energy region between 200 and 400 eV,
we observed an intensity peak at B300 eV that can be attributed
to Mg KLL Auger photoelectrons19 (Fig. 3b), which attests the
existence of Mg in Mg:MAPbCl3. Furthermore, XPS resolution
scans in Fig. 3c reveal a larger binding energy of Cl 2p electrons
in Mg:MAPbCl3 than that in pure MAPbCl3, which indicates that
the conceptualized p–p orbital interaction brings forth a tight
bonding in the Mg–Cl network. Meanwhile, we observed smaller
binding energy of Pb 4f electrons in Mg:MAPbCl3 (Fig. 3d), which
is consistent with moderate Pb–Cl connection by Mg-doping.
Also, Cl 2p electrons show a much smaller binding-energy
difference (0.04 eV) between Mg:MAPbCl3 and MAPbCl3 than
Pb 4f electrons, which is attributed to the more polarizable
p–p electrons in Mg–Cl bonds than the electrons in Pb–Cl bonds.
Lateral photoconductor-type electron-only devices (see
Fig. S4, ESI,† for the dimensions of interdigitated gold electrodes)
based on Mg:MAPbCl3 and pure MAPbCl3 single-crystals were
Fig. 2 (a) XRD patterns of Mg:MAPbCl3 and pure MAPbCl3 samples prepared
by grinding the single-crystals into fine powders. Insets are the photographs
of the original single-crystals. (b) UV-vis absorption spectra of Mg:MAPbCl3
and pure MAPbCl3 single-crystals. (c) Calculated DOS of Mg:MAPbCl3 and
pure MAPbCl3.
the Mg:MAPbCl3 single-crystals grow slowly. Pure MAPbCl3 single- fabricated for understanding the effect of Mg-doping on the
crystals were also grown without the addition of MgCl2. A more electronic properties. Under dark conditions, space-charge-
detailed experimental synthesis procedure is included in the ESI.† limited current (SCLC) measurements reveal a much lower
Fig. 2a compares the powder X-ray diffraction (XRD) patterns of trap-filled limit voltage (VTFL) for Mg:MAPbCl3 (0.337 V, Fig. 4b)
ground samples of pure MAPbCl3 and Mg:MAPbCl3 single-crystals with respect to MAPbCl3 single-crystal (1.460 V, Fig. 4a). Corres-
(inset shows photographs). Both samples show well-resolved XRD pondingly the trap density (nTrap) of electrons in Mg:MAPbCl3 is
%
peaks that can be fully indexed based on the cubic Pm3m three times lower than that in pure MAPbCl3 based on the
symmetry. Importantly, the characteristic (200) peak for following equation,20
Mg:MAPbCl3 shifts to higher 2y compared with that for pure
MAPbCl3, confirming a decrease of the lattice parameter (from
2ere0VTFL
ntraps
¼
;
(1)
qd2
a = 0.564 nm to a = 0.562 nm) of the cubic perovskite crystal
structure. This suggests that B2 at% of Pb2+ (ionic radius:
119 pm) ions are substituted by Mg2+ (ionic radius: 72 pm) in
the crystal structure according to the Vegard’s law.17,18 We also
studied the effect of Mg-doping content on the phase and
structure of Mg:MAPbCl3 single-crystals. As shown in Fig. S1
(ESI†), when the amount of Mg2+ precursors (MgCl2) in the
solution is increased, the lattice parameter is further reduced,
indicating incorporation of more Mg2+ in the crystal structure.
However, impurity phases are also identified in the resulting
crystals with higher Mg-doping content, which can be attributed
to phase segregation. In general, we do not observe a noticeable
effect of Mg-doping on the macroscopic shape of the as-grown
MAPbCl3, which is mostly related to the low doping level. The
UV-vis absorption spectra of Mg:MAPbCl3 and pure MAPbCl3 are
compared in Fig. 2b, exhibiting the same band edge at B430 nm.
The corresponding Tauc plots in Fig. S2 (ESI†) reveal the same
bandgap, 2.90 eV, in both Mg:MAPbCl3 and pure MAPbCl3 single-
crystals. We further calculated density of states (DOS) for
Mg:MAPbCl3 and pure MAPbCl3 as shown in Fig. 2c, which again
reveals the same bandgap for Mg:MAPbCl3 and pure MAPbCl3.
Fig. 3 XPS spectra of Mg:MAPbCl3 (red) and pure MAPbCl3 (black) single-
crystals: (a) survey scans and (b) zoomed-in view of survey scans. Higher
resolution scans of: (c) Cl 2p and (d) Pb 4f electrons.
