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Observation of an Internal p–n Junction in Pyrite FeS2 Single Crystals: Potential Origin of the Low Open Circuit Voltage in Pyrite Solar Cells
journal contribution
posted on 2020-06-22, 17:33 authored by Bryan Voigt, William Moore, Moumita Maiti, Jeff Walter, Bhaskar Das, Michael Manno, Chris Leighton, Eray S. AydilPyrite FeS2 has long been considered a potentially ideal
photovoltaic material, but solar cells utilizing pyrite exhibit low
open-circuit voltages (VOC) and have failed
to achieve conversion efficiencies >3%. The recent discovery of
a
conductive p-type surface layer on n-type pyrite single crystals raises the intriguing possibility that
the low VOC results from a leaky internal
p–n junction between the surface and interior. Here, we reveal
this internal junction, for the first time, through horizontal electronic
transport measurements on sulfur vacancy (VS)- and Co-doped
n-type pyrite single crystals. We observe a steep increase in resistance
upon cooling heavily VS-doped crystals below ∼200
K, as the dominant charge transport crosses over from interior to
surface conduction. The frequently employed two-resistor equivalent
circuit model for lightly-doped pyrite crystals cannot reproduce this
steep rise, but it can be accounted for, quantitatively, and with
high fidelity, by adding an internal Schottky junction resistance
between the surface and the interior. The average extracted Schottky
barrier height is 320 meV (varying from 130–560 meV), significantly
below expectations from band bending calculations (>750 meV) but
similar
in magnitude to VOC values reported for
pyrite heterojunction solar cells. This internal p–n junction
is thus implicated as the potential origin of the long-standing low-VOC problem in pyrite.
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Keywords
type pyriteLow Open Circuit VoltageSchottky barrier heightopen-circuit voltagestwo-resistor equivalent circuit modelPyrite Solar Cells Pyrite FeS 2pyrite heterojunctionSchottky junction resistancePyrite FeS 2 Single Crystalsphotovoltaic materialtype surface layerV OC valuesV Spyrite exhibitV OC resultscharge transportsurface conductionsulfur vacancyCo-doped n-type pyritetransport measurementsPotential Origin320 meVlightly-doped pyrite crystalsconductive pV OC problemV OC
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