Solvent-Mediated
Crystallization of CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> Films
for Heterojunction Depleted Perovskite
Solar Cells
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Abstract
Organo-lead halide perovskite solar
cells have gained enormous
significance and have now achieved power conversion efficiencies of
∼20%. However, the potential toxicity of lead in these systems
raises environmental concerns for widespread deployment. Here we investigate
solvent effects on the crystallization of the lead-free methylammonium
tin triiodide (CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub>) perovskite
films in a solution growth process. Highly uniform, pinhole-free perovskite
films are obtained from a dimethyl sulfoxide (DMSO) solution via a
transitional SnI<sub>2</sub>·3DMSO intermediate phase. This high-quality
perovskite film enables the realization of heterojunction depleted
solar cells based on mesoporous TiO<sub>2</sub> layer but in the absence
of any hole-transporting material with an unprecedented photocurrent
up to 21 mA cm<sup>–2</sup>. Charge extraction and transient
photovoltage decay measurements reveal high carrier densities in the
CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> perovskite device which
are one order of magnitude larger than CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>-based devices but with comparable recombination lifetimes
in both devices. The relatively high background dark carrier density
of the Sn-based perovskite is responsible for the lower photovoltaic
efficiency in comparison to the Pb-based analogues. These results
provide important progress toward achieving improved perovskite morphology
control in realizing solution-processed highly efficient lead-free
perovskite solar cells