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Fig. 4 SCLC plots for single-crystals: (a) pure MAPbCl3 and (b) Mg:MAPbCl3.
(c) On–off switching performance under 365 nm UV illumination of photo-
detectors based on pure MAPbCl3 (black) and Mg:MAPbCl3 (red) single-crystals.
Fig. 5 On–off switching performance under 450 nm light illumination with
where er, e0, VTFL, q, and d are the relative permittivity of MAPbCl3 different incidence power densities of thin-film photodetectors based on:
(23.9),1,4 vacuum permittivity (8.854 ꢁ 10ꢀ12 F mꢀ1), trap-filled
(a) pure MAPbCl3 and (b) Mg:MAPbCl3. Corresponding performance com-
parisons of the two types of thin-film photodetectors: (c) photocurrent
limit voltage (VTFL), elementary charge, and carrier drift length,
density and (d) photoresponsibility.
respectively. Furthermore, electron mobility of Mg:MAPbCl3 is
determined to be 34% higher than that of pure MAPbCl3 by fitting
the child regimes in Fig. 4a and b using the following equation,4
400 nm (Fig. S6, ESI†), it is evident that the Mg:MAPbCl3 thin film
retains more than 60% of the original absorbance after a period of
9ere0mV2
storage in the ambient air, as compared to only 25% for pure
MAPbCl3. The storage time is 3 h, which is sufficient for this
J ¼
(2)
8d3
stability comparison. In terms of device stability (Fig. S7, ESI†),
where m, V, and J are carrier mobility, voltage, and current density, the Mg:MAPbCl3 thin film photodetector has stabilized photo-
respectively. The photocurrent responses of photoconductor currents above 90% of the original magnitude during a 25 s
devices based on Mg:MAPbCl3 and pure MAPbCl3 single-crystals continuous output under 450 nm illumination at incident power
are compared in Fig. 4c. Upon the illumination of 365 nm UV densities of 0.02 W cmꢀ2 and 0.09 W cmꢀ2. For comparison, less
photons with a power density of 0.0524 mW cmꢀ2, Mg:MAPbCl3 than 80% of the original currents are retained for the pure
single-crystal exhibits a photocurrent of 55 mA, which is much MAPbCl3 device under the same test conditions. The observed
higher than MAPbCl3 (38 mA). This unambiguously confirms the material and device photocurrent stabilities corroborate the tighter
improved optoelectronic device performance due to the Mg-doping. atomic bonding as imparted by structural analyses mentioned
Vertical thin-film photodetector-type devices are further fabri- earlier.
cated by sandwiching the solution-processed halide perovskite
In summary, we have demonstrated Mg-doping as an effec-
layers with n-type and p-type charge extraction layers. The sche- tive method for enhancing electronic properties of MAPbCl3
matic illustrations showing the detailed device structures are perovskites without affecting the critical optical bandgaps,
included in Fig. S5 (ESI†). Near-UV (450 nm) light illumination which shows promising applications to high-performance
is used here. Fig. 5a and b show functional responses of photo- photodetectors. While it is currently challenging to gain more
detector devices based on Mg:MAPbCl3 and pure MAPbCl3 thin insights into the detailed crystal and defect structures using
films, respectively, under varied incident power densities. Fig. 5c high-resolution characterizations such as transmission electron
plots the photocurrent density as a function of incident power microscopy, synchrotron, and spectroscopies primarily due to
density for the photodetectors based on Fig. 5a and b. It is clear the soft nature of the as-synthesized perovskite materials,21 we
that the Mg:MAPbCl3-based photodetector outperforms the envision that future research effort in this direction may even-
MAPbCl3-based photodetector, particularly at higher power den- tually overcome such characterization obstacles and unveil the
sities. Fig. 5d compares the magnitude of the photo-response of exact character of metal doping in perovskites. Finally, this work
Mg:MAPbCl3-based and pure MAPbCl3-based photodetectors. points to a promising direction in influencing the atomic orbital
Once again, the Mg:MAPbCl3-based photodetector performs interaction and chemical bonding through rational metal-
significantly better at high power densities (40.13 W cmꢀ2).
doping for halide perovskites with tailored properties.
Mg-doping also improves the stability of MAPbCl3 perovskite,
This research performed at Kunming University of Science
which is important for the operation of electronic devices. We and Technology is supported by the National Key R&D Program
monitored the evolution of absorption spectra for Mg:MAPbCl3 of China (No. 2019YFB1503200), the National Natural Science
and MAPbCl3 thin films in a controlled environment (80% relative Foundation of China (grant No. 11504146 and No. 51762028),
humidity; 30 1C; air). By tracking the maximum absorbance near and Rare Metal Material Genetic Engineering Project of Yunnan
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9 J. Gong, X. Li, P. Guo, I. Zhang, W. Huang, K. Lu, Y. Cheng,
R. D. Schaller, T. J. Marks and T. Xu, J. Mater. Chem. A, 2019, 7,
13043–13049.
