Science - USA (2020-03-20)

S14). This saturated PCE was mainly limited
by the large interface trap density. If the in-
terface trap density was reduced to that in a
MAPbI 3 thin single crystal (2.0 × 10^15 cm−^3 ),
the PCE could be further enhanced to 25.4%,
which is near the PCE of 26.6% for a trap-free
MAPbI 3 thin-film solar cell (Fig. 4G). Simula-
tion of single-crystal solar cells also gave data
that were a good match to the experimental
data ( 27 ), which again showed that the PCE
of the MAPbI 3 single-crystal solar cell could be
further improved to 26.8% once the interface
trap densities are reduced to that of the bulk
trap density (fig. S15).
Lower–band gap perovskites are being studied
to harvest more sunlight, so we simulated
perovskite thin-film solar cells with band gaps
of 1.50 and 1.47 eV, which correspond to the
compositions of FA0.92MA0.08PbI 3 and FAPbI 3
(if it can be stabilized), respectively ( 34 , 35 ).
Assuming that these materials have the same
trap densities (a bulk trap density of 5.0 ×
1014 cm−^3 and interface trap density of 1.0 ×
1017 cm−^3 ) and capture cross sections as reg-
ular polycrystalline MAPbI 3 thin films, the
devices showed PCEs of 22.5 and 22.8%, respec-
tively (fig. S16). The efficiencies could be further
increased to 27.7 and 28.4%, respectively, when
the trap densities in the thin film are substan-
tially reduced to be the same as those in single
crystals.

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ACKNOWLEDGMENTS

Funding:This work was supported by the Center for Hybrid

Organic Inorganic Semiconductors for Energy (CHOISE), an Energy

Frontier Research Center funded by the Office of Basic Energy

Sciences, Office of Science within the U.S. Department of Energy.

The study of the silicon device was supported by the Solar Energy

Technologies Office (SETO) within the U.S. Department of Energy

under award no. DE-EE0008749. Partial study of the single crystal

growth was supported by Defense Threat Reduction Agency

under Grant HDTRA1170054.Author contributions:J.H., Z.N., and

C.B. designed the experiments. Z.N. and Y.L. synthesized the

perovskite single crystals. Q.J., W.-Q.W., S.C., and B.C. fabricated

the polycrystalline perovskite thin-film solar cells. B.H., Z.Y., and

Z.H. fabricated the silicon solar cells. Z.N. and C.B. carried out

the capacitance measurements for the devices. Z.N. conducted

the solar cell simulations. S.C. and X.D. carried out electron

microscope measurements for the perovskites. J.H. and Z.N. wrote

the paper, and all authors reviewed the paper.Competing

interests:None declared.Data and materials availability:All

data needed to evaluate the conclusions in the paper are present in

the paper or the supplementary materials.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/367/6484/1352/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S16

Tables S1 to S3

References ( 36 – 39 )

4 November 2019; accepted 25 February 2020

10.1126/science.aba0893

REPORTS

◥

GEOMORPHOLOGY

Latitudinal effect of vegetation on erosion rates

identified along western South America

J. Starke, T. A. Ehlers*, M. Schaller

Vegetation influences erosion by stabilizing hillslopes and accelerating weathering, thereby providing a

link between the biosphere and Earth’s surface. Previous studies investigating vegetation effects on

erosion have proved challenging owing to poorly understood interactions between vegetation and other

factors, such as precipitation and surface processes. We address these complexities along 3500

kilometers of the extreme climate and vegetation gradient of the Andean Western Cordillera (6°S

to 36°S latitude) using 86 cosmogenic radionuclide–derived, millennial time scale erosion rates and

multivariate statistics. We identify a bidirectional response to vegetation’s influence on erosion whereby

correlations between vegetation cover and erosion range from negative (dry, sparsely vegetated

settings) to positive (wetter, more vegetated settings). These observations result from competing

interactions between precipitation and vegetation on erosion in each setting.

T

he impact of vegetation on the shape and

evolution of Earth’s surface ranges (for

example) from the microscopic scale of

Mycorrhiza weathering for plant nutri-

tion to macroscopic scales where plants

retard hillslope erosion, stabilize environments

for sediment deposition, and affect precipita-

tion through evapotranspiration, interception,

and leaf phenology ( 1 – 6 ). However, defining the

influence of vegetation on catchment-averaged

erosion rates has proven difficult because of,

among other things, nonlinear interactions be-

tween vegetation type and cover with precipi-

tation, temperature, and solar radiation ( 7 – 10 ).

One approach for disentangling the effects of

vegetation and climate on landscape evolution

requires quantifying catchment erosion rates

over a large range of climate and biogeographic

conditions. The production of cosmogenic radio-

nuclides in the upper ~2 m of Earth’s surface

provides one means for quantifying millennial

time scale catchment-averaged erosion rates

( 11 , 12 ). We investigate the relationships be-

tween catchment-averaged erosion rates with

vegetation cover, climate, and topographic

slope along the climate and ecological gradi-

ent of the Andean Western Cordillera, South

America.

The Andean Western Cordillera between 6°S

and 36°S latitude extends 3500 km (Fig. 1A)

and crosses six climate zones, from hyperarid

to temperate ( 13 ), and four distinct biogeo-

graphic regions (Fig. 1C and 2A) ( 14 ). Cos-

mogenic radionuclide–derived erosion rates

and their controlling factors have been inves-

tigated along the Andean Western Cordillera,

often with conflicting results [e.g., ( 15 , 16 )].

The emphasis of previous studies ranged from

quantification of erosion rates in the vegetation-

limited Atacama Desert ( 16 , 17 ) and sediment

storage in hyperarid environments ( 18 ) to the

rates of canyon incision and hillslope erosion in

the Andean Western Cordillera ( 19 ). Few studies

( 15 , 20 ) have previously looked at systematic

1358 20 MARCH 2020•VOL 367 ISSUE 6484 SCIENCE

Department of Geosciences, University of Tuebingen, 72074,

Germany.

*Correspondingauthor. Email: [email protected]

RESEARCH