Fig. 3C, the Cu electrode of PSCs became
black after MPP tracking, indicating severe
degradation ( 23 ). Only 77.6% of the initial
PCE was retained in control PSCs after MPP
tracking for 600 hours. In PSCs with SST, no
obvious change in the Cu electrode was ob-
served. PSCs with SST retained 90.5% of their
initial PCE after continuous MPP tracking
for 1000 hours under illumination at 55° ± 5°C
(Fig. 3C, with non-normalized data in fig. S20A).
Thermal stability was also greatly improved in
PSCs with SST, and 91.8% of the initial PCE was
retainedafteragingat85°Cfor2200hours
(Fig. 3D), whereas control PSCs retained only
73.2% of the initial PCE after 1000 hours (non-
normalized data in fig. S20B).
In PSCs, ion migration or diffusion is a
major cause for device degradation that
cannot be avoided by encapsulation. Ion
diffusion in PSCs induced degradation of
perovskite and perovskite/PCBM heterojunc-
tions ( 22 ) and also corroded metal electrodes
( 23 ). The color variations of the Cu electrode
in control PSCs confirmed the electrode cor-
rosion, which should be caused by I−ion dif-
fusion during MPP tracking (inset of Fig. 3C).
To verify this point, we removed the Cu elec-
trode of PSCs after MPP tracking (fig. S21 and
movie S1) and measured the exposed PCBM
with SEM-EDX. In control PSCs, almost no
perovskite morphology was distinguished and
a strong I signal appeared in the PCBM layer
(Fig. 4A, with total element spectra in fig. S22A),
indicating the degradation of the perovskite
interface and severe ion diffusion. In PSCs with
SST, perovskite grains were still distinguished
and the I signal was very weak (Fig. 4B, with
total element spectra in fig. S22B), indicative
of a well-protected perovskite interface. We also
used Kelvin probe force microscopy (KPFM) to
investigate the electrical properties of the ex-
posed PCBM layer. In PSCs with SST, the PCBM
layer after MPP tracking showed a surface po-
tential similar to that of the PCBM layer in
fresh devices (Fig. 4C), whereas in control PSCs,
the surface potential showed large fluctuations
and low potential areas (dark regions in Fig. 4C)
appeared randomly after MPP tracking.
We therefore conclude that besides improving
device efficiency, SST also effectively inhibits
ion migration and protects the PCBM layer in
inverted PSCs, thus increasing device stability.
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AKNOWLEDGMENTS
Funding:This work was supported by the National Natural Science
Foundation of China (51903242, 52173161, 61974150, and
62104070), the Key Research Program of Frontier Sciences, the
Chinese Academy of Sciences (CAS) (QYZDB-SSW-JSC047), and
the Fundamental Research Funds for the Central Universities.
Author contributions:J.F. supervised the whole project. J.F. and
X.L. conceived the idea. X.L. designed the experiment and
characterized the devices. W.Z. and X.G. helped fabricate the
perovskite solar cells. C.L. and J.W. helped conduct the SEM and
EDX mapping characterization. X.L. and J.F. discussed and co-
wrote the paper.Competing interests:J.F. and X.L. are inventors
on patent (202110941804.1, China) submitted by East China
Normal University.Data and materials availability:All data are
available in the main text or the supplementary materials.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abl5676
Materials and Methods
Supplementary Text
Fig. S1 to S22
Table S1
Movie S1
22 July 2021; accepted 17 December 2021
10.1126/science.abl56762D MATERIALSSpin-orbitÐdriven ferromagnetism at half moiré
filling in magic-angle twisted bilayer graphene
Jiang-Xiazi Lin^1 , Ya-Hui Zhang^2 , Erin Morissette^1 , Zhi Wang^1 , Song Liu^3 , Daniel Rhodes^3 , K. Watanabe^4 ,
T. Taniguchi^4 , James Hone^3 , J. I. A. Li^1 *Strong electron correlation and spin-orbit coupling (SOC) can have a profound influence on the
electronic properties of materials. We examined their combined influence on a two-dimensional
electronic system at the atomic interface between magic-angle twisted bilayer graphene and a tungsten
diselenide crystal. We found that strong electron correlation within the moiré flatband stabilizes
correlated insulating states at both quarter and half filling, and that SOC transforms these Mott-like
insulators into ferromagnets, as evidenced by a robust anomalous Hall effect with hysteretic switching
behavior. The coupling between spin and valley degrees of freedom could be demonstrated through
control of the magnetic order with an in-plane magnetic field or a perpendicular electric field. Our
findings establish an experimental knob to engineer topological properties of moiré bands in twisted
bilayer graphene and related systems.T
he van der Waals (vdW) moiré structures
are an intriguing platform for exploring
the interplay of correlation, topology, and
broken symmetry in two-dimensional
(2D) electronic systems. The rotational
alignment between two sheets of vdW crystal
gives rise to a flat moiré energy band where
strong Coulomb correlation plays a dominating
role in a rich landscape of emergent quantum
phenomena ( 1 – 7 ). In a graphene moiré struc-ture, breaking theC 2 Tsymmetry was shown to
stabilize spontaneous orbital ferromagnetism
at quarter and three-quarter filling, which is
manifested in a robust anomalous Hall effect
(AHE) with hysteretic switching transitions
( 8 – 11 ). Unlike one- and three-quarter filling,
a potential orbital ferromagnetic state at a
half-filled moiré band would feature a spin-
unpolarizededgemodethatisabletoap-
proximate superconducting pairing along a
ferromagnet/superconductor interface ( 12 ).
Such a construction has been proposed to be
the key to realization of the Majorana mode.
However, an orbital ferromagnet is predicted
to be energetically unfavorable in twisted
graphene structures, owing to the intervalley
Hund’s coupling ( 13 – 16 ).SCIENCEscience.org 28 JANUARY 2022•VOL 375 ISSUE 6579 437
(^1) Department of Physics, Brown University, Providence,
RI 02912, USA.^2 Department of Physics, Harvard University,
Cambridge, MA 02138, USA.^3 Department of Mechanical
Engineering, Columbia University, New York, NY 10027, USA.
(^4) National Institute for Materials Science, 1-1 Namiki, Tsukuba
305-0044, Japan.
*Corresponding author. Email: [email protected]
RESEARCH | REPORTS