Using Mechanical Stress to Investigate the Rashba Effect in
Organic−Inorganic Hybrid Perovskites
Haomiao Yu,
†
Qi Zhang,
†
Yaru Zhang,
†
Kai Lu,
‡
Changfeng Han,
†
Yijun Yang,
†
Kai Wang,
†
Xi Wang,
†
Miaosheng Wang,
§
Jia Zhang,
§
and Bin Hu*
,†,§
†
Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University,
Beijing 100044, People’s Republic of China
‡
Wu Han National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei Province
430074, China
§
Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States
*
S
Supporting Information
ABSTRACT: Organic−inorganic hybrid perovskites simultaneously possess strong spin−
orbit coupling (SOC) and structure inversion asymmetry, establishing a Rashba effect to
influence light emission and photovoltaics. Here, we use mechanical bending as a convenient
approach to investigate the Rashba effect through SOC in perovskite (MAPbI
3−x
Cl
x
) films
by elastically deforming grains. It is observed that applying a concave bending can broaden
the line shape of the magnetophotocurrent, increasing the internal magnetic parameter B
0
from 121 to 205 mT, which indicates an enhancement on SOC. Interestingly, the PL lifetime
is found to be enlarged from 9.9 to 14.8 ns under this bending, which suggests that
introducing compressive strain can essentially increase the Rashba effect through SOC,
leading to an increase upon indirect band transition. Furthermore, the PL peak associated
with the Rashba effect is shifted from 776 to 780 nm under this mechanical bending.
Therefore, mechanical bending provides a convenient experimental method to approach the
Rashba effect in hybrid perovskites.
O
rganic−inorganic hybrid perovskites have demonstrated
attractive photovoltaic, light-emitting, lasing, and mag-
neto-optical properties, leading to interesting multifunctional
semiconducting materials.
1−7
Of these attractive properties, the
unique characteristics rely on the Rashba effect caused by
spin− orbit coupling (SOC) under structural inversion
asymmetry.
8−17
Essentially, the Rashba effect can be realized
in structural inversion asymmetry when the spin-up and spin-
down energies become nondegenerate in momentum space
under the influence of SOC. Within the framework of the
Rashba effect, optical absorption is fully allowed due to spin
selection rules, yielding strong absorption coe fficients.
However, upon optical absorption, an excited electron can
flip its spin due to SOC and transition to an opposite spin-
band structure with spin-forbidden recombination, leading to
an enlarged lifetime. As a result, strong optical absorption and
a long carrier lifetime can concurrently occur,
12
providing a
precondition to generate remarkable photovoltaics. On the
other hand, electron−phonon coupling provides a mechanism
of breaking the spin-forbidden rec omb ination, activating
downward electronic transition toward light emission.
17−19
Therefore, the Rashba effect plays an important role in
developing photovoltaic and light-emitting actions in organic−
inorganic hybrid perovskites. In this work, we mechanically
deformed the hybrid perovskite (MAPbI
3−x
Cl
x
) film coated on
a flexible PET substrate to morphologically vary the SOC by
introducing a vertically compressive stress by applying concave
bending. At the same time, we used photoluminescence (PL)
spectra to approach the Rashba effect. The structural
deformation was characterized by X-ray diffraction (XRD)
measurements under compressive bending. The SOC was
monitored by magnetophotocurrent and polarization-depend-
ent photocurrent. Our studies show that applying a vertically
compressive stress can mainly increase the Rashba effect by
introducing grain deformation and enhancing intrinsic SOC.
Consequently, the photoexcited states recombine through an
electron−phonon coupling mechanism with an enlarged
lifetime in organic−inorganic hybrid perovskites.
Here we applied mechanical bending onto a flexible
perovskite device to introduce grain deformation on the
perovskite film. The flexible substrate was bent with a concave
shape, as illustrated in Figure 1a. This concave bending
generated two different effects: tensile stress laterally and
compressive stress vertically. As a result, the observed bending
effects resulted from the combination of laterally tensile stress
and vertically compressive stress. To examine bending-induced
deformation in the perovskite film, out-of-plane XRD measure-
ments on the sample of PET/MAPbI
3−x
Cl
x
were performed.
To make a clear comparison, we present the (110), (310), and
Received: July 8, 2019
Accepted: August 28, 2019
Published: August 28, 2019
Letter
pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2019, 10, 5446−5450
© 2019 American Chemical Society 5446 DOI: 10.1021/acs.jpclett.9b01985
J. Phys. Chem. Lett. 2019, 10, 5446−5450
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