Probing molecular and crystalline orientation in solution-processed perovskite solar cells

Wenchao Huang, Fuzhi Huang, Eliot Gann, Yi-Bing Cheng, Christopher R. McNeill

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Abstract

The microstructure of solution-processed organometallic lead halide perovskite
thin fi lms prepared by the “gas-assisted” method is investigated with
synchrotron-based techniques. Using a combination of GIWAXS and NEXAFS
spectroscopy the orientational alignment of CH 3 NH 3 PbI 3 crystallites and
CH 3 NH 3 + cations are separately probed. The GIWAXS results reveal a lack
of preferential orientation of CH 3 NH 3 PbI 3 crystallites in 200–250 nm thick
fi lms prepared on both planar TiO 2 and mesoporous TiO 2 . Relatively high
effi ciencies are observed for device based on such fi lms, with 14.3% achieved
for planar devices and 12% for mesoporous devices suggesting that highly
oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner fi lms (≈60 nm) deposited on planar
TiO 2 (but not on mesoporous TiO 2 ) indicating that the formation of oriented
crystallites is sensitive to the kinetics of solvent evaporation and the underlying
TiO 2 morphology. NEXAFS measurements on all samples found that
CH 3 NH 3 + cations exhibit a random molecular orientation with respect to the
substrate. The lack of any NEXAFS dichroism for the thin CH 3 NH 3 PbI 3 layer
deposited on planar TiO 2 in particular indicates the absence of any preferential
orientation of CH 3 NH 3 + cations within the CH 3 NH 3 PbI 3 unit cell for
as-prepared layers, that is, without any electrical poling.
Original languageEnglish
Pages (from-to)5529 - 5536
Number of pages8
JournalAdvanced Functional Materials
Volume25
Issue number34
DOIs
Publication statusPublished - 2015

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