Anomalous
Band Gap Behavior in Mixed Sn and Pb Perovskites
Enables Broadening of Absorption Spectrum in Solar Cells
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Abstract
Perovskite-based
solar cells have recently been catapulted to the
cutting edge of thin-film photovoltaic research and development because
of their promise for high-power conversion efficiencies and ease of
fabrication. Two types of generic perovskites compounds have been
used in cell fabrication: either Pb- or Sn-based. Here, we describe
the performance of perovskite solar cells based on alloyed perovskite
solid solutions of methylammonium tin iodide and its lead analogue
(CH<sub>3</sub>NH<sub>3</sub>Sn<sub>1–<i>x</i></sub>Pb<sub><i>x</i></sub>I<sub>3</sub>). We exploit the fact
that, the energy band gaps of the mixed Pb/Sn compounds do not follow
a linear trend (the Vegard’s law) in between these two extremes
of 1.55 and 1.35 eV, respectively, but have narrower bandgap (<1.3
eV), thus extending the light absorption into the near-infrared (∼1,050
nm). A series of solution-processed solid-state photovoltaic devices
using a mixture of organic spiro-OMeTAD/lithium bis(trifluoromethylsulfonyl)imide/pyridinium
additives as hole transport layer were fabricated and studied as a
function of Sn to Pb ratio. Our results show that CH<sub>3</sub>NH<sub>3</sub>Sn<sub>0.5</sub>Pb<sub>0.5</sub>I<sub>3</sub> has the broadest
light absorption and highest short-circuit photocurrent density ∼20
mA cm<sup>–2</sup> (obtained under simulated full sunlight
of 100 mW cm<sup>–2</sup>)