posted on 2024-01-29, 09:30authored byJacob
M. Strain, Gabriella N. Ruiz, Sean T. Roberts, Michael J. Rose
Singlet fission produces a pair of
low-energy spin-triplet excitons
from a single high-energy spin-singlet exciton. While this process
offers the potential to enhance the efficiency of silicon solar cells
by ∼30%, meeting this goal requires overlayer materials that
can efficiently transport triplet excitons to an underlying silicon
substrate. Herein, we demonstrate that the chemical functionalization
of silicon surfaces controls the structure of vapor-deposited thin
films of perylenediimide (PDI) dyes, which are prototypical singlet
fission materials. Using a combination of atomic force microscopy
(AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS),
we find terminating Si(111) with either a thin, polar oxide layer
(SiOx) or with hydrophobic methyl groups
(Si–CH3) alters the structures of the resulting
PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an
“edge-on” orientation with respect to the silicon surface,
films grown on Si–CH3 contain large grains that
prefer to align in a “face-on” manner with respect to
the substrate. This “face-on” orientation is expected
to enhance exciton transport to silicon. Interestingly, we find that
the preferred mode of growth for different PDIs correlates with the
space group associated with bulk crystals of these compounds. While
PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse
crystalline columns, PDIs that inhabit triclinic (P1̅) space groups
afford films comprised of uniform, lamellar PDI domains. The results
highlight that silicon surface functionalization profoundly impacts
PDI thin film growth, and rational selection of a hydrophobic surface
that promotes “face-on” adsorption may improve energy
transfer to silicon.