10 L. Wang, G. D. Yuan, R. F. Duan, F. Huang, T. B. Wei, Z. Q. Liu,
J. X. Wang and J. M. Li, AIP Adv., 2016, 6, 045115.
11 D. Amgar, T. Binyamin, V. Uvarov and L. Etgar, Nanoscale, 2018, 10,
6060–6068.
12 S. Draguta, O. Sharia, S. J. Yoon, M. C. Brennan, Y. V. Morozov,
J. S. Manser, P. V. Kamat, W. F. Schneider and M. Kuno,
Nat. Commun., 2017, 8, 200.
Province (grant no. 2018ZE019). The research performed at
Brown University is supported by the US National Science
Foundation (grant No. OIA-1538893).
Conflicts of interest
There are no conflicts to declare.
13 W. A. Dunlap-Shohl, Y. Zhou, N. P. Padture and D. B. Mitzi,
Chem. Rev., 2019, 119, 3193–3295.
14 J. Autschbach, J. Chem. Phys., 2012, 136, 150902.
Notes and references
´
1 T. Matsushima, F. Bencheikh, T. Komino, M. R. Leyden, 15 N. Phung, R. Felix, D. Meggiolaro, A. Al-Ashouri, G. S. e Silva,
¨
A. S. D. Sandanayaka, C. Qin and C. Adachi, Nature, 2019, 572,
C. Hartmann, J. Hidalgo, H. Kobler, E. Mosconi, B. Lai, R. Gunder,
502–506.
M. Li, K.-L. Wang, Z.-K. Wang, K. Nie, E. Handick, R. G. Wilks,
J. A. Marquez, B. Rech, T. Unold, J.-P. Correa-Baena, S. Albrecht,
2 G. Xing, N. Mathews, S. S. Lim, N. Yantara, X. Liu, D. Sabba,
M. Gratzel, S. Mhaisalkar and T. C. Sum, Nat. Mater., 2014, 13,
476–480.
¨
F. D. Angelis, M. Bar and A. Abate, J. Am. Chem. Soc., 2020, 142,
2364–2374.
3 G. Maculan, A. D. Sheikh, A. L. Abdelhady, M. I. Saidaminov, 16 D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin,
M. A. Haque, B. Murali, E. Alarousu, O. F. Mohammed, T. Wu and
O. M. Bakr, J. Phys. Chem. Lett., 2015, 6, 3781–3786.
Y. Chen, S. Hoogland, A. Rothenberger and K. Katsiev, Science,
2015, 347, 519–522.
4 W. Wang, H. Xu, J. Cai, J. Zhu, C. Ni, F. Hong, Z. Fang, F. Xu, S. Cui, 17 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor.
R. Xu, L. Wang, F. Xu and J. Huang, Opt. Express, 2016, 24,
8411–8419.
5 E. Zheng, B. Yuh, G. A. Tosado and Q. Yu, J. Mater. Chem. C, 2017, 5,
3796–3806.
Gen. Crystallogr., 1976, 32, 751–767.
18 Y. Zhou, Z. Zhou, M. Chen, Y. Zong, J. Huang, S. Pang and
N. P. Padture, J. Mater. Chem. A, 2016, 4, 17623–17635.
19 A. Talapatra, S. K. Bandyopadhyay, P. Sen, P. Barat, S. Mukherjee
and M. Mukherjee, Phys. C, 2005, 419, 141–147.
6 D. Liu, C. Yang and R. R. Lunt, Joule, 2018, 2, 1827–1837.
7 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry: Principles 20 R. H. Bube, J. Appl. Phys., 1962, 33, 1733.
of Structure and Reactivity, HarperCollins College Publishers, New York, 21 (a) Y. Zhou, H. Sternlicht and N. P. Padture, Joule, 2019, 3, 641;
4th edn, 1993.
(b) Y. Zhou, H. Zhou, J. Deng, W. Cho and Z. Cai., Matter, 2020,
2, 360; (c) S. Cai and Y. Zhou, Joule, 2020, DOI: 10.1016/
j.joule.2020.11.014.
8 Y. Chen, M. He, J. Peng, Y. Sun and Z. Liang, Adv. Sci., 2016,
3, 1500392.
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